Reducing the Energy Demand of Bio-Ethanol through Salt-Extractive Distillation and Electrodialysis
Mohammed A. Hussain and Peter H Pfromm. Chemical Engineering, Kansas State University, 1005 Durland Hall, Manhattan, KS 66506
Bio-ethanol from corn currently consumes about 34,000 BTU in form of natural gas to produce one gallon of ethanol representing 76,000 BTU as lower heating value (U.S. industrial practice data, 2007). Separating ethanol from water consumes about 40% of the natural gas demand cited above. Saline extractive distillation of alcohol-water mixtures and fermentation broth has been considered elsewhere and fairly comprehensive experimental data, thermodynamic data, and simulations are available. Potentially very significant energy savings and process simplifications could be predicted. However, the recovery and recycling of the salt used to facilitate distillation has not been addressed beyond simple evaporation of the bottoms to recover pure dehydrated salt for recycling to the reflux (historic HIAG process) or recovery of a strong brine from the bottoms to feed a separate azeotrope breaking stage (IHI process). Electrodialysis is uniquely suited for salt recovery from the saline extractive distillation column bottoms for the case of ethanol recovery from fermentation broth since salt is selectively removed from the solution (no water is evaporated) and the electrodialysis membranes and overall fluid handling are tolerant to fermentation broth and even to entrained particulate matter. Concepts, modeling, and experimental data for electrodialysis-enabled salt extractive ethanol distillation will be shown. Aqueous/aqueous and aqueous/ethanol electrodialysis will be discussed.