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ZSM-5catalyst is reported by Valverde.20The catalyst used contains1.09 wt % Mg, has 35 wt % sodium montmorillionite as a binder,and has an average particle size of 0.75 mm. The five reactionsinvolved, together with their intrinsic and observed rate expres-sions,20are given in eqs 1−5. The reactions considered are(1) toluene methylation, (2) methanol dehydration, (3) toluenedisproportionation, (4) p-xylene dealkylation, and (5) p-xyleneisomerization, respectively.+→‐+ = prkpp T MXW 11 TM (1)→+ = rkp 2MGH2W 22 M2(2)→+‐ = prkp T /B / X 212133 T (3)‐→ + = ‐prkp XT /GH p 2144 X (4)‐→ ‐+‐ = ‐pmorkp X/(XX)p 2155 X (5)In the above equations, k denotes rate constant and p rep-resents partial pressure. Gaseous hydrocarbons (GH) representlight species (e.g., methane, ethane, ethylene, propane, propene,and butanes) that are produced during methanol dehydrationand p-xylene dealkylation reactions. GH is used to collectivelyrepresent gaseous hydrocarbons in the kinetic study byValverde.20In this study, GH is modeled as pure ethylene as itbalances the stoichiometric equations. In the p-xylene isomer-ization reaction, eq 5, equal amounts of m-xylene and o-xyleneare assumed to be formed as in Sotelo et al.19The reactions givenin eqs 1, 3, and 4 occur inside the pores of the catalyst, andthe effectiveness factors η1, η3, and η4 are used to account fordiffusion limitations.20The kinetic model parameters20are givenin Table 2, whereas the values of the effectiveness factors, η3 andη4 are given in Table 3 for three different temperatures: 460, 500,and 540 °C. The value of effectiveness factor η1 is 0.9820and canbe considered as equal to 1 as a good approximation.Using the kinetic parameters and model of Valverde,20anisothermal packed bed reactor is simulated using Aspen Plus.The simulation results are compared in Figures 1 and 2 with theexperimental data reported in Figures 6 and 7 in Sotelo et al.19The simulation results are found in good agreement with theexperimental data.19The effects of temperature, pressure,space time (weight of catalyst per unit feed flow rate of toluene,Wcat./FTo), and toluene-to-methanol feed ratio, FTo/FMo,ontoluene conversion, xylene selectivity, and p-xylene selectivityare analyzed using the sensitivity analysis feature in Aspen Plus.Conversion, C,isdefined as the percentage of toluene reacted,total xylenes selectivity, Sx, is the percentage of toluene reactedto form xylenes, and p-xylene selectivity, Sp, is the percentage ofp-xylene from total xylenes formed.Figure 3 shows the effect of temperature on conversion C andselectivities Sx and Sp. As the temperature increases, there is anincrease in toluene conversion but xylene selectivity drops anda more significant decline in p-xylene selectivity is noticed.This means that side reactions become more important as thetemperature increases; therefore, high p-xylene selectivity isfavored at low temperature. Figure 4 shows the effect of pressureon C, Sx, and Sp.With an increase in pressure toluene conversionincreases, but there is a drastic drop in p-xylene selectivity; thus,low pressure in the reactor is required to achieve high p-xyleneselectivity. The effects of space time on C, Sx, and Sp are studied.At low values of space time, there is high p-xylene selectivity butwith low conversion, which is consistent with the experimentalresults reported in Figure 4 in Sotelo et al.19The low space timemeans low contact time at the surface of the catalyst whichlessens the unwanted isomerization of p-xylene to m-xylene ando-xylene.8Low space time can be achieved with high toluene feedrate. The effect of toluene-to-methanol feed ratio is also studied,and the results are in agreement with the experimental resultsreported in Figure 5 in Sotelo et al.19With an increase in feedratio, there is not much of a change in selectivity, but there isa decrease in toluene conversion. This result is in agreementwith the experimental results in Figure 5 in Sotelo et al.19Lowtoluene-to-methanol feed ratio results inmoremethanol loss dueto methanol dehydration reaction. It is worth mentioning that ata toluene-to-methanol feed ratio of 1, a temperature of 500 °C,andWcat./FTo = 15 (g h)/mol, about 82%of the reactedmethanolis consumed by side reactions.Table 2. Kinetics of Toluene Methylation over Mg ModifiedZSM-5 Catalyst20parameterpre-exponential factor,Ai, mol/(g•h•atm2) or mol/(g•h•atm)activation energy,Ei, kJ/molk1 403 ± 5 45.7 ± 0.4k2 1346 ± 64 50.6 ± 0.5k3 96.2 ± 1 59.0 ± 0.5k4 0.3815 ± 0.05 19.6 ± 0.7k5 46.94 ± 0.5 48.9 ± 0.3Table 3. Effectiveness Factors for Toluene Methylation overMg
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