(782d) Semi-Continuous Syngas Fermentation in a Trickle Bed Reactor

Synthesis gas (syngas) fermentation involves the conversion of CO and H₂ to alcohols, organic acids and other chemicals using acetogens. A major challenge for advancing syngas fermentation is the mass transfer limitation associated with low solubility of CO and H₂ in the fermentation broth. An efficient syngas fermentation reactor should provide a mass transfer capability that is greater than the kinetic limitations of the acetogen used to obtain high gas conversion efficiency and productivity. Our recent mass transfer analysis of various reactors showed that the trickle bed reactor (TBR) can provide over three times more mass transfer than the stirred tank reactor. The TBR reduces the liquid resistance to mass transfer because a very thin liquid film is in contact with the gas which results in high mass transfer. The objective of the present study is to assess product formation and gas utilization by Clostridium ragsdalei in a TBR. The fermentation was performed at 37^oC using a syngas mixture of 38% CO, 28.5% CO₂, 28.5% H₂ and 5% N₂. The TBR was operated in semi continuous mode in which the gas was fed continuously and the liquid was circulated in the TBR. The effects of various gas and liquid flow rates on ethanol and acetic acid productivity and gas conversion efficiency were examined. Results showed that over 90% of the CO and 70% of the H₂ fed to the TBR were utilized by C. ragsdalei. CO uptake rates remained nearly constant during 1200 h of operation. However, H₂ uptake rates steadily increased as the biofilm in the TBR was establishing with time. This resulted in high CO utilization by C. ragsdalei and reduced CO inhibition to hydrogenase, which explains the increase in H₂ uptake rates. The production rate of ethanol and acetic acid almost doubled when the gas flow rate was doubled. Changing the liquid flow rates had a small effect on product formation and gas conversion efficiency. This study shows the potential for TBR use in the production of ethanol and acetic acid using syngas fermentation with high CO and H₂ conversion efficiencies.