Alkylation of Benzene by Propylene to Cumene 6

6.2 Reaction-Engineering Analysis 181Figure 6.4 The variation of selectivity with the molar ratio benzene/propylene.6.2.4Chemical EquilibriumChemical equilibrium indicates that more than 99% conversion of propylene maybe achieved for benzene/propylene ratios larger than three. However, the selectivityremains a problem. Figure 6.4 shows the variation of selectivity defined ascumene formed per mole of propylene, when only di - isopropylbenzene is thebyproduct. Increasing the ratio from 3 to 9 moles gives a significant selectivityimprovement from 82% to over 92%. From this point of view the performance ofbeta - zeolites reported in Table 6.6 seems to achieve its thermodynamic limit.Higher temperature is beneficial for getting higher yield, but the effect islimited.6.2.5KineticsThe examination of patents reveals that the operation conditions for the alkylationof benzene with propylene are temperatures between 150 and 230 ° C and pressuresbetween 25 and 35 bar. The catalyst productivity expressed as WHSV is in therange 1 – 10 (based on the reaction mixture) at benzene/propylene molar ratiosranging from 5 to 8.As mentioned, from the reaction kinetics viewpoint the behavior of zeolite catalystsshows large variability. In addition, the apparent kinetics can be affected bypore diffusion. The compilation of literature revealed some kinetic equations, buttheir applicability in a realistic design was questionable. In this section we illustratean approach that combines purely chemical reaction data with the evaluationof mass - transfer resistances. The source of kinetic data is a paper published byCorma et al. [7] dealing with MCM - 22 and beta - zeolites. The alkylation takes placein a down - flow liquid - phase microreactor charged with catalyst diluted with carborundum.The particles are small (0.25 – 0.40 mm) and as a result there are nodiffusion and mass - transfer limitations.