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

Abstract

Bioconversion of methane, a potent greenhouse gas, into biodegradable polyhydroxybutyrate (PHB), is an attractive option for methane management. Aerobic bioreactors designed for α-proteobacterial (Type II) methanotrophs can be operated to enable growth and PHB accumulation under nutrient-limiting conditions when fed CH4as 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 validation, we explored the impacts of gas delivery rate, reactor pressure, metabolic heat release, and pH on productivity 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.