Chiang Mai Journal of Science

 The continuous ethanol production from sweet sorghum juice in a continuous stirred tank bioreactor (STR) and a combined bioreactor was compared. The combined bioreactor comprised an STR and three tubular (plug flow) bioreactors (TB) in series with a total working volume of 3320 ml. The fermentation was carried out at 30°C by Saccharomyces cerevisiae NP 01. The sweet sorghum juice containing 250 g l^-1 of total sugar was introduced into the systems at total dilution rates (D) of 0.007, 0.02 and 0.04 h^-1. Our results clearly show that the degree of ethanol production depends on the residence time of the fermentation broth in the system. In the STR system at D = 0.007 h^-1, the ethanol concentration (P), yield (Y_P/S) and productivity (Q_P) were 67.28 ± 0.37 g l^-1, 0.52 ± 0.01 g g^-1 and 0.47 ± 0.00 g l^-1 h^-1, respectively. Using the combined bioreactor (CSTR + TB) could improve ethanol production efficiency. The sugar consumption and ethanol production of the combined bioreactor system were markedly higher than those of the STR system. The highest P value (106.01 ± 0.47 g l^-1) was obtained when using the combined system at D 0.007 h^-1; and under this condition, the Y_P/S and Q_P values were 0.50 ± 0.01 g g^-1 and 0.76 ± 0.02 g l^-1 h^-1, respectively.