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DIFFUSION MEASUREMENTS DURING PERVAPORATION THROUGH A ZEOLITE MEMBRANE
Citation:
Bowen**,TC, J. C. Wyss, M. Yu, J. Falconer, AND R. D. Noble. DIFFUSION MEASUREMENTS DURING PERVAPORATION THROUGH A ZEOLITE MEMBRANE. Presented at 8th International Conf. on Inorganic Membranes, Cincinnati, OH, July 18 - 22, 2004.
Impact/Purpose:
To inform the public
Description:
An isotopic-transient technique was used to directly measure diffusion times of H2O, methanol, ethanol, 2-propanol, and acetone in pure and binary mixture feeds transporting through a zeolite membrane under steady-state pervaporation conditions. Diffusivities can be determined from steady-state permeation if adsorption isotherms are known. Mixture adsorption isotherms for pervaporation components, however, are not known in general and are difficult to measure. The isotopic-transient measurements have potential to determine contributions of diffusion and adsorption using only transport through a membrane.
These measurements were performed with a germanium-substituted ZSM-5 zeolite (MFI structure) membrane. The membrane had an EtOH/H2O separation factor of 54 with a 120 g/m2.h flux for a 5 wt% EtOH/H2O feed at 313 K. This separation and other characterizations indicated that this was a good-quality membrane with few non-zeolite pores larger than the zeolite pores. Isotopically-labeled molecules were added to the feed during steady-state pervaporation, and their transient responses in the permeate were monitored with a mass spectrometer.
Pure component transient measurements showed that MeOH diffuses at approximately the same rate as H2O, but 3.3, 4.8, and 20 times faster than acetone, EtOH, and 2-PrOH, respectively. These relative diffusion rates correlate with molecular sizes and heats of adsorption. Diffusion rates increased exponentially with decreasing inverse temperature, as expected for activated diffusion. During mixture pervaporation, coadsorbed molecules in the membrane were directly shown to slow down or speed up the diffusion rates of each other. For EtOH/MeOH mixtures, EtOH diffused faster and MeOH slower than the corresponding pure component. Similarly, 2-PrOH diffused faster and MeOH slower in 2-PrOH/MeOH mixtures than as pure components. The faster molecule being inhibited and the slower molecule speeding up in a mixture is consistent with observations,[1] simulations,[1, 2] and modeling [2] of other mixtures in zeolites. Interestingly, acetone/MeOH mixtures exhibited qualitatively different behavior; both molecules in these mixtures diffused slower than pure acetone, which was the slower component.
References:
[1] R. Q. Snurr and J. Karger, Molecular Simulations and NMR Measurements of Binary Diffusion in Zeolites, J. Phys. Chem. B, 101 (1997) 6469.
[2] D. Paschek and R. Krishna, Diffusion of Binary Mixtures in Zeolites: Kinetic Monte Carlo versus Molecular Dynamics Simulations, Langmuir, 17 (2001) 247.