Separation of Methane and Nitrogen Using Heavy Reflux Pressure Swing Adsorption: Experiments and Modeling

Pressure swing adsorption (PSA) is commonly used for the challenging task of separating methane (CH4) and nitrogen (N2) gas mixtures. Previously we used pilot-scale tests and process simulations to demonstrate how PSA cycles can be optimized for methane-nitrogen separations by adjusting the feed flow, cycle step time, and desorption pressure for a given column size. However, to produce a high-value product stream, dilute feeds with <25% CH4 generally require greater enrichment than can be achieved with optimized conventional cycles. In this work, we investigated the effects of including a heavy product reflux/purge step in PSA cycles on the separation of CH4/N2 using ionic liquidic zeolites (ILZs) as adsorbents through both pilot plant tests and process simulations. In the pilot demonstrations, the use of a heavy purge step allowed the enrichment of feed mixtures with 5.6 and 25.1% methane to 27.4 and 85.5% with recoveries of 83 and 96%, respectively, which outperforms most reported studies under similar operational conditions. However, while the refluxes increased from 74 to 80%, the recovery of CH4 dropped from 79 to 75% as CH4 was lost into the light product stream. Optimum separation performance in terms of CH4 purity and recovery occurred at a bed capacity ratio for the purge step of CPU approximately equal to 0.87, which could help guide future selections of heavy purge flow rates for a given column size and adsorbent material.