Separation of carbon dioxide–methane mixtures by adsorption: Effects of geometry and energetics on selectivity

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Abstract
The adsorption of methane–carbon dioxide mixtures in micropores having either slit or cylindrical geometries, and different adsorption energies, has been studied using molecular simulation. The pore models were constructed from close packed layers of simple atoms whose Lennard-Jones interaction parameters could be varied. For realistic choices of the adsorbent parameters, adsorption selectivity outside the molecular sieving regime was found to vary over a range between about 4 and about 1/4, denoting a preference for either CO2 or for methane, depending on geometry and choice of parameters. The variations in selectivity are interpreted in terms of a balance between energetic factors, including adsorbate–adsorbent interactions and cooperative adsorbate interactions, and entropic factors associated with the constraints on rotational freedom of the CO2 molecule imposed mainly by the geometry of the pore spaces.