The window effect in zeolitic diffusion

Gorring’s 1973 experimental data, which showed an unusual oscillatory variation of the apparent intracrystalline diffusivity of n-alkanes in zeolite T with carbon number (termed the window effect), have been critically re-examined. Although the experiments were conducted under non-linear and non-isothermal conditions the uptake rates were interpreted according to the linear isothermal diffusion model. The unusual variation of apparent diffusivity with chain length appears to be an artifact of this analysis. It is shown that the observed pattern of variation in uptake rates can be accounted for by the intrusion of finite heat transfer resistance and isotherm non-linearity, assuming only the expected monotonic decrease of intrinsic diffusivity with carbon number.