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; 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 carbon dioxide at a fourth temperature of 700 DEG C to 900 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 mixtures 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 normally selective or reversely selective. Normally selective membranes selectively retain larger molecules while allowing smaller molecules to pass through the membrane. Conversely, reversely selective membranes selectively retain smaller molecules while allowing larger molecules to pass through. Therefore, a reversely selective membrane may be required to achieve gas separation of these larger molecules from smaller molecules. For reversely selective membranes, the average pore size is typically 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] Therefore, this paper discusses a method for producing a CMS membrane with the aforementioned benefits. Specifically, a CMS membrane formed according to one or more embodiments herein is provided with reverse selectivity through pyrolysis and oxidation of the copolymer at specific temperature and time thresholds. Furthermore, the CMS membrane according to one or more embodiments herein is provided with reverse selectivity for CO2-hydrogen separation, particularly through an additional annealing step between the pyrolysis and oxidation steps. Furthermore, an optional pretreatment of the copolymer, such as a 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 the 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; 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 carbon dioxide at a fourth temperature of 700° C. to 900° 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 / min.

[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 3A 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;

[0018] Figure 3B is a graphical illustration of three carbon dioxide adsorption isotherms for a microcapillary membrane prepared at an annealing temperature of 900° C. and without oxidation, as in embodiments herein;

[0019] Figure 4A 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;

[0020] Figure 4B As in the embodiments herein Figure 4A Graphical illustration of the effect of oxidation test time on the weight loss of microcapillary membranes;

[0021] Figure 5 is a graphical illustration of the amount of water adsorption at different humidity levels by a microcapillary membrane formed according to embodiments herein;

[0022] Figure 6A is a graphical illustration of the carbon dioxide-hydrogen separation performance, particularly the permeability to carbon dioxide, of membranes prepared according to embodiments herein;

[0023] Figure 6B is a graphical illustration of the carbon dioxide-hydrogen separation performance, particularly hydrogen permeability, of membranes prepared according to embodiments herein;

[0024] Figure 6C is a graphical illustration of the carbon dioxide-hydrogen separation performance of membranes prepared according to embodiments herein, particularly the selectivity of carbon dioxide relative to hydrogen;

[0025] Figure 7A is a graphical illustration of the carbon dioxide-nitrogen separation performance, particularly the permeability to carbon dioxide, of membranes prepared according to embodiments herein;

[0026] Figure 7B is a graphical illustration of the carbon dioxide-nitrogen separation performance, particularly the nitrogen permeability, of membranes prepared according to embodiments herein; and

[0027] Figure 7C is a graphical illustration of the carbon dioxide-nitrogen separation performance, particularly the selectivity of carbon dioxide relative to nitrogen, of membranes prepared according to embodiments herein. DETAILED DESCRIPTION

[0028]

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

[0029] 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.

[0030] 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 measurement is "permeability"

[0031] (P 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:

[0032] 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:

[0033] Finally, "selectivity" is defined herein 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:

[0034] 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, 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 carbon dioxide at a fourth temperature of 700°C to 900°C. Additionally or alternatively, one or more hollow fibers or one or more microcapillary membranes may be oxidized with a carbon dioxide-inert gas mixture.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 resin or molded film, as measured by differential scanning calorimetry (DSC) according to ASTM D3418. In embodiments, this level can range between 30% to 55% or 35% to 50%. Therefore, comprising comonomer generally helps to reduce precursor crystallinity, to ensure desired scope, and also helps to reduce melt temperature, thereby improves the processing properties of resulting copolymer. Generally, comprising monomers with larger volumes can tend to reduce overall copolymer crystallinity to a greater extent than comprising monomers with smaller volumes. Therefore, for example, compared with for example methyl acrylate or ethyl acrylate, butyl acrylate may tend to reduce crystallinity to a greater extent, assuming that methyl acrylate or / and ethyl acrylate are based on final copolymer composition and use with identical molar percentage (mol%).

[0041] One or more hollow fibers or microcapillary membranes may also include additional additives. Additives may include, but are not necessarily limited to, epoxidized oil stabilizers, such as epoxidized soybean oil, epoxidized linseed oil, and diglycidyl ethers of bisphenol A. Liquid plasticizers are also commonly used, such as aliphatic and aromatic esters, including, for example, dibutyl sebacate, acetyltributyl citrate, dioctyl phthalate, and combinations thereof. Other commonly used additives may include lubricants, such as polyethylene wax, paraffin wax, oxidized polyethylene wax, and combinations thereof. Lubricants may optionally be included, and may include, for example, high-density polyethylene, acrylate copolymers, and silicone polymers, and combinations thereof. Another group of additives that may be included is acid scavengers, such as epoxy compounds, magnesium hydroxide, magnesium oxide, tetrasodium pyrophosphate, calcium phosphate, magnesium phosphate, DHT 4A (a synthetic hydrotalcite-like halogen scavenger available from Kyowa Chemical Industry), calcium oxide, calcium carbonate, and combinations thereof. Antioxidants may also be incorporated, such as phenolic resins. Any or all of these types of additives may be included in one or more hollow fibers or one or more microcapillary membranes.

[0042] 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.

[0043] 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.

[0044] 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. As used herein, the term "at least 10% dehydrochlorination" means that the hollow fibers have been pre-treated by removing hydrochloric acid 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 dehydrochlorination level 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 can bear its own weight during further pyrolysis, where the copolymer structure may be further strengthened.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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 embodiments, pyrolysis may result in at least 90% by weight (such as at least 95% by weight or at least 99% by weight) of the copolymer being carbonized. As noted above, pyrolysis is also referred to as “carbonization” because the result is that the copolymer can be converted into a carbon-only or nearly carbon-only skeleton of its copolymer structure (that is, all or nearly all atoms except carbon are removed, but carbon-carbon bonds remain substantially intact), 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.

[0049] 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.

[0050] In embodiments, the inert gas may include carbon dioxide, nitrogen, any noble 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] As previously described, the method may further include oxidizing the one or more hollow fibers or the one or more microcapillary membranes with carbon dioxide at a fourth temperature of 700° C. to 900° C. The fourth temperature may also be 700° C. to 710° C., 710° C. to 750° C., 750° C. to 800° C., 800° C. to 850° C., 850° C. to 890° C., 890° C. to 900° C., or any combination of ranges or smaller ranges therein.

[0057] In embodiments, in the oxidation step, one or more hollow fibers or one or more microcapillary membranes may be substantially free of oxygen molecules. For example, and in embodiments, carbon dioxide may contain less than 5% by weight of oxygen molecules, such as 5% by weight of oxygen molecules to 1% by weight of oxygen molecules, 1% by weight of oxygen molecules to 0.1% by weight of oxygen molecules, 0.1% by weight of oxygen molecules to 0.01% by weight of oxygen molecules, 0.01% by weight of oxygen molecules to 0% by weight of oxygen molecules, or any combination thereof or a smaller range thereof, such as 0.01% by weight of oxygen molecules to 1% by weight of oxygen molecules. Without being limited by theory, the oxygen content of air may affect the oxidation rate of the one or more hollow fibers or the one or more microcapillary membranes. For example, at a higher oxygen content, the oxidation rate of the one or more hollow fibers or the one or more microcapillary membranes may increase accordingly. It is also known that steam and CO2 can be milder oxidants than air. Therefore, without being limited by theory, at a temperature higher than air, steam and / or CO2 can achieve similar levels of oxidation and pore opening.

[0058] 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 a range or a smaller range thereof. However, oxidation can also occur over a larger timeframe / range, such as 10 minutes to 7 days.

[0059] 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 to discharge 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 the hole can result in an overall increase in the permeability of all gaseous substances with a representative molecular diameter less than the 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), which is 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 %.

[0060] 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.

[0061] 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.

[0062] For the aforementioned oxygen and carbon content ranges, it is expected that some degree of oxidation may be required to achieve a specific level of pore opening. Specifically, during oxidation with carbon dioxide, two reactions may occur: the formation of oxygen-containing compounds on the carbon surface and the formation of CO gas. Therefore, the oxygen content of the CMS membrane may be correlated with the degree of pore opening and / or oxidation. In embodiments, the CMS membrane may contain an oxygen content of 5 atomic % to 17 atomic %.

[0063] 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 2As 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.

[0064] 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.

[0065] 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 reject smaller gas molecules from the membrane while accepting (and passing) larger gas molecules.

[0066] 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 or equal to 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.

[0067] 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.

[0068] 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.

[0069] In an embodiment, and at least because one or more hollow fibers or one or more microcapillary membranes utilize carbon dioxide instead of oxygen for oxidation, the one or more hollow fibers or one or more microcapillary membranes (or CMS membranes formed therefrom) after oxidation may include a Henry's adsorption constant for water that is reduced compared to an equivalent hollow fiber, microcapillary membrane, or CMS membrane formed using molecular oxygen as an oxidant. Specifically, as further explained in detail herein, the resulting hollow fibers, microcapillary membranes, and CMS membranes utilizing carbon dioxide as an oxidant may have an oxygen content that is reduced compared to oxygen as an oxidant. 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. This may have considerable benefits because low levels of water vapor are present in the feed in most CO2 capture processes. If water is strongly adsorbed, it may compete with CO2 for adsorption and permeation in the micropores, thereby reducing CO2 separation. For example, and in embodiments, the oxidized one or more hollow fibers or one or more microcapillary membranes (or CMS membranes formed therefrom) may comprise a Henry's adsorption constant for water of 0.25 to 2.5 grams of H2O per 100 grams of hollow fiber or microcapillary membrane at kilopascal pressure, assuming measurement at 35°C and 30% relative humidity.

[0070] As described above, embodiments herein also relate to methods for separating gases from a gas mixture. The gas mixture may comprise 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 an increased concentration of the first gas molecules compared to the second retentate stream, which in turn may have an increased concentration of the second gas molecules compared to the permeate first stream. In this way, the CMS membrane used in this method can be used to separate the first gas molecule and the second gas molecule from each other, and can be reverse selective.

[0071] 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-C8H 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."

[0072] 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).

[0073] As previously described, the mean / median pore size can be determined by gas adsorption. Partially graphitized nanosheets exhibit a slit pore structure. For example, a CMS membrane may have a total pore size distribution wider than 4 to 10 angstroms. In embodiments, a CMS membrane formed according to the methods herein may have a maximum pore size of approximately 10 angstroms.

[0074] 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.

[0075] Example

[0076] Hollow fibers and microcapillary membranes were formed according to embodiments herein for permeation testing, as explained in further detail below.

[0077] Hollow fiber preparation

[0078] Hollow fibers (6FDA hollow fibers) were 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 fibers had an outer diameter and inner diameter of 580 and 440 microns, respectively. The asymmetric hollow fiber wall thickness was also less than 5 microns.

[0079] Hollow Fiber Testing Procedure

[0080] Each hollow fiber in the hollow fiber 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 purged with 2 liters / minute (L / min) of air for crosslinking. No pretreatment was performed on 6FDA / BpDA-DAM hollow fibers or polyimide hollow fibers. The alumina tube bundle containing the fibers was pyrolyzed at an initial temperature of 250°C at a heating rate of 13.3°C / min and then increased at a heating rate of 3.85°C / min to a final pyrolysis temperature of 900°C. The fibers were then annealed at 910°C at a ramp rate of 3.85°C / min, followed by a final annealing temperature of 925°C at a ramp rate of 0.25°C / min, held for 2 hours, and then cooled in a 6" OD quartz tube furnace. An argon purge flow of 300 standard cubic centimeters per minute was used to maintain the furnace free of oxygen.

[0081] 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 300 standard cubic centimeters per minute of carbon dioxide. The furnace was heated at a ramp rate of 1°C per minute to a peak temperature between 700°C and 900°C, and then held for a specified time between 2 hours and 16 hours, as explained in further detail below. 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.

[0082] Microcapillary membrane preparation

[0083] 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.

[0084] 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 temperatures of the three zones of the extruder, elbow, and die were 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.

[0085] 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 from the bath by a winder. The film is stretched by increasing the speed of the winder. Stretching 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.

[0086] Microcapillary membrane testing procedure

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Some of the pyrolyzed or annealed microcapillary membranes were then placed in a quartz tube furnace purged with 300 standard cubic centimeters per minute of carbon dioxide 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 ramp rate of 3°C / min and held at the final temperature for the specified time before cooling. Unless otherwise noted, the resulting CMS membranes were stored in a nitrogen box and then fabricated into modules for permeation testing.

[0091] Penetration and selectivity testing

[0092] As previously described, each of the CMS membranes was then stored in a nitrogen-rich container until testing. Each of the previously formed and discussed CMS membranes was then tested for gas permeability and 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.

[0093] For hollow fibers, the two ends of the fiber according to the above example were inserted into two opposing half-inch wide openings on the wall of the annular unit. Teflon tape was used to create a cofferdam around the fiber in the hole. Epoxy resin (Scotch Weld DP100) was used to fill the space around the hollow fiber hole and form a seal.

[0094] For the microcapillary membrane, a similar custom-made ring osmosis 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 rod. Scotch Weld DP100 epoxy was then used to fill the top of the Gorilla Putty epoxy and seal the other end of the microcapillary membrane.

[0095] The assembly was used to perform mixed gas permeation tests. 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 permeation rate, 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 permeation is GPU: 1×10-6 cm3(STP) / (s.cm2.cm Hg). The results of the permeation tests are shown in Tables 1 and 2 below.

[0096] Table 1: Hollow fiber permeation test results

[0097]

[0098]

[0099] As shown in Table 1 above, an increase in carbon dioxide permeability was also observed with increasing oxidation time. However, for selectivity, a local maximum was observed for approximately 4 hours of oxidation time, for fiber 2. Without being limited by theory, this local maximum may be due to excessive oxidation and enlargement of the micropores of the fiber at 6 hours, resulting in increased hydrogen permeability.

[0100] Table 2: Microcapillary membrane permeation test results

[0101]

[0102] As shown in Table 2 above, for all membranes, an increase in carbon dioxide permeability with increasing oxidation temperature was observed. These same trends were also observed in hydrogen permeability.

[0103] Without being limited by theory, low overall permeance may not be desirable because a large volume of the carbon dioxide-hydrogen mixture needs to be separated to make the process economical relative to a comparative amine sweetening separation.

[0104] To determine why annealing specifically affects CO2-H2 selectivity, the samples annealed at 600 °C ( Figure 3A ) and 900℃ annealing ( Figure 3B ) were analyzed for their absorption behavior and mechanical properties over time. Specifically, Figure 3A and Figure 3B The microcapillary membranes in were subjected to carbon dioxide absorption at 50°C and 1000 kPa on three separate occasions. Between each adsorption test, the membranes were also placed in laboratory atmosphere for at least two days. Before each adsorption test, a vacuum treatment at 100°C was also applied to the membranes. Figure 3A As shown, Figure 3B In contrast, the carbon dioxide adsorption capacity of the CMS membranes decreases significantly over time for 600°C annealing relative to 900°C annealing, indicating that higher temperature annealing leads to a higher level of stability, likely due to a higher level of graphitization. This can also be demonstrated in Table 3, which shows that as the annealing temperature increases, membranes 4a-4c exhibit increased graphitization (lower oxygen content) and improved mechanical properties, until an observable peak somewhere between 900°C and 1500°C.

[0105] Table 4: Non-oxidized film and graphitization analysis

[0106]

[0107] like Figure 4A and Figure 4B As shown, the oxidation resistance of CMS membranes generally increases with increasing annealing temperature, which is consistent with the aforementioned conclusion that annealing reorders, shrinks, and strengthens the pore matrix during annealing. Without being limited by theory, membrane oxidation is generally considered an etching process, which may lead to matrix destruction or fragmentation in some cases. Therefore, higher annealing temperatures may help slow the oxidation process and reduce the likelihood of fiber / membrane fragmentation during oxidation.

[0108] Carbon oxidation by CO2 is an endothermic process. On the other hand, carbon oxidation by oxygen (air) is exothermic, which can produce a runaway reaction, which complicates the control of the oxidation level. Using CO2 as an oxidant and annealing the carbon at high temperatures are two ways to slow the oxidation rate so that it is easier to control. This may have some benefits in scaling up the process for commercial use, as the resulting CMS membrane can be minimally wasted to prevent CMS membrane breakage due to the exothermic nature (and runaway potential) of the oxidation process, while maximizing the overall permeability of the CMS membrane itself.

[0109] Furthermore, as previously discussed with respect to Table 4, the oxygen content of CMS membranes generally decreases with increasing annealing temperature, until a theoretical limit is reached. 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 additional benefit of being less susceptible to exposure to moisture during the absorption process. Furthermore, for CMS membranes using carbon dioxide rather than oxygen as the oxidant, the oxygen content can be further reduced, resulting in an additional degree of hydrophobicity. This is illustrated in Table 5 below, which shows the oxygen content of various membranes produced in the Examples herein.

[0110] Table 5: Comparison of oxygen content of membranes

[0111]

[0112] As shown in Table 5 above, membrane 5b (2467GPU, 84 selectivity) subjected to air oxidation at 350°C and 8 hours showed similar performance to membrane 2e (2810GPU, 51 selectivity) subjected to CO2 oxidation at 800°C and 8 hours. Therefore, it is expected that similar levels of pore enlargement may be achieved in the two membranes. The relative increase in the hydrophobicity of the CMS membrane is shown in Table 6 below. Specifically, membrane 2e exhibits a reduced Henry adsorption constant compared to membrane 5b subjected to air oxidation. A relative humidity of 30% was assumed in the measurement of the Henry adsorption constant. The lower water adsorption constant of membrane 2e means less interaction of water with the carbon membrane. As Figure 5 As shown, membrane 2e exhibits a slight capacity hysteresis during adsorption (solid line) and desorption (dashed line) when the relative humidity approaches 40%, indicating that the adsorption of water vapor on the membrane and its subsequent effect on CO2 permeability are reversible.

[0113] Table 6: Henry adsorption constants of various membranes

[0114]

[0115] In practical carbon dioxide capture and hydrogen separation applications (both before and after combustion), water vapor is present in the feed stream. While dryers can be installed in the process to remove water vapor, they add significant cost. Therefore, a more hydrophobic CMS membrane, such as that formed according to the embodiments herein, can reduce the reliance on such dryers and result in a significant cost reduction in the process.

[0116] To determine the long-term stability of hollow fiber and / or microcapillary membranes formed according to embodiments herein, various membranes were subjected to permeability testing for periods ranging from 70 hours to 150 hours. The results for membrane 1e and CO2 / H2 separation are shown in FIG. Figures 6A to 6C As shown, and the results for membrane 2d and CO2 / N2 separation are as follows 7A to 7C As shown. Figures 6A to 7C As shown, the membrane exhibited consistent permeability and preferential selectivity for carbon dioxide over the entire time range.

[0117] 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; 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 700° C. to 900° C.

[0118] 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.

[0119] 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.

[0120] A fourth aspect may include any previous aspect, wherein the one or more hollow fibers or the one or more microcapillary membranes after oxidation comprise a Henry's adsorption constant for water of 0.25 to 2.5 grams of H2O per 100 grams of hollow fiber or microcapillary membrane per kilopascal pressure.

[0121] A fifth aspect may include any previous aspect, 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.

[0122] A sixth aspect may include any previous aspect, wherein the copolymer comprises a polyimide copolymer.

[0123] A seventh 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.

[0124] An eighth 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.

[0125] A ninth aspect may include any of the previous aspects, 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 gauge and ambient temperature.

[0126] 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.

[0127] 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; and the first plurality of hollow fibers and the second plurality of hollow fibers are 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.

[0128] A twelfth aspect may include a method for 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.

[0129] 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.

[0130] 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.

[0131] A fifteenth aspect may include a method for separating a gas mixture comprising hydrogen and carbon dioxide utilizing 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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."

[0137] 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.

[0138] 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; as well as The one or more hollow fibers or the one or more microcapillary membranes are oxidized with carbon dioxide at a fourth temperature of 700°C to 900°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 a cooling step, an annealing step, or both, occurs 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. The method of any preceding claim, wherein the one or more hollow fibers or the one or more microcapillary membranes after oxidation comprise a Henry's adsorption constant for water of 0.25 to 2.5 grams of H2O per 100 grams of hollow fiber or microcapillary membrane at kilopascal pressure.

5. A method according to any preceding claim, wherein: The copolymer includes 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.

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

7. 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.

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

9. The method of any preceding claim, wherein the CMS membrane comprises: A carbon dioxide permeability of at least 2000 GPU (Gas Permeation Unit); and The carbon dioxide / hydrogen selectivity ranges from 80 to 200 at 350 kPa gauge and ambient temperature.

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; and 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 12, 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 for separating a gas mixture comprising hydrogen and carbon dioxide using a CMS membrane according to claim 13 or 14, the method comprising flowing 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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