Modified X Zeolites as Next Generation Carbon Dioxide Adsorbents

Next generation ion exchanged X zeolites have been evaluated as replacements for the zeolites used in the spacecraft carbon dioxide removal systems. Most recently, zeolites have been used in the International Space Station (ISS) Carbon Dioxide Removal Assembly (CDRA). The zeolites used in the ISS CDRA are termed ASRT 5A and have 50% improvement in adsorptive capacity for carbon dioxide compared to commercial zeolites. Relative to the ASRT 5A zeolites on ISS, the ion exchanged X zeolites provides a factor of 1.6 improvement in carbon dioxide capacity, and an improved isotherm shape. Further improvement in capacity, along with improved resistance to attrition or cracking is obtained if the X zeolite is immobilized using a polymeric binder into a porous composite which fills the adsorbent canister. Together, these improvements allow the zeolite to be used in an adiabatic pressure-swing mode, or in the temperature/pressure swing mode now used in ISS.

The systems implications of using the ion exchanged X zeolites result in performance optimization for the air revitalization system. Improvements in the adsorbent characteristics lead to a reduced equivalent weight for a carbon dioxide removal system relative to other system approaches. This decreased equivalent weight would result from combined reductions in hardware weight, electrical power consumption, and overboard losses of oxygen and nitrogen. Converse to reduced equivalent weight, reductions in the spacecraft cabin carbon dioxide partial pressure may be achieved with the ion exchanged X zeolites if the hardware weight were maintained.