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 {Eth}_ii for each species was taken to be a tenth of the pure component M-S diffusivity {Eth}_i. For the methanol/acetone mixtures, both species slowed down in the mixture. Adjusting the value of the self-exchange coefficient {Eth}_ii in the M-S equations did not provide an explanation of the observed experimental results; it appeared that the component diffusivities, {Eth}₁ and {Eth}₂, in the mixture were both lower than the values of the pure components, an effect that has not earlier been reported in the literature.