MULTI-LAYER COMPOSITE GAS SEPARATION MEMBRANES, METHODS FOR PREPARATION, AND USE

Abstract

Methods and systems for producing and using multi-layer composite co-polyimide membranes, one method for producing including preparing a microporous or mesoporous membrane support material for coating; applying a sealing layer to the membrane support material to prevent intrusion into the membrane support material of co-polyimide polymer; applying a first permselective co-polyimide layer atop and in contact with the sealing layer; and applying a second permselective co-polyimide layer atop and in contact with the first permselective co-polyimide layer.

Claims

1. A method for producing a multi-layer composite co-polyimide membrane, the method comprising the steps of: preparing a microporous or mesoporous membrane support material for coating; applying a sealing layer to the membrane support material to prevent intrusion into the membrane support material of co-polyimide polymer; applying a first permselective co-polyimide layer atop and in contact with the sealing layer; and applying a second permselective co-polyimide layer atop and in contact with the first permselective co-polyimide layer.

2. The method according to claim 1, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise block co-polymers.

3. The method according to claim 2, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise the same block co-polymers.

4. The method according to claim 3, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise block (6FDA-CARDO)/(6FDA-durene).

5. The method according to claim 1, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise random co-polymers.

6. The method according to claim 5, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise the same random co-polymers.

7. The method according to claim 1, where the first permselective co-polyimide layer and second permselective co-polyimide layer result in a thickness between about 1-3 μm.

8. The method according to claim 1, where the sealing layer comprises a solvent material being the same as a solvent material into which a first co-polyimide of the first permselective co-polyimide layer and into which a second co-polyimide of the second permselective co-polyimide layer are dissolved for the steps of applying the first permselective co-polyimide layer and second permselective co-polyimide layer.

9. The method according to claim 1, where the sealing layer further comprises a gutter layer.

10. The method according to claim 9, where the gutter layer comprises poly [1-(trimethylsilyl)-1-propyne] (PTMSP).

11. The method according to claim 1, where the microporous or mesoporous membrane support material for coating includes a support material selected from the group consisting of: flat sheet support material; hollow fiber support material; and combinations thereof.

12. The method according to claim 1, further comprising the step of crosslinking the first permselective co-polyimide layer and the second permselective co-polyimide layer.

13. The method according to claim 1, where the step of applying a sealing layer to the membrane support material includes the use of a component selected from the group consisting of: Chloroform (CHCl.sub.3); N,N-dimethylformamide (DMF); dichloromethane (DCM); N,N-dimethyl acetamide (DMAc); Acetone; Tetrahydrofuran (THF); N-Methyl-2-pyrrolidone (NMP); tetra chloromethane (CCl.sub.4)); and combinations of the same.

14. The method according to claim 1, where a step of drying is carried out after each of the steps of applying a sealing layer, applying a first permselective co-polyimide layer, and applying a second permselective co-polyimide layer.

15. The method according to claim 1, further comprising the step of formulating a first co-polyimide for the first permselective co-polyimide layer and formulating a second co-polyimide for the second permselective co-polyimide layer, where the steps of formulating comprise combining more than one monomer in a mixture of monomers, where the more than one monomer is selected from the group consisting of: 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA); 1,3-phenylenediamine (mPDA); durene diamine; 3,4,9,10-perylentetracarbonsauredianhydrid (PTCDA); pyromellitic dianhydride (PMDA); 1,4-bis(4-aminophenoxy)triptycene (BAPT); 4,5,6,7-Tetrabromo-2-azabenzimidazole (TBB); 4,4′-(9-Fluorenylidene)dianiline (FDA); and 4,4′-Oxydiphthalic anhydride (ODA).

16. The method according to claim 1, where the first permselective co-polyimide layer or second permselective co-polyimide layer comprises a polymer unit selected from the group consisting of: (6FDA-mPDA)/(6FDA-durene); (6FDA-PTCDA-FDA); (6FDA-TBB-FDA); (6FDA-BAPT-FDA); (PTCDA-FDA)/(PMDA-mPDA); (PMDA-FDA)/(PTCDA-mPDA); (ODA-FDA)/(PTCDA-mPDA); (6FDA-BAPT)/(6FDA-FDA); (PTCDA-mPDA)/(6FDA-FDA); (PTCDA-FDA)/(ODA-mPDA); (PTCDA-FDA)/(6FDA-FDA); (6FDA-TBB)/(6FDA-FDA); (6FDA-TBB)/(6FDA-durene); (6FDA-mPDA)/(6FDA-BAPT); (PTCDA-mPDA)/(6FDA-FDA); (6FDA-mPDA-BAPT); and (6FDA-FDA-mPDA).

17. The method according to claim 1, further comprising the step of formulating a first co-polyimide for the first permselective co-polyimide layer and formulating a second co-polyimide for the second permselective co-polyimide layer, where the steps of formulating comprise combining at least three distinct moieties polymerized together, the moieties including a 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) based moiety; a 9,9-bis(4-aminophenyl) fluorene (CARDO) based moiety; and 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) based moiety.

18. The method according to claim 1, further comprising the step of formulating a first co-polyimide for the first permselective co-polyimide layer and formulating a second co-polyimide for the second permselective co-polyimide layer, where the steps of formulating comprise combining at least three distinct moieties polymerized together, the moieties including a 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) based moiety; a 4,4′-(hexafluoroisopropylidene)dianiline (6FpDA) based moiety; and at least one component selected from the group consisting of: a 9,9-bis(4-aminophenyl) fluorene (CARDO) based moiety; a 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) based moiety; a 2,2′-bis(trifluoromethyl)benzidine (ABL-21) based moiety; a 3,3′-dihydroxybenzidine based moiety; and a 3,3′-(hexafluoroisopropylidene)dianiline based moiety.

19. The method according to claim 1, further comprising the step of formulating a first co-polyimide for the first permselective co-polyimide layer and formulating a second co-polyimide for the second permselective co-polyimide layer, where the steps of formulating comprise combining at least three distinct moieties polymerized together, the moieties including a 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) based moiety; a 2,4,6-trimethyl-m-phenylenediamine (DAM) based moiety; and at least one component selected from the group consisting of: a 4,4′-(hexafluoroisopropylidene)dianiline (6FpDA) based moiety; a 9,9-bis(4-aminophenyl) fluorene (CARDO) based moiety; a 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) based moiety; a 2,2′-bis(trifluoromethyl)benzidine (ABL-21) based moiety; a 3,3′-dihydroxybenzidine based moiety; and a 3,3′-(hexafluoroisopropylidene)dianiline based moiety.

20. The method according to claim 1, where the first permselective layer or second permselective layer comprises a 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) based moiety and a 2,2′-bis(trifluoromethyl)benzidine (ABL-21) based moiety.

21. The method according to claim 1, further comprising the step of formulating a first co-polyimide for the first permselective co-polyimide layer and formulating a second co-polyimide for the second permselective co-polyimide layer, where the steps of formulating comprise combining at least three distinct moieties polymerized together, the moieties including a dianhydride selected from the group consisting of: a 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) based moiety; a benzophenone-3,3′, 4,4′-tetracarboxylic dianhydride (BTDA) based moiety; and a pyromellitic dianhydride (PMDA) based moiety; a 2,4,6-trimethyl-m-phenylenediamine (DAM) based moiety; and at least one component selected from the group consisting of: a 4,4′-(hexafluoroisopropylidene)dianiline (6FpDA) based moiety; a 9,9-bis(4-aminophenyl) fluorene (CARDO) based moiety; a 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) based moiety; a 2,2′-bis(trifluoromethyl)benzidine (ABL-21) based moiety; a 3,3′-dihydroxybenzidine based moiety; and a 3,3′-(hexafluoroisopropylidene)dianiline based moiety.

22. A method of gas separation, the method comprising the step of: applying the multi-layer composite co-polyimide membrane of claim 1 to separate at least 2 components of a mixed gas stream.

23. The method according to claim 22, where feed pressure of the mixed gas stream to a feed side of the membrane is up to about 900 psig and H.sub.2S content of the mixed gas stream is up to about 22 volume percent.

24. A continuous system for producing a multi-layer composite co-polyimide membrane, the system comprising: a first reservoir comprising a sealing material for preparing a microporous or mesoporous membrane support material for coating by applying a sealing layer to the membrane support material; a second reservoir comprising a first co-polyimide solution for applying a first permselective co-polyimide layer to the membrane support material atop and in contact with the sealing layer applied at the first reservoir; and a third reservoir comprising a second co-polyimide solution for applying a second permselective co-polyimide layer to the membrane support material atop and in contact with the first permselective co-polyimide layer; and at least one drying device to dry separate layers applied to the membrane support material.

25. The system according to claim 24, where the second reservoir and third reservoir comprise block co-polymers.

26. The system according to claim 25, where the second reservoir and third reservoir comprise the same block co-polymers.

27. The system according to claim 25, where the second reservoir and third reservoir comprise block (6FDA-CARDO)/(6FDA-durene).

28. The system according to claim 24, where the second reservoir and third reservoir comprise random co-polymers.

29. The system according to claim 28, where the second reservoir and third reservoir comprise the same random co-polymers.

30. The system according to claim 24, where the sealing material in the first reservoir comprises a solvent material the same as a solvent material into which a first co-polyimide in the second reservoir and into which a second co-polyimide in the third reservoir are dissolved.

31. The system according to claim 24, where the microporous or mesoporous membrane support material includes a support material selected from the group consisting of: flat sheet support material; hollow fiber support material; and combinations thereof.

32. The system according to claim 24, further comprising a fourth reservoir with a cross-linking agent to crosslink the first co-polyimide solution and the second co-polyimide solution once disposed upon the microporous or mesoporous membrane support material.

33. The system according to claim 24, where the sealing material is selected from the group consisting of: Chloroform (CHCl.sub.3); N,N-dimethylformamide (DMF); dichloromethane (DCM); N,N-dimethyl acetamide (DMAc); Acetone; Tetrahydrofuran (THF); N-Methyl-2-pyrrolidone (NMP); tetra chloromethane (CCl.sub.4)); and combinations of the same.

34. The system according to claim 24, further comprising a fourth reservoir to apply a gutter layer to the microporous or mesoporous membrane support material.

35. A multi-layer composite co-polyimide membrane, the membrane comprising: a microporous or mesoporous membrane support material; a sealing material for coating a sealing layer on the membrane support material to prevent penetration of co-polyimides into pores of the microporous or mesoporous membrane support material; a first permselective co-polyimide layer atop and in contact with the sealing layer; and a second permselective co-polyimide layer atop and in contact with the first permselective co-polyimide layer.

36. The membrane according to claim 35, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise block co-polymers.

37. The membrane according to claim 36, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise the same block co-polymers.

38. The membrane according to claim 37, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise block (6FDA-CARDO)/(6FDA-durene).

39. The membrane according to claim 35, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise random co-polymers.

40. The membrane according to claim 39, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise the same random co-polymers.

41. The membrane according to claim 35, where the first permselective co-polyimide layer and second permselective co-polyimide layer result in a thickness between about 1-3 μm.

42. The membrane according to claim 35, where the sealing layer comprises a solvent material being the same as a solvent material into which a first co-polyimide of the first permselective co-polyimide layer and into which a second co-polyimide of the second co-polyimide layer are dissolved for applying the first permselective co-polyimide layer and second permselective co-polyimide layer.

43. The membrane according to claim 35, where the microporous or mesoporous membrane support material for coating includes a support material selected from the group consisting of: flat sheet support material; hollow fiber support material; and combinations thereof.

44. The membrane according to claim 35, further comprising crosslinking between the first permselective co-polyimide layer and the second permselective co-polyimide layer.

45. The membrane according to claim 35, where the first permselective co-polyimide layer and second permselective co-polyimide layer comprise at least one moiety selected from the group consisting of: a 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) based moiety; a 1,3-phenylenediamine (mPDA) based moiety; a 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) based moiety; a 3,4,9,10-perylentetracarbonsauredianhydrid (PTCDA) based moiety; a pyromellitic dianhydride (PMDA) based moiety; a 1,4-bis(4-aminophenoxy)triptycene (BAPT) based moiety; a 4,5,6,7-Tetrabromo-2-azabenzimidazole (TBB) based moiety; a 4,4′-(9-Fluorenylidene)dianiline (FDA) based moiety; a 4,4′-Oxydiphthalic anhydride (ODA) based moiety; a 9,9-bis(4-aminophenyl) fluorene (CARDO) based moiety; a 4,4′-(hexafluoroisopropylidene)dianiline (6FpDA) based moiety; a 2,2′-bis(trifluoromethyl)benzidine (ABL-21) based moiety; a 3,3′-dihydroxybenzidine based moiety; a 3,3′-(hexafluoroisopropylidene)dianiline based moiety; a 2,4,6-trimethyl-m-phenylenediamine (DAM) based moiety; a benzophenone-3,3′, 4,4′-tetracarboxylic dianhydride (BTDA) based moiety; and combinations of the same.

46. The membrane according to claim 35, where the membrane further comprises a gutter layer.

47. The membrane according to claim 46, where the gutter layer comprises poly [1-(trimethylsilyl)-1-propyne] (PTMSP).

48. The membrane according to claim 35, wherein the sealing material has a near-zero thickness as a layer from evaporation after drying of the first permselective co-polyimide layer and the second permselective co-polyimide layer, the sealing material operable to prevent intrusion of the co-polyimide layers into the microporous or mesoporous membrane support material during drying.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following descriptions, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the disclosure and are therefore not to be considered limiting of the disclosure's scope as it can admit to other equally effective embodiments.

[0060] FIG. 1 is a schematic representation of a continuous coating process to fabricate multi-layer composite co-polyimide membranes.

[0061] FIG. 2 is a schematic representation of a multi-layer composite co-polyimide separation membrane.

[0062] FIG. 3A is a scanning electron microscope (SEM) image of a cross-section of an interface between a substrate and a coating layer of a multi-layer composite thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane.

[0063] FIG. 3B is an enlarged SEM image of a surface of a coating layer of the composite, multi-layer thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane from FIG. 3A.

[0064] FIG. 3C is an enlarged SEM image of a cross-sectional view of the interface between the substrate and the coating layer of the multi-layer thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane of FIG. 3A.

DETAILED DESCRIPTION

[0065] So that the manner in which the features and advantages of the embodiments of apparatus, systems, and methods for production and use of multi-layer composite co-polyimide membranes for sour gas feed separations from natural gas, as well as others, which will become apparent, may be understood in more detail, a more particular description of the embodiments of the present disclosure briefly summarized previously may be had by reference to the various embodiments, which are illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the disclosure and are therefore not to be considered limiting of the present disclosure's scope, as it may include other effective embodiments as well.

[0066] Referring first to FIG. 1, embodiments here relate to methods for fabricating defect-free composite membranes and their use, particularly for gas and vapor separations including natural gas treatment. A consecutive coating process, as shown in FIG. 1, can include the steps of: (a) pre-wetting of a substrate using pure solvent to create an intermediate sealing pre-wetting layer, where the solvent can be used for application of polymer solution in one or more subsequent step (selective layers coating); (b) applying a gutter layer, where step (b) is optional; (c) deposition of a first permselective layer; (d) coating of second permselective layer directly on top of first permselective layer; and (e) immersion of the composite membrane formed from stages (a) to (d) in a cross-linking agent solution. In such a process, stages (a), (c), and (d) are generally required, and stages (b) and (e) are optional.

[0067] For example, in multi-layer composite co-polyimide continuous production apparatus 100, a first reservoir 102 contains a pure or substantially pure solvent to soak (or pre-wet) and coat microporous or mesoporous substrate 104 (forming an intermediate sealing pre-wetting layer 204 in FIG. 2). In a second reservoir 106, substrate 104 is coated with an optional gutter layer that can form a part of intermediate sealing pre-wetting layer 204, or be distinct from intermediate sealing pre-wetting layer 204. In one embodiment following a path “A,” substrate 104 coated with an optional gutter layer is sent to a first dryer 108, before proceeding to a third reservoir 110. Third reservoir 110 contains the coating to apply a first permselective co-polyimide layer either atop and directly in contact with the substrate without the optional gutter layer (route “B”) or atop and directly in contact with gutter layer (route “A”). The now coated substrate proceeds through a second dryer 112 and to a fourth reservoir 114 containing a second permselective co-polyimide layer in addition to or alternative to cross-linking agent. The substrate next passes through a third dryer 116 (optional in light of third reservoir 114 being optional), and ultimately onto a spool 118 as a thin film composite hollow fiber (“TFC HF”) multi-layer composite co-polyimide membrane. Tension control rollers 120, 122, 124, 126, 128, 130, 132, 134, and 136 control the speed of and tension applied to substrate 104 as it proceeds through multi-layer composite co-polyimide continuous production apparatus 100 to ensure adequate thickness of coatings from the reservoirs 106, 110, and 114 and to ensure adequate drying time in dryers 108, 112, and 116. In other embodiments, more reservoirs can be used to apply additional coating layers, for example a separate additional reservoir for application of crosslinking material or a third perm-selective layer.

[0068] Reservoir 102 contains solvent to pre-wet (or soak) the substrate to prevent pore plugging by polymer solutions and create an intermediate sealing pre-wetting layer 204. The substrate optionally can be coated with a gutter layer in addition to or alternative to the intermediate sealing pre-wetting layer to minimize plugging by taking route “A” in FIG. 1. Gutter layer can be formed using poly [1-(trimethylsilyl)-1-propyne] PTMSP, for example. However, this gutter layer is optional. The substrate can be sent directly to reservoir 110 through route “B” to form a perm-selective layer using co-polyimide directly atop the intermediate sealing pre-wetting layer. The substrate can follow either route “A” or “B”.

[0069] Reservoir 114 can contain either or both of crosslinking agent or a co-polyimide solution at a different concentration than reservoir 110. The co-polyimides in reservoirs 110, 114 can be the same co-polyimides at different concentrations or different co-polyimides at the same or different concentrations.

[0070] Drying temperature of the dryers 108, 112, 116 is generally 5-10° C. higher than the boiling point of solvent used in each solution in the reservoirs. Depending on chemical compatibility, other suitable substrates other than PAN include polyether ether ketone (PEEK), sulfonated polyether ether ketone (SPEEK), polysulfone (PSF), polyether sulfone (PESF), polyvinylidene fluoride (PVDF), polypropylene (PP), poly(tetrafluoroethylene) (PTFE), sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK), poly(phthalazinone ether amide (PPEA), polyether imide (PEI), Polyimide (PI) etc. Most of the substrates are commercially available.

[0071] Coating speed can vary from 10 m/min to 60 m/min. Coating speeds vary depending in part on the drying tower length. Gutter layer can be formed using PTMSP (about 0.1 to about 2.0 wt. % solution), and n-hexane in addition to or alternative to cyclohexane can be used for dissolving the PTMSP. PTMSP gutter layer (optional intermediate layer) thickness is generally in the about 0.2-1.0 micrometer range. Substrate thickness ranges from about 150 to 180 μm. First co-polyimide layer thickness ranges from about 0.2 to 1.0 μm and second co-polyimide layer thickness ranges from about 0.2 to 1.0 μm.

[0072] Different concentrations of co-polyimide solutions can be used for coating. About 2.0 to about 10 wt. % of co-polyimide can be dissolved in chloroform or in tetrahydrofuran (THF), for example. Optional crosslinking solution can be prepared by dissolving between about 0.2 to about 10 wt. % cross linking agent in water or organic solvents.

[0073] Two or more permselective layers can be fabricated on top of a porous substrate (flat sheet or hollow fiber) with precisely controlled membrane thicknesses. With methods applying systems similar to that shown in FIG. 1, intrusion of the coating solution(s) into the pores of substrate, which leads to dramatic decrease in selectivity and permeance, can be minimized. The resulting defect-free multi-layer composite membrane is particularly useful for sour gas feed separations from natural gas.

[0074] The membrane can ultimately include: (a) a microporous or mesoporous substrate; (b) an intermediate sealing pre-wetting layer; (c) a permselective layer; (d) a top permselective caulking layer; and (e) a crosslinking agent layer. The intermediate sealing pre-wetting layer is first deposited using a pure solvent directly onto a thicker porous non-selective support layer (reservoir 102). Then a gutter layer is applied, which is optional (reservoir 106). Then a first permselective layer (reservoir 110) is deposited from a solution directly onto the intermediate sealing pre-wetting layer coated on the porous support layer or onto the gutter layer. The intermediate sealing pre-wetting layer prevents penetration of the permselective layer reagents into the substrate pores during membrane preparation and provides a sealing layer in the finished membrane, thereby playing a critical role in decreasing the thickness of the selective layers. More or fewer layers may exist in the composite membranes. The optional gutter layer may form a separate layer or may comprise a portion of the intermediate sealing pre-wetting layer.

[0075] Examples of thin film sealing layers can include specific single or multi-component solvent systems, for example chloroform (CHCl.sub.3); N,N-dimethylformamide (DMF); dichloromethane (DCM); N,N-dimethyl acetamide (DMAc); acetone; tetrahydrofuran (THF); N-Methyl-2-pyrrolidone (NMP); and/or tetra chloromethane (CCl.sub.4)). The microporous or mesoporous support substrate can include polyacrylonitrile (PAN), for example. In one example, a thin film of block co-polyimide containing (6FDA-CARDO)/(6FDA-durene) (5000/5000) is coated onto the sealing layer already coated on a PAN support from a specific single or multi-component solvent system.

[0076] The next layer applied can be a thin film of the same block co-polyimide as used in the first permselective layer (or a difference block or random co-polyimide); and, the co-polyimide concentration in the solutions can be different in each of the layers (reservoirs 110, 114), or could be the same or about the same. The applied layer from reservoir 114 can be referred to as “the caulking layer” and can also be considered as a permselective layer coated on the first permselective layer to seal off any defect or pin-holes that may be present in the overall finished membrane. The optional fifth stage is application of a crosslinking agent solution (reservoir 114 or a separate reservoir, not pictured) in which the composite membrane formed from stages (a) to (d) is immersed. Crosslinking of the formed composite membrane can be achieved using different types and sizing of functional groups.

[0077] Examples include and are not limited to functionalization or grafting with polar or H.sub.2S-philic, in addition to or alternative to CO.sub.2-philic, groups that include Bromine (Br); sulfonate (SO.sub.3H); diallyl amine; acrylonitrile; jeffamines; and combinations thereof. Crosslinking can also be achieved using such cross-linkers as N,N-dimethylpiperizine, p-xylenediamine, m-xylenediamine, aliphatic diamine, polyethyleneimine, and 1,3-cyclohexane-bis(methylamine) for example. Co-polyimide solutions (reservoirs 110, 114) are formed by dissolving an appropriate amount of a polymer (block or random) in suitable solvent or multi-component solvent system such as chloroform (CHCl.sub.3); N,N-dimethylformamide (DMF); dichloromethane (DCM); N,N-dimethyl acetamide (DMAc); acetone; tetrahydrofuran (THF); N-Methyl-2-pyrrolidone (NMP); and/or tetra chloromethane (CCl.sub.4), for example. An effective film thicknesses of less than 1 micron can be achieved, and membranes can be used effectively as flat sheet membranes, as well as hollow fiber membranes, and can be applied in a plate and frame, spiral wound module, or hollow-fiber module arrangement. Resulting membranes have high gas fluxes and selectivities, and can be used, for example, to study gas transport properties of pure and sour mixed gas streams comprising H.sub.2S, CO.sub.2, He, CH.sub.4, N.sub.2 and C.sub.2H.sub.6 through the thin films of the co-polyimide membrane.

[0078] Plate and frame membrane systems utilize membranes laid on top of a plate-like structure, which in turn is held together by a frame-like support. Flat sheet membranes are bolted together with a frame around the perimeter; similar to a heat exchanger or filter press. There are two types of plate and frame membrane configurations; dead-end and cross flow. In dead-end plate and frame systems, the feed solution flows perpendicular into the membrane, while cross flow systems are made so that the flow is tangential to the membrane wall.

[0079] Spiral-wound elements include membranes, feed spacers, permeate spacers, and a permeate tube. First, a membrane is laid out and folded in half with the membrane facing inward. The feed spacer is then put in between the folded membranes, forming a membrane sandwich. The purpose of the feed spacer is to provide space for gas to flow between the membrane surfaces, and to allow for uniform flow between the membrane leaves. Next, the permeate spacer is attached to the permeate tube, and the membrane sandwich prepared earlier is attached to the permeate spacer using glue. The next permeate layer is laid down and sealed with glue, and the whole process is repeated until all of the required permeate spacers have been attached to the membranes. The finished membrane layers then are wrapped around the tube creating the spiral shape.

[0080] Hollow fiber filtration utilizes thousands of long, porous filaments ranging from about 1-3.5 mm wide, that are disposed in place in a PVC shell. Each filament is narrow in diameter and flexible. Hollow fiber membranes feature a very high packing density because of the small strand diameter. Because of the flexibility of the strands, certain filter configurations are possible that cannot be achieved in other filtration configurations.

[0081] Referring now to FIG. 2, a schematic representation is shown of a multi-layer composite co-polyimide separation membrane. For example, FIG. 2 can represent a membrane formed by the process and system of FIG. 1. Multi-layer composite co-polyimide separation membrane 200 includes a microporous or mesoporous support layer 202, for example PAN 350 Ultrafiltration (UF) membrane. Membrane 200 further includes an intermediate sealing pre-wetting layer 204 disposed atop and in intimate contact with microporous or mesoporous support layer 202. Intermediate sealing pre-wetting layer 204 includes a specific single or multi-component solvent system, for example chloroform (CHCl.sub.3); N,N-dimethylformamide (DMF); dichloromethane (DCM); N,N-dimethyl acetamide (DMAc); acetone; tetrahydrofuran (THF); N-Methyl-2-pyrrolidone (NMP); and/or tetra chloromethane (CCl.sub.4). Intermediate sealing pre-wetting layer 204 can optionally include or be replaced by a gutter layer such as poly [1-(trimethylsilyl)-1-propyne] (PTMSP) (not pictured).

[0082] Membrane 200 further includes a permselective block co-polyimide layer (for example (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide) 206 disposed atop and in intimate contact with intermediate sealing pre-wetting layer 204. Membrane 200 further includes a second permselective block co-polyimide layer (also referred to as a top caulking layer) (for example (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide) 208 disposed atop and in intimate contact with permselective block co-polyimide layer (for example (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide) 206. Optional crosslinking agent is embedded in the 3.sup.rd and 4.sup.th layers 206, 208, but doesn't necessarily form a separate distinct layer by itself. Layers 204, 206 may be the same or different block or random co-polyimides and can be applied with different concentrations in solution at different or similar thicknesses, or at similar concentrations in solution at similar or different thicknesses.

[0083] Polymers include random 6FDA-durene/CARDO and block (6FDA-durene)/(6FDA-CARDO). Other co-polyimides that can be used include random 6FDA-durene/6FpDA; block (6FDA-durene)/(6FDA-6FpDA); random 6FDA-CARDO/6FpDA; block (6FDA-CARDO)/(6FDA-6FpDA); random 6FDA-DAM/CARDO; block (6FDA-DAM)/(6FDA-CARDO); random 6FDA-DAM/6FpDA; block (6FDA-DAM)/(6FDA-6FpDA); random 6FDA-DAM/ABL-21; block (6FDA-DAM)/(6FDA-ABL-21) and combinations and mixtures thereof.

[0084] FIG. 3A is a scanning electron microscope (SEM) image of a cross-section of an interface between a substrate and a coating layer of a multi-layer composite thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane (for example an interface such as that between 202 and 204 in FIG. 2).

[0085] FIG. 3B is an enlarged SEM image of a surface of a coating layer of the multi-layer composite thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane from FIG. 3A (for example a surface such as 208 in FIG. 2).

[0086] FIG. 3C is an enlarged SEM image of a cross-sectional view of the interface between the substrate and the coating layer of the multi-layer thin film (6FDA-CARDO)/(6FDA-durene) (5000/5000) block co-polyimide membrane of FIG. 3A (for example an interface such as that between 202, 204, and 206 in FIG. 2). The interface appears intact and the perm-selective layer does not penetrate or bleed into the substrate.

[0087] Two parameters to assess intrinsic permeation and transport properties of membrane materials for gas separation are gas permeability, P.sub.i, which is also known as the permeability coefficient and selectivity (α.sub.ij). These can be expressed as shown in Equation 1:

[00001]$\begin{array}{cc}{P}_i=D_i{S}_i=\frac{j_i.Math.l}{p_{i\left(0\right)}-p_{i\left(l\right)}}& \mathrm{Eq}.1\end{array}$

[0088] (D.sub.iS.sub.i) is referred to as membrane permeability of component i, (P.sub.i), and it is a product of diffusion and solubility coefficients. The unit of permeability is Barrer, where 1 Barrer=10.sup.−10 (cm.sup.3(STP).Math.cm)/(cm.sup.2.Math.s.Math.cmHg).

[0089] Ideal selectivity (α.sub.ij) can be expressed as the permeability ratio of two single gases, as depicted in Equation 2:

[00002]$\begin{array}{cc}{\alpha }_{ij}=\frac{P_i}{P_j}& \mathrm{Eq}.2\end{array}$

[0090] Permeability coefficients of each gas species in the mixture, especially at low pressure, can be calculated from Equation 3 as follows:

[00003]$\begin{array}{cc}{P}_i=\frac{x_{i\left(l\right)}{J}_i.Math.l}{\left(P_f{x}_{i\left(0\right)}-P_p{x}_{i\left(l\right)}\right)}& \mathrm{Eq}.3\end{array}$

[0091] Permeance (Qi) is mostly used to evaluate membrane performance for composite membranes, and thus permeance can be expressed as Equation 4:

[00004]$\begin{array}{cc}{Q}_i=\frac{P_i}{l}& \mathrm{Eq}.4\end{array}$

[0092] A widely used unit for gas permeance is GPU (gas permeation unit), which is given as 1 GPU=10.sup.−6 (cm.sup.3(STP))/(cm.sup.2.Math.s.Math.cmHg).

[0093] The separation factor can be expressed as the ratio of the composition of the feed gas to the permeant gas, which is represented as Equation 5:

[00005]$\begin{array}{cc}{\alpha }_{i/j}^m=\frac{x_{i\left(l\right)}\text{/}{x}_{j\left(l\right)}}{x_{i\left(0\right)}\text{/}{x}_{j\left(0\right)}}& \mathrm{Eq}.5\end{array}$

[0094] For composite membranes, separation factor is usually used instead of selectivity, especially in mixed gas measurements. However for non-ideal gas mixtures, selectivity ∝.sub.i/j.sup.m,* is used and is expressed as Equation 6:

[00006]$\begin{array}{cc}{\propto }_{i/j}^{m,*}=\frac{P_i^{*}}{P_j^{*}}& \mathrm{Eq}.6\end{array}$

[0095] High gas permeance is obtained when a selective layer thickness is very thin (for example less than about 1.0 μm). Examples discussed here show development and fabrication of thin film composite membranes and investigation of their pure and sour mixed gas permeation properties for H.sub.2S, CO.sub.2, N.sub.2, He, CH.sub.4, and C.sub.2H.sub.6 mixed gas stream separations. An example block co-polyimide with (6FDA-CARDO)/(6FDA-durene) (5000/5000) backbone was employed in the selective layers. This was recently developed from co-polymerization of 6FDA-durene and 6FDA-CARDO) (U.S. Pat. Pub. No. 2018/0345229 A1). Embodiments tested here show enhancement in gas separation properties. Pure gases and mixed sour gases comprising CO.sub.2, CH.sub.4, N.sub.2, C.sub.2H.sub.6, He, and H.sub.2S were passed through the thin films of multi-layer composite co-polyimide membranes with (6FDA-CARDO)/(6FDA-durene) (5000:5000) for simultaneous separation of CO.sub.2, N.sub.2, He, and H.sub.2S from sour natural gas streams.

[0096] Resulting membranes, for example optionally produced from the system and process of FIG. 1, have high gas fluxes and selectivities. Permeation properties of simulated sour gas mixtures comprising 10; 55; 10; 3; and 22 vol. % of CO.sub.2, CH.sub.4, N.sub.2, C.sub.2H.sub.6, and H.sub.2S, respectively, through the membrane were studied at different gas feed pressures up to 700 psig. The CO.sub.2/CH.sub.4 and H.sub.2S/CH.sub.4 selectivities obtained for the TFC membrane are 10 and 20, respectively; while CO.sub.2 and H.sub.2S permeances are 78 and 149 GPU, respectively. These values and separation performances are surprising and unexpected. At high feed pressure (about 700 psig) and up to 22 vol. % H.sub.2S in a feed gas mixture, selectivities and permeances are still advantageous for separations in the membrane. Moreover, the CO.sub.2/CH.sub.4 selectivity of the co-polyimides does not degrade to anywhere near the same extent as was reported for cellulose acetate (CA), even under these much more aggressive environments. This stability at high pressures and high H.sub.2S concentration is impressive and unique.

EXAMPLES

[0097] The following Examples are given for the purpose of illustrating embodiments of the present invention, however, it is to be understood that these examples are merely illustrative in nature, and that the process embodiments are not necessarily limited thereto.

Example 1: Preparation of Multi-Layer Composite Block Co-Polyimide Membranes

[0098] Series of multi-layer composite block co-polyimide membranes (being represented by FIG. 2 and Table 1) were fabricated as follows. Block co-polyimides were synthesized using 2,2′-bis-(3,4′-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) (Alfa Aesar); 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene diamine) (Tokyo Chemical Industry); and 9,9-bis(4-aminophenyl)fluorene (CARDO) (Aldrich). The monomers were used as received without further purification, and as described in U.S. Pat. Pub. No. 2018/0345229 A1.

[0099] In some examples, poly(trimethylsilyl)propyne (“PTMSP” from Gelest Inc. USA) was used as received to form a gutter or sealing layer on top of mesoporous polyacrylonitrile ultrafiltration (UF) flat sheet membrane substrate (PAN350, molecular weight cut-off (MWCO)=20 kDa obtained from Sepro Membrane, USA). This PAN 350 was used as the support for the block co-polyimide composite membranes. Cyclohexane (99.5%, Sigma-Aldrich), chloroform (>99.8%, Sigma-Aldrich), n-hexane (97%, Sigma-Aldrich), m-cresol (Alfa Aesar) and methanol (ThermoFisher Scientific) were used as solvents as received.

[0100] Two reference composite membranes were initially developed as follows:

[0101] Reference I composite membrane was developed using a spin coater (KW-4A, Chemat Technology). Coating procedures were performed by first cleaning the PAN 350 UF membrane and then placing it on a vacuum chuck (rotating disk) by taping the PAN 350 UF membrane on the disk to form a flat surface. The spin coater's speed was set to a speed of about 3,000 rpm, and 1 mL of a chloroform sealing layer solvent was then dripped dropwise onto the UF support for about 10 seconds of coating time.

[0102] The membrane was then placed in an oven to dry at about at 80° C. overnight. Then 4 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same spinning procedure was then followed to coat the co-polyimide selective layer on top of chloroform-solvent-sealed PAN support. The co-polyimide coated composite membrane was dried in an oven at 60° C. overnight. The membrane coating thickness was determined from gas flux measurements. The coating conditions were set to obtain about 1-3 μm thickness of the selective layer.

[0103] Reference II composite membrane was developed using a similar procedure as described in the Reference I composite membrane. The spin coater's speed was set to a speed of about 3,000 rpm, and 1 mL of chloroform sealing layer solvent was then dripped dropwise onto the UF support for about 10 seconds of coating time. The membrane was then placed in an oven to dry at about 80° C. overnight. Then 8 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same procedure was then followed to coat the co-polyimide selective layer on top of chloroform-solvent-sealed PAN support. The co-polyimide coated composite membrane was dried in oven at 60° C. overnight. The membrane coating thickness was determined from gas flux measurements. The coating conditions were set to obtain about 1-3 μm thickness of the selective layer.

[0104] Multi-Layer Composite Membrane I.

[0105] After the two Reference membranes, the composite membranes were developed using a spin coater (KW-4A, Chemat Technology). For multi-layer composite membrane I, a PTMSP gutter layer was first formed on a clean PAN 350 UF membrane using a spin coater. Coating procedures were performed by first cleaning a PAN 350 UF membrane and then placing it on a vacuum chuck (rotating disk) by taping the membrane on the disk to form a flat surface. The spin coater's speed was set to a speed of about 3,000 rpm, and 1 mL of PTMSP gutter layer solution (1.5 wt. % of PTMSP dissolved in cyclohexane) was then dripped dropwise onto the UF support for about 10 seconds of coating time.

[0106] The membrane support with gutter layer was then placed in an oven to dry at about at 80° C. overnight. Then 4 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same procedure was then followed to coat the co-polyimide selective layer on top of PTMSP guttered PAN support. The co-polyimide coated composite membrane was dried in oven at 60° C. overnight. The membrane coating thickness was determined from gas flux measurements. The coating conditions were set to obtain about 1-3 μm thickness of the selective layer.

[0107] Multi-Layer Composite Membrane II:

[0108] Two additional composite membranes were developed using a similar procedure as described for the multi-layer composite membrane I, but without the gutter material. However in these cases, the cleaned PAN 350 UF membrane was first pre-wet using the spin coater with the same solvent (chloroform) used to dissolve the co-polyimide. The pre-wetting procedures were performed by first cleaning PAN 350 UF membrane, and then it was placed on a vacuum chuck (rotating disk) by taping the membrane on a disk to form a flat surface. The spin coater's speed was set to a speed of about 3,000 rpm, and 1 mL of chloroform sealing layer solvent was then dripped dropwise onto the UF support for about 10 seconds of coating time.

[0109] The membrane was then placed in an oven to dry at about at 80° C. overnight. Then 8 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same procedure was then followed to coat the co-polyimide selective layer on top of chloroform-solvent-sealed PAN support. The co-polyimide coated thin layer composite membrane was dried in oven at 60° C. overnight. Then another 5 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same spin coating procedure was then followed to coat co-polyimide selective/caulking layer directly on top of the 8 wt. % of the co-polyimide membrane for intimate contact. The co-polyimide coated multi-layer composite membrane was then dried in oven at 60° C. overnight. The membrane coating thickness was determined from gas flux measurements. The coating conditions were set to obtain about 1-3 μm thickness of the selective layer overall.

[0110] The thickness refers to both co-polyimide layers together. An intermediate sealing pre-wetting layer (for example, volatile solvent chloroform) contributes near-zero thickness, as it is expected to dry out or evaporate once a membrane is dried. An optional gutter layer thickness ranges from about 0.2 to about 1.0 μm. Substrate thickness can range from about 150 to about 180 μm. First co-polyimide layer thickness ranges from about 0.2 to 1.0 μm, and second co-polyimide layer thickness ranges from about 0.2 to 1.0 μm.

[0111] Multi-Layer Composite Membrane III:

[0112] A third composite membrane was developed using a similar procedure as described for multi-layer composite membranes I and II. A cleaned PAN 350 UF membrane was first pre-wet using the spin coater with the same solvent (chloroform) later used to dissolve the co-polyimide. The pre-wetting procedures were performed by first cleaning the PAN 350 UF membrane and then placing it on a vacuum chuck (rotating disk) by taping the membrane on a disk to form a flat surface. The spin coater's speed was set to a speed of about 3,000 rpm, and 1 mL of chloroform sealing layer solvent was then dripped dropwise onto the UF support for about 10 seconds of coating time.

[0113] The membrane was then placed in an oven to dry at about at 80° C. overnight. Then 3 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same spinning procedure was then followed to coat the co-polyimide selective layer on top of chloroform-solvent-sealed PAN support. The co-polyimide coated thin layer composite membrane was dried in an oven at 60° C. overnight. Then another 5 wt. % of (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide was dissolved in chloroform and filtered before use. The same procedure was then followed to coat the co-polyimide selective/caulking layer on top of the 3 wt. % of the initial co-polyimide membrane layer. The co-polyimide coated multi-layer composite membrane was then dried in oven at 60° C. overnight. The membrane coating thickness was determined from gas flux measurements. The coating conditions were set to obtain about 1-3 μm thickness of the selective layer.

[0114] The thickness refers to both co-polyimide layers together. An intermediate sealing pre-wetting layer (for example, volatile solvent chloroform) contributes near-zero thickness, as it is expected to dry out or evaporate once a membrane is dried. An optional gutter layer thickness ranges from about 0.2 to about 1.0 am. Substrate thickness can range from about 150 to about 180 am. First co-polyimide layer thickness ranges from about 0.2 to 1.0 am, and second co-polyimide layer thickness ranges from about 0.2 to 1.0 am.

TABLE-US-00001 TABLE 1 Pure gas permeances and ideal selectivity coefficients in produced membranes measured at 100 psig feed pressure and at 22° C., along with membrane production qualities. Membrane Pure gas permeance (GPU) Ideal selectivity Samples He N.sub.2 CH.sub.4 CO.sub.2 He/CH.sub.4 CO.sub.2/CH.sub.4 N.sub.2/CH.sub.4 Reference I: 174 35.2 45.4 197 3.83 4.33 0.78 Reference II: 71.2 14.4 18.6 80.5 3.83 4.33 0.77 Multi-layer 28.8 1.19 0.93 31.9 31.0 34.4 1.28 composite membrane I: Multi-layer 58.4 2.70 2.10 72.0 27.8 34.3 1.29 composite membrane II: Multi-layer 68.7 3.17 2.30 99.1 29.9 43.0 1.38 composite membrane III: Reference I Pre-wetting with solvent  4.0 wt. % polymer solution coating Reference II Pre-wetting with solvent  8.0 wt. % polymer solution coating Multi-layer PTMSP gutter layer coating  4.0 wt. % polymer composite coating membrane I Multi-layer Pre-wetting with solvent  8.0 wt. % polymer composite solution coating 5.0  wt. % polymer solution membrane II coating Multi-layer Pre-wetting with solvent 3.0  wt. % polymer composite solution coating 5.0 wt. % polymer solution membrane III coating

[0115] The morphologies of the multi-layer composite membranes were characterized by scanning electron microscopy (FEI QANTA 400F E-SEM). The membranes were freeze-fractured in liquid nitrogen and observed at 20 kV after gold sputtering.

[0116] The pure gas permeation properties of the composite membranes were analyzed for He, N.sub.2, CH.sub.4, and CO.sub.2 gases at a feed pressure of up to 100 psig using a constant pressure system. The sour gas mixture transport properties were also analyzed at 22° C. and at feed pressures of up to 700 psig using a sour gas mixture comprising 10, 55, 10, 3.0 and 22 vol. % of CO.sub.2, CH.sub.4, N.sub.2, C.sub.2H.sub.6, and H.sub.2S, respectively. For each experimental condition, three membranes were tested to ensure absence of any defect and to make sure measurements were reproducible. Any uncertainty on the measurements was generally less than +5% of the value shown.

[0117] As shown in Equations 1 and 3 supra, effective membrane thickness in composite or asymmetric membranes is important, however this property cannot be measured easily. Thus permeance, Qi as shown in Equation 4, is used to assess the permeation properties in TFC membranes rather than using permeability.

[0118] As depicted in FIGS. 3A-3C, a thin layer of co-polyimide selective layer was successfully coated on mesoporous PAN substrate. Approximately 2.0-2.5 μm of dense co-polyimide selective layer was observed. Even though this may look porous based on the cross-sectional image (FIG. 3A), the pores exist only up to half of the thickness of co-polyimide layer. Without being bound by any theory or practice, it is believed that the pores were formed, in part, as a result of the rapid solvent (chloroform) evaporation, which was used to dissolve the co-polyimide. Due to the dramatic increase of polymer concentration by fast evaporation of the solvent, the pores cannot be re-sealed. A first co-polyimide layer thickness ranges from about 0.2 to 1.0 μm and a second co-polyimide layer thickness ranges from about 0.2 to 1.0 μm. Pore sizes of substrate ranges between 2 nm and 50 nm for a mesoporous substrate and less than 2 nm for a microporous substrate. Optional gutter layer thickness ranges from about 0.2 to 1.0 μm.

Example 2: Evaluation of the CO.SUB.2./CH.SUB.4.; He/CH.SUB.4.; and N.SUB.2./CH.SUB.4 .Pure Gas Separation Performance of the Multi-Layer Composite (6FDA-CARDO) (6FDA-Durene) (5000/5000) Multi-Layer Composite Co-Polyimide Membranes Prepared in Example 1

[0119] The single gas transport properties of all the three multi-layer composite membranes (I, II and III) are shown in Table 1. Multi-layer composite membrane I exhibits CO.sub.2/CH.sub.4 and He/CH.sub.4 ideal selectivities of about 34.4 and 31.0, respectively, while composite membrane II shows CO.sub.2/CH.sub.4 and He/CH.sub.4 ideal selectivities of about 34.3 and 27.8, respectively. CO.sub.2/CH.sub.4 and He/CH.sub.4 ideal selectivities of about 43.0 and 29.9, respectively, are exhibited by composite membrane III. Thus, composite membrane III exhibits the highest permeances and ideal selectivities for all gases, when compared to the values obtained in composite membranes I and II. This can be attributed, in part, to the lower polymer concentration (3 wt. %) used in the first permselective layer of membrane III, which results in lower membrane thickness and thus higher gas permeance. In addition to this improved performance, a much smaller amount of polymer is required to fabricate it (membrane III), thus reducing cost.

[0120] Reference membranes I and II exhibit lesser selectivities for all gases with respect to methane. This is due to the membranes containing only 3 total layers (PAN, solvent, co-polyimide), which then results in membrane defects or pin holes, thereby rendering the membranes defective. This indicates that the second permselective layer is required to obtain defect-free composite membranes, as can be confirmed from the composite membranes (I, II & III), where much higher selectivities were obtained for all gases with respect to methane. Without being bound by any theory or practice, it is believed the 4.sup.th layer or second perm-selective layer acts in part as a gutter layer to block the defects or pinholes that may have been created during fabrication of the membrane.

[0121] Different results obtained in the dense versus composite membranes could be attributed, in part, to the transport behavior of TFC membranes in which a thin selective layer usually behaves differently from those of self-supporting dense membranes. This may be attributed to the different polymer chain arrangements in TFC and dense films, which then results in different permeation properties and separation performances.

Example 3: Evaluation of the CO.SUB.2./CH.SUB.4 .and H.SUB.2.S/CH.SUB.4 .Sour Mixed Gas Separation Performance of the Multi-Layer Composite Co-Polyimide (6FDA-CARDO) (6FDA-Durene) (5000/5000) Co-Polyimide Membranes Using Multi-Layer Composite Membrane H

[0122] In order to assess the real performance of composite membrane II, especially under an aggressive H.sub.2S environment (for example, H.sub.2S concentration of up to 22 vol. %), sour gas mixture tests were conducted. Thus, the permeation properties of simulated sour gas mixtures comprising 10; 55; 10; 3; and 22 vol. % of CO.sub.2, CH.sub.4, N.sub.2, C.sub.2H.sub.6, and H.sub.2S respectively, through the membrane were studied at different gas feed pressure of up to 700 psig, as shown in Table 2. The CO.sub.2/CH.sub.4 and H.sub.2S/CH.sub.4 selectivities obtained for the composite membrane are up to 10 and 20 respectively; while CO.sub.2 and H.sub.2S permeances are 78 and 149 GPU respectively measured at the feed pressure of 500 psig.

[0123] The effect of feed pressure was also studied. At 22 vol. % H.sub.2S, CO.sub.2 permeance slightly increases (from 48 GPU to 78 GPU), whereas H.sub.2S permeance increases within the range of 58.6 to 149 GPU, as the pressure rises up to 700 psig (Table 2). H.sub.2S rises faster due to higher H.sub.2S condensability, where it competes with CO.sub.2 for sorption sites. Since H.sub.2S has a better affinity for the sites, the sorption of CO.sub.2 is presumably greatly reduced, leading to lower CO.sub.2 permeance increase than H.sub.2S.

[0124] As shown in Table 2, the H.sub.2S/CH.sub.4 selectivity remains substantially constant within the pressure range of up to 700 psig. However CO.sub.2/CH.sub.4 selectivity declines with increasing pressure of up to 700 psig. These values and separation performances exhibited by the composite are advantageous as compared to the values obtained in certain other high performance polymeric membranes. Importantly, at high feed pressures (up to about 700 psig) and up to 22 vol. % H.sub.2S in a feed gas mixture, selectivities and permeances are still impressive in the composite membrane. Moreover, the CO.sub.2/CH.sub.4 selectivity of the membrane does not degrade to anywhere near the same extent as was reported for cellulose acetate (CA), even under these much more aggressive environments. These separation performances displayed by the membranes are surprisingly and unexpectedly advantageous, exceeding performance exhibited by some prior art high performance membranes. The feed conditions of sour gas applied here were aggressive as to H.sub.2S concentration.

TABLE-US-00002 TABLE 2 Sour mixed gas permeances and selectivity coefficients in the the multi-layer composite II (6FDA-CARDO)/(6FDA-durene) (5000/5000) co-polyimide membrane measured at 22° C. and using sour feed gas mixture containing 10; 55; 10; 3 and 22 vol. % of CO.sub.2, CH.sub.4, N.sub.2, C.sub.2H.sub.6, and H.sub.2S respectively. Total H.sub.2S C.sub.2H.sub.6 feed Selectivity vol. vol. pressure Permeance (GPU) CO.sub.2/ H.sub.2S/ % % (psig) N.sub.2 CH.sub.4 C.sub.2H.sub.6 CO.sub.2 H.sub.2S CH.sub.4 CH.sub.4 22.0 1.0 200 4.96 3.03 3.24 48.0 58.6 15.8 19.3 400 4.25 5.36 7.45 62.0 111 11.6 20.6 500 3.80 7.75 11.8 78.1 149 10.1 19.2 700 4.14 8.61 12.9 76.2 147 8.84 17.1

[0125] The term “about” when used with respect to a value or range refers to values including plus and minus 5% of the given value or range.

[0126] The singular forms “a,” “an,” and “the” include plural referents, unless the context clearly dictates otherwise.

[0127] In the drawings and specification, there have been disclosed embodiments of compositions, systems, and methods for production and use of multi-layer composite co-polyimide membranes for sour gas feed separations from natural gas, as well as others, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The embodiments of the present disclosure have been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the disclosure as described in the foregoing specification, and such modifications and changes are to be considered equivalents and part of this disclosure.