Energy-efficient heterogeneous extractive distillation system for the separation of close-boiling cyclohexaneicyclohexene mixture

Cyclohexane and cyclohexene are close-boiling compounds with relative volatility extremely close to unity, thus, separation via conventional distillation is impractical. In this paper, an energy-efficient heterogeneous extractive distillation system using ethylene glycol as heavy entrainer is proposed. By adding ethylene glycol into the system, large liquid-liquid envelope will form. However, the principle of separation is not exclusively the formation and split of heterogeneous azeotrope like that of a typical heterogeneous azeotropic distillation process, but also because of ethylene glycol greatly enhances the relative volatility of cyclohexane over cyclohexene when treated as a heavy entrainer. The proposed heterogeneous extractive distillation column includes a top decanter and no rectifying section. After establishing the base design of the proposed system, energy-saving schemes are also suggested to further reduce the total annual cost and the annual operating cost. The results show that the proposed heterogeneous extractive distillation system with a process-to-process heat exchanger has the predominant benefit of the lowest total annual cost. To compare the optimal result of this design flowsheet with a recently published separation method utilizing two reactive-distillation columns, our proposed design is much more economical with 61.4% savings in total annual cost and 64.6% savings in annual operating cost. (C) 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.