Browsing by Author "Kim, Se Hoon"
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Item Lime pretreatment and enzymatic hydrolysis of corn stover(Texas A&M University, 2005-08-29) Kim, Se HoonRenewable energy sources, such as lignocellulosic biomass, are environmentally friendly because they emit less pollution without contributing net carbon dioxide to the atmosphere. Among lignocellulosic biomass, corn stover is a very useful feedstock to economically produce environmentally friendly biofuels. Corn stover was pretreated with an excess of calcium hydroxide (0.5 g Ca(OH)2/g raw biomass) in non-oxidative and oxidative conditions at 25, 35, 45, and 55oC. The optimal condition is 55oC for 4 weeks with aeration, determined by yields of glucan and xylan. The overall yields of glucose (g glucan hydrolyzed/100 g original glucan) and xylose (g xylan hydrolyzed/100 g original xylan) were 91.3 and 51.8 at 15 FPU/g cellulose, respectively. Furthermore, when considering the dissolved fragments of glucan and xylan in the pretreatment liquors, the overall yields of glucose and xylose were 93.2 and 79.5 at 15 FPU/g cellulose, respectively. The pretreatment liquor has no inhibitory effect on ethanol fermentation using Saccharomyces cerevisiae D5A. At the recommended condition, only 0.073 g Ca(OH)2 was consumed per g of raw corn stover. Under extensive delignification conditions, 87.5% of the initial lignin was removed. Extensive delignfication required oxidative treatment and additional lime consumption. Deacetylation quickly reached a plateau within 1 week. Delignification highly depended on temperature and the presence of oxygen. Lignin and hemicellulose were selectively removed, but cellulose was not affected by lime pretreatment in mild temperatures (25 ?? 55oC). The delignification kinetic models of corn stover were empirically determined by three simultaneous first-order reactions. The activation energies for the oxidative delignification were estimated as 50.15 and 54.21 kJ/mol in the bulk and residual phases, respectively. Crystallinity slightly increased with delignification because amorphous components (lignin, hemicellulose) were removed. However, the increased crystallinity did not negatively affect the 3-d sugar yield of enzyme hydrolysis. Oxidative lime pretreatment lowered the acetyl and lignin contents to obtain high digestibility, regardless of crystallinity. The enzymatic digestibility of lime-treated biomass was affected by the change of structural features (acetylation, lignification, and crystallization) resulting from the treatment. The non-linear models for 3-d hydrolysis yields of glucan and xylan were empirically established as a function of the residual lignin fraction for the corn stover pretreated with lime and air.Item Relationships among Perceived Working Hours, General Stress, Work Centrality, Job Control, Job Demands, and Work Condition Constraints(2014-06-27) Kim, Se HoonAlthough working hours are generally regarded as a major indicator of stress, little exploration has been done about how working hours influence stress with the relevant individual and job characteristics. The purpose of this study was to examine the relationships among perceived working hours, general stress, work centrality, job control, job demands, and work condition constraints for full-time employees in the United States and Korea. A self-administered questionnaire survey approach was used to collect data which were utilized to reflect six variables of this study. The sample size was 805 (U.S.: 397, Korea: 408). As the results from the measurement invariance test indicated that the measurement model was not invariant by country, statistical examinations were conducted separately for each country to test hypotheses. In the path analysis, for the U.S. sample, Job Demands was significantly associated with Perceived Working Hours. Job Demands and Work Condition Constraints were significantly related to General Stress. There was a significant relationship between Work Condition Constraints and Job Demands. There were significant mediation effects for the relationships between Work Condition Constraints and Perceived Working Hours via Job Demands and between Work Condition Constraints and General Stress via Job Demands. For the Korean sample, in the path analysis, Work Centrality and Job Demands were significantly associated with Perceived Working Hours. Job Demands, Job Control, and Work Condition Constraints were significantly associated with General Stress. There was a significant relationship between Work Condition Constraints and Job Demands. All the hypothesized mediation effects were significant: from Job Demands via Perceived Working Hours to General Stress, from Work Condition Constraints via Job Demands to Perceived Working Hours, and Work Condition Constraints via Job Demands to General Stress. It was found that this study overall supported the selected theories and related research. The significance of this study includes contributions to work-life/work-family balance studies that are an emerging research area in HRD, practical implications for sound work-settings, and cultural validations of the theories related to working hours and stress. Limitations, implications for theories, research, and practices were also discussed.