A horizontal packed-bed bioreactor to reduce CO2 gas holdup in the continuous production of ethanol by immobilized yeast cells

Abstract

CO2 gas, evolved during alcohol fermentation using immobilized yeast, causes several undesirable problems in a packed-bed bioreactor installed vertically as it increases the dead space and causes hydrostatic pressure. In order to reduce this “CO2 gas phase effect” which lowers the efficiency of ethanol production, a shallow, horizontal packed-bed bioreactor has been developed with a free space above the gel bed. The horizontal packed-bed bioreactor was 1.5 times more productive than the vertical packed-bed bioreactor when operated continuously. Yeast cells immobilized in calcium alginate gel reached a steady state much quicker than those immobilized in polyacrylamide gels. In the horizontal packed-bed bioreactors, calcium alginate gel was also superior to polyacrylamide gel with respect to ethanol productivity. The profiles of both glucose and ethanol concentrations against axial sampling sites suggested that the horizontal packed-bed bioreactor was similar to a plug flow reactor. The mean gel size gradually increased upstream (1.9 mm to 3.3 mm). With the economic production of ethanol in view, the published data on different continuous alcohol production processes have been compared by plotting their productivities (y-coordinate) against the ethanol concentrations in the effluents (x-coordinate) for the dilution rate or space velocity at which the yield of ethanol from glucose was 95%. The horizontal packed-bed bioreactor has a very high performance which makes this bioreactor promising for the economic production of ethanol.