Biological conversion of methane to bioplastics: Kinetics, stoichiometry, and thermodynamic considerations for process optimization

Bioconversion of methane, a potent greenhouse gas, into biodegradable polyhydroxybutyrate (PHB), is an attractive option for methane management. Aerobic bioreactors designed for alpha-proteobacterial (Type II) meth-anotrophs can be operated to enable growth and PHB accumulation under nutrient-limiting conditions when fed CH4 as their sole source of carbon and energy. Using first principles, we develop and test a dynamic model for growth and PHB accumulation. The model includes the kinetics and stoichiometry of Type II methanotrophs and PHB accumulation, acid/base equilibria, metabolic heat release, and physicochemical transport of gaseous substrates through aqueous media. The model was then validated using an experimental dataset extracted from the literature. The numerical simulation accurately describes growth and PHB accumulation. After model vali-dation, we explored the impacts of gas delivery rate, reactor pressure, metabolic heat release, and pH on pro-ductivity and energy efficiency. Our results indicate that high mass-transfer rates and high-pressure operation are not needed to achieve significant PHB productivity, on the order of grams per liter per hour. The model also identifies operational windows that decrease energy inputs for cooling and mass transfer of oxygen and methane while still enabling significant PHB productivity.

Automated summary

Bioconversion of methane into PHB offers a promising solution for methane management. This study develops a dynamic model that accurately predicts the growth and PHB accumulation of Type II methanotrophs. The model explores the impact of various factors on productivity and energy efficiency and identifies operational windows for optimizing PHB production.