Membrane and Fluid Contactors for Safe and Efficient Methane Delivery in Methanotrophic Bioreactors
Jorge Luis Meraz, Kristian L. Dubrawski, Sahar H. El Abbadi, Kwang‐Ho Choo, Craig S. Criddle
Abstract
Methane (CH4), 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, CH4 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 CH4-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 CH4 and concerns about explosion hazards. To enable oxidation of CH4 at low levels, methane monooxygenase enzymes have evolved high affinities for CH4, as reflected in low half-saturation coefficients (Ks<0.1–6 mg/L). Specific rates of CH4 consumption can therefore become maximum at low levels of dissolved CH4. For such kinetics, high volumetric productivities can be achieved by increasing biomass concentrations. Historically, this has been achieved by pressurizing CH4 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.