Modeling and Simulation of Rate Induced PSA Separations

Abstract

Koss and coworkers (1), working with experimental data of adsorption kinetics of pure oxygen and nitrogen, found that some carbon molecular sieves deviated considerably from the generally found diffusion-controlled mechanism of adsorption. The molecular sieves that did deviate from the diffusion controlled mechanism presented a rate behavior that could be modeled quite accurately using a mass-action kinetic type of rate expression, such as the Langmuir adsorption kinetic expression, which is widely used in catalytic reaction modeling. Some of the characteristics of these ”Non-Fickian” molecular sieves studied were that the time constant for adsorption became shorter at higher pressures, following a performance mathematically predicted by Langmuir’s kinetic expression (1). The shape of the uptake curve deviated considerably from what would be expected from either Fickian diffusion or an activated diffusion controlled mechanism (see figure 1, and compare with the results of figure 2 for a Fickian CMS) The entire range of kinetic and equilibrium data obtained experimentally for single component gases at different pressures and temperatures of industrial practical interest was predicted accurately using Langmuir adsorption kinetics. In a second paper, Dominguez et al. evaluated two adsorption rate models to predict the adsorption dynamics of mixed gases in non-Fickian carbon molecular sieves. The Langmuir adsorption rate model (which in steady state reduces to Langmuir equilibrium) was found to give adequate prediction in the range of pressures and temperatures studied.

Separation with Carbon Molecular Sieves