The current research investigates the utilization of cotton gin waste as a feedstock to produce a value-added product - fuel ethanol. Cotton gin waste consists of pieces of burs, stems, motes (immature seeds) and cotton fiber, and is considered to be a lignocellulosic material. The three main chemical constituents are cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polysaccharides of primarily fermentable sugars, glucose and xylose respectively. Hemicellulose also includes small fractions of arabinose, galactose, and mannose, all of which are fermentable as well. The main issue in converting cotton gin waste to fuel ethanol is the accessibility of the polysaccharides for enzymatic breakdown into monosaccharides. This study focused on the use of steam explosion as the pretreatment method. Steam explosion treatment of biomass has been previously described to increase cellulose accessibility. The governing factors for the effectiveness of steam explosion are steam temperature and retention times. The two factors are combined into a single severity term, log(Ro). Following steam explosion pretreatment, cotton gin waste was subjected to enzyme hydrolysis using Primalco basic cellulase. The sugars released by enzyme hydrolysis were fermented by a genetically engineered Escherichia coli (Escherichia coli KO11). The effect of steam explosion pretreatment on ethanol production from cotton gin waste was studied using a statistically based experimental design. The results obtained from this study showed that steam exploded cotton gin waste is a heterogeneous material. Drying and milling of steam exploded cotton gin waste was necessary to reduce variability in compositional analysis. Raw cotton gin waste was found to have 52.3% fermentable sugars. The fiber loss during the steam explosion treatment was high, up to 24.1%. Xylan and glucan loss from the pretreatment was linear with respect to steam explosion severity. Steam explosion treatment on cotton gin waste increased the hydrolysis of cellulose by enzyme hydrolysis. Following 24 hours of enzyme hydrolysis, a maximum cellulose conversion of 66.9% was obtained at a severity of 4.68. Similarly, sugar to ethanol conversions were improved by steam explosion. Maximum sugar to ethanol conversion of 83.1% was observed at a severity of 3.56. The conclusions drawn from this study are the following: steam explosion was able to improve both glucose yields from enzyme hydrolysis and ethanol yields from fermentation. However, when analyzed on whole biomass, or starting material basis, it was found that the fiber loss incurred during steam explosion treatment negated the gain in ethanol yield. / Master of Science

In this study, the hydrolytic kinetics of mixtures of cotton gin waste (CGW) and recycled paper sludge (RPS) at various initial enzyme concentrations of Spezyme AO3117 and Novozymes NS50052 was investigated. The experiments showed that the concentrations of reducing sugars and the conversions of the mixtures increased with increasing initial enzyme concentration. The reducing sugar concentration and conversion of the mixture of 75% CGW and 25% RPS were higher than those of the mixture of 80% CGW and 20% RPS. The conversion of the former could reach 73.8% after a 72-hour hydrolysis at the initial enzyme loading of 17.4 Filter Paper Unit (FPU)/g substrate. A three-parameter kinetic model with convergent property based on enzyme deactivation and its analytical expression were derived. Using nonlinear regression, the parameters of the model were determined from the experimental data of hydrolytic kinetics of the mixtures. Based on this kinetic model of hydrolysis, two profit rate models, representing two kinds of operating modes with and without substrate recycling, were developed. Using the profit rate models, the optimal enzyme loading and hydrolytic time could be predicted for the maximum profit rate in ethanol production according to the costs of enzyme and operation, enzyme loading, and ethanol market price. Simulated results from the models based on the experimental data of hydrolysis of the mixture of 75% CGW and 25% RPS showed that use of a high substrate concentration and an operating mode with feedstock recycle could greatly increase the profit rate of ethanol production. The results also demonstrated that the hydrolysis at a low enzyme loading was economically required for systematic optimization of ethanol production. The development of profit rate model points out a way to optimize a monotonic function with variables, such as enzyme loading and hydrolytic time for the maximum profit rate. The study also investigated the ethanol production from the steam-exploded mixture of 75 wt% cotton gin waste and 25 wt% recycled paper sludge at various influencing factors, such as enzyme concentration, substrate concentration, and severity factor, by a novel operating mode: semi-simultaneous saccharification and fermentation (SSSF) consisting of a pre-hydrolysis and a simultaneous saccharification and fermentation (SSF). Four cases were studied: 24-hour pre-hydrolysis + 48-hour SSF (SSSF 24), 12-hour pre-hydrolysis + 60-hour SSF (SSSF 12), 72-hour SSF, and 48-hour hydrolysis + 12-hour fermentation (SHF). SSSF 24 produced higher ethanol concentration, yield, and productivity than the other operating modes. The higher temperature of steam explosion favored of ethanol production, but the higher initial enzyme concentration could not increase the final ethanol concentration though the hydrolytic rate of the substrate was increased. A mathematical model of SSSF, which consisted of an enzymatic hydrolysis model and a SSF model including four ordinary differential equations that describe the changes of cellobiose, glucose, microorganism, and ethanol concentrations with respect to residence time, was developed, and was used to simulate the data for the four components in the SSSF processes of ethanol production from the mixture. The model parameters were determined by a MATLAB program based on the batch experimental data of the SSSF. The analysis to the reaction rates of cellobiose, glucose, cell, and ethanol using the model and the parameters from the experiments showed that the conversion of cellulose to cellobiose was a rate-controlling step in the SSSF process of ethanol production from cellulose. / Ph. D.