Pure and Binary Adsorption Equilibria of Methane and Nitrogen on Activated Carbons, Desiccants, and Zeolites at Different Pressures

Several commercially available adsorbents that show promise for the separation of CH4 from N-2 including different activated carbons, desiccants, and zeolites were evaluated using single component adsorption isotherms of pure methane and nitrogen that were measured at 303 K for pressures of up to 10 atm using the gravimetric method. Following the screening, the adsorption of binary gas mixtures composed of methane and nitrogen were studied at 303 K and two different pressures on the adsorbents with the highest capacity for CH4 of each of the activated carbons, desiccants, and zeolites. These selected adsorbents included commercial zeolite 13X (Siliporite Nitroxy Pro from CECA Arkema Group), activated carbon (Xtrusorb A754 from Calgon Carbon Corp.), and silica gel (SG-B127 from Grace & Co). A concentration pulse chromatographic method was used to determine the pure component CH4 and N-2 Henrys Law constants, heats of adsorption, and the experimental binary adsorption isotherms for the three selected adsorbents. The effect of pressure on the binary CH4N2 system was examined at 1 and 4 atm total pressures. The experimental equilibrium binary adsorption isotherm behavior was compared to the predictions based on the Extended Langmuir Model and the Ideal Adsorbed Solution Theory using the parameters from the pure component adsorption isotherms. The experimental equilibrium data for each adsorbent were compared through XY phase diagrams and selectivity curves. Both binary models show only a modest ability to describe adsorption equilibria of the binary system on activated carbon, silica gel, and 13X. The experimental binary CH4/N-2 selectivity was shown to be different than the pure component data suggests for all of the adsorbents, indicating some competitive adsorption of CH4 and N-2 in the binary system. Results of this study showed that for this system, 13X zeolite had a combination of the lowest binary selectivity and the highest heats of adsorption. Silica gel had low heats of adsorption for both N-2 and CH4, but higher methane selectivity at high methane concentrations. Activated carbon provided high binary equilibrium selectivity and high capacity among the studied adsorbents, which has good implications for this gas separation application of natural gas treatment.