Improved estimation of zeolite diffusion coefficients from zero-length column experiments

Zero-length column (ZLC) diffusion experiments have been performed on dilute systems of benzene and ethylbenzene in silicalite in the temperature range of 325-485 K. The linear gas-solid exchange model with ideal vapor mixing in the sorption zone and Henry adsorption and Fick diffusion with concentration-independent coefficients was able to fit the full-time response curves. Systematic effects were accounted for in the fitted models, especially for crystal shape, size distribution, and apparatus. The effect of the surface barrier hypothesis at zeolite pore mouths is analyzed. It is shown that accounting for systematic effects and a thorough execution of the parameter estimation process lead to substantially better fits and affect parameter values. The newly established intracrystalline diffusivities are up to one order of magnitude higher than those in the literature. The transient ZLC adsorption coefficients are consistent with values determined independently under equilibrium conditions. (C) 1999 Elsevier Science Ltd. All rights reserved.