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An extractive dividing-wall column (EDWC) is simulated as a three-column system in this work. The design and control of the EDWC to separate ethyl acetate and isopropyl alcohol using ethylene glycol as the entrainer is studied in Aspen Plus and Aspen Dynamics. The optimum EDWC design with minimal total annual cost is screened first; then sensitivity analysis is conducted to investigate whether the entrainer flow rate and vapor split ratio can be used as control variables to hold the purity specifications; finally, three control structures for the EDWC are established, and the dynamic results of these control structures are compared. It is found that two improved control structures achieve satisfactory performance with much smaller deviation and shorter settling time by comparison to the basic control structure. Dynamic results also revealed that it is useful to adjust the entrainer flow rate or vapor split ratio to hold the purity specifications.

Benzene and cyclohexane (CYH) can be separated via extractive distillation using sulfolane (SULF) as entrainer. In this Article, the steady state of an extractive distillation system has been simulated using the RadFrac model in Aspen plus, and the controllability of this system with feed flow rate and feed composition disturbances has been studied in Aspen Dynamics. Sensitivity analysis is used to obtain optimal operation conditions. The common control scheme for extractive distillation is infeasible. Two control structures are demonstrated in this Article. Each control structure has three main temperature controllers, two in the extractive distillation column and one in entrainer recovery column. Although both control structures can solve all disturbance issues and maintain the product purities very close to the set points, the dynamic responses of initial control structure have large transient deviation when +20% feed flow rate disturbance is introduced, while the improved control structure can overcome this obstacle.

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