Separation of Methane and Nitrogen Using Heavy Reflux Pressure Swing Adsorption: Experiments and Modeling
Guoping Hu, Yalou Guo, Qinghu Zhao, Gongkui Xiao, Gang Kevin Li, Eric F. May
Abstract
Pressure swing adsorption (PSA) is commonly used for the challenging task of separating methane (CH 4 ) and nitrogen (N 2 ) 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% CH 4 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 CH 4 /N 2 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 CH 4 dropped from 79 to 75% as CH 4 was lost into the light product stream. Optimum separation performance in terms of CH 4 purity and recovery occurred at a bed capacity ratio for the purge step of C PU ≅0.87, which could help guide future selections of heavy purge flow rates for a given column size and adsorbent material.