Natural gas upgrading by selective separation on zeotype adsorbents

Abstract

In this study, three kinds of zeotype adsorbents (synthesized NaA and SAPO-34 in comparison with natural Clinoptilolite) were compared to find out their abilities of separating CO2 and N2 from CH4. The adsorbents were characterized by SEM (scanning electron microscope) for morphological property, powder X-ray diffraction for the phase structure, and BET (Brunauer, Emmett and Teller) for the pore textural properties. The adsorption experiments of CO2, N2, and CH4 by the adsorbents were carried out at three temperatures (278, 298, and 323 K) and for pressures up to 1000 kPa. The experimental adsorption equilibrium isotherms were well described by the Sips equation. Breakthrough curves were measured for the fixed bed. The adsorption heats for the gases on the adsorbents were calculated using the van't Hoff equation. The heats of adsorption at zero coverage of CO2, N2, and CH4 on SAPO-34 were found to be 36.74, 23.05, and 23.12 kJ mol−1, respectively, while the values for the other adsorbents were approximately lower than these. The ideal selectivity of carbon dioxide over methane for NaA, Clinoptilolite, and SAPO-34 were 22.7, 16.4, and 22.8, respectively, at atmospheric pressure and 278 K Henry's law regime. The ideal selectivity of the adsorbents for carbon dioxide over methane was 22.7, 16.4 and 22.8 for NaA, Clinoptilolite and SAPO-34, respectively, at atmospheric pressure and 278 K Henry's law regime. Under the same conditions, the selectivity of nitrogen over methane for NaA, Clinoptilolite, and SAPO-34 were 14.6, 7.9, and 8.8, respectively. The selectivity increased with increasing the pressure in all cases studied, especially in the SAPO-34. The effect of the moisture content in the adsorbents on the separation performance was investigated by dynamic experiments. The dynamic adsorption experiments of CO2/N2/CH4 gas mixture confirmed that for large amount of moisture in natural gas, the SAPO-34 is the best candidate among the studied adsorbents, and it has more promising performance for the separating N2 and CO2 from methane, while in the case of large amount of nitrogen in natural gas, the NaA is the best one.