Design and Control of the Cumene Process

The chemistry of the cumene process features the desired reaction of benzene with propylene to form cumene and the undesirable reaction of cumene with propylene to form p-diisopropylbenzene. Both reactions are irreversible. Since the second has a higher activation energy than the first, low reactor temperatures improve selectivity of cumene. However, low reactor temperatures result in low conversion of propylene for a given reactor size or require a large reactor for a given conversion. In addition, selectivity call be improved by using an excess of benzene to keep cumene and propylene concentrations low, but this increases separation costs. Therefore, the process provides all interesting example of plantwide economic design optimization in which there are many classical engineering trade-offs: reactor size versus temperature, selectivity versus recycle flow rate, and reactor size Versus recycle flow rate. Design optimization variables affect both energy costs and capital investment. They also affect the amount of reactants required to produce a specified amount of cumene product. The economic effect of reactant Consumption is very large, an order of magnitude greater than the impact of energy or capital. The process is presented in the design book by Turton et al. (Analysis, Synthesis and Design of Chemical Processes, 2nd ed.; Prentice Hall: Saddle River, NJ, 2003) and consists of a cooled tubular reactor and two distillation Columns. The liquid fresh feeds and the benzene recycle stream are vaporized, preheated, and fed into the vapor-phase reactor, which is cooled by generating steam. Reactor effluent is cooled and fed to the first Column that produces a distillate stream of benzene that is recycled back to the reactor. The second column separates the desired cumene product from the undesired p-diisopropylbenzene. The purpose of this paper is to develop the economically optimum design considering capital costs, energy costs, and raw material costs and then to develop a plantwide control Structure capable of effectively handling large disturbances in production rate.