Abstract
A modified version of the adsorption-diffusion model derived form the Maxwell−Stefan theory developed in a previous study (Pera-Titus, et al. Catal. Today 2006, 118, 73) is presented in this paper to describe the dehydration behavior of zeolite NaA membranes for pervaporation of ethanol/water mixtures. Compared to the former version, two additional contributions are included in the model: (1) the adsorbed solution theory of Myers and Prausnitz is used instead of the extended Langmuir isotherm to account for binary adsorption equilibria of water and ethanol on zeolite A, and (2) the explicit role of pressure-driven mechanisms in large intercrystalline defects (macrodefects) to permeation is considered. These refinements in the Maxwell−Stefan equations provide a superior description of solvent dehydration using zeolite NaA membranes. The fitted surface diffusivities at 323 K and at zero loading of water and ethanol for weak confinement show values in the order of 10-12 and 10-13 m2·s-1, respectively. The former values are 3−4 orders of magnitude higher than those that have been measured from water adsorption kinetics experiments. This difference might be ascribed to a certain role of nanosized grain boundaries between adjacent zeolite A crystals. Grain boundaries might behave as fast diffusion paths or nanoscopic shortcuts due to anisotropy of zeolite layers, resulting in higher apparent water surface diffusivities and lower apparent activation energies for surface diffusion.
Published Version
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