HIGH SELECTIVITY POLYIMIDE/PES BLEND HOLLOW FIBER MEMBRANE FOR GAS SEPARATIONS

Abstract

A low cost, high selectivity asymmetric polyimide/polyethersulfone (PES) blend hollow fiber membrane, a method of making the membrane and its use for a variety of liquid, gas, and vapor separations such as deep desulfurization of gasoline and diesel fuels, ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, CO.sub.2/CH.sub.4, CO.sub.2/N.sub.2, H.sub.2/CH.sub.4, He/CH.sub.4, O.sub.2/N.sub.2, H.sub.2S/CH.sub.4, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations. The polyimide/PES blend hollow fiber membrane is fabricated from a blend of a polyimide polymer and PES and showed surprisingly unique gas separation property with higher selectivities than either the polyimide hollow fiber membrane without PES polymer or the PES hollow fiber membrane without PES polymer for gas separations such as for H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2S/CH.sub.4, CO.sub.2/CH.sub.4 separations.

Claims

1. A hollow fiber membrane comprising a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I) ##STR00035## wherein X is ##STR00036##  or a mixture of ##STR00037##  and wherein Y is a mixture of ##STR00038##  a mixture of ##STR00039##  or a mixture of ##STR00040##  and wherein n is an integer from 20 to 2000.

2. The hollow fiber membrane of claim 1 wherein said polyimide and polyethersulfone are in a weight ratio from 5:1 to 1:5.

3. The hollow fiber membrane of claim 1 wherein X is ##STR00041## and wherein Y is a mixture of ##STR00042##

4. The hollow fiber membrane of claim 1 wherein X is ##STR00043## and wherein Y is a mixture of ##STR00044##

5. The hollow fiber membrane of claim 1 wherein said hollow fiber membrane has an asymmetric integrally skinned membrane structure comprising a thin selective skin layer on top of a porous support layer.

6. The hollow fiber membrane of claim 1 wherein said hollow fiber membrane has a H.sub.2 permeance of between 160 to 400 GPU and a H.sub.2/CH.sub.4 selectivity of from 100 to 220 at 50° C. under 6996 kPa feed pressure with 10 mol % H.sub.2 and 90 mol % CH.sub.4 in the feed.

7. The hollow fiber membrane of claim 1 wherein said hollow fiber membrane has a CO.sub.2 permeance of between 50 to 160 GPU and a CO.sub.2/CH.sub.4 selectivity of from 20 to 28 at 50° C. under 6651 kPa feed pressure with 10 mol % CO.sub.2 and 90 mol % CH.sub.4 in the feed.

8. The hollow fiber membrane of claim 1 further comprising a coating with a material selected from a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, or a UV radiation curable silicone rubber.

9. The hollow fiber membrane of claim 1 is cross-linked via UV radiation.

10. A process for separating at least one gas or vapor from a mixture of gases or vapors, the process comprising: (a) providing a polyimide/polyethersulfone blend hollow fiber membrane which is permeable to said at least one gas or vapor; (b) contacting the mixture of gases or vapors to one side of the membrane to cause said at least one gas or vapor to permeate the membrane; and (c) removing from an opposite side of the membrane a permeate gas or vapor composition comprising a portion of said at least one gas or vapor which permeated said membrane, wherein said polyimide/polyethersulfone blend hollow fiber membrane comprises a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I) ##STR00045## wherein X is ##STR00046##  or a mixture of ##STR00047##  and wherein Y is a mixture of ##STR00048##  a mixture of ##STR00049##  or a mixture of ##STR00050##  and wherein n is an integer from 20 to 2000.

11. The process of claim 10 wherein said mixture of gases is selected from CO.sub.2/CH.sub.4, CO.sub.2/N.sub.2, H.sub.2/CH.sub.4, He/CH.sub.4, O.sub.2/N.sub.2, and H.sub.2S/CH.sub.4.

12. The process of claim 10 wherein said mixture of gases is a biogas comprising CO.sub.2, H.sub.2S, and CH.sub.4.

13. The process of claim 10 wherein said mixture of gases is a natural gas comprising CO.sub.2, H.sub.2S, and CH.sub.4.

14. The process of claim 10 wherein said mixture of vapors is a gasoline or diesel fuel with sulfur compounds.

15. The process of claim 10 wherein said polyimide and polyethersulfone are in a weight ratio from 5:1 to 1:5.

16. A method of making a polyimide/polyethersulfone blend hollow fiber membrane comprising: (a) dissolving the polyimide and polyethersulfone in a mixture of solvents and non-solvents to form a hollow fiber spinning dope; (b) spinning the hollow fiber spinning dope and a bore fluid simultaneously via a phase inversion method using an annular spinneret to form a nascent hollow fiber membrane with a thin dense selective skin layer on the surface of the membrane and a porous non-selective support layer below the thin dense selective skin layer; (c) solvent exchanging the hollow fiber membrane with methanol; (d) annealing the solvent-exchanged hollow fiber membrane in a hot water bath; and (e) drying the membrane, wherein the polyimide/polyethersulfone blend hollow fiber membrane comprises a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I) ##STR00051##  wherein X is ##STR00052##  or a mixture of ##STR00053##  and wherein Y is a mixture of ##STR00054##  a mixture of ##STR00055##  or a mixture of ##STR00056##  and wherein n is an integer from 20 to 2000.

17. The method of claim 16 further comprises coating the outside surface of the polyimide/polyethersulfone blend hollow fiber membrane with a thin layer of material selected from a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, and a UV radiation curable silicone rubber.

18. The method of claim 16 wherein the solvents are NMP and 1,3-dioxolane and the non-solvents are acetone and isopropanol.

19. The method of claim 16 wherein the solvent exchange with methanol is at room temperature and the total methanol solvent exchange time is in a range of 30 minutes to 5 hours.

20. The method of claim 16 wherein the membrane drying temperature is in a range of 50° to 100° C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0018] The use of membranes for separation of both gases and liquids is a growing technological area with potentially high economic reward due to the low energy requirements and the potential for scaling up of modular membrane designs. Advances in membrane technology, with the continuing development of new membrane materials and new methods for the production of high-performance membranes will make this technology even more competitive with traditional, high-energy intensive and costly processes such as distillation. Among the applications for commercial gas separation membrane systems are nitrogen enrichment, oxygen enrichment, hydrogen recovery, removal of hydrogen sulfide and carbon dioxide from natural gas, biogas purification to remove acid gases, and dehydration of air and natural gas. Also, various hydrocarbon separations are potential applications for the appropriate membrane system. The membranes that are used in these applications must have high selectivity, durability, and productivity in order to be economically successful. Membranes for gas separations have evolved rapidly in the past 40-45 years due to their easy processability for scale-up and low energy requirements. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including nitrogen enrichment from air, carbon dioxide removal from natural gas and biogas and in enhanced oil recovery. The United States produces more than 70 million tons of organic waste each year. Organic wastes generate large amounts of methane as they decompose. Methane is a powerful greenhouse gas and will absorb 86 times more heat in 20 years than CO.sub.2. To reduce greenhouse gas emissions and the risk of pollution to waterways, organic waste can be removed and used to produce biogas, a renewable source of energy. United States currently has about 2,200 operating biogas systems, representing less than 20 percent of the total potential. The biogas feedstocks include food waste, landfill gas, livestock waste, wastewater treatment, and crop residues and normally contain about 30-40% CO.sub.2. Membrane technology together with other gas treating technologies can be used for the removal of CO.sub.2 and H.sub.2S from pre-treated biogas to produce purified renewable natural gas.

[0019] The present invention provides a new low cost, high selectivity asymmetric polyimide/PES blend hollow fiber membrane and a method of making the membrane. This invention also pertains to the application of asymmetric polyimide/PES blend hollow fiber membrane for a variety of low pressure and high pressure gas separations such as acid gas such as CO.sub.2 and H.sub.2S removal from natural gas or biogas (CO.sub.2/H.sub.2S/CH.sub.4), CO.sub.2 removal from flue gas (CO.sub.2/N.sub.2), olefin/paraffin separations (e.g. propylene/propane separation), H.sub.2 purification (H.sub.2/CH.sub.4), He recovery (He/CH.sub.4), air separation (O.sub.2/N.sub.2), iso/normal paraffins, polar molecules such as H.sub.2O, H.sub.2S, and NH.sub.3/mixtures with CH.sub.4, N.sub.2, H.sub.2, and other light gases separations.

[0020] The polyimide/PES blend hollow fiber membrane described in the present invention comprises a blend of polyethersulfone (PES) and a polyimide comprising a plurality of repeating units of formula (I)

##STR00007##

wherein X is

##STR00008##

or a mixture of

##STR00009##

and

[0021] wherein Y is a mixture of

##STR00010##

a mixture of

##STR00011##

or a mixture of

##STR00012##

and wherein n is an integer from 20 to 2000.

[0022] Some examples of the polyimide polymers comprising a plurality of repeating units of formula (I) may include, but are not limited to poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylenedianiline-2,4-toluenediamine) polyimide synthesized from 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-methylenedianiline (MDA), and 2,4-toluenediamine (2,4-TDA) monomers and the molar ratio of MDA to 2,4-TDA diamines is in a range of 1:10 to 10:1, poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylenedianiline-2,4-toluenediamine-2,6-toluenediamine) polyimide synthesized from BTDA, MDA, 2,4-TDA, and 2,6-toluenediamine (2,6-TDA) monomers and the molar ratio of MDA to 2,4-TDA to 2,6-TDA diamines is in a range of 1:5:5 to 10:1:1, poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-pyromellitic dianhydride-4,4′-methylenedianiline-2,4-toluenediamine) polyimide synthesized from BTDA, pyromellitic dianhydride (PMDA), MDA, and 2,4-TDA monomers and the molar ratio of BTDA to PMDA dianhydrides is in a range of 1:10 to 10:1 and the molar ratio of MDA to 2,4-TDA diamines is in a range of 1:10 to 10:1, poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate) polyimide synthesized from BTDA, 4,4′-methylene diphenyl diisocyanate (MDI), and toluene-2,4-diisocyanate (2,4-TDI) monomers and the molar ratio of MDI to 2,4-TDI diisocyanates is in a range of 1:10 to 10:1, poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-pyromellitic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate) polyimide synthesized from BTDA, PMDA, MDI, and 2,4-TDI and the molar ratio of BTDA to PMDA dianhydrides is in a range of 1:10 to 10:1 and the molar ratio of MDI to 2,4-TDI diisocyanates is in a range of 1:10 to 10:1, and poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate-toluene-2,6-diisocyanate) polyimide synthesized from BTDA, MDI, 2,4-TDI, and toluene-2,6-diisocyanate (2,6-TDI) monomers and the molar ratio of MDI to 2,4-TDI to 2,6-TDI diisocyanates is in a range of 1:5:5 to 10:1:1. The polyimide polymer comprising a plurality of repeating units of formula (I) in the current invention and PES can form molecular level blends at any weight ratio without phase separation. PES polymer has higher intrinsic gas permeabilities than the polyimide polymer comprising a plurality of repeating units of formula (I), therefore the addition of PES polymer to the polyimide polymer comprising a plurality of repeating units of formula (I) provides a new polyimide/PES blend with higher intrinsic gas permeability than the polyimide polymer comprising a plurality of repeating units of formula (I). Furthermore, the new polyimide/PES blend comprising the polyimide polymer comprising a plurality of repeating units of formula (I) showed surprisingly unique separation property with higher selectivities than either the polyimide polymer membrane comprising a plurality of repeating units of formula (I) or the PES polymer membrane for gas separations particularly for H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2S/CH.sub.4, and CO.sub.2/CH.sub.4 separations. The weight ratio of the polyimide polymer comprising a plurality of repeating units of formula (I) to PES in the polyimide/PES blend hollow fiber membrane can be in a range of 5:1 to 1:5. The new asymmetric polyimide/PES blend hollow fiber membrane described in the current invention has an asymmetric integrally skinned membrane structure comprising a thin selective skin layer on top of a porous support layer from the same polyimide/PES blend material. The spinning dope formulation for the preparation of the polyimide/PES blend hollow fiber membrane for gas separations in the present invention comprises N-methylpyrrolidone (NMP) and 1,3-dioxolane which are good solvents for the polyimide polymer comprising a plurality of repeating units of formula (I) and PES polymer. In some cases, the spinning dope formulation for the preparation of the polyimide/PES blend hollow fiber membrane for gas separations in the present invention also comprises acetone and isopropanol which are poor solvents for the polyimide polymer comprising a plurality of repeating units of formula (I) and PES polymer. It is believed that the proper weight ratio of the solvents used in the present invention provides the polyimide/PES blend hollow fiber membrane with <50 nm super thin selective skin layer which results in high permeance and high selectivity.

[0023] The present invention provides a method for the production of the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) by spinning the polyimide/PES blend hollow fiber spinning dope described in the present invention via a dry-wet phase inversion technique to form hollow fibers. This approach comprises: (a) dissolving PES and the polyimide comprising a plurality of repeating units of formula (I) in a mixture of NMP and 1,3-dioxolane, or a mixture of NMP, 1,3-dioxolane, and non-solvents for the polymers such as acetone, and isopropanol to form a polyimide/PES blend hollow fiber spinning dope; (b) spinning the polyimide/PES blend hollow fiber spinning dope and a bore fluid simultaneously from an annular spinneret using a hollow fiber spinning machine wherein said bore fluid is pumped into the center of the annulus and wherein said polyimide/PES blend hollow fiber spinning dope is pumped into the outer layer of the annulus; (c) passing the nascent polyimide/PES blend hollow fiber membrane through an air gap between the surface of the spinneret and the surface of the nonsolvent coagulation bath to evaporate the organic solvents for a certain time to form the nascent polyimide/PES blend hollow fiber membrane with a thin dense selective skin layer on the surface; (d) immersing the nascent polyimide/PES blend hollow fiber membrane into the nonsolvent (e.g., water) coagulation bath at a controlled temperature which is in a range of 0° to 50° C. to generate the porous non-selective support layer below the thin dense selective skin layer by phase inversion, followed by winding up the polyimide/PES blend hollow fibers on a drum, roll or other suitable device; (e) sequential solvent exchanging with methanol for one to three times and hexane for another one to three times at room temperature and each solvent exchange time is in a range of 30 min to 5 h. For some cases, the solvent exchange with hexane after methanol solvent exchange can be eliminated; (f) annealing the wet polyimide/PES blend hollow fibers in a hot water bath at a certain temperature which is in a range of 70° to 100° C. for a certain time which is in a range of 10 minutes to 3 hours; and (g) drying the polyimide/PES blend hollow fiber membrane at a certain temperature which is in a range of 50° to 100° C. It is worth noting that the order for the solvent exchanging step (e) and annealing step (f) is critical to achieve high membrane performance and prevent fire caused by flammable methanol solvent during the final membrane drying step. In some other cases a membrane post-treatment step can be added after step (g) to further improve the selectivity but does not change or damage the membrane or cause the membrane to lose performance with time. The membrane post-treatment step can involve coating the selective layer surface of the polyimide/PES blend hollow fiber membrane with a thin layer of material such as a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, or a UV radiation curable silicone rubber. The polyimide/PES blend hollow fiber membrane made using this approach contains a super thin defect-free dense selective skin layer which is less than 50 nm on a porous non-selective layer and both layers are made from the same polyimide/PES blend membrane material.

[0024] The new polyimide/PES blend hollow fiber membrane with high selectivity described in the current invention has an asymmetric integrally skinned membrane structure.

[0025] In some cases, the polyimide/PES blend hollow fiber membrane undergoes an additional crosslinking step, by chemical or UV crosslinking or other crosslinking process as known to one skilled in the art. The cross-linked polyimide/PES blend hollow fiber membrane can be prepared by UV crosslinking of the polyimide/PES blend hollow fiber membrane via UV radiation. The polyimide and PES polymers used for the preparation of the polyimide/PES blend hollow fiber membrane described in the current invention have UV cross-linkable benzophenone and sulfonyl functional groups. The cross-linked polyimide/PES blend hollow fiber membrane comprises polymer chain segments where at least part of these polymer chain segments are cross-linked to each other through possible direct covalent bonds by exposure to UV radiation. The cross-linking of the polyimide/PES blend hollow fiber membrane provides the membranes with improved selectivities and slightly decreased permeances compared to the corresponding uncross-linked polyimide/PES blend hollow fiber membrane.

[0026] The new polyimide/PES blend hollow fiber membrane has high selectivities for a wide range of gas separations such as for acid gas removal from natural gas or biogas, H.sub.2 recovery, He recovery, and air separations.

[0027] The invention provides a process for separating at least one gas from a mixture of gases using the new polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) described in the present invention, the process comprising: (a) providing the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) which is permeable to said at least one gas; (b) contacting the mixture on one side of the membrane to cause said at least one gas to permeate the membrane; and (c) removing from the opposite side of the membrane a permeate gas composition comprising a portion of said at least one gas which permeated said membrane.

[0028] The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) described in the present invention are especially useful in the purification, separation or adsorption of a particular species in the gas phase. The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) described in the present invention is especially useful in gas separation processes in air purification, renewable energy, petrochemical, refinery, and natural gas industries. Examples of such separations include separation of volatile organic compounds (such as toluene, xylene, and acetone) from an atmospheric gas, such as nitrogen or oxygen and nitrogen recovery from air. Further examples of such separations are for the separation of CO.sub.2 and/or H.sub.2S from natural gas or biogas, H.sub.2 from N.sub.2, CH.sub.4, and Ar in ammonia purge gas streams, H.sub.2 recovery in refineries, He recovery from natural gas, olefin/paraffin separations such as propylene/propane separation, and iso/normal paraffin separations. Any given pair or group of gases that differ in molecular size, for example nitrogen and oxygen, carbon dioxide and methane, hydrogen and methane or carbon monoxide, helium and methane, can be separated using the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I). More than two gases can be removed from a third gas. For example, some of the gas components which can be selectively removed from a raw natural gas using the membrane described herein include carbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide, helium, and other trace gases. Some of the gas components that can be selectively retained include hydrocarbon gases. When permeable components are acid components selected from the group consisting of carbon dioxide, hydrogen sulfide, and mixtures thereof and are removed from a hydrocarbon mixture such as natural gas, one module, or at least two in parallel service, or a series of modules may be utilized to remove the acid components. For example, when one module is utilized, the pressure of the feed gas may vary from 275 kPa to about 2.6 MPa (25 to 4000 psi). The differential pressure across the membrane can be as low as about 70 kPa or as high as 14.5 MPa (about 10 psi or as high as about 2100 psi) depending on many factors such as the particular membrane used, the flow rate of the inlet stream and the availability of a compressor to compress the permeate stream if such compression is desired. Differential pressure greater than about 14.5 MPa (2100 psi) may rupture the membrane. The operating temperature of the process may vary depending upon the temperature of the feed stream and upon ambient temperature conditions. Preferably, the effective operating temperature of the membranes of the present invention will range from about −50° to about 150° C. More preferably, the effective operating temperature of the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) will range from about −20° to about 100° C., and most preferably, the effective operating temperature of the membranes of the present invention will range from about 25° to about 100° C.

[0029] The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) described in the present invention is also especially useful in gas/vapor separation processes in chemical, petrochemical, pharmaceutical and allied industries for removing organic vapors from gas streams, e.g. in off-gas treatment for recovery of volatile organic compounds to meet clean air regulations, or within process streams in production plants so that valuable compounds (e.g., vinylchloride monomer, propylene) may be recovered. Further examples of gas/vapor separation processes in which the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) may be used are hydrocarbon vapor separation from hydrogen in oil and gas refineries, for hydrocarbon dew pointing of natural gas (i.e. to decrease the hydrocarbon dew point to below the lowest possible export pipeline temperature so that liquid hydrocarbons do not separate in the pipeline), for control of methane number in fuel gas for gas engines and gas turbines, and for gasoline recovery. The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) may incorporate a species that adsorbs strongly to certain gases (e.g. cobalt porphyrins or phthalocyanines for O.sub.2 or silver (I) for ethane) to facilitate their transport across the membrane.

[0030] The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) described in the present invention may also be used in the separation of liquid mixtures by pervaporation, such as in the removal of organic compounds (e. g., alcohols, phenols, chlorinated hydrocarbons, pyridines, ketones) from water such as aqueous effluents or process fluids. A membrane which is ethanol-selective would be used to increase the ethanol concentration in relatively dilute ethanol solutions (5-10% ethanol) obtained by fermentation processes. Another liquid phase separation example using the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) is the deep desulfurization of gasoline and diesel fuels by a pervaporation membrane process similar to the process described in U.S. Pat. No. 7,048,846, incorporated by reference herein in its entirety. The polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) that is selective to sulfur-containing molecules would be used to selectively remove sulfur-containing molecules from fluid catalytic cracking (FCC) and other naphtha hydrocarbon streams. Further liquid phase examples include the separation of one organic component from another organic component, e.g. to separate isomers of organic compounds. Mixtures of organic compounds which may be separated using the polyimide/PES blend hollow fiber membrane comprising a blend of PES and a polyimide comprising a plurality of repeating units of formula (I) include: ethylacetate-ethanol, diethylether-ethanol, acetic acid-ethanol, benzene-ethanol, chloroform-ethanol, chloroform-methanol, acetone-isopropylether, allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol, isopropylether-isopropanol, methanol-ethanol-isopropanol, and ethylacetate-ethanol-acetic acid.

EXAMPLES

[0031] The following examples are provided to illustrate one or more preferred embodiments of the invention but are not limited embodiments thereof. Numerous variations can be made to the following examples that lie within the scope of the invention.

Example 1

Preparation of poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate) polyimide (Abbreviated as BTDA-MDI-2,4-TDI)/polyethersulfone (PES) (1:1) Blend Hollow Fiber Membrane (Abbreviated as BTDA-MDI-2,4-TDI)/PES (1:1)) using BTDA-MDI-2,4-TDI and PES Polymer with a 1:1 Weight Ratio

[0032] A hollow fiber spinning dope containing 16.0 wt-% of poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate) polyimide synthesized from 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-methylene diphenyl diisocyanate (MDI), and toluene-2,4-diisocyanate (2,4-TDI) monomers and with 1:4 molar ratio of MDI to 2,4-TDI (abbreviated as BTDA-MDI-2,4-TDI), 16.0 wt-% of PES, 60.0 wt-% NMP, 5.8 wt-% 1,3-dioxolane, 1.1 wt-% acetone, and 1.1 wt-% isopropanol was prepared. The spinning dope was extruded at a flow rate of 4.0 mL/min through a spinneret at 35° C. spinning temperature. A bore fluid containing 20% by weight of water in NMP was injected to the bore of the fiber at a flow rate of 0.7 mL/min simultaneously with the extruding of the spinning dope. The nascent fiber traveled through an air gap length of 5 cm at room temperature with a humidity of <40%, and then was immersed into a water coagulant bath at 21° C. and wound up at a rate of 37.7 m/min. The water-wet fibers were cut into certain length and assembled together as a bundle. The water-wet hollow fiber bundle was solvent exchanged with methanol for three times and for 30 minutes each time and then was annealed in a hot water bath at 85° C. for 30 minutes. The annealed water-wet hollow fiber bundle was dried at 85° C. in an oven for 1.5 hours. The outside surface of the dried hollow fibers in the hollow fiber bundle was further coated with a thin layer of AF2400 fluoropolymer to form BTDA-MDI-2,4-TDI/PES (1:1) blend hollow fiber membrane.

Example 2

Preparation of BTDA-MDI-2,4-TDI/ PES (1:2) Blend Hollow Fiber Membrane (Abbreviated as BTDA-MDI-2,4-TDI)/PES (1:2)) Using BTDA-MDI-2,4-TDI and PES Polymer with a 1:2 Weight Ratio

[0033] A hollow fiber spinning dope containing 10.7 wt-% of poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-4,4′-methylene diphenyl diisocyanate-toluene-2,4-diisocyanate) polyimide synthesized from 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-methylene diphenyl diisocyanate (MDI), and toluene-2,4-diisocyanate (2,4-TDI) monomers and with 1:4 molar ratio of MDI to 2,4-TDI (abbreviated as BTDA-MDI-2,4-TDI), 21.3 wt-% of PES, 60.0 wt-% NMP, 5.8 wt-% 1,3-dioxolane, 1.1 wt-% acetone, and 1.1 wt-% isopropanol was prepared. The spinning dope was extruded at a flow rate of 4.0 mL/min through a spinneret at 35° C. spinning temperature. A bore fluid containing 20% by weight of water in NMP was injected to the bore of the fiber at a flow rate of 0.7 mL/min simultaneously with the extruding of the spinning dope. The nascent fiber traveled through an air gap length of 5 cm at room temperature with a humidity of <40%, and then was immersed into a water coagulant bath at 21° C. and wound up at a rate of 37.7 m/min. The water-wet fibers were cut into certain length and assembled together as a bundle. The water-wet hollow fiber bundle was solvent exchanged with methanol for three times and for 30 minutes each time and then was annealed in a hot water bath at 85° C. for 30 minutes. The annealed water-wet hollow fiber bundle was dried at 85° C. in an oven for 1.5 hours. The outside surface of the dried hollow fibers in the hollow fiber bundle was further coated with a thin layer of AF2400 fluoropolymer to form BTDA-MDI-2,4-TDI/PES (1:2) blend hollow fiber membrane.

Comparative Example 1

Preparation of BTDA-MDI-2,4-TDI Polyimide Hollow Fiber Membrane Using BTDA-MDI-2,4-TDI Polyimide

[0034] The BTDA-MDI-2,4-TDI polyimide hollow fiber membrane was prepared using a hollow fiber spinning dope containing 32 wt-% of BTDA-MDI-2,4-TDI polyimide, 60.0 wt-%

[0035] NMP, 5.8 wt-% 1,3-dioxolane, 1.1 wt-% acetone, and 1.1 wt-% isopropanol and the same spinning and coating conditions as described in Example 1.

Comparative Example 2

Preparation of PES Hollow Fiber Membrane

[0036] The PES hollow fiber membrane was prepared using a hollow fiber spinning dope containing 32 wt-% of PES, 60.0 wt-% NMP, 5.8 wt-% 1,3-dioxolane, 1.1 wt-% acetone, and 1.1 wt-% isopropanol and the same spinning and coating conditions as described in Example 1.

Example 3

Evaluation of BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4-TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES Hollow Fiber Membranes for H.SUB.2./CH.SUB.4 .Separation

[0037] The BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4-TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES hollow fiber membranes prepared in Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively, were tested for H.sub.2/CH.sub.4 separation at 50° C. under 6996 kPa (1000 psig) feed gas pressure with 10 mol % of H.sub.2 and 90 mol % of CH.sub.4 in the feed and the feed was introduced to the hollow fiber membrane modules from the shell side. The results are shown in Table 1. It can be seen from Table 1 that the PES hollow fiber membrane is defective with poor H.sub.2/CH.sub.4 selectivity. Both BTDA-MDI-2,4-TDI/PES (1:1) and BTDA-MDI-2,4-TDI/PES (1:2) hollow fiber membranes prepared from a blend of BTDA-MDI-2,4-TDI polyimide and PES showed higher H.sub.2 permeances and higher H.sub.2/CH.sub.4 selectivities than the BTDA-MDI-2,4-TDI polyimide hollow fiber membrane without PES blending.

TABLE-US-00001 TABLE 1 BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4- TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES hollow fiber membranes for H.sub.2/CH.sub.4 separation H.sub.2 permeance Membrane (GPU) H.sub.2/CH.sub.4 selectivity PES 221 3.3 BTDA-MDI-2,4-TDI 146 108 BTDA-MDI-2,4-TDI/PES (1:1) 213 224 BTDA-MDI-2,4-TDI/PES (1:2) 401 115 1 GPU = 10.sup.−6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing conditions: 50° C., 6996 kPa (1000 psig) feed gas pressure, 10 mol % H.sub.2 and 90 mol % of CH.sub.4 in the feed.

Example 4

Evaluation of BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4-TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES Hollow Fiber Membranes for CO.SUB.2./CH.SUB.4 .Separation

[0038] The BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4-TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES hollow fiber membranes prepared in Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively, were tested for CO.sub.2/CH.sub.4 separation at 50° C. under 6651 kPa (950 psig) feed gas pressure with 10 mol % of CO.sub.2 and 90 mol % of CH.sub.4 in the feed and the feed was introduced to the hollow fiber membrane modules from the shell side. The results are shown in Table 2. It can be seen from Table 2 that the PES hollow fiber membrane is defective with poor CO.sub.2/CH.sub.4 selectivity. Both BTDA-MDI-2,4-TDI/PES (1:1) and BTDA-MDI-2,4-TDI/PES (1:2) hollow fiber membranes prepared from a blend of BTDA-MDI-2,4-TDI polyimide and PES showed higher CO.sub.2 permeances and higher CO.sub.2/CH.sub.4 selectivities than the BTDA-MDI-2,4-TDI polyimide hollow fiber membrane without PES blending.

TABLE-US-00002 TABLE 2 BTDA-MDI-2,4-TDI/PES (1:1), BTDA-MDI-2,4-TDI/PES (1:2), BTDA-MDI-2,4-TDI and PES hollow fiber membranes for CO.sub.2/CH.sub.4 separation CO.sub.2 permeance Membrane (GPU) CO.sub.2/CH.sub.4 selectivity PES 90 1.3 BTDA-MDI-2,4-TDI 57 24.7 BTDA-MDI-2,4-TDI/PES (1:1) 93 28.4 BTDA-MDI-2,4-TDI/PES (1:2) 145 25.3 1 GPU = 10.sup.−6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing conditions: 50° C., 6651 kPa (950 psig) feed gas pressure, 10 mol % CO.sub.2 and 90 mol % of CH.sub.4 in the feed.

Specific Embodiments

[0039] While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

[0040] Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0041] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated, and wherein n is an integer from 20 to 2000.

[0042] A first embodiment of the invention is a hollow fiber membrane comprising a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I)

##STR00013##

wherein X is

##STR00014##

or a mixture of

##STR00015##

and wherein Y is a mixture of

##STR00016##

a mixture of

##STR00017##

or a mixture of

##STR00018##

and wherein n is an integer from 20 to 2000. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the polyimide and polyethersulfone are in a weight ratio from 5:1 to 1:5. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein X is

##STR00019##

and wherein Y is a mixture of

##STR00020##

An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein X is

##STR00021##

and wherein Y is a mixture of

##STR00022##

An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein there is an asymmetric integrally skinned membrane structure comprising a thin selective skin layer on top of a porous support layer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein said membrane has a H.sub.2 permeance of between 160 to 400 GPU and a H.sub.2/CH.sub.4 selectivity of from 100 to 220 at 50° C. under 6996 kPa feed pressure with 10 mol % H.sub.2 and 90 mol % CH.sub.4 in the feed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the membrane has a CO.sub.2 permeance of between 50 to 160 GPU and a CO.sub.2/CH.sub.4 selectivity of from 20 to 28 at 50° C. under 6651 kPa feed pressure with 10 mol % CO.sub.2 and 90 mol % CH.sub.4 in the feed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the membrane is further comprising a coating with a material selected from a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, or a UV radiation curable silicone rubber. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the membrane is further cross-linked via UV radiation.

[0043] A second embodiment of the invention is a process for separating at least one gas or vapor from a mixture of gases or vapors, the process comprising (a) providing a polyimide/polyethersulfone blend hollow fiber membrane which is permeable to the at least one gas or vapor; (b) contacting the mixture of gases or vapors to one side of the membrane to cause the at least one gas or vapor to permeate the membrane; and (c) removing from an opposite side of the membrane a permeate gas or vapor composition comprising a portion of the at least one gas or vapor which permeated the membrane, wherein the polyimide/polyethersulfone blend hollow fiber membrane comprises a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I)

##STR00023##

wherein X is

##STR00024##

or a mixture of

##STR00025##

and wherein Y is a mixture of

##STR00026##

a mixture of

##STR00027##

or a mixture of

##STR00028##

and wherein n is an integer from 20 to 2000.

[0044] An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the mixture of gases or vapors is selected from CO.sub.2/CH.sub.4, CO.sub.2/N.sub.2, H.sub.2/CH.sub.4, He/CH.sub.4, O.sub.2/N.sub.2, and H.sub.2S/CH.sub.4. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the mixture of gases is a biogas comprising CO.sub.2, H.sub.2S, and CH.sub.4. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the mixture of gases is a natural gas comprising CO.sub.2, H.sub.2S, and CH.sub.4. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the mixture of vapors is a gasoline or diesel fuel with sulfur compounds. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the polyimide and polyethersulfone are in a weight ratio from 5:1 to 1:5.

[0045] A third embodiment of the invention is a method of making a polyimide/polyethersulfone blend hollow fiber membrane comprising (a) dissolving the polyimide and polyethersulfone in a mixture of solvents and non-solvents to form a hollow fiber spinning dope; (b) spinning the hollow fiber spinning dope and a bore fluid simultaneously via a phase inversion method using an annular spinneret to form a nascent hollow fiber membrane with a thin dense selective skin layer on the surface of the membrane and a porous non-selective support layer below the thin dense selective skin layer; (c) solvent exchanging the hollow fiber membrane with methanol; (d) annealing the solvent-exchanged hollow fiber membrane in a hot water bath; and (e) drying the membrane, wherein the polyimide/polyethersulfone blend hollow fiber membrane comprises a blend of polyethersulfone and a polyimide comprising a plurality of repeating units of formula (I)

##STR00029##

wherein X is

##STR00030##

or a mixture of

##STR00031##

and wherein Y is a mixture of

##STR00032##

a mixture of

##STR00033##

or a mixture of

##STR00034##

and wherein n is an integer from 20 to 2000. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph further comprises coating the outside surface of the polyimide/polyethersulfone blend hollow fiber membrane with a thin layer of material selected from a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, and a UV radiation curable silicone rubber. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph wherein the solvents are NMP and 1,3-dioxolane and the non-solvents are acetone and isopropanol. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph wherein the solvent exchange with methanol is at room temperature and the total methanol solvent exchange time is in a range of 30 minutes to 5 hours. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph wherein the membrane drying temperature is in a range of 50° to 100° C.