(275a) High Pressure Adsorption Measurements with Excess Adsorption Described Using Modified Traditional Isotherms

High-pressure oxygen is an integral part of fuel cell systems, many NASA in situ resource utilization concepts including propulsion, and life support systems for extra vehicular activity. Due to limited information available for system designs over wide ranges of temperature and pressure, volumetric methods have been applied to measure adsorption isotherms of nitrogen and oxygen on various adsorbents covering temperatures from 105 K to 448 K and pressures up to 150 bar. Experimental data measured using two apparatuses with distinctly different designs show good agreement in a region of overlapping temperatures. High-pressure isotherms show maximum capacities for excess adsorption loadings as commonly observed for supercritical adsorption. The excess adsorption isotherms were modeled using traditional isotherm models modified to account for adsorbed phase volumes. All data over the entire temperature range are described well with a multi-temperature Toth model having only five parameters. We provide a simple approach to explain excess adsorption isotherms with conventional isotherm models developed originally for absolute adsorption without the need of developing high-pressure models or employing molecular simulations.