Separation of NF3 and CF4 using amorphous glassy perfluoropolymer Teflon AF and Hyflon AD60 membranes

Abstract

In this study, the pure and mixed gas permeabilities of Teflon AF2400, Teflon AF1600 and Hyflon AD60 membranes towards NF3 and CF4 were measured to determine whether membrane gas separation can be applied to purify NF3 of CF4. In accordance with literature results, it was shown that thermal annealing of the solution cast films was necessary to reach optimum performance, wherein all membranes studied had a preferential permeation of NF3 rather than CF4. The Teflon AF and Hyflon AD60 membranes displayed rather high pure and mixed gas selectivities (α(NF3/CF4)) considering the high free volume of the polymers. Furthermore, the α(NF3/CF4) increased with increasing diffusion selectivity of the glassy perfluoropolymers, of which the pure gas He/N2 ideal selectivities gave an indication, and which is related to the fractional free volume (FFV). As a result, Hyflon AD60 displayed the highest NF3/CF4 pure and mixed gas selectivity of just above 12, albeit with a rather low NF3 permeability of ca. 1.9Barrer. Although the membranes were sufficiently inert towards penetrant induced swelling, a Hyflon AD60 membrane swollen by residual casting solvent displayed an increase in the pure and mixed gas NF3 and CF4 permeabilites and reduced selectivity compared to that of an annealed, fully relaxed Hyflon AD60 membrane.