The cellulosic ethanol industry in the U.S. just built its first large plants. However, cellulosic ethanol is not widely considered economical. Previously, the Biomass Conversion Research Laboratory at Michigan State University created the RaBIT (Rapid Bioconversion with Integrated recycling Technology) process to improve cellulosic ethanol economics. The RaBIT process was successful in reducing capital cost, enzyme loading, and processing time while also increasing xylose consumption and ethanol productivity. However, cell recycling, a key component of the process, was not sustainable as xylose consumption decreased after each recycling event. The work presented in this dissertation investigated the cause of this decrease and through process changes eliminated the decrease.Four key variables were investigated for this work: strain suitability, nutrient deficiency, cell viability, and degradation product effects. Results showed that strains with sufficiently high specific xylose consumption rates were suitable for the RaBIT process. Studies of nutrient deficiency and cell viability showed that the specific xylose consumption rates were decreasing upon cell recycle and significant cell death was taking place during the xylose consumption phase. Degradation products were found to progressively accumulate within the cell. This accumulation was credited as the chief cause for decreasing cell performance upon recycle. Three process changes were implemented to improve RaBIT process fermentations. The combination of shortening fermentation time from 24 to 11 h and continuous feeding of hydrolysate eliminated the xylose consumption rate decrease. The new RaBIT fermentation process was capable of 0.8 g/L improved xylose consumption over 10 cycles. Previously, xylose consumption decreased by 3.6 g/L over just 1 cycle. The third process change allowed for the separation of cells based on age. The capability to selectively remove older cells showed benefit over non-selective removal of cells. However, cell removal over ten cycles was not sustainable as xylose consumption and cell mass decreased. Economic analysis was performed comparing the new RaBIT process to a traditional cellulosic ethanol process. The RaBIT process showed economic benefit over the traditional process, but was highly dependent on achieving an extended number of fermentation cycles. The RaBIT process does have clear benefits with regards to capital investment as initial investment and enzyme price sensitivity are low. Life cycle analysis showed that the RaBIT process was an improvement with regards to global climate change potential and acidification, but worse with regards to energy production and eutrophication when compared to the traditional cellulosic ethanol process.
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