Correlation and interpretation of zeolitic diffusion coefficients

Abstract

Kinetic data for the sorption of gases in zeolites are generally correlated in terms of Fickian diffusion coefficients. The diffusivities are, however, in general concentration dependent, indicating that the simple Fickian equation gives an inadequate description of the zeolitic transport process. A more convenient parameter for the correlation of kinetic data is the hydrodynamic drag coefficient k, which is related to the diffusivity by the expression : D=(RT/k)(∂ ln p/∂ ln c). Experimental data for the sorption of methane, ethane, propane and n-butane in 5A zeolite are presented. For these systems, the drag coefficients are effectively independent of concentration and the strong concentration dependence of the diffusivity can be entirely accounted for by the non-linearity of the sorption equilibrium isotherms. Correlation of the temperature dependence of the diffusivity in terms of an Eyring equation is unsatisfactory since the apparent activation energy is concentration dependent. A more satisfactory correlation may be obtained by considering separately the temperature dependence of the drag coefficients and the equilibrium isotherms. For the four sorbates studied, the drag coefficients may be expressed as a single function of reduced temperature. Comments on the mechanistic interpretation of the data are also included.