Continuous ethanol fermentation using immobilized yeast in a fluidized bed reactor

Continuous ethanol fermentation of glucose using fluidized bed technology was studied. Saccharomyces cerevisiae were immobilized and retained on porous microcarriers. Over two-thirds of the total reactor yeast cell mass was immobilized. Ethanol productivity was examined as dilution rate was varied, keeping all other experimental parameters constant. Ethanol yield remained high at an average of 0.36 g ethanol g−1 glucose (71% of theoretical yield) as the dilution rate was increased stepwise from 0.04 h−1 to 0.14 h−1. At a dilution rate of 0.15 h−1, the ethanol yield steeply declined to 0.22 g ethanol g−1 glucose (44% of theoretical yield). The low maximum percentage of theoretical yield is primarily due to an extended mean cell residence time, and possibly due to the inhibitory effect of a high dissolved carbon dioxide concentration, enhanced by the probable intermittent levels of low pH in the reactor. Constant ethanol production was possible at a high glucose loading rate of 840 g dm−3 day−1 (attained at a dilution rate of 0.14 h−1). Although the highest average ethanol concentration (97.14 g dm−3) occurred at the initial dilution rate of 0.04 h−1, the peak average ethanol production rate (2.87 g (g yeast)−1 day−1) was reached at a greater dilution rate of 0.11 h-1. Thus, the optimal dilution rate was determined to be between 0.11 h−1 and 0.14 h−1. Ethanol inhibition on yeast cells was absent in the reactor at average bulk-liquid ethanol concentrations as high as 97.14 g dm−3. In addition, zero-order kinetics on ethanol production and glucose utilization was evident.