Diffusion mechanism of N2 and CH4 in pelletized zeolite 4A, 5A and CaX

The adsorption kinetics and equilibria of N-2 and CH4 in pelletized zeolite 4A, 5A and CaX were studied theoretically and experimentally at the range of 273-293 K and 0.0-0.8 atm. Using the isothermal and nonisothermal adsorption rate models, the diffusion mechanism was investigated from the experimental uptake curves through a gravimetric method. While the diffusion rates of N-2 and CH4 were controlled by micropore diffusion in zeolite 4A, they were significantly affected by macropore diffusion in zeolite 5A. In the case of zeolite CaX, the controlling mechanism of diffusion rate was changed from macropore diffusion to micropore diffusion as the pressure increased. The diffusion rate in the N-2/4A system was faster than that in CH4/4A due to the sieving effect in the crystal pore. In both N-2 and CH4, the diffusion rates in zeolite 5A were faster than those in zeolite CaX because of the slope of adsorption isotherm. The diffusion rate obtained from the nonisothermal model was similar to the diffusion rate from the isothermal model in the range of tow pressure, but showed a significant difference at the high pressure because the slope of the isobar became great with pressure in the case of a linear isotherm. In the case of the systems controlled by micropore diffusion, the micropore diffusivity showed a moderately increasing trend with fractional coverage according to the Darken equation. In the systems controlled by macropore diffusion, the effective macropore diffusivity increased with pressure due to its favorable isotherm, and the macropore diffusivity kept a constant value in the domain of Knudsen diffusion.