BACKGROUND: A closed-circulating system for ethanol fermentation was constructed by coupling a cell-immobilized bed fermentor with pervaporation using a composite PDMS membrane. A continuous fermentation experiment was carried out for about 250 h in the system at 28 degrees C. RESULTS: The cell density in the immobilized bed was up to 1.76 x 10(10) cells g(-1) gel. The ethanol concentration in the broth was maintained at about 43 g L-1. The glucose utilization and ethanol productivity were 23.26 g L-1 h(-1) and 9.6 g L-1 h(-1), respectively. The total flux and the ethanol flux through the membrane pervaporation unit varied in the range 300-690 g m(-2) h(-1) and 61-190 g m(-2) h(-1), respectively. The average ethanol concentration in the permeate was 23.1% (wt%). The carbon recovery efficiency was 86.8% (wt%), determined by calculating the carbon balance kinetics. The effect of ethanol concentration in the broth on the ethanol productivity was analyzed by modeling product formation kinetics of the system. CONCLUSIONS: Compared with the traditional free cell fermentation system and packed bed fermentation system, the closed-circulating system has the promising features of higher glucose utilization and ethanol productivity, and cleaner production. (C) 2010 Society of Chemical Industry