(544c) Enhanced Mass Transfer in Syngas Fermentation

Syngas fermentation produces acetic acid and ethanol from CO and H₂ in gas produced from biomass and waste materials.  CO and H₂ components of the syngas are oxidized on enzymes inside the cell and provide electrons and protons that drive energy generation for cell function.  Studies of gas to liquid mass transfer in fermentation reactors using O₂ or CO into water or aqueous solution in the absence of fermentation have identified suitable fermenters on the basis of highest overall volumetric mass transfer coefficient, k_L(a/V_L).  These studies used a dissolved gas measurement method, such as dissolved O₂ probes or myoglobin analysis for dissolved CO, to define the rate of gas dissolution into the liquid phase.  In the present study, gas mass transfer capacity in fermenters was estimated from measured CO uptake rates with CO mass transfer limitation assumed during syngas fermentation.  Under mass transfer limitation, the k_L,CO(a/V_L) can be calculated from known CO partial pressure in the gas phase and the uptake rate of CO.  Syngas fermentation in stirred tank reactors showed an order of magnitude higher k_L,CO(a/V_L) than that predicted from mass transfer correlations derived from O₂ transfer into water.  This enhancement of mass transfer is well beyond that attained from the use of nutrient medium and is associated with the presence of fermenting cells.  Further, the k_L,CO(a/V_L) for fermentation in serum bottles exhibits a maximum at very low CO partial pressures.  The observed enhancement of k_L,CO(a/V_L), in active cell culture with constant interface area per liquid volume (a/V_L), requires an increase in k_L,CO.  Higher k_L,CO results from microbial culture activity, which is driven by the energy of the substrate gas. This presents a significant increase in energy efficiency and reduction in electrical power requirement for agitation.