Electrofuels are carbon-neutral, renewable transportation fuels generated using non-photosynthetic CO2 fixation. Biological conversion of H2, CO2 and O2 into alcohols is emerging as a promising strategy for producing electrofuels while reducing greenhouse gas emission. However, development of this process presents significant bioreactor-design challenges. First, these fermentations have unusually high demands for gas mass transfer. Second, the need to simultaneously deliver H2 and O2 creates safety issues, because these gases are incompatible, forming a flammable mixture over the range of 4 to 94% H2. Third, alcohols are typically inhibitory to the microbial biocatalysts, making their in situ removal desirable for continuous bioreactor operation. This dissertation describes development of a novel Bioreactor for Incompatible Gases (BIG) to address these challenges. The BIG features a hollow fiber configuration, in which the microbial biocatalysts are immobilized in the porous fiber walls. A liquid phase containing microbubbles of one gaseous reactant is pumped through the hollow fibers, and a gas phase containing another gaseous reactant(s) incompatible with the first is maintained outside the fibers. In this way, rapid mass transfer to the immobilized cells of both gaseous reactants is achieved without creating hazardous gas mixtures. In situ product removal can be achieved through the liquid stream (for nonvolatile products) and/or the gas stream (for volatile products). A prototype bench-scale BIG was designed, fabricated, and integrated with an Opto22-based control system that monitored and controlled the composition of the gas phase, dissolved oxygen concentration in the liquid phase, and temperature. The control system was programmed to make intelligent operational decisions in response to process contingencies. The BIG system was inoculated with an isobutanol (IBT)-producing strain of Ralstonia eutropha and operated stably for up to 19 days, during which continuous IBT production from H2, CO2 and O2 was achieved for the first time. A dynamic mathematical model was developed to describe the interplay of multispecies and multidirectional gas mass transfer, along with complex cellular kinetics involving mixed-gas substrates. The simulation results were used to interpret experimental findings and simulate BIG performance under a range of operating conditions. The experimental and modeling results established the unexpected finding that R. eutropha cells both produced and consumed IBT, depending on the local H2 concentration. The BIG prototype reactor and associated mathematical models are generic and could be used for a variety of biofuels based on incompatible gases.
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