(307d) Continuous Production of Ethanol from Syngas in a Trickle Bed Reactor By Clostridium Ragsdalei

The hybrid gasification and syngas-fermentation technology can be used in sustainable production of fuels and chemicals from biomass and wastes via gaseous feedstocks. The low solubility of CO and H₂ in the liquid medium makes mass transfer a major challenge for this technology. Gas fermentation reactors designed to provide mass transfer capacity to match the kinetic capabilities of the microbial cells will allow high gas conversion efficiency and productivity. A trickle bed reactor (TBR) provides high gas transfer into a thin liquid film with high surface area to liquid volume.  Clostridium ragsdalei was used for continuous syngas fermentation to produce ethanol in a 1-L TBR containing 6-mm glass beads. The syngas fed to the TBR contained 38% CO, 5% N₂, 28.5% CO₂ and 28.5% H₂ (by volume) with gas and liquid in counter-current and co-current flow modes. The effect of gas and liquid flow rates on ethanol productivity and gas conversion efficiency were examined. CO and H₂ conversion efficiencies reached over 88% in both counter-current and co-current flow modes. Gas utilization was more efficient, and about 50% higher ethanol production rate and yield were achieved during co-current flow compared to counter-current flow. The gas uptake in co-current mode was up to threefold higher than in counter-current mode from higher gas flow without flooding the TBR in the co-current mode. The molar ratio of ethanol to acetic acid obtained during co-current mode was 4:1. The CO and H₂ uptake rates and conversion efficiencies increased with an increase in dilution rate. The TBR shows good potential for use in the production of fuels and chemicals from gaseous feedstocks.