Maxwell-Stefan modelling of mixture desorption kinetics in microporous crystalline materials

In kinetically driven separations using microporous crystalline adsorbents, the more mobile component is recovered during the desorption phase of pressure swing adsorption (PSA) processes. The primary focus of this article is on the modelling of mixture desorption kinetics. The Maxwell-Stefan (M-S) formulation of intracrystalline diffusion is used to model the transient desorption of four different binary mixtures (N₂/CH₄, CO₂/C₂H₆, O₂/N₂, and C₃H₆/C₃H₈) in fixed beds packed with LTA-4A, DDR, CHA, or ZIF-8, that have cage-window structural topologies. The infra-crystalline flux of each species is coupled with that of partner species; the coupling effects are quantified by a 2 x 2 dimensional matrix of thermodynamic correction factors whose elements Gamma_ij can be determined from the model used to describe mixture adsorption equilibrium. For desorption from a single crystallite, thermodynamic coupling effects cause transient undershoots in the loading of the more mobile component. If the Γ_ij are assumed to equal δ_ij, the Kronecker delta, no undershoots are realized. The recovery of the more mobile component during the blowdown phase of PSA operations is significantly enhanced by the off-diagonal elements of Γ_ij. The major conclusion to emerge from this study is that modelling of mixture desorption kinetics need to properly account for thermodynamic coupling effects.