Buffer gases to blend with oxygen to produce breathing air are a requirement for Mars exploration. Before the recent announcement that water appears to exist in the Martian soil, the preferred source of oxygen was the Martian atmosphere. The Martian atmosphere, which contains approximately 95.3-percent carbon dioxide, 2.7-percent nitrogen, and 1.6-percent argon, is the source of the buffer gases. The source of oxygen would be from water electrolysis, either produced by the reactions of hydrogen with carbon dioxide, or water present on the surface of Mars. The need for carbon dioxide to produce fuels and other useful materials still exists, which means that it must be captured and separated from the nitrogen and argon. If it is assumed that carbon dioxide is captured by freezing at 193 kelvin (K), then a typical composition of gases in the chamber would be 30-percent carbon dioxide, 44-percent nitrogen, and 26-percent argon, which is the feed composition used in these studies. The target concentration of carbon dioxide in the buffer gas was 600 parts per million (ppm), the initial temperature was assumed to be 230 K, and the pressure was 1 kilopascal (kPa).

A test fixture was developed that allowed control of the feed gas composition, flow rates, and temperature. Pressure controllers were used to control the pressure drops across the membranes, and the volumetric flow rates of the permeate and raffinate were measured with wet test meters. When gas mixtures were used, a gas chromatograph measured the composition. Sufficient data were collected so mass balances could be calculated.

Preliminary tests with pure gases (carbon dioxide, nitrogen, and argon) were performed with four different commercial membranes over a temperature range from 230 to 298 K and pressure drops across the membranes that ranged from 0 to 28 kPa. The permeate flux and pressure drop data were used to calculate the pressure-normalized flux at different temperatures. The pressure-normalized flux, usually expressed in gas permeation units (GPU’s), was plotted versus temperature to show the temperature dependency of the membranes for various gases. Since the membrane film thickness was not known, the relative selectivity, which is the ratio of the GPU’s, was calculated for each of the gases as a function of temperature. Finally, mixtures of gases were passed through the membranes and the changes in composition were measured by gas chromatography. These data were then input to a solution-diffusion model and the design of a membrane system was generated. The final design used two hollow-fiber membranes produced by Air Products. These membranes are produced from brominated polysulfones and sold under the trade name Permea Prism Alpha Separators. The model used for the final design was the Permea PPA-22, Prism Alpha Separator, which has a surface area of 22 square feet. The results of the modeling are shown in the table and the system design is given in figure 1.

Stream 1 is the initial feed to the membrane and stream 5 is the product, which has a carbon dioxide concentration of 600 ppm and a flow rate that is approximately 50 percent of the feed.

Figure 1. Membrane Purification of Feed From the Capture
of Carbon Dioxide

Performance Data Generated by the Enerfex Model for 230 K

The membrane system, shown in figure 2, has a capacity that is approximately 12 times the flows used in the example given in the table or a product output at stream 5 of 1.2 standard liters per minute. Figure 2 shows the two membranes, pump required for stream 6, pressure controller for stream 6, and gas mixture supply.

Key accomplishments:

• 2001: Collection of data and design of a multimembrane system.
• 2002: Testing of the prototype membrane system to confirm capacity and purity of the product.

Contact: Dr. C.F. Parrish (Clyde.Parrish-1@ksc.nasa.gov), YA-C3, (321) 867-8763
Participating Organizations: Dynacs Inc. (L. Fitzpatrick, J.M. Surma, and T.R. Hodge), Florida Institute of Technology (P. Jennings and Dr. J. Whitlow), and Enerfex (R. Callahan)