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Circulating fluidized bed hydrodynamics in a riser of square cross-section Zhou, Jiahua 1995

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CIRCULATING FLUIDLZED BED HYDRODYNAMICS IN A RISEROF SQUARE CROSS-SECTIONByJiahua ZhouB.A.Sc., Zhejiang University, 1987M. A. Sc., Zhejiang University, 1989A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIES(Department of Chemical Engineering)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAApril 1995©JiahuaZhou, 1995In presenting this thesis In partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis br scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)__________Department of cHB’tLc/4L_IvrThe University of British ColumbiaVancouver, CanadaDate ,JVLYDE.6 (2/88)AbstractHydrodynamic experiments were carried out in a cold-model circulating fluidized bed riserof 146 mm x 146 mm square cross-section and 9.14 m height. Ottawa sand particles of meandiameter 213 urn, particle density 2640 kg/rn3 and loosely packed bed voidage 0.43 were used asthe bed material. Fibre optic probes were employed to measure both local time-mean voidage andparticle velocity.A core-annulus flow structure was found to exist in the square riser as for risers of circularcross-section investigated by earlier workers. The voidage generally decreases laterally from theaxis towards the wall. However, M-shaped lateral voidage profiles, which can also bedistinguished in earlier reported data from circular risers, were found in a number of cases, withvoidage first increasing from the wall, reaching a maximum, then decreasing somewhat towardsthe axis. In the corners, the voidage was lower while the descending particle velocity was higherthan for other near-wall locations at the same cross-section. Bimodal and tn-modal probabilitydistributions of particle concentration were found in some cases. The velocity of those particleswhich are ascending was found to be low at the wall and to increase towards the axis; there was amaximum in the lateral profile of descending particle velocity at a small distance from the wall.The velocity of particle downflow was found to be in the range 0.8 to 1.5 mIs. A theoreticalmodel has been established to predict the velocity of descending clusters near the wall. The modelsuggests that the aspect ratio of ellipsoidal clusters has only a limited influence on the clustervelocity. The thickness of the outer annular zone first decreased with height until a minimum wasreached 3 to 4 m above the distributor, then increased towards the top because net solids massflux was outward towards the wall near the bottom and inward towards the axis near the top ofthe column. Significant differences were found between core-annulus boundaries defined as thelocation of zero vertical solids flux and zero vertical mean particle velocity, with the annulusthickness based on the latter definition being smaller.11Wall roughness, introduced by affixing sand paper to the entire wall surface, was found tolead to an increase in voidage near the wall, while having little influence on the voidage near theaxis. More uniform lateral voidage profiles were obtained for the rough-walled riser. Neitherbimodal nor trimodal probability distributions of particle concentration were found for the rough-walled column, nor were there any M-shaped voidage profiles. Wall roughness increased theascending particle velocity somewhat, while having little influence on the descending particlevelocity near the wall. In a riser with simulated membrane walls, the valleys formed by the fin andtwo adjacent membrane tubes protect particles from the gas. Particle streamers near the wall thentend to move downwards in the fin region, leading to higher voidages and increase descendingparticle velocities than in the crest region or for flat smooth walls. It was found that wall featuressuch as roughness and membrane tubes have significant influence on the gas-solids flow structurenear the wall of circulating fluidized bed risers. Little influence of wall roughness and membranetube could be detected near the axis of the column.A sampling probe and piezoelectric probe were used to measure lateral solids mass fluxand momentum flux. Except at the very bottom of the riser, cross-flow fluxes were alwayssubstantially lower than (axial) net circulation fluxes, but high enough to assure considerableinterchange between the wall and core regions. Lateral fluxes were highest at the bottom of theriser, relatively constant at intermediate levels, then increased slightly near the top. The solidsmomentum flux was found to increase with height in the lower part of the column, and thendecrease in the upper part. The lateral particle velocity was as high as 2 to 3 mIs on the axis ofthe riser.111Table of ContentsAbstract iiList of Tables viiList of Figures viiiAcknowledgment xviChapter 1. Introduction 11.1. Fast Fluidization 11.1. Scope of Work 8Chapter 2. Experimental Set-up 112.1. Apparatus 112.2. Particles 142.3. CFB Systems Operation 152.4. Selection of Instrumentation 16Chapter 3. Voidage Profiles 183.1. Introduction 183.2. Instrumentation 213.2.1. Fundamentals of Fibre Optic Probe 223.2.2. Systems and Principle 253.2.3. Calibration Method 273.3. Results and Discussion 363.3.1. Basic Profiles 363.3.2. Probability Analysis 513.3.3. Intermittency Index Profiles 563.4. Summary 62Chapter 4. Velocity Profiles 644.1. Introduction 644.2. Fibre Optic Particle Velocity Probe 684.3. Experimental Results and Discussion 734.3.1. Lateral Velocity Profiles 744.3.2 Axial Velocity Profiles 784.3.3 Wall-Layer Particle Descending Velocity 80iv4.3.4 Core-Annulus Boundary 904.3.5 Exit Effect 1004.4. Summary 102Chapter 5. Influence of Wall Roughness on the Hydrodynamics 1045.1. Introduction 1045.2. Experimental Set-up 1055.3. Experimental Results and Discussion 1055.3.1. Voidage Profiles 1065.3.2. Intermittency Index Profiles 1135.3.3. Particle Velocity Profiles 1175.4. Summary 124Chapter 6. Influence of Membrane Wall 1266.1. Introduction 1266.2. Experimental Set-up 1276.3. Experimental Results and Discussion 1286.3.1. Voidage Profiles 1296.3.2. Intermittency Index Profiles 1336.3.3. Particle Velocity Profiles 1346.4. Summary 137Chapter 7. Particle Cross-Flow, Lateral Momentum Flux and Lateral Velocity 1407.1. Introduction 1407.2. Equipment and Instrumentation 1417.2.1. Sampling Probe 1417.2.2 Piezoelectric Probe 1437.3. Experimental Results and Discussion 1487.3.1. Solids Cross-Flow Mass Flux 1487.3.2. Lateral Momentum Flux 1597.3.3. Lateral Particle Velocity 1637.4. Summary 164Chapter 8. Conclusions and Recommendations 1678.1. General Conclusions 1678.2. Recommendations 171VNomenclature 172References 176Appendix I. Statistic Method in Analysis of Voidage and Particle Velocity Data 186I1.2Test 1861.1.1 Theory 1861.1.2. Sample Calculation 186I.2tTest 1881.2.1 Theory 1881.2.2. Sample Calculation 189Appendix II. Comparison Between Measured Lateral Solids Mass Fluxand Model Predictions by Senior and Brereton (1992) 191viList of TablesTable 2.1. Size distribution of sand particles. 15Table 3.1. Examples of fibre optic probes used to measure voidageand particle velocity. 23Table 4.1. Relative contributions of each force component for a clusterofE=1, E=0.8, e=O.7 and a=3 mm, assumingF0=1. 86Table 4.2. Descending particle velocities near the wall of CFB risersmeasured by various researchers. 89Table 7.1. Particle velocity and solids fluxes used in calibration ofpiezo-electric probe. 147Table 1.1. x2 test for voidage data from two different days at both walland axis of riser for Ug4.S mis and (3=20 kglm2s, z=0.79 m. 187Table 1.2. t test for particle descending velocities for Ug=5.mis andG=4O kglm2sat wall and on axis of riser. 190viiList of FiguresFigure 1.1 Fig. 1.1 Fluidization phase diagram for a fine powder, showingschematic diagrams of equipment suitable for the bubbling,turbulent and fast fluidization regimes (Yerushalmi et a!., 1976). 2Figure 1.2. Phase diagram showing fluidization regime for fluid crackingcatalyst (Yerushalmi et al., 1976). 3Figure 1.3. Qualitative representation of pressure gradients measured in amodified 152 mm system by Yerushalmi and Cankurt (1979).(a) across the bottom section (609 mm in height).(b) between pressure taps at heights of 609 mm and 2286 mmabove the bottom of the riser. 4Figure 1.4. Fluidization data for a fluid cracking catalyst: slip velocity vs.solid concentration obtained in a 152 mm i.d. riser (Yerushalmi& Cankurt, 1979). 5Figure 1.5. Qualitative fluidization map for fine solids due to Yerushalmi&Cankurt (1979). 5Figure 1.6 Fluidization regime map showing practical operating regions ofthe various hydrodynamic regime Grace (1986). Approximateboundaries between the different powder groups proposed byGeldart are shown at the bottom (Grace, 1990). 7Figure 2.1 Schematic of the circulating fluidized bed system. 12Figure 3.1. Configuration of reflective fibre probe. 24Figure 3.2 Type I: Detection of swarm of particles; Type II: Detection ofa single particle (Matsuno et al., 1983). 26Figure 3.3. Calibration method for fibre optic voidage probe employed byMatsunoetal, 1983. 28Figure 3.4. Calibration curve for fibre optic voidage probe by Matsunoet al. (1983). 29Figure 3.5. Calibration curve for fibre optic voidage probe obtained by QinandLiu, (1982). 31Figure 3.6. Calibration curve for fibre optic voidage probe employed byHartge et al. (1986). 32viiiFigure 3.7. Calibration curve for fibre optic voidage probe. 33Figure 3.8. Schematic of gas-solid system used to confirm a linearrelationship between voltage and solids concentration forthe fibre optic voidage probe. 34Figure 3.9. Verification of the linear relationship for the fibre optic voidageprobe using the gas-solid system shown schematically in Fig. 3.8 35Figure 3.10. Local instantaneous voidage versus time for a point near thetop of the riser. (Ug=S.5mis, G5=40 kg /m2s, xIX=0, y/Y=-1,z=8.98 m and 6=0.9). 37Figure 3.11. Axial profiles of local time-mean voidage for xIX=0, Ug=5.5 flhlSand G8=’40 kg /m2s. 38Figure 3.12. Axial profiles of time-mean voidage at the wall for two differentsolids circulation rates for xIX=0, yIY=-l and Ug=S.5 tillS. 39Figure 3.13. Axial profiles of local time-mean voidage at the axis for twodifferent solids circulation rates for xlXz0, y/Y=- 1 andUg”5.mIS. 40Figure 3.14. Axial profiles of local time-mean voidage at the wall with twodifferent superficial gas velocities for xIX=0, yIY=- 1 andG=40kgIm2s. 41Figure 3.15. Axial suspension density profiles compared with fitted curvescorresponding to equation (3.4) with ç=o.8 42Figure 3.16. Lateral profiles of local time-mean voidage near the bottom ofthe riser at four different gas velocities for xJX=0, z=0.79 andG=20 kglm2s. - 44Figure 3.17. Lateral profiles of local time-mean voidage for Ug7.O mis,G54O kglm2sand z6.20 m 45Figure 3.18. Lateral profiles of voidage for x/X0, Ug’5.5 mIs, G540 kglm2s. 46Figure 3.19. Pictures from Wei et al. (1993) showing a high voidage circulararea between center and wall regions. 48Figure 3.20. Comparison between simulation results and experimental datashowing similar lateral voidage profiles. 50ixFigure 3.21. Axial profiles of probability distribution of local time-meanparticle concentration, C = (1— s)/(1—) at the wall forUg=5.S mis, G=40 kglm2s, xlX=0 and y/Y=-1. C=1corresponds to packed bed particle concentration, i.e. 6=0.43. 52Figure 3.22. Axial profiles of probability distribution of local time-meanparticle concentration, CK (1 s)/(1—) at the axis forUg5.S mis, G40 kglm2s, x/X=O and yIY=O. C=1corresponds to packed bed particle concentration, i.e. 6=0.43. 53Figure 3.23. Lateral profile of probability distribution of local time-meanparticle concentration, C = (1 )/(1—) for Ug=5.5 mIs,G5=40 kg/m2s, x/X=0 and zO.79 m. C*1 corresponds topacked bed particle concentration, i.e. 6=0.43. 54Figure 3.24. Lateral profile of probability distribution of local time-meanparticle concentration, C = (1——) at the wall forUg5.S mis, G5=40 kglm2s, y/Y=-l, z=6.20 m. C1corresponds to packed bed particle concentration, i.e. e=0.43. 55Figure 3.25. Probability distribution of local time-mean particle concentration,= (1——) at the top of the riser for Ug5.mis, G5=40kglm2s, x!X=0, z=8.89 m. C*=1 corresponds to packed bedparticle concentration, i.e. 6=0.57. 57Figure 3.26. Lateral profiles of intermittency index at four levels for x/X=O,Ug5.m/s, G540 kglm2s. 59Figure 3.27. Axial profiles of intermittency index at the wall and at the axisof the column for xlX=0, Ug=S.5 m/s, G540 kg/m2s 60Figure 3.28. Lateral profiles of intermittency index for Ug=7.O mis,G54O kglm2sand z’6.2 m. 61Figure 3.29. Lateral profile of intermittency index near the top of the riserfor Ug5.m/s, G4O kg/m2s, x/X=0, z=8.98 m. 62Figure 4.1. Schematic of fiber optic and signal processing system used tomeasure particle velocity. 69Figure 4.2. Schematic of five-fibre optical particle velocity probe. 71xFigure 4.3. Particle velocity distribution for Ug=S .5 mIS, Gs=2O kglm2s,xIX=0, ylY=O. Mean particle velocities: upwards: 5.84 mIs,downwards: -0.97 mIs. Number of sampled particles:upwards: 1949, downwards: 51. 73Figure 4.4. Lateral profiles of local particle velocities for different solidsfluxes: Ug=S.S mis, z=6.2 m, x/X0. 74Figure 4.5. Lateral profiles of particle velocities for different superficialgas velocities: G5=40 kglm2s; z=6.2 m, x/X0. Forcoordinates see Fig. 2.1. 75Figure 4.6. Lateral profiles of fractions of particles travelling upwardsfor different solids fluxes: Ug= 5.5 mIs, z=6.2 m, xIX=0. 77Figure 4.7. Lateral profiles of fractions of particles travelling upwardsfor different superficial gas velocities: Gg4O kglm2s,z=6.2m;xIX=0. 78Figure 4.8. Vertical profiles of particle velocities and fraction of particlesdescending along wall of column: UgS.S m/s, G52O kg/m2s,xIX=0,yIY=1. 79Figure 4.9. Vertical profiles of particle velocities and fraction of particlesascending along axis of column: UgS.5 m/s, G5=2O kglm2s,xJX=0, yIYO. 80Figure 4.10. Vertical forces on spheroidal clusters near the wall. 83Figure 4.11. Shear stress on the wall of the riser by downflowing particlesvs. solids flux adapted from Van Swaaij et al. (1970). 84Figure 4.12. Simulation of the descending velocity of a spherical cluster,i.e. E=1, vs. diameter and cluster internal voidage for Ottawasand particles: Ug5.m/s and G=40 kg/m2s. 87Figure 4.13. Simulation of the descending velocity of a spherical cluster,i.e. E=1, vs. diameter and cluster internal voidage for FCCparticles: Ug5.rn/s and G=40 kglm2s. 88Figure 4.14. Predicted velocity of downflowing particle clusters near thewall vs. height-to-width ratio, E, for 0.75 and a6 mm. 90Figure 4.15. Schematic of the sampling system to measure net verticalsolids flux. 93xiFigure 4.16. Axial profiles of annular wall layer thickness for Ug=5.mIS,G8= 40 kglm2s, xIX=0. 94Figure 4.17. Lateral profiles of annular wall layer thickness for z=5. 13 m,Ug5.mis, G 40 kg/m2s. 95Figure 4.18 Lateral profiles of velocities and fraction of particles descendingnear the wall of column: Ug5.mis, G’40 kglm2s, z=5. 13 m,x/X=1. 96Figure 4.19. Lateral profiles of mean particle velocity: z=5. 13 m, Ug=5.mis,G8=40 kglm2s. 98Figure 4.20. Lateral profiles of annular wall layer thickness for: Ug=5.5 mIs,G=40 kglm2s. Insert shows shape of wall layer boundary attwo different heights assuming symmetry around axes. 99Figure 4.21. Lateral profiles of particle velocities and fraction of particlesascending at top of column: Ug=5.S mis, G’=40 kglm2s,z8.98 m, x/X=0. 100Figure 4.22. Lateral profiles of local particle velocities at top of column fordifferent solids fluxes: Ug=S.S mis, z=8.98 m, x/X=0. 101Figure 5.1. Comparison of axial profiles of voidages obtained fromdifferential pressure measurements for smooth- and rough-walled risers for UgS.S rn/s and G=40 kg/m2s. 107Figure 5.2. Axial profiles of time-mean voidage along the wall of the riserfor xIX=0, y/Y0, Ug5.mis, G=4O kg/m2s. 108Figure 5.3. Axial profiles of time-mean voidage along the axis of the riserfor x/X=0, y/Y0, Ug5.mis, G5=4O kglm2s. 108Figure 5.4. Lateral profiles of probability distribution of local time-meanparticle concentration, C’ = (1— 6)/(1—e) for Ug5.5 mis,G54O kg/m2s, x/Xr=0 and z0.79 m. C*l corresponds tothe packed bed particle concentration, i.e. 6=0.43. 109Figure 5.5. Lateral profiles of time-mean local voidage for x/X=0, z=z7.06 m,Ug5.mis, Gs4O kg/m2s. 111Figure 5.6. Lateral profiles of time-mean voidage near the exit of the riserfor xJXO, z8.98 m, Ug’5.mis, G540 kg/m2s. 112xliFigure 5.7. Lateral profiles of time-mean voidage showing corner effectfor z=6.2 m, Ug=7.O mis, G5=40 kglm2s. 114Figure 5.8. Axial profiles of intermittency index at the wall for x/X=0,y’Y=l, Ug=5.mIS, G540 kg/m2s. 115Figure 5.9. Axial profiles of intermittency index along the axis for x/X0,y/Y=O, Ug=5.mis, G5=40 kg/m2s. 116Figure 5.10. Lateral profiles of intermittency index near the riser exit forx/X=0, z8.98 m, Ug=5.mis, G=40 kglm2s. 116Figure 5.11. Lateral profiles of intermittency index for z6 .20 m,Ug=7.O mis, G=40 kglm2s. 118Figure 5.12. Lateral profiles of particle velocity for x/X=0, z=6.2 m,Ug5.S mis, G=40 kglm2s. 119Figure 5.13. Lateral profiles of fraction of particle which are ascending forx/X=0, z6.2 ffi, Ug=5.mis, G=40 kglm2s. 119Figure 5.14. Axial profiles of particle velocity at the wall for x/X=0,yfl, Ug=5.mis, G=40 kglm2s. 120Figure 5.15. Axial profiles of particle velocity along the axis for xlX=0,y/Y=0, Ug=S.5 mis, G=40 kg/m2s. 121Figure 5.16. Lateral profiles of velocities and fractions of rising particlesshowing corner effect for y/Y=-1, z=5. 13 m, Ug=5.m/s,G=40 kglm2s. 122Figure 5.17. Lateral profiles of particle velocity at the top for x/X=0,z8.98 m, Ug5.mis, Gr40 kg/m2s. 123Figure 6.1. Configuration of membrane wall. Tubes are normally vertical. 126Figure 6.2. Schematic of the window allowing measurements nearmembrane wall. 128Figure 6.3. Axial profiles of time-mean voidage near membrane wallshowing influence of operating conditions. 130Figure 6.4. Voidage profiles near membrane wall for z=6.7 m, Ug=5.S misand G5=40 kg/m2s. 131xliiFigure 6.5. Lateral voidage proffles for z=6.7 m, Ug=S.S rn/s andG=4O kg/m2salong the three parallel dashed lines shown inFig. 6.2. 132Figure 6.6. Fig. 6.6. Lateral profiles of intermittency Index for z=6.7 m,Ug=5.5 rn/s and G=4O kg/m2s. For positions of profiles seeFig. 6.2. 133Figure 6.7. Axial profiles of vertical particle velocity and fraction ofparticles which are descending for UgS.5 rn/s andG=40kg/m2s. 135Figure 6.8. Lateral profiles of particle velocities and fraction of descendingparticles near the membrane tube for z6.7 m, Ug=5.S tfl/S,G=40 kg/m2sand z=6.7 m. 137Figure 6.9. Lateral profiles of particle velocities and fraction of descendingparticles for z=6.7 m, Ug=5.rn/s and G40 kg/m2s. 138Figure 7.1. Schematic of the solids sampling probe. 142Figure 7.2. Schematic of the piezoelectric solids momentum flux probe 144Figure 7.3. Calibration system for the piezoelectric solids momentumflux probe. 145Figure 7.4. Calibration curve of piezoelectric probe. 147Figure 7.5. Axial profiles of solids cross-flow flux on the axis (x=y=O)showing the influence of operating conditions: (a) Ug7.O ni/s;(b) G540 kg/m2s. 149Figure 7.6. Axial profiles of outward solids cross-flow flux at wall(x/X=0, y/Y1) showingthe influence of operating conditions:(a) Ug7.O m/s; (b) G5=40 kg/m2s. 150Figure 7.7. Lateral profiles of solids cross-flow flux for xJX=0, z=6.2 m,Ug5.mis, G5=40 kg/m2s. 151Figure 7.8. Lateral profiles of solids cross-flow flux for xJX=0, z=6.2 m:(a) Ug7.O mis; (b) G5=40 kg/m2s. 153Figure 7.9. Lateral profiles of net cross-flow solids flux for x/X=0, z=6.2 m:(a) Ug7.O mis; (b) G5=40 kglm2s. 154xivFigure 7.10. Lateral profiles of net solids cross-flow flux at different heightsfor x/X=0, Ug5.mis, G54O kglm2s. 155Figure 7.11. Lateral profiles of solids cross-flow flux for z=6.2 m,Ug=5.miS, G540 kglm2s. 157Figure 7.12. Lateral profiles of net solids cross-flow flux for z=6.2 m,Ug=5.S mis, G54O kglm2s. 158Figure 7.13. Sensitivity of solids cross-flow to pressure difference forUg7.O mis, G5=40 kglm2s, xIX=0, z=0.77 m. 159Figure 7.14. Axial profiles of lateral solids momentum flux about the axis(x=y=0): (a) Ug7.O mis; (b) G5=4O kglm2s. 161Figure 7.15. Horizontal profiles of lateral solids momentum flux for xlX=0,z=6.2 m: (a) Ug7.O mis; (b) G5=40 kglm2s. 162Figure 7.16. Horizontal profiles of lateral solids momentum flux for z=6.2 m,Ug7.O mis, G5=4O kglm2s. 163Figure 7.17. Axial profiles of lateral particle velocity about the axis (x=y=0):(a) Ug7.O mis; (b) G5=40 kglm2s. 165Figure 8.1. Schematic of flow structure of a CFB riser of square cross-section. 170Figure 11.1. Comparison between measured net lateral solids mass fluxesand model predictions by Senior and Brereton (1992) forUg5.S mis and G=40 kglm2s 192Figure 111.1. Amplifying circuit with input resistance of 10122 for thepiezoelectric probe. 193Figure 111.2. A sample trace for piezoelectric probe. 193xvAcknowledgmentI would like to express my sincere gratitude to my main supervisor, Dr. J.R. Grace for hiscontinuous guidance and support over the entire course of this work. My appreciation also goesto the co-supervisors, Dr. C.H.M. Brereton and C.J. Lim for their ideas, discussion andassistance. I am indebted to Professor S. Qin of the Institute of Chemical Metallurgy, Beijing forhis input with respect to instrumentation. Appreciation is expressed to Dr. J.-X. Zhu and Dr. K.S.Lim for their discussion and assistance. The equipment owes a great deal to the ChemicalEngineering Workshop and Stores. I also wish to thank everybody in the Fluidization Group forhelpful discussions. Financial support from the operating funds of the Natural Sciences andEngineering Research Council of Canada is gratefully acknowledged.Finally I want to express my appreciation to my wife Yamei for her encouragement andsupport.xviChapter 1IntroductionGas-solid fluidization has been widely investigated in the latter half of this century becauseof its important role throughout the chemical process industries, as well as in diverse areas such aselectrical power generation, food processing, pharmaceuticals manufacture, mineral processing,etc. The beds are useful both as chemical reactors (gas-solid and solid-catalyzed), when gas andsolid particles must be brought into contact, and in certain physical processes (e.g. drying ofparticulate material, coating, quenching).Particles in fluidization systems for air at atmospheric pressure and temperature wereclassified by Geldart (1972, 1973) into four groups (A, B, C, and D) depending on their particlesize and particle density. Grace (1986) extended Geldart’s methods to gases other than air and topressures and temperature other than atmospheric. Different fluidization regimes, in particularhomogeneous, bubbling, turbulent, and fast fluidization have long been recognized and studied.1.1 Fast FluidizationThis thesis is concerned with the fast fluidization regime, which requires that thesuperficial gas velocity, Ug, be greater than the transport velocity, U (Bi, 1994). Beyond fastfluidization, there is dilute pneumatic transport at even higher superficial gas velocities. For fastfluidization and pneumatic transport, to keep the system operating, continuous feeding of particlesis necessary to replace particles being carried out of the system rapidly by the high-velocity gas.Yerushalmi et al. (1976) were the first to use the term “fast fluidization” to indicate aregime between turbulent fluidization and pneumatic transport. The onset of fast fluidization was1defined as the point above which bed density becomes strongly dependent on solids feed rate. Thisis shown in Figs 1.1 and 1.2. Yerushalmi and Cankurt (1979), as illustrated in Fig. 1.3, describedthe transport velocity as a superficial gas velocity below which a sharp change in pressure gradient(or choking) occurs. They further defined fast fluidization as the regime above the minimumtransport velocity for any solid circulation rate as indicated in Figs 1.4 and 1.5. However, Rhodesand Geldart (1986) and Schnitzlein and Weinstein (1988) could not identify a transport velocity intheir systems. Kato et al. (1992) reported that the transport velocity varies greatly with the designof apparatus. These conflicting reports on the transport velocity indicate that the transition to thefast fluidization regime is not well understood.dPlog(—)dzFig. 1.1 Fluidization phase diagram for a fine powder, showing schematicdiagrams of equipment suitable for the bubbling, turbulent and fast fluidizationregimes (Yerushalmi et al., 1976).H1G4ERlog(Ug)22. 3 4- 5675f0Gas Velocity, ft/sFig. 1.2. Phase diagram showing fluidization regime for fluid cracking catalyst(Yerushalmi et al., 1976).• Transition toFluidizationC,—S20 -Jo5-432FastFluidization24l5_1__1 I I I I I I -20 30 40A ‘fast fluidized bed” (termed circulating fluidized bed in this thesis) has been described(Yerushalmi et al, 1976) as a dense entrained suspension characterized by an aggregative state inwhich many of the solid particles are segregated in relatively large, densely packed clusters andstrands. The particle aggregation was used to explain the ability to operate with high relative gas-solids slip velocities, often much higher than single particle terminal velocities. Grace and Tuot(1979) explained the formation of the clusters and strands in terms of flow instability. Arena et al.(1989) described flow instability as the reason for clusters in circulating fluidized bed risers.3“ALBOTTOMSECTION(a)1APALNEARBOTTOM(b)SOLID RATEFig. 1.3. Qualitative representation of pressure gradients measured in a modified152 mm diameter system by Yerushalmi and Cankurt (1979).(a) across the bottom section (609 mm in height).(b) between pressure taps at heights of 609 mm and 2286 mm above the bottomof the riser.SOLID RATEGs4400(12C)00(123200 0.1 0.2 0.3 0.4 0.6 0.8 101.change in scaleFig. 1.4. Fluidization data for a fluid cracking catalyst: slip velocity vs. solidconcentration obtained in a 152 mm i.d. riser (Yerushalmi & Cankurt, 1979).BUBBLINGftUIDIZEDBEDTRANSPORTRISERREFLTOR1 -E 1mfFig. 1.5. Qualitative fluidization map for fine solids due to Yerushalmi & Cankurt (1979).5Since 1980, many flow regime diagrams for fast fluidization have been proposed, andthere have been various definitions of fast fluidization (Li and Kwauk; 1980, Yang, 1983;Takeuechi et al., 1986;. Grace 1986; Karri and Knowlton, 1990; Perales et a!., 1990). Here justtwo are introduced. As shown in Fig. 1.6, Grace (1986) indicated various regimes in whichcurrent industrial gas-solid reactors operate. From the diagram, one can see a region in whichcirculating fluidized beds usually operate. The boundaries shown in Fig. 1.6 are not intended tocorrespond fully to flow regime boundaries. When more experimental data are available, thediagram can be further expanded in the future.Although many flow regime diagrams have been proposed, fast fluidization has not beenconsistently defined because the mechanism of the transition from other regimes to fastfluidization has not been fully clarified. In this thesis no attempt is made to account for thetransition between flow regimes. This is the subject of a parallel study (Bi, 1994). Our goal here isto try to provide systematic and complete information on the flow structure in a circulatingfluidized bed.The term circulating fluidized bed (CFB) was first proposed by Reh in 1971. Fastfluidization is usually the flow regime in which CFB risers are operated. A CFB system consists ofa riser in which solid particles are suspended in the air flow, a gas-solids separator, and a solidsrecycling system to return solid particles from the separator back to the base of the riser.In the last two decades, CFBs, having been accepted worldwide as an advancedtechnology for combustion and other chemical reactions, have developed quickly because of manyadvantages. For example, in circulating fluidized bed combustion (CFBC) systems, the primaryadvantages are:6C,:10.02 1/3d = I/3 = dp(pgApg/iig)102Fig. 1.6. Fluidization regime map showing practical operating regions of thevarious hydrodynamic regime Grace (1986). Approximate boundariesbetween the different powder groups proposed by Geldart are shown atthe bottom (Grace, 1990).1 1071. Improved fuel combustion efficiency because of the recirculation of incompletelyburned fuel particles to the reaction chamber.2. Low SO emissions due to in situ sulphur capture by limestone or dolomite and thereutilization of fine sorbent particles which are collected by the gas-solids separators andreturned to the riser.3. Low NO emissions due to the staged introduction of primary and secondary air andrelatively low combustion temperatures.4. Good turndown ratio because suspension density can be varied readily by controllingsolids circulation rate, allowing the suspension-to-wall heat transfer coefficient to bechanged.5. Good gas-solids contacting and favourable heat and mass transfer.Because of their advantages, CFB reactors have also been employed in other industrialapplications such as gasification, ore roasting, calcination, and fluid catalytic cracking (FCC). In1992, over 250 FCCunits were processing one quarter of the worlds crude oil production and bythe end of 1995 some 400 CFB combustion units with a total thermal power of 23,000 MW willbe in operation (Werther, 1993).1.2 Scope of workMost CFB fundamental studies on such subjects as hydrodynamics and heat and masstransfer have been for idealized conditions far removed from those employed in industrialapplications. Although many applications, especially CFB combustors, involve risers ofrectangular cross-section, almost all available hydrodynamic data are from risers of circular crosssection, hence corner effects have been ignored. Different, even contradictory, phenomena have8been reported. There is also a lack of reliable hydrodynamic data on some subjects, especially onthe flow near the wall and solids lateral cross-flow. The lack of reliable experimental data leads todifficulties and poor accuracy in modelling hydrodynamics and heat and mass transfer in CFBrisers. Therefore, comprehensive experiments are needed to investigate systematically CFBhydrodynamics and, in particular, to provide as complete data as possible on various measures.Better and more complete data on hydrodynamics can improve CFB design and operation, as wellas providing a knowledge base for CFB scale-up.This work investigates hydrodynamics in a CFB riser of square cross-section. The studyinvolves measurements of local particle concentration, particle velocity and cross-flow fluxes,including both solids mass flux and momentum flux, all for the same experimental conditions,using recently developed measurement techniques. The influence of the corners onhydrodynamics, showing the difference between risers of square and circular cross-sections, isexamined. The influences of wall roughness and membrane walls are also investigated to providean even more comprehensive treatment. Operating conditions similar to those in commercial units(except for temperature and scale) were chosen. The purpose of this project is not to study thescaling rules for CFB, but to provide a comprehensive basis for both modelling and scale-up.Chapter 2 describes in detail the equipment in which most of the experimental work wascarried out. Since most of this work was experimental, measurement techniques are key to thesuccess of this work. In Chapter 2, the criteria for selection of instrumentation are alsointroduced. To make the thesis more readable, detailed information on instrumentation is givenseparately in the chapter where the specific technique is utilized.In Chapters 3 and 4, before showing experimental results on local particle concentrationand particle velocity profiles respectively, previous work is reviewed on the specific topics toprovide the background for the work, followed by a detailed description of the fibre optic probes9used to measure particle concentration and particle velocity and their calibration. Measurementsof particle concentration and particle velocity provide not only information on particle flow nearthe corners, but also on the configuration of the boundary between core and annulus. Theinfluence of operating conditions, in particular superficial gas velocity and solids circulation rate,on hydrodynamics is discussed in the light of the experimental results. Comparisons are also madebetween our experimental results and those from earlier studies.Chapters 5 and 6 discuss respectively the influence of wall roughness and membrane wallson the basic profiles, i.e. particle concentration and particle velocity profiles in the CFB riser.Sand-paper attached to the walls was used to simulate rough walls. Plexiglass half-round rodswere affixed to the inside surface of the riser to simulate membrane walls. Solids flow near boththe crest and the fin area is investigated.A detailed study of solids cross-flow is presented in Chapter 7. Lateral solids mass fluxwas measured by a solids sampling probe. Both the direction and the magnitude of the cross-flowmass flux are obtained. A piezoelectric probe has been developed and calibrated to measure thehorizontal component of lateral solids momentum flux. Lateral particle velocity at the axis of theriser is estimated by dividing the lateral solids momentum flux by the lateral solids mass flux.Combining information on solids cross-flow with the results from Chapters 3, 4 and 7 allowssolids flow in the CFB riser to be mapped.Conclusions of this work are provided in Chapter 8 together with recommendations forfuture investigation in the field of CFB hydrodynamics.10Chapter 2Experimental Set-up2.1 ApparatusTo investigate the influence of column configuration on hydrodynamics, a new riser ofsquare cross-section was designed and installed beside the one with circular cross-section used byBrereton (1987) to study the hydrodynamics in a CFB riser. Design criteria and constraintsprovided by Brereton (1987) were carefully considered. No change was made to the solidsseparation and recirculation systems used by Brereton (1987). The CFB system, illustrated in Fig.2.1, consists of the riser, two cyclones, a standpipe for storing recirculating solids and an L-valveto feed solids back into the riser.The column is constructed of carbon steel with wall thickness of 1/8 (3 mm), giving aninside cross-sectional area of 146 mm x 146 mm. The riser is made of seven sections of heights610, 610, 1219, 2438, 2438, 1219, 610 mm, respectively from bottom to top, giving a total heightof 9.14 m. Sections are connected to each other by flanges. Rubber gaskets are used betweenflanges for sealing. Smooth connections were assured. The length of the solids recirculationsystem was adjusted to fit the new riser, since the previous (152 mm diameter) circular cross-section riser used by Brereton was 9.3 m tall.Pairs of plexiglass windows, 4 (102 mm) in width, are mounted on facing surfaces forvisual observation. The windows are 20” (508 mm) high in the two bottom sections and one topsection, while the height of the windows in the other sections is 32” (813 mm). The plexiglassiizx1E Y Exit=VentSecondarySeparatorPrimary —Riser Separator— 6.08mSecondary9.14m Air—- StorageBed-1.23 ——1.21 - L-Va1vem 2.74m-— L-valve1.32m —___—Aeration—_______-PrimaryPrimary Air DistributorFig. 2.1. Schematic of the circulating fluidized bed system.12windows were specially designed to ensure a smooth inside surface, because roughness may affectthe hydrodynamics in the riser. There are measuring ports of inside diameter 1.5” (38 mm) andthickness 1/4” (6.4 mm) on some of the windows which can be mounted to sections of differentheight. Pressure taps of diameter 12.7 mm (0.5”) and measurement ports of diameter 38 nnri(1.5”) are fixed to the two facing steel walls of each section.Air is provided through a 3” (76.2 mm) pipe by a 0.15 m3/s, 34 kPa Sutorbuilt blower,model 7HV (Brereton, 1987), to the windbox at the base of the column. The blower air enters theriser through a multi-orifice distributor of 17% free area with 196 holes of 4.76 mm (3/16”)diameter and 9.53 mm (3/8”) pitch. The superficial gas velocity was measured with an orificemeter. The highest superficial gas velocity in the experiments was 8 mIs. Secondary air can beintroduced into the riser through two pairs (four ports at each level) of directly opposed ports of38 mm (1.5”) diameter located 1320, 2530 and/or 3760 mm above the distributor on all four wallsfor possible uiature study of the influence of secondary air injection on CFB hydrodynamics.However, no secondary air was employed in this study.At the top of the column, entrained solids are carried by the gas from the riser through a102 mm (4”) ID horizontal pipe into the primary cyclone, where most of the particles are capturedand fall into the standpipe for recirculation. Most fine particles not captured by the primarycyclone are removed by a secondary cyclone and returned to the standpipe. Air leaving thesecondary cyclone is vented outside the building. A butterfly valve was installed in the standpipeto measure the solids circulation rate. A detailed description of the standpipe and both the primaryand the secondary cyclones is provided by Burkell (1986) and Brereton (1987).Solids in the standpipe enter the vessel from the L-valve through a 146 mm ID pipecentered 114 mm (4.5”) above the distributor. The solids circulation rate is controlled by varyingthe flow of air supplied to the L-valve. Aeration was applied to the L-valve at two points, one13located at the axis and the other 121 mm above the axis of the horizontal section to achieve awide range of solids circulation rates. Rotameters are used to monitor the air flowrates. Pressureregulators are employed to stabilize the aeration flow rates, because the solids circulation rate isvery sensitive to the pressure and flowrate of air supplied to the L-valve.The time-of-descent technique (Burkell, 1986; Burkell et al., 1988) was employed tomeasure the solids circulation rate, defined here in terms of the amount of solids returned to theriser. This method involves measuring the time for identifiable particles to descend through aknown distance in a transparent section of a smooth-walled standpipe through which the solidsreturn in moving packed bed flow. Thus the solids circulation rate can be obtained from(2.1)where A1 is the cross-sectional area of the standpipe in which the identified particles aredescending, A is the cross-sectional area of the riser, L1 is the known distance, t is the time forthe identified particles to traverse the known distance, and Pbu is the loosely packed particledensity. The technique was compared with the butterfly valve method by Burkell et al. (1988) andagreement from the two techniques was generally good providing the location of the descentmeasurements was properly chosen.2.2 ParticlesOttawa sand of surface-to-volume mean diameter 213 .tm, particle density 2640 kg/rn3and loosely packed bed voidage 0.43 was used as the bed material. This sand has a minimumfluidization velocity of 0.048 mIs (Grace, 1982) and a terminal velocity of 1.41 mIs. The particlesize distribution from a sieve analysis is provided in Table 2.1.14Table 2.1. Size distribution of sand particles.Diameter Weight percentage(jim) (%)0-38 0.0338-53 0.0253-75 0.1075-90 0.5690-106 1.20106-125 2.72125-212 46.19212-250 22.65250-300 16.83300-355 6.47355-425 2.65425-600 0.592.3 CFB System OperationFor safety purposes, operating procedures for start-up and shut-down were established asfollows:Procedures for start-up:1. Open compressor bypass.152. Start compressor.3. Fully open valve on the primary air line.4. Gradually close the valve on the bypass line until desired superficial gas velocity isreached.5. Start feeding solids from the storage standpipe to the riser by turning on aeration to theL-valve until the desired solids circulation rate is reached.6. Adjust both superficial gas velocity and solids circulation rate until the desired operatingcondition is reached.While waiting for steady state conditions to be achieved (usually requiring around 20minutes), the system needs checking from the bottom to the top to make sure that there are noproblems.Procedures for shut-down:1. Terminate solids circulation by shutting off aeration to the L-valve.2. After the riser is emptied, open the valve on the bypass line.3. Stop the compressor.2.4 Selection of InstrumentationThe selection of a particular measuring method depends on specific data requirements, bedstructure, cost, environment and accessibility for sensor installation. Each of the techniques hascertain limitations. Some of the methods require fI.irther laboratory-scale evaluations. Eachtechnique must be assessed based on the requirements of the intended study and anticipatedparticle size, range, concentration and velocities.16After considering these factors, fiber optic probes, described in greater detail in Chapter 3and 4, were chosen to measure particle velocity in this study. With this technique, measurement oflocal particle concentration and particle velocity with high accuracy can be conducted either at thewall or inside the riser. The probes can be used both in the bottom region of the riser whereparticle concentration is high and in the upper dilute region.U-tube manometers are used to measure pressure profiles along the height of the riser.They provide reliable readings of pressure drops in the circulating fluidized bed: A sampling probeand a piezo-electric probe have been developed to measure solids cross-flow mass flux andmomentum flux respectively. They are simple in principle and straightforward to use.A detailed description of each measurement technique and its calibration is given in thechapter where the corresponding measurements are presented.17Chapter 3Voidage Profiles3.1 IntroductionBoth radial and axial voidage profiles have been widely studied in CFBs during the lastdecade. It was recognized in early studies of CFB hydrodynamics that there are axial variations invoidage, with a relatively dense region near the solids re-entry point located near the bottom ofthe riser (Yerushalmi et al., 1978). Li and Kwauk (1980), using a needle-type capacitance probe,found an S-shaped axial voidage profile with an inflection point which could serve to delineate adense phase at lower positions and a dilute phase in the upper portion of the riser. Hartge et al.(1986) measured the vertical voidage profile with pressure transducers, a needle capacitanceprobe, and fiber optic probes. Their results can be well described by an S-shaped voidage profile.Schnitzlein and Weinstein (1988), Cen et a!. (1988), and Li et al. (1988) confirmed this shape ofprofile.S-shaped voidage profiles are, however, not always found (Bai et al., 1992; Brereton andStromberg, 1986; Rhodes and Geldart, 1986; Li et a!., 1988). The exit configuration, solidsinventory, gas velocity and particle circulation rate are key factors that may change the shape ofthe voidage profile along the riser. An exponential decay function was found to be able torepresent the axial profiles voidage throughout the riser from the bottom to the top by Arena et al.(1986) who obtained a simple exponential distribution from direct voidage measurements, whilean S-shaped distribution was obtained from pressure drop signals neglecting solids acceleration.Brereton (1987) proposed that S-shape solids concentration profiles exist for relativelyhigh solids circulation rate, while profiles represented by an exponential decay function occur for18low solids circulation rates. Bai et a!. (1992) concluded that exponential axial voidage profilesexist for normal CFBs. When a very weakly restrictive entrance structure is present, the profileshave an S-shape. This conclusion is similar to that of Brereton (1987), because the tendency to be“restrictive” is measured by the ability to achieve high solids circulation rates.The exit structure was found by Brereton and Stromberg (1986) to influence the axialvoidage profile in CFB risers. In a riser with an abrupt exit, a C-shape axial voidage distributionwas found with low voidage at both the bottom and the top of the column and high voidage in themiddle. This was confirmed by Jin et al. (1988). Different means of measurement and differentcolumn configurations used by different authors may also be responsible for the discrepancies.The radial variation of particle concentration in a circulating fluidized bed was firstdetermined by Gajdos and Bierl (1978) who employed local solids flux probes and X-rays. Theyfound a lean core surrounded by a relative dense annulus. With an X-ray source and a chordalabsorptometer, Weinstein et al. (1986) confirmed this structure. A core-annulus distribution ofparticle concentration was also confirmed by Dry (1986) using a small heat pulse as a tracer, byBrereton (1987) with a capacitance probe, and by Horio et al. (1988) and Hartge et al. (1988)with fiber optic probes.These results clearly indicate the existence of a core-annulus flow structure with verysignificant radial concentration gradients and with the boundary between core and annulus notclearly defined. Various core/annulus two-zone models have been employed to describecirculating fluidized beds. Bader et a!. (1988) defined the core-annulus distribution in terms of alow density, high velocity gas-solids core surrounded by a slow-moving, high solids densityannular region. Brereton et al. (1987) proposed a core-annulus model in which gas is assumed totravel rapidly upwards in the core, while gas either moves upwards much more slowly or is pulled19downwards in the outer annular region. A similar assumption was also made by other workers,e.g. Bolton and Davidson (1988), Berruti and Kalogerakis (1989) and Rhodes (1990).Brereton and Grace (1993) introduced an intermittency index to compare the two-phasenature of flow in circulating fluiclized beds. Their study indicates that neither perfect cluster flownor perfect core-annulus flow exists in CFB risers. The flow structure tends to change withheight, with cluster-like structures more common near the bottom of the riser, and a core-annulusstructure more predominant towards the top. Some clusters or strands exist in the center of theriser, even at low solids concentration (Brereton and Stromberg, 1986; Horio et al., 1988).Solids concentration profiles depend not only on primary and secondary gas flow rates andthe solids circulation rate (Yerushalmi and Cankurt, 1979; Li and Kwauk, 1980; Brereton, 1987)but also on the properties of the particles (Weinstein et al., 1984). The geometry of the riser hasalso been found to have considerable influence on the hydrodynamics of circulating fluidized beds(Brereton and Stromberg, 1986; Jin et al., 1988; Schnitzlein and Weinstein, 1988; Wu et a!.,1990; Brereton and Grace, 1994).Almost all previous cold-model hydrodynamic research has been carried out on risers ofcircular cross-section. However, risers of square and rectangular cross-sections are widelyemployed in CFB applications such as combustion. In these risers, the corners are expected tohave considerable influence on both lateral and axial voidage profiles. This chapter presents bothlateral and axial profiles of voidage in a cold model circulating fluidized bed riser of a squarecross-section.203.2 InstrumentationVarious techniques have been used to measure voidage in fluidized beds. The three mostpopular techniques, i.e. X-ray or y-ray transmission, capacitance probes and optical fibre probes,are briefly reviewed here.An X-ray technique was used by Weinstein et al. (1986) in a circulating fluidized bed toobtain bed voidage profiles. X-ray and/or y-ray transmission techniques have been used for manyyears in conventional fluidized beds to detect bubbles and to measure bed voidage (e.g.Baumgarten and Pigforcl, 1960; Rowe and Partridge, 1965). A radioactive source emits a beam ofX-rays or y-rays across the vessel through its walls to the detector. The detector consists of aGeiger counter that produces an electrical impulse in response to each photon passing through thetube reaching the detector. These pulses are integrated and transformed into a DC signalproportional to the radiation received at the counter. In the case of a fluidized bed, theconcentration of particles along a chord is detected by the attenuation of the measured X-ray or yray signal.Significant differences in electrical properties such as capacitance between air and apacked bed of solid particles provide the basis for electrical properties measurement techniques.Local measurements of capacitance are related to the local state of the fluidized bed. Hartge et al.(1985) traversed a needle probe through a circulating fluidized bed riser and correlated theresulting capacitance with voidage.Capacitance probes have been employed often because they work well withnonconducting materials. For example, Brereton and Stromberg (1985) measured radial voidageprofiles in a circulating fluidized bed using a needle capacitance probe. Various probeconfigurations have been used, ranging from parallel plates to needle-shaped probes. Probe21configuration is an important consideration, as different geometries have varying responses. Also,the geometry can alter local flow conditions. Hence, configurations are required which minimizedisturbances.The basic mechanism of a capacitance probe is very simple. The capacitance of a solidmedium differs markedly from that of a gas. Therefore, changes of void fraction in the zone wherethe probe is located create changes in the dielectric constant, which in turn alter the systemcapacitance. A capacitance probe can provide a measure of local changes in instantaneous valuesof particle concentration. A capacitance measuring system can be capable of responding atfrequency of up to about 100 kHz (Brereton, 1987). The technique can also be used for hightemperature and high pressure systems. However, a capacitance probe needs calibration fordifferent bed materials and is not applicable for conductive bed materials. Also, the measuringvolume of the probe is not precisely defined. Another disadvantage is the sensitivity ofcapacitance measurements to moisture, which changes the dielectric constant and thus alters thecapacitance.3.2.1 Fundamentals of Fibre Optic ProbesFiber optic probes can be used to measure both voidage and particle velocity in circulatingfluidized beds. This technique has the advantages of simplicity, high accuracy and potential lowcost. Thus it is widely used for the study of hydrodynamics in fluidized beds. Some examples aresummarised in Table 2.1.Krohn (1982) divided fiber optic sensors into two basic classifications. First, for what isreferred to as an intrinsic fiber optic sensor, the transmission of the fiber is directly affected by thephysical phenomena being sensed. The second classification is for fiber optic position sensors22Table3.1.Examplesoffibreopticprobesusedtomeasurevoidageandparticlevelocity.WorkerBedTypeBedMateriald(mm)Ug(mis)G5(kgim2s)(kg/rn3)MeasuringParameterPatroseetal.FluidizedGlassBeads0.2-0.72.47etal.(1982)bedRandelmanetSpoutedGlassSpheres0.950.328val.(1983)BedNakajirnaTurbulentFCC,0.0640.13-1.33880BubbleFraction(1990)BedSilicaSand0.0870.27-1.591200Katoetal.CFBFCC0.742.4,3.0,1770BedVoidage(1990)4.0Rehetal.FluidizedGlassBeads,0.04-0.08BedVoidage(1990)BedAluminaParticles0-0.11BaietalCFBFCC0.0592-330-1801545v&Bed(1991)VoidageMorookaetFluidizedFCC0.065-0.15-5.0960-1110v,&BedVoidageal.(1990)Bed0.068Ishiietal.CFBFCC,0.06130.8,1.281780v,&BedVoidage(1990)FCC0.04640.4,0.641780Rhodesetal.CFBAluminum0.072-58-802430v&BedVoidage(1990)TrihydridePowderNowaketal.CFB98%FCC+0.0466-104.3-342300v,&BedVoidage(1990)2%Alumina3Kojimaetal.CFBFCC0.060.5-21000(1989)Horioeta!.CFBFCC0.061.1-1.31000ClusterVelocity&(1988)VoidageHartgeetal.CFBBedAsh,0.123.8-5.427-70&BedVoidage(1988)FCC0.0851.2-5.47-70which transit position change in a device (transducer) that is sensitive to physical propertychanges. Five types of sensors with different working principles — intensity modulated,transmissive, reflective, micro bending and intrinsic concepts are in common use. To measurelocal voidages and particle velocities in circulating fluidized beds in this work, the reflectiveconcept is used.Reflective fiber optic sensors are excellent position sensors, The configuration is shown inFig. 3.1. The sensor consists of two bundles of fibers, one of which transmits light to a reflectingtarget, while the other bundle traps reflected light and transmits it to a detector. The intensity ofthe detected light depends on how far the reflecting target is from the fiber optic probe (Krohn,1982).LightSourceDetector ReflectingSurfaceFig. 3.1 Configuration of reflective fibre probe.TransmittingBundleReceivingBundle24In fiber optic measuring systems, stable illumination light sources are needed to ensureaccurate measurements. Several illumination light sources have been used. Old et al. (1975) useda tungsten lamp as a light source, while Patrose and Caram (1982), Hartge et a!. (1988), Nakajimaet a!. (1990) employed lasers. Kojima et al. (1989) and Nowak et a!. (1990) used mercury lampsto produce ultraviolet light as their light source. A quartz lamp was utilized by Randelman et al.(1983), while Matsuno et al. (1983) employed an incandescent electric lamp and a halogen lampas light sources.3.2.2 System and PrincipleA fiber optic probe system used to measure bed voidage consists of a light source, opticalfibers, a photomultiplier, voltage integrator, data recorder, AJD converter and computer. Twobundles of fibers are used. One carries light from the light source and projects it onto a swarm ofparticles. The other transmits light reflected by the particles to a photo-multiplier where lightsignals are converted to electrical signals with the voltage output proportional to the intensity ofreflected light. There are usually two ways to measure particle concentration, depending on themethod of signal treatment after the light-receiver according to Matsuno et a!. (1983). Figure 3.2.shows the difference between these two methods.(1) When the particle diameter is smaller than the core diameter of the optic fiber, type Itreatment is applied. In this case, the output signals of pulses generated by all the particles existingwithin a dotted circle in Fig. 3.2 are integrated. The integrated value of voltage from the voltageintegrator is transmitted to a computer by an AID converter. The integrated values of voltage canbe correlated with the concentration of particles using a calibration. In this method, a swarm ofparticles is detected rather than single particles, so that instantaneous concentrations can bemeasured.25ri nrOutput SignalFig. 3.2 Type I: Detection of swarm of particles;Type II: Detection of a single particle (Matsuno et al., 1983)(2) When the particle diameter is greater than the core diameter of the optic fiber, type IItreatment is used. In this case, a single particle is detected by the optic fiber probe. The outputsignals from the light receiver are converted to the pulse at any threshold level V. Therefore, thepulse count corresponds to the number of particles. Using this method, particle velocity should beknown to convert the number of particles in a time interval to particle concentration. Thus thismethod requires a long time to measure particle concentration with high accuracy, and onlyaverage concentrations can be measured.Type IOutput SignalMeasuring Volume./•(••••Type IIParticle...26In this work, the former method is employed. The fiber optic probe system, similar to thatdescribed by Qin and Liu (1982), includes a light source, optical fibers, a photomultiplier, an A/Dconverter and a computer. Quartz fibers are used, each with a diameter of 0.015 mm. The outsidediameter of the probe is 3 mm. The duration of each measurement was 60 seconds at a samplingfrequency of 483 Hz. For each experimental point, at least four 60 s measurements were usuallytaken to obtain an average value.The size of the measuring volume depends on the local voidage. Tests, graduallyapproaching the tip of the fibre optic voidage probe with a small piece of stainless steel plate in acalibration beaker described latter, indicated that the measuring distance was approximately 7 mmfor 6 approaching 1 and about 4 mm for 60.9. It was clearly less than 4 mm in the wall regionwhere voidage changes most rapidly.3.2.3 Calibration MethodAs no method for direct conversion of integrated electric signals to particle concentrationexists, the system must first be calibrated. Two calibration methods are available:(1). Matsuno et al. (1983) used the system shown in Fig. 3.3 to calibrate their optical fiber.Particles from a sieve were made to fall uniformly by vibrating the sieve. The particleconcentration in the zone where particle terminal velocity is achieved can be calculated by:C= (3.1)Avtwhere z\W is the cumulative weight of particles passing through the cross-sectional area A withintime interval At, and vT is the terminal velocity of the particles. The optical fiber was set far27PaicIes• ::1ru..•-t--- SieveFibre OpticVoidage Probeiji 4 j?i c7..7.7V7/ .Fig. 3.3. Calibration method for fibre optic voidage probe employed byMatsuno et al, 1983.enough below the sieve so that the particle terminal velocity was achieved. The integration timewas set at two-second intervals. Four types of glass beads of average diameter 56.5 p.m anddensity 2520 kg/rn3 were used. Fig. 3.4 shows that the calibration curve was approximately linear.Scatter was said to be due to the manual vibration of the sieve. The particle size distribution mayalso be responsible for some data scatter, because the terminal velocities are different. Anothersystematic error of this method comes from the application of single-particle terminal velocity inEquation (3.1). Matsen (1982) demonstrates that the terminal velocity of particles for particleconcentration of 1% can be as much as twice the terminal velocity of single particles.2830200.2 0.4 0.6 0.800.0 1.0Voltage, VFig. 3.4. Calibration curve for fibre optic voidage probe by Matsuno et a!. (1983)29(2). For the second method, the calibration is carried out in a liquid-solid fluidized bed becauseparticles are quite uniformly distributed in such a system. Voidage can be obtained from theheight, H, of the expanded bed as:6=1— !(1_6) (3.2)where H0 is the height of a packed bed and is the voidage of the packed. bed. A set of bedvoidage data can be obtained by changing fluid velocity to achieve different bed heights, H. Toreduce the influence of an axially non-uniform voidage distribution, the calibration can be carriedout at different bed heights, and an average value can be obtained. This method of calibration forthe fibre optic voidage probe was used by Qin and Liu (1982) for glass beads of diameter 900 imand 300 p.m. As shown in Fig. 3.5, a plot of bed voidage vs output signal gives a nearly linearrelationship. A linear relationship was confirmed by Hartge et al. (1986) for sand particles ofdiameter 56 p.m as shown in Fig. 3.6 and by Boiarski (1985) for fine polystyrene beads. It isinteresting to note that in both Figs 3.5 and 3.6 the output of the fibre optic probe is not zero atunit voidage. It is suspected that dust and bubbles in the liquid-solid systems of Qin and Liu(1982) and Hartge et al. (1986) is probably responsible for the non-zero output signal of the fibreoptic probes at the unit voidage.Lischer and Louge (1992) simulated the calibration in liquid-solid systems by using a raytracing Monte Carlo algorithm. They showed that the accuracy of optical fiber measurementsincreases with a decreasing ratio of particle diameter to probe diameter, d/d. They also foundthat for transparent materials such as glass beads, water calibration may cause problems. Thesame particles should be used in the calibration to reduce errors. No other similar study on theperformance of optical fiber probes for determining particle concentrations has been reported.302.52.0>E 1.5-.1-’D8 i.o0.50.0 -Fig. 3.5. Calibration curve for fibre optic voidage probe obtained by Qin and Liu (1982).In our calibration, since d/d is only 0.07 and sand is opaque, the calibration should givereasonable results according to Lischer and Louge (1992).To further check the linear output of the fiber optic particle concentration probe,calibration using the same particles as in the experiments was conducted in two liquid-solidsystems. For voidages less than 0.8, the calibration was carried out in a liquid-solid fluidized bedsimilar to the one used by Qin and Liu (1982). Calibration results are illustrated by the openo d=O.3mm• d=O.9mmI I0.6 0.7 0.8 0.9Voidage,3186>D4200CoL-%Fig. 3.6. Calibration curve for fibre optic voidage probe by Hartge et a!., 1986.Blackened fibres are light transmiters, while open fibres are receivers.symbols in Fig. 3.7. Calibrations at voidages of 0.8 and above are difficult in liquid-solid fluidizedbeds because the bed surface is obscure. Therefore, calibrations at higher voidages were carriedout in a beaker. A known volume of solid particles was put into a beaker and mixed with water ofknown volume. The liquid-solid mixture was then stirred until the particles are uniformlydistributed in the water. Liquid-solid systems having different voidage can be achieved by mixingdifferent volumes of particles into the water. Before the tests, the liquid-solid systems werechecked to be free of bubbles and dust, so that the output of the fibre optic probe was zero at unit10 20 30 40 5032voidage to ensure the accuracy of each test. Results from the beaker are also shown by the closedsymbols in Fig. 3.7. It is seen from Fig. 3.7 that the calibrations from both systems were verynearly linear over the entire voidage range of interest.The linear relationship between the voidage and the output of the fibre optic voidageprobe in a gas-solid system was verified by a dropping test as indicated in Fig. 3.8. The solidsrecirculation system which is described in detail in Chapter 2 was employed to carry out the test.Particle velocity at the measuring location was measured by the fibre optic particle velocity probe1.0 • I I • I • I0.9.0.7• from Beaker where Partides are0.5 Uniformly Distributed in Liquid> 0 from Solid-Liquid Fluidized Bed0.4 I • I • I • I • I0.4 0.5 0.6 0.7 0.8 0.9 1.0Voidage from Fibre Optic ProbeFig. 3.7. Calibration curve for fibre optic voidage probe.33described in detail in Chapter 4. The solids flux was obtained by capturing and weighing theamount of particles descending through a known area over a known time interval. From thesemeasurements, the voidage was calculated. The test results, indicated in Fig. 3.9, provide furtherevidence of a linear relationship between voidage and the output of the voidage probe. Since theoutput of the fibre optic voidage probe has a linear relationship with the measured particleconcentration, in-situ calibration is required at only two points, one corresponding to the looselypacked bed voidage and the other to a voidage of unity.StorageBedL-ValveL-valveAerationFibre OpticVoidage ProbeBucketFig. 3.8. Schematic of gas-solid system used to confirm a linear relationshipbetween voltage and solids concentration for the fibre optic voidage probe.34C0Ca)C-)C0C)a)C)0Fig. 3.9. Verification of the linear relationship for the fibre optic voidage probeusing the gas-solid system shown schematically in Fig. 3.8.Measurements in the CFB riser were taken at heights of 0.25, 0.79, 1.83, 3.45, 5.13, 6.20,7.06, and 8.98 m above the distributor to obtain axial profiles of voidage. Lateral profiles involvedmeasurements at 0.0, 26.0, 47.0, 60.0, 66.5, and 73.0 mm from the riser axis. The coordinatesused on all figures appear in Fig. 2.1.Before starting each test, the windows near the measurement location were covered withblack plastic bags to prevent unwanted light from penetrating into the riser. A special fitting wasOutput, fts35used to mount the fibre optic probe to the riser so that the probe could be moved laterally withoutshutting down the CFB system. After the measuring system and a personal computer werehooked up, measurements were started.3.3 Results and Discussion3.3.1 Basic ProfilesFigure 3.10 shows a typical trace of local instantaneous voidage versus time. Theinstantaneous local voidage shows rapid fluctuations. Axial profiles of local time-averagevoidages under several different operating conditions are presented in Figs 3.11 to 3.14. Generallyspeaking, the time-mean voidage at the bottom of the riser is low and gradually increases towardsthe top. This is true both in the wall region and at the center. More significant changes in voidagewith height occur at the wall than in the central region.Figures 3.12 and 3.13 show that local time-mean voidage decreases with increasing solidscirculation rate. As portrayed in Fig. 3.14, the time-mean voidage increases with increasing gasvelocity. Superficial gas velocity and solids circulation rate exert more influence on the voidage atthe wall than in the core. From Figs 3.11, 3.12, and 3.14, we find that voidages near the wall arenot always lowest at the distributor. Forhigh circulation rates and/or low superficial gas velocitiesand in the wall region (0.8<y/Y<1), voidage decreases from just above the distributor to a heightjust above where the particles re-enter the riser. Voidage then increases along the column height.From Figs 3.11 to 3.14, one can also observe the influence of the riser exit at the top ofthe riser. Due to the abrupt exit, there is a strong exit effect. Because many particles hitting thetop of the riser tend to rebound, particle concentration near the exit becomes high. Compared361.0 0.9 0.8U) 3) 0.7 0.6 o.5o60SamplingTime,t (s)Fig.3.0.Localinstantaneousvoidageversustimefor apointnearthetopoftheriser.(Ug5.mis,G=40kg/m2s,x/X=0,1020304050y/Y-1,z=8.98mand=0.9).x/X0G=4O kg/m2sUg=5•mIs• y/Y-1• yIYO.82* yIYO* From iP Measuremento Integrated Cross-sectionalAverage ValueHeight, z(m)1.00.90.80.70.60.5Coa)-D1=.Ii2 4 6 80 10Fig. 3.11. Axial profiles of local time-mean voidage for x/X=O, UgZ=5.5 mISand G=’4O kg /m2s.with the exit effect in a riser of circular cross-section (Brereton and Grace, 1994), there appearsto be a more severe exit effect in the square column. Figures 3.11 to 3.14 indicate that the abruptexit influenced an extensive zone. The extent of the region of influence may be related to thecorners in the square column since corners can shelter particles rebounding from the top of theriser, while downward-moving particles are more difficult to strip from the corners than thosetraveling downwards along a smooth surface. Hence particles move downwards a longer distancein the corners of a square column than in a circular column. Similar reasoning may explain whyparticle concentration is higher in the lower part of the square riser for a given gas velocity andsolids circulation rate.381.00.9ai 0.8c,)C-)0.70.60.5Fig. 3.12. Axial profiles of time-mean voidage at the wall for two differentsolids circulation rates for xJXO, y/Yr=1 and Ug5.mIS.None of the profiles in Figs 3.11 to 3.14 is S-shaped. Thus the model of Li and Kwauk(1980) is not applicable to this study. The voidage profile in the lower part of the column from thelocation at which the solids re-enter the riser can be described by____c16 z—zo(33)Height, z (m)391.000.951(A)ci)go.o>0.850.80o 2 4 6 8 10Height, z (m)Fig. 3.13. Axial profiles of local time-mean voidage at the axis for two differentsolids circulation rates for xIX=O, y/Y=O and Ug=5. fl1I5.where z is the height above the distributor, is cross-sectional average voidage, is the cross-sectional average voidage at the solids re-entry location, z0 is the height of solids recirculationlocation and c is a decay constant. Rewriting Equation (3.3) in terms of solids suspension density,we obtain:=0(l-exp(- )) (3.4)z — z0where 15SUSp is the solids suspension density and is the solids suspension density at the solidsre-entry location. To find i5, the same approach as used by Senior and Brereton (1992) wasadopted here, i.e. assume that.• G4O kg’m2s• G2O kg/m2s401.02 4 6 80.9103cu 0.8c,)(T30> 0.7C)0.60.50 10Height, z (m)Fig. 3.14. Axial profiles of local time-mean voidage at the wall with two differentsuperficial gas velocities for xJX=O, y/Y=- 1 and G=4O kglm2s.Cl(1—0)=CU ( S )C2 (35)Pp(Ug— VT)where C0, C1, and C2 are constants, G5 is the solids circulation rate, Ug is the superficial gasvelocity, VT is the particle terminal Velocity, p, is the solids density, andu;=ug{p/[g(pp — pg)tg]}3 is the dimensionless Velocity given by Grace (1986) with g beingthe gravitational acceleration, Pg the gas density, and p.5 the gas viscosity. By correlating in41IEquation (3.5) and in Equation (3.3) against Ug and G, from the experiments, we obtainC02.3 x i04,C1=-1.4, C2=1.6, and =O.8 m.Figure 3.15 compares the predictions of to Equation (3.4) with solids suspension densitiesobtained from pressure drop measurements. The fitted equation generally fits well except near thetop of the riser where there is a significant exit effect.1600120080040000• Ug=5.mis, G=40 kg/m2s• Ug7.O mis, G=40 kg/m2sA Ug=5•mis, G=20 kg/m2sFitted Curve• •2 4 6 8Height, z (m)Fig. 3.15. Axial suspension density profiles compared with fitted curvescorresponding to equation (3.4) with O.8 m.42An axial profile of voidage calculated from differential pressure measurements along theheight of the riser is plotted as a dotted line in Fig. 3.11 in comparison with results from fiberoptic probe measurement. Results obtained from the optical fiber follow the same trend as thosefrom the measurement. To further check the results, local voidages from the fiber optic probewere integrated at two heights, 5.13 m and 6.20 m, assuming symmetry about the axis to obtaincross-sectional average voidages as indicated in Fig. 3.11. The results are seen to be in reasonableagreement.Lateral profiles of voidages 0.79 m above the bottom are shown in Fig. 3.16. The particleconcentration is high at the wall and low in the central region. Particles generally movedownwards in the wall region. These observations are very similar to those in risers of circularcross-section. They show that there is also a core-annulus structure in a riser of square cross-section. In the dilute core region, particles generally move upwards with the voidage usually 0.94to 0.99, depending on the operating conditions. In the annulus, particle concentration is high andparticles mainly move downwards, with occasional upward movement, with voidages from 0.88to 0.95 for the developed region. The lateral profile of voidage changes greatly near the wall andis more uniform in the core.Figure 3.17 shows cross-sectional voidage profiles. Voidage is seen to be higher at thecenter and lower at the wall, as in columns of circular cross-section. Particles in the corner regionof risers of square cross-section are protected by two walls and are more difficult to strip out ofthe corners than for risers of circular cross-section. Thus voidage becomes even lower towardsthe corner. From visual observations, particles in the corner region move downwards.As indicated in Fig. 3.18, the riser exit also causes an asymmetric lateral distribution ofvoidage near the top of the riser, with the time-mean voidage on the exit side being lower than onthe opposite side. There are two possible reasons. Because of the exit, gas and particles reaching431.000.951wo5 0.900> 0.850.800.75-1.0 0.0Fig. 3.16. Lateral profiles of local time-mean voidage near the bottom of theriser at four different gas velocities for x/XO, z0.79 and G=2O kglm2s.the top of the riser must have a horizontal component towards the exit. As a consequence,particle concentration is higher on this side. When a plexiglass horizontal duct was employed inthis study connecting the riser exit to the primary cyclone, particles were found to pile up on thelower surface of the horizontal duct. With enough accumulation of particles, some particles mayslip back periodically from the horizontal duct and move downwards along the near wall of theriser, thus, causing higher particle concentration on the exit side, as observed by Glicksman et al.(1993). Visual observations indicated that particles rebounding from the top tend to movedownwards in the corners of the riser where they are sheltered from the upfiowing gas.-0.8 -0.6 -0.4 -0.2Dimensionless Distance, yIY44I 1.00—— _I_—I I II iTh-..0.98IThE1096x/X=O0.6 —‘02o 0.8./0.0x!X=O.37\1à4jo°E/ 0.0 V.0peSSAxi•094x’X=O.72/ 0.0AxisFig. 3.17. Lateral profiles of local time-mean voidage for Ug=7.O mis,Gs4O kg/m2sand z=6.20 m.45Another interesting finding is that time-mean voidages are not always highest on the axisof the column. As seen in Fig. 3.18, the voidage sometimes reaches a maximum at a location ofabout 0.6 to 0.8 of the half-width of the column and then decreases slightly towards the center.From close scrutiny of the literature, a similar trend can also be found in risers of circular cross-section (e.g. Weinstein et al., 1986; Hartge et al., 1988; Kato et al., 1990; Nowak et al., 1990; Baiet al., 1991; Herb et a!., 1992). Since different measurement techniques such as X-rays,capacitance probes and fiber optic probes were used by these authors, it cannot be attributed tothe measuring technique. However, the Mshaped* profiles appear to be more distinguishable inour riser of square cross-section.Fig. 3.18. Lateral profiles of voidage for xJX=0, Ug=5.mis, G=r40 kglm2s.* W-shaped solids concentration profile’ is more accurate than “M-shaped voidage profile”. Since “M-shapedvoidage profile” was used in a published paper Zhou et a!. (1994), the term is also employed here.C*)a)Co- 0.850.80-1.0 -0.5 0.0 0.5 1.0Dimensionless Distance, yIY46The M-shaped profile is also in good accord with the radial gas velocity profiles obtainedby GeUperin et a!. (1976) using a total pressure probe and Tsuji et a!. (1984) employing a laser-Doppler velocimeter. Both studies found that the gas velocity was not a maximum at the center.Instead, it increased sharply at first until a maximum was reached and then decreased slowlytowards the center. The radial dimensionless position where the gas velocity reached its maximumwas similar to that where the voidage reached a maximum in our column.By using an optical fibre image system, Wei et al. (1993) obtained pictures which suggesta high voidage circular area between center and wall regions. Photographs obtained by Wei et al.(1993) are reproduced in Fig. 3.19. Particles are shown as black areas in the picture. Similarresults were obtained using a laser sheet technique by Horio et al. (1993) in a riser of circularcross-section, further validating our finding of M-shaped lateral voidage profiles.The M-shaped profiles may be due to cross-sectional non-uniformity of gas turbulenceintensity. In the core region, gas moves upwards, while in the annulus, descending particles draggas downwards (Brereton, 1987). At the interface between the core and annulus layers, the shearrate is high, at least 1000 times that in the core of the riser (Senior, 1992). Turbulence is likely tobe generated at the shear boundary. Hence, there may well be more radial particle diffusion in thisvicinity than near the axis of the column; causing the particle concentration to be a little higher atthe center of the riser. Yang et al. (1992) measured lateral profiles of gas and particle velocities ina circulating fluidized bed riser. They found a region between the axis and the wall where the gassolids slip velocity and gas fluctuation velocities reach maximum values. If this is correct, then thisphenomenon should be more distinguishable for small particles, e.g. FCC particles, because smallparticles are more likely to be influenced by gas motion and turbulence than by particle collisions47(Senior, 1992). Upon checking the bed materials in the literature for which this phenomenon wasshown, we find that in most cases when M-shaped profiles can be distinguished the particlediameters were smaller than 100 p.m.-- -r&’•1 25 —- -V-’: (:1.00—o :Q) -O75___‘.3.)-•-.. o.so--:.-—••— .0.25 .: 9:.00 C I I I I T0.0 0.5 1 .0Radial Position r/RFig. 3.19. Picture from Wei et al. (1993) showing a high voidage circular areabetween axis and wall regions.48Tung et al. (1988) reported that, in a circular riser, local voidage in the riser can becorrelated in terms of the cross-sectional average voidage and the dimensionless lateral position,with voidage independent of operating conditions, scale, particle properties and vertical position.Their correlation was(42+j9)for 4=rfR0.75 (3.6)= (3.62p647+O.191) for 4 = rfR 0.75 (3.7)where av is the cross-sectional average voidage. Zhang et al. (1991) used a single equation,= (O9I+425+3h1) (3.8)to represent the whole range. Equations (3.6), (3.7) and (3.8) do not fit the experimental data inthis study.Similar lateral voidage profiles, with local voidage only dependent on the cross-sectionalaverage voidage and the dimensionless radial position, were confirmed in a circular riser byRhodes et al. (1992) who proposed a semi-empirical equation‘— =2--’16av R)to simulate the lateral voidage profiles. Equation (3.9) also does not fit our experimental resultsvery well. One flaw of Equation (3.9) is that the calculated voidage at the axis of the riser isalways predicted to be unity. The lack of agreement in this study with the equations of Tung et al.(1988), Wang et al. (1991) and Rhodes et al. (1992) is probably related to the different bedmaterials used. Equations (3.6) to (3.9) were obtained for Group A particles, while Group Bparticles were used in this work. The different riser configuration is also likely to have been afactor, with the corners having some influence on the lateral voidage profiles.492Ev+l— /3lSav LyIC).)0CDFig. 3.20. Comparison between simulation results and experimental data showingsimilar lateral voidage profiles: xIX=0, z=5. 13 mAn empirical equation somewhat similar in form to the Rhodest semi-empirical equation(3.10)was written based on the experimental data from the square riser. Here Y is the half-width of thecolumn and y is the lateral distance from the axis. Figure 3.20 compares the calculation resultsfrom Equation (3.10) with experimental data obtained in the developed region. In the coreregion (y/Y 0.9), the empirically modified equation fits the experimental data very well.1.0 —_____0.9 -0.80.7 —0.0I K• Ug7.O mis, G=4O kgim2s• Ug5.mis, G=4O kg/m2sA Ug5.m/s, G=2O kg/m2sFitted curve0.2 0.4 0.6 0.8 1.0Dimensionless Distance, yIY50However, there are deviations in the wall region. Equation (3.10) gives reasonable results for thevoidage at the axis.3.3.2. Probabifity AnalysisProbability distributions of particle concentration for different heights at the center andwall regions are shown in Figs 3.21 and 3.22. At a high solids circulation rate and low superficialgas velocity, two peaks may be found as shown for the lowest levels in Fig. 3.21. This indicatesthat in the wall region over the lower part of the riser, there are two dominant particleconcentrations, both high. Rhodes et al. (1992), using high speed video photography, reportedthat at high solids fluxes, there was a substantial bulk downflow of particles at the wall, with thevoidage of this bulk downflow higher than that of the particle downflow in swarms. This mayaccount for the bimodal distributions. A bimodal distribution is found only in the lower region ofthe riser, with no bimodal distributions at the axis nor near the top. In the work of Rhodes et al.(1992) the bulk downflow disappeared at low solid fluxes. For the modest solids circulation rateand relatively high superficial gas velocity, only one peak can be seen in the probabilitydistribution of particle concentration at the wall.Louge et al. (1990), using a capacitance probe, did not find a bimodal probabilitydistribution of voidage at any location in their riser. The large measuring volume of the probe mayaccount for the lack of a bimodal distribution. The high measuring location may be another factorsince bimodal distributions were only found in the lower part of the riser in the present work.Figure 3.23 demonstrates that the particle concentration in the core is more uniform than518=0.778=0.56z=0.79m8=0.63z= 0.25 mFig. 3.21. Axial profiles of probability distribution of local time-mean particleconcentration, C (1——) at the wall for Ug5.mis, G=4O kg/m2s,xlX=O and y/Y=-1. C*=1 corresponds to packed bed particle concentration, i.e.6=O.43.6=0.90z= 5d3 m60.89z= 6.20 m60.88z3.45m8=0.91z= 7.06 m80.92z= 8.28 m6=0.90z= 8.98 m52c*Fig. 3.22. Axial profiles of probability distribution of local time-mean particleconcentration, C’ = (1——) at the axis for UgS .5 mIs, G=4O kg/m2s,x/X=O and y/Y=O. C*=1 corresponds to packed bed particle concentration, i.e.6=O.43.53Fig. 3.23. Lateral profile of probability distribution of local time-mean particleconcentration, C’ = (1——c) for Ug5.mis, G’=4O kg/m2s, xlX=O andz0.79 m. C*=1 corresponds to packed bed particle concentration, i.e. e=O.43.54near the wall. The locations of the peaks of the probability distribution curves for the annulusshift greatly from right to left. In the core, a much smaller shift is observed, indicating that theaxial profile of voidage is more uniform than at the wall. Trimodal distributions are observed atlocations a small distance inward from the wall, suggesting intermittent bulk downflow, particlestreamers and dilute suspension in this region.Figure 3.17 indicates that particle concentration is highest in the corner. As demonstratedin Fig. 3.24, the particle concentration in the corner has a wider distribution of particleconcentration than midway between opposite walls. At high solids circulation rate and lowsuperficial gas velocity, the instantaneous voidage in the corner can be as low as that of a loosepacked bed. This is not present at low solids circulation rate or high superficial gas velocity.Asymmetry of lateral voidage distributions was found at the top of the riser as shown inFig. 3.25. The probability distribution of voidage is wider at the wall than in the core. Only onedominant peak is observed at all lateral positions. A voidage equal to the packed bed value, asindicated by C*=1 in Fig. 3.25, is found near the wall at the particle exit side.3.3.3 Intermittency Index ProfilesAlthough the standard deviation provides a quantitative measure of the variability of localvoidage, it is difficult to explain the nature of the two-phase flow in circulating fluidized beds interms of the standard deviation, because the time-mean voidage varies from point to point. Anintermittency index was introduced by Brereton and Grace (1993) to provide a more usefulcomparison of the two-phase nature of the flow. It is defined as:55P(C*f=0.92xJX0=0.91x/X=O.37X072Fig. 3.24. Lateral profile of probability distribution of local time-mean particleconcentration, C = (1— 6)/(1—e) at the wall for Ug7.O mis, G=40 kg/m2s,y/Y-1, z6.20 m. C*=1 corresponds to packed bed particle concentration, i.e.60.43.56pFig.3.25. Probabilitydistributionoflocaltime-meanvoidageatthetopoftheriserforUgS.5mIS,G4Okglm2s,xIX=O,z=8.89m.C*=1correspondstopackedbedparticleconcentration,i.e.6O.43.(Standard deviation of density fluctuations— atagiven point ) a 311— Standard deviation of density fluctuations a *for fully segregated two — phase flow withidentical time — mean density at the same pointThe standard deviation for fully segregated two-phase flow, where the local voidage alternatesbetween only two values, one corresponding to the voidage at minimum fluidization and the otherto unity, can be shown to be= PmfJ (susp/Pmf) (1— susp/Pmf) (3.12)where p, is the bed density at minimum fluidization and is the time-average pointdensity.a* is a useful normalizing factor because it is the maximum possible value of the standarddeviation for a specific voidage.Lateral profiles of intermittency indices at different heights are shown in Fig. 3.26. Exceptat the bottom of the riser, y is lower in the interior, showing the flow to be more homogeneousthere than near the wall. Because of the entrance effect, ‘y increases towards the axis at a height of0.25 m. High y values near the wall indicate large fluctuations of voidage. According to theprobability profiles of voidage indicated in Figs 3.21 and 3.22, there are two dominant peaks nearthe wall at the bottom of the riser, while only one peak is found in the central region where y islower.Near the bottom of the riser, y increases from the wall until it reaches a maximum and thendecreases towards the axis. The location of the maximum y, indicating highest heterogeneity, is581.0 • I • I I—•——z=O.25 m—--—z=O.79 m0.8—A-—z=117m-—v---z=6.20m> 0.6_________.-———.—0.4’A0.2 I I • I • I-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, yIYFig. 3.26. Lateral profiles of intermittency index at four levels for x/X=O,Ug=S .5 mIs, G=40 kg/m2s.close to the boundary between the core and annulus where the shear rate is highest. The locationsof trimodal probability distribution of particle concentration, shown in Fig. 3.23 are quite close tothe locations of these maximum y values.Figure 3.27 shows the axial profiles of intermittency index ‘y both at the center and at thewall. It is seen that y gradually decreases with increasing height. The high value of y at the bottomshows high heterogeneity of flow with intermittent occurrence of dense phase and dilute phase.Flow becomes more homogeneous towards the top of the riser. At the bottom, as indicated in Fig.3.26, the cross-sectional distribution of y is more uniform than at higher locations.59I . I I4 6 8 10Height, z (m)Fig. 3.27. Axial profiles of intermittency index at the wall and at the axis of thecOlumn for xlX=O, Ug=5.mis, G4O kg/m2s.1.00.80.60.4• yIY=-1• y/Y=OCE0.20.00 2As shown in Fig. 3.28, the intermittency index in the corner is higher than at otherlocations at the same level. However, there is only a small difference between y in the corner andelsewhere near the wall, despite the much higher particle concentration in the corner, suggestingthat the heterogeneity of flow in the corner is not substantially different from that at otherlocations near the wall.The lateral profile of intermittency index at the top of the riser is asymmetric due to theexit effect as illustrated in Fig. 3.29, with y higher on the exit side than on the opposite side. Thismay occur because, as described above, particles pile up in the horizontal duct connecting the60rO.6j x/X0x/X=O.37x/X=O.72IFig. 3.28. Lateral profiles of intermittency index for Ug=7.O mis, G=4O kg/m2sand z=6.2 m.611.0 I I I0.80.6CI0___________________Dirrensionless Distance, yIYFig. 3.29. Lateral profile of intermittency index near the top of the riser forUg=5.mis, G4O kg/m2s, x/X=O, z=8.98riser exit to the primary cyclone intermittently slip backwards and descend along the wall on theexit side (Glicksman et al., 1993).3.4 SummarySimilar to results in risers of circular cross-section, the voidage at the bottom of the riseris low and gradually increases towards the top. Local time-mean voidage decreases withincreasing solids circulation rate andlor decreasing gas velocity. The superficial gas velocity andsolids circulation rate exert more influence on the voidage at the wall than in the core. The riserexit causes an asymmetric lateral distribution of voidage near the top of the riser with the timemean voidage on the exit side being lower than on the opposite side.62The lateral profiles of voidage show that time-mean voidages are not always highest onthe axis of the column, The voidage sometimes follows an “M-profil&’, reaching a maximum at alocation of about 0.6 to 0.8 of the half-width of the column and then decreasing slightly towardsthe center. Voidages in the corners of the column are lower than elsewhere.At lower locations, frequency distributions of voidage near the wall tend to be bimodal.Closer to the axis, only one dominant peak is observed. The voidage distribution becomesnarrower as one moves from the wall towards the axis, indicating that the particle concentration ismore uniform in the core. Trimodal distributions are observed a small distance inward from thewall, suggesting that bulk downflow, particle streamers and dilute suspension coexist in thisregion. Wider distributions of particle concentration are also found in the corner.The intermittency index, ‘y is used to characterize the heterogeneity of the flow. Flow inthe core tends to be more homogeneous than at the wall of the riser. Flow tends to become lessheterogeneous with increasing height.63Chapter 4Velocity Profiles4.1 IntroductionParticle velocities affect mixing, heat and mass transfer as well as erosion in circulatingfluidized beds. Bader et al. (1988) used a Pitot tube to measure local particle velocities. Both thesolids flux profile and the radial profile of axial particle velocity in the riser were found to beparabolic. However, because of the large gas-solid slip velocity and interference caused by arelatively large probe, a Pitot tube is not an accurate instrument for measuring particle velocities.It can also be easily blocked by fine particles.Monceaux et al. (1986) measured local solid fluxes in a circulating fluidized bed with asampling probe and detected axisymmetric mass flux profiles as well as downward solid fluxesnear the wall. Rhodes et al. (1988), using a sampling probe, found that there is a possibility ofdownward solids flux in the wall region. Both downward and upward solid fluxes decreased withheight in the riser. They explained this in terms of transfer of solids from the core to the wallregion. Measurements have been made by Bolton and Davidson (1988) using scoops to collectparticles in the falling film.Laser Doppler anemometry (LDA) has been used by many workers to detect particlevelocity in gas-solid systems. LDA employs frequency information from light scattered byparticles passing through a fringe or interference pattern to determine particle velocities. The lightbeam can be divided into two equal intensity beams by a beam splitter and surface mirror. Thetwo beams are made to cross in a small region causing fringes. Particles passing through the64intersection region scatter light towards the collecting optics. From the frequency detected by aphotomultiplier, the velocity can be calculated.The major advantage of LDA is that it can provide accurate and instantaneous localparticle velocities. This technique also has the advantage of being insensitive to the absoluteintensity of scattered light. However, in regions of high particle concentration, light beams may beblocked by particles. LDA measurements are usually restricted to flows with very low volumetricconcentrations (Yianneskis, 1987), limiting its utilization in circulating fluidized beds. The methodcan sometimes be used to measure particle velocities in the upper region of a riser where thevoidage is high. It cannot currently be employed to measure particle velocities in the bottomregion of circulating fluidized beds, except right at the wall, and it works best with particles oneor two orders of magnitude smaller than typical CFB particles which follow the fluid motionclosely. It is therefore not well suited to circulating fluidized beds. The laser Doppler techniquewas used by Yang et al. (1990) to measure local particle velocities in a dilute circulating fluidizedbed. They found that the average cross-sectional mean axial particle velocity increased withincreasing gas velocity and/or decreasing solid circulation rate. The laser Doppler techniqueprovides accurate and instantaneous local particle velocities. It is insensitive to the absoluteintensity of scattered light over a broad range.Photographic and video techniques are widely used to study particle behaviour in fluidizedbeds. These methods are based on the analysis of particle images. These techniques can providesuch information as particle velocity, direction of motion and acceleration. To obtain particlevelocity, the distance between successive images of the same particle is determined, and thedistance is divided by the time interval between the two images. However, the method needstedious and sometimes difficult analysis, even with the help of advanced image-analysis software.The particle acceleration can be obtained by a similar method.65Small, fast-moving particles are difficult to photograph, unless careful consideration isgiven to proper illumination as well as to the interpretation of the images produced. Effects toconsider are the illumination intensity, particle size and particle velocity. For contrast purposes,back illumination can be used so that recorded particles will appear as black dots or a dark dashon a bright background (Kirkman and Ryley, 1969). Photographic techniques can be used in two-dimensional risers or in risers of square cross-section. They cannot be employed in regions whereparticle concentrations are very high such as the bottom region of circulating fluidized bed risers.In circulating fluidized beds, highly concentrated downward moving particle streamers may alsoblock light from reaching the camera.High speed video techniques have been used to study particle motion at the wall of coldmodel circulating fluidized beds (Arena et a!., 1989; Rhodes et al., 1990; Zheng et al., 1992). Adescending velocity of 0.3-0.4 m/s was obtained for particle swarms in contact with the wall,while a velocity of about 1 m/s was found for particles descending a few millimeters from the wall(Rhodes et al., 1990). This method was also employed by Arena et al. (1989) a circulatingfluidized bed with a two-dimensional riser to investigate solids flow structures. Wirth et al. (1991)employed a video camera and tracer particles and found that the particle descending velocity atthe wall of their riser was about 2 mIs. The biggest advantage of the high speed video technique isthat there is no disturbance of the flow field in the riser. However, this technique is generallylimited to studies in two-dimensional cold model equipment. No on-line data can be obtained. Inaddition, photographic techniques are difficult to apply to the interior regions of circulatingfluidized beds.Zheng et al. (1992) developed a microcomputer-controlled multi-colour stroboscopicphotography system to study particle motion in the region adjacent to the wall in CFB risers.Successive red, blue and yellow images of white tracer particles in beds of black particles were66used to provide such information as particle velocity and their directions of motion in the wallregion in CFB risers.Tracer technology, employing, for example, radioactive particles, can also be used tomeasure particle velocities. Detecting sensors for the tracer particles are usually located atdifferent heights. Particle velocity can be obtained from the vertical distance between two sensorsdivided by the time interval that a tracer particle needs to travel through the distance. Largeparticles are usually required to obtain signals strong enough to be detected by sensors. Nolaterallradial particle velocities can usually be obtained.Fibre optic probes were used by Hartge et a!. (1988), Horio et al. (1988), Ishii et al.(1990), Nowak et a!. (1990), and Rhodes et al. (1990) to measure particle velocities in circulatingfluidized bed risers of circular cross-section. Particles generally move upwards in the core of theriser. Particle velocity decreases from the center towards the wall, with negative values of time-averaged velocity very near the wall, i.e. particles move mostly downwards adjacent to the wall.This technique has the advantages of simplicity, high accuracy and low cost. A fibre optic particlevelocity probe was therefore employed for our particle velocity measurements in a riser of squarecross-section.Previous research (e.g. Brereton et a!., 1986; Schnitzlein and Weinstein, 1988; Wu et al.,1991; Chapter 3) has indicated that the geometry of the riser has considerable influence on thehydrodynamics of CFBs. Although risers of square cross-section are widely used in industry,especially for CFB combustion, very little research has been carried out in risers of non-circularcross-section. In this chapter, results of our experimental study on both lateral and axial particlevelocity profiles are presented. This extends the work reported in Chapter 3 in which we havedetermined voidage profiles with the same particles and operating conditions in the same riser.674.2. Fiber Optic Particle Velocity ProbeParticle velocity in circulating fluidized beds can be measured using the fiber optic andsignal processing system shown in Fig. 4.1. The fibre or bundle B in the middle is used to projectlight to illuminate the particles. Fibers A and C serve as carriers of light reflected from theparticles to two photomultipliers, and the signals from the photomultipliers are then fed to acorrelation analyzer. The transit time tAC is read as the maximum of the cross-correlation function.With the fiber at an effective separation length, 1, the particle velocity is calculated as:vp=—1- (4.1)This method has been employed by Old Ct al. (1975), Patrose and Caram (1982), andKojima et al. (1989) to measure particle velocities in fluidized beds. The disadvantage of cross-correlation is its long computation time. High accuracy requires a high sampling frequency andlong computing times. For low particle velocity measurement, the maximum cross-correlationcoefficient is difficult to obtain (Hartge et al., 1988). For the measurement of high particlevelocities, less accuracy is usually obtained because of limited sampling frequency. The distancebetween fibers A and C, 1 also requires careful consideration. For small 1, measurement accuracy islow and high sampling frequency is required. For large 1, the maximum cross-correlationcoefficient is usually low. The diameter of the optic fibers is usually greater than the particlediameter. Cross-correlation is then often used to measure the velocity of a group of particles. Ifparticles are distributed uniformly in the riser, this method fails because no cross-correlationmethod can be obtained. This method can also be employed for the measurement of single largeparticles.Another signal processing method is to use peak detectors to obtain the time interval tACThis method can be used to measure the velocity of single particles. When a particle passes68LightProj ectorOpticalFiberFig. 4.1. Schematic of fiber optic and signal processing system used to measureparticle velocity.Light fromSource to Photo-multiplierto Photo-multiplierReceivers ofReflected LightFibre OpticProbe PhotoParticleAtAClation69through the tip of the probe, reflected light is carried to two photomultipliers by the pair of opticfibers. Each signal has a peak. Peak detectors can be used to determine the time interval betweenthe two peaks, allowing the particle velocity to be estimated. The advantage of this signalprocessing method is that the signals can be processed by a hardware-electric circuit instead ofsoftware, so that the processing time can be much shorter. To use this method, the diameter of theoptical fibers must be chosen carefUlly to ensure that the dominant signals are from singleparticles. This method works well for measurements in spouted beds (He et al., 1995) because ofthe large particle size and unique particle moving direction. However, in circulating fluidized beds,particles are smaller than in spouted beds and the optical fibers cannot be too small; moreoverthere is an influence of particles surrounding the measured particle. For example, light carried tophotomultipliers from probes A and C in Fig. 4.1 may not be reflected from the same particle. Toovercome this problem, a five-fibered probe has been developed in this work.As shown in Fig. 4.2, each of the five fibres is a silicon optical fibre having a diameter of200 Jim. The similarity between the fibre size and the particle diameter allows measurements fromsingle particles and optimizes the signal-to-noise ratio. The probe consists of a horizontalcylindrical portion of diameter 2 mm and length 0.3 m leading to a 10 mm long head of cross-section 0.5 mm wide by 1.8 mm high. Light is delivered via fibres B and D, and then reflected byparticles through fibres A, C and E to three separate photo-multipliers. Signals from the photomultipliers are carried to peak detectors. Separate measurements of particle velocity are availablefrom the A and C fibre signals and from the C and E signals. A velocity is only accepted if thesetwo measured velocities are within a certain tolerance, taken here to be 1% of each other, i.e.v=for 2 ‘0 0.01 (4.2)2 vAC+vCE70_AC _CE/where VAC—AACand VCE—/tCE(4.3)with 2AC and ?cE being the effective separation distance between A and C and between C and E,respectively, and tAC and tCE being the corresponding transit times determined from the peakdetectors.This probe and validation procedure minimize errors caused by light reflected totransmitting fibres from different particles, a problem encountered by three-fibre probes. Theto photomultiplierto photomultiplierFig. 4.2. Schematic of five-fibre optical particle velocity probe.71technique also ignores data from particles which are not travelling vertically upwards ordownwards, i.e. it determines the velocity of particles travelling in a vertical direction rather thanthe vertical component of all particles passing the probe.The system was calibrated by gluing some particles to a thin circular plate and thenrotating the plate at different angular velocities. An oscilloscope was used to find the actualvelocity of the measured particle. Thus, the effective optical distance between fibres can beobtained. The calibration indicated that the effective optical distance differs somewhat from thegeometric distance between fibres. Manufacturing flaws such as fibres not being perfectly paralleland differences in fiber diameter are probably responsible for this difference. Calibration isnecessary for accurate measurement.To provide comparisons with the voidage profiles, measurements of particle velocity weretaken at the same locations as the voidage measurements presented in Chapter 3, i.e. at heights of0.25, 0.79, 1.83, 3.45, 5.13, 6.20, 7.06, and 8.98 m above the distributor. Lateral profiles wereattained by performing measurements at 0.0, 26.0, 47.0, 60.0, 66.5, and 73 mm from the riseraxis. The coordinates for figures are illustrated in Fig. 2.1.Unlike the voidage measurements described in Chapter 3, a dark column is not requiredfor the measurement of particle velocity. As for the voidage measurements, a special fitting wasemployed to mount the fibre optic probe to the riser so that the probe could be moved laterallywithout shutting down the CFB system. After the measuring system and a personal computer arehooked up, measurements can be started. For each measurement, 2000 samples are usually takento obtain mean values for both ascending and descending particle velocities. Each measurementwas repeated six times to check the reproducibility.724.3 Experimental Results and DiscussionThe particle velocity distribution determined by the fibre optic particle velocity probe for atypical case is shown in Fig. 4.3. From the optical fibre measuring system, one can obtain thevelocities of both ascending and descending particles, as well as the relative numbers of particlestravelling upwards and downwards.Fig. 4.3. Particle velocity distribution for Ug5.mis, G=4O kg/m2s, xJX=O,y/Y=O, z=6.2 m. Mean particle velocities: upwards: 7.1 mIs, downwards: -1.6mis. Number of sampled particles: upwards: 1949, downwards: 51.I I Iz(I)ci)C.)C”0t3ci)C”C’)0ci)I16012080400--5—.—0Particle Velocity, v, rn/SI.15734.3.1 Lateral Profiles of Particle VelocityLateral profiles of vertical particle velocities are shown in Figs 4.4 and 4.5. As expected,the ascending particle velocity reached a minimum at the wall and gradually increased towards thecenter of the column. The non-uniformity no doubt occurs because of the no slip condition forgas, causing a lateral profile of gas velocity, and because the particle concentration is higher nearthe wall (Hartge et al., 1986; Weinstein et al., 1986; Hartge et a!., 1988; Chapter 3). Profiles weresymmetric about the axis at this height.The influence of solids circulation rate on particle velocity profile is shown in Fig. 4.4. Inthe central region of the riser, the ascending particle velocity increased with increasing solids8 •• 2 -,- v , V Descending ParticlesC]) A Ascending Particles> 010 08 06 04 02 0 0Dimensionless Distance, y/YFig. 4.4. Lateral profiles of local particle velocities for different solids fluxes:Ug5.m/s, z=6.2 m, xIX=O.74circulation rate. This is because the gas velocity increases with solids circulation rate in the centralregion of the riser (Yang et a!., 1993). Near the wall, however, the ascending particle velocitydecreased with increasing solids circulation rate. As reported in Chapter 3, particle concentrationincreased with increasing solids circulation rate, with greater changes near the wall. Since therewere more highly concentrated particles aggregates near the wall, the gas-solids slip velocity ishigher there. Because the gas velocity near the wall decreases with increasing solids circulationrate (Yang et al., 1993), the ascending particle velocity decreased, while the magnitude of thedescending particle velocity increased slightly with increasing solids circulation rate.8 I I • I64 .___U =5.5 rn/s2U:=7.O rn/sg Is A Ascending Particlesv —v—Descending Parti±s-2 -4 I • I I • I-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, y/YFig. 4.5. Lateral profiles of particle velocities for different superficial gas velocities:G=4O kg/m2s; z=6.2 m, xlXO. For coordinates see Fig. 2.175Figure 4.5 shows the influence of superficial gas velocity on the lateral particle velocityprofile. As expected, the ascending particle velocity increased with increasing superficial gasvelocity. However, the descending particle velocity changed very little as the superficial gasvelocity was varied over a limited range (5.5 to 7.0 mIs).The lateral profiles of particle velocity in Figs 4.4 and 4.5 can be compared with theresults of Hartge et al. (1988) obtained in a column of circular cross-section using an opticalprobe and those reported by Wang et al, (1993) using a particle dynamic analyzer. Similar trendswere recorded. Near the axis, the ratio of the particle velocity at the axis to the superficial gasvelocity vp/tJg in the developed region of the riser is around 1 to 1.3. For smaller particles oflower particle terminal velocity such as FCC, Vp/Ug can be considerably higher (Bader et al.,1988; Hartge et al., 1988; Yang et al., 1993). Measurements of local average particle velocities byYang et al. (1993) and Hartge et al. (1988) show that particle velocities decreased monotonicallywith radial distance, becoming negative (downwards) near the wall. This work shows that whendescending and ascending particle velocities were measured separately in a column of squarecross-section, the magnitude of the average velocity of descending particles increased from thewall to a maximum at around 0.7 of the half-width of the column and then decreased towards theaxis.Lateral profiles showing the local percentage of sampled particles which are being carriedupwards (rather than descending) are shown in Figs. 4.6 and 4.7. The percentage of ascendingparticles increased from the wall to the center. Almost all particles moved upwards in the centralregion of the riser, while most particles moved downwards near the wall. Given the lateral particleconcentration profiles presented in Chapter 3 for the same riser with the same particles, the lateralprofiles of particle velocities in Figs. 4.4 and 4.5, and the profiles in Figs. 4.6 and 4.7 showing theproportions of rising and falling particles, it is clear that a core-annulus flow structure exists in asquare CFB riser, as in risers of circular cross-section. In the dilute core of the riser, particles are76uniformly distributed and most particles travel upwards. Surrounding the core, there is an annulardense region near the wall where particle concentration is higher and most particles traveldownwards.As shown in Fig. 4.6, the fraction of particles which are ascending decreased significantlywhen the solids circulation rate was increased from 20 to 40 kg/m2s. More change was observednear the wall than in the central region. This is because the gas velocity near the wall decreaseswith increasing solids circulation rate (Yang et al., 1993), while the particle concentration in thesame region increased greatly with increasing solids circulation rate as shown in Chapter 3. Thethickness of the annular region, defined here as the distance from the wall to location where theC’)U)C.)eC4-.-CCP-0.8 -0.6 -0.4 -0.2Dimensionless Distance, y/Y0.0Fig. 4.6. Lateral profiles of fractions of particles travelling upwards for differentsolids fluxes: Ug= 5.5 m/s, z=6.2 m, x/X=0.10080600A G= 20 kg/m2s40 kg/m2s0-1.077100 I • I • I- IU) 80ci) --60—A— Ug=5•mis0------U=7.OmIs40 ‘7gP 120/C-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, y/YFig. 4.7. Lateral profiles of fractions of particles travelling upwards for differentsuperficial gas velocities: Gg=4O kg!m2s, z=6.2 m; xJX=0.vertical time-mean particle velocity is zero, also increased with increasing solids circulation rate.Therefore, more particles move downwards near the wall of the riser with increasing solidscirculation rate. Figure 4.7 indicates that the superficial gas velocity had little influence on theprofiles of fraction of particles travelling upwards for the conditions studied.4.3.2 Axial Profiles of Particle VelocityFigures 4.8 and 4.9 show axial profiles of particle velocities and sampled particle fractionsat the wall and at the center of the riser respcctively. The bars indicate the reproducibility of thevelocity measurements based on six measurement time intervals, each producing 2000 data points.78In the developing zone at the bottom of the riser, the magnitudes of both the ascending anddescending particle velocities increase with height. Similar results were obtained by Hartge et al.(1988) for a column of circular cross-section. As indicated in Fig. 4.8, there was also adeceleration zone near the exit of the riser where the magnitudes of both ascending anddescending particle velocities decreased. At the wall, the fraction of particles descending increasedwith height at the bottom of the riser, then decreased, and finally increased again at the top of theriser. These trends are consistent with the profiles of particle concentration presented in Chapter3.I I I3210Cl)00G)C-)0V , Descending ParticlesA Ascending Particles3)C-CU)0(I)U)U80Cl)a)70oDUO—30W2010—12 84 6Height, z (m)Fig. 4.8. Vertical profiles of particle velocities and fraction of particles descendingalong wall of column: Ug=5.5 mIs, G2O kglm2s, x/X=O, y/Y=- 1.79Fig. 4.9. Vertical profiles of particle velocities and fraction of particlesascending along axis of column: Ug=5.mis, G=2O kglm2s, xIX=O, ylY=O.6420-20()>.()0a)()Cu0100800ci)C) 0E‘J’J Cu c,)4O2o<2 4 6 8Height, z (m)Figure 4.9 indicates that along the axis of the riser, the fraction of particles travellingupwards hardly changed with height. This is consistent with particle concentration measurementsin the same column (see Chapter 3) which indicate that the particle concentration is relativelyuniform in the core of the riser.4.3.3 Wall-Layer Particle Descending VelocityThe velocities of particles descending at the wall are seen to be in the range of-0.8 to -1.5mis. This magnitude is similar to the magnitude of downward wall velocities summarized by Wuet al. (1990) and by Senior and Brereton (1992) for previous work by other authors with columns80of circular cross-section and is influenced very little by the operating conditions such as solidscirculation rate and superficial gas velocity. However, at a small distance, e.g. 3 mm, from thewall of the riser, it was found that the magnitude of the descending particle velocity could besignificantly greater than 2.5 rn/s.Downward moving clusters are generally believed (Yerushalmi et al., 1977; Brereton andStromberg 1986; Horio et al., 1988; Hartge et al., 1988; Rhodes et al., 1990; Li et al., 1991) toform wall layers in CFB risers. In spite of the fact that the structure and motion of the clustersplay very important roles with respect to such processes as heat transfer and erosion at the wall ofCFB risers, the size, velocity, density and other attributes of particle clusters near the wall are notwell known. Li et al. (1991) reported that the diameter of clusters is several millimeters. Clustersof dimension 2 to 3 mm* were found by Wei et al. (1993). Horio et at. (1993) illustrated, in ariser of 200 mm inner diameter, that the cluster size was roughly 1/20 to 1/10 the columndiameter. Clusters as large as 15 mm* have been found by Horio et al. (1993). As demonstratedin Chapter 3, with observed particle downflow near the wall, the voidage of the wall layer isusually in the range 0.6 to 0.9.As shown in Fig. 4.10, a cluster near the wall of a CFB riser can be modelled as aspheroidal assembly. One can then perform a force balance with five forces acting vertically:gravitational force (F0), drag (FD), wall friction (F), buoyancy (Fe), and a force Fe due tomomentum exchange between the cluster and the surrounding flow. If the cluster is approximatedas a spheroid with its symmetry axis oriented vertically, one can write equations for these forcesas follows.gravitational force:* Not clear whether this is a diameter or length.814itFG =——-—a2bgp(l )4it= ——a3Er gp(1—60)3(4.4)where a and b are the horizontal and vertical semiaxes of the spheroidal cluster, respectively,Erb/a, g is the gravitational acceleration, Pp iS the solids density, and is the internal voidage ofthe cluster.drag force:= CD Pga2u1FG2a44)ErbIa2b(4.5)Fig. 4.10. Vertical forces on spheroidal clusters near the wall.FBFwFDft82where Urel is the relative velocity between the gas and particles, Pg 15 the density of gas, and CD isdrag coefficient. Flow of gas through the cluster has been neglected, i.e. the cluster has beentreated as if it were a solid spheroid.An expression for drag on solid spheroidal bodies was developed by Militzer et al. (1989).The correlation can be considered as a modified version of an expression for drag on spheres(Militzer et al., 1989) in which the influence of variation in shape and flow regime are accountedfor:CDS (1+0.15 Re°687)+ 0.42 E1[Re 1+4.25xl0R&6] 5r1.00 + 0. 00096 — 0.000754 Re Er + 0.0924 + 0. 00276E 2 (4.6)[ Er Re2p aurel . . .where Re = g is the Reynolds number and g is the viscosity of gas. In this study,tgEquation (4.6) was used within its limits of applicability (0.2 Er 5.0 and 1 Re 200).Considering the influence of particle concentration (Wallis, 1969), CD can be estimated from thedrag coefficient of single spherical particle CDS as:CD = CDS 60Ut4’7 (4.7)where 5out is the effective outer voidage of clusters which can be calculated as(4.8)1—where 6 is the overall local voidage as measured by the fibre optic voidage probe, Since there is83no correlation available to calculate CD for spheroidal particles considering the influence ofparticle concentration, Equation (4.7) is employed here to estimate the effect of multiple clusters.wall friction forceFw = nEra2t (4.9)where t is shear stress on the cluster, N/rn2. Figure 4.11 shows the shear stress on the wall due todescending particle flow determined by Van Swaaij et al. (1970). Assuming u=v, a linearregression has been done to obtain the following correlation:= kp(1—6)u42Fig. 4.11. Shear stress on the wall of the riser by downflowing particles vs.solids flux adapted from Van Swaaij et al. (1970).(4.10)12108600 100 200 300 400 500Solids FIux kg/m2s84where t is shear stress, N/rn2;k0.016 is a constant, rn/s and u is the velocity of the cluster, rn/s.Assume gas velocity is zero, UcUreI.The rate of momentum transfer required to accelerate the cluster by collisions is equal inmagnitude to the decelerating force on the surrounding flow. Assume particles entering the clustercollide with all particles in the unit. The force on the cluster is then:Fc = GhO(uC — v)(itab)= GhO(u0— v)(ira2Er) (4.11)where Gh0 is the lateral solids flux from the adjacent dilute phase to the cluster flow, and is thevertical velocity of solids entering the cluster.buoyancyF =_a2bgpg(1_6c)=_a3Egpg(1_6) (4.12)When the cluster near the wall reaches its terminal velocity, we have:FG—F—Fw—Fc—FB=O (4.13)To solve the non-linear equation, Equation (4.13), for the cluster velocity, u0, the NewtonRaphson method was employed. in Equation (4.11) is assumed zero based on the fact that thevertical particle velocity at the boundary between the core and annulus can be taken as zero (seesection 4.3.4 below). According to cross-flow measurements presented in Chapter 7, the solidsflux entering the annulus from the core is of order 1/10 of the solids circulation rate. For both85Ottawa sand particles of particle density p= 2640 kg/rn3 and mean size d=213 urn and FCCparticles of p= 1600 kg/rn3 and d=70 aim, the cluster descending velocity was calculated fromthe force balance as a function of the voidage and the diameter of the cluster for Ug=5.5 rn/s andG=40 kg/m2s. The relative contributions with the various force terms for a cluster of Er=l, 6=0.8, 6=0.7 and a=3 mm are listed in Table 4.1.Table 4.1 Relative contributions of each force component for a cluster OfEr=1,6=0.8, =O.7 and a=3 mm, assuming FG=l.Ottawa Sand FCCFG 1 1F 0.47 0.56F 0.42 0.32FD 0.11 0.12FB <0.001 <0.001The model results for spherical clusters of both Ottawa sand and FCC particles are shownin Figs 4.12 and 4.13. It is seen that in addition to the particle density, the size and the internalvoidage of the cluster are also very important parameters influencing the descending velocity ofclusters. With increasing cluster diameter, the predicted descending velocity increases. The effectof decreasing internal cluster voidage is to increase the descending velocity of the cluster. Nearthe wall of CFB risers, clusters of large and/or dense solids have greater descending velocity.862.5EO.852.0D1.00U)>0.50.0 -0 5 10 15Cluster Size, 2a (mm)Fig. 4.12. Simulation of the descending velocity of a spherical cluster, i.e. Er=l,vs. diameter and cluster internal voidage for Ottawa sand particles: 6=0.9, Ug=S.5rn/s and G8=40 kg/m2s.Comparison of Figs 4.12 and 4.13 indicates that for the same size and voidage, Ottawa sandclusters have higher velocities of descent than FCC clusters.From the voidage measurements in the developed region given in Chapter 3, the time-mean voidage near the wall for Ug=5. rn/s and G=40 kg/m2s is around 0.9; and is roughly0.85. The measured particle descent velocity near the wall is in the range of 0.8 to 1.5 rn/s fromFig. 4.12. The corresponding cluster size 2a is in the range of 6.5 to 12 mm which is in reasonableaccord with the experimental data of Horio et al. (1993). In general, the model predictions for6O.80EO.65Er187I • I — • I2.01.6 -00.8C)0ci)> 0.40 6 9 12 15Cluster Diameter, 2a (mm)Fig. 4.13. Simulation of the descending velocity of a spherical cluster, i.e. Er=1,vs. diameter and cluster internal voidage for FCC particles: 6=0.9, Ug=5.rn/s andG=40 kg/m2s.both Ottawa sand and FCC particles agree well with experimental results of other researchers asindicated in Table 4.2.Since clusters in CFB risers are not spherical, simulation has been done to test thesensitivity of the descending cluster velocity to its aspect ratio. With clusters taken as spheroidalwith vertical axis of symmetry, the ratio of vertical semiaxis to horizontal semiaxis, Er, was variedfrom 0.5 to 5, covering the range in which the correlation for drag coefficient CD, i.e. Equation(4.6), is applicable. Calculations have been made for both Ottawa sand and FCC particles with6O.8O=O.75=O.7O6O.65.13Er188=O.75 and horizontal semiaxis a=6 mm. The simulation results are shown in Fig. 4.14. It is seenthat the descending cluster velocity increases with Er which means the longer the cluster, thefaster it descends because the drag coefficient CD decreases with increasing Er. However, forEr> 1, since F and F increase with Er because of the increase in surface area, no dramatic changein uc with Er is predicted.Table 4.2. Descending particle velocities near the wall of CFB risers measured byvarious researchers.Riser Measuring d p Ug Usize method (jim) (kg/rn3) (m/s) (kg/rn3) (m/s)Horio et al. 300mm Fibre optic 60 1000 1.17, 11.7, 11.25,(1988) id probe 1.29 11.75 0.5-1.8Bader et al. 305 mm Pitot tube 76 800 3.7 - 98 - 195(1988) id 6.1 0.5-1.8Hartge et al. 400 mm Fibre optic 85, 120 1500, 2.9, 30, 49(1988) id probe 2600 3.7 0.52.0Ishii et al. N.S. Fibre optic N.S. N.S. 1.29 N.S.(1989) probe 0.64.1Wuetal. 152mm High-speed 171 2650 7 N.S.(1989) id cinematography 1.26Wirth et a!. 170 x Video camera 50, 90, N.S. 1.9 N.S.(1991) 170 mm2 200 2Yang et a!. 140mm LDV 54 1550 1.8- 22-92(1992) id 4.3 0.5-1.2Rhodes et 305 mm High-speed 74.9 2456 3 - 5 2 - 80a!. (1992) id video camera 1.0N.S.=not specified.891.6__1.4Cl)IFig. 4.14. Predicted velocity of downflowing particle clusters near the wall vs.height-to-width ratio, Er, for =0.75 and a=6 mm.4.3.4 Core-Annulus BoundaryCFB risers of both circular and rectangular cross-section are commonly described in termsof a core-annulus flow structure, where there is a dilute core in which most particles travelupwards surrounded by a dense annular wall layer in which most particles move downwards.However, there is a lack of consistency in the way that the boundary between the core and the1 2 3 4 5Aspect Ratio, Er90annular wall layer has been defined by different researchers. To be able to compare results fromthe literature, it is important to adopt a common definition.There are two common ways of determining the boundary between the core and theannular downflow wall layer. One group of researchers (Hartge et al., 1988; Ishii et al., 1989;Yang et aL, 1992) defined the boundary between the core and the annular wall layer as thelocation where the time-average vertical particle velocity is zero. Others (Rhodes et al., 1988;Herb et al., 1992; Miller and Gidaspow, 1992) measured vertical solids fluxes, usually by meansof solids sampling probes, and defined the core-annulus boundary as the location where thevertical net solids flux is zeroThe time-mean vertical component of particle velocity and voidage are given by1 Jv =—i vdt (4.14)TO—I rTand 8 =— I Edt (4.15)TJOwhile the local vertical solids flux isG=!Pf v(1—e)dt (4.16)Only if vp and 6 are uncorrelated would it be possible to write(4.17)However, there is considerable evidence that there is a strong correlation between theinstantaneous local voidage, 6 , and the vertical component of particle velocity, vi,, especially inthe vicinity of the wall where periods of upward solids velocity (positive v) tend to be associated91with high local voidage while downward moving particles (negative v) tend to be associated withstreamers and clusters, and hence lower e. Given this, equation (4.17) is not expected to be valid,meaning that G and are not proportional to each other and are unlikely to reach 0 at the samelocation.Experiments with a sampling probe were carried out to measure the core-annulusboundary defined in terms of the location of zero vertical solids flux. The sampling system isillustrated in Fig. 4.15. The inside diameter of the sampling probe is 6.5 mm. A graduated cylinderserved as sample collector so that the amount of sample obtained in a certain time period can beread directly from the graduations. During sampling, two valves shown in Fig. 4.15 were fullyopen and particles are carried from the riser to the collector through the sampling probe due tothe pressure difference between the inside and the outside of the CFB riser. The upward anddownward solids fluxes were measured by pointing the open end of the probe downwards andupwards, respectively. The net vertical solids flux was determined from the difference between theupward and the downward solids fluxes.Bierl et al. (1980) found that isokinetic sampling is not necessary under conditions ofrelatively dilute flow of large CFB particles of 61 aim. Rhodes (1990), Miller and Gidaspow(1992) investigated the influence of the suction velocity of the sampling probe and found thatnonisokinetic sampling probe can be employed in CFB risers to measure the net vertical solidsflux. A non-isokinetic sampling system has been employed to measure the net vertical solid flux inthis study.The particle velocity data, coupled with the data for the voidages from Chapter 3 and thefractions of particles ascending and descending, allows the relative upward and downward fluxesand the thickness of the downflowing annulus layer to be determined. The axial profile of the92SamplingProbe15mm6.5 mm i.d. OpenAreaValve—VentParticlesFig. 4.15. Schematic of the sampling system to measure net vertical solids flux.annulus thickness for Ug= 5.5 mIs and G’ 40 kg/m2s is shown in Fig. 4.16. The core/annulusboundary has been taken as the location where the time-mean particle velocity is zero. Figure 4.16also shows the axial profiles of wall layer thickness obtained from solids flux measurements. It isseen that, along the entire height of the riser, the wall layer obtained from the flux measurement isIII GraduatedCylinder93always thicker than from the vertical velocity measurement. The difference between the two walllayer thicknesses decreases with height near the bottom and increases in the upper part of theriser. Recall that near the bottom of the riser, there is a particle acceleration region, while near thetop, there is a deceleration region.As indicated in Fig. 4.16, the wall layer thicknesses determined from both measurementtechniques first decreased with height from the bottom until a minimum was reached about 4 mabove the distributor, then increased towards the top. Near the bottom, the net lateral solids fluxmust have been outwards towards the wall of the riser, causing the annulus thickness to increaseas the wall layers descended towards the bottom. On the other hand, in the upper portion of the30 • I • I25 from flux measuremento from locity measurement20 7/5.0 • • I • I0 2 4 6 8 10Height, mFig. 4.16. Axial profiles of annular wall layer thickness for Ug5.mIS,G 40 kglm2s, xIX=0.94riser, many particles reaching the top are reflected downwards along the wall. Net solids fluxeswere then inwards towards the axis of the column as wall layers are stripped of particles as theydescend.Figure 4.17 plots the wall layer thickness versus lateral position along the wall. Thedifference between the measured wall layer thicknesses from the two methods is greatest near thecorner of the riser. Figures 4.16 and 4.17 clearly indicate that there is a significant differencebetween the zero vertical particle velocity boundary and the zero-vertical-net-solids-flux boundaryin CFB risers. This difference must be considered when comparing data obtained using the twodifferent techniques.5 • from flux measurementso from Iocity measurements0 • I I • I-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, y/YFig. 4.17. Lateral profiles of annular wall layer thickness for z5. 13 m, Ug=5.miS,G= 40 kg/m2s.95Although the axial and lateral distributions of the core-annulus boundary obtained fromthe two measurement methods exhibit similar trends, it is believed that the core-annulus boundaryis better defined as the location where the vertical net solids flux is zero because the core-annulusflow structure is usually described as a core where most particles travel upwards surrounded byan annular wall layer where particles generally travel downwards. However, in the followingparagraphs only the results based on measurements from the fibre optic particle velocity probe arepresented, because this chapter mainly discusses particle velocity.Figure 4.18 shows that the mean ascending particle velocity near the corner wassomewhat lower, while the magnitude of the descending particle velocity in the corner was higherthan mid-way between opposite walls. The same trend was obtained at a height of 6.20 m. Themagnitudes of the velocities of the descending particles at z=5. 13 and 6.20 m were around 1.8rn/s. Wang et al. (1993) found that instantaneous particle velocities in the corners of a square riserdid not exhibit any systematic trend, while the results given above are time-average values. Asnoted in Chapter 3 and consistent with the relative numbers of ascending and descending particles,the particle concentration in the corner was relatively high because descending particles are wellprotected in the corners where the gas velocity is reduced. Since the wall layer is thicker in thecorner, the effective cluster size is expected to be greater than elsewhere along the wall.According to the theoretical model, the descending velocities of clusters in the corner must behigher because of lower cluster voidages and larger cluster sizes.To check the accuracy of the experimental data, net solids fluxes were estimated byintegration using the voidage profile data from Chapter 3, together with particle velocity profilesand fractions of particles rising and falling from this chapter, all for Ug = 5.5 rn/s. The integratedvalues of net solids fluxes are 37.2 and 38.0 kg/m2s for heights of 5.13 and 6.2 m with G5= 4096>. “ CO )°0•v v Descending Particles 40Cl) A Ascending ParticlesC).20u0.4 0.6 0.8 1.8Lateral Position, xlXFig. 4.18. Lateral profiles of velocities and fraction of particles descending nearthe wall of column: Ug=5.5 mIs, G=40 kg!m2s, z=5. 13 m, y/Y= 1.kg / m2s, and 19.1 kg / m2s for z6.2 m with G5= 20 kg! m2s. The calculated fluxes aresufficiently close to the preset values of G that they add to the confidence in the data.Lateral profiles of mean vertical particle velocity appear in Fig. 4.19. Particles near thewall are seen to travel mostly downwards, while most particles are conveyed upwards in the core.In the corner, particles moved downwards faster and the thickness of the downflow wall layer wasgreater than mid-way between facing walls. Wall layer thicknesses based on the values of y!Ywhere v0 are shown in Fig. 4.20 for Ug=5. mIs and G=40 kg! m2s.The shape of the outerwall layer is indicated in the insert. Note the increased thickness of the wall layer in the corners.978Cl)>. 4.C).22CCuU)-4 --1.0 -0.8 -0.6 -0.4 -0.2 0.0Lateral Position, yIYFig. 4.19 Lateral profiles of mean particle velocity: z5.13 m, Ug=5.ITh’S,G=4O kg/m2s.The magnitudes of the average particle velocity in the core for the developed region havebeen compared with the predictions of a semi-empirical hydrodynamic model developed by Seniorand Brereton (1992). This model assumes a constant voidage and a constant downward particlevelocity in an outer annular layer, with a uniform dilute suspension in the core where particlestravel upwards at a constant velocity equal to the core gas superficial velocity minus the terminalsettling velocity. The wall layer thickness is assumed to be uniform at any height. The gas velocityis assumed to increase linearly from the wall in the annulus region and to be constant in the core.The predicted average particle velocity in the core of the developed region for Ug=5. mis andG=4O kglm2sis around 4.0 to 4.2 mIs for conditions corresponding to the riser employed in theA—e— x/X=0.37—A— x/X=0.7298—--•:.. -- Annulus60— 7.’ -. ..Coio-annulusboiiidaryE CoioE 50•,• z=6.20mci) 40 ‘WaIIofC flS9fCi30j:._z=5.EZZZZ• z=6.20m0 I • I • I • I-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, yIYFig. 4.20. Lateral profiles of annular wall layer thickness for: Ug=5.flhIS,G=40 kglm2s. Insert shows shape of wall layer boundary at twodifferent heights assuming symmetry around axes.present study, except near the top. The corresponding experimental average particle velocity inthe core was in range of 3.5 to 4.2 mIs, in good agreement with the predictions. On the otherhand, the thickness of the annular wall layer for the developed region calculated from the modelwas several times smaller than the experimental value.99Fig. 4.21. Lateral profiles of particle velocities and fraction of particles ascendingat top of column: Ug=5.mis, G=4O kg/m2s, z=8.98 m, xIX=O.4.3.5 Exit EffectLateral profiles of particle velocities and fraction of sampled particles ascending near thetop of the riser are presented in Fig. 4.21. Because of the presence of the exit, these profiles arenot symmetric around y/Y=O. Instead, the location of the maximum ascending particle velocity isshifted somewhat to the exit side. Also, the thickness of the annular wall layer obtained bymeasuring particle velocity was thinner on the exit side than on the opposite side. In the annularzone, the magnitude of the average velocity of descending particles was smaller on the exit sidethan on the opposite side. These results are consistent with the gas flow pattern for an abrupt exitproposed by Brereton (1987).8 100.80> fA A6O2—v—- Descending Particles A—A— Ascending Particles C>0 .2_Vy. -y1V_—y-----_yy/ 20-2: V—V04. I I I 0-1.0 -0.5 0.0 0.5 1.0Dimensionless Distance, y/Y100Figure 4.22 shows the influence of solids circulation rate on the lateral profiles of particlevelocities at the top of the riser. With increasing solids circulation rate, the ascending particlevelocity in the center of the riser increased, and the location of the peak corresponding to themaximum ascending particle velocity shifted slightly towards the exit side. The magnitude of thedescending particle velocity also increased somewhat with increasing solids circulation rate.8 •64. G2O kWm2sg 2- G=4O kWm2sv V Descending Particles(1) - - i —A—-Ascending Particles!Dimensionless Distance, yfYFig. 4.22. Lateral profiles of local particle velocities at top of column fordifferent solids fluxes: Ug=5.mis, z8.98 m, xlXO.1014.4 SummaryAn optical particle velocity measuring probe made of five 200 pm silicon optical fiberswas used to measure vertical particle velocity and the fractions of ascending and descendingparticles. The probe was 2 mm in diameter and 0.3 m in length with a 10 mm long head of cross-section 0.5 mm by 1.8 mm.As expected, the ascending particle velocity was lower at the wall and it graduallyincreased towards the center of the column. In the central region of the riser, the ascendingparticle velocity increased with increasing solids circulation rate. Near the wall, the ascendingparticle velocity decreased while the descending particle velocity increased slightly with increasingsolids circulation rate. The ascending particle velocity increased with superficial gas velocity.However, the descending particle velocity did not change much as the superficial gas velocity wasvaried from 5.5 to 7.0 mIs, showing a small drop.The particle velocity data, coupled with the data for the ascending and descendingparticles, allows the relative upward and downward fluxes and the thickness of the annulus layerto be determined. The annulus thickness first decreases from the bottom until a minimum isreached 3 to 4 m above the distributor, then increased towards the top. The wall layer thicknesswas greater when based on solids flux measurements than when based on the position where thetime-mean particle velocity was zero. The percentage of particles which are ascending increasedfrom the wall to the axis. The fraction of particles descending increased significantly when thesolids circulation rate was increased from 20 to 40 kglm2s. The annulus thickness also increasedwith solids circulation rate.In a developing zone at the bottom of the riser, both upward and downward particle102velocities were found to increase with height. The development length on the axis of the riser wasless than at the wall. The presence of the exit led to asymmetric profiles of particle velocities andof the fractions of sampled ascending particles at the top of the riser. The location of themaximum upward particle velocity shifted somewhat to the exit side, while the annular wall layerwas thinner on the exit side than on the opposite side. In the annular zone, the magnitude of thevelocity for descending particles was smaller on the exit side than on the opposite side.The ascending particle velocity was lower near the corner, while the descending particlevelocity was greater than mid-way between facing walls. Towards the corner, a greater fraction ofparticles moved downwards. The downward particle velocities at the wall were in the range 0.8 to1.5 mIs, influenced very little by solids circulation rate and superficial gas velocity. However, at asmall distance, e.g. 3 mm, from the wall of the riser, the downward particle velocity could besignificantly higher.A theoretical model has been established to predict the descending cluster velocity nearthe wall of the riser. The simulation results show that the descending velocity increases with thesize and the density of clusters. Clusters of Ottawa sand particles are predicted to have higherdescending velocities than clusters of FCC particles. Elongated clusters are predicted to descendmore quickly than spherical ones of similar cross-sectional area.103Chapter 5Influence of Wall Roughness on the Hydrodynamics5.1 IntroductionIn Chapters 3 and 4 basic hydrodynamics, i.e. voidages and particle velocities, in the CFBriser are discussed. All these results are for a riser with smooth inside wall surfaces. However, theinner surfaces of commercial CFB reactors can be quite rough (e.g. in CFB combustors withrefractories), with roughness elements sometimes exceeding particle diameter by several times.Roughness could have a significant influence on the hydrodynamics, which could in turn altermixing, gas-solids contacting and heat and mass transfer in circulating fluidized bed processes.Glicksman et al. (1991) reported that protrusions as small as one particle diameter cancause a change in the particle concentration in a circulating fluidized bed. Protrusions werebelieved to lead to an increase in solids concentration at the top of a hot column. No other earlywork has been reported on this topic.An experimental study has been carried out on hydrodynamics in a riser with rough innerwalls. Fiber optic probes were employed to measure particle concentrations and particlevelocities. Both probes could be moved horizontally at a series of ports along the riser wall. Theprobes are described in Chapters 3 and 4. In this chapter, the influence of wall roughness isdetermined by comparing the experimental results from a riser having rough inside wall surfaceswith corresponding results from the earlier chapters for the same riser with smooth inner surfaces.1045.2 Experimental Set-upSheets of sand-paper were affixed to all four inner walls of the riser, from the bottom tothe top, to provide a rough wall surface. The walls were completely covered, i.e. including theplexiglass windows. Double-sided tape was used to affix the sand-paper to the walls. The top ofthe column was also covered with sandpaper.Wirth et al. (1991), using a y-ray technique and Lints and Glicksman (1993), using aparticle impact probe, reported that a gas layer exists between downward-moving particle swarmsand the wall. They claim that the thickness of this gas layer is usually less than one particlediameter. The protrusions which were found by Glicksman et al. (1991) to influence thehydrodynamics were of order one particle diameter thick. In circulating fluidized bed combustors,the refractory roughness may be greater than 10 times the particle size. In large-scale industrialCFB units wall roughness is likely to influence hydrodynamics primarily near the wall. It seemslikely that roughness elements of the same order of size as the particle will have great influence onthe flow in the wall region. Therefore, wall roughness of the same order as particle diameter waschosen in the present work. The coarse sand-paper (Grit 40), had roughness elementsapproximately 0.45 mm in size, i.e. about twice the particle diameter.5.3 Experimental Results and DiscussionFrom initial tests, the roughness of fresh sand-paper changed slightly because of erosionduring the first hour of exposure to CFB conditions. However, after the first hour, no apparentfurther change in the roughness of the sand-paper was found. These tests indicate that particles domake contact with the walls to some extent, causing some erosion of the sand-paper. However,the degree of contact with the wall may not be very extensive, so that the findings do notnecessarily contradict the earlier work of Wirth et al. (1991) and Lints and Glicksman (1993)105described above. The circulating fluidized bed system was operated for 1.5 h before anymeasurements were taken to ensure that the roughness of the sand-paper did not changeappreciably during the experiments.In our experiments, net solids circulation rates of 40 and 60 kg/m2s and superficial gasvelocities of 5.5 mIs and 7.0 mIs were chosen. These operating conditions and the sand particleswere identical to those employed in Chapters 3 and 4. Each voidage data point again correspondsto five 60 s intervals of sampling at a frequency of 483 Hz, while each particle velocity datum wasobtained from five separate measurements of 2000 validated samples.Smooth-wall and rough-wall data were compared at the 90% and 95% confidence levelsusing the t-test. Confidence intervals were determined for both the bed voidage and particlevelocity measurements based on repeat measurements. The ranges of time-mean average valuesfor groups of repeated measurements are small as indicated by the bars on the points shownbelow. The confidence interval of each datum in this work is within the range shown by thereproducibility bars. The t-test is used below to examine the significance of differences betweenthe smooth-wall and rough-wall column.Axial profiles of differential pressures measured by U-tube manometers for both roughand smooth-walled riser are shown in Fig. 5.1. Except near the bottom and the top of the column,no distinguishable variation between the two curves is found. As illustrated in Fig. 5.1, thedifferent pressure drop for the column having rough walls seems to be slightly lower near thebottom, while wall roughness appears to increase the pressure drop somewhat near the top.However, the difference is so small that no firm conclusion can be drawn.106I • I5.3.1. Voidage Profiles4 6Height, z (m)Axial profiles of voidage are shown in Figs 5.2 and 5.3. Coordinates are the same as inFig. 2.1. The reproducibility shown by the bars indicates the maximum and minimum of fiverepeat measurements, each lasting 60 s and consisting of 28,980 data points, for each position andcondition. For all our data the 95% confidence interval is always smaller than the interval shownby the bars. Axial profiles of voidage near the walls of the riser with rough walls and with smoothwalls are compared in Fig. 5.2. The differences between the rough and smooth walls are clearlygreater than can be explained on the basis of experimental error. Except at the top of the riser, the1200010000-cu) • Srrooth Wall• Rough Wall600040002000I • I • I • I0 2 8 10Fig. 5.1. Comparison of axial profiles of voidages obtained from differential pressuremeasurements for smooth- and rough-walled risers for Ug=S.5 rn/s and G=40 kg/m2s.1071.00.9ci)c,)Co0.70-J0.60.510Height, z (m)Fig. 5.2. Axial profiles of time-mean local voidage at wall of the riser for xlX=O,y/Y-1, Ug=5.mis, G=’40 kg/m2s.voidage near the rough wall was greater than near the smooth surface. This may be because roughwalls cause more turbulence and more disruption of the particle flow near the wall.Figure 5.3 gives the axial profile of voidage along the axis of the riser. The t-test indicatesthat there is no distinguishable difference between the rough and smooth wall results from thebottom to the top of the riser. Figures 5.2 and 5.3 suggest that wall roughness only influencesvoidage in the wall region significantly.As illustrated in Chapter 3, a bimodal probability distribution of particle concentration cansometimes be obtained, with one peak likely corresponding to particle downflow in swarms and0 2 4 6 81081.000.95(A)a).G)0.850.800Bed Height, z (m)Fig. 5.3. Axial profiles of time-mean voidage along the axis of the riser for x/X=O,y/Y0, Ug5.mis, G4O kg/m2s.the other to bulk downflow of particles. As indicated in Fig. 5.4, particle concentrationmeasurements adjacent to a rough surface show that distributions of particle concentration wereneither bimodal nor trimodal. Instead, a single peak always appeared both at the wall and at thecolumn axis over the entire height. This indicates that wall roughness may change the character ofparticle downflow near the wall. The single peak in the probability distribution indicates a morehomogenous flow near the rough wall.2 4 6 8 10109Fig. 5.4. Lateral profiles of probability distribution of local time-mean particleconcentration, C = (1——e) for Ug5.mis, G54O kg/m2s, x/X=Oand zO.79 m. C*=1 corresponds to the packed bed particle concentration, i.e.6=0.43.1100 C 0 1G)c,)(U(U00-JLateral profiles of voidage appear in Figs 5.5 and 5.6 for z7.06 m and 8.98m,respectively. Wall roughness is seen to increase the voidage near the wall, but it has little influencenear the axis of the riser. More uniform profiles were obtained for the rough wall surface. Forsmooth walls, indicated by the solid curve in Fig. 5.5, the voidage is not always highest on theaxis of the riser. Instead, the voidage sometimes reaches a maximum at a location of about 0.6 to0.8 of the half-width of the column and then decreases slightly towards the center. As described inChapter 3, turbulence generated at the shear boundary is probably at the root of this M-shapeddistribution. The measurements for the rough wall surface indicate a significantly flatter profile in1.000.950..0.850.80-1.0 -0.8 -0.6Dimensionless Distance, yIYFigure 5.5. Lateral profiles of time-mean local voidage for x/X=0, z=7.06 m,Ug5.mis, G40 kg/m2s.Hithe interior of the column. Near the rough walls, because of turbulence generated by wallroughness, compared to the riser of smooth walls, particle downflows are likely to be moredisrupted and to have higher voidage.Because of the exit effect, asymmetric lateral profiles of voidage are found near the exitfor both the rough and smooth wall surfaces, as indicated in Fig. 5.6. The t-test at a 95%confidence level shows that wall roughness appears to cause a slight decrease in voidage at thewall near the top of the column. No significant influence of wall roughness on voidage in the corehas been found at z8.98.ci)0)0ci)Fig. 5.6. Lateral profiles of time-mean .voidage near the exit of the riser forxlX=O, z8.98 m, Ug5.mis, G=4O kglm2s.1.01.000.75-1.0 -0.5 0.0 0.5Dimensionless Distance, y/Y112Figure 5 7 shows the influence of wall roughness on the voidage near the corner of thecolumn. Towards the corner, the voidage for the rough walls was found to be somewhat higherthan for the smooth-walled column, with the changes becoming larger towards the corner. Itseems likely that this is due to augmented turbulence caused by the roughness of both wallsforming the corner.5.3.2 Intermittency Index ProfilesAs described in Chapter 3, the intermittency index introduced by Brereton and Grace(1993) may be used to characterize the homogeneity of the flow in circulating fluidized beds.Axial profiles of intermittency index at the wall and at the axis of the column are presented inFigs. 5.8 and 5.9 respectively. At the wall of the riser, except near the top, the intermittency indexfor the rough wall surface is lower than for the smooth surface. Wall roughness creates turbulencewhich tends to deform descending particle swarms, making the gas-particle flow near the wallmore uniform. This suggests that flow at the wall is more homogeneous with the rough wallsurfaces. As discussed in section 5.3.1, a similar conclusion has been obtained from the probabilitydistribution of voidage.The intermittency index along the axis of the riser, shown in Figure 5.9, does not changemuch except near the bottom and top of the riser. The rough surfaces lead to a small decrease inintermittency index near the bottom and an increase near the top. Therefore, except near thebottom and the top, wall roughness has no significant influence on either the magnitude ofvoidage or the flow structure near the axis of the riser.Near the top of the riser, however, higher values of intermittency index were obtained forthe rough wall surfaces, as indicated in Figs 5.8 and 5.9, suggesting a more heterogeneous flow.113I • I • I I0.96 x/X=O.720.92o Rough WallI I I I I0.96 -‘II0.92x/X=O.37I0.96- -0920.88 I • I I • I-1.0 -0.8 -0.6 -0.4 -0.2 0.0Dimensionless Distance, yIYFig. 5.7. Lateral profiles of time-mean voidage showing corner effect for z=6.2 m,Ug7.O mis, G4O kg/m2s.1140.8 I • • •—•— Smooth Wall06 Rough Wallx040.2 .....0 2 4 6 8 10Height, z (m)Fig. 5.8. Axial profiles of intermittency index at the wall for xlX=0, y/Y=-1,Ug=S.5 mis, G4O kg/m2s.Asymmetric lateral profiles of intermittency index near the exit of the riser are shown in Fig. 5.10.The intermittency index profile is flatter for the rough wall surfaces. The significant increase in ynear the exit shows a change in flow structure. For both rough and smooth walls, theintermittency index on the exit side is higher than on the opposite side. As noted in Chapter 3,this may be because of particles piling up in the horizontal duct connecting the riser exit to theprimary cyclone and intermittently slipping backwards to descend downwards along the wall onthe exit side (Glicksman et al., 1993).115I I I1.0 -0.8D 0.6-90.41)4-’ 0.2-90.0 -0 10Height, z (m)Fig. 5.9. Axial profiles of intermittency index along the axis for x/X=0, y/Y=0,Ug=5.mis, G40 kg/m2s.1.00.8 - • Smooth Wall0 Rough Wallxa)D 0.6 --9C0.4.1.a)4-’-90.0-1.0 -0.5 0.0 0.5 1.0Dimensionless Distance, y/YFig. 5.10. Lateral profiles of intermittency index near the riser exit for xIX=0,z8.98 m, Ug5.mis, G=40 kglm2s.• Smooth Wall0 Rough Wall2 4 6 8116As in Chapter 3, the intermittency index in the corner of the riser is higher than at otherlocations at the same level. Figure 5.11 shows that lower intermittency indices are obtained in thecorner of the riser with rough walls. This again suggests that flow is somewhat morehomogeneous with rough walls.5.3.3 Particle Velocity ProfilesLateral profiles of particle velocity 6.2 m above the distributor are illustrated in Fig. 5.12.At this level, the ascending particle velocity was higher for the rough wall. Similar results wereobtained for other levels. In the core, this is probably because of a higher gas velocity caused by athicker downflow wall layer for the rough wall surface at the same superficial gas velocity. Themagnitude of the velocities of particles descending close to the wall (yIY’-0.60 to -0.95) weresomewhat lower for the rough wall, probably due to the increase of the voidage of the particledownflow, while wall roughness had no significant influence on the velocity of descendingparticles at the wall. Near the axis, with a confidence level of 90%, descending particle velocitiesappear to be higher for the rough wall surface than for the smooth surface. However, theinfluence can be neglected at a confidence level of 95%, although it must be remembered thatthere are few descending particles there.Lateral profiles of the fraction of particles which are ascending appear in Fig. 5.13.Greater influence of wall roughness was found in the outer part of the riser. Little change wasfound right at the wall and near the axis of the column. If we ignore a possible bias in the resultsfrom the velocity probe (Chapter 4), the rough wall appears to produce a thicker wall layer wheremost particles move downwards.1170.3 • Smooth Wall (c)c Rough Wallc.2 I I I I I I I>I (b)(a)Dimensionless Distance, y/YFig. 5.11. Lateral profiles of intermittency index for z6.20 m, Ug7.O mIS,G=4O kglm2s.118Cl)>c00ci)C)Cu0.1)c,)cuci)C)ci)0Dimensionless Distance, yIYFig. 5.13. Lateral profiles of fraction of particle which are ascending for x/XO,z=6.2 m, Ug=5.mis, G=4O kg/m2s.86420-2-4—V-—,- -V--Descending Particles——,----Ascendmg ParticlesOpen Symbols--Rough WallSolid Symbols--Smooth Wall_—i-1.0 -0.8 -0.6 -04 -0.2 0.0Lateral Position, yIYFig. 5.12. Lateral profiles of particle velocity for x/X=0, z6.2 m, Ug=5.mis,G5=40 kglm2s.I I1008060402Q—A—Smooth Wall---A---Rough Wall-1.0 -0.8 -0.6 -0.4 -0.2 0.0119As for the smooth riser (Chapter 4), numerical integrations, involving the particleconcentrations, ascending and descending particle velocities, and fractions of the ascending anddescending particles, were carried out for the rough wall riser at the heights of 5.13 and 6.20 m tosee whether the calculated net particle flux matched the preset value of 40 kglm2s. The latter wasdetermined from the descent of identifiable particles in the standpipe as proposed by Burkell et al.(1987). Integration results of 36.8 and 38.2 kglmswere obtained, respectively, again within 10%of the true” value, helping to validate the experimental results.4 • • •v , v Descending Particles3 - A , Ascending ParticlesOpen Symbols--Rough Wall2 Solid Symbols--Smooth Wall>._-t.- f--!---. 10Height, z (m)Fig. 5.14. Axial profiles of particle velocity at the wall for x/X=0, yIY=-1,Ug5.mis, G8=40 kg/m2s.120Axial profiles of particle velocity at the wall and at the axis are compared for rough andsmooth walls in Figs 5.14 and 5.15, respectively. The reproducibility of the data was againdetermined by making five separate measurements for each position and condition, each of 2000data points. It is seen from Fig. 5.14 that the velocity of particle downflow at the wall, usually inthe range of 0.8 to 1.5 mIs, did not change appreciably with wall roughness. This is alsoconfirmed by t-tests at significance levels of both 10% and 5% illustrated by a sample calculationin Appendix I for the heights interval of 3.45 to 8.28 m, where particle velocity was found to bevirtually independent of height. It is not known whether roughness elements of size larger than0.45 mm would have a significant influence on the velocities of particle descending near the wall.As demonstrated in Appendix I, the t-test has been used to examine the influence of wall108 . -------..- -o , Ascending Particles2 Open Symbol--Rough WallSolid Symbol--Smooth WallHeight. z (m)Fig. 5.15. Axial profiles of particle velocity along the axis for x/X=0, y/Y=O,Ug5.m/s, G=40 kg/m2s.121roughness on the velocities of particles descending along the axis of the riser at z=3.5 to 8.3 m.The result of the test indicates that wall roughness influences the velocities of descending particlesat a confidence level of 90%; but the influence can be neglected at a confidence level of 95%.Figures 5.14 and 5.15 suggest that there are significant differences between the smoothand rough wall cases. Ascending particle velocities, both along the axis and at the wall, weresomewhat higher for the rough walls than for smooth walls. There was a deceleration zone nearthe top of the riser for both rough and smooth walls. The general trend for the riser with roughwalls is similar to that for the smooth-walled riser.2.0 • • 100::>-°• 600.0C.) . .v , V Descending Particles> . . A , z Ascending Particles- 1 0Open Symbols--Rough Wall-Solid Symbols--Smooth Wall 20O’5Dimensionless Distance, x/XFig. 5.16. Lateral profiles of velocities and fractions of rising particles showingcorner effect for y/Y=-1, z=5. 13 m, UgS.5 mis, G=40 kg/m2s.122Cl)>cC-)0ci)C)-tCu0The influence of wall roughness on particle velocity in the corner is shown in Fig. 5.16.The magnitudes of velocity of particles descending in the corner are slightly lower in the rough-walled riser. This is probably associated with the fact that the concentration of descendingparticles in the corner is lower for the rough wall. The influence of wall roughness on ascendingparticle velocities has not been found to be significant. Figure 5.16 also indicates the fractions ofascending particles near the corner of the rough-walled column. Wall roughness is seen to causelittle change.As shown in Fig. 5.17 asymmetric lateral profiles of particle velocity were obtained nearthe exit of the riser, similar to the riser with smooth walls. At a confidence level of 95%, the14 • •12 - v- - Descending Particles—*—,- ---Ascending Particles10 Open Symbols--Rough WailSolid Symbols--smooth Wall8-6 - - - - -4-..--2-‘ -0____--0.50O.5 1.0Dimensionless Distance, y/YFig. 5.17. Lateral profiles of particle velocity at the top for xIX=O, zzr8.98 m,Ug5.mis, G4O kg/m2s.123ascending particle velocities were higher for the riser with rough walls, while descending particlevelocities did not change significantly. A somewhat more symmetric profile was obtained for therough wall.5.4 SummaryRoughness elements were found to have some influence on the voidage and velocity ofparticles in a riser of square cross-section, especially near the wall:1. For rough walls, the voidage was higher near the wall, except near the top of the riser whererough surfaces led to a small decrease in voidage. Wall roughness had little influence on thevoidage near the axis of the column. More uniform lateral profiles of voidage were obtained forthe rough-walled riser. Wall roughness increased the voidage in the corner. No bimodalprobability distributions of particle concentration were found for the rough wall.2. Near the wall, the intermittency index for the rough wall was found to be lower than for thesmooth wall. Except near the bottom and top of the riser, wall roughness did not appreciably alterthe intermittency index on the axis of the column. However, the intermittency index was lowernear the bottom and higher near the top of the column for rough walls and was somewhat lowerin the corner of a column with rough walls.3. The t-test shows that ascending particle velocities both at the wall and the axis of the riser werehigher for the rough-walled riser. The magnitude of the particle velocity of descent near the walldid not change appreciably at confidence levels of 90% and 95%. On the axis, at a confidencelevel of 90%, the wall roughness was found to increase the velocities of descending particles atz=3.5 to 8.3 m, while the influence can be neglected at a confidence level of 95%. The velocity of124particle downflow in the corner of the riser was slightly lower with rough walls. Near the top ofthe riser, at a confidence level of 95%, the ascending particle velocities were higher, whiledescending particle velocities did not change significantly for the rough wall.125Chapter 66.1 IntroductionInfluence of Membrane WallMembrane walls are commonly employed as heat transfer surfaces to remove heat fromCFB combustors. Membrane walls are composed of parallel tubes connected with fins to form awall as indicated in Fig. 6.1. This geometry may well influence the dynamics of gas and particleflows in CFB risers. Wu et a!. (1990) reported that particles were stripped off a flat smooth wallby upflowing gas more readily than off a membrane surface where downflowing particles appearto be protected in the fin region. Local heat transfer coefficients near a membrane wall have beeninvestigated by Andersson and Leckner (1992) and Lockhart et al. (1995). Distributions of heatFinFig. 6.1. Configuration of membrane wall. Tubes are normally vertical.Tube126transfer coefficient near the membrane wall were found to be non-uniform. Andersson andLeckner (1994) observed highly concentrated particle downflow with a long residence time in thefin area.Despite the importance of the membrane wall geometry to CFB combustors, little researchhas been conducted to determine the influence of the membrane wall geometry on thehydrodynamics in CFB risers. No detailed local hydrodynamics near the membrane wall have beenreported. To better understand the heat transfer mechanism and erosion near the membrane wall,a better picture of local flow structure is needed. In this study, simulated membrane walls havebeen installed in the experimental cold model CFB riser described in detail in Chapter 3 toinvestigate the influence of this geometry on local hydrodynamics. Both voidage and particlevelocity near the membrane walls were measured using the fibre optic probes described in detail inChapters 3 and 4. Both probes could be inserted and moved horizontally using three ports aroundthe membrane tubes. Experimental results from the membrane-walled riser are compared with thesmooth flat-wall results obtained in the same riser.6.2 Experimental Set-upHalf-round plexiglass rods of diameter 1” (25.4 mm) were affixed by double-sided tape toall four inner walls of the CFB riser, extending from bottom to top, to simulate a membrane wallsurface. The ratio of rod diameter, d, to the pitch, p was set at 0.75, i.e. p’=339 mm, where p isthe distance between the centrelines of two adjacent fins. Measurements around the membranetube were conducted at points A, B and C in Fig. 6.2, where a.”48. 1 for point B. Lateral profilesof voidage and particle velocity were also measured along the three lines shown in Fig. 6.2 fromy=O to the wall through a specially designed window. The window can be moved to cover framesat four different heights, allowing measurements of voidage and particle velocities along theheight of the riser. The coordinates in Fig. 6.2 are the same as those defined in Fig. 2.1.1276.3 Experimental Results and DiscussionFor convenience of comparison, operating conditions identical to those in the same riser ofsmooth wall surface were adopted, i.e. superficial gas velocities of 5.5 and 7.0 mIs, and solidscirculation rates of 40 and 60 kglm2s. The solid particles were also the same as those in the earlierexperiments with the smooth flat-walled riser. Particle properties are described in Chapter 3.A’ B’. C’ x8.5 8.5leasuringLocationMembraneWall AFig. 6.2. Schematic of the window allowing measurements near membrane wall.128As in to Chapter 5, each data point for both voidage and particle velocity has beenobtained from the average of five separate measurements, each involving many individualdeterminations. The reproducibility of the data obtained for the membrane wall riser is indicatedby bars showing the range between the maximum and minimum of the sample. The 95%confidence interval of each datum in this chapter is within the range shown by the reproducibilitybars. The t-test has been employed to compare the membrane wall and flat wall results at aconfidence level of 95%.6.3.1 Voidage ProfilesAxial profiles of time-mean voidage near the membrane wall on the fin (a=O), on the side(cL=48.1) and on the crest (a=90.O) are shown in Fig. 6.3. There is a non-uniform voidagedistribution around the crest of the membrane wall. The voidage in the fin area is lower than nearthe crest and also lower than near the wall of the riser with a flat smooth wall surface (Chapter 3).The voidage near the crest is higher than for the flat wall (see Chapter 3) for the same operatingconditions, Ug’7.O rn/s and G=4O kg/m2s. Similar to the corners of the riser in Chapter 3,particles in the fin region are well protected in the valleys formed by the fin and two adjacent tubesurfaces and are therefore relatively difficult to strip off the wall. This protection is not relevantfor particles near the crest of the membrane tube. As a consequence, streamers of high particleconcentration are more easily formed in the fin region of the membrane wall, while on the crestsof the membrane wall, streamers are easily disturbed by upflowing gas, leading to higher voidagenear the crest. For the same reason, the particle renewal rate is higher near the crest region andlower along the fin of the membrane wall compared to the flat wall (Chapter 3) in the same riser.129Fig. 6.3. Axial profiles of time-mean voidage near membrane wall showinginfluence of operating conditions.-S1.00.81.0U)c,)Co0>0.8ci)1.00.80.60ctO(Point C)(fin)x=48.10(Point B)(side)c9O.O0(Point Ain Fig. 6.2)(crest).AUg5.mis, G=40 kg/m2sUg=7.O m/s, G=4O kg/m2sUg=7.O m/s, G=60 kg/m2s0 2 4 6 8Height, z (m)10130The influence of superficial gas velocity and solids circulation rate is also indicated in Fig.6.3. As for in the riser with flat walls investigated in Chapter 3, the voidage in both the fin and thecrest regions increased with increasing superficial gas velocity and decreasing solids circulationrate, showing that particle streamers are influenced by the operating conditions as expected.A polar plot of voidage profile around the membrane tube is given in Fig. 6.4. It is seenthat voidage increases with c from C in the fin region through B to A at the crest because, asdiscussed above, streamers in the fin region are well protected, while the crest region is totallyexposed. The disturbance of the upflowing gas on particle streamers near the membrane wallincreased with a..90018001.0 0.5 CVoidage00Fig. 6.4. Voidage profiles near membrane wall for z6.7 m, Ug5. rn/s andG3=40 kg/m2s.131Lateral profiles of voidage for z= 6.7 m, Ug=5. rn/s and G=40 kg/m2sare plotted in Fig.6.5. The voidage is always lower near the wall and increases towards the axis. The membranesurface only influences voidage significantly in the vicinity of the wall. From y=45 to 60 mm, forthe same lateral position y, near the wall, voidage increases from AA’ through BB’ to CC’. This isprobably because of the greater distance from CC’ to the wall. However, for the same distancefrom the wall, voidage decreases from AA’ to CC’. As indicated in Fig. 6.5, the voidage at y=0with membrane walls is around 0.96, very close to the value (0.96) obtained for the sameoperating conditions with flat walls in Chapter 3. Thus, the membrane wall has very little influenceon the voidage at the axis. No M-shaped lateral voidage profiles similar to those described inChapter 3 were found when the membrane wall riser was used.A’, B’, C’ci)c,)AB0.900.850.80• x=17.Omm(CC’)A x=8.5mm(BB’)• x=Omm(M’)0 6020 40 80Lateral Position y, (rrrn)Fig. 6.5. Lateral voidage profiles for z=6.7 m, Ug5.rn/s and G=40 kg/m2salong the three parallel dashed lines shown in Fig. 6.2.1326.3.2 Intermittency Index ProfilesIn Chapter 3, the intermittency index, ‘y, introduced by Brereton and Grace (1993), is usedto describe the flow heterogeneity in CFB risers. Lateral profiles of the intermittency index in themembrane-walled riser given in Fig. 6.6 indicate that y is generally higher near the outer wall andgradually decreases towards the interior. The lower y in the core indicates a more homogeneousflow. Near the membrane wall, ‘y is highest in the fin region suggesting that the largest relativefluctuations of voidage occur there.0.6Z0.50.40.30.2Lateral PosWon, y (mm)Fig. 6.6. Lateral profiles of intermittency Index for z=6 .7 m, Ug5.5 mIs andG=4O kglm2s. For positions of profiles see Fig. 6.2.80• x=17.Omm(CC’)• x=8.5mm(BB’)A xOmm(M’)A0 20 40 60133As indicated in Fig. 6.6, for the lateral intermittency index profile along CC’, i.e. forx=17.0 mm, ‘y increases from the fin of membrane wall until it reaches a maximum and thendecreases towards the center. The maximum y, indicating highest flow heterogeneity, occurs closeto the core-annulus boundary where there are large voidage fluctuations. As described in Chapter3, similar lateral y profiles were obtained near the bottom of a riser with flat walls.6.3.3 Particle Velocity ProfilesProfiles of axial ascending and descending mean particle velocities and the fraction ofparticles which were descending near the membrane wall are plotted in Fig. 6.7. Similar to particlevelocities for the riser with flat walls, the magnitudes of both ascending and descending particlevelocities increase first from the bottom with height and then decrease near the top because of theend effect discussed in Chapter 4. The fraction of descending particles decreases somewhat with znear the bottom and then increases near the top. Near the membrane wall, the ascending particlevelocity is highest in the crest region and lowest in the fin region. The fraction of descendingparticles is considerably higher in the valley of the membrane wall than at the crest.As discussed in Chapter 3, the descending particle velocity near the flat wall in the CFBriser is in the range of 0.8 to 1.5 mIs. This range is also valid for particles near the crest of themembrane tube; however the magnitude of the descending particle velocity in the fin area, asindicated in Fig. 6.7, was greater than 2 mIs. This is consistent with the elevated particleconcentration in the fin region. The measurements of both particle velocity and voidage indicatethat particle behaviour in the fin region is similar to that near the corners of the flat-walled riserwhere voidage and ascending particle velocity are low, while velocity and fraction of descendingparticles are relatively high.134(Point C) 80260I0 v v Descending ParticlesA Ascending Particles-I-220I I 0 :— 2• 801 (Point B) U)U.500f-1.o.-i= _n 1-20C—3 I I I I 0ct90.0°(Point A) 80 E!600:100Height, z (m)Fig. 6.7. Axial profiles of vertical particle velocity and fraction of particleswhich are descending for Ug5.rn/s and G=4O kg/m2s.135The voidage and particle velocity results confirm the explanation of heat transferphenomena near membrane walls. In particular for short surfaces (Lockhart et al., 1995) localheat transfer coefficients are higher on the fin while for long heat transfer surface (Andersson andLeckner, 1992) the heat transfer coefficient in the fin region is lower than on the crest because theparticle renewal rate along the fin is low, causing the particles there to approach the sametemperatures as the heat transfer surface.Lateral profiles of time-mean particle velocities and fraction of descending particles nearthe membrane tube for Ug=S.S rn/s and G4O kg/m2sare shown in Fig. 6.8. The magnitude of theparticle descending velocity and the fraction of descending particles decrease as one moves fromthe fin to the crest near the membrane tube. The ascending particle velocity, however, increaseswith cx.Fig. 6.9 illustrates the lateral profiles of time-mean particle velocities along AN, BB’ andCC’ at z=6.7 m, for Ug=5. mJs and G8=40 kglm2s. As for the lateral voidage profiles, theinfluence of the membrane wall on the particle velocity and on the fraction of descendingparticles, is only significant near the wall. The fraction of descending particles increasesmonotonically towards the wall. Near the wall, the magnitude of the descending particle velocityand the fraction of descending particles increased with x (see Fig. 6.2). Similar to the riser withflat walls (Chapter 4), the magnitude of the descending particle velocity first increases from theaxis with increasing y because the gas velocity decreases laterally (Yang et al., 1993); after amaximum is reached the magnitude of the descending particle velocity decreases towards the wall,probably because of wall friction, while the particle ascending velocity decreased monotonicallytowards the wall.1364 • I • • 100V v Descending ParticlesA Ascen60 C-)>- 08OU00 40 C.):x=48.1:PointB f0c0C-)I I • 0 c0 20 40 60 80 LLAngular Position, L (degrees)Fig. 6.8. Lateral profiles of particle velocities and fraction of descending particlesnear the membrane tube for z=6.7 m, Ug=5.mis, G=4O kg/m2sand z6.7 m.6.4. SummaryMembrane walls influence voidage and particle velocity in a CFB riser of square crosssection. Similar to corners, the valley or trough formed by the fin and two adjacent membranetubes protects particles from upflowing gas. There is very little influence of the membrane wallson the voidage and particle velocity in the core of the riser. Particle streamers tend to movedownwards along the fin in the protected valleys formed by the riser wall and two adjacent137ci)-; Cl) -G)1)>IDU)C-)•t:(‘30c3)CU)C-)Cl)c,)CU)C-)Cl)U)ID4-0C04-’C)U-Fig. 6.9. Lateral profiles of particle velocities and fraction of descending particles forz=6.7 m, Ug5.rn/s and G=4O kg/rn2s.U)>->,4-,0U)>IU)ci)C)(‘300 20 40 60Lateral Position, y (mm)80138membrane tube surfaces. Along the surface, the particle concentration is highest in the fin areaand lowest on the crest of the tube. The intermittency index is generally high near the membranewall and it decreases towards the axis. The magnitude of y in the fin region is higher than near thecrest, indicating more heterogeneous flow in the fin region. The magnitude of the downflowingparticle velocity is highest in the fin area and lowest in the crest area along the membrane tube.139Chapter 7Particle Cross-Flow, Lateral Momentum Flux and Lateral Velocity7.1 IntroductionA core-annulus distribution of particle concentration has been detected in CFB risers ofboth circular and rectangular cross-sections (Gajdos and Bier!, 1978; Weinstein et al., 1986; Dry,1986; Brereton, 1987; Horio et a!., 1988; Hartge et al., 1988; Chapters 3 and 4). Time-meanvertical solids fluxes in circulating fluidized bed risers are upwards in the core and usuallydownwards near the wall (Monceaux et al., 1986; Rhodes et al., 1988; Chapter 4).Solids are exchanged between the core and the annulus in CFB risers (Brereton et a!.,1987; Berruti and Kalogerakis, 1989; Leckner and Andersson, 1992; Herb et al., 1992). Solidscross-flow affects suspension-to-surface heat transfer by determining particle renewal rates.Cross-flow also influences the thickness of the annulus zone, wall coverage and the voidage ofstreamers near the wall. It may also affect erosion of the wall.Despite the importance of lateral flow, very little systematic research has been carried outon solids cross-flow. The only reported experimental data were obtained by Qi and Farag (1993)who used a sampling probe and reported that the radial particle flux near the wall was surprisinglylarge, even higher than the axial solids circulation flux in their riser. There is clearly a need toconfirm the magnitude of the cross-flux. In addition, no data on net lateral solids flux nor onhorizontal momentum have been reported for a riser of square cross-section.This chapter consists of two parts. In the first, a sampling probe was used to measurelateral solids mass fluxes in a circulating fluidized bed riser of square cross-section. In the second,140lateral solids momentum fluxes were determined by a piezoelectric probe. The combined resultsare then used to estimate lateral particle velocities along the riser axis.7.2 Equipment and InstrumentationExperiments were carried out in the same cold model circulating fluidized bed riser as inChapters 3 and 4. See Chapter 2 for details. Ottawa sand particles of mean diameter 213 rim,particle density 2640 kg/rn3 and loosely packed bed voidage 0.43 were again used as the bedmaterial.7.2.1 Sampling ProbeAs illustrated in Fig. 7.1, a sampling probe similar to that used by Qi and Farag (1993)was employed to measure the cross-flow solids mass flux. The sampling system consists of asampling probe, a sample collector and purging air. The inside diameter of the sampling probe is6.3 mm. It is inserted into the riser at an angle of 45 to the axis. Since the angle of repose of thesand particles is less than 35, sampled particles pass through the probe tube freely withoutblockage. The plane of the open area is vertical to ensure that only particles travelling laterally arecaptured. The upper part of the open area is covered as indicated in Fig. 7.1 to prevent particleshitting the upper inside surface of the probe from bouncing out. The open area of the probe is 38mm2. A modified graduated cylinder is employed to collect the sampled particles.Before each measurement, the solids sampling probe is purged by air to ensure that thechannel is free of solids. The system is then checked to make sure that it is well sealed becauseany leakage may lead to great error, as indicated below. When the CFB system is operating141kPressureGaugeFig. 7.1. Schematic of the solids sampling probe./ qPressureRelease ValveN AAPurgingAirSamplingProbeGraduatedCylinderSampled___ParticlesU L142steadily, the purging air is turned off to initiate sampling. After a sampling interval, which isusually 0.5 to 5 minutes, the purging air is re-established to stop the sampling. The cross-flow fluxis then simply(7.1)where V is the volume of particles collected, p, is the particle density, A is the open area of thesampling probe, 60 is the bulk voidage and t is the sampling time.7.2.2 Piezoelectric ProbeA piezoelectric probe was used to determine particle lateral momentum fluxes. When apiezoelectric crystal is deformed by mechanical forces, the crystal develops an electrical potential.When the piezoelectric probe is used to measure solids cross-flow in a circulating fluidized bed,the impact of cross-flow particles causes deformation of the piezoelectric crystal, and an electricalcharge is thus developed. The electrical charge produces an electrical potential from which themomentum flux of the cross-flow particles can be obtained. The electrical signal is logged by acomputer for data processing.The structure of the piezoelectric probe is shown in Fig. 7.2. Clamped by an 0-ring, thepiezoelectric crystal is placed at the mouth of a stainless steel enclosure. Since the crystal is veryfragile, a thin steel disk having the same area as the crystal is glued to its surface to absorb theimpacts. This steel disk was only 0.5 mm thick in order to minimize the reduction in sensitivity ofthe piezoelectric crystal. To prevent unwanted interference by particles hitting other parts of theprobe, the piezoelectric crystal is backed by soft sponge to absorb the energy of unwanted particle143Piezoelectric Steelcrystal enclosureFig. 7.2. Schematic of the piezoelectric solids momentum flux probe.impacts. To reduce the influence of unwanted particle impact further, the outside surface of thehead of the probe, except for the measuring surface, is covered by rubber. Two wires connectedto the piezoelectric crystal are protected by a steel tube. Signals are then transmitted to anamplifying circuit and thence to a computer for logging. The amplifying circuit and a typical tracefor the piezoelectric probe are provided in Appendix III.The cylindrical head of the probe has an outside diameter of 10 mm and a length of 18mm. The diameter of the circular particle impact area of the probe is 7.5 mm. The outsidediameter of the steel tube which supports the head of the probe is 3 mm. The total length of theprobe is 300 mm.A calibration system developed for the piezoelectric probe is illustrated in Fig. 7.3.Particles leaving the side of a small fluidized bed impinged on the piezoelectric probe through aStainless 0-ringsteel disk Rubberlayer/+Sponge/Steeltube144guiding tube. Previous studies (e.g. Martin and Davidson, 1983; Geldart et al., 1984) have shownthat the flow rate of gas-solids suspension leaving a fluidized bed through an orifice due topressure difference isP across the orifice can be estimated as:Q = AOCDcj(2 (7.2)where A0 is the cross-sectional area of the orifice, CDC is the discharge coefficient, and is theaverage suspension density given by[mIIIIuIuIIIKIIlImuIIIIJPiezoelectricprobeFig. 7.3. Calibration system for the piezoelectric solids momentum flux probe.FluidizedbedPipeGuidingtubeAeration 2Aeration IParticleCollector145P=(6)Pp+6Pg (7.3)Here s is the average voidage, Pg 1S the gas density and p is the particle density. The flowrate ofgas-solid flow is thus:m=QP= AOCDC /(2AP) (7.4)AP and/or j5 in Equation (7.4) can be varied by changing the superficial gas velocity, while theparticle velocities at the surface of the piezoelectric probe are controlled by changing the gasvelocity at the Aeration 2 location in Fig. 7.3. The characteristics of the system were determinedbefore it was used to calibrate the piezoelectric probe. The flow rate of particles striking thepiezoelectric probe was measured by weighing the particles delivered by the guiding tube in agiven time interval, while the particle velocity at the measuring location of the piezoelectric probewas measured by a fibre optic particle velocity probe, as described in detail in Chapter 4.A calibration curve for the piezo-electric probe has been obtained as shown in Fig. 7.4.The electrical potential, E, developed by the piezoelectric crystal is seen to be proportional to themomentum flow of solids striking the surface of the piezoelectric probe, i. e.E=Cmv (7.5)where C is a constant and v is the velocity of solids striking the probe normal to the surface. Alinear regression has been performed, yielding a calibration curve with a correlation coefficient of0.9985 and standard deviation of 0.0689. The detailed particle velocities and solids fluxes used inthe calibration of the piezo-electric probe are given in Table 7.1.1464IA B C D E F G HParticle 0.6 0.6 0.6 1.3 2.1 3.1 1.2 3.7 3.7 2.1velocity,misSolids 0.5 1.0 1.1 1.1 1.5 1.2 3.9 1.8 2.1 3.9flux,kg/m2s32I2 4 6 800Solids Momentum FIuç G* (kg/rns2)Fig. 7.4. Calibration cupve of piezo-electric probe.Table 7.1. Particle velocity and solids fluxes used in calibration of piezo-electric probe.1477.3 Experimental Results and Discussion7.3.1 Solids Cross-Flow Mass FluxAxial profiles of solids cross-flow mass fluxes at the axis and the wall of the riser areshown in Figs 7.5 and 7.6, respectively. Similar to axial particle concentration profiles in CFBrisers, the solids cross-flow flux was highest at the bottom of the riser. It decreased with heightbetween the bottom and 1.5 m above the distributor, remained nearly unchanged from z2 m to 6m, and finally increased slightly towards the top. The greatest change in flux was observed in thebottom region of the riser where particle concentration and particle velocity changed greatly. Asshown in Figs 7.5(b) and 7.6(b), the solids cross-flow flux decreased with increasing superficialgas velocity. A greater influence is observed at both the bottom and the top. Figures 7.5(a) and7.6(a) indicate that the solids cross-flow increases with solids circulation rate, with a greaterincrease near the bottom of the riser. The outward solids cross-flow flux is higher at the wall thanat the axis. The lateral solids flux at the axis of the riser is much smaller than the axial solidscirculation flux along the whole riser, while in the dense region at the bottom, the lateral flux atthe wall can be of the same order of magnitude as the solids circulation rate. The finding reportedby Qi and Farag (1993) that the lateral net solids flux near the wall can be higher than the (axial)solids circulation flux could not be verified in this work.Lateral profiles of solids cross-flow mass fluxes are shown in Fig. 7.7. Note that onlyoutward flux could be measured at y/Y=1, but the corresponding inward flux can be obtainedfrom the condition that the net flux at the wall is zero. Similar to lateral profiles of particleconcentration, both the inward and the outward solids fluxes reached a minimum at the axis andincreased laterally towards the wall. Near the wall, where particle concentration is highest,particle-particle collisions are more frequent, leading to higher solids cross-flow. The magnitude148• I • I • —(a) Z.1_____________________112jt__2Cl)c\J IE G 40 kglm2s- Zizz’(9><-039 (b)(1)- 2-’— ‘CtI Ug=55mla)-J•Ug=7:000 2 4 6 8 10Height,z(m)Fig. 7.5. Axial profiles of solids cross-flow flux on the axis (xzy0) showingthe influence of operating conditions: (a) Ug7.O mis; (b) G=4O kglm2s.14915 I I I I(a)12 0 G=40 kg/m2s-• G=60 kg/m2sC\I 9-6__.-c •---. -CDxI Ib= 20 - 0 U=5.5mlsoC/) • U=7.Ornls- 151ga)C 10-”_J5 ‘o o&---0---- --0 I I I I a0 2 4 6 8 10Height, z (m)Fig. 7.6. Axial profiles of outward solids cross-flow flux at wall (x/X0, y/Y1)showing the influence of operating conditions: (a) Ug7.O mis; (b) G8=40 kg/m2s.150Cl) 4• Outwards• irwartis2 NetFiuxLateral Position, y/YFig. 7.7. Lateral profiles of solids cross-flow flux for x/X=O, z=6.2 m, Ug=5.mIS,G=4O kg/rn25.of the net solids cross-flow flux, obtained by subtracting the outward solids flux from the inwardflux, first increases as one moves outwards from the axis, then decreases after a maximum isreached near the core-annulus boundary. Visual observations showed that near the axis of theriser, the sampled particles entered the sampling probe in a relatively regular manner, suggestinghomogenous solids cross-flow near the axis. Near the wall, however, the sampled particlesentered the probe in irregular clumps, indicating a heterogeneous flow structure. Near the bottomof the riser, the solids cross-flow flux was more heterogeneous than at higher locations, consistentwith the lateral intermittency index profiles obtained from particle concentration data by Brereton151and Grace (1993) in a riser of circular cross-section and data reported in Chapter 3 for the samesquare cross-section riser.As indicated in Fig. 7.8(a), the magnitude of both the inward and outward lateral solidsfluxes increased when the solids circulation rate was increased from 40 to 60 kg/m2s. Figure7.8(b) shows that both the outward and the inward solids cross-flow flux decreased withincreasing superficial gas velocity. The difference is greatest near the wall, presumably becauseparticle concentration varies most at the wall as the superficial gas velocity or solids circulationrate is varied.The influences of the superficial gas velocity and solids circulation rate on the lateralprofiles of net solids lateral fluxes 6.2 m above the distributor are shown in Fig. 7.9. At this level,the net solids flux was inwards towards the axis, except at the wall itself and at the axis where itwas 0. The magnitude of the net lateral solids flux increased with decreasing superficial gasvelocity and/or increasing solids circulation rate. However, the location of the maximum net solidsflux near the core-annulus boundary did not change appreciably with solids circulation rate orsuperficial gas velocity.Lateral profiles of net solids cross-flow fluxes at different heights are plotted in Fig. 7.10.It is seen that near the bottom of the riser, the net lateral solids fluxes are outwards from the axistowards the wall, while in the upper portion of the riser, the net lateral solids fluxes are inwardstowards the axis. This is consistent with the lateral profile of the wall layer thickness obtained inthe same riser for the same operating conditions (Chapter 4). In the lower part of the riser, sincethe net lateral solids fluxes are outwards towards the wall and particles in the annular wall layertravel downwards, wall layer thickness becomes thicker towards the bottom. In the upper part ofthe riser, however, wall layer thickness increases with height because the net lateral solids flux is152I I I ‘-A V G4O kg/m2si, V G=6O kg/m2s2-— -ALI)_:A, Outwards.4 - , v, v Inwards— I I I ILL 4 -, V Ug5.rn/s= A’ V Ug=7.Om/s /o 2-C/)L) LJI._J -v_s-2-I • I I • I0.0 0.2 0.4 0.6 0.8 1.0Lateral Position, yIYFig. 7.8. Lateral profiles of solids cross-flow flux for xJX=O, z=6.2 m:(a) Ug7.O mis; (b) G4O kglm2s.530.0S.__-OA rCl)0 G=40 kglrr?s .S.S.-0.8• G=60 kglrr?s.‘(9-1.2(a)00’ LCl)-g-ö-0.4 I—0 Ug=5.mlSa). Ug7.0mIS S. ‘-0.8(1) S.Z ‘-0-1.2(b)I I a I a I I0.0 0.2 0.4 0.6 0.8 1.0Lateral Posftion, yIYFig. 7.9. Lateral profiles of net cross-flow solids flux for x/X=O, z=6.2 m:(a) Ug7.O mis; (b) G=4O kglm2s.154IFig. 7.10. Lateral profiles of net solids cross-flow flux at different heightsfor x/X0, Ug=5.5 mIs, G4O kg/m2s.inwards towards the axis. At some intermediate level, between 3 and 4 m above the distributor forUg5.5 mIs and G=40 kg m2s, the wall layer thickness reaches a minimum and the net lateralsolids flux is expected to be zero, i.e. the lateral inward solids flux equals the lateral outward flux.Note that since limited data were obtained near the wall, the locations where the net lateral solidsmass fluxes reached maxima may not be exactly as shown in Figs 7.7, 7.9 and 7.10.The net lateral solids mass fluxes obtained near the core-annulus boundary at differentheights have been compared with the results of a semi-empirical hydrodynamic model developed0.4 0.6Lateral Position, yIY155by Senior and Brereton (1992). The model successfiully predicts that the direction of the netlateral solids mass flux is outward towards the wall near the bottom, while being inward towardsthe axis near the top of the riser. The detailed comparison is provided in Appendix II. Although anumber of papers (e.g. Harris and Davidson, 1994) have considered particle cross-flow, thereappear to be no other published predictive models with which to compare our experimental data.The local voidage, particle velocity and annulus thickness data presented in Chapters 3 and4 allow time-mean vertical solids flow rates in both the core and annulus to be determined at 5.13and 6.20 m above the distributor for Ug=5. rn/s and G=40 kg/m2s. By subtracting these two,the net solids exchange between the core and annulus between two levels can then be obtained.The estimated net solids cross-flux is 1.3 kglm2s outwards, which agrees very well with anintegrated mean value of about 1 kg/m2soutwards.Figure 7.11 shows that both the inward and outward solids cross-flow fluxes are highernear the corner of the column than midway between opposite walls. These profiles are quitesimilar to particle concentration profiles reported in Chapter 3 for the same height in the sameriser with the same operating conditions. It appears that lateral particle movement reaches amaximum in the corner region.The net lateral solids flux profiles near a corner are shown in Fig. 7.12. It is seen that thereis a greater lateral solids exchange towards the corner than elsewhere at the same cross-section.At this height, z6.2 m, the net lateral solids fluxes were inwards towards the axis. Experimentsconducted at a lower level where the net lateral solids fluxes were outwards towards the wallgave similar results, again showing higher fluxes near the corner.156I:-0.2 081.0Lateral Position, y/YFig. 7.11. Lateral profiles of solids cross-flow flux for z=6.2 m, Ug=5. flh!S,G=40 kg/m2s.In Figs 7.7 to 7.12, the net vertical solids flux was either 40 or 60 kg/m2sfor each of thecurves shown. In each case, the measured solids lateral flux (inwards, outwards or net) is seen tobe at least an order of magnitude smaller than the corresponding vertical net flux. This findingdiffers significantly from that of Qi and Farag (1993) who, as noted above, reported lateral fluxesof similar magnitude as, and sometimes in excess of, their vertical fluxes.In order to explain this discrepancy a test was carried out to check the sensitivity of thesampling probe to the pressure difference between the sample collector and the location inside theriser where samples were obtained. As shown in Fig. 7.13, the magnitudes of solids cross-flow157-200-2.5G):Co_J -..4-’a)z1.0Fig. 7.12. Lateral profiles of net solids cross-flow flux for z=6.2 m, Ug=5.flhIS,G=40 kg!m2s.fluxes obtained both near the wall and at the axis were very sensitive to this pressure difference.The measured value of the lateral solids flux at the axis with the pressure release valve, shown inFig. 7.1, fully open for Ug5. flh!5, G5=40 kg!m2sat 0.77 m above the distributor was as muchas 6.6 times the value with the system fully sealed, while near the wall the measured lateral solidsflux with the pressure release valve fully open was 3.6 times greater than with the system properlysealed. It is possible that even a small amount of air leakage from the receiver or connectingtubing employed by Qi and Farag could have caused much higher efflux of particles into theirsampling system, accounting for the substantial measured fluxes. However, as discussed in-0.5-1.0-1.5 ..AA..x/)(=0.35x/)@0.70A‘ I.A.-3.50.0 0.2 0.4 0.6 0.8Lateral Position, y/Y158301800AP, mm H20Fig. 7.13. Sensitivity of solids cross-flow to pressure difference for Ug=5.mIS,G=4O kglm2s, xIX=O, z=O.77 m.Chapter 4, the net solids flux obtained from the difference between inward and outward fluxes isnot much influenced by the leakage. Further measurements of cross-flow flux by other workers orwith other experimental techniques are needed to confirm the magnitude of the cross-flow flux. Inour view it is unlikely that lateral fluxes could be as high as the corresponding vertical fluxes.7.3.2. Lateral Momentum FluxLateral solids momentum fluxes were measured using the piezoelectric probe describedabove by dividing the momentum flow by the corresponding sensing area. Axial profiles of lateral2520150 200 400 600159momentum flux appear in Fig. 7.14. These indicate that the lateral solids momentum flux increasesfirst from the bottom of the riser and then decreases towards the top. Figure 7.14 also shows thatthe lateral solids momentum flux increased with increasing solids circulation rate and withdecreasing superficial gas velocity. These trends are the same as the influence of these operatingvariables on the lateral solids mass flux shown in Fig. 7.5.Figure 7.15 plots horizontal profiles of lateral solids momentum flux midway betweenopposite walls of the riser at a height of 6.2 m above the gas distributor. It is seen that the lateralsolids momentum flux increases slightly in moving away from the axis until a maximum is reachedat about 30 to 50% of the distance to the wall of the riser. It then decreased sharply towards thewall. Near the axis, there are relatively few particles present (Chapter 3) and this likely explainsthe minimum there. Particles move mostly downward in streamers at the wall, shielding the wallfrom impacts of horizontally moving particles. There are, however, occasional pulses when thepiezoelectric probe is flush with the wall indicating that some individual particles do penetratethrough to collide with the wall from inside the riser. Limited by the sensitivity of the piezoelectricprobe, the minimum solids momentum flux which can be detected is 0.12 kg/ms. The lateralsolids momentum flux is low at the wall, less than the 0.12 kg/rn s2 minimum level ofmeasurement.Horizontal profiles of lateral momentum flux are shown in Fig. 7.16 for three differentdistances from the axis (i.e. x/X0, 0.35 and 0.70), all at z=6.2 m. These measurementsdemonstrate that the lateral solids momentum flux is higher midway between parallel walls thannear one of the walls. The piezo-electric probe at the wall and near the corner showed that someparticles struck the measuring surface of the probe causing spikes in the output signals; however,the average output of the probe was essentially zero except near the very bottom of the riser,indicating lateral solids momentum fluxes less than the lower limit of measurement, 0.12 kg/rn s2.1601.0(‘4Cl) 082C)- 0.6E 0.80Cl) 0.60 0.4(‘5a)(‘5-J 0.010Height, z (m)Fig. 7.14. Axial profiles of lateral solids momentum flux about the axis(xy0): (a) G=4O kg!m2s; (b) Ug7.O flhJS.0 2 4 6 81611 .0(a)E0.6CD 0.4x2 U Ug=5.m/SL1_ 0.2-E Ug=7.Om/S— 0.0 I I I I IG)E (b)0.8Cl)0.60C’)0.41• G=4O kg/m2s0.2-• G=6O kg/m2s0.0 I I • I0.0 0.2 0.4 0.6 0.8Lateral Position, yIYFig. 7.15. Horizontal profiles of lateral solids momentum flux for x/X=O,z=6.2 m: (a) G=4O kglm2s; (b) Ug7.O 111/S.162EDE7.3.3. Lateral Particle VelocityIf one divides the lateral solids momentum flux by the corresponding lateral solids massflux, one obtains a quantity which we may calljjjPv(l — 6)dAdt(6)AiES1pp1_6)dAdtEbUFig. 7.16. Horizontal profiles of lateral solids momentum flux for z=6.2 m,Ug7.O mis, G5=4O kg!m2s.0.4 0.6Lateral Position, xfX163where T is sampling duration, Vh is the horizontal component of particle velocity normal to theprobe surface and A is the sensing area of the piezoelectric probe. If vh and 6 were to beuncorrelated, then for small measurement areas v*=vh. In general, we do not expect vh and 6 to beuncorrelated. However, the correlation should be minimal along the axis of the column, especiallyin the upper part of the riser.Hence, we can estimate the lateral component of particle velocity at xy=0 by dividing thelateral solids momentum flux by the lateral solids mass flux. The results are shown in Fig. 7.17.Similar to the axial velocity profiles of particles travelling vertically (Zhou et al., 1994) thehorizontal component of particle velocity on the axis increased with height near the bottom of thecolumn, then decreased near the top. More significant changes in lateral particle velocities werefound at the bottom of the riser where particles accelerated and at the top where particlesdecelerated. Between 3 and 7 m above the distributor, lateral particle velocities did not changevery much with height and averaged 2 to 3 mIs, somewhat less than half of the correspondingaverage vertical ascending particle velocity (Chapter 4). These lateral particle velocities are higherthan might be expected. Since these are the first experimental data for lateral particle velocities, itis important that other work be done to confirm values of this magnitude. As shown in Fig.7.17(a), the lateral particle velocity decreased when the solids circulation rate increased from 40to 60 kglm2s, presumably because of a stronger damping effect caused by an increase in particleconcentration. Not surprisingly, the lateral particle velocity increased as the superficial gasvelocity increased from 5.5 to 7.0 mIs (Fig. 7.17(b)).7.4 SummaryThe data in this chapter are especially useful in allowing estimates to be made of particleinterchange rates between the dilute core and the more concentrated wall regions and in providingdata for testing of future hydrodynamic models.1644(I)I321032I00Height, z (m)Fig. 7.17. Axial profiles of lateral particle velocity about the axis (xzy=O):(a) Ug7.O mis; (b) G=4O kg/m2s.2 4 6 8 10165Measurements of lateral solids mass fluxes, lateral momentum flux and lateral velocitieswere carried out in a circulating fluidized bed riser of square cross-section. Except at the verybottom of the riser, cross-flow fluxes were always substantially lower than (axial) net circulationfluxes, but high enough to assure considerable interchange between the wall and core regions.Lateral fluxes were highest at the bottom of the riser, relatively constant at intermediate heights,then increased slightly near the top. Higher fluxes were found near the corners than mid-waybetween opposite walls. Lateral fluxes tended to be higher at a higher solids circulation rate and ata lower superficial gas velocity for the range of conditions studied. The net solids mass flux wasoutwards towards the outer wall near the bottom and inwards in the upper part of the riser.The lateral solids momentum flux was found to increase with height at the bottom of theriser and then to decrease towards the top. A maximum lateral solids momentum flux was reachedat about 30 to 50% of the way from the axis to the wall of the riser. The lateral solids momentumflux increased with increasing solids circulation rate and with decreasing superficial gas velocity.The estimated average lateral component of particle velocity along the axis of the riserreached a maximum about half way up the column with a magnitude somewhat less than half thecorresponding velocity of vertically ascending particles measured in Chapter 4 for the samecolumn with the same particles.166Chapter 8Conclusions and Recommendations8.1 General ConclusionsMost previous research on circulating fluidized beds has been carried out in risers ofcircular cross-section, although square columns are widely used commercially in CFBcombustors. Despite the influence of column geometry on hydrodynamics in CFB risers,experimental and modelling results obtained from circular risers are commonly used in the designand operation of CFBs. To improve the understanding of the influence of column geometry onhydrodynamics, a CFB riser of 146 x 146 i2 square cross-section and 9.14 m height wasdesigned and constructed to study the hydrodynamics.The basic voidage and particle velocity profiles in a riser with flat and smooth walls havebeen determined to study the influence of corners on the square column on hydrodynamics. Theexperimental results in this thesis, giving local voidages, vertical and some horizontal particlevelocity data, vertical fluxes and horizontal fluxes, all for identical operating conditions, presentedin Chapters 3, 4 and 7, provide the most comprehensive mapping of CFB hydrodynamics for anygiven column. It is clear that there are complex interrelations between the various quantities andregions. The influences of wall roughness and membrane walls on hydrodynamics were alsoinvestigated by comparing the results with voidage and velocity profiles from the same columnwith flat smooth walls.In Chapters 3 and 4, particle concentration and particle velocity profiles have been studiedextensively. Fibre optic probes have been employed to measure both voidage and particle velocity.As in risers of circular cross-section, the voiclage is low near the bottom of the riser and increaseswith the height. The particle concentration is higher near the wall than towards the axis. End167effects, appear to be similar to those in circular columns. M-shaped lateral voidage profiles, alsoapplicable to in risers of circular cross-section by close scrutiny of data from previous studies,appear to be more prominent in the square riser. The particle concentration in the corners of thecolumn is higher than at other locations on the same cross-section. The probability distribution ofvoidage and intermittency index have been used to characterize the nature of flow in the CFBriser. Bimodal and trimodal probability distributions of voidage have been found. The particleflow has been found to be more heterogeneous with high fluctuation of voidage near the coreannulus boundary. -Ascending and descending particle velocities, together with the proportion of particlestravelling upwards and downwards were measured using a newly developed five-fibre opticalparticle velocity probe. A core-annulus structure was found to exist in the riser of square cross-section, but the shape of the boundary was no longer circular. The thickness of the wall layer, inwhich most particles were descending, decreased with height from the bottom of the riser and,after a minimum was reached, increased towards the top. There is a substantial difference betweencore-annulus boundaries defined in terms of the location where the time-mean vertical particlevelocity is zero and the position where the net vertical solids mass flux is zero. A model to predictthe descending velocity of wall clusters gives good results. Simulations for both Ottawa sand andFCC particles show good agreement with the experimental data. Particle density and theconfiguration of the cluster are both predicted to influence the velocity of clusters.The influence of wall roughness on the hydrodynamics in the CFB riser is elucidated inChapter 5. Statistical methods are used to analyze the data. The voidage near the rough wall hasbeen found to increase, while there was negligible change in voidage near the axis of the riser.Lateral voidage profiles for the rough walls were more uniform than for smooth walls. Wallroughness led to higher particle ascending velocities, while the velocity of particle downflow nearthe wall did not change appreciably with wall roughness.168Chapter 6 demonstrates that membrane walls influence voidage and particle velocities inthe CFB riser. As for the corners of the square riser where voidage is low and downflowingparticle velocity is high, the valleys or troughs formed by the riser wall and two adjacentmembrane tubes appear to protect wall-layer downflowing particles from the gas. Particles in thevalleys then descend further on average before being stripped off into the core upflow. This partof the work helps explain local heat transfer measurements for membrane surfaces in CFB risers.Solids lateral fluxes were measured using a sampling probe, and the results appear inChapter 7. Except at the very bottom of the riser, cross-flow mass fluxes were alwayssubstantially lower than (axial) net solids circulation fluxes, but high enough to assureconsiderable interchange between the wall and core regions. Lateral fluxes were highest at thebottom of the riser, relatively constant at intermediate heights, then increased slightly near the top.The solids mass flux is high near the wall, while even higher fluxes were found near the cornersthan mid-way between opposite walls. The net solids mass flux was outwards towards the outerwall near the bottom of the riser and inwards in the upper part of the riser. The direction changeof the net lateral solids flux is consistent with the measured axial profile of wall layer thicknesspresented in Chapter 4.A piezo-electric probe was developed to measure lateral solids momentum flux. It hasbeen found that the lateral solids momentum flux increased with height at the bottom of the riserand then decreased towards the top. There was a maximum in the lateral profile of lateral solidsmomentum flux at about 30 to 50% of the way from the axis to the wall of the riser. The lateralsolids momentum flux increased with increasing solids circulation rate and with decreasingsuperficial gas velocity for the range of conditions investigated.169Taken together, the various chapters provide a comprehensive picture of the flowstructure of the CFB riser of square cross-section. The various aspects are shown together inschematic form in Fig. 8.1.\SmoothWall))MembraneWallFig. 8.1. Schematic of flow structure of a CFB riser of square cross-section.170rCore/0•Core0RoughWall\\I :NJ8.2 RecommendationsProper terminology and clear definitions for many common concepts such as wall layerand particle aggregates are needed and should be unified to reduce misunderstandings betweenresearchers. To achieve this, a better understanding of CFB phenomena is needed.Until now, most hydrodynamic studies in CFBs have been concentrated on the solidsphase because of the development of electrical and optical techniques for solid phasemeasurements and the difficulty in determining gas phase behaviour. More work is needed tounderstand the gas-phase hydrodynamics.For solids-phase hydrodynamic studies, more work is recommended on newconfigurations and baffles to improve the performance of CFBs. The inf1uence of particle sizedistribution on the hydrodynamics in CFBs should also be investigated, although some work hasalready been done in conventional fluidized beds. More accurate, reliable and non-interferingmeasurement techniques are needed to further investigate particle aggregates and wall layerparticle downflows.With more experimental data available and a better understanding of CFB phenomena,theoretical analysis can also be improved. A better and comprehensive hydrodynamic model forCFBs is needed for the sake of both design and operation of CFB reactors. The results given inthis thesis may be useful to validating such a model.171Nomenclaturea, b Semiaxes of the spheroidal cluster, mA Open area of the sampling probe, m2A1 Cross-sectional area of the standpipe in which the identified particles are descending,m2A Cross-sectional area, m2A cross-sectional area of the riser, m3A,3 Cross-sectional area of the orifice, m2A.1, Sensing area of the piezoelectric probe, m2C, C0, C1, C2ConstantsCD Drag coefficientCDC Discharge coefficientCDS Single particle drag coefficientParticle concentration, kg/rn3C,,, Particle volumetic concentrationC* Local relative particle concentration= (p8-p9)‘(PmfPg)df Diameter of particle concentration probe,mmd Mean particle diameter, md Dimensionless particle diameter=A’3= d(p5Apg / p2)1/3Er Ratio of b to aE Electric potential developed by the piezoelectric crystal, mVFc Force on cluster because of momentum transfer, NFD Drag force, NFG Gravitational force, N172FB Buoyancy, NFw Wall friction force, Ng Acceleration due to gravity, rn/s2G* Lateral solids momentum flux, kg/m2.sGh Solids cross-flow mass flux, kg/m2.sGhO Lateral solids flux from the adjacent dilute phase to cluster flow, kg/m2sSolids circulation rate, kg/m2sH Bed height, mH0 Height of packed bed, mk Constant‘Ac Effective separation length, mL Height of expanded bed, mL Known distance, mL0 Height of packed bed, mth1, Solids mass flowrate, kg/sN Number of sampled particlesNu Number of sampled ascending particlesND0 Number of sampled descending particlesP(C*) Relative probability of C*Q Solids volumetric flow rate, m3/sRe Reynolds numbert Sampling time, st1 Time for the identified particles to traverse the known distance, stAC, tcE Transit time for particle to move from optical fibres A to C, C to E as shown inFig. 4.2, sT Sampling duration, su Velocity of cluster, rn/s173Urel Relative velocity between the gas and particles, rn/sU Output of the fibre optic voidage probe, voltUg Superficial gas velocity, rn/sU Minimum fluidization velocity, rn/sU- Transport velocity, rn/sDimensionless gas velocity’Ug(p /V Volume of sampled particles, m3v* Ratio of lateral solids momentum flux/lateral solids mass flux, misVh Horizontal component of particle velocity normal to probe surface, rn/svp Particle velocity, rn/sVertical velocity of solids entering cluster, misVT Particle terminal velocity, rn/sVAC, vcE Particle velocity obtained from fibres A and C, C and E, rn/sTime-mean of vertical component of particle velocity, rn/sV Threshhold, voltx, y Coordinates as shown in Fig. 2.1z Vertical coordinate measured from the primary air distributorX, Y Half-width of column cross-section, mDifference in height, rnPressure difference, N/rn2At Time interval, sAW Cumulative weight of particles passing through the cross-sectional area A, kgAp Difference in density=pp-pg, kg/rn3Angle, degreey Intermittency index defined by Equation (3.11)Dimensionless radius=r/R174Local voidageInternal voidage of the clusterPacked bed voidage6out Outer voidage of clustersTime-mean voidageav Cross-sectional average voidageCross-sectional average voidage at the solids re-entering locationVoidage at minimum fluidizationDecay constant, mAC’ CE Effective optical separation distance between fibres A and C, C and E, rnPbUUC loosely packed particle density, kg/rn3Average suspension density, kg/rn3Bed density at minimum fluidization, kg/rn3Pg Gas density, kg/rn3Particle density, kg/rn3Time-average point suspension density, kg/rn3a Standard deviation of density fluctuations for fully segregated two-phase flow withidentical time-mean density at the same point, kg/rn3Normalizing factor= PmfJ(Psusp “Pmf)(’ — Psusp‘1Pmf), kg/rn3Shear stress, N/rn2175ReferencesB. 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Nicklin), Engineering Foundation, New York, pp.781-789.185Appendix IStatistical Methods used to AnalyseVoidage and Particle Velocity Data1.1 x2 Test1.1.1 TheoryThe x2 test can be used to investigate the variance of the population. To perform the x2test, we compare values E and 0 drawn from two populations. The value of2 is obtained fromx2 (1.1)where 0 is an observed value and E is a hypothetical (expected) value.The calculated value is then compared with tabulated values of x2 given in standardstatistics texts. At a specified level of significance, if the latter values are not exceeded by thecalculated value, it is concluded that it is not possible to reject the hypothesis that the twopopulations are drawn from the same overall population.1.1.2. Sample CalculationResults of voidage measurements for Ug4.5 mis and G=2O kg/m2s obtained in twodifferent days at both wall and axis of the riser with smooth inner walls at height z0. 79 m abovethe distributor are shown in Table 1.1. Ten samples at duration 60 s for each day were recorded.186Table 1.1 x2 test for voidage data from two different days at both wall andaxis of riser for Ug=4.5mis, G=20 kg/m2s, z=0.79 m.At Wall On AxisDay 1 (O) Day 2 (012) Day 1 (021) Day 2 (E22)0.774 0.785 0.955 0.9510.793 0.757 0.96 0.9450.789 0.762 0.947 0.9490.761 0.785 0.957 0.9550.763 0.757 0.950 0.9530.780 0.782 0.952 0.9480.782 0.759 0.954 0.9520.764 0.769 0.958 0.9500.778 0.760 0.949 0.9550.767 0.779 0.959 0.947Mean(E) 0.775 0.770 0.954 0.950StandardDeviation 0.0112 0.0120 0.00448 0.00334Ca1cu1ated2 0.00146 0.00168 0.00019 0.00011Value of2 at a significance 2.088 2.088 2.088 2.088level of 99%Mean (E) of Days 1 and 2 0.773 0.952Calculated x2 1.62 x 1 o- 8.40 x 1 0-6Value of2 at a significance 1.57 io- 1.57 >< io-level of 99%Conclusion Over 99% sure that there is no significant differencebetween ten samples in each day, and data from Days 1and Day 2,187The x2 test is performed to examine whether there is a significant difference between the twopopulations.1.2 t Test1.2.1 TheoryThe t test is generally used to compare the mean values of two small populations (<30)whose variance a2 is unknown. Define the random variable t as(1.2)with z= (1.3)ai”Iwhere i is the population mean; 5 is the sample mean; n is the sample size, a is the standarddeviation, and v is the number of degrees of freedom = n-i.It can be shown (Kennedy and Neville, 1986) that from equation (1.2), the followingequation can be drawn:t=Ix12 (1.4)where Sd =+n2 (1.5)fl2188and = (n — 1)s? + (n2 — 1)s (1.6)n1 +n2 —2At a certain level of significance, usually 5% (confidence level 95%), if the calculatedvalue of t is greater than the corresponding tabulated value (available in standard statistics texts),it can be concluded that there is a significant difference between the two means, 5i and 2•1.2.2 Sample CalculationThe t test is conducted to examine the influence of wall roughness on the particledescending velocities at the wall and on the axis of the riser for UgS.S rn/s and G=4O kg/m2satheights of 3.45, 5.13, 6.20, 7.06 and 8.28 m above the distributor. It is shown in Table 1.2 that ata confidence level of 95%, wall roughness has no significant influence on particle descendingvelocity both at the wall and on the axis. However, at a confidence level of 90%, wall roughnessis found to influence descending particle velocities on the axis, while having no influence ondescending particle velocities at the wall.189Table 1.2. t test for particle descending velocities for Ug=5.S rn/s and G=40kg/m2sat wall and on axis of riser.Particle Descending Particle Descending Velocity,Velocity, vp, at Wall (mis) Vp, on Axis (mis)Height (m) Smooth Wall Rough Wall Smooth Wall Rough Wall3.45 -1.06 -0.92 -1.07 -1.615.13 -1.30 -1.34 -1.38 -2.166.20 -1.13 -0.99 -1.61 -2.207.06 -1.39 -1.08 -1.41 -1.798.28 -1.23 -0.83 -1.35 -1.54v 4 4Sc 0.166 0.256Sd 0.129 0.198‘p1’p2 0.190 . 0.496Calculated t Value 1.58 2.51TabulatedtValuefor 2.132 2.132significance level of10%Tabulated Value for 2.776 2.776significance level of 5%Conclusion Not significantly different at Different at confidence level ofa confidence level of 90% 90%, while not different at a95% confidence level190Appendix IIComparison Between Measured Lateral Solids Mass Fluxand Model Predictions by Senior and Brereton (1992)A comparison between the measured net lateral solids mass flux for Ug=5.5 ni/Sand G=40 kg/m2sand the results of a semi-empirical hydrodynamic model developed bySenior and Brereton (1992) for risers of circular cross-section is shown in Fig. 11.1. Thehydraulic radius of the square riser was used in the model calculation. Since secondary airmust be introduced in the model calculations, while no secondary air was employed in theexperiments, the ratio of primary to secondary air, Rair,was set at 9999, i.e. the secondaryair was only 0.0 1% introduced I m above the distributor for the predictions shown in Fig.11.1. The model successfi.illy predicts that the direction of the net lateral solids mass flux isoutward towards the wall near the bottom, while being inward towards the axis near thetop of the riser. While there is reasonable agreement over most of the height, there is asignificant difference between the experimental and modelling results near the bottom andthe top of the riser.Tests with Raji from 1 to 9999 at a constant total amount of air introduced into theriser show that the predicted net lateral solids flux near the bottom and the top of the riserdecreased only by 10%, indicating that the predicted lateral solids mass fluxes near thebottom and the top were insensitive to R. This is expected because the model considersturbulent flow near the bottom of the riser, while the measurements, as indicated inChapters 3 and 4, show a core-annulus flow structure even at the very bottom of thesquare riser. The rapid decrease of the percentage of core over the total cross-sectionalarea near the bottom and the top of the column appears to be responsible for the highpredicted lateral solids mass flux.1913020 Modelling predictions- --•-- Experimental resultso 10><0 --0U-10•2C-)-20I • I • I • I0 2 4 6 8 10Height, z (m)Fig. 11.1. Comparison between measured net lateral solids mass fluxesand model predictions by Senior and Brereton (1992) for Ug=5.flhJsand G=4O kg!m2s192Appendix ifiAmplifying Circuit and a Sample Trace for the Piezoelectric Probe0.01 lIEpin •c_1o1-’--4m 950 kFig. 111.1. Amplifjing circuit with input resistance of 1012 2 for the piezoelectric probe.>a)Fig. 111.2. Typical trace for piezoelectric probe for Ug7.O mis, G5=60 kg/m2s,x/X0, y/Y=0.3 and z6.2 m.Sampling Time, s193

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