Separation of Methane-Nitrogen Mixture by Pressure Swing Adsorption for Natural Gas Upgrading

A dynamic pressure swing adsorption simulation model has been developed that caters for a detailed transport mechanism of nitrogen and methane in the micropores of both ETS-4 and CMS adsorbents. Five adsorbents, namely, barium-exchanged ETS-4 dehydrated at 400 degrees C (Ba400), strontium-exchanged ETS-4 dehydrated at 190 degrees C (Sr190) and 270 degrees C (Sr270), Bergbau-Forchung carbon molecular sieve (BF CMS), and Takeda carbon molecular sieve (Takeda CMS), have been selected for the simulation study in order to compare their performances for upgrading natural gas. The transport mechanisms of gases in CMS and ETS-4 adsorbents are different. In pressure swing adsorption (PSA) simulation, the binary equilibrium and kinetics are represented by the models that have recently been experimentally verified for methane-nitrogen mixture in Ba400, Sr190, and Sr270 adsorbents [Majumdar, B.; Bhadra, S. J.; Marathe, R. P.; Farooq, S. Ind. Eng. Chem. Res. 2011, 50, 3021]. The multisite Langmuir isotherm is used for adsorption equilibrium. The kinetic model takes into account diffusion in both macropores and micropores and the concentration dependence of micropore diffusivity according to the chemical potential gradient as the driving force with constant limiting micropore diffusivity. It also has the provision to allow for the dual transport resistance and strong concentration dependence of the thermodynamically corrected micropore transport coefficients in CMS according to the published results [Huang, Q.; Farooq, S.; Karimi, I. A. Langmuir 2003, 19, 5722]. Operating conditions have been identified that favor high recovery while simultaneously meeting the required pipeline specification for methane purity. The performance of the best sample for methane-nitrogen separation by PSA found from the simulation study, Ba400, is compared with published performances of ETS-4 and clinoptilolite. It has been found that, in addition to meeting pipeline specification, Ba400 also gives higher recovery, thus making this adsorbent a promising candidate for further exploration.