Reactor design considerations for the conversion of particulate biomass to ethanol

Ethanol is rapidly becoming a significant transportation fuel additive in the US, with the use of cellulosic feedstocks being of increasing interest. Continuous processes have many attractive features for biomass processing, however continuous bioreactor systems have not been used under realistic conditions for cellulosic biomass ethanol production. This thesis reports development and use of experimental apparatus for continuous SSF along with results from modeling studies, and represents significant progress in the study of continuous biomass processing.; A variety of cellulosic substrates were fermented to ethanol by simultaneous saccharification and fermentation (SSF) with hydrolysis using cellulase from Trichoderma reesei and fermentation to ethanol by Saccharomyces cerevisiae. Continuous SSF using dilute acid pretreated mixed hardwood had a productivity of approximately 70% of the equivalent batch system at moderate substrate conversion (80%). Using Avicel as a substrate in solids retaining reactors increased reactor productivity up to 2.6 times that of batch SSF. Operation of these reactors using pretreated Poplar increased productivity to 1.5 times that of batch SSF on this substrate. Solids retaining reactors will reduce the cost of ethanol production from lignocellulose by 7.8 cents/gallon if the productivity benefits apparent from experimentation are attainable at full scale.; An analytical model was developed which accounts for biomass interaction with cellulase and the changing nature of biomass through the course of hydrolysis. In particular the declining specific reactivity of the substrate over the course of conversion, as observed by numerous investigators, was incorporated. Experimental data combined with modeling results underline the difference between the reaction of heterogeneous solids and typical soluble substrate reactions. Extension of the modeling framework to non-well-mixed reactor configurations predicted the observed productivity benefits from solids retaining continuous reactors. The benefits of solids retention diminish at high substrate conversion in a single continuous reactor; analysis and preliminary experimentation indicates the use of cascade reactors will alleviate this effect.