A comparison of process alternatives for energy-efficient bioethanol downstream processing

Ethanol is a biofuel used to reduce fossil fuel consumption and the greenhouse gases emissions associated with road transportation. Bioethanol is obtained from diluted fermentation broths, and various technological solutions are available for its recovery and dehydration prior marketing, usually blended with gasoline (gasohol). This work explores an ethanol purification approach where gasoline is used as a separation agent towards the direct production of gasohol. This alternative replaces part of the purification steps in common ethanol downstream processing, providing significant reductions on the operating costs required to produce bioethanol. Two configurations for the direct manufacture of E10 gasohol are analyzed. Process I incorporates a liquid-liquid extraction stage, while Process II uses an extractive distillation arrangement. The energy requirements of both configurations are compared against each other and with a benchmark process where ethanol dehydration is carried out by molecular sieves (Process MS). The results show that both alternatives are energetically advantageous since they can be optimized to reduce their equivalent energy requirements by 33% compared to Process MS, for ethanol concentrations in the fermentation broths between 2 and 10 wt%. Further improvements could be achieved with a multi-effect distillation system in the ethanol concentration step, allowing total energy savings of 68% relatively to the benchmark, for a fermentation broth containing 5 wt% ethanol. Estimated ethanol separation costs are approximately $0.15/kg ethanol, making these technological alternatives especially attractive for large scale industrial application.