Isotope exchange technique for measurement of gas adsorption equilibria and kinetics

The 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 N-2 as a pure gas were measured on commercial samples of a carbon molecular sieve and a 4-Angstrom zeolite using IET, as well as those of O-2 and N-2 from their binary mixtures. The carbon molecular sieve did not exhibit thermodynamic selectivity for air separation, but had a kinetic selectivity of O-2 over N-2. Mass-transfer resistances for self-diffusion of N-2 and O-2 on the carbon molecular sieve were controlled by pore mouth restrictions in the carbon, but those for adsorption of N, into the 4-Angstrom zeolite by Fickian diffusion inside the adsorbent. A linear driving force model described the uptakes of N-2 and O-2 in the carbon molecular sieve. The Fickian diffusion model described the N-2 uptake in the 4-Angstrom zeolite. Mass-transfer coefficients for both O-2 and N-2 on the carbon molecular sieve increased linearly with increasing gasphase partial pressure of these gases, and the pressure of O-2 did nor affect mass-transfer coefficients for N-2. The self-diffusivity of N-2 in the 4-Angstrom zeolite decreased with increasing adsorbate loading.