(329e) Energy-Saving Heterogeneous Extractive Distillation System for the Separation of Close-Boiling Cyclohexane/Cyclohexene Mixture

Cyclohexene is an important raw material frequently used to synthesize chemicals of high commercial value. However, the manufacturing of cyclohexene usually results in the presence of certain amounts of cyclohexane in the cyclohexene product stream. Therefore, the separation of these two components becomes a major concern in the relevant industrial process. Cyclohexane and cyclohexene are close-boiling compounds with extremely low relative volatility (1.074 at 1 atm). This is the main reason that makes the separation via conventional distillation very impractical. There is a recent study that deals with this separation task via reactive distillation [Yu et al., 2016], with a complex thermally coupled dividing-wall reactive distillation system. Although the total annual cost of this design is significantly reduced compared to the conventional single column process, it can be observed that the whole process is still energy intensive and the number of stages required in this complex structure is quite large.

Extractive distillation is one of the alternatives for the separation of close-boiling compounds. By adding a heavy entrainer into the system, the relative volatility of the original two components is enhanced to a certain degree that makes the separation process practical. In this work, ethylene glycol is selected as the heavy entrainer for the cyclohexane-cyclohexene close-boiling system. Like common extractive distillation systems, the lightest component (in this case, cyclohexane) is mainly drawn out as the distillate of the extractive distillation column, and cyclohexene will go with the heavy entrainer to the column bottoms. A second column is served as an entrainer recovery column to separate cyclohexene from the heavy entrainer (ethylene glycol), which is then recycled back to the extractive distillation column. A decanter on top of extractive distillation column is installed for the further purification of cyclohexane, which is an uncommon design in the extractive distillation process. The reason for this configuration is the introduction of ethylene glycol into the system. According to the cyclohexane-cyclohexene-ethylene glycol ternary diagram, there is a wide region where liquid-liquid phase split will occur. Although the composition of the top vapor from the extractive distillation column approaches cyclohexane, it is still inside the liquid-liquid phase split region. Hence, a decanter is used to perform the phase split to obtain the cyclohexane-rich phase and recycle the ethylene glycol-rich phase back to the extractive distillation column. Considering that the vapor-liquid-liquid equilibrium occurred inside the extractive distillation column, this process should be regarded as a heterogeneous extractive distillation system.

Very few studies have been proposed to illustrate the feasibility of heterogeneous extractive distillations, and the economic analysis of the separation process based on this kind of distillation is still rare in open literatures. In this work, a heterogeneous extractive distillation system for the separation of cyclohexane and cyclohexene is investigated from economical view point as well as operability aspect. Under the same product specifications, the heterogeneous extractive distillation system represents more than 60% savings of the total annual cost compared to the design in Yu et al. (2016).

References:

Yu, J.; Shi, L.; Yuan, Y.; Chen, H.; Wang, S.; Huang, K.; Thermally Coupled Reactive Distillation System for the Separations of Cyclohexane/Cyclohexene Mixtures, Ind. Eng. Chem. Res., 2016, 55, 311-322.