For the first time, a macroscopic method was used to measure directly diffusivity of fast diffusing species such as methane in silicalite (figure 14). A form of Wicke–Kallenbach technique was used to measure intracrystalline diffusivity. The technique uses a single embedded zeolite crystal as a membrane and a mass-selective detector to determine the transient mass response and hence the diffusion flux passing through the membrane. A concentration-difference driving force was used rather than a pressure-difference driving force. The diffusivities calculated from both adsorption and desorption transients were in excellent agreement, thus providing a check for the self-consistency of the data. The diffusivities of carbon dioxide, methane, ethane, propane and butane in silicalite between 30 and 70C were compared with those from the literature. The diffusivities reported here were between those measured with microscopic methods and those measured with other macroscopic methods.

Diffusion Measurements through Embedded Zeolite Crystals. M.S.Sun, O.Talu, D.B.Shah: AIChE Journal, 1996, 42[11], 3001-7