Ind. Eng. Chem. Res. 1993, 32,49-55
49
Kinetics of Toluene Disproportionation over Unmodified and Modified ZSM-5 Zeolites Maria A. Uguina,* Jos6 L. Sotelo, and David P. Serrano Chemical Engineering Department, Faculty of Chemistry, Complutense Uniuersity, 28040 Madrid, Spain
The kinetics of toluene disproportionation has been studied over both unmodified and Si-Mg modified ZSM-5 catalysts using an integral reactor and taking into account the reversioility of the reaction. It has been found that xylene dealkylation is the major secondary reaction, whereas toluene dealkylation can be considered negligible. For the unmodified ZSM-5, heterogeneous models based in the alkyl-transfer mechanism allowed the experimental results to be fit better than first and second pseudohomogeneous models, the best concordance being obtained when toluene adsorption is assumed as the rate-limiting step. The corresponding kinetic equation has been further applied to the modified zeolite (SiMg/ZSM-5), leading to the development of a kinetic model which includes the effect of the toluene and p-xylene intracrystalline diffusion rate and the presence of nonselective acid sites on the external zeolite surface. This model describes adequately the selective formation of p-xylene over modified ZSM-5, reproducing the experimental product distribution with an average relative error of 2.8%.
Introduction Toluene disproportionation is an industrial process for the production of benzene and xylenes. The reaction can be carried out over a variety of acid zeolites, such as mordenite, faujasite, and ZSM-5 (Aneke et al., 1979; Bhavikatti and Patwardhan, 1981; Meshram et al., 1983; Beltrame et al., 1985). The selective formation of p-xylene, the most valuable xylene isomer, is obtained when modified ZSM-5 zeolites are used as catalysts (Kaeding et al., 1981; Young et al., 1982; Meshram, 1987). Moreover, toluene disproportionation may also be a useful catalytic test for characterizing the shape selectivity of ZSM-5 samples since the kinetic diameters of the aromatic hydrocarbons involved in the reaction are very close to the ZSM-5 pore size. The mechanism and kinetics of toluene disproportionation over unmodified and modified ZSM-5 zeolites is still a matter of disagreement. Thus, three different mechanisms have been postulated in the literature (Gnep and Guisnet, 1981; Dooley et al., 1990; Fraenkel, 1990): alkyl transfer, dissociative and diphenylalkane mechanisms, which are shown in Figure 1. Likewise, with regard to the kinetics of the reaction over unmodified ZSM-5, Beltrame et al. (1985) proposed a second-order kinetic model assuming the bimolecular surface reaction between two adsorbed toluene molecules as rate-limiting step. However, Nayak and Riekert (1986) and Bhaskar and Do (1990) found a first-order dependence of the reaction rate on the toluene partial pressure. Recently, Dooley et al. (1990), collecting data from different authors and zeolites, concluded that most of the results can be adequately described by a second-order rate expression obtained with the assumption of toluene inhibition. Otherwise, it is known that ZSM-5 modification by treatment with a variety of agents allows enhancement of its para selectivity in different reactions with aromatic hydrocarbons as products: toluene disproportionation, xylene isomerization, toluene-methanol and toluene-ethan01 alkylations, etc. (Kaeding, 1977; Young et al., 1982; Olson and Haag, 1984; Ashton et al., 1986; Wang et al., 1989). The effect of the modifer agent was initially explained by an increase of the diffusional constraints due to its deposition within the zeolite channel system. Thus, the reaction over the modified ZSM-5 samples is controlled by the intracrystalline diffusion which leads to the for-
* To whom correspondence should be addressed.
mation of a primary product (the product just outside the zeolite channels) with a high proportion of the para isomer because of its higher diffusivity with regard to the other two isomers. The observed decline in para selectivity with the increase of the space time is related to the primary product isomerization by reentry into the zeolite pore structure. According to this approach, several kinetic models were developed, coupling the diffusion and intrinsic reaction rates in order to explain the relationship between catalytic activity and para selectivity over modified ZSM-5 zeolites (Wei, 1982; Theodorou and Wei, 1983; Do, 1985; Sundaresan and Hall, 1986). During the last few years, several authors have pointed out the role of the external surface of the ZSM-5 zeolite as the cause of nonselective transformations (Farcasiu and Degnan, 1988; Paparatto et al., 1988; Wang et al., 1989). In agreement with them, in spite of the low percentage of acid sites located on the external surface of the zeolite crystals, their catalytic effect has to be taken into account when comparing reactions with very different rates. In the case of toluene disproportionation over ZSM-5, Olson and Haag (1984) have reported that the ratio between the intrinsic rate constants of the secondary xylene isomerization and the main reaction is over 7000. Then, the rate of both reactions is comparable if xylene isomerization takes place only on the external zeolite surface. Therefore, the high para selectivity of the unmodified ZSM-5 samples should be related with a second role of the modifer agent: the removal, deactivation, or blockage of the nonselective external acid sites which avoids the primary product isomerization on the external zeolite surface. In this way, Fraenkel(1990) has developed a kinetic model for toluene disproportionation over modified ZSM-5 which includes the effects of the diffusional limitations and the external acid sites, the mathematic treatment being simplified by assuming that the reaction is irreversible with a pseudofmt-order rate. However, since toluene disproportionation is known to be reversible, such a kinetic model can only be applied to a differential reactor operating at low toluene conversions. The main goal of this work is to study the kinetics of toluene disproportionation over ZSM-5 zeolites, developing a kinetic model which includes the reversibility of the reaction; hence it can be used at high toluene conversions. Thereby, the work has been divided into two sections: (i) study of the kinetics of toluene disproportionation over unmodified ZSM-5 in absence of diffusional effects; (ii) development of a kinetic model for toluene dispropor-
0~88-5885/93/2632-0049$04.o0/0 0 1993 American Chemical Society
50 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
+6
6
+ZH
t Z H
CH3
acH2Q 0 4:; M3
CH
2-
$'
I +
CH3
Figure 1. Mechanism of toluene disproportionation over zeolites: alkyl-transfer (Ml),dissociative (M2), and diphenylalkane (M3) mechanisms.
tionation over a ZSM-5 sample modified with Si and Mg compounds, coupling the expression of the intrinsic reaction rate, obtained in the former section, with the rate of the intracrystalline diffusional process and taking into account the role of the external acid sites.
Experimental Section Catalyst Preparation. ZSM-5 zeolite was synthesized using ethanol as a template according to the procedure described elsewhere (Costa et al., 1987) with a Si02/A1203 molar ratio of 58. The nature and crystallinity (100%) of the product was determined by X-ray diffraction (XRD). The acid form was obtained from the as-synthesized sodium zeolite by ion exchange with an HC1 aqueous solution followed by calcination at 550 "C for 14 h. The zeolite was pelletized using 35 w t % sodium montmorillonite as binder. The effect of the activation and pelletization steps were studied in an earlier work (Uguina et al., 1991). The zeolite modification was carried out first by impregnation with an organic solution of a dimethylsilicone polymer (MERCK, GE SE-30) and subsequently with an aqueous solution of tetrahydrate magnesium acetate, yielding a catalyst with 2.5 and 1.08 wt 9'0 silicon and magnesium, respectively. The details of the ZSM-5 modification with both agents have been earlier reported (Uguina et al., 1992). Kinetic Measurements. Catalytic experiments were carried out in a fixed bed, continuous down flow reactor at atmospheric pressure. Before toluene or p-xylene (purity > 99.5%) were fed, the catalysts were treated in a nitrogen stream at 500 "C for 1 h. Then, the reactant was introduced using a syringe pump, and the gaseous and liquid products were collected for 1h after the system had reached steady state. In previous experiments, carried out by varying the time on stream, it was checked that the catalyst deactivation during the time of the kinetic measurements could be ruled out. The reaction product was analyzed by GC using 5%159'0 SP-1200fbentone 34 on a Supelcoport column for liquids and a Porapak Q column for gases. Results and Discussion (i) Toluene Disproportionation over Unmodified ZSM-5. Mass-Transfer Limitations. The effect of the
mass-transfer limitations was tested in two series of previous experiments, carried out by varying the toluene feed rate and the macroparticle size of the pellets, respectively. It was found that the mass transfer between the bulk gaseous phase and the external surface of the catalyst particles does not affect the overall process rate. Likewise, it was found that the resistance to the macropore diffusion through the binder matrix is negligible in the macroparticle size range used in this work (0.39-1.55 mm). Otherwise, the effect of the intracrystalline diffusional limitations on the toluene disproportionation rate has been estimated by calculating the observable Weisz modulus (a) and the effectiveness factor ( q ) in the way shown in a previous work (Uguina et al., 1991). From the value of 7 N 0.95 obtained, it is concluded that the intracrystalline diffusion slightly influences the rate of the main reaction and can be neglected in the study of the kinetics and mechanism over the unmodified ZSM-5. However, in the experiments carried out at low toluene conversions, a para selectivity (p-xylene percentage in the total of xylenes) higher than the thermodynamic equilibrium value (24%) has been obtained. This result shows the influence of the intracrystalline diffusion on the secondary reaction of xylene isomerization even in the unmodified zeolite. In order to avoid including the effect of the diffusional process in the first section of this work, the three xylene isomers have been grouped and considered as a single component (xylene). Reactions and Mechanism. The toluene conversion and benzenefxylene molar ratio obtained in a series of experiments conducted over the unmodified ZSM-5 at 475 "C and different space times are depicted in Figure 2. As expected, toluene conversion increases with the space time, but the benzenefxylene molar ratio deviates from 1and a maximum is observed in the xylene yield (not shown in Figure 2). This excess of benzene with regard to the stoichiometry of the reaction and the presence of different gaseous hydrocarbons in the effluent (see product distribution in Table I) reveal that secondary reactions of aromatic dealkylation take place on the catalyst in addition to toluene disproportionation. The maximum in the xylene yield could be due to xylene disproportionation, but it must be ruled out since trimethylbenzenes are present at very low concentrations. The production of gaseous hydro-
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 51 c(VO)
01 X
20
(%)
20 1. 2
10
i
/-
t
I
/
10
0
0.8
0
20
40 W/
Table I. Toluene and p-Xylene Conversion over the Unmodified ZSM-5 feed toluene p-xylene Operation Conditions temperature ("C) 475 47 5 space time (gh/mol) 70.7 8.1 0.6 0.5 2.0 1.3 0.3 0.3 10.6 0.1 72.1 9.2 2.5 0.2 0.3
carbons has been also observed by other authors, being usually assigned to toluene dealkylation, coupled in parallel with the main reaction (Aneke et al., 1979; Bhaskar and Do, 1990). However, in our experiments, the benzene/ xylene ratio approaches 1at low toluene conversions; i.e., selectivity toward toluene disproportionation tends to loo%, which suggests a relationship between the presence of xylene and the extension of dealkylation. In order to clarify this point, additional catalytic runs were carried out at the same temperature (475 "C)and several space times feeding pure p-xylene. A typical product composition has been summarized in Table I, whereas Figure 3 shows p-xylene conversion and the product distribution versus the space time. It can be observed that p-xylene undergoes isomerization to give nand o-xylene but, at the same time, a high proportion of toluene and gaseous hydrocarbons are produced which shows that p-xylene is dealkylated to a great extent. Likewise, it is remarkable the low benzene concentration even at high conversions. Since some benzene has to be produced by toluene disproportionation, it can be proposed that toluene dealkylation is negligible. This conclusion has also been supported by further kinetic analysis: if toluene dealkylation is included, a negative and low value is obtained for the corresponding kinetic constant. Therefore, two reactions have been considered in the kinetic study of toluene disproportionation over the unmodified ZSM-5 2T ?r? B + X toluene disproportionation xylene dealkylation
X
+
T
+ GH
2
4
6
a WIFpx, (g.h/md)
5, (4. h /mol)
Figure 2. Toluene disproportionation over unmodified ZSM-5 (T = 475 "C): (0)toluene conversion (C); (0) benzene/xylene molar ratio ( B I X ) .
Product Distribution (mol W ) 0.5 methane 0.2 ethane ethylene 0.5 0.2 propane propene 14.2 benzene 72.9 toluene ethylbenzene 3.7 p-xylene 5.5 m-xylene 2.1 o-xylene ethyltoluene 0.1 trimethylbenzene 0.1
lo 0
0
60
Figure 3. p-Xylene conversion over unmodified ZSM-5 (T= 475 OC): (0) p-xylene conversion; ( 0 )m- + o-xylenes; (A)toluene; (0) benzene.
where T, B, X, and GH denote toluene, benzene, xylene, and gaseous hydrocarbons, respectively. Figure 1shows the three different mechanisms proposed for toluene disproportionation over zeolites. The dissociative mechanism (M2) involves in the first step the toluene adsorption on an acid site which leads to the This species formation of a methyl carbocation (CH,+). is known by its high instability and hence in many cases should lead to the formation of gaseous hydrocarbons instead of the subsequent alkylation step. Then, toluene dealkylation should be also observed. Since we have concluded above that this reaction does not happen, the dissociative mechanism has to be discarded. Otherwise, in the diphenylalkane mechanism, the formation of this voluminous intermediate may be hindered by a restricted transition-state selectivity because of the ZSM-5 structure without large cavities and with narrow 10-ring channels. Furthermore, the different kinetic equations based on this model which have been proposed earlier (Gnep and Guisnet, 1981; Dooley et al., 1990) include the hydrogen partial pressure, species involved in two steps of the M3 mechanism. Taking into account that our experiments have been carried out in the absence of hydrogen, we have considered the alkyl-transfer mechanism (Ml) more likely for toluene disproportionation over ZSM-5 under our operating conditions. Kinetics and Models. Since the experimental data have been obtained in an integral reactor, the different kinetic models tested include the rate of the reverse reaction in agreement with the reversible nature of toluene disproportionation. Likewise, in all cases, it has been supposed that the secondary xylene dealkylation is an irreversible reaction with pseudo-first-order kinetics. Taking into account the alkyl-transfer mechanism, three Langmuir-Hinshelwood rate expressions have been derived assuming that each one of the mechanism steps is ratecontrolling: toluene adsorption on an acid site
surface reaction between two toluene molecules, one adsorbed and the other in the gaseous phase, to give xylene (adsorbed) and benzene (gaseous phase)
52 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
xylene desorption of the acid site
Likewise, different pseudohomogeneous models have also been tested corresponding to first- and second-order rates: TD = k D ( P T n l - P B ~ ~ P x ~ ~ / K D ) (4) The fit between the experimental data and the kinetic models was performed using a computer program which combines a fourth-order Runge-Kutta method to integrate the rate expressions with a nonlinear regression method based on Marquardt's algorithm to estimate the kinetic and adsorption constants. These parameters were calculated by minimizing the objective function
F=C l=l
[(Xi)calc
-(x~)obs~~
Table 11. Toluene Disproportionation over Unmodified ZSM-5. Fitting of the Kinetic Models homogeneous models nl n7 n, F (90) 1 0 1 1.540 18.8 1 1 0 1.360 17.6 1 1 1 0.383 9.3 2 1 1 0.217 7.1 2 0 1 0.204 6.8 2 1 0 0.197 6.7 heterogeneous models F f (%) toluene adsorption 0.065 4.0 surface reaction 0.075 4.3 xylene desorption 0.072 4.2
t
(5)
(XI)&'
with (XJcalcand (Xl)obsbeing the molar fraction of the different species, calculated and observed, respectively. The average relative error (4 in the reproduction of the product distribution and the value of the objective function obtained with the different kinetic models tested have been summarized in Table 11. It can be seen that the heterogeneous models provide a much better fit of the experimental data than the pseudohomogeneous models. The best fit is achieved with the expression rate deduced by assuming toluene adsorption as the rate-limiting step, although from a statistical point of view it is difficult to establish differences between this model and that which considers the rate controlled by the xylene desorption. Nevertheless, in the three heterogeneous models, a strong correlation between the toluene and xylene equilibrium adsorption constants were observed; i.e., infinite combinations of them led to the same value of the objective function. In order to obtain reliable values of the parameters, the heterogeneous models were simplified by considering that both constants are the same. This simplification almost did not change the fit to the experimental data, the best correlation being again obtained with the model corresponding to toluene adsorption as the ratelimiting step. The values calculated for the parameters included in this model are kD = 0.0047 mol/(g.h.atm)
kd = 0.0070 mol/(gh-atm) KT = Kx = 9.72 atm-' (ii)Para-SelectiveToluene Disproportionationover Si-Mg Modified ZSM-5. Catalyst. The catalyst used in this section was a ZSM-5 zeolite modified by impregnation with silicon and magnesium compounds (SiMg/ ZSM-5). The silicon modification was carried out using a dimethylsilicone polymer as precursor, which, due to its large molecular size, cannot prenetrate into the zeolite channel system, being deposited on the external surface of the crystah and deactivating the nonselective acid sites. In the other hand, the size and nature of the precursor used in the magnesium modification allow this agent to be located inside the zeolite pore, leading to an enhancement of the diffusional limitations. The contributions of both modifier agents are necessary to obtain an optimum relationship between para selectivity and catalytic activity, their contents in the catalyst having been chosen in a previous work (Serrano, 1990) by means of three factorial
0 - 1
t 40 Go
I 0
1 100
200
3 00
1.2
4 00
W/F,,
( g h/rnol)
Figure 4. Toluene disproportionation over modified SiMg/ZSM-5 (2' = 510 OC): (0) toluene conversion (C);(0) benzene/xylene molar ratio ( B I X ) ;(A)para selectivity (Ps).
designs of experiments which included the effect of the operation conditions (space time, temperature, and toluene partial pressure). Reactions and Mechanism. The kinetic runs over the SiMgf ZSM-5 catalyst were carried out at three temperatures (470, 510, and 550 "C)and different space times. Figure 4 shows toluene conversion, benzenelxylene molar ratio, and para selectivity versus space time for the experiments at 510 "C. It can also be observed that in the modified catalyst the benzenelxylene molar ratio deviates from 1 as the space time increases: the dealkylation reaction takes place only if there are xylenes in the reaction mixture. The high para selectivity exhibited by this catalyst due to the shape-selectivity enhancement caused by the Si and Mg modifications can be noted. Likewise, the para selectivity is increased at lower space times, being remarkable that a value of 100% is obtained by space time approaching zero. This finding shows that p-xylene is the only xylene isomer which can be detected just outside the channel system of the modified zeolite. The internal xylene isomerization is completely controlled by the intracrystalline diffusion due to the contraints caused by the modifier agents. Although m-and o-xylenes may be initially formed on the active sites, they diffuse very slowly through the modified channel system, p-xylene being the unique isomer able to leave the zeolite. Therefore, the further decline on para selectivity with the space time can only be explained by the p-xylene isomerization over the acid sites still remaining on the external surface of the zeolite even after the Si and Mg modifications, whereas the isomerization by reentry into the channel system cannot be considered. According to this, the system is described by the following reactions: overall toluene disproportionation to benzene and pxylene, including in an unique step the toluene conversion into the initial product and the subsequent transformation
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 53 of this into the primary product as a consequence of the diffusion-isomerization coupling (PDk
2T eB
+ PX
parameter
p-xylene dealkylation
kD kd
(rd)cb
PX-TTGH
KT Deif kl
p-xylene isomerization
PX
MX
The two first reactions take place mainly within the channel system of the zeolite; hence their kinetics have to be described by taking into account that the intrinsic reaction rates can be influenced by the hindered toluene and p-xylene diffusion through the modified channel of the SiMg/ZSM-5. However, as it has been concluded, p-xylene isomerization is carried out on the external zeolite surface which implies that its rate is not altered by the diffusional steps. Kinetic Model. In agreement with the mechanism and reactions proposed, the rates of toluene disappearance and benzene and p-xylene formation are given by
(7)
where the observed reaction rates are related with the intrinsic rates by means of the correspondingeffectiveness factors: = ?DrD
(9)
(rd)oba = vdrd
(10)
(rD)obs
Table 111. Para Selective Toluene Disproportionation over SiMg/ZSM-5. Parameters of the Kinetic Model
The intrinsic toluene disproportionation rate has been described using kinetic equation 1 obtained in the first section of this work, whereas p-xylene dealkylation and isomerization have been considered irreversible first-order reactions. Although xylene isomerization is known to be reversible, the former assumption is valid for our experiments over SiMg/ZSM-5 because in all experiments very low concentrations of m-and o-xylenes were obtained, as a consequence of the high para selectivity observed for this catalyst. The effectiveness factors can be related with the different kinetic parameters and reaction conditions using the generalized expression proposed by Froment and Bischoff (1979):
where co may be assumed to be zero (irreversible reactions) or the equilibrium concentration (reversible reactions) in the presence of strong diffusional limitations. If eq 11 is applied to the intrinsic rate expressions of toluene disproportionation and p-xylene dealkylation, a mathematic model is obtained which allows one to calculate the corresponding effectiveness factors. This model has been summarized in the Appendix. In order to avoid including an excessive number of parameters, the kinetic model has been simplified assuming
preexponential factor 4280 mol/(g.h-atm) 3.5 X IO7 mol/(gh.atm) 1.45 X IO-* atm-' 2.44 X IO-' mz/h 74.9 mol/ (gheatm)
activation energy (kJ/mol) 99 176 -54 57 69
-
that the diffusion coefficients of toluene and p-xylene have approximately the same value (DT,eff DpX,eff),a supposition based in the same minimum molecular diameter of both compounds. The fit between the model and the experimental data have been performed using simultaneously the results obtained at the three different temperatures, the regression and integration methods being the same as in section i. The influence of temperature was described using the Arrhenius equation for the kinetic constants and the diffusion coefficient, whereas the van't Hoff equation was used for the equilibrium adsorption constants. The values obtained for the different parameters of the model are summarized in Table 111. The activation energy calculated for toluene disproportionation is close to the values reported by other authors also over ZSM-5 (Beltrame et al. (1985), 85 kJ/mol; Bhaskar and Do (1990), 100-121 kJ/mol). This fact shows that the kinetic model developed here has been able to isolate the effects of the chemical and physical steps on the overall reaction rate. In contrast with the value reported by Olson and Haag (1984) for the ratio kI/kD over unmodified ZSM-5 (-7000), from Table I11 values close to 2 are obtained for the Si and Mg modified sample. It is concluded that both modifications have sharply reduced the p-xylene isomerization rate, whereas their effects on the toluene disproportionation rate have not been so important. Also remarkable is the high activation energy obtained for p-xylene dealkylation, which shows how this secondary reaction is very favored by the temperature increase. The relatively high activation energy calculated for the effective diffusion coefficient of toluene and pxylene suggests that the diffusion of these compounds through the modified channel system is a process strongly dependent on the temperature: the increase of this variable allows toluene and p-xylene molecules to have the necessary energy to save the constraints imposed by the modifier agents. Finally, the value obtained for the equilibrium adsorption constant has been checked to satisfy the thermodynamic requirements postulated by Mears and Boudart (1966)and Vannice et al. (1979). Parts a-c of Figure 5 show a comparison between the experimental toluene, benzene, and p-xylene molar fractions and those calculated with the kinetic model. A very good fit can be observed with an average relative error of 2.8%. Thus, the proposed kinetic model can describe the maximum obtained in the p-xylene molar fraction at 510 and 550 "C with the increase of space time. Conclusions When toluene disproportionation is carried out over ZSM-5 in the absence of hydrogen, a benzene/xylene molar ratio higher than 1 is observed in the product due to the secondary reaction of xylene dealkylation, toluene dealkylation being negligible in the temperature range studied. For the unmodified ZSM-5, heterogeneous models based on the alkyl-transfer mechanism allowed a much better fit of the experimental results than first- and second-order
54 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
= toluene and p-xylene effective diffusion coefficient, respectively, m2/h kD = rate constant of toluene disproportionation, mol/(g hsatm) kd = rate constant of xylene dealkylation, mol/(gh.atm) k I = rate constant of p-xylene isomerization, mol/(gh-atm) KD = thermodynamic equilibrium constant of toluene disproportionation KT, Kx,KPX= equilibrium adsorption constants of toluene, xylene, and p-xylene, respectively, atm-' L = catalyst diffusional half-width, m PB,PT,Px,PpX= partial pressure of benzene, toluene, xylene, and p-xylene, respectively, atm P T , ~Ppx:, , = partial pressure of toluene and p-xylene corresponding to the equilibrium composition of toluene disproportionation, atm rD, rd, rI = intrinsic reaction rates of toluene disproportionation, p-xylene dealkylation, and p-xylene isomerization, respectively, mol/ (gh) (rD)obs,(rd),bs = observed reaction rates of toluene disproportionation and p-xylene dealkylation, respectively, mol/ (gh) rv = reaction rate referred to the catalyst volume, mol/ (m3.h) R = constant of the ideal gas law, atmm3/(mobK) R g , R T , RPX = formation rate of benzene, toluene, and pxylene, respectively, mol/ (gh) XB,X T , Xpx = molar fraction of benzene, toluene, and pxylene, respectively Greek Symbols t = average relative error 4 = observable Weisz modulus q = effectiveness factor qD, qd = effectiveness factors for toluene disproportionation and p-xylene dealkylation, respectively pp = catalyst density, g/m3 n = global pressure, atm DT,eff, DyX,eff
100
1
I 0
0
100
200
300
400
c)
m
WIF,, (g h i m o l )
Figure 5. Comparison between the observed (0,A, 0)and calculated (-) product distributions over modified SiMg/ZSM-5 toluene (a), benzene (b), and p-xylene (c); (0) 470 OC, (A)510 "C, and (0) 550 "C.
Appendix The following assumptions have been carried out to obtain the expressions of the effectiveness factors from eq
pseudohomogeneous models, the best concordance being obtained with the rate expression corresponding to toluene adsorption as the rate-limiting step. When a ZSM-5 modified with Si and Mg compounds is used as the catalyst, p-xylene is the only xylene isomer just outside the channel system; hence the further decline on para selectivity with the space time has been assigned to p-xylene isomerization over the acid sites still remaining on the external zeolite surface after both modifications. The expression rate obtained for toluene disproportionation over the unmodified ZSM-5 has been applied to the data of the modified ZSM-5, taking into account the rates of the intracrystalline toluene and p-xylene diffusion and the presence of nonselective external acid sites. The kinetic model developed fits very well the experimental product distribution and can be applied at high toluene conversions since the reversibility of toluene disproportionation is taken into account. The relatively high activation energy calculated for toluene and p-xylene effective diffusion coefficient shows the great influence of the temperature on the aromatic hydrocarbon diffusion through the modified channel system of ZSM-5.
(a) Deffdoes not depend on the concentration. (b) Mole concentration (ci) has been related to the using the ideal gas law. partial pressure (Pi) (c) There are strong diffusional limitations hence the concentration in the center of the zeolite crystal (c,) is assumed to be the equilibrium concentration. (d) The shape of the zeolite crystal is considered spherical. With these assumptions and with the introduction of the intrinsic rate expressions, the integration of eq 11 leads to effectiveness factor for toluene disproportionation
Nomenclature c = concentration, mol/& c, = concentration in the external surface of the particle, mol/m3 c, = concentration in the center of the particle, mol/m3 Deff= effective diffusion coefficient, m2/h
11:
(2~T,ef&D/(RTpp))''2
ID =
LrD
(12)
(I)1'2
being
I, =
I, =
[In 2A 1
(APT2
+ BPT + C)- -BI 3 ] 2A
p~ Pr..
(15)
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 55
(19)
effectiveness factor for p-xylene dealkylation
Registry No. PhCH,, 108-88-3; CH3CsH4-p-CH3,106-42-3; CHI, 74-82-8; CZHB, 74-84-0; CHznCHz, 74-85-1; C3H8, 74-98-6; CHZ=CHCH,, 115-07-1; P h H , 71-43-2; E t P h , 100-41-4; CH3CBH4-rn-CH3, 108-383; CH3C6H4-o-CH3,9547-6; ethyltoluene, 25550-14-5; trimethylbenzene, 25551-13-7; magnesium acetate, 142-72-3.
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Received for review April 20, 1992 Revised manuscript received August 28, 1992 Accepted September 30, 1992
