Alkylation of Benzene by Propylene to Cumene 6

6.2 Reaction-Engineering Analysis 179Table 6.5 Selectivity obtained with different zeolite catalysts in cumene synthesis [4] .Zeolite - beta Mordenite MCM - 22 Zeolite - YOverall selectivity on propylene (%) 99.87 98.61 98.74 98.30Table 6.6 Selectivity and DIPB distribution at different temperatureand propylene conversions [6] .Catalyst T ( ° C) X propylene a)(%)b)Selectivity (%) Iso/n DIPBs distribution (%)ratioCumene DIPB Oligo ortho meta paraMCM – 22 180 76.05 92.12 7.34 0.32 1650 10 30 6097.97 90.56 9.03 0.27 830 8 32 60220 91.70 90.78 8.84 0.18 790 7 33 6096.28 89.54 9.60 0.11 460 5 38 57Beta 180 76.25 92.16 6.96 0.41 920 6 42 5297.34 90.76 8.33 0.25 900 5 44 51220 89.90 89.34 10.07 0.21 720 5 46 4998.34 88.67 10.58 0.15 460 3 51 46Reactions conditions: total pressure, 3.5 MPa; B/P mole ratio, 7.2. Catalyst with Si/Al ratio about 16.a) Different propylene conversion were achieved by changing the WHSV.b) Selectivity referred to propylene.in cumene around 90%. Other studies prefer MCM - 22 because of better stabilityagainst deactivation [7] . As Table 6.6 shows, the selectivities of zeolite - beta andMCM - 22 are similar in the range of temperature of 180 – 220 ° C and benzene/propyleneratios of 3.5 – 7.2. Modified Y - type zeolites were found capable of selectivityover 97% at lower temperature [8] , and are therefore recommended for catalyticdistillation. Recent patents show that the new superactive zeolite catalysts are suitablefor both alkylation and transalkylation reactions.At quasiequal selectivity the differences in performance of catalysts can be justifiedby the amount of trace impurities produced. Because operating at lower temperatureis more favorable, material efficiency seems to be in contradiction withheat integration.Table 6.7 displays some physical properties of zeolites. A study issued fromindustry [8] demonstrates the significant role of mass - transfer resistances, evenfor small particles below 1 mm, reporting that pore diffusion may decrease theeffectiveness from low to very low values (0.4 to 0.06). The external mass - transferresistance is much less important. In consequence, in commercial operation onlya small part of the catalyst is effectively used, typically less than 10%. Since the