TRANSPORT IN MICROPOROUS SOLIDS PART II: MEASUREMENT OF MICROPORE DIFFUSIVITIES

摘要

The experimental methods that have been developed for measuring diffusion in microporous adsorbents are reviewed with emphasis on macroscopic and single crystal techniques. Comparative diffusion data for selected systems are discussed and the role of crystal defects is considered as a possible explanation for the apparent correlation of macro diffusivities with the scale of the measurement. A wide range of different experimental techniques have been used for studying diffusion in microporous solids; a chronological summary is given in figure 1. In Table 1 these techniques are classified according to the scale of the measurement, the nature of the diffusion process and whether the measurement is carried out under transient or steady state conditions. A full bibliography is included. The largest category (transient, macroscopic measurements) comprises techniques that measure rates of adsorption/desorption, either directly or indirectly, in assemblages of microporous crystals or dual-porosity particles. The macroscopic steady state category includes catalyst effectiveness and membrane permeation measurements. In their simplest form all these techniques yield the transport diffusivity, but variants involving the use of isotopically tagged tracers can be devised to measure the self-diffusivity. Techniques that depend on measurements with a single zeolite crystal but measure transient uptake or permeation, as in the single crystal membrane, are referred to as 'mesoscopic' techniques. The information obtained is essentially similar to that derived from the macroscopic techniques although the data refer to an individual crystal rather than to averages over many crystals.