(592g) The Influence of Partial Pressure On the Fermentation of Syngas to Ethanol

Many fermentation processes have been used to convert biomass to various fuels. Currently, the fermentation of syngas, which is a mixture of CO, CO2, and H2, is being studied for the production of ethanol. In our studies, an anaerobic microbial catalyst Clostridium P11 was used. The mass transfer rate, which is a function of the gas partial pressure, is an important issue in anaerobic fermentation because it affects the availability of the carbon source from the syngas substrate, especially for the low soluble gaseous substrate CO. On the other hand, gas-substrate enzymes in the metabolic pathway are also sensitive to the gas partial pressure due to Michaelis-Menton type kinetics. Since the fermentation process utilizes CO and/or CO2 as the primary carbon substrate, it's important to distinguish how partial pressures affect cell growth and product formation in relation to either mass transfer limitations or metabolic pathway (reaction) limitations.

In all studies, microbial catalyst was observed to first produce acetic acid as a fermentative product while in the growth phase. As expected, a switch was observed from acetic acid to ethanol as the cell concentration reached steady state. In this work, partial pressures and mass transfer rates were adjusted to assess the effects on cell growth and product formation. Mass transfer rates were significantly varied using either bubbling of the syngas through the media or delivering the syngas to the media through a hollow fiber system. The mass transfer variation allowed for changing the mass transfer rate while maintaining the same partial pressure or changing the partial pressure while maintaining the same mass transfer rate. This allowed for the determination of how the metabolic pathway was affected by partial pressure in the absence of mass transfer limitations. Additionally, studies conducted under mass transfer limitations showed how mass transfer can mask the partial pressure effects on the metabolic pathway. This work provides significant insights on how varying partial pressures from gasifiers (as a result of gasifier operation or varying biomass) can affect the syngas fermentation process.