Ethanol production in an immobilized cell reactor coupled with the recycling of effluent from the bottom of a distillation column

An ethanol fermentation system consisting of an immobilized cell reactor and a distillation column was constructed for recycling the effluent from the bottom of a distillation column. The effluent contained medium components and Ca ion, which were needed to stabilize the activity level of immobilized cells. We studied the durability of the ethanol production system in which only glucose aqueous solution was added and ethanol was harvested from the top of the distillation column without waste during the cultivation. We confirmed that the ethanol production could continue for 500 h, and that the experimental ethanol production yield was equivalent to 92% of the theoretical yield. It was assumed that the major portion of the glucose was consumed for ethanol production in this system, while the other components were used primarily for the growth and maintenance of the cells. The cost of the medium and the wastewater treatment for ethanol production by the proposed system is much lower than that of the conventional ethanol production process, because the amount of the medium components, except glucose, added during the process was much less than in the conventional alcohol production process. Furthermore, the amount of wastewater was around 6% of that produced in the conventional process. A preliminary automatic control sytem for the maintenance of the ethanol production was also constructed in order to avoid manual operation in nighttime. The glucose feed rate and the time interval of the distillation were kept constant at night. The computer controlled the on-off operation of the feeding pump in order to avoid excess feeding, which caused the liquid volumes in the bioreactor and the bottom vessel of distillation system to be above the critical volume levels. From the culture experiments, we confirmed that our control system would be effective for the reliable operation of an ethanol fermentation system and would eliminate manual operation in nighttime.