Optimal batch distillation control based on specific measures

Abstract

Two independent measures characterise a single batch distillation run: the degree of separation difficulty, which indicates the difficulty at the start and the degree of exhaustion, which indicates bottom exhaustion at the end of the run. If one of both measures remains within bounds, then constant quality control appears to be the best control policy. It has been proven in the literature that the application of slop recycling increases the production rate. One of the goals of this study is to derive simple scheduling models based on specific measures for the optimal operation of batch distillation with slop recycling. Simulation studies for a cyclic pseudo-steady state operation over a broad range of degrees of difficulty for binary and ternary distillations are performed. Also the influence of the tray hold-up has been studied. All simulations show that at maximum production rate the degree of separation difficulty balances the degree of exhaustion (dependent variable in the optimum). The slop recycling strategy keeps the degree of exhaustion during the production phase and the degree of difficulty during the slop phase within bounds. As a result, the improvement of slop recycling at constant quality control compared to constant reflux control is 15–20% for difficult separation and can amount to more than 35% for relatively easy separations. The resulting production time appears to have a linear relation with the average separation difficulty. This relation has been experimentally verified.