Gas mass transfer with microbial CO2 fixation and poly(3-hydroxybutyrate) synthesis in a packed bed bioreactor

Gas mass transfer is the rate-limiting step in CO2 fixation by a hydrogen-oxidizing bacterium Cupriavidus necator under chemolithoautotrophic conditions. The mass transfer rates (i.e. the gas uptake rates) were measured in a packed bed bioreactor under the conditions of microbial growth on CO2. The k(L)a values of O-2 and H-2 were determined under either O-2 or H-2 limitation, respectively, and the ratio was proportional to a square root ratio of the molecular diffusivities in water, indicating that a surface renew model is suitable for the gas mass transfer in the packed bed. At a low gassing rate, the O-2 mass transfer coefficient was to different extent affected by packing materials, bed geometry, liquid velocity, and liquid distribution. The liquid distributor and/or bed void space exhibited a high effect on the mass transfer coefficient. With microbial growth, the gas fermentation was becoming limited either by dissolved O-2 or H-2, depending on gas composition, but neither O-2 nor H-2 limitation could trigger the formation of a substantial amount of poly(3-hydroxybutyrate) (PHB) under nutrient-rich conditions. Instead, when a nitrogen nutrient control strategy was applied, PHB synthesis was significantly improved and the PHB content reached up to 67 wt% of cell mass. (C) 2017 Elsevier B.V. All rights reserved.