Plantwide control: the search for the self-optimizing control structure

Plantwide control is concerned with the structural decisions involved in the control system design of a chemical plant (C.S. Foss, Critique of chemical process control theory, AIChE Journal 19(2), 1973) 209±214; ``Which variables should be controlled, which variables should be measured, which inputs should be manipulated, and which links should be made between them?'' In particular, the ®rst issue about which variables to control has received little attention. It is argued that the answer is related to ®nding a simple and robust way of implementing the economically optimal operating policy. The goal is to ®nd a set of controlled variables which, when kept at constant setpoints, indirectly lead to near-optimal operation with acceptable loss. This is denoted ``self-optimizing'' control. Since the economics are determined by the overall plant behavior, it is necessary to take a plantwide perspective. A systematic procedure for ®nding suitable controlled variables based on only steady-state information is presented. Important steps are degree of freedom analysis, de®nition of optimal operation (cost and constraints), and evaluation of the loss when the controlled variables are kept constant rather than optimally adjusted. A case study yields very interesting insights into the control and maximum throughput of distillation columns. # 2000 Elsevier Science Ltd. All rights reserved.