Investigation into the Performance of Membrane Separator Technologies used in the International Space Station Regenerative Life Support Systems: Results and Lessons Learned

The Volatile Removal Assembly Flight Experiment (VRAFE) was performed in May of 1999, on board Shuttle Flight STS-96 to support the development of the International Space Station (ISS) Water Recovery System (WRS). The objective of this experiment was to address concerns in the performance of a two-phase, catalytic reactor in a microgravity environment. During the experiment, an unexpected finding was discovered when the VRAFE Gas/Liquid Separator (GLS) failed to separate gas from the reactor outlet stream. The VRAFE GLS was a two-membrane (flat sheet hydrophobic and hydrophilic membrane) gas trap. Flight data as well as the post-flight failure investigation determined that the GLS hydrophobic membrane failed as a result of very fine hydrophilic catalyst particles from the VRAFE reactor that had contaminated the surface of the hydrophobic membrane. These particles allowed a water layer to wick across the surface of the hydrophobic membrane and effectively block the pores from passing gas. Microgravity magnified the failure effect since there was no gravity to free drain the water layer from the hydrophobic membrane.

Though this GLS design was not the VRA flight design, a hydrophobic membrane separator is used in the VRA and other membrane separators are used throughout the ISS life support systems. As a result of the GLS failure, an assessment of all the membrane separators in the water recycling and oxygen generation systems was performed to determine if membrane separation was acceptable in each application. This paper summarizes the results and lessons learned from this investigation.