Process Modifications on Enzymatic Saccharification for Improved Conversion and Concentration of Sugars for Bioethanol Production

Decreasing fossil fuel inventory and increasing energy demand have raised concerns over fuel crisis in the past decades. Biofuel derived from lignocellulosic biomass was believed to be one of the sustainable solutions to fuel crises without competing with food and feed.;One challenge in making bioethanol production economical is to increase total solids in hydrolysis system while maintaining sugar conversion efficiency. In chapter 2, a mixing strategy was developed to solve the mixing problem that occurs at high total solids enzymatic hydrolysis, and therefore yield as high sugar conversion as in the low total solids scenarios. Since the water involved in the high total solids process is much lower compared with low total solids cases, the sugar concentration was increased from 26g/L to 121g/L. This would tremendously decrease the capital expenditure for large equipment and operation cost for distillation of excess water after fermentation. A patent has been granted to NCSU for this concept and licenses have been granted to various companies.;Effective enzymatic hydrolysis at low enzyme loading requires extended treatment of substrate to decrease the enzyme cost for biofuel production. In the third chapter, oxygen delignification and refining was thus introduced into this project as post treatment methods for the green liquor (GL) pretreated hardwood pulp. It was found that sequence of post treatment affected pulp digestibility and splitting refining intensity before and after oxygen delignification has a linear relationship between the refining intensity before oxygen delignification and lignin removal efficiency. Too intensive fibrillation could harm high solids enzymatic hydrolysis since they generate agglomerate at low moisture content, however, for low solids, this problem would not occur. Kneading could improved high solids enzymatic hydrolysis especially at low enzyme loading but at high enzyme loading the improvement was marginal, since high enzyme loadings are capable of breaking agglomerate at early stage of hydrolysis.;Enzymes recycling can be an important method to reduce the overall bioethanol production cost. In the third and forth study (chapter 4 and chapter 5), hydrolysate was used as the source of recycled enzymes. Six recycling processes with and without surfactant were developed, evaluated for enzyme recycling percentage and enzymatic hydrolysis efficiency in chapter 4 with both green-liquor pretreated softwood and hardwood. In chapter 5, a recycling process was developed and evaluated to achieve higher sugar concentration at low solids enzymatic hydrolysis. In this process the pretreated substrate is pressed to 20%+ solids using a commercially available press and then diluted with recycled sugar solution to various solids content (5-15%). With the process in chapter 5, sugar concentration can be increased from 35 g/L to 141 g/L with almost no sugar yield loss at steady state. Some chemicals created during hydrolysis seemed to increase enzymatic hydrolysis efficiency, compared to glucose and xylose simulated hydrolysate. Recycled enzymes increased sugar yield up to 8% in new cycle saccharification. The results suggested that it is highly recommended to recycle the hydrolysate as dilution water for fresh substrates in order to avoid mixing problems and build up sugar concentration.;Lignin filtration represents a significant cost for a bioethanol plant because it is inherently difficult to filter due to its small particle size. The process in Chapter 6 will enable the elimination of the lignin filter and decrease the retention volumes in enzymatic hydrolysis, thus resulting in significant capital savings. This chapter introduces a concept that makes use of a multi-stage enzymatic hydrolysis process along with the use of inexpensive gravity clarifiers to decrease the required operation volume and to separate the lignin, while increases the overall sugar conversion at the same total enzyme loading.