Membrane and Fluid Contactors for Safe and Efficient Methane Delivery in Methanotrophic Bioreactors

Methane (${\mathrm{CH}}_4$), a potent greenhouse gas, is globally available as both natural gas and biogas for residential, commercial, and industrial use. Though an excellent source of heat and power, ${\mathrm{CH}}_4$ is often flared or released into the air due to the lack of economically attractive end use options. One promising option is its use as a low-cost feedstock for growth of ${\mathrm{CH}}_4$-oxidizing microorganisms (methanotrophs) and production of single cell protein, methanol, bioplastics, and other bioproducts. However, such opportunities are impeded by the low aqueous solubility of ${\mathrm{CH}}_4$ and concerns about explosion hazards. To enable oxidation of ${\mathrm{CH}}_4$ at low levels, methane monooxygenase enzymes have evolved high affinities for ${\mathrm{CH}}_4$, as reflected in low half-saturation coefficients ($K_s<0.1–6\mathrm{mg}/\mathrm{L}$). Specific rates of ${\mathrm{CH}}_4$ consumption can therefore become maximum at low levels of dissolved ${\mathrm{CH}}_4$. For such kinetics, high volumetric productivities can be achieved by increasing biomass concentrations. Historically, this has been achieved by pressurizing ${\mathrm{CH}}_4$ feedstock. New methods include coupling high media recirculation rates with in-line mass transfer devices (static mixers, gas permeable membranes); recirculating fluid contactors, such as polymers or oils; and modifying fluid media with hydrophilic additives, such as electrolytes and alcohols. These new methods ensure that a flammable mixture is not created and provide many opportunities for innovation.