MOLTEN SALT HYDRATE MEMBRANES FOR THE SEPARATION OF GASES

211-P-US03677 MOLTEN SALT HYDRATE MEMBRANES FOR THE SEPARATION OF GASES TECHNICAL FIELD OF THE INVENTION The present invention relates to gas separation processes using membranes which are highly permeable yet selective to the passage of a particular component of a gaseous mixture.

BACKGROUND OF THE INVENTION Numerous approaches to the separation of a gas from a mixture of gases by differential permeation have been investigated. Robb, et al., in U.S. Patent 3,335,545, have proposed use of a liquid, entrapped in a porous or permeable support, to separate mixtures of gases. The use of a so-called "quasi-liquid film" for example, diethylene glycol, in a support has permitted separation of carbon dioxide from nitrogen, hydrogen or oxygen, the selectivity being somewhat higher than would be predicted on the basis of the molecular weights of the gases as the sole factor influencing separation. Ward, III, has proposed, in U.S. Patent 3,503,186, a similar procedure for separating sulfur dioxide from other gases.

The use of facilitated transport liquid membranes has been reviewed by Way, et al., J. Membrane Science, vol. 12 (1982), pages 239-259.

Another typical disclosure of gas separation using a facilitated trans¬ port immobilized membrane is that of Bassett, at al., Biochemica et Biophysica Acta, vol 211 (1970), pages 194-215. Facilitated transport of gases through liquid membranes is also recited for example, in the following U.S. Patents:

3,396,510 Ward, III, et al.

3,676,220 Hard, III 3,819,806 Ward, III, et al.

4,014,665 Steigelmann 4,015,955 Steigelmann, et al.

4,119,408 Matson 4,147,754 Hard, III 3() 4,174,374 Matson Ward, III, et a). '510 recite using an immobilized liquid film as a permeable membrane, wherein the liquid film contains at least one sol¬ uble, non-volatile, dissolved carrier species, which is reversibly chemically reactive with a selected component of a gaseous mixture.

Ward, III, et al. '806 discloses the formation of an immobilized liquid membrane for the separation of an acid gas from other components of a gas stream. Transport of the acid gas through the immobilized liquid membrane is facilitated by the addition of a water-soluble salt to the liquid membrane.

Steigelmann '665 discloses a membrane-liquid barrier system having a complex-forming, silver-containing ion component in an aqueous solu¬ tion. The membrane-liquid barrier combination can be used to separate olefinically-unsaturated hydrocarbons from mixtures containing them and other components, such as alkanes.

Kimura et al. (U.S. Patent 4,318,714) have r'ecited using an ion- exchange membrane to accomplish facilitated separation of a gas from a mixture of gases.

Yamamoto et al. (U.S. Patent 3,155,467) have disclosed separation and purification of hydrogen, using a palladium alloy as a permeable wall structure.

Solid and molten salt electrolytes have been disclosed, in the fuel cell or electrochemical arts, in the following representative patents:

3,400,054 Ruka et al.

3,432,363 Gillis 3,527,618 Bushnell 3,727,058 Schrey 4,317,865 Trocciola et al.

U.K. 2,082,156 Yoshisato et al.

A limiting feature in many of these disclosures is that a gas should not completely permeate the electrolyte or electrode, since complete penetra¬ tion could short circuit the device.

Batigne et al., U.S. Patent 4,396,572, recite using a porous ceramic barrier, having a plurality of superimposed layers of pastes of varying composition, to separate uranium hexafluoride isotopes by ultrafiltration.

Fleck, U.S. Patent 2,983,767, teaches separating a fluid mixture of organic compounds by passing the mixture over a diffusion barrier con¬ taining a specified Werner metal complex in the form of a crystalline solid which forms a clathrate inclusion compound with one or more compo- nents of the fluid.

Erickson, U.S. Patent 4,132,766 teaches a process for separating oxygen from air by contacting the air with an oxygen acceptor comprised of a molten solution of alkali nitrite and nitrate salts at elevated temperature and pressure to react with the oxygen. The oxygen is subse- quently recovered by reducing the pressure while supplying heat to the salt solution.

Trocciola, et al., U.S. Patent 3,847,672 discloses a fuel cell system comprising a gas separator. The gas separator comprises a tile or block of salt in a molten or solid state having opposed reactive sur- faces. A gas stream containing hydrogen and carbon dioxide is fed to one surface of the salt at which surface the CO- is chemically taken up by the salt. A sweep or stripping gas is maintained at the downstream surface of the salt tile at which surface CCL is released.

Dounoucos, U.S. Patent 3,447,286 discloses a totally enclosed liquid membrane for the separation of gases. The membrane comprises a porous body having direct channels extending therethrough from face to face and providing an open area completely enclosed together with a selected liquid filling the channels between non-porous layers of solid perm- selective membrane material.

Pez, et al., U.S. Patent 4,617,029 teaches a process for separating a gas from a mixture of gases by passing the gas mixture over a membrane selectively permeable by the gas being separated, owing to the occurrence of one or more reversible oxidation-reduction reactions between the gas being separated and a continuous layer of active molten material im- mobilized in a rigid, porous, inert support.

BRIEF SUMMARY OF THE INVENTION The present invention is a process for separating at least one component from at least one other component in a gaseous mixture by passing the gaseous mixture over a membrane selectively permeable to the _ 4 - gas being separated, which membrane comprises a thin film of a molten salt hydrate. The molten salt hydrate may be immobilized within the pores of a thin, porous inert support or alternatively, may be encap¬ sulated in a non-porous, gas permeable polymer or polymer blend material.

Generally, the molten salt hydrates comprise a cationic species, an anionic species and water, and membranes comprising such hydrates are typically well suited for separations such as the separation of C02 from CH- and H2, H2S from CH4 and H2, olefin gases from alkanes and also for the separation of water vapor from air or nitrogen.

DETAILED DESCRIPTION OF THE INVENTION The present invention involves the use of molten salt hydrates supported in porous supports or encapsulated in polymeric materials as membranes for gas separations.

Molten salt hydrates can be used to fabricate immobilized and/or encapsulation molten salt hydrate membranes. The molten salt hydrate material either as a single component or with a limited amount of added water can be immobilized within the pores of a thin, porous inert sup- port, or a film of the hydrate may simply be layered on a relatively gas permeable polymer. Alternatively, the hydrate material may be encapsu¬ lated in a non-porous, gas permeable polymer or polymer blend material such as poly(4-methyl-l-pentene), poly(trimethylsilylpropyne) or silicone rubber.

Typically, a salt can be defined as a substance which is comprised of cations and anions and which is characterized by overall electrical neutrality. A salt hydrate is a salt which contains bound water. The composition of a salt hydrate is usually represented by a formula of the form AxBy«»nH20 where A and B are ionic species of opposite charge and n is the number of moles of bound water per mole of salt. The term "salt hydrate" does not indicate the manner or degree of tenacity by which the water molecules are held to the salt. However, it has been suggested that salt hydrates do not contain water in excess of the primary hydration sphere of the ions.

Many salts exist as hydrates and upon heating, melt to yield liquid systems which contain bound water. Such systems are known as "molten salt hydrates" or "hydrate melts" (see Angel 1, C. A.; Green, D. M. 3. Am. Chem.

Soc., 88, 5192 (1966)) and may be viewed as comprising a special class within the broad area of molten salts. Examples of substances which yield molten salt hydrates are MgCa2»6H20 (melting point (m.p.) IWC), Ca(N03)2»4H20 (m.p. 390C) and CaCft296H20 (m.p. 290C).

It is important to emphasize that "hydrate melts" are not merely concentrated aqueous solutions. As described by H. H. Emons, pester, chem. jL, 87, 3-9 (1986), hydrate melts supply "a link between (aqueous) electrolyte solutions and salt melts." Melt hydrates are viewed as containing the minimum quantity of water to fill the primary hydration sphere of the ions. Hence, hydrate melts are generally considered to contain approximately 0.1 to 0.25 mole fraction of salt or, conversely, approximately 3-9 moles of H20 per mole of salt. In view of this, the existence of molten salt hydrates with a range of 1-12 moles of water per mole of salt is conceivable. Since water in excess of the primary hydration spheres is absent, direct water-ion interactions dominate and, unlike aqueous solutions, exclusive H20-H2O interactions are absent.

such fundamental differences are reflected 1n various thermodynamic and other properties such as AH of evaporation, conductivity and partial molar dilution enthalpy.

The present invention relates to the use of molten salt hydrates in membrane systems i.e., in immobilized and/or encapsulated melt hydrate membranes for gas separation processes. We have found such melt hydrate membranes to be particularly useful for the separation of C02 from CH4 and H2, H2S from CH4 and H2, olefin gases from alkanes, and of water vapor from fL and other permanent gases, although they are also well suited for separating a wide variety of other gases, such as S02, NH3, HF, HC1, HBr, H2S, olefins such as C2H4 and C3H6, and mixtures thereof from a wide variety of feed gases.

Molten salt hydrate membranes are typically used for separating the components of a gas mixture by passing the gas mixture over the feed side of the membrane and recovering a gas stream which is enriched with the - 6 desired product at the permeate side. The permeate gases may be collected directly or alternatively their recovery may be facilitated by sweeping the permeate side with an inert gas. In most cases, for the effective operation of the membrane, it may be necessary to have at least a minimal partial vapor pressure of water; i.e., a minimal level of humidity in either the feed or permeate sweep streams to the membrane in order to retain the active molten salt hydrate composition.

There are at least two advantages to the use of molten salt hydrates in membranes over simple molten salts for certain gas separations.

First, the melting point of the hydrate is generally much lower than that of the anhydrous salt. For example, the melting points of FeCa3 and FeC2.»6H20 are 306oC and 370C, respectively. Secondly, many gases react reversibly with water, undergo reversible chemical reactions in the presence of water or are quite soluble in water. Hence, a selective permeation of such gases can be achieved by using molten salt hydrate membranes. Carbon dioxide is one such gas. C02 is quite soluble in water and reacts with water via equation (1) as follows:

H20 + C02 > HCO" + H+ (1) However, the thermodynamics and kinetics of the reaction of C02 with OH" (reaction (2)) are more favorable than those of reaction (1).

oh" + co, > hco: <2> d< J In molten salt hydrate media, OH" can be generated by reaction of H20 with basic anions:

b~ + H20 > BH + OH (3) Hence, a molten salt hydrate containing a basic anion B should facilitate transport of C02 via equations (1) and (2) and act as a "salt water" barrier to gases which are inert with respect to H20 (e.g., CH4, N2, H2, etc.).

It is envisaged that molten salt hydrate membranes containing moderately basic anions such as acetate, trifluoroacetate, formate, benzoate, glycinate, - 7 fluoride, carbonate, sulfite, sulfide and hydrogen sulfide (HS~) may be particularly useful. The use of tetramethylammonium fluoride tetrahydrate and tetraethylammonium acetate tetrahydrate in the fabrication of immobilized molten salt hydrate membranes for the separation of C02 from CH4 and CC from H? is described In Examples 1-5 below. One major advantage of the present invention is that gases such as CH and, particularly, H2 can be retained at feed pressures thus eliminating recompression costs. By contrast, conventional polymeric membranes readily permeate H2 and the gas must be recompressed. Some moisture-saturated cellulose-based membranes permeate CCL more rapidly than H2 through essentially a water film barrier but the ratio of the CCL to H2 permeabilities are quite low <~10> in comparison with the present invention.

In addition to C02 and water vapor, other gases such as S02, NH3, HF, HC1, HBr, HCN, and H2S which react or interact reversibly with H20, iS undergo reversible chemical reactions in water, or have high solubilities in H20 may be well suited to separations via immobilized molten salt hydrate membranes. For example, SCL is quite soluble in water and undergoes the hydration reaction in (4):

H20 + B + S02 > HS03 + BH <4> Roberts and Friedlander, AIChE Journal, 26, 593-610 (1980) have investi¬ gated the transport of S02 through films of water and neutral aqueous solutions and have shown that 83 to 95% of the flux of S02 results from hydration as in Equation (5).

2H20 + S02 < ; HSO" + O* • (5) In basic media, such as a solution containing NaOH, NaHSO and NaoS0o, the flux of S0o was found to be 28 times as great as in pure water. Hence, immobilized molten salt hydrate membranes with weakly basic anions appear ideally suited for separations involving S02.

Accordingly, the molten salt hydrates suggested for use involving separations of COp may be used for SCL separations as well.

As a second example, hydrogen sulfide may be expected to react reversibly with molten salt hydrates containing weakly basic anions (B~) by the equation:

H2S + B" > BH + HS <6> thus resulting in a facilitated transport of the gas.

Immobilized molten salt hydrate membranes can also be applied to separ¬ ations which exploit the relatively low melting point of salt hydrates rather than the reactivity (or solubility) of gases with waters of hydration. An example of such a membrane utilizes the silver ions contained in a molten salt hydrate to reversibly complex and thus facilitate transport of olefins to effect a separation from saturated hydrocarbons, such as the separation of C2H4 from C2H6.

Molten salt hydrates react (or interact) reversibly with H20 and, thus, are ideal materials for the fabrication of membranes for the separation of water vapor from nitrogen, air, hydrocarbons or other non-polar gases. The use of tetraethylammonium acetate tetrahydrate for this purpose is described in Examples 4 and 5 below..

2Q The disadvantage of prior art immobilized aqueous solutions which renders them impractical is their tendency to dehydrate, even at room temperature, and become ineffective. It has been shown that molten salt hydrate membranes, even at 50°C, retain their liquidity and are effective when exposed to feed and sweep gases with dew points of O-ZO'C. Under analogous conditions, one would expect an immobilized aqueous solution membrane to dehydrate. The difference between the two systems is believed to arise from a greater reduction of the water vapor pressure above a molten salt hydrate than above an aqueous solu¬ tion.

Calculations and Data Presentation The flux of a particular gas passing through the membrane is determined from its concentration in the controlled flow of helium which sweeps the permeate side of the membrane. Membrane performance is expressed in terms of the standard permeability, Po, for each gas. Po is defined as the experi- -1 2 mental flux 3, (seem sec ), per unit area A (cm ) of membrane surface and 1BÎ1426 unit pressure differential AP (cmHg) of the particular gas between the two interfaces of the membrane multiplied by the thickness (8.) of the membrane.

Values of Po have been corrected for the porosity and the tortuosity of the polymeric porous support.

J«P»tortuosit.y Po = A»ûp»porosity in units of sccm»cm/cm2os»cmHg.

Values of Po are expressed in Barrers; 1 Barrer = 1X10" seem» cm/cm »s«>cmHg. The thickness of the active layer was used in the cal¬ culation of Po for encapsulated membranes. This assumes that the gas permeance of the encapsulating polymer is much greater than that of the active membrane element, which in most cases is a valid assumption.

The following examples are presented to illustrate the present invention and are not meant to be limiting.

Example 1 An immobilized Tetramethylammonium Fluoride Tetrahydrate, (CH3)4NF»4H20 (TMAF> Molten Salt Hydrate Membrane for the Separation of CCL from CH..

Tetramethylammonium fluoride tetrahydrate (obtained from Aldrich Chemical Co.) in the molten state (M.P. = 39-420C> was used to fabricate an immobilized molten salt hydrate membrane to separate COj from CH4. This membrane was prepared in the following manner. An aque¬ ous solution of (CH3>4NF4»4H20, tf4M, was applied to a film of Celgard 3501 (Celanese Corp.). The impregnated polymer was dried in vacuum at rbom temperature for about one hour. The resulting film was placed in a holder and sealed in a permeation test cell and heated to 500C. Humidified C02/CH4 feed and He permeate sweep gases were used. Experimental conditions and membrane performance data are listed in Table I below.

Table I PERFORMANCE OF IMMOBILIZED TMAF/*CELGARD MOLTEN SALT HYDRATE MEMBRANES FOR THE SEPARATION OF CO2 FROM CH4 at 50°C FEED GAS PERMEATE GAS FLOW: 20 SCCM He FLOW: 10 SCCM; P=76 cm P P P (CO,) PJW.) CO CH o 2 o 4 2 4 (Barrers) (Barrers) ccCOp/CH/ (cmHg) (cmHg) d * cc = selecti vi ty The observed drop in the standard permeability of C02 with increasing partial pressure of carbon dioxide in the feed is consistent .with a C02 carrier mediated facilitated transport mechanism. The reversible interaction of C02 with a species in the melt and transport of the CO -carrier complex across the membrane is indicated by the above results.

Example 2 An encapsulated Te tramethylammonium Fluoride Tetrahydrate, (CH ) NF-AFLO, Molten Salt Hydrate Membrane for the Separation of C02 from CH4 .

The salt hydrate, tetramethylammonium fluoride tetrahydrate, (CH ) NF-4H90, (TMAF), in the molten state (>39°C), was used to fabricate an encapsulated membrane for separating C02 from CH4. This membrane was prepared in the following manner. Molten TMAF (at ca. 50oC) was applied to a slightly moistened piece of Celgard 3501 and the excess *Trade mark n salt was removed. This TMAF/Celgard was placed between two films of polytrimethylsilylpropyne (PTMSP) (130 and 112 pm). The membrane was placed in a holder, sealed in a permeation cell and heated to 50oC. Since TMAF solidifies if dehydrated, both feed and sweep gases were humidified by passage through water bubblers at room temperature. Experimental conditions and results are detailed in Table II below. Values of the standard permeability, Po, were calculated assuming that only the TMAF/ Celgard layer was responsible for gas permeation.

It is fairly certain that under the conditions of this and the previous membrane experiment (Ex. 1) that at 50oC using moist feed and permeate sweep gases, the salt remains as a molten hydrate. In fact, heating (CH3>4NF»4H20 to lOOT under a flow of dry N2, results in loss of only 1.8 moles H20 per mole of salt.

Table II PERFORMANCE OF ENCAPSULATED TMAF/CELGARD MEMBRANES FOR THE SEPARATION OF CO2 FROM CH4 AT 500C FEEDFLOW: 20 GASSCCM PERMEATE GASHe FLOW: 10 SCCM; P=76 cm pco2 PCH4 P0(C02) Po(CH4) (cmHg) (cmHg) (Barrers) (Barrers) «CO2/CH4 43 41 755 6.1 124 81 77 638 6.4 100 119 114 545 6.7 81 199 191 421 7.3 58 244 234 381 12.2 31 371 355 318 13.3 24 As in Example 1, the decrease in the standard permeability of C02 with increasing partial pressure of C0? in the feed is consistent with, and thereby indicates, a ŒL carrier mediated facilitated transport mechani sm.

Example 3 An Immobilized Tetraethylammonium Acetate Tetrahydrate (CoHc>/lN+CHC0:»4Ho0, (TEAA), Molten Salt Hydrate Mem- brane for the Separation of C02 from CH4.

Tetraethylammonium acetate tetrahydrate in the molten state was used to fabricate a membrane which separates C02 from CH4. The membrane was prepared in the following manner. Molten TEAA was applied to a piece of Celgard 3501 and the excess salt was removed. The TEAA/Celgard was placed in a holder and sealed in a permeation cell and heated to 50oC.

The feed gas, 1:1 C02:CH4 and the sweep gas, He, were passed through water bubblers at ZS'C. Experimental conditions and membrane performance data are listed in Table III below.

Table III PERFORMANCE OF IMMOBILIZED TEAA/CELGARD MOLTEN SALT HYDRATE MEMBRANES FOR THE SEPARATION OF CO2 from CH4 at 50oC PERMEATE GAS FEED GAS FLOW: 20 SCCM prco2 pCH4 (cmHg) (cmHg) He FLOW: 10 SCCM; P=76 cm co2/cy PoC02 P0(CH4) (Barrers) (Barrers) 38.8 37.2 157 8 The above results show a good separation factor (selectivity) for C02 and a low standard permeability (P ) for CH4.

Example 4 An Immobilized Tetramethylammonium Fluoride Tetrahydrate, (CH3)NF»4H201 (TMAF), Molten Salt Hydrate Membrane for the Separa¬ tion of C02 from CH4 and H2.

The salt hydrate, tetramethylammonium fluoride tetrahydrate, (CH3)4NF»4H20, (TMAF), in the molten state was used to fabricate a membrane which separates C02 from CH4 and H2. This membrane was prepared in the following manner. An aqueous solution of - 13 <CH3)4NF4-H20, s4M( was applied to a film of Celgard 3401. The impregnated support was dried under vacuum at room temperature for about one hour and subsequently soaked In molten TMAF at sSO'C for minutes. Following removal of the excess molten salt, the membrane was placed in a membrane test cell and heated to 50aC.

Feed and sweep gases were humidified by passage through water bubblers at 230C. Experimental conditions and membrane performance data are listed in Table IV below.

Table IV PERFORMANCE OF IMMOBILIZED TMAF/CELGARD MOLTEN SALT HYDRATE MEMBRANES FOR THE SEPARATION OF CO2 FROM CH4 AT 50oC FEED GAS PERMEATE GAS TS FLOW: i 20 SCCM He FLOW: 10 SCCM; P= 76 cm X(cmHg) pco2(cmHg) pCH4(cmHg) 0(H2)(Barrers P0o(C02)) (Barrers) P„0(CH4)(Barrers) <*cc )2/H2 œC02/CH4 43.196.9 21.147.3 20.145.0 5166 452320 1112 95 4127 171.8 83.9 79.9 58 230 12 4 19 301.8 147.3 140.3 40 162 12 4 14 Much of the permeability of H2 may arise from permeation through the non-porous portion of Celgard 3401. The permeability of H2 through polypropylene (Celgard 3501) is 41 Barrers. As in the above examples, this example shows a good separation of C02.

Example A Supported Molten Tetramethylammonium Fluoride Tetrahydrate Film for the Separation of C02 from CH4 and H,,.

The salt hydrate, tetramethylammonium fluoride tetrahydrate, was melted upon the surface of a film of poly(trimethylsilylpropyne) (PTMSP) - 14 secured in a suitable holder. The holder and membrane were sealed in a membrane test cell and heated to 50oC. Feed and sweep gases were humidified by passage through water bubblers at 50C. Experimental conditions and membranes performance data (2 runs) are listed in Table V below. P/a values rather than Po are reported since the thickness of the salt layer is unknown.

Table V Performance of Molten TMAF on the Surface of PTMSP For Separation of CO2 from CH4 and H2 at 50oC.

1st RUN FEED GAS PERMEATE GAS FLOW: 20 SCCM He FLOW: 10 SCCM; P=76 cm F>H2 pC02 pCH4 P/Q.x]08(cm3/cm2«cmHg»sec) 0CCO2/H2 «CO2/CH4 (cmHg) (cmHg) (cmHg) H2 CO2 CH4 47.1 23.0 21.9 N.O.* 2970 8.4 100 48.7 46.7 43 2120 11 167 81.6 77.6 52 1580 11 97 • 2nd RUN 44.2 21.6 20.5 N.O. 2040 N.O.

189 92.4 88.0 20 1410 5.8 69 240 321 156 149 35 1070 6.7 31 160 'N.O. = not observed Example 6 An Immobilized Tetraethylammonium Acetate Tetrahydrate, (CoH(-)/1N+CH,C0:«4H„0, (TEAA), Molten Salt Hydrate Mem- brane for the Separation of Water Vapor from N2.

The salt hydrate, tetraethylammonium acetate tetrahydrate, <CoH[-)/,N+CHoC0l«4H„0, (TEAA), in the molten state (>42,,C> was 2 D 4 6 2 à used to fabricate a membrane which separates H20 vapor from N2.

This membrane was prepared in the following manner. Molten TEAA was applied to a piece of Celgard 3501 and the excess salt was removed. The TEAA/Celgard was placed in a holder and sealed in a membrane test eel! and heated to 50oC.

The feed gas, was passed through a water bubbler at 250C. The results are reported in Table VI below.

Table VI FEED GAS PERMEATE GAS FLOW: 20 SCCM He FLOW: 10 SCCM; P=76 cm PN2 PH20 prH20 W P0N2 ~h20/N2 (cmHg) (cmHg) (cmHg) (Barrers) (Barrers) 73 2.35 1.35 22,400 7 4700 It is evident that there is a high flux of H20 through the membrane which results in a significant depletion of H20 from the feed gas stream (i.e., high H20 recoveries in the permeate stream). Hence, the approximation that the pressure of water in the permeate is near zero is not valid and APU n cannot be set equal to the pressure of water n2U in the feed, P,. Since the change in the pressure of water as a func¬ tion of distance from the feed or sweep inlet is unknown, an average driving force, APH20, was used in the calculation of Po(H20).

Example 7 Encapsulated Tetraethylammonium Acetate Tetrahydrate, (TEAA), Molten Salt Hydrate Membranes for the Separation of Water Vapor from N2.

This example describes the preparation and performance of a Celgard immobilized TEAA molten salt hydrate membrane which was encapsulated in polytrimethylsilylpropyne (PTMSP) (Run A), and in silicone rubber (Run B). A Celgard 3501 membrane was prepared as in Example 6. The membrane was placed between two films of PTMSP (134 and 140 \xm thick), or two films of silicone rubber (127 m thick), placed in a holder, sealed in a test cell and heated to 50oC. The feed gas, N2, was passed through a water bubbler at 250C. Dew point data was obtained as in Example 6. Flow rates of H20 and N2 and the partial pressures of these components are reported in Table VII. Po values were calculated assuming only the TEAA/Celgard layer was responsible for gas permeation.

As described in the previous example, an "average" àP2Q was used to calculate Po. The performance of the above encapsulated membrane systems and, as comparative examples, the corresponding data for PTMSP and silicone rubber polymer membranes, are listed in Table VII.

!-• 3S p {—j H- M S &1 M- O •XI n O tl) o o 3 — (DO n> U3 ÎO W W g- SL& i ii K (D SU n> M ta rf (D B cr> 3 to *Q 3 to a o §a to O o -J O K 3 to f 0 1-3 cr (D to O vo o o o o <71 o o U1 o TJ ES w O (D B> 1 » "J 1 M. r1 (D O (a .« CO OY >*» U) Ul o to o o o to O to S ii o It 1s evident that encapsulated melt hydrate membranes can be prepared which are very effective for the separation of water vapor from nitrogen. The polymers PTMSP and silicone rubber also have very high permeabilities for water vapor, but are much poorer barriers for the permeation of N2 than the melt hydrate membranes as can be seen by comparing the selectivity measurements set out above.

Example 8 An immobilized molten salt hydrate membrane consisting of Calcium Nitrate Tetrahydrate plus and Silver Nitrate; Ca<N03)2»4H20 + 0.2 AgNOs for the separation of CH2=CH2 from CH3CH3.

Calcium Nitrate tetrahydrate (obtained from Fisher Scientific Company) in the molten state <M.P. = 39-430C> and silver nitrate (obtained from Alfa Products) were used to fabricate an immobilized molten salt hydrate membrane to separate CH?=CH2 from CH3CH3.

This membrane was prepared in the following manner. A 0.2 molar amount of silver nitrate was added to molten calcium nitrate tetrahydrate.

Upon cooling to room temperature, a molten mixture was obtained. Molten Ca(N03)2»4H20/0.2 AgNOs was applied to a piece of Celgard® 3401 and the excess salt was removed. The Ca(N03)2»4H20/0.2 AgNCh/Celgard® was placed in a holder and sealed in a permeation cell and heated at 230C. The feed gas, 1:1 ethylenerethane, and the sweep gas, He, were passed through water bubblers at 230C. Experimental conditions and membrane performance data are listed in Table VIII below.

TABLE VIII Performance of Immobilized Calcium Nitrate Tetrahydrate/ AgNOa/Celgard® molten salt hydrate, membranes for the separation of ethylene from ethane at 230C.

Flow:

Feed Gas seem Permeate Gas He Flow: 10 seem P = 76 cmHg CHn=CH« (cmHg) 132.1 CH3CH3 (cmHg) 132.1 Po, CH2=CH2 Po CHqCn «CH2=CH2/CH3CH3 (Barrers) (Barrers) 68.2 14.4 4.7 Example 9 An immobilized molten salt hydrate membrane consisting of Tetraethyl- ammonlum Acetate Tetrahydrate (TEAA) for the separation of H2S from CH.

Tetraethylammonium Acetate Tetrahydrate In the molten state was used to fabricate a membrane which separates H2S from CH4. The membrane was prepared In the follolng manner. Molten TEAA was applied to a piece of Celgard®3401 and the excess salt was removed with a glass slide. The TEAA/Celgard® was placed in a holder and sealed in a permeation cell and heated at SCTC. the feed gas, 5% H2S:95% CH4, was fed to the membrane 1° dry. The sweep gas, helium, was passed through a water bubbler at room temperature. Experimental conditions and membrane performance data are listed in Table IX.

TABLE IX Performance of Immobilized Tetraethylammonium Acetate Tetrahydrate/ AgNOa/Celgard© Molten Salt Hydrate Membranes for the separation of H2S from CH4 at 50oC.

Feed Gas Permeate Gas Flow: 20 seem He Flow: 10 seem P = 76 cmHg PH2S(cmHg)3.8 (cmHg)72.2 P0H2S(Barrers)2710 Example POCH4 OCH2S/CH4 (Barrers) 6.5 420 An immobilized molten salt hydrate membrane consisting of Tetramethyl- ammonium Fluoride Tetrahydrate (TMAF) for the separation of H2S from CH4.

Tetramethylammonium Fluoride Tetrahydrate in the molten state was used to fabricate a membrane which separates HS from CH4. The membrane was prepared in the following manner. Molten TMAF was applied to a piece of Celgard®3401 and the excess salt was removed with a glass slide. The TMAF/Celgard® was placed in a holder and sealed in a permeation cell and heated at SOT. The feed gas, 57= H2S:95% CH4, was fed to the membrane dry. The sweep gas, helium, was passed through a water bubbler at room temperature. Experimental conditions and membrane performance data are listed in Table X.

TABLE X Performance of Immobilized Tetramethylammonium Fluoride Tetrahydrate/Ce1gard« Molten Salt Hydrate Membranes for the separation of H2S from CH4 at 50oC Feed Gas Permeate Gas Flow: 20 seem He Flow: 10 seem P = 76 cmHg PH2S PCn4 PoH2S PoCH4 «HzS/OU (cmHg) (cmHg) Barrers Barrers 3.8 72.2 2430 2 1200 Having thus described the present invention what is now deemed appro¬ priate for Letters Patent is set out in the following appended claims.

0482K