Membranes on Mars for In-Situ Resource Utilization Processes

The goal of this project is the development and characterization of synthetic membranes for the separation and purification of CO₂ from the Martian atmosphere for in-situ resource utilization (ISRU) applications such as in-situ propellant production. Candidate materials should have high selectivity for carbon dioxide over nitrogen and argon, and a glass transition temperature of -40 °C or less to remain in rubbery state at low temperature for high permeance (flux/driving force). Membrane materials we identified include the rubbery polymers poly(dimethyl siloxane) (PDMS) and the copolymer poly(dimethyl, methylphenyl siloxane) (PMPS). Pure and mixed gas permeation experiments with CO₂, N₂ and Ar were performed with these membrane materials in the temperature range -25 to 21 °C. In experiments with the commercially available PDMS membranes, the pure gas CO₂ permeability increases from 1932 Barrers to 2755 Barrers as the temperature decreases from 22 to -30 °C. The ideal CO₂/N₂ separation factor (ratio of pure gas permeabilities/permeances) increases from 7.5 to 17.5 over the same temperature range. However, in mixed gas experiments, the CO₂/N₂ separation factor for PDMS was much lower, increasing from 4.5 to 6 as the temperature decreased from 22 to -30 °C. Pure gas permeation results with PMPS membranes also show an increase in CO₂ permeability from 1450 Barrers to 1650 Barrers as the temperature decreases from 21 °C to -10 °C for differential feed pressure of 20 psi. The CO₂/N₂ ideal separation factor increased from 12 to 27 over the same range of temperature. Unlike PDMS, the mixed gas CO₂/N₂ selectivities are nearly the same as the pure gas values. We will also present reasons for these differences and discuss the design of membrane modules.