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

- Exposure of microbial cells to gas phase facilitated gas substrates consumption.
- A packed bed with high void space exhibited the highest mass transfer coefficient.
- Surface renewal model is more suitable for gas mass transfer in packed bed bioreactor.
- PHB up to 67 wt% of cell mass was formed from CO2 under nitrogen limitation.

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 kLa values of O2 and H2 were determined under either O2 or H2 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 O2 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 O2 or H2, depending on gas composition, but neither O2 nor H2 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.