With the aim of saving energy and capital cost, in this work we propose a novel extractive distillation strategy by varying pressure for the separation of pressure-sensitive azeotropic mixtures. Proceeding from the thermodynamic insight of ternary systems in extractive distillations, the considerable energy-saving potential by changing operating pressures is observed. The separation of acetone−methanol with a minimum boiling azeotrope using a heavy entrainer chlorobenzene is chosen as an illustrative case, and it belongs to a scarce classification 1.0-1a-m2 in which the component with higher boiling point is withdrawn as a product. Through the analysis of a ternary residue curve map and isovolatility curves, it could be observed that the minimal amounts of entrainer feed and extractive feasible regions in the ternary diagram are sensitive to pressures. A 3 atm pressure is preliminarily selected for the comparison with atmospheric pressure. The optimal pressure is found through sensitivity analysis. The results showed that 33.9% and 30.1% reductions in energy consumption and total annual cost, respectively, are achieved compared with that operated at atmospheric pressure. The discussion is then carried out by analyzing residue curve maps, relative volatility profile, and extractive efficiency indicators. Finally, heat integration was considered to further reduce costs. The methodology proposed in this work may provide some new theoretical guidance for the design and optimization of azeotrope separations through extractive distillation.

h i g h l i g h t s We optimize an extractive distillation process with preconcentrator column. The case study belongs to the usual 1.0-1a class; min T + E heavy. Lowering the pressure enhances relative volatility in all columns. Lowering the pressure improves energy consumption and reduces costs. Too much entrainer recycle impurity limits distillate purity and recovery.

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There is rich literature on the separation of binary azeotropic mixtures, whereas few studies exist on the separation of biazeotropic ternary mixtures. In this work, we propose a systematic approach for energy-efficient extractive distillation processes for the separation of a biazeotropic mixture that involves thermodynamic insights via residue curve maps and the univolatility line to find the optimal entrainer and operating pressure, global optimization based on a proposed two-step optimization procedure, and double-effect heat integration to achieve further saving of energy consumption. An energy-saving reduced-pressure extractive distillation (RPED) with a heat integration flowsheet is then proposed to achieve the minimum total annual cost (TAC). The results show that the TAC, energy consumption, and exergy loss of the proposed RPED with heat integration are reduced by 75.2%, 80.5%, and 85.8% compared with literature designs.

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