Modelling gas mixture adsorption in active carbons: DFT vs AST

A slit-pore model and a fast density functional theory (a "slab"-DFT) for predicting gas mixture adsorption in active carbons were presented. The DFT parameters were fitted to reproduce adsorption isotherms of each pure gas in graphitic slit pores generated by Monte-Carlo simulation, and gas-surface interactions were calibrated to a high surface area carbon, rather than a low surface area carbon. These models were used to predict the adsorption of mixtures of CO₂, CH₄, and N₂ up to reasonably high pressure in active carbons based on an analysis of one or more probe isotherms. The activity coefficient of the CO₂/N₂ mixture was dependent on the pore-size distribution (PSD) of the material. The ideality of this mixture increased with increasing pore width. However, for mixtures that were close to ideal even in the smallest pores, e.g., CO₂/CH₄ mixture was much more ideal than CO₂/N₂, the PSD would have less influence and it would be difficult for any theory, no matter how complex, to outperform IAST. Ideal adsorbed solution theory was much quicker than the slab-DFT, but the slab-DFT was much more versatile in that it was not limited to pressures and temperatures defined by the input pure component isotherms. This is an abstract of a paper presented at the AIChE 2004 Annual Meeting (Austin, TX 11/7-12/2004).