Measurement of intracrystalline diffusion in NaX zeolite by capillary column gas chromatography

Abstract

Chromatographic HETP (height equivalent to a theoretical plate) measurements are reported for a series of hydrocarbon sorbates ( n-butane, n-hexane, cyclohexane, n-octane, benzene and decalin) in a capillary column (150 μm diameter) packed with 100-μm crystals of NaX zeolite. Such a column is superior to a conventional-packed column in terms of both heat transfer and external mass transfer characteristics. Dimensionless plots of reduced HETP versus reciprocal Peclet numbers show clearly that, for the more strongly adsorbed species, the HETP can be quantitatively accounted for by external mass transfer resistance and axial dispersion so that only lower limits for intracrystalline diffusivities can be obtained. The HETP data for butane, cyclohexane and decalin show clearly the intrusion of internal (micropore) diffusional resistance so, for these species, reasonably reliable intracrystalline diffusivities can be derived. The diffusivities so obtained (for butane and cyclohexane) are smaller than previously reported nuclear magnetic resonance (NMR) self-diffusivities but larger than the zero-length column (ZLC) values.