Carbon molecular sieve membranes, methods of manufacture and uses thereof

Abstract

A method of making a carbon molecular sieve (CMS) membrane may include forming a copolymer selected from one or more of a polyvinylidene chloride (PVDC) copolymer, a polyimide copolymer, a polyetherimide copolymer, a polyacrylonitrile copolymer, and a polyphenylene ether copolymer into one or more hollow fibers or one or more microcapillary membranes; pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes with an inert gas or under vacuum at a second temperature of 600 DEG C to 700 DEG C; and annealing the one or more hollow fibers or the one or more microcapillary films with an inert gas or under vacuum at a third temperature of 900 DEG C to 1500 DEG C; the one or more hollow fibers or the one or more microcapillary membranes are oxidized with air at a fourth temperature of 300 DEG C to 400 DEG C.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of and priority to U.S. application serial number 63 / 485,320, filed February 16, 2023, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to the field of gas separation using carbon membranes. More specifically, the present disclosure relates to methods of producing carbon membranes for separating gases, and methods of separating gases from gaseous mixtures, such as carbon dioxide-hydrogen gas mixtures, by passing the mixture through the carbon membranes, as described in detail herein. Background Art

[0004] In the chemical industry, decarbonization technologies are continuously being developed because carbon dioxide (CO2) emissions are a source of global climate change. One potential decarbonization strategy is to develop hydrogen fuel gas from fuel gases such as methane. However, in this process, carbon dioxide remains a by-product, leading to the need for strategies to capture and sequester carbon from mixed hydrogen-carbon dioxide streams. A common separation method is to capture carbon dioxide in an amine sweetening process, where an aqueous solution of amine is passed through the gas stream, thereby absorbing the carbon dioxide and producing relatively pure hydrogen for use as fuel. However, thermal energy is generally required to evaporate the aqueous amine stream and decompose the carbon dioxide, the fuel of which can be derived from a hydrocarbon source. Secondly, amines themselves are toxic and corrosive, requiring special handling and additional safety procedures, thereby increasing costs.

[0005] Therefore, it is desirable to be able to separate carbon dioxide from hydrogen without the need for methods of amine or hydrocarbon source heat energy. Carbon molecular sieves (CMS) and CMS membranes are one such means that have traditionally been used to separate gas mixtures. CMS can be prepared from various resins that are pyrolyzed at different temperatures and / or under different conditions. Pyrolysis reduces the resin to carbon, but generally maintains at least some porosity in the pyrolysis product in the form of micropores. Subsequently, the CMS formed in this way can be used in conventional gas separation equipment (such as packed beds, chromatographic columns, etc.) that employ specific gas adsorption, wherein the size of the micropores determines which gas in the gas mixture is adsorbed and which gas is not adsorbed. Adsorption and desorption techniques can be alternately used for separation, according to, for example, conventional pressure swing adsorption methods or temperature swing adsorption methods. CMS membranes are also used to separate gases by passing a gas mixture through a CMS membrane.

[0006] Using CMS to accomplish separations generally assumes that the micropores are at least as large as or larger than the specific molecules that will enter the micropores. However, preparing CMS with micropores of the correct size for certain gas separations is a particular challenge. Summary of the Invention

[0007] CMS membranes can also be subdivided based on selectivity. Specifically, CMS membranes can be classified as either forward selective or reverse selective. Forward selective membranes selectively retain larger molecules while allowing smaller molecules to pass through the membrane. In contrast, reverse selective membranes selectively retain smaller molecules while allowing larger molecules to pass through. Therefore, reverse selective membranes may be desired to achieve gas separation of these larger molecules from smaller molecules. For reverse selective membranes, they typically have an average pore size that is larger than the size of the larger molecules. Larger molecules adsorb more strongly in the micropores and prevent the adsorption / permeation of smaller molecules. Selectivity depends on the micropore size and the adsorbate-adsorbent surface interaction.

[0008] However, complicating matters is that hydrogen molecules are so small that even if a membrane is designed to be reverse selective, the molecule can often pass through. Therefore, it is desirable to produce reverse selective CMS membranes that provide high reverse selectivity for gas separations, particularly for the separation of carbon dioxide and hydrogen.

[0009] Thus, this document discusses methods for producing CMS membranes with the aforementioned benefits. Specifically, CMS membranes formed according to one or more embodiments herein are provided with reverse selectivity through pyrolysis and oxidation of the copolymer at specific temperature and time thresholds. Furthermore, CMS membranes according to one or more embodiments herein are provided with reverse selectivity for CO2-hydrogen separation, particularly through an additional annealing step between the pyrolysis and oxidation steps. Furthermore, optional pretreatment of the copolymer, such as polyvinylidene chloride copolymer, can increase its melting temperature, thereby at least partially stabilizing the copolymer and preventing it from collapsing or melting during the subsequent pyrolysis. Pyrolysis further increases the degree of carbonization of the copolymer, thereby preventing the copolymer from being destroyed during subsequent oxidation when carbon atoms are expelled from the CMS membrane as CO2 / CO and oxygenates are formed on the surface of the CMS membrane. The annealing step reorders and stabilizes the carbon structure of the CMS membrane via graphitization, thereby shrinking the pores on average and producing a more consistent pore size, which can then be expanded by the oxidation step. Thus, a stable carbon structure with consistent pore size can be achieved, allowing CO2-hydrogen separation without significant hydrogen permeability.

[0010] According to one embodiment, a method for manufacturing a carbon molecular sieve (CMS) membrane may include: forming a copolymer into one or more hollow fibers or one or more microcapillary membranes, the copolymer being selected from one or more of a polyvinylidene chloride (PVDC) copolymer, a polyimide copolymer, a polyetherimide copolymer, a polyacrylonitrile copolymer, and a polyphenylene ether copolymer; pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes at a second temperature of 600° C. to 700° C. using an inert gas or under vacuum; and annealing the one or more hollow fibers or the one or more microcapillary membranes at a third temperature of 900° C. to 1500° C. using an inert gas or under vacuum; and oxidizing the one or more hollow fibers or the one or more microcapillary membranes using air at a fourth temperature of 300° C. to 400° C.

[0011] According to another embodiment, a method for separating a gas mixture comprising hydrogen and carbon dioxide may include manufacturing a CMS membrane and flowing the gas mixture through the CMS membrane to produce a permeate first stream having an increased carbon dioxide concentration and a retentate second stream having an increased hydrogen concentration.

[0012] According to another embodiment, the carbon molecular sieve (CMS) membrane may comprise one or more hollow fibers or one or more microcapillary membranes, wherein: the one or more hollow fibers or the one or more microcapillary membranes comprise a copolymer selected from one or more of polyvinylidene chloride (PVDC) copolymers, polyimide copolymers, polyetherimide copolymers, polyacrylonitrile copolymers, and polyphenylene ether copolymers; and the CMS membrane comprises a carbon dioxide permeability of at least 1000 GPU (gas permeation unit) and a carbon dioxide / hydrogen selectivity of 50 to 200 at 350 kPa gauge pressure and ambient temperature.

[0013] Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will become apparent to those skilled in the art from that description or may be learned by practicing the described embodiments, including the detailed description and claims provided below. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The following detailed description of specific embodiments of the present disclosure is best understood when read in conjunction with the accompanying drawings, in which:

[0015] Figure 1 is a graphical illustration of the weight loss over time of PVDC fibers of different diameters under a temperature ramp of approximately 5°C per minute;

[0016] Figure 2 A carbon molecular sieve membrane composed of a woven matrix of a plurality of hollow fibers as in the embodiments herein is shown;

[0017] Figure 3 is a scatter plot of permeability versus selectivity for various hollow fiber and microcapillary membranes prepared as described in embodiments herein;

[0018] Figure 4A is a graphical illustration of the carbon dioxide-hydrogen separation performance, particularly the permeability to carbon dioxide, of four membranes prepared according to the procedure of Example 8, as in the embodiments herein;

[0019] Figure 4B is a graphical illustration of carbon dioxide-hydrogen separation performance, particularly permeability to hydrogen, according to embodiments herein;

[0020] Figure 4C is a graphical illustration of carbon dioxide-hydrogen separation performance, particularly the selectivity of carbon dioxide relative to hydrogen, according to embodiments herein;

[0021] Figure 5 is a graphical illustration of the micropore size distribution of membranes according to embodiments herein, membrane 17 (annealed at 1200° C., no oxidation) and membrane 8 (oxidized in air at 1200° C.);

[0022] Figure 6A is a graphical illustration of three carbon dioxide adsorption isotherms for a microcapillary membrane prepared at an annealing temperature of 600° C. and without oxidation, as in embodiments herein;

[0023] Figure 6B is as in the embodiments herein with Figure 6A Graphical illustration of three carbon dioxide adsorption isotherms for similar microcapillary membranes but prepared at an annealing temperature of 900°C and without oxidation;

[0024] Figure 7A is a graphical illustration of time versus temperature oxidation testing of three microcapillary films annealed at 900° C., 1200° C., and 1500° C., according to embodiments herein; and

[0025] Figure 7B As in the embodiments herein Figure 7A Graphical illustration of the effect of oxidation test time on the weight loss of microcapillary membranes. DETAILED DESCRIPTION

[0026]

[0014] Embodiments described herein relate to methods of making carbon molecular sieve (CMS) membranes and methods of utilizing CMS membranes.

[0027] As used herein, "dehydrochlorination" can refer to an elimination reaction that removes hydrogen, chlorine, or a hydrogen halide from a substrate. For example, the dehydrochlorination of polyvinylidene chloride can involve a 1,2 elimination via an ion pair or a highly polarized four-center transition state. In another example, dehydrochlorination involves rearrangement of the chloroallyl structure to a cis-allyl configuration, which then loses HCl via a six-center concerted process.

[0028] The gas permeation characteristics of a membrane (such as a CMS membrane described in further detail herein) can be determined by a gas permeation test. Two intrinsic properties are used to evaluate the separation performance of a membrane material: its "permeability" (a measure of the intrinsic productivity of the membrane); and its "selectivity" (a measure of the separation efficiency of the membrane). It is usually measured in Barrer The unit of permeability is i ), which is based on the flux (n i ) divided by the partial pressure difference between the upstream and downstream sides of the membrane (Δp i ) and multiplied by the thickness of the membrane (1) to calculate. In the embodiments herein, the thickness of the membrane can generally be expressed as the wall thickness of the hollow fiber (1) (OD-ID)* / 2:

[0029] Another term "permeability" is defined herein as the productivity of a CMS membrane or individual hollow fibers and is typically expressed as a gas permeation unit (GPU). is measured by dividing the permeability by the effective membrane separation layer thickness:

[0030] Finally, "selectivity" is defined in this article as the permeability of one gas through a membrane, or the ratio of the permeability of another gas of the same nature. It is measured as a unitless ratio:

[0031]

[0032] As previously described, embodiments herein relate to methods for manufacturing carbon molecular sieve (CMS) membranes and processes utilizing CMS membranes. The method may initially include forming the copolymer into one or more hollow fibers or one or more microcapillary membranes. The method may also optionally include pretreating the one or more hollow fibers or the one or more microcapillary membranes by heating at a first temperature of 120°C to 200°C using air, using an inert gas, under vacuum, or a combination thereof. The method may also include pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes using an inert gas or under vacuum at a second temperature of 600°C to 700°C. The method may also include annealing the one or more hollow fibers or the one or more microcapillary membranes using an inert gas or under vacuum at a third temperature of 900°C to 1500°C. The method may also include oxidizing the one or more hollow fibers or the one or more microcapillary membranes using air at a fourth temperature of 300°C to 400°C.

[0033] As previously described, the method may initially include forming the copolymer into a hollow fiber or microcapillary membrane. The copolymer may be selected from polyvinylidene chloride (PVDC) copolymers, polyimide copolymers, polyetherimide copolymers, polyacrylonitrile copolymers, and polyphenylene oxide copolymers. In some embodiments, the copolymer may be a PVDC copolymer. In other embodiments, the copolymer may be a polyimide copolymer.

[0034] In an embodiment, the copolymer can be formed by copolymerization of the copolymer and a comonomer. The copolymerization method may include, but is not limited to, bulk polymerization, suspension polymerization, or emulsion polymerization. It is generally preferred that the copolymerization be carried out at a temperature such as 10° C. to 120° C., 20° C. to 100° C., or 30° C. to 90° C., which ensures that thermal degradation of all components is avoided, such as with respect to PVDC copolymers.

[0035] As previously described, the copolymer can include at least one of the following comonomers: vinyl monomer, vinyl chloride monomer, acrylate monomer, methacrylate monomer, styrene monomer, acrylonitrile, methacrylonitrile, itaconic acid and pyrolyzed chlorotrifluoroethylene. Based on the total weight of the copolymer, in an embodiment comprising a PVDC copolymer, the PVDC copolymer can include at least 60 wt % or alternatively at least 70 wt % of vinylidene chloride. The polyvinylidene chloride copolymer can include up to about 97 wt % of vinylidene chloride, and therefore the polyvinylidene chloride copolymer can include at least 3 wt % of the comonomers previously stated in this paragraph. The polyvinylidene chloride copolymer can include 3 wt % to 40 wt %, 3 wt % to 30 wt % or 3 wt % to 20 wt % of the comonomer. The polyvinylidene chloride copolymer can also include 3.5 wt % to 15 wt %, 4 wt % to 12 wt %, 7 wt % to 28 wt % or 9 wt % to 25 wt % of the comonomer.

[0036] After copolymerization, copolymer can be formed into one or more hollow fibers or one or more microcapillary membranes by any suitable method known in the art. For example, copolymer can be melt-extruded or solution-spun so that copolymer is formed into hollow fibers. Fiber can be produced by uniaxial stretching using known fiber processes for copolymer, and the fiber can be a circular or shaped hollow fiber, or any other desired hollow fiber form. Microcapillary membrane can be produced by biaxial stretching using known membrane processes for copolymer. It is also conceivable that precursor membrane and / or fiber can be coextruded with multiple copolymers and / or other polymers.

[0037] It should be noted that the fiber preparation method can optionally include stretching (such as stretching of resin) to form melt-extruded fiber or film. In a particular embodiment, this stretching can be particularly effective in inducing faster crystallization and increase, and therefore is particularly effective in improving the crystallite arrangement of one or more hollow fibers. Desirably, the draw ratio range is 1 to 8, such as 1 to 6, 1 to 4 and 2 to 4.

[0038] Generally, it is useful that one or more hollow fibers or microcapillary membranes have a certain amount of crystallinity. In embodiments herein, this crystallinity is generally 25% to 75% of the resin or molded film, as measured by differential scanning calorimetry (DSC) according to ASTM D3418. In embodiments, this level range is also 30% to 55% or 35% to 50%. Therefore, comprising a comonomer generally helps to reduce the precursor crystallinity, to ensure the desired range, and also helps to reduce the melting temperature, thereby improving the processing properties of the resulting copolymer. Generally, comprising a monomer with a larger volume can tend to reduce the overall copolymer crystallinity to a greater extent than comprising a monomer with a smaller volume. Therefore, for example, compared with, for example, methyl acrylate or ethyl acrylate, butyl acrylate can tend to reduce crystallinity to a greater extent, assuming that methyl acrylate or / and ethyl acrylate are used based on the same molar percentage (mol%) of the final copolymer composition.

[0039] In an embodiment, the total amount of all additives combined may be no more than 15% by weight of the one or more hollow fibers or the one or more microcapillary membranes, such as no more than 8% by weight or no more than 3% by weight. However, in many applications, a combined amount of at least 2% by weight of all additives may be typical, so that the range of use is from 2% to 8% by weight or from 2% to 3% by weight of the one or more hollow fibers or the one or more microcapillary membranes. Those skilled in the art will be aware of the use of such additives and their indications and contraindications without further guidance herein.

[0040] One or more hollow fibers can include internal diameter and external diameter respectively. One or more hollow fibers can also include length. In other words, one or more hollow fibers can be considered as tube. The external diameter of one or more hollow fibers can be 50 microns (micrometers) to 5000 microns. External diameter can also be any narrower range within the range of 50 microns to 5000 microns. For example, one or more hollow fibers can have an external diameter of any combination of 50 microns to 100 microns, 100 microns to 1000 microns, 1000 microns to 2500 microns, 2500 microns to 4000 microns, 4000 microns to 5000 microns or any endpoints of these scopes. One or more hollow fibers can also have a thickness between internal diameter and external diameter. Thickness can be 10 microns to 100 microns. For example, one or more hollow fibers can have an external diameter of 50 microns and a thickness of 10 microns, so that an internal diameter is 30 microns. The one or more hollow fibers may alternatively have an outer diameter of 5000 microns and a thickness of 100 microns, resulting in an inner diameter of 4800 microns.

[0041] As previously described, in at least some embodiments, the method may further include pre-treating the one or more hollow fibers or the one or more microcapillary membranes by heating at a first temperature of 120° C. to 200° C. using air, using an inert gas, under vacuum, or a combination thereof. In embodiments, pre-treating the one or more hollow fibers or the one or more microcapillary membranes may be used to stabilize or “lock” the copolymer structure prior to pyrolysis / carbonization thereof, such as when the copolymer is a PVDC copolymer. In at least some embodiments, in this step, the one or more hollow fibers or the one or more microcapillary membranes may be heated to a temperature below the melting temperature of the PVDC copolymer to dehydrochlorinate the fibers to a degree of at least 10%, such as 10% to 15%, 15% to 20%, or 20% to 30%, 30% to 50%, or any combination of ranges or smaller ranges therein.

[0042] As used herein, the term "at least 10% dehydrochlorination" means that the hollow fibers have been pre-treated by removing hydrochloride to a point at which the copolymer hollow fibers are no longer fusible and, in fact, may begin to become non-fusible. Without being limited by theory, such changes in molecular dynamics may begin to occur at a point of approximately 10% dehydrochlorination and may be complete or maintained as the level of dehydrochlorination increases beyond that point. In embodiments, this "locking" of the copolymer structure may prevent further deformation or bending during the pyrolysis and oxidation steps that follow the pretreatment. In other words, due at least in part to the fact that the copolymer is no longer fusible, the copolymer may be considered self-supporting, i.e., the copolymer may bear its own weight during further pyrolysis, wherein the copolymer structure may be further strengthened.

[0043] The first temperature may also be any temperature range between 120° C. and 200° C. For example, the one or more hollow fibers or the one or more microcapillary membranes may be heated at a first temperature of 120° C. to 130° C., 130° C. to 150° C., 150° C. to 160° C., 160° C. to 180° C., 180° C. to 190° C., 190° C. to 200° C., or any combination of ranges or smaller ranges therein. In embodiments, heating at any of these temperatures may also crosslink the interior of the one or more hollow fibers or the one or more microcapillary membranes.

[0044] In an embodiment, the one or more hollow fibers or the one or more microcapillary membranes can be pretreated for a period of 10 minutes to 72 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 10 hours, 10 hours to 20 hours, 20 hours to 40 hours, 40 hours to 60 hours, 60 hours to 72 hours, or any combination of ranges or smaller ranges therein.

[0045] Pretreating the one or more hollow fibers or the one or more microcapillary membranes may further comprise contacting the one or more hollow fibers or the one or more microcapillary membranes with air, an inert gas, or both. Contacting the one or more hollow fibers or the one or more microcapillary membranes with air or an inert gas may be performed at a rate sufficient to purge pretreatment gas products (such as methane, hydrogen, carbon monoxide, and carbon dioxide) to prevent side reactions of the dehydrochlorination gas products on the carbon surface.

[0046] As previously described, the method may also include pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes at a second temperature of 600° C. to 700° C. using an inert gas or under vacuum. In an embodiment, pyrolysis can cause at least 90% by weight (such as at least 95% by weight or at least 99% by weight) of the copolymer to be carbonized. As noted above, pyrolysis is also referred to as "carbonization" because the result is that the copolymer can be converted into a skeleton (that is, all or nearly all atoms except carbon are removed, but carbon-carbon bonds are substantially intact) of its copolymer structure, and the one or more hollow fibers or the one or more microcapillary membranes can now be referred to as "carbonaceous". Pyrolysis can be performed using any means generally known to those skilled in the art.

[0047] The second temperature may also be any narrower temperature range within 600° C. to 700° C. For example, the one or more hollow fibers or the one or more microcapillary membranes may be pyrolyzed at a second temperature of 600° C. to 625° C., 625° C. to 650° C., 650° C. to 675° C., 675° C. to 700° C., or any combination of ranges or smaller ranges therein, such as 625° C. to 675° C. As previously described, pretreating the one or more hollow fibers or the one or more microcapillary membranes at the second temperature may further include contacting the one or more hollow fibers or the one or more microcapillary membranes with an inert gas.

[0048] In an embodiment, the inert gas may include carbon dioxide; nitrogen; any inert gas (including but not limited to argon); or a combination thereof. Contacting the one or more hollow fibers or the one or more microcapillary membranes with air or an inert gas may be performed at a rate sufficient to purge the pyrolysis gas products, thereby preventing side reactions of the pyrolysis gas products on the carbon surface.

[0049] In an embodiment, the one or more hollow fibers or the one or more microcapillary membranes may be pyrolyzed at the second temperature for a period of 10 minutes to 72 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 10 hours, 10 hours to 20 hours, 20 hours to 40 hours, 40 hours to 60 hours, 60 hours to 72 hours, or any combination of ranges or smaller ranges therein.

[0050] As previously described, the method may further include annealing the one or more hollow fibers or the one or more microcapillary membranes using an inert gas or under vacuum at a third temperature of 900° C. to 1500° C. The third temperature may also be 900° C. to 1000° C., 1000° C. to 1100° C., 1100° C. to 1200° C., 1200° C. to 1300° C., 1300° C. to 1400° C., 1400° C. to 1500° C., or a combination of the foregoing ranges or smaller ranges thereof, such as 1200° C. to 1500° C.

[0051] In an embodiment, the one or more hollow fibers or the one or more microcapillary membranes may be annealed at the third temperature for a period of 10 minutes to 72 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 10 hours, 10 hours to 20 hours, 20 hours to 40 hours, 40 hours to 60 hours, 60 hours to 72 hours, or any combination of ranges or smaller ranges therein.

[0052] In at least some embodiments, the method may further include cooling the one or more hollow fibers or the one or more microcapillary membranes to a temperature of less than or equal to 60° C. between pyrolysis and annealing. Without being limited by theory, in the case of a batch pyrolysis furnace, this cooling step may allow for removal of pyrolysis gas products prior to the annealing step. However, in embodiments that do not include a batch pyrolysis furnace, such as a continuous process, the method may proceed from the pyrolysis step to the annealing step without cooling the one or more hollow fibers or the one or more microcapillary membranes.

[0053] In at least some embodiments, the cooling step, the annealing step, or both can occur after observing a weight loss of greater than or equal to 70% for the one or more hollow fibers or the one or more microcapillary membranes during the pyrolysis step. Without being limited by theory, and as Figure 1 As shown, the weight loss of the one or more hollow fibers or the one or more microcapillary membranes begins to plateau upon reaching 70% weight loss or greater during pyrolysis, indicating that the reaction has begun to progress from pyrolysis to annealing, where reordering and stabilization of the carbon structure are typically observed. Figure 1 The weight loss of hollow fibers is shown, but the same trend should also apply to microcapillary membranes. Figure 1 As shown, this trend is consistent regardless of the thickness of the hollow fiber, which is also expected for microcapillary membranes.

[0054] As previously described, the method may further include oxidizing the one or more hollow fibers or the one or more microcapillary membranes using air at a fourth temperature of 300° C. to 400° C. The fourth temperature may also be 300° C. to 310° C., 310° C. to 350° C., 350° C. to 375° C., 375° C. to 400° C., or any combination of ranges or smaller ranges therein.

[0055] In embodiments, air can comprise 10 wt % to 30 wt % oxygen by the weight of air, such as approximately 20 wt % oxygen, or such as 10 wt % to 12 wt %, 12 wt % to 18 wt %, 18 wt % to 22 wt %, 22 wt % to 28 wt %, 28 wt % to 30 wt % or any combination of ranges or smaller ranges therein, such as approximately 20 wt % oxygen. Without being limited by theory, the oxygen content of air can affect the oxidation rate of the one or more hollow fibers or the one or more microcapillary membranes. For example, under higher oxygen content, the oxidation rate of the one or more hollow fibers or the one or more microcapillary membranes can increase accordingly. It is also known that steam and CO can be milder oxidants than air. Therefore, without being limited by theory, at temperatures higher than air, steam and / or CO can be able to achieve similar levels of oxidation and pore opening.

[0056] In an embodiment, the one or more hollow fibers or the one or more microcapillary membranes can be oxidized at the fourth temperature for a period of 10 minutes to 40 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 10 hours, 10 hours to 20 hours, 20 hours to 30 hours, 30 hours to 40 hours, 40 hours to 42 hours, or any combination of ranges or smaller ranges therein. However, oxidation can also occur over a larger time frame / range, such as 10 minutes to 7 days.

[0057] Without being limited by theory, it is expected that the oxidation of the one or more hollow fibers or the one or more microcapillary membranes can be operated to increase its pore volume. Specifically, oxidation can be used for discharging carbon atoms (such as with CO and / or CO2 gas) from the edge of the pore wall previously produced from the one or more hollow fibers or the one or more microcapillary membranes, thereby expanding them. The expansion of pore can result in an overall increase in the permeability of all gaseous substances with a representative molecular diameter less than pore size. However, oxidation can also result in a relatively large increase in the permeability of more strongly adsorbed gases such as (CO2) compared to less strongly adsorbed gases (e.g., H2), mainly due to the gas absorption effect in the pores of the one or more hollow fibers or the one or more microcapillary membranes. In embodiments, the oxidation step can stop when the loss of the one or more hollow fibers or the one or more microcapillary membranes in the oxidation step is less than 20 weight %.

[0058] As explained in further detail below, annealing the one or more hollow fibers or the one or more microcapillary membranes at a temperature greater than or equal to 900° C. can be used to reorder and stabilize the carbon structure of the CMS membrane, thereby shrinking the pores on average and producing a more consistent pore size that can be subsequently expanded by the oxidation step. Annealing can also have a secondary benefit of making the CMS membrane more resistant to typical carbon expulsion during oxidation. Without being limited by theory, this increase in the electrical resistance of the one or more hollow fibers or the one or more microcapillary membranes can be used to concentrate carbon expulsion around the pores, rather than on the support structure of the one or more hollow fibers or the one or more microcapillary membranes. The result can be that the pore throats expand during subsequent oxidation without the one or more hollow fibers or the one or more microcapillary membranes breaking up.

[0059] As previously discussed, the CMS membranes formed according to the methods herein can be self-supporting. In other words, and in one or more embodiments, the CMS membrane does not include a support structure for the CMS membrane. The CMS membranes formed according to the methods herein may have an oxygen content of 5 atomic % (atomic ratio) to 17 atomic %, such as 5 atomic % to 7 atomic %, 7 atomic % to 9 atomic %, 9 atomic % to 12 atomic %, 12 atomic % to 16 atomic %, 16 atomic % to 17 atomic %, or any combination of ranges or smaller ranges therein. The CMS membranes formed according to the methods herein may also have a carbon content of 82 atomic % to 94 atomic %, such as 82 atomic % to 83 atomic %, 83 atomic % to 86 atomic %, 86 atomic % to 90 atomic %, 90 atomic % to 92 atomic %, 92 atomic % to 94 atomic %, or any combination of ranges or smaller ranges therein.

[0060] With respect to the aforementioned oxygen and carbon content ranges, it is anticipated that a certain degree of oxidation may be required to achieve a specific level of pore opening. Specifically, during oxidation, two reactions may occur: the formation of oxygen-containing compounds on the carbon surface and the formation of CO / CO2 gas. Therefore, the oxygen content of the CMS membrane may be correlated with the degree of pore opening and / or the degree of oxidation. In embodiments, the CMS membrane may contain an oxygen content of 5 atomic % to 15 atomic %.

[0061] As previously discussed, embodiments herein may also relate to CMS membranes. The CMS membrane may be any CMS membrane formed according to the methods previously discussed. In embodiments, and as Figure 2 As shown, one or more fibers of the CMS membrane can be a first plurality of hollow fibers and a second plurality of hollow fibers. The first plurality of hollow fibers and the second plurality of hollow fibers can also be arranged in a woven grid structure, such as Figure 2 As shown. The first plurality of hollow fibers and the second plurality of hollow fibers can also be arranged in a woven grid structure for the pretreatment step, the pyrolysis step, the cooling step, the annealing step, the oxidation step or a combination thereof. It is expected that this woven breathable structure will help to reach a more uniform temperature and gas composition at the solid / gas interface. This is especially critical for the oxidation step to achieve the same degree of oxidation. The woven grid structure can preferably be formed in a polymerized state before thermal conversion. Without being limited by theory, it is expected that the flexible polymer structure is more easily woven into a structure.

[0062] In other embodiments, the one or more hollow fibers or the one or more microcapillary membranes can be bundled together to form a CMS membrane, such as by using an adhesive or bonding mechanism.

[0063] As previously discussed, CMS membranes can have a permeability, which is expressed as the permeability of a gas flowing through the membrane layer thickness (the thickness of the wall of the individual hollow fibers). However, CMS membranes can have different permeabilities for gases of different sizes. As previously discussed, the ratio of these different permeabilities can be expressed as selectivity for a given gas. For example, a CMS membrane can selectively separate different gases from each other. As discussed in further detail below, this can allow the CMS membrane to act as a preferential separator for gases of different sizes. The CMS membranes herein can also be reverse selective. In other words, they can preferentially expel smaller gas molecules from the membrane while accepting (and passing through) larger gas molecules.

[0064] In an embodiment, in an environment of at least carbon dioxide and hydrogen, the CMS membranes herein may have a carbon dioxide permeability of at least 1000 GPU or at least 2000 GPU, such as 1000 GPU to 1500 GPU, 1500 GPU to 2000 GPU, 2000 GPU to 2500 GPU, 2500 GPU to 3000 GPU, or any combination of the foregoing ranges or smaller ranges therein, such as 1000 GPU to 3000 GPU. The CMS membranes herein may also have a hydrogen permeability of less than 40 GPU, such as 40 GPU to 30 GPU, 30 GPU to 20 GPU, 20 GPU to 10 GPU, 10 GPU to 1 GPU, or any combination of the foregoing ranges or smaller ranges therein, such as 1 GPU to 40 GPU.

[0065] The CMS membranes herein may also have a carbon dioxide / hydrogen mixed gas selectivity greater than 50 at ambient temperature, such as 50 to 70, 70 to 80, 80 to 100, 100 to 120, 120 to 140, 140 to 160, 160 to 180, 180 to 200, or any combination of the foregoing ranges or smaller ranges therein, such as 80 to 200 or 50 to 200. Without being limited by theory, such selectivities may allow the CMS membrane to preferentially separate heavier carbon dioxide gas molecules from lighter hydrogen gas molecules.

[0066] Without being limited by theory, permeability and selectivity may depend on temperature and pressure. Thus, the permeabilities and selectivities described above may be understood to occur at ambient temperature (20° C.) and 350 kPa gauge pressure, and thus may also be understood to vary with respect to different temperatures and / or pressures. Specifically, CO2 permeability may be understood to increase if measured at temperatures above ambient temperature and to decrease at temperatures below ambient temperature. CO2 / H2 selectivity may be understood to increase at lower temperatures and decrease at higher temperatures.

[0067] As described above, embodiments herein also relate to a method for separating a gas from a gas mixture. The gas mixture may include a first gas molecule and a second gas molecule. The second gas molecule (i.e., hydrogen) may have a smaller representative molecular diameter than the first gas molecule (i.e., carbon dioxide). The method may include: forming a CMS membrane, and passing the gas mixture through the CMS membrane to produce a permeate first stream and a second retentate stream. The CMS membrane used in this method may be any CMS membrane previously discussed. The permeate first stream may have a higher concentration of first gas molecules than the second retentate stream, which in turn may have a higher concentration of second gas molecules than the permeate first stream. In this way, the CMS membrane used in this method can be operated to separate the first gas molecule and the second gas molecule from each other, and can be reverse selective.

[0068] As previously mentioned, determining the micropore / molecular size of a CMS membrane is important for determining the suitability of the CMS membrane for a particular separation. Different ways of determining molecular size have been developed. One commonly used method is to determine the "kinetic diameter" of a given molecule. A reference that lists various of these kinetic diameters based on their use in zeolite applications is DW Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, John Wiley & Sons, Inc. (New York, NY 1974), 636 and these determinations are often used for non-zeolites, carbon molecular sieves, known to have slit-shaped pores. In view of the foregoing and for the purposes of the present invention, the following kinetic diameters taken from the Breck reference cited above are used herein as representative molecular diameters for the following molecules: He (2.6 angstroms, ), H2 N2 CO2 CH4 C2H4 C3H8 i-C4H 10 SF6 (sulfur hexafluoride) and i-8H 18 (Isooctane) However, because this reference table lacks the kinetic diameter for ethane and the kinetic diameter given therein for propylene is considered by at least some researchers to be inaccurate for the CMS material itself, for these two materials, the Lennard-Jones collision diameter is used in this paper rather than the Breck kinetic diameter. These Lennard-Jones collision diameters are and C3H6 See, eg, Staudt-Bickel C., Koros WJ, "Olefin / paraffin gas separations with 61-DA-based polyimide membranes," J. Membr. Sci. (2000) 170(2), 205-214 for further discussion. The kinetic diameter and the Lennard-Jones collision diameter are collectively referred to as the "representative molecular diameter."

[0069] In an embodiment, the CMS membrane may have an average and / or median pore size that is larger than the representative molecular diameter of the first gas molecule (i.e., carbon dioxide), such as at least twice as large. The average / median pore size of the CMS membrane can be determined by gas adsorption techniques using gas probe molecules of different sizes. The CMS membrane can have an average / median pore size of about 4.5 angstroms to about 7 angstroms, such as about 4.5 angstroms to 5 angstroms, about 5 angstroms to about 6 angstroms, about 6 angstroms to about 6.6 angstroms, about 6.6 angstroms to about 7 angstroms, or any combination of the foregoing ranges or smaller ranges therein, such as 5 angstroms to 7 angstroms. Without being limited by theory, the average / median pore size of 6.6 angstroms to 7 angstroms can theoretically allow for the absorption and surface flow of two layers of carbon dioxide molecules while leaving no room for hydrogen molecules to penetrate, as the representative molecular diameter of carbon dioxide is estimated to be 3.3 angstroms. Furthermore, an average / median pore size of 4.5 to 6 angstroms could theoretically allow for the absorption and surface mobility of carbon dioxide molecules while leaving no room for the penetration of hydrogen molecules, since the representative molecular diameter of carbon dioxide is estimated to be 3.3 angstroms and the representative molecular diameter of hydrogen is estimated to be 2.9 angstroms (approximately 6.2 angstroms combined).

[0070] As mentioned previously, the mean / median pore size can be determined by gas adsorption. Partially graphitized nanosheets exhibit a slit pore structure. For example, Figure 5 As shown, CMS membranes may have a broader total pore size distribution than 4 to 10 angstroms. In embodiments, CMS membranes formed according to the methods herein may have a maximum pore size of approximately 10 angstroms.

[0071] In addition to the average micropore size, it is generally desirable in the art to optimize the total micropore volume, which can be measured via the Brunauer-Emmett-Teller (BET) method at liquid N2 temperatures. This can be further confirmed by helium (He) pycnometry and mercury (Hg) intrusion methods. For most separation applications, a total micropore volume of at least 0.10 mL / g, preferably at least 0.15 mL / g, and more preferably at least 0.20 mL / g according to the BET method at liquid N2 temperatures may be required to ensure commercially efficient adsorption of the desired gas.

[0072] Example

[0073] Hollow fiber and microcapillary membranes were formed according to embodiments herein for permeation testing, as explained in further detail below.

[0074] Hollow fiber preparation

[0075] According to embodiments of the present invention, three different types of hollow fibers are prepared by melt extrusion or solution spinning, as explained in more detail below. Specifically, the first type of hollow fibers (PVDC hollow fibers) are formed by melt extruding a PVDC copolymer of vinyl chloride. The fibers have an outer diameter (OD) and inner diameter (ID) of 164 microns and 76 microns, respectively.

[0076] The second type of hollow fiber (6FDA hollow fiber) was formed by solution spinning of a polyimide 6FDA / BpDA-DAM polymer using the method described in the publication (Xu et al., Physical aging in carbon molecular sieve membranes. Carbon 2014, 80, 155-166.). The OD and ID of the fiber were 580 microns and 440 microns, respectively. The thickness of the asymmetric hollow fiber wall was also less than 5 microns.

[0077] The third type of hollow fiber (Matrimid hollow fiber) is formed by solution spinning of Matrimid polyimide polymer using the method specified in the publication (Xu et al., Matrimid (R) derived carbon molecular sieve hollow fiber membranes for ethylene / ethane separation. J. Membr. Sci. 2011, 380 (1-2), 138-147.). The OD and ID of the fiber are 400 microns and 230 microns, respectively. The thickness of the asymmetric hollow fiber wall is also less than 1 micron.

[0078] Hollow Fiber Testing Procedure

[0079] Each hollow fiber in the hollow fiber bundle was individually passed through an alumina tube. The alumina tube kept the individual hollow fibers straight and separated during pyrolysis. For PVDC hollow fibers, the precursor fibers were pretreated at 130°C in a low temperature oven with a 2 liter / minute (L / min) air purge for crosslinking. No pretreatment was performed on 6FDA / BpDA-DAM hollow fibers or polyimide hollow fibers. The alumina tube bundle containing the fibers was then pyrolyzed at 600°C and annealed at 900°C at a rate of 3°C / min, then held for 2 hours before cooling in a 6" OD quartz tube furnace. A nitrogen purge flow of 5 L / min was used to keep the furnace free of oxygen.

[0080] A bundle of each pyrolyzed fiber was then inserted into a 0.25" ID alumina tube, which was then placed in a quartz tube furnace for oxidation. The furnace was continuously purged with air. The furnace was heated at a ramp rate of 1°C / min to a peak temperature between 275°C and 400°C and then held for 8 hours. The furnace was then allowed to cool to room temperature before unloading the samples. Unless otherwise stated, the resulting CMS membrane fibers were stored in a nitrogen box and then made into modules for permeation testing.

[0081] Microcapillary membrane preparation

[0082] According to embodiments herein, microcapillary films are extruded using a PVDC resin obtained from SK Global SARAN, specifically SBR711 (a PVDC copolymer resin containing approximately 8.5% by weight of methyl acrylate). Specifically, the microcapillary film die has a simple, split-body design with a two-inch-wide air intake manifold insert containing 42 parallel hollow pins positioned near the die outlet for introducing air into the polymer melt forming the microcapillaries. The extruder pump rate and air flow rate are adjusted to achieve the desired microcapillary diameter.

[0083] PVDC microcapillary film samples were extruded using a 0.75 inch diameter single screw extruder with three barrel temperature zones. An elbow adapter was made to position the microcapillary film die so that the extruded ribbon would be directed downward into a water bath. The elbow and die were heated using a metal heating element clamped in place. The temperature of the three zones of the extruder, elbow, and die was raised from 155°C to 170°C until no unmelted resin was seen in the extruded film. The temperature was kept as low as possible to avoid thermal decomposition of the PVDC resin. Polyethylene (PE) resin was periodically fed into the extruder to periodically rinse off the accumulated char.

[0084] When extruded from the die, the microcapillary film is quenched in a room temperature water bath, then wound around a guide roller at the bottom of the bath and subsequently pulled out of the bath by a winder. The film is stretched by increasing the speed of the winder. Stretch occurs near the exit of the die, reducing film thickness and film width. The extruded microcapillary film is then cut into approximately 3-foot strips and placed on top of a flat laboratory bench under atmospheric conditions to allow the PVDC to fully crystallize for approximately one week.

[0085] Microcapillary membrane testing procedure

[0086] A 7.5 cm long microcapillary tape was cut from the melt-extruded film. The tape was then placed between two honeycomb ceramic plates. Two sheets of Whatman filter paper (Whatman 1003-125) were placed between the PVDC microcapillary membrane and the porous ceramic plate (each weighing approximately 100 grams) as a liner. The tape / filter paper / ceramic plate sandwich was preconditioned in an oven with an air purge (5 liters / min). The oven temperature was raised to 130°C at a heating rate of 1°C / min and then maintained at 130°C for 24 hours. After the oven cooled to below 60°C, the sandwich was removed.

[0087] The pretreated microcapillary membrane, along with the filter paper and porous ceramic plate, was then placed in a nitrogen-purged quartz tube furnace. The furnace was first heated to 250°C at 0.1°C / min and then to a final pyrolysis temperature in the range of 600°C to 900°C at a heating rate of 3°C / min. The final temperature was maintained for two hours before cooling. The sample was removed after the furnace cooled to below 60°C.

[0088] Some of the pyrolyzed microcapillary membranes were then placed in a nitrogen-purged alumina tube furnace for an annealing step. The remaining microcapillary membranes were retained as a non-annealed control for comparison. The furnace was first ramped at 5°C / min to a final temperature of 900°C to 1500°C and then held at the final temperature for 120 minutes before cooling. The samples were removed after the furnace cooled to below 60°C.

[0089] Some of the pyrolyzed or annealed microcapillary membranes were then placed in an air-purged quartz tube furnace for an oxidation step. The remaining microcapillary membranes were retained as non-oxidized controls for comparison. The furnace was first heated to the final temperature at a rate of 3°C / min and held at the final temperature for 8 hours before cooling. Unless otherwise noted, the resulting CMS membranes were stored in a nitrogen box and then fabricated into modules for permeation testing.

[0090] Penetration and selectivity testing

[0091] As previously described, each of the CMS membranes was then stored in a nitrogen-rich container until testing. Subsequently, each of the previously formed and discussed CMS membranes was tested for gas permeability and gas selectivity. This was accomplished by constructing a custom annular permeation cell "module." The annular cell had a five-inch outer diameter, a three-inch inner diameter, four half-inch wide openings with 9 / 16-inch O-ring fittings in the wall, and a quarter-inch thick cover with O-ring seals on both sides. The O-rings were provided by SAE / MS.

[0092] For the hollow fibers, two fibers from the above examples were inserted into a half-inch wide opening in the wall of the annular unit. Teflon tape was used to create a cofferdam around the fibers in the hole. Epoxy resin (Scotch Weld DP100) was used to fill the space around the hollow fiber hole and form a seal.

[0093] For the microcapillary membrane, a similar custom-made ring permeation unit "module" was used. One end of the microcapillary membrane was inserted into one of the half-inch-wide openings. Similar to the hollow fiber module, a dam was then formed using a Gorilla Universal Putty epoxy stick. Scotch Weld DP100 epoxy was used to fill the top of the Gorilla Putty epoxy and seal the other end of the microcapillary membrane.

[0094] The module was used to test mixed gas permeation. The mixed gas was first cleaned by passing through an activated carbon guard bed and then fed into a reservoir within the annular unit. The feed was an equal molecular weight 52 psi gauge carbon dioxide / hydrogen feed to reflect the feed generated from hydrogen fuel gas. The permeation rate was used to calculate the permeability, normalized by the transmembrane pressure difference and the total membrane surface area. For microcapillary membranes, the membrane area is the product of the unsealed membrane length, width, and 2 (each microcapillary membrane has two surfaces). The unit of permeability is GPU: 1×10-6cm3(STP) / (s.cm2.cm Hg). The results of the permeation tests are shown in Tables 1 and 2 below.

[0095] Table 1: Hollow fiber permeation test results

[0096]

[0097] As shown in Table 1 above, fibers 1-3b were pyrolyzed at 600°C and subsequently annealed at 900°C under a nitrogen purge. The hollow fibers were then oxidized at a rate of 3°C / min for 8 hours. The oxidation temperatures are shown in Table 1 above. For fibers 1, 2, and 3, ten fibers, three fibers, and three fibers were formed per batch, respectively. As shown in Table 1 above, only fiber 2a, oxidized at 275°C, showed preferred results, with fiber 2a exhibiting sufficient selectivity and carbon dioxide permeability, but also increasing hydrogen permeability. For fibers 1, 3a, and 3b, the fibers did not exhibit sufficient selectivity or fragmented during most of the formation period. Figure 3 Scatter plots of carbon dioxide permeability (x-axis) versus carbon dioxide-hydrogen selectivity are shown for each of the aforementioned fibers and the microcapillary membrane described below. Figure 3 As shown, PVDC microcapillary membranes exhibit the best compromise between high carbon dioxide permeability while also achieving sufficient selectivity.

[0098] Table 2: Microcapillary membrane permeation test results

[0099]

[0100] As shown in Table 2 above, the membranes were pyrolyzed at 600°C and subsequently annealed under nitrogen purge at 900°C (membranes 1-4b), 1200°C (membranes 5-9), and 1500°C (membranes 10-13). The hollow fibers were then oxidized at a rate of 3°C / min for 8 hours. The oxidation temperatures are shown in Table 1 above. For membranes 1-13, three membranes were formed per oxidation batch.

[0101] As shown in Table 2 above, for all membranes, an increase in carbon dioxide permeability was observed with increasing oxidation temperature. These same trends were not observed for hydrogen permeability, as large hydrogen permeability jumps (and associated decreases in selectivity) were only observed when the oxidation temperature exceeded a given threshold for a given annealing temperature. Specifically, for membranes annealed at 900°C (1-4b), there was an observable drop in selectivity when oxidation increased beyond 325°C, as shown in membranes 4a and 4b. For membranes annealed at 1200°C (5-9), there was an observable drop in selectivity when oxidation increased beyond 350°C, as shown in membrane 9. For membranes annealed at 1500°C (10-13), there was an observable drop in selectivity when oxidation increased beyond 375°C, as shown in membrane 13.

[0102] Without being limited by theory, high hydrogen permeability may not be desirable for the intended separation because hydrogen intended for hydrogen fuel may pass through the membrane, thereby limiting the economic viability of the separation process. For similar reasons, low carbon dioxide permeability may not be desirable because a large amount of carbon dioxide-hydrogen mixture must be separated to make the process economical relative to a comparative amine sweetening separation. Fragmentation results may not be desirable because they may indicate that the process is not easily repeatable, thereby reducing the ability of the CMS membrane to be manufactured on a large scale.

[0103] In order to test the performance consistency of the membrane, four membranes of Example 8 were randomly selected to make four modules for gas permeation testing. Figures 4A to 4C As shown, permeation testing of membrane 8 showed that the membrane achieved permeability and selectivity metrics with standard deviations of less than 8%, indicating that the membranes formed are reproducible and theoretically scalable for commercial use. Brunauer-Emmet-Teller (BET) analysis using a two-dimensional non-local density functional theory model (2D-NLDFT) was performed to determine the micropore distribution of membranes 8a-8d as shown in Figure 1. Figure 5 Specifically, membranes 8a-8d were determined to have an average pore size of approximately 5 angstroms.

[0104] To isolate the effects of oxidation, the previously discussed non-oxidized PVDC microcapillary membrane was also exposed to the same feed and conditions, and the permeation results are shown in Table 3 below.

[0105] Table 3: Non-oxidative permeation test results

[0106]

[0107] For the membranes without oxidation (membranes 14-17), hydrogen permeability was actually observed to slightly exceed carbon dioxide permeability. As a trend, permeability was observed to generally decrease with increasing annealing temperature, likely due to overall reordering and shrinkage of the pore matrix, as discussed previously.

[0108] In order to determine why annealing specifically leads to an effect on the stability of the carbon structure, the absorption behavior and mechanical properties of membranes 14 and 15 (films without oxidation) were analyzed over time. Specifically, membranes 14 and 15 were subjected to carbon dioxide absorption at 50°C and 1000 kPA on three separate occasions, as shown in FIG. Figures 6A to 6B As shown. Between each adsorption test, membranes 14 and 15 were placed in the laboratory atmosphere for at least two days. Before each adsorption test, membranes 14 and 15 were also pretreated using a vacuum treatment at 100°C. Figure 6A As shown, Figure 6BIn contrast, for film 14, the carbon dioxide adsorption capacity of the CMS membranes decreased significantly over time compared to film 15, indicating that the higher temperature annealing resulted in a higher level of graphitization. This can also be demonstrated in Table 4, which shows that as the heat treatment temperature increases, films 14-17 exhibit increased graphitization (lower oxygen content), decreased resistivity, and improved mechanical properties until an observable peak somewhere between 900°C and 1500°C.

[0109] Table 4: Non-oxidized film and graphitization analysis

[0110]

[0111] like Figure 7A and Figure 7B As shown, the oxidation resistance of CMS membranes generally increases with increasing annealing temperature, which is consistent with the above conclusion that annealing reorders, shrinks, and strengthens the pore matrix during the annealing process. Oxidation of membranes is generally considered an exothermic process, which in some cases can lead to matrix destruction, see previous membranes / fibers that broke during oxidation. Therefore, higher annealing temperatures can help slow the oxidation process and reduce the likelihood of fiber / membrane breakage during oxidation. In addition, as previously described with respect to Table 4, the oxygen content of CMS membranes generally decreases with increasing annealing temperature. This trend also occurs in CMS membranes obtained after oxidation, see membranes 4, 8, and 11 above. Without being limited by theory, as the oxygen content decreases, the resulting CMS membrane may also become more hydrophobic due to the removal of polar oxygen species. A more hydrophobic CMS membrane may have the benefit of being less affected by exposure to moisture during the absorption process, which is an additional benefit.

[0112] According to a first aspect, a method for manufacturing a carbon molecular sieve (CMS) membrane may include: forming a copolymer into one or more hollow fibers or one or more microcapillary membranes, the copolymer being selected from one or more of a polyvinylidene chloride (PVDC) copolymer, a polyimide copolymer, a polyetherimide copolymer, a polyacrylonitrile copolymer, and a polyphenylene ether copolymer; pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes at a second temperature of 600° C. to 700° C. using an inert gas or under vacuum; and annealing the one or more hollow fibers or the one or more microcapillary membranes at a third temperature of 900° C. to 1500° C. using an inert gas or under vacuum; and oxidizing the one or more hollow fibers or the one or more microcapillary membranes using air at a fourth temperature of 300° C. to 400° C.

[0113] A second aspect may include the first aspect, and may further include cooling the one or more hollow fibers or the one or more microcapillary membranes to a temperature of less than or equal to 60° C. between pyrolyzing and annealing.

[0114] A third aspect may include any of the previous aspects, wherein the cooling step, the annealing step, or both occur after observing a weight loss of greater than or equal to 70% for the one or more hollow fibers or the one or more microcapillary membranes during the pyrolysis step.

[0115] A fourth aspect may include any of the previous aspects, wherein the copolymer comprises a polyvinylidene chloride (PVDC) copolymer; and the method further comprises pretreating the one or more hollow fibers or the one or more microcapillary membranes by heating at a first temperature of 120°C to 200°C with air, with an inert gas, under vacuum, or a combination thereof prior to pyrolysis.

[0116] A fifth aspect may include any previous aspect, wherein the copolymer comprises a polyimide copolymer.

[0117] A sixth aspect may include any previous aspect, wherein the oxidizing step is stopped when the loss of the one or more hollow fibers or the one or more microcapillary membranes in the oxidizing step is less than 20 weight percent.

[0118] A seventh aspect may include any previous aspect, wherein the CMS membrane comprises: a carbon dioxide permeability of at least 1000 GPU (gas permeation units); and a carbon dioxide / hydrogen selectivity of 50 to 200 at 350 kPa gauge and ambient temperature.

[0119] An eighth aspect may include any previous aspect, wherein the CMS membrane comprises: a carbon dioxide permeability of at least 2000 GPU (gas permeation units); and a carbon dioxide / hydrogen selectivity of 80 to 200 at 350 kPA and ambient temperature.

[0120] A ninth aspect may include any of the previous aspects, wherein the CMS membrane comprises a median pore size of about 4.5 angstroms to about 7 angstroms.

[0121] A tenth aspect may include any previous aspect, wherein the copolymer comprises at least one of the following comonomers: vinyl monomers, vinyl chloride monomers, acrylate monomers, methacrylate monomers, styrene monomers, acrylonitrile, methacrylonitrile, itaconic acid, and pyrolyzed chlorotrifluoroethylene.

[0122] The eleventh aspect may include any previous aspect, wherein the one or more hollow fibers are a first plurality of hollow fibers and a second plurality of hollow fibers; the first plurality of hollow fibers and the second plurality of hollow fibers are arranged in a woven grid structure for use in the pretreatment step, the pyrolysis step, the cooling step, the annealing step, the oxidation step, or a combination thereof.

[0123] A twelfth aspect may comprise a method of separating a gas mixture comprising hydrogen and carbon dioxide using a CMS membrane manufactured according to any of the previous aspects, the method comprising: manufacturing a CMS membrane according to any of the previous aspects; and passing the gas mixture through the CMS membrane to produce a first permeate stream having an increased carbon dioxide concentration and a second retentate stream having an increased hydrogen concentration.

[0124] A thirteenth aspect may include a carbon molecular sieve (CMS) membrane comprising one or more hollow fibers or one or more microcapillary membranes, wherein: the one or more hollow fibers or the one or more microcapillary membranes comprise a copolymer selected from one or more of polyvinylidene chloride (PVDC) copolymers, polyimide copolymers, polyetherimide copolymers, polyacrylonitrile copolymers, and polyphenylene ether copolymers; and the CMS membrane comprises a carbon dioxide permeability of at least 1000 GPU (gas permeation unit) and a carbon dioxide / hydrogen selectivity of 50 to 200 at 350 kPa gauge pressure and ambient temperature.

[0125] A fourteenth aspect may include the membrane of the previous aspect, wherein the one or more hollow fibers include a first plurality of hollow fibers and a second plurality of hollow fibers, and the first plurality of hollow fibers and the second plurality of hollow fibers are arranged in a woven grid structure.

[0126] A fifteenth aspect may include a method for separating a gas mixture comprising hydrogen and carbon dioxide using the CMS membrane of claim 13 or 14, the method comprising flowing the gas mixture through the CMS membrane to produce: a first stream of permeate having an increased carbon dioxide concentration; and a second stream of retentate having an increased hydrogen concentration.

[0127] It should be noted that, unlike statements of intended use, statements in this disclosure that a component of the present disclosure is "operable" or "sufficient" in a particular manner to exhibit a particular characteristic or function in a particular manner are structural statements. More specifically, references in this disclosure to a component being "operable" or "sufficient" in a manner that indicates an existing physical condition of the component are to be considered as explicit statements of a structural feature of the component.

[0128] It should also be noted that reference herein to “at least one” component, element, etc. should not be used to create an inference that alternative use of the article “a” should be limited to a single component, element, etc. The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

[0129] Ranges are provided throughout the disclosure. It is anticipated that each discrete value encompassed by these ranges is also included. Additionally, ranges formed by each discrete value encompassed by the explicitly disclosed ranges are also anticipated.

[0130] It should be noted that the terms "preferably," "generally," and "typically" as used herein are not used to limit the scope of the claimed invention or to imply that certain features are critical, necessary, or even essential to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of the disclosed embodiments or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the disclosed invention.

[0131] It should be noted that one or more of the appended claims utilize the term "wherein" as a transitional expression. For purposes of defining the present invention, it should be noted that this term is introduced in the claims as an open transitional phrase that is used to introduce a recitation of a series of features of the structure and should be interpreted in the same manner as the more commonly used open-ended term "comprising." It should be noted that the use of the terms "having" or "including" or their grammatical variations in this disclosure should also be interpreted in a manner similar to the more commonly used open-ended term "comprising."

[0132] As used in this disclosure, terms such as "first" and "second" are arbitrarily designated and are intended only to distinguish between two or more instances or components. It should be understood that the words "first" and "second" have no other purpose and are not part of the component names or descriptions, nor do they necessarily define the relative positioning, location, or order of the components. Furthermore, it should be understood that the mere use of the terms "first" and "second" does not require the presence of any "third" component, although such a possibility is contemplated within the scope of this disclosure.

[0133] The subject matter of the present embodiments herein has been described in detail and by reference to specific embodiments thereof, it should be noted that the various details disclosed herein should not be construed as implying that these details relate to elements that are essential components of the various embodiments described herein, even when particular elements are shown in each of the drawings accompanying this description. Furthermore, it will be apparent that modifications and variations can be made without departing from the scope of the present embodiments, including but not limited to the embodiments defined in the appended claims. More specifically, although some aspects of the invention are identified herein as preferred or particularly advantageous, it is contemplated that the present embodiments are not necessarily limited to these aspects.

Claims

1. A method for manufacturing a carbon molecular sieve (CMS) membrane, the method comprising: forming a copolymer into one or more hollow fibers or one or more microcapillary membranes, the copolymer being selected from one or more of a polyvinylidene chloride (PVDC) copolymer, a polyimide copolymer, a polyetherimide copolymer, a polyacrylonitrile copolymer, and a polyphenylene ether copolymer; pyrolyzing the one or more hollow fibers or the one or more microcapillary membranes at a second temperature of 600° C. to 700° C. using an inert gas or under vacuum; annealing the one or more hollow fibers or the one or more microcapillary membranes at a third temperature of 900° C. to 1500° C. using an inert gas or under vacuum; and The one or more hollow fibers or the one or more microcapillary membranes are oxidized using air at a fourth temperature of 300°C to 400°C. 2 . The method of claim 1 , further comprising cooling the one or more hollow fibers or the one or more microcapillary membranes to a temperature of less than or equal to 60° C. between pyrolyzing and annealing.

3. The method of any preceding claim, wherein the cooling step, the annealing step, or both occur after observing a weight loss of greater than or equal to 70% for the one or more hollow fibers or the one or more microcapillary membranes during the pyrolysis step.

4. A method according to any preceding claim, wherein: The copolymer comprises a polyvinylidene chloride (PVDC) copolymer; and The method further includes pretreating the one or more hollow fibers or the one or more microcapillary membranes by heating at a first temperature of 120° C. to 200° C. with air, with an inert gas, under vacuum, or a combination thereof prior to pyrolysis.

5. A method according to any preceding claim, wherein the copolymer comprises a polyimide copolymer.

6. The method according to any preceding claim, wherein the oxidation step is stopped when the loss of the one or more hollow fibers or the one or more microcapillary membranes in the oxidation step is less than 20% by weight.

7. The method of any preceding claim, wherein the CMS membrane comprises: a carbon dioxide permeability of at least 1000 GPU (Gas Permeation Unit); and The carbon dioxide / hydrogen selectivity ranges from 50 to 200 at 350 kPa gauge and ambient temperature.

8. The method of any preceding claim, wherein the CMS membrane comprises: a carbon dioxide permeability of at least 2000 GPU (Gas Permeation Unit); and CO2 / H2 selectivity of 80 to 200 at 350 kPA and ambient temperature.

9. The method of any preceding claim, wherein the CMS membrane comprises a median pore size of about 4.5 angstroms to about 7 angstroms.

10. The method of any preceding claim, wherein the copolymer comprises at least one of the following comonomers: vinyl monomers, vinyl chloride monomers, acrylate monomers, methacrylate monomers, styrene monomers, acrylonitrile, methacrylonitrile, itaconic acid, and pyrolyzed chlorotrifluoroethylene.

11. The method according to any one of claims 1 to 10, wherein: The one or more hollow fibers are a first plurality of hollow fibers and a second plurality of hollow fibers; The first plurality of hollow fibers and the second plurality of hollow fibers are arranged in a woven grid structure for use in the pretreatment step, the pyrolysis step, the cooling step, the annealing step, the oxidation step, or a combination thereof.

12. A method for separating a gas mixture comprising hydrogen and carbon dioxide using a CMS membrane manufactured according to any one of claims 1 to 11, the method comprising: Manufacturing a CMS membrane according to any preceding claim; as well as The gas mixture is passed through the CMS membrane to produce a permeate first stream having an increased carbon dioxide concentration and a retentate second stream having an increased hydrogen concentration.

13. A carbon molecular sieve (CMS) membrane comprising one or more hollow fibers or one or more microcapillary membranes, wherein: The one or more hollow fibers or the one or more microcapillary membranes comprise a copolymer selected from one or more of polyvinylidene chloride (PVDC) copolymers, polyimide copolymers, polyetherimide copolymers, polyacrylonitrile copolymers, and polyphenylene ether copolymers; The CMS membrane comprises a carbon dioxide permeability of at least 1000 GPU (gas permeation units) and a carbon dioxide / hydrogen selectivity of 50 to 200 at 350 kPa gauge pressure and ambient temperature.

14. The membrane of claim 13, wherein: The one or more hollow fibers include a first plurality of hollow fibers and a second plurality of hollow fibers; and The first plurality of hollow fibers and the second plurality of hollow fibers are arranged in a woven grid structure.

15. A method of separating a gas mixture comprising hydrogen and carbon dioxide using a CMS membrane according to claim 13 or 14, the method comprising passing the gas mixture through the CMS membrane to produce: a permeate first stream having an increased carbon dioxide concentration; and A second stream of the retentate has an increased hydrogen concentration.

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