MEASUREMENTS OF EFFECTIVE SELF-DIFFUSION COEFFICIENTS IN A GEL-TYPE CATION-EXCHANGER BY THE ZERO-LENGTH-COLUMN METHOD

The zero-length column (ZLC), conventionally developed for measuring t he diffusivity of gases and liquids in zeolite crystals, has been exte nded to the measurement of intraparticle diffusivities in ion exchange rs. A theoretical model which considers the ZLC cell as a continuous s tirred tank. reactor is developed. The model allows the use of nonline ar ion-exchange equilibrium isotherms, describes ion fluxes with Nerns t-Planck equations, and includes the resistances to mass transfer both in the particle and in the film. Simulations have been carried out to analyze the effect of model parameters on the system response and to choose experimental operating conditions. The model validity has been tested by comparison with the experimental results of the ZLC for a st ep perturbation in the binary systems Na+-H+, K+-H+, and Na+-K+.