Design and Control of Ethanol/Benzene Separation by Energy-Saving Extraction–Distillation Process Using Glycerol as an Effective Heavy Solvent

An energy-efficient extraction–distillation process for separating ethanol/benzene is investigated in this paper. Compared with other separation methods (i.e., azeotropic distillation, extractive distillation and pressure-swing distillation), solvent extraction remains to be one of the most preferable separation methods because the separation principle is achieved by their relative solubilities in two different immiscible liquid phases without energy usage. Thus, lower operating cost by utilizing an extraction–distillation separation process is expectable with the finding of an effective solvent. In this work, glycerol as a heavy solvent is used to extract ethanol from benzene in the extraction column. Since the extract phase inevitably contains some benzene and also ethanol is not a stable node in the operating distillation region, two alternative distillation sequences can be considered to obtain ethanol product at the purity specification and also purify glycerol for recycling back to the extraction column. Experimental data from ternary liquid–liquid equilibrium experiments conducted in a previously published paper is used to verify the simulation model used in the extraction column, while the simulation model for other distillation column/flashers in the proposed separation system can be verified by vapor–liquid equilibrium experiments conducted in another published paper. The results show that a significant reduction of 65.1% in total annual cost and 68.3% in annual operating cost can be obtained from the proposed system compared with that of the conventional extractive distillation system published recently in the open literature. Furthermore, dynamic control of our proposed separation system is also investigated based on the results of open-loop and closed-loop sensitivity tests. It is found that both ethanol and benzene products can be maintained at high purity despite having large throughput and feed composition disturbances.