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The overall goal of this research is to develop an integrated process for syngas fermentation to ethanol with simultaneous recovery of acetic acid. <P>
The specific objectives are: <OL> <LI> Quantify the mass transfer coefficients of syngas (CO and H2) in the aqueous phase using both hollow fiber membrane and stirred-tank reactors. <LI>Optimize the mass transfer rate in a HFM-based reactor to maximize CO and H2 solubility. <LI>Test the effects of various operating conditions on the growth of syngas-fermenting anaerobes in a membrane-based reactor. <LI> Optimize the growth conditions of syngas-fermenting anaerobes for maximum ethanol yields using Bana grass-derived syngas. <LI> Examine the adsorption and recovery of acetic acid using MPS nanoparticle materials. <LI>Develop design parameters and conduct an economic analysis for scale-up.
NON-TECHNICAL SUMMARY: The overarching goal of this research is to efficiently convert biomass-derived synthesis gas (syngas) into ethanol through anaerobic fermentation. Lignocellulosic biomass is a renewable non-food energy feedstock abundantly available in the United States. Ethanol can be produced from biomass through a biochemical route but it requires extensive pretreatment to obtain fermentable sugars. Another approach is through gasification of biomass to produce syngas (CO and H2) followed by its fermentation into ethanol. Current approaches for syngas fermentation, however, suffer from low ethanol yield largely due to the gas/liquid mass transfer limitation of syngas, kinetic limitation due to low biomass level and the generation of inhibitory acetic acid during fermentation. To overcome these challenges this research proposes two novel concepts: use of microporous hollow fiber membrane (HFM) to improve the solubility of CO and H2 in aqueous phase, and use of mesoporous silica (MPS) nanoparticle materials for selective adsorption and recovery of acetic acid. Bana grass (Pennisetum purpureum) is a high yield fiber crop of Hawai'i and will be used as a feedstock for syngas production. A series of batch assays with three major syngas-fermenting microbes will be conducted at mesophilic conditions to select a high ethanol yield anaerobe. The selected microbe will then be tested in a HFM bioreactor, where the microbes attached to the membrane will ferment syngas to ethanol and acetic acid. Following ethanol recovery, the media will be passed through an MPS bed for selective acetic acid recovery. The proposed membrane-based system will result in a significant improvement in the mass transfer of syngas constituents and lead a path towards commercialization of syngas-to-ethanol technology
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APPROACH: This research will employ innovative hollow fiber membranes (HFM) in which syngas is delivered to the attached microbes (on HFM surface) by diffusion through the walls of microporous HFM. HFM also act as support media for biofilm growth. We hypothesize that the use of HFM will eliminate the mass transfer limitation and can maintain a higher cell density in the bioreactor than traditional bioreactor designs. Mesoporous silica (MPS) functionalized with an amine group has a significantly higher surface area-to-mass ratio and can selectively adsorb acetic acid from the fermentation broth. The adsorbed acid can be recovered readily as a byproduct. The acetic acid can provide an additional revenue stream as it is a starting material for the synthesis of various polymers which traditionally use petroleum-based feedstocks. The MPS nanomaterials can then be regenerated through pH control. The acetic acid-free broth is perpetually recycled back to the fermenter to supplement the needed nutrients.