Leveraging Surge Capacity for Sustainable Process Operation: Control Structure

The impact of the material balance control structure on economic and sustainable process operation is evaluated using linear analysis as well as two rigorous case studies on an ester purification process and an acetone manufacturing process. Three alternative control structures with the throughput manipulator (TPM) at the fresh feed (CS1), at the feed to the first major unit operation inside the recycle loop (CS2), and at the bottleneck capacity constraint that limits maximum throughput (CS3) are evaluated. Dynamic results for backed-off operation at a given throughput (Mode I) and maximum throughput (Mode II) limited by a capacity constraint are obtained for realistic disturbances. The plantwide results show that CS3 and CS2, in that order, achieve significant Mode I energy savings (up to 13%) and higher Mode II maximum throughputs (up to 7%) compared to CS1 (conventional structure) as the active constraint control is significantly tighter due to the propagation of plantwide flow transients and nonlinearity out of the recycle loop. The use of supervisory MPC on top of CS1 recovers the economic loss only partially compared to CS2 and CS3 with their TPM inside the recycle loop. Based on the case studies, locating the TPM at the bottleneck constraint inside the recycle loop is strongly recommended.