Reactor design issues for synthesis-gas fermentations

Synthesis gas is readily obtained by gasifying coal, oil, biomass, or waste organics and represents an abundant, potentially inexpensive, feedstock for bioprocessing. The primary components of synthesis gas, carbon monoxide and hydrogen, can be converted into methane, organic acids, and alcohols via anaerobic fermentations. Bioconversion of synthesis gas is an attractive alternative to catalytic processing because the biological catalysts are highly specific and often more tolerant of sulfur contaminants than inorganic catalysts. However, because the aqueous solubilities of carbon monoxide and hydrogen are low, synthesis-gas fermentations are typically limited by the rate of gas-to-liquid mass transfer. Consequently, a major engineering challenge in commercial development of synthesis-gas fermentations is to provide sufficient gas mass transfer in an energy-efficient manner. This paper reviews recent progress in the development of synthesis-gas fermentations, with emphasis on efforts to increase the efficiency of gas mass transfer. Metabolic properties of several microbes able to ferment synthesis gas are described. Results of synthesis-gas fermentations conducted in various bioreactor configurations are summarized. Recent results showing enhancement of synthesis-gas fermentations using microbubble dispersions are presented, and studies of the mass-transfer and coalescence properties of microbubbles are described.