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

Plantwide control is concerned with the structural decisions involved in th e control system design of a chemical plant (C.S. Foss, Critique of chemica l process control theory, AIChE Journal 19(2), 1973) 209-214; "Which variab les should be controlled, which variables should be measured, which inputs should be manipulated, and which links should be made between them?" In par ticular, the first issue about which variables to control has received litt le attention. It is argued that the answer is related to finding a simple a nd robust way of implementing the economically optimal operating policy. Th e goal is to fmd 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 determi ned by the overall plant behavior, it is necessary to take a plantwide pers pective. A systematic procedure for finding suitable controlled variables b ased on only steady-state information is presented. Important steps are deg ree of freedom analysis, definition of optimal operation (cost and constrai nts), and evaluation of the loss when the controlled variables are kept con stant rather than optimally adjusted. A case study yields very interesting insights into the control and maximum throughput of distillation columns. ( C) 2000 Elsevier Science Ltd. All rights reserved.