Modeling transient permeation of polar organic mixtures through a MFI zeolite membrane using the Maxwell–Stefan equations

Isotopic transient permeation of methanol/ethanol, methanol/2-propanol, and methanol/acetone mixtures through a MFI zeolite membrane was investigated experimentally. For the methanol/2-propanol and methanol/ethanol mixtures, the more mobile species (methanol) was slowed down in the mixture and the tardy species (2-propanol or ethanol) was speeded up. The extent of slowing down and speeding up depended on the mixture composition. The Maxwell–Stefan (M–S) diffusion equations reproduced the observed mixture permeation results, at least qualitatively, provided the self-exchange coefficient Ðii for each species was taken to be a tenth of the pure component M–S diffusivity Ði. For the methanol/acetone mixtures, both species slowed down in the mixture. Adjusting the value of the self-exchange coefficient Ðii in the M–S equations did not provide an explanation of the observed experimental results; it appeared that the component diffusivities, Ð1 and Ð2, in the mixture were both lower than the values of the pure components, an effect that has not earlier been reported in the literature.