Control of extractive distillation processes with intermediate impurities removed as a side withdrawal from the extractive distillation column

BACKGROUND: The design and control of extractive distillation (ED) for separating azeotropes has been widely investigated. However, most researches only deal with simple binary azeotropes without considering impurities. In fact, in industrial practices, intermediate impurities can be removed as a side withdrawal from the extractive distillation column. The dynamic control of such processes has not been investigated. Furthermore, there are many heat‐integrated alternatives of basic ED sequence. When considering impurities withdrawn, the dynamic control of these alternatives may face some difficulties. Thus, this article aims to establish the effective control structures of ED processes for separating azeotropes with intermediate impurities, including basic sequence and heat‐integrated alternatives. Ethanol dehydration is selected to be explored, including a basic sequence (BS), a heat‐integrated sequence (HIS) which combines the pre‐concentration column and the entrainer recovery column into one column and the thermodynamic equivalent sequence of the HIS (TEHIS). Overall assessment of the dynamic performances of these sequences is implemented. RESULTS: For all the sequences, the dynamic performance with entrainer recycling stream being proportional to the extractive distillation column (EDC) feed is better than being proportional to the fresh feed. Meanwhile, large negative transient deviations in the water product purity and intensive fluctuations of sensitive‐stage temperatures can be observed for the HIS, but not for the TEHIS. CONCLUSIONS: The TEHIS is the best sequence considering both economics and dynamics. These sequences and the analyses of their dynamic controllability are valuable in practice for multicompoent azeotropes purification. © 2020 Society of Chemical Industry