Permeation and separation of SO2, H2S and CO2 through thermally rearranged (TR) polymeric membranes

The permeabilities of sulfur dioxide and hydrogen sulfide are reported here for two copolymer based TR membranes, as a function of feed partial pressure and temperature. The two TR membranes are formed from the precursor co-polyimide 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4,4′-diaminobiphenyl (HAB) and 2,4,6-trimethyl-m-phylenediamine (DAM) (6FDA-HAB-DAM), where one of the membranes undergoes crosslinking with 3,5-diaminobenzoic acid (DABA) during TR. It was observed that SO2 permeability decreased with increasing partial pressure and temperature in both TR membranes. This is indicative that SO2 permeability is dominated by SO2 solubility within the TR membranes, and is attributed to the high condensability of SO2. Both polymeric membranes displayed selectivity for SO2 against CO2, with separation performance similar to other classes of polymeric membranes. H2S permeability was observed to decrease with increasing partial pressure and temperature for the cross-linked TR (XTR) membrane. The non-cross-linked TR membrane displayed constant H2S permeability with pressure and permeability increased with increasing temperature. Hence, permeability within the XTR membrane was H2S solubility dominated, while in the non-crosslinked TR membrane permeability was H2S diffusivity dominated. This was attributed to the difference in free volume and cavity size distribution in the TR membrane as a result of crosslinking. It was observed that both membranes displayed H2S/CO2 selectivity of less than one, indicative that they favor CO2 over H2S.