Self-supporting carbon molecular sieve membranes and methods of use thereof

Abstract

A method of making a self-supporting carbon molecular sieve (CMS) membrane may include: forming a polyvinylidene chloride (PVDC) copolymer into one or more hollow fiber or microcapillary membranes; pretreating the one or more hollow fibers or the microcapillary membrane by heating with air, an inert gas, or both at a first temperature of 120 DEG C to 200 DEG C; pyrolyzing the one or more hollow fibers or the microcapillary membrane with the inert gas at a second temperature of 600 DEG C to 1100 DEG C; and oxidizing the one or more hollow fibers or the microcapillary membrane with air at a third temperature of 300 DEG C to 500 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 gas separation, and methods for separating gases from gas mixtures by passing the gas mixtures through carbon membranes, as detailed herein. Background Art

[0004] Carbon molecular sieves (CMS) and CMS membranes are currently used to separate gases. 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 usually maintains at least some porosity in the pyrolysis products in the form of micropores. Subsequently, the CMS formed in this way can be used in traditional gas separation equipment (such as packed beds, chromatographic columns, etc.) that adopt 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 used alternately for separation according to, for example, traditional pressure swing adsorption methods or temperature swing adsorption methods. CMS membranes are also used to separate gases by causing a gas mixture to flow through the CMS membrane.

[0005] The use of CMS to perform 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. Previously, polyvinylidene chloride copolymers have been pyrolyzed to form CMS membranes, but have a tendency to form large pores. Lamond TG et al., "Research on the Properties of SARAN-type Carbons" Molecular sieve characteristics ( In another example, a CMS membrane is formed using a polyvinylidene chloride copolymer that is pyrolyzed while being supported on a macroporous carbon substrate. T. A. Centeno et al., "Molecular sieve gas separation membranes based on poly(vinylidene chloride-co-vinyl chloride)," Carbon (2000) 38, 1067-1073. The selectivity is described as being particularly high for the O2 / N2 system, resulting in a micropore size of 0. With N2 However, Centeno’s CMS membranes are prepared by at least partially melting the PV DC copolymer prior to pyrolysis, which means that the CMS structures cannot be prepared in an unsupported form. Summary of the Invention

[0006] 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, reversely selective membranes may be required to achieve gas separation of these larger molecules from smaller molecules.

[0007] For reverse selective membranes, their average pore size is typically larger than that of larger molecules. Larger molecules are more strongly adsorbed in the micropores and prevent the adsorption / permeation of smaller molecules. Selectivity depends on the micropore size and the adsorbate-adsorbent surface interaction. However, the formation of larger pores during the production of the membrane may negatively affect stability. If the mechanical stability of the membrane drops below a certain threshold, the membrane may fail (i.e., break / break). To address this limitation, reverse selective PVDC CMS membranes have previously been prepared by inert pyrolysis of PVDC membranes deposited on porous inorganic supports (graphite or alumina). However, as the process scales up, these individual supports become too expensive, and membranes with very low reverse selectivity have previously been produced, see TACenteno et al., as previously described. Previously, the present inventors have found that self-supporting PVDC CMS membranes can be prepared by inert pyrolysis to reduce costs. However, these self-supporting PVDC CMS membranes have normal selectivity (smaller molecules penetrate faster than larger molecules).

[0008] Therefore, methods for producing reverse selective CMS membranes that do not require supports and provide high reverse selectivity to achieve gas separation are desired. These CMS membranes without supports may also be referred to as self-supporting CMS membranes. Therefore, methods for producing CMS membranes having the above benefits are discussed herein. In particular, the CMS membranes formed according to one or more embodiments of the present invention are made self-supporting and reverse selective by pretreatment of the polyvinylidene chloride copolymer, pyrolysis of the pretreatment product, and oxidation of the pyrolysis product at specific temperature and time thresholds. Pretreatment of the polyvinylidene chloride copolymer can increase its melting temperature, at least partially stabilize the copolymer during subsequent pyrolysis and prevent it from collapsing or melting. Pyrolysis further increases the degree of carbonization of the polyvinylidene chloride copolymer, preventing damage to the polyvinylidene chloride copolymer during oxidation when carbon atoms are expelled from the CMS membrane as CO2 / CO and form oxygenates on the surface of the CMS membrane.

[0009] According to one embodiment, a method of manufacturing a self-supporting carbon molecular sieve (CMS) membrane may include: forming a polyvinylidene chloride (PVDC) copolymer into one or more hollow fibers or microcapillary membranes; pretreating the one or more hollow fibers or the microcapillary membranes by heating with air, an inert gas, or both at a first temperature of 120°C to 200°C; pyrolyzing the one or more hollow fibers or the microcapillary membranes with the inert gas at a second temperature of 600°C to 1100°C; and oxidizing the one or more hollow fibers or the microcapillary membranes with air at a third temperature of 300°C to 500°C.

[0010] According to another embodiment, a method for separating a gas from a gas mixture may include: manufacturing a self-supporting CMS membrane according to the previous embodiment; and flowing the gas mixture through the self-supporting CMS membrane to produce a permeate first material stream having an increased concentration of first gas molecules and a second permeate material stream having an increased concentration of second gas molecules, wherein the second gas molecules have a smaller representative molecular diameter than the first gas molecules.

[0011] 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

[0012] The following detailed description of specific embodiments of the present disclosure is best understood when read in conjunction with the accompanying drawings, in which:

[0013] Figure 1showing a graph of pore size versus observed incremental area for CMS membranes formed without and with oxidation, as in embodiments herein;

[0014] Figure 2A A graph showing weight percent during oxidation of hollow fibers according to embodiments herein at 350°C;

[0015] Figure 2B a graph showing weight percent during oxidation of another hollow fiber at 400° C. according to embodiments herein;

[0016] Figure 2C a graph showing weight percent during oxidation of another hollow fiber at 450° C. according to embodiments herein; and

[0017] Figure 3 Shown is a comparison of the oxygen content to the carbon content of a plurality of hollow fibers according to embodiments herein. DETAILED DESCRIPTION

[0018]

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

[0019] As used herein, the term "C# hydrocarbons" (where "#" is a positive integer) is intended to describe all hydrocarbons having # carbon atoms. Additionally, the term "C#+ hydrocarbons" is intended to describe all hydrocarbon molecules having # or more carbon atoms. Thus, the term "C 2+ "Hydrocarbon" is intended to describe a mixture of hydrocarbons having 2 or more carbon atoms. The term "C 2+ "Alkanes" therefore relate to alkanes having 2 or more carbon atoms. The term "C1-C8 hydrocarbons" is intended to describe mixtures of hydrocarbons having from 1 to 8 carbon atoms (inclusive).

[0020] 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.

[0021] 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 (ni ) 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:

[0022] 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:

[0023] 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:

[0024] As previously described, embodiments herein relate to methods for manufacturing self-supporting carbon molecular sieve (CMS) membranes and methods for utilizing self-supporting CMS membranes. The method may initially include forming a polyvinylidene chloride (PVDC) copolymer into a hollow fiber or microcapillary membrane. The method may also include pretreating one or more hollow fibers or microcapillary membranes by heating at a first temperature of 120°C to 200°C using air, an inert gas, or a combination thereof. The method may also include pyrolyzing one or more hollow fibers or microcapillary membranes at a second temperature of 600°C to 1100°C using an inert gas. The method may also include oxidizing one or more hollow fibers or microcapillary membranes using air at a third temperature of 300°C to 500°C.

[0025] As previously described, the method may initially include forming a polyvinylidene chloride (PVDC) copolymer into a hollow fiber or microcapillary membrane. In an embodiment, the PVDC copolymer can be formed by copolymerization of a vinylidene chloride copolymer with 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.) that ensures that thermal degradation of all PVDC components in the PVDC component is avoided.

[0026] The polyvinylidene chloride may have the general formula I, where n is an integer from 1 to 1000:

[0027]

[0028] As previously described, the polyvinylidene chloride copolymer can include vinylidene chloride and 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, the polyvinylidene chloride 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 thus 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.

[0029] After copolymerization, the polyvinylidene chloride copolymer can be formed into one or more hollow fibers or microcapillary membranes by any suitable method known in the art. For example, PVDC can be melt-extruded or solution-spun so that PVDC is formed into hollow fibers. Fibers can be produced by uniaxial stretching using known fiber processes for PVDC copolymers, and the fibers can be round or shaped hollow fibers, or any other desired fiber morphology. Microcapillary membranes can be produced by biaxial stretching using known membrane processes for PVDC copolymers. It is also conceivable that precursor films and / or fibers can be coextruded with multiple PVDC copolymers and / or other polymers.

[0030] 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.

[0031] 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%).

[0032] 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 frequently 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 lubricants 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 fiber or microcapillary membranes.

[0033] In an embodiment, the total amount of all additives combined can be no more than 15% by weight of the one or more hollow fibers or 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 can be typical, so their use ranges from 2% to 8% by weight or 2% to 3% by weight of the one or more hollow fibers or 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.

[0034] 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.

[0035] As previously described, the method may further comprise pre-treating the one or more hollow fibers or microcapillary membranes by heating at a first temperature of 120° C. to 200° C. using air, an inert gas, or both. In embodiments, pre-treating the one or more hollow fibers or microcapillary membranes may serve to stabilize or “lock” the copolymer structure prior to pyrolysis / carbonization thereof. In this step, the one or more hollow fibers or microcapillary membranes may typically be heated to below the melting temperature of the PVDC in order to dehydrochlorinate the fibers to an extent 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 hydrogen chloride to a point at which the PVDC copolymer hollow fibers no longer melt and may, in fact, 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 this point. In embodiments, this "locking" of the copolymer structure can prevent further deformation or bending during the pyrolysis and oxidation steps following pretreatment. In other words, due at least in part to the fact that the copolymer is no longer molten, the copolymer can be considered self-supporting, i.e., the copolymer can bear its own weight during further pyrolysis, wherein the copolymer structure can be further strengthened.

[0036] The first temperature can also be any temperature range from 120° C. to 200° C. For example, the one or more hollow fibers or microcapillary membranes can 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 therein or smaller. In embodiments, heating at any of these temperatures can crosslink the interior of the one or more hollow fibers or microcapillary membranes in addition to dehydrochlorination.

[0037] In an embodiment, one or more hollow fibers or microcapillary membranes can be pretreated for a period of 2 hours to 48 hours, such as 2 hours to 5 hours, 5 hours to 12 hours, 12 hours to 18 hours, 18 hours to 22 hours, 22 hours to 24 hours, 24 hours to 26 hours, 26 hours to 36 hours, 36 hours to 42 hours, 42 hours to 48 hours, or any combination of ranges or smaller ranges therein, such as 22 hours to 26 hours.

[0038] Pretreating the one or more hollow fibers or microcapillary membranes can also include contacting the one or more hollow fibers or microcapillary membranes with air, an inert gas, or both. Contacting the one or more hollow fibers or microcapillary membranes with air or an inert gas can 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.

[0039] As previously described, the method may further comprise pyrolyzing one or more hollow fibers or microcapillary membranes at a second temperature of 600° C. to 1100° C. using an inert gas. In embodiments, pyrolysis may result in at least 90% by weight (e.g., 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, as a result, the copolymer is converted into a carbon-only or nearly carbon-only skeleton of its copolymer structure (i.e., all or nearly all atoms other than carbon are removed, but carbon-carbon bonds remain substantially intact), and the one or more hollow fibers or microcapillary membranes may now be referred to as “carbon-containing”. Pyrolysis may be performed using any means generally known to those skilled in the art.

[0040] The second temperature may also be any narrower temperature range within 600° C. to 1100° C. For example, the one or more hollow fibers or microcapillary membranes may be pyrolyzed at a second temperature of 600° C. to 650° C., 650° C. to 700° C., 700° C. to 800° C., 800° C. to 850° C., 850° C. to 900° C., 900° C. to 950° C., 950° C. to 1000° C., 1000° C. to 1100° C., or any combination of ranges or smaller ranges therein, such as 900° C. to 1100° C. or 600° C. to 900° C. As previously described, pyrolyzing the one or more hollow fibers or microcapillary membranes at the second temperature may also include contacting the one or more hollow fibers or microcapillary membranes with an inert gas.

[0041] In embodiments, 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 microcapillary membranes with the inert gas may be performed at a rate sufficient to purge the pyrolysis gas products, thereby preventing side reactions of the dehydrochlorination gas products on the carbon surface.

[0042] In an embodiment, the one or more hollow fibers or microcapillary membranes can be pyrolyzed at the second temperature for a period of 10 minutes to 48 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 3 hours, 3 hours to 4 hours, 4 hours to 12 hours, 12 hours to 24 hours, 24 hours to 48 hours, or any combination of ranges or smaller ranges therein, such as 1.5 hours to 2.5 hours or 10 minutes to 4 hours.

[0043] As previously described, the method may further include oxidizing the one or more hollow fibers or microcapillary membranes with air at a third temperature of 300° C. to 500° C., such as greater than 400° C. to 500° C. The third temperature may also be 300° C. to 310° C., 310° C. to 350° C., 350° C. to 375° C., 375° C. to 400° C., 400° C. to 405° C., 405° C. to 410° C., 410° C. to 425° C., 425° C. to 450° C., 450° C. to 470° C., 470° C. to 490° C., 490° C. to 500° C., or any combination of ranges or smaller ranges therein.

[0044] 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 therein or smaller ranges (such as approximately 20 wt %) oxygen. Without being limited by theory, the oxygen content of air can affect the oxidation rate of one or more hollow fibers or microcapillary membranes. For example, at higher oxygen content, the oxidation rate of one or more hollow fibers or microcapillary membranes can correspondingly increase. 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 achieve similar levels of oxidation and open pores.

[0045] In an embodiment, one or more hollow fibers or microcapillary membranes can be oxidized at the third temperature for 10 minutes to 12 hours, such as 10 minutes to 30 minutes, 30 minutes to 1 hour, 1 hour to 2 hours, 2 hours to 3 hours, 3 hours to 4 hours, 4 hours to 8 hours, 8 hours to 10 hours, 10 hours to about 11.5 hours, about 11.5 hours to 12 hours, or any combination of ranges therein or smaller ranges (such as 4 hours to 12 hours or about 8 hours). However, oxidation can also occur over a larger timeframe / range, such as 10 minutes to 7 days.

[0046] Without being limited by theory, it is contemplated that oxidation of one or more hollow fibers or microcapillary membranes can be used to increase their pore volume. In particular, oxidation can be used to expel carbon atoms (such as in the form of CO and / or CO2 gas) from one or more hollow fibers or microcapillary membranes along the edges of previously created pore walls, thereby enlarging them. The enlargement of the pores can result in an overall increase in permeability for all gas species having a molecular diameter smaller than the representative molecular diameter of the pore size. However, oxidation can also result in a relatively greater increase in permeability for hydrocarbon species and non-hydrocarbon gases, primarily due to gas absorption effects within the pores of the one or more hollow fibers or microcapillary membranes.

[0047] As explained in further detail below, pyrolyzing the one or more hollow fiber or microcapillary membranes at a temperature of 900° C. or greater can be used to carbonize and / or harden the CMS membrane, thereby making it more resistant to typical carbon expulsion during oxidation. Without being limited by theory, this increased resistance of the one or more hollow fiber or microcapillary membranes can be used to focus carbon expulsion around the pores, rather than on the support structure of the one or more hollow fiber or microcapillary membranes. The result can be an enlargement of the pore throats during subsequent oxidation without breaking the one or more hollow fiber or microcapillary membranes.

[0048] In embodiments, the method may further include, after oxidizing the one or more hollow fibers or microcapillary membranes, pyrolyzing the one or more hollow fibers or microcapillary membranes again at a second temperature using an inert gas. Without being limited by theory, the pyrolysis after oxidation may be used to further shrink and adjust pore size. Furthermore, the pyrolysis after oxidation may potentially produce a surface with fewer oxygenates through inert thermal decomposition of oxygenate species.

[0049] As previously discussed, the CMS membrane formed according to the method 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 carbon molecular sieve membrane. The CMS membrane formed according to the method herein can have an oxygen content of 5 wt % to 17 wt %, such as 5 wt % to 7 wt %, 7 wt % to 9 wt %, 9 wt % to 12 wt %, 12 wt % to 16 wt %, 16 wt % to 17 wt % or any combination of ranges or smaller ranges therein. The CMS membrane formed according to the method herein can also have a carbon content of 82 wt % to 94 wt %, such as 82 wt % to 83 wt %, 83 wt % to 86 wt %, 86 wt % to 90 wt %, 90 wt % to 92 wt %, 92 wt % to 94 wt % or any combination of ranges or smaller ranges therein.

[0050] 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 open pores. Specifically, during oxidation, two reactions may occur: the formation of oxygenates on the carbon surface and the formation of CO / CO2 gas. Therefore, the oxygen content of a CMS membrane can be correlated with the degree of open pores and / or the degree of oxidation.

[0051] 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.

[0052] In an embodiment, in an environment of at least propylene and hydrogen, the CMS membranes herein may have a propylene (C3H6) permeance of 280 GPU to 3000 GPU, a hydrogen permeance of 3 GPU to 36 GPU, and a propylene / hydrogen selectivity of 40 to 260. In an environment of at least n-butane and hydrogen, the CMS membranes herein may have an n-butane (C4H6) permeance of 700 GPU to 3800 GPU.10 ) permeability, a hydrogen permeability of 6 GPU to 21 GPU, and an n-butane / hydrogen selectivity of 90 to 420. In addition, in at least a propylene and methane environment, the CMS membrane herein may have a propylene permeability of at least greater than 1000 (such as 2000 GPU to 2400 GPU or about 2170 GPU), a methane permeability of 30 GPU to 60 GPU (such as about 47 GPU), and a propylene / methane selectivity of 40 to 80. In at least a propylene and nitrogen environment, the CMS membrane herein may have a propylene permeability of 1600 GPU to 2200 GPU (such as about 2000 GPU), a nitrogen permeability of 10 GPU to 18 GPU (such as about 14 GPU), and a propylene / nitrogen selectivity of 80 to 200.

[0053] Without being limited by theory, permeability and selectivity may be temperature dependent. Thus, the permeability and selectivity described above can be understood as occurring at 35°C, and therefore can also be understood to vary with respect to different temperatures. In particular, it is expected that if measured at a temperature below 35°C, the permeability can be understood to increase, and if measured at a temperature above 35°C, the permeability can be understood to decrease. This may or may not be associated with an increase or decrease in the reverse selectivity of the membrane.

[0054] Without being limited by theory, the above selectivity allows the CMS membrane to preferentially separate heavier propylene and n-butane gas molecules from lighter hydrogen, methane, and nitrogen molecules. It is expected that this behavior can also be extended to other heavy and light gas molecule combinations, such that the CMS membrane can preferentially separate heavier hydrocarbon gas molecules (those with a carbon content equal to or greater than 2, i.e., C 2+ hydrocarbons) from lighter gas molecules, including but not limited to hydrogen, carbon dioxide, carbon monoxide, nitrogen, oxygen or methane.

[0055] As described above, embodiments herein also relate to methods for separating gases from a gas mixture. The gas mixture may contain a first gas molecule and a second gas molecule. The second gas molecule may have a smaller representative molecular diameter than the first gas molecule. The method may include forming a CMS membrane, and flowing the gas mixture through the CMS membrane to produce a permeate first stream and a second retentate stream. The CMS membrane used in the 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 the method can be used to separate the first gas molecules and the second gas molecules from each other, and can be reverse selective.

[0056] 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 measuring molecular size have been developed. A 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, New York 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 of the following molecules: He (2.6 angstroms, ), H2 N2 CO2 CH4 C2H4 C3H8 i-C4H 10 SF6 (sulfur hexafluoride) and 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, for example, 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."

[0057] In an embodiment, the CMS membrane may have an average pore size that is larger than the representative molecular diameter of the first gas molecule. The average 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 pore size of less than about 5 angstroms or less than about 4.3 angstroms. The CMS membrane can have an average pore size of about 3.9 angstroms to about 4.0 angstroms, about 4.0 angstroms to 4.1 angstroms, about 4.1 angstroms to about 4.3 angstroms, about 4.3 angstroms to about 4.5 angstroms, about 4.5 angstroms to about 4.7 angstroms, about 4.7 angstroms to about 4.9 angstroms, about 4.9 angstroms to about 5 angstroms, or any combination of ranges therein or smaller ranges thereof (such as about 4.0 angstroms to about 4.3 angstroms). The average pore size can be determined by gas adsorption. For example, as Figure 1 As shown, the CMS membrane may have a broader total pore size distribution than 3.9 angstroms to 5 angstroms. In embodiments, the CMS membrane formed according to the methods herein may have a maximum pore size of about 9 angstroms.

[0058] 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.

[0059] In embodiments, the gas mixture may include olefins, paraffins, or both. The gas mixture may also include carbon dioxide, nitrogen, carbon monoxide, methane, nitrogen, ethane, propane, ethylene, propylene, butane, butene, or combinations thereof. In other words, the gas mixture may include C 2+ Hydrocarbons. The first gas molecule and the second gas molecule can include any of the aforementioned gases, as long as the second gas molecules each have a smaller representative molecular diameter than the first gas molecule. For example, in an embodiment where the first gas molecule is propylene, the second gas molecule can be hydrogen, carbon dioxide, carbon monoxide, nitrogen, oxygen, methane, or a combination thereof, i.e., any gas molecule or combination of gas molecules, each of which has a smaller representative molecular diameter than propylene. Similarly, if the first gas molecule is butane, the second gas molecule can be hydrogen, carbon dioxide, carbon monoxide, nitrogen, oxygen, methane, or a combination thereof.

[0060] Example

[0061] According to embodiments herein, hollow fibers and microcapillary membranes were formed by melt extrusion of a PVDC copolymer of vinyl chloride obtained from Asahi Kasei. The properties of the hollow fibers are shown in Table 1 below.

[0062] Table 1 :Hollow fiber characteristics

[0063]

[0064]

[0065] Microcapillary films were extruded using commercial PVDC obtained from SK Global SARAN (SARAN 711) according to the following procedure. The microcapillary film die had a simple two-piece design with a two-inch-wide air manifold insert containing 42 parallel hollow pins located near the die outlet and used to introduce air into the PVDC copolymer melt, thereby forming the microcapillaries. The extruder pump rate and air flow rate were adjusted to achieve the desired microcapillary diameter.

[0066] 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 also periodically fed into the extruder to periodically rinse off accumulated char.

[0067] Upon extrusion from the die, the microcapillary film is quenched into a deep, room-temperature water bath, where it is wound around guide rollers at the bottom of the bath and then pulled from the bath using a winder. The film is then stretched by increasing the winder speed. Stretching occurs near the die exit, reducing both film thickness and film width. The stretched microcapillary film is then cut into approximately 3-foot strips and left flat under atmospheric conditions for a week to fully crystallize.

[0068] Subsequently, CMS membranes were formed from the hollow fibers and microcapillary membranes according to embodiments herein. The resulting membranes were used for permeation testing, as described in further detail in Example Sets 1 and 2 below.

[0069] Example Group 1 :

[0070] Manufacturing of CMS membranes

[0071] The CMS membrane using hollow fibers is formed by passing hollow fibers of about 9 inches in length through alumina tubes (1 fiber per tube). Alumina tubes are used to keep the CMS fibers straight and avoid sintering and bending during pyrolysis. The hollow fibers are then pretreated at a temperature of about 130°C, purged with 2L / min of air to crosslink the fiber surface. This pretreatment is carried out for about 24 hours. The hollow fibers are then pyrolyzed to 600°C, 900°C, or 1100°C with a temperature ramp of 3°C / min, and then kept at the peak temperature for 2 hours. The pyrolysis furnace is also continuously purged with 5L / min of nitrogen to maintain an oxygen-free environment. Finally, in another furnace, at a final temperature of 300°C to 475°C, with a gas flow of 5L / min, the pyrolyzed hollow fibers are subjected to oxidation. The temperature ramp is 1°C / min, and the final temperature is kept for about 8 hours. The results of these different formation parameters are shown in Table 2 below. Before the permeation test, the samples were kept in a nitrogen box. After oxidation, the film 4 was also subjected to a second pyrolysis at 1100°C.

[0072] Table 2: CMS membrane formation using hollow fibers .

[0073] Example# Pyrolysis temperature (℃) Oxidation temperature (℃) Structural integrity Hole size Selectivity Comparative Example Fiber 1 600 NA Self-supporting Small hole normal Comparative Example Fiber 2 900 NA Self-supporting Small hole normal Comparative Example Fiber 3 600 300 Self-supporting Small hole normal Comparative Example Fiber 4 900 300 Self-supporting Small hole normal Comparative Example Fiber 5 600 350 Self-supporting Small hole normal Comparative Example Fiber 6 900 350 Self-supporting Small hole normal Comparative Example Fiber 7 600 400 Self-supporting Small hole normal Comparative Example Fiber 8 900 400 Self-supporting Small hole normal Comparative Example Fiber 9 600 450 Broken fibers NA NA Comparative Example Fiber 10 600 475 Broken fibers NA NA Fiber 1 900 450 Self-supporting Large hole Reverse Fiber 2 900 475 Self-supporting Large hole Reverse

[0074] The CMS membrane using microcapillary membrane was formed by cutting 3-foot strips of microcapillary membrane into strips with a length of 5 cm. Two pieces of Whatman filter paper (Whatman 1003-125) were placed between the PV DC microcapillary membrane and a porous ceramic plate (about 100 grams each) as a cushion. The tape / filter paper / ceramic plate sandwich was then placed in an oven with an air purge (5 liters / minute) for pretreatment. For pretreatment, the oven temperature was raised to 130°C at a ramp of 1°C / minute and then maintained at 130°C for 24 hours. The sandwich was then removed and cooled to below 60°C. The sandwich was then pyrolyzed in a quartz tube furnace (6" diameter, 24" length) with nitrogen purged at 5 liters / minute to a temperature between 600°C and 1100°C. During pyrolysis, the furnace was first raised to 250°C at 0.1°C / minute, then ramped to the final temperature at 3°C / minute and held at the final temperature for approximately 120 minutes before cooling to below 60°C. An initial lower pyrolysis temperature was used to allow sufficient time for HCl gas to escape from the microcapillary membranes. It was found that if this stage did not occur, the mechanical integrity of the fiber could be compromised, possibly due in part to the formation of undesirable bubbles and foam in the microcapillary membranes.

[0075] Finally, the sandwich structure was then oxidized in another furnace at a final temperature of 300°C to 475°C. The temperature was raised to the final temperature at 1°C / minute and held for 8 hours. Without being limited by theory, and as previously mentioned, longer times and lower temperatures can also be used to achieve the same degree of oxidation. In at least some embodiments, a longer oxidation time range may be preferred to avoid over-oxidation. For actual laboratory operations, an 8-hour hold at the final temperature was used. The results of these different formation parameters are shown in Table 3 below.

[0076] Table 3: CMS membrane formation using microcapillary membranes

[0077]

[0078] As shown in Tables 2 and 3 above, it was found that oxidation temperatures greater than 400°C were necessary to achieve reverse selectivity for the CMS membranes at the specific air composition and treatment time used. Simultaneously, it was found that pyrolysis temperatures of 900°C were necessary to ensure the structural integrity of the CMS membranes at oxidation temperatures above 400°C, in other words, to allow the CMS membranes to be self-supporting and not break apart. Without being limited by theory, the foregoing tends to indicate that small differences in fiber and membrane processing can lead to relatively large differences in membrane / fiber performance. More specifically, only specific combinations of pretreatment, pyrolysis, and oxidation temperature and time ranges (such as those discussed in the embodiments herein) can produce self-supporting CMS membranes with reverse selectivity.

[0079] Permeation testing of CMS membranes

[0080] After formation, each CMS membrane is then stored in a nitrogen-rich container until testing. The gas permeability and gas selectivity of each CMS membrane previously formed and discussed are then tested. This is accomplished by building a customized annular permeation unit "module". The annular unit has a five-inch outer diameter, a three-inch inner diameter, four and a half inch wide openings with 9 / 16 inch O-ring fittings on the wall, and a quarter inch thick cover with O-ring seals on both sides. O-rings are provided by SAE / MS. For hollow fibers, ten hollow fibers according to the above-described embodiment are inserted into the half inch wide openings on the wall of the annular unit at approximately 3 o'clock and 9 o'clock positions. Teflon tape is used to make a cofferdam around the fiber bundle in the hole. Epoxy resin (Scotch Weld DP100) is used to fill the space around the hollow fiber hole and form a seal.

[0081] 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 at approximately the 9 o'clock position. Similar to the hollow fiber module, a dam was then formed using a Gorilla Universal Putty epoxy stick. 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.

[0082] Subsequently, the module was used to conduct a mixed gas permeation test at 35°C. Initially, the various gas mixtures shown in Table 4 were injected at 100 cm 3 / min to 200cm 3 / min. The gas mixture enters the module through a half-inch wide opening in the wall of the annular cell at approximately the 12 o'clock position, passes through the CMS membrane or fiber bundle positioned across the center of the module, and exits the module as a retentate (result) stream through a half-inch wide opening in the wall of the annular cell at approximately the 6 o'clock position. The retentate stream is maintained at 52 psig (psi gauge).

[0083] Then, use about 10cm 3 / min and a continuous helium purge of 14.7 psia carried the permeate stream (the gas mixture trapped within the hollow fiber bundle) from the 9 o'clock position to the 3 o'clock position through the capillaries of the hollow fibers to the area for gas chromatography (GC) analysis. 3 / min and a continuous argon purge of 16.7 psig carried the permeate stream (the gas mixture trapped within the microcapillary membrane) through the capillaries of the microcapillary membrane to the 9 o'clock position to the area for gas chromatography (GC) analysis.

[0084] The permeation flux is calculated using the purge gas flow rate and the permeation gas concentration measured by GC. As previously mentioned, the permeability of hollow fibers is calculated by using the standardized permeation flux by total membrane area, which is the product of exposed fiber length, fiber number and hollow fiber OD. The permeability of microcapillary membranes is calculated by using the standardized permeation flow rate by transmembrane pressure difference and total surface area, which is the product of unsealed capillary length, width and the number of plies multiplied by 2 (each microcapillary membrane has two surfaces).

[0085] As mentioned previously, penetration is measured in GPUs as follows: Selectivity was determined by taking the ratio of the second gas permeability divided by the first gas permeability.The results of the permeability testing are shown in Tables 4 to 8 below.

[0086] Specifically, Tables 4 through 8 show the permeabilities and selectivities for a range of hollow fiber and microcapillary membranes for specific gas separations. Each replicate of a fiber or membrane indicates a subsequent newly generated module of Example # fiber / membrane. Table 4 below shows the permeabilities and selectivities for a mixture of propylene and hydrogen. Table 5 shows the permeabilities and selectivities for a mixture of n-butane and hydrogen. Table 6 shows the permeabilities and selectivities for a mixture of n-butane and isobutane. Table 7 shows the permeabilities and selectivities for a mixture of carbon dioxide, hydrogen, and methane. Table 8 shows the permeabilities and selectivities for a mixture of propane and propylene. Table 9 shows the permeabilities and selectivities for the remaining gas mixtures tested.

[0087] Table 4 : Selectivity of propylene relative to hydrogen

[0088]

[0089] As shown in Table 4 above, the fibers and membranes exhibit high selectivity for preferentially separating propylene from hydrogen, indicating their usefulness in commercial applications for separating hydrogen from olefins, such as recovering olefins from the output side of cracking units, and for purifying hydrogen-containing hydrocarbon streams for recycle in hydroprocessing (hydrocracking / hydrotreating) units.

[0090] Table 5 : Selectivity of n-butane relative to hydrogen

[0091]

[0092] As shown in Table 5 above, the fibers and membranes also exhibit high selectivity for preferentially separating n-butane from hydrogen, indicating their usefulness in commercial applications for separating hydrogen from paraffins, such as recovering paraffins from the output side of cracking units, and for purifying hydrogen-containing hydrocarbon streams for recycle in hydroprocessing (hydrocracking / hydrotreating) units. Furthermore, in combination with the olefin separation properties noted in Table 4, the fibers and membranes can also be used to recover hydrogen from paraffin dehydrogenation units.

[0093] Table 6 : Selectivity of n-butane relative to isobutane

[0094]

[0095] As shown in Table 6 above, the fibers and membranes exhibited low selectivity for preferentially separating n-butane from isobutane. This indicates reverse selectivity of the membranes after the formation methods according to the embodiments herein. These differences can be further contrasted with the comparative fibers / membranes in Table 8 and below.

[0096] Table 7 : Hydrogen, carbon dioxide and methane selectivity

[0097]

[0098]

[0099] surface 8 : Selectivity of propane over propylene

[0100]

[0101] As shown in Tables 7 and 8 above, the fibers and membranes exhibit low selectivity for preferentially separating carbon dioxide from hydrogen, carbon dioxide from methane, methane from hydrogen, and propane from propylene, compared to the relatively high selectivity for the comparative fibers and membranes. As previously described, this tends to show that small differences in the processing of the fibers and membranes can lead to relatively large differences in the properties of the membranes and fibers. More specifically, only specific combinations of pretreatment, pyrolysis, and oxidation (such as those discussed in the embodiments herein) can produce self-supporting CMS membranes with reverse selectivity. Furthermore, as shown in Tables 2 and 3, certain thresholds of pretreatment time, pyrolysis temperature (900° C. and above), and oxidation temperature (greater than 400° C.) must be reached to allow the CMS membrane to be self-supporting and reverse selective. As previously described, these thresholds may also be affected by the composition of the air and the oxidation time used. Therefore, the aforementioned thresholds can be understood to occur at oxidation times of 8 hours or less.

[0102] Table 9 : Remaining test gas mixture

[0103]

[0104] As shown in Table 9 above, the fibers and membranes also exhibited high selectivity for preferentially separating propylene from nitrogen, which may indicate the membrane's usefulness in recovering nitrogen from nitrogen purge streams used in polymerization reactors for producing polyolefins (polypropylene, polyethylene, etc.). Additionally, the fibers and membranes exhibited high selectivity for preferentially separating propylene from methane, which may indicate the membrane's usefulness in recovering natural gas liquids (hydrocarbons with a carbon content greater than one) from hydrocarbon product streams containing methane.

[0105] Example Group 2

[0106] Further permeation tests of the fibers obtained from Asahi Kasei were also performed, this time also taking into account the weight loss and resulting molecular composition of the resulting CMS membranes.

[0107] First, the CMS membrane using hollow fibers was passed through an alumina tube in a manner similar to Example 1. The hollow fibers were then pre-treated at a temperature of approximately 150°C with a 300 cm 3 / minute of air purge to crosslink the fiber surface. Pretreatment was carried out for about 24 hours. Then 3The fibers were pyrolyzed at 600°C or 900°C with a temperature ramp of 3°C / min under an argon purge of 0.5°C / min and then held at the peak temperature for 2 hours.

[0108] The fibers were then subjected to oxidation at different temperatures, 300°C, 350°C, 400°C, and 450°C, for 11.5 hours to measure the effects of different oxidation temperatures on the composition and properties of the resulting membranes. 3 / min of air (approximately 21% oxygen content). The temperature ramp was 1°C / min, and the final temperature was held for approximately 11.5 hours, as previously mentioned. Figure 2A 、 Figure 2B and Figure 2C The weight loss of the fibers over the time span of the oxidation process is shown for oxidation temperatures of 350°C, 400°C, and 450°C, respectively. Data was recorded for approximately 11.5 hours, but a cutoff of 8 hours was used to compare weight losses. The weight loss at approximately 8 hours of oxidation is shown in Table 10 below.

[0109] Table 10 : Weight loss of oxidized fibers at eight hours

[0110]

[0111] Additionally, the molecular composition of the fibers was measured by X-ray photoelectron spectroscopy (XPS) on the resulting fibers. The results of these tests are shown in Table 11 below. As previously described, oxidation of PVDC hollow fibers serves to expel carbon from the hollow fibers and replace the expelled carbon with a higher oxygen content. Therefore, the degree of fiber oxidation can be classified by the fiber's oxygen-to-carbon ratio. Figure 3 Shown is a comparison of the fibers formed in Example Set 2 compared to the original Asahi Kasei PVDC fibers after initial inert pyrolysis and without oxidation.

[0112] Table 11 :Molecular composition of oxidized fibers

[0113]

[0114]

[0115] The standard deviation (σ) of the XPS was calculated from three independent points for each fiber. The elemental composition was calculated assuming that the detected elements accounted for 100% of the species on the surface. Hydrogen content was also not considered, as these species are not sensitive to XPS. After oxidation, each CMS membrane was then stored in a nitrogen-rich container until testing. The gas permeability and gas selectivity of each CMS membrane previously formed and discussed were then tested in a manner similar to Example Set 1. Specifically, by placing each fiber in the ring permeation unit "module" described above.

[0116] Tables 12 through 15 show the permeabilities and selectivities across a range of fibers for the specified gas separations. Fibers obtained by pyrolysis at 900°C and oxidation at 475°C were also tested. Table 12 below shows the permeabilities and selectivities for a mixture of hydrogen, carbon dioxide, and methane. Table 13 shows the permeabilities and selectivities for a mixture of propylene and propane. Table 14 shows the permeabilities and selectivities for a mixture of n-butane and isobutane. Table 15 shows the permeabilities and selectivities for the remaining gas mixtures tested.

[0117] Table 12 : Hydrogen, carbon dioxide and methane selectivity

[0118]

[0119] As shown in Table 12 above, the fibers exhibited low selectivity for preferential separation of carbon dioxide from hydrogen, carbon dioxide from methane, and methane from hydrogen, consistent with previously tested fibers. A trend toward lower selectivity for carbon dioxide from methane separation was also observed at pyrolysis temperatures above the threshold temperature of 900°C and when the oxidation temperature was increased above 400°C (if the oxidation time was 8 hours or less). As previously discussed, this tends to show that small differences in the processing of the fibers and membranes can lead to relatively large differences in the performance of the membranes and fibers. More specifically, only specific combinations of pretreatment, pyrolysis, and oxidation (such as those discussed in the embodiments herein) can produce self-supporting CMS membranes with reverse selectivity for certain gas separations. Furthermore, as further shown in Table 12 above, certain thresholds of pretreatment time, pyrolysis temperature (900°C and above), and oxidation temperature (greater than 400°C) must be reached to allow the CMS membrane to be self-supporting and reverse selective.

[0120] Table 13 :Propylene and propane selectivity

[0121]

[0122] Table 14: Selectivity of n-butane and isobutane

[0123]

[0124] As shown in Tables 13 and 14 above, the fibers also exhibited low selectivity for preferentially separating propylene from propane and n-butane from isobutane, consistent with other fibers and membranes previously tested. This indicates reverse selectivity of the membrane after forming according to embodiments herein.

[0125] Table 15 :Other gas selectivity

[0126]

[0127] As shown in Table 15 above, the fibers exhibited high selectivity for preferentially separating hydrogen from heavier species, such as n-butane and propylene, consistent with previous testing and indicative of the fiber's reverse selectivity. The ability of the fibers to preferentially separate hydrogen from heavier species may indicate the usefulness of the fibers and membranes in recovering hydrogen from hydrocarbon-containing streams. This may include, for example, recovering unused hydrogen feed from a hydrotreater or hydrocracking upgrading unit.

[0128] According to a first aspect, a method for manufacturing a self-supporting carbon molecular sieve (CMS) membrane may include: forming a polyvinylidene chloride (PVDC) copolymer into one or more hollow fibers or microcapillary membranes; pretreating the one or more hollow fibers or the microcapillary membranes by heating with air, an inert gas, or both at a first temperature of 120°C to 200°C; pyrolyzing the one or more hollow fibers or the microcapillary membranes with the inert gas at a second temperature of 600°C to 1100°C; and oxidizing the one or more hollow fibers or the microcapillary membranes with the air at a third temperature of 300°C to 500°C.

[0129] The second aspect may include any of the preceding aspects, and may further include wherein the self-supporting CMS membrane has a propylene / hydrogen selectivity of 40 to 260; an n-butane / hydrogen selectivity of 90 to 420; a propylene / methane selectivity of 40 to 80; a propylene / nitrogen selectivity of 80 to 200; or a combination thereof.

[0130] A third aspect may include any of the preceding aspects, and may further include wherein the one or more hollow fibers or microcapillary membranes are pretreated for a period of 24 hours to 48 hours; the one or more hollow fibers or the microcapillary membranes are oxidized for a period of 10 minutes to 7 days; the air comprises 10 weight percent to 30 weight percent oxygen measured by the weight of the air; or a combination thereof.

[0131] A fourth aspect may include any of the preceding aspects, and may further include wherein the second temperature is 900°C to 1100°C; the third temperature is greater than 400°C to 500°C; and the one or more hollow fibers or microcapillary membranes are oxidized for a period of 10 minutes to 12 hours.

[0132] A fifth aspect may include any of the foregoing aspects, and may further include wherein after oxidizing the one or more hollow fibers or microcapillary membranes, the one or more hollow fibers or microcapillary membranes are again pyrolyzed at a second temperature using an inert gas.

[0133] A sixth aspect may include any of the preceding aspects, and may further include wherein the inert gas comprises argon, nitrogen, or both.

[0134] A seventh aspect may include any of the preceding aspects, and may further include wherein the polyvinylidene chloride copolymer comprises vinylidene chloride and 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.

[0135] An eighth aspect can include any of the preceding aspects, and may further include wherein forming the PVDC copolymer into one or more hollow fibers or microcapillary membranes is performed by melt extrusion.

[0136] A ninth aspect may include any of the preceding aspects, and may further include a carbon molecular sieve membrane manufactured according to any of the preceding aspects, wherein: the CMS membrane is reverse selective; the CMS membrane does not include a support structure; the CMS membrane has a carbon content of 82 wt% to 94 wt%; and the CMS membrane has an oxygen content of 5 wt% to 17 wt% measured by weight of the CMS membrane.

[0137] The tenth aspect may include any of the preceding aspects and may further include a method for separating a gas from a gas mixture comprising first gas molecules and second gas molecules, wherein the method comprises: manufacturing a self-supporting CMS membrane according to any of the preceding aspects; and flowing the gas mixture through the self-supporting CMS membrane to produce a permeate first stream having an increased concentration of the first gas molecules and a second retentate stream having an increased concentration of the second gas molecules, wherein the second gas molecules have a smaller representative molecular diameter than the first gas molecules.

[0138] An eleventh aspect can include any of the preceding aspects, and may further include wherein the self-supporting CMS membrane has an average pore size greater than a representative molecular diameter of a first gas molecule as determined by gas adsorption using gas probe molecules of varying sizes.

[0139] A twelfth aspect may include any of the preceding aspects, and may further include wherein: the self-supporting CMS membrane has a carbon content of 82 wt% to 94 wt%; and the self-supporting CMS membrane has an oxygen content of 5 wt% to 17 wt% measured by weight of the CMS membrane.

[0140] A thirteenth aspect may include any of the preceding aspects, and may further include wherein the first gas molecules include C 2+ hydrocarbons; and the second gas molecules include carbon dioxide, nitrogen, carbon monoxide, methane, hydrogen, hydrogen sulfide, or a combination thereof.

[0141] A fourteenth aspect may include any of the preceding aspects, and may further include wherein the first gas molecules include n-butane; the second gas molecules include hydrogen, methane, nitrogen, or a combination thereof; and the carbon molecular sieve has an n-butane / hydrogen selectivity of 90 to 420.

[0142] The fifteenth aspect may include any of the aforementioned aspects, and may also include wherein the first gas molecule comprises propylene; the second gas molecule comprises hydrogen, methane, nitrogen, or a combination thereof; and the carbon molecular sieve has: a propylene / hydrogen selectivity of 40 to 260; a propylene / methane selectivity of 40 to 80; a propylene / nitrogen selectivity of 80 to 200; or a combination thereof.

[0143] 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.

[0144] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0145] 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.

[0146] As used in this disclosure and the appended claims, the words "comprises," "has," and "includes" and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0147] 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.

[0148] While the subject matter of the present disclosure has been described in detail and with reference to specific embodiments, it should be noted that the various details disclosed in this disclosure should not be construed as implying that these details relate to elements that are essential components of the various embodiments described in this disclosure. Furthermore, it will be apparent that modifications and variations are possible without departing from the scope of this disclosure, including but not limited to the embodiments defined in the appended claims.

Claims

1. A method for producing a self-supporting carbon molecular sieve (CMS) membrane, the method comprising: forming a polyvinylidene chloride (PVDC) copolymer into one or more hollow fibers or microcapillary membranes; pretreating the one or more hollow fibers or the microcapillary membrane by heating at a first temperature of 120° C. to 200° C. with air, an inert gas, or both; pyrolyzing the one or more hollow fibers or the microcapillary membrane at a second temperature of 600° C. to 1100° C. using the inert gas; as well as The one or more hollow fibers or the microcapillary membrane are oxidized at a third temperature of 300°C to 500°C using the air.

2. The method according to any preceding claim, wherein the self-supporting CMS membrane has: Propylene / hydrogen selectivity of 40 to 260; n-butane / hydrogen selectivity of 90 to 420; Propylene / methane selectivity of 40 to 80; a propylene / nitrogen selectivity of 80 to 200; or A combination of them.

3. A method according to any preceding claim, wherein: pretreating the one or more hollow fibers or the microcapillary membrane for a period of 24 hours to 48 hours; oxidizing the one or more hollow fibers or the microcapillary membrane for a period of 10 minutes to 7 days; The air comprises from 10% to 30% by weight oxygen, measured by weight of the air; or A combination of them.

4. A method according to any preceding claim, wherein: The second temperature is 900° C. to 1100° C.; The third temperature is greater than 400°C to 500°C; and The one or more hollow fibers or the microcapillary membrane are oxidized for a period of 10 minutes to 12 hours.

5. The method according to any preceding claim, wherein after oxidizing the one or more hollow fibers or the microcapillary membrane, the method further comprises pyrolyzing the one or more hollow fibers or the microcapillary membrane again at the second temperature using the inert gas.

6. A method according to any preceding claim, wherein the inert gas comprises argon, nitrogen or both.

7. The method of any preceding claim, wherein the polyvinylidene chloride copolymer comprises vinylidene chloride and 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.

8. The method of any preceding claim, wherein forming the PVDC copolymer into the one or more hollow fibers or the microcapillary membrane is performed by melt extrusion.

9. A carbon molecular sieve membrane manufactured according to any preceding claim, wherein: The CMS membrane is reverse selective and does not include a support structure; and The CMS membrane has a carbon content of 82 wt% to 94 wt% and an oxygen content of 5 wt% to 17 wt% measured by weight of the CMS membrane.

10. A method for separating a gas from a gas mixture, the gas mixture comprising first gas molecules and second gas molecules, the method comprising: Manufacturing a self-supporting CMS membrane according to any preceding claim; as well as The gas mixture is flowed through the self-supporting CMS membrane to produce a permeate first stream having an increased concentration of the first gas molecules and a second retentate stream having an increased concentration of the second gas molecules, wherein the second gas molecules have a smaller representative molecular diameter than the first gas molecules.

11. The method of claim 10, wherein the self-supporting CMS membrane has an average pore size larger than the representative molecular diameter of the first gas molecules as determined by gas adsorption using gas probe molecules of different sizes.

12. The method according to any one of claims 10 to 11, wherein the self-supporting CMS membrane has a carbon content of 82 to 94 wt% and an oxygen content of 5 to 17 wt%, measured by the weight of the CMS membrane.

13. The method according to any one of claims 10 to 12, wherein The first gas molecules contain C 2+ hydrocarbons; and The second gas molecules include carbon dioxide, nitrogen, carbon monoxide, methane, hydrogen, hydrogen sulfide, or a combination thereof.

14. The method according to any one of claims 10 to 12, wherein: The first gas molecules include n-butane; The second gas molecules include hydrogen, methane, nitrogen, or a combination thereof; and The carbon molecular sieve has an n-butane / hydrogen selectivity of 90 to 420.

15. The method according to any one of claims 10 to 12, wherein: The first gas molecules include propylene; The second gas molecules include hydrogen, methane, nitrogen, or a combination thereof; and The carbon molecular sieve has: Propylene / hydrogen selectivity of 40 to 260; Propylene / methane selectivity of 40 to 80; a propylene / nitrogen selectivity of 80 to 200; or A combination of them.

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