Co-adsorption of N2 in the presence of CH4 within carbon nanospaces: evidence from molecular simulations

Molecular simulations were performed to study the separation of CH4 and N2 from mixtures of composition xCH4/xN2 = 5/95 and xCH4/xN2 = 10/90 at 50 bar and 298 K on prototype carbon materials with different pore structures. The studied carbon structures include a slit and a tubular pore, that represent the simplest form of activated carbon and carbon nanotubes, respectively, in addition to a realistic porous carbon model with disordered pore structure and a recently introduced carbon foam model, which has a three-dimensional pore structure. The results indicate that, depending on the pressure and composition, the pore structure influences both the CH4/N2 selectivity and the adsorption behaviour of the fluid molecules. The selectivity was decided by the interactions between CH4 and N2 molecules within the pore structure, in addition to the solid–fluid interactions. The simulation results indicate that, at least for the case of activated carbons (slit and random pores), it would not be appropriate to predict the binary adsorption behaviour of methane and nitrogen by means of pure component information. Regardless of the pore structure, the simulation results indicate that carbon materials show a CH4/N2 (thermodynamic) selectivity of only 2–3 up to 2 bar at 298 K, and above this pressure, at equilibrium, none of the carbon materials is adequate for the efficient separation of this mixture.Support from the Ministerio de Ciencia e Innovación (Joint Japan–Spain project PLE2009-0052) is acknowledged. KVK would like to thank Ministerio de Ciencia e Innovación (Spain) for a Juan de la Cierva contract