Alkylation of Benzene by Propylene to Cumene 6

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196 6 Alkylation of Benzene by Propylene to CumeneFigure 6.14 Catalytic distillation column for cumene synthesis.recovery viewpoint the pressure should be selected to get the highest possibletemperature in top. From the reaction viewpoint the temperature should favorboth the reaction rate and the desired selectivity. Better selectivity is obtained atlower temperature (see Table 6.6 ). The above aspects are contradictory and a compromisehas to be found.If, from the heat - saving viewpoint the advantage of reactive distillation overfixed - bed reactor technology seems obvious, from reaction - engineering viewpointthere are some physical constraints that may be seen as disadvantages. Firstly,when a reactant is highly volatile the driving force for reaction in the liquid phaseis diminished by the vapor – liquid equilibrium. This is the case with the propylene.The reaction rate is about r IPB = kx B Py P / K P , in which P is the column pressure andK P is the VLE constant of propylene, As a result, the concentration of the propylenein liquid at the operating pressure (11 bar) is about an order of magnitude lowerwith respect to the fully dissolved gas at higher pressure (25 bar). Secondly, theflow of reactants and products in the countercurrent favors the formation of secondarypolyalkylation species with respect to a cocurrent PFR. This negative effecton selectivity is amplified by the fact that in a RD setup the amount of benzene islimited by the optimum reflux rate needed for heat integration.Therefore, adopting the solution of reactive distillation instead of separate reactionand separation units does not lead automatically to a more efficient process.Matching the conditions of separation and reaction in the same device requirescareful design. The element with the highest impact is the chemical reaction. Thekey condition for an efficient and competitive process by reactive distillation is theavailability of a superactive catalyst capable to compensate the loss in the drivingforce by phase equilibrium, but at the same time ensuring a good selectivitypattern.Figure 6.15 presents a compact flowsheet based on catalytic distillation, as simulatedwith Aspen Plus TM [9] . Benzene and propylene are fed in countercurrent in
6.7 Reactive Distillation Process 197Figure 6.15 Flowsheet for cumene synthesis making use of catalytic distillation.
Page 5 and 6: 6.2 Reaction-Engineering Analysis 1
Page 11 and 12: 6.3 Reactor/Separator/Recycle Struc
Page 13 and 14: 6.4 Mass Balance and Simulation 185
Page 15 and 16: 6.5 Energy Integration 187of propan
Page 17 and 18: 6.5 Energy Integration 189Figure 6.
Page 19 and 20: 6.5 Energy Integration 191Figure 6.
Page 21 and 22: 6.6 Complete Process Flowsheet 193F
Page 23: equivalent to 1090 kg steam/ton - c
Page 27 and 28: 6.8 Conclusions 199Figure 6.16 Temp

Alkylation of Benzene by Propylene to Cumene 6

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