Optimal batch distillation control based on specific measures

Two independent measures characterise a single batch distillation run: the degree of separation difficulty, which indicates the difficulty at the star t and the degree of exhaustion, which indicates bottom exhaustion at the en d 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 i n the literature that the application of slop recycling increases the produ ction rate. One of the goals of this study is to derive simple scheduling m odels based on specific measures for the optimal operation of batch distill ation with slop recycling. Simulation studies for a cyclic pseudo-steady st ate operation over a broad range of degrees of difficulty for binary and te rnary distillations are performed. Also the influence of the tray hold-up h as been studied. All simulations show that at maximum production rate the d egree 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 recyclin g at constant quality control compared to constant reflux control is 15-20% for difficult separation and can amount to more than 35% for relatively ea sy separations. The resulting production time appears to have a linear rela tion with the average separation difficulty. This relation has been experim entally verified. (C) 1998 Elsevier Science S.A. All rights reserved.