Self-optimizing Control Configuration for Two-product Distillation Columns

The choice of control structures for distillation columns is an important issue for practical industrial operation. There is no single “best ” structure for all columns, so some authors feel that each column should be treated independently. Nevertheless, the objective of this work is to find for a structure that is “reasonable ” for all columns. In this paper, we consider the steady-state deviations in product composition, assuming that we only have available flows and temperatures for control. By using local methods, including the exact local method and the minimum singular value rule, we search for two “self-optimizing ” variables, which when held constant result in small deviations in the presence of disturbances. We find that for most columns, a good choice is to keep a constant reflux to feed ratio L/F and keep a constant temperature in the middle of the bottom section of the column. Especially for multicomponent separations, it does not help to control two temperatures