Poly(3-hydroxybutyrate) (PHB) production from CO2: Model development and process optimization

• A calibrated and validated mechanistic model for autotrophic PHB is presented.
• Biomass growth continues under O2 stress, leading to increased PHB production.
• Nitrogen stress further increases PHB production in case of O2 inhibition.

The biosynthesis of poly(3-hydroxybutyrate) (PHB) directly from carbon dioxide (CO2), is a sustainable alternative for non-renewable, petroleum-based polymer production. The conversion of CO2 implies a reduction of greenhouse gas emissions. Hydrogen oxidizing bacteria such as Cupriavidus necator have the ability to store PHB using CO2 as a carbon source, i.e., through an autotrophic conversion. In this study, a mathematical model based on mass balances was set up to describe autotrophic PHB production. The model takes into account the stoichiometry and kinetics of biomass growth and PHB formation as well as physical transfer from the gas phase to the liquid fermentation broth. The developed model was calibrated and validated based on independent experimental datasets from literature, obtained for C. necator. The obtained simulation results accurately described the dynamics of autotrophic biomass growth and PHB production. The effect of oxygen (O2) and/or nitrogen stress conditions, as well as of the gas mixture composition in terms of O2 and hydrogen (H2) was investigated through scenario analysis. As major outcome, a higher maximum PHB concentration was obtained under oxygen stress conditions compared to nitrogen stress conditions. At high O2 fractions in the gas mixture, which would result in H2 limitation before O2 limitation, PHB production can be increased by applying nitrogen stress. The effect of the reactor type was assessed through comparing a continuous stirred tank reactor (CSTR) with an air-lift fermentor. The developed model forms the basis for future design with minimum experimentation of suitable control strategy aiming at a high PHB production.