Pure and mixture adsorption equilibria of methane and nitrogen onto clinoptilolite: effects of Cs+ and Fe3+ exchanged cations on separation performance

Cation exchanged clinoptilolite has emerged as an adsorbent that may be used for the treatment of natural gas type mixtures containing excess amounts of nitrogen. In this study, natural clinoptilolite was modified through Fe 3+ and Cs + cation exchange. The resultant mineral phase and elemental composition were verified and the extra framework cation type, charge, and distribution is shown to have a profound effect on the observed adsorptive properties. Single gas adsorption isotherms were conducted at 15 °C and/or 30 °C for CH 4 and N 2 using a microgravimetric adsorption analyser, and the ideal equimolar CH 4 /N 2 selectivity values were determined for the natural and modified clinoptilolites. Natural clinoptilolite presents selectivity values close to 1. However, we demonstrate that clinoptilolite cation-exchanged with Cs + and Fe 3+ cations is rendered preferentially adsorptive for CH 4 over N 2 with a higher CH 4 /N 2 selectivity than many adsorbents reported in the literature. The experimental CH 4 –N 2 binary adsorption behavior was evaluated for the first time on raw clinoptilolite and was compared to the modified clinoptilolites with Cs + and Fe 3+ exchanged cations using the concentration pulse chromatographic technique. The experimental binary isotherms showed non-ideal behavior with competitive adsorption between CH 4 and N 2 . In most cases, the theoretical models could not adequately describe the experimental adsorption behavior. Within the temperature range studied of 15 °C and 30 °C, the binary CH 4 /N 2 separation factors range from 2.7 to 5.3, and 3.9 to 7.3 for Cs + and Fe 3+ exchanged clinoptilolite, respectively. These results are comparable or better than those found in literature for activated carbon which typically ranges from 2.1 to 5.5 and may serve as an alternative adsorbent for this type of separation which warrants further investigation.