Enhanced ethanol production by continuous fermentation in a two-tank system with cell recycling

• A cell recycling two-tank system was used for continuous production of bioethanol.
• Each tank fermentor equipped with a settler for recycling of flocculent yeast.
• The system was effective for ethanol production at high dilution rates.
• The productivity was tremendously enhanced compared with batch fermentation.

An experimental method for producing ethanol continuously was designed and tested with a cell-recycling two-tank system, which was composed of two fermentors, each of which was individually equipped with a settler for recycling flocculent yeast. This system was effective for the continuous fermentation of ethanol from sucrose at high cell-recycling (r = 0.8–0.9) and dilution (up to 0.48 h−1) rates. The system has several advantages; the high cell concentration in the fermentors and relief of substrate and product inhibition. Thus, the enhanced productivity using this continuous fermentation with the two-tank cell-recycling system was significantly higher compared with that of the batch fermentation. The results indicate that increased recycling ratios caused an increase in biomass concentration and subsequently, product concentration in the tank. The ethanol productivity increased with the dilution rate, but higher dilution rates could render increasing amounts of sugar unconverted. Continuous fermentation with the sugar feed concentration of 160 g/l at r = 0.9 and dilution rate of 0.2 h−1 achieved the highest productivity with less than 2% of the unconverted sugar in the product steam. Under the same cell recycling ratios a productivity range of 6.9–7.5 g/l h−1 could be achieved with feeding concentrations of 80–200 g/l, while batch fermentation at these sugar concentrations led to productivities of 3.85–4.48 g/l h−1.