You are here:
SEPARATION AND CONCENTRATION OF ETHANOL BY PERVAPORATION
Citation:
VANE, L. M. AND Y. HUANG. SEPARATION AND CONCENTRATION OF ETHANOL BY PERVAPORATION. Presented at International Fuel Ethanol Workshop, Milwaukee, WI, June 20 - 23, 2006.
Impact/Purpose:
To inform the public
Description:
A significant issue affecting widespread acceptance of bioethanol as a sustainable fuel is the energy used to grow the feedstock, ferment the feedstock to ethanol, and separate dry ethanol from the fermentation broth. For the latter, the best current technology is two-step distillation to produce 190 proof ethanol, followed by molecular sieve drying to remove the remaining water. For the past two years, MTR, working with the USEPA's National Risk Management Research Laboratory, have been developing a membrane process called pervaporation for this application. In pervaporation, a warm feed solution flows across one side of a selective membrane. One of the components of the feed preferentially permeates the membrane as vapor. Transport through the membrane is induced by maintaining a vapor pressure on the permeate side of the membrane that is lower than the vapor pressure of the feed liquid. The pressure difference is achieved by cooling the permeate vapor to a temperature lower than the feed stream, causing the permeate vapor to condense and create a vacuum. Pervaporation is inherently energy-efficient because only the small fraction of the feed liquid that passes through the membrane must be vaporized. Alternatives such as distillation require vaporization of a much larger fraction of the feed to achieve the separation. Membranes can be made that selectively permeate ethanol or that selectively permeate water. In principle, an all-membrane ethanol/water separation process could be devised to replace the distillation/molecular sieve system. The pervaporation process would use ethanol-permeable membranes on the front end to remove ethanol from the fermentation broth, and water-permeable membranes on the back end to remove the remaining water from the first-step ethanol concentrate, producing dry ethanol. In practice, the first commercial plants using membranes for ethanol/water separation are likely to be combined with current distillation technology. Early targets for pervaporation membrane technology in ethanol plants include replacement of back-end molecular sieve units in conventional distillation plants and front-end separation of ethanol from continuous fermentation processes for increased plant capacity. In this review paper, the current state-of-the-art for ethanol/water separation using membranes will be discussed including recent progress from the MTR/USEPA collaborative pervaporation project. Opportunities for energy savings and options for integration into ethanol plants will be described.