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Continuous spouted bed process for sulphur-coating urea Tsai, Bobby S. E. 1986

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CONTINUOUS SPOUTED BED PROCESS FOR SULPHUR-COATING UREA by BOBBY S.E. TSAI A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES Chemical E n g i n e e r i n g We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1986 © Bobby S.E. T s a i , 1986 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the The U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree that permission f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Chemical E n g i n e e r i n g The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: June, 1986 ABSTRACT Sulphur-coated urea i s a c o n t r o l l e d - r e l e a s e , n i t r o g e n f e r t i l i z e r which has proven to be s u c c e s s f u l commercially. Current p r o d u c t i o n technology used by Canadian I n d u s t r i e s L t d . and other firms i s based on the Tennessee V a l l e y A u t h o r i t y r o t a r y drum pr o c e s s . T h i s process i s mech a n i c a l l y complex and the sulphur coated urea r e q u i r e s an a d d i t i o n a l wax coat to achieve the d e s i r e d s l o w - r e l e a s e p r o p e r t i e s . By c o n t r a s t , the spouted bed c o a t i n g process developed at UBC promises to be m e c h a n i c a l l y s i m p l e r . A f e r some i n i t i a l d i f f i c u l t i e s , s u c c e s s f u l continuous c o a t i n g was achieved i n the present t h e s i s p r o j e c t by means of a mo d i f i e d l a b o r a t o r y spouted bed f a c i l i t y . The f a c i l i t y can produce 9.6 kg/h of sul p h u r - c o a t e d urea f o r up to 3 hours. Coating i s performed i n a spouted bed, which c o n s i s t s of a c y l i n d r i c a l column (0.154 m I.D. by 0.91 m high) and a 60° c o n i c a l base. Urea i s f e d i n t o the bed through a feed tube i n s e r t e d i n t o the bed annulus. Molten sulphur i s sprayed through a nozzle i n t o the base of the bed c o - c u r r e n t l y with the spouting a i r . Product i s d i s c h a r g e d from s l o t s around the perimeter of the spouting column and c o l l e c t e d i n a product c o o l e r . P a r t i c u l a t e s from the o f f - g a s l e a v i n g the spouted bed and the c o o l e r are removed by a cyc l o n e and water scrubber. i i Product q u a l i t y i s determined by a r a p i d d i s s o l u t i o n t e s t developed at UBC and a 7-day d i s s o l u t i o n t e s t developed by the TVA and m o d i f i e d at UBC. The product q u a l i t y i s found to depend on the bed temperature and the bed depth. In the range of 55 to 90°C the d i s s o l u t i o n value decreases up to a bed temperature of approximately 80°C and then i n c r e a s e s a g a i n . C oating performed at two d i f f e r e n t bed depths, 0.15 m and 0.25 m, show that s u p e r i o r product q u a l i t y i s achieved with the shallower bed. A set of optimal c o a t i n g c o n d i t i o n s i s found: c o a t i n g temperature of 80°C, bed depth of 0.15 m, spouting a i r flow 3 r a t e of 0.65 m /min and a t o m i z i n g a i r pressure of 208 kPa. Under these c o n d i t i o n s , m o d i f i e d 7-day d i s s o l u t i o n values of about 30% are found f o r the UBC product compared to 88% f o r the CIL product with s i m i l a r sulphur content and no wax. When coated with 30 wt% or more sulphur, the UBC product i s able to meet the i n d u s t r i a l standard of 25% of urea d i s s o l v e d i n 7 days. T a b l e o f C o n t e n t s A B S T R A C T i i L I S T OF T A B L E S v i L I S T OF F I G U R E S v i i A c k n o w l e d g e m e n t x i 1 . INTRODUCTION 1 1 . 1 THE T V A PROCESS 2 1 .2 T H E UBC SPOUTED BED PROCESS 5 1 .3 O B J E C T I V E S OF T H I S T H E S I S 7 2 . L I T E R A T U R E REVIEW 9 2.1 T H E TVA PROCESS f o r SULPHUR C O A T I N G o f UREA 9 2 . 1 . 1 T V A PROCESS DEVELOPMENT 9 2 . 1 . 2 TVA DEMONSTRATION S C A L E P L A N T 22 2 . 2 THE UBC PROCESS 24 2 . 2 . 1 THE IMPROVED UBC PROCESS 38 2 . 3 S E L E C T P H Y S I C A L P R O P E R T I E S OF SULPHUR 45 3 . E X P E R I M E N T A L APPARATUS 49 3.1 THE SPOUTED BED 49 3 . 2 SULPHUR SUPPLY S Y S T E M 52 3 . 3 N O Z Z L E ASSEMBLY 59 3 . 4 UREA F E E D I N G D E V I C E 62 3 . 5 PRODUCT WITHDRAWAL D E V I C E 64 3 . 6 PRODUCT C O L L E C T O R S 66 3 . 7 DUST C O L L E C T O R S 67 3 . 8 A I R , S T E A M , a n d WATER S U P P L I E S 67 4 . E X P E R I M E N T A L PROCEDURES 72 4 .1 C O A T I N G PROCEDURES 72 4 . 1 . 1 ST A RT UP 72 i v 4 . 1 . 2 C O A T I N G 73 4 . 1 . 3 SHUT DOWN 75 4 . 2 MEASUREMENTS OF O P E R A T I N G V A R I A B L E S 75 4 . 3 PRODUCT Q U A L I T Y A N A L Y S I S 77 4 . 3 . 1 T O T A L SULPHUR CONTENT 77 4 . 3 . 2 M O D I F I E D 7 - D A Y D I S S O L U T I O N T E S T 77 4 . 3 . 3 R A P I D D I S S O L U T I O N T E S T 81 5 . O P E R A T I N G E X P E R I E N C E 85 5.1 SULPHUR SUPPLY SYSTEM 85 5 . 2 F E E D I N G D E V I C E 88 5 . 3 WITHDRAWAL D E V I C E S 89 5 . 4 A T O M I Z I N G AIR L I N E 90 6 . R E S U L T S AND D I S C U S S I O N 91 6.1 PRODUCT Q U A L I T Y 92 6 . 1 . 1 I N A D E Q U A T E L Y COATED PRODUCT 92 6 . 1 . 2 H E A V I L Y COATED PRODUCT 100 6 . 2 O P T I M A L O P E R A T I N G TEMPERATURE 103 6 . 3 E F F E C T OF BED D E P T H 106 6 . 4 COMPARISON BETWEEN UBC AND C I L PRODUCTS 109 7 . CONCLUSIONS AND RECOMMENDATIONS 114 7.1 LABORATORY T E S T S 114 7 . 2 RECOMMENDATIONS 114 8 . R E F E R E N C E S 118 A P P E N D I X I 120 A P P E N D I X I I 1 29 v LIST OF TABLES Table 2.1 Optimum Operating C o n d i t i o n s f o r Sulphur C o a t i n g of Urea i n the TVA 900 kg/hr P l a n t 20 Table 2.2 Summary of D i s s o l u t i o n Rates f o r Pneumatic and H y d r a u l i c Atomizations 21 Table 2.3 Main Process V a r i a b l e s and Product C h a r a c t e r i s t i c s of TVA Demonstration P l a n t (Cap. 9.07 t/hr) 25 Table 2.4 P h y s i c a l and Chemical P r o p e r i t i e s of Sulphur.... 46 Table 2.5 P r o p e r t i e s of Common Sulphur A l l o t r o p e s 47 Table 4.1 F l u i d Flow Measurement Equipment- Rotameters.... 75 Table 6.1 Runs from the UBC F a c i l i t y Which Had 7-Day D i s s o l u t i o n Values of 25% or Less 113 Table 1.1 D i s s o l u t i o n Test R e s u l t s 121 Table 1.2 Operating C o n d i t i o n s 124 v i LIST OF FIGURES F i g u r e 1.1 Flowsheet of TVA Process f o r Sulphur C o a t i n g of Urea (C a p a c i t y 900 kg/hr) 3 F i g u r e 1.2 UBC Spouted Bed 6 F i g u r e 2.1 TVA Process f o r Sulphur Coating of Urea (136 kg/hr F a c i l i t y ) 12 F i g u r e 2.2 I n t e r n a l s of the C o a t i n g Drum (a) Pneumatic Spraying 16 (b) H y d r a l i c Spraying 16 F i g u r e 2.3 Sulphur Spray Header System and Spray Nozzle D e t a i l (a) Pneumatic Spraying 17 (b) H y d r a u l i c Spraying 17 F i g u r e 2.4 E f f e c t of Sub s t r a t e Temperature on Urea D i s s o l u t i o n Rates f o r TVA Process 19 F i g u r e 2.5 TVA Demonstration Scale P l a n t f o r Sulphur Coating of Urea (9.07 t/hr) 23 F i g u r e 2.6 UBC Spouted Bed f o r Sulphur C o a t i n g of Urea (designed by Mathur and Meisen) 26 F i g u r e 2.7 UBC Spouted Bed f o r Sulphur C o a t i n g of Urea (designed by Mathur, Meisen and Zee) 28 F i g u r e 2.8 Nozzle Arrangement (Mathur, Meisen & Zee) 30 F i g u r e 2.9 UBC Spouted Bed f o r Sulphur Coating of Urea (designed by Mathur, Meisen & Lim) 32 F i g u r e 2.10 Steam Heated Sulphur M e l t e r (Mathur, Meisen & Lim) 34 F i g u r e 2.11 Nozzle Arrangement v i i (Mathur, Meisen & Lim) 35 F i g u r e 2.12 S i m p l i f i e d Diagram of UBC Spouted Bed F a c i l i t y f o r Batch-mode Coating (designed by Meisen and Weiss) 39 F i g u r e 2.13 E f f e c t of Bed Temperature on T>^^ 41 F i g u r e 2.14 E f f e c t of Sulphur Flow on D 2 5 43 F i g u r e 2.15 E f f e c t of Atomizing A i r Flow Rate on 44 F i g u r e 2.16 Sulphur V i s c o s i t y as a F u n c t i o n of Temp 48 F i g u r e 3.1 S i m p l i f i e d Diagram of UBC Spouted Bed F a c i l i t y f o r Continuous Coating (designed by Meisen and T s a i ) 50 F i g u r e 3.2 Spouted Bed Column 51 F i g u r e 3.3 Shutter Assembly (Mathur, Meisen & Lim) 53 F i f u r e 3.4(a) S e c t i o n a l View of Sulphur M e l t e r 54 F i g u r e 3.4(b) Sulphur Melter Top View 55 F i g u r e 3.5 S e c t i o n a l View of Sulphur F i l t e r 57 F i g u r e 3.6 General View of Sulphur Rotameter 58 F i g u r e 3.7 P e r f o r a t e d P l a t e , Upper Flange and Nozzle S e c t i o n a l View 60 F i g u r e 3.8 S e c t i o n a l View of Nozzle Arrangement and General Assembly 61 F i g u r e 3.9 Urea Feeding Arrangement.. 63 F i g u r e 3.10 S e c t i o n a l View of Withdrawl Device 65 F i g u r e 3.11 Dust C o l l e c t i o n System 68 F i g u r e 3.12 A i r L i n e s 69 F i g u r e 3.13 Steam and Condensate System 71 F i g u r e 4.1 Thermocouple L o c a t i o n 78 v i i i F i g u r e 4.2 Test Tube Used i n M o d i f i e d 7-Day D i s s o l u t i o n Test 80 F i g u r e 4.3(a) General View of the Apparatus Used i n the RDT 82 F i g u r e 4.3(b) F r i t t e d G l a s s Tube Used in the RDT 83 F i g u r e 6.1 D i s s o l u t i o n Values Measured i n the RDT f o r UBC Run #10 and CIL Products 93 F i g u r e 6.2 Sulphur Content D i s t r i b u t i o n f o r UBC Run #10 and CIL 94 F i g u r e 6.3 D i s s o l u t i o n Values Measured i n the RDT fo r UBC Run #19A and CIL Products 96 F i g u r e 6.4 Sulphur Content D i s t r i b u t i o n f o r UBC Run #25c and CIL 98 F i g u r e 6.5 D i s s o l u t i o n Values Measured i n the RDT f o r UBC Run #25C and CIL Products 99 F i g u r e 6.6 Sulphur Content D i s t r i b u t i o n f o r Samples of 1,2 and 3 h of Continuous Operation 102 F i g u r e 6.7 D i s s o l u t i o n Values as a Fun c t i o n of Bed Temperature Measured i n the RDT 104 F i g u r e 6.8 D i s s o l u t i o n Values as a Function of Bed Temperature Measured i n the M7T 105 F i g u r e 6.9 D i s s o l u t i o n Values as a Function of Bed depth Measured i n the RDT 107 F i g u r e 6.10 D i s s o l u t i o n Values as a Fun c t i o n of Bed Depth i n the M7T 108 F i g u r e 6.11 Product Q u a l i t y Comparison f o r UBC Run #25B, #25C, #3!b and CIL Products ix Measured i n the RDT 110 Fi g u r e 6.12 Product Q u a l i t y Comparison f o r UBC Runs #25B, #25C, #30B, #31A, #31B and CIL Products Measured i n the M7T 112 Fi g u r e II.1 C a l i b r a t i o n Curve f o r C o o l i n g A i r Rotameter 130 Fi g u r e II.2 C a l i b r a t i o n Curve f o r Spouting A i r Rotameter 133 Fi g u r e II.3 C a l i b r a t i o n Curve f o r Atomizing A i r Rotameter 132 Fi g u r e II.4 C a l i b r a t i o n Curve f o r Sulphur Rotameter 133 Fi g u r e II.5 Abbe RefTactometer C a l i b r a t i o n Curve f o r Aqueous S o l u t i o n s of Urea 134 x Acknowledgement I wish to thank Dr. Axel Meisen, under whose s u p e r v i s i o n and guidance t h i s i n v e s t i g a t i o n was undertaken, f o r h i s a d v i c e and encouragement in a l l stages of t h i s work. I a l s o wish to thank Mr. Van Quang Le, Mr. V i c t o r Lee and Mr. Samson Tom f o r t h e i r a s s i s t a n c e i n o p e r a t i n g the experimental apparatus and t e s t i n g the product q u a l i t y . The a s s i s t a n c e p r o v i d e d by the personnel of the Workshop and the S t o r e s of the Chemical E n g i n e e r i n g Department i s h i g h l y a p p r e c i a t e d . The f i n a n c i a l support p r o v i d e d by the N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l of Canada and Canadian I n d u s t r i e s , L t d . i s g r e a t f u l l y acknowledged, and s p e c i a l thanks are due to Mr. T e r r y Lynch, T e c h n i c a l Manager of the A g r i c u l t u r a l D i v i s i o n of CIL f o r h i s i n v a l u a b l e a s s i s t a n c e . x i 1 . INTRODUCTION Urea i s used e x t e n s i v e l y as a f e r t i l i z e r because of i t s h i g h n i t r o g e n content (46.6% by weight). Most n i t r o g e n f e r t i l i z e r s are h i g h l y water s o l u b l e and s u s c e p t i b l e to l o s s by r u n - o f f or l e a c h i n g before they can be a s s i m i l a t e d by c r o p s . In the case of urea, t e s t s have shown that as much as 75% of the s u p p l i e d n i t r o g e n may be l o s t i n r e gions with high, i n t e r m i t t e n t r a i n f a l l . ^ F e r t i l i z e r r u n - o f f not only r e s u l t s i n i n c r e a s e d c o s t s , but i t a l s o c o n t r i b u t e s to the u n d e s i r a b l e growth of weeds and algae i n l o c a l waters. Although urea l o s s e s can be minimized by repeated a p p l i c a t i o n s of s m a l l e r f e r t i l i z e r dosages, t h i s approach i s not u s u a l l y economical due to high labour c o s t . An a l t e r n a t i v e approach i s to develop f e r t i l i z e r s with c o n t r o l l e d n u t r i e n t r e l e a s e r a t e s which match crop requirements. T h i s can be achieved by e n c a p s u l a t i n g the f e r t i l i z e r granules with c o a t s of low water p e r m e a b i l i t y . Such c o a t s r e t a r d the r e l e a s e of n u t r i e n t s and t h e r e f o r e g i v e p l a n t s more time to a s s i m i l a t e them. (2) (3) Rindt et a l . , B l o u i n et a l . , and M c C l e l l a n and (4) Scheib of the Tennessee V a l l e y A u t h o r i t y (TVA) have examined v a r i o u s f e r t i l i z e r / c o a t combinations, but only s u l p h u r - c o a t e d urea has proven to be s u f f i c i e n t l y promising f o r l a r g e s c a l e commercial p r o d u c t i o n . Sulphur i s an 1 2 a t t r a c t i v e c o a t i n g m a t e r i a l because of i t s w a t e r - r e s i s t a n c e , b i o - d e g r a d a b i l i t y , abundant supply, low c o s t , and c a p a b i l i t y of forming a good coat under c e r t a i n c i r c u m s t a n c e s. Furthermore, sulphur i s an e s s e n t i a l p l a n t n u t r i e n t which i s l a c k i n g i n many s o i l s . However, sulphur a l s o has two disadvantages: high f r i a b i l i t y and poor adhesion to urea. Two processes have been s t u d i e d f o r manufacturing s u l p h u r - c o a t e d urea, i . e . the TVA r o t a r y drum process and the UBC spouted bed pro c e s s . B r i e f d e s c r i p t i o n s of both processes are given below with f u r t h e r d e t a i l s being presented i n the next c h a p t e r . 1.1 THE TVA PROCESS The Tennessee V a l l e y A u t h o r i t y (TVA) developed the f i r s t l a r g e - s c a l e sulphur c o a t i n g process which was commercialized by Canadian I n d u s t r i e s L i m i t e d (CIL) i n O n t a r i o , Canada and by Ag I n d u s t r i e s Manufacturing Corp., Alabama, USA. The TVA process c o n s i s t s e s s e n t i a l l y of a r o t a r y drum i n t o which preheated (approx. 60 to 80°C) urea i s f e d . Molten sulphur (approx. 150°C) i s sprayed onto the urea p a r t i c l e s and s o l i d i f i e s r a p i d l y thereby forming s o l i d c o a t s . The product l e a v i n g the drum proceeds through a waxing f a c i l i t y to s e a l i m p e r f e c t i o n s i n the sulphur c o a t . The product i s then c o o l e d and c o n d i t i o n e d t o prevent agglomeration (see F i g . 1.1). G R A N U L A R U R E A P R E H E A T I N G D R U M ( R A D I A N T H E A T ) S U L F U R F R O M T A N K C A R S U L F U R F E E D T A N K S U L F U R C O A T I N G D R U M I 5 4 . 4 ° C ( I 5 4 . 4 ° C ) t|-l__L I 1_4.J_^  i I A. A A /?•• /)• i I 4 9 c I 8 - S P R A Y 7 4 - 7 7 ° C N O Z Z L E S ( 7 4 - 9 0 ° C ) S U L F U R P U M P H E A T E D A T O M I Z I N G A I R ( N O A I R U S E D W I T H H Y D R A U L I C S P R A Y N O Z Z L E S ) W A X C O A T I N G D R U M 7 7 - I I O ° C ( 7 7 1 1 0 ° C ) P R E H E A T E D W A X I 6 - 3 8 ° C ( 1 6 3 8 ° C ) I N L E T ^ A I R 6 8 - 7 l ° C ( 6 8 8 2 ° C ) D I S C H A R G E A I R C O O L E R LJ 4 I ° C ( 4 I ° C M A X M A X ) C O N D I T I O N I N G A G E N T L, C O N D I T I O N I N G D R U M 1 S C A L P I N G S C R E E N P R O D U C T Figure 1.1: Flowsheet of TVA Process f o r Sulphur Coating of Urea (Capacity= 900 kg/hr)(13) (Figures i n pa r e n t h e s i s i n d i c a t e process temperature for h y d r a l i c spraying) 4 The q u a l i t y of sulphur coated urea may be e v a l u a t e d by l a b o r a t o r y or f i e l d t e s t s . In l a b o r a t o r y t e s t s , the product q u a l i t y i s u s u a l l y expressed i n terms of the 7-day d i s s o l u t i o n (D__), i . e . the percentage of urea which l. b d i s s o l v e s when 50 g of sample c o n t a i n i n g 25 wt% sulphur are p l a c e d i n 250 mL of water at 37.8°C f o r 7 days. D i s s o l u t i o n v a l u e s of s u l p h u r - c o a t e d urea produced by the TVA process range from about 17.5% to 35%. The d i s s o l u t i o n depends p r i m a r i l y on the c o a t i n g temperature, sulphur a t o m i z a t i o n , coat t h i c k n e s s , and, to a l e s s e r extent, the s e a l a n t . Sulphur-coated urea has been t e s t e d i n many s o i l s and crops ( A l l e n ^ ^ , L i e g e l ^ ^ , Sherma^^, J a r a m i l l o ^ ^ ) and the r e s u l t s have shown s u b s t a n t i a l y i e l d i n c r e a s e s . For example, when a p p l i e d to Bermuda g r a s s , a more uniform uptake of n i t r o g e n and a 13% higher grass y i e l d were achieved with (7) s u l p h u r - c o a t e d urea than uncoated urea . I t has a l s o been shown that s u l p h u r - c o a t e d urea can be used e f f e c t i v e l y f o r t u r f , p asture and hay p r o d u c t i o n where high l e a c h i n g and decomposition l o s s e s occur. S i m i l a r e f f e c t i v e n e s s has a l s o been observed with long term crops such as p i n e a p p l e , sugar-cane and timber. 5 1.2 THE UBC SPOUTED BED PROCESS A sulphur c o a t i n g process u s i n g the spouted bed mode (9) was developed by Mathur and Meisen at the U n i v e r s i t y of B r i t i s h Columbia (UBC) s t a r t i n g i n 1975. The f a c i l i t y c o n s i s t e d mainly of a spouted bed, i . e . a c y l i n d r i c a l column with a c o n i c a l base f i l l e d with urea p a r t i c l e s . A high v e l o c i t y a i r j e t entered the bed from the bottom and c a r r i e d p a r t i c l e s l o c a t e d i n the c e n t r a l r e g i o n of the bed (spout) upwards u n t i l they reached the top of the bed ( f o u n t a i n ) whence they f e l l back i n t o the annulus. The annulus behaved as a slowly descending packed bed. P a r t i c l e s r e - e n t e r e d the spout near the bottom of the bed and were c a r r i e d upwards once again (see F i g . 1.2). Coating was accomplished by s p r a y i n g molten sulphur through an ato m i z i n g n o z z l e i n t o the bottom of the column. Each time a urea granule passed through the "spray zone", i t a c q u i r e d a l a y e r of sulphur which s o l i d i f i e d by the time i t reached the top of the bed. Then the granule f e l l back and descended i n the annulus before r e c e i v i n g another l a y e r of sulphur as i t passed through the spray zone a g a i n . Repeated passage through the spray zone helped to minimize coat i m p e r f e c t i o n s . The product q u a l i t y i n terms of D valu e s was found to 25 be h i g h l y dependent on bed temperature and sulphur flow 6 F i g u r e 1 . 2 : U.B.C. Spouted Bed (Arrows i n d i c a t e d i r e c t i o n of s o l i d s 7 r a t e . I n i t i a l experiments and d i s s o l u t i o n t e s t s demonstrated poor r e p r o d u c i b i l i t y . O p e r a t i o n a l problems such as nozzle p l u g g i n g , sulphur f i l t r a t i o n problems and molten sulphur h a n d l i n g d i f f i c u l t i e s r e q u i r e d a complete r e - d e s i g n and m o d i f i c a t i o n of the equipment. A m o d i f i e d c o a t i n g f a c i l i t y was d e v i s e d by Weiss and M e i s e n ^ 1 ^ i n 1981. To prevent n o z z l e p l u g g i n g due to sulphur s o l i d i f i c a t i o n and i m p u r i t i e s , a steam-jacketed sulphur l i n e and a c a r t r i d g e f i l t e r were added. S u c c e s s f u l batch-mode c o a t i n g was achieved. The product q u a l i t y i n terms of D-_ v a l u e s was comparable to the CIL product. A q u a l i t i v e model of the c o a t i n g process was proposed by Weiss and Meisen^ 1 ^ . They suggested that coat q u a l i t y was s t r o n g l y a f f e c t e d by the sulphur d r o p l e t s i z e , the spreading of sulphur drops and the nature of the urea s u r f a c e . The e f f e c t of s i l i c o n e a d d i t i v e s was a l s o i n v e s t i g a t e d , and the i n i t i a l f i n d i n g s i n d i c a t e d that s i l i c o n e improved product q u a l i t y s l i g h t l y . 1.3 OBJECTIVES OF THIS THESIS The main o b j e c t i v e s of t h i s t h e s i s are as f o l l o w s : 1. Modify the e x i s t i n g batch-wise l a b o r a t o r y spouted bed f a c i l i t y to permit continuous o p e r a t i o n . 2. Compare the q u a l i t y of the product produced by t h i s 8 process and the TVA process by using l a b o r a t o r y t e s t s . 3 . I n v e s t i g a t e the r e l a t i o n s h i p between product q u a l i t y and o p e r a t i o n a l v a r i a b l e s such as bed temperature and bed depth. 2. LITERATURE REVIEW 2.1 THE TVA PROCESS FOR SULPHUR COATING OF UREA 2.1.1 TVA PROCESS DEVELOPMENT (2) Rindt, B l o u i n , and Ge t z i n g e r performed the i n i t i a l s t u d i e s on sulphur c o a t i n g of urea and developed a batch-wise process i n which the urea was p l a c e d i n a r o t a r y pan and sprayed with molten sulphur. Most of the experiments were performed with pan-granulated urea a l s o produced by TVA. However, other f e r t i l i z e r s u b s t r a t e s were a l s o t e s t e d . Among them were diammonium phosphate c r y s t a l s and g r a n u l e s , ammonium phosphate n i t r a t e g r a n u l e s , and potassium c h l o r i d e g r a n u l e s . Two types of sulphur were examined: " b r i g h t " and "dark" grades of commercially produced sulphur; both y i e l d e d s i m i l a r d i s s o l u t i o n t e s t r e s u l t s . The product q u a l i t y was e v a l u a t e d by determining the d i s s o l u t i o n of 2 g of sample i n 10 mL of water at 37.8°C a f t e r one and f i v e days. The one-day t e s t gave an i n d i c a t i o n of the p r o p o r t i o n of p a r t i c l e s with imperfect c o a t i n g s . The f i v e - d a y t e s t gave an i n d i c a t i o n of moisture p e n e t r a t i o n i n t o w e l l coated p a r t i c l e s by measuring the average d i s s o l u t i o n r a t e f o r the next four days. (2) Based on t h e i r f i n d i n g s , Rindt et a l . concluded that c o a t i n g s of sulphur alone were not s u f f i c i e n t l y 9 10 w a t e r - r e s i s t a n t due to c r a c k s and pores. Two approaches were s t u d i e d to s o l v e t h i s problem. One was to use a d d i t i v e s to minimize the presence of c r a c k s and pores i n the sulphur s h e l l . The other was to s e a l the coa t s with another l a y e r of wax. The f i r s t approach was s t u d i e d e x t e n s i v e l y and v a r i o u s a d d i t i v e s (e.g. org a n i c p o l y s u l f i d e s , i n o r g a n i c o x i d e s , and naphthalene) were t e s t e d . However no s i g n i f i c a n t improvements i n product q u a l i t y were achieved. Rindt et (2) a l . t h e r e f o r e c o n c e n t r a t e d on the second approach i n which s e a l i n g the sulphur coat with biodegradable wax was t r i e d . However, two problems a r o s e : product agglomeration and m i c r o b i a l a t t a c k . Diatomaceous e a r t h was used t o overcome agglomeration and the a d d i t i o n of m i c r o b i c i d e s i n h i b i t e d m i c r o b i a l a t t a c k . (2) A c c o r d i n g to Rindt et a l . , the product q u a l i t y was i n f l u e n c e d by the temperature of molten sulphur, f e r t i l i z e r s u b s t r a t e and atomizing a i r d u r i n g the c o a t i n g o p e r a t i o n . The optimal temperature f o r the sulphur and atomizing a i r was found to l i e between 135 to 140°C. The d i s s o l u t i o n v a l u e s were found to decrease r a p i d l y with s u b s t r a t e temperature up to 65.5°C and then they i n c r e a s e d a g a i n . The best product q u a l i t y was o b t a i n e d i n a temperature range of (2) 65.5 to 76.6°C. Rindt et a l . o f f e r e d the f o l l o w i n g e x p l a n a t i o n : At temperatures lower then 65.5°C, the c o a t i n g 11 was rough and d i f f i c u l t to s e a l because of premature f r e e z i n g of the sulphur d r o p l e t s . At temperatures higher than 76.6°C, the sulphur d i d not s o l i d i f y r a p i d l y enough and ran o f f the s u b s t r a t e . The e f f e c t of s u b s t r a t e temperature on wax c o a t i n g was a l s o s t u d i e d and the optimal temperature range was found to be 65.5 to 71.1°C. The e f f e c t s of sulphur and s e a l a n t content were a l s o i n v e s t i g a t e d and i t was found that the d i s s o l u t i o n decreased with coat t h i c k n e s s . The optimal combination of c o a t i n g m a t e r i a l s was proposed to be: 16 wt% sulphur, 3 wt% wax, 0.5 wt% m i c r o b i c i d e s , and 1 wt% c o n d i t i o n e r . Based on the batch-wise pan c o a t i n g f a c i l i t y , B l o u i n et (3) a l . developed a continuous process (see F i g . 2.1). I t c o n s i s t e d mainly of a r o t a r y drum 0.914 m I.D. by 1.219 m long i n which urea was coated c o n t i n u o u s l y at a r a t e up to 136 kg/h. The drum was d i v i d e d i n t o three compartments. The f i r s t one served as a urea preheater where the urea temperature was e l e v a t e d to about 71°C by hot a i r j e t s (137.8°C). In the second compartment, molten sulphur at 148.9-154.4°C was sprayed onto the urea through three pneumatic a t o m i z i n g n o z z l e s . In the f i n a l compartment, the sulphur-coated urea was then sprayed with molten wax (93.3 to 104°C) c o n t a i n i n g a m i c r o b i c i d e (8% by weight). The product was d i s c h a r g e d i n t o a second drum where i t was Preheating Coating Sealing Coal Tar =O-G»O Metering Pump Atomizing A i r i — I48.9°C Molten Sulfur \7 ooling 71.1 ° C 65 .6°C 137.8 °C 1 — 'P rehea t Air Cooling A i r I48.9°C Metering Pump Conditioner Feeder Screen^ Product ( 3 6 % NM 7 8 % Urea 1 7 % Su l fu r 3 % Wax 0 2 % Coal Tar 1.8% Condit ioner "Oversize (3) Figure 2 . 1 : TVA Process for Sulphur Coating of Urea ( 1 3 6 kg/hr F a c i l i t y ) 13 c o o l e d to about 38°C and c o n d i t i o n e d with diatomaceous e a r t h or c l a y . (3) B l o u i n , Rindt and Moore a l s o developed a b e t t e r l a b o r a t o r y t e s t f o r determining product q u a l i t y . T h i s t e s t r e q u i r e d 50 g of sample to be immersed i n 250 mL of water at 37.8°C f o r a s p e c i f i e d p e r i o d of time. The amount of urea d i s s o l v e d i n the water was determined by measuring the s p e c i f i c g r a v i t y of the s o l u t i o n . Product with l a r g e l y imperfect c o a t s d i s s o l v e d r a p i d l y r e s u l t i n g i n h i g h d i s s o l u t i o n (2.5% d a i l y ) d u r i n g the f i r s t seven days of t e s t i n g . Slow l e a c h i n g from w e l l - c o a t e d m a t e r i a l was i n d i c a t e d by subsequent lower d i s s o l u t i o n (0.5% d a i l y ) . P r e v i o u s f i n d i n g s were confirmed, i . e . the d i s s o l u t i o n decreased with i n c r e a s i n g coat t h i c k n e s s . At sulphur contents of 27 wt% and 30 wt%, the urea d i s s o l u t i o n ranged from 4 to 27% over a p e r i o d of 10 days. The urea p a r t i c l e s ranged from 99%+3.36mm (+6 mesh) to 75%-2.00 mm (-10 mesh). The e f f e c t of sulphur spray c o n d i t i o n s was a l s o i n v e s t i g a t e d f o r g r a n u l a r and p r i l l e d urea. I t was observed that the d i s s o l u t i o n decreased with i n c r e a s i n g atomizing a i r p r e s s u r e f o r both types of urea. I t was a l s o noted that the spray p a t t e r n s produced by d i f f e r e n t n o z z l e types had an e f f e c t on product q u a l i t y . The best q u a l i t y was obtained with an e x t e r a l - m i x i n g spray n o z z l e having a d i a m e t r i c a l l y 14 opposed wing t i p a i r j e t . A 7-day d i s s o l u t i o n of 5% was achieved when urea was coated with 28 wt% of c o a t i n g m a t e r i a l . However, more t y p i c a l products had a 7-day d i s s o l u t i o n of 15% and contained 17% sulphur, 3% wax, 0.2% c o a l t a r ( m i c r o b i c i d e s ) and 1.8% a n t i c a k i n g agent. The storage and h a n d l i n g c h a r a c t e r i s t i c s of the f i n i s h e d product which had s a t i s f a c t o r y d i s s o l u t i o n c h a r a c t e r i s t i c s were a l s o t e s t e d . I t was found that normal storage and hand l i n g procedures d i d not r e s u l t i n s e r i o u s product d e g r a d a t i o n . Moisture a b s o r p t i o n was 5% to 12% of that of s i m i l a r l y exposed uncoated m a t e r i a l . TVA subsequently designed and b u i l t a 900 kg/h demonstration f a c i l i t y based on experience gained with the 136 kg/h p l a n t ^ 1 ^ 1 4 \ In t h i s f a c i l i t y , separate equipment was used f o r p r e h e a t i n g , sulphur c o a t i n g and wax c o a t i n g . The urea was f i r s t heated to about 80°C i n a r o t a r y drum heater and then fed i n t o the sulphur c o a t i n g drum where molten sulphur was sprayed onto the urea by e i t h e r pneumatic or h y d r a u l i c n o z z l e s . The s u l p h u r - c o a t e d urea was then t r a n s f e r r e d to a wax c o a t i n g drum by a bucket e l e v a t o r . A m i c r o c r y s t a l l i n e wax was sprayed onto the coated p a r t i c l e s to s e a l i m p e r f e c t i o n s . However, the use of m i c r o b i c i d e s was d i s c o n t i n u e d when i t was found that they were not needed. The product l e a v i n g the 15 wax drum was c o o l e d i n a f l u i d i z e d bed c o o l e r to 15.6-37.8°C and f i n a l l y d i s c h a r g e d i n t o a c o n d i t i o n i n g drum. Diatomaceous e a r t h was used as a c o n d i t i o n e r to prevent product c a k i n g d u r i n g s t o r a g e . The l a s t p r o c e s s i n g step was sc r e e n i n g to remove any o v e r s i z e d p a r t i c l e s . (13) S h i r l e y and Meline used t h i s f a c i l i t y to compare the e f f e c t i v e n e s s of using pneumatic and h y d r a u l i c sulphur atomizing systems. Pneumatic n o z z l e s were used i n i t i a l l y . The i n t e r n a l s of the c o a t i n g drum are i l l u s t r a t e d i n F i g . 2.2a. The molten sulphur was passed through three f i l t e r s b efore s p r a y i n g to prevent p l u g g i n g . The i m p u r i t i e s i n the sulphur are mainly CARSUL, which i s a s o l i d carbon-sulphur complex formed by hydrocarbons r e a c t i n g with molten sulphur. (Hydrocarbons are u s u a l l y presented i n sulphur produced d u r i n g petroleum r e f i n i n g . ) The sulphur i s then d i s t r i b u t e d through a steam heated header equipped with e i g h t a i r - a t o m i z i n g n o z z l e s (see F i g . 2.3a). The n o z z l e s are p o s i t i o n e d to spray v e r t i c a l l y downward and d i r e c t l y onto the f a s t e s t moving p a r t of the r o t a r y bed. The f o l l o w i n g o p e r a t i n g parameters were examined by (13) S h i r l e y and Meline : t o t a l c o a t i n g weight of sulphur, wax, and c o n d i t i o n e r ; type and s i z e of pneumatic n o z z l e s ; number and p o s i t i o n of sulphur n o z z l e s ; r o t a t i o n a l speed of the c o a t i n g drum; ato m i z i n g a i r flow r a t e ; and c o a t i n g temperature. They observed a pronounced e f f e c t of s u b s t r a t e D i s c h a r g e Re t a i n i n g R i n g D i s c h a r g e R e t a i n i n g R i n g L i f t i n g F l i g h t s S p r a y i n g D i s t a n c e 5 1/2" D e f l e c t o r P l a t e F a l l i n g C u r t a i n o f U r e a G r a n u l e s ( C u r t a i n H e i g h t 17 " ) S p r a y i n g H e i g h t 2" S p r a y i n g D i s t a n c e 5 1/2' (a) Pneumatic Spraying (b) H y d r a u l i c Spraying (13) Figure 2.2: I n t e r n a l s of the Coating Drum, TVA Process (900 kg/hr F a c i l i t y ) 1 7 Atomiz ing A i r M o l t e n S u l f u r t o m i z i n g A i r Header ^ C o n d e n s a t e S u l f u r Header Condensate Steam M o l t e n Su l fu r Molten i 1 Sulfur H E Z ^ " 1 Sulfur Flow Control Orif ice Atomizing A i r Heated Atomizing Ai r Wings (a) Pneumatic A t o m i z a t i o n <T > Flat Spray Pat te rn Nozzle T ip A l u m i n u m Gaskets S t r a i n e r A s s e m b l y Nozzle Body (b) H y d r a u l i c A t o m i z a t i o n F i g u r e 2 . 3 : Sulphur Spray Header.System and Spray Nozzle D e t a i l (TVA Process 900 kg/hr P l a n t ) ( l 3 ) 18 temperature on the d i s s o l u t i o n (see F i g . 2.4). The d i s s o l u t i o n decreased with s u b s t r a t e temperature up to 95.5°C and then i n c r e a s e d a g a i n . From 70 to 95.5°C, the d i s s o l u t i o n values dropped from 40% to 5%. No c l e a r e x p l a n a t i o n was o f f e r e d f o r t h i s behaviour. The optimal c o a t i n g temperature found i n t h i s case was higher then the (2) p r e v i o u s study done by Rindt et a l . , which was at between 65.5 to 76.6°C. However, t h i s d i s c r e p e n c y might be due to the d i f f e r e n t temperature d e t e c t i n g l o c a t i o n s f o r the two s t u d i e s . (13) S h i r l e y and Meline were able to produce a c c e p t a b l e product without wax i n t h e i r s t u d i e s . A 7-day d i s s o l u t i o n of 19.7% was obtained f o r a product c o n t a i n i n g 20 wt% c o a t i n g m a t e r i a l . Table 2.1 summarizes the optimum o p e r a t i n g c o n d i t i o n s f o r both pneumatic and h y d r a u l i c a t o m i z a t i o n systems. As i n d i c a t e d by Table 2.2, pneumatic a t o m i z a t i o n gave b e t t e r product q u a l i t y than h y d r a u l i c a t o m i z a t i o n ; but TVA decided to search f o r an a l t e r n a t i v e to minimize dust e l u t r i a t i o n from the c o a t i n g drum. T y p i c a l dust l o a d i n g s i n 3 the a i r l e a v i n g the drum ranged from 0.150 to 0.359 g/m /kg of sulphur sprayed when us i n g pneumatic n o z z l e s . By comparison, h y d r a u l i c n o z z l e s y i e l d e d dust l o a d i n g s of 0.007 / 3 , to 0.018 g/m /kg of sulphur sprayed. The l a r g e s t dust / 3 c o n c e n t r a t i o n measured was 3.2 g/m which i s w e l l below 35 F i g u r e 2.4: E f f e c t of S u b s t r a t e Temperature on Urea D i s s o l u t i o n Rates f o r TVA Process ( S h i r l e y & Meline) Table 2.1: Optimum Operating C o n d i t i o n s f o r Sulphur Coating of Urea i n the TVA 900 kg/hour P l a n t Pneumatic Atomization Hydraulic Atomization With Sealant Without Sealant With Sealant Without Sealant Preheated Urea Li q u i d Sulphur Atomizing A i r Sulphur Coating Drum Sulphur Coating Drum Ex i t Liquid Wax Wax Coat Drum ex i t Cooling A i r Cooler E x i t 7 day Diss o l u -t i o n Product with 20% t o t a l coating Production Rate 62.8-65.6°C 79.4-82.2°C 154.4°C 154.4°C 143.3-148.9°C 146.1-148.9°C 68.3-71.1°C 73.9-76.7°C 76.7-110.0°C 68.3-71.1°C 15.6-37.8°C 40.6°C Max. 12.5% Max. 95.5°C 90.6-93.3°C 19.7% 51.7-79.4°C 154.4°C 148.9°C 93.3°C 73.9-90.6°C 76.7-110.0°C 68.3-82.2°C 15.6-37.8°C 54.4°C Max 40.6°C Max 15.7' 60-73.9°C 154.4°C 148.9°C 93.3°C Max 73.9-85.0°C 54.4°C Max ;i.2« 907 kgs/hour 454 kgs/hour 3628 kgs/hour 1814 kgs/hour 21 Table 2.2 Summary of D i s s o l u t i o n Rates f o r Pneumatic and H y d r a u l i c A t o m i z a t i o n s (with and without s e a l a n t ) T o t a l Coat Weight % Sulphur, % Pneumatic H y d r a l i c Atomization A t o m i z a t i o n with without with without s e a l a n t s e a l a n t s e a l a n t s e a l a n t 16.0 20.0 19.5 23.0 1 1 .0 20.0 14.5 23.0 7 Day D i s s o l u t i o n , % 12.5 19.7 15.7 31.2 P r o d u c t i o n Rate, kg/hr 907 454 3628 1814 22 g/m , the lower e x p l o s i v e l i m i t of sulphur i n a i r . The h y d r a u l i c system was q u i t e s i m i l a r to the pneumatic one except f o r the i n t e r n a l s of the c o a t i n g drum and the sulphur spray p a t t e r n (see F i g . 2.2b and 2.3b). The sulphur was steam heated and had three i n l i n e f i l t e r s to remove i m p u r i t i e s as w e l l as a s t r a i n e r upstream of each n o z z l e . F i l t r a t i o n was more important due to sma l l e r n o z z l e openings, i . e . 178 urn to 381 Mm, compared with 457 Mm to 711 Mm i n the pneumatic n o z z l e s . As a r e s u l t of s u c c e s s f u l p i l o t p l a n t work and the i n c r e a s i n g demand f o r sulphur - c o a t e d urea, TVA decided to (16) design and b u i l d a 9.070 t/h demonstration p l a n t 2.1.2 TVA DEMONSTRATION SCALE PLANT T h i s p l a n t was d e r i v e d from the p r e v i o u s 0.9 t/h p i l o t p l a n t with the f o l l o w i n g m o d i f i c a t i o n s (see F i g . 2.5): - F l u i d i z e d bed preheater f o r urea; - D i r e c t f e e d i n g from the TVA pan-granulator to e l i m i n a t e i n t e r m e d i a t e storage and reduce h e a t i n g c o s t (the product e x i t s the g r a n u l a t o r at 115°C); - New p o l l u t i o n c o n t r o l equipment s i n c e higher dust c o n c e n t r a t i o n s were expected; - Improved, h i g h - p r e s s u r e sulphur i n j e c t i o n ; ( t h e o r e t i c a l l y , the n o z z l e e f f i c i e n c y improves with i n c r e a s i n g p r e s s u r e To Atm st nam bright stock oil storage lank Su l fur F i l ler rO comp air for normal operation primary sulfur f i lter high pressure sulfur pump 50 100 isig oi steam molten sulfur storage B transfer pump j[ J-steam sulfur feed fank return fo granulation unit ,3^™ ^ j i l e d t n k pump pump urea from pan granulation' unit Oversize urea feed elevator process elevator Product Elevator F i g u r e 2.5: TVA Demonstration Scale Plant f o r Sulphur Coating of Urea (9.07 m ton/hr)( 16) to to 24 and d e c r e a s i n g nozzle s i z e ^ 1 < ^ . Operating p r e s s u r e s were chosen as 6.8-10.4 MPa and n o z z l e openings ranged from 178 urn to 381 urn.); - High degree of i n s t r u m e n t a t i o n to provide b e t t e r c o n t r o l f o r a smoother and s a f e r o p e r a t i o n and a more unifrom product; - Improved bulk h a n d l i n g f a c i l i t i e s f o r a l l raw m a t e r i a l s ; - U t i l i z e g r a v i t y flow f o r the whole process from the preheater to the f i n a l s c reens; - Hot sweeping a i r was p r o v i d e d to both sulphur c o a t i n g and s e a l a n t c o a t i n g drums. (The a i r i n the sulphur c o a t i n g drum reduced the sulphur dust c o n c e n t r a t i o n and removed dust, thereby minimizing e x p l o s i o n problems. A i r i n the s e a l a n t drum d i l u t e d any hydrocarbon vapors present f o r the same c o n s i d e r a t i o n s . ) The demonstration p l a n t a p p l i e d three c o a t i n g s to the urea: sulphur, s e a l a n t (70% b r i g h t s t o c k o i l and 30% p o l y e t h y l e n e ) , and c o n d i t i o n e r (diatomaceous e a r t h ) . The major process v a r i a b l e s are summerized i n Table 2.3. The f i n a l product had a 7-Day d i s s o l u t i o n ranging from 25% to 30%. 2.2 THE UBC PROCESS Mathur and Meisen began the design of a batch-wise spouted bed c o a t i n g process i n 1975 (see F i g . 2.6). The bed Table 2.3: Main Process V a r i a b l e s and Product C h a r a c t e r i s t i c s o f TVA Demonstration P l a n t ( C a p a c i t y 9.07 t/hr) T o t a l C o a t i n g , \ 7 days D i s s o l u t i o n , \ S u b s t r a t e : Type S i z e ( T y l e r nesh) \ .7 -7-8 -8*9 -9 Urea Feed Rate, metric Ton/hour F l u i d i i e d Bed Preheater: E n t e r i n g A i r temp., °C Su b s t r a t e e x i t temp., *C Re t e n t i o n time, min Sulphur Coating Drum: Sulphur feed r a t e : per N o z z l e , kg/h T o t a l , kg/h Sulphur feed temp., °C Spray Nozzles Type: Number Spray Tip opening, um H y d r a u l i c press at Nozzle t i p s , kPa Rev o l u t i o n s per minute R e t e n t i o n time, min Temperature of Sulphur-Coated urea l e a v i n g , "C Sealant Coating Drum Temperature of coated product l e a v i n g •c 23 25-30 Pan Cranulated Urea 6 21 43 30 7.S3 66.1 6S.0 1.8 39.S 1733.3 1S4.4 H y d r a u l i c atomizing 44 330 7579 11.8 7 81.1 77.2 Type of sealant Temperature o f sealant at a p p l i c a t i o n , "C 2000 Mol wt. Polyethylene 30". wt B r i g h t s t o c k 79* wt Rev o l u t i o n s per minute Ret e n t i o n time, min Sealant a p p l i e d , wt % of t o t a l prod. F l u i d i z e d Bed Cooler Temperatures M a t e r i a l e n t e r i n g , °C Co o l e r A i r e n t e r i n g , °C M a t e r i a l l e a v i n g , °C C o n d i t i o n i n g Drum R e v o l u t i o n per minute R e t e n t i o n time. Bin C o n d i t i o n i n g agent, wt % of t o t a l prod. 123.9 11.S 0.7 2.1 13.9 24.4 10 1.5 2.4 To Vent 1 \ \ Air Flow /<a CD Atomized Sulphur Urea V ~ f M — - /—C e r r j m i c Tube ^-—Heating Tape Sulphur Flow X X Spouted Bed Spouting Air I 1 3 KW Heaters ={ .75 KW f Atomizing Air Sulphur C yclones Compressed Sulphur Melter F i g u r e 2 . 6 : UBC S p o u t e d Bed f o r S u l p h u r C o a t i n g o f U r e a ( i n i t i a l p r o t o t y p e d e s i g n e d by Ma thu r and M e i s e n ) 27 was a c y l i n d r i c a l v e s s e l with a c o n i c a l base c o n t a i n i n g urea p a r t i c l e s . The p a r t i c l e s were spouted by warm a i r . Sulphur was melted i n a e l e c t r i c a l l y heated v e s s e l and i n j e c t e d through an a i r atomizing n o z z l e by compressed n i t r o g e n . A l l l i n e s were e l e c t r i c a l l y heated and i n s u l a t e d . The no z z l e arrangement c o n s i s t e d of a ceramic tube with two c o n c e n t r i c tubes p l a c e d i n the middle. The i n s i d e tube was the sulphur l i n e surrounded by the atom i z i n g a i r l i n e . The spouting a i r and the atomizing a i r were heated by 3 KW and 0.75 KW e l e c t r i c a l h e a t e r s , r e s p e c t i v e l y . Two c y c l o n e s were i n s t a l l e d to remove e l u t r i a t e d f i n e s from the bed. S e v e r a l problems were experienced with t h i s equipment: low sulphur m e l t i n g r a t e , i n a b i l i t y to maintain a constant sulphur flow, poor alignment of the atomizing n o z z l e and noz z l e t i p plugging due to sulphur s o l i d i f i c a t i o n . M o d i f i c a t i o n s were made by Mathur, Meisen, and Z e e ^ 1 1 ^ d u r i n g the summer of 1976 to overcome some of these problems (see F i g . 2.7) . Prominent changes i n v o l v e d the i n s t a l l a t i o n of a new melter, n o z z l e arrangement and sulphur t r a n s f e r l i n e s . The new melter was e s s e n t i a l l y a s h e l l and tube heat exchanger with hot a i r p a s s i n g through the tubes. The melter had a c a p a c i t y of 31 kg of sulphur. Molten sulphur was t r a n s f e r r e d from the melter by a s t a i n l e s s s t e e l gear pump. To maintain the d e s i r e d o p e r a t i n g temperature of 130-158°C, the pump Figure 2.7: UBC Spouted Bed f o r Sulphur Coating of Urea ( M o d i f i c a t i o n designed by Mathur, Meisen, and Zee) To Vent S p o u t i n g Air - * — A tomiz ing Ai r Su lphu r Flow = = = = = A i r Flow Hea t ing Tapes Asbestos Lagg ing 29 head was wrapped with e l e c t r i c a l h e a t i n g tape and asbestos i n s u l a t i o n . The temperature of the pump was monitored by a thermocouple f i t t e d i n t o the pump body. An e l e c t r i c a l l y heated f i l t e r with two c o n s e c u t i v e 80 mesh s t a i n l e s s s t e e l screens was i n s t a l l e d to remove i m p u r i t i e s . The m o d i f i e d n o z z l e assembly (see F i g . 2.8) was heated by hot atomizing a i r p a s s i n g through the gap between the noz z l e and the cap. An " e x t e r n a l - m i x i n g " type nozzle was used, which had a sulphur opening of 380 um I.D. The spouting v e s s e l was made from a 0.15 m I.D. m i l d s t e e l pipe with viewing g l a s s e s f i t t e d at the f r o n t and back. Sampling p o r t s were f i t t e d to both the c y l i n d r i c a l and c o n i c a l s e c t i o n s of the column. O r i f i c e p l a t e s of d i f f e r e n t openings were used. The main purposes of Z e e ' s ^ 1 1 ^ work were: to produce s u l p h u r - c o a t e d urea and compare i t s d i s s o l u t i o n with t h a t produced commercially by CIL, to study the i n f l u e n c e s of s u b s t r a t e s i z e , c o a t i n g time, c o o l i n g time, and type of c o o l i n g on product q u a l i t y . V a r i a b l e s such as bed temperature and sulphur flow r a t e were not i n v e s t i g a t e d . A l l experiments were performed at high bed temperatures (^90°C). Two types of urea ( f o r e s t and a g r i c u l t u r a l grades) were s t u d i e d , and high-grade sulphur s l a t e was used. The F i g u r e 2 . 8 : N o z z l e A r r a n g e m e n t ( M a t h u r , M e i s e n & Zee) 3/4' 1/2' T 7//\ I Y///////m*± -5"4> -2 1/4 "oi 2 1/4 5/8" , A Nozzle B Nozzle Cop C Locking Ring 0 Nozzle Base E Nozzle Housing F Orifice Plate a Atomizing Air Inlet b Atomizing Air Distribution Well c Atomizing Air Passage d Atomizing Air Gap e Sulphur Inlet f Sulphur Passage 31 product was e v a l u a t e d by a s l i g h t l y m o d i f i e d 7-day t e s t and by an a c c e l e r a t e d d i s s o l u t i o n t e s t developed by Zee. The f a s t d i s s o l u t i o n t e s t employed a mechanical shaker. Sulphur-coated, g r a n u l a r urea c o n t a i n i n g 28 wt% sulphur c o u l d be produced and had a 7-day d i s s o l u t i o n of 89%. The CIL product c o n t a i n i n g 39% sulphur and 2% wax had a s i m i l a r d i s s o l u t i o n v a l u e . Zee, t h e r e f o r e , concluded that the spouted bed process c o u l d produce a s u p e r i o r product than the i n d u s t r i a l p r o c e s s . I t was a l s o d i s c o v e r e d that c o o l i n g the product i n the spouted bed had l i t t l e e f f e c t on product q u a l i t y . Although Zee's study p r o v i d e d no e x t e n s i v e q u a n t i t a t i v e i n f o r m a t i o n , i t demonstrated the f e a s i b i l i t y of the spouted bed c o a t i n g process and i n d i c a t e d t hat i t warranted f u r t h e r study. C o n s i d e r a b l e o p e r a t i o n a l problems s t i l l remained i n Zee's equipment. The main ones were p l u g g i n g due to sulphur s o l i d i f i c a t i o n i n the n o z z l e t i p and p i p i n g , d i f f i c u l t i e s with temperature c o n t r o l and an inadequate m e l t i n g system due to the need f o r l a r g e amounts of hot a i r to melt the sulphur. Mathur, Meisen and Lim attempted to s o l v e the aforementioned problems (see F i g . 2.9). Major changes and F i g u r e 2 . 9 : UBC S p o u t e d Bed f o r S u l p h u r C o a t i n g of U r e a ( m o d i f i c a t i o n d e s i g n e d by M a t h u r , M e i s e n & L im) Cyclone R o t a m e t e r s A i r + -W a t e r Sulphur Pot Sleam Supply. From Main ( -75 psig) ROTAMETERS 1. Cooling Air 2. Spouling Air 3. Atomizing Air 4. Water St irrer L Main Heater Cooling Column Spouting Column fFf Spouling Air ^ •To Drain Atomizing Air Line Shutter Qrjfjce i 3 Steam Trap To Drain Steam Heated Pump Steam Traps To Drain To Drain Asbestos Lagging Heating Tape +- + Sulphur Flow Dust Free Exhaust Scrubber To Drain GJ M 33 a d d i t i o n s i n c l u d e d a new sulphur melter (see F i g . 2.10), steam j a c k e t s f o r a l l sulphur l i n e s , a steam-heated sulphur metering pump, improved steam-heated n o z z l e arrangement (shown i n F i g . 2.11), e l e c t r i c a l h e a t i n g tape f o r p r e h e a t i n g the atomizing a i r , a g l a s s spouted bed column, a s h u t t e r mechanism f o r v a r i a b l e o r i f i c e openings and a water scrubber f o r f i n e s removal and odour e l i m i n a t i o n . A 0.15 m I.D. by 0.91 m high pyrex g l a s s column with a 60° s t a i n l e s s s t e e l c o n i c a l base served as the spouting column. A sampling p o r t was l o c a t e d at the s i d e of the c o n i c a l bottom. Temperature measurements were p r o v i d e d by three iron-308 Constantan thermocouples p l a c e d at v a r i o u s bed l e v e l s . The new melter was a pyrex g l a s s v e s s e l equipped with a s t a i n l e s s s t e e l c o i l and had a c a p a c i t y of 27.2 kg of molten sulphur. Molten sulphur was pumped from the melter to the spouting column by a steam heated metering pump. The steam heated n o z z l e assembly i s shown i n F i g . 2.11. The a i r atomizing n o z z l e was of the " i n t e r n a l - m i x i n g " type from Spraying Systems Inc., Wheaton, 111 ( F l u i d Cap # 2050, A i r Cap #67147) . (1 8) In the runs performed by Lim the e f f e c t s of the f o l l o w i n g parameters were examined: c o a t i n g temperature, spouting a i r flow r a t e , a t o m i z i n g a i r flow r a t e , sulphur 35 •5.0"<p 8.l3"<p-»-| 4.5 "<p A Nozzle a Sulphur Passage B Nozzle Cap b Atomizing Air Passage C Nozzle Cap Retainer Ring c Atomizing Air Distribution Well D Nozzle Base d Steam Inlet E Nozzle Housing e Sulphur Inlet F Plug f Steam Outlet g Atomizing Air Inlet F i g u r e 2.11: Nozzle Arrangement (Mathur, Meisen & Lim) 36 flow r a t e , bed depth, a d d i t i v e s , and types of urea. The standard 7-day d i s s o l u t i o n t e s t with s l i g h t m o d i f i c a t i o n s was used to t e s t the product q u a l i t y . The bed temperature seemed to have a strong e f f e c t on product q u a l i t y . The product d i s s o l u t i o n decreased with i n c r e a s i n g temperature up to 8 0 ° C and then i n c r e a s e d a g a i n . At low bed temperatures i t was b e l i e v e d that s p r a y i n g molten sulphur i n t o a c o o l bed caused the sulphur d r o p l e t s to s o l i d i f y p a r t i a l l y before they c o l l i d e d with the urea p a r t i c l e s thus g i v i n g a poorer bond; hence the d i s s o l u t i o n was h i g h . At e l e v a t e d temperatures, the urea-sulphur bond was improved, p o s s i b l y due to the i n c r e a s e d presence of m o n o c l i n i c and polymeric sulphur a l l o t r o p e s . The product had a b r i g h t - y e l l o w and shiny appearance whereas low temperature o p e r a t i o n r e s u l t e d i n a w h i t i s h - y e l l o w , mat product suggesting orthorhombic sulphur. At temperatures above 8 0 ° C the sulphur c o a t s had c r a c k s which l e d to higher urea d i s s o l u t i o n . These cr a c k s were probably due to c o n t r a c t i o n of sulphur as i t changed from the m o n o c l i n i c sulphur a l l o t r o p e of lower d e n s i t y to the orthorhombic sulphur of higher d e n s i t y upon c o o l i n g . Thus, a minimum product d i s s o l u t i o n o c c u r r e d at about 8 0 ° C . I t was a l s o observed that i n c r e a s i n g the sulphur flow rate and hence d r o p l e t s i z e improved the product q u a l i t y . T h i s f i n d i n g supported the suggestion that s m a l l e r sulphur 3 7 d r o p l e t s s o l i d i f i e d p a r t i a l l y before impact on the urea granules thereby l e a d i n g to poorer q u a l i t y . The spouting a i r flow r a t e seemed to have l i t t l e i n f l u e n c e on product q u a l i t y i n the range t e s t e d . The a t o m i z i n g a i r flow r a t e was found to have a d i s t i n c t i n f l u e n c e on product d i s s o l u t i o n . The d i s s o l u t i o n i n c r e a s e d with i n c r e a s i n g a t o m i z i n g a i r flow r a t e , but no c l e a r e x p l a n a t i o n was o f f e r e d f o r t h i s behaviour. The e f f e c t of bed depth was demonstrated by the o b s e r v a t i o n that shallower beds gave a somewhat b e t t e r product. T h i s e f f e c t was probably due to a more r a p i d s o l i d s c i r c u l a t i o n r a t e i n a shallower bed which g i v e s the granules a b e t t e r chance of a c q u i r i n g uniform c o a t . Lim used s e v e r a l a d d i t i v e s (gaseous c ° 2 ' ammonia, n i t r o g e n and l i q u i d d i c y c l o p e n t a d i e n e ) i n h i s c o a t i n g o p e r a t i o n s , but found no major improvements. Based on these f i n d i n g s , an optimum set of o p e r a t i n g c o n d i t i o n s was d e r i v e d by Lim which i n v o l v e d p r e h e a t i n g the urea to about 80°C and m a i n t a i n i n g an a p p r o p r i a t e combination of sulphur and a t o m i z i n g a i r flow r a t e s (18.1 kg 3 of s u l p h u r / m of a t o m i z i n g a i r ) . Under these circumstances, a D value of l e s s then 20% was a c h i e v e d . 38 Improvements were s t i l l r e q u i r e d to overcome problems such as sulphur p l u g g i n g i n the t r a n s f e r l i n e s , lengthy p r e h e a t i n g time and c o l d spots i n the sulphur pump. 2.2.1 THE IMPROVED UBC PROCESS The main o b j e c t i v e s of a subsequent study by Meisen and . (10) . Weiss i n 1981 were to improve the l a b o r a t o r y equipment to achieve t r o u b l e - f r e e o p e r a t i o n and to determine q u a n t i t a t i v e l y the r e l a t i o n s h i p s between product q u a l i t y and the p r i n c i p a l o p e r a t i n g v a r i a b l e s . F i g u r e 2.12 shows a s i m p l i f i e d flow sheet of the new p l a n t . The spouted bed column was mounted on a post and c o u l d be r o t a t e d upon d i s c o n n e c t i n g i t from the spouting a i r l i n e . I t was p o s s i b l e then to p l a c e the bed i n e i t h e r " c o a t i n g " or " c o o l i n g " p o s i t i o n . Furthermore, the urea p a r t i c l e s c o u l d be e a s i l y dumped out of the bed when i t was p l a c e d at an i n t e r m e d i a t e p o s i t i o n . The pyrex g l a s s sulphur melter was r e p l a c e d by a steam-heated s t a i n l e s s s t e e l tank. The tank was s t i r r e d c o n t i n u o u s l y with a v a r i a b l e speed motor to ensure sulphur remained in a molten s t a t e . Sulphur was f o r c e d out by p r e s s u r i z e d n i t r o g e n . T h i s arrangement was found to be more r e l i a b l e than the p r e v i o u s metering pump. The sulphur passed through a s t a i n l e s s s t e e l Rigimesh c a r t r i d g e f i l t e r with 0.149 mm openings before e n t e r i n g a custom-made, steam F i g u r e 2 . 1 2 : S i m p l i f i e d D i a g r a m o f UBC S p o u t e d Bed F a c i l i t y f o r Ba t ch -mode C o a t i n g ( d e s i g n e d by M e i s e n and W e i s s ) Water -cxi-Additi ves Line Cooling Air Line Spouting Air Line —co-Air Nitrogen Stirrer Sulfur Melter Filter iL=—I, ROTAMETERS I. Cooling Air 2 Spouting Air 3 Atomizing Air 4 Sulfur Electric .Heater Cyclone Cooling Position-Coat Pd~5i tion \/ ng C/Shutter -cxy-_ A10 m|z i ng_ A j r_ L !M Sulfur Line Steam Heated Electrical Tape Dust Free Exhaust to Fume Hood to Scrubber Y D r o i n 40 heated rotameter. Since the sulphur flow r a t e was an important v a r i a b l e , a rotameter p r o v i d e d d i r e c t i n d i c a t i o n and measurement of sulphur flow. The product q u a l i t y was s t u d i e d as a f u n c t i o n of bed temperature (48-86°C), sulphur flow r a t e (34-260 g/min), 3 atomizing a i r flow r a t e (0.402-0.785 m /h) and bed depth (0.28-0.47 m). Weiss and M e i s e n ^ 1 ^ again found the f a m i l i a r r e l a t i o n s h i p between product d i s o l u t i o n and bed temperature: an i n i t i a l decrease i n d i s s o l u t i o n with temperature f o l l o w e d by an i n c r e a s e with a minimum at approximately 80°C (see F i g . 2.13). D i s s o l u t i o n v a l u e s of 60%, 30%, and 40% were found f o r bed temperatures of 60, 80, and 90°C, r e s p e c t i v e l y . E l e c t r o m i c r o g r a p h s of coated urea supported ( 1 9 ) the p r e v i o u s e x p l a n a t i o n s advanced by Meisen and Mathur Coats formed at e l e v a t e d temperature had c r a c k s which were probably caused by the c o n t r a c t i o n of sulphur as i t changed from the m o n o c l i n i c a l l o t r o p e of lower d e n s i t y to orthorhombic form of higher d e n s i t y upon c o o l i n g . T h i s sulphur t r a n s i t i o n r e s u l t e d i n coat i m p e r f e c t i o n s which reduced the product q u a l i t y . At lower bed temperatures, the sulphur d r o p l e t s p a r t i a l l y s o l i d i f i e d before c o l l i d i n g with the urea p a r t i c l e s and t h e r e f o r e l e f t gaps i n sulphur c o a t s . (15) F i g u r e 2.13: E f f e c t of Bed Temperature on D 42 The D values were found to decrease with i n c r e a s i n g 2 5 sulphur flow r a t e and to i n c r e a s e with i n c r e a s i n g atomizing a i r flow r a t e (see F i g s . 2.14 & 2.15). Sulphur and atomizing a i r flow r a t e s a f f e c t e d the sulphur d r o p l e t s i z e produced by (17) the pneumatic n o z z l e : the d r o p l e t s i z e i n c r e a s e d with i n c r e a s i n g l i q u i d flow r a t e and decreased with i n c r e a s i n g a tomizing a i r flow r a t e . T h e r e f o r e , these o b s e r v a t i o n s supported the p r e v i o u s f i n d i n g t h a t the product q u a l i t y was i n f l u e n c e d by the sulphur d r o p l e t s i z e . A q u a l i t a t i v e model f o r c o a t i n g was proposed by W e i s s ^ 2 ^ which i n v o l v e d the sulphur d r o p l e t s i z e , sulphur f i l m spreading and the i n t e r f a c i a l t e n s i o n between sulphur and urea. No r e l i a b l e experimental r e s u l t s were obtained to v e r i f y the model. N e i t h e r the i n i t i a l bed depth or i t s corresponding minimum spouting v e l o c i t y had any a p p r e c i a b l e i n f l u e n c e on the product q u a l i t y . A set of optimum o p e r a t i n g c o n d i t i o n s was d e r i v e d from these experimental r e s u l t s , and D,__ values 2 b of l e s s than 25% were a c h i e v a b l e . The i m p l i c a t i o n s of the l a b o r a t o r y f i n d i n g s f o r commercial s c a l e p r o d u c t i o n were d i s c u s s e d . According to the (9) theory presented by Mathur and Meisen , which i s based on the number of times urea p a r t i c l e s pass through the spray zone and o v e r a l l heat balances, experimental data were I I 4 8 ° C 5 8 67 86 78 I I l Weiss ( 1 5 ) LIM'S DATA 86 ° C X 8 2 • 7 6 , O 7 8 ° C • _ 67.. A 6 8 • 58 ' o 59 • 48 * 51 • J L 0 .0022 j 0 . 0 0 4 4 0.0066 | 0.0II0 |0.0I54! 0.022I 0 . 0 3 3 0 0.0033 0.0088 0.0I32 O0 I76 I N V E R S E OF S U L P H U R FLOW R A T E , m i n / g F i g u r e 2.14: E f f e c t of Sulphur Flow Rate on D (15) 25 45 extended to commercial s i z e u n i t s . Simple economic a n a l y s i s of two f u l l - s c a l e p l a n t s with p r o d u c t i o n r a t e s of 196.3 t/year and 314.8 t/year were presented; labour accounted f o r a major f r a c t i o n (15% to 21%) of the f i n a l p r o d u c t i o n (20) c o s t . T h e r e f o r e , a continuous spouted bed c o a t i n g process w i t h lower labour requirement and higher output c a p a c i t y i s the l o g i c a l next step, and that i s the main o b j e c t i v e of the present t h e s i s . 2.3 SELECT PHYSICAL PROPERTIES OF SULPHUR Se v e r a l sulphur p r o p e r t i e s are presented here s i n c e they are important background i n f o r m a t i o n f o r t h i s t h e s i s . Table 2.4 l i s t s some b a s i c p h y s i c a l and chemical p r o p e r t i e s of sulphur. Table 2.5 g i v e s the p r o p e r t i e s of orthorhombic and m o n o c l i n i c sulphur a l l o t r o p e s . The a l l o t r o p i c t r a n s i t i o n temperature of 95.5°C i s an important f a c t o r i n determining the product q u a l i t y . F i g u r e 2.16 shows the sulphur v i s c o s i t y as a f u n c t i o n of temperature. At 159°C, sulphur polymerizes and becomes very v i s c o u s . Thus, d u r i n g c o a t i n g o p e r a t i o n , temperatures should be kept below 159°C to a v o i d the v i s c o u s sulphur r e g i o n . Table 2.4: Physical and Chemical Properties of Sulph Physical State (21°C-1 atm) Bulk density, kg/m3 B o i l i n g point, °C Melting point, °C Odour Flash Point, °C COC Auto i g n i t i o n Temperature (Dust i n a i r , °C) Vapor pressure, 20°C, mm Hg Explosive l i m i t s (Dust i n A i r ) g/m3 S o l i d 1200-1394 Lumps 560-960 Powder 444°C 119°C None 188°C 190°C Less than 0.0001 35 (Lower) 1400 (Upper) 47 Table 2.5: Property Common Name Recommended Name Molecular Formula C r y s t a l l i n e Form U n i t C e l l S t a b i l i t y C o lor Density , g/cm3 Shore B-2 Hardness T e n s i l e Strength, p s i P r o p e r t i e s o f Common Sulphur A l l o t r o p e s S a Orthorhombic Sulphur Orthorhombic (a) Sulphur S128 Orthorhombic 16 Molecules o f S, (S ) A 8 < 95.5°C Opaque Yellow at 24°C 2.07 90 48 S 6 Monoclinic Sulphur Mono c l i n i c (8) Sulphur S48 Monocl i n i c 6 Molecules of S, (S D) A 8 95.5°C to 119°C Between Yellow and Orange 1.96 95 60 48 03 Supercoo led S u l p h u r Normal L iqu id Su lphur O E x t r a p o l a t e d X Calc. Fa r r Q MacLeod Data Q D e t e r m i n e d , R. Fane l l i — D e t . , R. Fane l l i a R.F. B a c o n O \ \ O V _L 40.0 60.0 80.0 100.0 120.0 Temperature, C 140.0 160.0 (22) F i g u r e 2.16: Sulphur V i s c o s i t y as a F u n c t i o n of Temp. 3. EXPERIMENTAL APPARATUS The main components of the present experimental apparatus are (see F i g . 3.1): the spouted bed, the sulphur supply system, the n o z z l e assembly, the urea f e e d i n g and the product withdrawal d e v i c e s , the product c o l l e c t o r s , steam, a i r , and water supply systems. 3.1 THE SPOUTED BED The c o a t i n g o p e r a t i o n takes p l a c e i n the spouted bed, which c o n s i s t s of a 0.152 m I.D., 0.91 m high, 6.35 mm t h i c k c a s t a c r y l i c column, a 60° s t a i n l e s s s t e e l c o n i c a l base, and a s h u t t e r mechanism s e r v i n g as a v a r i a b l e - s i z e o r i f i c e (see F i g . 3.2). E x i s t i n g equipment from the p i l o t p l a n t designed by Meisen and W e i s s ^ 1 ^ was used with the e x c e p t i o n of r e p l a c i n g the o r i g i n a l pyrex g l a s s column with a p l e x i g l a s s column. The p l e x i g l a s s column c o u l d operate c o n t i n u o u s l y up to a temperature of 110°C. Two s e r i e s of e i g h t i d e n t i c a l 50.8 mm by 12.7 mm s l o t s each were d r i l l e d around the perimeter of the column at h e i g h t s of 0.15 and 0.25 m f o r product withdrawal. Depended on which depth the bed was operated a t , one s e r i e s of s l o t s would be open, while the other s e r i e s would be taped shut. Three iron-Constantan thermocouples were p l a c e d at d i f f e r e n t bed l e v e l s to monitor the bed temperatures. 49 F i g u r e 3.1: S i m p l i f i e d Flowsheet of UBC Spouted Bed F a c i l i t y Used i n t h i s Work (designed by Meisen and Ts a i ) LABORATORY AIR INSTRUMENTATION AIR NITROGEN -tXh SULPHUR MELTER UREA STORAGE DRUM COOLING AIR LINE SPOUTING AIR LINE |OF,LTER FEEDER ELECTRIC HEATER 5= ATOMIZING AIR LINE -{Xr-f SULPHUR_ LINE WATER CYCLONE SPOUTED BED EXHAUST SCRUBBER V D R A I N ROTAMETERS , C 0 O L I N G AIR 3 ATOMIZING AIR STEAM JACKETED 2 SPOUTING AIR 4 SULPHUR ELECTRIC HEATING TAPE 51 3 Long Iron - Constantin Thermocouple jC )22 To Cyclone 1 Sampling Port — B P ^/SA YU Top Plate TlO" dia. x I 1/4" S.S.! V J VA zn 6 dia. x 3 x 1/4 Pyrex Glass Vessel Conical Base Shutter Assembly 52 The s h u t t e r mechanism c o n t r o l s the s i z e of the o r i f i c e opening at the base of the bed. As shown i n F i g . 3.3, the s h u t t e r c o n s i s t s of f i v e S-shaped, o v e r l a p p i n g s t a i n l e s s s t e e l l e a v e s arranged i n a c i r c l e . The range of openings i s 3.2 mm to 40 mm. 3.2 SULPHUR SUPPLY SYSTEM The e x i s t i n g system designed by Meisen and W e i s s ^ 1 ^ i s used with minor m o d i f i c a t i o n s . Major components of t h i s system are the sulphur melter, f i l t e r , rotameter, flow c o n t r o l v a l v e , sulphur l i n e , and n i t r o g e n supply. a) Sulphur m e l t e r : A steam-jacketed, s t a i n l e s s s t e e l v e s s e l i s used as the sulphur melter (see F i g . 3.4). The dimensions of the v e s s e l are 0.203 m I.D. and 0.457 m h i g h . The approximate c a p a c i t y i s 25 kg of s u l p h u r . A v a r i a b l e - s p e e d , motor-driven s t i r r e r (Model GT 60-10, manufactured by Sargent Welch, Chicago, 111.) i s used to ensure sulphur u n i f o r m i t y . A pressure r e l i e f and a p r e s s u r e r e g u l a t i n g v a l v e s are added on top of the melter to prevent pressure b u i l d - u p . b) Sulphur f i l t e r : To prevent n o z z l e p l u g g i n g due to p a r t i c u l a t e s , a s t a i n l e s s s t e e l 316 c a r t r i d g e f i l t e r (screen s i z e #100 or 149ium) i s used (Rigimesh, manufactured by P a l l Canada L t d . , 53 54 Steam Inlet 18" .Mixer Shaft Seal Housing Sulphur Fitting Port 3/8 1/4 13/8 I \ 1 Viton 0 Ring I.D. 8 1/2 O.D. 8 3/4 1/2 " j 10 3/4-\ Sulphur Outlet _Sulphur Melter i Condensate j Outlet ^PJ^-rp///////////A\ \f////////TT^w-A 8 o r t o m P ! a t e 1/2 3/8 / / / / t Drainage Plug 4 : ' Condensate Outlet (15) F i g u r e 3 . 4 ( a ) : S e c t i o n a l V i e w o f S u l p h u r M e l t e r ' ( 1 5 ) F i g u r e 3 . 4 ( b ) : S u l p h u r M e l t e r TOP V i e w 56 M i s s i s s a u g a , Ont.). The f i l t e r element i s p l a c e d w i t h i n a steam-jacketed c a s i n g to prevent sulphur s o l i d i f y i n g . See F i g . 3.5 f o r i t s s e c t i o n a l view. c) Sulphur rotameter: As shown i n F i g . 3.6, a standard rotameter tube (Brooks, H a t f i e l d , Penn., Model R-6M-25-A) i n s i d e a steam heated brass block i s used as the sulphur rotameter. Two 316 s t a i n l e s s s t e e l p i e c e s on top and bottom of the brass c a s i n g are used to h o l d the rotameter tube i n p l a c e and v i t o n 0 - r i n g s are used to s e a l the ends. Two polycarbonate windows with heat r e s i s t a n t gaskets are p l a c e d i n the f r o n t and the back of the brass block allow a c l e a r view of the rotameter tube. A set of g l a s s and s t a i n l e s s s t e e l f l o a t s i s used. The advantage of u s i n g a rotameter i s that any stoppage of sulphur flow can be d e t e c t e d immediately. d) Sulphur l i n e : The main sulphur l i n e i s a 6.35 mm, 316 s t a i n l e s s s t e e l tube e n c l o s e d by a 9.53 mm steam heated tube. The l i n e i s then i n s u l a t e d with f i b e r g l a s s . A l l f i t t i n g s are s u p p l i e d by Swagelock. A sulphur flow c o n t r o l v a l v e l o c a t e d upstream of the rotameter i s i n s t a l l e d t o p r o v i d e s t e a d i e r sulphur flow. The v a l v e was a l s o steam heated. e) N i t r o g e n supply: I n d u s t r i a l grade p r e s s u r i z e d n i t r o g e n i s used to f o r c e F i g u r e 3 . 5 : S e c t i o n a l V i e w o f S u l p h u r F i l t e r Condensate ^Outlet \ • Sulphur Outlet Steam Inlet / L . Steam Inlet II 1/8 T I T ^ UL 1 11/2 ,{2 r -Filter Casing -Filter Element Sulphur Inlet / / Condensate Outlet 58 B 14 o o o o o Sulphur Outlet Steam Inlet 3/8" Tubing -3/8" Pipethread •1/16 Bolts B Polycarbonate Window Plug 1/8 MPT Hole 3/8" Sulphur Inlet F i g u r e 3.6: General View of Sulphur Rotameter (15) 59 sulphur out of the m e l t e r . The sulphur flow r a t e i s c o n t r o l l e d by the U p r e s s u r e which can be set by a d j u s t i n g the r e g u l a t o r on the gas c y l i n d e r . 3.3 NOZZLE ASSEMBLY The assembly c o n s i s t s of a p e r f o r a t e d p l a t e , the "bayonet" and the spray n o z z l e . a) P e r f o r a t e d p l a t e : The p e r f o r a t e d p l a t e serves as a flow s t r a i g h t e n e r and a i r d i s t r i b u t o r f o r the spouting a i r ; i t i s a l s o used to p o s i t i o n and a l i g n the n o z z l e and the bayonet (see F i g . 3.7). b) Bayonet: The bayonet i s c o n s t r u c t e d from a 25.4 mm I.D. s t a i n l e s s s t e e l tube, which c o n t a i n s separate sulphur, atomizing a i r , and steam l i n e s (see F i g . 3.8). Sulphur and atomizing a i r l i n e s are welded at the top underneath the spray n o z z l e . Steam i s used t o heat the whole bayonet. At the bottom, the tube f l a r e s out to p r o v i d e a d d i t i o n a l space f o r a thermocouple and a condensate o u t l e t . Thermal expansion problems are overcome by a l l o w i n g the bottom of the bayonet to expand f r e e l y . 60 Air Cap < Re ta ine r R ing ! - « — F l u i d Cap 3/16 0.545" J A 0.455" _r — P e r f o r a t e d Plate >*-l5 Holes 1/4"« . A x i s L ine 2 5 / 8 * -15 Holes 1/4 $ -15 Holes 1/4"* 15 Holes 1/4"* Atomizing Air Line 1/4 "(A Tube •Sulphur Line 1/4 "0 Tube • Steam 1 / 4 " 0 Tube Axis Line 2 3 / 8 * , Axis Line 2 * , Axis Line I 1/2 * F i g u r e 3.7: P e r f o r a t e d P l a t e , Upper Flange and Nozzle S e c t i o n a l View(15) 61 Nozzle 24 '////////////////////////Li// 5 3/4 "7-Bottom Seal Steam Inlet 21 -Top Flange •Gasket Spouting Air Line 3" I.D. Copper Tubing Bayonet I 1/8" O.D. S.S. Tubing Base Plate / / ; ' / / / / / / / / / / / / / / / ' / / "Bottom Flange -Bottom Tee Spouting Air Inlet -Atomizing Air Line Sulphur Line F i g u r e 3.8: S e c t i o n a l View of Nozzle Arrangement and General Assembly (15) 62 c) N o z z l e : An " i n t e r n a l - m i x i n g " type pneumatic n o z z l e i s used f o r s p r a y i n g sulphur (see F i g . 3.7). I t c o n s i s t s of three p a r t s : the f l u i d cap, the a i r cap, and the r e t a i n e r r i n g ( F l u i d Cap 67147, Spraying System Co., Wheaton, 111.). Molten sulphur flows through the f l u i d cap, which narrows i n t o a f i n e t i p (640/xm I.D.). Atomizing a i r e n t e r s through three e q u a l l y spaced holes i n t o the gap between the a i r cap and the f l u i d cap. The a i r and sulphur streams converge j u s t above the n o z z l e t i p thus forming sulphur d r o p l e t s . 3.4 UREA FEEDING DEVICE Urea p e l l e t s are s t o r e d i n a 0.305 m I.D. and 0.710 m high p l a s t i c drum. The drum i s p l a c e d at an e l e v a t i o n of 0.750 m above the spouted bed. From the bottom of the storage b i n , urea p a r t i c l e s f a l l i n t o a v i b r a t i n g magnetic feeder mounted d i r e c t l y underneath the b i n (Model F-10, manufactured by FMC, Homer C i t y , Penn.). T h i s feeder i s c o n t r o l l e d by a v i b r a t i o n c o n t r o l l e r (Model CS3DT, FMC, Homer C i t y , Penn.) and can feed up to 300 g/min of urea (see F i g . 3.9). V a r i o u s combinations of v i b r a t i n g feeder and f e e d i n g tube l o c a t i o n s were t e s t e d . The f i n a l f e e d i n g arrangement 63 F i g u r e 3.9: Urea Feeding Arrangement 64 was as f o l l o w s : At the top of the spouting bed, the v i b r a t i n g feeder i s j o i n e d to a 25.4 mm I.D. feeding tube by a l e n g t h of f l e x i b l e hose. The fe e d i n g tube i s i n s e r t e d i n t o the annulus of the bed. Urea p a r t i c l e s can t h e r e f o r e be fed d i r e c t l y i n t o the bed at v a r i o u s h e i g h t s . 3.5 PRODUCT WITHDRAWAL DEVICE The product withdrawal mechanism depends on g r a v i t y . Sulphur-coated urea d i s c h a r g e d through the column s l o t s are de p o s i t e d i n a p l e x i g l a s s r i n g (see F i g . 3.10). An elongated, 50.8 mm I.D. hole i s d r i l l e d at the bottom of the p l e x i g l a s s r i n g . Product f a l l s through t h i s hole i n t o the product c o o l e r . Two d i f f e r e n t methods were t r i e d to t r a n s f e r product from the p l e x i g l a s s r i n g to the product c o l l e c t o r s . The f i r s t method uses a motor to d r i v e four sweepers to move the product to the o u t l e t h o l e . Four p l e x i g l a s s p l a t e s with t e f l o n p i e c e s a t t a c h e d to the bottom and the s i d e are used as sweepers. They are mounted on a c i r c u l a r sweeper p l a t e . The p l a t e i t s e l f i s r o t a t e d by a v a r i a b l e - s p e e d motor and rode on a p l e x i g l a s s s u p p o r t i n g r i n g . T h i s minimizes wear on the t e f l o n p i e c e s . The second method u t i l i z e s j u s t g r a v i t y flow: the sweeping mechanism i s removed, and seven of the e i g h t s l o t s i n the p l e x i g l a s s column are taped shut. T h e r e f o r e , product 65 F i g u r e 3.10: S e c t i o n a l View of Withdrawl Device 66 e x t r a c t i o n from the column occurs only through a s i n g l e s l o t l o c a t e d d i r e c t l y above the o u t l e t hole i n the p l e x i g l a s s r i n g . The o u t l e t hole and the product c o l l e c t o r s are j o i n e d by a 76.2 mm I.D. f l e x i b l e hose. Copper f i t t i n g s i n s t a l l e d at the end of the hose and at the tops of the product c o l l e c t o r s allow easy c o n n e c t i n g and d i s c o n n e c t i n g . 3.6 PRODUCT COLLECTORS The product l e a v i n g the spouted bed can be d i v e r t e d e i t h e r i n t o a " s t a r t - u p product r e s e r v o i r " or a "product c o o l e r " . Both d e v i c e s are made of pyrex g l a s s . The s t a r t - u p product r e s e r v o i r (0.23 m I.D. and 0.30 m high) can s t o r e approximately 10 kg of s t a r t - u p product. The product c o o l e r reduces the product temperature by pa s s i n g a i r from the l a b o r a t o r y a i r supply through i t . The product c o o l e r (0.30 m I.D. and 0.45 m high) has a c a p a c i t y of 25 kg of product. The product c o o l e r has a 50.8 mm c o o l i n g a i r i n l e t a t the bottom. A double l a y e r of wire mesh screens i s used to support the product and to a c t as an a i r d i s t r i b u t o r . Product i s removed from the c o o l e r u s i n g vaccum s u c t i o n . 67 3.7 DUST COLLECTORS Urea and sulphur f i n e s e l u t r i a t e d from the top of the spouted bed pass through a f l e x i b l e exhaust hose and i n t o a c y c l o n e . A water scrubber f u r t h e r downstream of the exhaust l i n e removes a d d i t i o n a l f i n e dust and odors. The t r e a t e d a i r i s then vented d i r e c t l y i n t o the l a b o r a t o r y exhaust system. The same set-up i s used f o r the product c o o l e r exhaust l i n e (see F i g . 3.11). 3.8 AIR, STEAM, AND WATER SUPPLIES As shown i n F i g . 3.12, a l l a i r l i n e s are connected to the l a b o r a t o r y compressed a i r supply using a common m a n i f o l d (maximum pr e s s u r e of 240 kPa). Instrumentation a i r i s a l s o added to the atomizing a i r l i n e t o pr o v i d e c l e a n e r a i r . Furthermore, a pr e s s u r e r e g u l a t o r i s added to the atomizing a i r l i n e to ensure steady a i r flow. Rotameters are used f o r a l l a i r flow measurements. The spouting a i r i s heated by a 3 KW e l e c t r i c heater c o n t r o l l e d by a p r o p o r t i o n a l - i n t e g r a l c o n t r o l l e r s u p p l i e d by Omega E n g i n e e r i n g Inc., Stamford, Conn. (Model No. 49J, range of 0-200°C). The atomizing a i r i s heated by s i l i c o n rubber e l e c t r i c h e a t i n g tapes (manufactured by Thermolyne Corp., Dubuque, Iowa) wrapped around the a i r l i n e . The temperature i s c o n t r o l l e d a l s o by an Omega model PI F i g u r e 3 . 1 1 : Dust C o l l e c t i o n System Fines Not C o l l e c t e d by Cyclone + A i r CTi CO Water 69 c o w <u c •rH u •r-i < CN cn CD 1-1 Cn •rH r H o a CO D u in xi td ••-> /—Qj Spouting A i r Heater j ^ Spouted Bed 1-1 1-1 •f-l < V-i < •rH cn < Cn c c -rH cn •r-i N c -U •rH •rH 6 r-1 O O O a -M o CO < 70 c o n t r o l l e r (Model No. 49J, range of 0-200°C) and monitored with an i r o n - c o n s t a n t a n thermocouple. The c e n t r a l l a b o r a t o r y steam supply (approximately 550 kPa pressure) i s used to heat the sulphur melter and l i n e s . A pressure r e g u l a t o r i s i n s t a l l e d to set the steam p r e s s u r e to the d e s i r e d v a l u e . The l a y o u t of a l l steam and condensate l i n e s i s shown i n F i g . 3.13. A l l steam t r a p s d i s c h a r g e i n t o a common atmospheric header which d r a i n s i n t o the main sewer system. Cold water from the l a b o r a t o r y supply i s used i n the water scrubber. F i g u r e 3.13: Steam and Condensate System 4. EXPERIMENTAL PROCEDURES 4.1 COATING PROCEDURES The experimental procedures f o l l o w e d d u r i n g the c o a t i n g o p e r a t i o n are d e s c r i b e d below. 4.1.1 START UP The f o l l o w i n g steps have to be performed p r i o r t o c o a t i n g : 4. F i l l the melter with g r a n u l a r sulphur. 5. F i l l the urea storage b i n with urea p a r t i c l e s . 6. S t a r t the water flow to the scrubber. 7. Open the main a i r and steam v a l v e s . Set the steam pr e s s u r e r e g u l a t o r to the d e s i r e d value ( u s u a l l y about 380-415 KPa). 8. Slowly open the steam supply v a l v e u n t i l the pressure reaches the d e s i r e d v a l u e . 9. Switch on the main power supply and the d i g i t a l temperature readout. Check temperatures a t v a r i o u s p o i n t s of the equipment. 10. Switch on the s t i r r e r i n the melter a f t e r approximately two hours of h e a t i n g . I f the s t i r r e r does not tu r n due to incomplete m e l t i n g , wait f o r awhile and t r y a g a i n . Once the s t i r r e r does t u r n , set the speed c o n t r o l l e r to the mid-point of the lower range. T y p i c a l l y , a l l the sulphur w i l l be melted w i t h i n three hours. 72 73 11. Weigh a s p e c i f i c amount of urea ( t y p i c a l l y 4.5 kg) and t r a n s f e r i t to the spouting column while the column i s at the i n t e r m e d i a t e p o s i t i o n . Make sure the sampling port and the s h u t t e r are c l o s e d . 12. Screw i n the spray n o z z l e at the center of the p e r f o r a t e d p l a t e , and t i g h t e n i t with a wrench. Be sure the n o z z l e t i p i s c l e a n . 4.1.2 COATING 1. Open the atomizing a i r v a l v e and turn on the atomizing a i r h e a t e r . Set the temperature to 145°C. 2. Wait u n t i l the atomizing a i r temperature reaches the d e s i r e d v a l u e , then move the spouting column to the " c o a t i n g " p o s i t i o n . Secured the column with b o l t s . 3. Connect the exhaust trunks to the spouting column and the product c o o l e r . Connect the f e e d i n g tube to the feeder and the withdrawl tube to the s t a r t - u p product r e s e r v o i r . 4. Simultaneously open the s h u t t e r and the spouting a i r flow v a l v e u n t i l spouting s t a r t s and the f o u n t a i n reaches a height of about 0.20 m above the bed. T h i s height i s t y p i c a l f o r good, steady c i r c u l a t i o n . 5. Switch on the urea feeder, and set the c o n t r o l l e r to the d e s i r e d feed r a t e , t y p i c a l l y about 120 g/min. 6. Turn on the withdrawal sweeper motor i f t h i s method of withdrawal i s used. 7. Turn on the spouting a i r heater and set the a i r 74 temperature to the d e s i r e d v a l u e . 8. Continue to monitor the temperatures throughout the equipment. Make sure the spouting a i r temperature does o not exceed 110 C, which w i l l cause p l e x i g l a s s to deform. 9. Close the sulphur f i l l i n g p o rt and the p r e s s u r e r e l i e f v a l v e . Open the n i t r o g e n supply v a l v e to about 210 KPa to p r e s s u r i z e the m e l t e r . 10. Open the sulphur flow c o n t r o l v a l v e slowly u n t i l a steady sulphur flow r a t e i s reached. 11. Set the d e s i r e d sulphur flow r a t e by changing the n i t r o g e n p r e s s u r e a p p l i e d to the m e l t e r . 12. Monitor a l l o p e r a t i n g c o n d i t i o n s . Switch the withdrawl o u t l e t from the s t a r t - u p r e s e r v o i r to the product c o o l e r a f t e r the steady s t a t e i s achieved; which normally takes about an hour. Open the c o o l i n g a i r v a l v e to the product c o o l e r . 13. Samples are c o l l e c t e d every hour from the withdrawl o u t l e t by d i s c o n n e c t i n g i t from the product c o o l e r . 14. When the experiment i s completed, c l o s e the n i t r o g e n supply v a l v e and open the p r e s s u r e r e l i e f v a l v e on top of the m e l t e r . 15. Turn o f f the sweeper motor and the urea f e e d e r . 16. A f t e r most of the sulphur has flowed back i n t o the m e l t e r , c l o s e the sulphur flow v a l v e . 17. Switch o f f both the a t o m i z i n g a i r and the spouting a i r h e a t e r s . 18. Reduce the atomizing a i r flow to a minimum v a l u e . 75 Simultaneously c l o s e the s h u t t e r and the spouting a i r v a l v e . 19. Disconnect and swing out the spouted bed. Open the s h u t t e r and unload the r e s i d u a l product from the bed. 20. Remove and weigh the product from the c o o l e r . Weigh a l s o the dust c o l l e c t e d i n the c y c l o n e hopper. 4.1.3 SHUT DOWN 1. Remove the spray n o z z l e and i n s p e c t i t f o r any p l u g g i n g . Soak the n o z z l e i n a 50% NaOH s o l u t i o n u n t i l the next o p e r a t i o n . 2. Turn o f f the s t i r r e r i n sulphur m e l t e r . 3. Close the steam supply v a l v e . 4. Stop the water flow to the scrubber. 5. Switch o f f the main power s w i t c h . 6. C l o s e the steam and a i r v a l v e s at the p l a n t l i m i t s . 7. Clean up the column and the c y c l o n e dust hopper. 4.2 MEASUREMENTS OF OPERATING VARIABLES a) F l u i d Flow Measurements: A l l flow r a t e s were measured by c a l i b r a t e d , f u l l view rotameters. P a r t i c u l a r s of the instruments are presented i n Table 4.1. b) Temperatures Measurements: A l l temperatures were measured with c a l i b r a t e d Table 4.1 F l u i d Flow Measurement Equipment- Rotameters Stream Range 3 , Atomizing A i r 0-1.572 m /hr 3 , . Spouting A i r 0-1.434 m /min . ~ . 3 . . C o o l i n g A i r 0-2.538 m /min Sulphur 0-100 g/min Type Brooks Tube S i z e : R-7M-25-1/ Glass FLoat Brooks Tube S i z e : R-12M-25-4/ F l o a t S i z e : 12-RS-221 Brooks Tube S i z e : R-12M-127-3/ F l o a t S i z e : 12-RS-221 Brooks Tube S i z e : R-6M-25-1/ Glass & S t a i n l e s s S t e e l F l o a t s 77 ir o n - c o n s t a n t a n thermocouples connected to a d i g i t a l d i s p l a y . See F i g . 4.1 f o r the l o c a t i o n s of thermocouples throughout the equipment. 4.3 PRODUCT QUALITY ANALYSIS Sulphur coated urea was analyzed f o r sulphur content and d i s s o l u t i o n i n water. 4.3.1 TOTAL SULPHUR CONTENT The t o t a l amount of sulphur d e p o s i t e d on the urea was determined by c r u s h i n g a known amount of sulphur - c o a t e d urea (about 20 g) i n t o f i n e powder and adding i t to d i s t i l l e d water (200 mL). The s o l u t i o n i s allowed to s i t f o r two hours to ensure that a l l urea has d i s s o l v e d . The s o l u t i o n i s then f i l t e r e d and i t s r e f r a c t i v e index measured with an Abbe refTactometer (Model JB7150, Bausch & Lomb O p t i c a l Co., Rochester, N.Y.). The urea c o n c e n t r a t i o n i s determined from a refTactometer c a l i b r a t i o n curve f o r aqueous s o l u t i o n s of urea. The urea content i n the s o l u t i o n can be c a l c u l a t e d once i t s c o n c e n t r a t i o n i s known. The sulphur content i s then determined by d i f f e r e n c e . 4.3.2 MODIFIED 7-DAY DISSOLUTION TEST The standard 7-day t e s t developed by TVA was m o d i f i e d s l i g h t l y . The standard 7-day t e s t r e q u i r e s a known weight (approx. 10 g) of sul p h u r - c o a t e d urea to be p l a c e d i n 50 mL F i g u r e 4.1: Thermocouple L o c a t i o n 3 ' f i " ro 0 1 2 3 4 5 6 7 8 9 10 1 1 1 2 Ambient Temperature Spouted Bed Column 1 Spouted Bed Column 2 Spouted Bed Column 3 Atomizing A i r Temperature Spouting A i r Heater E x i t Nozzle Arrangement Spouting A i r P r i o r to Bed Sulphur Melter Sulphur F i l t e r Sulphur Rotameter Product Cooler Scrubber L i q u i d E x i t 79 of water i n a capped t e s t tube. The t e s t tube i s then kept i n a constant temperature water bath at 37.8°C f o r seven days. P r i o r t o a n a l y s i s , the t e s t tube i s g e n t l y a g i t a t e d . The urea c o n c e n t r a t i o n i s determined from i t s r e f r a c t i v e index. A p r a c t i c a l problem i s encountered with t h i s t e s t . Since the s o l u t i o n i s stagnant d u r i n g the t e s t p e r i o d , the r e g i o n c l o s e r t o the p e l l e t s has a higher urea c o n c e n t r a t i o n than the region f u r t h e r away from the p e l l e t s . In order to get an sample with the average s o l u t i o n c o n c e n t r a t i o n , the t e s t tube must be g e n t l y shaken before t a k i n g a sample. However, the sulphur c o a t s were extremely f r a g i l e and even g e n t l e a g i t a t i o n may damage the coats and r e s u l t i n a d d i t i o n a l urea r e l e a s e . I t was t h e r e f o r e d i f f i c u l t to ensure r e p r o d u c i b l e r e s u l t s . A m o d i f i e d 7-day d i s s o l u t i o n t e s t (M7T) mwas de v i s e d to overcome t h i s problem. A t e s t tube with a f r i t t e d g l a s s p l a t e and a stopcock at i t s base was used i n s t e a d of a standard t e s t tube (see F i g . 4.2). The procedure i s s i m i l a r to the standard 7-day t e s t : 10 g of s u l p h u r - c o a t e d urea are p l a c e d i n 50 mL of water, and the whole assembly kept i n a water bath at 37.8°C f o r 7 days. The s o l u t i o n i s subsequently d r a i n e d from the assembly and s t i r r e d to achieve uniform urea c o n c e n t r a t i o n . The amount of urea d i s s o l v e d i s determined by r e f r a c t i v e index. Shaking of the F i g u r e 4.2: Test Tube Used in M o d i f i e d 7-Day D i s s o l u t i o n Test 81 t e s t tube i s no longer necessary, and the problem of damaging the coat s i s t h e r e f o r e e l i m i n a t e d . 4.3.3 RAPID DISSOLUTION TEST A r a p i d d i s s o l u t i o n t e s t was a l s o developed to t e s t the q u a l i t y of sulphur-coated urea. I t i s very inconvenient and i m p r a c t i c a l to wait f o r seven days before o b t a i n i n g a measurement of product q u a l i t y by us i n g the 7-day d i s s o l u t i o n t e s t . The Rapid D i s s o l u t i o n Test (RDT) i n i t i a l l y (24) proposed by Meisen and Woolley p r o v i d e d a f a s t e r t e s t f o r q u a l i t y a n a l y s i s . Rather than p l a c i n g s ulphur-coated urea p a r t i c l e s i n stagnant water, the RDT employs a continuous r e c i r c u l a t i o n of s o l u t i o n through a bed of p a r t i c l e s . The apparatus f o r the RDT i s shown i n F i g . 4.3. A b r i e f summary of the procedure i s : Place 30 g of sulphur-coated urea and 150 mL of d i s t i l l e d water i n t o a t e s t tube (35.0 mm I.D. and 0.26 m high) which has a f r i t t e d g l a s s p l a t e as a base. The t e s t tube i s then kept i n a water bath ( I m p e r i a l III Water Bath, Model 18100, Lab Li n e Instrument Inc., Melrose Park, 111.) at 37.8°C. The s o l u t i o n i s c o n t i n u o u s l y r e c y c l e d by a p e r i s t a l t i c pump (M a s t e r f l e x Pump, Model WZ1R057, 7523-00 with pumphead, Model 7018-20, Cole-Palmer Instrument Co., Chicago, 111.). P l a s t i c t u b i ng ( M a s t e r f l e x 6409-18, Tygon R3603) of 9.53 mm I.D. and 2 m long i s connected to the t e s t tube at the top and the bottom to c i r c u l a t e the s o l u t i o n . 82 F i g u r e 4 . 3 ( a ) : G e n e r a l View o f t h e A p p a r a t u s Used i n t h e RDT 83 F i g u r e 4 . 3 ( b ) : F r i t t e d G l a s s T e s t Tube Used i n RDT 84 L i q u i d samples are taken from the s o l u t i o n i n the t e s t tube and t h e i r urea c o n c e n t r a t i o n i s determined from r e f r a c t i v e index measurements. 5. OPERATING EXPERIENCE V a r i o u s problems were encountered with the continuous c o a t i n g u n i t and d i f f e r e n t approaches to s o l v e these problems were t r i e d . In t h i s chapter the problems and s o l u t i o n s are d i s c u s s e d . 5.1 SULPHUR SUPPLY SYSTEM One of the most s e r i o u s problems of the sulphur supply system was p l u g g i n g . Plugging o c c u r r e d e i t h e r i n the sulphur l i n e or at the n o z z l e t i p and was due to e i t h e r s c a l i n g or sulphur f r e e z i n g . To prevent p l u g g i n g due to s c a l i n g , the sulphur supply system was taken apart and c l e a n e d thoroughly. The melter and the f i l t e r were found to be coated with f i l m s of s o l i d sulphur and d i r t . The f i l m s were chipped away with a c h i s e l , and the melter and the f i l t e r were then scrubbed with sandpaper, The sulphur l i n e was f l u s h e d f i r s t with sulphur then with steam. T h i s c l e a n i n g procedure was f o l l o w e d a f t e r each prolonged shutdown. The e n t i r e sulphur l i n e was steam j a c k e t e d to a v o i d sulphur s o l i d i f y i n g . J o i n t s without steam j a c k e t i n g were h e a v i l y i n s u l a t e d to a v o i d c o l d s p o t s . 85 8 6 The i n - l i n e f i l t e r was used to remove p a r t i c u l a t e s to prevent the nozzle t i p from p l u g g i n g . The e n t i r e nozzle was di s c o n n e c t e d from the system a f t e r each experiment and soaked i n a strong c a u s t i c s o l u t i o n (normally 50% NaOH) to d i s s o l v e any remaining s o l i d s u l p h u r . When the nozzle t i p plugged d u r i n g c o a t i n g , the o p e r a t i o n had to be t e m p o r a r i l y d i s c o n t i n u e d . The nozzle i t s e l f was removed from the u n i t and heated with an e l e c t r i c gun to melt any sulph u r . Sometimes h e a t i n g was i n s u f f i c i e n t and the t i p had to be clea n e d by poking a s t i f f wire through i t to remove m a t e r i a l s other than sulphur. The no z z l e was then r e p l a c e d and the o p e r a t i o n r e s t a r t e d . The cause of sulphur s o l i d i f i c a t i o n at the no z z l e t i p was o f t e n due to sulphur f l o o d i n g d u r i n g the i n i t i a l sulphur flow. Excess sulphur c o o l e d and s o l i d i f i e d above the a i r c a p opening and stopped the sulphur flow. T h e r e f o r e , the sulphur flow should be i n c r e a s e d g r a d u a l l y d u r i n g s t a r t - u p . A p o s s i b l e reason f o r sulphur f r e e z i n g at the no z z l e t i p was thought to be inadequate h e a t i n g p r o v i d e d by the ato m i z i n g a i r . The temperature at the t i p was measured t o make sure that i t was not below the sulphur m e l t i n g p o i n t . A drop of onl y 5°C between the no z z l e t i p and the atomizing a i r l i n e was observed (145 to 140°C); so the at o m i z i n g a i r temperature at the no z z l e t i p was adequate to prevent 87 sulphur s o l i d i f i c a t i o n . N i t rogen l e a k i n g from the melter was another reason why sulphur flow would cease. Leaks arose mainly at the f i l l i n g p o r t and the s t i r r e r s h a f t . An a d d i t i o n a l 0 - r i n g was t h e r e f o r e added between the f i l l i n g p o r t cap and the melter top to provide a b e t t e r s e a l . S e v e r a l attempts were made to r e p l a c e the mechanical s e a l of the s t i r r e r s h a f t and to r e a l i g n the s h a f t to ensure gas t i g h t n e s s ; s m a l l amounts of n i t r o g e n leakage c o u l d , however, not be avoided. T h i s caused the sulphur flow r a t e to be unsteady and, at low n i t r o g e n p r e s s u r e s , the l e a k s would become q u i t e s i g n i f i c a n t and r e s u l t i n complete stoppage of the sulphur flow. The s t i r r e r was t h e r e f o r e removed and the s h a f t housing s e a l e d with s i l i c o n rubber. T h i s measure f i n a l l y stopped a l l n i t r o g e n leakages. Steam condensate l e a k i n g from the bottom of the bayonet presented another problem. Condensate quenched the sulphur l i n e and s o l i d i f i e d the s u l p h u r . T h i s problem was overcame by s i l v e r s o l d e r i n g the s e a l s between the bayonet bottom and i t s i n l e t p i p e s . However, due to the d i f f i c u l t y of s i l v e r s o l d e r i n g of s t a i n l e s s s t e e l , three weldings had to be attempted before a c h i e v i n g s u c c e s s . M o d i f y i n g the bayonet assembly by r e p l a c i n g the weldered j o i n t s with compression f i t t i n g s was c o n s i d e r e d . 88 However, s i n c e compression f i t t i n g s r e q u i r e d more space than weldings and the s u r f a c e area at the bottom of the bayonet was l i m i t e d , there was not s u f f i c i e n t room to i n s t a l l compression f i t t i n g s f o r a l l i n l e t p i p e s . T h e r e f o r e , t h i s means of m i d i f i c a t i o n was abandoned. O c c a s i o n a l l y , i t was observed that sulphur sprayed from the n o z z l e even though no n i t r o g e n pressure was a p p l i e d to the m e l t e r . T h i s phenomenon was thought to be due to the sulphur vapor pressure i n the m e l t e r . A r e g u l a t o r v a l v e was i n s t a l l e d upstream of the rotameter, which c o u l d shut o f f a l l sulphur flow when the u n i t was not i n the c o a t i n g mode. Furthermore, a pressure r e g u l a t o r and a f a s t p r essure r e l i e f v a l v e were added to the melter as s a f e t y measures. A l l v a l v e s were steam t r a c e d to a v o i d sulphur p l u g g i n g . 5.2 FEEDING DEVICE The main problem with the urea f e e d i n g arrangement was the placement of the urea storage drum. Because of a concrete beam l o c a t e d above the spouting column, the urea drum had to be mounted to the s i d e of the column i n s t e a d of d i r e c t l y above i t . The v i b r a t i n g feeder was f i r s t mounted below the c o n c r e t e beam and above the spouting column, and a f l e x i b l e duct was used to connect the drum and the feeder. However, the i n c l i n e angle was not s u f f i c i e n t between the drum and the feeder. T h e r e f o r e , t h i s arrangement r e s u l t e d i n 89 low and i n c o n s i s t e n t urea feed r a t e s . To s o l v e t h i s problem, the v i b r a t i n g feeder was removed from i t s o r i g i n a l p o s i t i o n and l o c a t e d d i r e c t l y underneath the urea drum. Urea granules c o u l d f a l l e a s i l y and s t e a d i l y from the drum to the v i b r a t i n g f e e d e r . A f l e x i b l e duct s t i l l needed to connect the o u t l e t of the feeder and the i n l e t of the f e e d i n g tube. However, urea granules r o l l e d down t h i s 37° i n c l i n e d f l e x i b l e duct r e a d i l y . The new arrangement s o l v e d the problem of i n c o n s i s t e n t urea f e e d i n g . O r i g i n a l l y , the f e e d i n g tube was designed to be i n s e r t e d only 12.7 mm deep i n t o the annulus. However, e x c e s s i v e amounts of urea s h o r t - c i r c u i t e d the spray zone and t h e r e f o r e l e f t the bed p o o r l y coated with sulphur. The f e e d i n g tube was then extended to the bottom of the c y l i n d r i c a l s e c t i o n of the column. In a d d i t i o n , b r i d g i n g of urea p a r t i c l e s o c c u r r e d i n s i d e t h i s 12.7 mm I.D. f e e d i n g tube. The tube diameter was t h e r e f o r e e n l a r g e d to 25.4 mm I.D. T h i s r e v i s e d design of the f e e d i n g arrangement allowed t r o u b l e - f r e e , continuous c o a t i n g o p e r a t i o n . 5.3 WITHDRAWAL DEVICES The motor c o n t r o l l e r used f o r the withdrawal sweepers f a i l e d a f t e r a short p e r i o d of o p e r a t i o n . I t c o u l d not be r e p a i r e d due to the u n a v a i l a b i l i t y of spare p a r t s . The 90 m a j o r i t y of the experiments were t h e r e f o r e conducted u s i n g the simple g r a v i t y withdrawal scheme without sweepers. In t h i s withdrawal arrangement only one s l o t was opened f o r product e x t r a c t i o n . Since the column diameter was q u i t e s m a l l , bed l e v e l was f a i r l y c o n s t a n t . The product withdrawal rate was a l s o adequate. 5 . 4 ATOMIZING AIR LINE I t was observed t h a t the sulphur flow r a t e was s i g n i f i c a n t l y a f f e c t e d by the atomizing a i r flow r a t e and/or atomizing a i r p r e s s u r e . A p r e s s u r e r e g u l a t o r was added to the atomizing a i r l i n e to e l i m i n a t e p r e s s u r e f l u c t u a t i o n s and thus p r o v i d e s t e a d i e r flows of a t o m i z i n g a i r and sulphur. 6. RESULTS AND DISCUSSION The c o n v e r s i o n of the e x i s t i n g batch-mode spouted bed c o a t e r to a continuous one was s u c c e s s f u l . The c o n v e r s i o n process i n c l u d e d : a d d i t i o n of a urea feeder, product withdrawal d e v i c e s and product c o l l e c t o r s , as w e l l as m o d i f i c a t i o n to the sulphur supply system, atomizing a i r l i n e , and temperature c o n t r o l s . A f t e r some i n i t i a l d i f f i c u l t i e s , continuous sulphur c o a t i n g of urea was achieved. The spouted bed c o a t e r c o u l d operate c o n t i n u o u s l y f o r up to three hours at a p r o d u c t i o n r a t e of 9.6 kg/hr of su l p h u r - c o a t e d urea. The d u r a t i o n of continuous o p e r a t i o n was l i m i t e d by the s i z e of the sulphur m e l t e r . When the sulphur tank was empty, c o a t i n g had to be stopped to allow r e f i l l i n g and m e l t i n g of s o l i d s u l p h u r . A l l experiments were run to produce a product c o n t a i n i n g approximately 25 wt% s u l p h u r . The product q u a l i t y was compared to the CIL product of s i m i l a r sulphur content by using both the Rapid D i s s o l u t i o n Test (RDT) and the M o d i f i e d 7-day Test (M7T). 91 92 6.1 PRODUCT QUALITY 6.1.1 INADEQUATELY COATED PRODUCT F i g u r e 6.1 shows RDT r e s u l t s of a t y p i c a l product from the UBC continuous spouted bed process (Run #10). The UBC and CIL products had a urea d i s s o l u t i o n value a f t e r 1 minute of 17% and 9%, r e s p e c t i v e l y . A f t e r 45 minutes the corresp o n d i n g v a l u e s were 39% and 25%. These r e s u l t s i n d i c a t e t h at the UBC product was of i n f e r i o r q u a l i t y ; i n p a r t i c u l a r , the hig h i n i t i a l urea r e l e a s e was u n d e s i r a b l e . When analyzed i n M7T, the UBC and CIL products had D of 46% and 88%, r e s p e c t i v e l y . D e s p i t e higher i n i t i a l r e l e a s e as i n d i c a t e d by the RDT, the UBC product had a lower o v e r a l l d i s s o l u t i o n v alue than the CIL product a f t e r 7 days. T h e r e f o r e , the i n i t i a l d i s s o l u t i o n of the UBC product r e q u i r e s p a r t i c u l a r a t t e n t i o n . I t was suspected that the UBC product was unevenly coated and the sulphur content of 100 randomly s e l e c t e d p a r t i c l e s was t h e r e f o r e determined. The r e s u l t s are p l o t t e d on p r o b a b i l i t y paper as shown i n F i g . 6.2. A s i m i l a r a n a l y s i s was performed on 100 CIL p a r t i c l e s f o r comparison purposes. Although the mean sulphur contents f o r both products (UBC Run#l0 and CIL) were approximately 25 wt%, as i n d i c a t e d by F i g . 6.2, 27% of the UBC product c o n t a i n e d l e s s 93 1 1 — I T 1 1 — T T T UBC #10 CIL J _ L _L_L 1 _ L l l 10° 7 101 TIME (MIN) ioa F i g u r e 6 . 1 : D i s s o l u t i o n V a l u e s M e a s u r e d i n t h e RDT f o r UBC Run #10 ( B e d D e p t h = 0 . 1 5 m, B e d T e m p = 8 5 ° C , S p o u t i n g A i r F l o w = 0 . 7 5 m 3 / m i n , A t o m i z i n g A i r F l o w = 0 . 5 5 m-Vh) a n d C I L P r o d u c t s 94 1 — i i i , 1 1 . i — i r i i i i 1 1 1 I I i i I ' > o 00 O X — o v D CO I-LU O LO z o i 0 ^ o DC ZD O 1 « Q_ _ J ZD CO o CM UBC #10 CIL • 1 '„• 1 1 1 L J L 1 L J — J . » j i i I i I L J 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 FRACTION WITH SULPHUR CONTENT LESS THAN X F i g u r e 6.2: Sulphur Content D i s t r i b u t i o n f o r UBC Run #10 and CIL 95 than 10% sulphur; these p o o r l y coated p a r t i c l e s undoubtedly caused the high i n i t i a l r e l e a s e of urea. The CIL product, on the other hand, had only 5% of i t s p a r t i c l e s with l e s s than 10% sulphur. I t was suspected t h a t , i n case of the UBC product, the sulphur d i s t r i b u t i o n was dependent on the l o c a t i o n of the urea f e e d i n g tube i n the spouted bed. I n i t i a l l y , the tube was i n s e r t e d only 12.5 mm i n t o the bed annulus, and some f r e s h urea p a r t i c l e s might bypass the sulphur spray and be d i s c h a r g e d from the bed without being c o a t e d . The f e e d i n g tube was t h e r e f o r e extended i n order to i n t r o d u c e the urea c l o s e r to the spray zone. U n f o r t u n a t e l y , lowering the urea feed tube d i s t u r b e d the bed symmetry. The f o u n t a i n was found to be d e f l e c t e d away from the feed tube. P a r t i c l e s were t h e r e f o r e f r e q u e n t l y d i s c h a r g e d from the f o u n t a i n r a t h e r than the annulus. Such short c i r c u i t i n g was, of course, a l s o u n d e s i r a b l e and r e s u l t e d i n uneven sulphur c o a t i n g . To reduce s h o r t - c i r c u i t i n g , a l l but one s l o t was taped shut on the spouted bed column. The product, which was ob t a i n e d a f t e r these m o d i f i c a t i o n s were made, y i e l d e d improved RDT valu e s (see F i g . 6.3). The 1 min d i s s o l u t i o n v alue decreased from 17% to 12.5%, and the 45-minute urea d i s s o l u t i o n value dropped from 96 q d o CO o CO CM o d o o d ~i r i — r i i i ~ ] — T Q > O 6 CO w CO ^ o UBC #19A CIL i i i i i i i i i i 10° 5 7 101 TIME (MIN) 102 F i g u r e 6.3: D i s s o l u t i o n Values Measured i n the RDT f o r UBC Run #19 (Bed Depth=0.15 m, Bed Temp=75°C Spouting A i r Flow=0.80 m 3/min, Atomizing A i r Flow=0.45 nW/h) and CIL Products 97 dropped from 39% to 25.6%. The urea d i s s o l u t i o n a f t e r 45 minutes f o r the UBC and CIL products was t h e r e f o r e s i m i l a r . However, the i n i t i a l urea r e l e a s e was s t i l l higher f o r the UBC product. T h e r e f o r e , the q u a l i t y of the UBC product should be f u r t h e r improved. In the M7T, the UBC product had a D._ value of 37%, which was s i g n i f i c a n t l y l e s s than the CIL product (88%). I t was suspected t h a t the s i z e of the sulphur spray cone produced by the nozzle might not be s u f f i c i e n t ; some p a r t i c l e s c o u l d pass through the spray zone without being coated. T h e r e f o r e , experiments were performed by i n c r e a s i n g the atomizing a i r pressure from from 105 KPa to 208 KPa; t h i s should c r e a t e a l a r g e r spray zone and improve c o a t i n g . However, i t was found that the atomizing a i r pressure a f f e c t e d the sulphur flow r a t e , and the n i t r o g e n pressure had to be r a i s e d to pr o v i d e a constant sulphur flow. There was some i n i t i a l concern about the a b i l i t y of the sulphur rotameter to withstand the p r e s s u r e , but no problems were encountered a t n i t r o g e n p r e s s u r e s up to 240 kPa. F i g u r e 6.4 shows the sulphur d i s t r i b u t i o n of the UBC product (Run #25C) produced at i n c r e a s e d atomizing a i r pr e s s u r e . I t i s evident t h a t only 9% of the product c o n t a i n e d l e s s than 10% sulp h u r . T h i s i s a s i g n i f i c a n t r e d u c t i o n i n p o o r l y coated product and the RDT va l u e s were a l s o lowered (see F i g . 6.5). The UBC product has 1 min 98 i i r 1 1 — r LB3 25C CIL • • • • 5 10 20 30 40 50 60 70 80 90 95 FRACTION WITH SULPHUR CONTENT LESS THAN~~X 98 99 F i g u r e 6.4: Sulphur Content D i s t r i b u t i o n f o r UBC Run #25C and CIL 99 o 6 o i d CO o CO C\2 Q > O o CO w CO 3 h w 2 PS o d o i d o d n r i—i i i i -i 1—i—i—rnr CIL UBC #25C i i i i i 10° io» i i • 7 10s TIME (MIN) F i g u r e 6.5: D i s s o l u t i o n Values Measured i n the RDT f o r UBC Run #25C (Bed Depth=0.15 m, Bed Temp=77°C, Spouting A i r Flow=0.65 m 3/ m i n' Atomizing A i r Flow=0.45 m-Vh) and CIL Products 100 d i s s o l u t i o n v a l u e s i n the range of 10% which were comparable to the CIL product. Moreover, the UBC product has 45 min d i s s o l u t i o n v a l u e s of about 20%, which were l e s s than the CIL product. S i m i l a r r e d u c t i o n i n 7 day d i s s o l u t i o n v a l u e s were a l s o found; the UBC product r e l e a s e d only 25% of i t s urea a f t e r 7 days as measured by the M7T t e s t . 6.1.2 HEAVILY COATED PRODUCT Apart form inadequately coated product, the UBC product a l s o c o n t a i n e d a s i g n i f i c a n t f r a c t i o n of h e a v i l y coated p e l l e t s . S ince spouted beds behave somewhat l i k e w e l l (26) s t i r r e d v e s s e l s , i t i s t h e o r e t i c a l l y p o s s i b l e f o r a small f r a c t i o n of p a r t i c l e s t o stay i n the bed f o r very long p e r i o d s of time. Such p a r t i c l e s would become h e a v i l y coated with s u l p h u r . In p r a c t i c e , some o v e r s i z e d p a r t i c l e s were produced i n the spouted bed. A f t e r o p e r a t i n g c o n t i n u o u s l y f o r 2.5 to 3 hours, p e l l e t s up to 10 mm i n diameter appeared; the mean p e l l e t s i z e was about 2.5 mm. The l a r g e r p a r t i c l e s are expected to have a s h o r t e r c i r c u l a t i o n path i n the bed because they are heavy and are not c a r r i e d a l l the way to the top of the f o u n t a i n . S i n c e they may not reach the top of the annulus, t h e i r chances of (26) d i s c h a r g i n g from the column are a l s o decreased 101 Sulphur content d i s t r i b u t i o n s of samples taken a f t e r 1, 2 and 3 h of o p e r a t i o n are shown in F i g . 6.6. As time progressed, the slope of the p r o b a b i l i t y curves i n c r e a s e s which i n d i c a t e s l a r g e r p o r t i o n s of p o o r l y coated and overcoated product. The p o o r l y coated f r a c t i o n g i v e s a high i n i t i a l d i s s o l u t i o n , whereas the overcoated f r a c t i o n d i s s o l v e s only very s l o w l y . Both of these tendencies are u n d e s i r a b l e . The e x i s t i n g withdrawal mechanism c o u l d not s e l e c t i v e l y withdraw o v e r s i z e d p a r t i c l e s . I n c r e a s i n g the spouting a i r flow r a t e should improve the mixing of small and l a r g e p a r t i c l e s and l e a d to the d i s c h a r g e of o v e r s i z e d p e l l e t s . Experiments were performed with higher spouting a i r flows up 3 . . to 1.10 m /min. However, prolonged use of h i g h spouting a i r flow r a t e s a l s o caused an e x c e s s i v e d i s c h a r g e of small p a r t i c l e s from the f o u n t a i n , and decreased the bed l e v e l i n the column. The decreased bed depth coupled with the high spouting a i r flow r a t e i n c r e a s e d the f o u n t a i n height which l e d t o f u r t h e r p a r t i c l e d i s c h a r g e , and the bed depth co n t i n u e d to drop. Such i n s t a b i l i t y r e s u l t s i n unsteady o p e r a t i o n . T h e r e f o r e , continuous experiments c o u l d not be run with e x c e s s i v e spouting a i r flows. Since the d u r a t i o n of the experiment was l i m i t e d by the s i z e of the sulphur melter to 3 hours, i t was not p o s s i b l e to i n v e s t i g a t e f u r t h e r on the bed i n s t a b i l i t y due to the 1 0 2 X r-I — ~Z. LU r-o o QC Z> X Q_ _J Z) c/) co • 1 HR B 2 HR o A 3 HR o CD O LO o o co o CVJ a A a A a • A " • a • A • J L 5 10 20 30 40 50 60 70 80 90 95 FRACTION WITH SULPHUR CONTENT LESS THAN X 98 99 F i g u r e 6.6: Sulphur Content D i s t r i b u t i o n f o r Samples of 1, 2 & 3 h of Continuous O p e r a t i o n 1 03 h e a v i l y coated p a r t i c l e s beyond 3 hours. However, w i t h i n the 3-hour l i m i t , c o a t i n g was f a i r l y s t a b l e . 6.2 OPTIMAL OPERATING TEMPERATURE A f t e r a d d r e s s i n g the problem of inadequately coated product, experiments were performed to f i n d the optimal . . (10) o p e r a t i n g c o n d i t i o n s . Previous s t u d i e s by Weiss with the batch UBC spouted bed process showed that the bed temperature had the g r e a t e s t e f f e c t on product q u a l i t y and that the lowest urea d i s s o l u t i o n v a l u e s occured f o r product obtained at bed temperatures of approximately 80°C. The e f f e c t of bed temperature i n continuous c o a t i n g was examined f o r the range of 55 to 90°C. F i g u r e s 6.7 and 6.8 show the 45 min d i s s o l u t i o n v a l u e s i n the RDT and the 7 day d i s s o l u t i o n v a l u e s i n the M7T as f u n c t i o n s of bed temperature. Both f i g u r e s i n d i c a t e that the d i s s o l u t i o n decreased i n i t i a l l y with i n c r e a s i n g temperature. T h i s was fol l o w e d by an i n c r e a s e beyond the minimum of about 80°C. V i s u a l i n s p e c t i o n s of the samples c o l l e c t e d showed that product had a b r i g h t e r yellow coat at higher bed temperatures than at lower temperatures. T h i s i s a l s o c o n s i s t e n t with Weiss' f i n d i n g s f o r batch o p e r a t i o n . F i g u r e 6.7: D i s s o l u t i o n Values as F u n c t i o n s of Bed Temperature Measured i n the RDT 7> q i i i i i i i i i i I o 50.0 60.0 70.0 80.0 90.0 100.0 BED TEMPERATURE (C) F i g u r e 6 . 8 : D i s s o l u t i o n Values as F u n c t i o n s of bed temperature Measured i n the M7T 106 F i g u r e s 6.7 and 6.8 both i n d i c a t e minimum d i s s o l u t i o n v alues at approximately 80°C r which i s about 15°C below the phase t r a n s i t i o n temperature of S to S„ (95.5°C). When a a p thermocouple was i n s e r t e d i n t o the c e n t e r of the spout 0.10 m above the n o z z l e , a temperature of 90°C was measured compared to the mean bed temperature of 80°C. T h e r e f o r e , the experimental data seem to support Weiss' e x p l a n a t i o n f o r the bed temperature e f f e c t . 6.3 EFFECT OF BED DEPTH Experiments were a l s o performed to study the e f f e c t of bed depth on product q u a l i t y . In Weiss' work^ 1 <^ i t was found that bed l e v e l had very l i t t l e e f f e c t on d i s s o l u t i o n r a t e s i n the range of 0.15 to 0.25 m. To t e s t the e f f e c t of bed l e v e l i n continuous c o a t i n g , bed l e v e l s of 0.15 m and 0.25 m were used. For the same urea and sulphur flow r a t e s , experimental runs were conducted over a temperature range of 55 to 90°C f o r the two d i f f e r e n t bed depths. The r e s u l t s are p l o t t e d i n F i g s . 6.9 and 6.10. The d i s s o l u t i o n values were higher f o r the 0.25 m than f o r the 0.15 m bed. Although the d i f f e r e n c e was not g r e a t , i t was d i s t i n c t . Part of the problem encountered with the deeper beds was that i n s u f f i c i e n t a i r was a v a i l a b l e f o r s p o u t i n g . Greater bed depth c o u l d a l s o r e s u l t i n more p a r t i c l e s h o r t - c i r c u i t i n g and by-passing the spray zone. I t i s F i g u r e 6 . 9 : D i s s o l u t i o n Values as F u n c t i o n s of Bed Depth Measured i n the RDT 108 F i g u r e 6.10: D i s s o l u t i o n Values as F u n c t i o n s of Bed Depth Measured i n the M7T 109 t h e r e f o r e p l a u s i b l e f o r the product to c o n t a i n more inadequately coated m a t e r i a l and to g i v e r i s e to higher d i s s o l u t i o n v a l u e s . Furthermore, s i n c e g r e a t e r bed depth r e q u i r e s a higher minimum spouting v e l o c i t y , the s o l i d c i r c u l a t i o n r a t e i s a l s o i n c r e a s e d and thereby leads to l e s s (27) sulphur d e p o s i t i o n per urea p a r t i c l e per pass . More p a r t i c l e s t h e r e f o r e were p o o r l y coated when d i s c h a r g e d from the bed. Co a t i n g should be performed i n the 0.15 m bed f o r b e t t e r product q u a l i t y . 6.4 COMPARISON BETWEEN UBC AND CIL PRODUCTS With the present l a b o r a t o r y f a c i l i t y , the best product q u a l i t y was found under the f o l l o w i n g o p e r a t i n g c o n d i t i o n s : mean bed temperature of 80°C, bed depth of 0.15 m, spouting 3 3 a i r flow of 0.65 m /min, and atomizing a i r flow of 0.45 m /h at p r essure of 208 kPa. D i s s o l u t i o n v a l u e s f o r samples produced under these c o n d i t i o n s had b e t t e r d i s s o l u t i o n c h a r a c t e r i s t i c s than the CIL product. F i g . 6.11 p r o v i d e s a comparison between products produced under the optimal c o n d i t i o n s i n the UBC f a c i l i t y and the CIL f a c i l i t y . Although the 1 min RDT d i s s o l u t i o n values f o r both the UBC and the CIL products were s i m i l a r (approximately 9%), t h i s value i s s t i l l q u i t e h i g h . However, f u r t h e r r e d u c t i o n of the d i s s o l u t i o n has not yet been achieved with the e x i s t i n g UBC f a c i l i t y . In batch-wise T 1 1—I I I I CD CIL ( 25 wt% Sulphur) • UBC #25B ( 29 wt% Sulphur) O UBC #25C (28 wt% Sulphur) — X UBC #3 IB (25 wt% Sulphur) 3 5 7 101 3 5 7 10a TIME (MIN) igure 6.11: Product Q u a l i t y Comparison f o r UBC Runs #25B, #25C, #31B and CIL Measured in RDT 111 o p e r a t i o n (Run #16), the spouted bed process y i e l d e d a product which had a f i r s t minute r e l e a s e r a t e of 3%. UBC product showed a slower r e l e a s e a f t e r 45 minutes i n RDT than the CIL product. D i s s o l u t i o n v a l u e s of 15% or l e s s were ach i e v e d with the UBC product compared with 24% f o r the CIL product. F i g u r e 6.12 p r o v i d e s a comparison of product q u a l i t y i n terms of the M o d i f i e d 7-day T e s t . The UBC product had d i s s s o l u t i o n v a l u e s i n the range of 25 to 35% whereas the CIL product had a value of 88%. Although none of the products w i t h 25 wt% of sulphur from the UBC spouted bed process have achieved 7-day d i s s o l u t i o n v a l u e s of 25%, which i s the i n d u s t r i a l requirement, Runs #25A, #25B, and #29 with sulphur content of approximately 30 wt% have d i s s o l u t i o n v a l u e s below 25% (see Table 6.1). Since i t may be l e s s expensive t o coat urea with 30 wt% sulphur than with 25 wt% sulphur and wax, the UBC spouted bed process has demonstrated promising p o t e n t i a l f o r commercial p r o d u c t i o n . 1 12 F i g u r e 6.12: Product Q u a l i t y Comparison f o r UBC Runs #25B,#25C, #30B, #31A, #31B and CIL Measured i n M7T 1 1 3 Table 6.1: Runs from the UBC F a c i l i t y Which Had 7-Day D i s s o l u t i o n Values of 25% or Less Run # Sulphur 7-day D i s s o l u t i o n Value, % Content, wt% 25A 30.7 25.0 25B 29.2 21.0 2 6A 45.0 17.8 26B 46.5 21 .9 26C 42.0 20.0 27 37.5 22 .9 29 32.2 22.3 7. CONCLUSIONS AND RECOMMENDATIONS 7.1 LABORATORY TESTS S u c c e s s f u l continuous sulphur c o a t i n g of urea was achieved i n a m o d i f i e d l a b o r a t o r y - s c a l e spouted bed f a c i l i t y , which can produce 9.6 kg/hr of SCU f o r up to 3 hours. D i s s o l u t i o n t e s t a n a l y s e s showed that the product q u a l i t y i s comparable t o , i f not b e t t e r , than CIL's commercial product. Some spouted bed product has met the i n d u s t r i a l d i s s o l u t i o n standard of 25% i n 7 days. The best product q u a l i t y i s obt a i n e d when o p e r a t i n g with a bed temperature of 80°C and a bed depth of 0.15 m. From experimental measurements, the product q u a l i t y was found to be a f u n c t i o n of bed temperature and, to a l e s s e r degree, bed depth. 7.2 RECOMMENDATIONS Fu r t h e r s t u d i e s should be done with the e x i s t i n g l a b o r a t o r y f a c i l i t y to determine the e f f e c t s of urea feed and sulphur flow r a t e s on product q u a l i t y . The maximum c a p a c i t y of the present p l a n t should a l s o be determined f o r sca l e - u p c o n s i d e r a t i o n s . A new p i l o t p l a n t should be b u i l t to e l u c i d a t e and e l i m i n a t e the occurence of uncoated and overcoated p r o d u c t s . Some form of s i e v e s may be i n s t a l l e d i n 1 14 115 the spouting column to remove o v e r s i z e d p a r t i c l e s , to r e c y c l e u n d e r s i z e d ones, and to withdraw on l y s p e c i f i c s i z e d p a r t i c l e s as f i n a l p roduct. Instead of using p r e s s u r i z e d n i t r o g e n to t r a n s f e r molten sulphur, a submerged c e n t r i f u g a l pump should be c o n s i d e r e d f o r t h i s purpose. R e f i l l i n g s o l i d sulphur i n t o the melter becomes f e a s i b l e with t h i s system because i t i s not under p r e s s u r e , and the s i z e of the sulphur melter w i l l not l i m i t the o p e r a b i l i t y of the f a c i l i t y . Thus, the e f f e c t s of h e a v i l y coated p a r t i c l e s on bed s t a b i l i t y and on product q u a l i t y can be f u r t h e r i n v e s t i g a t e d beyond the 3-hour l i m i t . Another design c o n s i d e r a t i o n f o r the f u t u r e p i l o t p l a n t i s the feed l o c a t i o n of the f r e s h urea. I f urea p a r t i c l e s are fed i n t o the the spouted bed from the bottom r a t h e r than from the top, i t w i l l no longer r e q u i r e a submerged f e e d i n g tube i n the bed annulus. Furthermore, i t i s observed that the amount of p o o r l y coated p a r t i c l e s was reduced with the feed l o c a t i o n nearer to the spray zone. T h e r e f o r e , f e e d i n g f r e s h urea p a r t i c l e s d i r e c t l y i n t o the c o n i c a l bottom should minimize p a r t i c l e s bypassing the spray zone. S t u d i e s should a l s o c ontinue on d e v e l o p i n g the r a p i d d i s s o l u t i o n t e s t and determining a c o r r e l a t i o n between i t and the i n d u s t r i a l l y accepted 7-day d i s s o l u t i o n t e s t as w e l l as the m o d i i f i e d 7-day d i s s o l u t i o n t e s t . In t h i s t h e s i s , no 116 attempt was made to analyze product q u a l i t y i n the standard 7-day t e s t ; t h e r e f o r e the c o r r e l a t i o n s among v a r i o u s d i s s o l u t i o n t e s t s are not known. In the f u t u r e , product q u a l i t y should be determined i n a l l three t e s t s to e s t a b l i s h c o r r e l a t i o n s among them. (15) In Weiss' work i t was noted that the product q u a l i t y decreased a f t e r a p e r i o d of time. T h i s aging e f f e c t was not s t u d i e d e x t e n s i v e l y i n t h i s t h e s i s ; however, t h i s aspect of the product q u a l i t y should be examined i n d e t a i l i n the f u t u r e . The e f f e c t of d i f f e r e n t urea s u b s t r a t e s a l s o bears s i g n i f i c a n t importance. A l l experiments were done i n t h i s t h e s i s with g r a n u l a r urea, the use of p r i l l e d urea should a l s o be c o n s i d e r e d . The uses of a d d i t i v e s to strengthen the sulphur coat and s e a l a n t s to s e a l the pores and cr a c k s on the sulphur coat should be i n v e s t i g a t e d . Furthermore, s e v e r a l runs from the present p i l o t were found to have m o d i f i e d 7-day d i s s o l u t i o n v a l u e s of 25% or l e s s with a sulphur content of 30 wt% or more. A q u a l i t y comparison between products with more than 25 wt% sulphur and products with 25 wt% sulphur and wax should be done. I f a s u p e r i o r or comparable product can be produced without s e a l a n t or a d d i t i v e s but with g r e a t e r sulphur content, i t o f f e r s an a l t e r n a t i v e t o be 1 1 7 c o n s i d e r e d f o r commercial p r o d u c t i o n . 8. REFERENCES 1. Anonymous, Sulphur I n s t . J . 8 ( 4 ) , ( 1 972). 2. B l o u i n , G.M., Rindt, D.W. and Moore, O.E. J . Agr. Food Chem. 19(5), (1971). 3. Rindt, D.W., B l o u i n , G.M., and G e t s i n g e r . J.G., J . Agr Food Chem. 16(5), (1968) . 4. M c C l e l l a n , G.H. and Scheib, R.M., Sulphur I n s t . J, 9(3-4), (1973). 5. A l l e n , E.R. and Mays, D.A., J . Agr. Food Chem. 16, (1971) . 6. L i e g e l , E.A. and Walsh, L.M., J . Agr. Food Chem. 68, (1976) . 7. Sharma, G.C., P a t e l , A .J. and Mays, D.A., J . Am. Soc H o r t i c , S c i . 101 (2), ( 1 976). 8. J a r a m i l l o C e l i s , R. and Bazan, R., T u r r i a l b a 26(1), (1976) . 9. Mathur, K.B. and Meisen, A., P r i v a t e communication, (1975). 10. Weiss, P.J. and Meisen, A., The Canadian Jour, of Chem. Engr., 6J_(6), ( 1983). 11. Zee, C.J., B.A. Sc. T h e s i s , U.B.C., (1977). 12. Anonymous, Sulphur I n s t . J . 8 ( 4 ) , (1972). 13. S h i r l e y , A.R. J r . , and Mel i n e , R.S., New Uses of  Sulphur, Advances i n Chemistry S e r i e s v. 140, ACS, Washington, D.C. (1975). 14. Anonymous, Sulphur I n s t . J . 8 ( 4 ) , (1974). 118 119 15. Weiss, P.J., M.A. Sc. T h e s i s , Dept. of Chemical E n g i n e e r i n g , U.B.C., (1981). 16. TVA B u l l e t i n , TVA DEMONSTRATION SCALE PLANT. 17. Nukiyama,S., and Tanasawa, Y., Trans. Soc. Mech. Engrs. (Japan), 4-6, Reports 1-6 (1938-40). T r a n s l a t e d by E. Hooc f o r Defence Research Board, Dept. of N a t i o n a l Defence, Canada, 10 M-9-47(393), H.Q. 2-0-264-1 (March 18, 1950). 18. J e f f r e y Lim, B.A. Sc. T h e s i s , Dept. of Chemical E n g i n e e r i n g , U.B.C., (1978). 19. Meisen, A. and Mathur. K.B., Paper presented at the 2nd I n t e r n a t i o n a l Conference on F e r t i l i z e r s . Proceedings of the B r i t i s h Sulphur C o r p o r a t i o n - Part I, December 3-6 (1978) . 20. Meyer, B., Chem. Rev. 64, 429, (1964). 21. F r e e p o r t Sulphur Co., Sulphur Data Book, McGraw-Hill (1954). 22. S t a u f f e r Chemical Co., B u l l e t i n , (1967). 23. Donahue, J . and Meyer,B., The Naming of Sulphur  A l l o t r o p e s , Elemental Sulphur, B Meyer, by John W i l l e y and Sons Inc. (1965). 24. Dale, J.M. and Ludwig, A.C., Mechanical P r o p e r t i e s of  Sulphur A l l o t r o p e s , Mat. Res. & Stand., (Aug. 1965). 25. Woolley, J . and Meisen, A., P r i v a t e communication, (1984) 26. P i c c i n i n i , N., Bernhard, A., Campagna, P. and V a l l a n a , F., The Canadian Jour, of Chem. Engr., 55(4), (1977). 27. Mathur, K.B. and E p s t e i n , N. Spouted Beds, Academic Press, New York (1974). APPENDIX I 1.1 D i s s o l u t i o n Test R e s u l t s 1.2 Operating C o n d i t i o n s 120 121 Table 1.1 D i s s o l u t i o n Test R e s u l t s Urea D i s s o l u t i o n Rate, % Run Sulphur Rapid Modi f i e d No. Content, D i s s o l u t i o n Test 7-Day Tes wt% 1 minute 45 minutes 7 days 1 22.4 17.4 25.7 35.9 2 24.7 14.3 23.0 70.5 3 17.2 33.9 58.3 76.0 4 17.9 44.0 67.2 83.4 5 35.5 6.4 12.8 30. 1 6 33.0 15.1 26.4 36. 1 7 32.0 13.5 31.2 39.3 8 17.9 21.1 38.3 52.6 9 17.9 16.5 34.2 50.7 1 0 25.4 16.9 38.7 46.3 1 1 26.2 15.9 37.9 47.4 1 2 15.5 31.2 66.4 71.9 -13 28.5 12.2 20.7 31 .9 1 4 22.4 16.4 31 .9 58.6 15 • 19.4 12.3 43.3 56.2 16 23.2 2.9 53.9 70.5 17 24.2 13.5 37. 1 42.2 122 Run Sulphur No. Content, wt% 18 26.6 19A 23.2 19B 17.9 20 17.2 21A 39.8 21B 26.2 22A 31.5 22B 29.2 23A 26.2 23B 24.7 24A 23.2 24B 22.4 24C 23.2 25A 30.7 25B 29.2 25C 28.5 26A 45.0 26B 46.5 26C 42.0 27 37.5 28A 24.0 28B 26.2 Urea D i s s o l u t i o n Rate, % Rapid M o d i f i e d D i s s o l u t i o n Test 7-Day Test 1 minute 45 minutes 7 days 13.0 29.3 38.5 12.5 25.6 37.0 13.3 34.9 46.5 21.4 45.0 56.8 6.9 20.7 31 .9 12.8 29.2 33.2 7.9 19.5 43.9 6.8 18.1 43.2 15.3 32. 1 53.0 17.2 30.4 52.5 17.2 3.0.4 41 .7 16.5 • 26.7 36.9 15.8 26. 1 51 .5 9.2 19.6 25.0 6.9 12.2 21 .0 1 0 s 0 18.4 28.0 6.8 15.1 17.8 6.8 15.2 21.9 4.5 14.3 20.0 7.8 15.1 22.9 6.9 18.8 29.8 10.2 24.0 33.2 123 Run Sulphur No. Content, wt% 29 32.2 30A 28.5 30B 27.2 30C 32.2 30D 21.0 31A 24.0 31B 25.4 31C 26.9 31D 25.5 31E 27.0 32A 18.0 32B 21.7 32C 24.0 32D 23.2 33A 26.2 33B 27.7 34A 21.7 34B 23.2 35A 26.2 35B 22.4 36A 28.4 36B 26.9 37 26.2 Urea D i s s o l u t i o n Rate, % Rapid M o d i f i e d D i s s o l u t i o n Test 7-Day Test 1 minute 45 minutes 7 davs 7.2 15.6 22.3 8.9 19.5 34.0 9.4 21.3 31 .8 7.7 18.4 32.3 12.2 25.8 39.1 8.9 20.8 34.8 8.6 17.8 34.4 12.1 22.6 33.6 12.0 24. 1 37.0 12.9 27.9 39.3 16.1 30.9 35.9 14.4 28.2 37.5 10.4 22. 3 38.6 9.8 20.1 39.7 17.2 30. 1 36.2 15.3 32.2 46.9 12.0 22.6 36.1 11.7 21.6 42.2 18.4 27.5 43.4 11.7 25.7 43.6 8.9 21.6 52. 1 9.2 23.9 58.3 17.9 29.6 64.2 T a b l e 1.2 Summary o f O p e r a t i n g C o n d i t i o n s S p o u t i n g A i r R u n B e d H t B e d Temp F l o w Temp m ' C m 3 / m m " C 1 0 . 1 5 54 1 .00 68 2 0 . 1 5 52 1.05 65 3 0 . 1 5 50 0 . 7 0 65 4 0 . 1 5 50 1 .00 65 5 0 . 1 5 70 0 . 8 0 74 6 0 . 1 5 6 5 , 0 . 8 5 70 7 0 . 1 5 57 1 .10 61 8 0 . 1 5 65 0 . 7 5 71 9 • 0 . 1 5 74 0 . 7 5 78 10 0 . 1 5 85 0 . 7 5 90 " ^ 11 0 . 1 5 79 0 . 7 5 88 12 0 . ' 15 67 0 . 7 5 75 13 0 . 1 5 82 0 . 9 5 89 14 0 . 1 5 76 0 . 7 5 85 A t o m i z i n g A i r F l o w Temp P r e s s . m 3 / h r " C k p a 0 . 5 5 145 104 0 . 6 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 5 5 145 104 0 . 4 5 145 104 0 . 3 5 145 104 0 . 3 5 145 104 U r e a S u l p h u r F e e d F1ow Temp g/m i n g /m i n " C 100 30 150 100 30 150 100 2 2 150 100 22 150 100 35 150 100 36 150 100 36 150 100 28 150 100 . 3 1 150 100 30 150 100 30 150 100 2 0 150 100 37 150 100 35 150 S p o u t i n g A f r R u n B e d Ht B e d Temp F l o w Temp m ' C m J / rn i n ' C 15 0 . 1 5 68 0 . 8 0 76 16 0 . 2 5 85 0 . 8 5 80 17 0 . 1 5 8 1 0 . 7 5 80 18 0 . 1 5 8 0 0 . 8 0 80 19A 0 . 1 5 75 0 . 8 0 76 19B 0 . 1 5 71 0 . 8 0 70 2 0 0 . 1 5 62 0 . 7 0 90 21A 0 . 1 5 7 0 1 .00 50 21B 0 . 1 5 7 0 1 .00 50 22A 0 . 1 5 66 0 . 9 5 40 22B 0 . 1 5 76 0 . 9 5 68 23A 0 . 1 5 6 0 0 . 9 0 35 2 3 B 0 . 1 5 64 0 . 9 0 41 24A 0 . 15 63 0 . 9 0 40 24B 0 . 1 5 6 0 0 . 7 0 30 A t o m i z i n g A i r U r e a S u l p h u r F l o w Temp P r e s s . F e e d F l o w Temp m / h r ' C k P a g / r a i n g /m i n C 0 . 3 5 145 104 100 22 150 0 . 4 5 145 104 6 0 150 0 . 4 5 145 104 100 38 150 0 . 5 0 145 104 100 37 150 0 . 4 5 145 104 100 37 150 0 . 4 0 145 104 3 0 0 65 150 0 . 3 5 145 104 3 0 0 76 150 0 . 5 5 145 104 9 0 54 150 0 . 5 5 145 104 9 0 46 150 0 . 5 0 145 104 9 0 39 150 0 . 5 0 145 104 9 0 39 150 0 . 5 0 145 104 100 33 150 0 . 5 0 145 104 100 33 150 0 . 3 7 145 104 100 33 150 0 . 3 7 145 104 100 33 150 S p o u t i n g A i r -Run B e d Ht B e d Temp F l o w Temp m ' C m 3 / r a i n ' C 24C 0 . 1 5 56 0 . 7 0 26 25A 0 . 1 5 85 0 . 6 0 76 25B 0 . 1 5 8 0 0 . 6 5 68 25C 0 . 1 5 77 0 . 6 5 6 0 26A 0 1 5 77 0 . 6 0 6 0 26B 0 . 1 5 72 0 . 7 0 52 26C 0 . 1 5 7 0 0 . 70 45 27 0 . 1 5 8 0 0 . 6 0 69 28A 0 . 1 5 7 ! 0 . 6 0 48 28B 0 . 1 5 69 0 . 6 0 48 2 9 0 . 1 5 7 9 0 . 9 5 68 30A 0 1 5 74 0 . 6 5 68 SOB 0 . 1 5 74 0 . 6 5 68 30C 0 . 1 5 78 0 . 6 5 76 3 0 0 0 . 1 5 78 0 . 6 5 76 ^ A torn i 2 i n g A i r F1ow Temp P r e s s . n i 3 / h r ' C kPa 0 . 3 7 145 104 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 0 145 2 1 0 0 . 4 0 145 2 1 0 0 . 4 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 3 7 145 2 1 0 0 . 4 0 145 2 1 0 0 . 4 Q 145 2 1 0 0 . 4 0 145 2 1 0 0 . 4 0 145 2 1 0 U r e a S u l p h u r F e e d F1ow Temp g / m i n cj/in i n " C 100 33 150 100 33 150 100 33 150 100 33 150 100 33 150 100 33 150 100 33 150 6 0 32 150 6 0 3 0 150 6 0 30 150 100 64 150 120 42 150 120 42 150 120 4 2 150 120 42 150 Run B e d Ht B e d m " c 31A 0 . 1 5 78 31B 0 . 1 5 81 31C 0 . 1 5 83 31D 0 . 1 5 87 3 1 E 0 . 1 5 87 32A 0 . 2 5 79 32B 0 . 2 5 81 32C 0 . 2 5 82 32D 0 . 2 5 83 33A 0 . 2 5 85 33B 0 . 2 5 85 34A 0 . 2 5 77 34B 0 . 2 5 78 35A 0 . 2 5 73 35B 0 . 2 5 72 S p o u t i n g A i r Temp F l o w Temp m 3 / m i n ' 0 0 . 6 5 78 0 . 6 5 78 0 . 6 5 88 0 . 6 5 93 0 . 6 5 94 0 . 9 0 88 1 .00 87 1 .00 88 1 .10 88 1 .10 89 1.10 89 0 . 8 5 76 0 . 9 0 76 1 .00 62 1.05 62 A torn i z i n g A i n F1ow Temp P r e s s . n i 3 / h r " C kPa 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 4 5 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 U r e a S u l p h u r F e e d F1ow Temp g/m i n g / m i n ' C 120 42 150 120 42 150 120 42 150 120 42 150 120 42 150 1235 43 150 135 43 150 135 46 150 135 49 150 135 49 150 135 4 9 150 135 45 150 135 45 150 135 45 150 135 45 150 Run B e d Ht B e d m ' C 36A 0 . 2 5 67 36B 0 . 2 5 6 7 37 0 . 2 5 6 0 S p o u t i ng A i r Temp F l o w Temp m J / m i n ' C 1 .00 48 1.05 48 1 .10 4 0 A tain i z i n g A i r F l o w Temp P r e s s . n i 3 / h r ' C kPa 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 0 . 5 0 145 2 1 0 U r e a S u l p h u r F e e d F I ow Temp g/m i n g/in i n ' C 135 45 150 135 45 150 135 45 150 APPENDIX II I I . 1 C a l i b r a t i o n Curves f o r Rotameters II.2 C a l i b r a t i o n Curve f o r Abbe RefTactometer 129 F i g u r e II 1 : C a l i b r a t i o n Curve f o r C o o l i n g A i r Rot (Temp.: 20°C, P r e s s . : 101.28 kPa) 131 d 0.0 0.3 0.6 0.9 1.2 1.5 AIR FLOW RATE (m3/min) F i g u r e I I . 2 : C a l i b r a t i o n Curve f o r Spouting A i r Rotameter (Temp.: 20°C, P r e s s . : 101.28 kPa) 132 F i g . I I . 3 : C a l i b r a t i o n Curve f o r Atomizing A i r Rotemeter (Temp.: 20°C, P r e s s . : 101.28 kPa) 133 F i g u r e I I . 4 : C a l i b r a t i o n Curve f o r Sulphur Rotameter Temp.: 148°C (Glass F l o a t ) 134 0.0 20.0 40.0 60.0 80.0 100.0 Urea Concentration (g/L) F i g . I I . 5 : Abbe RefTactometer C a l i b r a t i o n Curve f o r Aqueous S o l u t i o n s of Urea 

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