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Abstract.
Plantwide control is concerned with the structural decisions involved
in the control system design of a chemical plant (Foss 1973); 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 first issue about which variables to
control has received little attention. It is argued that the answer is
related to finding a simple and robust way of implementing the
economically optimal operating policy. The goal is to find 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 finding suitable controlled
variables based on only steady-state information is presented.
Important steps are degree of freedom analysis, definition 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.