OPTIMAL-CONTROL OF BINARY BATCH DISTILLATION IN TRAY OR PACKED-COLUMNS

The minimum time problem for batch distillation in tray or packed columns is solved using Pontryagin's maximum principle. It is assumed that the boiler hold-up is dominant. The model equations are discretized in time using orthogonal collocation. The resulting equations are solved in the order of a few seconds using an IBM 3033. The results for tray columns with reflux ratio as the control variable show an improvement of up to 5% over constant reflux operation, in agreement with earlier work. The results for packed columns with constant mass transfer coefficients show a similar improvement when either the reflux ratio or the boil-up are controlled optimally. When both variables are controlled optimally a time saving of 10% is obtained compared with conventional operation with constant controls.