Isotope exchange technique for measurement of gas adsorption equilibria and kinetics

Abstract

AbstractThe isotope exchange technique (IET) can be used to simultaneously measure multicomponent gas adsorption equilibria and self‐diffusivities of the components in a single isothermal experiment without disturbing the overall adsorbed phase. An experimental protocol for the IET and corresponding data analysis procedures is described. Isotherms and self‐diffusivities for adsorption of N2 as a pure gas were measured on commercial samples of a carbon molecular sieve and a 4‐Å zeolite using IET, as well as those of O2 and N2 from their binary mixtures. The carbon molecular sieve did not exhibit thermo‐dynamic selectivity for air separation, but had a kinetic selectivity of O2 over N2 Mass‐transfer resistances for self‐diffusion of N2 and O2 on the carbon molecular sieve were controlled by pore mouth restrictions in the carbon, but those for adsorption of N2 into the 4‐Å zeolite by Fickian diffusion inside the adsorbent. A linear driving force model described the uptakes of N2 and O2 in the carbon molecular sieve. The Fickian diffusion model described the N2 uptake in the 4‐Å zeolite. Mass‐transfer coefficients for both O2 and N2 on the carbon molecular sieve increased linearly with increasing gas‐phase partial pressure of these gases, and the pressure of O2 did not affect mass‐transfer coefficients for N2. The self‐diffusivity of N2 in the 4‐Å zeolite decreased with increasing adsorbate loading.