The fermentation of syngas (a mixture of CO, CO2 and H2) to produce ethanol is of interest as an alternative fuel. Clostridium carboxidivorans, has been found to produce higher than average amounts of ethanol and butanol from CO-rich mixtures. This project sought to determine the effects of the redox level in the solution, partial pressures in the headspace and mass transfer limitations on the products obtained through fermentation of syngas. It was determined that cysteine sulfide has a greater effect on the redox level of the media used to grow bacteria, than does the gas composition. Therefore, changing gas composition during the process will have little effect on the redox. However, addition of cysteine sulfide may vary the redox level. When cells were first inoculated, the redox level dropped and leveled at -200 mV SHE for optimal growth. In addition, cells switch from acetic acid to ethanol production after a drop of 40-70 mV in the redox level. Different sizes of reactors were used, including 1 liter reactors (non-pressurized), 50 mL bottles (20 psig) and 100 mL bottles (20 psig). The 50 mL bottles have more than double the growth rate than the 100 mL bottles (0.57 day-1 compared to 0.20 day-1). Partial pressures were measured in these two sizes to determine the different consumptions and the effect of partial pressure on both growth and production of acetic acid/ethanol. It is clear that re-gassing the bottles every 12 hours to keep the pressure higher in the 100 mL bottles makes a significant difference in the growth, making them very similar to the 50 mL bottles. Both the 50 mL and 100 mL bottle were found to have essentially the same mass transfer rate (0.227 L/hr vs. 0.255 L/hr). However, because of headspace differences, there was more CO available for the 50 mL bottles (on a per liter basis) as compared to the 100 mL bottles. Mass transfer analysis proved useful in pointing out that all three reactors likely experienced mass transfer limitations such that mass transfer effects are critical to address when performing studies involving syngas fermentation.

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Biofuel production via fermentation is produced primarily by fermentation of simple sugars. Besides the sugar fermentation route, there exists a promising alternative process that uses syngas (CO, H2, CO2) produced from biomass as building blocks for biofuels. Although syngas fermentation has many benefits, there are several challenges that still need to be addressed in order for syngas fermentation to become a viable process for producing biofuels on a large scale. One challenge is mass transfer limitations due to low solubilities of syngas species. The hollow fiber reactor (HFR) is one type of reactor that has the potential for achieving high mass transfer rates for biofuels production. However, a better understanding of mass transfer limitations in HFRs is still needed. In addition there have been relatively few studies performing actual fermentations in an HFR to assess whether high mass transfer rates equate to better fermentation results. Besides mass transfer, one other difficulty with syngas fermentation is understanding the role that CO and H2 play as electron donors and how different CO and H2 ratios effect syngas fermentation. In addition to electrons from CO and H2, electrodes can also be used to augment the supply of electrons or provide the only source of electrons for syngas fermentation. This work performed an in depth reactor comparison that compared mass transfer rates and fermentation abilities. The HFR achieved the highest oxygen mass transfer coefficient (1062 h-1) compared to other reactors. In fermentations, the HFR showed very high production rates (5.3 mMc/hr) and ethanol to acetic acid ratios (13) compared to other common reactors. This work also analyzed the use of electrons from H2 and CO by C. ragsdalei and to study the effects of these two different electron sources on product formation and cell growth. This study showed that cell growth is not largely effected by CO composition although there must be at least some minimum amount of CO present (between 5-20%). Interestingly, H2 composition has no effect on cell growth. Also, more electron equivalents will lead to higher product formation rates. Following Acetyl-CoA formation, H2 is only used for product formation but not cell growth. In addition to these studies on electrons from H2 and CO, this work also assessed the redox states of methyl viologen (MV) for use as an artificial electron carrier in applications such as syngas fermentation. A validated thermodynamic model was presented in order to illustrate the most likely redox state of MV depending on the system setup. Variable MV extinction coefficients and standard redox potentials reported in literature were assessed to provide recommended values for modeling and analysis. Model results showed that there are narrow potential ranges in which MV can change from one redox state to another, thus affecting the potential use as an artificial electron carrier.