Ethanol can be produced by using continuous flow fermentors arranged in a series with complete sugar utilization or high ethanol concentra­tion. With two fermentors arranged in a series, the retention time can be chosen so that the sugar is only partially utilized in the first, with fermentation completed in the second. Ethanol inhibition is reduced in the first fermentor, allowing a faster throughput. The second, lower — productivity fermentor can now convert less sugar than if operated alone. For high product concentration, productivity of a two-stage system has been 2.3 times higher than that of a single stage [47, 75].

A two-stage continuous ethanol fermentation process with yeast recir­culation is used industrially by Danish Distilleries Ltd., Grena, for molasses fermentation (see Fig. 3.8). Two fermentors with 170,000-L volume produce 66 g/L ethanol in 21-h retention time [76].