(313c) Reducing Water Demand by Increasing Energy Efficiency for Bio-Ethanol

The U.S. has a significant commitment to bio ethanol production and other countries like Brazil will continue to produce large amounts of this bio fuel via fermentation. Although the advantages and disadvantages of bio ethanol by fermentation are debatable it is clear that fermentation-based ethanol will be produced on a very large scale for some time to come and therefore merits efforts to clarify and improve any technological and environmental aspect of it. The water consumption for fermentation-based bio ethanol has recently begun to attract the attention of the public, legislators, engineers, and scientists in the U.S. and elsewhere. Widely varying water needs ranging from zero to perhaps more than 10 gallons of water per gallon of ethanol are reported, excluding the water needs to grow the needed plants. We first show a minimum non-recyclable consumption of about 2.85 gal water/gal of ethanol produced (for processing of corn to fuel grade ethanol, gate-to-storage) assuming zero liquid discharge from the process and otherwise current industrial practice data for yeast fermentation-based bio ethanol. Reduction of the thermal energy input to the process is vital to reduce this irretrievable water consumption. The direct relation of the minimum water demand of bio-ethanol production to the energy used in the process will be shown based on the well established chemical engineering concepts of unit operations and mass and energy balances. We will then discuss a possible reduction of the energy demand and thereby the water consumption for water/ethanol separation by salt extractive distillation and electrodialysis.