Self-optimizing control of fully electrified heat pump assisted distillation process by flash vapor circulation

Abstract

A fully electrified flash vapor circulation (FVC) distillation system for methanol/water separation is investigated, with a focus on identifying suitable self-optimizing controlled variables for minimizing electric power consumption. Starting from two published schemes (CS1 and CS2), four new control structures (CS3 – CS6) are developed to explain and remove the strongly asymmetric closed-loop behavior observed in the original designs. Closed-loop dynamic simulations are performed under ± 20% step disturbances in throughput and methanol composition. Control performance is assessed using product quality deviations, energy indicators, settling time, and integrated absolute error. The results show that controlling compressor discharge pressure in CS1 and CS2 is a poor choice because it over-constrains the series pressure loop of the FVC configuration, which drives excessive recycle, asymmetric responses, and COP deterioration. Eliminating this redundant pressure constraint in CS3 restores the available degrees of freedom and yields more symmetric behavior with lower compression power. Building on self-optimizing control, CS4 and CS5 select condenser outlet conditions as controlled variables to coordinate the trade-off between pressure lift and circulating vapor flow, giving similar energy performance. Finally, CS6 adds a distillate composition controller on top of CS4 to enforce the methanol specification and avoid both under and over purification, with only longer transients due to the slower composition loop. To generalize these findings, a three-step workflow is proposed as a transferable design procedure for heat pump assisted distillation systems.