(110i) Energy And Water Optimization In Ethanol By Digestion Of Solubles

Authors: 
Powers, M., Cargill, Inc
Zullo, L., Cargill Incorporated


Water is an increasingly scarce commodity. Midwest ethanol producers have zero water discharge, yet intake rates are considerable, exceeding 3 gallons per gallon of ethanol. Energy also is an increasingly important component of manufacturing costs. Moreover, in the case of ethanol production, it is clear that there is a direct coupling of the energy and water problems ? for every energy unit spent in heat, it is approximately true an energy unit is spent in evaporation, pulling water into the plant. Therefore, one-sided energy solutions such as simply burning feed products to meet energy needs without proper attention to energy (=water) efficiency in a process is not desirable.

Feed byproducts still carry considerable value in the majority of Midwest American ethanol operations, and should not generally be viewed as simply biomass to be consumed as energy. Ideally, a process would purify and recycle water at low temperatures, producing energy as a byproduct, without destroying any more of the feed byproduct stream than is necessary to achieve the greatest energy gains. Ultra-low residence time anaerobic digestion technology offers favorable opportunities to increase the energy efficiency in a fermentation process without minimal impact upon feed byproduct streams. Anaerobic digestion of thin stillage allows recycle of water to a fermentation process with zero discharge without requiring all stillage water be vaporized.

While the economic case for anaerobic digestion is clear compared to other stillage processing options three areas need development in order to put the technology into practice in a large scale corn ethanol plant. Firstly, the performance of the digestion process itself needs to be quantified to determine biogas yields and residence times. Secondly, the water recycle from AD may have a significant effect upon the fermentation process, and this must be quantified in terms of purge requirements or increased fermentation capital requirements. Finally, the engineering of the plants energy integration will need to be reworked to incorporate utilization of dryer waste gas to replace to some degree the role of evaporator steam. If these three design challenges can be met the profitability and energy yield of the ethanol production process can be potentially increased, while simultaneously greatly decreasing the associated environmental footprint of a plant.