NAMS 2002 Workshop - ICOM 2008

Gas Separation I – 6
Monday July 14, 12:15 PM-12:45 PM, Kaua’i
Solubility and Diffusivity of Organic Vapors in Mixed Matrix Membranes
Formed by High Free Volume Glasses Loaded with Fumed Silica
M. Ferrari, University of Bologna, Bologna, Italy
M. De Angelis, University of Bologna, Bologna, Italy
M. Galizia, University of Bologna, Bologna, Italy
T. Merkel, MTR- Membrane Technology and Research, Menlo Park, CA, USA
G. Sarti (Speaker), University of Bologna, Bologna, Italy, giulio.sarti@unibo.it
Solubility and diffusivity of mixed matrix membranes based on amorphous
Teflon® AF 2400 or PTMSP loaded with different amounts of nanoscale fumed
silica (FS) have been studied at 25°C. For such systems filler addition induces
variations in density as well as in sorptive capacity that do not obey an additive
rule, at any filler contents. The solubility isotherms of different hydrocarbons in
mixed matrices (MM) with different FS loadings up to 40 wt % are presented and
discussed in some detail, together with the dependence of the apparent
diffusivity on penetrant concentration and filler loading. Remarkably, with
increasing the FS content the penetrant diffusivity increases as well as the
apparent solubility in the polymer phase, in spite of the fact that the filler is
impermeable and endowed with an adsorption capacity smaller than that of the
pure polymers considered. In view of the complex behavior observed, the
characterization of the permeability of the mixed matrix and its selectivity towards
components in mixed gases seem to require an extensive experimental work in
which use is made of different penetrants as well as of different matrices at
various filler contents. Remarkably, analysis of the experimental data collected
clearly indicates that the study of the mixed matrix behavior, i.e. solubility,
diffusivity and permeability, can be greatly simplified and rationalized as follows:
i) consider first a test penetrant and measure the solubility isotherms for the
different mixed matrices at different filler contents; ii) calculate the solubility
isotherms in the polymer phase alone of the MM, by considering that the
adsorption contribution onto the FS surface remains the same as on the pure FS
particles; iii) calculate the effective density of the polymer phase of the MM, or
equivalently calculate the fractional free volume (FFV) of the polymer phase, by
using the NELF model for the solubility in glassy polymers; iv) finally, use the
latter value (density of the polymer phase or FFV) to calculate in a predictive way
the solubility isotherms in the same MM of other penetrants of interest. The
procedure indicated above has been followed in some detail, using n-butane as
test penetrant; the effects of filler content on the polymer FFV and density of the
different MM considered was calculated. From those values the solubility
isotherms of other hydrocarbon vapors (CH4, C2, C3, and C5) and of gases (N2)
were calculated from the Nelf model and the predicted values match in a rather