(365f) A Comparative Modeling Analysis of Electromicrobial Production Process Strategies

Electromicrobial production strategies, in which electrochemically-generated mediator molecules provide energy for microbes to fix CO₂ and generate value-added products, have received significant attention for the sustainable production of commodity chemicals. Here, we describe the development and application of comprehensive process models for three proposed electromicrobial production systems: a Knallgas bacteria-based system, a formatotroph-based system, and an acetate-mediated system in which acetate is first produced by the anaerobic conversion of H₂ and CO₂ via an acetogenic organism and is subsequently consumed for the generation of target products via heterotrophic growth. We consider the formation of several products including biomass, industrial enzymes, and lactic acid.

The lactic acid productivity is highest for the formatotroph system, ~30% higher than the Knallgas bacteria system and ~200% higher than the acetate-mediated system; this is due primarily to the high solubility of formate, which avoids coupling growth substrate availability to the gas-liquid mass transfer rate of sparingly soluble gases such as H₂. Notably, restricting the Knallgas bacteria system to nonflammable gas mixtures reduces the lactic acid productivity to equal that of the acetate-mediated system. However, the Knallgas bacteria system achieves the highest overall efficiency, ~4x greater than the formatotroph system and ~5x greater the acetate-mediated system. This difference is due, in the former case, to the substantial energy penalty associated with concentrating the formic acid effluent from the CO₂ electrolyzer to achieve industrially-relevant lactic acid titers (100 g/L) and the relative inefficiency of CO₂ electrolysis to formate. In the latter case, the lactic acid titer is limited by salt build-up, so the low system efficiency is caused by the energy penalty associated with concentrating the lactic acid effluent. Regardless of the system, the single-pass conversion efficiency of CO₂ is low (~10% in all cases), so gas recycle would be necessary to avoid high energy costs in upstream CO₂ capture devices in a fully integrated process. In sum, our analysis reveals trade-offs between productivity and efficiency in major electromicrobial production schemes, indicates the need for an integrated ecodesign framework relying on life cycle impacts that can evaluate these trade-offs, and demonstrates the utility of process models towards this ecodesign paradigm.