Overcoming the limits of methane bioconversion : computational tools for the industrialization of polyhydroxybutyrate (PHB) from methane
Keywords: Engineering.

Humanity's dependency on fossil-derived products has led to global pollution across our land, water, and air environments. In particular, petroleum-based plastic pollution is pervasive across all environments, with significant contributions to global oceanic plastic pollution and greenhouse gas emissions. Current trends in conventional fossil carbon-based plastics manufacturing continue polluting at an alarming rate, requiring sustainable alternatives to chemical-based plastics. Sustainable material needs, both in quantity and quality, can be met via use of biological organisms. Methanotrophic organisms are a promising biotechnology that can address environmental concerns of plastics pollution and greenhouse gas emissions. Methanotrophs grow by consuming a potent greenhouse gas, methane, as their sole source of carbon and energy. As these organisms grow, they can eventually transform excess methane into a biodegradable plastic, polyhydroxybutyrate (PHB). PHB has similar material properties to conventional chemical-based plastics, showing promise as a replacement to fossil fuel-derived plastics. Using methane as a carbon feedstock for PHB is attractive due to its relative abundance, low-cost, and climate mitigation potential. Methane has a global warming potential that is over 20 times that of carbon dioxide (CO2), removing it before it enters the atmosphere is essential mitigate continued climate change impacts. This dissertation investigates the potential of methanotrophic PHB production via a critical review and a series of computational studies that include equilibrium, dynamic, and techno-economic analysis (TEA) models. This thesis comprises 3 research chapters that synthesize and evaluate the potential of methane based PHB production. Chapter 2 highlights methane's potential as a feedstock, synthesizing current trends in engineered systems that utilize or have the potential to utilize methane effectively as a substrate. In addition, chapter 2 summarizes methanotrophic organisms' metabolic diversity and broad range of observed microbial kinetic parameters (e.g., specific growth rate, yield). Finally, in chapter 2, using fundamentals of biotechnology and chemical engineering, an equilibrium model is developed to assess rate limitations of methanotrophic growth across a range of observed mass transfer and volumetric consumption rates. Next, in Chapter 3, a dynamic first principles PHB accumulation model is developed. The model considers physical, chemical, and biological kinetics of a methanotrophic bioreactor and is used to evaluate PHB productivity and energy efficiency considering a broad range of bioreactor operating conditions. Chapter 4 is techno-economic analysis model that investigates how the design of industrial scale bioreactor systems impact PHB cost, in addition to considering the social cost of carbon (SCC) from the PHB production process. The model is used to analyze cost and SCC impacts considering various physical design approaches that include the size of the fermenter, the rate of mixing, whether the system is pressurized, and the size and type of centrifuge and dryer. Additionally, the model incorporates the process impacts of methanotrophic microbial kinetics. Together, these research chapters highlight the potential of methane and methanotrophs as a robust technology platform that can produce sustainable, biodegradable alternatives to chemical-based plastics, while mitigating the release of a potent greenhouse gas