Continuous ethanol extraction by pervaporation from a membrane bioreactor

In order to obtain a high productivity of ethanol, a membrane bioreactor consisting of a fermentor and a pervaporation system was applied to the continuous alcoholic fermentation process. A microporous hydrophobic polytetrafluoroethylene membrane was used for pervaporation. Glucose medium and baker's yeast were used for the fermentation. Three types of continuous fermentation experiment were carried out: conventional free-cell fermentation as the standard process; a fermentation in which product ethanol was extracted continuously by pervaporation from the membrane bioreactor; and a fermentation in which ethanol was extracted by pervaporation and part of the culture broth was simultaneously removed from the fermentation system.

The fermented ethanol was continuously extracted, and simultaneously concentrated by pervaporation, from the membrane bioreactor, and the extracted ethanol concentration was 6 to 8 times higher than in the broth. A high concentration of microorganisms was realized by immobilizing cells in the membrane bioreactor. When the ethanol concentration in the broth was kept low by pervaporation, the specific rate of ethanol production increased. However, the fraction of viable cells decreased because of the accumulation of inorganic salts fed as a nutrient, of nonvolatile by-products and of aged cells, which were not extracted by pervaporation from the fermentation solution. In order to achieve a high ethanol productivity, part of the fermentation broth must be removed from the membrane bioreactor.