Process optimization of a butane/butene alkylation unit

The paper presents the optimization of an isooctane production unit by butane - butene alkylation, for which a Reactor-Separation-Recycle model of the plant is developed and solved in Matlab. Equipment design is performed and utilities consumption is evaluated. The objective function is the total annual cost (TAC), consisting of capital and operating expenditures. The decision variables are the reactors size, the distribution of raw materials to reactors, the reaction temperature and the component recoveries in the distillation columns. The minimum cost is obtained for a series of reactors operated at the lowest temperature allowed by the coolant availability, reactants fed to the first reactor of the series, and low recoveries of the inert impurities. Use of the TAC as objective function leads to a much lower cost, compared to the optimization aiming recycle minimization. The results of the rigorous process simulation performed in Hysys differ by less than 10% from the prediction of the Reactor - Separation - Recycle model.