Robert Quinn

New facilitated transport membranes for the separation of carbon dioxide from hydrogen and methane

New facilitated transport membranes which selectively permeate carbon dioxide from hydrogen and methane have been prepared and examined under a variety of conditions. Membranes consist of melts of the salt hydrates tetramethylammonium fluoride tetrahydrate, [(CH3)4N]F·4H2O, or tetraethylammonium acetate tetrahydrate, [(C2H5)4N]CH3CO2·4H2O, immobilized in films of Celgard 3401®. Operating at 50°C, both membranes exhibited CO2 permeabilities which increased with decreasing feed partial pressure of CO2, characteristic of a facilitated transport membrane. Selectivities of CO2H2 and of CO2CH4 increased with decreasing feed pressure since H2 and CH4 permeances were independent of feed pressure. Selectivities of CO2H2 were disappointingly low due to permeation of H2 through the dense phase of Celgard®. With some difficulty, the microporous support was eliminated by construction of membranes consisting of liquid [(CH3)4N]F·4H2O on the surface of a film of poly (trimethylsilylpropyne). Selectivities of CO2H2 as high as 360 were then observed at low feed partial pressures of CO2. Modeling of membrane properties is consistent with permeation of CO2 by a facilitated transport mechanism with reasonable derived diffusivities for the chemically bound CO2 carrier species.

Polyelectrolyte membranes for acid gas separations

Abstract New facilitated transport membranes have been synthesized and shown to selectively permeate carbon dioxide and hydrogen sulfide from methane and hydrogen. The membranes are based on the polyelectrolyte poly(vinylbenzyltrimethyl-ammonium fluoride), PVBTAF, and exhibit exceptional permselective properties. For example, at 23°C and 32 cmHg CO 2 , a PVBTAF composite membrane displayed a CO 2 permeance of 6×10 −6 cm 3 /cm 2 s cmHg and CO 2 /H 2 and CO 2 /CH 4 selectivities of 87 and 1000, respectively. The CO 2 /H 2 selectivity is the highest reported for any membrane. The permeance of both CO 2 and H 2 S increased with decreasing feed partial pressure of the respective gases, a characteristic of facilitated transport membranes. The permselectivity is also dependent on the hydration state of the membrane and is optimal at a gas stream relative humidity in the range 0.25−0.50. The membranes show no deterioration after 30 days of continuous operation but react with trace level sulfur-containing contaminants common to cylinder H 2 S.

Polyelectrolyte-salt blend membranes for acid gas separations

Abstract The CO 2 CH 4 and CO 2 H 2 permselectivity of poly(vinylbenzyltrimethylammonium fluoride), PVBTAF, polyelectrolyte membranes can be significantly improved by blending in certain fluoride-containing organic and inorganic salts. For example, the CO 2 permeance of a PVBTAF-4CsF (4 mol CsF/mol repeat unit) composite membrane was more than four times that of a simple PVBTAF composite membrane while CO 2 CH 4 and CO 2 H 2 selectivities were comparable. Surprisingly, the blends are at least macroscopically homogeneous even with as much a 6 mol salt/mol PVBTAF repeat unit. The optimal salt loading appears to be approximately 4 mol CsF/mol polyelectrolyte repeat unit. Membrane performance is strongly dependent on the relative humidity of the gas streams and is maximized in the range of 30–50% relative humidity. Membranes containing choline fluoride exhibited improved membrane performance at relative humidities below 30%. Permselective data suggests that CO 2 transport is kinetically limited in 10 μm thick films. The blends are stable in CO 2 /CH 4 /H 2 streams for more than 30 days of continuous operation, however, the membranes suffer an irreversible degradation due to reaction with trace level sulfur-containing contaminants common to cylinder H 2 S.

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.

Selective Permeation of Ammonia and Carbon Dioxide by Novel Membranes

Abstract Experimental results are presented on membranes of novel composition which selectively permeate ammonia and carbon dioxide from mixtures containing hydrogen. The CO2-selective membrane, which consists of a thin liquid film of the salt hydrate tetramethylammonium fluoride tetrahydrate, exhibits a CO2 permeance of 4-1 × 10−5 cm3/cm2·s·cmHg with selectivity, α(CO2/H2), ranging from 360-30. The NH3-selective membrane, poly(vinylammonium thiocyanate), displays a high NH3 permeance, 5−20 × 10−5 cm3/cm2·s·cmHg, with α(NH3/N2) as high as 3600 and α(NH3/H2) as high as 6000. Such membranes, which retain H2 at pressure in the feed stream, may offer new opportunities in the design of separation processes.

A repair technique for acid gas selective polyelectrolyte membranes

Abstract The fabrication and permselective properties of polyelectrolyte and polyelectrolyte-salt blend membranes and their utility for acid gas separations have been described. Such membranes permeate acid gases by a facilitated transport mechanism and, in addition, act largely as barriers to the permeation of non-acidic gases. Fabrication of defect free polyelectrolyte membranes is difficult and even a relatively low defect rate can result in a substantial decrease in selectivity. A technique to repair polyelectrolyte membranes is described that surprisingly results in both improved CO2 permeances and selectivities. The technique involves casting an additional polyelectrolyte layer upon a precast polyelectrolyte membrane. Membranes consisting of a layer of poly(vinylbenzyltrimethylammonium fluoride) (PVBTAF) cast upon poly(diallyldimethylammonium fluoride) (PDADMAF) exhibited CO 2 H 2 selectivities and CO2 permeances, which were 2–18 and 1.3–2.3 times greater, respectively, than for those consisting of a single layer of PDADMAF. This is surprising since defect repair is expected to improve selectivity but not permeance. Although the cause of the unexpectedly high CO2 permeances remains unknown, it is perhaps related to a mutual dissolution of the two polyelectrolytes at their interface.

Ion exchange resins as reversible acid gas absorbents

Strongly basic anion exchange resins containing quaternary ammonium functionality and fluoride or acetate anions were found to remove carbon dioxide and hydrogen sulfide from gas streams. The absorption/desorption isotherms, heats of absorption, and gas separation properties were determined for a series of such resins. The fluoride form of Amberlyst® A-26, for example, absorbed CO2 and H2S reversibly, 0.23 mol CO2/mol F− and 0.24 mol H2S/mol F− at 100 kPa and 22 and 30°C, respectively. Absorption of CO2 was fast compared to its desorption. Characterization by NMR indicated that bicarbonate was formed by reaction of CO2 with F− containing resins. Heats of CO2 absorption by F− Amberlyst® A-26 were pressure dependent and ranged from −5.0 to −3.2 kcal/mole CO2 for pressures of 50 to 1000 kPa. The fluoride and acetate containing resins were effective for removal of CO2 and H2S from gas mixtures. Passage of a gas mixture containing 1% CO2 or 5% H2S through a packed column of F− Amberlyst® A-26 at 22°C reduced the CO2 or H2S concentration to less than 25 ppm. Regeneration of the absorbents was accomplished by purging with inert gas at 50°C. Removal of CO2 from gas streams containing substantial water vapor concentrations was achieved using F− Amberlyst® A-26 resin.

Liquid Salt Hydrate Acid Gas Absorbents: An Unusual Desorption Of Carbon Dioxide And Hydrogen Sulfide Upon Solidification

Abstract It was recently reported that certain salt hydrate melts can function as pressure swing absorbents for acid gases. The utility of these salt hydrates derives from their large and reversible acid gas absorption capacities. Typical is the salt hydrate tetramethylammonium fluoride tetrahydrate which as a melt absorbs 0.30 mol CO2/mol salt at 50[ddot]C and 100 kPa CO2. It has now been discovered that the reactivity of some of these salt hydrate melts with CO2 and H2S exhibits an unusual and unexpected temperature dependence. When certain specific salt hydrate melts containing absorbed CO2 were cooled to temperatures which resulted in solidification, CO2 was spontaneously desorbed. For example, a sample of tetraethylammonium acetate tetrahydrate (TEAA) containing 0.15 mol CO2/mol salt at 50[ddot]C and 102 kPa desorbed 90% of its bound CO2 upon cooling to 26[ddot]C. Gas absorption and desorption are completely reversible, and the absorbent can be cycled by simply raising or lowering the temperature thr...