Ionic-Liquid-Based Contactors for Carbon Dioxide Removal from Simulated Spacecraft Cabin Atmospheres

The ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim][Ac]), was used to remove carbon dioxide (CO2) from a simulated spacecraft cabin atmosphere (2 mm Hg (267 Pa)) partial pressure of CO2 with balance of nitrogen (630 mm Hg total pressure). Three gas–liquid contactor configurations were experimentally characterized to measure the rates of CO2 removal from the simulated atmosphere with [bmim][Ac]. Two of the contactors, a parallel path flat plate and hollow fiber membrane, used hydrophobic porous membranes (silicone and polypropylene, respectively) to separate the gas and ionic liquid streams. The third, a three-dimensional printed interior-corner capillary-driven contactor, was configured for the crossflow of gas directly over the free liquid surface. At circa 90–100 mL/min liquid flow and gas flow rate (0.24 slpm), inlet and outlet CO2 concentration differentials in the gas flow were 990, 2420, and 2680±140 ppm for the flat plate, hollow fiber, and interior-corner contactors, respectively. For these same contactors, the CO2 removal rates were (18±2), (41±3), and (60±3) g⋅m−2⋅day−1, respectively. Overall mass transfer coefficients k were (5.2±0.3), (16.8±1.3), and (25.0±1.9)×10−5 m⋅s−1 for each contactor, respectively. The coefficients generally decreased in direct proportion to increased gas flow. These performance metrics were nearly insensitive to variations in the flow of ionic liquid. The maximum uptake of CO2 by [bmim][Ac] was measured at 7.45 w% at 630 mm Hg and room temperature (23°C). The loading was 1.67 w% when exposed to the simulated spacecraft cabin atmosphere. In addition, desorption with thermal vacuum and thermal sparge processes were studied at temperatures from 20 to 80°C, the latter using dry inert gases (argon and nitrogen) to remove CO2 from the ionic liquid. After 4 h at 71°C under rough vacuum (0.5 mm Hg) without stirring, gravimetric measurements indicated a decrease in loading of CO2 from 1.75 w% to 1.29 w%. In comparison, for a 95 mL/min gas sparge flow through CO2-saturated [bmim][Ac] at 65°C, the loading decreased from 1.67 w% to 0.75 w% over the same period. The results suggest the importance of elevated temperature coupled with agitation to increase the rate of CO2 desorption from the ionic liquid.