Intensification of bioethanol production by simultaneous saccharification and fermentation in an oscillatory baffled reactor

Abstract

Bioethanol is an alternative fuel produced mainly by biochemical conversion of biomass. This can be carried out efficiently and economically by simultaneous saccharification and fermentation (SSF) of sugarcane, corn, wheat, cellulose, etc., a process which integrates the enzymatic saccharification of the complex, polymeric sugars to glucose with the fermentative synthesis of ethanol by yeasts (Saccharomyces cerevisiae). However, the SSF unit operation still contributes nearly 50% to the cost of ethanol production. In SSF it is essential that a high sugar yield is obtained in the saccharification of cellulose. This yield is affected by factors such as inhibition of enzyme action by heat and other degradation products, enzyme and substrate concentrations, speed of enzyme action, adsorption of cellulase to cellulose, and degree of agitation. SSF was investigated in an intensified form of plug flow reactor, called the Oscillatory Baffled Reactor (OBR). The effect of agitation on saccharification of microcrystalline cellulose was correlated with the mean strain rates in the reactors. After 168 h of saccharification at 200 Wm-3 (Watts per cubic meters), 91% conversion of the cellulose (~25 g L-1 glucose) was observed in the OBR, whereas in the STR 74% conversion (~21 g L-1 glucose) was observed. At 120 Wm-3, the conversion in the OBR was 69% (~19 g L-1 glucose) within the first 24 h of saccharification and 88% conversion (24 g L-1 glucose) after 168 h. At the same power density the conversions in the STR were 55% (15.3 g L-1 glucose) and 67% (~18.6 g L-1 glucose), differences of 14 and 21% respectively. At 200 Wm-3 the ethanol concentration in a Stirred Tank Reactor (STR) after 72 h was 10.9 g L-1 (80.3% of theoretical yield) equivalent to production yield Yp/s = 0.55 g.g-1 cellulose and a volumetric productivity Qp of 0.15 g L-1 h-1. In the OBR at 200 Wm-3 the final concentration of ethanol after 72 h SSF was 12.5 g L-1 (93.8% of theoretical yield) equivalent to production yield Yp/s = 0.63 g.g-1 cellulose and a volumetric productivity Qp of 0.2 g L-1 h-1. It is hypothesised that the reason for these differences is the differing extents of cellulase deactivation in the two reactors. The OBR has a more uniform shear field than the STR, so the enzyme and yeasts would be exposed to fewer pockets of high shear.