(75c) PSA Separation of CH4/N2 Mixture with Carbon Molecular Sieve

Coalbed methane (CBM) is a kind of unconventional natural gas, and liquefaction is a good alternative to make use of it. In China, CBM often contains air when it comes out of the well. Considering issues regarding safety, it is natural that oxygen must be removed firstly. Then, the effective enrichment of methane from methane-nitrogen mixture is one of the most important issues in the liquefaction of CBM. In this paper, a two-bed pressure swing adsorption (PSA) experimental apparatus was used to separate CH₄/N₂ mixture on a selected type of carbon molecular sieve (CMS) adsorbent. Owing to the kinetic separation mechanism on CMS, nitrogen is adsorbed on CMS, while methane with certain pressure is delivered continuously to the next process of liquefaction. The influence of adsorption pressure, feed gas velocity and cycle time on the overall performance of the adsorption process was discussed. Feeding a mixture of 50% CH₄/50% N₂, methane purity of 95.45% was obtained; and 94.89% purity was obtained while feeding a mixture of 30% CH₄/70% N₂. The results show that all three parameters may affect the separation performance, and the later two have greater influences. It is easier to achieve methane purity above 90% at low adsorption pressure and low feed gas velocity. And the shorter the cycle time, the higher the methane purity is, yet the lower the methane recovery rate is.