Enrichment of Oxygen-Containing Low-Concentration Coalbed Methane with CMS-3KT as the Adsorbent

A type of carbon molecular sieve (CMS-3KT) was used as the adsorbent for the CH₄ enrichment of a methane/oxygen/nitrogen (CH₄/O₂/N₂) mixture using micro-positive pressure vacuum pressure swing adsorption (∼120 kPa). The adsorption isotherms of individual CH₄, O₂, and N₂ on CMS-3KT were studied and fitted. The results indicated the important influence of the adsorbent surface heterogeneity on the adsorption equilibrium process. In addition, the interaction of adsorbent-adsorbate in this process was studied from the measured adsorption heat. The adsorption uptake curves were fitted linearly with a classical micropore model for evaluating the kinetics-based separation possibility of CH₄/O₂/N₂, and the corresponding diffusion time constants of CH₄, O₂, and N₂ were calculated. Based on the results of adsorption equilibrium and kinetics, breakthrough experiments were employed to explore the upper limit value of methane concentration in feed gas such that methane can be enriched feasibly but difficultly. The breakthrough experiments were performed on the CH₄/O₂/N₂ mixture with CH₄ concentration ranging from 1 to 30%. Regarding industrial application, the O₂ removal and CH₄ enrichment performance of ultra-low-concentration methane (CH₄ < 5%) were evaluated according to the results of the breakthrough experiment. The results indicated that the proposed method was promising for enriching O₂-containing ultra-low-concentration CBM.