Hydrogen sulfide separation from gas streams using salt hydrate chemical absorbents and immobilized liquid membranes

The salt hydrate tetramethylammonium fluoride tetrahydrate, [(CH3)4N]F·4H2O, in the liquid state reversibly absorbs large quantities of hydrogen sulfide, for example 0.30 mol H2S per mole of salt at 50°C and 100 kPa. Gas absorption likely occurs by deprotonation of H2S to form bisulfide, HS− , and bifluoride, HF2 −. The equilibrium constant for the reaction of H2S with [(CH3)4N]F·4H2O is 1.4 × 10− 2 kPa− 1 at 50°C as determined from absorption/desorption data. The heat of H2S absorption was surprisingly low, −0.78 kcal mol− 1. A second salt hydrate, tetraethylammonium acetate tetrahydrate, [(C2H5)4N]CH3CO2·4H2O, also reversibly absorbs H2S but with a lower affinity. However, at higher pressures, its H2S capacity increases more than does that of [(CH3)4N]F·4H2O making it more suitable for use as a pressure swing absorbent. Absorption/desorption data are consistent with reaction of one mole of [(C2H5)4N]CH3CO2·4H2O for each mole of H2S and an equilibrium constant of 6.4 × 10− 4 kPa− 1 mol H2S per mole of salt. Membranes consisting of liquid [(CH3)4N]F·4H2O immobilized in a microporous support exhibit selective permeation of H2S from CH4 and CO2. Permeabilities of H2S increased with decreasing feed pressure, consistent with facilitated transport of H2S. The H2S/CH4 selectivities ranged from 140 to 34 and decreased with increasing feed pressure while H2S/CO2 selectivities were 8–6. The presence of H2S in the feed tends to suppress permeation of CO2, implying that both gases compete for the same carrier species.