Date of Award

December 2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Ali Alshami

Abstract

In the quest for sustainable and renewable energy sources, microbial gas fermentation has emerged as an alternative method for producing bioethanol and other valuable chemicals. Bioethanol is notable for its extensive application as a biofuel and its capacity to reduce greenhouse gas emissions. Conventional ethanol production predominantly depends on sugar and starch feedstocks, which compete for food resources. Microbial C1 gas fermentation presents a sustainable alternative by using industrial waste gases, or gases from gasification of municipal solid wastes, biomass, agricultural residues, animal wastes, and energy crops, which are all nonfood-based. Microbial gas fermentation is influenced by temperature, pH, gas liquid mass transfer and redox. Kinetics analysis experiments were performed, which provided more understanding of the physiology as well as kinetic parameters of the microorganism. Understanding how process parameters and growth medium components influence growth and end-product formation when cells are grown on carbon monoxide (CO) as the primary carbon source is vital for successful process optimization. Experiments conducted showed that increasing CO gas pressure, low pH, high cysteine concentration, and low yeast extract concentration increased the yield of ethanol production. The redox potential has also been identified as a key factor that affects the efficiency and yield of microbial C1 gas fermentation. Acetogenic bacteria are constrained by cellular energy limitations, and their production is regulated by redox and thermodynamics. Titanium (III) nitrilotriacetic acid was used as a redox mediator to promote production of reduced fermentation end products in Clostridium ljungdahlii. While excess H2 did not affect the metabolic end-product profiles observed, Ti(III)NTA resulted in an increased production of reduced metabolites such as ethanol and 2,3-butanediol, beyond those reported when CO was used as sole carbon and energy source, as well as lactic acid. These results reveal a possible correlation between the redox potential of supplied electrons and their metabolic effect. Understanding the significance of these factors is essential for optimizing and designing fermentation operations to improve production of biofuels and valuable chemicals from microbial gas fermentation.

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