CHARACTERIZATION OF CERAMIC MEMBRANES .2. MODIFIED COMMERCIAL MEMBRANES WITH PORE-SIZE UNDER 40 ANGSTROM

A methodology was developed for characterizing a series of membranes w ith pore size less-than-or-equal-to 40 angstrom. These membranes were modified from commercial microporous gamma-Al2O3 membranes (40 angstro m) by pore size reduction. SEM/EDAX analysis showed a modified layer w ith a thickness of approximately 1.5 mum deposited within the existing porous structure. A flow-weighted pore size distribution analyzer was used to determine pore size versus flow contribution for the pore siz e ranging from 40 angstrom down to approximately 15 angstrom. Size exc lusion with selected gaseous molecules was employed to determine pore size < 6 angstrom. Pore size not accounted for by the above two method s was assigned to 15-6 angstrom. Gas permeation of helium and nitrogen through the pore size ranging from > 40 to less-than-or-equal-to 5 an gstrom at 25-degrees-C was investigated in detail. Selectivity decreas ed slightly along with the pore size reduction in the Knudsen regime ( i.e., 40 angstrom down to approximately 20 angstrom), then increased d ramatically in the molecular sieving regime. The unexpected decrease o f the selectivity in the Knudsen regime was likely to have resulted fr om the contribution of surface diffusion of nitrogen.