(120d) Reducing the Energy Demand of Cellulose Based Fuel Ethanol Through Salt Extractive Distillation Enabled by Electrodialysis

The Renewable Fuels Standard (RFS2), established under the Energy Independence and Security Act (EISA) of 2007, mandates the production of 36 billion gallons/year of renewable fuels in the U.S. in 2022, out of which 16 billion gallons/year must be produced from cellulosic sources. The fermentation based corn-to-fuel ethanol process is widely used to produce fuel ethanol in the U.S. While fermentation can be used to produce fuel ethanol from cellulosic sources, the concentration of ethanol in the fermentation broth may be less than 5 wt% compared to 10-15 wt% in case of corn-ethanol. Similar to corn-ethanol production, distillation can be used for separating and concentrating ethanol from fermentation broth followed by molecular sieve adsorption to achieve fuel grade purity, but the separation energy demand increases due to the drastic increase in distillation energy demand for ethanol concentrations less than 5wt%. 25,100 Btu/gal for 5 wt% ethanol compared to 15,100 Btu/gal for 15 wt% ethanol has been reported.¹ Dilute ethanol from the fermentation broth can be separated and concentrated aided by salt extractive distillation to directly produce fuel ethanol leading to significant energy savings. Techniques other than evaporation and drying for recovering salt, which is used to facilitate distillation, have rarely been considered. Since evaporative salt concentration/crystallization and solids drying techniques are energy intensive, reducing the energy demand for the salt recovery step becomes essential for reducing the overall energy demand. In this study, a combination of electrodialysis and spray drying is investigated for recovering salt. The salt extractive column bottoms stream is pre-concentrated by electrodialysis followed by spray drying. In electrodialysis, the dilute salt solution is concentrated by selectively removing the salt ions from the solution rather than evaporating water; therefore, requiring less energy than an evaporative process. Final recovery of dry salt is achieved in a spray dryer. Salt extractive distillation, with salt recovery enabled by a new scheme of electrodialysis and spray drying, is here conceptually integrated in the water-ethanol separation train of a fermentation based cellulosic fuel ethanol plant and investigated through process simulation using Aspen Plus^® V7.2. Concepts, modeling, and results showing potential energy savings will be discussed.

¹- Côté P, Noël G, Moore S. The Chatham demonstration: From design to operation of a 20 m³/d membrane-based ethanol dewatering system. Desalination. 2010;250:1060-1066.