The impact of trace amounts of CO2 on the high-pressure adsorption of CH4 on 5A zeolite

The impact of trace amounts of CO2 on CH4 adsorption in a zeolite at elevated pressure was examined using experimental, binary adsorption isotherms, and molecular simulations. CH4 upgrading is important for a variety of applications, and the use of zeolites for CH4 upgrading has been considered previously but, to the best of our knowledge, not at pressures between 60 and 70 bar. The non -ideality of the adsorption was examined by comparing the experimental loadings to predicted loadings from the Ideal Adsorbed Solution Theory (IAST). When adsorption occurs at a total pressure of 58-66 bar, and a CO2 fraction below 0.42 mol %, there was less than 5 % error in the CH4 IAST predicted loading compared to the experimental value, but the deviation in CO2 loading between IAST and experiments, even at these low CO2 mole percentages, varied nominally from 4 to 13 %. Above 0.42 mol % deviations from IAST were more severe. Grand Canonical Monte Carlo simulations captured the trends in the adsorption data and illustrated how CO2 molecules align in the pore when adsorbed such that oxygens on both ends of the molecule can interact with cations in the zeolite. The results highlight the applicability of adsorbent materials to separate CO2 from process streams at elevated pressure and provide molecular insights that can aid the design of adsorbent structures.