(326c) Gas to Liquid Flow Reactor for Conversion of Biogas or Waste Methane to Organic Acids with Hydrogel-Encapsulated Methanotrophs As Fixed Biocatalysts

Small-scale and disparate sources of methane continue to increase in abundance with the growth of biogas generated from waste processing, including digestion of solid wastes and waste-water treatment. Methane is expensive to transport and a greenhouse gas with a warning potential 25 times that of CO₂. Biological upgrading of methane and/or biogas by methanotrophic microorganisms is a promising approach for efficient conversion to value-added products. Metabolic engineering of methanotrophs like Methylococcus capsulatus has been successful in converting methane to a variety of organic acids including muconic, lactic, and succinic acids. However, liquid fermenters utilizing methanotrophs are fundamentally mass transfer limited due to the poor solubility of methane in water. Gas-sparged fermentation broths are ineffective for these bioconversions, and an alternative bioreactor design to overcome these limitations is presented herein.

Whole cell methanotroph biocatalysts are encapsulated within a fixed hydrogel support to contact a continuous gas phase in a flow reactor. This approach imagines hydrogel-encapsulated cells as a fixed catalyst bed similar to thermochemical gas flow reactors. Methanotrophs encapsulated in biocompatible hydrogels such as cross-linked PEGs are shown to be viable and function as effective biocatalysts for months. Here, hydrogel biocatalysts are mechanically supported within 3D printed lattice scaffolds, allowing for controlled catalyst geometries for diffusion and reaction. This gas flow reactor was utilized for the conversion of simulated and real biogas feedstocks with these hydrogel-encapsulated biocatalysts. Early work demonstrated that reducing feature sizes to sub-mm scale achieves more than an order of magnitude increase in volumetric productivity. Initial demonstrations at mL-scale were scaled up by several orders of magnitude in flow reactor operation. Wild type methanotrophs were utilized to study methane consumption and process parameters, and engineered strains were studied for organic acid productivity. Various 3D printed scaffold geometries were tested and tradeoffs between mass transfer improvements and overall biocatalyst loading were assessed. The effects of process variables including feed composition, pressure, and gas flow rates on flow bioreactor performance were evaluated. Finally, product recovery approaches and reactor configurations are addressed. This work demonstrates an flow bioreactor for flexible biogas upgrading incorporating advances in additive manufacturing, biocatalyst immobilization, methanotroph metabolic engineering, and reactor design.