Plantwide Control for Economic Operation of a Recycle Process

Plant-wide control system design for economically optimum operation of a recycle process with side reaction is studied. The process consists of a liquid phase CSTR followed by two simple distillation columns. The exothermic irreversible reactions A + B → C (main reaction) and C + B → D (side reaction) occur in the CSTR. The reactor effluent is distilled in the recycle column to recycle the light reactants (A and B) back to the CSTR. The column bottoms is further distilled in the product column to produce nearly pure C as the overhead product with side-product D leaving from the bottoms. For a base-case design, the steadystate operating degrees of freedom are optimized to maximize operating profit for two modes of operation – Mode I: Given fresh A feed rate and Mode II: Maximum through-put. The set of active constraints at the economic optimum significantly simplifies the plant-wide control design problem by forcing structural decisions for process operation close to and where possible, at the active constraints. The economic performance of the control structure so synthesized is compared with other reasonable regulatory structures with and without a supervisory optimizing constraint controller. Quantitative process operation back-off results show that the incorporation of economic considerations in plantwide control system design can significantly improve profitability.