Graduate Thesis Or Dissertation
 

Modeling and optimization of the dilute-sulfuric acid pretreatment of lignocellulosic

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  • Environmental concerns about urban air quality, global climate change, energy security and economic considerations motivate a growing interest in alternative fuels for the transportation sector. Ethanol, a fermentation-derived fuel, can be produced by bioconversion of renewable materials, such as wood, grass, and waste. Combustion of ethanol fuel, in both neat and blended form, can improve the engine efficiency, and lower the emission of CO, NO[subscript x], and volatile organic compounds (VOC), hence reducing the urban ozone level. Moreover, enhanced agricultural activities for production and collection of lignocellulosic feedstocks and industrial developments for production of ethanol will help the economic growth by creating new jobs and new income sources. Bioconversion of lignocellulosic feedstocks into ethanol requires a pretreatment process to increase the digestibility of cellulose by cellulolytic enzymes. The dilute-sulfuric acid pretreatment can hydrolyze hemicelluloses (xylan), disrupt lignin structure, and increase the yield of ethanol production from fermentation of monomeric units of cellulose (glucose). In this study, herbaceous (corn stover and switchgrass) and woody (poplar chips) feedstocks were pretreated with dilute sulfuric acid (0.6, 0.9, and 1.2% w/w) in a batch reactor at relatively high temperatures (140, 160 and 180°C). A unifying kinetic model including reaction time, temperature and acid concentration was developed, and pertinent kinetic parameters were determined. This model can predict the percentages of xylan remaining in the pretreated solids, net xylose yield in the liquid prehydrolysate, and xylose loss after pretreatment of a feedstock at a certain set of reaction conditions. Using this model, four optimum reaction conditions for obtaining maximum net xylose yield in the liquid prehydrolysate were identified. The yield and rate of ethanol production from the optimum prehydrolysates by the pentose fermenting yeast, Pichia stipitis, were determined. It was found that pretreating the selected feedstocks at 170-180°C with 1.0-1.2% sulfuric acid for 1-3 min resulted in the recovery of 80-85% of the original xylan in the liquid prehyrolysate. It was also found that feedstocks with higher neutralizing capacity (e.g., corn stover) produced lower sugar yields as a result of acid neutralization. Pretreatment of feedstocks at conditions beyond the optimum reaction conditions would increase the extent of xylose degradation, and lower the yield and rate of ethanol production due to loss of fermentable sugars and formation of toxic byproducts. The optimum prehydrolysates of corn stover produced the highest yields of ethanol (0.39-0.47 g ethanol/g xylose) followed by switchgrass (0.36-0.45) and poplar (0.26-0.44). The inhibitory effects of byproducts (e.g., acetate) was more pronounced in poplar prehydrolysates.
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