On the profit of optimum control in batch distillation

Abstract

Using a profit function, in which the start-up time and several cost factors are taken into account, optimum reflux policies for binary batch distillation processes with negligible hold-up are calculated. The optimum in controlling is compared with the conventional constant-overheads composition and constant reflux policies. Calculations are made for a wide variety of cases. It is possible to systemise the results by introducing two dimensionless numbers. The first number, containing only process parameters, is a measure of the difficulty of a separation. The preference for one of the three reflux policies considered will be found to depend strongly on this number. The second dimensionless number combines the influences of the cost factors and the start-up time. By means of these numbers a simple rule of thumb is formulated, which predicts when optimum control is worth while.