Active Constraint Regions for Optimal Operation of Distillation Columns

Abstract

When designing the control structure of distillation columns, with optimal operation in mind, it is important to know how the active set of constraints changes with disturbances. This issue has received little attention in the literature. This paper applies a procedure presented in an earlier paper, to find how the active constraints for distillation columns change with variations in energy cost and feed flow rate. The production of the most valuable product is maximized by keeping its purity on the minimum allowed, that is, by keeping the valuable product on spec. This is the avoid product giveaway rule, which means that the purity specification of the valuable product is always an active constraint. We find that this rule generally holds for distillation for the common case when the price of the on-spec product is independent of purity. The paper includes three case studies: a single distillation column with constant product prices, a single column where the price of the most valuable product is dependent on purity, and two distillation columns in series. In all three case studies, there is a bottleneck, corresponding to a feed flow rate above which the column(s) cannot operate without breaking constraints.