Friction Between Gas-Solid Suspension and Circulating Fluidized Bed Downers

Transcription

1 Refereed Proceedin The 12th International Conference on Fluidization - New Horizon in Fluidization Enineerin Enineerin Conference International Year 2007 Friction Between Ga-Solid Supenion and Circulatin Fluidized Bed Downer Xiaobo Qi Hui Zhan Jee Zhu Univerity of Wetern Ontario, qi1011@hotail.co The Univerity of Wetern Ontario, hzhan1@uwo.ca Univerity of Wetern Ontario, zhu@uwo.ca Thi paper i poted at ECI Diital Archive. xii/10

2 FLUIDIZATION XII 95 Qi et al.: Friction Between Supenion and CFB Downer FRICTION BETWEEN GAS-SOLID SUSPENSION AND CIRCULATING FLUIDIZED BED DOWNERS Xiao-Bo Qi, Hui Zhan, Jee Zhu # Departent of Cheical and Biocheical Enineerin, The Univerity of Wetern Ontario, London, Ontario, Canada, N6A 5B9 #Tel: ; Fax: ; Eail: jzhu@uwo.ca ABSTRACT Friction between co-current downflow a-olid upenion and the colun wall wa invetiated. A new odel to predict preure drop due to friction between the aolid upenion in the fully developed ection and the downer wall wa developed. The reult how that the friction between the a-olid upenion and the downer wall caue a inificant deviation of the apparent olid concentration fro the actual one, epecially for thoe operatin condition with hiher uperficial a velocitie and olid circulation rate. When the uperficial a velocity i reater than 8 /, the actual olid concentration in the fully developed reion of the downer can be up to 2~3 tie of the apparent value. After the frictional preure drop i conidered, the predicted actual olid concentration by the propoed odel aree well with the experiental value. INTRODUCTION Differential preure eaureent have uually been ued to etiate axial profile of cro-ectional averae olid concentration in circulatin fluidized bed (CFB) rier/downer, auin that the preure drop due to a-olid upenion to wall friction and particle acceleration are neliible. Thi ethod ha been accepted by any reearcher, ince it i non-intruive, inexpenive and iple. However, any experiental reult howed that the contribution of friction and acceleration to the total preure drop can not be nelected under certain operatin condition (1-5). Coparin the actual olid concentration directly eaured by a erie of quick-cloin valve with the apparent value inferred fro preure radient, Arena et al (1) found that even in the fully developed zone of the rier, the friction between a-olid upenion and the rier wall can till lead to inificant deviation between the apparent and actual olid concentration. Van Swaaij et al (2) found the frictional preure drop to be 20~40% of the total preure drop in dilute flow. Wirth et al (3) found the deviation of the apparent olid concentration fro the actual one to be about 20%. Harte et al (4) found ood areeent between the apparent olid concentration and the actual value, but particle-wall friction i inificant at hih a velocitie. Under hih-denity operatin condition, the axiu contribution of friction preure lo to the total preure drop wa le than 20% (5). Nuerou particle-wall friction factor correlation are available in the literature for predictin preure drop in dilute phae vertical upward a-olid flow (6-14). Mot Publihed by ECI Diital Archive,

3 96 QI, ZHANG, ZHU of the The ainly 12th International concentrated Conference on with Fluidization dilute - phae New Horizon vertical in Fluidization upward Enineerin, pneuatic Art. 10 [2007] conveyin in relatively all pipe. However, up to now few work ha been conducted to invetiate the friction between co-current downflow a-olid upenion and CFB downer wall. Due to a inificant ditinction between CFB rier and downer (15) and the fact that all the correlation are erely epirical rereion of experiental data, the correlation obtained in the CFB rier cannot be afely extrapolated outide the rane of the experiental data. In thi tudy, yteatic experiental tet on the deviation of the apparent olid concentration fro the actual one in a lon downer were carried out to characterize the friction between the a-olid upenion and the downer wall. At the eantie, a new odel that predict the preure drop due to friction ha been developed for the fully developed reion of the downer. Experiental data fro the literature are alo ued to validate the odel. GAS-SOLID SUSPENSION TO WALL FRICITION MODEL Apparent and Actual Solid Concentration For a teady co-current downward a-olid two-phae flow and on the bai of the oentu equation, the preure drop i expreed a follow: dp d 2 2 dp = [ ρ (1 ε ) V + ρ p ε Vp ] + [ ρ (1 ε ) + ρ pε ] (1) dz dz dz total When the a-olid flow reache fully developed tate, the acceleration ter d 2 2 [ ρ (1 ε) V + ρpεvp ] (2) dz hould be zero. So, the total preure drop in the fully developed zone conit of only two part: the tatic head of the a-olid upenion and the preure lo due to the friction between the a-olid upenion and the downer wall. If takin the friction preure lo into account, the actual olid concentration, ε act, can be evaluated by ε 1 dp dp act = ρ + (3) ( ρ p ρ ) dz total dz f However, if the frictional preure lo i nelected, one obtain the apparent olid concentration, ε app, by inferrin fro the eaured total preure radient. ε 1 dp app = ρ (4) ( ρ p ρ ) dz total Coparin Eq (3) with Eq (4), it can be noted that for co-current downward a-olid flow, the actual olid concentration ut be underetiated by the apparent one, ince the friction tre exerted on the downer internal wall i contrary to the direction of the a-olid flow. Preure Drop due to Friction between Ga-olid and Bed Wall Different for the coon approach to eparately evaluate the a-wall and particle-wall frictional preure drop, thi tudy treat the a-olid two-phae flow in CFB downer a a one-dienional peudo-hooeneou flow, due to the experiental fact that exitence of particle ha inificant influence on the a flow field of a-olid flow. A with ot invetiator,, in thi tudy, the Fannin friction equation for inle fluid flow a pipe wa ued to define a cobined friction factor 2 f

4 FLUIDIZATION XII 97 between a-olid upenion Qi et al.: Friction and Between CFB downer Supenion wall, and CFB f + Downer, a 2 P 2 f+ ρ u = (5) z f D where ρ and u are the cro-ectional averae a-olid upenion denity and it velocity, repectively. The cro-ectional averae a-olid upenion denity ρ i known a: ρ = ρ 1 ε + ρ ε = ρ + ( ρ ρ ε (6) ( ) p p ) wherea the upenion velocity u can be defined differently dependin on different purpoe. a definition of u accordin to a conervation i propoed in thi tudy, that i G (1 ε ) ρu + G u = = (7) ρ ρ + ( ρ ρ ) ε The friction factor, f +, can alo be defined followin the Blaiu correlation a 0.25 f + = Re (8) where Dρu D ((1 ε ) ρu + G ) Re = = (9) µ µ Subtitutin Eq (6) and (7) into Eq (5), one can obtain the frictional preure drop between the a-olid upenion and colun wall in the fully developed zone of downer p ((1 ε ) ρu + G ) ( ρ + ( ρ ρ ε ) 0.25 P µ = (10) 1.25 z f D p ) Obviouly, if there i no particle in the downer (G =0), Eq (10) reduce to the Fannin equation for predictin friction preure drop in a pipe with a alone. Conequently, cobinin Eq (3) and (10), one can predict the actual olid concentration in the fully developed zone of CFB downer baed on the eaured axial total preure radient, with iven downer diaeter, a and olid propertie and operatin condition. EXPERIMENTAL APPARATUS The experient were carried out in a cold odel CFB downer. The experiental etup i illutrated cheatically in Fiure 1. The acrylic downer i 9.3 lon and 100 i.d. In order to iniize the electrotatic found in the downer colun, a all trea of tea wa introduced into the ain air pipeline to huidify the de-oiled fluidization air to a relative huidity of 70-80%. Thi ha been hown to be very effective. The fluidization air upplied by a blower i at 20 C. An orifice plate wa eployed to eaure the uperficial a velocitie. The particulate aterial were pent FCC (Sauter ean diaeter d p =67 µ, Publihed by ECI Diital Archive, 2007 Fiure 1. Scheatic diara 3 of CFB downer etup 1.75

5 98 QI, ZHANG, ZHU particle The denity 12th International ρ p =1500 Conference k/on 3 ) Fluidization particle. - New Solid Horizon circulation in Fluidization rate Enineerin, were Art. reulated 10 [2007] by a butterfly valve and were eaured by a eaurin pipe (16). A erie of differential preure tranducer were utilized to eaure preure drop alon the downer colun. Apparent olid holdup were inferred fro the eaured preure radient. A ulti-fiber optical probe wa choen to eaure the actual olid concentration in the CFB downer. The precie calibration procedure of the olid concentration probe and other detail of the probe can be found in Zhan et al (17). Actual olid concentration were obtained by interatin the local value at different radial poition. RESULTS AND DISCUSSION Preure Drop due to Friction Fiure 2 copare the cro-ectionally averaed apparent olid concentration eaured by the fiber optical probe with the apparent one in the downer under typical operatin condition. Fro Fiure 2, one can find that the apparent olid concentration in the downer are far lower than the actual one not only in the acceleration zone but alo in the fully developed reion. In the acceleration zone of the downer, the cloer to a ditributor, the reater the deviation. Thi could be explained a: ince the acceleration direction of a-olid upenion i the ae a ravity, the particle in the acceleration zone of the downer are not fully upended by the a and therefore the eaured preure radient in the acceleration zone i only a all part of the tatic head of the a-olid upenion. Conequently, the eaured preure radient in the acceleration zone of the downer can not be totally ued to etiate the actual olid concentration. Moreover, a hown in Fiure 2(b), even in the fully developed zone of the downer, the actual olid concentration are till ore than twice of the apparent value. When the differential preure eaureent ethod i utilized to etiate the olid concentration in the downer, nelectin the frictional preure lo in the downer would lead to ubtantial deviation of the actual olid concentration fro the apparent value ince the deviation in the fully developed zone of the downer ainly coe fro the friction between the a-olid upenion and the downer wall Thi i in line with the deduction of Zhu et al (18). That i, it hould be very careful to ue the differential preure eaureent to etiate the actual olid concentration in a dilute downer iven the lower preure radient and the relatively hih upenion-to-wall friction in the downer. ε ε FCC d p = 67 µ ρ p = 1500 k -3 ε app z / ε act ε cal (a) U =3.7 /, G =49 k/ 2 FCC d p = 67 µ ρ p = 1500 k. -3 ε act ε cal ε app z / (b) U =10.2 /, G =205 k/ 2 Fiure 2. A coparion of apparent, actual and predicted olid holdup in the downer 4

6 FLUIDIZATION XII 99 To further invetiate the Qi et effect al.: Friction of Between the olid Supenion and 0.12 CFB Downer concentration on the friction, Fiure 3 U (/) copare the apparent olid concentration and the actual value in the fully developed zone of a hih-denity downer under different uperficial a velocitie (data fro Liu et al Gla bead 0.06 ρ (19)). When U =5.44/, the actual olid p = 2500 k/ 3 d concentration can be up to 3 tie of the p = 332 µ 0.04 apparent value, indicatin that the friction preure lo i a ore iportant part of the preure balance in the hih-denity downer % 0.00 than that in the low-denity downer Extenively exainin Fiure 3, one can find ε act that, for a iven uperficial a velocity, the Fiure 3. A coparion of apparent and abolute deviation of the actual olid actual olid holdup in the fully concentration fro the apparent one developed zone of a hih-denity increae linearly with the olid downer (data fro Liu et al (19)) concentration in the downer. Thi uet 4.0 (dp/dz) that the friction between the a-olid tatic /(dp/dz) total (dp/dz) upenion and downer wall i not only a f /(dp/dz) total 3.0 (dp/dz) function of the particle velocity but alo the fp /(dp/dz) total (dp/dz) olid concentration. A uch, ot f /(dp/dz) total correlation in the literature are le accurate 2.0 ince they are only a function of olid G =100 k/ 2. velocity. To quantitatively exaine the extent of the preure drop coin fro a-wall and particle-wall friction, Fiure 4 how the relative contribution of the four preure drop in the downer. Since the frictional preure drop ainly chane with actual olid velocity, V, Fiure 4 ive the variation of the four preure drop with the actual olid velocitie in the downer. The particle-wall frictional preure loe are uch reater than thoe due to a-wall friction, conitent with the reult of Rautiainen and Sarkoaa (14). Mot frictional preure drop coe fro the particle-wall friction. Conequently, when hih denity operation i preent in a downer, the particle-wall friction would lead to a ore inificant deviation of the apparent olid holdup fro the actual value, a indicated by Fiure 4. It alo een fro Fiure 4 that the preure drop due to particle-wall friction i relatively low and only lihtly increae with the actual olid velocitie for V < 10 /. When V i reater than 10 /, however the preure drop increae harply with V. Becaue the actual olid velocitie under ot operatin condition in lab and pilot aolid CFB yte are le than 10 /, the Publihed by ECI Diital Archive, 2007 frictional preure loe ay be le ε app Preure radient ratio (-) V (/) Fiure 4. Variation of preure drop ratio with actual olid velocity in the downer. ( P/ z) f (Pa/) FCC d p = 67 µ ρ p = 1500 k/ 3 U (/) / G (k/ 2 ) 10.2/ / /50 8.1/ / / /49 5.7/ / / /49 0.0/ / / V (/) Fiure 5. Effect of operatin condition on the frictional preure drop in the downer 5

7 100 QI, ZHANG, ZHU inificant. The 12th On International the other Conference hand, on Fluidization for the - operatin New Horizon condition in Fluidization Enineerin, under which Art. 10 [2007] the actual olid velocitie are reater than 10 /, the deviation between the actual and apparent olid concentration due to frictional preure drop can be inificant in indutrial FCC rier reactor. Since the axial preure radient in the downer are lower than thoe in the rier (15), the frictional preure drop in the downer i relatively hiher than that in the rier. Therefore, a uch ore inificant error ay occur if the apparent olid concentration inferred fro the preure radient are ued to dein, cale up and operate downer reactor. Effect of Operatin Condition Fiure 5 further how the effect of operatin condition on the frictional preure drop in the fully developed zone of the downer. Obviouly, the operatin condition have inificant influence on the frictional preure drop. It i expected that when olid circulation rate reain contant, the frictional preure drop increae with uperficial a velocity. At a iven uperficial a velocity, the frictional preure drop increae with olid circulation rate. And, with increain uperficial a velocity, the effect of olid circulation rate on the friction preure drop radually becoe ore inificant. Thu, the effect of uperficial a velocitie on the frictional preure lo are different than that of olid circulation rate. But, for the ae olid velocity, there would be any et of uperficial a velocitie and olid circulation rate. Conequently, it i not enouh to correlate the olid friction factor with the only one paraeter of olid velocity, and thu operatin condition paraeter (i. e. uperficial a velocity and olid circulation rate) hould be included in the correlation of friction factor. Fiure 3 alo copare the apparent olid concentration with the actual olid concentration in the fully developed ection of the hih-denity downer under different uperficial a velocitie. The operatin condition have reat influence on the difference between the apparent and actual olid concentration. For the ae actual olid concentration, the difference between the apparent and actual olid concentration increae with uperficial a velocity. And, with increain olid concentration, the effect of uperficial a velocity on the difference between the apparent and actual olid concentration becoe ore noticeable. At a iven uperficial a velocity, the deviation linearly increae with the olid concentration. A a conequence, the frictional preure lo i a function of both the olid velocity and the olid concentration. Influence of Particle Propertie Fiure 6 preent the effect of particle diaeter on the frictional preure drop in the fully developed ection of the hih-denity downer. Under the ae operatin condition in the downer, the frictional preure lo with aller particle i reater than that with coarer particle. Thi trend can alo be inferred fro the correlation of Klinzin and Mathur (13). However, thi concluion need further verification with ore experiental reult. Validation of the Model Fiure 2 alo copare the eaured actual olid concentration and the predicted value under typical operatin condition. A hown in Fiure 2, after the frictional preure drop i added to the teted total preure drop, the predicted actual olid 6

8 FLUIDIZATION XII 101 concentration in the fully Qi et developed al.: Friction Between zone Supenion of and 1000 CFB Downer the downer fit well with the teted one d p (µ) interated fro the local olid holdup. In fact, thi i enerally the cae for all experiental reult obtained in thi work. Fiure 7 copare the predicted olid concentration with the actual one obtained in thi work and the literature (19). Obviouly, the eaured actual olid 200 concentration aree well with the predicted value. Given the experient of Liu et al (19) in a wide rane of operatin condition, the excellent fit of the experiental data with the predicted value further how the reliability of the new odel. CONCLUSIONS The tudy on the friction between co-current downflow a-olid upenion in the fully developed zone and the downer wall ha led to a new odel that can uccefully predict the preure drop due to friction between a-olid upenion and the downer wall in the fully developed zone. By coparin the apparent olid concentration with the actual value, it i found that the friction between a-olid upenion and the wall caue a inificant deviation of the actual olid concentration fro the apparent one o that it cannot be nelected under certain operatin condition, epecially for the downward a-olid flow with hiher uperficial a velocitie and/or olid circulation rate. For the downward a-olid downer flow in the downer, the actual olid concentration can be up to 2~3 tie of the apparent value under certain operatin condition. The friction preure lo decreae with increain particle diaeter. The predicted actual olid concentration by the propoed odel aree well with the experiental value. In eneral, the friction between the a-olid upenion and the downer wall i an iportant factor that ut be taken into account in the odelin, dein and operation of the CFB downer reactor. ACKNOWLEDGMENT U = 1.02/ ρ p = 2500 k/ G (k/ 2. ) The author are rateful to the National Natural Science Foundation of China and the Natural Science and Enineerin Reearch Council of Canada for financial upport. ( P/ z) f (Pa/) Fiure 6. Effect of particle diaeter on the friction preure drop for la bead (data fro Liu et al, (19)) ε cal Particle d p (µ) ρ p (k/ 3 ) D () FCC Gla bead Gla bead data fro Ref. [19] data fro Ref. [19] ε act +20% -20% Fiure 7. A coparion of teted actual olid holdup with predicted value in the fully developed ection of the REFERENCES Publihed by ECI Diital Archive,

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