(398ao) Plantwide Control for Maximum Throughput Operation of an Ester Purification Process

Plantwide control for maximum throughput operation of an ester purification process consisting of a liquid−liquid extractor (LLX), a distillation column, and material recycle is evaluated. The extractor uses water solvent and is necessary to cross a distillation boundary making pure ester product recovery feasible. Extraction capacity is the bottleneck constraint. A steady state analysis reveals the occurrence of steady state multiplicity induced infeasibility for too high fresh feed rates and for too low water feed rates. To avoid infeasibility due to the multiplicity behavior, we recommend using the extractor feed rate as the throughput manipulator (TPM) with the fresh feed rate adjusted as a makeup stream (CS1). The other control loop pairings are conventional. Rigorous closed loop dynamic results show that CS1 is significantly more robust than the conventional control structure (CS2) with the fresh feed rate as the TPM. In particular, CS1 achieves more than 25% higher maximum throughput compared to CS2 for large feed composition changes in specific directions. This is attributed to the self-regulatory nature of the recycle inventory control in CS1. In contrast, CS2 is susceptible to failure due to overfeeding-induced steady state infeasibility. The presentation will highlight the central role of control structure/strategy design for robust plant operation.