Critical Role of Zeolite Adsorbents in a Displacement–Adsorption Separation Process of Methane–Nitrogen Gas

CO2 displacement by pressure-swing adsorption has been considered a highly efficient method for CH4 enrichment from low-concentration coalbed methane. In this work, the displacement-adsorption separation process of zeolite adsorbents and the effect of selectivity and adsorption capacity of different adsorbents on the displacement-separation process were considered. The displacement-separation process was analyzed by the evolution of breakthrough gas and bed distribution at different displacement times, whereas the optimal purity, recovery, and bed utilization distribution of different zeolite adsorbents were intensively investigated. The results showed that for 20% methane feed gas, the large selectivity difference between 5A (1.8) and K-Chabazite (5.3), which possessed similar CH4 adsorption capacity, resulted in a 26% difference (from 73 to 99%) in the maximum CH4 product concentration. In this case, the best enrichment was obtained for K-Chabazite, which increased the CH4 concentration from 20 to 96.8% while maintaining a recovery of 90%. The difference in CH4 adsorption capacity between SAPO-11 (6.8 cm3/g) and SSZ-13 (25.9 cm3/g), which have similar selectivity, resulted in a six times difference (from 8.9 mmol/cycle to 55.1 mmol/cycle) in treatment capacity. Therefore, the selectivity of the adsorbent was more critical in the displacement-adsorption separation process, and the adsorption capacity played an auxiliary role, which was conducive to the production and application of the adsorbent.