Cation Exchange Modification of Clinoptilolite Zeolite for Applications in Nitrogen Rejection

Abstract

Natural gas and biogas are increasingly important resources to the global energy mix. With the growth in the development of unconventional resources, contaminate gases within natural gas mixtures are growing challenges for industry. Inert gases such as N2 cannot be more than 4%vol. in order to meet pipeline specifications. This is done in an effort to increase the heating value and reduce the compression volume of the natural gas. While approximately 15% of natural gas reserves in the United States do not meet this threshold, additional sources including fugitive methane emissions from gas wells and coal bed gas also contain large amounts of N2 relative to CH4. For these kinds of gases to be used commercially they need to be treated. Conventional treatment methods such as cryogenic distillation are energy intensive; requiring large economies of scale and intense process integration to improve efficiency. Therefore, the development of some unconventional natural gas resources proves to be very difficult particularly for small and more remote operations. As the easier and better quality fuel stocks become more difficult to extract, the need to improve overall processing techniques by exploring energy efficient and cost effective separation systems becomes more important. The integration of adsorption technology within existing natural gas upgrading processes has the potential to achieve a more economical separation of N2 from hydrocarbon mixtures. However, such technologies have shown limited commercial success as almost all adsorbents are only moderately selective for CH4 over N2, with only a few adsorbents being N2 selective. The screening and characterization of new adsorbent materials is thus necessary prior to implementing such separation technologies on a greater scale. Following an extensive adsorbent screening study, clinoptilolite was found to be a material of interest due to both favourable kinetic and equilibrium separation properties. The objective of this research was thus to modify clinoptilolites through cation exchange to improve the separation of N2 and/or CO2 from natural gas or biogas resources. Clinoptilolite is a naturally occurring aluminosilicate zeolite with a very narrow effective pore size distribution. This adsorbent is unique as it may be either CH4 selective or N2 selective. Since the framework charge of clinoptilolite is negative, charges balancing extra framework cations occupy the channels of the zeolite. This zeolite may be modified through cation exchange to induce favourable separation properties by altering the morphology of the material. Following screening, the adsorption characteristics of CO2, CH4, and N2 on the adsorbents were evaluated and their performances were related to the underlining morphology and elemental composition of these materials. Temperature effects and experimental CH4 and N2 mixture adsorption measurements were compared to multicomponent extensions of single gas adsorption isotherms. These adsorbent materials are promising candidates for the application of natural gas refining. Their implementation into various adsorption technologies are considered. Findings from this research showed the possibility of improved CH4/N2 equilibrium selectivity over natural clinoptilolite using Cs+ and Fe3+ exchanged varieties. Additionally, the possibility of N2/CH4 selective kinetic separations were demonstrated for Ni2+ and Li+ exchanged clinoptilolites. Ag+ exchanged clinoptilolite was shown to be N2 selective over CH4 for potential equilibrium adsorption separations. Finally, the idea of membranes formed of clinoptilolite and a glass binder supported by a porous ceramic alumina support were presented for the first time.