Ind. Eng. Chem. Res. 1991, 30, 281-286
281
KINETICS AND CATALYSIS Kinetics of Deactivation of Methylation of Toluene over H-ZSM-5and Hydrogen Mordenite Catalysts Ramakrishna Mantha, Subhash Bhatia, and Musti S. Rao* Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, U.P., India
A systematic and detailed kinetics and deactivation study on the alkylation of toluene on H-ZSM-5 and hydrogen mordenite (HM) catalysts has been carried out. T h e reaction was carried out in an atmosphere of hydrogen in a fixed-bed flow reactor at atmospheric pressure. T h e effects of temperature, product inhibition, and to1uene:methanol feed ratio on the rate of reaction were studied. From the product distribution pattern, plausible mechanisms for the main and deactivation reactions have been proposed by using the Langmuir-Hinshelwood approach. T h e kinetic parameters in the rate equation were determined by use of nonlinear regression analysis, and the activation energies for the main and deactivation reactions were found out. The activation energies for the main and deactivation reaction have been found to be 79.83 f 6.31 and 50.40 f 6.25 kJ/mol, respectively. Introduction Owing to considerable industrial demands, the selective preparation of p-xylene by the acid-catalyzed alkylation of toluene has been the subject of a number of investigations. An alternative route for xylenes is toluene disproportionation. p-Xylene is an important raw material for the production of dimethyl terephthalate and purified terephthalic acid (Chen and Garwood, 1978). The mechanism for the alkylation of toluene was given by Kaeding et al. (1981). Yashima et al. (1972), Zielinski and Sarbak (19811, and Bhat (1982) suggested the role of acidic centers in the selective production of p-xylene. Itoh et al. (1980), Lacroix et al. (1984), and Engelhardt et al. (1987) studied the side-chain alkylation of toluene by methanol. Shape-selective catalysis on ZSM-5 type zeolites was studied by Bhat (1983) and Meshram et al. (1984). Recently, Bhat et al. (1989) proposed a reaction mechanism for alkylation based on product distribution pattern. Deactivation is the irreversible loss of catalyst activity. Activity of the catalyst in the presence of coking is given by Corella and AsOa (1982): (-TA) = (-r.JOa = f ( T , p J a (1) -da/dt = \k(T,pi)ad (2) Wei (1982) explained the decline of p-xylene selectivity under increasing conversion. Zeng and Zheng (1987) observed a rise in the selective pattern when the catalyst was poisoned with nitrogenous base. Coughlan et al. (1983) observed rapid deactivation with increasing time onstream. The literature on the kinetics of deactivation of this industrially important reaction is rather scarce. The present investigation was ca;ried out with the following objectives: (i) to study the kinetics of toluene methylation on H-ZSM-5 catalyst; (ii) to propose a reaction mechanism and develop a kinetic model based on the mechanism; (iii) to study the kinetics of deactivation of toluene methylation on H-ZSM-5 and hydrogen mordenite (HM) catalysts; (iv) * To whom correspondence should be addressed.
to propose a deactivation mechanism by coking and develop a kinetic model based on the mechanism; (v) to estimate the kinetic as well as the deactivation parameters.
Experimental Section The catalyst H-ZSM-5 as prepared by Rawtani et al. (1989) from rice-husk ash was used in the present investigation. The other catalyst used in the present study was hydrogen mordenite (Zeolon 900H), which was obtained from M/S Norton Chemicals, England. Analytical grade toluene and methanol were obtained from M/s Ranbaxy Laboratories Limited, New Delhi. Technical grade hydrogen and nitrogen were supplied by M/S Indian Oxygen Limited, Kanpur. Experimental Procedure. Methylation of toluene was carried out in a fixed-bed flow reactor at atmospheric pressure. The catalyst was placed in an electrically heated stainless steel reactor (1.5 cm X 25 cm), reduced in an atmosphere of H, (40 mL/min a t STP) at 723 K for 5 h, and brought to the reaction temperature in situ. The catalyst bed was supported on glass wool over a wire mesh. To ensure proper heat and mass transfer, hollow cylindrical inert ceramic beads of size 1.5 mm 0.d. and 3.0 mm long were mixed with the active zeolite. The reactant mixture of toluene and methanol was pumped to the reactor through a preheater by a metering pump (Model RPG-20; range 0-1.36 mL/min), supplied by Fluid Metering Instruments corporation, New York, NY. The temperature of the preheater was always kept at 393 K. The vapor was carried by hydrogen to the reactor. The product was cooled by circulating ice-cold water through a glass tube condenser, and samples for analysis were collected periodically. Product Analysis. Liquid samples were analyzed by using a gas chromatograph supplied by M / S Chromatography and Instruments Co., Baroda, India. The gas chromatograph, fitted with a flame ionization detector, was operated at a column temperature of 393 K. N2 was used as the carrier gas (1.1kg/cm2 input pressure). The flow rates of Nz and Hz were kept at 40 mL/min. The components of the sample mixture were separated over a 1.6
0888-%85/ 91 12630-0281$02.5010 0 1991 American Chemical Society
282 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 xiu
I
I
Temp.=ll3 K
--
-30gcot/mol/hr FAo TIM = 3/1
0
FA 0
H2 f l o w r a t e . reactant flow rate
2 2.0
1.0
Temp ~ 6 7 3K
-=30gcot/mol/hr
c
=40
z 3.01.0
0
0.5
1.0
1.5
2.0
2.5
Flow r a t e Of MQOH FAO m o l I h r
3.0
5.0
2.0 3 Toluene 2.5 3.0 I Methanolmole 3.5 , 6.0 r a t Li o5
1.5
2
3.5, x 16'
Figure 2. Effect of toluene/methanol mole ratio on rate of reaction.
F i g u r e 1. Test for external diffusional resistance.
mm X 3 m long stainless steel column packed with 5% Bentone-34 and 5 % diisodecyl phthalate on 60-80-mesh Chromosorb W.
Kinetic Study Kinetic data were taken a t different temperatures ranging from 623 to 748 K at a constant W / F b of 30 g of catalyst/(mol.h). In each kinetic run 0.5 g of catalyst was charged in the reactor. Depending on the effective reactant mixture density and molecular weight, its volumetric flow rate was controlled so as to send always 0.0167 mol/h of reactant (on methanol basis). Hydrogen gas was used as a carrier gas in all the experiments. In order to study the deactivation kinetics, time-on-stream experiments were carried out at different temperatures and concentrations of the reactants. Mass-Transfer Considerations. In any kinetic study, it is important that mass-transfer resistances are negligible during the reaction. The catalyst particle size (6.5 pm) used in the present investigation was so small that intraparticle diffusional resistances were neglected. Effect of external diffusion on rate of formation of xylenes (0- + p + m-xylenes)was studied by conducting runs at a constant W / F b (30 g of catalyst/(mol.h)) but varying flow rates of reactant mixture and hydrogen. The results are shown in Figure 1. The molar ratio of toluene to methanol was kept at 3:1, while the molar ratio of reactant mixture to hydrogen was kept at 1:4. For a H2flow rate above 100 mL/min (or a methanol flow rate of 0.0167 mol/h), external diffusional effects were found to be negligible. As the experiments were carried under excess of toluene, the performance of the catalyst was defined as yield = moles of xylenes formed/mole of methanol fed (3)
Effect of Molar Ratio. The effect of molar ratio of to1uene:methanol on the rate of reaction (rate of formation of 0- + p - + m-xylenes) was studied in the range of 1:l-5:1 and is shown in Figure 2. The rates of formation of xylenes increased with the molar ratio of toluene to methanol. It was observed that the rate of reaction was of negative order with respect to the methanol concentration. This is in conformity with the findings of Yashima et al. (1972). Effect of Carrier Gas. The effect of carrier gas on the catalyst activity has been studied with hydrogen and nitrogen gases. Time-on-stream studies were carried out on
0.61
0.3 0
F T
2
L
6
8
10
12
Time t , h r
Figure 3. Effect of carrier gas on activity of H-ZSM-5 catalyst.
H-ZSM-5 catalyst under constant contact time (30 g of catalyst/(mol-h)) and temperature (673 K) and ratio of reactants (toluene:methanol3:1) using both the gases independently. This is shown in Figure 3. The catalyst deactivation was more rapid in N2atmosphere as compared with H, atmosphere. To account for this kind of behavior, one can assume either a difference in coke distribution or a difference in the nature of gaseous atmosphere. Mass spectrum studies by Minachev and Garanin (1971) revealed rapid deactivation of the catalyst due to coke deposition on the outer surface of the catalyst. Further, they observed that the coke deposited under the nitrogen atmosphere consisted of higher molecular weight compounds than the coke deposited under the hydrogen atmosphere. Carbonium ions were found to be the intermediates for coking (Streitwieser and Reif, 1964; Gnep et al., 1980). It is believed that a zeolite which has very strong acid sites is capable of activating molecular hydrogen to enable it to reduce the concentration of adsorbed carbonium ion species on the catalyst surface, thus minimizing the coke formation (Streitwieser and Reif, 1964). Effect of Temperature. Under constant contact time and for different ratios of reactants, the effect of temperature in the range of 623-773 K on the alkylation of toluene with methanol on H-ZSM-5 has been studied. The results are shown in Figure 4. The rate of reaction increased with temperature until 723 K. A t temperatures
~
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 283 Table I. Kinetic Constants Estimated Using Nonlinear Regression (Catalyst H-ZSM-9 Dual-Site Mechanism) mol/(h.g of temperature, K catalyst. atm2) 623 648 673 698 723 773
18
0.306 0.309 0.613
kt
k; k3
"'
FA0 0
1.51 600
I
TIM5511
A
T I M = 311
I
TIM
i
2.531 0.712 1.726
I
c
I
Temperature, H
I
"..600
4.950 0.912 2.435
7 50
Figure 4. Effect of temperature on rate.
90
4.520 0.800 2.176
111
700
650
1.762 0.683 1.214
clusion that p-xylene is a primary product of alkylation. Because of different steric encumberance of the three isomers (meta > ortho > para) and the microporosity of the catalyst, an excess of para isomer is formed, which is in conformity with earlier findings (Kaeding et al., 1981). The mechanism proposed is similar to the one proposed by Bhat et al. (1989):
w =30gcat/mol/hr
2.5
1.215 0.621 1.102
I
&H3
k-3
&
H3
(5)
=30 g c o t l m o l l h r FA0 T I M ; 311
650
700
750
Temperature. K
Figure 5. Effect of temperature on para selectivity.
higher than 723 K, decomposition of the alkylating reagent (methanol) occurred instead of alkylation, decreasing the effective rate of reaction.
Kinetic Modeling From the product distribution, it is found that p-xylene is the main product of alkylation-the para selectivity increases with temperature, as seen from Figure 5. Here, para selectivity is defined as para selectivity = moles of p-xylene formed/total moles of xylenes formed (4) According to Cavallaro et al. (1987), it is possible that the formation of trimethylbenzene (TMB) may take place with a bimolecular mechanism at high methanol concentrations. A t low methanol concentrations and small contact times, however, the production of TMB is hindered. These observations, along with earlier works (Young et al., 1982) on a similar type of zeolite catalyst, have led to the con-
With the use of this mechanism along with the Langmuir-Hinshelwood approach, and the experimental data, various reaction rate models (adsorption, desorption, and surface-reaction controlling) have been tested. The adsorption and desorption models gave negative constants (not shown). Hence they are assumed not plausible. On the basis of a dual-site mechanism, the following rate equations are obtained. rate of disappearance of methanol or toluene = r M = rT = klKTKMPTPM/p (6) rate of formation of p-xylene = rpx = [ ~ ~ K T K M P T-Ph(KpxPpx M - Kmxf'rnx/KJI / p ( 7 ) rate of formation of m-xylene = rmx= [k2(Kp,Pp,KmxPmx/K2) - k,(KmxPmx - KoxPox/KJI / p (8) rate of formation of o-xylene = r,, = [k,(KmxPmx - KoxPox/KdI/p (9) where Z = 1 KTPT + KMPM + KmxPmx + K,,P,, + KprPpx (10) The rate of formation of trimethylbenzene (TMB) is not considered here in the present model. The equilibrium constants are given by the equations K2 = kZ/k-z (11)
+
K3 = k 3 / k - 3 (12) These equilibrium constants used in the parameter estimation are calculated from thermodynamic data. The kinetic parameters in (6)-(9) are evaluated by using nonlinear regression (BSOLVE) based on Marquardt's algorithm (Kuester and Mize, 1973). Tables I and I1 respectively give the kinetic and adsorption constants thus estimated by nonlinear regression method. From the numerical values of these constants, it can be seen that, with an increase in temperature, the values of kinetic constants increase. Also, the values of adsorption constants decrease with increasing temperature which, in
284 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 Table 11. Values of Estimated Adsorption Constants at Different Temperatures (Dual-Site Mechanism) const, temperature, K atm-' 623 648 673 698 723 773 0.284 0.241 0.183 0.136 0.098 0.361 KT 0.417 0.400 0.800 0.611 0.502 0.900 KM 5.712 4.310 3.714 3.142 2.876 K,, 6.354 K,, 26.123 20.312 17.965 14.821 13.67 12.31 K", 51.79 46.527 40.131 35.00 32.20 31.017
-3
x 10
-s
o p-xylene
42-
A I
?LO-
O
A
5
(+
A '
9-
r
f< I
k, fi, KT
KM KPX
Krnx K O ,
E , kJ/mol 79.83 f 6.31 28.28 f 2.91 43.22 f 3.43 -38.24 f 3.34 -26.61 f 2.23 -25.73 f 1.77 -22.47 f 1.38 -16.65 f 1.72
2.158
x
?
>
- 3.6-
637 K
Temp
= 30 g c o t l m o l l h r FA 0 T / M = 311
k0
3.27 X lo6 9.0 x 10' 2.70 x 103 1.72 x 10-4 5.38 x 10-3 4.79 x 10-2 3.37 x 10''
A
0
A
O
Table 111. Values of Estimated Activation Energies and Preexponential Factors rate const
m-xylene o-xylene
3.2
3.0
t C
8
4
12 Mol 'I. xylene
16
20
Figure 6. Effect of product inhibition on rate of reaction. x 10'
3.0-
fact, is the right trend for these constants. The kinetic constants evaluated a t different temperatures were used to determine the activation energies and frequency factors by using the Arrhenius relation. The kinetic constants obtained in the present study using H-ZSM-5 are in close agreement with the reported data by Bhat et al. (1989). A least-squares fitting algorithm was employed in order to evaluate the slope and intercept from In k vs 1/T values. The apparent activation energies as well as the preexponential factors thus obtained are tabulated in Table 111. Our value of the apparent activation energy of 79.83 f 6.31 kJ/mol is comparable to that of 71.5 kJ/mol reported by Wang et al. (1986). We obtained an activation energy of 28.28 f 2.91 kJ/mol for the isomerization reaction. This is comparable to the activation energy of 35.37 kJ/mol, as reported by Bhat et al. (1989). It is believed that, for alkylation, both toluene and methanol will have to move to adjacent sites for the reaction to take place, while isomerization can take place on the same site where xylenes are formed (Bhat et al., 1989). The present findings are in agreement with those of earlier works (Young et al., 1982; Hsu et al., 1988; Bhat et al., 1989). Also, from a comparison of the activation energies for the isomerization of p-xylene to m-xylene, and that of m-xylene to o-xylene, we note that the former is preferred over the latter. This is manifested in the product distribution, too (para > meta > ortho).
Kinetics and Deactivation Deactivation by coking is a common phenomenon in zeolites. Active site coverage and pore blockage by coke may result in deactivation. The deactivation phenomenon can be represented by = (-rAl0a (13) Le., activity, a = rate of reaction at any time t/rate of reaction at time = 0 (14) X-ray diffraction of H-ZSM-5 and HM catalysts before and after the reaction (not shown here) showed that there were no structural changes in the catalyst, indicating their thermal stability. To determine the mode of deactivation, (series or parallel) experiments were conducted with varying amounts of o-, m-, or p-xylene (0-, p - , m-xylenes taken one at a time) along with the feed. The effect of product inhibition on rate is shown in Figure 6. There was no appreciable
2.5 -
d.
m.h-l m
E
1.5
( 0 - 0 . 5 - 1 . 0 ) v ~ t.
2.0-
w
= 30gcatlmollhr
Fa0
Figure 7. Test for finding the deactivation order d.
product inhibition on the rate of reaction. These results point toward a parallel-type deactivation. The reaction scheme can be represented as methanol + toluene xylenes (15) methanol + toluene coke4 (16) To study the kinetics of deactivation, time-on-stream data were taken at different temperatures and concentrations at a fixed contact time. To describe the deactivation by coke, use is made of (2). For a differential reactor, the input concentration is nearly same as the output concentration. Hence, we have -da/dt = \k(T,P,,)ad for Pi= Pi, (17)
--
It has already been established that the number of sites involved in the controlling step of the main reaction is 2 (i.e., m = 2). In order to determine the order of deactivation d , we proceed assuming different number of active sites involved in deactivation reaction (i.e., h) and obtain the corresponding value of d. Different relations of a vs t obtained from (17) are tested for linearity, in order to determine the value of d. With the experimental data, it is found that, for d = 1.5, a linear plot is obtained (Figure 7). In describing the deactivation function, use is made of the different deactivation mechanisms (dl, d2, d3, d4, d5, and d6) proposed by Corella and Asha (1982) and presented in Table IV. By use of a Langmuir-Hinshelwood
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 285 Table IV. Different Deactivation Mechanisms
mechanism dl
steps
~
adsorption M+I=MI
first
second
d2
Table V. Kinetic Deactivation Constants temDerature. K 623 648 673 698
equations"
coking sequence
first
adsorption
P1Ih 7P2Ih
P3Ih
KT* KM*
...
kd
M+I=M*I second
d3
P1Ib 7P2Ih adsorption M+I=MI
first
third
P31h ...
P1Ih ePZIh adsorption MtI=M*I
first
P3Ih
P,Ih
adsorption
second
coke precursor formation nM(g) + hMI P1Ih coking sequence
MtI=MI
+MI pllh
+MI
PZIh+leP31h+2
adsorption
second
coke precursor formation nM(g) + hMI + PiIh t Ibloc,.4 coking sequence PIIh pPIh+l PA+*
M+I=MI
"fl=
KT* KM*
-
first
third
0.3897 0.8314 1.9050
0.2976 0.8001 2.3460
0.1982 0.1471 0.7281 0.6235 0.5073 0.2345 3.1570
3.8210
Catalyst HM
5.910
0.1492
0.3126 0.7931 4.1890
0.4496 6.4350
Catalyst H-ZSM-5 50.40 f 6.25 -30.90 f 5.63 -16.68 f 1.83
Catalyst HM 25.40 f 3.47 -33.48 f 3.34 -19.04 f 1.38
1.82 x 104 1.14 X 3.84 X
3.78 X lo2 7.39 x 10-4 2.47
X
Mk)
first
third
kd
=PJh =P3Ih ... M(s)
0.9437 2.7310
KT* KM*
t..
coke precursor formation nM(g) + hM*I PiIh coking sequence
third
0.4578
kd
-
second
d6
Mk)
0.4007 0.9216 0.9310
Table VI. Activation Energies and Preexponential Factors for the Deactivation Reaction E, kJ/mol km
-
Mk)
d5
kd
coke precursor formation nM(g) + hMI PIIh coking sequence
second
d4
KT* KM*
coke precursor formation nM(g) + hM*I + PiIh coking sequence
third
773
723
Catalyst H-ZSM-5
coke precursor formation nM(g) + hMI PlIh
third
~~
0, 1, 2; h = 1, 2.
*"
mental data were similarly analyzed and the parameters tabulated in Table V. Apparent activation energies were determined by a least-squares fitting program from In k vs 1/T plots and tabulated in Table VI. The activation energy for the deactivation reaction on H-ZSM-5 catalyst was 50.4 f 6.25 kJ/mol, which is less than that for the main reaction (79.83 f 6.31 kJ/mol). The heats of adsorption for both the catalysts are comparable, indicating a similar kind of deactivation on the catalyst sites. The activation energy of the deactivation reaction on HM zeolite has been found to be 25.4 f 3.47 kJ/mol, which is nearly half of that on H-ZSM-5. The pore structure of HM is more susceptible to pore blockage by coking. Results show that there was an almost 50% decrease in the catalyst activity and diffusivity for a 4% coke deposition on mordenites (Hughes, 1984). Rapid deactivation of mordenites was observed during the transalkylation of toluene (Gnep et al., 1980), confirming our findings.
kind of approach, deactivation rate equations are derived for all six mechanisms, assuming the coke precursor formation step as the rate-governing step. The coking sequence as a rate-controlling step has not been considered here, as in that case, the total number of active sites, L , is given as Conclusions L = CI KTPTCI KMPMCI hKIKMhPMhCt H-ZSM-5 zeolite prepared from rice-husk ash proved hc,~,+ ~ K ~ K ~ K M (18) ~ P M ~toCbe~ a very active catalyst. A comparison of activation energies revealed that the deactivation reaction had a lower and it cannot be solved explicitly for values of h other than activation energy in comparison to that of the main re1. In the present work, we have established that h = 2. action. Hence, H-ZSM-5 seems to be a better catalyst in The deactivation kinetic parameters are estimated by comparison to HM zeolite. Heterogeneous modeling of the employing the BSOLVE nonlinear regression algorithm. main reaction as well as the deactivation reaction shows Except for the model shown below all other models showed the plausibility of a dual-site mechanism. negative parameters. Hence, these other models are rejected. Following is the scheme of deactivation: Nomenclature M I MI* (19) a = catalyst activity T I * TI* (20) d = deactivation order E = activation energy, kJ/mol MI* TI* * P,I I (21) FAd = feed rate of reactant (methanol basis), mol/h PI1 P2I ?=PJ ... (22) h = number of active sites involved in the controlling step of deactivation reaction The deactivation function is determined for the above I = active sites reaction scheme and is k , , k,, k 3 = rate constants, mol/(h.g of catalyst.atm2) kd = deactivation rate constant, mol/ (h-g of catalyst.atm2) kdO, KO = preexponential factors KT, K,, K p x ,K,,, KO, = adsorption and desorption rate (23) constants, atm-l To make a quantitative study of the deactivation mechaKT*,KM*= adsorption and desorption rate constants in the nism, another catalyst, HM, was employed; the experideactivation reaction, atm-'
+
+
+
+
+ +
+
+
Ind. Eng. Chem. Res. 1991,30, 286-291
286
m = number of active sites involved in the rate-controlling step of the main reaction M = methanol mx = m-xylene ox = o-xylene P,I, PJ, ... = coke precursors PT, PM,P,,, Pox, P,, = partial pressures of different components, atm P, = partial pressure of ith component, atm px = p-xylene r M ,rpx,rmx,rox = rates, mol/(h.g of catalyst) T = absolute temperature, K T = toluene t = time, h W = weight of the catalyst, g Greek Letter \zI = deactivation function defined by (2) Registry No. Toluene, 108-88-3; methanol, 67-56-1; p-xylene, 106-42-3; o-xylene, 95-47-6 m-xylene, 108-38-3: nitrogen, 7727-37-9 hydrogen, 1333-74-0: coke, 7440-44-0.
Literature Cited Bhat, S. G. T. Selectivity for Xylene Isomers in the Reaction of Alkylation of Toluene with Methanol on Zeolite Catalysts. J . Catal. 1982, 7*5,196-199. Bhat, S. G. T. Alkylation of Toluene with Methanol over L-Type Zeolite Catalysts. Indian J . Technol. 1983, 21, 65-69. Bhat, Y. S.; Halgeri, A. B.; Prasada Rao, T. S. R. Kinetics of Toluene Alkylation with Methanol on HZSM-8 Zeolite Catalysts. Ind. Eng. Chem. Res. 1989, 28, 890-894. Cavallaro, S.; Pino, L.; Tsiakaras, P.; Giordano, N.; Rao, B. S. S. Alkylation of Toluene with Methanol. 111: Para-selectivity on Modified ZSM-5 Zeolites. Zeolites 1987, 7, 408-411. Chen, N. Y.; Garwood, W. E. Some Catalytic Properties of ZSM-5, a New Shape Selective Zeolite. J . Catal. 1978, 52, 453-458. Corella, J.; AsCa, J. M. Kinetic Equations of Mechanistic Type with Non-separable Variables for Catalyst Deactivation. Ind. Eng. Chem. Process Des. Deu. 1982, 21(1), 55-61. Coughlan, B.; Carroll, W. M.; Nunan, J. Alkylation Reactions over Ion-exchanged Molecular Sieve Zeolite Catalysts. Parts I, 11, J . Chem. Soc., Faraday Trans. 1983, 79(2), 281-296. (Chem. Abstr. 1983, 98, 574, 178425d). Engelhardt, J.; Szanyi, J.; Valyon, J. Alkylation of Toluene with MeOH on Commercial X Zeolite in different Alkali Cation Forms. J . Catal. 1987, 107, 296-306. Gnep, N. S.; Martin de Armando, M. L.; Marcilly, C.; Ha, B. H.; Guisuet, M. Catalyst Deactiuation; Delmon, B., Froment, G. F.,
Eds.; Amsterdam: Elsevier, 1980; p 79. Hsu, Y. S.; Lee, T. Y.; Hu, H. C. Isomerization of Ethyl-benzene and M-xylene on Zeolites. Ind. Eng. Chen. Res. 1988,27, 942-947. Hughes, Ronald. Deactivation of Catalysts; Academic Press: London, 1984. Itoh, H.; Miyamoto, A.; Murakani, Y. Mechanism of the Side-chain Alkylation of Toluene with MeOH. J . Catal. 1980,64,284-294. Kaeding, W. W.; Chu, C.; Young, L. B.; Weinstein, B.; Butter, S. A. Selective Alkylation of Toluene with Methanol to Produce Paraxylene. J . Catal. 1981, 67, 159-174. Kuester, J. I.; Mize, J. H. Optimization Techniques with Fortran; McGraw-Hill: New York, 1973; p p 240-250. Lacroix, C.; Eluzarche, A.; Kiennemann, A.: Boyer, A. Promotion Role of Some Metals (Cu,Ag) in the Side Chain Alkylation of Toluene by Methanol. Zeolites 1984, 4, 109-111. Meshram, N. R.; Kulkarni, Suneeta, B.; Ratnasamy, P. Transalkylation of Toluene with C9 Aromatic hydrocarbons over ZSM-5 Zeolites. J . Chem. Technol. Biotechnol. 1984, 34A, 119-126. Minachev, Kh.; Garanin, V. Molecular Sieve Zeolite-11. Adu. Chem. Ser. 1971, 102, 441-450. Rawtani, A. V.; Rao, M. S.; Gokhale, K. V. G. K. Synthesis of ZSM-5 Zeolite Using Silica from Rice-Husk Ash. Ind. Eng. Chem. Res. 1989, 28, 1411-1414. Streitwieser, A., Jr.; Reif, L. Mechanism of Transalkylation of Ethylbenzene with Gallium Bromide-hydrogen Bromide. J . Am. Chem. SOC. 1964,86, 1988-1993. Wang, Q.; Meng, Y.; Han, Q.; Miao, G.; Liu, N. Pulse Catalytic Reaction Kinetics of Toluene on HCeY Zeolite, Gaodeng Xuexiao Huaxue Xuebao 1986, 7(10), 912-916 (in Chinese) (from Chem. Abstr. 1987, 106, 215824q). Wei, J. A Mathematical Theory of Enhanced Para-Selectivity in Molecular Sieve Catalysts. J . Catal. 1982, 76, 433-439. Yashima, T.; Keiichi, S.; Tomoki; Nobuyoshi, H. Alkylation on Synthetic Zeolites. 111: Alkylation of Toluene with Methanol and Formaldehyde on Alkali Cation Exchanged Zeolites. J . Catal. 1972,26, 303-312. Young, L. B.; Butter, S. A,; Kaeding, W. W. Shape Selective Reactions with Zeolite Catalysts. 111. Selectivity in Xylene Isomerization, Toluene-methanol Alkylation, and Toluene Disproportionation over ZSM-5 Zeolite Catalysts. J . Catal. 1982, 76, 418-432. Zeng, Z.; Zheng, C. Shape-selective Catalysis of ZSM-5 Zeolites Modified with Alkali Compounds. Caodeng Xueico Huaxu Xuebao 1987,8(2),97-102 (in Chinese) (from Chem. Abstr. 1987, 107, 60974h). Zielinski, S.; Sarbak, Z. Alkylation of Toluene with Methanol over Rare Earth Forms of Na-X, FAU and Na-Y, FAU. React. Kinet. Catal. Lett. 1981, 16(2-3), 119-122.
Receiued for review March 12, 1990 Accepted August 16, 1990
Optimization of Catalyst Distribution in a Tubular Reactor Chengjun Du and Richard Turton* Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506
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Optimization of catalyst distribution in a single tubular reactor for the simple series reaction A B C in which each reaction step requires a specific catalyst has been carried out. Optimal catalyst distribution and maximum final mole fraction of product C are presented graphically for some simple cases with elementary reaction kinetics and reactor conditions. T h e methodology for solving such problems, illustrated in this work, can be used by a process engineer in retrofitting old facilities.
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Introduction This paper considers the simple series reaction A B C in which the first reaction step requires a catalyst and the second step requires a different catalyst. The question that this paper addresses is what is the optimal distribution of catalyst in a single reactor that optimizes the production of final product C during a single-pass operation? The production of C in a single reactor will in general be lower than in a two-reactor scheme in which the con-
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ditions of each reactor may be adjusted to optimize both B and B C. Multithe reaction steps, namely, A ple-reactor schemes have been studied by a variety of workers (Aris, 1965; Chartrand and Crowe, 1969), and an excellent discussion of the various cases considered has been given by Doraiswamy and Sharma (1983). We concentrate rather on the case when only a single reactor is available for both the reaction steps. Such a case is usually suboptimal compared with a multiple-reactor configuration
0888-5885/91/2630-0286$02.50/0 0 1991 American Chemical Society
