260
Ind. Eng. C h e m . Res. 1989, 28, 260-264
Hydrogenation of Ethylbenzene over a Nickel/Mordenite Catalyst. Catalytic Decay by Thiophene Poisoning Xos6 L. Seoane* and Adolfo Arcoya I n s t i t u t o d e Catcilisis y Petroleoquimica, Serrano 119, 28006 Madrid, S p a i n
Jose A. GonzPlez and Nestor Travieso Centro d e Investigaciones Quimicas, Washington 169, Cerro, Habana, Cuba
Hydrogenation of ethylbenzene to ethylcyclohexane on a Ni/mordenite catalyst was studied in a fixed bed tubular reactor at 3 MPa, molar H,/ETB = 10,373-563 K, and liquid space velocity between 1.5 and 20 h-l. The effect of time on stream was also analyzed using a feed containing 100 ppm of thiophene. The formation of hydrogenation products and the incidence of thiophene on their selectivity is explained through a scheme of consecutive steps in the adsorbed state. T h e adsorption-desorption rates of the intermediates on the catalyst surface is altered by the presence of thiophene. The catalytic activity decay caused by sulfur poisoning was examined by using the model of Levenspiel for the “side-by-side” poisoning, and an equation for the rate of deactivation is given. Catalytic hydrogenation is the conventional method to remove or reduce the aromatic hydrocarbons from petroleum derivatives such as kerosene, medium distillates, paraffinic solvents, etc. The supported Pt-, Pd-, or Nibased catalysts exhibit the highest hydrogenation activity at moderate temperatures (Watson, 1976), but they are very sensitive to the sulfur compounds present in the feed (Maxted, 1951). Nevertheless, it was reported that metallic catalysts with very good resistance toward sulfur poisoning were obtained using zeolites as support (Maxwell, 1982). Several comprehensive and well-documented reviews concerning the deactivation of metals by sulfur compounds have appeared in the last few years (Bartholomew et al., 1982; Hughes, 1984). Recent papers are specifically devoted to the sulfur poisoning of nickel-based catalysts (Bartholomew, 1987; Ahmed et al., 1987; Boitiaux et al., 1987). Regarding the kinetics of the catalytic reaction with deactivation, Szepe and Levenspiel (1971) and Levenspiel (1972) developed a simple but general mathematical approach starting from two separate power law kinetic equation, one for the catalytic reaction and other for the deactivation of the catalyst. This model has been successfully used in a number of reactions with poisoning (Fuentes et al., 1981; Fuentes, 1985; Radovic and Vannice 1987). However, relatively few kinetic data have been reported on the nickel poisoning by sulfur in the hydrogenation of aromatic compounds (Weng et al., 1975; Masagutov et al., 1988). To our knowledge no work specifically devoted to ethylbenzene is available. This paper deals with the hydrogenation of ethylbenzene on a Ni/mordenite catalyst. The influence of space time, temperature, and presence of thiophene in the feed, on both the catalytic activity and product distribution, is discussed. Kinetic equations for catalyst deactivation by sulfur based on the Levenspiel model are analyzed.
Experimental Methods A Ni/sodium mordenite catalyst containing 8 w t 70 of Ni was prepared by soaking the powdered sodium mordenite (molar ratio Si02/A1,03 = 12) with a solution of Ni(N03)2.6H20.After drying, the solid was pelleted and then crushed to a particle size between 1.41 and 2.38 mm and later calcined at 773 K for 2 h in a dry air stream. The reduction was accomplished in situ prior to reaction at 523 K and 1 MPa for 3 h in a hydrogen stream. Kinetic measurements were carried out in a conventional pressure system with fixed bed down flow reactor, con0888-5885/89/2628-0260$01.50/0
taining 10 mL of catalyst diluted with inert S i c of similar size, in order to get a more isothermal profile. The reactant are mixed and preheated in the upper zone of the reactor (35 cm of length), containing inert S i c particles, before they come into contact with the catalyst bed. The reactor was heated with a silicone oil bath, and the temperature in the catalytic bed was measured with a thermocouple located in a coaxial thermowell. Reaction conditions were 3 MPa and molar ratio H,/ETB = 10, while temperature and liquid space velocity were changed between 373 and 563 K and 1.5 and 20 h-l, respectively. We made preliminary experiments to check that under these conditions both external and inside the pores diffusional limitations were absent. Liquid ethylbenzene was diluted with decalin (70 wt %), and in the poisoning runs 100 ppm of thiophene was added to the feed. Purified hydrogen was measured and controlled with a fine valve and a thermal conductivity flowmeter. Reaction products were collected at steady state during 1 h when thiophene was absent. In the experiment with poison, samples were periodically collected and analyzed by temperature-programmed GLC, with a column of 10 wt % tris(cyanoethoxy)propane (TCEP) on Chromosorb P.
Results and Discussion Effect of Space Time. Figure 1 shows conversion of ethylbenzene and yields of the reaction products as a function of the space time a t 525 K. Since the yield of ethylcyclohexane is practically equal to the total conversion, the two curves are coincident in all the experimental range. Besides ethylcyclohexane formation, a small amount of ethylcyclohexene intermediate was observed. Its yield reaches the maximum value for a conversion close to 45%. Additionally, a secondary and less important reaction of cracking also takes place, with methylcyclohexane and methane as the main products. In a few runs, traces of cyclohexane in the liquid could also be detected by GC, but not ethane in the corresponding exit gas, probably due to excess dilution. The formation of these products is explained from the scheme in Figure 2. Ethylbenzene is hydrogenated following a pathway of consecutive reactions in the adsorbed state on metallic sites. An adsorption-desorption equilibrium is established between the catalyst surface and the homogeneous phase, which permits the detection of ethylcyclohexene as a hydrogenation intermediate product. On the contrary, because of the high reactivity of the di‘01989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 3, 1989 261
s3
l
75
d
!I 7 .%50
43
/
L l \
0.75
I,
"k,il -
0.50
R
363
413
463
513
563
temperature (k)
Figure 3. Influence of temperature on the hydrogenation of ethylbenzene. Conversion of ethylbenzene (0).Selectivity to ethylcyclohexane (e),ethylcyclohexene (A),and cracking products 0
0.4
0.8
(A).
space time (h)
Figure 1. Hydrogenation of ethylbenzene as a function of space time at 525 K. Conversion of ethylbenzene or yield of ethylcyclohexane (0). Yields of ethylcyclohexene (A)and cracking products
-5-
0
-6-
(0).
- _ x
Adsorption Desorption
11
=
-8-
-9surface -10I
Figure 2. Scheme of the reaction.
olefinic bond, ethylcyclohexadiene cannot be detected. This type of reaction scheme was proposed some year ago by Hartog and Zwietering (1963) and Siege1 and Ku (1965) in the hydrogenation of aromatic hydrocarbons on solid catalysts. From a similar analysis to that mentioned above, Schoenmaker-Stolk et al. (1987) adopted this stepwise scheme to describe the reaction network of the hydrogenation of benzene over Ru/Si02. On the other hand, ethylcyclohexane can crack to methylcyclohexane or cyclohexane plus methane or ethane, respectively. But also ethylbenzene can give toluene or benzene, and these can be hydrogenated as suggested in the reaction pathway. We think that, in our experimental conditions, the sequence hydrogenation plus cracking is more probable than cracking plus hydrogenation. In the last case, benzene should preferentially be formed, which by hydrogenation would give a concentration of cyclohexene higher than that of methylcyclohexane. On the contrary, we have obtained these products in a reverse ratio, and neither benzene nor toluene was ever detected. The cracking products can be formed through a bifunctional mechanism which requires both acid and metallic centers on the catalyst surface. However, taking into account the hydrogenolysis activity of nickel and the weak acidity of our catalyst, those compounds could also be formed by a rupture of the lateral hydrocarbon chain on the metallic sites. Effect of Temperature. The effect of reaction temperature on conversion and selectivities for 6 = 0.33 h is
I
I
1
depicted in Figure 3. Since ethylbenzene hydrogenation is an exothermic reaction (AHozss= -48.17 kcal-mol-'), the equilibrium conversion diminishes as the temperature increases. From thermodynamic data, we have calculated the standard free enthalpy and the conversion at equilibrium, as a function of temperature (Ludeiia, 1986). It appears that below 600 K the equilibrium conversion remains close to loo%, so there is no limitation in our experimental conditions. Even at lower temperatures, Ni/mordenite behaves as a good catalyst for total hydrogenation. For example, at 393 K, when the conversion is only 7%, the ethylcyclohexane is already formed with 98% selectivity. Moreover, in all cases, the formation of cracking products is limited to about 0.3% only. As can be concluded from Figure 1,the kinetics of the hydrogenation of ethylbenzene on our catalyst is acceptably represented, at least up to a conversion of about 75%, by a zero-order law in agreement with Yoshinaga and Okada (1975). In consequence, the reaction rate becomes -70
x cox = --
V/Fo
e
- lz
= A exp(-E/RT)
(1)
which allows us to calculate straightforwardly the rate constant from the conversion data of Figure 3. Here, Co = 2.5 mol of ETB-L-l. In an Arrhenius plot (Figure 4), these zero-order overall rate constants closely fall on a straight line, until about 476 K ( x 0.5). From this line, an average activation energy of 11.7 f 1.5 kcal-mol-l has been estimated. Effect of Thiophene. In Figure 5 we can see the catalytic activity decay with the time on stream for 6 = 0.5
262 Ind. Eng. Chem. Res., Vol. 28, No. 3, 1989
-
loot
~
with thiophene
-1
f\
-
C
J
-2
-3
!I
I
1
I
I
0
2
4
6
8
time
(hl
I
1
1 0 1 2
1
,
14
Figure 7. Fitting of kinetic eq 7.
g 25 1 4
1
0+'3b
tl
0
2
I 6
of hydrogenation are irreversible; then the selectivity to an intermediate product is given by the expression 8
10
12
14
S=
time on stream l h )
Figure 5. Catalytic activity decay versus time on stream in the presence of 100 ppm of thiophene. A
I
25
50
100
75
conversion (molar
%I
Figure 6. Yields of the reaction products versus conversion. (A) Ethylcyclohexene: pure feed ( 0 )and poisoned feed (0). (B)Methylcyclohexene: without sulfur (@), with sulfur (O), and cyclohexane in the presence of poison ( A ) .
h and T = 523 K in the presence of 100 ppm of thiophene. A sharp, continuous drop of activity is observed by the effect of the poison on the metal sites. The half-deactivation time, i.e., the time required for the conversion to diminish to half its initial value (+), can be a very useful parameter for a first quantitative evaluation of catalyst thio resistance a t a given set of conditions. In our case, t1/2 is 4.2 h. Previously, we have verified that, with an uncontamined feed, a steady state is reached after a transient period of 1-2 h during which the activity decreases about 2% for conversions higher than 90%. The apparent low efficiency of mordenite support to protect the nickel against sulfur, as compared with other zeolites (Landau et al., 1976), is mainly due to the fact that we have used an untreated sodium mordenite. Thus, its acidic centers are shown by Chukin et al. (1977),the metal does not interact with the weakly acid protonic sites of the untreated zeolite. We have also verified that, when the acidic function of mordenite was enhanced by proton exchange or previous thermal treatment, the resultant nickellmordenite catalyst exhibits a higher thio resistance at the same conditions (Arcoya et al., 1986). The yields of different products change with time in a similar way to that shown in Figure 1. This fact allows us to compare yields as a function of conversion by changing either the space time or the time on stream. The maximum of ethylcyclohexane is found for the same conversion values (approximately 50%), though the yields in the presence of thiophene are lower (Figure 6A). This suggests that sulfur modifies the selectivity of the catalyst toward the product of partial hydrogenation, acting probably on the adsorption-desorption rates of the pathway in Figure 2. Let us assume that the successive steps
-rD -
-rr
Due to its electronegative character, thiophene reinforces the surface acidity of the catalyst and consequently decreases the desorption rate of the ethylcyclohexene which has a donor character. The transfer of T electrons from the cycloolefin to the empty d orbitals of nickel and the partial transfer of the electrons from the metal to the antibonding orbitals of the olefin (Martin, 1982) have a double effect, namely, the surface bond becomes stronger and the equilibrium is displaced toward the adsorbed species. Consequently, the hydrogenation of ethylcyclohexene is easier and its selectivity decreases. Furthermore, the presence of thiophene in the feed also modifies the yield of cracking products (see Figure 6B). Now, the cyclohexane appears and the yield of methylcyclohexane is higher than that obtained in the absence of sulfur. These results indicate that such products are formed rather by cracking via acid centers on the support and not by monofunctional metallic hydrogenolysis. In fact, if the latter were to be the case, the yield to cracking products would decrease in the presence of thiophene, due to the strong poisoning of the nickel atoms. The H2S produced increases the surface acidity of the support, transforming part of its Lewis acid sites into Bronsted sites (Ribeiro et al., 1986). Deactivation Kinetics. For the kinetic analysis of deactivation, we have followed the model of Levenspiel (1972) for the "side-by-side'' poisoning. Catalyst deactivation takes place simultaneously to the principal reaction ETB Th
+ Hz -k ETCH
+ Cat
kd
(3)
ThCat
(4) Thus, assuming separable kinetics and taking into account (l), the overall rate of disappearance of ethylbenzene can be written as dx -r = Co - a = ka (5) d% since hydrogen is in large excess. The rate at which the active sites are poisoned is given by
-da/dt = kdCThmaP= kdraP
(6)
= constant. Then, for p = 1, we obtain k6' In x = In - - kdrt
(7)
-rd =
where
CTh
co
Figure 7 provides the test for this equation. A straight line was obtained from our experimental data (circles) using the least-squares method. The calculated rate constants were k = 5.82 mol of ETB-L-' of catalyst-h-' and kd = 0.23 h-'. For the half-deactivation time, we get tIl2 = 4.0 h, in good agreement with the experimental value ( t = 4.2 h).
Ind. Eng. Chem. Res., Vol. 28, No. 3, 1989 263 Only a slight deviation was observed a t the beginning of the reaction ( x > 0.85). We have verified that other kinetic equations derived for p = 1 and n = 1, 2 do not account equally well for our experimental data. Furthermore, other possible equations corresponding to n = 0 , l and p = 0, 2, 3 (Fuentes and Figueras, 1978) do not fit them a t all. An order of deactivation p = 1 indicates that poison diffusion resistance inside the pores can be neglected (Khang and Levenspiel, 1973). One of the greater criticisms which is frequently made about the Levenspiel model is related to the separable kinetics. Although this approach is generally accepted for catalyst coking (Butt et al., 1978; Shum et al., 1987), its use is questioned to some extent when the poison is incorporated into the feed. For Butt et al. (1978), the separable model is only valid for an ideal solid surface. We think it is a simple and useful first approach to the kinetics of catalyst poisoning, in the same way that the Langmuir adsorption isotherm is. However, the separability does not account for deactivation on surfaces with different sites. Weng et al. (1975) showed for hydrogenation of benzene over supported Ni in the presence of thiophene that a separable linear model fits the data of activity versus time, but it is not able to predict the propagation of the catalytic activity zone in a nonisothermal fixed bed reactor. To make such a prediction, they have to use a semiempirical equation based on a two-sites model, one type of site needed for thiophene adsorption alone and the other for the adsorption and hydrogenation combined. Separable kinetics do not allow us to explain either changes in selectivity with the degree of surface poisoning, or preferential poisoning in bifunctional catalysts, which are due to surface heterogeneity (Bakshi and Gavalas, 1975; Gavalas, 1971). The satisfactory good fitting of our experimental results with the rate equation used is probably due to the monofunctional character of the catalyst and the absence of either significant temperature gradients or changes in selectivity. The power rate equation that we get can be but a rough approximation to a more detailed kinetic model operating in the reaction, for example, a Langmuir-Hinshelwood type of mechanism. In deactivation processes, however, the real difficulty lies in the choice of an adequate model from which the rate equation can be derived. Furthermore, it is known that different equations based on similar or different mechanisms, or even empirical equations, can satisfactorily fit a given set of data, provided enough terms and parameters are included. We consider, therefore, that it was not needed within the context of our work to look for a very elaborated mechanism and corresponding rate equation that, with enough terms and parameters, could possibly account for better the deactivation of our Ni catalyst in a wide range of experimental conditions.
Acknowledgment The authors gratefully acknowledge financial support from Comision Mixta Hispano-Cubana.
Nomenclature a = activity factor A = frequency factor C = concentration of ethylbenzene, mo1.L-l Co = concentration of ethylbenzene in the liquid feed Cat = catalyst pellet CTh = concentration of thiophene in the feed, ppm
E = activation energy ETB = ethylbenzene ETCH = ethylcyclohexane Fo = feed flow of ethylbenzene, mol-h-' k = rate constant for the main reaction AH = enthalpy change kd = rate constant for the deactivation reaction kd' = kdCThm m = order of reaction of thiophene n = order of reaction of ethylbenzene p = order of reaction of deactivation -r = reaction rate of ethylbenzene -ro = reaction rate with the fresh catalyst -rA = adsorption rate -rD = desorption rate -r, = hydrogenation rate in Figure 2 -rd = rate of disappearance of active sites R = gas constant, kcal.mol-'.K-' 5' = selectivity t = time, h tllz = half-deactivation time T = temperature, K Th = thiophene V = volume of catalyst, L x = fractional conversion Greek Letters 8 = liquid space time (h) = (space velocity)-' T = molecular orbital Registry No. Ni, 7440-02-0;ethylenebenzene, 100-41-4; thiophene, 110-02-1;ethylcyclohexane, 1678-91-7;ethylcyclohexene, 1453-24-3;methycyclohexane, 108-87-2;cyclohexane, 110-82-7.
Literature Cited Ahmed, K.; Chadwick, D.; Kershenbaum, L. S. Mechanisms for thiophene poisoning of nickel catalysts: Effect of crystallite size. In Catalyst deactiuation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1987;p 513. Arcoya, A.; Gonzilez, J. A.; Ludeiia, P.; Seoane, X. L. Hidrogenacidn de etilbenceno en presencia de tiofeno con catalizadores de niquel. In Actas del X Simposio Iberoamericano d e CatMsis; Mlrida: Venezuela, 1986;Vol. 11, p 1146. Bakshi, K. R.; Gavalas, G. R. Effects of nonseparable kinetics in alcohol dehydration over poisoned silica-alumina. AIChE J. 1975, 21 (3),494. Bartholomew, C. H. Mechanisms of nickel catalyst poisoning. In Catalyst Deactiuation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1987;p 81. Bartholomew, C. V.; Agrawal, P. K.; Katzer, J. R. Sulfur poisoning of metals. Adu. Catal. 1982,31, 135. Boitiaux, J. P.; Cosyns, J.; Verna, F. Poisoning of hydrogenation catalysts. How to cope with this general problem. In Catalyst deactiuation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1987;p 105. Butt, J. B.; Wachter, C. K.; Billimoria, R. N. On the separability of catalytic deactivation kinetics. Chem. Eng. Sci. 1978,33,1321. Chukin, G. D.; Landau, M. V.; Kruglikov, V. Ya.; Agieuskii, D. A.; Smirnov, S. V.; Belozerov, A. L.; Asrieve, V. D.; Goncharova, S. N.; Radchenko, E. D.; Konovalchikov, 0. D.; Agafonov, A. V. Investigation into the nature of the interaction between the metal component and support in metal zeolite catalyst. In Proceedings of the S i x t h International Congress o n Catalysis; Bond, G. G., Wells, P. B., Trompkins, F. C., Eds.; The Chemical Society: London, 1977;p 668 Fuentes, G. 2. Catalyst deactivation and steady-state activity: a generalized power-law equation model. Appl. Catal. 1985,15,33. Fuentes, S.; Figueras, F. Kinetics of self-poisoning of Pd/A1203 catalysts in the hydrogenolysis of cyclopentane: Influence of the dispersion of palladium and sulfate poisoning. J. Catal. 1978,54, 397. Fuentes, S.; Figueras, F.; Gdmez, R. Deactivation by coking of rhodium catalysts of widely varing dispersion. J. Catal. 1981,68,419. Gavalas, G.R. Properties of Partially Deactivated Reactors with Applications in Bifunctional Catalysis. Ind. Eng. Chem. Fundam. 1971,10(4),621.
Ind. Eng. C h e m . R e s . 1989,28, 264-271
264
Hartog, F.; Zwietering, P. Olefins as intermediates in the hydrogenation of aromatic hydrocarbons. J . Catal. 1963,2, 79. Hughes, R. Deactivation of Catalysts; Academic Press: London, 1984. Khang, S. J.; Levenspiel, 0. The Suitability of an nth-Order Rate Form To Represent Deactivation Catalyst Pellets. Ind. Eng. Chem. Fundam. 1973, 12, 185. Landau, M. V.; Kruglikov, Ya.; Goncharova, N. V.; Donoval'chikov, 0. D.; Chukin, G. D.; Smirnov, B. V.; Malevich, V. I. Nature of the interaction of the metallic component in Pd-zeolite hydrogenation catalysts 11. Sulfur resistance of the metallic component in Pd-zeolite hydrogenation catalysts. Kinet. Katal. 1976,17(5), 1281. Levenspiel, 0. J. Experimental search for a simple rate equation to describe deactivation porous catalyst particles. J . catal. 1972,25, 265. Ludeiia, P. Efecto del tiofeno en la hidrogenaci6n de etilbenceno con catalizadores de niquel soportado. Tiorresistencia. Thesis, Universidad Complutense de Madrid, 1986. Martin, G. A. Modification of catalytic and chemisorption properties of iron and nickel by additives. In Metal-support and metal-additiue effects in catalysis; Imelik, B., Naccache, C., Goudurier, G., Praliaud, H., Meriaudeau, P., Gallezot, P., Martin, G. A., Vedrine, J. C., Eds.; Elsevier: Amsterdam, 1982; p 315. Masagutov, R. M.; Kovaleva, L. V.; Morozov, B. F.; Spivak, S. I. Deactivation of nickel catalysts by thiophene in benzene hydrogenation. React. Kinet. Catal. Lett. 1988, 36(1), 177. Maxted, E. B. Deactivation of catalysts. Adv. Catal. 1951,3, 129. Maxwell, I. Nonacid catalysis with zeolites. Adu. Catal. 1982, 31, I.
Radovic, L. R.; Vannice, M. A. Sulfur tolerance of methanol synt.hesis catalysts: Modelling of catalyst deactivation. Appl. Catal.
1987, 29, 1. Ribeiro, M.; Ribeiro, F.: Dufresne, P.; Marcilly, C. Effect of nickel poisoning with sulphur or lead on the activity, selectivity and stability of Ni-H mordenite catalysts in the disproportionation of toluene. J . Mol. Catal. 1986, 35, 227. Schoenmaker-Stolk, M. C.; Verwijs, J. W.; Don, J. A.; Scholten, J. J. F. The catalytic hydrogenation of benzene over supported metal catalysts I. Gas-phase hydrogenation of benzene over ruthenium-on-silica. Appl. Catal. 1987, 29, 73. Shum, V. K.; Sachtler, W. M. H.; Butt, J. B. A Deactivation Correlation for PtRe/A120s in n-Hexane Conversion. Ind. Eng. Chem. Res. 1987,26, 1280. Siegel, S.; Ku, V. Stereochemistry and the mechanism of hydrogenation of aromatic hydrocarbons; cycloalkene intermediates. In Proceedings of the Third International Congress on Catalysis; Sachtler, W. M. H., Schuit, G. C. A,, Zwietering, P., Eds.; NorthHolland Publishing: Amsterdam, 1965; p 1199. Szepe, S.; Levenspiel, 0. Catalyst deactivation. In Proceeding of the 4 t h European Symposium on Chemical Reaction Engineering, Brussels, 1968; Pergamon: London, 1971; p 265. Watson, A. Catalysts can help optimize profit picture in ethylene plant. Oil Gas J . 1976, 8(11), 179. Weng, H. S.; Eigenberger, G.; Butt, J. B. Catalyst poisoning and fixed bed reactor dynamics. Chem. Eng. Sci. 1975, 30, 1341. Yoshinaga, K.; Okada, M. Catalytic behavior of alumina- and chromia-supported nickel catalysts for ethylbenzene hydrogenation. Kumamoto Daigaku Kogakubu Kenkyu Hokoku 1975, 24(1), 1.
Received f o r review April 4, 1988 Revised manuscript received October 12, 1988 Accepted November 22, 1988
Catalytic Cracking Kinetic Models. Parameter Estimation and Model Evaluation Laura L. Oliveira* P E T R O B R A S I C E N P E S I D I C A T , I l h a d o FundBo, Quadra 7, 21910 Rio de Janeiro, Brazil
Evaristo C. Biscaia, Jr. C O P P E , UFRJ, Ilha d o FundBo, Centro de Tecnologia, Bloco G, 21945 Rio d e Janeiro, Brazil
Four kinetic models for catalytic cracking of gasoline were proposed based on catalytic cracking and catalyst deactivation models presented in the literature. In three of them, it was assumed that activity decayed with catalyst time-on-stream; in the last model, the decay in activity was related t o the amount of coke deposited on the catalyst. Experiments for estimating the parameters of the models were performed in a bench-scale fixed bed reactor. One of the kinetic models was selected as the one that best describes the catalytic cracking of gasoline. The selected model was further incorporated into a gas oil range paraffinic compounds cracking model, which also had its parameters estimated. Finally, some coke-on-catalyst profiles along the axial direction of a fixed bed reactor during the catalytic cracking of gas oil range paraffinic compounds were predicted. The study of catalytic cracking reactions has followed the lumping methodology. In the first reaction scheme, proposed by Weekman (1968), only two lumps were considered: the feed and the products. Later, since the prediction of the gasoline yield was as important as the prediction of total conversion, some models were proposed in which total products were divided into two lumps: one corresponding to gasoline and the other to the sum of gas (C,-C,) and coke (Weekman and Nace, 1970; Pachovsky and Wojciechowski, 1971). The number of lumps of the proposed models was increased in order to obtain a more detailed prediction of products composition and to ensure a higher homogeneity inside each lump. Among the models proposed for the catalytic cracking of gas oil, the 10-lump model developed by Jacob et al. (1976) is of 0888-5885/89/2628-0264$O1.50/0
particular interest as its rate constants do not depend on the feedstock composition (Figure 1). This is a highly desirable characteristic in the industrial process simulation, due to the high frequency of change of feed quality. John and Wojciechowski (1975) proposed a reaction scheme for the catalytic cracking of gas oil (Figure 2) in which the gas composition was detailed. Propylene, nbutane, and butenes were considered primary and secondary products, which means that they can be formed directly from gas oil cracking and also from gasoline. Coke, methane, ethane, ethylene, propane, and isobutane were considered only secondary products (formed only from gasoline cracking). Corma et al. (1984) changed part of this scheme by considering propane and isobutane as primary and secondary products (Figure 3). 0 1989 American Chemical Society
