Effect of Particle Size Distribution on the Performance of a Catalytic Fluidized Bed Reactor

Transcription

1 Refereed Proceedins The th International Conference on Fluidization - New Horizons in Fluidization Enineerin Enineerin Conferences International Year 007 Effect of Particle Size Distribution on the Performance of a Catalytic Fluidized Bed Reactor Sanjib das Sharma Todd S. Pusley Uniersity of Saskatchewan Uniersity of Saskatchewan, todd.usley@usask.ca This aer is osted at ECI Diital Archies. htt://dc.enconfintl.or/fluidization xii/80

2 FLUIDIZATION XII 655 das Sharma and Pusley: Effect of PSD on the Performance of a Fluid Bed Reactor ABSTRACT EFFECT OF PARTICLE SIZE DISTRIBUTION ON THE PERFORMANCE OF A CATALYTIC FLUIDIZED BED REACTOR Sanjib das Sharma and Todd Pusley Deartment of Chemical Enineerin The Uniersity of Saskatchewan 57 Camus Drie Saskatoon S Canada T: ; F: ; E: todd.usley@usask.ca Comutational Fluid Dynamics (CFD) has been used to model ozone decomosition in a two-dimensional fluidized bed of.0 meters heiht and 0. m width. Simulations were carried out with mono-size, bimodal, and wide catalyst article size distributions with the same mean diameter to model the effect of fines on the reaction. CFD redictions fall within 0% of the exerimental data. INTRODUCTION Fluidized bed technoloy facilitates the effectie use of fine, hihly actie catalysts. Howeer, imroer desin and/or oeration of fluidized bed reactors can lead to conersions that fall well below the theoretical lower limit of erfectly mixed flow. It has been shown [] that this is due to byassin of reactant as traed inside bubbles, resultin in oor as-solid contactin. Preious exerimental works [e..,, 4, 5] hae established that the addition of fines can increase conersion in fluidized bed reactors. Sun and Grace [] and Grace and Sun [] hae reorted that, aart from the imroement in the reactor erformance in a fluidized bed with a broad article size distribution, catalytically actie fines are more effectie in boostin conersion than catalytically actie coarse articles. The heteroeneous ozone decomosition reaction has been used by researchers for many years to quantify as-solids contactin in fluidized bed catalytic reactors [e..,, 6, 7]. This method is attractie because the reaction requires only low concentrations of the reactant, detection is raid and accurate, and there is a measurable reaction rate at ambient conditions. Furthermore, owin to the low reactant concentrations, density and temerature chanes due to reaction can be nelected and, since reactants are re-mixed, the influence of as-solid contactin within the reactor is isolated. Published by ECI Diital Archies, 007

3 656 SHARMA, PUGSLEY While The ozone th International decomosition Conference in on Fluidization a fluidized - New bed Horizons reactor in Fluidization has been Enineerin, extensiely Art. 80 [007] studied in seeral exerimental works, the only reious comutational fluid dynamics (CFD) modelin effort is that of Syamlal and O Brien [8]. The catalyst in their model was a Geldart B owder and, while areement was ood between their model and exerimental data, they used a sinle mean article diameter rather than the article size distribution in the model. In the resent work, we use CFD to model the catalytic decomosition of ozone in a fluidized bed reactor containin a Geldart A catalyst owder. The main objectie of the study is to model the influence of article size distribution on the reaction. The model will be alidated by comarison with the exerimental data of Sun and Grace [] and Grace and Sun []. COMPUTATIONAL FLUID DYNAMICS (CFD) MODEL In the resent work, we use the Eulerian CFD modelin aroach in which the as and solid hases are treated as fully interenetratin continua. This inoles the solution of the fundamental equations of mass and momentum conseration subject to suitable constitutie equations that describe the rheoloical roerties of each hase and the interactions between the hases. The oernin equations are summarized in Table for the eneral case of M solid hases. For M =, the model reerts to the mono-size article case (i.e. a sinle solid hase with size corresondin to the mean diameter). For a two-dimensional model such as the one deeloed in the resent study, there are M+ deendent ariables to be soled: oidae, as ressure, and M elocity comonents ( er hase). This is done by numerically solin the couled non-linear set of artial differential equations, as will be described later. Constitutie relations for the as-hase stress ( τ ) ranular stress ( S ) as-solids dra (F m ) and solids-solids dra (F l ) are summarized in Table. The only interaction forces considered between the as and solid hases are dra and buoyancy; the article-article collision and frictional forces are roued into the ranular stress term. The constants m to 4m in Equations (8) to () are defined as follows: m d = m m = ( + emm ) 0mm 4d ( + e m mm 0mm m = m π ) π 8 [ + + ] + + 0mm 0.4( emm )(emm ) 0mm ( emm ) 5 π htt://dc.enconfintl.or/fluidization_xii/80 ( e ) (5) (6) ( e ) (7) mm 0mm 4m = (8) d m mm

4 FLUIDIZATION XII 657 das Sharma and Pusley: Effect of PSD on the Performance of a Fluid Bed Reactor The ranular enery equation (Equation (4)) may be soled to obtain the followin exression for the ranular temerature: Θ m = m tr ( ) ( ) D + mtr D + 4 4m mtr ( D ) 4m + m tr D Table Goernin equations for the as and solid hases. Descrition Equation M Volume fraction + = () Gas hase m= R () ( ) ( ) continuity +. = Solid hase continuity Secies balance Gas-hase momentum Solid hase momentum N t n = N s R () ( ) + ( ) = t n = ( X n ) + ( X n ) = Rn. (4) t ( ) + ( ) = P + τ + t (5) M F ( ) + m= t S m ( ) + ( ) F m M ( ) + + F ( ) n sn = P l= l + sl (6) (9) The key assumtions in the model are as follows: The fractional scale factors C and C are introduced into the dra function equations as er Mceen and Pusley [9]. The alue of C is adjusted to ensure no sereation of the catalyst in the bed. The alue of C is 0.5 [9]. Gas-hase turbulence is damened by the resence of the articles [0]. The critical ackin ( *) and the close-acked oidae ( c ) are equal to the minimum fluidization oidae. GEOMETRY, BOUNDARY CONDITIONS, AND NUMERICAL SOLUTION The two-dimensional model eometry defined in the resent study is based on the exerimental test bed of Sun and Grace [] and Grace and Sun [], which was m in heiht and 0. m inside diameter. The bed mass modeled in the resent study was Published.75 by k, ECI corresondin Diital Archies, 007to a static bed heiht of 0.4 m.

5 658 SHARMA, PUGSLEY The th International Conference on Fluidization - New Horizons in Fluidization Enineerin, Art. 80 [007] Table Constitutie equations for the as and solids hases Descrition Equation 7..8 Gas-solid dra F = Re m C (7) m d m Solidsolid ( e )( π lm C π + + ) ( ) flm 8 sl sl d l + d m 0lm sl Fl = C π ( sld l + d m ) dra (8) Gas stress τ = µ D µ tr( D )I (9) Granular stress Solids ressure (lastic reime) Stress tensor (lastic reime) nd inariant of the strain rate tensor Rate of strain tensor Solids ressure (iscous reime) Stress tensor (iscous reime) nd coefficient of iscosity S S P P = P I + τ = P I + τ * P * if (0) if () = () = 0 () 5 ( c )0 τ = µ D (4) P sinφ I D µ = (5) I D + D D = 6 + D = [( D D ) + ( D D ) + ( D D ) ] + D [ + ( ) ] T (6) (7) P = Θ (8) m τ = λtr D I + µ D (9) µ = (0) m Θ m λ = () m Θ m s o = + [mono-size] Radial dist. () functions M d ld m 0lm = + ( d l + d m ) m= d m [hard shere mix] () Granular enery * ( P I + τ ) γ = 0 (4) In the exerimental studies [, ], three catalyst mixtures of different size distributions (denoted as monosize, bimodal, and wide), but the same mean htt://dc.enconfintl.or/fluidization_xii/80 4

6 FLUIDIZATION XII 659 diameter (60 µm) das Sharma were and examined Pusley: Effect for of PSD their on the influence Performance on of a reactor Fluid Bed erformance. Reactor In the resent modelin study, the catalyst mixtures are reresented numerically by one or more solid secies. For the monosize case, there is one solid secies with a 60 µm diameter. The bimodal mixture consists of 40 and 80 µm secies with resectie mass fractions of 0.47 and 0.5. For the wide size distribution, nine secies with the same mass fractions as those ien in the oriinal exerimental studies are defined in the model. The inlet boundary condition was defined as uniform distribution of the fluidizin as at a temerature of 7 C; the outlet boundary condition was a secified constant ressure (atmosheric). At the walls, the no sli condition was alied for the ashase while the artial sli condition was used for the solids. These conditions are consistent with our earlier modelin work []. The bed was assumed to be at minimum fluidization oidae conditions initially. The solid elocity was initially set to zero, while the as elocity inside the bed was set to the interstitial as elocity at minimum fluidization conditions. From the uer bed surface to the exit of the test essel, the as elocity was set equal to the minimum fluidization elocity. The oernin artial differential equations were discretized and soled numerically usin the CFD code MFIX ( deeloed in the National Enery Technoloy Laboratory (NETL) of the United States Deartment of Enery. There are many different schemes aailable within MFIX for discretizin the oernin artial differential equations. The second order Suerbee is used in the resent study as it has been reorted to ie better resolution of bubbles []. All numerical solution arameters (i.e., under-relaxation, swee direction, linear equation solers, and residual tolerances) were left as their default alues in MFIX. Adatie time stein was used in all the simulations, which alters the time ste deendin uon the stability and conerence of the solution. The MFIX simulations were run usin a distributed-memory arallel ersion of MFIX on the Bioinformatics and Comutational Bioloy Research Laboratory (BIRL) Beowulf comuter cluster at the Uniersity of Saskatchewan. From the initial conditions, fluidizin air was introduced uniformly into the bottom of the bed, which initiated a transient eriod of bed exansion and increasin oidae. For all the suerficial as elocities used in this study, the initial transient eriod lasted for 5 s of real time. An additional 5 s of real time was modeled after the initial transient. The 0 s of real-time fluidization modelin required week of comutation time for the monosize articles and about weeks for the bimodal articles. Howeer, with a wide article distribution (nine different solid secies), the simulation time was months for 0 seconds of real-time. RESULTS AND DISCUSSION The time-aeraed ozone conersion was obtained by aerain the outlet concentration of ozone between 7 seconds and 0 seconds. The CFD model redictions so obtained were comared with the exerimental test data of Sun and Grace [] for a catalyst mass of.75 k and the three different catalyst size Published by ECI Diital Archies, 007 5

7 660 SHARMA, PUGSLEY distributions. The th International The results Conference are on resented Fluidization - New in terms Horizons ozone in Fluidization conersion Enineerin, as Art. a 80 function [007] of the dimensionless reaction rate constant k' f : k f k = r H mf ( ) where k r is the first-order reaction rate constant. U mf (0) Fiure resent the CFD redictions of ozone conersion for the three different article size distributions. The model redicts the exected trend of increasin conersion with the hiher alues of the rate constant. The model also redicts the trends obsered by Sun and Grace [] that, for a ien alue of the rate constant, the catalyst consistin of a wide size distribution ies the hihest conersion, followed by the bimodal and then the narrow PSD. Predicted Ozone Conersion k f ' narrow PSD bimodal PSD wide PSD Fiure CFD redictions of ozone conersion as a function of dimensionless rate constant for the three different size distributions. Fiure resents a quantitatie comarison of model redictions and exerimental data. It can be seen that the model systematically redicts hiher conersion alues than the exerimental data. Still, areement is ery ood, with the model redictions bein within 0% of the data in all cases. To demarcate the effect of the fines from that of the coarse articles, searate simulations were carried out for the bimodal distribution with only the fines as catalytically actie and with only coarse articles as actie. The results are resented in Fiure where it can be seen that, when only the fines are actie, conersion is much lower. Howeer, when the coarse articles are made actie, the conersion achieed is only slihtly lower than that obtained when all the articles htt://dc.enconfintl.or/fluidization_xii/80 6

8 FLUIDIZATION XII 66 are actie. This das trend Sharma could and Pusley: simly Effect be of PSD due on the to Performance the lower of mass a Fluid Bed fraction Reactor of the catalyst bein actie, howeer it is contrary to the results reorted by Sun and Grace [] who exlained their obserations of increased conersion with the fines actie as bein due to increased concentration of finer catalyst articles inside the bubbles. Such a difference between the exerimental obseration and CFD redictions could be attributed to the fact that, althouh reorted bimodal, the actual article size distribution had a sinificant amount fines (less than 0 microns). In the CFD simulations, the fine articles were 40 microns. The resence of fines less than 0 microns increases the fine article concentration within the bubbles containin the reactin ozone. The rest of the bed which contains the inactie coarser articles does not hae much effect. For the same reason when the fine articles are made inactie, the conersion falls dramatically. Predicted Ozone Conersion % -0% narrow bimodal PSD wide Exerimental Ozone Conersion Fiure Parity lot comarin the model redictions with exerimental ozone conersion data of Sun and Grace []. CONCLUSION Reaction kinetics for the decomosition of ozone were incororated into a CFD model of a bubblin fluidized bed. The model redictions of ozone conersion aree to within 0% of the exerimental data of Sun and Grace []. When only the fines fraction in the bimodal catalyst size distribution is made actie, the trend in ozone conersion redicted by the model is oosite to that reorted exerimentally. This is likely due to the absence of fines inside the bubble in the CFD model redictions. NOMENCLATURE Due to sace constraints, the reader is referred to our recent aer in the AIChE Journal [] for a comlete list of symbols and their definitions. Published by ECI Diital Archies, 007 7

9 66 SHARMA, PUGSLEY The th International Conference on Fluidization - New Horizons in Fluidization Enineerin, Art. 80 [007] Predicted Ozone Conersion k f ' fines actie coarse actie all actie Fiure CFD redictions of ozone conersion as a function of dimensionless rate constant showin the influence of actie fines on conersion. REFERENCES. Leensiel, O., Chemical Reaction Enineerin, rd Ed., Wiley, New York, NY (999), Sun, G. and Grace, J.R., The effect of article size distribution on the erformance of a catalytic fluidized bed reactor, Chem. En. Sci., 45, (990).. Grace, J. R. and Sun, G., Influence of article size distribution on the erformance of fluidized bed reactors, Can. J. Chem. En., 69, 6-4 (99). 4. Yates, J. G. and Newton, D., Fine article effects in a fluidized-bed reactor, Chem. En. Sci., 4, (986). 5. Zenz, F. A and Othmer, D.F., Fluidization and Fluid-Particle Systems, Reinhold Publishin Cor., New York, NY (960), Fryer, C. and Potter, O. E., Exerimental inestiation of models for fluidized bed catalytic reactors, AIChE J.,, 8 (976). 7. Ouyan, S. J., Lin, J., and Potter, O. E, Ozone decomosition in a 0.54 m diameter circulatin fluidized bed reactor, Powder Technol., 74, 7 (99). 8. Syamlal, M. and O'Brien, T. J., Fluid dynamic simulation of O decomosition in a bubblin fluidized bed, AIChE J., 49, (00). 9. Mceen, T. R. and Pusley, T., Simulation and exerimental alidation of a freely bubblin bed of FCC catalyst, Powder Technol., 9, 9 5 (00). 0. Enwald, H., Peirano, E., and Almstedt, A. E., Eulerian two-hase flow theory alied to fluidization, Int. J. Multih. Flow, (sul.), - 66 (996).. Das Sharma, S., Pusley, T., and Delatour, R., Three-dimensional CFD model of the deaeration rate of FCC articles, AIChE J. 5, (006).. Guenther, C. and Syamlal, M. The effect of numerical diffusion on isolated bubbles htt://dc.enconfintl.or/fluidization_xii/80 a as-solid fluidized bed, Powder Technol., 6, 4-54 (00). 8