Modeling Pervaporation of Ethanol/Water Mixtures within ‘Real' Zeolite NaA Membranes

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 m 2 ‚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.

Water adsorption in hydrophilic zeolites: experiment and simulation

“…The rest of the experimental data presented in Figs. 3 and 4 8,47,48 deviates from the matching isotherms, and we will neglect them in the present study.…”

Reactivity of Brönsted acid ionic liquids as dual solvent and catalyst for Fischer esterifications

“…Whereas when zeolite 4A (a zeolite that is believed to only adsorb water from the above esterification system [36,37]) was used to adsorb the formed water, the yield of acetate slightly increased by ca 3%, compared to the corresponding reactions carried out in the general 'flask-reflux' reactors (entries 27 vs. 29 and 28 vs. 30). Based on our results, we therefore conclude that the removal of water from the ionic liquid-catalyzed esterification reaction system is necessary in order to obtain a higher yield of ester, although ionic liquid is generally viewed as a kind of water-sequester solvent and can be used to tailor the equilibrium conversion of esterification [38,39].…”

Mass transfer simulation on pervaporation dehydration of ethanol through hollow fiber NaA zeolite membranes

“…In general, the PV process through zeolite membrane is described by the adsorption-diffusion mechanism. Pera-Titus et al 10,11 developed an adsorption-diffusion model derived from the Maxwell-Stefan theory to describe the PV separation of water/ ethanol mixture through NaA zeolite membranes. They revealed that the adsorption of water and ethanol at the liquid feed/membrane interface made great contribution to the PV performance.…”