Selection of Control Structure and Temperature Location for Two-Product Distillation Columns

The choice of control structures for distillation columns is important 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 a structure that is ‘reasonable’ for all columns. In this paper, we consider the steady-state deviations in product compositions in response to disturbances, assuming that only flows and temperatures are available for control. For most columns a good choice is to fix reflux and a temperature. For binary separations, the temperature should be located where the temperature slope is steep. For multicomponent mixtures, the same rule applies except that one should avoid column sections with large changes in non-key components, for example at the column end and at the feed. Control of two temperatures is better for some columns, but not all.