(396f) Modeling of Diffusion and Chemical Reactions in Ion-Exchange Resins during Swelling and Shrinking

Ion-exchange resins used in chromatographic separation, ion exchange, and heterogeneous catalysis are elastic polymeric materials. The extent of swelling of the polymer, and thus the dimensions of resin particles, change in response to changes in, for example, the solvent composition and salt content of the surrounding liquid. This is reflected to the intraparticle mass transfer rates through the mesh width of the apertures in the polymer network.

Mass transfer and chemical reactions in sulfonated PS?DVB ion exchange resins during shrinking and swelling of the particles were investigated experimentally and with computer simulations. A specially constructed flow-through cell and an optical microscope were used to monitor the diffusion induced volume changes of the resins. Esterification of acetic acid with ethanol and binary water?acetic acid mixtures were used as reactive and non-reactive model systems.

The applicability of particle diffusion models based on the Fick's law and the Maxwell?Stefan approach, as well as two-point approximations of these, was investigated. In the Maxwell?Stefan approach, the chemical potentials of the mobile species were calculated with models derived from thermodynamics of polymer solutions and gels by taking into account the elastic nature of the polymer. The effect of swelling ratio on the diffusion coefficients was described with a geometric obstruction model for the sake of simplicity. The governing equations were written and solved in polymer mass coordinates, which enables faster and more accurate numerical solution.

The choice of the mass transfer model was found to have a large impact on the predicted swelling behavior of the resins. It was shown that, owing to the elastic nature of the resins, diffusion coupled with volume changes of particles was best described with a model that explicitly takes the cross-links in the polymer into account.