Evaluation of syngas mass transfer and its impact on syngas fermentation and development of a novel enhanced gas to liquid mass transfer bioreactor
The ability to utilize lignocellulosic biomass, an abundantly available renewable material, provides an immense opportunity to produce significant quantities of renewable bio-based fuels and chemicals. However, challenges in processing this material have limited the scale and commercial feasibility of this production pathway.
Syngas fermentation provides an avenue that combines thermochemical processing (gasification) of lignocellulosic biomass with the biological process of fermentation to potentially utilize all the carbon contained in lignocellulosic biomass to generate liquid fuels. The biological conversion of syngas generated from the gasification of lignocellulosic biomass has several advantages including the relatively mild conditions required by biological catalysts, specificity of product compounds, and the inherent robustness of biological systems with contaminating compounds in syngas.
A limiting factor in this technology is the low gas to liquid mass transfer rates of syngas components, specifically CO and H2, which leads to low microbial productivity and product yields. Research present in this study explored the impacts of gas to liquid mass transfer on syngas fermentation at a fundamental metabolic level within the cell as well as its impact on the distribution of products generated. Additionally, this research was extended at an engineering level to develop a novel syngas fermentation bioreactor to achieve significantly higher gas to liquid mass transfer rates and production rates over traditional fermentation systems.
Fundamental studies on the impact of mass transfer resulted in a deeper understanding of how syngas is assimilated in the cell’s metabolism and mass transfer impacts on different stages of the culture’s metabolism, specifically the critical step of alcohol production. At the engineering scale, a bioreactor capable of reaching mass transfer rates (KLa) of 2.28 sec-1 using oxygen as a model gas and up to 0.5 – 0.8 sec-1 with an integrated packed bed region was developed. Production rates of ethanol achieved were measured at 746 mg/L/hr within the immobilized biofilm region of the reactor.
These results provide a deeper understanding of the syngas fermentation process and provide an opportunity to further develop the unique bioreactor developed in this study to create a more effective and efficient process to produce biofuels via syngas fermentation.