Method for producing hollow fiber carbon membrane

Abstract

A method for producing a hollow fiber carbon membrane includes heating a polymer precursor to a pyrolysis temperature of 900° C. or more and 1200° C. or less, and pyrolyzing the polymer precursor at the pyrolysis temperature in a pyrolysis atmosphere containing oxygen in an amount greater than 0 ppm and less than 200 ppm.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application is a PCT application claiming priority to U.S. Provisional Patent Application No. 63 / 224,085, filed July 21, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE DISCLOSURE

[0002] Embodiments of the present disclosure relate generally to hollow fiber carbon molecular sieve (CMS) membranes for use in gas separation, and more particularly to methods for making highly selective hollow fiber CMS membranes. [Background technology]

[0003] Membranes are widely used for gas and liquid separation, including, for example, separation of acid gases such as CO2 and H2S from natural gas, and removal of O2 from air. Gas transport through such membranes is generally modeled by a sorption-diffusion mechanism. Currently, polymeric membranes are well studied and widely available for gas separation because they are easy and inexpensive to fabricate. However, CMS membranes have been shown to have attractive separation performance properties that exceed those of polymeric membranes.

[0004] CMS membranes are typically produced through pyrolysis of polymer precursors. For example, it is known that defect-free hollow fiber CMS membranes can be produced by pyrolyzing cellulose hollow fibers. In addition, many other polymers have been used to produce CMS membranes in fiber and dense film form, among which polyimides are favored. Polyimides have high glass transition temperatures, are easy to process, and perform better than most other polymer membranes, even before pyrolysis. Summary of the Invention

[0005] One class of separation applications in which CMS membranes may find utility is olefin-paraffin separation, which requires the separation of olefins from paraffins as well as from lighter gases such as H2, CO2, and CH4. New CMS membranes and methods for making these CMS membranes are needed.

[0006] According to an embodiment, a method for producing a hollow fiber carbon membrane includes heating a polymer precursor to a pyrolysis temperature of 900° C. or more and 1200° C. or less, and pyrolyzing the polymer precursor at the pyrolysis temperature in a pyrolysis atmosphere containing oxygen in an amount greater than 0 ppm and less than 200 ppm.

[0007] It should be understood that both the foregoing general description and the following detailed description present embodiments of the technology and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed technology. The accompanying drawings are included to provide a further understanding of the technology, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments and, together with the description, serve to explain the principles and operation of the technology. Moreover, the drawings and description are intended to be merely illustrative and are not intended to limit the scope of the claims in any manner.

[0008] Additional features and advantages of the described embodiments are set forth in the detailed description which follows, and in part will become readily apparent to those skilled in the art from the description, or may be learned by practice of the described embodiments, including the following detailed description and drawings and claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] CMS membranes according to embodiments disclosed and described herein advantageously separate olefins from paraffins, as well as CO2 from tail gas and natural gas (CH4) and from H2 and other light gases used in gasification and syngas to olefins conversion processes. Of particular interest is olefins from paraffins separation, separating ethylene from ethane. Methods according to embodiments disclosed and described herein have economic advantages over current separation technologies such as cryogenic C2 distillation (C2 splitters), including in terms of both CAPEX and OPEX.

[0010] CMS membrane separation properties are primarily influenced by factors such as (1) pyrolysis precursor, (2) pyrolysis temperature, (3) heat soak time, and (4) pyrolysis atmosphere. For example, it has been shown that an increase in both temperature and heat soak time increases the selectivity for CO2 / CH4 separation but decreases the permeability. In addition, precursor polymers with a rigid and densely packed structure tend to result in CMS membranes with higher selectivity compared to less rigid precursor polymers. The effect of pyrolysis atmosphere gas has not been studied in detail, nor has the stability of the membrane with respect to long-term use of CMS membranes and maintenance of permeability and selectivity for specific gas molecules of interest.

[0011] According to one or more embodiments described herein, a method of making a hollow fiber carbon membrane according to embodiments includes heating a polymer precursor to a pyrolysis temperature of 900° C. or more and 1200° C. or less, and pyrolyzing the polymer precursor at the pyrolysis temperature in a pyrolysis atmosphere containing oxygen in an amount greater than 0 ppm and less than 200 ppm. Exemplary polymer precursors are now described.

[0012] The polymer precursor may be any useful polymer for making hollow fiber CMS membranes, such as, for example, polyimides. In one or more embodiments, the polymer precursor is 2,4,6-trimethylene-1,3-phenylenediamine (DAM), oxydianaline (ODA), dimethyl-3,7-diaminodiphenyl-thiophene-5,5'-dioxide (DDBT), 3,5-diaminobenzoic acid (DABA), 2.3,5,6-tetramethylene-1,4-phenylenediamine (Durene), meta-phenylenediamine (m-PDA), 2,4-diaminotoluene (2,4-DAT), tetramethylmethylenedianaline (TMMDA), 4,4'-diamino-2,2'-biphenyldiamine (BMP ... The polymer comprises a polymer formed from one or more monomers selected from the group consisting of phenyldisulfonic acid (BDSA), 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandione (6FDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-oxydiphthalic anhydride (ODPA), and benzophenonetetracarboxylic dianhydride (BTDA).

[0013] The polymer precursor may be any useful polymer for making hollow fiber CMS membranes, such as, for example, a polyimide. If a polyimide is used, the polyimide may be a conventional or fluorinated polyimide. In an embodiment, the polymer precursor is a monomer A X , B Y and C Z where X, Y, and Z are the respective mole fractions of A, B, and C present in the polymer. In embodiments, X+Y+Z=1. In other embodiments, X+Y+Z<1 and other monomers are present in the polymer.

[0014] A, B and C are 2,4,6-trimethyl-1,3-phenylenediamine (DAM), oxydianaline (ODA), dimethyl-3,7-diaminodiphenyl-thiophene-5,5'-dioxide (DDBT), 3,5-diaminobenzoic acid (DABA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (Duren), meta-phenylenediamine (m-PDA), 2,4-diaminotoluene (2,4-DAT), tetramethylmethylenedianaline (TMMDA), 4,4'-diamino-2,2'-biphenyldisulfonic acid (BDSA), 5,5'-[2,2 ,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandione (6FDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-oxydiphthalic anhydride (ODPA), 5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane (DAPI), and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). In embodiments, the polyimide may contain at least two different moieties selected from DAM, ODA, DDBT, DABA, durene, m-PDA, 2,4-DAT, TMMDA, BDSA, 6FDA, BPDA, PMDA, NTDA, and BTDA.

[0015] In an embodiment, A is a monomer selected from the group consisting of 6FDA, ODPA, and BTDA. B is DAM and C is a monomer selected from the group consisting of BPDA and PMDA. In an embodiment, A is 6FDA, B is DAM, and Z is 0. In an embodiment, the polyimide may be MATRIMID™ 5218 (Huntsman Advanced Materials), a commercially available polyimide in which A is BTDA, B is DAPI, and Z is 0.

[0016] In an embodiment, the polyimide may comprise, consist essentially of, or consist of 6FDA / BPDA-DAM as shown in formula (1), which may be synthesized via a thermal or chemical process from a combination of three commercially available monomers, namely, DAM, 6FDA, and BPDA. In an embodiment of formula (1), X+Y may be 0.1 to 0.9, and Z may be 0.1 to 0.9. In an embodiment of formula (1), X+Y may be 0.1 to 1, and Z may be 0 to 0.9. In an embodiment of formula (1), X may be 0, and Y+Z may be 1. In an embodiment, X and Z may be 0.25, 0.3, or 0.4 to 0.9, 0.8, or 0.75. In an embodiment, X+Y is 0.5, and z is 0.5. The following formula (2) shows a representative structure of 6FDA / BPDA-DAM, where the ratio of X to Z can be adjusted to tailor the polymer properties. In embodiments, a 1:1 ratio of component X to component Z may be abbreviated as 6FDA / BPDA(1:1)-DAM. [ka] [ka]

[0017] In embodiments, the polyimide may be formed by the reaction of a diamine with a dianhydride. In such embodiments, at least one of A, B, and C is a diamine and at least one other of A, B, and C is a dianhydride. In embodiments, the total diamines and dianhydrides may be in a molar ratio of diamine to dianhydride of from 49:51 to 51:49. In embodiments, the diamines and dianhydrides may be in a molar ratio of diamine to dianhydride of about 50:50.

[0018] In embodiments, two or more dianhydrides may be used with one diamine. In such embodiments using two dianhydrides, dianhydride 1 and dianhydride 2, the molar ratio of dianhydride 1 to dianhydride 2 may be from 20:80 to 80:20. For example, the molar ratio may be from 25:75 to 75:25, from 30:70 to 70:30, from 35:65 to 65:35, from 40:60 to 60:40, or even from 45:55 to 55:45. In embodiments, the molar ratio of dianhydride 1 to dianhydride 2 may be about 50:50. In embodiments, one dianhydride may be used with two or more diamines. In such embodiments using two diamines, diamine 1 and diamine 2, the molar ratio of diamine 1 to diamine 2 may be from 20:80 to 80:20. For example, the molar ratio may be 25:75 to 75:25, 30:70 to 70:30, 35:65 to 65:35, 40:60 to 60:40, or even 45:55 to 55:45. In an embodiment, the molar ratio of diamine 1 to diamine 2 may be about 50:50. In an embodiment, the polymer precursor membrane produced but not pyrolyzed is substantially defect-free. By "defect-free" is meant that the selectivity of a gas pair through a hollow fiber membrane is at least 90 percent of the selectivity for the same gas pair through a dense film prepared from the same composition used to make the polymer precursor membrane. By way of illustration, 6FDA / BPDA(1:1)-DAM polymer has an O2 / N2 selectivity (also known as "dense film selectivity") of 4.1.

[0019] In embodiments, the precursor polymers can be formed into hollow fibers or films. Conventional procedures for making these can be used. For example, hollow fibers can be made using coextrusion procedures, including dry-jet wet spinning processes (where there is an air gap between the tip of the spinneret and the coagulation or quench bath) or wet spinning processes (where the air gap distance is zero).

[0020] Despite the use of the above-mentioned polymer precursors, it can still be difficult to obtain a CMS membrane capable of separating olefins from paraffins while still providing high performance. For example, the most common separation mechanism for gas separation is based on the size difference between gas pairs. For ethylene / ethane separation, the size difference between the two gas molecules is relatively small (0.1 Å) compared to other gas pairs. As a result, it is very difficult to achieve very high selectivity using membrane-based separation techniques. CMS membranes have a very narrow size cutoff and provide better selectivity for size-based selection when compared to polymer membranes. However, it is still difficult to achieve high selectivity, especially for the ethylene-ethane gas pair. In addition to achieving high selectivity, it is even more difficult to achieve high selectivity and permeability simultaneously. Permeability is important for reducing the membrane area.

[0021] One method of producing ultraselective asymmetric CMS membranes according to embodiments disclosed and described herein is by pyrolysis of polymer hollow fibers at temperatures above 900 degrees Celsius (°C). Conventional wisdom has been that increasing pyrolysis temperature correspondingly increases selectivity, but increasing pyrolysis temperature decreases permeability. For example, Koros et al. (Adv. Mater. 2017, 29, 1701631) report the synthesis of ultraselective dense CMS membranes by pyrolyzing MATRIMID-based precursors at high temperatures (900°C). This trend is consistent with Pinnau et al. (https: / / doi.org / 10.1016 / j.memsci.2019.05.020) reporting that membrane permeability decreases with increasing membrane selectivity for selected gas pairs. Thus, conventional wisdom is that permeability is inversely related to ultrahigh selectivity.

[0022] However, an unexpected anomaly in the above trend (i.e., the inverse correlation between permeability and selectivity) was observed, where for fibers pyrolyzed at a particular temperature, a gradual increase in permeability accompanied by a step change in selectivity was observed. This makes it possible to synthesize highly selective CMS asymmetric membranes without sacrificing too much permeability when forming CMS membranes under certain pyrolysis conditions.

[0023] Next, pyrolysis conditions are described. Any suitable support means for holding the hollow fiber CMS membrane during pyrolysis can be used, including sandwiching it between two metallic wire meshes or using a stainless steel mesh plate in combination with stainless steel wires, as described in U.S. Patent No. 8,709,133, column 6, line 58 to column 7, line 4, which is incorporated by reference.

[0024] The precursor polymer can be pyrolyzed under various inert gas purge or vacuum conditions (e.g., pressures of 0.1 mbar or less) to form hollow fiber CMS membranes (i.e., to carbonize the precursor polymer). U.S. Patent No. 6,565,631 describes a heating method for pyrolyzing polymer fibers to form hollow fiber CMS membranes and is incorporated herein by reference. According to an embodiment, the pyrolysis temperature of the method for producing hollow fiber carbon membranes can be 900° C. or higher and 1200° C. or lower. The pyrolysis temperature can be adjusted in combination with the pyrolysis atmosphere to tailor the performance properties of the resulting hollow fiber CMS membrane. In an embodiment, the pyrolysis temperature is 925°C to 1200°C, 950°C to 1200°C, 975°C to 1200°C, 1000°C to 1200°C, 1025°C to 1200°C, 1050°C to 1200°C, 1075°C to 1200°C, 1100°C to 1200°C, 1125°C to 1200°C, 1150°C to 1200°C, 1175°C to 1200°C, 1180°C to 1200°C, 120 ... 00℃ to 1175℃, 925℃ to 1175℃, 950℃ to 1175℃, 975℃ to 1175℃, 1000℃ to 1175℃, 1025℃ to 1175℃, 1050 ℃ to 1175℃, 1075℃ to 1175℃, 1100℃ to 1175℃, 1125℃ to 1175℃, 1150℃ to 1175℃, 900℃ to 1150℃, 925℃ and above Above 1150℃, above 950℃ below 1150℃, above 975℃ below 1150℃, above 1000℃ below 1150℃, above 1025℃ below 1150℃, above 1050℃ below 1150℃, above 1075℃ 1150℃ or less, 1100℃ or more and 1150℃ or less, 1125℃ or more and 1150℃ or less, 900℃ or more and 1125℃ or less, 925℃ or more and 1125℃ or less, 950℃ or more and 1125℃ or less, 975℃ or more and 1125 ℃ or less, 1000℃ or more and 1125℃ or less, 1025℃ or more and 1125℃ or less, 1050℃ or more and 1125℃ or less, 1075℃ or more and 1125℃ or less, 1100℃ or more and 1125℃ or less, 900℃ or more and 1100℃ or less Below, 925℃ to 1100℃, 950℃ to 1100℃, 975℃ to 1100℃, 1000℃ to 1100℃, 1025℃ to 1100℃, 1050℃ to 1100℃,1075℃ to 1100℃, 900℃ to 1075℃, 925℃ to 1075℃, 950℃ to 1075℃, 975℃ to 1075℃, 1000℃ to 1075℃, 1025℃ to 1075℃, 105 0℃ to 1075℃, 900℃ to 1050℃, 925℃ to 1050℃, 950℃ to 1050℃, 975℃ to 1050℃, 1000℃ to 1050℃, 1025℃ to 1050℃, 900℃ and above The pyrolysis temperature may be 1025°C or less, 925°C to 1025°C, 950°C to 1025°C, 975°C to 1025°C, 1000°C to 1025°C, 900°C to 1000°C, 925°C to 1000°C, 950°C to 1000°C, 975°C to 1000°C, 900°C to 975°C, 925°C to 975°C, 950°C to 975°C, 900°C to 950°C, 925°C to 950°C, or 900°C to 925°C. A pyrolysis temperature below 900°C will not provide the desired selectivity, and a temperature above 1200°C will damage the structure of the CMS membrane. It is contemplated that the acceptable pyrolysis temperature range may be any of the temperatures described herein or greater and any of the temperatures described herein or less.

[0025] The pyrolysis soak time (i.e., duration at pyrolysis temperature) may vary (and may not include the soak time), but may be, for example, 1 to 24 hours, 2 to 8 hours, or 4 to 6 hours. An exemplary heating protocol may include (1) starting at a first set point of about 50° C., (2) heating at a rate of about 13.3° C. per minute to a second set point of about 250° C., (3) heating at a rate of about 3.85° C. per minute to a third set point of about 535° C., and (4) heating at a rate of about 0.25° C. per minute to a fourth set point of about 550° C. The fourth set point may then be maintained for the determined soak time.

[0026] As mentioned above, the precursor polymer may be pyrolyzed under various inert gas purge or vacuum conditions. In embodiments, the precursor polymer may be pyrolyzed under a vacuum at low pressure (e.g., 0.1 mbar or less). In embodiments, the pyrolysis utilizes a controlled inert purge gas atmosphere with a small amount of an oxidant, such as oxygen. Thus, in one or more embodiments, the pyrolysis atmosphere includes an inert gas and oxygen. In embodiments, the inert gas is selected from the group consisting of nitrogen, helium, argon, or a combination thereof. In one or more embodiments, the inert purge gas is argon, and thus the pyrolysis atmosphere includes, in embodiments, argon and oxygen. It has been found that it is possible to further increase the permeability of films pyrolyzed at high temperatures (e.g., 900°C or higher) by carrying out the pyrolysis in a trace amount of oxygen in the inert gas mixture. In contrast, conventional wisdom believes that the presence of oxygen reduces the permeability of films pyrolyzed at temperatures of 550°C or higher (see U.S. Patent Application Publication No. 2013 / 030592).

[0027] The pyrolysis disclosed and described herein utilizes a controlled purge gas atmosphere in the presence of low levels of an oxidizing agent, such as oxygen, with the purge gas acting as a carrier gas. By using any suitable method, such as a valve, an inert gas containing a specific concentration of oxygen can be introduced into the pyrolysis atmosphere.For example, the amount of oxidizing agents such as oxygen in the purged atmosphere may be greater than 0 ppm and less than 200 ppm, greater than 10 ppm and less than 200 ppm, greater than 15 ppm and less than 200 ppm, greater than 20 ppm and less than 25 ppm and less than 200 ppm, greater than 50 ppm and less than 200 ppm, greater than 75 ppm and less than 200 ppm, greater than 100 ppm and less than 200 ppm, greater than 125 ppm and less than 200 ppm, greater than 150 ppm and less than 200 ppm, greater than 175 ppm and less, greater than 0 ppm and less than 175 ppm, greater than 10 ppm and less than 175 ppm, greater than 15 ppm and less than 175 ppm less than, 20ppm to 175ppm, 25ppm to 175ppm, 50ppm to 175ppm, 75ppm to 175ppm, 100ppm to 175ppm, 125ppm to 175ppm, 150ppm to 175ppm, greater than 0ppm to 150ppm, 10ppm to 150ppm, 15ppm to 150ppm, 20ppm to 150ppm, 25ppm to 150ppm, 50ppm to 150ppm, 75ppm to 150ppm, 100ppm to 150ppm, 12 5ppm or more but not exceeding 150ppm, over 0ppm but not exceeding 100ppm, 10ppm or more but not exceeding 100ppm, 15ppm or more but not exceeding 100ppm, 20ppm or more but not exceeding 100ppm, 25ppm or more but not exceeding 100ppm, 50ppm or more but not exceeding 100ppm, 75ppm or more but not exceeding 100ppm, over 0ppm but not exceeding 75ppm, 10ppm or more but not exceeding 75ppm, 15ppm or more but not exceeding 75ppm, 20ppm or more but not exceeding 75ppm, 25ppm or more but not exceeding 75ppm, 50ppm or more but not exceeding 75ppm, 75ppm or more but not exceeding 75ppm, It may be more than 0 ppm but not more than 50 ppm, 10 ppm or more but not more than 50 ppm, 15 ppm or more but not more than 50 ppm, 20 ppm or more but not more than 50 ppm, 25 ppm or more but not more than 50 ppm, 50 ppm or more but not more than 50 ppm, more than 0 ppm but not more than 25 ppm, 10 ppm or more but not more than 25 ppm, 15 ppm or more but not more than 25 ppm, 20 ppm or more but not more than 25 ppm, more than 0 ppm but not more than 20 ppm, 10 ppm or more but not more than 20 ppm, more than 0 ppm but not more than 15 ppm, more than 10 ppm or more but not more than 15 ppm, or more than 0 ppm but not more than 10 ppm.In the embodiments disclosed and described herein, it has been found that increasing the oxygen content to 100 ppm or even 200 ppm unexpectedly increases the permeability of the CMS membrane. However, as the oxygen content increases beyond 100 ppm, the selectivity begins to decrease, and as the oxygen content exceeds 200 ppm, the selectivity becomes undesirable. It is contemplated that the acceptable concentration range of the oxidant in the pyrolysis purge gas atmosphere may be any of the concentrations described herein or higher, including 0 ppm, and any of the temperatures described herein or lower.

[0028] In embodiments, the oxidant added to the purge gas atmosphere used in pyrolysis may be selected from the group consisting of gaseous oxygen, CO2, nitric oxide ozone, hydrogen peroxide, water vapor, and air.

[0029] After pyrolysis, the formed hollow fiber CMS membrane is cooled to a temperature near room temperature, such as 50° C. or less. Cooling may be at any useful rate, such as passive cooling (e.g., turning off the furnace and allowing it to cool naturally). Alternatively, it may be desirable to cool more rapidly, such as by using known techniques to achieve faster cooling, including, but not limited to, employing cooling fans or water-cooled jackets, or opening the furnace to the ambient environment.

[0030] In an embodiment, the hollow fiber CMS membrane may be asymmetric. As used herein, the term "asymmetric" refers to the property of a hollow fiber CMS membrane that the hollow fiber CMS membrane has at least one relatively denser layer and at least one relatively less dense layer. For example, in an embodiment, one layer of the hollow fiber CMS membrane may be 1 μm or more and 10 μm or less and may be denser than the second layer. The second layer may be thicker than the first layer, such as 20 μm or more and up to 200 μm. An asymmetric membrane may be defined as an entity composed of a very thin dense skin on a thick porous substructure, which may be the same or different material as the material of the dense skin layer. Asymmetric membranes may be fabricated by phase inversion to fabricate in one step, or thin layers may be coated on a pre-prepared porous support using a dip coating method. These layers in an asymmetric membrane may be physically created by coating or may be created by chemical modification. Asymmetric membranes may be in the form of hollow fiber or film structures. Asymmetric membranes may optionally contain a third layer of the same or different material to improve membrane performance.

[0031] CMS membranes according to embodiments are also characterized by unique hydrogen to ethylene (H2 / C2H4) selectivity properties. For example, Koros et al. report that H2 / C2H4 selectivity increases with increasing pyrolysis temperature. This is consistent with the general trend of increasing selectivity with pyrolysis temperature observed for most gas pairs. However, it was unexpectedly found that a decrease in H2 / C2H4 selectivity was observed for samples pyrolyzed at high temperatures (e.g., 900°C or higher). In addition, it has also been found that membrane permeability can also be improved by pyrolyzing membranes with reduced skin thickness at higher temperatures, as disclosed and described herein.

[0032] According to embodiments, the hollow fiber carbon membrane has a hydrogen to ethylene (H2 / C2H4) selectivity (calculated as defined below) of 50 or less, such as 45 or less, 40 or less, 35 or less, 30 or less, 25 or less, 20 or less, 15 or less, or 10 or less, when processing a stream containing equal amounts of hydrogen and ethylene. EXAMPLES

[0033] The following examples are illustrative in nature and should not be construed as serving to limit the scope of the present application.

[0034] Preparation of CMS

[0035] CMS membranes were made using 6FDA:BPDA-DAM polymer. 6FDA:BPDA-DAM was obtained from Akron Polymer Systems, Akron, Ohio. The polymer was vacuum dried at 110° C. for 24 hours and then formed into a dope. The dope was made by mixing 6FDA:BPDA-DAM polymer with the solvents and compounds in Table 1 and roll mixed in a glass bottle sealed with a polytetrafluoroethylene (TEFLON™) cap at a rotation speed of 5 revolutions per minute (rpm) for a period of about 3 weeks to form a homogenous dope.

[0036] [Table 1]

[0037] NMP is N-methyl-2-pyrrolidone and THF is tetrahydrofuran.

[0038] The homogeneous dope was loaded into a 500 milliliter (mL) syringe pump and the dope was degassed overnight by heating the pump to a set-point temperature of 50 °C to 60 °C using a heating tape.

[0039] The bore fluids (85 wt. % NMP and 15 wt. % water, based on the total bore fluid weight) were loaded into separate 100 mL syringe pumps, and the dope and bore fluids were then coextruded through a spinneret operating at a flow rate of 180 milliliters per hour (mL / hr) for the dope and 60 mL / hr for the bore fluid, with both the bore fluid and the dope filtered in the line between the delivery pump and the spinneret using 40 μm and 2 μm metal filters. Temperature was controlled at a set point temperature of 70° C. using thermocouples and heating tapes located on the spinneret, dope filter, and dope pump.

[0040] After passing through a 15 centimeter (cm) air gap, the nascent fibers formed by the spinneret were quenched in a water bath (50° C.) to phase separate the fibers. The fibers passed over a TEFLON guide and were collected using a 0.32 meter (m) diameter polyethylene drum operating at a take-up speed of 30 meters per minute (m / min).

[0041] The fibers were cut from the drum and rinsed at least four times in separate water baths over a 48 hour period. The rinsed fibers in the glass container were solvent exchanged three times with methanol for 20 minutes and then hexane for 20 minutes before the fibers were collected and either dried under vacuum at a set point temperature of 110° C. for 1 hour or dried under vacuum at 75° C. for 3 hours.

[0042] The precursor fibers were pyrolyzed in a pyrolysis chamber with oxygen content kept between 5 and 10 ppm at room temperature (using Ar as an inert purge gas), except for sample 6 (108), where the oxygen content was increased to 30 ppm by introducing a premix of 30 ppm oxygen and Ar. After pyrolysis of the membranes, single fiber modules were fabricated and tested for permeability of CO2 / N2 and C2H4 / C2H6 gas pairs. One separate measurement was then performed for H2 / C2H4 gas pair. To test the gas separation performance, the pyrolyzed and / or oxidized CMS hollow fibers were placed in a stainless steel casing. The membrane module is housed in a temperature-controlled oven (Quincy Lab, Chicago, IL). The test gas flow rate was controlled by a mass flow controller (Brooks Instrument, Hatfield, PA), and the pressure was monitored and controlled by a pressure transducer. In these experiments, the single fiber CMS fiber module was maintained under a constant upstream pressure at 35 °C. Argon was used as the sweep gas to carry the permeate to a downstream flow meter and gas chromatograph (GC). A Maxum II process GC (Siemens, Munich, Germany) was used to measure the composition of the permeate and sweep mixtures, and a Mesalabs Bios Drycal flow meter (Mesa Labs, Butler, NJ) was used to measure the permeate flow rate. The volumetric flow rate from the Bios DryCal flow meter and the composition from the GC were used to analyze the permeability and selectivity of the fibers in the test gas system.

[0043] Table 2 summarizes the performance data of 6FDA-BPDA-DAM CMS membranes pyrolyzed at different temperatures.

[0044] [Table 2]

[0045] Table 2 shows that an initial decrease in permeability for both CO2 and ethylene was observed with increasing temperature up to 800°C. This is consistent with literature teachings which suggest that membrane gas permeability decreases significantly with increasing pyrolysis temperature. However, an increase in permeability was observed as the temperature was increased beyond 800°C to 925°C. A corresponding sharp increase in gas pair selectivity was observed for both the CO2 / N2 and C2 pairs. In the investigation of the H2 / C2H4 gas pair, selectivity increased with increasing pyrolysis temperature up to 800°C. A significant decrease in selectivity was observed above 800°C.

[0046] Possible examples for 6FDA-PMDA-DAM CMS fibers are shown in Table 3. Pyrolysis was carried out at high temperatures with different oxygen levels in Ar gas. An increase in permeability was observed for both gas pairs when pyrolyzed in the presence of higher oxygen content.

[0047] [Table 3]

[0048] Formulas and test details for the parameters measured in the examples are provided below.

[0049] In evaluating the separation performance of membrane materials, two intrinsic properties are useful: "permeability", which is an index of the intrinsic productivity of hollow fiber CMS membranes, and "selectivity", which is an index of the separation efficiency of hollow fiber CMS membranes. Typically, the Barrer (1 Barrer = 10 -10 [cm 3 (STP)cm] / [cm 2 The "permeability" is determined by the partial pressure difference Δp i Flux n i It is calculated as:

number

[0050] Another term "permeability" is defined herein as the productivity of an asymmetric hollow fiber membrane, typically expressed in Gas Permeability Units (GPU) (1 GPU = 10 -6 [cm 3 (STP)] / [cm 2 The permeability is measured in millisieverts (mS / cmHg) and is calculated by dividing the permeability by the effective membrane separator thickness.

number

[0051] Finally, "selectivity" is defined herein as the ability to compare the permeability or transmission rate of one gas through a hollow fiber CMS membrane with the same property of another gas. It is measured as a unitless ratio.

number

[0052] Table 1 shows the gas separation properties of the control and oxidized air-exposed samples made in accordance with the subject matter described herein. Samples 1 and 7 were not oxidized and are comparative examples. Samples 2-6 were exposed to air at the specified initial air exposure temperature.

[0053] The results shown in Tables 2 and 3 demonstrate the effect of pyrolysis temperature and oxygen content in the pyrolysis atmosphere on permeability and selectivity. As can be seen in the examples, the formation of CMS membranes using the pyrolysis methods disclosed and described herein results in an unexpected balance of selectivity and permeability.

[0054] It will be apparent to those skilled in the art that various modifications can be made to the described embodiments without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover modifications and variations of the described embodiments provided such modifications and variations come within the scope of the appended claims and their equivalents.

Claims

1. A method for manufacturing a hollow fiber carbon membrane, comprising: heating a polymer precursor to a pyrolysis temperature of 900 °C or higher and 1200 °C or lower; and pyrolyzing the polymer precursor at the pyrolysis temperature in a pyrolysis atmosphere containing oxygen in an amount exceeding 0 ppm and less than 200 ppm.

2. The method according to claim 1, wherein the hollow fiber carbon membrane is asymmetric.

3. The method according to claim 1, wherein the pyrolysis temperature is 900 °C or higher and 1000 °C or lower.

4. The method according to claim 1, wherein the pyrolysis temperature is 925 °C or higher and 975 °C or lower.

5. The method according to claim 1, wherein the pyrolysis atmosphere contains oxygen in an amount exceeding 0 ppm and less than 150 ppm.

6. The method according to claim 1, wherein the pyrolysis atmosphere contains oxygen in an amount exceeding 5 ppm and less than 150 ppm.

7. The method according to claim 1, wherein the pyrolysis atmosphere contains oxygen in an amount exceeding 0 ppm and less than 100 ppm.

8. The method according to claim 1, wherein the pyrolysis atmosphere contains an inert gas and oxygen.

9. The method according to claim 1, wherein the pyrolysis atmosphere contains argon and oxygen.

10. The method according to claim 1, wherein the polymer precursor contains polyimide.

11. The method according to claim 1, comprising a polymer formed from one or more monomers selected from the group consisting of 2,4,6-trimethyl-1,3-phenylenediamine (DAM), oxydianiline (ODA), dimethyl-3,7-diaminodiphenyl-thiophene-5,5'-dioxide (DDBT), 3,5-diaminobenzoic acid (DABA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene), meta-phenylenediamine (m-PDA), 2,4-diaminotoluene (2,4-DAT), tetramethylmethylenedianiline (TMMDA), 4,4'-diamino-2,2'-biphenyldisulfonic acid (BDSA), 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandione (6FDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-oxydiphthalic anhydride, and benzophenonetetracarboxylic dianhydride (BTDA). [

12. ] The polymer precursor is monomer A Ｘ , B Ｙ and C Ｚ The polymer comprises X, Y, and Z are the respective molar fractions of A, B, and C, the sum of X + Y + Z is 1 or more, The method according to claim 1, wherein A, B, and C are each monomers selected from the group consisting of 2,4,6-trimethyl-1,3-phenylenediamine (DAM), oxydianiline (ODA), dimethyl-3,7-diaminodiphenyl-thiophene-5,5'-dioxide (DDBT), 3,5-diaminobenzoic acid (DABA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene), meta-phenylenediamine (m-PDA), 2,4-diaminotoluene (2,4-DAT), tetramethylmethylenedianiline (TMMDA), 4,4'-diamino-2,2'-biphenyldisulfonic acid (BDSA), 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandione (6FDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-oxydiphthalic anhydride (ODPA), 5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane (DAPI), and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). In an embodiment, the polyimide may contain at least two different moieties selected from DAM, ODA, DDBT, DABA, durene, m-PDA, 2,4-DAT, TMMDA, BDSA, 6FDA, BPDA, PMDA, NTDA, and BTDA.

13. A is a monomer selected from the group consisting of 6FDA, ODPA, and BTDA, B is DAM, C is a monomer selected from the group consisting of BPDA and PMDA, the method according to claim 12.

14. A hollow fiber carbon membrane produced by the method according to any one of Claims 1 to 13, wherein when the hollow fiber carbon membrane treats a stream containing equal amounts of hydrogen and ethylene, the hydrogen-to-ethylene (H ２ / C ２ H ４ ) selectivity is less than 50, the hollow fiber carbon membrane.

15. A hollow fiber carbon membrane produced by the method according to any one of claims 1 to 13, which has a hydrogen / ethylene (H ２ / C ２ H ４ ) selectivity of 30 or less when treating a stream containing equal amounts of hydrogen and ethylene.