A COMBINED MILP AND LOGIC-BASED APPROACH TO THE SYNTHESIS OF OPERATING PROCEDURES FOR BATCH PLANTS

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Abstract
Synthesizing a set of operating procedures for the safe and efficient transient operation of chemical plants is a difficult problem owing to the enormous number of possible combinations of actions in a typical plant. In most current industrial plant design practice, there are no formal methods for systematically transforming process specifications into operating procedures for the plant operators and into sequence control instructions for the control computers. There is much scope for a formalized computer-based procedure synthesis methodology to assist the design engineer/plant operator with both the formulation and assessment of procedures off-line and, eventually, with the on-line problem of procedure synthesis in response to unexpected situations A recently developed approach for operating procedure synthesis for multipurpose batch plants is considered. The modelling formalism used includes the separate definition of process operations, as State Task Networks, and of physical plant, at the level of detail of a piping and instrumentation diagram. In this paper, a subgoaling procedure is developed using the State Task Network representation which decomposes the procedure synthesis goals into simpler subgoals by means of an efficient Mixed Integer Linear Programming (MILP) technique. Detailed control sequences are then generated for each subgoal using a set of rules and algorithms specific for each type of subgoal. The procedure sequences thus generated are validated by simulation on a plant model with checking of physical and operational constraints at each new plant state. We have found that this hierarchical approach to the procedure synthesis problem greatly reduces the problem complexity The usefulness of the general approach and of the subgoaling procedure in particular are demonstrated through a multiproduct batch plant example.