Effects of main and cross term diffusivities on kinetic separations by pressure swing adsorption

A simple solution to the pressure swing adsorption (PSA) cyclic steady state for a kinetically controlled two-step isothermal process is derived. This solution uses the linear driving force (LDF) model for diffusion rates, non-linear mixed gas isotherms and is general for any number of adsorbing components. The solution permits the calculation of the maximum possible purity for simple PSA cycles. The LDF model is also modified for binary diffusion by including main and diagonal terms of the diffusivities. The concentration dependence of diffusivity and coupling or interference introduced by the addition of these terms affects PSA performance predictions of purity, which improves, and recovery, which diminishes. At fixed low pressures, the reduction in recovery and productivity on inclusion of cross terms may be related to a phenomenon of counter-diffusion.