Adsorption of gases on carbon molecular sieves used for air separation. Spherical adsorptives as probes for kinetic selectivity

The adsorption of oxygen, nitrogen, and a series of noble gases (neon, argon, and krypton) on a carbon molecular sieve were studied over a range of temperatures above the critical temperature of the adsorptives as a function of pressure in order to understand further the mechanism of air separation. The noble gases were used as probes for the selective porosity in the carbon molecular sieve. The uptakes of all gases studied were virtually linear at low equilibrium pressures in agreement with Henry's law, but deviation occurred at higher pressures. The isosteric enthalpies of adsorption were calculated from the variation in the Henry's law constant with temperature. The adsorption kinetics were studied with different amounts of preadsorbed gas for pressure increments in the range 1-100 kPa. The adsorption kinetics obey a linear driving force mass transfer model for oxygen, nitrogen, argon, and krypton for the experimental conditions studied. The adsorption kinetics for neon deviate from this model, and the data fit a kinetic model which combines diffusion and barrier resistance characteristics. The ratios of the rate constants (k(O-2)/k(N-2)) for each pressure increment in the pressure range 0-9 kPa over the temperature range 303-313 K were typically 25, and this clearly demonstrates the molecular sieving characteristics. The activation energies for the adsorption process were in the order krypton > argon > nitrogen > oxygen similar to neon. The results are discussed in terms of the mechanism of gas separation using carbon molecular sieves.