Abstract :
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?ʹʹ 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-optimizingʹʹ
control. Since the economics are determined by the overall plant behavior, it is necessary to take a plantwide perspective. A sys-
tematic 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 max-
imum throughput of distillation columns.
