Novel Heat-Pump-Assisted Extractive Distillation for Bioethanol Purification

TLDR

Bioethanol purification is energy‑intensive due to the need to concentrate diluted fermentation streams and overcome the ethanol–water azeotrope, typically requiring a three‑column distillation sequence that incurs high energy costs. This study introduces a heat‑pump‑assisted extractive distillation process within a dividing‑wall column to address these energy challenges. The process recompresses the ethanol top vapor to over 3.1 bar to drive the side reboiler, uses ethylene glycol as a mass‑separating agent, and is evaluated through Aspen Plus simulations. Simulations show a 40 % reduction in specific energy use—from 2.07 to 1.24 kWh kg⁻¹ ethanol—and a 24 % annual cost savings, despite a 29 % capital cost increase, compared to the conventional sequence.

Abstract

The purification of bioethanol fuel involves an energy-intensive separation process to concentrate the diluted streams obtained in the fermentation stage and to overcome the azeotropic behavior of the ethanol–water mixture. The conventional separation sequence employs three distillation columns that carry out several tasks, penalized by high-energy requirements: preconcentration of ethanol, extractive distillation, and solvent recovery. To solve this problem, we propose here a novel heat-pump-assisted extractive distillation process taking place in a dividing-wall column (DWC). In this configuration, the ethanol top vapor stream of the extractive DWC is recompressed from atmospheric pressure to over 3.1 bar (thus to a higher temperature) and used to drive the side reboiler of the DWC, which is responsible for the water vaporization. For a fair comparison with the previously reported studies, we consider here a mixture of 10 wt % ethanol (100 ktpy plant capacity) that is concentrated and dehydrated using ethylene glycol as mass-separating agent. Rigorous process simulations of the proposed vapor recompression (VRC) heat-pump-assisted extractive DWC were carried out in AspenTech Aspen Plus. The results show that the specific energy requirements drop from 2.07 kWh/kg (classic sequence) to only 1.24 kWh/kg ethanol (VRC-assisted extractive DWC); thus, energy savings of over 40% are possible. Considering the requirements for a compressor and use of electricity in the case of the heat-pump-assisted alternative, it is possible to reduce the total annual cost by approximately 24%, despite the 29% increase of the capital expenditures, for the novel process as compared to the optimized conventional separation process.