HYDORDYNAMIC MODEL OF FLUID CATALYTIC CRACKING (FCC) RISER REACTOR. Abstract

Transcription

1 HYDORDYNAMIC MODEL OF FLUID CATALYTIC CRACKING (FCC) RISER REACTOR Jun You, Chao Zhu, Department of Mechanical Enineerin, New Jerey Intitute of Technoloy, Newark, NJ,712 Abtract While commercial FCC rier reactor convert heavy hydrocarbon petroleum fraction into a late of more uable product, ma tranfer, momentum tranfer, heat tranfer, catalytic crackin reaction and phae tranfer are entanled and occur imultaneouly inide the rier reactor. Since the FCC proce i in nature tron inter-coupled with multiphae flow hydrodynamic, in order to predict accurate behavior of the rier reactor, all thee procee need to be comprehenive modeled. Unfortunately mot of publihed literature focued too much on the FCC proce itelf without payin enouh attention to inificant influence on the crackin kinetic from multiphae flow hydrodynamic, uch a local catalyt concentration, local catalyt velocity and even catalyt averae ize. The aim of thi work i to develop a eneric modelin approach which can fully incorporate multiphae flow hydrodynamic with FCC proce. The emphai of thi model i to develop a framework to imultaneouly imulate the multiphae flow hydrodynamic, crackin reaction and their inter-couplin characteritic in rier reactor. Thi modelin approach open up a new dimenion for makin eneric model uitable for the analyi and control tudie of FCC unit. Prediction of the model will compare with the yield pattern of indutrial cale plant data reported in literature. Keyword: Fluid Catalytic Crackin, Multiphae flow hydrodynamic, Crackin Kinetic, Rier Reactor Introduction Fluid catalytic crackin (FCC) i one of the mot important refinery unit operation. It i the primary converion proce in the modern refinery indutry. FCC rier reactor i deined to ue acidic catalyt to decompoe heavy oil, uch a vacuum a oil, into more valuable lihter hydrocarbon at certain rane. The hydrocarbon feed enter reactor rier throuh feed atomizin nozzle and come in contact with the hot catalyt comin from the reenerator. The feed et vaporized and crack down to the lihter molecule a it travel upward alon with the catalyt. A a reult of crackin, the velocity of the vapor increae alon the rier heiht. Coke, the byproduct of crackin reaction, et depoited on the catalyt urface thu cauin the catalyt to looe it activity. The cracked hydrocarbon vapor are eparated from the deactivated catalyt in a eparator; Effort are continuouly made to improve the proce in order to increae the productivity of the refinery indutry and alo to reduce pollutant emiion to the environment. However detailed modelin of the rier reactor i a challenin tak for theoretical invetiator not only due to complex hydrocarbon in the FCC feed but alo becaue of the involvement of different type of reaction takin place imultaneouly. Correpondin Author: Tel: 1-(973) ; chao.zhu@njit.edu

2 Numerou article are found on catalytic crackin reaction in the publihed literature. Mot of them are baed on repreentation of oil in few lump (like 4 lump, 1 lump, 14 lump and o on). Weekman and Nace [1] were firt to develop a three-lump crackin model to tudy aoline production of a FCC unit; the three lump conidered were a oil, aoline and liht ae plu coke lump. Thee model led to a relationhip between the aoline yield and the a oil converion. However, becaue liht a and coke are taken a one lump, the coke yield cannot be determined eparately. Since thee tudie, more complex kinetic model for the FCC proce have been developed. Thee include: the four-lump model [2]; the five-lump model [3, 4]; the ix-lump model [5]; and the ten-lump model [6]. All thee model conidered iothermal plu flow in the rier reactor. However, when a a vertical rier tranport the particle, both experimental and theoretical tudie have hown they are non-uniformly ditributed all over the cro-ection of the rier. Momentum tranfer, heat tranfer, ma tranfer, and catalytic crackin reaction are interrelated and occur imultaneouly in commercial FCC rier reactor. To predict accurate behavior of the rier reactor, all thee procee need to be modeled. In recent year, many effort have been made in order to develop a comprehenive mathematical model which can incorporate chemical reaction kinetic, multiphae hydrodynamic and their inter-couplin proce [7, 8, 9]. Unfortunately mot of publihed literature didn t pay enouh attention to inificant influence on the crackin kinetic from multiphae flow hydrodynamic, uch a local catalyt concentration, local catalyt velocity and even catalyt averae ize, which are believed to place inificant effect on the reaction kinetic. The aim of thi work i to develop a eneric modelin approach which can fully incorporate multiphae flow hydrodynamic with FCC reaction kinetic. The emphai of thi model i to develop a framework to imultaneouly imulate the multiphae flow hydrodynamic, crackin reaction and their inter-couplin characteritic in rier reactor. In order to avoid the mathematical complexitie, four lump kinetic cheme are bein ued in thi work. Thi modelin approach open up a new dimenion for makin eneric model uitable for the analyi and control tudie of FCC unit. Prediction of the model will compare with the yield pattern of indutrial cale plant data reported in literature. Model decription The aim of thi work i to develop a modelin approach which can incorporate a-olid multiphae flow hydrodynamic with FCC proce with the conideration of the effect of local temperature and catalyt local concentration. In order to implify the equation derivation, followin commonly ued aumption were made. 1. It i aumed that the mall amount of team and by-product H 2 are nelected in the mixture of vapor, o all the a phae will only compoed by the component in the four-lump model. 2. it i aumed that, at the rier inlet, hydrocarbon feed come in contact with the hot catalyt comin from the reenerator and intantly vaporize (takin away latent heat and enible heat from the hot catalyt). The vapor thu formed move upward in thermal equilibrium with the catalyt. Under thi aumption, the complex multiphae flow which include a, olid and liquid become aolid flow. 3. There i no lo of heat from the rier and the temperature of the reaction mixture (hydrocarbon vapor and catalyt) fall only becaue of the endothermicity of the crackin reaction. 4. Deal a law i aumed to hold while calculatin a phae denity variation on account of molar expanion due to crackin and a phae temperature.

3 5. Temperature of a phae and olid phae i aumed to be identical, and heat and ma tranfer reitance are aumed a neliible. In thi way, it i much eaier to calculate total enery balance inide the rier reactor. 6. In addition, all coke (one of the production of the crackin reaction) i aumed to attach on the catalyt particle urface and the chane of catalyt particle dimenion i nelected. Fiure 1 ive the typical four-lump model ued in our modelin approach. It i aumed that the vacuum a oil i cracked into the mot deired aoline, by-product of ae and coke. Since the FCC reactor i operatin at hih temperature, the econdary crackin reaction occur for aoline to from coke and ae. There i no inter-reaction between coke and ae. The olid catalyt and a oil wa fed from the bottom of the rier reactor and the overnin equation are dicued below. The ma balance of a and olid phae can be decribed by equation (1) & (2). Solid phae: d( α ρu) = ( r3 + r5) M4 (1) Ga phae: d( α ρu) = ( r3 + r5) M4 (2) Here the averae a denity can be calculated from the ideal a law a: 3 P ( Ci Mi) i= 1 ρ = 3 (3) RT C i= 1 i The term in the left hand ide of the above equation repreent the coke ma amount, which i the ma tranfer between a and olid phae when coke depoit on the urface of the catalyt olid. Baed on the force balance of each control volume, the momentum equation of olid and a phae can be derived a, dp du = α ρ + α ρ u + FD (4) Which how that, preure radient i balanced for ravitational force, acceleration of a phae and particle-fluid interfacial force between a and olid. The momentum equation of olid phae can be expreed baed on (5); du F G dz D = α ρ + + γ (5)

4 The particle-fluid interfacial force balance for ravitational force, olid acceleration and the olid axial compact momentum due to inter particle colliion. In equation (4), F D i the dra force per unit volume. It i obtained by multiplyin the dra force on a inle particle by the number of particle per unit volume. In thi model, Richard-Zaki equation i till utilized to conider the averaed dra force. 18μ α FD = ( U ) 2 4 U (6) d (1 α ) In the olid acceleration reime, the tabilized wake effect become too important [24, 24], which lead to reduction in dra force of trailin particle of colliion pair. Hence, the modified dra force may be expreed by: FD = k1fd (7) K1 i the coefficient of wake effect of the neihborin particle on the particle-fluid interfacial force [22], which i repreented a, π k1 = 1 (1 A)exp B 3 1 6α (8) where B Then the corrected dra force hould be: A = 1 exp( Re Re ) (9) = Re Re ) (1) F = K F (11) ' D With the aumption of intantaneou vaporization of the Ga Oil, the teady-tate continuity equation for the component and lump in the a phae can be written a equation (12), (13) and (14); dc du 1 U + C1 = ( k1+ k2 + k3) C1 (12) dc du 2 U + C2 = mkc 1 1 ( k4 + k5) C2 (13) dc du 3 U + C3 = nk2c1+ qk4c2 (14) While the continuity equation for the coke on the catalyt i repreented by dc4 du U + C4 = k3c1+ k5c2 (15) where m, n and q tand for the molar expanion of each component, which mean, durin the crackin proce, each molar of reactant will become m, n or q molar of production. The temperature dependence of kinetic parameter appearin in Equation (12)~(15) wa decribed by the Arrheniu expreion: 1 D 1 1 ( ) ( c E j R T T = ) Φ (16) ki L ki e o

5 Intead of uin a uniform catalyt to oil ratio, a local catalyt to oil ratio, which i directly coupled with local catalyt concentration and reactant concentration, i ued in thi approach. Thi local catalyt to oil ratio can be repreented a, c αρ ( ) L = (17) o CM 1 1 Beide, the catalyt activity coefficient Φ repreent catalyt deactivation becaue of coke depoition. Thi coefficient depend on the coke concentration on the catalyt, and followin correlation propoed by Pitault et al.[1] will be adopted in thi model. B + 1 Φ = (18) B+ exp( A wtcoke%) ycoke w and wtcoke% = 1 (19) mc Since it ha been aumed that the a phae i alway in thermal equilibrium with the olid catalyt all alon the rier, and nelectin the heat tranfer reitance between the catalyt and a phae, the evolution of the temperature alon the rier i obtained from the enthalpy balance equation, dt αρc U + M C C U + αρc U = r Δ H (2) ( ) p, coke coke p, coke p, 1 r The enery equation repreent the combined chane of internal enery of a and olid phae. In ummary, for a complete decription of thi a-olid flow coupled reaction model, we have totally 1 independent equation ((1), (2), (3),(4), (5), (12), (13), (14), (15), (2)) for totally 1 independent variable (, α U, U, C, C, C, C,, ρ T ), hence, the problem i cloed. Reult and dicuion In thi propoed model, the material balance equation were combined with reaction kinetic and the hydrodynamic model equation to obtain the mole of each lump at the any heiht level of the rier, thu, thi model can not only predict the yield pattern alon the rier heiht, but alo can ive the prediction the temperature, preure, phae velocity and phae concentration alon the whole rier. A a preliminary tudy of the propoed model, we ued one operatin cae reported in literature [11]. The followin table 1 ive the baic parameter of the operatin condition. Table 1 Operatin condition of indutrial FCC rier reactor Parameter Derouin et al. (1997) Catalyt flux (k/m 2 ) 47 CTO 5.5 VGO feed rate (k/) 85 Feed temperature (K) 65 Catalyt inlet temperature (K) 96 Rier inide diameter (m) 1 Rier heiht (m) 35 Droplet diameter (μm) 1

6 Fiure 2 ive the model prediction of axial profile of the a and olid velocitie. It i intereted to notice that, becaue of the molar expanion of the crackin reaction, the a velocity keep increain alon the whole rier, which alo caue the increae of the olid velocity due to the aolid interactin force. Ri er Hei ht ( m) Solid velocity Ga vel oci t y Phae Vel oci t y ( m/) Fi. 2 Model prediction of axial profile of a and olid velocity Fi. 3 demontrate the averae a denity chane tendency alon the rier. With the VGO crackin into Gaoline and a oil, the averaed a denity alo decreae ince the averaed molecular weiht decreae. However the fiure 3 alo illutrate the fact that with the increae of the rier heiht, the averae a denity chane radient alo decreae. It how that at the lower part of the rier, the crackin reaction are much more violent than thoe in the upper part of the rier, which i becaue that in the lower part of the rier, the temperature, reactant molar concentration and contact area with catalyt all preent lare value. 12 Ga Den i t y ( K/ m^3) Ga deni t y Ri er Hei ht ( m) Fi. 3 Model prediction of averae a denity

7 Ri er Hei ht ( m) Lump mol ar concent r at i on Fi. 4 Model prediction of axial profile of lump molar concentration In Fi. 4, it how the model prediction of molar concentration chane of all four lump. Baed on the fiure, we can notice with the a oil keep crackin into other production, it molar concentration decreae alon the rier, and all other three lump, includin aoline, ae and coke molar concentration increae. Baed on the model prediction, we can not only predict the production yield at the exit of rier, but alo find the detailed concentration profile at any heiht of rier. Concluion A comprehenive mechanitic model i propoed in thi paper, which decribe the mechanim to conider the inter-couplin between a-olid flow hydrodynamic with chemical reaction kinetic. The emphai of thi model i to develop a framework to imultaneouly imulate the multiphae flow hydrodynamic, crackin reaction and their inter-couplin characteritic in rier reactor. In order to avoid the mathematical complexitie, four lump kinetic cheme are bein ued in thi work. Thi modelin approach open up a new dimenion for makin eneric model uitable for the analyi and control tudie of FCC unit. Prediction of the model are compared with the yield pattern of indutrial cale plant data reported in literature. Nomenclature C i molar concentration of lump (c/o) catalyt oil ratio [ cat/ oil] d averae ize of olid particle F D dra force G flow flux k i rate contant for the crackin of ith peudo-component M i molecular weiht of component i M coke molecular weiht of coke P preure r i rate of diappearance of ith peudo-component R ideal a contant T a and olid temperature U olid velocity U a velocity wt concentration of coke on catalyt urface coke C1 C2 C3 C4

8 ΔH r ρ ρ α α ε γ μ heat of reaction olid denity averae denity of a phae olid volume fraction olid volume fraction catalyt poroity Solid momentum diipation vicoity Reference 1. Weekman, V.W., Nace, D.M., 197. Kinetic of catalytic crackin electivity in fixed, movin, and fluid bed reactor. AICHE Journal 16 (3), Lee, L., Chen, Y., Huan, T., Pan, W., Four lump kinetic model for fluid catalytic crackin proce. Can. J. of Chem. En., 67, Larocca, M., N,., de Laa, H., Crackin catalyt deactiviation by nickel and vanadium contaminant, Ind. En. Chem. Re., 199, 29, Corella, J., France, E., Analyi of the rier reactor of a fluid catalytic crackin unit: model baed on kinetic of crackin and deactiva tion from laboratory tet. In: Occelli, M.L. (Ed.), Fluid Catalytic Crackin II: Concept in Catalyt Dein, ACS Sympoium Serie, vol American Chemical Society, Wahinton, pp Takatuka, T., Sato, S., Morimoto, Y., Hahimoto, H., A reaction model for catalytic crackin of reidual oil, Int. Chem. En., 1987, 27, Jacob, S.M., Gro, B., Voltz, S.E.,& Weekman, 1976, V.W., A lumpin and reaction cheme for catalytic crackin, AICHE Journal, 22, Subramanya V. Nayak, Saket L. Johi, Vivek V. Ranade, Modelin of vaporization and crackin of liquid oil injected in a a-olid rier, Chem. En. Sci., 6, (25), Gupta, R.K., Kumar, K., Srivatava, V.K., A new eneric approach for the modelin of fluid catalytic crackin (FCC) rier reactor, Chem. En. Sci., 62, (27), Han, I.S., Chun, C.B., Dynamic modelin and imulation of a fluidized catalytic crackin proce, Chem. En. Sci., 56, (21), Pitault, I., Nevicato, D., Blaetti, A.P., Delaa, H.I., Fluid catalytic crackin catalyt for reformulated aoline-kinetic modelin. Ind. En. Chem. Re, 1994, 33, 353

9 11. Derouin, C., Nevicato, D., Foriier, M., Wild, G., Bernard, J., 1997, Hydrodynamic of rier unit and their impact on FCC operation. Indu. & En. Chem. Re. 36,