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Fluid mechanics of high velocity fluidised beds Brereton, Clive 1987

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FLUID MECHANICS OF HIGH VELOCITY  FLUIDISED BEDS  by C L I V E BRERETON B . A . S c . ( H o n s . ) , T h e U n i v e r s i t y Of B r i t i s h  Columbia,1982  A THESIS SUBMITTED IN PARTIAL FULFILMENT  OF  THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY  OF GRADUATE  D e p a r t m e n t Of C h e m i c a l  We a c c e p t to  this  thesis  the required  THE UNIVERSITY  Clive  Engineering  as  conforming  standard  OF BRITISH COLUMBIA  December  ©  STUDIES  1987  Brereton,  1987  3 9  In  presenting  degree  at  this  the  thesis in  University of  partial  fulfilment  of  British Columbia, I agree  freely available for reference and study. I further copying  of  department publication  this or of  thesis for by  his  or  her  representatives.  requirements that the  for  an advanced  Library shall make  It  is  granted  by the  understood  that  this thesis for financial gain shall not be allowed without  Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3  it  agree that permission for extensive  scholarly purposes may be  permission.  DE-6(3/81)  the  head of copying  my or  my written  ABSTRACT  This relating beds. dia.  thesis t o the  project  solids  the m a c r o s t r u c t u r e  descriptor,  return.  I t was  solids  generated  exit  found  that  s e p a r a t i o n from  strong internal  and  the  .15  m  important upon  location  abrupt  exits  the c o n v e y i n g  circulation  fluidised m high x  the d e n s i t y p r o f i l e ,  o f the g a s / s o l i d s  inertial  of a 9.3  showed a s t r o n g dependence o f one  macroscopic geometry  aspects  f l u i d , m e c h a n i c s of c i r c u l a t i n g  S t u d i e s of riser  s t u d i e d a number of  the of  the  promoted gas  which  p a t t e r n s and  high  slip  velocities. Microstructural macrostructural capacitance develops  strongly  investigation,  probe,  radially of  the  and the  by  an  non-uniform  radial  t o a more d i l u t e  the needle  density  d e n s i t y decay.  at a l l l o c a t i o n s  present  core-annular  profile The  " i n t e r m i t t e n c y index,"  column w i t h p r o n o u n c e d  structures  of  using a  a t the h i g h e s t d e n s i t i e s .  cluster-like  at  type  i n the  aggregation However,  the base  was lower  or  the  rapidly  flow s l i g h t l y  gave  further  way up  column. This  a  showed how  characterised  clustering  the  i n support  with height causing a gradual  structure,  regions  studies,  number  effects  radially  non-uniform  of m a c r o s c o p i c of e x i t  type,  structure  phenomena.  solids  These  was  used  to  explain  i n c l u d e d the  return location,  secondary  air  -  addition  and  studies,  drawn t o g e t h e r ,  defined  gas  mixing.  t e n t a t i v e l y with  choking,  pneumatic  iii  The  -  r e s u l t s of  allow  fast  respect  transport,  the  various  fluidisation  to  be  to i t s r e l a t i o n s h i p s  and  other  to  fluidisation  regimes. Separate and  the  studies  t r a n s i t i o n to  were p e r f o r m e d turbulent  residence  time d i s t r i b u t i o n was  different  from p l u g  by  a two-zone m o d e l .  be  gradual,  but  flow The  and  developed  turbulent  transport  conditions.  fluidisation. found  could  turbulent  nonetheless zone d i d  t o examine gas  be  t o be  gas  substantially  characterised  t r a n s i t i o n was  a transition, not  The  mixing  although  exist until  well  crudely found a  beyond  to  - iv TABLE OF CONTENTS Page ABSTRACT  i  L I S T OF TABLES  v i i  L I S T OF FIGURES  viii  ACKNOWLEDGEMENTS  xxi  1.  2.  3.  INTRODUCTION  1  1.1  Initial  Concepts  1.2  H i s t o r i c a l and C u r r e n t  1 Industrial  Perspectives  11  1.3  F a s t F l u i d i s a t i o n and D e n s i t y P r o f i l e s  23  1.4  Objectives of the Present  52  Study  APPARATUS  54  2.1  Design Considerations  54  2.2  The R i s e r Column  58  2.3  The G a s - S o l i d s  2.4  Storage  2.5  D a t a Measurement and A c q u i s i t i o n  Separation  and R e c i r c u l a t i o n  System  63  Systems  69  EXPERIMENTAL RESULTS 3.1  D e n s i t y P r o f i l e s and E n t r a i n m e n t R a t e s i n C i r c u l a t i n g F l u i d i s e d Beds - M a c r o s c o p i c Aspects 3.1.1 3.1.2 3.1.3 3.1.4  C o n s i d e r a t i o n s r e g a r d i n g use o f pressure data I n i t i a l s t u d i e s with alumina I n i t i a l s t u d i e s of the e x i t e f f e c t E n t r a n c e e f f e c t s and more on e x i t effects  74 82  82 82 84 95 100  - v Page 3.1.5 3.1.6 3.1.7  3.2  M i c r o s t r u c t u r a l Aspects of F l u i d i s e d Bed 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7  4.  Low v e l o c i t y entrainment t e s t s The imposed pressure drop, a s i g n i f i c a n t i n f l u e n c e on c i r c u l a t i n g bed s t r u c t u r e ? Secondary a i r a d d i t i o n - impact upon c i r c u l a t i n g f l u i d i s e d bed d e n s i t y profiles the  110 116 120  Circulating 124  Scope of m i c r o s t r u c t u r a l study Design c r i t e r i a for a m i c r o s t r u c t u r a l probe Capacitance probes - a general description Development of a capacitance probe f o r t h i s study Response and c a l i b r a t i o n of the c a p a c i t a n c e probe Experimental s t u d i e s with the c a p a c i t a n c e probe Treatment of r e s u l t s from the microstructural investigation  124 124 127 130 137 148 151  DISCUSSION  160  4.1  160  4.2  Microstructural Results 4.1.1 Development of the gas and s o l i d s flow p r o f i l e s 4.1.2 P o s s i b l e mechanisms for s o l i d s motion.. 4.1.3 Nature of the l o c a l s o l i d s flow structure 4.1.4 Analogies with low v e l o c i t y regimes.... 4.1.5 Fast f l u i d i s a t i o n and the s a t u r a t e d carrying capacity 4.1.6 Fast f l u i d i s a t i o n and choking 4.1.7 F a s t - - f l u i d i s a t i o n and c i r c u l a t i n g beds - d e f i n i t i o n s 4.1.8 Scale i n f l u e n c e s 4.1.9 The imposed pressure drop phenomenon... D i s c u s s i o n of the M a c r o s t r u c t u r a l and t h e i r I m p l i c a t i o n s . . . . 4.2.1 4.2.2  160 166 173 183 185 191 195 199 203  Results  Exit effects in c i r c u l a t i n g f l u i d i s e d beds E f f e c t s of secondary a i r i n t r o d u c t i o n and s o l i d s return l o c a t i o n  210 210 219  -  viPage  5.  6.  THE TRANSITION TO TURBULENT FLUIDISATION, A BRIEF EXPERIMENTAL AND CONCEPTUAL STUDY...  224  5.1  Introduction  224  5.2  A Brief  5.3  Experimental  5.4  Results  History  of Turbulent  Design  and D i s c u s s i o n  6.1  Introduction  6.2  The E x p e r i m e n t a l  6.2.3 6.2.4  235 FLUIDISED BED  Data A n a l y s i s  6.4  Results,  253 253  Study  258  General considerations D e s i g n o f s a m p l i n g and i n j e c t i o n systems Detector/sampling system characterisation Riser characterisation  6.3  225 230  AXIAL GAS MIXING IN A CIRCULATING  6-2.1 6.2.2  Fluidisation  Discussion  258 259 270 270 274  and M o d e l l i n g  286  7.  SUMMARY AND CONCLUSIONS  302  8.  RECOMMENDATIONS  305  NOMENCLATURE  307  REFERENCES  311  APPENDIX 1. APPENDIX 2.  APPENDIX 3.  Sample O u t p u t from Time S e r i e s R o u t i n e BMD: 02T  Analysis 322  Estimation of the F l u c t u a t i n g V e l o c i t y Component f o r A i r on t h e C e n t r e l i n e o f a S i n g l e - P h a s e P i p e Flow (Ug = 6 . 5 ra/s, D i a . = .152 m, NTP)  329  Computation of P s e u d o - D i s p e r s i o n C o e f f i c i e n t s from F - C u r v e D a t a  331  -vii  -  LIST OF TABLES Page Table 2.1 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 3.5 Table 5.1  Table 6.1  T r a n s i t time f o r sand p a r t i c l e s 300 mm s e c t i o n of the L - v a l v e  over a 78  P r o p e r t i e s of alumina used i n high velocity f l u i d i s a t i o n studies  85  P r o p e r t i e s of Ottawa sand used i n high velocity f l u i d i s a t i o n studies  96  R e s u l t s from entrainment t e s t s at low velocity  115  Pressure drops over the c i r c u l a t i n g f l u i d i s e d bed loop  118  Techniques used f o r determining l o c a l s o l i d s hold-up and p a r t i c l e v e l o c i t y  126  References f o r t u r b u l e n t f l u i d i s a t i o n s t u d i e s and methods used to i d e n t i f y the turbulent t r a n s i t i o n  232  Test c o n d i t i o n s f o r d i s p e r s i o n measurements i n the c i r c u l a t i n g bed  275  fluidised  - viii  -  LIST OF FIGDRES Page Figure Figure Figure  Figure  Figure  Figure Figure  1.1 1.2 1.3  1.4  1.5  1.6 1.7  Schematic gas-solid  diagram of a v e r t i c a l transport line  Flow r e g i m e s f o r g a s - s o l i d f l o w t o Leung (1980) Photograph of a f a s t through the w a l l  fluidised  2 according 4 bed v i e w e d 8  S l i p v e l o c i t i e s i n high v e l o c i t y f l u i d i s a t i o n a c c o r d i n g t o Y e r u s h a l m i and C a n k u r t (1978)  9  Regime d i a g r a m f o r g a s - s o l i d c o n t a c t i n g a c c o r d i n g t o Grace (1986). Ap i s t h e d i f f e r e n c e between p a r t i c l e and gas densities  12  A t y p i c a l c i r c u l a t i n g bed combustor ( K u l l e n d o r f and A n d e r s s o n , 1985)  16  C o n t r o l o f a c i r c u l a t i n g bed combustor by v a r i a t i o n o f h e a t t r a n s f e r r a t e t o a membrane w a l l . Q represents the t o t a l heat a b s o r p t i o n , H the o v e r a l l heat t r a n s f e r c o e f f i c i e n t above t h e s e c o n d a r y a i r p o r t s , and p c mean s u s p e n s i o n d e n s i t y above t h e s e c o n d a r y a i r p o r t s  18  Heat t r a n s f e r c o e f f i c i e n t s i n f a s t f l u i d i s a t i o n as a f u n c t i o n o f s u s p e n s i o n d e n s i t y and t e m p e r a t u r e ( K o b r o and B r e r e t o n , 1985)  22  Density p r o f i l e s f o r fast ( L i and Kwauk, 1980)  25  Q  t  n  e  s e  Figure  Figure Figure  Figure  1.8  1.9 1.10  1.11  fluidisation  The L i and Kwauk (1980) model f o r d e n s i t y p r o f i l e s i n f a s t f l u i d i s e d beds  28  C o r r e l a t i o n s f o r parameters i n the L i e t a l . (1982) d e n s i t y d i s t r i b u t i o n model  29  -  ix  Page  Figure  Figure  Figure  Figure  Figure  Figure  Figure  Figure  1.12  1.13  1.14  1.15  1.16  1.17  2.1  2.2  E f f e c t i v e c l u s t e r diameters i n high v e l o c i t y f l u i d i s e d beds (Yerushalmi e t a l . , 1978)  36  P l o t o f v o i d a g e v e r s u s gas v e l o c i t y f o r f l u i d c r a c k i n g c a t a l y s t o v e r a wide range of gas v e l o c i t y showing r e g i m e t r a n s i t i o n s ( A v i d a n , 1980)  38  Flow r e g i m e s o b s e r v e d i n v e r t i c a l g a s l i q u i d f l o w (Soo, 1982), and a f l o w p a t t e r n map ( H e w i t t and R o b e r t s , 1969)  40  R a d i a l s o l i d s f l u x , v e l o c i t y and p r o f i l e s i n a r i s e r according to e t a l . (1980)  42  density Bierl  C a t a l y s t d e n s i t y d i s t r i b u t i o n s over c r o s s - s e c t i o n of a c o m m e r c i a l r i s e r (Schuurmans, 1980)  the 46  Gas v e l o c i t y p r o f i l e , c a t a l y s t d e n s i t y d i s t r i b u t i o n and s o l i d s f l u x p r o f i l e measured by van B r e u g e l et a J . (1969-70) i n a 0.3 m d i a . r i s e r . . .TT.TT  48  S c h e m a t i c of the c i r c u l a t i n g f l u i d i s e d bed t e s t r i g . Numbers d e s i g n a t e p r i n c i p a l p r e s s u r e measurement l o c a t i o n s f o r l o o p p r e s s u r e measurement s t u d i e s  57  Detail the  of  the  secondary  a i r nozzles  for  r i s e r column  60  Figure  2.3  D i a g r a m of  Figure  2.4  Detail  Figure  2.5  Figure  2.6  Primary cyclone d e t a i l , a l l dimensions i n mm Secondary cyclone d e t a i l , a l l dimensions i n mm  Figure Figure  2.7 2.8  pressure  of a column  Tertiary i n mm  cyclone  tap/probe port support  detail  bracket  62 64  66 67  a l l dimensions  Modified b u t t e r f l y valve for s o l i d s c i r c u l a t i o n r a t e measurement, d i m e n s i o n s i n mm  68  72  -  x Page  Figure  Figure  Figure  Figure Figure  Figure  2.9  2.10  2.11  3.1 3.2  3.3  A e r a t i o n p o i n t s on t h e L - v a l v e and a t y p i c a l o p e r a t i n g mode from K n o w l t o n and Hirsan (1978). D i m e n s i o n s i n mm  75  A t y p i c a l pressure trace f o r build-up of a l u m i n a on t h e b u t t e r f l y v a l v e f o r a c i r c u l a t i o n r a t e measurement. A straight l i n e i s s k e t c h e d t o show t h e l i n e a r i t y of t h e b u i l d - u p  77  Manifold construction f o r d i f f e r e n t i a l p r e s s u r e measurements showing how 14 r i s e r locations are manifolded  80  Scanning e l e c t r o n micrograph of alumina particles. M a g n i f i c a t i o n 400 x  87  C i r c u l a t i n g f l u i d i s e d bed as i n i t i a l l y constructed. A l u m i n a c o u l d be c i r c u l a t e d i n t h i s short L-valve design with s i n g l e point aeration  88  Longitudinal density distributions f o r a l u m i n a i n a c i r c u l a t i n g f l u i d i s e d bed, Ug = 5.4 m/s, G = 18, 41, and 95 kg/m s  90  s  2  Figure  3.4  Longitudinal density distributions for a l u m i n a i n a c i r c u l a t i n g f l u i d i s e d bed, U = 4.3 m/s, G = 21 and 42 kg/m s.... 2  g  Figure  3.5  s  Longitudinal density distribution f o r a l u m i n a i n a c i r c u l a t i n g bed, Ug = 2.6 m/s, G = 25 k g / m s  93  Longitudinal density distributions f o r a l u m i n a i n a c i r c u l a t i n g bed, G a p p r o x i m a t e l y c o n s t a n t (20 k g / m s ) , U = 2.6, 4.3 and 5.4 m/s  94  2  s  Figure  3.6  92  s  2  g  Figure  Figure  3.7  3.8  Longitudinal density p r o f i l e s obtained f o r sand i n a c i r c u l a t i n g bed as t h e base a p p e a r e d v i s u a l l y choked, gas v e l o c i t i e s between 3.7 and 9.2 m/s R i s e r column c o n f i g u r e d t o r e t u r n s o l i d s 1.98 m above t h e d i s t r i b u t o r plate f o r entrance effect studies  99  101  -  xi Page  Figure  3.9  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 4.9 m/s, G = 24 and 26 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above t h e gas d i s t r i b u t o r  102  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, U = 6.1 m/s, G = 35, 45, and 59 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above t h e gas d i s t r i b u t o r  103  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 7.1 m/s, G = 45 and 73 k g / i n s , a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above the gas d i s t r i b u t o r  104  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, U = 8.1 m/s, G = 66, 71, and 82 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above t h e gas d i s t r i b u t o r  105  D e t a i l s of three e x i t s s t u d i e d f o r c i r c u l a t i n g bed a p p l i c a t i o n  106  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, U = 7.1 m/s, G = 36, 73, 93, and 116 kg/m s, smooth e x i t , s o l i d s r e t u r n 1.98 m above t h e gas d i s t r i b u t o r  108  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 7.1 m/s, G = 73 kg/m s, smooth and a b r u p t e x i t s , s o l i d s r e t u r n 1.98 m above t h e gas d i s t r i b u t o r . Triangles r e p r e s e n t smooth e x i t p r o f i l e , c i r c l e s abrupt  109  2  s  Figure  3.10  g  s  2  Figure  3.11  2  s  Figure  3.12  g  s  2  Figure Figure  3.13 3.14  g  s  2  Figure  3.15  2  s  Figure  3.16  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 7.1 m/s, G = 73 kg/m s, a b r u p t and e x t e n d e d e x i t s , s o l i d s r e t u r n a t 1.98 m above t h e gas d i s t r i b u t o r . T r i a n g l e s r e p r e s e n t extended e x i t p r o f i l e , c i r c l e s abrupt I l l 2  s  Figure  Figure  3.17  3.18  Column c o n f i g u r a t i o n f o r 1.5 entrainment t e s t s (a) I n i t i a l configuration (b) Final Configuration  m/s  Pressure versus e l e v a t i o n p l o t s f o r a r i s e r w i t h f l u i d i s e d and packed bed s t o r a g e zones. Numbers r e f e r t o l o c a t i o n s on F i g u r e 2.1  113  119  -  xii Page  Figure  3.19  Figure  3.20  Figure  Figure  Figure Figure Figure  Figure  Figure  Figure  3.21  3.22  3.23 3.24 3.25  3.26  3.27  3.28  D e n s i t y p r o f i l e s measured i n c i r c u l a t i n g beds of sand a t a t o t a l gas v e l o c i t y o f 8.6 m/s and a s o l i d s c i r c u l a t i o n r a t e o f 2 45 kg/m s w i t h d i f f e r e n t p r i m a r y t o s e c o n d a r y (P/S) a i r r a t i o s . Secondary a i r i n t r o d u c e d t h r o u g h opposed p o r t s . C i r c l e s , zero secondary a i r ; t r i a n g l e s , P/S = 1.36; s q u a r e s , P/S = 0.83 D e n s i t y p r o f i l e s measured i n c i r c u l a t i n g beds of sand a t a t o t a l gas v e l o c i t y o f 8-5 m/s and a s o l i d s c i r c u l a t i o n r a t e o f 45 kg/m s w i t h d i f f e r e n t p r i m a r y t o s e c o n d a r y (P/S) a i r r a t i o s . Secondary a i r i n t r o d u c e d through s w i r l p o r t s . C i r c l e s , zero secondary a i r ; t r i a n g l e s , P/S = 1.39; s q u a r e s , P/S = 0.85  122  123  Block diagram of a c a p a c i t a n c e probe system i l l u s t r a t i n g p r i n c i p a l system components  128  Variables influencing the c a p a c i t a n c e of a c o a x i a l c y l i n d r i c a l c a p a c i t o r ( T i p l e r , 1976)  132  A typical probe  134  simple  needle  A p h o t o g r a p h of the probe d e s i g n  capacitance  final  capacitance 136  P l o t of the r e l a t i v e p e r m i t t i v i t y of u n i f o r m sand a i r s u s p e n s i o n s a c c o r d i n g to Wiener (1912)  141  Output from c a p a c i t a n c e p r o b e g r a d u a l l y immersed i n t o a f i x e d bed of sand showing l i n e a r i t y of v o l t a g e w i t h i m m e r s i o n d e p t h . .  143  C o m p a r i s o n of r a d i a l l y a v e r a g e d densities o b t a i n e d u s i n g an i n t e g r a t e d c a p a c i t a n c e p r o b e s i g n a l , and the same d e n s i t i e s calculated from p r e s s u r e drop measurements  145  C a p a c i t a n c e p r o b e t r a v e r s i n g r i g mounted on a s e c t i o n of the c i r c u l a t i n g f l u i d i s e d bed  149  - xiii  Page  Figure  3.29  Longitudinal and r a d i a l d e n s i t y d i s t r i b u t i o n s i n a c i r c u l a t i n g bed o f s a n d , Ug = 6.5 m/s, G = 62 kg/m s. A l s o shown a r e r a d i a l d i s t r i b u t i o n s o f the s t a n d a r d d e v i a t i o n o f d e n s i t y fluctuations  154  Longitudinal and r a d i a l d e n s i t y d i s t r i b u t i o n s i n a c i r c u l a t i n g bed o f sand, Ug = 6.5 m/s, G = 48 kg/m s. A l s o shown a r e r a d i a l d i s t r i b u t i o n s o f the s t a n d a r d d e v i a t i o n o f d e n s i t y fluctuations  155  Longitudinal and r a d i a l d e n s i t y d i s t r i b u t i o n s i n a c i r c u l a t i n g bed o f sand, U = 6.5 m/s, G = 42 kg/m s. A l s o shown a r e r a d i a l d i s t r i b u t i o n s o f the s t a n d a r d d e v i a t i o n o f d e n s i t y fluctuations  156  s  Figure  3.30  2  s  Figure  3.31  2  g  Figure  3.32  s  Radial variations i n density fluctuations, power s p e c t r a l d i s t r i b u t i o n o f d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y f l u c t u a t i o n s , Ug = 6.5 m/s, G = 62 kg/m s, Z = 533 mm 157 2  s  Figure  3.33  Radial variations i n density fluctuations, power s p e c t r a l d i s t r i b u t i o n o f d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y f l u c t u a t i o n s , Ug = 6.5 m/s, = 62 kg/m s, Z = 1448 mm  158  Radial v a r i a t i o n s i n density fluctuations, power s p e c t r a l d i s t r i b u t i o n o f d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y f l u c t u a t i o n s , Ug = 6.5 m/s, G„ = 62 kg/m s, Z = 2362 mm  159  V a r i a t i o n of l o c a l v e r t i c a l s u p e r f i c i a l gas v e l o c i t y w i t h r a d i a l p o s i t i o n and h e i g h t as c a l c u l a t e d by t h e m o d i f i e d Kozeny e q u a t i o n . Density p r o f i l e s are measured v a l u e s a t v e r t i c a l l o c a t i o n s o f 0.533 m and 2.362 m f o r a gas v e l o c i t y of 6.5 m/s and a s o l i d s c i r c u l a t i o n r a t e o f 62 kg/m s  165  2  s  Figure  3.34  2  Figure  4.1  2  -  xiv Page  Figure  4.2  F i g u r e 4.3  Figure  Figure  4.4  4.5  I m p l i c a t i o n s of a simple d i f f u s i o n a l model f o r s o l i d s m o t i o n f o r gas and s o l i d s density p r o f i l e s . Lower f i g u r e shows a t y p i c a l e x p e r i m e n t a l r e s u l t , upper f i g u r e the r e q u i r e m e n t s f o r " a d i f f u s i o n a l model t o be c o n s i s t e n t Streamfunction flow.  profiles  i n a developing  The s t r e a m f u n c t i o n i s p l o t t e d as a f u n c t i o n o f r a d i u s f o r a u n i f o r m gas v e l o c i t y p r o f i l e and a p a r a b o l i c p r o f i l e showing how, as t h e p r o f i l e changes w i t h h e i g h t due t o r e d i s t r i b u t i o n and decay o f d e n s i t y , t h e r e i s a b u l k f l o w o f gas towards the w a l l . Arrows j o i n p o i n t s o f c o n s t a n t s t r e a m f u n c t i o n showing t h e d i r e c t i o n o f gas f l o w  170  I n t e r m i t t e n c y i n d i c e s as a f u n c t i o n o f r a d i u s f o r f u l l y developed core-annular and " i d e a l c l u s t e r f l o w . "  177  I n t e r m i t t e n c y index p l o t t e d as a f u n c t i o n of r a d i u s a t t h r e e v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g bed o f sand f o r Ug = 6.5 m/s, G = 62 kg/m s  178  I n t e r m i t t e n c y i n d e x p l o t t e d as a f u n c t i o n of r a d i u s a t three v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g bed o f sand f o r U = 6.5 m/s, G = 48 kg/m s  179  I n t e r m i t t e n c y i n d e x p l o t t e d as a f u n c t i o n of r a d i u s a t t h r e e v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g bed o f sand f o r Ug = 6.5 6.5 m/s, G = 43 kg/m s  180  A d e p i c t i o n o f a smooth e x i t r i s e r f o r i l l u s t r a t i o n of the concepts i n v o l v e d w i t h f l o w s t r u c t u r e s above and below t h e saturated carrying capacity. To t h e r i g h t i s a t y p i c a l d e n s i t y p r o f i l e below saturation  188  2  0  s  F i g u r e 4.6  2  F i g u r e 4.7  2  s  Figure  4.8  168  -  XV  -  Page Figure  Figure  Figure  Figure  Figure  4.9  4.10  4.11  4.12  4.13  A s c h e m a t i c d i a g r a m showing s o l i d s f l u x e s i n a r i s e r o p e r a t i n g above t h e s a t u r a t e d c a r r y i n g c a p a c i t y but below c h o k i n g . On t h e l e f t i s a s c h e m a t i c i n w h i c h arrows i n d i c a t e approximate d i r e c t i o n s o f s o l i d s f l o w and show t h e d e v e l o p m e n t of the w a l l l a y e r . On t h e r i g h t i s a s e c o n d d i a g r a m where t h e w i d t h o f up and downflow a r r o w s g i v e s an i d e a o f how up and downflow f l u x e s v a r y w i t h h e i g h t i n the u n i t t o g i v e a net p o s i t i v e f l u x  192  A s c h e m a t i c showing t h e c o n c e p t o f c h o k i n g i n a r i s e r as a p p l i e d i n t h i s t h e s i s , and as o b s e r v e d i n t h e o v e r a l l density p r o f i l e  194  A s c h e m a t i c d i a g r a m showing a h i g h v e l o c i t y r i s e r o p e r a t i n g at a s o l i d s c i r c u l a t i o n r a t e g r e a t e r than c h o k i n g . On t h e l e f t i s a s c h e m a t i c i n w h i c h a r r o w s i n d i c a t e approximate d i r e c t i o n s of s o l i d s f l o w and show t h e development o f t h e w a l l l a y e r up t o i t s maximum s t a b l e ( c h o k e d ) thickness. On t h e r i g h t a s e c o n d d i a g r a m shows how up, down, and c r o s s f l u x e s v a r y w i t h h e i g h t , and shows how t h e c r o s s - f l u x i s i n e q u i l i b r i u m i n t h e choked zone  196  F a s t f l u i d i s a t i o n d e f i n e d i n terms o f a r e g i o n o f p o t e n t i a l gas v e l o c i t i e s and solids circulation rates  198  Existence different  of d i f f e r e n t flow height r i s e r s .  regimes i n  In the l e f t hand r i s e r , which i s t a l l , a t c i r c u l a t i o n r a t e s G i and G 2 t h e r i s e r i s s u b s t a n t i a l l y choked and s m a l l changes i n t h e c i r c u l a t i o n r a t e do n o t c a u s e a l a r g e f r a c t i o n a l change i n t h e inventory. In t h e r i g h t hand r i s e r o f the same d i a m e t e r , which i s s h o r t , t h e same c i r c u l a t i o n r a t e s p r o d u c e d r a m a t i c f r a c t i o n a l i n v e n t o r y changes. This s i t u a t i o n , where changes i n c i r c u l a t i o n r a t e p r o d u c e l a r g e and c o n t r o l l a b l e changes i n o v e r a l l h o l d up, i . e . , where a f a s t f l u i d i s e d bed o c c u p i e s t h e whole column, i s u t i l i s e d i n c i r c u l a t i n g f l u i d i s e d beds s  S  200  -  xvi  Page  Figure  Figure  Figure  Figure  4.14  4.15  4.16  4.17  Density p r o f i l e for a large c i r c u l a t i n g f l u i d i s e d bed combustor (32 m h i g h x 8 m d i a . ) i n f e r r e d from d a t a p r o v i d e d by Wein and F e l w o r ( 1 9 8 6 ) . D i s c o n t i n u i t i e s i n gas v e l o c i t y a r e p o i n t s of a i r a d d i t i o n ; the g r a d i e n t r e f l e c t s a f u r n a c e expansion  202  The p r o p o s e d form o f a d e c a y l e n g t h versus diameter f u n c t i o n f o r f a s t f l u i d i s a t i o n b a s e d upon e x a m i n a t i o n o f d a t a from s m a l l and l a r g e u n i t s  204  A c o n v e n t i o n a l l a b o r a t o r y c i r c u l a t i n g bed r e c y c l e loop with a f u l l y f l u i d i s e d r e t u r n c o n t r o l l e d by a m e c h a n i c a l ( e . g . , slide) valve  207  A p o s s i b l e e x p l a n a t i o n f o r the a p p a r e n t i n f l u e n c e of imposed p r e s s u r e d r o p upon riser operation. When t h e d e c a y l e n g t h i s s h o r t compared to the column h e i g h t , s m a l l changes i n the e x i t d e n s i t y and e x t e r n a l c i r c u l a t i o n r a t e r e s u l t from l a r g e changes i n t h e column i n v e n t o r y ( p r e s s u r e d r o p ) . Hence, s i n c e the p r e s s u r e drop a c r o s s the v a l v e i s a f u n c t i o n o n l y of the s o l i d s c i r c u l a t i o n r a t e , according to the p r e s s u r e b a l a n c e , column p r e s s u r e d r o p a p p e a r s t o depend upon r e t u r n l e g p r e s s u r e drop  Figure  Figure  Figure  4.18  4.19  4.20  208  A b u b b l i n g f l u i d i s e d bed i l l u s t r a t i n g t h e phenomenon of c o e x i s t e n c e of s t a b l e s t a t e s at choking. The b u b b l i n g bed on the l e f t , c h a r g e d w i t h a wide r a n g e of i n v e n t o r i e s (medium and h i g h a r e shown h e r e ) , w i l l show c o e x i s t e n c e o f dense and d i l u t e p h a s e s , and c i r c u l a t i o n a t the c h o k i n g flux, p r o v i d e d the s p e c i f i e d c o n d i t i o n s a r e met  211  Diagram showing how s o l i d s a r e s e p a r a t e d i n e r t i a l l y by an a b r u p t e x i t p r o m o t i n g internal circulation  213  Heat t r a n s f e r s u r f a c e l o c a t i o n s f o r d i f f e r e n t c o m m e r c i a l combustor d e s i g n s  217  -  xvii  Page  F i g u r e 4.21  V a r i a t i o n of s o l i d s fluxes i n a c i r c u l a t i n g f l u i d i s e d bed w i t h s o l i d s r e t u r n some d i s t a n c e above t h e gas distributor. The d e n s i t y p r o f i l e on t h e l e f t hand s i d e , t y p i c a l o f t h e f a s t bed w i t h e l e v a t e d s o l i d s r e t u r n , i s t h o u g h t t o be c a u s e d by up, down, and c r o s s f l u x e s a s shown on t h e r i g h t - h a n d s i d e . There i s a lower zone o f z e r o s o l i d s f l u x , an upper zone o f n e t upward f l u x , and a complex c r o s s f l o w p a t t e r n , p a r t i c u l a r l y a t t h e r e t u r n l o c a t i o n where d o w n f l o w i n g s o l i d s a r e d i s p l a c e d from t h e w a l l i n t o the c o r e .  Figure  5.1  D i m e n s i o n l e s s p r e s s u r e f l u c t u a t i o n s and o v e r a l l bed d e n s i t y p l o t t e d a g a i n s t gas v e l o c i t y t o i l l u s t r a t e the onset of the t u r b u l e n t t r a n s i t i o n , U , and t h e f u l l y t u r b u l e n t s t a t e , Ufc ( T u r n e r 1978)  222  c  Figure  Figure  Figure  Figure  5.2  5.3  5.4  5.5  228  Traces of the d i f f e r e n t i a l pressure f l u c t u a t i o n , a s i n d i c a t e d by t r a n s d u c e r v o l t a g e , v e r s u s time i n s e c o n d s o v e r a 400 mm s e c t i o n o f a bed o f sand a t d i f f e r e n t gas v e l o c i t i e s a - 0.11 m/s, b - 0.19 m/s, c - 0.40 m/s, d - 0.57 m/s, e - 1.25 m/s, f - 2.14 m/s, g - 2.9 m/s, h - 3.9 m/s  236  P l o t o f a p p a r e n t s o l i d s volume f r a c t i o n v e r s u s s u p e r f i c i a l gas v e l o c i t y f o r a f l u i d i s e d bed o f sand i n a 0.152 m diameter r e a c t o r  239  P l o t of s t a n d a r d d e v i a t i o n o f d i f f e r e n t i a l p r e s s u r e f l u c t u a t i o n s o v e r a 460 mm l e n g t h of a f l u i d i s e d bed o f sand v e r s u s s u p e r f i c i a l gas v e l o c i t y  240  P l o t of the standard d e v i a t i o n of d i f f e r e n t i a l pressure fluctuations n o r m a l i s e d w . r . t . mean d i f f e r e n t i a l p r e s s u r e , o v e r a 460 mm s e c t i o n o f a f l u i d i s e d bed o f sand, v e r s u s s u p e r f i c i a l gas v e l o c i t y  241  -  xviii  Page  Figure  Figure  Figure  Figure  Figure  Figure Figure Figure Figure  5.6  5.7  5.8  6.1  6.2  6.3 6.4 6.5 6.6  P l o t o f t h e maximum p e a k - t o - p e a k p r e s s u r e f l u c t u a t i o n o v e r a 460 mm s e c t i o n o f a f l u i d i s e d bed o f sand v e r s u s s u p e r f i c i a l gas v e l o c i t y  242  Absolute slugging Davidson i s not a  244  pressure f l u c t u a t i o n s i n a bed a c c o r d i n g t o Kehoe and (1973) showing a b s o l u t e p r e s s u r e bimodal f u n c t i o n  Standard d e v i a t i o n of a d i f f e r e n t i a l p r e s s u r e s i g n a l measured o v e r a s m a l l length versus expansion f o r s l u g flow, s h o w i n g a maximum d e s p i t e no l o s s o f t h e two p h a s e n a t u r e  248  T r a c e r p r o f i l e s measured 50 mm u p s t r e a m of a c e n t r e l i n e i n j e c t i o n p o i n t a t d i f f e r e n t gas v e l o c i t i e s ( C a n k u r t and Yerushalmi, 1978). C/Co i s t h e r a t i o o f t r a c e r c o n c e n t r a t i o n a t t h e measurement point to the i n j e c t i o n concentration. The lower g r a p h r e p r e s e n t s f a s t bed c o n d i t i o n s (Ug > 1.8 m/s). T u r b u l e n t f l u i d i s a t i o n e x i s t s between 0.6 m/s and 1.7 m/s  254  V a r i a t i o n of a x i a l d i s p e r s i o n c o e f f i c i e n t w i t h gas v e l o c i t y i n p a s s i n g from slugging to turbulent f l u i d i s a t i o n ( Y e r u s h a l m i and A v i d a n , 1985). T r a n s i t i o n to t u r b u l e n c e r e p o r t e d l y o c c u r s a r o u n d 0.7 m/s  256  V a r i a t i o n o f a x i a l P e c l e t number w i t h gas v e l o c i t y ( Y e r u s h a l m i and A v i d a n , 1985)  257  Tracer i n j e c t i o n studies  263  system  f o r gas RTD  P h o t o g r a p h o f t r a c e r s a m p l i n g and d e t e c t o r s y s t e m f o r RTD s t u d i e s  265  E f f e c t of sampling r a t e through thermal c o n d u c t i v i t y c e l l upon d e t e c t o r dispersion. (Dead t i m e removed from F - c u r v e s )  267  - xix Page Figure  Figure Figure  6.7  6.8 6.9  E f f e c t of thermal c o n d u c t i v i t y c e l l type upon d e t e c t o r d i s p e r s i o n showing how d e s i g n m o d i f i c a t i o n s (new c e l l type d r i l l e d out) reduced d i s p e r s i o n . (Dead time removed from F - c u r v e s )  268  Test c o n f i g u r a t i o n to e s t a b l i s h detector linearity  271  Graph o f i n t e g r a t e d d e t e c t o r o u t p u t v e r s u s i n j e c t i o n volume t o d e m o n s t r a t e detector l i n e a r i t y  272 273  Figure  6.10  Optimised  detector  F-curve  response  Figure  6.11  Combined r i s e r / d e t e c t o r F - c u r v e r e s p o n s e for a x i a l mixing determinations i n c i r c u l a t i n g beds o f sand a t Ug = 7.1 m/s. Abrupt e x i t , G = 0, 37, 49, 60 kg/m s Smooth e x i t , G = 0, 65, 41, 33, 43 kg/m s 2  s  s  2  Figure  6.12a D e f i n i t i o n s o f d e v i a t i o n v a r i a b l e s f o r t r a n s f o r m E q u a t i o n 6.1. 6.12b  Figure  Figure  Figure  276  6.13  6.14  6.15  P r o c e d u r e f o r i s o l a t i n g t h e column RTD from t h e i n d i v i d u a l s t e p r e s p o n s e s f o r the d e t e c t o r and d e t e c t o r column combinations using deconvolution. IFT represents inverse Fourier transform  281  Smoothed segmented F - c u r v e r e s p o n s e s s u i t a b l e f o r t r a n s f o r m i n g (dead time has been removed)  283  E - c u r v e s f o r t h e d e t e c t o r ( c i r c l e s ) and d e t e c t o r / r i s e r c o m b i n a t i o n ( s q u a r e s ) from the i n v e r s e t r a n s f o r m s o f t h e t r a n s f o r m e d step responses  285  E x t r a p o l a t i o n o f t h e r e a l and i m a g i n a r y components o f t h e F o u r i e r t r a n s f o r m o f t h e r i s e r RTD beyond t h e i r r e g i o n o f accuracy to approximate higher frequency components b e f o r e i n v e r s i o n  287  - xx Page Figure  Figure Figure  6.16  6.17 6.18  Comparison of the e x p e r i m e n t a l riser/ d e t e c t o r combination F-curve with the c o n v o l u t i o n o f c a l c u l a t e d r i s e r and d e t e c t o r RTD's s u b j e c t e d t o a s t e p i n p u t . Dead t i m e has been removed. Comparison i s favourable  288  RTD f o r r u n D i s 3 computed by deconvolution. Dead time removed  289  Two zone model f o r gas m i x i n g i n a c i r c u l a t i n g f l u i d s e d bed. C = c o n c e n t r a t i o n i n annulus, C = c o n c e n t r a t i o n i n core, r = core r a d i u s , R = column r a d i u s , k = mass t r a n s f e r (crossflow) c o e f f i c i e n t  291  a  c  c  Figure  6.19  C o m p a r i s o n o f t h e RTD f o r t h e r i s e r f o r run D i s 3 w i t h b e s t f i t p r e d i c t i o n s o f the two zone model. a - c o n t i n u i t y obeyed, r = 0.059 m, k = 0.11 m/s b - c o n t i n u i t y r e l a x e d , r = 0.059 m, k = 0.08 m/s, U = 8.55 m/s c  295  P l o t o f pseudo v e s s e l d i s p e r s i o n number (D/UgL) f o r a x i a l m i x i n g a g a i n s t p r e s s u r e d r o p i n a c i r c u l a t i n g bed o f s a n d , Ug = 7.1 m/s  297  c  c  Figure  Figure  Figure  6.20  6.21  A3.1  V a r i a t i o n of the standard d e v i a t i o n o f a b s o l u t e p r e s s u r e f l u c t u a t i o n s near the base o f t h e c i r c u l a t i n g f l u i d i s e d bed w i t h t o t a l p r e s s u r e drop o v e r t h e u n i t for d i f f e r e n t e x i t geometries, c i r c l e s r e p r e s e n t a b r u p t e x i t , s q u a r e s smooth  299  F - c u r v e r e s p o n s e o f s y s t e m and d e t e c t o r p l o t t e d on normal p r o b a b i l i t y p a p e r (Yu, 1985), ' (Ug = 3.5 m/s, G = 30 kg/m s, a l u m i n a )  333  s  1.  1.1  INTRODUCTION  I n i t i a l Concepts The  short  history  but  cracking many  of  turbulent.  processes and  as  as  little  agreement  appears  fluidisation  and  were late  they  to  the  be  generally  themselves,  contacting  above,  one  of  the  most  with  circulating  fluidised  exactly  what  i s meant  the  gas-solids  loop  of  the  unit  unit  i s charged velocity  10  m/s,  At  low  then  to  a  for  new  have  been  and  there  a  nearly  or  has  circulating The  only  fast,  scheme  for  through with  an  a  a  sequence  velocities,  several  times  raised  first this  to  For  material  cyclone  flow  is  Figure  line.  inventory  of  difficult  beds  term.  entrained  i s gradually  then gas  by  transport  i n which  base  gas  favour  characterised.  circulating,  working  closed  for  renewed  constitutes be  been  many  reactions.  implied  vertical  of  them  discussions  might  excellent  a  has  1940s  out  brought  what  that  beds  early  1 9 7 0 s when  time,  about  how  i s an  in  contactors  consensus  bed  fluidised  interest  that  the  fluidised  gas-solid  the  academic  Since  turbulent  they  until  prominence.  As  Created  operations,  industrial  been  circulating  and of  from  a  1.1  illustrates solids  returned  zero  to  particles to  will  minimum  fluidisation,  the  a  the  If  the  and  the  approximately  regimes  value,  a  i t is  standpipe.  solid  of  describing  the is  aspects  be  familiar  recorded.  packed  and bed,  - 2  Figure  1.1  -  Schematic diagram of transport l i n e .  a vertical  gas-solid  -  bubble-free only,  fluidization  o r o v e r an  regimes  will  familar  regime,  observed. are  less  Leung regarding  right  abrupt This in  less  well  possible  types of behaviour  by  such  In t h e s e c o n d transition;  may  from a d i l u t e  structures  for cocurrent  "non-slugging  as t h e t h e bed  and  (1975) and  velocity  Cankurt,  for gas-solid  of p r i n c i p a l  such  interest  is  system.  1978),  sharp  a  contacting. study.  from  regime 1980)  or  fast  state suspended  of s o l i d s ,  an e x c e l l e n t  to t h i s  a  (1978).  i s reduced  enters a  c o n c e n t r a t i o n , v i g o r o u s backmixing  characteristics  the  t h e r e i s an  Smith  g r a d i e n t s i n the  velocites,  Two  known as c h o k i n g , d e s c r i b e d  gradually  longitudinal  slip  rate,  dense p h a s e f l o w " ( L e u n g ,  (Yerushalmi  gas-solid  velocity  t o a dense phase  t h e r e i s not gas  1.2).  In t h e f i r s t ,  i f t h e gas  a u t h o r s as Yang  transport,  by  occur.  circulation  type of system  characterised  regime  transport i s  flow regimes  o f F i g u r e 1.2,  instead,  fluidisation  solids  another  understood.  i s t h e phenomenon commonly  called  velocities  f l o w i n terms o f a f l o w c h a r t ( F i g u r e  transition  pneumatic  fluidisation  extremes the flow  at a constant s o l i d s  detail  high  phase pneumatic  the d i f f e r e n t  hand b r a n c h  reduced  A t t h e h i g h e s t gas  dilute  and  bubbling  point  (1980) d e s c r i b e s t h e c o n v e n t i o n a l wisdom  gas-solids different  r a n g e ) and  Between t h e s e two clear  -  ( a t t h e minimum f l u i d i z a t i o n  extended  be s e e n .  3  and  combination T h i s i s the  of  -  4 -  Dilute (lean) Phase Flow Fuzzy T r a n s i t i o n  Sharp Transition (Choking Transition)  Non-slugging Dense Phase Flow  Slugging  Fuzzy T r a n s i t i o n  Dense  Slugging Dense Phase Flow Sharp T r a n s i t i o n  High Gas Velocity  Phase Flow Sharp T r a n s i t i o n T r a n s i t i o n from F l u i d i z e d Flow to Packed Bed Flow Packed Bed (Moving Bed) Flow Low Gas Velocity  Figure  1.2  Flow r e g i m e s f o r g a s - s o l i d Leung ( 1 9 8 0 ) .  flow according  to  -  An regime  a l t e r n a t i v e way i s by  gradually  5  -  of a p p r o a c h i n g i n c r e a s i n g the  the gas  fast  velocity.  gives  a somewhat d i f f e r e n t  i n s i g h t i n t o the  small  diameter  appropriately  the  gas  velocity is raised, f i r s t  occur.  These are  transition the  slugging  regime,  introductory  and  longer  transition  in this  has  thesis.  For  the  two  regimes,  neither a  to a  turbulent  and  a dilute  There are regime t h a t  to bubbling gas  and  slugs.  (Massimilla,  1973)  be  is  and  of  through  purposes of defined  i s improved  Although  the  the  1978),  and  a  bubbling surrounds  but  such  the  be  that,  because  where  s a i d to with  no  bed  of p a r t i c l e s  truly  this  uniformity.  be  the small,  contacting  there  is less  p a r t i c u l a t e s i n the  there  evidence  as  phase c o n t i n u i t y a r e  slugging,  to bypass  the  phase can  the  controversy  increased  or p a c k e t s  as  slugging  s t a t e , b u b b l e s as  Cankurt,  a  turbulent  implications associated scales  solids  f o r gas  b u b b l e s or  c o n d i t i o n of  r e f l u x i n g strands  continuous.  to  phase s t r u c t u r e of  fluidised  (Yerushalmi  dense nor  familiar  the  may  In  then  where a dense phase e m u l s i o n  "turbulent"  of  and  r a i s e d much  breakdown of  consists  tendency  i . e . the  process.  velocity is raised  transition  exist  between  the  Less  the  new  compared  as  transition  phase v o i d s ,  this  documented.  This  sized particles,  bubbling  section  slugging  dilute In  This  i s considered  gradual  well  which o c c u r s  fluidisation. and  column w i t h  fluidisation  i s some e v i d e n c e  i s f o r improved  form for  of this  contacting  -  6  -  relative  t o a system  d e s c r i b e d by e m u l s i o n  phases.  The s t u d i e s  to date  initial  concept  of turbulent  suspension of aggregates and  reforming.  evidence  However, m i c r o s t r u c t u r a l confuse  the b a s i c  generally lower  more u n i f o r m  of the apparent  clusters  turbulent  ( i n a time  regime  without  turbulent regime  units.  s h o u l d not be a l l o w e d t o  than  f l u i d i s a t i o n as a  i s found  at  immediately  regime  i s that,  r e g r o u p i n g of p a r t i c l e s mean s e n s e ) , s l i p  than  o f an i n d i v i d u a l  can s t i l l  circulating Just  bed s u r f a c e ,  fast  i s entered from  i s g r a d u a l , so t h i s  not be d r a m a t i c .  i n the than  and  this  as a s t a t i c  albeit  somewhat  velocities.  gas v e l o c i t y  as t h e t r a n s i t i o n  strands  Therefore the  be c h a r a c t e r i z e d  fluidisation,  bed r e g i m e ,  fluidisation  velocities  particle,  entrainraent.  a t lower  the s u p e r f i c i a l turbulent  into  be an o r d e r o f magnitude h i g h e r  w i t h an i d e n t i f i a b l e  As beyond  of the t u r b u l e n t  substantial  regime  more d i f f u s e  need  may  terminal velocity  occurs  end.  1983) p r o v i d e s  velocities.  because  the  (Abed,  of t u r b u l e n t  state  One c h a r a c t e r i s t i c  or  c o n t i n u o u s breakdown  non-uniformity i n small  details  concept  support the  f l u i d i s a t i o n as a u n i f o r m  at least  f o r strong radial  bubble  do not c o m p l e t e l y  i n gas w i t h  One s t u d y  and  Rather,  i s gradually  raised  fluidisation,  or the  from  the low  velocity  slugging to turbulent entry to f a s t  the r a t e  fluidisation  of entrainment  from  -  the  turbulent  that, the  bed g r a d u a l l y  unless  entrained  bed e m p t i e s  very  -  becomes so h i g h  material  (> 15kg/m s)  i s continuously  replaced,  rapidly.  Although a fast to a turbulent  7  fluidised  bed i s v i s u a l l y  bed, a t l e a s t i n i t s d e n s e s t  very  similar  regions,  i t has  some s u b s t a n t i a l l y d i f f e r e n t p r o p e r t i e s : (i)  While  separating profile  the t u r b u l e n t  t h e dense bed from a f r e e b o a r d  in a circulating  transition contain  Therefore velocity  a fast  and s o l i d rate.  many i n d u s t r i a l Like  of  a circulating  Figure  1.4.  development  by g a s  but a l s o by t h e s o l i d s dependence i s c r i t i c a l f o r  fluidised  a transparent  A photograph  the turbulent  velocities  not o n l y  bed, t h e c i r c u l a t i n g bed wall  appears to c o n s i s t of  of r e f l u x i n g p a r t i c l e s continuously  reforming.  like  final  reactor.  applications.  through  many s t r a n d s and  properties, This  low s o l i d s  bed i s a t r a n s p o r t  i s characterised  the t u r b u l e n t  when v i e w e d  gradual  and may n o t even  bed has f a i r l y  fluidised  i t s structure  circulation  the d e n s i t y  extreme.  While a t u r b u l e n t  entrainment,  zone,  surface  bed u n d e r g o e s an even more  from dense t o d i l u t e s t a t e s ,  either  (ii)  bed has a d i f f u s e  bed, s l i p  bed may  by an o r d e r Together  i s shown i n F i g u r e velocities  exceed  of t h e o r i e s  1.3.  This  observations  for circulating  Also,  i n t h e dense  individual particle  of magnitude. these  coalescing  regions terminal  i s illustrated in  l e d to the fluidised  bed  fluid  - 8 -  F i g u r e 1.3  Photograph of a f a s t through the w a l l .  f l u i d i z e d bed  viewed  - 9 -  t  1  r  i  «  1 i i i i  i  SOLID RATE, 6, (Kg/m* s) L. SLIP VELOCITY;  195 146  SOUDS*FCC W  FAST FLUIDIZED REGIME  € (m/s)  TRANSPORT VELOCITY TURBULENT REGIME '4^  I -19.5  /BREAKDOWN OF SLUGS _ %  BUBBLING REGIME fm1  l*l O.I  0.2  0.3  i-e  Figure  1.4  S l i p v e l o c i t i e s i n high according to Yerushalmi  I I II  0 . 4 0 . 6 0 . 8l.O ^ c h a n g e i n scale  velocity fluidization and C a n k u r t ( 1 9 7 8 ) .  - 10 -  m e c h a n i c s b a s e d upon t h e e x i s t e n c e of  particles,  dispersed  cross-section. single  Each c l u s t e r  larger particle  evidently  uniformly  the r e a c t o r  was assumed t o behave  like a i t was  s t a t e o f f o r m a t i o n and  I t was t h e s e  w h i c h gave t h e f a s t  over  or c l u s t e r s  i n a t i m e mean s e n s e , a l t h o u g h  i n a continuous  destruction.  of strands,  large, "effective  bed i t s h i g h  slip  agglomerates"  velocities  and a  number o f p o t e n t i a l a d v a n t a g e s : (i)  The a b i l i t y fine  particles  throughput (ii)  to maintain  Intense  i n small  allow  Therefore particle (iv)  allowing  promoting  uniformity.  the s u r f a c e  area  the  of each  individual  remains a c t i v e .  bed a s s o c i a t e d w i t h  change t h e r i s e r  external  features,  circulating  packed,  throughflow.  There a r e p o t e n t i a l c o n t r o l advantages  rapidly  circulation  together  bed c o u l d  with  to the  the a b i l i t y to  inventory  by c h a n g i n g  rate of s o l i d s .  others,  be an e f f e c t i v e  suggested  that  medium f o r many  reactions. Having placed hydrodynamic  high  cross-section units.  substantial convective  circulating  the  d e n s e beds o f  The a g g l o m e r a t e s a r e i n no s e n s e c l o s e and  These  velocities,  backmixing of s o l i d s ,  temperature (iii)  at high  relatively  fast  context,  fluidisation  i t i s valuable  in a qualitative t o see i t d e f i n e d  -  11  somewhat more q u a n t i t a t i v e l y . been u s e d  for this  r e g i m e d i a g r a m due occupies The  a region  velocity  l o w e r end, (slip  velocity  limitations,  in  this  group C et  to  (1986a).  Grace limited  Figure  imposed by  for fast  (Geldart,  proposed  fluidisation  and  particle  the  velocity). this  (>  1.5  particles,  are  sizes. the  uper  end  Particle  size  regime diagram,  c l e a r as y e t w h e t h e r v e r y  finer  1973)  shows a  suspensions at  i n d i c a t e d on  fluidisation  r e g i m e , and  often  l a c k of t r a n s p o r t a t  approaches t e r m i n a l  I t i s not  1.5 Fast  velocities  overly dilute  although  more t e n u o u s . particles  and  of  by  Regime d i a g r a m s have  purposes,  l i m i t s are  and  -  mm)  can  falling  c u r r e n t l y under  be  in  are  large contacted  Geldart's  study  (Brereton  a l . , 1987).  1.2  H i s t o r i c a l and Current Circulating  since  the  efforts  e a r l y 1940s when, as  Gilliland  velocities  stream  part  of  cracking process  (1950) examined  early research  upflow c a t a l y t i c  i n 1943;  gas  lost  favour  dust  collection  l e d t o the cracker  however, t h i s  been known  Standard  solid  between minimum f l u i d i s a t i o n and  This  commercial  Perspectives  beds have a r g u a b l y  to develop a c a t a l y t i c  L e w i s and  1985).  fluidised  Industrial  and  contacting  riser  solids  inventory  control  at  (Squires,  development  of  placed  a on  crackers  because of o p e r a t i o n a l d i f f i c u l t i e s , and  for o i l ,  3 m/s  w h i c h was  other  Oil's  soon  notably  (Turner,  - 12  -  dp-.Ar'/'.^Vs//.*]'' F i g u r e 1.5  3  Regime diagram f o r g a s - s o l i d c o n t a c t i n g a c c o r d i n g t o Grace (1986a). Ap i s the d i f f e r e n c e between p a r t i c l e and gas d e n s i t i e s .  - 13 -  1979). bed  Therefore  as  a standard  regeneration,  velocity  time,  with  shorter solids  published  this  fast  transport,  i n the  the  t h e r e had low  maintaining  intermediate  fast  the  velocity  riser  permitted  with  advances i n  surprisingly the  late  work on  largely gas  little  1970s. pneumatic  concentrations the  related  s t u d i e s tended  high  which  to f o c u s  latter;  suspension  the  on  region  densities,  unstudied. velocities,  in a  context  c r a c k i n g , a p p e a r s t o have o r i g i n a t e d w i t h  L u r g i Chemie und developed  solids  work on  and  intermediate  at  been  zeolite  gas-solid  t h e r e was  a v o i d i n g the  Contacting  of  Lurgi  gas  this  standard.  of upflow  suspended  f o r m e r and  By  a fast  Huttenteknik fluidised  of  c r a c k i n g c o u l d become a  been s u b s t a n t i a l  but  early  concept  activity  and  subject u n t i l  had  than  the  o i l i n d u s t r y , i n a regime  fluidisation,  other Reh  the  until  and  industry  also substantial  phenomenon of c h o k i n g ,  of  riser  fluidisation,  at very and  times,  long h i s t o r y  r e s e a r c h on  to t h i s  (oKO.Ol),  systems,  operations  arguably  Prior  more d u r a b l e  C u r r e n t l y i t i s the  contacting  and  e_t aJU , 1 9 5 9 ) .  i n t r o d u c t i o n of u l t r a - h i g h  control  Despite  fluidised  a p p e a r s t o have r e i n t r o d u c e d t h e  w h i c h were b o t h  flow  cracking  units proliferated  v a p o u r phase r e s i d e n c e  reality.  is  these  t o the b u b b l i n g  catalytic  c r a c k i n g (Rehbein  the  catalysts,  f o r both  and  1950s when S h e l l high  i n d u s t r y turned  bed  (Reh  e t al_. , 1 9 8 0 ) .  alumina  calciner  and  -  from  this  (Reh  et^ a_l. , 1980).  processes  progressed  into  14 -  combustion  transfer  and S 0  Low p r e s s u r e  gasification  gasification  o f low g r a d e f u e l s  firing  (iv)  to  (Kobro  and S t r o m b e r g ,  - t o produce  from  and o t h e r  solid  coal  Metallurgical process  f o r p r e r e d u c t i o n of i r o n  similar  process  minimising  gases  Both  as wood  f o r lime  liquid  ores.  processes  energy  c o s t s f o r subsequent  kiln  fuels fuels.  t h e ELRED  o r e , and a  f o r p r e r e d u c t i o n of  nickel  lateritic  d r a m a t i c a l l y reduce process  steps (Hirsch  a l . , 1985) . of these  difficult  circulating  flue  - for  carbonaceous  processes  have been d r i v e n by t h e need sound  use o f s t o c k s o f  sulphur, high ash, or s t r o n g l y caking  proved  such  processes - including  make e c o n o m i c , e n v i r o n m e n t a l l y  high  from  surface  1986).  processes  Several  2  processes  Pyrolysis  et  mass  1985).  waste t o p r o d u c e a gas s u i t a b l e  (iii)  high  and l a r g e a v a i l a b l e  a d s o r p t i o n o f HF, HG1  (Reh, (ii)  coefficients  bed  including:  Gas c l e a n i n g p r o c e s s e s - u t i l i s i n g  for  applications  A l a r g e number o f c i r c u l a t i n g  a r e now under d e v e l o p m e n t  (i)  and o t h e r  to process  using other  c o a l s which  techniques.  have  The  bed p r e s e n t s a t u r b u l e n t c o n t a c t i n g medium problems with  caking  and s i m u l t a n e o u s l y p r o v i d i n g  -  the o p p o r t u n i t y  T h i s may  90%  ratios  have r e m o v a l  as  (Reh  bed  combustion  culms,  s u l p h u r petroleum cokes  sands p r o c e s s i n g ,  units;  formed  Greenfield  p r o c e s s (Stromberg  new in  unit  offer  Europe,  States  The  et a l . ,  strongly  1985).  and  typical 1.6.  traditional  a  Commercial  technology f o r the have been  built  i n the United  as s m a l l  as 2.5  l a r g e as 100  thesis,  notably  o f gas v e l o c i t i e s  towards  conclusions  technologies, velocities  as  even  the Carver  numbers o f u n i t s  and  of t h i s  the range  the g e n e r a l  Figure  1985)  and  MW-th  MW  e  1985).  specifics  and  directed  in  large  and Canada w i t h c a p a c i t i e s  (Schweiger,  A  et a l . ,  S c a n d i n a v i a , and more r e c e n t l y  (Stromberg  solids  and  as a b y p r o d u c t o f  by  an e x t r e m e l y c o m p e t i t i v e  market,  has  anthracite  h i g h m o i s t u r e wood w a s t e s ,  sewage s l u d g e powder p r o d u c e d  now  to burn c o a l s  these i n c l u d e  dehydrated  units  1980).  number of o t h e r f u e l s i n  circulating  tar  approaching  et a l . ,  of these d i f f i c u l t  t o t h e use o f a l a r g e  high  i n combustion  efficiencies  low as 1.5:1  Success with combustion led  -  f o r i n - b e d s u l p h u r removal  processes. a t Ca:S  15  valid  n u m e r i c a l v a l u e s may solids  t y p e s which  circulating It offers  pulverised  employed,  the combustion  remain  have  application.  been While  for a l l circulating  be s p e c i f i c were  fluidised  fired  to the  bed  gas  examined.  bed  the f o l l o w i n g  or s t o k e r  the types o f  combustor advantages  combustion  i s shown over a system.  -  16  -  GAS  HOT WATER STEAM  LIMESTONE WATER /STEAM  PRIMARY AIR 40-80%  Figure  1.6  A t y p i c a l c i r c u l a t i n g bed and A n d e r s s o n , 1 9 8 5 ) .  combustor  (Kullendorf  -  (i)  Decreased ability rough  (ii)  fuel  Decreased  h i g h ash,  form  costs,  associated  -  p r e p a r a t i o n c o s t s because  t o burn  crushed  17  minus 25  - 40mm.  and o p e r a t i n g ,  w i t h removal  of s u l p h u r from  desulphurisation  In-bed  using  l i m e s t o n e o r d o l o m i t e as a s o r b e n t expensive  desulphurisation  can  flue  equipment  be  the  gas.  than r e q u i r i n g  the  high sulphur coals i n  - typically  capital  of  flue  accomplished rather  gas  such  as wet  or  dry  production  due  scrubbers. (iii)  Decreased  thermal  to decreased All  and  operating  these advantages  bubbling f l u i d i s e d  circulating  fluidised Improved without beds,  turndown and  by  virtue  W/m K) t h r o u g h 2  Simpler  (iii)  Improved u t i l i s a t i o n  turndown and  further  Lower NOx  however,  advantages  multifuel  still:  capability,  compartmentalised to vary  heat  o v e r a wide r a n g e control  of  control  practices.  (e.g.  solids  of in-bed sorbents f o r  capture without  classification, (iv)  more  rate.  (ii)  sulphur  by  combustors;  of the a b i l i t y  coefficients  circulation  X  offered  t h e need t o p r o v i d e  transfer - 250  bed  beds o f f e r  N0  temperatures.  are a l s o  traditional  (i)  fuel  external  treatment  e m i s s i o n s than  and  ash  recycle  loops.  f o r b u b b l i n g beds  by  50  -  virtue at (v)  18 -  of the a b i l i t y  two o r even  three  Lower c o s t s f o r f u e l improved  to stage  solids  a i r introduction  levels. f e e d i n g systems  mixing  decreases  since  t h e number o f  feed points required. (vi)  Fuel savings  through  improved  combustion  efficiency. (vii)  Smaller gas  To a fair  unit  c r o s s - s e c t i o n s by v i r t u e  velocities.  qualify  these  comments and p r e s e n t  p e r s p e c t i v e i t i s important  beds c a n p r o v i d e efficiency cyclone  as the best  circulating  can be r e d u c e d  However, t h i s  t o note  e q u a l l y good s u l p h u r  ash r e c y c l e loops  emissions  of higher  tends  that  capture  units,  a r e employed. by s e c o n d a r y  bubbling  beds i n  bubbling  and c o m b u s t i o n  provided Also,  N0  that  X  a i r addition.  t o take p l a c e a t the expense o f s u l p h u r  capture. Hence, b u b b l i n g the  circulating  and  w e l l understood  argument the  bed w i t h  i n favour  simplicity  capability.  bed t e c h n o l o g y  competes s t r o n g l y w i t h  the advantage of proven  ash c h a r a c t e r i s t i c s . of a c i r c u l a t i n g  The  principal  bed i s then  o f turndown and an i m p r o v e d  I t i s not p o s s i b l e a t t h i s  reliability  reduced  multifuel  stage  t o make an  e c o n o m i c c h o i c e between t h e two i n a g i v e n  size  assessing  bed w i t h a s h  recycle  the r e l a t i v e  c o s t s of a bubbling  and a c i r c u l a t i n g  unit.  r a n g e by  to  -  The the  circulating  following (i)  19  -  f l u i d i s e d bed  perceived  merits  of  Temperature u n i f o r m i t y solids  (ii)  fast due  of  fluidisation: to  intensive  internal  mixing.  Intimate gas-solid  (iii)  combustor makes use  contacting  gas-solids  slip  A b i l i t y to  control  control  circulation  of  velocities heat  by  and  virtue  no  transfer rates,  of  high  bubbles. coefficients  and  hence  by  density  profiles. It the  is this first  study.  third two,  w h i c h i s not  w h i c h has  T h i s has  Brereton  point,  entirely  m o t i v a t e d much of  been d i s c u s s e d  (1985) w i t h t h e  in detail  e s s e n c e of  the  divorced  the by  from  present  Kobro  argument  and  given  below. Our  ability  fluidised  bed  to  be  able to  we  consider  to  control  combustors predict  the  maintain a desirable furnace  of  controlling  surfaces. be  the  This  accomplished  recirculation since  the  to  first  design  combustion  surfaces) for  design  transfer  or  chamber w h i c h has  transfer  to  is a direct  heat  control  and  function and  amount of  objective  temperature  then heat  the  having  remove h e a t reduces  the  to  use  f r o m the boiler  as  of  our  ability  mechanics. being  in a  into  heat  combustion and  one  these  this  e x c e s s a i r or  efficiency,  to  riser  p r o b l e m becomes  Ideally  If  able  (water c o o l e d  transferred  i s shown i n F i g u r e 1.7. without  fluid  membrane w a l l s  sides,  circulating  should  flue  chamber the  gas  -  20 -  Fixed Inventory C F B A  Secondary Zone  Q=H AAT 0  Cooling  •—  Secondary Air  Density  p  High U - * H i g h / 9 7 + H i g h H g  Low Ug  e(  Low/9  sec  -*Low  H  Q  Q  Primary Air  g u r e 1.7  C o n t r o l o f a c i r c u l a t i n g bed c o m b u s t o r by v a r i a t i o n o f h e a t t r a n s f e r r a t e t o a membrane wall. Q r e p r e s e n t s the t o t a l heat a b s o r p t i o n , H the o v e r a l l heat t r a n s f e r c o e f f i c i e n t above t h e s e c o n d a r y a i r p o r t s , and P the mean s u s p e n s i o n d e n s i t y above t h e s e c o n d a r y a i r ports Q  s e c  - 21  second  i s mechanically  In o r d e r  to maintain  independent of of  that  and  provides  at  converse  control  transfer  "fixed here;  this  be  illustrated  inventory unit". the  reader  area  there  substantially distributed  i s fixed  by  design,  the heat  circulating  transfer  bed  typifying  of c i r c u l a t i n g what i s c a l l e d  This i s s u f f i c i e n t  i s referred  t o Kobro and type,  inventory c i r c u l a t i n g i s very  a l l o f an  zone.  to secondary  F i g u r e 1.7  different  type  little initial  hold-up  f o r our  Brereton  bed a  purposes (1985)  the " v a r i a b l e  gas  The  fluidised i n the  bed  recycle leg  i n v e n t o r y of m a t e r i a l i s  between a dense p r i m a r y  velocity  primary  This implies  design."  In a f i x e d combustor  removal i s  furnace  to vary The  one  d i s c u s s i o n of a second u n i t  inventory  by  firing  ability.  discussion, just  will  l o a d , or f o r  holds.  force i s fixed  furnace w a l l .  precisely  low  temperature,  a s m a l l amount o f h e a t  the heat  to the  In t h i s  with  then  undesirable.  furnace  f o l l o w i n g r e q u i r e s "an a b i l i t y  coefficient  high  constant  l o a d the  s i n c e the d r i v i n g  load  for  fuels  at high  temperature,  economically  l o a d or f u e l ,  high moisture  necessary;  and  -  zone and  a less  amount i n e a c h d e p e n d s upon  air split  and  the  total  gas  showing a p p r o x i m a t e d i s t r i b u t i o n s  velocities  dense  representing f u l l  conditions.  Now,  F i g u r e 1.8  coefficients  for a c i r c u l a t i n g  and  bed  boiler  velocity, for  half  shows measured h e a t (Kobro  the  two  load  transfer and  so  -  Brereton, linear  1985),  function  range.  indicating  22 -  that  of suspension  I f these  results  over  the measured  a r e combined  with  the  o f F i g u r e 1.7,  heat  surface i s located  secondary scenario  a i r ports, will  coefficient. control  The which  be  and  variation  holdup,  exit  heat  1.3  bed  and  for  load to  in velocity  bed  combustor  promoting  o n l y be  and  profiles  a  simple  utilised i f profiles  and  other o p e r a t i n g on  a macroscopic  microscopic level  temperature  level,  will  uniformity, furnace  other parameters  essential  to  the  designer.  Fast F l u i d i s a t i o n and D e n s i t y P r o f i l e s Up  what  transfer,  load  techniques.  be made o f d e n s i t y  on a macro and  temperature  circulating  density  to t h i s  i s meant by  beds have found  point fast  this  introduction  fluidisation,  application  as  the  changes i n p r i m a r y  However, i t can can  long  trnasfer  p r o v i d e s f o r extremely  with v e l o c i t y  The  the m i x i n g  determine  high  of the c i r c u l a t i n g  from a d e c r e a s e  accurate predictions  and  other  turndown  i n shaft  parameters.  the h i g h v e l o c i t y ,  f i n e - t u n e d by  as  above  tendency  turndown p r a c t i c e .  their  principally  Hence t h e r e i s a n a t u r a l  natural  decrease  i t i s evident that,  heat  air split  results  then  density  promote a h i g h e r o v e r a l l  which may  secondary  approximately  density  distributions transfer  t h e s e a r e an  thus  has  f o c u s s e d upon  where f a s t  f a r , and  why  fluidised it is  -  Figure  1.8  12,3 -  Heat t r a n s f e r c o e f f i c i e n t s i n f a s t f l u i d i s a t i o n as a f u n c t i o n of s u s p e n s i o n d e n s i t y and t e m p e r a t u r e (Kobro and B r e r e t o n , 1985).  -  important order one is  able  to c h a r a c t e r i s e  t o p r e d i c t how  a  fast  p a r t i c u l a r instance also  other In  t o be  important  cases,  these  reactor the  extent  and  why  of  this  the the  density  bed  will  a CFB  to p r e d i c t  catalyst density gas  solid  profile  perform  It  profiles  reactions profile  contact-time  in  in  combustor.  density  where c a t a l y t i c  contacting. will  fluidised  able  f o r example,  determines  the  - turndown o f  t o be  instances,  -  24  in  occur.  along  history  the and  It i s e s s e n t i a l to understand  change as  the  catalyst circulation  how  rate  is  changed. Because of received  i t s importance,  considerable  the  attention,  density  both  from  profile the  of  i t s macrostructure,  and  i t s microstructure.  is  still  the  density  not  c l e a r how  t a k e s c e r t a i n shapes, p a r a m e t e r s s u c h as rate, the  and  how  the  s i z e and  microstructural  of  view  However, i t  forms,  i t i s a f f e c t e d by  particle  point  why  it  variations in  solids  circulation  e l e m e n t s combine  to  form  macrostructure. Experimental  (Yerushalmi a l . , 1981; Li  how  profile  has  and  and  studies  Cankurt,  W e i n s t e i n £t  such as 1978;  circulating  fluidised  approximately from a high  "S"  beds may  shaped  density  Turner,  al_. , 1983)  Kwauk (1980) show t h a t  and  density  conducted 1979;  at  W e i n s t e i n e_t  profiles  Figure the  a t CUNY  similar studies  generally  profile,  asymptote  those  be  base of  in  described  1.9.  by  They the  by  an  stretch  column,  or  - 25 -  Sol ids IRON CONCENTRATE circulation, kg/M • sec top  ALUMINA  FCC CATALYST  PYRITE CINDER  16  2  4J .e 60 •H <U  rc  «  bottom  Figure  1.0 O.ft  1.9  0.9 1.0 0.« V o i d a g e ,  Density p r o f i l e s Kwauk, 1980).  f o r fast  fluidisation  ( L i and  - 26  at  some i m a g i n a r y p o i n t  inflection asymptote similar  point,  below t h e b a s e ,  or p o r t i o n s  Rhodes and  Geldart  Different  Li  included  solids  length  bed, of  B r e r e t o n and  have been f o u n d  i n a model by L i and The  density,  gas  circulation  "imposed  variables density,  rate,  and  in a later  gas  (1985). the  Many o f  these  particle  viscosity,  by  size,  gas  velocity, the  i s treated  at  section.  interprets The  the voidage  the c i r c u l a t i n g  profile  characteristic  upon e a c h c l u s t e r .  Each  i n terms  density  to a balance of " d i f f u s i v e "  by  Stromberg  l a s t parameter  and  characterised  e t al_. ( 1 9 8 5 ) ,  to influence  include  L i and Kwauk model t r e a t s  acting  shape  Kwauk (1980) e x t e n d e d  The  ascribed  recently,  more c o n t r o v e r s i a l l y ,  pressure drop." This  particles.  Fusey  o f t h e shape o f t h e p r o f i l e .  et a l . (1982).  particle  More  characteristic  e t a_l. ( 1 9 8 5 ) ,  (1985) and  factors  details  of t h i s  an  density  a t o r beyond t h e t o p o f t h e u n i t .  have been f o u n d by A r e n a  are  through  t o what seems t o be a low  profiles,  specific  -  and  profile  fluidised  of  profile  clusters  shape i s  "buoyancy"  forces  i s then  the e q u a t i o n  -1  (Z -  Z.)  (1.1)  o  where t h e f i t t e d significance  constants e  a >  as shown i n F i g u r e  e*  and  1.10.  Z±  have a  The  terms  physical e  a  and  e*  - 27 -  have been c o r r e l a t e d solids  and a s i n g l e  by L i e t a l _ . unit  shown i n F i g u r e 1.11. length  f o r the f a s t  of  the i n f l e c t i o n  et  a l . (1982).  uniform,  geometry, t h e s e was termed  Q  fluidisation,  of the s t r u c t u r e  dispersion  of aggregates  circulating  and t h e l o c a t i o n  correlated  or  approximately  p r o v i d e s a s i m p l e way o f such  as h i g h  velocities  (Yerushalmi  contacting  with  (Wainright  and Hoffman, 1974; DeLasa and Gau, 1 9 7 3 ) .  structure through  corresponds  The  process  a n a l y s e s on u n i f o r m  gas-solid  within  visual  slip  clusters The  observations i n other  below.  c a n b e g i n by c o n s i d e r i n g  fluid-solid  such a l i n e a r  suspensions.  stability  analysis,  f o r s m a l l p e r t u r b a t i o n s , and r a t i o n a l i s e d t h e of bubbles  a uniform  perturbations. velocities who  with  These a r e c o n s i d e r e d  (1971) p e r f o r m e d  formation that  flows.  rationalisation  stability  valid  resistance  a w a l l and a l s o has some p r e c e d e n t s  multiphase  Jackson  e t a_l. , 1978) and good  very well  by L i  of the c i r c u l a t i n g  bed phenomena  no d i f f u s i o n a l  being  the c h a r a c t e r i s t i c  and t h i s ,  bed i n terms o f a u n i f o r m ,  interpreting  correlations  marked by Z i were a l s o  Interpretation fluidised  Z  (1982) f o r a number o f  gas-solid  suspension  The c o n c e p t  and more d i l u t e  concluded  including  i n bubbling f l u i d i s e d  that  beds by s h o w i n g  was u n s t a b l e t o v o i d a g e  was e x t e n d e d  to higher  s y s t e m s by G r a c e  and Tuot  a l l homogeneous f l u i d - s o l i d  liquid-solid  systems,  (1979)  systems,  a r e u n s t a b l e t o such  - 28 -  Typical Voidage Distribution  Proposed Physical Modal  DKNSB CUISTEHSl , volume f r a c t i o n of dense clusters, f solids concn. i n dense phase, 1 - £ &  DILUTE CONTINUUM: volume f r a c t i o n of d i l u t e phase, 1 - f solids concn. i n d i l u t e phase, 1 - £* 4  S E G R E G A T I O N  D I P P U S I O N  voidage t  Figure  1.10  FLUX  F L U X  gas  The L i and Kwauk (1980) model f o r d e n s i t y p r o f i l e s i n f a s t f l u i d i s e d beds.  - 29 -  U'_ Re  Figure  1.11  s  G  =  C o r r e l a t i o n s f o r parameters i n the L i e t a l . (1982) d e n s i t y d i s t r i b u t i o n model. ( R e ^ ~ ~ i s t h e p a r t i c l e R e y n o l d number b a s e d on t h e r e l a t i v e v e l o c i t y between g a s and s o l i d . True v e l o c i t i e s , not s u p e r f i c i a l v e l o c i t i e s a r e used in the d e f i n i t i o n . Ar i s t h e A r c h i m e d e s number.) -  -  perturbations segregation growth o f compared  and  therefore  into multiple  the  homogeneous and s u c h as can  like  G r a c e and systems are  liquid  stable  leadshot  behave  which  entropic  non-linear  al.  density  of  p r e d i c t the  support  (1964) f o u n d  to each o t h e r ,  are  systems  ratios  which  beds.  is  important  heterogeneity.  are  limited  n a t u r e of  instability. governed  tended  that  and  spheres,  falling  velocities  higher  individual  spheres.  flow  concept  For  to  by  this  the The  system final  energetic  form and  released  through  than  using  theory.  forces; the  This the  the  terminal result  Jayaweera  in close  liquids  at  arrays  low due  velocities also  be  reflections  et  proximity Reynolds to  c l u s t e r s had  could  method o f  o f a c l u s t e r can  example,  to c l u s t e r i n r e g u l a r  hydrodynamic  mathematically  f o r the  studies.  b a l a n c e of  fluid  system,  considerations.  i n a number of  numbers,  Exceptions  perturbation,  processes  the  appear  fluidised  analyses  of  small  that a l l f l u i d - s o l i d  existence  a result  of  systems i s  systems  high  to voidage  not  rate  i t t r a v e l s through  to observers.  the  t h e y do  Theoretical found  liquid  fluidised  stability  i s formed as  depends on  i n most  Tuot's c o n c l u s i o n ,  unstable  toward  However, the  gas-solid bubbling  linear  conclusion;  show a t e n d e n c y  i n water, with  because i t e x p l a i n s However,  may  r a t e at which  so most p r a c t i c a l  -  phases.  perturbation  t o the  30  of  a  terminal the  shown and  viscous  be  -  At  higher  postulated or  R e y n o l d s numbers Zenz and Othmer  a "wake mechanism".  t u r b u l e n t wake b e h i n d  region  o f low p r e s s u r e ,  concentration the  distance  behind  fluid  will  fall  velocity fall  so s m a l l ,  laminar  constitutes a "when t h e  that  t h e wake  downstream p a r t i c l e , t h e  i n t o t h e wake o f t h e u p s t r e a m will  present  a  diameter which the p r e v a l e n t  can then c e r t a i n l y still  a  becomes s o g r e a t , o r  t h e two p a r t i c l e s  composite or e f f e c t i v e  reaction  not s u s t a i n , so t h a t t h e  farther, giving rise  to a s o r t of chain  which c o l l a p s e s the e n t i r e bed."  In t h i s  instance,  wake mechanism i s s e e n as a means o f e x p l a i n i n g a  choking to  suspension  (1960)  that  t h e o r i s e d that  r e a c h e s t h e next  and t h e r e b y  particles  the  they  between p a r t i c l e s  downstream p a r t i c l e  larger  Recognising  a single particle  of a d i l u t e  a particle  particle,  31 -  transition.  explain  fast  the t r a n s i t i o n  fluidisation Evidence  Ford  An i d e n t i c a l  from d i l u t e  a t some c r i t i c a l  f o r t h e wake e f f e c t  (1950) who d r o p p e d c h a i n s  glycerol. collapsed chain  mass, b u t beyond appeared  formed by t h e f i r s t length  since  there  this  t o exceed  link,  c a n be p o s t u l a t e d  phase t r a n s p o r t t o  solids  loading.  i s cited  of various  Up t o a c e r t a i n l e n g t h  length  effect  lengths  a chain  critical  fell  was s u f f i c i e n t  fell  drag  through  as a  length,  the length  the chain  i n t h e work o f  where t h e  o f t h e wake  as a  straight  on t h e f i n a l  extend i t . Plausible  t h e o r i e s s u c h as t h e wake mechanism,  links to  - 32  substantiated  by  a l a r g e number o f  (Razumov e t a l . , 1968; Turner,  1979;  cluster  theory  -  Reh,  Kwauk, 1 9 8 0 ) , for fast  models f o r c l u s t e r  1971;  " s i g h t i n g s " of Y o u s f i and  and  hence,  i n nature  effective  cluster  construed  as  providing  v a l u e s f o r p a r a m e t e r s w h i c h can  predictive  using experimental  validating  models.  velocity uniform  suspensions  fluidisation. the  a cluster  The  data  theory,  to  but  a  hypothesise  used  widely  and  i n sedimentation equation  an  the  as  in  studies  expansion  successfully and  be  simply  be u s e d  b a s i s f o r most o f  (1954) e q u a t i o n ,  correlation  of  tentative  h e n c e none o f them s h o u l d  The  the R i c h a r d s o n - Z a k i  1974;  A l l o f t h e s e m o d e l s were  empirical  sizes;  Gau,  l e d to general acceptance  fluidisation  sizes.  clusters  was  versus for  liquid  i s p u r e l y e m p i r i c a l and  takes  form  U  n  where Vj_ i s t h e m o d i f i e d velocity, settling  differing velocity  (1.2)  single  for liquid  i n an  particle  terminal  fluidisation  infinite  medium by a  from  the  column  correction,  log  V  =  log V  ±  +  d D  P  (1.3)  - 33 -  and  'n' i s a R i c h a r d s o n - Z a k i  with  the s i n g l e  particle  (R-Z) e x p o n e n t w h i c h  free  fall  Reynolds  varies  number  d n = 4.65  + 20 ^  (  n =(4.4  + 18  n =(4.4  + 18 -gfi) R e "  R  e  t  <  0  ,  2  )  d  n = 4.4 R e " * 0  Re"  (0.2< R e < 1) (1 < R e < 200)  0 , 1  and  Davies,  1969)  (200 <Re < 500) ( 1 . 7 )  1  t  (  1966; G o d a r d and R i c h a r d s o n , that  this  equation  particulate  fluidised gas-solid  solids  above minimum  just  bubbling,  by  Z a k i exponent  the Richardson-Zaki  8.85  found  found  correlation,  by C r o w t h e r and W h i t e h e a d  t h e normal  system  particles was used and  5  0  0  range  i n the s i z e to c a l c u l a t e  Richardson  1968; Mogan e t a l . ,  larger  with  minimum  than  predicted  v a l u e s a s h i g h as  (1968) and a v a l u e o f 32 Mogan e t  phenomenon, and  reduced  (R-Z) c o r r e l a t i o n  close  (2.4 - 4 . 6 5 ) , by a s s u m i n g  was d o m i n a t e d  (1*8)  )  ( t y p i c a l l y group A  (1978).  i n the Richardson-Zaki  behaviour  >  but the v a l u e s of t h e  'n' were much  al_. (1970/71 and 1969) e x p l a i n e d t h i s  to  t  f l u i d i s a t i o n , b u t below  by Godard and R i c h a r d s o n  the exponent  e  c o u l d be a p p l i e d t o  systems  and h i g h p r e s s u r e s y s t e m s ) ,  Richardson  R  work (Capes and M c l l h i n n e y , 1968;  has s u g g e s t e d  (1.6)  t  n = 2.4 Some e a r l y  (1.5)  t  by t h e l a r g e s t  that the cut of  distribution.  When t h i s  size  V t an R e t , an  R-Z  o f 4.88  an a c c u r a t e e x p a n s i o n  correlation  index  were o b t a i n e d .  fraction  -  34  -  However, Capes ( 1 9 7 4 ) t r e a t e d Mogan's d a t a somewhat differently. floe  U s i n g an a p p r o a c h  w h i c h had  been u s e d  to  f o r m a t i o n i n s e d i m e n t a t i o n ( M i c h a e l s and B o l g e r ,  Scott,  1 9 6 8 ) , Capes f o u n d  c o u l d a l s o be r e d u c e d an a p p a r e n t  t h a t the expansion  t o the normal  v o i d a g e and an a p p a r e n t  R-Z  treat 1962;  correlation  f o r m by  terminal  introducing  velocity.  T h e s e c o u l d be c o n s i d e r e d as t h e v o i d a g e o f t h e s y s t e m t h e a g g r e g a t e s t r e a t e d as h a r d s p h e r e s , and an aggregate  velocity  respectively.  with  effective  Using these  "apparent  v a l u e s " , e x c e l l e n t p r e d i c t i o n s of expansion r e s u l t e d , w i t h s t a n d a r d d e v i a t i o n of p r e d i c t e d  f r o m o b s e r v e d v a l u e s 4%  t h a n u s i n g Mogan's a p p r o a c h .  In a d d i t i o n ,  fell  satisfying  i n the range  3.0  t o 3.5,  technique which i s p h y s i c a l l y with  liquid It  use  systems,  t h e R-Z  index  more p l a u s i b l e ,  by  analogy  t h a n Mogan's method. s t u d i e s of  o f t h e R i c h a r d s o n - Z a k i e x p r e s s i o n i n some d e t a i l  beds.  valid  because  t o the study of f a s t  and  A l s o i t i s i m p o r t a n t to note t h a t  the  a p p l i c a t i o n s are f o r p a r t i c u l a t e characteristic  the  cases i n which a g g l o m e r a t i o n i s  c o n s i d e r e d , a concept c e n t r a l turbulent  less  values for a  i s v a l u a b l e to consider these e a r l y  they are the f i r s t  a  o f most l i q u i d  fluidisation,  fluidised  systems  and  u n d e r c o n d i t i o n s o f dense phase e x p a n s i o n .  uniformity  i m p l i e d by  those s t a t e s , a u n i f o r m i t y  considered  n e c e s s a r y t o a p p l y t h e R-Z  only The  generally  correlation,  i s also  - 35  implied  by  a p p l y i n g t h e R-Z  -  correlation  to higher  velocity  states  and  i s s u g g e s t i v e o f a p e r c e i v e d breakdown o f  bubble  and  dense p h a s e s t r u c t u r e  of  lower  the  velocity  fluidisation. Yerushalmi apply  t h e R-Z  Using  a concept  they  computed  Figure  1.12  obtained without  effective  and  cluster  diameters  which  of v o i d a g e . using force expansion  a l l gas  Avidan  by  even  an R-Z  type  to the i d e a  of l e t t i n g ' n ' e x c e e d  Richardson-Zaki  l i m i t s , and  c o n s i d e r e d i t as  index which  10  i n c r e a s e d w i t h an The  i n t h e s l u g g i n g regime  developed  turbulent  identical  t o the R-Z  Ut  i n the o r i g i n a l  by  an  effective  Yerushalmi  value of  but  and  index  'n' was  gave r e s u l t s  et a l . (1978).  used  a two  4.5-5  in  i s almost  suspensions.  e x p r e s s i o n was  terminal velocity,  the  'n' c o u l d be  approached  v a l u e f o r homogeneous  Richardson-Zaki  The  diameters  increased  f l u i d i s a t i o n , a v a l u e which  cluster  experimentally. cluster  techniques  f l u i d i s a t i o n regimes,  c h a r a c t e r i n the system. as  (1974),  (1983)  correlation.  correlated  bed.  a r e shown i n  Stromberg balance  to  reverted  nonuniformity  with  t o Capes  s l u g g i n g , c o u l d be He  fluidised  identical  o f an  that  authors  not  results  application  original  but  as a f u n c t i o n  expression.  high  similar  similar  bubbling  first  expression to a c i r c u l a t i n g  (1980) p r o p o s e d  phase  e t a l _ . (1978) were the  replaced  Vt*» found  to f i n d  which a g r e e d  effective favourably  as  -  F i g u r e 1.12  Effective fluidised  36  -  c l u s t e r diameters i n high v e l o c i t y beds ( Y e r u s h a l m i e t a l . , 1 9 7 8 ) .  - 37  Figure  1.13  illustrates  a p p r o a c h f o r one plot  of  w i t h i n a r e g i m e , and The  larger particles Geldart  has  by  one  aggregation  has  fluidisation  distinct  method may  breaks c l e a r l y not  s u c h b r e a k may  be v a l i d ,  expressions  some o t h e r  proliferated  literature.  formation  as an  the  transition transition  concept  of  velocity upon m o d i f i c a t i o n s  however, o t h e r For  empirical  example, Matsen  integral  though, the a u t h o r s  by  p a r t of  diagram.  a p p e a r t o have relative  coalescence  and  time-average sense, the packets  as p o r o u s a s s e m b l a g e s .  This  turbulent fluidised  There are  his  a d y n a m i c e q u i l i b r i u m where p a c k e t s  p a r t i c l e s undergo c o n t i n u o u s  gas-liquid  and  of a g e n e r a l i s e d g a s - s o l i d s f l o w r e g i m e  uniformity created  and  t h e same  the e x i s t e n c e of a c o n d i t i o n of  where, i n a  however, f o r Rhodes  focussed  expression;  index  delineate  turbulent  i n the h i g h  I t has  R-Z  a  authors.  have a l s o been d e v e l o p e d .  In a l l cases, envisaged  be  d i s c u s s i o n shows how  (1982) uses c l u s t e r explanation  not  Avidan's  S t r a i g h t l i n e s on  (Canada e t a _ l , 1 9 7 8 ) , and  the Richardson-Zaki  fast  case.  l o g i o U show a c o n s t a n t  been o b s e r v e d by  This brief  of  good f i t g i v e n by  ( 1 9 8 6 ) have s u g g e s t e d t h a t t h e  indicated as  the  fine-particle  logio e against  transitions.  -  convenient  i s the bed  cluster  breakup can  be  of  but treated  explanation  of  phenomena.  analogies  between g a s - s o l i d  systems which are u s e f u l f o r v i s u a l i s i n g  and  the  - 38  Figure  1.13  -  P l o t of v o i d a g e v e r s u s gas v e l o c i t y f o r f l u i d c r a c k i n g c a t a l y s t o v e r a wide range o f gas v e l o c i t y showing r e g i m e t r a n s i t i o n s ( A v i d a n , 1980).  -  39  v a r i o u s regimes which can e x i s t .  -  F i g u r e 1.14 shows t h e f l o w  regimes which a r e found i n v e r t i c a l g a s - l i q u i d w i t h a regime diagram i n d i c a t i n g  flow together  phase b o u n d a r i e s as a  f u n c t i o n o f d i m e n s i o n a l g a s and l i q u i d k i n e t i c  energy  parameters. There a r e d i r e c t  a n a l o g i e s b e t w e e n b u b b l e and s l u g  flows i n the g a s - l i q u i d  s y s t e m , and b u b b l i n g and s l u g g i n g  fluidisation  i n the gas-solid  fluidisation  i s v i s u a l l y comparable  Discounting  longitudinal  system.  Similarly,  t o churn  turbulent  flow.  g r a d i e n t s and c o n s i d e r i n g a  cluster  a p p r o a c h t h e n w i s p y - a n n u l a r f l o w w o u l d seem t o be a good analogue to f a s t  fluidisation.  Particularly in a  large  d i a m e t e r s y s t e m w h e r e t h e a r e a o c c u p i e d by t h e a n n u l u s  would  be a s m a l l f r a c t i o n o f t h e t o t a l and t h e w i s p y c l u s t e r - l i k e core would  dominate.  At t h i s p o i n t school of thinking hydrodynamics.  w i t h r e g a r d t o c i r c u l a t i n g f l u i d i s e d bed  A number o f a u t h o r s m i g h t  annular gas-liquid fluidisation  i t i s c o n v e n i e n t t o i n t r o d u c e a second  boundary  that  flow i s a f a r s u p e r i o r analogue t o f a s t  than wispy-annular flow.  w h i c h show p r o n o u n c e d turbulent  contend  Citing  experiments  r a d i a l g r a d i e n t s i n f a s t and even  f l u i d i s e d beds, w i t h s t r o n g e v i d e n c e f o r a layer,  solids  those authors contest the idea of c l u s t e r  f o r m a t i o n and e x p l a i n many c i r c u l a t i n g bed phenomena u s i n g a core-annular  model.  - 40  Figure  1.14  -  Flow regimes observed i n v e r t i c a l g a s - l i q u i d f l o w ( S o o , 1 9 8 2 ) , and a f l o w p a t t e r n map ( H e w i t t and R o b e r t s , 1 9 6 9 ) .  - 41  Strong first  radial  gradients in fast  n o t e d by B i e r l e t a_l ( 1 9 8 0 ) .  solids  flux  probe  solids  flux  profiles  combined  profiles  upstream  and  1.15.  and  The  vertical  particle  particle  of  large  a r e combined  range  o f gas  directly  velocities,  comparable  profiles  and  = 50  um,  lm/s  t o 6m/s.  kg/m  used  ) and  However B i e r l  gas  used  which,  i n the  o n l y a 75  in  mm  which  can  have a marked e f f e c t  characteristics,  particularly  of  e t ajL. a r e  catalysts  such  as  (dp32 range mm  performed  fines  fluidisation  dense phase  and  et a l . (1978).  velocities  factors  this  on  by B i e r l  cracking  the  core, with  of the type of s o l i d  et; a l . used  Other  profiles.  profile,  r i s e r , whereas t h e s t u d i e s a t CUNY were dia. unit.  the  shown i n  flux  diameter a 150  i n both  show a  density  to those of Yerushalmi  a u t h o r s a t some p o i n t s  particle  However, when  an a n n u l u s  conditions  a  g r a d i e n t s i n the  velocity.  In t e r m s  Both  Pp = 1100  to i n f e r  between an u p f l o w  suspension density, i s i n downflow.  o b t a i n e d from  the s o l i d s  to generate a r a d i a l  shows a s t r o n g d e m a r c a t i o n  When t h e s e were  profiles  radial  a  gave  these are a l s o  velocity  component o f t h e gas  average,  density  g r a d a t i o n as do  are i n d i c a t i v e  uniform  downstream  be u s e d  downstream d i r e c t i o n s ;  continuous r a d i a l  two  and  shown i n F i g u r e 1.15.  they c o u l d  beds were  Measurements made w i t h  p o i n t e d upstream  probe,  velocity  Figure  fluidized  with s t a g n a t i o n pressure p r o f i l e s  momentum f l u x  These  -  viscosity  content  - 42 -  CALCULATED DENSITY PROFILES (THIRTEEN FEET FROM ENTRANCE) CALCULATED PARTICLE VELOCITY PROFILES  2/3  (r/R)  Figure  1.15  1  O  1/3  2/3  (r/R)  Radial s o l i d s f l u x , v e l o c i t y and d e n s i t y p r o f i l e s i n a r i s e r according to B i e r l et a l (1980).  -  43  -  ( M a t h e s o n e t a l . , 1949)  are  different,  u n l i k e l y to produce such  but  would be  macrostructural  also  confirm  the  there  was  of  increased  noting  that X-ray  core-annular  height  towards i n c r e a s e d  i n the  d i r e c t i o n of  and  that  surrounding  d i l u t e cores,  that  had  one  previously.  large  and  observed of  industry,  small  interpreted  as  circulating  bed  units. being was  was  a new  risers,  that  with  However,  with  of  Bierl's  dense  generally  as  related  structures risers,  a totally  r e g i m e due  principally  to  lower o p e r a t i n g  Yerushalmi  and  (1978) e x p r e s s  sense  structure to  the  shown s i m i l a r s t r u c t u r e s  considered  annuli  f i n d i n g i n the  flow  However, t h e s e  the  decreased  remained.  beds c o n s i s t  not  to  f i n d i n g , and  flow.  c h a r a c t e r i s t i c of  Cankurt  incomplete  homogeneity  structure  t h i s type of  had  be  associated  fast fluidised  Studies  petrochemical  also  a core-annular  conclusion,  no  gross  t e c h n i q u e s were u s e d  flow s t r u c t u r e  h o m o g e n e i t y was  densities,  t o have been  B i e r l ' s r e s u l t s would  a tendency  increasing  likely  changes.  Discussion without  also  in  had  and  both  been  the  different  flow  velocities.  this difference  as  follows: " D e m i x i n g of is  the  gas  and  gross i n comparison with  fast  bed  condition.  pass every  inch  or  A probe so  from a  catalyst  the  i n the  riser  f i n e - s c a l e demixing  traversing lean  void  the  f a s t bed  to a c l u s t e r  reactor of  the  would or  -  strand  of  contours  solid, across  segregation, solid  or  vice versa.  the  high  -  44  riser  In c o n t r a s t ,  r e a c t o r r e v e a l extreme  density  z o n e s most  f l o w i n g downward a p p e a r a l o n g  remains r e l a t i v e l y Early  ideas  likely the  wall, while  that  risers  were e n t i r e l y  different  presence  substantial wall effects,  bed  operations.  Studies  ( 1 9 8 5 ) , H a r t g e e t a l . (1985) and (1985) a l l show the annulus  capacitance cases in  the  the  distinctions  than  of o p t i c a l  study.  regime  g e n e r a l l y be  lower  solids  In view of  limited  backmixing  these  and  permit,  temperature  gradients.  acceleration  region,  would  longitudinal  These d i s t i n c t i o n s  pressure  are  solids  absorption, absorption,  pronounced  and  these than  fluidised  perceptions a  beds rather  riser  higher  gas  velocities,  limit  the  degree  Apart  a l s o expect gradients  discussed  dense  Specifically,  f o r example,  longitudinal  uniform  Stromberg  f i n d i n g s , the  to q u a l i t a t i v e  transition.  one  less  circulating  d e n s i t i e s which  and  et a l .  However, i n each o f  c h a r a c t e r i s e d by  suspension  and  gamma r a y  d e m a r c a t i o n was  the  circulating  Weinstein  using X-ray  p r o b e s and  between r i s e r s clear,  by  Brereton  probes r e s p e c t i v e l y .  a distinct  would and  less  core  from  to accept  even i n  of a r e l a t i v e l y  operations  core-annulus  Bierl  become  presence  l a y e r i n CFB  combinations  the  dilute.  beds have r e c e n t l y been m o d i f i e d  fluidised  solid  representing  circulating of  density  of  appreciable from an  initial  approximately  in a riser  f u r t h e r i n Chapter  column. 4.  -  However, the beds a r e riser  similarities  sufficiently  industry, riser  strong  their  use  a number o f  installations.  diameter.  of  Neither  except  to  cases,  gamma r a y  say  c o n t o u r s and walls.  Saxton  that  Worley  risers  and  diameter  absorption  indicated 1.16  of  the  region.  The  t h o s e of  was  strong  in a  typical which  occupy  shows t h a t  e x p e r i m e n t s are  not  the  provide  the  a  to  Schuurmans  which t h e y size.  of  studied  In a l l  the  density  catalyst the up  In  this to  toward  Schuurmans  wall  core  1.1m  to  four  case  there  the  annulus  (1952) a r e  very  similar  convincing  evidence  large diameter This  artifacts  in of  the  compared  i s a valuable  e f f e c t s observed simply  over  from 0.5  to detect  segregation  et a l  equipment.  the  contours  a s u b s t a n t i a l f r a c t i o n of  which has  laboratory  Schuurmans  c e n t r e l i n e value.  Schuurmans and  reactor  (1970) and  riser  to  commercial  (1970) nor  used  i n the o i l  (1957),  shows a r e s u l t from  r e s u l t s o f Hunt  a n n u l u s can  even  of  Casagrande  commercial  indicating densities close t h a n the  cracking  ranging  Worley  a l s o a d e f i n i t e t r a n s i t i o n from  the  d e t a i l e d review  catalyst density  t h e y were o f  Figure  times higher  to  circulating  have been made on  Bartholomew and  commercial  (1980) i n d i c a t e the  paper  and a  for catalytic  et a l _ . (1957) , S a x t o n and  cross-sections  is  to j u s t i f y  studies  (1980) have a l l p r e s e n t e d  the  between r i s e r s  literature. B e c a u s e of  Hunt  -  45  small  unit, to  insight,  laboratory the  that  - 46 -  2 Q o  CATALYST  DENSITY,  kg/m  3  / / / "  / / /  lOOr-  • i  \  #  / •  * >  •1.0  /  #  0  / /•  1.0 R  Figure  1.16  C a t a l y s t d e n s i t y d i s t r i b u t i o n s over the c r o s s - s e c t i o n of a commercial r i s e r (Schuurmans, 1980 ) .  - 47  equipment with  size  correspondingly Finally,  Worley that  c a u s e d by  breaking  l a r g e boundary  phenomena may  In b o t h  a t one  cases  the  Whether or  depend upon t h e  nature  and  s u c h as  bends i n t h e  catalyst  of  feed  riser  density  not the  be  this  can  by  upward and profiles  i n a 0.3  flux  of  6.3  m/s.  create gas law. then  Breugel  400  ID  study  where s e l e c t i v i t y  engagers  upon  s t r u c t u r e has  the  are  the  i n the  been  measured  gas  velocity  shown i n  Figure  a l u m i n a a t a mean mass gas  velocity  presence of normally  f o l l o w i n g approximately  of  solids be the  lower v e l o c i t y  i m p l i c a t i o n s for conversion  i s an  phenomenon e q u i v a l e n t  um  i n what would  i s mirrored  dramatic  Results  a superficial  shows how  profile  If this  will  catalyst  mass f l u x e s , and  t r a n s p o r t o f 40  gradients  velocity  reactor  in a l l instances  have a marked e f f e c t  unit.  2  steep  i t has  m  kg/m s and  The  show  riser  e t a l . (1969-70) who  downward s o l i d s  for riser  (1957),  discontinuous,  but  baffles,  and  distribution.  van  1.17  Saxton  is beneficial  More f u n d a m e n t a l work upon r i s e r performed  by  the  process,  ratios  effects.  i n f l u e n c e d by  annulus i s  nozzles,  t o volume  Casagrande  o r more p o i n t s a r o u n d  circumference.  structures  layer  presented  Bartholomew and  core-annular  internals.  large surface area  density contours  (1970) and  -  can  a turbulent 1/7 CFB  power regime,  i n systems  i s s u e , because i t i n t r o d u c e s  to T a y l o r d i s p e r s i o n i n laminar  a flow.  - 48 -  Figure  1.17  Gas v e l o c i t y p r o f i l e , and upward and downward s o l i d s f l u x p r o f i l e s m e a s u r e d by v a n - B r e u g e l e t al. (1969-70) i n a 0.3 m dia. riser. —'  - 49 -  It  i s t h e r e f o r e important  of  the s o l i d  impact also  of s o l i d s  The van B r e u g e l  flux  profiles  determined  i n the B i e r l et  of a core-annulus  flow  s t r u c t u r e can be  undertaken  from a more f u n d a m e n t a l v i e w p o i n t  of  flow.  cluster  structure,  past  The s t r o n g e r research  into  definition gas f l o w  defined  p o r o u s media, and t h e a b i l i t y  balance  equations  Nakamura and Capes  diameter  they  obtained  riser.  developed  "intermediate  They  flow  model e x c e p t  with  various  not be s a i d  regimes".  negative  shear  i n a 76 mm  shear  they The  c a l l e d the core-annular uniform  c l o s e t o the s i n g l e  where t h e r e  on t h e r i s e r  a r e on a v e r a g e  This negative  core-annular  t o be s u p e r i o r t o a  a t gas v e l o c i t i e s  stress  a  solids  t r a n s p o r t i n what  terminal velocity  low.  t o w r i t e momentum  f l o w model t o p n e u m a t i c t r a n s p o r t  particle  particles  geometrically  tenable.  and t u r b u l e n t f l o w  model c o u l d  through  s t u d i e d c o n d i t i o n s between c h o k i n g and  pneumatic  flow  modelling  of t h e f l o w  (1973) a p p l i e d b o t h  model and a u n i f o r m which  than  on a w e l l d e f i n e d a n n u l u s w a l l combine t o  make f u n d a m e n t a l a p p r o a c h e s  well  study  (1980).  Modelling  data  have an  p r o v i d e s u s e f u l c o n f i r m a t i o n of the q u a l i t a t i v e  study  flow  understanding  mechanics s i n c e these  upon t h e gas phase b e h a v i o u r .  validity al.  phase f l u i d  t o gain a thorough  i s consistently a  w a l l because the  i n downflow and t h e gas v e l o c i t y i s stress  c o n d i t i o n may a l s o h o l d i n  -  fast  50 -  f l u i d i s a t i o n where high suspended s o l i d s  ensure  downflow at the w a l l .  One of the most  densities interesting  c o n c l u s i o n s of the Nakamura and Capes study was that a core annular flow model w i l l always p r e d i c t a lower p r e s s u r e drop than a uniform flow model.  overall  T h i s has important  i m p l i c a t i o n s i f the s t r u c t u r e i s c o n s t r a i n e d to minimise pressure  drop.  Shimizu  (1965) presents a model f o r core-annular  flow  which i s c o n s i d e r a b l y simpler than the model of Nakamura and Capes i n one sense:  i t assumes a gas v e l o c i t y p r o f i l e ,  based  upon a measured value of the c e n t r e l i n e gas v e l o c i t y , whereas Nakamura and Capes determine  gas v e l o c i t i e s i n the  core and annular s e c t i o n s by combinations balances and the c o n s t r a i n t minimised. valid it  of momentum  that pressure drop i s  Despite t h i s s i m p l i f i c a t i o n ,  the model i s more  f o r a p p l i c a t i o n s i n many c i r c u l a t i n g bed cases because  takes i n t o account  to the annulus.  bulk t r a n s f e r of s o l i d s from the core  T h i s i n turn p r e d i c t s a decaying d e n s i t y  p r o f i l e while the Nakamura-Capes model i s only v a l i d f o r fully  developed  flow.  Both of the above models are w r i t t e n f o r s p e c i f i c s c e n a r i o s where the authors have observed phenomena, and attempted  core-annular  to c o r r e l a t e t h e i r r e s u l t s i n terms  of a p h y s i c a l l y c o n s i s t e n t s o l i d s flow p a t t e r n . sense  flow  In t h i s  they are true core-annular models, which r e l y upon a  -  s p e c i f i c gas v e l o c i t y  51  -  p r o f i l e f o r annulus formation and  where each term has a r e a d i l y  apparent p h y s i c a l meaning.  Both models represent a u s e f u l and v a l i d s t a r t i n g  pointfor  any model of a c i r c u l a t i n g bed based upon a core-annulus conception, which might be used to show how could e x p l a i n c e r t a i n hydrodynamic  t h i s flow regime  c i r c u l a t i n g bed r e a c t i o n and  phenomena.  However, there a l s o e x i s t  which could be c a l l e d general upflow/downflow  models.  i n c l u d e van Deemter's "countercurrent flow model" Staub's " t u r b u l e n t flow model"  models These  (1967) and  (1979), both of which  c o n s i d e r phases flowing i n d i f f e r e n t  directions,  but n e i t h e r  of which c o n s i d e r s the s p a t i a l d i s t r i b u t i o n of the phases. Thus they could e q u a l l y w e l l be used f o r an a p p r o p r i a t e l y conceived c l u s t e r  flow as f o r a core annulus model.  Staub model i s a m o d i f i c a t i o n of van Deemter's model, was  w r i t t e n with low v e l o c i t y  an e f f e c t i v e exchange  diffusion  regimes i n mind.  coefficient,  The which  It replaces  used to express s o l i d s  between upflow and downflow phases, with a  t u r b u l e n t mixing length, and uses the Richardson-Zaki e x p r e s s i o n to represent expansion behaviour of both dense and d i l u t e  phases.  In summary, a number of models e x i s t w i t h i n the l i t e r a t u r e which with m o d i f i c a t i o n s could be a p p l i e d to a c i r c u l a t i n g f l u i d i s e d bed which i s b e l i e v e d to c o n s i s t of an upflowing d i l u t e core and a descending dense annulus.  -  There i s strong evidence characteristic  of both  -  52  that t h i s flow s t r u c t u r e i s  large and small diameter  riser  r e a c t o r s , which operate at higher gas v e l o c i t i e s circulating  than  beds, and there i s a l s o strong evidence  that there i s some form of denser bed o p e r a t i o n s .  to show  wall region i n c i r c u l a t i n g  However, i t i s not c l e a r what kind of  s t r u c t u r e s are present  i n a CFB core - whether i t i s d i l u t e  or c o n s i s t s of a s t r o n g l y backmixed c l u s t e r core and annulus i n t e r a c t  phase - or how  to produce the c h a r a c t e r i s t i c  d e n s i t y p r o f i l e s and other phenomena a s s o c i a t e d with velocity  1.4  high  fluidisation.  O b j e c t i v e s o f the Present Study At the outset of t h i s study  understanding  there was a need f o r  of c i r c u l a t i n g bed phenonmena at two l e v e l s ,  the m a c r o s t r u c t u r a l and m i c r o s t r u c t u r a l . T h i s was r e q u i r e d from a fundamental viewpoint  to determine the  natures of regimes and regime t r a n s i t i o n s , a p p l i e d viewpoint  and from an  to improve design c o r r e l a t i o n s f o r  pressure drop and heat conceptual  understanding  transfer coefficients.  understanding  of the c i r c u l a t i n g  regime, and a l l high v e l o c i t y  regimes,  A better  f l u i d i s e d bed  was a l s o r e q u i r e d t o  b e t t e r comprehend the advantages and l i m i t a t i o n s of circulating  beds as r e a c t o r s .  T h i s i n turn should  more r e l i a b l e scale-up and more cost e f f e c t i v e improved p r e d i c t i v e models.  lead t o  units  through  -  53  -  With these three broad o b j e c t i v e s as a background, more specific (i)  r e a l i s a b l e o b j e c t i v e s were e s t a b l i s h e d : To c h a r a c t e r i s e how gas  and  v e l o c i t y a f f e c t the macrostructure of a  circulating (ii)  s o l i d s c i r c u l a t i o n rate  f l u i d i s e d bed  unit.  To e s t a b l i s h what m i c r o s t r u c t u r a l  changes  accompany changes i n macrostructure. ( i i i ) To  formulate a conceptual  circulating  fluidised  the c i r c u l a t i n g  the  which e s t a b l i s h e s  f l u i d i s e d bed  p a t t e r n of the other and  bed  model of  fits  into  how  the  regimes, i n c l u d i n g choking,  to ensure that t h i s model i s c o n s i s t e n t  a v a i l a b l e m i c r o s t r u c t u r a l and  with  macrostructural  data from u n i t s of a l l s i z e s . In a d d i t i o n to t h i s main focus upon the general mechanics of the c i r c u l a t i n g bed arose from the work. the c i r c u l a t i n g bed  regime, two  smaller  These were s t u d i e s of gas  detailed  studies  mixing i n  regime, something which i s r e l a t e d  c l o s e l y to the s o l i d phase flow s t r u c t u r e , and from slugging  fluid  to t u r b u l e n t  breakdown  fluidisation.  These aspects  i n separate s e c t i o n s at the end  of the t h e s i s .  are  APPARATOS  2.  2.1  Design C o n s i d e r a t i o n s The  e x p e r i m e n t a l measurements i n t h e p r e s e n t s t u d y were  conducted unit  almost  fabricated  describes  the c o n s t r u c t i o n  and  the point The  (i) portion  criteria  Papers  this  and  the  systems.  of such u n i t s  section accompanying  References  periodically  g r e a t e r than  o f a CFB  tall  could  was  through  to  the  are given at  designed with  enough t h a t the  transport  the  substantial  acceleration ( S h i m i z u elb a l . ,  developed  acceleration  the r e s u l t s mm  fully  flow" i s taken  the apparent then  a  mean e n t r y l e n g t h s g r e a t e r t h a n  i f "developed  column,  (1981) s u g g e s t  unit  to achieve a t r u l y  lengths  (ii)  unit  o p e r a t e s beyond  However,  possible  bed  on p n e u m a t i c  pipe diameters  than  work. T h i s  bed  i n mind:  o f the u n i t  show t h a t  longer  appear  circulating  reference.  state.  base  of t h i s  dimensions  I t s h o u l d be  length. 1978)  relevant of  for this  data a c q u i s i t i o n  circulating  following  150  and  p i e c e s of apparatus  report  on a s i n g l e  specifically  instrumentation other  exclusively  that  a 152  unit  must have as  to  imply  zone a t  t o make the r e s u l t s  the  of W e i n s t e i n et a l .  ID column must be  substantially  2m. The  flow  large a diameter  amenable t o s c a l e  up.  as  -  (iii)  The u n i t  superficial minimum  only  fluidization velocity  the t y p i c a l  bed c o m b u s t i o n  which i s t h e  m a t e r i a l s u c h as  and as h i g h  pneumatic t r a n s p o r t v e l o c i t y  circulating  as 10 m/s, a  f o r a f i n e m a t e r i a l and  operating  systems.  range o f  T h e s e may use c o a r s e  m a t e r i a l s up t o 750 um d i a . . (iv)  type,  to operate at  for a fine  cracking catalyst,  s l i g h t l y beyond  inert  be d e s i g n e d  gas v e l o c i t i e s as low as 1 mm/s,  50um d i a . f l u i d typical  should  55 -  The u n i t  with  Brereton, research changes  controlled 1985).  than  i s more s u i t e d inventory unit  solids  and d r a i n a g e  be o f t h e v a r i a b l e  c i r c u l a t i o n of s o l i d s  This  a fixed  in total  addition  should  inventory  independently  o f gas v e l o c i t y .  (v)  should  The u n i t  observation (vi) allowing  of the flow The u n i t  flexibility  exit  type,  upon  circulating  the  fluid  air  staged  r e l i e s upon controlled  solids  be t r a n s p a r e n t  should  circulation  to permit  visual  be modular i n c o n s t r u c t i o n  i n geometric diameter  p a r a m e t e r s such so t h a t  bed c h a r a c t e r i s t i c s  Accommodation  introduction  which  phenomena.  and even u n i t  (vii)  ( K o b r o and  t o fundamental  through  to manipulate  inventory  should  the e f f e c t  c a n be  o f each  determined.  be made t o p e r m i t  o f t h e a i r a t more than  mechanics of combustion  as h e i g h t ,  one l e v e l  to simulate  s y s t e m s which o p e r a t e  a t two, and sometimes t h r e e  levels.  with  -  The  design  56  -  of the u n i t was  constrained  by the  following  factors: (i)  A maximum u n i t height  headroom i n the (ii)  of 10 m,  to the r i s e r of 10 Nm  scfm) a v a i l a b l e at a d e l i v e r y pressure  basic  An  (ii)  A s o l i d s separation  (iii)  S o l i d s storage  and  which comprises  i n t o the  r i s e r at i t s base.  c o n t r o l l e d by varying one  or more p o i n t s .  the  s o l i d s up and  systems. i n any  fluidised  of  bed  the typically  P a r t i c l e s are fed at a constant  large diameter storage  column, through an  The  rate  L-valve  r a t e of s o l i d s flow i s  the r a t e of a e r a t i o n to the L-valve High v e l o c i t y motive gas  discharges first  a l . (1976) has  them through an e x i t at the  of two  cyclones. u n i t and  no cone.  at  then c a r r i e s  the r i s e r , at the same rate at which they  c o a x i a l with the storage et  return  conditions  from the  i n t o the  and  was  system,  regimes the c i r c u l a t i n g  operates as f o l l o w s :  top  (5 p s i g ) .  instrumented high v e l o c i t y r i s e r ,  Under steady o p e r a t i n g  supplied,  of 34 kPa  (324  sections:  (i)  transport  /min  the framework of these c o n s t r a i n t s a u n i t  designed which i s shown i n Figure 2.1 three  by  laboratory.  An a i r supply  Within  dictated  This f i r s t  are  riser  cyclone  is  f o l l o w i n g Yerushalmi  Hence large f l u x e s of  which might choke the c o n i c a l e x i t of a more  solids,  conventional  (  -  57 •  A i r Out  Primary and Secondary Cyclones Exit  Riser Column  Modified Butterfly Valve  Storage Bed  B u b b l i n g (Storage) Bed A e r a t i o n  L-Valve  Secondary  Tangential  Air  Opposed  H.P. A i r  B lower Air  Figure  2.1  Schematic of t h e c i r c u l a t i n g f l u i d i s e d bed rig. Numbers d e s i g n a t e p r i n c i p a l pressure measurement l o c a t i o n s f o r l o o p pressure measurement s t u d i e s .  test  -  cyclone, they  can  begin  which are second  rain  not  caught  This  of  by  the  conventional  returns  captured  external, aerated  discharged  either  described  2.2  storage  cycles.  Gas  zone from where and  separator  any  high  particles  to the  circulating  a  storage  Gas  bed  to  efficiency  is  cyclonic separators  the  solids  pass  Stairmand  recycle line.  to t e r t i a r y  Each p a r t of  the  primary  t h r o u g h an  atmosphere.  -  downwards i n t o  further circulation  cyclone  design.  58  zone  then or  to  system  is  now  separately.  The R i s e r Column The  152  mm  circulating  (6")  ID  polyacrylic  165  o f 457  sections,  combined  9.3  m  mm  tubing.  multiples  fluidised  mm  (6  1/2")  (18")  introduced  into  plate  the  This  drilled  on  with  identical  screen  could  in  low  the  19%  be  and  6.3  velocity  and  exit  column h e i g h t mm  heights.  air  was  a p e r f o r a t e d p l a t e at  the  and  was  The  formed The  mm  a  12.7  7.9  mm  (1/4") h o l e s  on  lower p l a t e w i t h A 200 between  regimes,  of  (18")  Aluminium p l a t e s .  centres.  of  sections, i n  i n 457  f r e e area  the  sandwiched  constructed  transparent  inlet  of d i f f e r e n t  (1/2") t h i c k D u r a l  was  and  reduced  column t h r o u g h had  (1/2") s q u a r e p i t c h holes  lengths  be  to g i v e a u n i t  mm  cast  was  t o g i v e a maximum t o t a l  increments  12.7  OD  riser  Individual flanged  (366") which c o u l d  column b a s e .  bed  reducing  mm (5/16")  steel  plates, for the  two  upper  mesh s t a i n l e s s the  from  effective  operation open  -  area the  to approximately windbox.  screen with  8% and e l i m i n a t i n g  In the high  was g e n e r a l l y  59 -  velocity fluidization  omitted  because  f i n e s and tramp m a t e r i a l  deterioration  o f t h e blower  Air  be s u p p l i e d  could  s o l i d s leakage  to plug  this both  by slow  filters. to the r i s e r  compressor with a c a p a c i t y  studies  i t tended  generated  into  column  from e i t h e r a  o f 0.03 Nm /s a t 200 kPag (64 3  SCFM a t 30 p s i g ) , o r a S u t o r b i l t model 7 HV b l o w e r  providing  0.15 Nm /s o f a i r a t 34 kPag  A i r from  (324 SCFM a t 5 p s i g ) .  3  the  compressor  was m e t e r e d  r o t a m e t e r s and e n t e r e d compressor  a i r provided  gas v e l o c i t i e s  drop n o z z l e .  superficial  the unit,  nozzles.  also  These n o z z l e s ,  distributor 2.2.  I t was s u i t a b l e up t o a maximum  t o the primary  a i r could  This  interchanged  i n Figure  be i n t r o d u c e d  located  762 mm  2.1, a r e d e p i c t e d  any o t h e r  10 m/s.  a i r , introduced  location i sa r b i t r a r y since with  t h e windbox a t  (2 1/2") d i a . , low  gas v e l o c i t y o f a p p r o x i m a t e l y  In a d d i t i o n of  63.5 mm  port;  up t o 1.65 m/s i n t h e  Blower a i r e n t e r e d  base t h r o u g h a l a r g e ,  pressure  parallel  t h e windbox t h r o u g h a s i d e  152 mm d i a m e t e r r i s e r . its  by one o f t h r e e  section  a t t h e base  through  secondary  ( 3 0 " ) above t h e i n detail  i n Figure  t h e y may be  of the r i s e r  t o produce  g e o m e t r i e s o f i n t e r e s t ; however, t h e c o n f i g u r a t i o n shown g i v e s combustion  a dense bed d e p t h applications.  typical  Two s e t s  o f some  which i s  industrial  o f secondary a i r nozzles  -  Figure 2.2  60  -  D e t a i l of the secondary a i r n o z z l e s f o r the r i s e r column.  -  -  61  are a v a i l a b l e , both using a i r s u p p l i e d by the blower and nozzle  metered using a sharp edged o r i f i c e .  The  set c o n s i s t s of four d i r e c t l y opposed 25.4  dia. radial  i n l e t s spaced at 90°  circumference.  The  set uses four  f i n d use  (1")  the  identically  t a n g e n t i a l l y i n t o the r i s e r .  of secondary a i r i n l e t  upper  mm  i n t e r v a l s around  lower nozzle  sized i n l e t s f i r i n g  air  Sutorbilt  Both types  commercially, although  swirl  i s l i m i t e d to small u n i t s . A d d i t i o n a l f e a t u r e s of the r i s e r  inlet  tee, with a standard  s e c t i o n are the  r i g h t angle tee  construction  permitting  s o l i d s to flow f r e e l y from the L-valve  riser,  the e x i t .  and  abrupt t r a n s i t i o n a 95.3  mm  centreline  In Figure 2.1  from the 152  (3 3/4")  mm  ID v e r t i c a l  ID h o r i z o n t a l e x i t .  located 76 mm  i n t o the  t h i s i s shown as (6")  T h i s e x i t has  (3") below the r i s e r  geometries are d e s c r i b e d  i n Chapter  To allow measurement of s t a t i c and d i f f e r e n t p o i n t s along  the  permit i n s e r t i o n of v a r i o u s ports/pressure  top.  the  length of the r i s e r .  ports  i s shown i n Figure 2.3.  The  t r a n s f e r probes and  This  l a s e r doppler  was  Other  mm  and  to  access  (18") i n t e r v a l s  c o n s t r u c t i o n of  these  For more s p e c i a l i s e d  measurements, a n t i c i p a t e d i n the  a  dynamic pressures  i n t r u s i v e probes,  along  to  3.  length of the r i s e r ,  taps were l o c a t e d at 457  an  riser  the most common of s e v e r a l e x i t geometries t e s t e d . exit  solids  f u t u r e , such as heat s i g n a l s , the cost of  at  - 62 -  C O T T O N WOOL S O L I O S FILTER  1/4 NPT TO CONNECT TO T U B E FITTING W A L L  F i g u r e 2.3  Diagram of p r e s s u r e tap/probe p o r t .  - 63  modification  has  interchangeable be  readily  without  2.3  the  brackets  (54")  test  remainder  lengths  the  of  to f i x the  of  column  four  of which  can  instrumentation section  the  column. at the  i n F i g u r e 2.4  the  the  s e c t i o n s , any  of  riser.  of  use  structurally  detailed  allowing vertical  The  on  each of  brackets  provide  column, but rigidly  movement due  base,  are  and the  designed  i n a h o r i z o n t a l plane to thermal  expansion.  The G a s - S o l i d s S e p a r a t i o n System Gas  and  horizontal  solids  cyclone.  Sch.  pipe,  40  requiring to a  This  to a l l o w  design,  solids storage  serves fluxes,  zone and  the  enter  a short  the  The  m o d i f i c a t i o n s to the  300  without  cyclone mm  omitted  8"  was  for a as  extended standard  noted  by Y e r u s h a l m i  above.  et a l .  p u r p o s e of p e r m i t t i n g e x t r e m e l y  t o p a s s upward and  unduly  cyclone  modified  f r o m a p i e c e of  allowing aeration a i r , provided  cyclone  the  and  the  with  t o t h a t used  the  reduce  body o f  cone was  L-valve,  without  hosing  compared  the d u a l and  pass through  constructed  The  mm, but  riser  considerable erosion  replacement.  design,similar  (1976),  flexible  T h i s was  l e n g t h of 508  Stairmand  l e a v i n g the  s e c t i o n of  primary  to  by  a specialised  f o r the w e i g h t  principally while  (18")  column i s s u p p o r t e d  mm  support  mm  r e p l a c e d by  support  1372  457  affecting  The by  been m i n i m i s e d  -  affecting  conventional  efficiency  the  outward  solids  design  to  do  significantly,  high the  through  discharge. not  appear  i t s capture  - 64  -  PLATE  WASHER  COLUMN BRACKET  II  'I  COMPRESSION SPRING  IE H ll II •i .J  HEX M i> li I' li  11/4 SQ  NUT  TUBULAR S T E E L SUPPORT  3/8"  Figure  2.4  Detail  o f a column  support  bracket.  BOLT  -  efficiency  typically  interest.  Details  65  exceeding  -  98%  of the cyclone  f o r the c o n d i t i o n s of design  a r e shown  i n Figure  2.5. Gas primary  from  cyclone  tertiary Like  the f a s t  pipe.  However,  design. inlet  gas  duct  allow  the i n l e t  flow  one  into  storage  a  superficial used  because course are  hopper.  detailed  was  days  i n Figure  the sometimes  atmosphere. shown i n  seamless  steel  conventional located  towards  pressure  i n the  the cyclone to  wall  enhance This  drop  had  t o be  would  cause  discharge l e g . cyclone  tertiary  could  mm  flowrates  and  was  riser)  two. used  tended  2.7.  one  passed  to  discharging  (< 4  These  m/s such  cyclone  cyclones  were  i n the c i r c u l a t i n g  to break  of o p e r a t i o n .  be  cyclones  At low v e l o c i t i e s  alumina  friable  of several  a  i n t h e 152  and a t h i g h e r  i t was  S c h . 40  t o be v a r i e d ,  too high  parallel  when  vane  to  cyclone,  o f more  solids  the secondary  velocity  and  a t low gas f l o w r a t e s .  since  o r two  necessary  was  leaves  discharge  an 8"  guide  velocity  the s o l i d s  leaving  from  gas and  efficiency  up  before  cyclone  to d i r e c t  either  only  this  judiciously  Gas  was  fabricated  unit  secondary,  the secondary  A variable pitch  collection used  was  cyclone  storage  through  separators  the primary 2.6,  and  and p a s s e s  cyclone  Figure  bed and  The  down  over  tertiary  bed,  the cyclones  -  66  -  203  102  GAS GAS/SOLIDS INLET  EXHAUST  ±  K O  MOUNTING FLANGE  SOLIDS  F i g u r e 2.5  EXHAUST  P r i m a r y c y c l o n e d e t a i l , a l l dimensions i n mm.  -  67  h«  -  203  H  -•j 1271«GAS EXHAUST GAS/SOLIDS INLET 8  T  [  11  MOUNTING BRACKETS  o  3  SOLIDS DISCHARGE  F i g u r e 2.6  Secondary cyclone d e t a i l , a l l dimensions i n mm.  -  Figure  2.7  Tertiary  cyclone  68  -  d e t a i l , a l l dimensions  in  mm.  -  2.4  Storage and R e c i r c u l a t i o n Systems Solids  through  captured  a 343  mm  by  (13  the  1/2")  column o n t o a dense bed are  f e d by The  flanged length 1067  356  mm  (14")  mm  the  major  (42").  of  fifth  the  unit  further high  of  nearly  is filled 180  kg  inventory  extremely  320 with  i n the  riser  head.  Secondly,  recirculation  rate using  (Yerushalmi the  et_ al_. , 1976;  availability  Thirdly,  i t allows  riser,  up  of  return  the  of a  t o 229  mm  mm  (54"),  of  length  section contains  an  impact  Approximately  solids,  The  and  constituting a the L - v a l v e ,  L-valve  object  Firstly  1200  contains  a  of p r o v i d i n g i t provides  such  do  not  substantially affect  B u r k e l l , 1985)  at a  s i n c e changes i n  measurement of  "butterfly  valve  technique" upon  solids.  f u t u r e date to a without  the  the  which r e l i e s  fluidised  i n diameter,  a  for  L-valve,  i t permits  total when  the  the  five  two  riser,  of  expansion  system.  riser.  same  l a r g e mass of  (9")  they  column" c o n s i s t s o f  (1986).  The  threefold.  polyacrylic  base o f t h e  e x c l u s i v e of  sand.  OD  downward  the  the  with  kg,  stable operation  inventory  L-valve  Burkell  of m a t e r i a l . was  t o the  flanged  column i s f i l l e d  inventory  by  (14")  1372  s e c t i o n s of  The  mm  spiral  column b a s e , f r o m h e r e  length  mm  the  the  L-valve  described  of  356  cyclone  d i a . "storage  s e c t i o n s , two as  at  a polyacrylic  primary ID  flowmeter  the  -  69  larger  modification  -  The  storage  fluidised the  plate  from  12.7  mm  dia.  holes  with  on a 12.7 mm  as a l o w - v e l o c i t y  Fluidisation  the storage  (1/2") t h i c k ,  a i r provided  zone t h r o u g h  1.6 mm  The d i s t r i b u t o r i s  pitch  plates respectively.  A 200 mesh s t a i n l e s s  each i s  ( 1 / 16") and 3.2 mm  (1/2") s q u a r e  by  a perforated  Aluminium p l a t e ,  steel  t h e two p l a t e s t o p r e v e n t  drilled  (1/8") through  These p r o v i d e  screen  solids  1.2%  i s sandwiched  from weeping  into  windbox. The  with  zone.  a t t h e column b a s e .  u p p e r and lower  between  -  be r u n e i t h e r  two p i e c e s o f D u r a l  open a r e a .  the  enters  distributor  formed  the  s e c t i o n may  or non-aerated  compressor  70  vertical  the storage  distributor location  of the L-valve  zone and p a s s e s  directly  p l a t e s and t h e windbox;  i s concentric through  but p r e c l u d i n g gas  fluidization  (i)  To i n c r e a s e t h e s o l i d s  f o r three  To v a r y  high  solids  rates.  t h e "imposed  circulating  purposes:  head on t h e L - v a l v e ,  making i t e a s i e r t o g e n e r a t e  (ii)  t o be moved  leakage.  a i r i s used  circulation  both  i t i s s e a l e d at each  by an o - r i n g a l l o w i n g t h e L - v a l v e  vertically The  standpipe  pressure  drop"  a c r o s s the  bed as d e f i n e d by W e i n s t e i n  et a l .  (1983) . (iii) For  To p e r m i t  circulation  circulation  r a t e measurements.  rate determinations,  a f l u i d i s e d bed  - 71 -  storage  zone i s u s e d  pressure middle  drop,  i n combination  modified  butterfly  valve  are depleted  1976).  pressure  circulation  remains approximately  circulation  r a t e c a n be measured  change o f p r e s s u r e butterfly above.  When t h i s p r e s s u r e  itself)  i s equated  butterfly  preferentially  up t h i s  (less  below t h e  that across  of s o l i d s  bed l e v e l  l i n e , both  1 metre the valve  per u n i t  area, a  Opening t h e  t o normal o p e r a t i o n ; i t d r o p s below t h e gas b y p a s s e s  eliminating fluidisation  v a l v e and r e d u c i n g  the secondary  cyclone  efficiency. t h e r i s e r column,  at the d i s t r i b u t o r  Also  l i k e the r i s e r  removal o f d i f f e r e n t  is  drop  t h e lower  both  The  the r a t e of  point approximately  r e t u r n l i n e , a t which p o i n t  above t h e b u t t e r f l y  Like  Solids  by m o n i t o r i n g  r e t u r n s the system  cyclone  e t a_l. ,  for solids  constant.  t o the weight  must be opened b e f o r e  separation  (Yerushalmi  head  valve  up upon i t  r a t e measurement c a n be o b t a i n e d .  valve  secondary  build  d r o p between a p o i n t j u s t  v a l v e , and a s e c o n d  circulation  When t h i s  solids  i n t h e r e g i o n below  However t h e t o t a l  a p o r o u s p l a t e , low l o c a t e d between t h e  two f l a n g e s i n t h e r e t u r n z o n e .  ( F i g u r e 2.8) i s c l o s e d , f l u i d i s e d and  with  final  the L-valve,  t h e r e t u r n column  and w i t h  column,  brackets  i s supported  along  i t may be r e d u c e d  i t s length.  i n l e n g t h by  sections.  element  of the c i r c u l a t i n g  which c o n t r o l s the feed  fluidised  bed l o o p  rate of solids  into  Figure  2.8  Modified b u t t e r f l y valve f o r solids c i r c u l a t i o n r a t e measurement, d i m e n s i o n s i n mm.  -  the  riser.  Knowlton  L-valves  and H i r s a n  transport  lines,  Energiteknik circulating as a t y p e small  (1978) as f e e d e r s  and t h e i r  so  that  f o r pneumatic  use has been p a t e n t e d  flows  bed c o m b u s t i o n  'Properly  aeration  systems.  Studsvik  They can be c l a s s i f i e d  solids  flow  designed  not d e s i g n e d with  they  valve,  up t h e L - v a l v e .  i n "fluoseal"  type  t o c o n t r o l the s o l i d s a mechanical  valve,  using a  o f a l a r g e mass  also provide  c o n t r a s t s markedly with  which a r e used  conjunction  by  AB i n Sweden as c o n t r o l l e d r e c y c l e l e g s f o r  gas does n o t f l o w  partial  a gas s e a l  The use o f the f u l l y  fluidised  arrangements. flow,  unless  but they  These  used i n  do p r o v i d e  a  seal. Laboratory  their  has  circulating  catalytic  solids  returns  cracker  aeratable  t o be n o t e a s i l y  valves,  t h e UBC c i r c u l a t i n g  L-valve  concept.  would  particle  valve.  materials,  c o n t r o l l e d using  applications.  In t h e e v e n t using  of i n s t a l l i n g a s l i d e  fine  valve  t h e r e c y c l e l e g and p r o v i d i n g  show  fluidised  Although  this  which a r e  non-mechanical  bed was d e s i g n e d  t h e use o f L - v a l v e s  not f u n c t i o n p r o p e r l y  remained  fully  t o use t h e  The r a t i o n a l e was m e c h a n i c a l  a d e s i r e to study  fine  beds commonly  by u s i n g  c o n t r o l l e d by a s l i d e  advantages f o r f i n e  and  fluidised  heritage  reputed  of  e x t e n s i v e l y by  amount o f a e r a t i o n t o c o n t r o l t h e f l o w  solids.  gas  have been examined  of p a r t i a l l y aerated  of  are  -  73  simplicity  more f u l l y f o r that  the L-valve  material,  the o p t i o n  i n the v e r t i c a l  sufficient  aeration  section over  -  the  r e m a i n d e r of  the  fully  number o f  aeration points  showing  (6") of  the  typical  ID,  1/2")  the  mm  (6  vertical  zone.  A  711  riser  mm  feed  transmission  For  OD  tee  of  was  cast  around  together  75  mm  a neoprene  v i b r a t i o n s between  with  a  with  sleeve  152  The  the  mm height  upper  storage  connected  which  riser  diagram  loop.  from  i n t o the  the  recycle  tubing.  (96")  to  large  the  constructed  acrylic  a  recycle  h o r i z o n t a l s e c t i o n was  using  section  reason  f o r the  2438 mm  approximately  (28")  this  2.9,  flow  L-valve  s e c t i o n was  protruding  vertical  were p r o v i d e d  r e g i m e of the  165  flow.  shown i n F i g u r e  Mechanically,  portion  the  fluidised  These a r e  -  h o r i z o n t a l and  establish  leg.  74  to  prevented  and  return  column.  2.5  Data Measurement and A c q u i s i t i o n Characterisation  circulating pressure  profile  velocity, The (i) using  fluidised  solids  the  bed  along  the  used  orifice  greater  than  1.5  were used  u n i t at  are  R i s e r S u p e r f i c i a l Gas an  metre,  at  of  the  known v a l u e s  r a t e and  riser  summarised  of  the riser  below:  V e l o c i t y - This  and  was  measured  s u i t a b l e f o r gas  velocities.  with  velocities  Rotameters c a l i b r a t e d with  lower  gas  geometry.  f a b r i c a t e d to ASME s t a n d a r d s ,  orifices, m/s.  macrostructure  r e q u i r e s measurement o f  circulation  techniques  interchangeable  meter  of  a dry  test  - 75  6000  Ib/hr  STAGNANT S O L I D S REGION  RETURN COLUMN  Figure  2.9  A e r a t i o n p o i n t s on t h e L - v a l v e and a t y p i c a l o p e r a t i n g mode from K n o w l t o n and H i r s a n (1978). D i m e n s i o n s i n mm.  - 76 -  (ii)  Solids Circulation  was measured by  either  R a t e - The s o l i d s  using  tracking individual  particles  L-valve  and a s s u m i n g p l u g  Burkell  (1986) compared  they  c o u l d be used  calibration constant correct  flow  these  voidage  transducer  pressure  with  for different  f o r u s i n g one t e c h n i q u e  second  i n the second  individual the is  measurements  of  t o assume  over  51 D 20 c a p a c i t a t i v e  has i n t e r c h a n g e a b l e  differential f o r sand  pressure  trials. case  and a  t o t h e ease o f i d e n t i f y i n g  with  time  a designated  that i s o l a t e d  itself.  measurements  The  pressure  on t h e b u t t e r f l y times f o r  300 mm  length of  o f F i g u r e 2.10 i n d i c a t e s t h a t i t  the modified  the c i r c u l a t i o n  the repeated  alumina  a t a b u l a t i o n of t r a n s i t  The l i n e a r i t y  using  of  i n the f i r s t  up o f a l u m i n a  sand p a r t i c l e s  justifiable  In g e n e r a l , t h e  F i g u r e 2.10 shows a t y p i c a l  2.1 p r o v i d e s  L-valve.  affect  i s related  particles.  Table  that a  The c a l i b r a t i o n  velocity.  t r a c k i n g was u s e d  f o r the b u i l d  that  f o r t h e moving bed used t o  f o r measurement  reason  valve;  case.  This transducer  diaphragms s u i t a b l e  individual  l e g of the  t o r e c o r d t h e change o f d i f f e r e n t i a l  time.  Particle  technique, or  and showed  provided  r a t e s , u s i n g a Disa type  pressure  rate  the standpipe.  techniques  measured t o mean r a d i a l  circulation  trace  across  was made i n t h e l a t t e r  v a l v e was used  valve  i n t h e downflow  interchangeably  was an a p p a r e n t  butterfly  ranges.  the b u t t e r f l y  circulation  circulation  butterfly  v a l v e do n o t  The low s t a n d a r d  of Table  rate  deviation  2.1 shows t h a t  -  Figure  2.10  77 -  A t y p i c a l pressure trace f o r build-up of a l u m i n a on t h e b u t t e r f l y v a l v e f o r a c i r c u l a t i o n r a t e measurement. A straight i s s k e t c h e d t o show t h e l i n e a r i t y o f t h e build-up.  line  - 78 -  Table  2.1  T r a n s i t Times f o r P a r t i c l e s o v e r a 300 mm V e r t i c a l S e c t i o n of the L-Valve Determined V i s u a l l y ' a Typical Result  Particle  Number  Time ( s )  1  5.48  2  6.96  3  6.79  4  6.28  5  6.64  Average Standard  time deviation  6.43 0.587  -  particle  tracking  79 -  p r o v i d e s a s t r a i g h t f o r w a r d and  r e p r o d u c i b l e method o f d e t e r m i n i n g individual (iii)  particles  are easily  Riser Pressure P r o f i l e s  determined  using the pressure  increments  along  the r i s e r  c o u l d be c o n n e c t e d F i g u r e 2.11.  circulation  r a t e s when  identifiable, - Pressure  taps  profiles  l o c a t e d a t 457 mm  length.  between any 2 a d j a c e n t  (18")  Any 14 of t h e s e  i n t o a s y s t e m o f two m a n i f o l d s  Hence d i f f e r e n t i a l  were  pressures  p o i n t s t o generate  taps  shown i n  c o u l d be measured the r i s e r  pressure  profile. E a c h m a n i f o l d was manometer Accurate  i n t o one end o f e i t h e r  or the Disa c a p a c i t a t i v e pressure p r o f i l e  transducer,  together  below) t o r e g i s t e r time.  connected  with  determination a data  these  using transducer  calibrations  demanded  i n output  voltage signal  a v e r a g e v a l u e w h i c h c o u l d be c o n v e r t e d pressure  tranducer. use of t h e  l o g g i n g system ( d e s c r i b e d  the v a r i a t i o n s  I n t e g r a t i o n of t h i s  pressure  a  calibration  voltage  then  into a curves.  with  yielded  an  differential Linearity  of  made t h e i n t e g r a t i o n / c o n v e r s i o n p r o c e s s  straightforward. Logging determination suitable example their  signals  i n this  manner  of a v e r a g e p r e s s u r e  i n c a s e s where t r a n s i e n t s i n s t u d i e s of p r e s s u r e  mean v a l u e s .  attenuated  In these  was  drops,  suitable for but was not  were o f i n t e r e s t , f o r  fluctuations  cases  signals  by t h e l e n g t h s of p r e s s u r e  rather  than  would be  lines,  and t h e v a l v e s  -  -  80  R iser 14 13 12 11  Pressure Transducer  ~i  10  ~i  r  9  0  8 7  ±  6  1  5 4  i  i  2  "IL  1  I  i i - n i l  I  xh-  - C X -  -00-Dx}-  -CXr-  ""1 i  3  X H  Manifolds  — C X r -  -tXr-  --txj.  --tx-  -CXr-  -txj-  -CXr-  I  L___  Manometer  Figure  2.11  Manifold construction for d i f f e r e n t i a l pressure measurements showing how 14 r i s e r l o c a t i o n s a r e manifolded.  -  in  the m a n i f o l d .  separate located  Therefore,  pressure as  81  lines  -  for transient  were s e t up  and  measurements,  the  transducer  c l o s e as p o s s i b l e t o t h e p r e s s u r e  was  fluctuation  source. The time  datalogging  variations  s y s t e m w h i c h was  in transducer  number o f p o i n t s i n t h i s signals. (1986).  I t has The  programmable run  at  r a t e s up  pressure and  datalogging  v o l t a g e , was  in detail  by  board an  IBM  software  operating XT  presented  t o 30,000 p o i n t s p e r sampling  by  i n the  r a t e s o f 100  d u r a t i o n s of 30  mode  and  in  (1986), to -  the 12V  l o g g i n g of  p o i n t s per  s were t y p i c a l l y  voltage  Operating  Burkell  For  a  D/A  r a n g e + 12V  second.  at  Burkell  in bipolar  computer.  the  used  to record d i f f e r e n t  could a c q u i r e data  signals,  sampling  t o measure  s y s t e m c o n s i s t s o f a Tecmar A/D,  f a s h i o n , with  Tecmar b o a r d  study  been d e s c r i b e d  i n c o n j u n c t i o n with  this  output  used  used.  second  - 82 -  3.  3.1  EXPERIMENTAL  D e n s i t y P r o f i l e s and Entrainment  Rates -  Macroscopic  Aspects  3.1.1  C o n s i d e r a t i o n s r e g a r d i n g use of pressure As  noted  circulating by  fluidised  the v a r i a t i o n  along  profiles  gas-solid  catalyst  can a s s i s t  determine  i n corabustors.  influences  ( K o b r o and B r e r e t o n , Density  (i.e.  profiles  bed.  t o the weight  i t slocal  heat  transfer  level,  partially value  profoundly  coefficients  pressure  profile,  pressures  The p r e s s u r e  of the s o l i d s  over  p o r t i o n i s accounted  from t h e  o r from  direct  regular  intervals  drop  i s then  and f l u i d  t o t h e h y d r o s t a t i c p r e s s u r e ) , assuming  negligible  This  and p o l l u t a n t  are g e n e r a l l y i n f e r r e d  of d i f f e r e n t i a l  a circulating  bed s y s t e m .  1985).  o f the a b s o l u t e  measurement  totally  the e v o l u t i o n of  density w i l l  while  suspension-to-wall  to p r e d i c t  On a more m e c h a n i c a l  solids  fan requirements,  height  i n p r e d i c t i o n s of conversion  and o f c a r b o n  i n t e g r a t e d suspended  along  density with  t o be a b l e  i f one i s t o s t u d y  systems,  concentrations  gradient  solids  I t i s important  history  of  beds has g e n e r a l l y been c h a r a c t e r i s e d  c o n t a c t i n g i n any c i r c u l a t i n g  contacting  the  1, t h e m a c r o s t r u c t u r e  o f suspended  t h e column.  density  in  i n Chapter  data  ascribed  per u n i t  area,  that a  f o r by t h e combined  effects  -  of  gas-wall  friction,  acceleration.  83  -  solids-wall  T h i s assumption,  friction,  and s o l i d s  leading to  has been shown t o be a c c u r a t e  t o w i t h i n 10% by T u r n e r  152 mm  (Turner,  ID column u s e d  a t CUNY  et  a l . (1985) found  that the error  in  a s m a l l e r column  (41 mm  this  study  runs  of i n t e r e s t ,  as were drop.  gives  t  was c a l c u l a t e d  velocity  order  Using  i n t h e Rose  kg/m3.  flow  a value  (10  i n a 150 mm  flux  o f 70 kg/m s a t a gas 2  An a d d i t i o n a l  w i t h a d e n s i t y o f 100  , a c c e l e r a t e d o r d e c e l e r a t e d between  over  a 3 m d i s t a n c e , adds o r s u b t r a c t s 15 kg/ra  approach  this  drop  h o l d up o f 1.3  d i a . column.  by e q u a t i o n  factor  pressure  kg/m  density given  crude,  frictional  ), frictional  shows t h a t a s u s p e n s i o n  apparent  pressure  o f Rose and  admittedly of  small  by Soo (1982) f o r a  as e q u i v a l e n t t o a s o l i d s  T h i s was f o r a s o l i d s  calculation  _ l +  were  the c o r r e l a t i o n  indicated  correlation,  o f 7 m/s  drops  of magnitude e s t i m a t e  In  i n a l l the  frictional  (1957) was a p p l i e d w h i c h , w h i l e  a useful  effects.  estimate,  o f 7 m/s.  3.1 s i n c e ,  gas p r e s s u r e  o f t h e two-phase  For the l a t t e r  Barnacle  equation  frictional  However A r e n a  c o u l d be a s h i g h a s 70%  ID) a t v e l o c i t i e s  we have employed  estimates  1978).  fora  3.1.  magnitude a t e n t r a n c e s  rest  Hence,  and e x i t s .  and 6 m/s to the errors  may  -  3.1.2  Initial At  the outset  the p a r t i c u l a r proceed did  studies with  circulating density  different  solid.  scanning  closely  spherical  Early  attempts to o b t a i n  column shown  high  circulation  valve  circulation  Figure  with  situation  reproducible  3.1.  A  that the  mass f o r m i n g  experience,  at i d e n t i c a l  a violent  a  i n a small downward  i t was n o t p o s s i b l e alumina using  there  38 mm  solids  flow  a  High flow  i n the  had been no  (1.5") d i a . t e s t velocity.  i n the set-up  vertical  profiles  w h i c h used a  stick-slip  although  problems with  remedied  density  r a t e s of the f i n e  w h i c h had a s h o r t e r was  i n Table  a single aeration point.  behaviour  were t h e r e 3.2  distribution,  each c r y s t a l  (6") d i a . L - v a l v e ,  o f such  running  Neither  Our f i r s t  r a t e s promoted  152 mm  fluidisation  3.1, shows  i n F i g u r e 2.1, showed t h a t  customary L - v a l v e  evidence  with  o f 64um,  particle.  were u n s u c c e s s f u l .  obtain  mean d i a m e t e r  size  Figure  authors.  a l u m i n a as t h e  by a s i e v e a n a l y s i s , i s g i v e n  e l e c t r o n micrograph,  We  were  of e a r l i e r  The c o m p l e t e  then t o  of the flow.  and a measured minimum  i s polycrystalline  large  those  which  had a S a u t e r  alumina  to  profiles  from  This  o f 10.5 mm/s.  determined  to c h a r a c t e r i s e  the m i c r o s t r u c t u r e  s t u d i e s were c o n d u c t e d w i t h  o f 3500 kg/m  velocity  simply  CFB column i n w h i c h we were w o r k i n g ,  not a n t i c i p a t e f i n d i n g  Initial  alumina  t h e i n t e n t i o n was  r a p i d l y to study  significantly  84 -  L section.  by j u d i c i o u s a e r a t i o n  of The  around the  -  85 -  Table 3.1 P r o p e r t i e s of Alumina Used i n High V e l o c i t y F l u i d i s a t i o n Studies Property  Value  Mean P a r t i c l e Diameter, d P  64  um  P a r t i c l e Density, kg/m  p  3,500  p  3  Bulk Density, kg/m  1,140  3  Loose Packed Voidage,  e L P a r t i c l e Terminal V e l o c i t y , V , based on gas p r o p e r t i e s at 25°C m/s  *0.67  T  0.24  t  Archimedes Number  31  U „ Experimental mf m/s  0.010  Bulk Density at Minimum F l u i d i s a t i o n , kg/m  1,100  3  Bed  Voidage at Minimum F l u i d i s a t i o n ,  e^  Angle of Repose •includes internal  m  *0.69 30°  voids of p a r t i c l e s .  Table  3.1  cont'd  - 86 -  Size Analysis Sieve Interval  P a r t i c l e Diameter um  Weight Fraction  710-600  655  600-500  550  500-425  462.5  425-355  390  355-300  327.5  300-250  275  250-212  231  -  212-180  196  -  180-150  165  1.1  150-125  137.5  2.8  125-106  115.5  5.8  106-90  98  11.8  90-75  82.5  24.4  75-63  69  37.0  63-53  58  3.6  53-45  49  4.4  45-38  41.5  3.3  38-0  19  5.7  -  Figure  3.1  87  -  S c a n n i n g e l e c t r o n m i c r o g r a p h of a l u m i n a particles. M a g n i f i c a t i o n 400 x.  -  88  -  A i r Out  Exit  Modified Butterfly Valve  Riser Column Storage Bed  B u b b l i n g (Storage] Bed A e r a t i o n  L-Valve Aeration  Blower Air  Figure  3.2  C i r c u l a t i n g f l u i d i s e d bed as i n i t i a l l y constructed. A l u m i n a c o u l d be c i r c u l a t e d i n t h i s short L-valve design with s i n g l e point aeration.  -  L-valve  seal.  T h i s appeared  characteristics corner  t o improve  of the alumina,  which remained  circumstances  89 -  totally  except  the flow i n the r e g i o n of t h e  deaerated.  a s m a l l amount o f m o t i v e  Under  these  gas d i r e c t e d  the  a x i s of the h o r i z o n t a l s e c t i o n of the L - v a l v e ,  the  corner  s e c t i o n , provided  circulation  excellent control,  runs,  the r e s u l t s  were p l o t t e d  were somewhat  shown i n F i g u r e 3.3 f o r a s e r i e s velocity  was h e l d  circulation half  substantially  r a t e v a r i e d over  higher  from  results  up t h e column,  density  or a steady  rising  toward  observed 3.2. lack  constant  i n early  runs  of previous  there  In t h e lower  and t h e h e i g h t  isa with  However, decay o f dramatic the d e n s i t y  In r e t r o s p e c t ,  because a s i m i l a r  However, a t t h a t p o i n t  i n which gas  and t h e s o l i d s  authors.  of the p r o f i l e ,  work p e r f o r m e d  They a r e  behave a s one would  the top of the u n i t .  surprising  of height  of three  density value,  i n the curvature  less  surprising.  instead of a continuous  change  were  even a t h i g h  f o r a number o f  a wide r a n g e .  o f t h e column t h e p r o f i l e s  anticipate  aerating  rates.  When t h e d e n s i t y p r o f i l e s steady  along  behaviour  these  had been  i n t h e column o f F i g u r e  i t had been a t t r i b u t e d  to a  o f t h e column had been  extended. Three  f u r t h e r runs  were a t v e l o c i t i e s first  lower  were made w i t h than  the alumina.  the o r i g i n a l  4.8 m/s.  two were a t 3.8 m/s and a t c i r c u l a t i o n  rates  These The  -  90 -  O  5  m on t-  6 -  {/) Q  > O m < x o  0  Figure  3.3  40 80 120 160 SUSPENDED SOLIDS DENSITY , k g / m 3  200  L o n g i t u d i n a l d e n s i t y d i s t r i b u t i o n s f o r alumina i n a c i r c u l a t i n g f l u i d i s e d bed, Ug = 5.4 m/s, G = 18, 41, and 95 kg/m s. 2  s  - 91 -  equivalent  to the higher  made a t a s u p e r f i c i a l permissible  the  higher  close  rate  run  velocity  r u n s made a t c o n s t a n t  2  Cursory  examination  alumina  shows t r e n d s  consistent 3.5 a  with published  Figure  at the lowest  effect  which,  solids  circulation  demanded  earlier  considerable  upon c o m m e r c i a l  regarding  the nature  joints.  measured a t a 3.5 shows t h e f i n a l  circulation  rate  profiles  data.  of the (« 20  dilute  design,  of the d i l u t e  However, exit  gas v e l o c i t i e s and t o occupy  previously  attention  Figure  zone f o l l o w e d by  i s a dramatic  of high  This  o f t h e column  freeboard.  there  generated  F o r example,  acceleration  can s t r e t c h  of the u n i t .  impacting  at the  could  column  i n t h e lower h a l f  at combinations  upper h a l f  effect  slugging  3.6 i s a p l o t  of the d e n s i t y  velocity,  rates,  rates at  gas v e l o c i t i e s .  shows t h e c h a r a c t e r i s t i c  except  a value  limited the  profiles  solids  dense phase and a d e c a y i n g  the  which  o f 3.8 m/s; F i g u r e Lastly,  r u n was  of the c i r c u l a t i o n  "choking" behaviour  of density  kg/m s) but a t d i f f e r e n t  with  velocity,  cause breakage a t the f l a n g e d  a t Ug = 2.6 m/s.  three  for this  The f a c t o r  This  3.4 i s a p l o t  superficial  The f i n a l  a t 2.3 m/s was a v i o l e n t  b a s e o f t h e column. occasionally  rate  cases.  o f 2.3 m/s a t t h e h i g h e s t  t o the lowest  velocities.  circulation  Figure  velocity  circulation  fortuitously  velocity  since,  t h e whole o f  unobserved i n addition to  i t has i m p l i c a t i o n s phase.  -  0  20  92 -  40  60  80  KM)  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.4  L o n g i t u d i n a l d e n s i t y d i s t r i b u t i o n s f o r alumina i n a c i r c u l a t i n g f l u i d i s e d bed, Ug = 4.3 m/s, G = 21 and 42 kg/m s. 2  s  - 93 -  0  200  400  600  800  1000  SUSPENDED SOLIDS DENSITY , kg/m3  F i g u r e 3.5  L o n g i t u d i n a l density d i s t r i b u t i o n f o r alumina i n a c i r c u l a t i n g b e d , Ug = 2.6 m/s, G = 25 k g / m s . s  2  - 94 -  200  400  600  800  1000  SUSPENDED SOLIDS DENSITY , kg/m3  F i g u r e 3.6  L o n g i t u d i n a l d e n s i t y d i s t r i b u t i o n s f o r alumina i n a c i r c u l a t i n g bed, G approximately constant (= 20 kg/m s), U = 2.6, 4.3 and 5.4 m/s. s  2  g  -  Further because  so t h a t  of m a t e r i a l .  therefore  filled  diameter,  and  given  A full  i n T a b l e 3.2.  typical bed  w i t h Ottawa sand  combustion  operation without  systems.  required  to  column  was  \im S a u t e r mean  o f t h e sand  used  Also,  rate  chosen  because  in circulating  it is  fluidised  permits  gas v e l o c i t i e s  requirements  with  properties i s  the m a t e r i a l  at higher s u p e r f i c i a l  the c i r c u l a t i o n  o f 156  sand was  o f t h e bed m a t e r i a l  The  underwent  e x p e r i m e n t s were p e r f o r m e d  listing  A fine  possible  alumina  distribution.  a l l subsequent  material.  The  c o n s i d e r a b l e make-up was  maintain a constant size  this  -  e x p e r i m e n t s w i t h a l u m i n a were not  of a p a u c i t y  attrition  95  (6 - 10  m/s)  becoming  excessive.  3.1.3  S t u d i e s of the e x i t To  sand,  establish  the e x i t  as w i t h alumina, a s e r i e s  conceived. at  that  effect  These  regular  velocity "choked  dense  Because  choking i s a contentious  series. dense  phase"  circulation  bed  and  to d e f i n e  was  750  was  velocity and  was  set  a t each  gas  mm  up  the  until  a  column.  phenomenon i n t h e  transport  i t as a v i s u a l  a t the base  was  increased  literature,  phenomenon  However, i n a l l c a s e s t h e v i s u a l  phase  runs"  9 m/s,  rate  visible  pneumatic  present with  o f "choked  i n c r e m e n t s between 4 and  circulating  was  were r u n s where t h e gas  the s o l i d s  necessary  effect  of the column  i t was  for this  appearance  coincided  test  of a  with a  -  96  Table Properties  -  3.2  o f Ottawa Sand Used i n H i g h Fluidisation Studies  Property  Value  Mean P a r t i c l e  Diameter,  d P  um Particle kg/m  Velocity  148  Density, p p  2,650  3  Bulk Density, kg/m  1,550  3  Loose  Packed  Voidage,  e  0.42  L  P a r t i c l e Terminal V e l o c i t y , V , b a s e d on gas p r o p e r t i e s a t 25°C m/s t  Archimedes U  mf  E  x  P  e  r  i  Number m  e  n  t  a  i  0.99  290 0.021  m/s B u l k D e n s i t y a t Minimum F l u i d i s a t i o n , kg/m  1,500  Bed  0.43  3  Voidage  Angle  a t Minimum F l u i d i s a t i o n ,  o f Repose  29°  Table  3.2 cont'd  - 97 -  Size Analysis Sieve Interval  Particle  vva  Diameter  Weight Fraction  710-600  655  0.1  600-500  550  0.2  500-425  462.5  0.3  425-355  390  0.7  355-300  327.5  1.4  300-250  275  0.9  250-212  231  12.7  212-180  196  11.5  180-150  165  31.5  150-125  137.5  22.4  125-106  115.5  8.9  106-90  98  4.6  82.5  2.9  .75-63  69  1.5  63-53  58  0.3  53-45  49  0.1  45-38  41.5  0.1  38-0  19  0.1  90-75  -  situation a  where a s m a l l  dramatic  intended riser  rise  i n the  to imply  was One  that  slugging,  increase fast the  only  bed  wished  up  at  to determine  each gas  that  the  velocity.  sufficiently  long,  sufficiently  short,  fraction  of the  limiting  density  and  column  inventory.  these  I t was  This the  "choked dilute  hoped  entrance  at  their  their  for  length  base.  combustion  This  length.  The  fact  represents  processes  of h i g h e r  providing expense  and  that  phase s o l i d s  hold  the  of  tall,  Figure  decay  3.7  series.  exit  effect  the  circulation  sand  a  limiting  For which  runs.  over  small  the  diameter  existing  lengths  drop.  The  profiles  i s enhanced by  the  case  power  situation  column  there  higher  The  small,  t r a n s f e r at  obtained  f o r alumina,  form  hold-ups.  s i n c e , as  u s u a l l y exceed  sand, as  fan  over  to  practical  solid  heat  was  some'  finding  r a t e s would be  different  shows s i x d e n s i t y  test  column  lengths  reached  much h i g h e r  units, is entirely  the  that  exit  value  minimal a d d i t i o n a l o v e r a l l  Chapter 4,  base o f  i n such u n i t s , s i n c e  i n terms o f p r e s s u r e  diameter  not  i f a choked phase were a l l o w e d  r e q u i r e m e n t s would p r e v e n t benefits  is  tests"  columns would p r e s u m a b l y have such a d e n s i t y much of  caused  was  that  t h i s would be  l a y i n the  rate  such a dense phase e x i s t e d .  limiting  the  that  in circulation  dense phase o f  r a t i o n a l e f o r running  we  -  93  for  high large  discussed  in  lengths. i n the is a  choked  dramatic  velocities  of  - 99 -  0  1  0  Figure  3.7  ' 100 200 300 400 SUSPENDED SOLIDS DENSITY , k g / m 3 1  1  1  1  500  L o n g i t u d i n a l d e n s i t y p r o f i l e s o b t a i n e d f o r sand i n a c i r c u l a t i n g bed as t h e base a p p e a r e d v i s u a l l y choked, gas v e l o c i t i e s between 3.7 and 9.2 m/s.  - 100 -  The effect it  significance  a r e noted  should  i n subsequent  be n o t e d  phase v a r i e d  t h e n a t u r e o f t h e choked  considerably  with  velocity,  with  carryover,  flow dominated  from a  slugging  a t t h e low end, t o a  visually  by s t r a n d s  exit  o f t h e column,  significant. return  at the  to  8.1 m/s.  of  circulation  i n which  Density rates  3.8.  geometry  c h o i c e of e x i t  A detailed  make t h e s i m p l e s t  circulating  i s also  effect  geometry  was v a r i e d  bed t o t h e c y c l o n e .  by p r e s s u r e  drop  These  profiles upon  exit,  design,  by t h e d e s i r e  from t h e  The a c t u a l  constraints  set of  effect.  f o r the i n i t i a l  transition  4.9  f o r a number  o f gas v e l o c i t y entrance  was  from  The c o m p l e t e  3.13, was m o t i v a t e d  possible  plate.  s e t of t e s t s  were e s t a b l i s h e d  a solids  i n Figure  was  t h e column was r e a s s e m b l e d t o  a t each gas f l o w .  the important  shown i n d e t a i l  found a t  s e t of experiments  t h e gas v e l o c i t y  profiles  what i s e v i d e n t l y  dictated  profiles  i s shown i n F i g u r e s 3.9 t o 3.12.  demonstrate  The  purpose  effects  1.98 m above t h e gas d i s t r i b u t o r  i s shown i n F i g u r e  then performed  t h e next  density  whether e n t r a n c e  For t h i s  the s o l i d s  profiles  and more on e x i t  by t h e i n t e r e s t i n g  aimed a t d e t e r m i n i n g  to  high  Entrance e f f e c t s , Prompted  and,  Here  a t t h e h i g h end.  3.1.4  This  of the t h e s i s .  that  smooth r e f l u x i n g  the  sections  of the e x i t  though  type c o n d i t i o n  wall  and t h e i m p l i c a t i o n s  which  sizing  was  required  then  that  - 101 Air Out P r i m a r y and Secondary Cyclones  Modified Butterfly Valve  Storage Bed  B u b b l i n g (Storage) Bed A e r a t i o n L-Valve  Jl(*  L-Valve Aeration  H.P.Air  Figure  3.8  R i s e r column c o n f i g u r e d t o r e t u r n s o l i d s 1.98 m above t h e d i s t r i b u t o r p l a t e f o r e n t r a n c e e f f e c t studies.  -  0  Figure  3.9  102 -  100 200 300 400 SUSPENDED SOLIDS DENSITY , k g / m 3  500  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, U = 4.9 m/s, G = 24 and 26 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above t h e gas distributor. g  s  -  103  -  O  5  go on  6  Q Ui  > O m <  0  150  300  450  600  750  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.10  Longitudinal density profiles f o r ^ a n d , U = 6.1 m/s, G = 3 5 , 4 5 , a n d 59 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m a b o v e t h e g a s distributor. g  s  - 104 -  0  100  200  300  400  500  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.11  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, U = 7.1 m/s, G = 45 and 73 kg/m s, abrupt e x i t , s o l i d s r e t u r n at 1.98 m above the gas distributor. g  2  s  -  105 -  9 -  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.12  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 8.1 m/s, G = 66, 71, and 82 kg/m s, a b r u p t e x i t , s o l i d s r e t u r n a t 1.98 m above t h e gas distributor. 2  s  - 106  Figure  3.13  D e t a i l s of t h r e e bed application.  -  exits  studied  for  circulating  - 107 -  the  entrance  velocity  approximately the  152 mm  choice  the  (6") r i s e r ) .  industrial generally  that  curved  the exit  second  in  could  was d e s i g n e d a smooth  of the e x i t  be o b s e r v e d  to follow the we p r o p o s e d  geometry.  which minimised  This  radius  exit  exit  Therefore  r e f l e c t i o n and  v i a a long  of t e s t s  was r u n w i t h  bend  i s also  used  from  shown  later for  t h e smooth e x i t  o f 7.1 m/s and a t c i r c u l a t i o n  kg/m s.  These  compared  directly  which  could  purposes.  series  results,  and d e p i c t e d  immediately  would  and u l t i m a t e l y  velocity  exit  units  considerable  channel,  be a l t e r e d ,  3.13 t o g e t h e r w i t h a t h i r d  to  current  i n which  the e x i t  transition  f o r the  transition.  of s o l i d  column t o t h e c y c l o n e .  comparative A  effect  exit  upflow Figure  results,  into  by t h e c u r v a t u r e  incorporated the  with  t h e t o p end p l a t e  gas p a t h  (9 m/s i n  circulating  However, i n d u s t r i a l  the i n i t i a l from  rate  the entrance v e l o c i t y  consistent  due t o t h e i n a b i l i t y  controlled a  velocity,  have a somewhat more g r a d u a l  visually,  below  rationalization  a t 7 m/s, a t y p i c a l  CFBC p r a c t i c e .  reflection  sharply  The f i n a l  i s 15 m/s, a l s o  Following solid  was t h a t  bed combustor  cyclone  be k e p t  20 ra/s a t t h e maximum gas f l o w  of s i z e  fluidised  to the cyclone  with  the previous  i n Figure  apparent.  visually  shown i n F i g u r e  With  i t appeared  rates  a t a gas  from 36 t o 116  3.14, can be  tests  made on t h e o l d  3.11. The d i f f e r e n c e i s t h e new " z e r o  reflection  t o be, t h e d e n s i t y  profile  exit",  -  108 -  Legend  0  K  K  )  2  0  0  3  0  0  4  0  0  5  0  0  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.14  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand. U = 7.1 m/s, G = 36, 73, 93, and 116 kg/m s, smooth e x i t , s o l i d s r e t u r n 1.98 m above t h e gas distributor. g  s  -  Figure  3.15  109  -  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 7.1 m/s, G = 73 kg/m s, smooth and a b r u p t e x i t s , s o l i d s r e t u r n 1.98 m above t h e gas d i s t r i b u t o r . T r i a n g l e s r e p r e s e n t smooth e x i t p r o f i l e , c i r c l e s abrupt. s  - 110  continues  to  taking  a  on  literature  decay  form more s i m i l a r by  with  the  effect from  and  gas  a  the  with  with  initial  the  close  occur be  additional  on  initial  variant for  exit  to  the  two but  exit  right  top  exit  design.  plotted  3 of  The  profiles  tee.  allowing  distance  The  conditions  deviations  return  Figure  in Figure "  identical  small  solids  deceleration  for  the  with  are  almost  of  is  rates,  shows t h a t  geometry.  influence  the  exit.  same, a l t h o u g h  little short  the  profiles  i t s entirety,  conducted  this  abrupt  the  seems t o  gas  results  substantially two  to  in  comparison  circulation  figure  exit,  presented  density  through  the  solids  specific  column  variant  obtained  A  This  to  profiles  solids  e x p e r i m e n t was  slight  the  and  exits.  entrance  together  to  which d e p i c t s  velocities  different  final  results 3.16  3.15  influences  A 3.13,  gas  the  entrance  p r e v i o u s workers.  shown i n F i g u r e identical  from s o l i d s  -  the  are  between  Hence solids  above the  there an  exit  offtake.  3.1.5  Low  velocity  After what  finding  s u c h marked  is effectively  decided p r o v e at  to  entrainment  "entrained"  investigate  much  Bubbling  lower and  gas  how  effects at  of  high  significant  exit  geometry  velocity, exit  upon  we  geometry  could  velocities.  turbulent  have been c o r r e l a t e d  tests  using  fluidised  bed  entrainment  a wide number of  models  data  which  -  Figure  3.16  I l l-  L o n g i t u d i n a l d e n s i t y p r o f i l e s f o r sand, Ug = 7.1 m/s, G = 73 kg/m s, a b r u p t a n d e x t e n d e d e x i t s , s o l i d s r e t u r n a t 1.98 m above the gas d i s t r i b u t o r . Triangles represent extended e x i t p r o f i l e , c i r c l e s abrupt. s  -  -  112  have grown i n c o m p l e x i t y w i t h t i m e . of  this  thesis  to d i s c u s s  common f o r e n t r a i n m e n t correlations  t o be  that  "variations  unusual."  One  experimental  geometric  An m/s This of  velocity  a single  measurable not  was um  i s 1.5  particle  as  initial  literature  conducted  notes  not  i s the geometry o f t h e  account  times  Grace  changes w i d e l y from  one exit;  whether  f o r some o f  the  studies  correlations.  and  a t a gas  velocity  the c a l c u l a t e d  o f mean s i z e ,  in relatively  brief  substantial  of  1.5  Ottawa  terminal  sufficient  sand.  velocity  to provide  experiments,  material  series  was  run w i t h  loss  but  i n our  diameter  efficiency  from  a i r p o r t s have been p l u g g e d  of the  entrained  the apparatus  Here the s o l i d s  l o o p s have been removed e n t i r e l y  The  pot.  test  the s e c o n d a r y  inside  high  it is  F o r example,  made t o d e t e r m i n e  shown i n F i g u r e 3.17a.  storage and  that  different  d i a . S a u t e r mean d i a m e t e r  entrainment  purpose  column. An  up  was  so h i g h as t o p r o d u c e  tall  that  to another  could  the  o r d e r s of magnitude are  between d i f f e r e n t  u s i n g the 156  from  some o f the p e r t i n e n t  attempt  experiment  to note  different.  of the f a c t o r s  effects  discrepancies  predictions  o f two  set-up  hence a b r i e f  these except  radically  (1982) i n s u m m a r i s i n g  I t i s not  return the  set  and  system,  flush  with  the  column.  solids  Stairmand  were c a p t u r e d by a 178 cyclone discharging  They were r e t u r n e d t o the system  after  mm  into each  diameter a catch test.  -  Figure  3.17  113  -  C o l u m n c o n f i g u r a t i o n f o r 1.5 tests (a) Initial configuration (b) Final Configuration  m/s  entrainment  -  Examination discharge 100%  o f an  absolute  l i n e showed  capture  of the  -  114  filter  that t h i s  l o c a t e d i n the  cyclone  cyclone  provided  e n t r a i n e d m a t e r i a l at  the  effectively  gas  velocity  employed. Despite solids  precautions  hold-up  measurement  i n the  of  saltation  and  identical pressure end  of  duct  with  the  exit  t o the  each t e s t the  line.  saltated  90% of  equal and the  rates.  means can  the  at  the  too high  swirl  this,  were same  because  the  of  a i r into  f i v e minutes.  With  c o u l d be  was  This i s  a i r ports.  admitting  of  system  a d d i t i o n of a  were blown out  At the  high the exit  this  this  obtained;  The be  results  these  showed  of  statistically the  entrainment. although  d i f f e r e n c e i s not  exit  geometry  two  t h a t the  r e j e c t e d at a c o n f i d e n c e  total  the  were a n a l y s e d  for equivalence  however, t h e  of  was  f o r the  solids  results  95%;  fraction  results  3.3.  entrainment  Therefore, small  of  s y s t e m by  u s i n g a T - t e s t to t e s t  of  accurate  shown i n , F i g u r e 3.17b.  reproducible results  shown i n T a b l e  entrainment  ensure  remedy  except  t o one  cyclone  To  for approximately  modification  The  same e x i t  set-up  a i r source  into  and  v a r i a n c e between r u n s  t o F i g u r e 3.17a  upper p o r t  are  The  i n the  reconfigured  system,  to e l i m i n a t e p o i n t s f o r  entrained material, i n i t i a l  irreproducible. conditions  designed  can  mean hypothesis  level  between  large, only  account  discrepancies in various  7%  f o r some  entrainment  - 115 -  Table 3.3 R e s u l t s from Entrainment Tests at Low V e l o c i t y Gas V e l o c i t y =1.5 m/s I n i t i a l Bed Height = 0.91 m T o t a l U n i t Height = 9.3 m  Run Designation  Exit Geometry  Run Time (mins)  Mass of S o l i d s Entrained (kg)  la  Abrupt  5  0.68  lb  Abrupt  5  0.74  lc  Abrupt  5  0.77  Id  Abrupt  5  0.74  2a  Smooth  5  0.74  2b  Smooth  5  0.80  2c  Smooth  5  0.80  2d  Smooth  5  0.80  -  correlations and  at t h i s  in a tall  116  relatively  column,  -  modest  superficial  t h e r e must be o t h e r more  velocity  substantial  factors.  3.1.6  The imposed  p r e s s u r e drop - a s i g n i f i c a n t  on c i r c u l a t i n g A  large  bed s t r u c t u r e ?  number o f p a r a m e t e r s  density  profile  include  t h e gas v e l o c i t y ,  solids  physical  geometry.  in a circulating  This  circulation  has r e c e i v e d  "imposed  the  system.  c o n c e p t was  al•  (1981) whose e x p e r i m e n t s a p p e a r  given  system a t a f i x e d  rate,  an i n f i n i t e  depending contains phases  different  l e g . With  performed For  by t h e need this  zone,  gas and  the u n i t  relatively  t o show t h a t , and s o l i d s profiles  of s t a b l e  little  across  circulation  can e x i s t  Each  dense  for a  profile  and  dilute  t h e amount o f each b e i n g  to b a l a n c e the p r e s s u r e drop over the i n mind, a b r i e f  t o examine t h e "imposed t h e "imposed  rate,  by W e i n s t e i n e t  p r e s s u r e drop.  fractions  and a t r a n s i t i o n  determined return  of d e n s i t y  upon t h e imposed  These  p r e s s u r e drop"  introduced  gas v e l o c i t y  number  bed.  and, as shown above,  which  i s t h e so c a l l e d  a r e known t o i n f l u e n c e t h e fluidised  solids  properties,  A parameter  attention  influence  series  of t e s t s  p r e s s u r e drop"  p r e s s u r e drop" experiments  was  configured  as shown i n F i g u r e  was  t h e n measured o v e r each  the  two s e t s  2.1.  portion  of c i r c u m s t a n c e s .  phenomenon. the system  The p r e s s u r e d r o p  of the s o l i d s  In each  was  loop  c a s e t h e gas  under  -  velocity  and s o l i d s  However,  i n the f i r s t  maintained mm  fully  the  second  storage  imposed  plots  results  of absolute  o f t h e imposed  From t h e s e  The happened.  was  for  pressure  i n t a b u l a r form versus  results  such  versus  height  diagrams  as f o r a  as a s l i d e  absolute  gas f l o w s  to the s o l i d s  pressure  drop.  On  that  and  t h e r e was  no  density  o f 6.9  m/s  and a  drop  indicate  over  what  mechanical  or b u t t e r f l y  valve, the  a c t s as a p r e s s u r e  storage  pressure  reducer  vessel i s fluidised,  a t the b u b b l i n g  bed  down t h e L - v a l v e  at a  which  counterbalancing  produces a  the c o n t r a r y ,  fluidisation  of t h e s t o r a g e  and  are s u b s t a n t i a l l y  the o p e r a t i o n  i t seems  3.4  Hg.  relative  upon  was  level i n  2  When t h e s o l i d s  a high  gas f l o w  +117  mercury  i n Table  upon t h e r i s e r  s e c t i o n of t h e L - v a l v e  distributor,  -1mm  vertical  f o r a gas v e l o c i t y  In t h e same manner  t h e gas.  creating  drop of  r a t e o f 29 kg/m s t h e p r e s s u r e  13 mm  constriction, vertical  was  s u p p l i e d t o the s o l i d s  pressure  In each c a s e  circulation  riser  vessel  In c o n t r a s t , i n  drop o f o n l y  pressure  impact  the  upon t h e l o o p .  a r e shown  3.18.  solids  storage  t h e same b e d .  Figure  profile.  identical.  so t h a t a p r e s s u r e  v e s s e l and a p r e s s u r e  The  r a t e s were  run the s o l i d s  r u n no a e r a t i o n was  measured o v e r  as  circulation  fluidised  o f m e r c u r y was  -  117  i n the absence of  vessel,  of the u n i t  velocity  the a b s o l u t e  reduced. was  The  pressure  only  impact  t h a t , i n the l a t t e r  case,  - 118 -  Table Pressure  Drops Over t h e C i r c u l a t i n g  Run C o n d i t i o n s : R i s e r Gas V e l o c i t y Solids Circulation  Differential Pressure Designation  3.4 Fluidised  Bed  Loop  - 6.9 m/s R a t e - 29 kg/m s 2  P r e s s u r e Drop w i t h Fluidisation Air i n S t o r a g e Zone (cm H 0 ) 2  P r e s s u r e Drop Fluidisation Storage (cm H  2  1-2  17 .8  17 .8  2-3  32 .2  29.9  3-4  -159.7  1.4  4-5  65.6  -86 .0  5-1  48 .1  44.5  •Pressure  Tap D e s i g n a t i o n ^ ar° i l l u s t r a t e d i n F i g u r e  Without Air in Zone 0)  2.1.  T  J  119 -  1  1  1  1  1  1  1  1  1  l  1  l  l  l  0  40  80  r  l  120  160  G a u g e Pressure (mmK^O)  •  r  1  1  10  1  1  r  1  Packed Bed  8 6  © X  4  0  40  80  120  160  G a u g e Pressure (mm H2O) Figure  3.18  Pressure versus e l e v a t i o n plots f o r a r i s e r w i t h f l u i d i s e d and packed bed s t o r a g e z o n e s . Numbers r e f e r t o l o c a t i o n s on F i g u r e 2.1.  - 120 -  the L - v a l v e a e r a t i o n n e c e s s a r y motive  had t o be i n c r e a s e d 15% t o p r o v i d e t h e  gas f o r t h e r e q u i r e d  horizontal  solids  transport. Our system  results  t h e gas v e l o c i t y  determine is  indicated  that  and s o l i d s  the p r e s s u r e p r o f i l e  rationalised  f o r a g i v e n c i r c u l a t i n g bed circulation  i n the r i s e r  i n terms o f t h e s e e m i n g l y  section.  o f W e i n s t e i n e_t a_l. (1981) i n C h a p t e r  3.1.7  The i m p a c t bed  density  Industrial are than  primary  a i r , secondary  conveying  air  promotes staged fuel  lower zone.  NOx,  higher density  which  fuels,  fraction  f o r mixing  However, d e s p i t e i t s i m p o r t a n c e  to p r a c t i c a l  behaviour  a i r introduction  i s not w e l l  Staged  or primary of h i g h l y  f o r turndown p u r p o s e s i n (Kobro  of staged  of a i r used  i n formation of a  some " c o n s t a n t i n v e n t o r y " u n i t s  effect  at Duisburg  formation of  bed i n t h e l o w e s t  and i s e s s e n t i a l  a i r a t more  1986).  reduces  and r e s u l t s  systems  locations,  (Wein and F e l w o r ,  T h i s dense bed i s a d v a n t a g e o u s  reactive  unit  at three v e r t i c a l  combustion,  NOx and t h e r m a l  bed c o m b u s t i o n  the L u r g i  a i r and a t h i r d  of the feed  velocity  circulating  of the f l u i d i s i n g  F o r example,  a i r introduction  for  both  a i r a d d i t i o n on  fluidised  by a d d i t i o n  one l o c a t i o n .  utilises  4.  profiles  circulating  characterised  This  contradictory  result  of secondary  rate uniquely  understood.  and B r e r e t o n ,  1985).  operation, the  upon c i r c u l a t i n g bed  -  In  a combustion  particle operate study,  size,  this  maintained split. to  and t h e f u e l  This  of t e s t s  shown i n F i g u r e  Both  ports  four  2.1.  firstly  circulation  from  and 3.20.  with  boiler  context  constant  load.  the unit  and t h r o u g h  (39") f u r t h e r  (swirl)  the t o t a l  and d e n s i t y  s e t up a s  up t h e column. were  and l a t e r t h e  nozzles.  gas v e l o c i t y constant profiles  to secondary  four  a i r introduction  opposed p o r t s ,  the d i f f e r e n t  was  t h e column a t two v e r t i c a l  r a t e s were m a i n t a i n e d  each o f t h r e e p r i m a r y  3.19  990 mm  horizontal  2  Results  while maintaining  the d i r e c t l y  kg/m s r e s p e c t i v e l y ,  gas v e l o c i t y  i n an i n d u s t r i a l  distributor  located  In t h i s  the p r i m a r y - t o - s e c o n d a r y a i r  A i r entered  each geometry  total  mean  zone c a n  regimes.  while  methods o f s e c o n d a r y  tangential For  the primary  was c o n d u c t e d  the primary  available  tested,  by v a r y i n g  i s equivalent  series  secondary  type,  of f l u i d i s a t i o n  the a i r s p l i t  locations,  d e p e n d i n g upon t h e l o a d ,  regime was v a r i e d , constant  varying A  system,  i n a variety  -  121  and  solids  a t 8.6 m/s and 45 were r e c o r d e d a t  a i r splits.  r u n s a r e shown i n F i g u r e s  They a r e r a t i o n a l i s e d  i n Chapter  4.  - 122 -  0  100  200  300  400  SUSPENDED SOLIDS DENSITY , kg/m3  F i g u r e 3.19  Density p r o f i l e s measured i n c i r c u l a t i n g beds of sand at a t o t a l gas v e l o c i t y of 8.6 m/s and a s o l i d s c i r c u l a t i o n r a t e of 45 kg/m s with d i f f e r e n t primary to secondary (P/S) a i r ratios. Secondary a i r introduced through opposed p o r t s . C i r c l e s , zero secondary a i r ; t r i a n g l e s , P/S = 1.36; squares, P/S = 0.83.  -  123  -  SUSPENDED SOLIDS DENSITY , kg/m3  Figure  3.20  D e n s i t y p r o f i l e s measured i n c i r c u l a t i n g beds of sand a t a t o t a l gas v e l o c i t y o f 8^5 m/s and a s o l i d s c i r c u l a t i o n r a t e of 45 kg/m s w i t h d i f f e r e n t p r i m a r y t o s e c o n d a r y (P/S) a i r ratios. Secondary a i r i n t r o d u c e d through swirl ports. C i r c l e s , zero secondary a i r ; t r i a n g l e s , P/S = 1.39; s q u a r e s , P/S = 0.85.  -  3.2  3.2.1  approach  circulating  taken  fluidised  towards u n d e r s t a n d i n g  bed  and  variables,  t o t r y and  behaviour  then by  how  was  macrosturcture,  studying  first  i t varied  This section  of  a c a p a c i t a n c e probe  to study  Results  design are  presented.  3.2.2  Design The was  criteria  from  (i)  To  (iii)  To  The the  combination circulating  d e s c r i b e s the local  development  suspended the  solids  final  probe  of  the  circulating  fluidised  intended:  determine  the  determine  time  the  concentration  at  the  time-mean and  suspended  the  local  solids  locations.  t i m e - v a r i a t i o n of  local  measure  averaged  at v a r i o u s  instantaneous  To  macroscopic  f o r a m i c r o s t r u c t u r a l probe  concentration (ii)  the  experimental  this  t e s t s with  m i c r o s t r u c t u r a l study originally  key  model o f  thesis  concentration. then  with  the m i c r o s t r u c t u r e .  the  the  to study  rationalise  s t u d i e s leads to a conceptual  bed.  bed  Bed  Scope The  of  -  Experimental Study of the M i c r o s t r u e t u r e of the Circulating Fluidised  of  124  suspended  the  solids  same p o i n t s . solids  instantaneous  velocity basis.  both  on  a  -  (iv)  To  125  -  o b t a i n t h e s e measurements w i t h o u t  either  the  gas  or  the  solids  measurement d e v i c e s h o u l d measured  flow,  not  signal. have been i n t r o d u c e d  circulating  fluidised  of determining 1986).  bubbling  v a r i o u s parameters  In a d d i t i o n ,  been u s e d  techniques  beds w i t h (Grace  f o r the  suspended  i n T a b l e 3.5.  shows t h a t of the  v a r i o u s types  optic  variety  by O k i  developed  easily  a p r o b e has  U n f o r t u n a t e l y , the  g i v e the  instantaneous  local  the  suited  t o our  application  Of was  instantaneous  velocity  local data  at  low  fibre  and  optic on  the  to  values. cannot  be  the  one  of  obtained  the  high  p r o b e does  that  such  a  best  probe. provide  solids  i n t e g r a t e d to  from  not  options  t h a t was  could  local  It i s unfortunate be  fibre  distinguish  that t h i s  measurements o f then  this  an  the c a p a c i t a n c e  accurate  These would  of  c a p a c i t y t o measure  these,  c o n s t r u c t e d , i t appeared  average  of p r o b e s ,  important  Properly  concentration.  These  h y d r o d y n a m i c models; a l t e r n a t i v e  were t h e r e f o r e c o n s i d e r e d .  have  of  Examination  density variations  b a s i s which a r e  between d i f f e r e n t  Baeyens,  e t a l _ . (1980) i s one  most o f t h e r e q u i r e d p a r a m e t e r s b o t h temperatures.  and  concentration.  table  such  the o b j e c t i v e  determination  solids  a r e summarised  most p r o m i s i n g ;  into  several non-intrusive techniques  specifically  point-averaged  i . e . the  i n f l u e n c e the  A l a r g e number o f p r o b e s and  disturbing  yield  particle probe.  T a b l e 3.5 T e c h n i q u e s Used  f o r D e t e r m i n i n g L o c a l S o l i d s Hold-Up and Particle Velocity ( m o d i f i e d from G r a c e and Baeyans, 1986)  Method  Some R e f e r e n c e s  Quantity(ies) Measured  A d d i t i o n of tagged tracer particles  Geldart (1972)  S o l i d s c i r c u l a t i o n and velocity (instantaneous)  X  W e i n s t e i n e t a l . (1985) B i e r l e t aTT T T 9 8 0 )  R a d i a l voidage ( t i m e mean)  Impact o f p a r t i c l e s on a piezo e l e c t r i c c r y s t a l  Heertjes  Local solids velocity (instantaneous)  T r a c k i n g of particles  Un e t a l . (1980) Masson e t j a l . (1978)  Local solids velocity (instantaneous)  M e r r y and D a v i d s o n (1973) Rao & V e n k a t e s w a r k u (1973)  Local solids velocity (instantaneous)  T r a n s i t time o f p a r t i c l e s v i e w e d by o p t i c a l f i b r e light detectors  Ohki e t a l .  Local solids velocity and h o l d - u p ( i n s t a n t a n e o u s v e l o c i t y , mean h o l d - u p )  Thermistor  G l i c k s m a n and McAndrews (1985)  Local (time  solids mean)  Werther & Molerus Almstedt & Olsson  Local  solids  or y-ray  Tracking tracers  photography  radioactive  of r a d i o p i l l  Capacitance  probe  probe  I s o k i n e t i c and momentum f l u x probes  Bierl  and  Cranfield  et a l .  et a l .  (1970/71)  (1980)  (1973) (1982)  (1980)  distribution  velocity h o l d up  L o c a l s o l i d s h o l d up ( t i m e mean) and s o l i d s f l u x ( t i m e mean upward and downward)  -  127 -  However, t h e s e d a t a a p p e a r e d initial  discrimination  circulating  3.2.3  beds.  techniques f o r measuring  fluidised  beds  using  since  b r o a d c a s t i n g made t h e b a s i c  capacitor 10 mm.  Werther  Their  w i t h each p l a t e  This r e l a t i v e l y  to t r u l y  work o f Morse  frequency modulation i n s t r u m e n t components w i d e l y and  initial 13 mm  p r o b e was a p a r a l l e l  needle probes  surface  plate  s q u a r e and s e p a r a t e d by  (1973) ( 2 . 3 mm  n e e d l e d i a . ) and even Fitzgerald  variations i n  cumbersome d e s i g n h a s s i n c e  miniaturised  and M o l e r u s  porosity  the pioneering  (1951) s o o n a f t e r  cheaply a v a i l a b l e .  description  c a p a c i t a n c e p r o b e s have been i n  c o n t i n u o u s development and B a l l o u  important f o r  between t h e v a r i o u s models f o r  Capacitances probes - general The  way  t o be t h e l e a s t  given  such as those o f  n e e d l e l e n g t h x 0.4 mm  probes  s u c h as t h o s e o f  ( 1 9 7 6 ) ; however t h e b a s i c  circuitry  probes are i d e n t i c a l ,  and o n l y  modern t r a n s i s t o r i s e d  components h a s a l l o w e d m i n i a t u r i s a t i o n  and  substantial  g a i n s i n probe  Most c a p a c i t a n c e p r o b e components which (i)  the  s t a b i l i t y of  performance.  systems  consist  a r e shown s c h e m a t i c a l l y  of three  basic  i n F i g u r e 3.21.  The p r o b e : This  considered  the improved  o f many  i s made o f two p a r t s ,  electrically,  measuring  section  i s a fixed  itself,  which  the support  system  which,  c a p a c i t a n c e , and  i s typically  either a  -  128  -  Probe  5  Variable Capacitance  Figure  3.21  Fixed C  FM Signal  Resonant Oscillating Circuit  Voltage Signal  Demodulator  B l o c k diagram of a c a p a c i t a n c e probe system i l l u s t r a t i n g p r i n c i p a l s y s t e m components.  - 129  -  p a i r of p a r a l l e l p l a t e s , or a needle p r o t r u d i n g support.  E i t h e r geometry forms a second, v a r i a b l e  capacitance, The  from the  i n p a r a l l e l with  the  first.  probe i s i n s e r t e d i n t o the f l u i d i s e d  bed  with  measuring s e c t i o n located i n the region of i n t e r e s t ;  the  changes  of void f r a c t i o n i n t h i s zone create changes i n the dielectric  constant  capacitance.  which i n turn a l t e r the system  Hence, capacitance  becomes a continuous  i n d i c a t o r of the s o l i d s loading i n the region of the probe tip.  (ii)  The  resonant o s c i l l a t o r  The  capacitance  transistorised  circuit:  probe forms part of a  tuned o s c i l l a t i n g c i r c u i t whose o s c i l l a t i n g  frequency v a r i e s as a f u n c t i o n of the probe capacitance. our  case the o s c i l l a t o r was  of the Clapp type,  commercially a v a i l a b l e transducer Other types, 1951)  e.g.,  could a l s o have been used.  i n capacitance  part of a  system marketted by  the H a r t l e y o s c i l l a t o r  In  (Morse and  Disa. Ballou,  In each case the v a r i a t i o n  at the probe t i p i s converted  to a frequency  modulation; t h i s i s e s p e c i a l l y s u i t a b l e f o r s i g n a l transmission attenuation.  s i n c e i t i s s e n s i t i v e n e i t h e r to noise nor  to  -  (iii) The  The  demodulating  frequency  130  circuit:  modulated  signal  give  a v o l t a g e or c u r r e n t o u t p u t  from  the base  converter" give  frequency.  or f r e q u e n c y  an o u t p u t Although  must be  in a  discriminator  system  type  of c a p a c i t a n c e measurement  source,  and  discharge  a fixed  c a p a c i t a n c e changes by  the c a p a c i t o r o v e r  Development of a probe f o r t h i s  study  constructed  c a p a c i t a n c e probe developed  t h e D i s a 51E01  51E02 o s c i l l a t o r  and  compatible  discrimination  with  frequency  c o u p l i n g the A.C.  cycle  available  for this  study  reactance  to a  51E02 t u n i n g p l u g .  for capacitative f o r resonant  the  was  conversion  r e a c t a n c e c o n v e r t e r combined  components a r e d e s i g n e d transducers  other  circuit.  upon a c o m m e r c i a l l y  system, type  from  used  of an  The based  measured  (1976) has  one  worked  half  suitably  3.2.4  Fitzgerald  the  it is  at l e a s t  d e v i c e has  to  requirements.  s e t up,  that  shift  designed  d e s c r i b e d above i s p r o b a b l y  to note  to  "reactance  with data-logging  f o r completeness  successfully.  to the  circuit  most common t y p e o f c a p a c i t a n c e p r o b e important  demodulated  proportional  This occurs  compatible the  -  with  a  These  pressure  t u n i n g and  frequency  plug/oscillator/convertor  combination. Given  such  a system,  the d e s i g n p r o c e s s  f o r the  probe  - 131 -  requires  the manufacture of a u n i t  capacitance,  and  specifications line. 30  capacitance  variations  both  p f , and a c a p a c i t a n c e  fixed  transducer  probe c a p a c i t a n c e  between  variation,  dielectric  due  to  according  t o t h e r e f e r e n c e manual f o r t h e D i s a  reactance  convertor.  coaxial may  zero  and  a t t h e p r o b e t i p , not t o e x c e e d 1 p f  changes  The  base  w i t h i n the  of the c a p a c i t a t i v e p r e s s u r e  These are a t o t a l  constant  with  v a r i a b l e s which cylindrical  be summarised  51E01  type  i n f l u e n c e the c a p a c i t a n c e  of a  c a p a c i t o r a r e shown i n F i g u r e 3 . 2 2  by t h e  and  equation 2ire  DL  C  (3.2) *n{-}  Figure  3.23  shows a t y p i c a l  capacitance coaxial the  cylindrical  inner  grounded around changes and  of the support  "live" sheath  probe d e s i g n . can s i m p l y  capacitance  wire  t h e measurement  Calculations acceptable  shown below  field  with  indicate  the with  the outer  variable capacitance  between  few m i l l i m e t r e s o f grounded  design  and  t i p i s c r e a t e d by d i e l e c t r i c  i n a much more complex  the f i n a l  of F i g u r e 3 . 2 2  model  The  fixed  be d e s c r i b e d by  t r e a t e d as t h e c o r e  as t h e s h e l l .  The  that  the needle t i p  sheath.  i n order  a base c a p a c i t a n c e  constant  to b u i l d  between  0 and  an 30  -  Cylindrical capacitor consisting of an inner wire of radius a and a concentric outer shell of radius b. The electricfieldis calculated from Gauss' law using a cylindrical gaussian surface of radius r and length L, between the conductors.  F i g u r e 3.22  132 -  2nDe L Q  V a r i a b l e s i n f l u e n c i n g the c a p a c i t a n c e of a c o a x i a l c y l i n d r i c a l c a p a c i t o r ( T i p l e r , 1976).  - 133  pf,  a number o f r e s t r i c t i o n s  significant  i f the p r o b e  temperature  work  insulating  length  i s t o be  i n such  compared  o f probe  apply.  (a p r e r e q u i s i t e  ceramics  permittivity  -  These are  used  subsequently  f o r our  probes  to p l a s t i c s  particularly for high  s t u d y ) because  have h i g h  the  relative  or a i r ; t h i s  restricts  'L' w h i c h g i v e s the maximum 30  the  pf  capacitance. C o n s i d e r a probe live (3  wire  mm  What would be material  permittivity  of  Application length  an  t h e maximum probe  of Equation  152  3.2  80 mm  insufficient  must c o n s i s t  mm.  to a larger  away from  the t i p .  The  larger 80 mm  the d i e l e c t r i c The  sheath  both  a  l e n g t h can  by  away  t h e measurement p o i n t .  using s u f f i c i e n t l y  to decrease  probe  by  sheath  probe  close  to  the zone,  centimetres  section virtue  can  of the  then  diameter  However, t h e  with  increasing  be  larger  become v i r t u a l l y  large  be  across a  i s p a r t i a l l y a i r and  unlimited  gain begins  that  several  diameter  ceramic.  length  relative  flow d i s t u r b a n c e i n t h i s  partially  from  mm  i s the maximum w h i c h c o u l d  diameter  longer than  because  indicates  of a small diameter  expanding  and  with a  Therefore a p r a c t i c a l  volume, t o m i n i m i s e  ID,  2  dia.  length i f the  value for t r a v e r s a l  measuring  considerably  ceramic  mm  6?  column o f d i a m e t e r design  ID of a p p r o x i m a t e l y  i s a typical  of a p p r o x i m a t e l y  tolerated,  c o n s t r u c t e d w i t h a 0.75  i n a s h e a t h w i t h an  OD).  insulating  t o be  sheathing relative diameter,  -  LIV/E NEEDLE UJIRE 0.7S mm DIA  134 -  GROUNDED SHEATH 3 mm DIA  GROUNDING LUIRE COAX COUPLING  TEFLON INSULATION  Dimensions i n  Figure  3.23  A typical  simple  needle  mm  capacitance  probe.  - 135 -  because  of the nature of the  denominator  of Equation  Finally, minimise improve the  the probe's  probe  due  order  therefore  study,  each  smaller  mm  OD  the  sheath  end.  This  to increase  (1 3/8")  from  s h e a t h can be  the t i p .  f r o m changes  integrity  over the c o u r s e of until  finally  f r o m 3.2  steel  tubing  i s press f i t t e d  fabricated  a  and  mm at  rigidity 3.24.  (1/8")  from 6.4 tubing. strand, 6 mm  mm The  mm  (1/4")  the remainder  (1/4") live  stainless  OD  the  i n t o a 6.4  t i p , so t h a t  (0.025") d i a . w i r e , p r o t r u d i n g  OD,  4.6  of of mm  probe steel,  (3/16") f r o m  I t r u n s a l o n g the s h e a t h a x i s  t h e s h e a t h by a s i n g l e ,  is  this  the sheath diameter at a d i s t a n c e  single  tip.  i n the  i s shown i n F i g u r e  stainless  the probe  is a rigid,  and  o f t h e same  Structural  i s constructed  component  from  static  of  o f the p r o b e d e s i g n .  probe  steel  the p r o b e  under  c a p a c i t a n c e changes  (0.180") ID s t a i n l e s s  at  bending  those r e s u l t i n g  final  to  Changes i n t h e geometry  than the p r e v i o u s ,  (0.85") ID 316  bushing  35 mm  The  probe  measurement  i t i s valuable  compromise between m i n i a t u r i s a t i o n  obtained. The  i n the  diameter sheath to  p r o b e s were f a b r i c a t e d  satisfactory  2.2  can p r o d u c e  important part  Several  was  rigidity.  c o n s t a n t around an  that  of the s m a l l e r  o f m a g n i t u d e as  dielectric  noted  to imperceptible  dynamic f o r c e s  term  3.1.  i t s h o u l d be  the length  logarithmic  the  1  mm  sheath  insulated  c o n t i n u o u s , magnesium o x i d e  - 136  g u r e 3.24  -  A photograph of the f i n a l design.  capacitance  probe  -  thermocouple insulator, ( 1 / 8 " ) OD  insulator.  and  the  sheath  thermocouple electrical  using  cement.  compatible using  R e s p o n s e and In o r d e r  circulating  frequencies observed system  lower  employed the  than  measurement no  system.  i n the  KHz  problem.  measurement  to  limit  site.  the  plug  than  Disa  the  high  are  three  datalogging  up  conversion  to a frequency  resonant (4.5  of do  frequency  - 5.5  response  at  MHz).  in for  magnitude not  pose a  capacitance  n e i t h e r mechanical  of  frequency  interested  orders  s i n c e the  at  frequencies  reactance  signals  probe  in a  responding  cut o f f frequency, Also,  made  tuning  responses  of  extremely  are  was  the D i s a  oscillating circuit  moving p a r t s , t h e r e  hysteresis  higher  measurements, 100  this  mm  fitting  o f the c a p a c i t a n c e  capable  The  a 6.3  fitting.  f r e q u e n c i e s which we  bed the  the  convenient  t h e m e a s u r i n g and  of m e a s u r i n g  b e c a u s e of  circulating  be  of  transient  bed,  significantly  i n the  Therefore  the  should  i s capable  KHz,  band  connection  mm ceramic  compression  connector;  calibration  fluidised  instrumentation  100  a BNC  to study  3.2  non-measurement end,  (1/8") f e m a l e NPT  the RF  the  temperature  to provide a  a BNC-to-RF t r a n s i t i o n a l  3.2.5  r i g i d i n the  "Omegatite" high  to accept  with  i s held r i g i d i n  i s held  at the  mm  -  wire  Lastly,  connection  modified  The  insulator  ( 1 / 4 " ) t u b e t o 3.2 was  137  lags the  probe nor  has  -  The D.C.  electronic  voltage  to +lpf signal  and  between +6V -1  identical from the  the  high  sensitivity  magnitude f a s t e r Having  the  next  generate be  with  Hz  (i)  t h a t the  first  bed  test  process  the v o i d  to  2.5, this  second, 2 orders  of  requirements. capacitance  probe system i s range  ( i ) t h a t the  friction  available), could  signal  i n the  interfere  (assumed  probe  ( i i ) t h a t the  p r o b e d i d not  could  measurement  significantly volume.  repoducibility: to e s t a b l i s h  signal  a calibration,  t e s t e d i n packed simply  in Section  m e c h a n i c s i n the measurement  Signal  attached  f u r t h e r i n f o r m a t i o n became  against  fluid  noted  is  signals  s y s t e m s were i n t r o d u c e d ,  a reproducible signal,  in developing  was  As  transducer  a  The  for logging similar  pressure  This  using  probe s i g n a l  i n the d e s i r e d f r e q u e n c y  until  In o r d e r assist  capacitance  s t e p s were t o d e t e r m i n e  (iii) the  the  is a  respectively  b i t conversion.  e s t a b l i s h e d t h a t the  calibrated  zone,  than  converter  IBM-XT computer  t o 30,000 p o i n t s per  of measuring  0-100  the  converter.  l o g up  reactance  corresponding  twelve  b a s i c measurement  p r o g r a m can  the  i n the probe c a p a c i t a n c e .  t o the program used  where the  t o be  with  -  -6V,  f o r l o g g i n g the  same r e a c t a n c e  capable  and  f o r l o g g i n g on  interface  p r o g r a m used  from  pf v a r i a t i o n s  i s ideal  Tecmar A/D  output  138  and  c o n s i s t e d of  the  bubbling  immersing  reproducibility, capacitance beds. the  The  probe packed  probe t i p i n a  and  -  packed This  bed'of  established  consistently and  s a n d , moving that  by moving probe  one  was  constant  In a d d i t i o n ,  the probe around  then removing i t .  reproducible  value  since i t  i n a packed  the dynamic  d i d not a p p e a r  forces  to d e f l e c t  bed, caused the  tip significantly. Tests  packed fell  i t a r o u n d , and  the s i g n a l  attained  another i n a i r .  -  139  i n bubbling  bed  result.  the p r o b e  provided  In a t y p i c a l  to a c o n s i s t e n t  passed  beds  dilute  t i p before  confirmation  trace,  phase  value  returning  of the  the probe  signal  each t i m e a b u b b l e  t o i t s dense  phase  value.  (ii)  Probe The  resolved to a  after  establish linear  and  calibration:  problem of probe c a l i b r a t i o n a number of e x p e r i m e n t s .  that  function  therefore a  Fortunately  It i s f i r s t  be useful  t h e o u t p u t from the r e a c t a n c e c o n v e r t e r i s of the c a p a c i t a n c e  linear  this  can o n l y  function  i s a feature  i n t h e measurement  of the d i e l e c t r i c  zone  constant.  of the r e a c t a n c e c o n v e r t e r  design.  Theoretically  t h e n , by  establishing  between  the d i e l e c t r i c  constant  of t h e g a s - s o l i d s u s p e n s i o n  and  the s o l i d s  loading,  uniformly  dispersed  establish  a probe  and  assuming  that  o v e r t h e probe volume,  calibration.  This  a  relationship  the s o l i d s  are  i t i s possible  p r o b l e m has  been  to  - 140 -  treated the  by  Bakker and  f u n c t i o n due  approximation  Heertjes  t o Weiner  t o the  ( 1 9 5 9 ) who  (1912) g i v e s  true d i e l e c t i c  e s t a b l i s h e d that  the  best  constant  versus  voidage  curve.  2D  D +  (3.3) 2  where: D  = relative  permittivity  of u n i f o r m l y  dispersed  Di =  relative  permittivity  of  the  discontinous  D2 =  relative  permittivity  of  the  continuous  e  voidage  =  This  relationship (D\  system,  = 7,  i s plotted in Figure D  =  2  1)  is  slightly  non-linear  is  c l o s e to  linear  fluidisation Our narrow  own  and  the  The  uniform  suspensions  by  output  p l o t t e d as a  was  then  a  linear  voidage,  at  to  p r o b e t i p was  were m a i n t a i n e d  confirm  region  least  vigorous  e =  this  the  function  e = 0.35,  it  for  linearity  immersed of  sand/air  1.0).  fine  a  absolute  Reactance  f u n c t i o n of  region  in  over  polythylene  agitation.  narrow  for a  interest  dependence of d i e l e c t r i c i n the  phase  e = 0 to  r e g i o n of  (e = 0.5  phase  that although  experiments confirmed  range.  alcohol  i n the  over  studies  showing  3.25  mixture  solids  converter  hold  constant  e = 0.97  powder  to  up  to  upon e =  1.0.  -  Figure  3.25  141  P l o t of the r e l a t i v e sand a i r s u s p e n s i o n s (1912) .  -  p e r m i t t i v i t y of uniform a c c o r d i n g to Wiener  - 142 -  A second 3.26.  The  measured  calibration  probe  depths  converter  t i p was  experiment immersed  i n a packed  output  was  The  Molerus  (1973) when a p r o b e  than  first  that  dielectric linearly  field  and  i t i s no  solids  experimental  then  solids then  the f i e l d  stated  extend  by  the p r o b e  t i p , and  output  i s linear  with  the f a c t  that  with  i n a packed  fraction  bed  i s known. to s o l i d s  c o n c l u d i n g the that  insensitive The  that  t o the latter  a linear  measuring  result  hold  both  comments on  interpolation  by  is  up.  theoretical a  needle  output  of i n t e r e s t  I n t e r m e d i a t e probe fractions  the  t o assume u n i f o r m l y  the probe  o f the m a t e r i a l  that  distribution  the mean s o l i d s  establishes  the  varies  i s led to conclude  longer necessary  first  more  of  c y l i n d r i c a l and  to c a l c u l a t e  and  cyclohexane,  does not  results  one  immersed  of Werther  into  the end  of  j u s t i f i c a t i o n s for calibrating  probe  converted In  field  volume.  preceding discussion  capacitance air,  i s essentialy  within  distributed  hold-up,  change i s f a i r l y  because  The  these  as a f u n c t i o n  dipped  of probe  Reactance  constant of the uniform suspension  capacitance solids  was  beyond  Combining  with s o l i d s  the probe  useful  1 mm  sand.  to a r e s u l t  the probe  the v a r i a t i o n  submergence.  of  that  approximately  second  analogous  to v a r i o u s a c c u r a t e l y  of f i n e  plotted  depth.  indicates  result,  then  bed  i s shown i n F i g u r e  outputs  linear  whose are  interpolations.  calibration procedure  in  i t must  has  been  be  -  Figure  3.26  143  -  Output from c a p a c i t a n c e probe g r a d u a l l y immersed i n t o a f i x e d bed of sand s h o w i n g l i n e a r i t y of v o l t a g e with immersion depth.  - 144 -  commonly used 1983;  C a r o t e n u t o e_t a l . ,  h a s been the  given  solids  resolve  1974).  However  to j u s t i f i c a t i o n ,  finally  I t should  also  made.  Radial  cross-section  density gave  w i t h mean b u l k d e n s i t i e s measurements.  This  little  zone;  attention  i t was n e c e s s a r y t o  commencing  be n o t e d t h a t  the probe c a l i b r a t i o n  ( e . g . Abed,  o r t o t h e p r o b l e m o f where  both these i s s u e s before  check  column  probe c a l i b r a t i o n  l i e i n t h e measurement  experiments. to  f o r capacitance  time-consuming i t was  possible  p r o c e d u r e when t h e r u n s were profiles  densities derived  comparison  integrated  which  over the  corresponded well  from p r e s s u r e  i s illustrated  drop i n Figure  3.27.  (ii)  Reliabilty  o f probe  Reliability probe with  requires  that  p r o b l e m has been only  They  the  rise  immersed  of bubbles i n t h e i r  of  disturbance  was, s u r p r i s i n g l y ,  of  probe  and a " r e l a t i v e l y  size,  disturbance."  volume.  This (1981), but  i n gently  bubbling  probes can i n t e r f e r e  and t e n d t o b o t h deform  velocity  significantly  by Rowe and Masson  of bubble parameters  found t h a t  substantially  not i n t e r f e r e  i n t h e measurement  investigated  f o r measurement  beds.  o f d a t a t a k e n by t h e c a p a c i t a n c e  the probe  the hydrodynamics  data:  t h e shape,  and a l t e r  vicinity.  The d e g r e e  not n e c e s s a r i l y massive  probe  a  function  caused  little  - 145  0  100  200  -  300  400  500  600  D e n s i t y F r o m C a p a c i t a n c e P r o b e (kg/nr*)  F i g u r e 3.27  Comparison of r a d i a l l y averaged d e n s i t i e s o b t a i n e d u s i n g an i n t e g r a t e d c a p a c i t a n c e probe s i g n a l , and the same d e n s i t i e s c a l c u l a t e d from p r e s s u r e drop measurements.  - 146 -  Of  the nine probes t e s t e d  supported  vertically  by Rowe and Masson e i g h t  were  and one was s u p p o r t e d h o r i z o n t a l l y  like  the probe used i n t h i s  study.  found  f o r measurement o f b u b b l e  t o be p r e f e r a b l e  p a r a m e t e r s because decelerates  "the h o r i z o n t a l l y  the bubbles markedly  Unfortunately relating be  forthcoming  uncertain  large  penetrate  contentious  provide  occur  through  boundary  measurement will  layer,  phase  b e c a u s e when a p a c k e t  volume  flow  a t present the beds  r e m a i n s an  i s s u e and even a doppler  anemometry  After  there  the remaining s i g n a l  and a r o u n d t h e v e r y  scale  rejecting  a  because of i n a b i l i t y t o  t o w a r d s measurement o f d i l u t e would  work  Rowe  and f i n e r  a solution.  of the s i g n a l  that  splitting."  nor i s i t l i k e l y t o  Therefore,  t e c h n i q u e such as l a s e r  the s o l i d s  probability  beds;  of probes f o r c i r c u l a t i n g  not n e c e s s a r i l y fraction  beds.  phase  any s i m i l a r  t o t h e complex  and p o t e n t i a l l y  non-intrusive does  fluidised  in circulating  geometry  and p r o m o t e s  the x-ray t e c h n i q u e s which  a r e not s u i t e d  structures optimum  since  p r o b e s were  supported probe  t h e r e has n o t been  to c i r c u l a t i n g  applied,  The v e r t i c a l  small  i n general  will  will  i s a strong be b i a s e d  characteristics. of p a r t i c l e  heavily This  i s passing  m e a s u r i n g volume, t h e be s h i e l d e d  and no s i g n a l  be r e g i s t e r e d . Hence,  there  t o make l o c a l  i s no i d e a l  way t o d e s i g n a p r o b e  measurements i n a c i r c u l a t i n g  bed.  f o r , or  Probes  - 147 -  must the  be d e s i g n e d results In  probe be  must  physical  had t o e n t e r either  on-line while  from  upward  removal  directed  with  constraints  probe,  i n both  probe  downward  moving  s o l i d s , and an upward  solids,  radius,  since  centerline.to When needle  the  because,  flow  these  not i n h i b i t The probe  volume large  structure  over  should  a parallel  was made  probe  horizontal  Since  probe  appears the will  the near from t h e  be m i n i m i s e d .  plate  i n a c i r c u l a t i n g bed.  a  probe  movement  as s m a l l  would  a  upward  as  through  possible  bubbles, a  to i d e n t i f y finer  probe  with  measurement o f  d i d not d i s t u r b  i s desirable  sense  of the probe  circumferential  a  flow i n  time.  probe  and geometry  over  Also,  are primarily  effects  the s i z e  prevent  s o l i d s movement  particle fluxes  although a large  and a  Radial  preferred  volume.  measuring  demixing, the  was  facing  b u t by m e a s u r i n g  the wall,  i t does  probe  small  radial  considering  probe  because  compromise.  be a f f e c t e d ,  with  directed  not  v e l o c i t i e s with  certainly disturb  the h o r i z o n t a l l y  acceptable  certainly  would  would  of s o l i d s  and i n an i n s t a n t a n e o u s s e n s e  the  and c o u l d  a time-mean  facing  most  rise  the d i r e c t i o n  downward  moving  this  that  maintenance.  bubble  then  prudence.  dictated  since  for regular  t o measure  and even  o f t h e column  o r downward  c i r c u l a t i n g bed changes  position,  interference appropriate  the side  required  i t i s logical  downwards  minimal  be t r e a t e d  our case,  bent  the  to cause  likely  scale  disturb  -  3.2.6  Experimental In  probe fact the  order  itself  while the  the  test  circulation  mm,  1448  distributor. 30s  was  a low  rigidly,  bending  of a c o a x i a l  changes.  of  To  examined  column was gas  of  radial  and  At  s i n c e any  s i d e of  noise.  frequency  i s 50  three  stability  the  column  frequency  so  of  of  At  57"  these  detail.  each  circulation  probe the  93") was  p o i n t s per dual  was  near w a l l  vertical and  solids  signal  100  from logged  second;  hi/lo  can  the  the for it  filter  l o g g i n g c o n d i t i o n s the data  to  locations,  t o remove s p u r i o u s  that meaningful  2.1,  the  signal  Hz  to  m/s,  the  model 442 100  conducted  shown i n F i g u r e  of 6.5  (21",  each p o s i t i o n  Hz,  as  different  to  was  conditions in  rates.  produce  column.  p o s i t i o n s from  2362 mm  Under  the  s e t up  three  pass frequency  then  geometric  cable,  ensure t h i s  capacitance  with a Rockland  frequency  was  velocity  the  a t each of mm  tests  set at three  at a sampling  conditioned for  the  a t seven  centreline, 533  series  variation  recorded  the  In  the p r o b e t i p t o  clamped  of d e n s i t y a l o n g  for a constant  the  from  series  decay  Specifically,  rate  circuit,  t r a v e r s i n g r i g shown i n F i g u r e 3.28 A  the  tuned  p r o b e i s t r a v e r s e d from one  examine t h e  traces  i n s e c t i o n 3.2.4.  the  as  capacitance  probe  as n o t e d  capacitance  constructed.  and,  the c a p a c i t a n c e  rigid,  must be  such  other,the  The  studies with  must be  oscillator,  measurable  -  to o b t a i n r e l i a b l e  the whole of  variations,  148  be  was  set  high Nyquist  obtained  -  F i g u r e 3.28  149  -  C a p a c i t a n c e p r o b e t r a v e r s i n g r i g mounted on s e c t i o n of t h e c i r c u l a t i n g f l u i d i s e d bed.  a  -  up  to  this  frequency  limit.  the  signal digitally,  EMI  model SE  the  merit  to  6150  i t was  Finally,  i n a d d i t i o n to  recorded  i n analogue  Mark 2 u l t r a v i o l e t  of p e r m i t t i n g  check t h a t  -  150  the  processing Only  of  capacitance  on  a purged  the  perfect  signal  be  had  over  be  fitting  at  t a k e n as  tended The  some t i m e  a zero,  or  wall.  It  was  not  a baseline  bearing the  capacitance  the  so  the  probe  to s e a l the  probe  although  surface  required  probe u n l e s s  achieve  that a b a s e l i n e  period. 30  i s also  This  s sampling hold  appeared  each p o i n t  probe  to  for  perfect  mimimum c a p a c i t a n c e  i n the  this  as  3.28),  t o make a b a s e l i n e at  i t also  the  to s h i f t  (Figure  insignificant  the  possible  signal  despite a l l  need  measurement  even a t  the  that,  were u n s u c c e s s f u l ,  volume; v i s u a l l y  because  the  has  without  during  Numerous a t t e m p t s  measurement  establishing  was  column.  each s p e c i f i c  that,  will  This  a d e f l e c t i o n on  t o be  signals  encountered  obtained.  alignment  effect  was  the  the  This  an  data.  sand o f f t h e  creates  can  each p o i n t  there  across  for keeping  alignment  of  probe b a s e l i n e  compression  traversing,  in  difficulty  traversed  ideal  digitized  probe t e s t s .  precautions, was  the  one  of  probe i s f u n c t i o n i n g p r o p e r l y ;  permits q u a l i t a t i v e evaluation  f o r m on  oscillagraph.  immediate o b s e r v a t i o n  logging  i n an  up  in  t o be  probe  assumes interval  the the  case,  correction empty  temperature  at  by  column  -  sensitive. preheated Although in  circulation  the b l o w e r ,  and  one  can c o r r e c t  for this  alters  from  that  represent  o c c a s i o n s , but  t h e gas, which the b a s e l i n e  by m e a s u r i n g  This  assumption  voidage.  the r e s u l t  i s that  One  solids  slightly.  temperature  p r o c e d u r e was  was  limitation  l o a d i n g s on  was  assumed  of a d o p t i n g  lower p e r c e n t a g e a c c u r a c y t h a n a t t h e  since  absolute error  error.  However,  structure  and  causes h i g h e r  the g e n e r a l c o n c l u s i o n s  differentiation  remain  valid.  3.2.7  Treatment  of r e s u l t s  this are wall,  proportional  regarding  between t h e f l o w  from the  to  the c e n t r e l i n e  measured w i t h a small  adopted  indistinguishable  o b t a i n e d i f t h e minimum s i g n a l 100%  is  f o r p u r e a i r i n t h e windbox,  c o m p l e x i t y i s not w a r r a n t e d . several  -  cools  by  the measurement volume, and  the on  Solids  151  flow  regimes  microstructural  investigation Before  results  measurements, capacitance was  probe  be  computed  that  have means and through  are both  different  stationary  analysed using  c a p a c i t a n c e probe  sections  variance.  and  signals  It establishes  v a l u e s from a s i n g l e  the  This  (1971)  o f t h e same time than  from  ergodic.  o f Bendat  v a r i a n c e s no d i f f e r e n t  sampling  from  n e c e s s a r y t o show t h a t  shown a c c o r d i n g t o t h e c r i t e r i a  requiring  be  i t was  could  history  explicable that  the d a t a  continuous  signal;  can  - 152  there  i s no  implication  periodic  components.  analysed  t o g i v e the  t h a t the  The  mean s o l i d s  (ii)  The  standard  the  mean  The  power s p e c t r u m and  The  generated second),  two  this  microcomputer.  the  data  software is  are  The  the  (30  a n a l y s i s was The  deviation  of  from  these  the  on  points  points  per  IBM-XT  autocovariance  were computed and  t o 3.34.  longitudinal  radial  data  by  by  making use from  uploading of  this  the package  t h e m i c r o s t r u c t u r a l s t u d i e s can  voidage  voidage  a specific  first  variation  three  final  radial  vertical  three  three at  combined  i n the mean and  a t the The  The  These are  variations  which were c o n s i d e r e d . concentrate  the  1.  i n F i g u r e s 3.29  the  about  readily  p e r f o r m e d upon t h e  A sample o u t p u t  i n Appendix  results  a t 100  power s p e c t r a and  more complex;  show the  of  was  f u n c t i o n of  3000 d i s c r e t e  three conditions studied.  plots  density  computed  s of data  p a c k a g e BMD:02T.  summarised these  upon the  be  t o t h e m a i n f r a m e computer  presented  solids  autocovariance  p a r a m e t e r s can  f o r each r u n  functions  location  density  d e v i a t i o n o f the  operations  so  from each  possess  density.  first  arithmetic  does not  f o l l o w i n g data:  The  solids  signal  signal  (i)  (iii)  -  of  each  of  with  the  standard locations  figures  distribution  be  each  f o r the  high  -  circulation  rate  153  -  r u n , and show t h r e e  s e v e n measurement  c u r v e s f o r each of the  volumes:  (i)  A trace  of the density  (ii)  The power  spectral  fluctuation  with  d i s t r i b u t i o n of the  time, density  fluctuations, (iii)  The a u t o c o v a r i a n c e  of the d e n s i t y  fluctuations.  HEIGHT A B O V E DISTRIBUTOR . m  c CD  M  & cna. o p - tr  C CO D  pr+  ><  l-h  rt 01 3 H-  1  0  II p> rt iQ tr <Tt t o HO £ n  rt  X O  I—' H - LQ,  0  Point D e n s i t y  c o o 3 P>  to rt rt cn HC 3 P) O LQ C i rt H i HO 3  01  ro o o  o o  co o o  (kg/m ) 3  o o  O O  CD O O  r t n t r n  cn CD cu fD O & P i Hii) tn O pi rt t-h (-•  pi3 5O cn a. > CD P> 3 p 3 3 QJ  h{  pj PJ cn  CD  rt  CD H . L Q  <  p> H- a II HP> W rt P) <Tl rt M • H HU l HO tr rt o Hi cn H-  Standard Deviation Density Fluctuations  O  cn  -  S'ST -  Of (kg/m ) 3  O o  HEIGHT ABOVE DISTRIBUTOR . m  -O—O-  p-  cQ  0  *c<n 3  CD  CT O  c  en  oX  o  |-  (V  m  W a  O  O. Q PCD P- cn 3  t O 3  3 cn cn rtII QJ  w  -1  pO rt co pi-S 0 c »-h rt X O O. PI— P3 C O \ P> PJ g pj | _ i o rt cn NJ rt d cn P- PJ PJ O • 3 3 cQ p . rt t-h P- I"!  lO  "i*  3 Oi  C+ H -  1  Point D e n s i t y  (kg/m ) cn o o 3  CD  o o  O  o  P-  O rt > cr n 3 tr H (D PJ cn CD cn Oi Oi Pcn o o PJ rt PJ cn Hi pj 0 cn O.  c/> ~<  OLIO  & 3 PJ CD 3 3 3 ft) »-{ OJ cn Oi  P-  (o —  o  CO O ^  CO  Cn  to 00 CO  cn rt  01  p-  <Tt  O PJ • ^ 3 P Ul HO i-ti  3  cr  c  \ rt cn P-  -  o  cr  o_  pj •» pH rt  O. ro O K ; (D cQ < Oi P- i-{ PJ II PPJ  rt  3  Standard Deviation Of Density F l u c t u a t i o n s (kg/m ) 3  3 cn  -  SSI  -  X  2.  CQ'  IT  HEIGHT ABOVE DISTRIBUTOR . m  P-  iQ  0 P.  CD  a a O H- tr CD P-Cn 3 O 3 cn CO rt I I pj i£) P- n prt P- 4^ o rt p- d 0 n Ml rt X O p3 h0 P - L Q 0 P> O 3 P> rt rort C cn p- p> 1  1  Point Density (kg/m ) to co A cn o o o o o o o o 3  o  p)  3 3  rt p-  o> o o  -vi  o o  Oi  O  tr i-i  3  CD P) Cb Oi PP)  cn cn rt P)  3  cn Oi  l-i  Oi cn  t>no  P> CD  Oi p)  3 3  Oi P) -  l-t  OJ  to  ro  S3  o  cOoA 6i"  prt  S3  oo co  3 cr  o X °. t2o" 3"  C <D Oi P- P> II PP) O. cn rt p- <y\ rt ii P- P> PO  3  tr  O  rt p-  i-h  0  Standard Deviation Of Density Fluctuations (kg/m ) 3  O  3  CO o o  cn  cn O o  o o  X to —• o O -N CO rO CO CO  -  9QT -  2. i  Distance From Wall, mm  Z =  533  rnm  8  13  23  33  43  53 0.5  0-2 04 06 0-8 1-0 Time ,s F i g u r e 3.32  1.0  1.5  TIME LAG (»)  Radial variations i n density fluctuations, power s p e c t r a l d i s t r i b u t i o n of d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y f l u c t u a t i o n s , U = 6.5 m/s, G = 62 kg/m s Z = 533 mm. g  8  -  157 -  4  (  FREQUENCY (H>)  Distance From Wall, mm  Z =1448 m m  8  13  23  33  43  53 10  Figure  3.33  1.5  TIME LAG (i)  T i m e ,s  Radial variations i n density f l u c t u a t i o n s , power s p e c t r a l d i s t r i b u t i o n o f d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y f l u c t u a t i o n s , U = 6 . 5 m/s, G = 62 kg/m s, Z = 1448 mm. g  -  s  158 -  Z = 2362 m m  Distance From Wall, mm  8  13  23  33  43  53 1.0  TIME LAG (t)  T i m e ,s  Figure  3.34  Radial v a r i a t i o n s i n density f l u c t u a t i o n s , power s p e c t r a l d i s t r i b u t i o n o f d e n s i t y f l u c t u a t i o n s , and a u t o c o v a r i a n c e o f d e n s i t y fluctuations, U = 6.5 m/s, G = 63 kg/m s, Z — 2362 mm. g  -  159 -  -  4.  4.1  4.1.1  -  160  DISCUSSION  Microstructural Results  Development of Gas The  and S o l i d s Flow P r o f i l e s  o b j e c t i v e of the m i c r o s t r u c t u r a l study  circulating  fluidised  bed was  and  3.30  3.31  and  to e x p l a i n these  distributions.  c i r u l a t i o n rates.  The  radially in  Figures  3.29,  et a J . (1985) obtained  X-ray absorption  different  f i g u r e s are very s i m i l a r i n  appearance to r e s u l t s presented  and  how  show the v a r i a t i o n of the time-mean r a d i a l  loading d i s t r i b u t i o n with height f o r the  Weinstein  the  to develop a p i c t u r e of  s o l i d s d i s t r i b u t e themselves l o n g i t u d i n a l l y and a riser,  of  by Hartge et a l . (1985) and using a f i b r e o p t i c probe  respectively.  The  r e s u l t s confirm a  r a d i a l d i s t r i b u t i o n of d e n s i t y which i n c r e a s e s from the c e n t r e l i n e to the w a l l , without core and The  annular  regions.  l o n g i t u d i n a l and  three cases  allow one  One  s o l i d s concentration  r a d i a l d e n s i t y p r o f i l e s f o r the  to propose a gross  s o l i d s i n the important concentration.  a sharp demarcation between  flow p a t t e r n f o r  region of decaying  p o t e n t i a l explanation  solids f o r t h i s decay i n  i s that s o l i d s move from an  zone i n the centre of the r i s e r  to a downflowing zone at  r i s e r w a l l where they cascade downward i n sheets base of the column.  upflowing the  towards the  -  It appears  161  -  that there are two  l e v e l s to the problem of  e l u c i d a t i n g the flow s t r u c t u r e s i n a c i r c u l a t i n g bed.  The  patterns. interact and  first The  is identification second  fluidised  of the gas and s o l i d s  i s attempting  to e x p l a i n how  the  flow two  to produce the d r i v i n g f o r c e s f o r changes i n gas  s o l i d s d i s t r i b u t i o n s as the two  p r o f i l e s develop.  In  view of the l i m i t e d number, and type, of measurements which could be a t t a i n e d i n t h i s study, i t i s not p o s s i b l e to e s t a b l i s h c o n c l u s i v e l y what happens at e i t h e r  level.  However, i t i s p o s s i b l e to p o s t u l a t e some p o s s i b l e mechanisms, and suggest Our  own  d i r e c t i o n s for future research.  measurements have examined the s o l i d s phase  e x c l u s i v e l y , and are l i m i t e d to f i n d i n g s about the d i s t r i b u t i o n of that phase.  I t i s not p o s s i b l e to  e s t a b l i s h , f o r example, the d i r e c t i o n of s o l i d s flow at a point e i t h e r on an instantaneous, or time-mean b a s i s using a s i n g l e capacitance probe. suggested  earlier,  s o l i d s upflow near  However, the s o l i d s flow p a t t e r n  proposing a developing p r o f i l e with net  i n a low d e n s i t y c e n t r a l zone and  net downflow  to the r i s e r w a l l s , i s a l o g i c a l extension of the  observed (i)  d e n s i t y p r o f i l e s f o r the f o l l o w i n g reasons: It i s supported  by v i s u a l obervations of the w a l l  region. (ii)  It i s c o n s i s t e n t with r e s u l t s f o r steady riser  flows obtained by other authors  state  (e.g. van  -  162  -  Breugel, 1969-70; Monceaux et al_. , 1985)  who  measured downflow at the w a l l using mass f l u x probes. (iii)  It can be r a t i o n a l i s e d by a simple f l u i d  mechanic  e x p l a n a t i o n presented below. An i n c r e a s e d understanding  of the s o l i d s p r o f i l e  development can be gained by p o s t u l a t i n g gas phase.  The experimental  what happens to the  solids r a d i a l density p r o f i l e s  were taken at d i f f e r e n t h e i g h t s and s i m p l i f i e d models were applied  to estimate the v e r t i c a l component of gas  at each l o c a t i o n .  I f i t i s assumed that  velocity  the r a d i a l pressure  g r a d i e n t i s very much s m a l l e r than the l o n g i t u d i n a l  over a  given c r o s s - s e c t i o n ,  can  then the v e r t i c a l gas v e l o c i t y  estimated provided that  a pressure drop-voidage-gas  relationship  In p r a c t i c e ,  suitable  i s known.  relationship;  there i s no  be  velocity  such  the s o l i d s aggregation phenomena and  complex s o l i d s c i r c u l a t i o n p a t t e r n s render c o n v e n t i o n a l correlations  inapplicable  in a strict  the s i m p l i f i c a t i o n i s made that substantially velocity  sense.  However, i f  s l i p v e l o c i t i e s are  higher than s o l i d v e l o c i t i e s , so that  approximates  can be  Three d i f f e r e n t  gas  the s l i p v e l o c i t y and c o n v e n t i o n a l  forms of pressure drop vs voidage then some i n s i g h t  the  correlations  are  applied,  achieved.  forms of c o r r e l a t i o n  were used,  upon the Kozeny (1927), Burke-Plummer (1928),  and  based  -  Richardson-Zaki  163  -  (1954) e q u a t i o n s .  Each s u g g e s t s  a  different  form  f o r the p r e s s u r e  drop-voidage  dependence which  can  used  to c a l c u l a t e  gas  profiles,  with  the  velocity  original  constant  constant  whose m a g n i t u d e i s f i x e d  gas  velocity.  i n each e q u a t i o n  For  example,  Ap Al  Now gradient local  150  =  A  imposing  the  suggests  that  voidage  i n the  r e p l a c e d by a t o g i v e the  the Kozeny  nil 2 d P  albeit  (1  -  e)  the  fitted  correct  equation  be  total  gives  2 (  4  >  1  )  3 e  c o n d i t i o n of constant local  vertical  pressure  velocity  varies  with  f o l l o w i n g manner.  ,_v3  U  (r) = K  {—^12  ,}  (l-e(r)) Evaluating  K to give the  TJ  U  (4.2)  2  correct  *  {  £  (r) =  (  R  >  mean v e l o c i t y  }  3  > e(r)  2  R  2  (4-3)  e ( r  0  r  R,  gives  ,  J  .  (l-e(r)) The results t o be  value of will  be  strictly  mathematically strong  radial  this  valid,  approach s i n c e one  applicable, what one  i s not  but  expects  that  does not  simply  quantitative  expect  that i t demonstrates  intuitively,  gradients in solids  equations  d e n s i t y are  i . e . that linked  to  - 164  strong  radial  gradients  suggests another vertical  gas  velocity.  simultaneous  velocity,  and  solids  that  correlations.  t h e gas  the  Figure and  4.1  the w a l l  uniform the  centreline  column base This  gas  distribution  i n one  less  i n most i n s t a n c e s .  density  decays  In  c a s e an  this  centreline  part  o f t h e bed  proportionally initial  faster  approach  does not e n f o r c e t h e n o - s l i p may  be  the  velocity).  o f t h e gas  velocity  than  can  centreline  that  o f gas  at the from  voidage  i t s h o u l d be  important  at  behavior i s  for certain  Also,  Here  B i e r l et a l .  when t h e  dependencies.  development.  case.  pronounced  redistribution  can be p r e d i c t e d  which  such  However, some a n o m a l i e s  velocity  a condition  t o match t h e more  t h e mean s u p e r f i c i a l gas  but u s u a l l y  as  decreases  i s e s t i m a t e d as 26 m/s  profile  lower  profile  maximum  increases  i n t h e e s t i m a t e d development  i n the  these  decays.  evident  occur  distribution  nature of  t o t h e m a g n i t u d e f o u n d by  A similar,  p r o f i l e are  velocity  the c e n t r e l i n e  velocity  that  h e i g h t a l o n g t h e column  gradually  solids  (4 t i m e s  i s similar  (1980).  shows how  is  also  voidage-velocity  s o l i d density  velocity  radial  case  with i n c r e a s i n g  r a d i a l l y averaged  which  density  the q u a l i t a t i v e o f the  For each  approach  changes i n the r a d i a l  c h a n g e s i s t h e same f o r each  becomes f l a t t e r  The  i n t u i t i v e l y logical result  changes i n t h e r a d i a l  matched by of  i n gas  -  wall.  wall  to  versus  noted  that  condition  at the  i n the t r u e  profile  this wall,  165  0  20  40  60  76  D i s t a n c e From Wall (mm)  0  20  40  60  76  Distance From Wall (mm)  Figure  4.1  Variation o f l o c a l v e r t i c a l s u p e r f i c i a l gas v e l o c i t y with r a d i a l p o s i t i o n and h e i g h t a s calculated by t h e m o d i f i e d K o z e n y e q u a t i o n . D e n s i t y p r o f i l e s a r e measured values a t vertical l o c a t i o n s o f 0.533 m a n d 2.362 m f o r a g a s v e l o c i t y o f 6.5 m/s a n d a s o l i d s c i r c u l a t i o n r a t e o f 62 k g / m s . 2  -  166  -  To summarise, e s t i m a t i o n of v e r t i c a l v e l o c i t y  profiles  using a v a r i e t y of v o i d a g e - v e l o c i t y c o r r e l a t i o n s , shows that the p o i n t s of highest v e l o c i t y on a r a d i a l p r o f i l e c h a r a c t e r i s e d by the  lowest  d e n s i t i e s , and  are  visa versa.  The  same computations suggest that the gas v e l o c i t y p r o f i l e develop with height  from a s t r o n g l y curved  p r o f i l e i n the  high d e n s i t y zone at the base of the u n i t to a more p r o f i l e i n the lower d e n s i t y regions at greater  4.1.2  uniform  heights.  P o s s i b l e mechanisms f o r s o l i d s movement The  c a l c u l a t i o n s of S e c t i o n 4.1.1  gas v e l o c i t y p r o f i l e with height, but  imply  p l a c e , i . e . why  changes i n the  i n themselves they  not approach the complex problem of why  such changes  to decrease with We  do  take  do s o l i d s r e d i s t r i b u t e themselves and  why  does the high o v e r a l l d e n s i t y at the base of the u n i t  tend  height?  suggest that t h i s i s at l e a s t p a r t i a l l y due  n a t u r a l tendency f o r s o l i d s to move i n t o the low  to a  gas  v e l o c i t y region c l o s e to the w a l l generated by the no c o n d i t i o n which must e x i s t at t h i s p o i n t . downward once i n t h i s  Solids will  low v e l o c i t y r e g i o n , and,  unless  are r e e n t r a i n e d , the c r o s s - s e c t i o n a l mean d e n s i t y w i l l with  may  slip fall they decay  height. It i s p o s s i b l e to p o s t u l a t e a number of mechanisms to  account f o r r a d i a l s o l i d s motion. movement i s not a simple  I t i s c l e a r that  d i f f u s i o n a l process  radial  based upon a  -  solids  density  solids  at  inward  from r e g i o n s  the  centreline. t o w a r d s the further  up  velocity  driving force. base o f  the  If t h i s  of  high  density  A profile  with  high  the  column,  profile  at  combined  of  i n excess of  choking  density,  the  be  the  to agglomerate  would  the  species.  capacities transport lower  with  choking  suggest  that,  by  at  inference  at on  predicted  the  bed  This  glance  is  i s to  of  local  conditions.  and  could  require trends  be  the  on  terms  an  low  the  appropriately This  like  to  agglomerate-like  carrying  diffusional  to high  would  require  v e l o c i t y than at t h o s e of  macroscopic the  in  a substantial rethinking  density  microscopic,  or  for  introduced.  to e x p l a i n  at high  the  This  tendency  in saturated  c o r r e l a t i o n s such as least  in  d r i v i n g f o r c e as  i n terms of  velocity, since,  the  gas  motion again  the  (cluster),  concentrations  Current  decaying  a complicated  as a measure of  regarding  from r e g i o n s  choking  density  a p l a u s i b l e model f o r t h o s e who  circulating  ideas  lower  saturation carrying capacity,  However, i t would  current  to d i f f u s e  would be  first  with  c l u s t e r gradient  represent  consider  of  considered  radial  with  for solids  prevalent  t u r n might  defined  wall  at  movement, but  density  system  t o the  then  4.2.  driving force  a diffusional  tend  case  showing a c e n t r e l i n e minimum.  in Figure  the  were the  centreline density  the  A more a t t r a c t i v e c o n c e p t consider  -  column would  downflow s i n k  illustrated  167  Matsen  level,  opposite  is  and  low. (1982) hence  true.  - 168  High density causing down-  -  Density gradient for diffusional flux to wall  Form of density profile required to justify simple diffusional model  Gas Velocity or Solids Density  Gas Velocity Solids Density  Gas Velocity or Solids Density  Typical form of density I profile  R  F i g u r e 4.2  _ Radius  o  I m p l i c a t i o n s of a simple d i f f u s i o n a l model f o r s o l i d s motion f o r gas and s o l i d s d e n s i t y p r o f i l e s . Lower f i g u r e shows a t y p i c a l e x p e r i m e n t a l r e s u l t , upper f i g u r e the requirements f o r a d i f f u s i o n a l model t o be consistent.  -  Matsen  169  -  finds: U  a ch = 0.0003  a ch = 1.26  x 10"**  /V  <  t  3.41 /V )  (U  1.29  (4.4)  , U /V.> ' g' t  t  1.29  (4.5)  where, a  ch  solids  =  volume f r a c t i o n  Ug  = superficial  Vt  = solids  Rejection  of  gas  (m/s)  terminal velocity  (m/s)  these  mechanisms f o r t r a n s f e r  gas  f l o w and  specifically  profile  develops  with  velocity  profile  parabolic flatter  as  higher  base of  the  up,  plots  and  is  convective  illustrated  potential  gas  flow  i n F i g u r e 4.3.  radial  velocity  b a s e d on  changes i n the  and  profiles  flat  over  the order  o f 0.12  section.  Therefore  solids than the  transfer  m/s  may  to d i f f u s i v e nature  over  o f any  column t o a  the  the  stream  is a  transition  for  the  small  height. of  This  the  i n v o l v e d , which  1 m,  a t some p o i n t s on  processes.  suggest  the  the  were  gas  If this  modelling  values  radial  radially  to convective  subsequent  substantially  f u n c t i o n between p a r a b o l i c  a d i s t a n c e of  mixing  gas  streamlines  Crude e s t i m a t e s  components  due  velocity  approximately  that there  some p o r t i o n of be  gas  f e a t u r e s of  c o n s i d e r s the  from an  flow development, i t i s apparent radial  the  I f one  developing  of m a t e r i a l  o f more complex  o f how  height.  shape a t t h e  profile  choking  velocity  t o t h e w a l l prompts e x a m i n a t i o n the  for  flow  cross  outward rather  i s correct  processes  of  would  then be  - 170 T  1  1  "  1  1  r  Distance From Wall (mm) F i g u r e 4.3  Streamfunction  p r o f i l e s i n a developing  flow.  The s t r e a m f u n c t i o n i s p l o t t e d as a f u n c t i o n of r a d i u s f o r a u n i f o r m gas v e l o c i t y p r o f i l e and a p a r a b o l i c p r o f i l e showing how, as the p r o f i l e changes w i t h h e i g h t due t o r e d i s t r i b u t i o n and decay of d e n s i t y , t h e r e i s a bulk f l o w of gas towards the w a l l . Arrows j o i n p o i n t s o f c o n s t a n t s t r e a m f u n c t i o n showing the d i r e c t i o n of gas f l o w .  -  substantially valuable of  altered.  radial  flow  strong  convective  that  t h e same o r d e r  diffusion  due t o gas  movement  caused  intensity. solids  flow  This  which  of s o l i d s ,  the  by r a d i a l  component then  variations  i n the s o l i d s radial  solids  radial  t o be l e s s  linear suggest  relying  upon  density i s  i n gas  i t s radial  motion.  because  Intuition  of s o l i d s  than  the  turbulence  motion  gradient  influenced  reinforce  diffusional  phase.  gradient  particularly  of turbulence  (1969-70) would  Thus,  f l u x e s of at  could  i n gas  gradient,  up and down s o l i d s  the almost  solids  i s not a t r u e d i f f u s i o n a l  intensity,  primarily  t o be r e d u c e d  i s an e f f e c t i v e  to generate a strong  i s likely  single-phase  (Soo, 1982).  mechanism, which  that a strong  turbulence  fluctuation) of  However t h e r e i s  i s likely  generate  dimensional  turbulence.  non-continuum e f f e c t s  likely  could  a r e d r i v e n up a d e n s i t y  suggests  components  o f m a g n i t u d e as t u r b u l e n t  A second p o t e n t i a l convective  velocity  of a  (Appendix 2 ) .  the l a t t e r  flows  i t is  (r.m.s. v e l o c i t y  by t h e p r e s e n c e o f s o l i d s  radial  purposes  a typical  on t h e c e n t r e l i n e  a t Ug = 6 . 5 m/s  evidence  convective  compare w i t h  intensity  0.1 m/s  substantially  least  radial  0.13 m/s  turbulence  approximately gas  For comparative  t o note t h a t  approximately  -  171  component  t h e v e r t i c a l by I f the r a d i a l  i s damped by t h e p r e s e n c e o f s o l i d s , velocity higher  profiles radial  of van-Breugel  turbulence  intensities  -  on  the c e n t r e l i n e  shear  i n the wall region, although  i s approximately  section.  constant  This i s a different  where a h i g h in  than  172 -  shear  the r a d i a l  r a t e near  are considered  albeit  with  the high  intensity  collection  to single  (Hinze,  phase  1959).  t o f o l l o w t h e gas phase  naturally  effects,  flow  Now i f  turbulence,  the gradient i n  c r e a t e s a s o l i d s motion  t o t h e low i n t e n s i t y  of p a r t i c l e s  cross  t h e w a l l c r e a t e s a weak maximum  p h a s e l a g s due t o d r a g intensity  the r a d i a l  situation  turbulent intensity  solids  turbulence  over  t h e gas  zone,  a t t h e w a l l enhanced  from  with by  inelastic  collision. In summary, capacitance density visual other  the experimental  p r o b e show s t r o n g r a d i a l  distribution evidence  authors  solids  of the s o l i d s .  i n steady with  structure.  f r o m an u p f l o w i n g  results  riser  flows,  h e i g h t may  core  i n a d e n s i t y which  This potential other  types  occur  i n the core  suggest  t h a t decay o f radially movement o f  annulus  height,  f o r by c o n v e c t i v e  with the  gas f l o w and  intensity.  motion.  F o r example,  due t o a g g l o m e r a t i o n  their  and r e s u l t s o f  be due t o a  decays with  with  mechanism f o r d e c a y does n o t p r e c l u d e  of s o l i d s  with  layer  t o downflowing  gradients i n turbulence  clusters,  T h i s , combined  I t appears that r a d i a l  movement p e r h a p s a c c o u n t e d radial  non-uniformities i n the  o f downflow a t t h e w a l l  the d e n s i t y p r o f i l e non-uniform  s t u d i e s using the  subsequent  decay may  of s o l i d s  downward m o t i o n  into due t o  also  -  increased the  wall  e f f e c t i v e mass. may  be  aided  by  173  Also,  strong  radial  predominantly not  possible  present and  to v e r i f y  turbulent  until  (Soo  non-uniformities  core-annular  study;  movement o f  electrostatic  developed pneumatic t r a n s p o r t the  -  the  fields  ideas  mechanics of  do  suggest  in However,  a  Unfortunately  with  radial  fluctuation gradients  e f f e c t i v e measurement  as  e t a_l. , 1964).  structure.  these  s o l i d s towards  the gas  must  it is  r e s u l t s of and  solids  remain  the flow,  conjecture  techniques  f o r these  s o l i d s flow  structure  parameters  become a v a i l a b l e .  4.1.3  Nature of To  the  gain  two  local  a better  phase flow,  density, point  the  understanding  in addition  which i n d i c a t e the  i n the  column,  traces  of  density,  quantitatively about  to the  This  Finally,  i s r e f l e c t e d i n the  autospectral  density  time t r a c e  changes with the  height.  the  of  local  behaviour at t o examine  - 3.34,  the  of  a  the  time  and  the  of  more  density  p e r i o d i c i t y of  autocovariance  density  and  functions. of  the For  point the  l o w e s t measurement p o i n t  distributor),  3.32  deviation  fluctuations  The  nature  i s r e f l e c t e d i n the  standard  i t s mean v a l u e .  local  profiles  important  shown i n F i g u r e  i n the  the  time-averaged  i t i s also  instantaneous behaviour.  of  density  highest (« 533  centreline density  shows  dramatic  circulation  mm  above  rate  at  the  fluctuates  dramatically  - 174  between v a l u e s kg/m  .  loose  The  as  upper  p a c k e d bed  structure  at  character. rapidly  high value  point  the  height  914  mm  density  has  decayed given  p a c k e t s of  particles  914  mm  with  and  up  kg/m  two  20-40  close  to  that  phase,  the  the  bed  cluster-like  peak d e n s i t i e s decay these wall  measurement p o i n t  local  to b a r e l y  30  the  kg/m ,  and  3  t o a much more homogeneous smaller  through  column b r i n g s s t r u c t u r e as  and  the  l e s s dense  p r o b e volume.  remarkably the  to a constant  a saturated  as  and/or d i f f u s e t o t h e  from 150  passing  the  low  showing  3  c e n t r e l i n e as  first  way  a p p e a r s t o have d e c a y e d with  the  as  a density  a strong  occasional  c e n t r e l i n e flow  associated  1325  t o b r e a k up  flow  the  has  along  centreline  in  of  above the  s t r u c t u r e has  further  kg/m  However, b o t h mean and  a g g l o m e r a t e s appear  bulk  1200  represents  density  this  with  region.  as  -  local  density  little  A change  density w h i c h may  carrying capacity,  as  be  discussed  below. In basis, the  order  to  standard  i t i s useful  deviation.  and  3.31.  the  variability  point.  Although of  Those a r e  Not  local  s u r p r i s i n g l y , the decreases  time-mean d e n s i t y  phase s t r u c t u r e profiles  shown i n F i g u r e s  these provide the  two  t o examine r a d i a l  density,  to  distribution.  the  curves are  also varies  standard  from w a l l  3.29,  of  of the 3.30  a q u a n t i t a t i v e measure  i n t e r p r e t e d because d e n s i t y  fluctuations the  on a more q u a n t i t a t i v e  g i v i n g more i n s i g h t i n t o the  system,  easily  compare p r o f i l e s  not  from p o i n t  d e v i a t i o n of  of  to  density  centreline, following  - 175 -  To p r o v i d e nature index  a more v a l i d  of the flow may  comparator of the  at d i f f e r e n t  two-phase  p o i n t s , an i n t e r m i t t e n c y  be d e f i n e d a s : Standard D e v i a t i o n of D e n s i t y F l u c t u a t i o n s a t a ^Y ^ fStandard D e v i a t i o n of D e n s i t y Fluctuations for fully segregated Two Phase Flow w i t h I d e n t i c a l Mean d e n s i t y as a t P o i n t P  Intermittency = Y = y  e  Y  P o :  n t  p  (4.6) The  denominator i n t h i s  considering as  the f u l l y  intermittently  packed  assumed  gas v e l o c i t i e s  the  that t h i s  composed  to apply  (Davidson  of a  structure segregated probe  or " i d e a l "  finds  into  The s t a n d a r d  either  density  one o f two  clusters.  At h i g h  cluster  i n these  analytically  and does not depend  1963).  values,  loose  theory beds a t low  T h i s does not  upon  I t can be shown t o be:  zero  that  or the  velocities  this  f l o w " where s o l i d s  flow are  o f dense phase and t h e  or i n gas a t any  d e v i a t i o n of d e n s i t y  can be computed  and  form o f c o n t a c t i n g , o n l y  l o o s e packed c l u s t e r s  itself  voids  fluidised  and H a r r i s o n ,  "ideal  two p h a s e s y s t e m  two phase  for bubbling  l o o s e p a c k e d bed.  represents  i s d e r i v e d by  of s o l i d s - f r e e  i s an optimum  d e n s i t y can have o n l y  density  flow  segregated  r e g i o n s as i n t h e well-known  commonly  imply  expression  fluctuations  f o r any a v e r a g e the s i z e  instant. for this  local  or frequency  of  -  p  o =  where  h J ^  (  p = local P  "  1  a  p  L  }  (  solids  of- s o l i d s  = standard  This  /  average  = hold-up  L  p  176 -  3  hold-up  normalising factor t h e maximum  deviation  for a particularly  local  representing segregated As index, and  varies  (kg/m ) .  for local  standard  p o s s i b l e standard  density.  Therefore the  between z e r o and u n i t y  the extremes of p e r f e c t l y  homogeneous and  fully  (non-homogeneous) f l o w s .  an example o f t h e p o s s i b l e i n t e r p r e t a t i o n c o n s i d e r t h e extremes of f u l l y  ideal  cluster  flow,  F i g u r e 4.4.  developed  of t h i s  core-annular  Core-annular  flow,  despite  t h e s t e p change i n d e n s i t y a t some r a d i u s , h a s a  uniform  i n t e r m i t t e n c y index  cross-section, index  while perfect  of zero over cluster  t h e whole  f l o w has a c o n s t a n t  of u n i t y . The  plotted  i n t e r m i t t e n c y i n d i c e s have been c a l c u l a t e d and f o r each o f t h e measured  profiles.  shown i n F i g u r e s 4.5 t o 4.7 f o r t h e t h r e e fluxes. case  )  i n l o o s e p a c k e d bed (kg/m )  because i t i s a l s o  index  7  (kg/m )  deviation  intermittency  *  3  holdup  d e v i a t i o n of s o l i d s  i s a useful  4  The i n d e x  profiles  These p l o t s a r e circulation  f o r the highest  ( F i g u r e 4.5) g i v e s some i n t e r e s t i n g  circulation  insights  rate  i n t o the  - 177 CLUSTERING  clusters  CORE-ANNULAR  annulus  core  Intermittency Index  V  Radius  Figure 4.4  Radius  I n t e r m i t t e n c y i n d i c e s as a f u n c t i o n o f r a d i u s f o r f u l l y d e v e l o p e d c o r e - a n n u l a r and " i d e a l cluster flow."  -  0.7  06  1  178  -  —"o^o •— — 1  r  O  •f  i  05  S y m b o l Height ( 0 533 1-448 2.362  O  0-1  • A  •  •  0  20  J  I  40  60  D i s t a n c e From Wall  Figure  4.5  L  76 (mm)  I n t e r m i t t e n c y i n d e x p l o t t e d as a f u n c t i o n of r a d i u s at t h r e e v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g bed of sand f o r U = 6.5 m/s, G = 62 kg/m s. g  2  s  - 179  F i g u r e 4.6  -  I n t e r m i t t e n c y i n d e x p l o t t e d as a f u n c t i o n o f radius at three v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g bed of sand f o r U = 6.5 m/s, G = 48 kg/m s. g  2  s  - 180 -  0.7  ~i  1  1  1  1  r  Symbol Height (m)  06  O • A  05  0-533 1.448 2.362  x <D  •g 0 4 o §  E  Ic  0-3 0 2  0-1  i  J  0  20  i  i  40  *  •  60  •  76  Distance From Wall (mm)  Figure  4.7  Intermittency index p l o t t e d as a f u n c t i o n of radius at three v e r t i c a l l o c a t i o n s i n a c i r c u l a t i n g b e d o f s a n d f o r Ug = 6.5 m/s, G = 43 k g / m s . 2  s  -  181 -  nature  of the o v e r a l l  density p r o f i l e  lowest  point, despite the strong r a d i a l  the  i n t e r m i t t e n c y index  the  c r o s s - s e c t i o n ; there  halfway of  remains s u b s t a n t i a l l y i s a weak maximum  0.65 i n d i c a t e s  with  intermittent  near  constancy  what i s s e e n  packets  constant  on t h e i n s t a n t a n e o u s bodies  However, 914 mm h i g h e r  inhomogeneity  over  over  over  approximately The h i g h  inhomogeneous  o f d e n s e and d i l u t e  of the index  cluster-like  dramatically  a very  At the  gradient of density,  between t h e w a l l and t h e c e n t r e l i n e .  approximately  i.e.,  development.  phase.  value  flow The  the c r o s s - s e c t i o n confirms time  traces of density,  t h e whole c r o s s - s e c t i o n .  up t h e column t h e p i c t u r e i s  different.  A rapid  decay  i s e v i d e n t , presumably  i n the c e n t r e l i n e  due t o c l u s t e r  breakdown and c o n v e c t i v e t r a n s p o r t t o t h e w a l l r e g i o n . inhomogeneity the  i n c r e a s e s towards the w a l l a l o n g  agglomerate  t r a n s p o r t and r e a c h e s  t h e w a l l r e g i o n , 20mm f r o m inhomogeneity decayed decrease  from  a t the w a l l remains high,  Finally  measurement, t h e i n d e x , t o have r e a c h e d  developed  with  value along  continues  the w a l l as the p r o f i l e fully  together  a constant  However, i n h o m o g e n e i t y  final  although  at the h i g h e s t  continues  shape.  0.47 i n  Thus, t h e  i t s v a l u e a t t h e column b a s e w i t h  i n density.  the path of  a maximum o f  the wall i t s e l f .  The  i t t o o has  the o v e r a l l  l e v e l of  the density,  appears  the c e n t r e l i n e .  t o decay  i n the v i c i n i t y o f  to r e c t i f y  I t i s important  towards i t s to r e a l i s e  -  that  the  final  geometry,  The  which i s r a t i o n a l i s e d  other p r o f i l e s that  intermittency  probably lower  due  The  a t the  solids  a complete  The  signals  i n these  i s related  f o r heat  transfer  power s p e c t r a  and  signals  little  from  each  perceived  o f one  They a r e r e m a r k a b l e  periodicity,  phases,  knowledge  Although traces, renewal  run  were  primarily  even where t h e r e i s a  the w a l l s .  velocities  and  the  requires  point  column and  t h e s e gas  densities  studies.  inhomogeneous f l o w s t r u c t u r e  at  periodicity.  This i s especially  relatively  i n some o f t h e s e  the  autocovariance functions for  shown i n F i g u r e s 3.32-3.34. showing very  to signal  of the flow s t r u c t u r e  cases  clustering.  study of  between d i f f e r e n t  hence  latter  initial  of  This i s  time-averaged  o f phase r e n e w a l .  similar  near-constancy  i n d e x gave i n f o r m a t i o n about  picture  c a p a c i t a n c e probe  peaks  exit  a  r a t e s and  a s p e c t of the s t a t i s t i c a l  of d e n s i t y  periodicity  important  the  in a  level.  circulation  t h e mean v a l u e s i n d i c a t e d  distribution  for  lowest  t h e need f o r a p r o n o u n c e d  final  vary  the i n i t i a l  suspension d e n s i t i e s  the inhomogeneity  of  lack  index  c a p a c i t a n c e probe While  by  i n terms o f  of inhomogeneity  they  t o lower  initial  precluding  i s dictated  model i n S e c t i o n 4.1.5.  manner, e x c e p t the  -  shape o f t h e p r o f i l e  a fact  conceptual  182  - at the base of  the  t h e power s p e c t r a  show weak  the g e n e r a l f i n d i n g  i s that  takes place f a i r l y  randomly  -  with  a power s p e c t r u m  band  noise  of  (Bendat,  modelling  velocities t h e heat well  d e f i n e d by  4.1.4  the with  conceptual  decay  zone of  bubbling  as e a r l y  disperse  of  freeboard  solids  by  the  low  as  proposed  particles "but  They  velocity  a model  rates  r a t e s are approach  scenario.  f o r s o l i d s movement i n bed  bears  strong  freeboard  beds appear  analogy  flow  lower  with  densities also  " g r o u p s or  c o n c e n t r a t i o n " (than  d e s c r i b e the t o the  disengaging  i n the  action  f r e e b o a r d based  the of  moving  d i s p e r s e phase  "streamers"  column w a l l . "  the  than  t r a n s p o r t of upward  to areas  the  been These  a t a c o n c e n t r a t i o n much lower  f o r the  in  t o have  L e w i s e t aJ.. ( 1 9 6 2 ) .  at a h i g h e r  near  govern  C l u s t e r s t r u c t u r e s i n the  by  "due  form  regimes  "projectiles"  also  zone as  f o r wide  low  statistical  models o f  fluidised 1962  at  renewal  velocity  t o downflow," i n c l u d i n g  gas  A  circulating  systems.  than  where t h e s e  model p r o p o s e d  turbulent eddies  favorable  high  seen  a different  t h a t of t h e dense phase and  phase),  dense p h a s e . "  and  low v e l o c i t y  describe solids  approximately  of  in this  of b u b b l i n g  streamers  processes  processes.  some d e s c r i p t i o n s and  authors  the  bubble  The  observed  spectrum  T h i s suggests  transfer  Analogies with  freeboard  t o the  coefficient,  more v a l i d  velocity  -  where, f o r s m a l l p a r t i c l e s ,  transfer  would be  similar  1980).  of h e a t  183  and  "regions  L e w i s e_t a l .  upon  mechanistic  -  184  considerations  of s o l i d s  model does not  consider s p a t i a l  downward  upflow  including  agglomerates  wakes of b u b b l e s ,  elutriation  data.  a model  transfer  between e a c h .  completely  moving  able  these  early  complex bed.  the  be  The  into  freeboard  projected  between p h y s i c a l beds and  and  the  stream  agglomerates the  structure in  f r e e b o a r d , one  might  Unfortunately this case.  solids  dimensional  Being unable  evident  nature  movement i n t o  realistic  (1969)  solids  Given  expect  The  drawback  of  i n the  the  to a c t u a l  the  circulating 1985)  which  freeboard, wall  and  zone  application  i s that, although  s o l i d s movement mechanisms,  a l . (1980) p r o p o s e a r a d i a l  not  to p r e d i c t  a descending  bed  does  bed  one-dimensional,  ( e . g . , H o r i o jet a l . , 1980,  better.  noses  f r e e b o a r d models t o t h e c i r c u l a t i n g  m o t i o n of  particle  from  Levenspiel  agglomerates.  f r e e b o a r d models a r e  two  the  t h r e e p h a s e s were a gas  models t o a c i r c u l a t i n g  represent et  upward  of m u l t i p l e p h a s e s ,  example, K u n i i and  completely  L a t e r models  consider should  of  the  been common i n c o n s i d e r a t i o n of  modification.  radial  develop  these  to apply  t o be  has  descending  fluidised  little  appear  distributions  dispersed solids,  similarity  circulating  with  However,  i n c o r p o r a t i n g t h r e e p h a s e s and  upward, and  apparent  t o be  downflow.  concept  thrown  For  proposed  with  and  fluxes.  In s u b s e q u e n t work t h e  and  -  they  (e.g.,  s o l i d s movement due  of  to  Horio  - 185  turbulence  intensity  simplified  t o g i v e a w e l l mixed  g r a d i e n t s ) , the m o d e l s a r e  annulus.  T h i s may  freeboard  turbulence generated  of  bubbles"  "ghost  core may  and  annular  also  be  circulating density  bed  demarcation,  and  a  the  starting  point  velocity  replace i t with conditions.  capacitance  to d e v e l o p  flows  probe  concept  the  model  model  of s a t u r a t e d c a r r y i n g  a  two  represents  model o f  the  fast  of t h e t u r b u l e n t  characteristic  bubble  more a p p r o p r i a t e with  to  some  6.  pressure  f o r the  bed.  high  i s probably  saturated carrying  model  fluidised  the  Nonetheless,  nature  and  It  core-annular  i n Chapter  tests  a conceptual  in a c i r c u l a t i n g  of  Such a model i s used  and  between  structure varies  annulus)  something  mixing  Fast f l u i d i s a t i o n  used  the  fluxes.  t h a t the  energy  However, i n t h e  clear  t o remove t h e  t o d e s c r i b e gas  The  no  of t h e  the  properties.  section  bed,  where  demarcation  for a mechanistic  zone, p r o v i d e d  and  success  solids  bed  constant  top.  c o r e , w e l l mixed  frequency  the  t o w a r d s the  r a d i u s , with  i s modified  4.1.5  to a c l e a r  fairly  subsequently  a w e l l mixed  dissipation  i n the more d i l u t e  structure  high  by  a true two-dimensional  (well-mixed  decay  and  i n the b u b b l i n g  gives rise  to r e p r e s e n t  useful  bed  core  r e g i o n of a c i r c u l a t i n g with  zone  valid  riser  continuously  needed  be  zones w i t h  valid  -  With  capacity  profiles  local  were  solids  introduction  capacity, this  can  be  of  - 186  extended  i n t o more g e n e r a l  circulating For there flow the  bed  the  radially  value  then,  turbulent layer,  averaged  they  are  Consequently radially the  any  critical  It  can  suspension  and, argued  reach  the  pipe  could  that  in tall  but  this such  as wall  the  particle  boundary  gas  downflow.  or  equal  layer i s  turbulent  downward, due than  If  critical  i s l e s s than,  gas to  terminal  statistically zone and  occur.  likely finite  velocity.  eddies  that  average velocities,  a v e r a g e , move upwards.  falling  not  gas-solids  However, i f the  has  without tall  some f i n i t e ,  vertical  Whether or  t o be  every p a r t i c l e  reentrainment  sustain pneumatically  academic q u e s t i o n Profiles  by  that  t o move i n t o the  decays.  lower  in a sufficiently  d i s e n g a g e m e n t would  processes  i s a bulk  gas  often  wall  p r o b a b i l i t y of  eventually,  tall  the  the  gas  exceeds  density  to f a l l  on  for  numerous t o m o d i f y  mean d e n s i t y  tending  be  density  there  layer v e l o c i t i e s  eventually small  that  explain  consider  density  a given  transport  density,  reentrained  at  cause p a r t i c l e s  sufficiently  particles  boundary are  the  averaged  penetrated  line  sufficiently  l a y e r such  to  d i s c u s s i o n we  suspension  solids  when s o l i d s  diffusion  boundary  to  transport  which h e l p  behaviour.  this  i s a maximum s t a b l e in a tall  ideas  macroscopic  p u r p o s e s of  -  not  an  conveyed  resolved  columns do  pipe,  by  appear  will  although so  that  complete  infinitely solids  is  an  experiment. to  approach  - 187  limiting  values  capacities just  as  so  that non-zero s a t u r a t e d c a r r y i n g  a p p e a r t o be  the  concept  reasonable  which s o l i d  flux  useful  i n studying  entrainment  defined  that  here  the  does not  saturated flow.  gas  and  solids  developed" If  the  accepted, into  or  two  of  imply  and  height  of h e i g h t ,  freeboard  (TDH),  has  been  phenomena.  saturated carrying capacity  anything  It only  velocity  and  purposes,  about  the  s t r u c t u r e of  i m p l i e s t h a t both concentrations  the  should  time-mean be  "fully  stable.  e x i s t e n c e of a s a t u r a t e d c a r r y i n g c a p a c i t y i s  vertical  gas-solid  distinctly  circulation it  i s independent  concept  the  for practical  of a t r a n s p o r t d i s e n g a g i n g  beyond  Note  -  two  different  r a t e s lower  than  phase f l o w s  types, this  those  can  be  with  divided  solids  c a p a c i t y , and  those  where  i s higher. To  flow  illustrate  i n the  gradually this  curved  study  gradual  riser  the  differences, consider a  o f F i g u r e 4.8 exit,  similar  (see Chapter  3).  enough c u r v a t u r e  separative particles  that  f o r c e s a c t i n g on which  No.  2 used  there are  no  through  The  only  other  notable  diameter  ratio  of a p p r o x i m a t e l y  study.  exit  smooth  i s assumed  and  in this  a very  This exit  particles  upward  particles  t o the  reactor,  used  no  travel  which has  gas-solid  this  50,  to  have  inertial  approaching plane  a-a'  p a s s downward t h r o u g h  f e a t u r e of  in  riser  a similar  i t . leave  this  Hence the  plane.  i s a height value  to  to that  -  188  -  SMOOTH EXIT PROMOTING ZERO SOLIDS REFLECTION ACROSS PLANE A-A  EXIT PLANE  DEVELOPED FLOW ZONE X 10  REGION OF SOLIDS ACCELERATION, REDISTRIBUTION, AND BACKFLOW SOLIDS FEED DENSITY  GAS  IN  SCHEMATIC OF A SMOOTH EXIT RISER  Figure  4.8  TYPICAL DENSITY PROFILE FOR TRANSPORT BELOW THE SATURATED CARRYING CAPACITY  A d e p i c t i o n o f a smooth e x i t r i s e r f o r i l l u s t r a t i o n of t h e c o n c e p t s i n v o l v e d w i t h f l o w s t r u c t u r e s above and below t h e s a t u r a t e d carrying capacity. To the r i g h t i s a t y p i c a l d e n s i t y p r o f i l e below s a t u r a t i o n .  - 189 -  Gas-solids rate  i s lower t h a n  relatively and of  i n this  riser,  the saturated  straightforward.  2 of Figure  where t h e c i r c u l a t i o n  carrying capacity, are  They a r e t y p i f i e d by p r o f i l e s 1  3.14 r e p r e s e n t i n g  solids  circulation  rates  36 kg/m s and 73 kg/m s r e s p e c t i v e l y a t t h e p r e v a i l i n g 2  gas  2  velocity  length is  flows  o f 7.1 m/s.  of approximately  F o r each o f t h e s e p r o f i l e s , a 2 m above t h e s o l i d s  c h a r a c t e r i s e d by g r a d i e n t s  density  Gradually  decreasing  are accelerated  (ii)  Local  redistributed return  i n the apparent  circulation  pressure  final  patterns  f r o m an i n i t i a l  Apparent  slip  to their  system t o a s t a b l e  (iii)  radial  density  as t h e  velocity. as t h e s o l i d s a r e imposed  by t h e  pattern.  increases  by c o n s t a n t  suspension  accuracy  l i m i t s o f t h e measurement  Visually  one s e e s o c c a s i o n a l  down t h e w a l l "stable"  velocities  distribution  however, a s t a b l e p r o f i l e  characterised  due t o a c c e l e r a t i v e  over  profiles,  density,  system  strands  the region  i s reached  but the s t r a n d s  within the  ( ± 5 kg/m ). 3  of p a r t i c l e s  corresponding  which i s  falling  t o these  are rapidly  reentrained  do n o t s u r v i v e . Now  into  suspension  drop.  Eventually,  and  point  c a u s e d by:  (i) solids  injection  consider  the s c e n a r i o  the r i s e r at a r a t e  when s o l i d s  greater  are introduced  than t h e s a t u r a t e d  carrying  -  capacity order  f o r t h e gas v e l o c i t y  to obtain  invariant  with  190 -  and column  a longitudinal profile time,  the r a t e equal  take  the d e n s i t y  although  critical  value  resolved  when a d e n s i t y  greater are  given  sufficient  located  to t h e i r  sufficient  than c r i t i c a l , solids  Profiles  feed  leave  rate.  height.  a scenario  must  to i t s  The s i t u a t i o n i s at the e x i t  that  3.14  which i s  when  the e x i t  i s continuing  3 t o 5 of F i g u r e  the  This  which o c c u r s  i n the r i s e r  there  becomes  t o decay  at c i r c u l a t i o n  respectively, density  each  r a t e s o f 93 kg/m  successive  at the e x i t  plane  2  s and 116 kg/m  profile requiring  to s a t i s f y  with  i l l u s t r a t e this  2  effect  In  remains  tends t o decay  i s established  i n a zone where d e n s i t y  height.  which  a t which s o l i d s  r e a c t o r must become place  i n question.  a  t h e imposed  s  higher circulation  rate. To  clarify  essential solids is  to r e l a t e  i s happening  capacity,  Consider  a t , or j u s t and a s m a l l  increase  understanding  these  move c o n v e c t i v e l y ,  when t h e r i s e r carrying  i s made i n t h e r a t e According  of s a t u r a t i o n will  towards t h e w a l l  at which  to our  of the i n t e r n a l  additional solids  i ti s  to the m i c r o s t r u c t u r a l  the s i t u a t i o n  movement, and our u n d e r s t a n d i n g capacity,  instances,  below t h e s a t u r a t i o n  a r e f e d to the r i s e r .  microstructural  i n these  macrostructure  circulation.  running  solids  what  solids carrying  gradually  where  they  diffuse,  begin to  or  -  form a downflowing instantaneous However, as riser  core  the  newly  riser  material.  decays over  layer  and  core The  the  required  the  the  change,  shown i n F i g u r e  greater  situation,  saturation  4.1.6.  Fast  described  "choking  by  capacity  increments  way  above,  rate  flux"  the  the  upflow  upflow  exit.  net  Rhodes and  net  Geldart  has  r e s u l t of of  an  constant  capacity.  transport  the  zone  development  carrying  for  wall  reinforcement  developing  is a  are  fresh  r e i n f o r c i n g the  The  the  solids  density  where t h e r e  since  riser.  combine w i t h  i s the  no  layer reaches  downflowing  and  This  (1985), below  whole column  is  the now  choking solids circulation  capacity  can  be  with a developing column t o  i s increased  in this  of  is  profile.  i n the  carrying  base o f  circulation  4.9,  saturated  fluidisation  saturation  the  the  a developing  the  from  riser  d i f f e r e n t than t h a t  carrying  as  Small  then  top  even f u r t h e r , a the  pattern,  also portrayed  dramatically  operates  at  the  length,  until  upflux  the  r e s u l t i n g higher  internal circulation  circulation  Hence t h e r e  where t h e y  a greater  density  at  point,  p r o c e s s which c o n t i n u e s  internal  layer.  formed d o w n f l o w i n g  s o l i d s feed  i n t o the  upflowing  s o l i d s wall  e f f e c t upon o u t p u t  base and  forced  -  191  accommodated profile  i t s exit. beyond  discussion,  rate  beyond in  the  extending  However, i f  some v a l u e ,  the  called  a dense phase w i t h  the  stable  -  192  -  DOWNFLOW  Figure  4.9  UPFLOW CORE  A s c h e m a t i c diagram showing s o l i d s f l u x e s i n a r i s e r o p e r a t i n g above the s a t u r a t e d carrying c a p a c i t y but below c h o k i n g . On t h e l e f t i s a s c h e m a t i c i n w h i c h arrows i n d i c a t e approximate d i r e c t i o n s of s o l i d s flow a n d show t h e d e v e l o p m e n t of the wall l a y e r . On the r i g h t i s a second d i a g r a m where the w i d t h o f up a n d d o w n f l o w a r r o w s g i v e s a n i d e a o f how up a n d d o w n f l o w f l u x e s v a r y w i t h h e i g h t i n t h e u n i t to g i v e a net p o s i t i v e flux.  - 193 -  density  a p p e a r s a t t h e base o f t h e column.  here  implies a suspended"solids  vary  appreciably  4.10. in  with  P r o f i l e 5 of F i g u r e  o u r own  clearly Figure  work.  1.9.  column  with  density  a t t h e column  value.  To s a t i s f y  stable  dense phase  rate.  characteristic described  begins  formation  i s also  occurs  sufficient  feature  shown  when t h e density  the high  exit  density  forms t o w h a t e v e r  This  to give process  by t h e model  requirement,  height  rise  of f a s t  the  a  i s necessary f o r imposed  to the fluidisation  o f L i and Kwauk (1980) and  by W e i n s t e i n  at the  "choking"  the r e q u i r e d gives  required  decay p r o f i l e ,  base e x c e e d s a c r i t i c a l  ' s ' shaped p r o f i l e s  experimentally  t o show t h i s  region  a conventional  n o r m a l decay p r o c e s s  circulation  which does n o t  as i l l u s t r a t e d i n F i g u r e  3.14  rate necessitates that,  density  o f L i and Kwauk (1980) r e p r o d u c e d i n  Dense phase  exit,  concentration  A stable density  i n the r e s u l t s  circulation  the  height,  Stable  e t a l _ . (1980) a t h i g h  observed solid  fluxes. A possible microstructural explanation phenomenon i s a s s o c i a t e d downflowing  solids  circulation  rate  solids  movement  and  denser wall  the  l a y e r may  wall  with  the development  layer.  i s gradually  of t h i s  As  toward t h e w a l l  of t h e  t h e imposed  increased,  then  and downflow  solid the p r o c e s s  create  l a y e r s a t the base of t h e u n i t .  be s u f f i c i e n t l y  t h i c k , and  choking  of  thicker  Eventually,  t h e r a t e o f gas  -  194  -  saturated carrying capacity  increasing  stable dense phase choking Density Figure  4.10  A s c h e m a t i c showing t h e c o n c e p t of c h o k i n g r i s e r as a p p l i e d i n t h i s t h e s i s , and a s observed i n the o v e r a l l d e n s i t y p r o f i l e .  in a  - 195 -  flow  through  the core  amount o f s o l i d s point  by  are s t r i p p e d  process  and s o l i d s Briens  4.11.  from  that  the w a l l  and t h e growth p r o c e s s  flow  to give  this  This  (1986),  logical  a substantial  region.  At this  between t h e  resulting i n stable  scenario,  also  i s illustrated  of d i f f e r e n t  different  becomes a v e r y In  patterns.  and Bergougnou  The b u i l d - u p  phase  high,  a dynamic e q u i l i b r i u m c a n be c o n c e i v e d  stripping gas  sufficiently  i n Figure  amounts o f t h i s  d e n s i t i e s a t the r e a c t o r and r e a d i l y e x p l a i n e d  conceived  "stable" exit  then  scenario.  d i s c u s s i o n c h o k i n g has been d e f i n e d  as t h e  a p p e a r a n c e o f dense p h a s e a t t h e base o f t h e t r a n s p o r t line.  This  general  is a definition  concept  which  o f c h o k i n g as a p p l i e d  s y s t e m s where, a t some s o l i d s incremental the  pressure  circulation  anything example  about that  i s consistent  rate  increases  the nature  i t should  t o pneumatic  circulation  drop r e q u i r e d  transport  rate, the  f o r a small  rapidly.  increment i n  I t does n o t  o f t h e dense phase  be s l u g g i n g  with the  imply  formed, f o r  or produce  large  pressure  fluctuations.  4.1.7  Fast Dilute  important  fluidisation  and c i r c u l a t i n g  phase t r a n s p o r t  extreme s t a t e s  riser  where t h e l e n g t h  decay  from choked  and choked  flow  for a gas-solids  required  to d i l u t e  beds - d e f i n i t i o n s represent  flow.  f o r the s o l i d s  i s much s h o r t e r  two  I n any density to  than  the r i s e r  -  196 -  DOWNFLOW WALL LAYER  UPFLOW CORE  Net external solids flux Solids layer with zero thickness J  Exit plane, upf low only due to zero reflection  tttt  Developing solids layer  Fully developed solids layer has maximum stable thickness • choked zone  Solids  WW  I  f e e d a t  ^-s^ r—' external flux  Turn around at base  Figure  4.11  A s c h e m a t i c d i a g r a m showing r i s e r operating at a solids g r e a t e r than c h o k i n g .  a high v e l o c i t y circulation rate  On t h e l e f t i s a s c h e m a t i c i n which a r r o w s i n d i c a t e approximate d i r e c t i o n s of s o l i d s flow and show t h e d e v e l o p m e n t o f t h e w a l l l a y e r up t o i t s maximum s t a b l e ( c h o k e d ) t h i c k n e s s . On t h e r i g h t a s e c o n d d i a g r a m shows how up, down, and c r o s s f l u x e s v a r y w i t h h e i g h t , and shows how t h e c r o s s - f l u x i s i n e q u i l i b r i u m i n t h e choked zone.  -  itself,  then  the r i s e r  susbstantially  will  i t will  latter.  F o r such a r i s e r ,  range of s o l i d s saturated  value  these  substantial value, to  limits  density,  transitional  (density  sufficiently  within  t h e zone.  An  This  flux  density  at the e x i t , phase  f l u i d i s a t i o n corresponds and can o n l y  solids  to t h i s  exist  where t h e r a t e o f decay  corollary  i s that  between  fluidisation  The l a t t e r  solids  a lower  with a  above  of d e n s i t y  motion  i s i l l u s t r a t e d i n F i g u r e 4.12.  fluidisation  bed.  column.  i n the r i s e r  low t o promote v i g o u r o u s  interesting  fast  the e n t i r e  from  where  t o an u p p e r  t o the choked  decay) r e g i o n  some minimum gas v e l o c i t y is  gradient  over a  a lower v a l u e ,  fills  t h e imposed  Fast  carrying  exists  i s exceeded,  corresponding  lower down.  from  i s a region  density  to assure  a higher value,  fluidisation  rates  capacity  there  a d i l u t e , or  and above i t can be t h e  dense phase  vertical  needed  fast  circulation  carrying  in either  Below t h e s a t u r a t i o n  be t h e f o r m e r ,  where a s t a b l e  Between  operate  choked mode.  capacity  the  197 -  there  implies  of t h i s  definition  i s an i m p o r t a n t  distinction  and t h e c i r c u l a t i n g a configuration  and o p e r a t i n g  providing  substantial  external  circulation  rates,  with  i n a gradual  manner w i t h  height  as t h e c i r c u l a t i o n  rate is  i s varied.  little  The l a s t  o r no choked  feature  solids  fluidised  conditions  varying  f o r vigorous  of f a s t  will  phase p r e s e n t ,  refluxing local  o n l y be seen i.e.,  and densities  i f there  i f most o r a l l  -  198  -  REGION  OF  FAST  BOUNDARIES  FLUIDISATION  SOMEWHAT  ARBITRARY  I  I MINIMUM GIVE  U  1  TO  6  MAXIMUM  HIGH  U  AT  Q  REASONABLE  CIRCULATION  CAN  OCCUR  WHICH  DENSITY AT  PRACTICAL  G_ IMINIMUM  G  S STRONG  FOR  1  , REFLUXING  LOW DENSE  MEDIUM  Ug  PHASE  BUBBLING  IS  OR  DENSE  Ug  HIGH  PHASE  IS  TURBULENT  DENSE VERY  Ug  V.HIGH  PHASE  IS  TURBULENT  DENSE DOES  Ug  PHASE NOT  FORM  SLUGGING DECAY  ZONE  VERY  IS  DECAY  SHORT  ZONE  OF  INTERMEDIATE LENGTH  DECAY  ZONE  IS  OF  DECAY  SUBSTANTIAL  AN  LENGTH  ZONE  WITH  ZONE  IS  ACCELERATION  VIGOROUS REFLUXING  G  s  LOW IS  S  A  T  IS  AND  VERY  REACTOR  SUBSTANTIALLY  CHOKED  6  S SAT • INTERMEDIATE/ 1  A OF IS  8  LARGE THE  PART REACTOR  STILL  CHOKED  Figure  4.12  F a s t f l u i d i s a t i o n d e f i n e d i n terms o f a of p o t e n t i a l gas v e l o c i t i e s and s o l i d s circulation rates.  region  -  of  the  the  although  column o p e r a t e s  fast  circulation a  circulating  bed  I f the  height  height  of  decay  circulation create  a  in  a  criteria The  would be  it  impacts  is  desirable  desirable  and  pressure  the  1979)  may  and  regime  be  increase  of  with  the  top  i s not the  of  choked  capacity  fast  However, the  circulating  "transport  the  and  by  because  bed:  it  with  i t may  often  be  phase b e c a u s e o f important  velocity"  the  external  the to  (Yerushalmi  b e c a u s e of  bounds of  measured  above  4.13.  academic  hold-up  will  fluidised  meeting  It i s also  unit-specific  itself,  just  the  in  portion.  bed  change o f  results.  therefore  a  of  the  i s shown i n F i g u r e  f o r c o n t r o l purposes,  d r o p which  operation  geometry o f  small  reactor  two  a build-up  on  s u b s t a n t i a l l y exceeds  then a  This  t h a t measurement o f  Cankurt,  effective  circulating  formed.  to a v o i d  Hence,  saturation carrying  a fully  between  rate  fluidisation  4.1.8  length,  unit  t o have a g r a d u a l  considerations,  are  the  upon c o n t r o l l a b i l i t y of  circulation  realise  of  some f r a c t i o n  reactor  distinction  high  velocities,  s u b s t a n t i a l l y choked  short  regime.  i s a regime d e p e n d e n t  r a t e above the  regime occupying  fast  depends a l s o upon the  unit.  the  gas  -  i n the  fluidisation  r a t e s and  199  height  fast methods,  affected.  Scale The  influences  notion  that  operate a c i r c u l a t i n g  height bed  can  i n f l u e n c e our  i s very  important  ability  when  to  considering  - 200 -  Tall Riser  Exit plane Region of fast fluidisation  Short Riser  si  Exit plane  Choked region Region of fast fluidisation  Density  Density  gure  4.13  Existence different  of d i f f e r e n t flow height r i s e r s .  regimes i n  In t h e l e f t hand r i s e r , which i s t a l l , a t c i r c u l a t i o n r a t e s G i and G 2 t h e r i s e r i s substantially choked and s m a l l changes i n t h e c i r c u l a t i o n r a t e do not cause a l a r g e f r a c t i o n a l change i n t h e i n v e n t o r y . In t h e r i g h t hand r i s e r o f t h e same d i a m e t e r , which s h o r t , t h e same c i r c u l a t i o n r a t e s p r o d u c e dramatic f r a c t i o n a l inventory changes. This s i t u a t i o n , where changes i n c i r c u l a t i o n r a t e p r o d u c e l a r g e and c o n t r o l l a b l e changes i n o v e r a l l h o l d up, i . e . , where a f a s t f l u i d i s e d bed o c c u p i e s t h e whole column, i s . u t i l i s e d i n c i r c u l a t i n g f l u i d i s e d beds s  S  -  results  from  l a b o r a t o r y s c a l e u n i t s w h i c h may  h e i g h t - t o diameter operations diameter  there  32  m/s  Felwor,  this unit  i s greater  compared w i t h  (Profile influence  that  the  than  the  at a s i m i l a r 3.14).  32  of  scale  increased decay  lengths.  such u n i t ,  which o p e r a t e s  effect  of  decay  m height  the  gas  order  at a  the  of  length the  o f 4.5  velocity  Although  of temperature here,  (1982) show l i t t l e  f r o m one  E v i d e n t l y the  lengths  3 of F i g u r e  large  a gas  a temperature of approximately  1986).  decay  operating  combustor  and  (Wein and  unit  evidence  in proportionally increased  8 m diameter  o f 6.4  have  However, f o r i n d u s t r i a l  i s the d e n s i t y p r o f i l e  m high,  velocity  ratios.  i s strong  results  F i g u r e 4.14  -  201  and  there  results  temperature  m  could  upon  for  riser,  as  i n our  solids  of  850°C  own  flux  be  some  Stromberg density  profiles. It larger  i s p o s s i b l e to r a t i o n a l i z e  d i a m e t e r u n i t s b a s e d upon t h e  developed the  core  larger  earlier.  S o l i d s must  r e g i o n to the  diameter  profile,  the  intensities increased, decay  longer  initial remain  then  length  one  travel  I f the  solids  constant  longer  for  model  distances  the  velocity  distribution,  and  turbulence  the  from  a downflow zone i n of  as  slope  column d i a m e t e r  would a n t i c i p a t e a  upon d i a m e t e r .  lengths  s o l i d s movement  annulus to reach  columns.  decay  linear  Unfortunately,  is  dependence  there  are  few  of  -  Gas  202  -  Velocity  (m/s)  0  2  4  6  8  10  I  1  1  1  1  1  500 Density  Figure  4.14  (kg/m ) 3  Density p r o f i l e for a large circulating f l u i d i s e d b e d c o m b u s t o r (32 m h i g h x 8 m d i a . ) i n f e r r e d f r o m d a t a p r o v i d e d by W e i n a n d Felwor (1986). D i s c o n t i n u i t i e s i n gas v e l o c i t y are p o i n t s of a i r a d d i t i o n ; the g r a d i e n t r e f l e c t s a furnace expansion.  - 203 -  d a t a showing the e f f e c t s Simple  visual  profiles  suggests  somewhat  less  finds who  some s u p p o r t  suggests  turbulent at  that  regime  low d i a m e t e r  Taking  that  this  decay  linearly  diffusivity  intensity i s also  over  diameter  i n a large vessel,  one.  4.1.9  infinite  number  of f a s t  rate,  and t h a t  which  t h e "imposed  i s less  argues  i n the with  but l e s s  vary  and  recognising  solids  motion,  linearly  than  linearly  that  circulating  important  bed p r e s s u r e p r o f i l e s c a n and  i s an i m p o r t a n t  practically  i s matched  an  circulation  depends upon t h e need t o  a g i v e n p r e s s u r e drop system  in a  phenomenon  o f gas v e l o c i t y  i s found  with  i n F i g u r e 4.15.  fluidised  drop"  diameter  constant.  argument o f W e i n s t e i n e t a_l. (1983) t h a t  a t any c o m b i n a t i o n  It  length w i l l  p r e s s u r e drop  exist  satisfy  linearly  (1980)  a s a measure o f a mean  This i s i l l u s t r a t e d  The imposed The  diffusivities  responsible for radial  t h e decay  which  w i t h t h e work o f A v i d a n  a cross-section,  that  average  a result  become a p p r o x i m a t e l y  suggests  small  with diameter,  axial  and t h e n  density  l e n g t h s i n c r e a s e on  by a n a l o g y solids  scale.  of l o n g i t u d i n a l  vary approximately  the a x i a l  turbulence  comparison that  than  of reactor  academic  as shown below. over  the return  issue.  Weinstein l e g of a  by an a p p r o p r i a t e d i s t r i b u t i o n  -  F i g u r e 4.15  204  -  The proposed form of a decay l e n g t h versus diameter f u n c t i o n f o r f a s t f l u i d i s a t i o n based upon examination of data from s m a l l and l a r g e units.  -  of dense phase, d i l u t e column w i t h of  system u s i n g immediate the  an L - v a l v e  r i s e r pressure  while  f o r design  state  that  side.  to apply  difficulty.  drop a d j u s t s  of L-valves,  3.1.6), w i t h  function  o f gas v e l o c i t y  first  called  produce  This total  and s o l i d s  support  that loop,  (1978)  the l a t t e r ,  drop a unique  circulation  f o r the apparent  a fully  rate.  anomaly  circulation  lies in  of d e n s i t y  hold-up at the e x i t  plane.  dense p h a s e  i n the f r e e b o a r d ,  of the f a c t  i s required  explanation.  i n the  approximately  rate c l o s e to the saturated  i s a reflection  rate  i n what we have  developed  because o f t h e  s u b s t a n t i a l changes  hold-up  i s an  t o t h e needs o f t h e r i s e r  the r i s e r pressure  with  then,  decay  circulation  can  of a  t o match t h e r e c y c l e  Two e l e m e n t s a r e key t o t h i s  flow,  lower r e g i o n s ,  in  to design  i s t h a t when a column o p e r a t e s  exponential  rate.  leg, there  t o make s u f f i c i e n t l y a c c u r a t e  choke  functions  c o n s t r u c t i o n s o f t h e u n i t s and g e n e r a l  determinations. The  phases  e t a_l. (1983) s t a t e  results  A possible explanation  inability  model  i n the r i s e r  K n o w l t o n and H i r s a n  adjusts  Our own e x p e r i m e n t a l  different  this  i n the return  (Section  the  circulation  Weinstein  the L-valve  length  i n dense and d i l u t e  and s o l i d s  When a t t e m p t i n g  -  p h a s e and decay  the hold-up  t h e gas v e l o c i t y  205  small  carrying  i n the d e n s i t y  changes capacity  profile.  t h a t a l a r g e change i n  t o produce a small The s e c o n d  change  element  i n the  i s related to  -  the  recycle  around  a  bed l o o p .  valve or s l i d e  situation  when a c o n s t a n t  the  equipment  i s loaded  and  the unit  recycle  and r i s e r  over  control  length,  circulation  these  control  balance  and where t h e p r e s s u r e  themselves  compared  will  constant  i n F i g u r e 4.17.  drop  drop with  i n each  I t would  take  circulation  r a t e measurement  increased  circulation  r a t e of the higher  cause  such  Although presented  here  there  of Weinstein  is little  and t h e t h e o r y  element.  over  the s o l i d s  the f r e s h  leg. an  solids  to provide This i s  extremely  to d i s t i n g u i s h the  an i n c r e a s e has o c c u r r e d .  f o r the f i n d i n g s  t o t h e decay  and r e c y c l e  sensitive  although  then  be e s t a b l i s h e d a t a l m o s t  between r i s e r  increase i n pressure  illustrated  to the s o l i d s  by Rhodes and G e l d a r t  the p r e s s u r e  v a l v e remains almost  equal  over the  r a t e because of the f i r s t  circumstances  distributing  drop  i n v e n t o r y i s now i n c r e a s e d ,  a new p r e s s u r e  consider  e q u i l i b r a t e at  valve i s related  that the r e a c t o r i s t a l l  identical  Now  with  i s maintained,  Eventually i t will  T h i s s c e n a r i o has been m o d e l l e d  provided  fluidised  inventory of s o l i d s ,  sides are balanced  (1986b). I f t h e t o t a l  an  valve setting  r a t e where t h e p r e s s u r e  the s o l i d s  c i r c u l a t i n g bed  i s fully  with a given  i s started.  some c i r c u l a t i o n  Under  A typical  balance  valve f o r control.  the  flux.  f o r a pressure  l o o p , shown i n F i g u r e 4.16,  butterfly  drop  -  s y s t e m and t h e need  the c i r c u l a t i n g  recycle  206  inventory This  system,  i s a possible  e t a l . (1983).  difference  between t h e t h e o r y  of Weinstein  e t a l . f o r many  -  Riser i  207  -  Mechanical Valve  Return Leg  Sufficient air for fluidisation  <hpair<  High Velocity Air Figure  4.16  A c o n v e n t i o n a l l a b o r a t o r y c i r c u l a t i n g bed r e c y c l e loop with a f u l l y f l u i d i s e d r e t u r n c o n t r o l l e d by a m e c h a n i c a l ( e . g . , s l i d e ) v a l v e .  -  208  -  Small change in G s  Short decay length cf. column height  Density  Pressure Balance:  APcolumn AR valve' AP.return! t  Figure  4.17  return2  = 0  A p o s s i b l e e x p l a n a t i o n f o r the apparent i n f l u e n c e o f imposed p r e s s u r e drop upon operation.  riser  When t h e decay l e n g t h i s s h o r t compared t o t h e column h e i g h t , s m a l l changes i n t h e e x i t d e n s i t y and e x t e r n a l c i r c u l a t i o n r a t e r e s u l t from l a r g e changes i n the column i n v e n t o r y (pressure drop). Hence, s i n c e t h e p r e s s u r e drop a c r o s s the v a l v e i s a f u n c t i o n o n l y of the s o l i d s c i r c u l a t i o n r a t e , a c c o r d i n g to the p r e s s u r e b a l a n c e , column p r e s s u r e drop a p p e a r s t o depend upon r e t u r n l e g p r e s s u r e d r o p .  -  practical  purposes,  distinction. solids This and  there  209  -  i s an i m p o r t a n t  Here we p o s t u l a t e  f l u x there  i s only  that  a t any gas v e l o c i t y and  a s i n g l e unique s t a b l e  i s d i l u t e phase below t h e s a t u r a t e d dense phase above.  capacity propose  i n fast  as  t h e imposed  fluidisation  there  distributions theory  Such a t h e o r y  pressure in a riser  does n o t .  and  fluidisation model  state  theory  returning  typical  profile and  such  phase state  s i m i l a r to that of postulate  v e l o c i t y freeboard  to s i t u a t i o n s without f l u o s e a l type  material  that  phenomenon  In t h i s  over  reactor  is still  case  and r e t u r n  legs.  compared  control A  be a  r a t e must be  balance i s  However,  the d e n s i t y  of the c i r c u l a t i o n  cases,  over  bubbling  d i p l e g would  a pressure  function  In many s u c h  which a r e t a l l  close  the c i r c u l a t i o n  so t h a t  a unique  the unique  returns.  through a cyclone  by i t e r a t i o n  gas v e l o c i t y .  reactors  at the  some f a c t o r  can a r i s e when a p p l y i n g  or with  example.  established achieved  i s very  1986b) who a l s o  i s a high  et a l .  i t as such.  recirculation, bed  requires  rates  whereas, a s i n g l e s t e a d y  (1985,  Some c o n f u s i o n stable  carrying  a r e two s t a b l e  to determine  Our own t h e o r y  Rhodes and G e l d a r t fast  drop  capacity  However, W e i n s t e i n  p h a s e s f o r many d i f f e r e n t s o l i d s c i r c u l a t i o n same gas v e l o c i t y .  state.  carrying  Only a t the s a t u r a t e d  can two p h a s e s c o e x i s t . that  academic  rate  particularly in  t o t h e decay  length,  then  -  the  stable  carrying that  circulation  capacity,  two s t a b l e  imposed This  states  pressure  reactor  d r o p ) would  whether  other  internal  a  dilute  undergo  further  stability  discussion  be e q u a l  to the  where we  saturated  hypothesize  and where i n v e n t o r y  influence  the r i s e r  (or  profile.  4.18.  i s extremely  difficult  o r n o t two s t a b l e  than a t the choking  creating stable  would  can e x i s t  i s shown i n F i g u r e  conclusively  -  t h e one s i t u a t i o n  Unfortunately,it  of  rate  210  recirculation  phase d e n s i t y ,  can e x i s t  Exit  create  exit  in this  so f a r has c o n s i d e r e d  the appearance of  column.  instance  only  in a  phenomena  when t h e same d e n s i t y  decay i n a smooth  become c r u c i a l  states  flux.  may  t o show  would  Definitions  so t h e  the n o n - r e f l e c t i v e  exit.  4.2  Macrostructural  4.2.1  Exit The  strong  fluidised This  exit the  effect  compares  and s o l i d s  configurations. exit  density  in circulating  bed d e n s i t y  figure  velocity  effects  R e s u l t s and t h e i r  geometry profile,  of e x i t profiles  two p r o f i l e s circulation  fluidised  geometry was  Implications  upon  beds circulating  shown i n F i g u r e obtained  rate  at i d e n t i c a l  but with  two  E x a m i n i n g t h e two p r o f i l e s  i s important not j u s t  3.15. gas  different shows  that  b e c a u s e i t can i n f l u e n c e t h e  i n t h e immediate v i c i n i t y  of t h e  - 211 -  •  stable dilute phase  stable dilute phase  •  TDH V  »•  *  TDH  j  stable dense phase  o °o stable dense phase  o © ° Density  Unit with medium inventory  Density Unit with high inventory  Two stable states form when: column height > T D H + dense phase height and circulation rate is not controlled Figure  4.18  A b u b b l i n g f l u i d i s e d bed i l l u s t r a t i n g t h e phenomenon o f c o e x i s t e n c e o f s t a b l e s t a t e s a t choking. The b u b b l i n g bed on t h e l e f t , c h a r g e d w i t h a wide range o f i n v e n t o r i e s (medium and h i g h a r e shown h e r e ) , w i l l show c o e x i s t e n c e o f dense and d i l u t e p h a s e s , and c i r c u l a t i o n a t t h e c h o k i n g f l u x , provided the s p e c i f i e d c o n d i t i o n s are met.  -  exit,  -  212  but throughout the column.  In the case  this  c o n s t i t u t e s a d i s t a n c e of 9.4 m, or 60 e q u i v a l e n t diameters.  E i t h e r measure i s s i g n i f i c a n t  on an i n d u s t r i a l  s c a l e i f the phenomenon i s found to a l s o occur  i n large  equipment. V i s u a l observations effect.  i n d i c a t e the nature  They i n d i c a t e an i n e r t i a l  gas at the top of the abrupt  s e p a r a t i o n of s o l i d s  f o r c e d to turn i n a short  The s e p a r a t i o n phenomenon i s shown i n F i g u r e 4.19;  l i k e c y c l o n i c separation i t i s highly v e l o c i t y Separated  solids  cascade downwards along  the t o t a l d e n s i t y of the suspension; but  from  e x i t column as the gas i s  a c c e l e r a t e d and simultaneously radius.  of the e x i t  some continue  dependent.  the w a l l , adding t o  some are r e e n t r a i n e d ,  downward to i n f l u e n c e the p r o f i l e at a  d i s t a n c e tens of diameters below the e x i t . The  second and t h i r d e x i t s  which were t e s t e d , which can  be denoted as "smooth" and "extended" r e s p e c t i v e l y , give p r o f i l e s which support an i n e r t i a l  separator.  the idea of the abrupt  The second e x i t was designed to  minimise s e p a r a t i o n of s o l i d s gradual area  curvature  reduction.  causing solids  from the gas.  than the f i r s t  There was no v i s u a l  d e n s i t y with height Finally,  I t has a more  combined with a gradual evidence  s e p a r a t i o n , and the continuous  the case.  e x i t a c t i n g as  of t h i s  exit  decay of suspended  a l s o suggests that t h i s may be  slip velocities  c a l c u l a t e d from Figure  -  213  -  Crossf lowing solids layers on roof  I Gas streamlines Solids streamlines  Figure  4.19  D i a g r a m s h o w i n g how s o l i d s a r e s e p a r a t e d i n e r t i a l l y by a n a b r u p t e x i t p r o m o t i n g internal circulation.  -  3.14 of  for the  this  exit  column.  approach  This  214  -  the  i s also  terminal  indicative  velocity of  at  small  the  top  amounts  of  reflection. The  "extended"  constructed  believing  were s e p a r a t e d an  extension  shown  that  the  gas  and  in Figure  i f particles  inertially,  section,  encountering might  exit,  were g i v e n then  exit,  slip  practice,  the  p r o f i l e s were n e a r l y  small  but  the  geometry  separation region,  reason  and  downflowing  that  wall  solids  much of  the  with  for  exit  much of  solids  No. for  ending  result  similar  1.  In  the In  two  this  inertial  downflow f l u x The  before  higher  clear.  the  in  velocities  to  identical  which  decelerate  downward  lead  absolutely  zone.  layer  up  in a  i s due  for  both  structure,  wall  to exits  and  is  hence  characteristics.  might are  than  separated  reentrainment  separated  be  less  observed likely  to  in a large travel  unit  a l l the  where way  to  wall. The  two  in  annular  A difference  the  i s not  i t seems t h a t  results  downflow i n an  similar  velocities  to  downward  T h i s might  and  was  clusters,  time  higher  densities  cases,  or  accelerate  their  reduce reentrainment.  3.13,  importance  of  the  exit  effect  can  be  discussed  levels: (i)  Impact  upon d e n s i t y  modelling  profile correlations  and  on  a  (ii) The  Impact  upon  importance  design.  of the e x i t  e f f e c t upon  correlations  i s that  must  some c h a r a c t e r i s t i c o f t h e e x i t  can  reflect influence  introduces  the nature  even t h e b a s i c  o f any p r e d i c t i v e  an a d d i t i o n a l d e g r e e o f d i f f i c u l t y  mathematical makes decay  Stromberg  increases  densities  the  even appear  that  A  t o be a f f e c t e d .  the simple  fluid  This  enters  mechanics.  i n t o any  It also (1980)  i n c a s e s where t h e d e n s i t y and s u b j e c t t o limiting  Most s i g n i f i c a n t l y , i t  mechanical  i s based  which  i n t o the  i n c a s e s where i t does n o t s i n c e  model upon  which  i s not s u i t a b l e f o r a l l  configurations.  s i m p l e method which  effects,  and which  consider  reflection 0  unsuitable  L i and Kwauk model  geometric  to  (1982),  geometry,  s u c h as t h o s e o f L i and Kwauk  at the top of the r e a c t o r ,  correction  implies  condition  d e s c r i p t i o n of the f l u i d equations,  equations  shape o f t h e p r o f i l e .  p r o b l e m b e c a u s e a complex boundary  and  density  could  i s valuable  each e x i t  geometry  c o e f f i c e n t , a.  and would be d e f i n e d  This  be used  to describe  for illustrative as b e i n g could  as f o l l o w s :  purposes i s  characterised  ideally  exit  vary  by a  from  °° t o  - 216 -  p  = Suspension  e  density at e x i t  Ug  = Superficial  V-t  = Single particle  G  = Solids  s  gas  level  velocity terminal  circulation  velocity  rate  A high value of a i m p l i e s a high potentially possible of  to separate  internal  internal  due t o c l u s t e r i n g  mixing,  exit  velocity  and/or r e f l e c t i o n .  t h e two, and b o t h making  slip  imply  I t i s not  a high  degree  a a measure o f t h e r a t i o o f  to external c i r c u l a t i o n  rates.  At the other  e x t r e m e , an a v a l u e o f z e r o r e p r e s e n t s t h e c o n d i t i o n o f no exit  reflection  stream  leaving  exits,  the s l i p  with  t h e column. velocity  terminal  velocity.  The  potential  coefficient, any  exit  of  or  i s apparent  fluidised  utilise  the heat  surface  value  may be e i t h e r  case,  promoted  by smooth  i s approximately  equal  of the parameter  a, t h e r e f l e c t i o n  to the  when c o n s i d e r i n g t h e s u i t a b i l i t y o f situation.  bed c o m b u s t i o n  different  transfer  pneumatic t r a n s p o r t  In t h i s  f o r a given design  circulating designs  an u n d i s t u r b e d  locations  surface. inside  F o r example, i n  systems  different  for different  fractions  F i g u r e 4.20 shows how the c i r c u l a t i n g  this  bed c o m b u s t o r ,  o u t s i d e i n t h e r e t u r n l o o p , d e p e n d i n g upon t h e c o n t r o l  and  turndown p h i l o s o p h y .  fraction from  o f heat  The f i g u r e  also  shows how t h e  exchange s u r f a c e i n t h e combustor  u n i t y i n some d e s i g n s  to zero  i n others, with  can vary others  -  217  -  Convection Pass Heat removal fraction - F3  Optional Fluid Bed Heat Exchanger Heat removal fraction - F2  Design Type  F3  Reference  0.5  0.0  0.5  Engstrom e t a l . ( 1 9 8 5 )  Battelle-Rlley  0.0  0.5  0.5  Jones e t a l . ( 1 9 8 2 )  0.5  Reh e t a l . (1980)  0.5  Stromberg  Studsvik/B&W  4.20  F2  Ahlstrom/Pyropower  Lurgi/CE  Figure  F1  0.0-0.5 0.0-0.5 0.5  0.0  Heat t r a n s f e r s u r f a c e l o c a t i o n s commercial combustor designs.  (1982)  for different  -  still  operating  selection the  of  at  exit  relative  an  amounts o f  for effective  external  heat  and  external  heat  considerations. r a t e must be of  external  In  exit  should  mixing  the  The  external  selected  of  recycle  these  transfer.  On  circulation  the  the  hand,  other  substantial internal  establish  temperature  uniformity  coefficient  required it is  mixing  the  units  different  to permit  to maintain  reflection  However,  external  within  to  external  desirable  Hence i n t e r m e d i a t e  maximum  require  c a s e s the  no  tending  loops.  transfer surface  by  circulation  In a u n i t w i t h  while minimising  cost  optimum  is dictated  geometry w i t h be  s u f f i c i e n t l y high heat  and  operation.  coefficient  circulation  point.  i n each c a s e  internal  exchange an  maximise i n t e r n a l  with  intermediate  configuration  required  reflection  -  218  amount still  to  combustor.  would a p p e a r  to  be  optimum. Similar exit  lines  configurations  processes.  For  of  thinking  f o r other  example  where p l u g  flow  reflection  exit  be  later,  this  In  adversely  summary, v a r y i n g  fluidised relative  may  bed  exit  amounts of  different  circulating  in catalytic  pyrolysis,  might  suggest  of  solids  bed  cracking  chemical or  the  presents internal  gas  opportunity  and  as  zero shown  dispersion.  geometry of a an  flash  is desirable, a  most s u i t a b l e a l t h o u g h , affect  optimum  external  circulating to  vary  the  circulation  in  the  -  unit.  This  pressure  i n turn  drop  influences  f o r a given  g a s - s o l i d s mass and h e a t appears units,  likely,  at least  different velocity  primary  and r e t u r n  can be e x p l a i n e d  of secondary  to secondary  anticipate,  zone d e n s i t y ,  solids  with  directly  density  air splits  high  above  decay  primary  profiles for  at constant  in density effect  total  from t h e  becomes  discontinuity  p r o f i l e at the secondary lower  opposed  The p r o f i l e s a r e  T h e r e a p p e a r s t o be a s l i g h t  would  and  of s o l i d s  a i r with  t o where t h e e x i t  of the d e n s i t y  flow at  i n terms o f t h e r a d i a l l y  3.19.  by a c o n t i n u o u s  distributor  plug  o f t h e c i r c u l a t i n g bed.  as shown i n F i g u r e  evident.  flow  phenomena a s s o c i a t e d  t o graphs of l o n g i t u d i n a l  characterised primary  effect,  structure  scale  flow a t the o t h e r .  a i r introduction,  leads  primary  t o the s o l i d s  I f , as  f o r a given  p r o d u c i n g RTD's a p p r o x i m a t i n g  the e x i t  Introduction  one  respect  location  distributor,  slope  i n large  return  non-uniform  and t h e  characteristics.  of secondary a i r i n t r o d u c t i o n  secondary  ports  transfer  rate,  Effects  Like  the  with  extreme and mixed  4.2.2  circulation  mixing, the  geometry c a n be " c u s t o m i z e d "  characteristics, one  the s o l i d s  t h e same phenomena o c c u r  the e x i t  service,  219 -  velocity  i n the  a i r ports. promotes  As  higher  but a t t h e t o p of t h e r e a c t o r t h e  -  profiles  are i d e n t i c a l .  considering secondary annular  air  profiles  cases  there  profile  secondary  was  i t was  and a h i g h  different,  core  Forcing  manner  what  effectively  and i n t o  Thus, w h i l e  and a n n u l a r  the annular  solids  by v i r t u e  solids  density  decreased  of f a s t  decay  t r a n s f e r to the w a l l a r e a .  layer  the downflowing ports,  t o be exchanged a i r prevented  t o t u r n around i n  r e g i o n a t t h e base o f zone.  tending  the s w i r l a i r  to i n t e n s i f y  fluidisation.  lengths  of the  of the r e a c t o r  the s w i r l  becomes a s e p a r a t e d  phenomena  of the  that the  not p o s s i b l e t o d e t e r m i n e whether  core-annulus  In  f o r the s w i r l  the c e n t r e  a substantial cyclonic effect,  solids  apparent  c a s c a d e between t h e opposed  promotes a h i g h  was  3.20.  the downflowing  t h e upper and l o w e r z o n e s ,  anticipate  Figure  zone where t h e s e c o n d a r y  i t was  forcing  reentrained.  this  had  and d o w n f l o w i n g  (tangential entry)  by t h e t e n d e n c y  itself,  both  latter.  It  swirl  density  Visually  caused  annulus could  permitting between  by  j o i n e d at the  i s a d i s c o n t i n u i t y i n the gradient  velocities,  upward a g a i n s t  solids  core  a i r t o p i c k up a n n u l u s p a r t i c l e s  angular  where  with  a i r are remarkably  difference  the  obtained  i s introduced.  high  can be e x p l a i n e d  on t o p o f one a n o t h e r  a i r p o r t s by an u p f l o w i n g  secondary  density  The p r o f i l e s  exchange o f s o l i d s .  The  these  two r i s e r s  220 -  One  might  due t o i n c r e a s e d However,  the  rates of  when s w i r l a i r  -  is  introduced  i t i s not p o s s i b l e  in  a m e a n i n g f u l way w i t h  If  the s w i r l  promoting  t o compare decay  the c o n v e n t i o n a l  a i r was used  be made s i n c e  location  -  i n t h e s e c o n d a r y mode,  reentrainment,  could  221  as p r i m a r y  solids  will  by t h e d i s t r i b u t o r  air distribution.  a i ra valid  be r e e n t r a i n e d  irrespective  lengths  comparison  at t h i s  of the type of a i r  introduction. Introduction distributor  of s o l i d s  plate  introduction.  some d i s t a n c e  i s i n some ways l i k e  Both a f f e c t  are  solids  formed.  high  This  can be seen  situation  Solids fall  the wall,  An i n t e r n a l  established a  transfer.  Above is  just  characteristic  solids  i n Figures  in a distillation  downflow  from  pattern  decay  of d e n s i t y  picture with  return  height port  than  i s a net v e r t i c a l  flux  column. section,  a t t h e base o f t h e  appears  t o be  circulating  bed, w i t h  due t o r a d i a l  t h e upward and downward  i s shown i n F i g u r e  a second  t h e lower  The  analogous to  t h e upper  with height  o f how  zones  3.9 t o 3.12.  of s o l i d s ,  as i n a c o n v e n t i o n a l  vary  the s o l i d s  flux  circulation  A likely  fluxes  two s e p a r a t e  and change d i r e c t i o n  more c o n v e n t i o n a l  there  flux  a r e f e d by a n n u l a r  down  unit.  of t o t a l  solids  By r e t u r n i n g t h e  up i n t h e column,  lower bed has no net v e r t i c a l the  swirl a i r  t h e normal exchange o f  between u p p e r and lower bed z o n e s . recycled  above t h e  4.21.  zone i s formed  which  zone i n t h e s e n s e  of s o l i d s .  This  flux,  that  and t h e  - 222 -  DOWNFLOW LAYER  UPFLOW CORE |  Gas in  F i g u r e 4.21  V a r i a t i o n of s o l i d s f l u x e s i n a c i r c u l a t i n g f l u i d i s e d bed with s o l i d s r e t u r n some d i s t a n c e above the gas d i s t r i b u t o r . The d e n s i t y p r o f i l e on the l e f t hand s i d e , t y p i c a l of the f a s t bed with elevated s o l i d s r e t u r n , i s thought to be caused by up, down, and cross f l u x e s as shown on the r i g h t - h a n d side. There i s a lower zone of zero s o l i d s f l u x , an upper zone of net upward f l u x , and a complex c r o s s f l o w pattern, p a r t i c u l a r l y at the return l o c a t i o n where downflowing s o l i d s are d i s p l a c e d from the wall i n t o the core.  - 223 -  fact  that  forcing at  returning  solids disturb  s o l i d s t o the core,  t h e base o f t h i s  upper  the downflowing  contribute  zone.  t o the high  annulus, densities  -  -  224  5 . THE TRANSITION TO TURBULENT FLUIDISATION - A BRIEF STUDY  5.1  Introduction In  fluid  addition  to providing  mechanics of f a s t  circulating  bed u n i t  fluidisation turbulent  fluidised  also  velocities  regimes.  an o p p o r t u n i t y t o s t u d y t h e the design of the  a l l o w s o p e r a t i o n a t low  i n t h e b u b b l i n g and  Examination  of the l i t e r a t u r e  differing  regime  i n particular,  regime  and t o c h o k i n g .  Avidan  (1980) and Yong e t a_l. (1986) s u g g e s t  particle contend this  that,  Our first  such as  that the  suspension of  (1986a)  f l u i d i s a t i o n may o c c u r i n power c a t a l y s t s ,  by many a u t h o r s of s o l i d s  to a  the  i s not a refluxing  zone. own work i n t h i s  169 pm d i a m e t e r  investigation,  the  turbulent  but a t r a n s f e r  was t o i d e n t i f y  dramatic  to a stable  observed  t o the s l u g g i n g  while authors  beds o f f i n e  fluidisation  transition  freeboard  Notably,  Rhodes and G e l d a r t  manner i n l a r g e  regime  and i t s r e l a t i o n s h i p  although  reveals  the nature of the t u r b u l e n t  corresponds  clusters,  turbulent  the  regime  about  so-called  widely  turbulent  views  beds,  what  sand  low v e l o c i t y  used  and t o i d e n t i f y  transition  circulating  a r e a had two o b j e c t i v e s .  exist f o r  d u r i n g most o f t h i s whether o r n o t t h e r e i s a  between t h e s e  bed s t a t e .  regimes  The  low v e l o c i t y  The second  r e g i m e s and  was t o c r i t i c a l l y  -  examine t h e regime t r a n s i t i o n own  -  225  literature  i n the l i g h t  o f our  results.  5.2  A B r i e f H i s t o r y o f Turbulent The  first  o b s e r v a t i o n of a t r a n s i t i o n  or  slugging fluidised  is  generally attributed  are  Fluidisation from a  bed t o a more homogeneous t o Lanneau  (1960).  early the  line,  at v e l o c i t i e s  as 1949 ( Z e n z ,  1949).  suggest  in a  s l u g g i n g , as  T h i s paper mentions voidages i n  becomes " e x t r e m e l y  t h a t Zenz o b s e r v e d  there  fluidisation  somewhere beyond  r a n g e 0.9-1.0, and t h e f a c t  particles  suspension  However,  r e f e r e n c e s t o a smooth d i s p e r s e phase  transport  bubbling  t h a t t h e movement o f  turbulent", observations  at least  which  Geldart's turbulent  transition. Despite homogeneous defining  these  early  observations  state,  which  i n Lanneau's case  a heterogeneity  capacitance  f o r another  studying  s l u g g i n g beds,  regime breaks  coalescence fluid  down i n t o  - virtually  called  from b u b b l i n g  this  literature  Kehoe and D a v i d s o n that at high  "a s t a t e o f  (1971),  velocities  continuous  a c h a n n e l l i n g s t a t e with  d a r t i n g i n a z i g zag f a s h i o n through They  it  observed  detailed  no f u r t h e r  decade u n t i l  more  went as f a r as  and making  p r o b e measurements,  appeared  this  index  o f an a p p a r e n t l y  tongues o f  the bed."  the " t u r b u l e n t regime" to d i s t i n g u i s h  o r s l u g g i n g and t o i m p l y  a very  different  -  modus o p e r a n d i It the  i s important  turbulent  intended with  the  to  imply,  well  to reproduce  structure  a bubbling  voidage  frequency  what was  slugging  slugging  bed  with  observed  to  early description  stable  will  be  from  and  Whether or left  The of  same s e n s e distinct  i n the  sense  not  is  slugging  i n the  i s stable,  of  fluidisation  imply a r e o r d e r i n g  i s unstable  height.  state-  inhomogeneities.  state,  s t r u c t u r e which  increases  this  i . e . , a regime d i s t i n c t  t o a new  or  ordered  i n d i c a t e what t u r b u l e n t  scale higher  freeboard  fact  bed  t o the  t r a n s i t i o n seems i n t e n d e d  bed  that a  to  finer  regime  compared  -  226  this  unanswered a t  from  that is in  this  point. The  work by  observations Massimilla  of  Kehoe and turbulent  i t s nature,  slugging  r e g i m e by  structure. Massimilla  found  the  that  dramatically  that  offers better  v i r t u e of  Studying  two  (1971) was  fluidisation  (1973) e s t a b l i s h e d  whatever  flow,  Davidson  the  by  other  contacting  phase models,  underpredict  regime,  than  the  oxidation  two  of  the  transition,  coalescence, high  degree  but of  Carotenuto provide  a  suggesting  there  i s no  uniformity  was  phase  propylene,  bed  s l u g breakdown  slug  conversion.  C a p a c i t a n c e p r o b e s i g n a l s gave some i n d i c a t i o n o f of  the  suitable for  turbulent  by  workers.  turbulent  a breakdown of  catalytic  followed  the  nature  and  indication in this  study  that  obtained.  et a l . (1974) and  c l e a r e r i n s i g h t i n t o the  Crescitelli nature  of  et a l . (1975) the  turbulent  a  -  transition.  Both a u t h o r s  from beds of  fine  transition an  bubbling  and  from  signals,  to i d e n t i f y  d i v i d e d by  said  to r i s e  and  r a t h e r than of  average t o t a l  on  flow  fluidisation.  gas  13 78;  and  of  using probe regime. absolute  freeboard This  decrease.  pressure  ratio  was  peak  was  The  the  turbulent transition,  fully  turbulent f l u i d i s a t i o n  was  said  ratio  attain  this  and  Uk  and  the  ratio  taken  introduced fully  Avidan regime,  a constant  from T u r n e r ' s  to denote  the  turbulent state  (1980) s t u d i e d the  notably  bed  expansion,  to e x i s t  F i g u r e 5.1  value.  of  when  the  the  and  the  is a typical  work w i t h  onset  of  Avidan,  turbulent  velocity.  then  study  (peak t o peak  bed  found  (1977)  of  of  onset  phases  Potter  capacitance  fluctuations  with  o c c u r r i n g which  concept  the  the  identified  observations  the  signals  show t h a t  dilute  and  a t CUNY ( T u r n e r ,  against  the  probe  l e d t o a more f o c u s s e d  t o a peak and  to s i g n i f y  be  densities  Theil  the onset  pressure  d r o p ) were p l o t t e d found  can  dense and  (1978) i n t r o d u c e d  fluctuations,  Dimensionless  local  data  papers  pressure  pressure  d i a . ) and  s l u g g i n g to t u r b u l e n t  fluidisation  Turner  of  s l u g g i n g beds.  These i n i t i a l  1980).  ym  between t h o s e  experimental  transitions  turbulent  300  capacitance  to t u r b u l e n t f l u i d i s a t i o n  intermediate  presented  examined (<  increased probability  are in  solids  -  227  symbols  plot U  c  transition  respectively. p r o p e r t i e s of  the  and  transition  defined a  turbulent  -  228  -  SOLIDS *HFZ- 20  GAS  Figure  5.1  VELOCITY  U  (m/$)  D i m e n s i o n l e s s p r e s s u r e f l u c t u a t i o n s and overall bed d e n s i t y plotted against gas v e l o c i t y to i l l u s t r a t e the o n s e t of the turbulent transition, U , and the f u l l y turbulent s t a t e , Uk (Turner 1978). c  -  based  on  a dramatic  (1954) p l o t , turbulent in  gas,  and  approach  composed  Richardson-Zaki clusters  log U).  by  Capes  regime) are s i m i l a r  visualised  for clusters  (1974) u t i l i s i n g  expression.  diameters  Avidan  of a uniform d i s p e r s i o n  s i z e s were computed  proposed  -  change i n s l o p e o f a R i c h a r d s o n - Z a k i  ( l o g e vs.  bed  229  These s i z e s  of the o r d e r of 4 to s i z e s  (1978) u s i n g a d i f f e r e n t  mm  computed  approach.  clusters  based  on  (equivalent i n the  turbulent  by Y e r u s h a l m i mixing  was  u s i n g an a x i a l  suggestive  o f a much more homogeneous random s t r u c t u r e  finds  used  m i x i n g models s u c h  (1963) o r K u n i i  i n these  lower  Possibly  because  and  velocity of  p r e s s u r e measurements can authors  fluidisation.  These  pressure  phase, and  typically  w i t h which  with  include  subsequent  a decrease  the onset  the  in  of  Canada e t a l . ( 1 9 7 8 ) ,  (1986). G e l d a r t (1986a) have q u e s t i o n e d  using pressure f l u c t u a t i o n s  transition  ease  in identifying  turbulent  of  than  of D a v i d s o n  be made, a number of  of p r e s s u r e f l u c t u a t i o n  Rhodes and  is  regimes.  the r e l a t i v e  have f o l l o w e d T u r n e r  Yong e t a J .  as t h o s e  two  L e v e n s p i e l (1969) a r e  some form  and  in itself  i n the b u b b l i n g o r s l u g g i n g r e g i m e s ;  multi-parameter Harrison  which  et a l .  correlated  one  an  a modified  Solids  diffusivity,  of  a  criterion.  fluctuations  They  a l o n e as a  suggest  does not  that  in reality  the  validity  turbulent a decrease denote  a  in  -  fundamental stable  by  -  r e s t r u c t u r i n g o f t h e bed t o a new r e g i m e w i t h  cluster  pressure  230  suspension.  signal  which  they  Rather, observed  argue t h a t t h e  c a n be e x p l a i n e d  simply  c o n s i d e r i n g the i n c r e a s i n g t r a n s f e r of m a t e r i a l t o the  freeboard,  and hence t h e d e c r e a s i n g  increasing  superficial  (Geldart  and Rhodes,  1985), t h e y  data,  e x t e n s i v e l y t o support  explained  regime.  by r a d i a l  with  In a s e p a r a t e  a l s o argue a g a i n s t  of c l u s t e r s  used  bed l e v e l ,  gas v e l o c i t y .  suspension  be  they  a  on t h e g r o u n d s t h a t s l i p cluster  non-uniformities  These have been f o u n d  paper a stable  velocity  theory,  can a l s o  i n the t u r b u l e n t  experimentally  by Abed  (1983). To  summarise,  a t t h e t i m e when we made o u r t u r b u l e n t  transition  measurements,  literature  a v a i l a b l e and two a p p a r e n t l y  which  sought  seemed  to e x p l a i n the reported  d e s i r a b l e to design  resolve  clarify  5.3.  Experimental The  with  changes  initial  observations.  It  which a t best  would  system.  Design fluidisation  t h e equipment  experiments  were r u n w i t h  were  s e t up as shown i n F i g u r e 3.8.  i n c o n f i g u r a t i o n were n e c e s s a r y  tests  theories  t h e two t h e o r i e s , o r a t  what happens i n o u r own  low v e l o c i t y  conducted  conflicting  an e x p e r i m e n t  t h e d i f f e r e n c e s between  least  The  t h e r e was a body o f e x i s t i n g  the s o l i d s  b e c a u s e , when  returned  to the base  - 231 -  of  the u n i t ,  the L-valve  tended  to discharge  solids  i f the  column was r u n i n t h e s l u g g i n g mode. W i t h t h e column s e t up w i t h bed  level,  solids the  flow  leakage  velocity which  was no p r o b l e m and a s m a l l  c o u l d be m a i n t a i n e d  elutriation  rate.  through  was d e t e r m i n e d  by a t r i a l  was i n c r e a s e d u n t i l  measure o f t h e bed i n v e n t o r y under  distributor  pressure  drop  and c y c l o n e s  was s m a l l  t h e bed o f s o l i d s Table  method  when t h i s  fluctuations. et  (typically  pressure  on t u r b u l e n t  was  et  overall  which,  column p r e s s u r e  differential  identified,  (1978) and Canada  regime t r a n s i t i o n s depending  drop.  pressure  to absolute  upon t h e a u t h o r , by d i v i s i o n by  On t h e o t h e r  a l . (1986) and Rhodes and G e l d a r t  used  to the  i n v o l v e d measurement o f p r e s s u r e  may o r may n o t have been made d i m e n s i o n l e s s the  pressure  9 kPa.  a t t e n t i o n has been g i v e n  related  fluctuations  0.5 kPa and  to the d i f f e r e n t i a l  The t a b l e shows t h a t T u r n e r  a l . (1978) b o t h  was a  through the  by w h i c h t h e t u r b u l e n t t r a n s i t i o n  especially  absolute  c o n d i t i o n s where t h e  of approximately  Particular  a t each gas  a desired  5.1 t a b u l a t e s t h e work t o d a t e  fluidisation.  t o match  This pressure  f o r gas f l o w  0.2 kPa r e s p e c t i v e l y ) compared over  the L-valve  and e r r o r p r o c e s s i n  was a t t a i n e d i n t h e windbox.  fractional  controlled  The r e q u i r e d c i r c u l a t i o n  the c i r c u l a t i o n  pressure  a r e t u r n p o i n t above t h e  (1986a)  fluctuations  hand, Yong both  between a p o i n t i n  - 232 Table References for Turbulent Methods Used t o I d e n t i f y  Technique(s)  Reference Lanneau  (1960)  5.1 F l u i d i s a t i o n S t u d i e s and the T u r b u l e n t T r a n s i t i o n  Used  C a p a c i t a n c e probe s i g n a l s used t o c a l c u l a t e a h e t e r o g e n e i t y index equal to the mean l o c a l d e v i a t i o n o f bed d e n s i t y about the l o c a l average d e n s i t y . H e t e r o g e n e i t y maximised i n t h e s l u g g i n g regime. and X - r a y breakdown o f  Kehoe & D a v i d s o n (1971)  C a p a c i t a n c e probe s i g n a l s p h o t o g r a p h s used t o i n f e r the s l u g g i n g regime.  Massimilla (1973)  C a p a c i t a n c e p r o b e s i g n a l s and d e v i a t i o n o f bed e x p a n s i o n d a t a from s l u g f l o w t h e o r y used t o i n f e r breakdown o f t h e s l u g g i n g regime.  Carotenuto (1974)  et a l .  Probability density functions for c a p a c i t a n c e p r o b e s i g n a l s were p l o t t e d and found t o become u n i m o d a l a t transition.  Thiel & Potter (1977)  Visual  Turner  Measurement o f manometer p r e s s u r e f l u c t u a t i o n s between t h e base of t h e bed and the top of the f r e e b o a r d g i v e p l o t s o f peak t o peak f l u c t u a t i o n s d i v i d e d by t o t a l bed p r e s s u r e drop w h i c h showed maxima a t t h e " b e g i n n i n g of the t r a n s i t i o n to t u r b u l e n c e . " Effectively an a b s o l u t e p r e s s u r e f l u c t u a t i o n .  (1979)  observation  of breakdown of  slugs.  Canada e t a l . (1975)  As per T u r n e r ( 1 9 7 8 ) , but u s i n g p r e s s u r e t r a n s d u c e r r a t h e r than manometer.  a a  C r e s c i t e l l i jet a l (1978)  L o s s o f p e r i o d i c i t y of a u t o c o r r e l a t i o n f u n c t i o n of c a p a c i t a n c e p r o b e s i g n a l s and gradual decrease in absolute pressure f l u c t u a t i o n s used to s i g n i f y breakdown.  -  Table  5.1  233  -  Cont'd  Reference  Technique(s)  Avidan  Change i n s l o p e of R i c h a r d s o n - Z a k i p l o t , ( l o g e vs l o g u) used t o i d e n t i f y transition. V i s u a l d e t e r m i n a t i o n o f bed h e i g h t gave e.  Abed  (1980)  (1983)  Used  Changes i n t h e v a r i a n c e and skewness o f the p r o b a b i l i t y d e n s i t y f u n c t i o n s of c a p a c i t a n c e probe s i g n a l s at d i f f e r e n t r a d i i used t o i n f e r t r a n s i t i o n .  S a t i j a & Fan (1985)  A b s o l u t e p r e s s u r e f l u c t a t i o n s measured by t r a n s d u c e r s were m o n i t o r e d above t h e bed, and t h e t u r b u l e n t t r a n s i t i o n d e f i n e d as the p o i n t where t h e r e was a z e r o p r o b a b i l i t y of zero p r e s s u r e .  Yong e t a l . (19861  I d e n t i f i e d the b e g i n n i n g of the t u r b u l e n t t r a n s i t i o n using d i f f e r e n t statistical m a n i p u l a t i o n s of d i f f e r e n t i a l p r e s s u r e f l u c t u a t i o n d a t a measured w i t h transducers. T h e s e were c a l l e d a " n o n u n i f o r m i t y c o e f f i c i e n t i n the time domain" and a " n o n u n i f o r m i t y c o e f f i c i e n t i n t h e f r e q u e n c y domain.  Rhodes & G e l d a r t (1986a)  D i f f e r e n t i a l pressure f l u c t u a t i o n s measured u s i n g manometers used t o i n f e r " p s e u d o - t r a n s i t i o n " to t u r b u l e n c e .  a  -  the  bed  and  identify below,  234  a second p o i n t ,  what  j u s t above t h e  i s presumably  i t i s not  -  the  c l e a r that  d i s t r i b u t o r , to  same t r a n s i t i o n .  these  two  As  techniques  shown  are  identical. In  this  study  differential mm  apart,  pressures  with  distributor  a slightly  the  were measured between two  lower p o i n t  plate.  This  measurement of a b s o l u t e differential v o i d a g e and upper It  bed  be  this  the  estimated  at  fallen  considered  give  below the  a l l accurately  preferable  i n t e g r a t i o n of a measure of  mean bed i n the  a c c e l e r a t i o n and  Davidson,  1973).  the  not  upper p r e s s u r e  however, t h a t  this  experimental i n which the  This  t o make measurements. i t was  first  upper p r e s s u r e  tap  were i n b a l a n c e corresponded  to a  m/s  to  the  the  bed  the tap.  voidage  slugging  regime  deceleration i s shown  s e r i e s of  that  at a wind loose  gas  t o 4 m/s, At  established and  a  superficial  intervals velocity  from 0.05  design  raised  This  above  460  clearly  results.  performed  rates  mm  taken;  points  e s t a b l i s h whether or  gradually  the  230  because  s i g n a l would  to note,  (Kehoe and  With was  pressure  method b e c a u s e o f  effects in  had  located  s e t - u p was  more i m p o r t a n t l y  i s important  cannot by  pressure  surface  d i f f e r e n t a p p r o a c h was  velocity  stopping  each  that  packed  dense phase  pressure bed  was  at  regular  measurement  elutriation box  experiments  rates of  height  10 of  covered and  return  kPa. 850  mm  of  -  solids.  Pressure  measurement transducer through at  were  logged high  over the  sensitivity  diaphragm) connected  t h e Tecmar b o a r d .  Each s i g n a l  i n analogue  format  pressure  t o t h e IBM-XT  was l o g g e d  r a t e o f 100 p o i n t s p e r s e c o n d  f o r 60 s  and was  on t h e UV c h a r t  recorded  recorder.  R e s u l t s and D i s c u s s i o n Each data  fluidisation mean s o l i d s standard visual into  -  the Disa  ( w i t h a 0.2 mm  simultaneously  5.4.  fluctuations  section using  a sampling  235  file  from t h e low v e l o c i t y  s t u d i e s was i n t e g r a t e d t o o b t a i n t h e a p p a r e n t hold-up,  deviation.  observations  the behavior Figures  signal  obtained  and t h e n  analysed  The r e s u l t s  could  of the experiments  be combined  t o g a i n some  5.2a-h a r e t r a c e s o f t h e d i f f e r e n t i a l  Beside  observed  then  with  insights  of the system.  f o r t h e bed f o r s e l e c t e d c a s e s  range.  t o o b t a i n the  each  figure  over  the o p e r a t i n g  i s a description  i n t h e column a t t h e time  pressure  o f what was  when t h e t r a c e was  recorded. At of  velocities  the f l u i d i s e d  bubbling  visual  regime.  1 m/s  bed i s e x a c t l y as one would  the behaviour anticipate;  g i v e s way t o s l u g g i n g a t 0.12 m/s and from h e r e t o  approximately and  up t o a p p r o x i m a t e l y  1 m/s b o t h  observations  At higher  the d i f f e r e n t i a l  pressure  are c o n s i s t e n t with  velocities  there  signals  the s l u g g i n g  i s evidence  o f some  - 236 -  a.  >  1  0-11 m/s  VIGOROUS BUBBLING,  CO  VERGING ON SLUGGING  W  T-  T C>  5  10  15  w  20  b. 0-19 m/s  V  DEVELOPED SLUGGING WITH CLEAR TWO  6-  PHASE STRUCTURE AND STRONG  4-  PERIODICITY  2-  0  5  10  '  15  "  20  'T  w  c-  V  0-40 m/s  AGAIN, DEVELOPED SLUGGING WITH  6-  1i  CLEAR TWO PHASE STRUCTURE  4-  2-  0  Vi  5  "  10  "  15  20  ' T  r  d. 0-57 m/s  6-  SOME VISUAL EVIDENCE FOR A SMALL AMOUNT OF SLUG BREAKDOWN  0  F i g u r e 5-2  2  4  6  8  10  T  Traces of the d i f f e r e n t i a l pressure fluctuation, a s i n d i c a t e d by t r a n s d u c e r v o l t a g e , v e r s u s t i m e i n s e c o n d s o v e r a 400 mm s e c t i o n o f a bed o f sand a t d i f f e r e n t g a s veloci ties a - 0.11 m/s, b - 0.19 m/s, c - 0.40 m/s, d - 0.57 m/s, e - 1.25 m/s, f - 2.14 m/s, g - 2.9 m/s, h - 3.9 m/s  -  237  -  e. 1-25 m/s IRREGULAR INTERMITTENT PERIODS OF A MORE UNIFORM NATURE, VISUALLY LIKE FAST FLUIDISATION 6  8  10  T  f. 2-14m/s INCREASINGLY REGULAR PERIODS OF UNIFORMITY WITH INCREASED LENGTH, BUT STILL PERIODIC SLUGGING  9. 2-9 m/s CONTINUOUS SOLIDS ADDITION REQUIRED AT A LOW RATE, FURTHER INCREASED UNIFORMITY  h.3-9 m/s SUBSTANTIAL ENTRAINMENT, MOSTLY UNIFORM, BUT STILL SOME SLUGGING CHARACTER  238  -  slugs  breaking  down; p e r i o d s a r e o b s e r v e d  developed  s l u g s d u r i n g which  registers  smaller fluctuations  periods be  corresponded  cascading  patterns  circulating highest  regime.  call  of h i g h e r t o times  t o those  found  gas v e l o c i t y  the f r a c t i o n  o f time  corresponding  "turbulence" gradually increasing  was o c c u r i n g , w i t h measured  than  w h i c h was v i s u a l l y the pressure  identical  of the c i r c u l a t i n g  m/s  before  both  The solids  a stable  fluctuation  standard  respectively. because function  regime.  are also  versus  increasing  substantial  that this  Velocities  rates  component, evident  was t h e low  had t o e x c e e d  t a p s and showed no s l u g g i n g  h o l d up, s o l i d s  normalised  t o what we  5  dense p h a s e c o u l d be e s t a b l i s h e d w h i c h  pressure  results  tests,  t o s l u g g i n g , was s t i l l  t r a c e d e s p i t e the f a c t  end  velocity  circulation  However, a p e r i o d i c  2  seemed t o  with  At the h i g h e s t v e l o c i t y  12 kg/m s.  These  up t o t h e  s t u d i e d i n these  entrainment  covered  when s o l i d s  persisted  velocity.  in  frequency.  i n the higher  superficial  greater  transducer  t h e w a l l s , between s l u g s , i n  These s i g n a l s  superficial  4.0 m/s, w i t h shall  similar  between w e l l  the pressure  visually  downwards a l o n g  very  -  plotted  hold-up  as g r a p h s o f  The f i r s t  i t indicates  graph,  pressure  i n F i g u r e s 5.3 t o 5.6 F i g u r e 5.3, i s i m p o r t a n t  t h a t t h e mean p r e s s u r e  o f gas v e l o c i t y  apparent  standard d e v i a t i o n ,  d e v i a t i o n , and peak t o peak gas v e l o c i t y  tendency.  c o n f i r m i n g , what  drop  i s a smooth  i s observed  - 239  0  -  1  Superficial Gas Velocity  Figure  5.3  2  (m/s)  P l o t o f a p p a r e n t s o l i d s volume f r a c t i o n v e r s u s s u p e r f i c i a l gas v e l o c i t y f o r a f l u i d i s e d bed o f sand i n a 0.152 m d i a m e t e r r e a c t o r .  - 240 -  (0  a.  o —  —  a  2  (0 3 (0  TJ C 4->  a)  1  0 Superficial  Figure  5.4  2 G a s Velocity  (m/s)  P l o t of standard d e v i a t i o n of d i f f e r e n t i a l p r e s s u r e f l u c t u a t i o n s o v e r a 460 mm l e n g t h o f a f l u i d i s e d bed o f sand v e r s u s s u p e r f i c i a l gas velocity.  -  1  241  1  1  o 0-8  -  o o  o o  1  1  o  °  o  o o  o o  -  06 c o  o  o  -  .2 '>  o Q c  o  0-4  "a c  o  CO  o  -  0-2 - o o  _  0  1  5.5  1  1  0 Superficial  Figure  1  1  1  2 Gas Velocity  (m/s)  P l o t o f the s t a n d a r d d e v i a t i o n of d i f f e r e n t i a l p r e s s u r e f l u c t u a t i o n s n o r m a l i s e d w . r . t . mean d i f f e r e n t i a l p r e s s u r e , o v e r a 460 mm s e c t i o n o f a f l u i d i s e d bed o f sand, v e r s u s s u p e r f i c i a l gas veloci ty.  - 242 -  0  1  Superficial  Figure  5.6  2  Gas Velocity  (m/s)  P l o t of t h e maximum p e a k - t o - p e a k p r e s s u r e f l u c t u a t i o n o v e r a 460 mm s e c t i o n o f a f l u i d i s e d bed of sand v e r s u s s u p e r f i c i a l gas velocity.  -  visually, pressure  t h a t the bed  level  has  realise  turbulent  results  transition will  are  not  transition  to a s t a b l e c l u s t e r  probe s i g n a l s  I f the  are  analysed  ideal  and  i t s a u t o c o r r e l a t i o n which  et aJ.  and  of s i m i l a r  Interpretation  of a b s o l u t e For  example,  at a p o i n t  been d e v e l o p e d  (1974) a s s u m i n g  then  illustrated  i n F i g u r e 5.7,  capacitance The  signal  from  a  a well defined  low p.d.f.  autospectrum i n  been d e m o n s t r a t e d  by  fluid  cracking catalyst  of  by  Crescitelli  et a J .  (1978)  diameter. pressure  signals  consider  the  s l u g g i n g bed.  waves from  i s seen  i s much  absolute  by Kehoe and  effectively  a triangular  a  density function  shift  in a freely  that pressure  point are  approximately  being  has  theoretically  measurement  is  for a  a Ludox c a t a l y s t  signal  the  i s s e e n as  frequency  This result  (1974)  more c o m p l i c a t e d .  has  with  a their  s l u g g i n g r e g i m e t o a more u n i m o d a l  s u r f a c e mean d i a m e t e r  pressure  upper  important  to i n d i c a t e  indicators.  should  less well defined, higher  Carotenuto  for  bimodal p.d.f.  i n the  i t is  envisages  for i t s probability  turbulent state.  um  study  suspension,  transition  be  characteristic  both  transition  can  60  this  depend upon what one  t o be.  frequency  of  necessarily identical;  transition  the  d r o p p e d below t h e  t h a t the v a r i o u s methods used  suitability  with a  not  tap.  In d i s c u s s i n g t h e to  -  243  Davidson  s l u g s above  damped. to r i s e  This  the  The  signal,  and  fall  waveform; p r e s s u r e  i n what  rises  as  - 244 -  Absolute pressure f l u c t u a t i o n s i n a slugging bed a c c o r d i n g t o Kehoe and D a v i d s o n (1973) showing a b s o l u t e p r e s s u r e i s not a b i m o d a l function.  -  the  slug  245  -  goes by and s u b s e q u e n t l y  probability  density  function  falls  i s calculated  waveform, i t does n o t y i e l d a b i m o d a l strong  of  of the s i g n a l s  maximum p r e s s u r e  function,  fluctuations,  i s limited  fluctuation,  If a  f o r such a  two phase c h a r a c t e r s o f t h e s y s t e m .  interpretation the  back.  despite the  Hence,  t o observations of  or the standard  w h i c h do n o t have  deviation  unambiguous  interpretations. In to  this  study,  as noted  , a d i f f e r e n t i a l rather  pressure  signal  measured  o v e r a 460 mm  the  was measured.  distributor.  from  just  while  Here,  section  I n Yong's c a s e ,  above t h e d i s t r i b u t o r  The  just  fluctuations  of measuring  i s that  they  the pressure  affect  was u s e d .  d i f f e r e n t i a l pressure be r e l a t i v e l y  acceleration  insensitive  zone, p a r t i c u l a r l y i n  b e c a u s e s u c h downstream f l u c t u a t i o n s taps  and hence  the d i f f e r e n t i a l pressure  the s o l i d s  h o l d - u p between affects,  which  above  e x p e r i m e n t s a 100 mm  should  t h e upper and lower  Therefore,  230 mm  t o the middle of the u n i t ,  what happens above t h e measurement  deep beds,  d r o p was  t h e measurement was made  above t h e d i s t r i b u t o r  merit  t h a n an a b s o l u t e  o f bed b e g i n n i n g  i n t h e Rhodes and G e l d a r t  interval  of  fashion  t h e s t u d i e s o f Yong ejt a_l. (1986) and Rhodes and G e l d a r t  (1986a)  to  e a r l i e r , and i n s i m i l a r  cancel.  signal  t h e two t a p s ,  tends  equally  i s a time combined  towards a t r u e  trace with  measurement  - 246  of  the  h o l d - u p as  infinity.  The  fluctuations relatively in  statement  might  few  only  solids  relative  factor upper  tap  tap  strong  absolute authors their  very  the  v a r i e d from evidence 1973)  t o be  were a b l e own  fluctuation  height 100  bed  mm.  Staffin,  height  In  the  to p r e d i c t absolute  s l u g g i n g b e d s and  those  s u r f a c e was  pressure  tap  were damped o u t .  c o n s i s t e n t l y over  i n the  developed  tap,  such In  as  these  i n c r e a s e d by  Kehoe  affect latter  a  the  d e e p e r beds  1967;  does not  fluctuations.  are  s u r f a c e above  For  or  downstream  experiments.  amplitude  of t h e  t o 450  to zero  t r u e when t h e r e  Geldart  ( F i a c c o and  t h a t they  to  above the u p p e r p r e s s u r e  t h a t bed  pressure  tends  sensitivity  there  and  even  the  study  the  pressure f l u c t u a t i o n s of F i a c c o and  w e l l u s i n g a model w h i c h d i s r e g a r d e d  assuming bed  as  separation  regarding fail  pressure  o f 10.6  Davidson,  in  the  a number of t h e Rhodes and  cases  is  either  -  upstream  In our  400  mm  s l u g flow  own  effects  studies  above t h e and  Staffin  the  upper  turbulent  regimes. The  preceeding  differences  between a b s o l u t e  fluctuations. precautions the  to  Below we  are  amplitude  true  and  i t i s not  of e i t h e r  clearly  transition.  F i g u r e 5.8.  This  there  differential  show t h a t , u n l e s s  taken,  hydrodynamic  consider  d i s c u s s i o n shows t h a t  clear  way  pressure  certain  that a decrease  signifies One  are  to  shows t h a t  the  beginning  illustrate the  standard  in of  this  a is  - 247 -  Point Density Fluctuations In An ' I d e a l ' T w o - P h a s e System  Figure  5.8  Standard d e v i a t i o n of a d i f f e r e n t i a l pressure s i g n a l measured o v e r a s m a l l l e n g t h v e r s u s e x p a n s i o n f o r s l u g f l o w , showing a maximum d e s p i t e no l o s s o f the two phase n a t u r e .  -  -  248  d e v i a t i o n of a d i f f e r e n t i a l pressure s i g n a l , measured with c l o s e l y spaced  pressure taps, w i l l reach a maximum at some  v e l o c i t y where the s l u g length i s equal to the s l u g spacing, even i f there i s no breakup whatsoever of the s l u g structure.  Another p o s s i b l e means of o b t a i n i n g a peak i n  f l u c t u a t i o n amplitude,  i f t h i s amplitude  was  measured u s i n g  a simple manometer, would be to have a s l u g frequency c l o s e to the n a t u r a l frequency of the manometer. circumstances  Under these  a resonant peak would be seen, again without  a  true breakdown of the two phase s t r u c t u r e . Our measurements avoided the major p o i n t s of contention.  Specifically,  they were made at the base of a  deep bed which should e l i m i n a t e the problem of a pressure f l u c t u a t i o n s i g n a l which v a r i e s with the weight of above the measurement taps. response  transducer was  solids  A l s o , a high s e n s i t i v i t y ,  rapid  used which allows a complete t r a c e  of the f l u c t u a t i o n to be made.  T h i s permitted us to  e s t a b l i s h not only the maximum f l u c t u a t i o n , but a l s o s t a t e of the bed between the peaks.  the  F i n a l l y the pressure  transducer e l i m i n a t e s any p o s s i b i l i t y of measurement system resonance,  the resonant  frequency  of the transducer being of  the order of 2 kHz. The  graphs can now  compare F i g u r e s 5.4 f l u c t u a t i o n s and  be examined i n d e t a i l .  and 5.6,  Firstly,  graphs of peak-to-peak pressure  the standard d e v i a t i o n s of the same  -  fluctuations.  249  I t i s apparent  which developed  -  that,  slugging i s established  m/s,  where t h e r e i s a s u b s t a n t i a l  drop  i n the amplitude  although  and  fluctuations  regularity  visually:  The  There  turbulent  turbulent although from et  measurement  (1986a). low  volume but Avidan  A transition  entrainment  there  behavior  (1973), In our  (1978),  not  can  i n the  bed  oscillations.  mass t o c a u s e The  latter  over  of  t h e same sees  interspersed  be  upon  definition,  seen  i n some  a_l. (1978) and  sense  traces  Carotenuto  does o c c u r implied  i n the  either  by  o r Rhodes and G e l d a r t  to a s t a b l e  is definitely  the  the n o t i o n of a  a transition  (1980),  unchanged,  i n a developing  support  Canada ert view  However,  out what one  not  cluster  suspension  applicable,  a p s e u d o - t r a n s i t i o n w h i c h depends upon a  overlying  t h e r e i s no  fluid.  depends s t r o n g l y  very s i m i l a r  to 3  s l u g g i n g , somewhat  found  phase  these data  transition  a l • (1974).  Turner  developed  flow of a s i n g l e  Massimilla  gradually  are p e r i o d s of t u r b u l e n c e  Whether o r not  mm/s)  the s t a n d a r d d e v i a t i o n  decreases  to the i n t e r m i t t e n c y  at  amplitude  p r e s s u r e t r a c e s bear  between p e r i o d s o f w e l l analoguous  remains  of these high  d i m i n i s h e s , so t h a t  range.  ( « 130  entrainment,  fluctuation  the p r e s s u r e f l u c t u a t i o n s velocity  the v e l o c i t y  of p r e s s u r e f l u c t u a t i o n s .  the peak-to-peak  frequency  from  decreased  undoubtably  but  with  neither  decreased  peak-to-peak o c c u r s i n the  upper  is  - 250  part  of  the  bed,  and  at high  -  enough v e l o c i t y  downward t o  the measurement  this  effect  was  circumvented.  loss  of  two  phase c h a r a c t e r i n p e r i o d s  the  intermittency. well  into  what  Turbulent simply  zone, but What  by  would  u s i n g a deep  i s seen  is a  fluidisation  for this  system  c h a r a c t e r i z e d regime w i t h  fast  velocities  flow  smooth t r a n s p o r t f l o w  and  has  interspersed  with  fluctuations, velocity  be  concluding  slugging and  the  alone  this  and  breakdown  they  indicative  standard  because  relatively  a  of a  slug  and  amplitude  as  the  that t h i s  gas  picture i s  of Rowe e t a l .  (1980).  number of p o i n t s  a transition  occurs  Peak-to-peak transition;  should from  semantics  fluctuations  however,  time  d e v i a t i o n measurements show t h a t  i s occurring.  observe  large  depends s t r o n g l y upon b o t h  technique.  work of Rhodes and  which  section, not  a  "turbulence"  slug-like  observations  Whether o r  measurement not  of  It i s notable  to turbulence  are  traces  the  with X-ray  reiterated.  b u r s t s of  between  intermediate  f o r m e r becoming dominant  i s increased.  consistent In  the  periods  until  steady p r o p e r t i e s .  gas  character with  remains  i s t h e r e f o r e not  i t denotes a range of  intermittent  gradual  fluidisation.  Instead, and  bed  of  However, a s l u g g i n g component i s c o n v e n t i o n a l l y termed  work  will  Secondly  i t is essential  Geldart  (1986a).  occur  under many  shallow  beds a r e u s e d .  The  to  heed  transition  circumstances However, t h i s  is  -  not  a real  character (1978). both  i n d i c a t o r o f a breakdown o f t h e two phase i n the sense  Finally,  implied  by Y e r u s h a l m i  the r e s u l t s of t h i s  t o t h e equipment  There  251 -  and t o t h e s o l i d  i s some e v i d e n c e  any  unit  stable these bed  s i z e so t h a t  circumstances Grace  limited limit gas  where t h e bed s i z e i s s e v e r a l  voidage  slugging  a t which a s t a b l e  times  and l i k e l y i n t h e maximum  i s precluded. that  Under t h e maximum  b u b b l i n g bed c a n s u r v i v e i s  by c l o s e p a c k i n g o f t h e b u b b l e s ,  on o v e r a l l bed v o i d a g e  leading  of approximately  t o an u p p e r  0.7.  Higher  f l o w s would n a t u r a l l y r e s u l t i n a d i s c o n t i n u o u s  i s an a p p r o a c h  points the  1973),  (1986b) has n o t e d  p h a s e and a s i t u a t i o n e q u i v a l e n t This  p a r t i c l e s used.  bed i n beds o f f i n e s o l i d s  e t a l . , 1978; M a s s i m i l l a ,  bubble  are s p e c i f i c  i n favour of a t u r b u l e n t t r a n s i t i o n  t o a homogeneous low e n t r a i n m e n t (Crescitelli  study  and C a n k u r t  to turbulent  particulate influenced  transition".  by p a r t i c l e  shape  catalyst  i n turn  influence  i s the e f f e c t i v e i s strongly  f a c t o r and f i n e s c o n t e n t  This  might  explain  t r a n s i t i o n behaviour  found  by C r e s c i t e l l i  (1978) f o r two v i r t u a l l y catalyst,  which c o u l d  One o f t h e s e  phase v i s c o s i t y which  ( M a t h e s o n e t a J . , 1949). different  fluidisation.  which demands more a t t e n t i o n b e c a u s e i t  t o w a r d s a number o f p a r a m e t e r s  "turbulent  solid  dp = 60 ym p dp = 60 ym p  identical  = 1400 kg/m , and a f l u i d = 940 kg/m .  et a l .  Ludox  3  p  3  p  materials,  the widely  The f o r m e r  cracking  showed  -  dramatic  252  -  breakdown to turbulence, without  slug  formation,  while the l a t t e r broke down slowly from s l u g g i n g , i n a f a s h i o n a p p r a r e n t l y s i m i l a r to the breakdown observed i n this  study.  -  6.  6.1  -  MIXING IN A CIRCULATING FLUIDISED  AXIAL GAS  BED  Introduction Early  d i s c u s s i o n about  fluidised  bed  perceived  merits  al.,  1976).  backmixing the  high  interest  influenced  cases  residence  flow  (1978) and  bed,  several except  seemed  the  (Yerushalmi  small  v i r t u e of  backmixing  i s of  processing may  be  strongly  gas  to approach plug  gas  backmixing  perceived  contactor.  notion  helium  wall.  the  This  a small Cankurt  differed  there  and  CFB  as  that  of  obtained  measured  t r a c e s of  there  tracer  circulating  was  helium  was  in a wall the  a  Yerushalmi  profile  Yerushalmi's  from d a t a  systems  when h e l i u m  were no  except  fraction  the  bed  c e n t r e l i n e of a  suggested  p h y s i c a l backmixing  and  that  concentration  upstream,  of  gas.  flow.  in fast  Both Cankurt  at  gas  r e g i m e s by  i t i s g e n e r a l l y d e s i r a b l e f o r the  s t u d i e s of  et  amount of  the  t o occupy  6.1,  number of i t s  time d i s t r i b u t i o n  Yang e_t a l _ . (1983) f o u n d  cross-section. Figure  Gas  selectivity  residence  centimetres  negligible  fluidised  circulating  of  a radial  at  reported  employed.  injected continuously and  upon a  i n c e r t a i n chemical  t o r e i n f o r c e the  near p l u g  was  the  a  to other  time d i s t r i b u t i o n  Previous appear  was  where p r o d u c t  by  b e n e f i t s of  for gas-solid contacting  velocities  applications  the  focussed  Among t h e s e  gas  these  technology  relative  particular  In  253  l a y e r , which  reactor results, i n the  shown i n  somewhat  - 254  -  U(m/s)  -7.6  -6.1  -2.5  0  2.5  5.1  7.6  Distance From Centre (cm)  Figure  6.1  T r a c e r p r o f i l e s measured 50 mm u p s t r e a m of a centreline i n j e c t i o n p o i n t a t d i f f e r e n t gas v e l o c i t i e s ( C a n k u r t and Y e r u s h a l m i , 1978). C/Co i s the r a t i o o f t r a c e r c o n c e n t r a t i o n a t t h e measurement p o i n t t o t h e i n j e c t i o n concentration. The lower g r a p h r e p r e s e n t s f a s t bed c o n d i t i o n s (Ug > 1.8 m/s). Turbulent f l u i d i s a t i o n e x i s t s between 0.6 m/s and 1.7 m/s.  -  lower in  velocity  the  steady  turbulent  upstream w a l l  At  the  passing  6.2)  with  number on the  data  velocity, be  countercurrent the  fast  number,  modelled  even beyond  insufficient  gas  Yang e t aA. occurs  interior  the  m a g n i t u d e of  is  small.  residence an  attempt  nor  6.3).  and  dispersion  coefficient  dependence of the  effectively  using  a  axial  Peclet  regime  simple  1986).  increase  i n turbulent  (Figure  turbulent  (Yerushalmi,  t o show an  Avidan  fluidisation  In  for  transport  Yerushalmi  in  While  Peclet  fluidisation, a  seemed a p p r o p r i a t e ,  although  Cankurt  (1983) d e m o n s t r a t e wall  riser,  turbulent do  the  to occur  apparently  twodue  to  backmixing.  near the  of  expected;  that  longer  Unfortunately,  only  linear  change  found  at  axial  two-phase model  no  was  turbulent  (Figure  regime c o n t i n u e d  p h a s e model  and  to  approximately  could  A dramatic  injection  i n the  from s l u g g i n g an  regime.  lower v e l o c i t i e s  (1985) n o t e a d e c r e a s e in  -  concentration  centreline tracer  velocity.  255  they  Therefore  in fast  t h e y do  not  dispersion demonstrate  and  that  Yerushalmi physical  fluidisation,  to q u a n t i f y  the  coefficient that  not  in  might  downflow a t actual  were u n d e r t a k e n  mixing  downflow  i n d i c a t e what s o r t  some measurements of  time d i s t r i b u t i o n s  (1978)  phenomena.  the of  be the  wall  gas  in this  work i n  -  Figure  6.2  256  -  V a r i a t i o n of a x i a l d i s p e r s i o n c o e f f i c i e n t w i t h gas v e l o c i t y i n p a s s i n g from s l u g g i n g t o t u r b u l e n t f l u i d i s a t i o n ( Y e r u s h a l m i and A v i d a n , 1985). T r a n s i t i o n to t u r b u l e n c e r e p o r t e d l y o c c u r s a r o u n d 0.7 m/s.  - 257  Figure  6.3  -  V a r i a t i o n of a x i a l P e c l e t number w i t h gas v e l o c i t y ( Y e r u s h a l m i and A v i d a n , 1985).  -  6.2  The Experimental Study  6.2.1  General  considerations  One o f t h e s i m p l e s t measuring inject pulse the  258 -  residence  and most  time d i s t r i b u t i o n s i n flow  a t r a c e r i n t o the f l u i d with  entering  systems i s t o  the system  a known waveform; t h e c o n c e n t r a t i o n  t r a c e r i s then m o n i t o r e d  residence  time d i s t r i b u t i o n  and  waveforms.  inlet  common methods o f  The t r a c e r must  satisfy  h i s t o r y of  a t the system e x i t , c a n be i n f e r r e d from  certain criteria  be s u i t a b l e f o r u s e i n s u c h a t e s t ; (i)  Be r e a d i l y d e t e c t a b l e ,  (ii)  Mix i n t i m a t e l y w i t h  in a  and t h e the e x i t  i n order  to  i t must:  preferably  the f l o w i n g  in real  fluid  RTD o f t h e t r a c e r i s r e p r e s e n t a t i v e  time.  so t h a t t h e  of the bulk  fluid. (iii)  E i t h e r have p h y s i c a l p r o p e r t i e s flowing  fluid,  flowing  fluid  in  a small  fluid For  flow  circulating helium  so t h a t  enough c o n c e n t r a t i o n  of a i r a t h i g h  and c a r b o n  could  be d e t e c t e d  using  a cheap,  i t may r e p l a c e  velocities  present  the bulk  through the  gaseous t r a c e r s , hydrogen,  were c o n s i d e r e d .  a t low c o n c e n t r a t i o n s  commerically  of t h e  are unaffected.  bed s e v e r a l  dioxide  that  to the  part  when i t i s i n j e c t e d , o r be  properties  fluidised  identical  A l l of these  i n a i r i n real  a v a i l a b l e , thermal  time  conductivity  -  detector  salvaged  only  the helium  that  the bulk  acceptable  259  -  from an o l d gas c h r o m a t o g r a p h .  could  flow  be used  i n small  enough q u a n t i t i e s  p r o p e r t i e s were u n a f f e c t e d  signal-to-noise ratio  However,  a t an  i n the d e t e c t o r  (Yu,  1986).  The t r a c e r f l o w r a t e s  required  f o r e i t h e r hydrogen or  carbon  d i o x i d e were t o o h i g h  for this  study  f l o w ) where t h e extreme s e n s i t i v i t y conductivity high  throughflow  through the  cell  the c e l l ;  riser  column.  together  with  The  cause  only  helium  was  create these  be u s e d , noise  Therefore  a thermal  buoyancy.  taken  6.2.1  S a m p l i n g and i n j e c t i o n  withdrawal  to design  that  for detection. weight of  of a i r t h a t  there  at the i n j e c t i o n  to minimise  this  might  point  due  by j u d i c i o u s  system.  systems  suitable tracer injection  to  understand  of  t h e u n i t , and t o a p p r e c i a t e  Specifically,  employed  the molecular  systems f o r the c i r c u l a t i n g  interpretation  pulsations i n  t r a c e r was  that  than  of the i n j e c t i o n  order  helium  was  design  In  b e c a u s e m o d i f i c a t i o n s and  from p r e s s u r e  tracer segregation C a r e was  of the thermal  conductivity cell  f o r concern  total  due t o f l o w f l u c t u a t i o n s  result  so much s m a l l e r  be s i g n i f i c a n t to  cannot  (> 2%  the mixing p r o c e s s e s  of t h e r e s u l t s  how  and  bed, i t i s  which  occur  these  affect  important  a t e i t h e r end  i n any s u b s e q u e n t  i f the o b j e c t i v e of the experiment  the models. i s to  -  determine  true residence  characterise  the  260  time d i s t r i b u t i o n s ,  s y s t e m i n terras of  o r m u l t i p a r a m e t e r model, t h e n closed into  so  the  that  t r a c e r gas  system.  system boundaries the  circulating  additional (i)  must be  distributed  Gas  l e a v i n g the  the  a l l flow  to  single be  of  or  across  the  Because  there  this  are  also  distribution  the  exit  or a mixing  RTD  e n t e r i n g gas,  this  gas  column,  this  be  way,  by  of  sampled to g i v e  a  must be  i t was entry  into  adding  error i s small  developed  average.  satisfied,  then  it is  data.  point ensuring  error i s introduced in this  concentration  be  duct,  cup  t r a c e r enters  the  i t is  p o i n t average t r a c e r  accurate  the mouth o f  an  across  that  upstream  either  same i n s t a n t a n e o u s  t h a t the  i n the  a uniform  immediately  above c o n s t r a i n t s can  t u r b u l e n t at  the  out  i n s u c h a way  v e s s e l must  instantaneous  to generate  windbox a t  of  should  convective.  with  position  entrance.  ensure  Although  entirely  injected  the  to give  very  f i t to a  either  i s non-uniform r a d i a l l y  concentration,  dispersed  diffuse  be  each r a d i a l  true  To  cannot  should  isokinetically  possible  best  than  problems:  at  a l l of  rather  the boundaries  alternatively,  initially  If  the  Stated  bed  Tracer  (ii)  -  the  column  uniformly  injected  into  nozzle.  The  rapid the  the  the flow  is  mixing.  residence  time  windbox RTD  to  (mean r e s i d e n c e  that  time i n  - 261 -  t h e windbox time).  t o use m u l t i p o i n t  immediately  each  t h a n 2% o f t h e o v e r a l l  Windbox i n j e c t i o n  having or  less  point  proportional  cup  problem. 152  mm  duct  point  Both  injection  to the  sampling average  Reynolds  acts  number  riser  are e l i m i n a t e d  single exit,  sampling  need  to produce  an  cup  delta,  or u n i t  which  i s measured  exits  radial  duct  to p r o v i d e not  converge mm  a from  (4")  mixer  i n the m i x i n g  a  exit This  at the h i g h experiments.  c o n c e n t r a t i o n at  the  of c o n t r a c t i o n , as p o s s i b l e  and  i s dictated  helium pulse with a  for this  function;  impulse  In p r a c t i c e ,  however, i t was  delta  input  for this  simplest  this  the r e s p o n s e  the  is a curve  i s the r e s i d e n c e time  windbox/riser/sensor not p o s s i b l e  system,  by  reproducible  form  of a s u b s t a n t i a l  system.  to approximate  because  this  amount  a  to the  average.  impulse  the i n t r o d u c t i o n  at  velocity.  p l a c e d as c l o s e  f o r the combined  require  with a rate  t o a 100  in tracer  distribution  helium  below  the c y c l o n e e n t r a n c e .  In i t s t h e o r e t i c a l l y  Dirac  gas  d e s i g n of the i n j e c t o r input  immediately  plate  at the p o i n t  location,  physical  waveform.  diameter  profiles  gives a mixing The  abrupt  ( » 1 0 0 , 0 0 0 ) used  radial  of  c o n c e n t r a t i o n was  as an e x c e l l e n t  Therefore, exit  local  gas  i s compatible with  contraction  either  the p r o b l e m  a c r o s s the e x i t  t h e smooth and  (6") column i n s i d e  which  eliminates  above the d i s t r i b u t o r  Representative a mixing  also  mean r e s i d e n c e  would  a  -  of  helium  i n a time which was s h o r t  residence without  time i n the r i s e r .  This  d i s t u r b i n g the flow;  change i n h e l i u m  input,  n o - f l o w a t time 0 . be o b t a i n e d  preferable rotameter  using  i n the helium  results  transform.  preferable  line  6.4.  Note  helium  complex  results  a n a l y s i s of the  i t would have  of the s o l e n o i d  development  of  time.  length  The  i n Figure  to the d i s c h a r g e  i s closed,  to d i f f u s e  has no  been  c y c l e squarewave i n p u t  and d e t e c t o r  problem  form of p u l s e  there  slowly  the sharpness of the step  than the i n j e c t o r  considerable  this  i s minimal  i n t o the  pulse.  systems are c o n s i d e r a b l y  and were t h e s u b j e c t effort.  of a  The s y s t e m  complexity  i n the e x i t i n g  gas s t r e a m  from:  (i)  The s o l i d s  (ii)  The s h o r t  loading residence  circulating  bed.  times  was a  unstably  The o n l y  the mathematical  i n the n o z z l e  sampling  was  oscillate  s y s t e m i s shown s c h e m a t i c a l l y  s y s t e m and b l u n t The  shut-off  and t h i s  to the r e a c t o r r e s i d e n c e  the p r o x i m i t y  step  a t t i m e 0~ t o  instantaneous  impulsively.  so t h a t when t h e s o l e n o i d  residual  flow  which would  In r e t r o s p e c t  equal  injector  nozzle  i s that  t o use a s i n g l e  approximately  accomplished  i n t r o d u c t i o n because there  i s more complex b e c a u s e  Fourier  helium  input  t o t h e 1.3 s  not be  a solenoid valve,  to instantaneous  the step  could  from a c o n s t a n t  when t h e s o l e n o i d was opened with  compared  t h e r e f o r e we c h o s e a  Essentially  +  could  -  262  f o r t h e gas i n t h e  more  -  263  -  Riser  Distributor  Solenoid Valve  •130m rn Windbox  4  mm  In  He  Figure  6.4  Tracer  injection  system  for  gas  RTD  studies.  -  The put  first  factor  necessitates  t h r o u g h an a b s o l u t e  delicate second  264 -  filter  before  filaments  of a thermal  f a c t o r makes  the design  the  detector  the  gas r e s i d e n c e  The  any sample  contacting  conductivity  s t r e a m be  the  detector.  of the sampling  The  s y s t e m and  more complex b e c a u s e o f t h e need t o m i n i m i s e time  promote d i s p e r s i o n dispersion  that  i n t h i s system.  i n the detector  Long  residence  system which  times  can mask t h e  phenomena i n t h e column.  final  sampling-system/detector  6.5.  welded  o n t o a 6.3 mm ( 1 / 4 " OD), 4.6 mm (.180") ID, 40 mm  length  s t a i n l e s s s t e e l tube.  sample  cell.  filter  effect, 15 ym  sampling  cell  tube  system d i s p e r s i o n .  into  conductivity  3  sampling  rate  rate,  i n order t o  Each v a r i a b l e  filter  and 60 ym l e v e l s ) s a m p l i n g  had some  pore s i z e ( t e s t e d a t (tested  a t 1667  l e v e l s ) and c o n d u c t i v i t y  3  design. The  pore  filter  i n turn  length,  d e s i g n were a l l v a r i e d  mm /s, 3333 mm /s and 5000 mm /s cell  metal  r e s u l t s from a s e r i e s o f e x p e r i m e n t s i n  t h e most n o t a b l e b e i n g  3  i s fitted  Bowmac t h e r m a l  pore s i z e , sampling  conductivity  minimise  This  of a modified  The d e s i g n  which and  side  o f a 60 ym s i n t e r e d  i s shown i n  Figure  the  It consists  design  decrease  i ndispersion  s i z e i s r a t i o n a l , while  w i t h an i n c r e a s e d  t h e need  parameters a t a l l i s a r e f l e c t i o n thermal  conductivity  cell  to vary  of the f a c t  was b e i n g  used  filter  t h e l a s t two that t h e  i n a service f o r  - 265  F i g u r e 6.5  -  P h o t o g r a p h o f t r a c e r s a m p l i n g and s y s t e m f o r RTD s t u d i e s .  detector  - 266 -  which  i t was not  operation, the  intended.  recommended  I n normal gas c h r o m a t o g r a p h y  flowrates  r a n g e 416 - 833 mm /s s o t h a t  heat  3  filament  i s t o an e s s e n t i a l l y  flowrate  v a r i a t i o n s do not  gas f o l l o w s a c o n v o l u t e d  the  cell  not  affect  not  are i n the  and s m a l l stability. drop path  t o minimise d i s p e r s i o n s i n c e However, i n  Also, through  this  therefore  3  the  runs,  gas f l o w r a t e  and a c e l l  minor m o d i f i c a t i o n , a l l o w e d Figures  our  straight  dramatically.  of sampling  chosen which,  tracer  concentration  Figure  6.7 shows how c h a n g i n g t o a modified  changes improve  t o a step  gas f l o w i n g the  with  throughflow.  these  response  i n the  t o 4167  F i g u r e 6.6 i l l u s t r a t e s  effect  Beckman d e s i g n  upon t h e  was i n c r e a s e d  design  6.6 and 6.7 show t h a t  operation  cell  from a  filter.  convoluted  (drilled  in  the  change i n  i n t o the  Bowmac u n i t  throughflow) i s also e f f e c t i v e  out t o  minimising  dispersion. After  making s u c h  construction conducivity output  will  we r e q u i r e minimum d i s p e r s i o n and h i g h  mm /s i n normal  improve  cell  pressure  i n t e g r a t e d peak a r e a s .  application flowrates,*  designed  gas  the  high  such a c e l l  t r a n s f e r from  stagnant  affect  the  cell  through  substantial modifications  and o p e r a t i n g cell,  o f the  concentration.  cell  c o n d i t i o n s o f the  i t was n e c e s s a r y bridge  This  remained  i s the  usual  t o the  thermal  to e s t a b l i s h that the linear case  for  with  helium  thermal  the  -  Figure  6.6  267  -  E f f e c t of sampling rate through thermal c o n d u c t i v i t y c e l l upon d e t e c t o r dispersion. (Dead time removed from F-curves).  -  268  -  modified original  0  5  10  15  Time(s)  Figure  6.7  E f f e c t o f t h e r m a l c o n d u c t i v i t y c e l l t y p e upon detector d i s p e r s i o n s h o w i n g how design modifications (new c e l l t y p e d r i l l e d out) reduced d i s p e r s i o n . (Dead time removed from F-curves).  -  conductivity transfer  cells,  test  in  F i g u r e 6.8  mm  but may  for this  /s and  linearity  and  the mixing  each  the  largest  different  was  changing  150  was  mA.  c o n f i g u r e d as  The  integrated  volume o f h e l i u m  flow of  4167  cell  output  curve  the output  volume i n j e c t e d  ( F i g u r e 6.9),  as p r o o f o f  concentration  f o r the t r a c e r  over  and  linearity  the  required  the  for  peak  the  tests.  plotted  shown  injected  ensuring that,  injected,  (mV.s) was  heat  gas  to a c o n c e n t r a t i o n h i g h e r than  o f t h e peak a r e a  the  c o n v e c t i v e component.  volumes o f h e l i u m were  then  c o n c e n t r a t i o n s proposed  taken  by  tee f o r a constant c a r r i e r  condition  corresponded  was  affected  the c e l l  a b r i d g e c u r r e n t of  for  graph  be  mode t o g i v e a s u b s t a n t i a l  To  into  -  269  Finally  a g a i n s t the  linearity  a helium  of t h i s  of c e l l  output  (mV)  range.  T h i s i s shown by  plot  with  (1986) • The system,  study and  a sensor  approximately column. performed  to t h i s  equal  T h i s was since  point  developed  which e x h i b i t e d  an  injection  dispersion  i n m a g n i t u d e t o the d i s p e r s i o n  adequate  f o r the e x p e r i m e n t s  sensor/sampling  acounted  for  effectively  provided  that  i t i s of a  magnitude  had  system  dispersion  using mathematical lower  t o the d i s p e r s i o n  to  i s t o be  be can  be  techniques,  or c o m p a r a b l e o r d e r  which  i n the  of  measured.  Yu  - 270 -  6.2.3  D e t e c t o r / s a m p l i n g system To  eliminate  t h e combined requires by  characterisation  d e t e c t o r / s a m p l i n g system  riser/detector/sampling  system  measurement o f t h e d e t e c t o r RTD.  inserting  detector  the i n j e c t i o n  m/s  i n the r i s e r ,  the v e l o c i t y  used  upstream  and t h e s a m p l i n g  voltage  t h e b r i d g e was a m p l i f i e d  from  "Amplivolt"  DC a m p l i f i e r  the u l t r a v i o l e t  t h e computer. the h e l i u m recorder,  chart  F-curve  o f 7.1  Output  3  100 t i m e s u s i n g a CRC  of the s o l e n o i d , an e v e n t  computation  response  velocity  i n analogue  r e c o r d e r and i n d i g i t a l  flow, t r i g g e r e d  flow.  was 4166 mm /s.  and r e c o r d e d b o t h  Activation  permitting  rate  of the  i n subsequent  experiments,  The  T h i s was o b t a i n e d  and making a s t e p change i n h e l i u m t r a c e r  a i r f l o w was c o n s t a n t a t a s u p e r f i c i a l  from  response  lance immediately  The  on  dispersion  which  format  form on  shut o f f  marker on t h e UV  o f mean r e s i d e n c e t i m e s .  o b t a i n e d under  these c o n d i t i o n s i s  shown i n F i g u r e 6.10.  6.2.4  Riser The  initial  experiments m/s  characterisation characterisation  at a constant s u p e r f i c i a l  and a t d i f f e r e n t  different whether virtue  exit  solids  of c r e a t i n g  different different  A summary  l e d to a s e r i e s of  gas v e l o c i t y  circulation  configurations  t h e s e cause  patterns.  tests  rates.  were s t u d i e d  o f 7.1  Two  to establish  amounts o f gas m i x i n g by internal  of the t e s t  solids  conditions  mixing appears  i n Table  - 271 -  sample  tee for pulse helium injection  r-r£b  o  sample stream  conductivity cell reference stream  ^>hpair)>-  Figure  6.8  Test c o n f i g u r a t i o n to e s t a b l i s h detector linearity.  -  272  -  Volume Of He Injected (mm ) 3  Figure  6.9  Graph o f i n t e g r a t e d d e t e c t o r o u t p u t v e r s u s i n j e c t i o n volume t o d e m o n s t r a t e d e t e c t o r linearity.  -  -  273  Time (s)  Figure  6.10  Optimised  detector  F-curve  response.  -  6.1;  t h e column c o n f i g u r a t i o n  exits a  isation;  the o u t p u t  the F-curve total  by  sensor, the  from  detailed  relatively  of  of  i n turn  data  at  exit.  This i s  smooth  exit  through  t o the The  7,  identical  as v o l t a g e  transfer.  formed  can  shapes  be  The  the  variations  high  pressure  the b a s i s o f a  further  a n a l y s e d most e a s i l y  i n both  s m a l l (D/UL  two  o f C and  and  In t h i s  F curves are  conditions.  study  The  from  curves.  and  i f the  detector i s  case  (Levenspiel,  insensitive  to  the  C curves both f o r  the d e t e c t o r / r i s e r  computed  response  the r i s e r  < 0.02).  the boundary  readily  these  6.11.  Dis 4 to Dis  i n the  b r i d g e due  approximately Gaussian, be  test  flow v a r i a t i o n s  of heat  t h e d e t e c t o r a l o n e and  can  making  Analysis  amount o f d i s p e r s i o n  nature  cause  themselves  Dispersion  the  of  below.  Data  1972)  for a  runs  w i t h an a b r u p t  conductivity  component  fluctuations  then  than  which a r e m a n i f e s t e d thermal  consisted  c h o k i n g a t the base o f t h e column.  fluctuations  convective  6.3  a smooth e x i t ,  smooth  smooth  f o r the d e t e c t i o n c h a r a c t e r -  large pressure fluctuations  resulting  pressure  test  and  c u r v e s a r e shown i n F i g u r e  column p r e s s u r e drop  caused  in  i s less  Each  f l o w as  When t h e column has  case  f o r the a b r u p t  i s shown i n F i g u r e 2.1.  s t e p change i n t r a c e r  -  274  combination  the d i s p e r s i o n  the d i f f e r e n c e  i n the  are riser  i n the v a r i a n c e s  Table Test  Run Designation  6.1  C o n d i t i o n s f o r D i s p e r s i o n Measurements i n t h e C i r c u l a t i n g F l u i d i s e d Bed  Top Geometry  Gas Velocity (m/s)  Solids Flux in R i s e r (kg/m s )  Total Riser P r e s s u r e Drop (mm Hg)  DisO  Abrupt  7.1  0  0  Disl  Abrupt  7.1  37  24  Dis2  Abrupt  7.1  49  50  Dis3  Abrupt  7.1  60  72  Dis4  Smooth  7.1  0  0  Dis5  Smooth  7.1  65  27  Dis6  Smooth  7.1  41  12  Dis7  Smooth  7.1  33  7  Dis8  Smooth  7.1  43  17  - 276 -  Ug = 71 m/s Gs = 0 kg/m s Smooth Exit  -F»1  2  4T  F=0  F«1  Ug=7.1m/ G =65kg/m s S  2  s  Smooth Exit F=0  Ug=71m/s G  F = 1  = 4 1 kg/m s 2  s  Smooth Exit F=0  Ug = 7-1 m/s G = 33kg/m s Smooth Exit  F=1  2  s  F=0-  T  4  Ug = 7-1 m/s G = 43 k g / m s Smooth Exit 2  s  F=0  r  0  F i g u r e 6.11  T "  I  4  3  Combined r i s e r / d e t e c t o r F - c u r v e r e s p o n s e f o r a x i a l m i x i n g d e t e r m i n a t i o n s i n c i r c u l a t i n g beds o f sand a t Ug = 7.1 m/s. A b r u p t e x i t , G = 0, 37, 49, 60 kg/nTs 33, 43 kg/nTs Smooth e x i t , G = 0, 65. 41 s  s  (Time i n seconds)  Cont.  - 277 -  Ug =7-1 m/s G = 0 kg/m s Abrupt Exit  "r - 1  ~  /  2  s  /  F:0  i  1  0  1  2  I  3  4  T  4  T  Ug =7-1 m/s G =37 kg/m s Abrupt Exit 2  s  F=0  ^  —  /  ~  1  i  0  1  Ug =71 m/s G =49kg/m s Abrupt Exit  I  2  i  3  2  F=1  s  F=0  >  ^ — ^ ^ / ^ " ^  6  *  Ug =71 m/s G =61 kg/m s Abrupt Exit  i  4  3  -v  T  -F-1  2  s  /  F= 0 •  i  0  1  •  2  1  3  4  T  -  Unfortunately, can be plug  although  c r u d e l y approximated  graph  response  paper,  curves. higher  than  the f o r m e r  to  of the  techniques, either  which o t h e r models can  t h e measured by a f i n i t e  applied  f i t dispersion  F-curve  and  tracer  to f i t the  be  derived.  coefficient  difference  dispersion  the r e s u l t s case  of t h e  i t i s not  model.  the  first  necessary  RTD.  of  the  is  first  squares  same  technique generating and  f o r the d e t e c t o r . t o assume a model f o r and  the d e t e c t o r  transform RTD  can  a model a p p r o p r i a t e t o i t s shape and characteristics  In  f o r the r i s e r  signal  The  or,  curve  combination  part  using Fourier  true r i s e r  The  coefficient  I n s t e a d , t h e combined  are deconvoluted  of  a theoretical  to the r i s e r / d e t e c t o r  best-fit  latter  physical  Gaussian  f o r t h e d e t e c t o r , m i n i m i s i n g t h e sum  generate  using  from  case, a best  system.  signal  shows d i s p e r s i o n c o n s i d e r a b l y  time  utilising  the  response  residence  second  the  t r u e f o r t h e combined  i s u n s u i t a b l e , to generate  generated  In  probability  dispersion,  generated  a  normal  the  for large  the former  then  on  of  coefficient  distributions  is  linearity  Therefore r i g o r o u s treatment  data with a d i s p e r s i o n  between  t h e near  curve  d a t a r e q u i r e s more complex  this  i n the d e t e c t o r  the recommended maximum f o r t h e  approximation.  if  the m i x i n g  when i t i s p l o t t e d  the same i s not  Even  -  by a s m a l l - d i s p e r s i o n d i s p e r s e d  f l o w model, as shown by  F-curve  278  system.  techniques  t h e n be  t h e known  fitted  -  Of  the  relative such is  as  techniques,  simplicity the data  and  generated  ability with  models o f t e n p r o v i d e The  eventual  was  data  of data.  evaluation  of  transform  a n a l y s i s used simple  The  the RTD  probability  t h a t may  two  the  be  other  techniques.  which p e r m i t t e d  second  technique case  was  The a  rapid a rigorous  using  the F o u r i e r  g r a p h s of  paper  F value  analysis: the  of a p p r o x i m a t e l y  i g n o r i n g the. r e m a i n d e r  calculation  is illustrated  ignores  extended  the  response  followed a Gaussian  pseudo d i s p e r s i o n c o e f f i c i e n t  have a s u b s t a n t i a l pseudo-dispersion  tail  0.05  curves  the  t o 0.7.  the  calculated  3.  response  coefficient under  l e a d to q u a l i t a t i v e  closely  Therefore  distribution.  i n Appendix of  normal  distribution  c o u l d be of  on  f o r any  curve,  which  could  However,  c o n d i t i o n s , and i t s  conclusions.  F  This  i s a u s e f u l number f o r  different  a  Effectively i t  i n f l u e n c e upon s e l e c t i v i t y .  c o m p a r i s o n of m i x i n g trends  that  a n a l y s i s b a s e d upon  for a single  Non-rigorous The  curve  second  techniques.  (i)  f r o m an  The  assumption  assumption  of  data,  f o r l a r g e d i s p e r s i o n when  coefficient,  comparison  noisy  smooth e x i t .  implicit an  the advantages  better physical representations.  a non-rigorous  pseudo-dispersion  has  to handle  the  i s no  true, especially  -  first  i s d i s p e r s i v e i n nature,  from  first  the  the  s u p e r i o r i n that there  mixing far  two  279  the  - 280  (ii)  R i g o r o u s RTD Rigorous  requires the  -  computation:  computation  of  the RTD  combination.  Under s i m p l e r  circumstances this  involves  computing  of  division  of  responses,  inversion  of the r e s u l t i n g  However, n e i t h e r since  response  t h e p u l s e does not  transformations Luyben  the t r a n s f e r  —  finite  Fourier  and  the  RTD.  transform  to zero; t h e r e f o r e  i f a p r o c e s s has domain, and  t o g i v e a time G(iw),  a  be  J Q  the d e t e c t o r t o an  is  the  same a r b i t r a r y  Q  i s subjected to  domain o u t p u t x ( t ) ,  i s given  by  of t h e input,  and G ( t )  (t) e  (6.1) w  dt  x  a r e d e f i n e d i n F i g u r e 6.12a.  theory  of  response  treated.  transfer  ^  , Q and  the r i s e r ,  transforms,  b e f o r e the d a t a can  =  Convolution  of  transforms  - i w t ,. Jr°° x * /... (t) e dt  •  Q - iw  x  a  return  function,  x - IW  where x,  the two  frequency  a step type pulse Q(t)  G(iw)  has  are r e q u i r e d  i n the  the F o u r i e r  transform to y i e l d  (1973) shows t h a t  f u n c t i o n G(iw)  then  test  d e c o n v o l u t i o n o f t h e F c u r v e s f o r t h e d e t e c t o r and  riser/detector  both  for a pulse  shows t h a t  arbitary  i f Q(t)  i s the  response  s t e p i n p u t I ( t ) , and  riser/detector then G(iw)  combination  i s the  i s the time  frequency  response  t o an  to  i f x(t) the  response impulse  -  281  -  a.  xlt)  1 b.  Step Change  Column RTD=R1  *(  C4  •Sensor  Measured output  RTD=R2 Transform according to Tliwh x - iwj x*|tlexp(-iwt)dt ^7 and take IFT  Ttt)  T(t) >  A  T(t)  •  R1  R1.R2  t T(t)  For the sensor alone  A.  *L A T(t)  R2  From the transformed step responses for the isolated detector,R2, and detector-column combination, R1*R2, deconvolution gives: R1=|R1»R2I*J_ R2 Figure  6.12a  D e f i n i t i o n s of d e v i a t i o n t r a n s f o r m E q u a t i o n 6.1.  variables for  6.12b  P r o c e d u r e f o r i s o l a t i n g t h e c o l u m n RTD f r o m t h e i n d i v i d u a l s t e p r e s p o n s e s f o r t h e d e t e c t o r and d e t e c t o r column c o m b i n a t i o n s u s i n g deconvolution. IFT r e p r e s e n t s i n v e r s e F o u r i e r transform.  -  forcing  function; i . e . G(t)  distribution. that  This  deconvolution  transformed riser  of  the  responses  noise  not  the  m a g n i t u d e and  and  the  band-limited.  inaccurate  as  combination  is sufficient  was  curves  manually  digitised  frequency  This  t h e U.V.  versions  showing  and  to generate  the  filter.  6.13.  band-limited  by  recorder  and  the  by  and  segmented  that  using  the  POLFT, a v a i l a b l e i n the  especially  s u i t e d to non-band-limited  aliasing.  This  accurate  individual  G(iw)  routine  = x - iw  and  functions  J°° x * ( t )  e"  l w t  dt  and  using  high  pulsations,  using  versions  the  Dis  3,  computer  functions  inverse  and  of are  are  Fourier  gave what a p p e a r e d ,  transforms  response  UBC  output  to removing  transforms  a special  The  discrete Fourier  f o r run  subroutine,  shown t o be,  manually,  pressure  response  problem  solved  by  the  the  of  increasingly  i s approached.  smoothing  by  transforms  computation  transform  to generate  Smoothed  The  was  the  i s complicated  Fourier  i s p h y s i c a l l y equivalent  response,  Figure  6.1  the  frequency  minimised  from  that  of  f l u c t u a t i o n s , caused  pass  detector  fact  phase a n g l e  response  transforms.  Equation  T h i s makes a c c u r a t e  the N y q u i s t  problem of noise  the  time  i n F i g u r e 6.12b,  transformed  a p p l i c a t i o n of  are  in  residence  RTD.  measurement  low  i s the  is illustrated  detector  Practical  a  -  282  shown  not transform  library to  and  avoid  were  later  transforms  of  -  F i g u r e 6.13  283  -  i  i  i  1  0  1  2  3  1 4  1 5  1 6  Smoothed segmented F-curve responses s u i t a b l e f o r t r a n s f o r m i n g (dead time has been removed).  -  -  284  and G(iw)  These are  = Q -  iw  recognised  However, a l t h o u g h  i n the the  to generate  divided.  l w t  i . e . the E curves  and time  d e t e c t o r , and  useful  RTD's when t h e y  are  inverted*,  frequency,  result  and  Once t h i s  p r o b l e m was  i f they  high  becomes  in either  extremely  transform.  r e c o g n i s e d , a good a p p r o x i m a t i o n  coefficients  of  the q u o t i e n t at h i g h  extrapolating  the  real  are  imaginary  zero at  of t h e d i v i s i o n  to small a b s o l u t e e r r o r s  o b t a i n e d by  frequency  real  approach  sensitive  be  6.14. sufficiently  transforms  the F o u r i e r  be  are  T h i s o c c u r s because both  the  the  transforms  becomes i n a c c u r a t e a t h i g h  and  of  f o r the  can  domain, F i g u r e  individual  components of b o t h  could  dt.  e q u i v a l e n t to the t r a n s f o r m s  combination  as such  result  e~  of the F c u r v e s ,  detector/riser  the  Q*(t)  individually  derivatives  accurate  Q  and  to  frequency imaginary  •Whenever a F o u r i e r t r a n s f o r m was i n v e r t e d , i t was assumed t h a t the i n v e r s e t r a n s f o r m ( t h e RTD) was r e a l . Under t h e s e c i r c u m s t a n c e s the second h a l f o f the F o u r i e r c o e f f i c i e n t s a r e t h e complex c o n j u g a t e s o f the f i r s t h a l f . I f t h e r e i s any n o i s e , t h i s w i l l not o c c u r n a t u r a l l y when t h e d a t a a r e m a n i p u l a t e d i n any way ( e . g . , d i f f e r e n t i a t i o n i n the time domain which i s e q u i v a l e n t t o m u l t i p l i c a t i o n by 'iw' i n t h e f r e q u e n c y d o m a i n ) . T h e r e f o r e the c o n d i t i o n was enforced.  - 285  -  UJ  Time (s)  F i g u r e 6.14  E - c u r v e s f o r the d e t e c t o r ( c i r c l e s ) and d e t e c t o r / r i s e r combination ( s q u a r e s ) from the i n v e r s e t r a n s f o r m s of the transformed s t e p responses.  -  parts high  f r o m where t h e y frequency  zero.  Both  Figure  6.15,  Finally,  establish  the  was  the  6.16.  6.4  the  This  the  rigorous  resulting  comparison  RTD  RTD  of C a n k u r t  valid  in indicating  i n d i c a t e that  impression  of  the  riser.  Dispersion  density  of  column time  run  length.  total can  distribution  could  in this  was  the  made f o r run 6.17.  result shown i n to  Dis  Although  (1978) and  Yang  100  the kg/m  approximation  to  high  to  false our  suspension  over the  are  our  give a  i f applied  3.  the  (1983)  upstream d i s p e r s i o n ,  s u b s t a n t i a l at  give  To  manner,  helping  e a r l y r e s u l t s would  flow  alone.  Modelling  D i s 3 which a v e r a g e s A plug  riser  f u n c t i o n as  amount of m i x i n g be  suffice.  approach.  little  these  to  r e s p o n s e and  response  Yerushalmi very  approach  i n t h i s manner, i s  i s favourable,  the  to  zero r a p i d l y ,  obtained  determination  and  parts  f o r the  i s shown i n F i g u r e  tests  results  RTD  combined  of  RTD  detector  R e s u l t s , D i s c u s s i o n and A  frequency,  imaginary  corrected  the  the  validity  low  to approach  i s an  of  with  compared w i t h  establish  result  accuracy  and  at  e x t r a p o l a t i o n appeared  transform,  convoluted  Figure  The  linear  when the then  was  real  components a p p e a r so a  -  accurate,  where b o t h  inverted,  it  are  286  the  total  residence  large errors in conversion  and  selectivity. The using  RTD  of  Figure  6.17  a d i s p e r s i o n model.  i s not The  suitable for description  large standard  deviation  -  287  -  1.6  O  Re Im  t c o  x-1  •  1.2  c o a £ o  TRANSFORM EXTRAPOLATION REQUIRED IN THIS AREA BECAUSE SUBSTANTIAL ERRORS RESULT FROM DIVISION OF SMALL NUMBERS  o  LL  0.8  >» mm  CO  c 5S  CO  E ?  CO  0.4  "co <D  FREQUENCY =  (0.1  SECONDS)(ADDRESS-1)  4 8 1^ 16 P o i n t A d d r e s s In D i s c r e t e F T Figure  6.15  ?0  E x t r a p o l a t i o n of the r e a l and imaginary components of the F o u r i e r transform of the r i s e r RTD beyond t h e i r region of accuracy to approximate higher frequency components before inversion.  -  288  -  Time (s)  Figure  6.16  C o m p a r i s o n of the e x p e r i m e n t a l riser/detector c o m b i n a t i o n F - c u r v e w i t h the c o n v o l u t i o n o f c a l c u l a t e d r i s e r and d e t e c t o r RTD's s u b j e c t e d to a step i n p u t . Dead time has been removed. Comparison i s f a v o u r a b l e .  -  T  I  0  1  289  i  1  1  1  1  1  1  1  I  I  I  I  1  2  3  Time  Figure 6.17  -  r  L  4  (s)  RTD f o r run D i s 3 computed by d e c o n v o l u t i o n . Dead time removed.  -  290  -  about the mean would r e q u i r e a small a x i a l P e c l e t number, but t h i s i s i n c o n s i s t e n t with the small amount of dispersion.  A more a p p r o p r i a t e model, e.g.  a two-phase  model, s i m i l a r i n form to the model a p p l i e d by (1986) i n the bubbling and  forward  t u r b u l e n t regimes,  Yerushalmi would appear  to be needed. In a p p l y i n g the two the two  phase model to the f a s t bed  phases which are assumed are a core of  c r o s s - s e c t i o n through stagnant  annulus;  constant  which a l l of the gas passes,  this i s illustrated  regime,  and  i n F i g u r e 6.18.  a Both  the core and annular regions are assumed to be well mixed radially.  While t h i s r e p r e s e n t a t i o n i s o b v i o u s l y a  tremendous s i m p l i f i c a t i o n of the true flow s t r u c t u r e , there are a number of f a c t o r s which support in  this (i)  modelling the system  way: R a d i a l d e n s i t y d i s t r i b u t i o n s measured i n S e c t i o n 3.2.6  show a flow s t r u c t u r e with core and  s e c t i o n s where, at the f a i r l y  low  annular  densities  p r e v a i l i n g over much of the column (< 50 kg/m ), 3  the d e n s i t y i s d i s t r i b u t e d q u i t e uniformly radially  to w i t h i n approximately  20 mm  of the  wall. (ii)  Bierl mixing  et a l . (1980) i n d i c a t e that r a d i a l  gas  w i t h i n a core region i s r a p i d at high  solids fluxes,  supporting the approximation  well mixed core zone.  of a  -  291  -  Each Zone Well Mixed Radially  R<«-  1  AZ  C  /  \  c  i  u9  Stagnant Annulus  Figure  6.18  Plug Flow Core  Two zone model f o r gas m i x i n g i n a c i r c u l a t i n g f l u i d s e d bed. C = concentration i n annulus, C = c o n c e n t r a t i o n i n core, r = core r a d i u s , R = column r a d i u s , k = mass t r a n s f e r (crossflow) c o e f f i c i e n t . a  c  c  - 292 -  (iii)  Separate  experiments,  i n which  injected  continuously  a t t h e w a l l , and m e a s u r e d  at  p o i n t s on t h e w a l l u p s t r e a m and downstream,  showed up  t h a t gas f l o w a t t h e w a l l was  and down f o r s o l i d s  kg/m .  With  3  simplicity as (iv)  little  loadings  greater  information  suggested  modelling  periodically than  30  available, the annular  zone  stagnant.  A gas f l o w model o f t h i s requiring regions,  interchange  The model Horio  although not  (1986) f o r a c h o k i n g i s similar  and  annular  s u c c e s s f u l l y by B r i e n s and model,  to the freeboard  e t al_. ( 1 9 8 5 ) , d i s c u s s e d  w h i c h was r a t i o n a l i s e d quite  type,  between c o r e  has been used  Bergougnou (v)  t r a c e r was  i n S e c t i o n 4.1.4,  at that point  s u i t a b l e f o r the d i l u t e  model o f  regions  as b e i n g of a  c i r c u l a t i n g bed. Weaknesses o f t h e two zone model a r e : (i)  I t i s unsuited residence  times  gradients  i n the high  of  caused  by s u b s t a n t i a l v e l o c i t y density region  t h e bed and p e r h a p s o v e r  fraction (ii)  t o d e s c r i b i n g the spread i n  a t t h e base  a substantial  of i t s length,  As a p p l i e d i n t h i s characterised  study,  t h e two zone model i s  by two p a r a m e t e r s :  a crossflow  - 293  coefficient  and  the a n n u l u s . generated over  the  fractional  S i n g l e best  which o p t i m i s e  the e n t i r e  these  -  along  the  f i t of  the  However, i n  parameters almost  height  occupied  f i t parameters the  column.  area  certainly  to  are model  practice,  vary  column a c c o r d i n g  by  with  local  conditions. The with  equations  definitions  f o r the  of  the  two-zone model a r e  v a r i o u s symbols  given  given  below,  in Figure  6.18.  9C 3t  8 C  9  °_  a  ,  £K  +  r  2  k  ( c v  c  in  (R  that  both  (C  c  9 t  ) a'  c  rj  +  - r ) c  2  the v o i d a g e  c o r e and  C  (6.2)  0  v  '  impulse  column, u s i n g an residence and  time  a d d i t i o n of finite  distributions  annulus  the  approximately overall  were s o l v e d , w i t h  explicit  tracer  the  unity  low initial  t o the  base  of  difference routine  compared  the with  two  to  f o r v a r i o u s c o n d i t i o n s of  area.  p r e d i c t i o n s of  parameters are  (6.3)  annulus r e f l e c t i n g  o f an  The  =  i s assumed t o be  condition  crossflow  c 3Z  a  These e q u a t i o n s  give  c  - C ) = 0  density.  the  3C  c  r  a  Note  c  _  zone model w i t h  the measured  best f i t  residence  time  -  294  distribution  i n F i g u r e 6.19;  first  i s the best s o l u t i o n which could be obtained i f  result  a c o n t i n u i t y c o n d i t i o n was UgirR  2  = U  c l  rr  two  -  r e s u l t s are shown.  The  imposed, i . e . (6.4)  2 c  A b e t t e r r e s u l t could not be obtained because the requirement  that the t r a c e r begins to appear at  approximately conducive  1 second  demands a small annulus  to a l a r g e spread i n residence time.  the c o n t i n u i t y requirement, model somewhat, the second  which i s not By  removing  but i n doing so compromising r e s u l t was  the  obtained which shows  e x c e l l e n t agreement with the experimental  result.  The  agreement i s a c t u a l l y enhanced by some numerical d i s p e r s i o n i n the numerical s o l u t i o n causing some apparent  forward  d i s p e r s i o n which would not be evident with a more accurate numerical a stagnant  scheme. annulus,  Hence the simple core annular model, with i s not a p a r t i c u l a r l y good  r e p r e s e n t a t i o n of mixing p a t t e r n s i n the c i r c u l a t i n g  bed.  However, i t i s s u b s t a n t i a l l y b e t t e r than a plug flow approximation  or a simple d i s p e r s i o n model and  s t a r t i n g point f o r f u r t h e r  i s a useful  developments.  Major improvements i n the model can be v i s u a l i s e d i n c l u s i o n of d i s p e r s i o n i n the core and annulus v e l o c i t y g r a d i e n t s i n one  or both zones.  could i n c r e a s e the amount of backmixing  Any  by  and/or  one of these  without  compromising  - 295 -  Time (s)  F i g u r e 6.19  Comparison of the RTD f o r the r i s e r f o r run D i s 3 with best f i t p r e d i c t i o n s of the two zone model• a - c o n t i n u i t y obeyed, r = 0.059 m, k = 0.11 m/s b - c o n t i n u i t y r e l a x e d , r = 0.059 m, k = 0.08 m/s, U = 8.55 m/s c  c  c  -  the  continuity condition.  rewritten  t o account  below a r e v e r y coefficients  296 -  I n a d d i t i o n , t h e model s h o u l d  for local  important,  be  c o n d i t i o n s , which a s shown  r a t h e r than  using  lumped  t o d e s c r i b e t h e performance of the whole  column. A only  detailed  performed  residence  time  f o r run Dis3.  distribution Other  gas m i x i n g  analysed  i n terms o f t h e p s e u d o - d i s p e r s i o n  compared  on t h i s  coefficient for  runs  plotted  DisO  constant  basis.  and t o t a l  data  were  coefficient  and  F i g u r e 6.20 i s a g r a p h o f t h i s  against the t o t a l  through  a n a l y s i s was  to Dis7,  pressure  column p r e s s u r e  where gas v e l o c i t y  drop  was h e l d  d r o p and column e x i t  geometry  were v a r i e d . The  pseudo-dispersion  increasing exits to  mean s o l i d s  suspension  w h i c h were employed.  i n c r e a s e the suspension  wall  coefficient  as d e s c r i b e d  increases  d e n s i t y f o r both  Increased  mean h o l d - u p  d e n s i t y i n the v i c i n i t y  i n Chapter  i n c r e a s e d gas downflow and b a c k m i x i n g .  is  the dramatic  coefficient results to  of the e x i t  at constant  i n a dramatic  the abrupt  design  type  mean h o l d - u p .  increase i n axial under  of the appears of the  4, which i n t u r n i s r e f l e c t e d  in  effect  with  More  surprising  upon t h e d i s p e r s i o n The smooth dispersion  c o n d i t i o n s of i d e n t i c a l  exit relative total  hold-up. This general  observation  riser  c o u l d have i m p o r t a n t  reactor design.  implications for  In S e c t i o n 4.2 i t was  -  297 -  20  Total Pressure Drop (mm Hg)  Figure  6.20  P l o t of pseudo v e s s e l d i s p e r s i o n number (D/UgL) f o r a x i a l m i x i n g a g a i n s t p r e s s u r e d r o p i n a c i r c u l a t i n g bed o f sand, Ug = 7.1 m/s.  -  established ratios of  of  that the  exit  design  -  c o u l d be  used  internal-to-externalcirculation,  contactor requiring  performance. influences overall  298  different  I t i s now  the  gas  clear  mixing,  ratios  control  different  which  design  i n turn  also affects  r e a c t o r performance.  In an  effort  to understand  the  impact  of e x i t  geometry  upon d i s p e r s i o n , measurements were made o f a b s o l u t e fluctuations the D i s a total held  constant of  riser  difference two  m/s.  pressure  rises  In  mm  the  rapidly  total  of p r e s s u r e  t o be  different  gas  velocity  shows the plotted a  using  was  standard  against  the  dramatic  case  to a high  the  standard  value  d e n s e phase f o r m i n g abrupt  exit  distribution pressure  of  drop,  at the base  i s used,  solids  with  over  a lower  of  there i s the standard  fluctuations.  These o b s e r v a t i o n s  appear  for  illustrates  However, when the  The  distributor  The  fluctuations  smooth e x i t  w i t h a choked  phase m i x i n g .  transducer column.  d r o p and  much more u n i f o r m  deviation  the  above t h e  pressure  c o n d i t i o n s a t t h e base of t h e column f o r  very  column a t a g i v e n  which  533  F i g u r e 6.21  the p r e s s u r e  column.  very  a t 7.1  between  associated  a  drops over  exits.  deviation  the  riser  c a p a c i t a t i v e pressure  deviation  the  i n the  pressure  total  types  f o r optimum  t h a t the e x i t  a factor  to  appear  upward  and  primarily  t o be  related  to the  downward movements of r e s p o n s i b l e f o r the  net  gas  solids,  pressure  - 299  g u r e 6.21  -  V a r i a t i o n o f the s t a n d a r d d e v i a t i o n of a b s o l u t e p r e s s u r e f l u c t u a t i o n s near the base o f t h e c i r c u l a t i n g f l u i d i s e d bed w i t h t o t a l p r e s s u r e drop o v e r the u n i t f o r d i f f e r e n t exit geometries, c i r c l e s r e p r e s e n t abrupt e x i t , s q u a r e s smooth.  -  300  -  f l u c t u a t i o n s , a l s o generate s u b s t a n t i a l gas A l t h o u g h t h i s mechanism i s not the two  mixing.  completely consistent  with  zone model of m i x i n g developed e a r l i e r , which  assumes r a d i a l t r a n s f e r p r o c e s s e s to be r e s p o n s i b l e  for  m i x i n g , the d e f e c t s of the two  zone model were noted,  this result i s consistent  the d e f e c t s .  with  independent of the v a l i d i t y of the  and  However,  two-zone model, p r e s s u r e  f l u c t u a t i o n s can be r a t i o n a l i s e d as a means f o r g e n e r a t i n g dispersion, non-linear  and  i t appears t h a t the r e l a t i o n s h i p i s  under the c o n d i t i o n s  studied  here.  Large  f l u c t u a t i o n s at the base of the smooth e x i t column, caused by the b u i l d - u p of the choked dense phase, c r e a t e dispersion, are  few On  large  perhaps because they remain undamped —  there  s o l i d s above to p r o v i d e a damping e f f e c t . the o t h e r hand, w i t h  well distributed, creating  the abrupt e x i t , s o l i d s  damping of p r e s s u r e  are  fluctuations  which remain at t h e i r l o c a l l e v e l r e l a t e d to l o c a l s o l i d s motion. Gas  mixing i s evidently  considerably  more a t t e n t i o n .  r a i s e d some i n t e r e s t i n g (i)  a s u b j e c t which This preliminary  study  has  issues:  I t i s c l e a r t h a t gas  may  flow in a c i r c u l a t i n g (ii)  requires  E x i t geometry can  not be c l o s e to p l u g  bed.  s u b s t a n t i a l l y a f f e c t gas  because of i t s impact upon hydrodynamics.  mixing  - 301  (iii)  -  A two-zone model, m o d i f i e d velocity  f o r the  annulus or with  corrections,  may  w o r k a b l e gas  mixing  with model fast  solids  flow  t o i n c l u d e a phase  be  a viable  other  starting  model which  i s consistent  patterns, particularly  i s a p p l i e d t o t h e more d i l u t e bed.  point for a  i f the  regions of  a  -  7.  302 -  SUMMARY AND CONCLUSIONS  In summarising the r e s u l t s of t h i s t h e s i s we c o n s i d e r elements o f the I n t r o d u c t i o n and examine how our views have been m o d i f i e d  by the p r e s e n t experiments.  At the o u t s e t o f  the study i t was c l e a r t h a t s m a l l s c a l e c i r c u l a t i n g beds a r e i n f l u e n c e d by the w a l l where a l a y e r of h i g h d e n s i t y s o l i d s forms, but i t was u n c l e a r what the nature o f core was, how d r a m a t i c was the t r a n s i t i o n between core and a n n u l u s , and t o what e x t e n t  a g g l o m e r a t i o n accounted f o r many c i r c u l a t i n g bed  properties. Our  r e s u l t s , the measurements o f r a d i a l  density  p r o f i l e s and the development of an i n t e r m i t t e n c y  index,  suggest t h a t the w a l l i s s u b s t a n t i a l l y r e s p o n s i b l e c h a r a c t e r i s t i c d e n s i t y decay p r o f i l e s .  for the  Comparison w i t h  d e n s i t y p r o f i l e s from l a r g e u n i t s suggests t h a t t h i s may a l s o h o l d on s c a l e - u p .  The r e s u l t s c o n f i r m  strong  radial  v a r i a t i o n s i n d e n s i t y i n our 152 mm d i a . u n i t , w i t h t h e character  of the time-mean r a d i a l d e n s i t y p r o f i l e , and t h e  i n s t a n t a n e o u s f l u c t u a t i o n s i n d e n s i t y , changing markedly with height  and average s o l i d s l o a d i n g .  High l o a d i n g s a t  the base o f the v e s s e l produced a s t r o n g two phase character,  o r i n t e r m i t t e n c y , even on the c e n t r e l i n e of  r i s e r , which i s s u g g e s t i v e  o f aggregate f o r m a t i o n .  Also the  power spectrum was i n d i c a t i v e of a f a i r l y random s e r i e s of  -  aggregate f o r m a t i o n and  -  303  d e s t r u c t i o n processes  i m p l i e d i n the c l u s t e r models of h i g h fluidisation.  of the  velocity  However, t h i s two phase c h a r a c t e r on  c e n t r e l i n e r a p i d l y g i v e s way  type  the  t o a more homogeneous s t r u c t u r e  h i g h e r up the r e a c t o r as the s o l i d s d e n s i t y decays.  We  suggest t h a t c o m b i n a t i o n s of r a d i a l c o n v e c t i v e gas f l o w s turbulence  i n t e n s i t y g r a d i e n t s may  and  be r e s p o n s i b l e f o r  r e c t i f i c a t i o n of the s o l i d s p r o f i l e s to the more t y p i c a l l y core-annular  s t r u c t u r e found s e v e r a l metres above the  solids  r e t u r n , and have d e f i n e d f a s t f l u i d i s a t i o n as the decay zone where the r e c t i f i c a t i o n process  takes p l a c e .  From t h i s p e r s p e c t i v e , n e i t h e r a c o r e - a n n u l a r a c l u s t e r model can adequately d e s c r i b e a c i r c u l a t i n g bed.  model nor  be used c o n c e p t u a l l y to  However, i n view of  the  apparent importance of the r a d i a l n o n u n i f o r m i t y  to describe  many f a s t f l u i d i s e d bed phenomena, v a r i a n t s of  core-annular  models may The  be p r e f e r a b l e i n many  circumstances.  d e f i n i t i o n of a f a s t f l u i d i s e d bed as the d e n s i t y  r e c t i f i c a t i o n r e g i o n between a s y m p t o t i c  l i m i t s of choked  dense and d i l u t e phases d e l i n e a t e s f a s t f l u i d i s a t i o n from the c i r c u l a t i n g f l u i d i s e d bed. f l u i d i s a t i o n may  We  consider that  e x i s t as a r e g i o n of an o t h e r w i s e  r e a c t o r whereas a c i r c u l a t i n g bed,  fast choked  w i t h the p r o p e r t y  of  c o n t r o l l a b i l i t y , can o n l y e x i s t when f a s t f l u i d i s a t i o n o c c u r s over most of i t s l e n g t h .  Hence, we would d i s p u t e  the  - 304  -  d i s t i n c t i o n between choking and non-choking fundamental properties.  systems from a  s t a n d p o i n t based upon s o l i d s and However, an o p e r a t o r may  gas  observe c h o k i n g i n a  system w i t h l a r g e h e i g h t - t o - d i a m e t e r r a t i o when a dense phase of some n a t u r e , b u b b l i n g , s l u g g i n g or t u r b u l e n t ^ w i l l almost  fill  the r e a c t o r at a c r i t i c a l  solids  flux.  The phenomena of c h o k i n g , the s a t u r a t e d c a r r y i n g c a p a c i t y , e x i t e f f e c t s and gas mixing may  a l l be e x p l a i n e d  based on r a d i a l n o n - u n i f o r m i t y o r i g i n a t i n g at the w a l l .  In  the f i r s t two cases t h i s leads i n d i r e c t l y t o the i d e a of a unique  s t a b l e s t a t e f o r a g i v e n gas v e l o c i t y and  f l u x , o t h e r than at the choking f l u x i t s e l f ; cases r a d i a l n o n u n i f o r m i t y p r o v i d e s one  solids  i n the l a s t  plausible  e x p l a n a t i o n f o r t r e n d s and p h y s i c a l e f f e c t s .  E x i t s were  found t o have a s u b s t a n t i a l i n f l u e n c e on both the gas solid fluid  two  and  mechanics, and gas m i x i n g s t u d i e s showed RTD's  s u b s t a n t i a l l y d i f f e r e n t from p l u g f l o w . F i n a l l y , the t u r b u l e n t t r a n s i t i o n was to  s t u d i e d and  found  be a g r a d u a l t r a n s i t i o n to a more homogeneous s t a t e .  H o w e v e r , s u b s t a n t i a l dense phase homogeneity was u n t i l w e l l i n t o a t r a n s p o r t regime.  not a c h i e v e d  -  Discussion  tentative  in  their  were a v a i l a b l e ideas.  to  the  b a s e and  to  known t o  influence  The (i)  and  Summary  conclusions either  Hence, the  e x t e n s i o n s of  present  make use  of  unit.  fast  vary  optic  and  These data  at  the  patterns bed  (iii)  To  vary  height  of  otherwise the  diameter  now  constant of  ratio.  to  locations  to  established  within  regions,  and  mechanisms,  validity  their  in  determine  circulating  the  of  bed  of  at  UBC  the  dependence  upon  ratios,  diameter  variation  data  factors  components  choked  decay  the  establish  diameter  the  upon  beds.  help  are  and  c h o k i n g arguments w i t h to  the  existing  different  should  more c o n c r e t e  hence t o  height  the  technology  vertical  particles  fluidised  establish  and  focus p r i m a r i l y expand  are  data  d i s p r o v e many of  fluidised  fiber  radial for  thesis  are:  what c i r c u l a t i o n  To  or  work, t o  circulating  velocity  (ii)  this  e x p e r i m e n t s which c o n t r o l  determine  the  of  because i n s u f f i c i e n t  validate  recommendations  the  sections  recommendations  design  To  -  RECOMMENDATIONS  8.  The  305  of  the  circulating  conditions  decay  length  and with  to  bed  under  establish  length  to  -  (iv)  306  -  To p e r f o r m a l l t e s t s under exit  can be  conditions  readily characterised  (preferably  smooth) and where t h e s o l i d s f e e d characterized. unsuitable  current  feed  can a l s o geometry  be i s very  f o r v a l i d a t i n g mathematical  descriptions which  The  where t h e  o f t h e CFB  because  s o l i d s a r e f e d does  t h e manner i n  not g i v e  a known  initial distribution, (v)  To examine t h e p o s s i b i l i t y exits  using  a reflection  significance (vi)  c o e f f i c i e n t , and  of e x i t e f f e c t s i n l a r g e  be  under  controlled  introduced  To f u r t h e r  conditions  the  decay  where i t can  as t h e p r i m a r y a i r f o r t h e  the s t u d i e s  and m o d e l l i n g  system,  of the  m i x i n g phenomena t o p r o d u c e more r e a l i s t i c and  coefficients valid  scale  systems.  for local  of  units,  To examine t h e i m p a c t o f s w i r l a i r upon length  (vii)  of c h a r a c t e r i s a t i o n  use  gas models  i n large  -  307 -  NOMENCLATURE  A  Area  f o r heat  transfer  m  Ar  A r c h i m e d e s number d ( p ( p - p ) / P ) P g P g  a  Live wire radius  b  Sheath i n n e r  C  Capacitance of c o a x i a l capacitor  2  3  C  a  Tracer  C  c  Tracer  f o r c a p a c i t a n c e probe  radius  f o r capacitance  concentration concentration  probe  cylindrical i n annulus  i n core  m f mol/m  Diameter of c y l i n d r i c a l  D  Relative  D  Axial dispersion  dp  Mean p a r t i c l e d i a m e t e r  m  dp32  S a u t e r mean p a r t i c l e d i a m e t e r  m  E  Response t o u n i t function  impulse  F  Response t o u n i t  step  s  g H  Q  Solids  3  mol/m  D  G  column  m  m  permittivity coefficient  forcing  function  circulation flux  kg/m s 2  Acceleration  due t o g r a v i t y  Overall  transfer  heat  k  Mass t r a n s f e r  L  Total  1  Length c o o r d i n a t e  n  Richardson-Zaki  p  Pressure  Q  Heat  transfer  coefficient  (crossflow)  length  m/s  coefficient  m/s W/m  m/s m m  index  rate  K  W  2  -  308  -  R  R a d i u s o f c y l i n d r i c a l column  Re  R e y n o l d s number  r  Radius of core  m  t  Time  s  U  Local  c  U  fluid  m  velocity  m/s  Superficial  fluid  Ug  Superficial  gas v e l o c i t y  U f  Minimum f l u i d i s a t i o n v e l o c i t y  m/s  U  L o c a l v e r t i c a l component velocity  m/s  c  m  z  U'  Turbulent  TJ  Superficial  Vi  Modified  V  Single  r  velocity  fluctuating fluid  i n core  m/s  i n column  m/s  of f l u i d  velocity  velocity  p a r t i c l e terminal  particle  terminal  component  m/s  i n column  m/s  velocity  m/s  velocity  m/s  w  Frequency  H  Z  Height  m  Z  Q  Decay  coordinate length  defined  by L i and Kwauk  m  (1980)  Zj[  I n f l e c t i o n point height and Kwauk ( 1 9 8 0 )  a  Reflection  a  S o l i d s volume  fraction  S o l i d s volume  fraction  Y  Intermittency  index  E  Average voidage a column  e(r)  Local  a  ch  defined  by L i  coefficient  voidage  at choking  i n a cross  at radius r  s e c t i o n of  m  z  -  309 -  e  a  Limiting dilute  phase  e  Q  P e r m i t t i v i t y of f r e e space  e*  L i m i t i n g dense  y  V i s c o s i t y o f gas  p  Gas d e n s i t y  p P  e  L  Suspended Loose  phase  solids  packed  voidage N/m  2  voidage Pa.s  density  at exit  plane  bed d e n s i t y  pp  Particle/density  Psusp  Suspended s o l i d s d e n s i t y over cross s e c t i o n  kg/m  3  kg/m  averaged  kg/m  3  kg/m  3  o  p  Local  t i m e mean suspended  0  Standard  1  Prandtl  Annulus  c  Core  c ch  Cluster Choking  e  Exit  g or G  Gas  i  Modified  L  Liquid  mf  Minimum  o  Overall  p  Particle  density  kg/m  deviation mixing  length  Subscripts a  solids  m -  (letters)  w i t h column c o r r e c t i o n  fluidisation  - 310  s  Solids  sec  Above  t  Terminal  z  Vertical  secondary  a i r ports  component Subscripts -  0  -  Free  space  (numbers)  (permittivity)  REFERENCES Abed,  R. 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Further studies of t h e r e g i m e s o f f l u i d i z a t i o n , Powder T e c h n o l . , V o l . 24, 187-205. Y e r u s h a l m i , J . , C a n k u r t , N.T., G e l d a r t , D., and L i s s , B. (1978). Flow r e g i m e s i n v e r t i c a l g a s - s o l i d c o n t a c t s y s t e m s , A . I . C h . E . Symp., S e r . , No. 176, V o l . 74, 1-13. Y e r u s h a l m i , J . , T u r n e r , D.H., and S q u i r e s , A.M. (1976). The f a s t f l u i d i z e d bed, I n d . Eng. Chem., P r o c e s s , Des. Dev., V o l . 15, No. 1, 47-53. Yong, J . , Yu, Z., Wang, Z., and P i n g , C. (1986). A c r i t e r i o n f o r t r a n s i t i o n from b u b b l i n g t o t u r b u l e n t f l u i d i s a t i o n , i n " F l u i d i z a t i o n V," K. O s t e r g a a r d and A. S o r e n s o n e d s . , E n g i n e e r i n g F o u n d a t i o n , New Y o r k . Y o u s f i , Y., and Gau, G. (1974). Aerodynamique de l ' e c o u l e m e n t v e r t i c a l de s u s p e n s i o n s c o n c e n t r e e s g a z - s o l i d e s - I . Regimes d ' e c c u l e m e n t e t s t a b i l i t e a e r o d y n a m i q u e , Chem. Eng. S c i . , V o l . 29, 1939-1946. Yu,  J . (1986). A x i a l gas d i s p e r s i o n i n a c i r c u l a t i n g f l u i d i z e d bed, B . A . S c T h e s i s , UBC Dept. o f C h e m i c a l E n g i n e e r i n g , Vancouver.  Zenz,  F.A. (1949). Two-phase f l u i d - s o l i d f l o w , Industrial and E n g i n e e r i n g C h e m i s t r y , V o l . 41, No. 12, 2801-2806.  Zenz,  F.A., and Othmer, D.F. (1960). F l u i d i z a t i o n and f l u i d - p a r t i c l e systems, R h e i n h o l d P u b l i s h i n g Corp., York.  New  - 322 -  APPENDIX 1  Sample Output from Computer Program BMP:02T  BMOO2T-AUT0COVARIANCE HEALTH  SCIENCES  AND  POWER  COMPUTING  SPECTRAL  FACILITY.  P  INPUT  DATA  INPUT  SERIES  TO  BE TO  ANALYSIS  -  REVISED  JUNE  19.  1972  UCLA  R  O  PRINTED BE  B  L  E  M  N  U  M  B  E  R  TR9A  OUT  PLOTTED  OUT--.-  0ETRENDIN8 PREWHITENING VALUE  OF  CONSTANT  C •  USED  IN  NUMBER  OF  SERIES  NUMBER  OF  OATA  POINTS  -  1000  NUMBER  OF  LAGS  CHOSEN  -  50  THE  PREWHITENING  TRANSFORMATION  Z ( T ) - X ( T * 1 ) - C X ( T )  - - -  0.0  1 I 03  to NUMBER USE  OF  PREVIOUS  CONSTANT  NUMBER  VARIABLE  FORMAT  SELECTION  IS  (F12.7)  OF  TIME  CARDS  -  1  £0  DATA  I  INTERVAL  VARIABLE  •  FORMAT  0.02000  CARDS  •  SECOND  1  AUTOCOVARIANCE  LAG  OF  (SECONO)  SERIES  17743.6  0 . 0 0.0200  11450.3  0.0400  8288.98  0.0600  6659.96  O.OBOO  5710.32  0.  5318.36  1000  0.1200  4478.52  O.  1400  3831.08  0.  1600  3209.43 2406.84  O.1800  2088.13  0.2000 O.  1956.59  2200  1174.17  0.2400  969.828  0.2600  708.401  0.2800  778.518  0.3000 O.  934.609  3200  564.531  0.3400  398.910  0.3600  245.367  0.3B00  222.414  0.4000 0.4200 0.4400 0.4600 0.4800  102.781 -219.187 -833.458 -1218.93  0.5000  -  0.5200  -916.927  0.5400  -781.072  1141.97  0.5600  -981.011  0.5800  -  0.6000  -930.207  0.6200  -587.962  0.6400  -880.839  0.6600  -829.666  0.6800  -1224.45  0.7000  -  0.7200  -1146.63  1140.38  1121.99  0.7400  -546.295  0.7600  -35.6170  0.7800  -391.193  0.8000 0.8200 0.8400 0.8600 0.8800 0.9000 0.9200 0.9400 0.9600  -612.881 -376.483 -330.808 -716.045 -597.059 -463.235 -361.000 -672.605 -451.567  0.9800  68.4087  1.0000  -221.269  1  GRAPH  OF AUTOCOVARIANCE  FUNCTION OF SERIES  2000.000 0.0  1 PLOTTED  AGAINST  6000.000 4000.000  TIME  UP TO A LAOOF  10000.000 8000.000  1.0000  SECOND  14000.000 12O00.O00  18000.000 16000.000  .•....+....+....+....+....+....+....+....+....•....+....+....+....+....•....*•....+....+....+....+. - 0 . 0  - 0 . 0  -0.020  - 0 . 0 2 0  -0.040  -O.040  -0.060  -0.060  -0.080  - 0 . 0 8 0  -o.too  -O.100  120  -O -O -0 -O -O -O -O -O -O -O  140 160 180 -0.200 -0.220 -O.240 -O.260 -O.280 .300  .. 1 2 0 .. 1 4 0 .. 1 6 0 .. 1 8 0 .. 2 0 0 .. 2 2 0 .. 2 4 0 .. 2 6 0 .. 2 8 0 . .300  . 320  - 0 . 3 2 0  . 340  - 0 . 3 4 0  .360  - 0 . 3 6 0  .380  - 0 . 3 8 0  .400  - 0 . 4 0 0  .420  - 0 . 4 2 0  .440  - 0 . 4 4 0  . 460  - 0 . 4 6 0  .480  -0.480  .500  - 0 . 5 0 0  .520  - 0 . 5 2 0  .540  - 0 . 5 4 0  .560  -0.560  .980  -0.580  .600  - 0 . 6 0 0  .620  -0.620  .640  -0.640  .660  -0.660  .680  -0.680  .700  - 0 . 7 0 0  -0.720  - 0 . 7 2 0  -0.740  - O . 7 4 0  .760  -0.760  .780  -0.780  .800  - 0 . 8 0 0  .820  - 0 . 8 2 0  .840  - 0 . 8 4 0  .860  - 0 . B 6 0  .880 -0.900  - 0 . 8 8 0 •  - 0 . 9 0 0  -0.920  -0.920  -0.940  -0.940  -0.960  -O.960  -0.980 -1.000  - 0 . 9 8 0 •  -l.OOO  2000.000 0.0  6000.000 4000.000  10000.000 8000.000  14000.000 12000.000  18000.000 16000.000  00  FREQUENCY (CYCLES/SECOND) 0.0 0.500 1 .000 1 .500 2.000 2.500 3.000 3.500 4.000 4.500 5.000 5.500 6. OOO 6. SOO 7.000 7. SOO 8.000 8.500 9.000 9.500 to.000 10.500 11.000 1 1 .500 12.000 12.500 13.000 13.500 14.000 14.500 15.000 15.600 16.000 16.500 17.000 17.500 18.000 18 .500 19.000 19.500 20.000 20. BOO 21 .000 21 .500 22.000 22.500 23.000 23.500 24.000 24.500 23.000 THE CHECK SUM OF POWER SPECTRAL ESTIMATES IS THE DIFFERENCE IS-O.3476563  POWER SPECTRAL ESTIMATES OF SERIES 1 780.2102 825.6116 706.3047 448.7271 313.6892 270.6157 218.6600 158.8460 116.6621 1tO.8536 133.2542 117.364 1 100.4470 101.1807 95 .08351 92 .33142 83 .73883 70 .81042 88 .69572 93 .05856 66 . 18443 SO .28069 50 .96480 50 .74872 50 .61479 45 . 30302 52 .12030 47 .39372 39 .26118 41 .44556 34.03842 32 .89920 31 .51137 28 .52795 27..24739 29..98529 36. 88815 40. 94727 37. 36729 37. 59845 31 .02040 25. 02356 25. 48636 27. 85072 27. 36684 32. 53157 28. 86046 23. 25368 24.94315 23. 90150 20. 87764 17743.30  AND SHOULD BE  17743.64  GRAPH  O F T H E POWER  SPECTRAL  100.000 0.0  ESTIMATES  OF SERIES  1 AGAINST  300.000 200.000  FREQUENCY  500.000 400.000  (CVCLES/SECOND)  700.000 600.000  900.000 800.000  ..+....+....+....+....+....+....+....+.... ....+.... .... .... '.... ....*.-..*-... .... .... .. +  +  +  +  +  +  +  4  - 0 . 0  - 0 . 0  -O.  -O.  500  500  -1  .000  - 1 . 5 0 0  -1  .500  - 2 . 0 0 0  - 2 . 0 0 0  -1  .000  - 2 . 5 0 0  - 2 . 5 0 0 - 3 . 0 0 0  - 3 . 0 0 0  - 3 . 5 0 0  - 3 . 5 0 0  - 4 . 0 0 0  -4  - 4 . 5 0 0  - 4 . 5 0 0  - 5 . 0 0 0  -5.OOO  .000  - 5 . 5 0 0  - 5 . 5 0 0  - 6 . 0 0 0  - 6 . 0 0 0  -6 . 500  - 6 . 5 0 0  - 7 . 0 0 0  -7.OOO  - 7 . 5 0 0  - 7 . 5 0 0  -8.OOO  - 8 . 0 0 0  - 8 . 5 0 0  - 8 . 5 0 0  - 9 . 0 0 0  - 9 . 0 0 0  - 9 . 5 0 0  - 9 . 5 0 0  -10.000  - 1 0 . 0 0 0  - 1 0 . 5 0 0  - 1 0 . 5 0 0  -11.000  -11.  -11.500  - 1 1 . 5 0 0  -12.000  - 1 2 . 0 0 0  -12.500  - 1 2 . 5 0 0  -  13.000  -  13.500  -  14.000  -13.  - 1 4 . 0 0 0 - 1 4 . 5 0 0 - 1 5 . 0 0 0  - 15 . 0 0 0  - 1 5 . 5 0 0  -15.500  -16.OOO  16.000  - 1 6 . 5 0 0  -16.500  - 1 7 . 0 0 0  -17.000  - 1 7 . 5 0 0  -17.500 -  OOO  - 1 3 . 5 0 0  -14.500  -  OOO  - 1 8 . 0 0 0  18.000  - 1 8 . 5 0 0  -18.500  -19.OOO  - 19 . 0 0 0  - 1 9 . 5 0 0  -19.500  - 2 0 . 0 0 0  -20.000  - 2 0 . 5 0 0  -20.500  - 2 1 . 0 0 0  -21.000  - 2 1 . 5 0 0  -21.500  - 2 2 . 0 0 0  -22.000  - 2 2 . 5 0 0  - 2 2 . 5 0 0  - 2 3 . 0 0 0  -23.000  - 2 3 . 5 0 0  -23.500  - 2 4 . 0 0 0  - 2 4 . 0 0 0  - 2 4 . 5 0 0  - 2 4 . 5 0 0  - 2 5 . 0 0 0  - 2 5 . 0 0 0  100.000 0.0  300.000 200.000  500.000 400.000  700.000 600.000  900.000 800.000  bo  POWER SPECTRAL ESTIMATES OF SERIES  3.200  3600  1  4.000  PLOTTED IN A LOG SCALE AGAINST FREQUENCY (CYCLES/SECONO) 4.400  4.800  5.200  5.600  6.000  6.800  6.400  .. ....•....+....+....+....+....•....+....•....+....•....+....+....+....+....+....+....•....+. +  -0 0 -O 50O - 1 .000 - 1 . 500 -2 .000 -2 .500 -3 .000 -3 .500 -4 .000 -4 .500 -S OOO -9 .500 -6 .000 -6 .500 -7 .000 -7 .500 • -8 .000 -8 .500 -9 .000 -9. .500 -10. .000 • -10. .500 -11. .000 -11. .500 -12. .000 . -12. .500 < -13. 000 . -13. 500 . -14. 000 . - 14 . 500 . 000 < - 15. 500 . - 15. OOO . - 16. SOO . 16 . OOO . 17 . 500 * 17 . 000 . 18. 500 . 18 . 000 . t9. 500 . 19. 000 • 20. 500 . 20 000 . 21 . 500 . 21 . 000 . 22. 500 • 22. 000 . 23. 500 . 23. 000 . 24. 500 . 24 . 000 + 25 .  •  +  -0.0 - O . 500 -1 . OOO -1 .500 -2 ..000 -2 ..500 -3. .000 -3 ..500 -4 .000 . -4 ..500 -5. .000 -5.500 -6.000 -6.500 -7.000 -7. SOO -8.000 -8.500 " -9.000 -B.500 -10.000 -10.500 -11.000 -11.500 -12.000 -12.500 -13.000 -13.500 -14.000 -14.500 -15.000 -15.500 -16.000 -16.500 - 17.000 - 17.500 -18.000 -18.500 -19.000 -19.500 -20.000 -20.500 -2 1.OOO -21.500 -22.000 -22.500 -23.000 -23.500 -24.000 -24.500 -25.000  .. + . . . . • . . , . • . . . . + . . . . + . . . . + . . . . + ....•....+ . . . . + . . . . • . . . . • . . . . + . . . . + . . . . - « . . . . . + . . . . • . 3.600 4.400 5.200 6.000 6.800 4.000 4.800 S.600 6.400  w to oo  -  329  -  APPENDIX 2  Estimation on  of the  F l u c t u a t i n g V e l o c i t y Component  the C e n t r e l i n e of a Single-Phase (Ug  = 6.5  m/s,  Assuming P r a n d t l ' s relatively we  isotropic  D =  mixing  turbulence  .152  length on  m,  Pipe  Flow  NTP)  theory  the  for Air  to hold  for  c e n t r e l i n e of  a  the  pipe  write  (U'(r))  At power  the  low  2  and  mixing  position,  (A2.1)  £ ( 2  prevailing  (Schlicting, the  =  the  but  result  1979).  length  R e y n o l d s number, 66,000, b o t h  The  the  of N i k u r a d s e h o l d  approximately  latter  f o r smooth  i s dependent  independent  states that on  diameter  and  1/7  pipes  radial  of R e y n o l d s number o v e r a wide  range o f R e y n o l d s numbers.  On  the  pipe  c e n t r e l i n e 2i/D  =  0.14. The  1/7  power  law  gives  U (r) z  U  (A2.2)  c  where n = 7 and  where U  c  i s the  centreline velocity  given  -  330 -  by XT ~ (n + l ) ( 2 n + 1)  (A2.3)  n  Substituting U  = 6.5 m/s,  g  n = 7 g i v e s U = 7.96 m/s  and  7?56Now  <S> '  =  (A2.4)  d i f f e r e n t i a t i n g gives  dU ( r ) -fr—  -6/7  1  7 7177- * < '  =  7  9 6  >  <  A 2  ' > 5  and s e t t i n g r = R = .076 m g i v e s  dU ( r ) \ T  i  Q  7  = j ( T^)  Finally, substituting  (U'(r))  a  = 14.96 m/s  (A2.6)  2  i n t o A2.1,  - M524) (0.14) (2")  and U ( r ) = 0 .16 m/s. 1  ,  6  J  2  2  (14.96)  2  - 331 -  APPENDIX 3  Computation  of Pseudo-Dispersion C o e f f i c i e n t s F-Curve  The  this  Data  F - c u r v e s can be d e s c r i b e d  distribution  i n t h e range F = 0.1  a p p r o x i m a t e l y by a normal t o F = 0.7  i s shown i n F i g u r e A3.1 below  response paper. normal  for a typical Assuming  from  i n which  run i s p l o t t e d  (Yu, 1985); the F-curve  on normal  probability  the remainder of the response to f o l l o w a  distribution  gives  the standard  deviation  for this  d i s t r i b u t i o n as:  c  (detector  since and  & riser)  t h e r e a r e 1.91  70th p e r c e n t i l e s  same a s s u m p t i o n s in  residence  =  t  (  F  =  * i  standard  g"i  ?  t ( F  =  '  deviations  o f a normal  (A3.1)  )  between t h e 1 0 t h  distribution.  f o r the detector  t i m e s due t o r i s e r  1  alone allows  dispersion  Making t h e the v a r i a n c e  t o be  calculated  as: 2 a (riser) Finally, dispersion  = a  2  (detector  following  + riser)  Levenspiel  number" i s r e l a t e d  2 - a" ( d e t e c t o r ) 1  (1972), the ' v e s s e l  t o t h e v a r i a n c e by:  (A3.2)  -  / D v _ a  (riser)  JTL g  ~=2  {  )  ~  332  -  (A3.3)  2t  where t i s the mean v e s s e l  r e s i d e n c e time.  The a x i a l  number i s the i n v e r s e of the " v e s s e l d i s p e r s i o n  Peclet  number."  - 333  99.9  1  i  1  -  i  i  i  i  G G  7  95 98  0°°  IN OUTLET  95  So  G eo  <  TO  O  HELIUM  63  / G  SO  to  20  10  s  p  /  G  -  / G / / G  20  O  .0  /  / / ~ G /  -  G  -  2 f  2-ZS  .25  Figure  A3.1  i  TIME l i 3• s e c  >hi3  i  IOU  i  IZ-13  i  /*X3  F - c u r v e r e s p o n s e o f : s y s t e m and d e t e c t o r on normal p r o b a b i l i t y paper (Yu, 1985), 3.5 m/s, G = 30 kg/m s, a l u m i n a ) . s  /t-2_J  plotted (Ug =  

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