Highlighting the Influence of Thermodynamic Coupling on Kinetic Separations with Microporous Crystalline Materials

The main focus of this article is on mixture separations that are driven by differences in intracrystalline diffusivities of guest molecules in microporous crystalline adsorbent materials. Such “kinetic” separations serve to over-ride, and reverse, the selectivities dictated by mixture adsorption equilibrium. The Maxwell–Stefan formulation for the description of intracrystalline fluxes shows that the flux of each species is coupled with that of the partner species. For n-component mixtures, the coupling is quantified by a n × n dimensional matrix of thermodynamic correction factors with elements Γij; these elements can be determined from the model used to describe the mixture adsorption equilibrium. If the thermodynamic coupling effects are essentially ignored, i.e., the Γij is assumed to be equal to δij, the Kronecker delta, the Maxwell–Stefan formulation degenerates to yield uncoupled flux relations. The significance of thermodynamic coupling is highlighted by detailed analysis of separations of five different mixtures: N₂/CH₄, CO₂/C₂H₆, O₂/N₂, C₃H₆/C₃H₈, and hexane isomers. In all cases, the productivity of the purified raffinate, containing the tardier species, is found to be significantly larger than that anticipated if the simplification Γij = δij is assumed. The reason for the strong influence of Γij on transient breakthroughs is traceable to the phenomenon of uphill intracrystalline diffusion of more mobile species. The major conclusion to emerge from this study is that modeling of kinetic separations needs to properly account for the thermodynamic coupling effects.