Continuous Ethanol Production from Dilute-acid Hydrolyzates: Detoxification and Fermentation Strategy

Doctoral thesis, 2006

The production of fuel ethanol from cellulosic biomass is of growing interest around the world. Lignocellulosic residuals can be used to produce transportation fuel, with the overall process having little net production of greenhouse gases. Lignocellulosic materials are available as a by-product of many industrial processes and agricultural materials, or can potentially be produced from dedicated energy crops. The production of ethanol from lignocellulosic materials includes hydrolysis which breaks the cellulose and hemicellulose polymers to fermentable sugars, followed by cultivation which converts the sugars into ethanol, and finally a separation process where ethanol purification is carried out to produce fuel ethanol. However, some by-products such as furan compounds are released during chemical hydrolysis and inhibit the yeast during cultivation. This work contributes a solution to overcome these problems especially for a continuous process which is economically superior. Hydrolyzate detoxification using lime (‘overliming’) in concert with the capability of yeast to carry out in-situ detoxification is focused upon in the work. The kinetics of the overliming process were studied, where both sugars and furan compounds are degraded through a formation of complex ion. The sudden addition of lime in a batch process shows severe degradation of sugars together with furan compounds. This knowledge leads to development of a continuous detoxification process where gradual addition of lime can save 25% of the initial sugar with similar detoxification effects under certain conditions. Cell immobilization and cell flocculation have been studied to develop a high cell density system. High cell density is effective in carrying out in-situ detoxification. This study shows a good combination of continuous detoxification and cell immobilization where continuous ethanol production of 5.14 g/L•h can be carried out at a high feeding rate of 0.648 h(-1). In addition, the application of a serial bioreactor has been found to increase the utilization of substrates. A gain in substrate assimilation of 11.6% has been achieved when using a serial bioreactor at residence time of 2.32 h. Furthermore, a cell flocculating system has been studied and developed. In a steady-state condition, the cell flocculation system could cultivate fresh hydrolyzates at a high feeding rate of 0.52 h(-1) without any additional chemical detoxification, while sugar assimilation and ethanol productivity were 96% and 7.4 g/L•h respectively. In conclusion, this study proposes a concept of rapid continuous production of ethanol where inhibitory obstacles can be overcome by chemical detoxification and/or in-situ detoxification by yeast.