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In this paper, the energy-saving potential of an extractive dividing-wall column (EDWC) is investigated. One potential drawback on the EDWC design is that two reboilers in the original design need to be combined into one reboiler. Since a heavy entrainer is used in the extractive distillation system, often cases show that the total reboiler duty is reduced but with adversely increasing the total steam cost. Three industrial separation systems of isopropyl alcohol-water, dimethyl carbonate-methanol, and acetone-methanol have been used as demonstrating examples for critical assessment of the energy-saving potential of the EDWC design. It is found that although the savings of the overall reboiler duty can be made by using the EDWC design, only the acetone-methanol system using water as an entrainer actually saves on the steam cost. The control performance of the EDWC design is also hampered because of losing one important control degree-of-freedom.

In this paper, design and control of an isopropyl alcohol (IPA) dehydration process via extractive distillation have been investigated. The heavy-boiling entrainer used to aid the separation is dimethyl sulfoxide (DMSO). The design flowsheet includes an extractive distillation column and an entrainer recovery column with the top product of the extractive distillation column to be IPA and the top product of the entrainer recovery column to be water. The bottom product of the entrainer recovery column is the recovered DMSO which is recycled back to the extractive distillation column. The optimal design flowsheet of this complete process has been established showing that the total annual cost and the needed steam cost of this design flowsheet is significantly less than a competing design flowsheet via heterogeneous azeotropic distillation. A very simple overall control strategy has also been proposed which requires only one tray temperature control loop in each column to hold the high-purity specifications of the two products. Dynamic simulations reveal that fixing of the reflux ratio is not a suitable control strategy. Instead, the strategy to fix the two reflux flow rates should be used to reject feed disturbances.

Dimethyl carbonate is an environmentally benign and biodegradable chemical. It can be produced by a transesterification reaction of methanol with ethylene carbonate, coproducing another useful product, ethylene glycol. A reactive distillation column can be utilized for the complete conversion of ethylene carbonate with methanol in excess. The coproduct, ethylene glycol, is the bottom product while the top product of this reactive distillation column is a mixture of dimethyl carbonate and methanol close to the azeotropic composition. In this paper, an economical separation process via extractive distillation is proposed to obtain pure dimethyl carbonate product and also pure methanol to be recycled to the reactive distillation column. A very simple procedure is proposed in this paper for the quick comparison of alternative entrainer candidates before rigorous process simulation is conducted. Aniline is found to be a very effective entrainer to enhance the relative volatility between methanol and dimethyl carbonate. A problem with small heavy-boiler impurities in the feed stream of the extractive distillation process is also pointed out, with a practical solution given for this problem. Simple overall control strategy of this process is also proposed to maintain product purity despite various feed disturbances. Only one tray temperature control loop is required for each of the columns.

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