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Gas-permeable membranes that continuously transfer carbon dioxide (CO2) from air to water were investigated in an effort to bypass the operational limitations of expendable solid absorbents currently used for CO2 control in closed-circuit underwater breathing apparatus (UBA). Rebreather UBA CO2 control requirements and known membrane properties were used to create a functional hierarchy of membrane types and CO2 transfer mechanisms, from which one membrane configuration was selected for evaluation. This Direct Contact Membrane Separator (DCMS) employs microporous hydrophobic Hollow Fiber Membrane (HFM) modules to create large membrane areas in small volumes for air-water phase contact without intermixing. Since the micropores in the hydrophobic walls of the hollow fibers are air-filled, gas permeation rates through this membrane are far higher than for any solid or liquid membrane.

Gas-liquid interphase mass transfer operations, such as gas-liquid phase separation, gas absorption into liquid or dissolved gas separation from liquid, gas humidification and drying via liquid contact, and evaporative cooling are readily accomplished on the Earth with settling/spray chambers, packed towers, or bubble columns. The inability of these gravity-dependent phase contact and/or separation devices to function in microgravity has almost completely precluded the use of gas-liquid mass transfer as a available unit operation in spacecraft and spacesuit fluid processing systems. Recent advances in hollow fiber membrane performance have made this technology suitable for gas-liquid mass transfer operations in microgravity, in which the walls of many small hollow fibers provide very high volume-specific-surface-area for fluid phase separation or contact without gravity dependence or phase intermixing.