NAMS 2002 Workshop - ICOM 2008

pressure, and permporosimetry with benzene present a consistent picture of
membrane properties.
Permporosimetry, temperature-programmed desorption, and pervaporation of
mixtures clearly show that propane, n-butane, i-butane, n-pentane, n-hexane, noctane,
and SF6 all decrease the size of defects in MFI membranes by swelling
MFI crystals when they adsorb. Although XRD and optical microscopy studies
show that crystal expansion is less than 0.5% linearly for MFI crystals, such
expansion can essentially seal 2-nm membranes. For the membrane with 90% of
its helium flow through defects, n-hexane adsorption decreased the helium flow
almost three orders of magnitude. This membrane also had a 2,2-dimethylbutane
flux during pervaporation that was 160 times its n-hexane flux; that is, the larger
molecule permeated 160 times faster because the defects that it permeated
through were almost sealed off by n-hexane adsorption. The defects were sealed
at much less than saturation loadings in the MFI crystals. The loading required to
decrease the flux through defects by more than two orders of magnitude
increased as the number of carbon atoms in the alkane decreased. These results
may explain many inconsistencies for MFI permeation in the literature. They also
indicate that flow through defects can be more important at higher feed
concentrations, and show that characterizations at high feed concentrations
provide a better indication of membrane quality.