Alkylation of Benzene by Propylene to Cumene 6

6.5 Energy Integration 187of propane with less than 100 ppm benzene: 16 theoretical stages with feed on 5and a temperature of 150 ° C, and a reflux of 4300 kg/h.The recycle column (C - 2) is characterized by a very large distillate/feed ratio.Therefore, the design should minimize the reboiler duty. In addition, the pressureshould be low enough to avoid excessive reboiler temperature. High recovery ofbenzene in top is desirable (over 99.9%) but small amounts of cumene are tolerated.In a first attempt we consider a column of 25 stages operated at atmosphericpressure, with feed location in the middle and a reflux ratio of 0.26.The column (C - 3) for cumene distillation operates under vacuum to avoid anexcessive bottom temperature. A number of 30 stages and a reflux ratio of 1.2 aresufficient to ensure good - purity cumene with less than 100 ppm benzene.In order to focus on the main issues of process integration, we disregard thedistillation column for heavies, as well as the transalkylation section. A preliminarysimulated flowsheet in Aspen Plus [9] is shown in Figure 6.8 , with values of temperatures,pressures and heat duties. The fresh feed of propylene is 110 kmol/h.Note that design specifications are used for the fine tuning of the simulation blocks.The fresh benzene is added in the recycle loop as makeup stream so as to keepthe recycle flow rate constant. This approach makes the convergence easier.6.5Energy IntegrationBased on the preliminary flowsheet a table of streams for heat integration are built(Table 6.10 ). On this basis a pinch - point analysis can be done by using a specializedsoftware, such as SuperTarget [10] . In this way, targets for energy by can bedetermined for ∆ T min of 10 ° C, as shown in Figure 6.8 . The minimum energyrequirements are Q h = 9143.4 kW and Q c = 11 063.9 kW. By taking advantage onlyfrom process/process heat exchange a saving in heat up to 43% in hot utility and40% in cold utility can be achieved. Since the reaction is highly exothermal, weexpect possible export of energy too.Table 6.10 Hot and cold stream table.Cold streamsHot streamsName T s ( ° C) T t ( ° C) Duty (kW) Name T s ( ° C) T t ( ° C) Duty (kW)Reactor_in 72 170 3 657.5 Reactor_out 236.6 150.0 − 3 781.4Feed_C - 2 197.9 90.0 − 4 116.9Reb_C - 1 198.9 197.9 2 318.1 Cond_C - 1 36.3 35.3 − 399.1Reb_C - 2 162.1 163.1 7 870.8 Cond_C - 2 83.4 82.4 − 7 572.9Reb_C - 3 161.1 162.1 1 979.6 Cond_C - 3 99.3 98.3 − 2 413.6Total 15 826.6 − 18 283.9