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Modified ZSM-5 Catalyst20temp, °C η3 η4460 0.7781 ± 0.117 2.977 ± 1.10500 0.5355 ± 0.170 3.212 ± 0.59540 0.4470 ± 0.320 3.201 ± 0.97 3. REACTOR MODELING AND OPTIMIZATIONThe parameters affecting the toluene methylation reactor areadjusted so that the process is operable and high p-xyleneselectivity is achieved. Sensitivity analysis shows that highp-xylene selectivity is favored at low temperature, low pressure,and low contact time. Although low pressure is advantageous,a pressure of 3 bar is selected to account for the pressure dropin the reactor and downstream heat exchangers. Typicallythe pressure drop in packed bed is less than 10% of the inletpressure21and the pressure drop in heat exchangers is in therange 0.1−0.5 bar.22Pressure higher than 3 bar lowers thep-xylene selectivity as shown in Figure 4. Aspen Plus optimiza-tion tool21is used to optimize the values of temperature, toluene-to-methanol feed ratio, and space time in the reactor. The sequentialquadratic programming (SQP) algorithm is selected to solve theoptimization problem. The objective function, given by eq 6, ismaximum p-xylene selectivity. It is defined in the optimizationtool input form using a Fortran statement. The toluene feedflow rate is set to 1000 kmol/h while values of catalyst loading,Wcat., and methanol feed flow rate, FMo, are used as adjustablevariables for optimizing values of space time, Wcat./FTo,andmethanol-to-toluene feed ratio, FTo/FMo. A methanol loss to sidereactions of 40% is used as a constraint; it is calculated as shown ineq 7 and input in the optimization tool as a Fortran statement.Table 4 shows the starting values and the results of Aspen Plusoptimization tool. The p-xylene selectivity is increased from58.0% to 97.7% by optimizing the reactor inlet temperature,toluene-to-methanol feed ratio, and space time; the optimumvalues are found to be 400 °C, 2, and 2.5 (g h)/mol, respectively.Furthermore, methanol loss to side reaction is decreased from 82.4% to 40%. The optimized values of temperatureand space time are lower than the starting values, which showsthat the optimization results are in agreement with the sensitiv-ity analysis findings. Based on these results, a downstreamprocess is developed to separate p-xylene from the reactionproducts.Objective: maximize=++× ‐‐‐‐⎛⎝⎜ ⎜⎞⎠⎟ ⎟ SFFFF100ppmopXXXX (6)Objective constraint:=‐×≤p% methanol lossmethanol reacted to form other than xylenetotal methanol reacted100 40 (7)4. PROCESS FLOW DIAGRAM DEVELOPMENTBased on the optimization results of section 3, a process isdeveloped for the production of high purity p-xylene bymethylation of toluene. The Peng−Robinson (PR) equation ofstate has been widely used inmodeling hydrocarbon systems. TheAspen Plus process simulator is used with the Peng−Robinsonproperty method for simulation as it is one of the recommendedproperty methods by Aspen Plus Property Method SelectionAssistant23for hydrocarbons. The gas phase contains water andmethanol in addition to hydrocarbons. For the two polarcompounds, water and methanol, the Peng−Robinson equationof state (PR-EOS) interaction parameters are available in theAspen Property Data Bank.23For modeling liquid−liquidseparation in the decanter, the nonrandom two-liquid (NRTL)property method is used as the Aspen Plus Property Data Bank23has all the interaction parameters among polar compounds (waterandmethanol) and the nonpolar aromatics. RadFrac block is used for modeling distillation columns. The process flow diagram isshown in Figure 5, and the stream table is given in Table 5.Toluene and methanol feeds, at atmospheric pressure andambient temperature, are pumped by PMP-100 and PMP-101instruments pumps and then heated to the reactor temperature of400 °C by HX-100 and HX-101 heaters. At 3 bar and 400 °C,both toluene and methanol are in the gas phase. They are mixedwith recycled methanol and toluene stream S-21 and fed to thereactor as streamS-5. The fresh feeds of toluene andmethanol arespecified using the Aspen Plus design specifications tool so thatthe reactor feed (stream S-5) is a mixture of 1000 kmol/h tolueneand 500 kmol/h methanol. This results in a toluene-to-methanolratio, FTo/FMo, of 2. The reactor product streamS-6 has unreactedtoluene and methanol along with xylenes product and reactionbyproducts such as water, benzene, and gaseous hydrocarbons(GH). Light compounds such as gaseous hydrocarbons areseparated by cooling stream S-6 to 50 °C. The temperature of50 °C is chosen as it results in a significant removal of 85mol% ofGH from stream S-7; furthermore, this cooling can be easilyachieved by cooling water. The cooled stream (S-7), which hascondensed heavy components, is then flashed at a pressure of2 bar in vessel V-100. Flash vessel V-100 separates 85 mol % ofthe light hydrocarbons present in stream S-7. The liquid stream isfed to a decanter vessel, V-101,
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