378947 Enhanced Mass Transfer in Syngas Fermentation

Wednesday, November 19, 2014: 4:05 PM
International B (Marriott Marquis Atlanta)
John R. Phillips, Hasan K. Atiyeh and Raymond L. Huhnke, Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, OK

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, kL(a/VL).  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 kL,CO(a/VL) 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 kL,CO(a/VL) 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 kL,CO(a/VL) for fermentation in serum bottles exhibits a maximum at very low CO partial pressures.  The observed enhancement of kL,CO(a/VL), in active cell culture with constant interface area per liquid volume (a/VL), requires an increase in kL,CO.  Higher kL,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.

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