CARBON DIOXIDE SEPARATION MEMBRANES AND PROCESS

Abstract

This invention discloses a thin-film composite membrane and process for the separation of carbon dioxide from non-hydrophilic gases such as methane, hydrogen, and nitrogen. The thin-film composite membrane has a gas-separation layer and a nonporous high-diffusion-rate layer, and has carbon dioxide to non-hydrophilic gas selectivity that is greater than the intrinsic selectivity of the gas-separation layer alone.

Claims

1. A thin-film composite membrane comprising: a) a porous-layer support; and b) a nonporous high-diffusion rate layer that is in direct contact with said porous-layer support; and c) a gas-separation layer that is nonporous and in direct contact with said nonporous high-diffusion rate layer; and wherein the gas separation layer comprises a fluorinated ionomer having non-silver ionic groups and a perfluorinated polymer backbone.

2. The thin-film composite membrane of claim 1 in which the non-silver ionic groups are selected from the group consisting of ammonium sulfonate lithium sulfonate, sodium sulfonate and potassium sulfonate.

3. The thin-film composite membrane of claim 1 in which the fluorinated ionomer comprises repeat units derived from a monomer of structure CF.sub.2CFOR.sub.fSO.sub.2F wherein R.sub.f is perfluoroalkyl or perfluoroalkoxy containing 2 to 10 carbon atoms.

4. (canceled)

5. The thin-film composite membrane of claim 3 in which the monomer is CF.sub.2CFOCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.2F or CF.sub.2CFOCF.sub.2CF.sub.2SO.sub.2F.

6. The thin-film composite membrane of claim 1 in which the fluorinated ionomer comprises repeat units derived from a monomer selected from the group consisting of tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), and perfluoro(2,2-dimethyl-1,3-dioxole).

7. The thin-film composite membrane of claim 1 in which the nonporous high-diffusion-rate layer comprises a copolymer comprising perfluoro(2,2-dimethyl-1,3-dioxole).

8. The thin-film composite membrane of claim 1 in which the nonporous high-diffusion-rate layer comprises a copolymer comprising perfluoro(2,2-dimethyl-1,3-dioxole) and tetrafluoroethylene.

9. The thin-film composite membrane of claim 1 in which the gas-separation layer thickness is less than 1 m.

10. A process for separating a gaseous composition comprising carbon dioxide and a non-hydrophilic gas using a thin-film composite membrane; the process comprising: a) providing a thin-film composite membrane having a feed side and a permeate side and comprising; i) a porous-layer support; and ii) a nonporous high-diffusion rate layer that is in direct contact with said porous-layer support; and iii) a gas-separation layer that is nonporous and in direct contact with said nonporous high-diffusion rate layer; and b) exposing the feed-side to a flowing feed-side composition comprising carbon dioxide and a non-hydrophilic gas, and; c) providing a driving force and producing a permeate-side composition having a higher ratio of carbon dioxide to non-hydrophilic gas than the feed-side composition; and wherein the gas separation layer comprises a fluorinated ionomer having non-silver ionic groups and a perfluorinated polymer backbone.

11. The process of claim 10 in which the non-silver ionic groups are selected from the group consisting of ammonium sulfonate, lithium sulfonate, sodium sulfonate, and potassium sulfonate.

12. The process of claim 10 in which the fluorinated ionomer comprises repeat units derived from a monomer selected from the group consisting of: tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), and perfluoro(2,2-dimethyl-1,3-dioxole).

13. The process of claim 10 in which the fluorinated ionomer comprises repeat units derived from a monomer of structure CF.sub.2CFOR.sub.fSO.sub.2F wherein R.sub.f is perfluoroalkyl or perfluoroalkoxy containing 2 to 10 carbon atoms.

14. (canceled)

15. The process of claim 13 in which the monomer is CF.sub.2CFOCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.2F or CF.sub.2CFOCF.sub.2CF.sub.2SO.sub.2F.

16. The process of claim 10 in which the nonporous high-diffusion-rate layer comprises a copolymer comprising perfluoro(2,2-dimethyl-1,3-dioxole) and tetrafluoroethylene.

17. The process of claim 10 in which the non-hydrophilic gas is selected from a group consisting of: methane, ethane, oxygen, nitrogen, or hydrogen.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0022] The accompanying FIG. 1 is included to provide a further understanding of the invention, is incorporated in, and constitutes a part of this specification. FIG. 1 illustrates embodiments of the invention and together with the description serve to explain the principles of the invention. FIG. 1 shows a cross-sectional view of an exemplary high-permeance and high-selectivity thin-film composite membrane 10 comprising a nonporous gas-separation layer 30, nonporous high-diffusion-rate layer 50, and a porous-layer support 70. The layer surfaces are coplanar and in direct contact to each other. This may also be referred to as laminated or bonded together in the field of membrane technology although usually no separate adhesive is employed.

[0023] Corresponding reference characters indicate corresponding parts throughout the view of the FIGURE and are not to be construed as limiting the scope of the invention in any manner. Furthermore, FIG. 1 is not necessarily to scale; some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] As used herein, the terms comprises, comprising, includes, including, has, having or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. In addition, use of a or an are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

[0025] Certain exemplary embodiments of the present invention are described herein and are illustrated in the accompanying figures. The embodiments described are only for purposes of illustrating the present invention and should not be interpreted as limiting the scope of the invention. Other embodiments of the invention, and certain modifications, combinations and improvements of the described embodiments, will occur to those skilled in the art and all such alternate embodiments, combinations, modifications, improvements are within the scope of the present invention. Certain additional terms are also used and some of them are further defined within the following detailed description of the invention:

[0026] Ionomers are useful materials for fabrication of the gas-separation layer of the thin-film composite membrane of the invention. An ionomer is a copolymer that comprises covalently bound ionic groups such as sulfonic acid, sulfonate, carboxylic acid, carboxylate, phosphate, phosphonium, or ammonium. Ionic groups may be hydrophilic and sulfonic acid or sulfonate salts are preferred ionic groups and the ionomer equivalent weight is the weight of ionomer containing one mole of ionic group. The ionomer equivalent weight is preferably less than 5000 grams per mole, more preferably less than 2000, and very preferably between 500 and 1000-g/mole. Preferred ionomers are fluoropolymers that comprise repeat units A and B in which A is a polymerized derivative of a fluorinated monomer and B contains hydrophilic ionic groups. Especially preferred ionomers are fluoropolymers in which there are no carbon-hydrogen groups in the polymer-backbone repeating units. Examples of the latter ionomers are well known in the art and include copolymers comprising repeat units of a perfluorovinyl ether, having a pendant sulfonate group, such as for example Nafion (Chemours, Wilmington, Del.) or Aquivion (Solvay, Houston, Tex.).

[0027] Solution casting is a preferred film forming technique to fabricate the gas-separation layer of the composite membrane. Therein, a dilute solution of the ionomer is first prepared at concentrations that are preferably less than 5%, more preferably less than 2%, and very preferably between 0.1% and 1%. Suitable solvents or solvent mixtures are those that will dissolve the ionomer and evaporate at an appropriate rate to form the gas-separation layer in a timely manner. Residual or trace solvent remaining in the gas-separation layer should not interfere with subsequent processing steps. For example, suitable solvents include but are not limited to lower alcohols such as ethanol, isopropanol, n-propanol, certain ketone, ether, amide, and ester solvents, and mixtures therefrom. Certain mixtures of the preceding solvents with fluorinated solvents such as Novec HFE7200, and HFE7300 are also suitable.

[0028] Preferred casting techniques for the ionomer solution include but are not limited to ring casting, dip-coating, spin-coating, slot-die coating, and Mayer rod coating. The solution is cast and the solvent(s) are evaporated to form the dry gas-separation layer. The gas-separation layer thickness has a significant influence on the membrane permeance and cost. The gas-separation layer is thin and preferably has a thickness of 0.01-m to 1.0-m, more preferably 0.05-m to 0.5-m. The gas-separation layer may be fabricated by casting directly onto a substrate comprising a high-diffusion rate layer. The high-diffusion rate layer also enables fabrication of the thin gas-separation layer by preventing the ionomer solution from penetrating into additional layers such as a porous-layer support.

[0029] Preferred materials for a high-diffusion rate layer include copolymers comprising repeat units from perfluoro-2,2-trifluoromethyl-1,3-dioxole (PDD), particularly if it is a component of a perfluoropolymer. In general, high molar percentages of PDD are desirable and consistent with being able to process the copolymers into a high-diffusion rate layer. In any PDD copolymer material, it is preferred that at least about 50 mole percent of the total repeat units are derived from PDD, more preferably at least 80 mole percent. These materials may also comprise functional groups that include perfluoroether, ester, carboxylate, and chloro. Very preferred copolymers comprise PDD with tetrafluoroethylene, available as Teflon AF (The Chemours Co., Wilmington, Del.) and for further information about Teflon AF, see P. R. Resnick et al. in Teflon AF Amorphous Fluoropolymers, J. Schiers, Ed., Modern Fluoropolymers, John Wiley & Sons, New York 1997 397-420, which is hereby incorporated by reference. A preferred grade of Teflon AF is AF 2400, which is reported to contain 83 mole percent PDD and 17 percent tetrafluoroethylene.

[0030] The high-diffusion rate layer of the invention may also be fabricated by solution casting and preferred casting techniques include but are not limited to ring casting, dip coating, spin-coating, slot-die coating, and Mayer rod coating. Dilute solutions are prepared at concentrations that are preferably less than 1%, and more preferably between 0.05% and 0.5%. Suitable solvents or solvent mixtures are those that dissolve the layer material and evaporate at an appropriate rate to form the layer in a timely manner. Residual or trace solvent remaining in the layer should not interfere with subsequent processing steps. For example, suitable solvents for a fluorinated layer material include but are not limited to fluorinated solvents such as Novec FC770, HFE7200, and HFE7300. The solution is cast onto a suitable substrate such as a porous-layer support and the solvent(s) are evaporated to form the high-diffusion rate layer. The layer is very thin and preferably about 0.01-m to about 0.5-m, and more preferably 0.01-m to 0.1-m. The layer preferably has a carbon dioxide permeance of at least 5000-GPU at 25 C., more preferably at least 10,000-GPU, and most preferably at least 10 times greater than the gas-separation layer. Permeance, which is pressure normalized flux, is typically reported in gas permeance units or GPU and has units of 10.sup.6cm.sup.3(STP)/cm.sup.2/sec/cmHg. Permeability is further normalized for thickness and has units of 10.sup.10cm.sup.3(STP)cm/cm.sup.2/sec/cmHg and reported in Barrer.

[0031] The porous-layer support reinforces the gas-separation and high-diffusion rate layers and helps to strengthen the composite membrane as a whole such that the membrane may be fabricated into more complex geometries such as spiral-wound or hollow-fiber membrane modules. The porous-layer support may be in the form of a flat sheet, hollow fiber, or tube. Suitable materials for a porous-layer support include but are not limited to polyvinylidine fluoride, expanded polytetrafluoroethylene, polyacrylonitrile, polysulfone, and polyethersulfone. The porous-layer support may also comprise an even stronger backing material such as porous non-woven polyester or polypropylene. Porous inorganic substrates such as silica or alumina are also suitable materials for the porous-layer support. Permeate gases should flow relatively unobstructed through the usually much thicker porous-layer support having a preferred porosity that is 40% or greater. The average pore size is preferably less 0.1-m and more preferably between 0.01 and 0.03-m.

[0032] The thin-film composite membrane may be subjected to a thermal treatment step annealed to further improve mechanical durability, long-term permeance and selectivity, and resistance to degradation from contact with liquid water. The ionomer in the gas-separation layer is annealed by heating the composite membrane to near or above the glass transition temperature of the ionomer. The exact glass transition temperature will be dependent on the ionomer composition and the associated counter ion. Generally, annealing temperatures for the preferred ionomers are between 50 and 200 C., and preferably between 75 and 150 C. The composite membrane is preferably heated for 0.1 to 10 minutes, more preferably for 1 to 5 minutes. The appropriate annealing temperature and time should not degrade the other components of the composite membrane.

[0033] The thin-film composite membrane is highly useful for the separation of carbon dioxide from compositions comprising a non-hydrophilic gas. A non-hydrophilic gas is a gas that has a low solubility in water that is approximately 100-mg/L or less at 1-bar and 20 C. Examples of non-hydrophilic gases include hydrogen (H.sub.2, 1.6-mg/L), oxygen (O.sub.2, 43-mg/L), nitrogen (N.sub.2, 19-mg/L), methane (CH.sub.4, 23-mg/L), and ethane (C.sub.2H.sub.6, 62-mg/L). The solubility of carbon dioxide for comparison is approximately 1700-mg/L at 1-bar and 20 C. The membrane is exposed to a flowing gaseous feed-composition comprising carbon dioxide and a non-hydrophilic gas. A driving force is provided in which the carbon dioxide pressure on the membrane feed-side is higher than on the permeate side. This may be accomplished by applying a vacuum on the membrane permeate-side and may be preferred for the carbon dioxide separation from flue gas due to the lower energy consumption. Separation of carbon dioxide from the gaseous feed-mixture occurs through the membrane producing a membrane permeate-side composition having a higher concentration of carbon dioxide than the feed composition. Separation may also be enhanced by having water vapor in the feed mixture and optionally as a sweep gas on the membrane permeate-side, which functions to further reduce the carbon dioxide concentration.

EXAMPLES

[0034] Examples of certain representative embodiments of the invention are as follows. Proportions and percentages are by weight unless otherwise indicated. All units of weight and measure not originally obtained in SI units have been converted to SI units. Unless otherwise indicated, pressure values disclosed are gage pressures (i.e. relative to atmospheric pressure). Certain abbreviations used in the examples are defined by their Chemical Abstracts Names or structures as follows:

TABLE-US-00001 VF fluoroethene PPSF 1,1,2,2-tetrafluoro-2-[(1,2,2-trifluoroethenyl)oxy]- ethanesulfonyl fluoride PDD 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole HFPO dimer CF.sub.3CF.sub.2CF.sub.2OCF(CF.sub.3)C(O)OOC(O)CF(CF.sub.3)OCF.sub.2CF.sub.2CF.sub.3 peroxide

Example 1

[0035] Synthesis and Hydrolysis of PDD/VF/PPSF (Feed Ratio: 1/2/1.5) Terpolymer:

[0036] Into a 150-mL stainless steel pressure vessel, after argon purging for 5 minutes, were added a magnetic stirring bar, 10.5-g PPSF, 6.1-g PDD, 32-mL VertrelXF, 0.6-mL and HFPO dimer peroxide (0.15 M) in VertrelXF. The pressure vessel was sealed, initially cooled to 0 C. then charged with 2.3-g of vinyl fluoride gas. The reaction mixture was stirred at room temperature in a water bath overnight. The reaction vessel was brought to ambient atmospheric pressure, opened, and 50 mL of methanol was added to the reaction mixture. The precipitated gel was transferred to a glass dish and dried in a fume hood at ambient temperature to remove the majority of volatile components, and then in a forced air oven at 80 C. for 6 hours to yield 10.7 g of PDD/VF/PPSF terpolymer as a pale color solid. Glass transition temperature (Tg)=55 C.

[0037] 10.7-g of PDD/VF/PPSF terpolymer synthesized in the above procedure, 200-mL methanol, 3.3-g ammonium carbonate, and a magnetic stirring bar were added to a glass 500 mL round-bottom flask. The reaction mixture was stirred over the weekend at 50-60 C. as the terpolymer dissolved. The methanol was removed by evaporation to isolate the terpolymer. The terpolymer was acid exchanged twice using 100-mL of 2.0-M hydrochloric acid and stirring for 15 minutes. Excess acid was removed by rinsing three times with 100-mL of de-ionized water. The terpolymer was dried in a forced-air oven at 100 C. overnight. The yield of acid-form terpolymer was 10.7-g as a brownish solid having a Tg at 92 C.

Example 2

[0038] Membrane Fabrication from the PDD/VF/PPSF Terpolymer:

[0039] Substrates comprising a nonporous high-diffusion rate layer were first prepared by ring casting 0.1 wt. % solutions of Teflon AF 2400 in Novec FC770 onto asymmetrically porous sheets of either polyacrylonitrile (PAN) or polyvinylidene fluoride (PVDF) microfiltration membrane and drying at ambient temperature. The PAN substrate with the high-diffusion rate layer had a CO.sub.2 permeance of 23,000 GPU. The PVDF substrate with the high-diffusion rate layer had a CO.sub.2 permeance of 7,900-GPU.

[0040] The sulfonic-acid-form PDD/VF/PPSF terpolymer from Example 1 was dissolved at room temperature in isopropanol to make 0.7 and 1.0-wt % solutions. Separate fractions of this solution were stirred with 3 equivalents of ammonium, lithium, sodium, or potassium carbonate to form the corresponding sulfonate salts. The solutions were filtered to remove excess carbonate salt and/or prior to further use (1-m). The solutions were ring cast onto the high-diffusion rate layer surface of the substrates. The wet substrate was held vertically to drain the excess casting solution and was then dried in an oven at 65 C. for 30 minutes to form the thin-film composite membrane.

Example 3

[0041] Membrane Fabrication from a Commercial Sulfonic-Acid (or Sulfonate) Ionomer:

[0042] Aquivion D72-25BS dispersion (25-wt %) in water (Solvay, Houston Tex.) and having a 720-g/mole equivalent weight was purchased from Aldrich (Milwaukee Wis.). The dispersion was diluted with isopropanol to make 0.25, 0.50, and 1.0-wt % concentrations. Separate fractions of these dispersion concentrations were stirred with 3 equivalents of ammonium or lithium carbonate to form the corresponding sulfonate salts. The solutions were filtered to remove excess carbonate salt and/or prior to further use (1-m). The dispersions were ring cast onto the high-diffusion rate layer surface of the PAN or PVDF substrates, as prepared in Example 2. The wet substrate was held vertically to drain the excess casting dispersion and was then dried in an oven at 65 C. for 30 minutes to form the thin-film composite membrane.

Example 4

[0043] Gas-separation layer thickness estimation: An estimate of the thickness for the gas-separation layer (GSL) of the composite membranes prepared in Example 3 was calculated from the wet substrate dispersion weight, the dispersion percent concentration ([% Dispersion]), the membrane cast-surface area (38.3-cm.sup.2), and the ionomer density (p), which was reported at 2.06-g/cm.sup.3. The GSL thicknesses were calculated from the following equation (1) and the results are shown in Table 1.

[00001]$\begin{array}{cc}\mathrm{GSL}.Math..Math.\left(.Math..Math.m\right)=\frac{\mathrm{Wet}.Math..Math.\mathrm{film}.Math..Math.\left(g\right)\left[\%.Math.\mathrm{Dispersion}\right]100}{.Math..Math.\left(g.Math.\text{/}.Math.{\mathrm{cm}}^3\right)38.3.Math.\left({\mathrm{cm}}^2\right)}& \left(1\right)\end{array}$

TABLE-US-00002 TABLE 1 [Dispersion] % Wet film (g) GSL thickness (m) 0.25 0.347 0.11 0.50 0.426 0.27

Example 5

[0044] Membrane Fabrication from a Commercial Sulfonic-Acid (or Sulfonate) Ionomer:

[0045] Aquivion D79-25BS dispersion (25-wt %) in water (Solvay, Houston Tex.), having a 790-g/mole equivalent weight, was purchased from Aldrich (Milwaukee Wis.). The procedure of Example 3 was repeated to a form a 1.0-wt % sulfonic-acid-form dispersion and corresponding composite membrane.

Example 6

[0046] Membrane Fabrication from a Commercial Sulfonic-Acid (or Sulfonate) Ionomer:

[0047] The procedure of Example 3 was repeated to a form a 1.0-wt % sulfonic-acid-form dispersion and corresponding composite membrane using Liquion LQ1105 dispersion (5-wt %) in a water/alcohol mixture (Ion Power, New Castle Del.) and having an 1100-g/mole equivalent weight.

Example 7

[0048] General Procedure for Membrane Gas-Separation Measurement:

[0049] The thin-film composite membranes were separately tested in a stainless-steel cross-flow permeation cell having a 13.85-cm.sup.2 active area. Feed gas mixtures were humidified by bubbling through water and fed to the cell at flow rates between 0.8-2.5 standard liters per min. The permeate flow was measured using an acoustic flow meter and concentrations of carbon dioxide and methane in the permeate were measured using a Varian 450 gas chromatograph or Landtec Biogas 5000 meter. Feed flow rates were adjusted such that the stage cut (i.e. flow of permeate relative to feed flow) was maintained below 3.5%. Permeance was calculated for each component independently using the log mean partial pressure difference across the membrane. Selectivity was calculated as the ratio of carbon dioxide permeance to nitrogen or methane permeance.

Example 8

[0050] Carbon Dioxide Separations from Nitrogen:

[0051] Membrane samples from Examples 2, 3, 5, and 6 were tested for CO.sub.2/N.sub.2 mixed gas separation at ambient temperature (20-25 C.) at a feed pressure of 30-psig and with CO.sub.2 concentrations of 40% and 20%. The permeate pressure was close to atmospheric pressure. Table 2 shows high CO.sub.2 permeance and selectivity over nitrogen for all tested membranes.

TABLE-US-00003 TABLE 2 40% CO.sub.2 feed 20% CO.sub.2 feed Membrane Coating CO.sub.2 CO.sub.2/N.sub.2 CO.sub.2 CO.sub.2/N.sub.2 from Cation solution Permeance Selec- Permeance Selec- Example form conc. (GPU) tivity (GPU) tivity 2 H.sup.+ 1.0% 696 46.7 2 NH.sub.4.sup.+ 0.7% 532 50.2 3 H.sup.+ 1.0% 1955 45.0 5 H.sup.+ 1.0% 1473 45.6 6 H.sup.+ 1.0% 1027 36.3 3 H.sup.+ 0.5% 2950 36.8 3 Li.sup.+ 0.5% 1507 36.8 3 NH.sub.4.sup.+ 0.5% 1750 49.7 3 H.sup.+ 0.25% 4130 32.9 5187 40.7 3 NH.sub.4.sup.+ 0.25% 2823 39.2 3376 41.2

Example 9

[0052] Carbon Dioxide Separations from Nitrogen at Low Feed and Permeate Pressure:

[0053] Composite membrane samples from Example 3 were tested for CO.sub.2/N.sub.2 mixed gas separation at ambient temperature (20-23 C.) at feed pressure of 14.9-16.2-psia and a 20% CO.sub.2 concentration. The permeate pressure was 2.5-3.2-psia. Table 3 shows the high permeance and high selectivity for the composite membranes under close to real-world and commercially attractive conditions.

TABLE-US-00004 TABLE 3 Coating CO.sub.2 Cation solution Permeance CO.sub.2/N.sub.2 form conc. (GPU) Selectivity H.sup.+ 0.25% 7885 37.8 NH.sub.4.sup.+ 0.25% 7338 43.8

Example 10

[0054] Composite Membrane Aging Performance for Carbon Dioxide Separations from Nitrogen:

[0055] TCM samples from Example 3 were tested initially for CO.sub.2/N.sub.2 mixed gas separation performance at ambient temperature and at a feed pressure of 30-psig, 40% CO.sub.2 feed concentration, and near atmospheric permeate pressure. The composite membranes were tested again after 1 week of exposure to air. Table 4 shows that the permeance for the composite membrane having an ammonium cation in the gas-separation layer may have slightly decreased by 5% but appeared to be more stable than the acid. The high selectivity for both membranes was effectively unchanged.

TABLE-US-00005 TABLE 4 Initial Performance After 1 week Coating CO.sub.2 CO.sub.2 Cation solution Permeance CO.sub.2/N.sub.2 Permeance CO.sub.2/N.sub.2 form conc. (GPU) Selectivity (GPU) Selectivity H.sup.+ 0.25% 5127 31.0 2111 38.8 NH.sub.4.sup.+ 0.25% 2823 39.3 2682 39.8

Example 11

[0056] Carbon Dioxide Separations from Methane:

[0057] Composite membrane samples from Examples 2 and 3 were tested for CO.sub.2/CH.sub.4 mixed gas separation at ambient temperature (20-25 C.) at a feed pressure of 60-psig, 40% CO.sub.2 feed concentration, and near atmospheric permeate pressure. Table 5 shows high CO.sub.2 permeance and high selectivity over methane for most of the membranes.

TABLE-US-00006 TABLE 5 Membrane Coating CO.sub.2 from Cation solution Permeance CO.sub.2/CH.sub.4 Example form conc. (GPU) Selectivity 2 H.sup.+ 1.0% 730 32.6 2 NH.sub.4.sup.+ 0.7% 347 44.0 2 Na.sup.+ 1.0% 777 30.8 2 Li.sup.+ 1.0% 744 29.9 2 K.sup.+ 1.0% 491 46.0 3 H.sup.+ 1.0% 1592 25.0 3 NH.sub.4.sup.+ 0.5% 1276 32.1

Example 12

[0058] Carbon Dioxide Separations from Methane at Varied Feed Pressures and CO.sub.2 Concentrations:

[0059] Composite membrane samples from Example 2 containing ammonium cations in the gas-separation layer were tested for CO.sub.2/CH.sub.4 mixed gas separation at ambient temperature (20-23 C.), feed pressures between 30 and 90 psig, CO.sub.2 feed concentrations between 10 and 40%, and near atmospheric permeate pressure. The feed flow was 1000-scc/min.

[0060] Table 6 shows high CO.sub.2 permeance and higher CO.sub.2/CH.sub.4 selectivity at lower feed pressures and lower CO.sub.2 concentrations. The composite membranes were directly exposed to manure gas for 1 month and no significant performance change was observed.

TABLE-US-00007 TABLE 6 Feed CO.sub.2 CH.sub.4 Pressure permeance permeance CO.sub.2/CH.sub.4 [CO.sub.2] (psig) (GPU) (GPU) Selectivity 40% 30 595 11.8 50.3 60 399 9.6 41.7 90 327 8.0 40.7 20% 30 708 14.4 49.2 60 629 14.9 42.0 90 492 12.2 40.5 10% 30 1102 15.4 71.7 60 612 12.8 47.9 90 432 11.1 38.8

[0061] It will be apparent to those skilled in the art that various modifications, combinations and variations can be made in the present invention without departing from the scope of the invention. Specific embodiments, features and elements described herein may be modified, and/or combined in any suitable manner. Thus, it is intended that the present invention cover the modifications, combinations and variations of this invention provided they come within the scope of the appended claims and their equivalents.