Kinetics and equilibrium of adsorption on 4A zeolite

Abstract

Adsorption isotherms for Ar, 0 2, N 2, CO, CO 2, CH 4, and C 2H 6 on 4A zeolite at three or more temperatures were determined. An adsorption equation based on a 2-dimensional virial equation in terms of integer powers of the reciprocal of ( A - σ) was shown to fit the equilibrium data accurately with three constants for C 2H 6 and two constants for other gases. Here A is the area per molecule and σ is the area of the molecule in a close-packed situation. Rates of adsorption and desorption of Ar, N 2, CO, CH 4, and C 2H 6 on 4A zeolite were determined over ranges of temperature in which the rate was moderately fast. Electron microscopy showed that the particles were cubes, and their size-distribution was determined. The conventional Fick's law rate equation for cubes was used to produce a generalized rate curve for the particle size distribution of the adsorbent. This curve was applied to the last 20% of the rate curve to obtain a diffusivity that could be related to the final amount adsorbed. This procedure also avoids the initial rapid portion of the adsorption, in which large variations of adsorbent temperature from that of the bath often occur. The diffusivities increased with amount adsorbed by a small extent for Ar and CH 4 and by larger amounts for N 2, CO, and C 2H 6. The activation energy for diffusion, as well as the heat of adsorption, were nearly independent of amount adsorbed for Ar and CH 4, but these quantities decreased substantially with coverage for N 2, CO, and C 2H 6. The dependence upon amount adsorbed of diffusivity and activation energy seemed related to the shape of the adsorption isotherm; those for Ar and CH 4 were nearly linear, whereas isotherms for the other gases had large curvatures. The activation energy for diffusion varied with coverage in the same way as heat of adsorption.