Butanol fermentation process from sugarcane juice by clostridium acetobutylicum immobilized on silk cocoons

Abstract

Butanol, a kind of alcohol, can be used as a fuel oil in forms of gasoline-alcohol blend (gasohol) for automobiles and other vehicles. The gasohol basically demotes gasoline usage, but promotes the efficiency of the combustion engine. Besides butanol is recently derived from petroleum, it is obtained from fermenting with Clostridium strain as butanol-producing bacteria in the presence of sugar or starch as a carbon source. Butanol produced from the fermentation generally presents in the mixture of acetone, butanol, and ethanol (ABE). However, currently, the particular ABE fermentation yields low butanol productivity. Therefore, this study aimed to improve the butanol production to achieve higher productivity in the industrial scale, and to reduce the production cost by using the sucrose-rich sugarcane juice as a cheaper carbon source. In the present study, the batch fermentation was performed using Clostridium acetobutylicum (ATCC824). In addition, thin shell silk cocoon (TSC), a residual from the silk industry, was used as a nitrogen source as well as a support material for the immobilization of C. acetobutylicum. The ABE fermentations were operated under an anaerobic condition at the controlled temperature of 35 °C, pH of 4.5-5.0, and the rotational speed of 200 rpm in all batches. In the study of effect of nitrogen sources which were yeast extract and thin-shell silk cocoon fragment on the fermentation, it was found that those nitrogen sources were a great combined nitrogen source providing ABE products at the highest concentrations of approximately 8.03, 12.72, and 1.50 g/L, respectively. Furthermore, in the immobilized culture (IC-TSC), the initial concentration of sugarcane juice (80, 90, 100 g/L) affected the ABE concentrations. The optimum initial sugar concentration and fermentation period were 80 g/L and 132 h yielding ABE at the greatest concentrations of 4.2, 15, and 1.6 g/L, respectively (productivity based on the total ABE concentration of 0.15 g/L/h). As compared the IC-TSC to the suspended culture (SC) under the identical condition, SC provided the lower contents of acetone and butanol (3.46 and 14.03 g/L, respectively). Whereas, the similar concentration (1.4-1.6 g/L) of ethanol in both culture systems was shown. In the repeated batch of IC-TSC, butanol at 5.8 g/L was appeared in the first cycle at 72 h of fermentation period, but 13.0, 12.7 and 12.8 g/L were obtained in the second, third and fourth cycle, respectively after 48 h of the fermentation (productivity based on the total ABE concentration of 0.43 g/L/h). In addition, the evaluation of the environmental impact, in terms of CO2 emissions was performed associated with three processes: raw material preparation, fermentation process, and electric utility in the production. A Life Cycle Assessment (LCA) based on the Ecoinvent database was utilized as a tool for this study. According to the information from the experimental data, 1000 g of the main product (butanol) operated in batch and repeated batch systems were set as the basis of the LCA. Since butanol and acetone productivities are considerably enhanced by using immobilized culture (IC) on TSC in repeated batch fermentation, the total CO2 emissions per butanol production significantly decreased. The evaluation reveals that the total CO2 emissions from batch and repeated batch systems are 32.07 and 3.36 kg CO2/kg butanol, respectively.