Control of fully heat-integrated pressure-swing distillation with strict pressure manipulation: A case study of separating a maximum-boiling azeotrope with small pressure-induced shift

Abstract

Pressure-swing distillation (PSD) is an alternative configuration used to separate pressure-sensitive azeotropes and avoid product contamination(s) in extractive or heterogeneous azeotropic distillation via the entrainer(s). Full heat integration between high- and low-pressure columns (HPC and LPC) can provide significant energy savings compared to the conventional case, while it also loses the control degree of freedom, trim condenser or reboiler heat load, via the integrated condenser/reboiler exchanger. Typically, the pressure of HPC is left uncontrolled, and additional pressure-compensated control strategies are implemented, which may create safety issues. To address this, the study proposes adding an auxiliary cooling-water-driven condenser to the fully heat-integrated PSD system, providing an extra manipulated variable to regulate the pressure of HPC. Compared to the conventional PSD system, the new modified process could achieve a 26.31% reduction in total annual cost. The research uses the maximum-boiling azeotropic mixture of acetone/chloroform as a case study which makes the distillation system characterized by high reflux ratios and a large recycle flow rate, leading to the unwelcome snowball effect. To achieve effective process control, robust control structures are proposed that rigorously regulate the pressure of the column by manipulating the heat removal in the trim condenser, attenuating or eliminating potential overpressure issues. Product purity and other evaluation indicators, such as product transient deviations, offsets, integral absolute error, and oscillation degrees, are also considered. Results show that the proposed control schemes can efficiently control the pressure of HPC, weaken the snowball effect, and maintain product purity. This study highlights the importance of pressure control for fully heat-integrated PSD systems, particularly for pressure-sensitive azeotropes with small pressure-induced shifts.