Ethanol Production in Semi-Continuous and Continuous Syngas Fermentation in Trickle Bed Reactor

The hybrid thermochemical-biochemical conversion process involves gasification of renewable feedstocks to syngas (primarily CO, CO₂ and H₂), which is fermented to alcohols and other chemicals. A major challenge in gas fermentation is the mass transfer limitation due to the low solubility of gaseous substrates in the liquid medium. Efficient gas fermentation reactors should provide mass transfer capabilities that are greater than the kinetic limitations of the microorganisms used to ensure high gas conversion efficiency and productivity. A trickle bed reactor (TBR) minimizes 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 ability of Clostridium ragsdalei to produce ethanol during semi-continuous and continuous syngas fermentations in a TBR was investigated. Fermentations were performed in a 1-L TBR at 37°C in counter-current and co-current flow modes. H₂ conversion efficiency was doubled during semi-continuous fermentation with periodic medium replacement after formation of biofilms on the 6-mm beads. The periodic medium replacement in semi-continuous fermentation favored acetic acid production and resulted in a molar ratio of ethanol to acetic acid of 1:2. However, continuous fermentation favored ethanol production with a molar ratio of ethanol to acetic acid of 4:1.  A more efficient gas utilization by C. ragsdalei and about 50% higher ethanol production rate and yield were measured in both semi-continuous and continuous fermentations during co-current flow compared to in counter-current flow.  This shows the potential for TBR use in syngas fermentations for production of fuels and chemicals.