Mass transfer rates of oxygen, nitrogen, and argon in carbon molecular sieves determined by pressure-swing frequency response

Abstract

The adsorption rates of pure O2, N2, and Ar have been measured at pressures from 0.125bar to 1bar on two carbon molecular sieve materials, Shirasagi MSC-3R type 162 and type 172, using pressure-swing frequency response. For each material, O2 adsorbs much faster than the other two gases, with Ar being the slowest of the three. Adsorption rates of N2 and Ar on both materials obey the linear driving force rate equation, indicating that a barrier resistance is rate limiting. Adsorption of O2 on both materials is best described using a combined resistance model, which treats a barrier resistance in series with a micropore diffusion resistance. However, the contribution of micropore diffusion to O2 adsorption rates is small. The experimental barrier resistance coefficients for all gases on both materials increase with pressure. The pressure dependence of the barrier coefficient can be explained by the existence of either Langmuir kinetics with a distribution of pore sizes or shell diffusion where the adsorbent with constricted pores in the shell has different equilibrium characteristics than the adsorbent in the core of a particle. The two materials investigated in this work demonstrate high kinetic selectivity for O2 over N2 and Ar, suggesting that they could be useful in pressure-swing adsorption processes designed to generate a high-purity O2 product.