Methods for separating hydrogen and carbon-containing compounds

Abstract

Hydrogen (H2) and a carbon containing compound may be separated by a process that includes introducing a mixed gas to a first membrane, the mixed gas comprising a target chemical species and a secondary chemical species; a first retentate and a first permeate at the first membrane, wherein the first retentate comprises an increased concentration of the target chemical species and the first permeate comprises an increased concentration of the secondary chemical species; introducing the first retentate to a second membrane; and separating a second retentate and a second permeate at the second membrane, wherein the second retentate comprises an increased concentration of the secondary chemical species and the second permeate comprises an increased concentration of the target chemical species, wherein the target chemical species is H2 or the carbon-containing compound, the secondary chemical species is H2 or the carbon-containing compound, and the target chemical species is different than the secondary chemical species.

Description

METHODS FOR SEPARATING HYDROGEN AND CARBON-CONTAINING COMPOUNDSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 63 / 579,624 filed August 30, 2023, the contents of which are incorporated in their entirety herein.TECHNICAL FIELD

[0002] The present disclosure relates to gas separation and, more specifically, to separation of hydrogen and ethylene using a combination of reverse-selective and ultra- selective membranes.BACKGROUND

[0003] Gas separation is a vital process in cracker plants that usually involves employing cryogenic distillation or occasionally membrane separation to extract and refine gases such as ethylene and hydrogen from mixed feedstocks. Cryogenic distillation capitalizes on differences in boiling points, resulting in high-purity outputs. Nonetheless, cryogenic distillation’s energy-intensive character arises due to the necessity of sustaining extremely low temperatures, leading to substantial refrigeration needs and intricate infrastructure in addition to high-compression and associated costs. This culminates in noteworthy operational expenditures and a significant initial investment. In contrast, membrane separation offers energy efficiency and occupies less space than cryogenic distillation. Nevertheless, achieving stringent purity standards can pose difficulties, and the power-demanding compression essential for this technique contributes to costs. In essence, both cryogenic distillation and membrane separation serve gas separation well, yet the former's refrigeration requisites and the latter's compression demands amplify their individual costliness and intricacy within cracker plants. Therefore, the development of new and more efficient methods for separating gases will be needed to address the aforementioned challenges.SUMMARY

[0004] Described herein are methods for separating gases, such as hydrogen and a carbon- containing compound (e.g., ethylene and / or carbon dioxide). In the embodiments described herein, a mixed gas that includes a target chemical species and a secondary chemical speciesmay undergo separation utilizing a two-stage membrane system. Each stage may utilize a reverse-selective membrane or an ultra-selective membrane, depending on the composition of the mixed gas. A reverse-selective membrane may be utilized in the first stage when the target chemical species has a lesser representative molecular diameter than the secondary chemical species. For example, the reverse-selective membrane may be utilized in the first stage when hydrogen is the target chemical species and ethylene is the secondary chemical species. Alternatively, an ultra-selective membrane may be utilized as the first membrane when the secondary chemical species has a lesser representative molecular diameter than the target chemical species. For example, the ultra-selective membrane may be utilized in the first stage when ethylene is the target chemical species and hydrogen is the secondary chemical species. The second membrane may separate the retentate produced from the first membrane separation, which is still at high pressure. The second membrane may be a reverse- selective membrane when the first membrane is an ultra-selective membrane, and vice versa. As is described herein, the utilization of a reverse-selective and ultra-selective membrane combination system may serve to efficiently separate gases, such as carbon-containing compounds and hydrogen.

[0005] According to one or more embodiments described herein, a process for separating hydrogen and a carbon-containing compound may comprise introducing a mixed gas to a first membrane, the mixed gas comprising a target chemical species and a secondary chemical species. The process further comprises separating a first retentate and a first permeate at the first membrane, wherein the first retentate comprises an increased concentration of the target chemical species and the first permeate comprises an increased concentration of the secondary chemical species. The process further includes introducing the first retentate to a second membrane, and separating a second retentate and a second permeate at the second membrane, wherein the second retentate comprises an increased concentration of the secondary chemical species and the second permeate comprises an increased concentration of the target chemical species. Additionally, the target chemical species is hydrogen or the carbon-containing compound, the secondary chemical species is hydrogen or the carbon- containing compound, and the target chemical species is different than the secondary chemical species.

[0006] It is to be understood that both the foregoing general description and the following detailed description present embodiments of the described technology and are intended toprovide an overview or framework for understanding the nature and character of the described technology as it is claimed. The accompanying drawing is included to provide a further understanding of the described technology and are incorporated into and constitute a part of this specification. The drawing illustrates various embodiments and, together with the description, serve to explain the principles and operations of the described technology. Additionally, the drawing and descriptions are meant to be merely illustrative, and are not intended to limit the scope of the claims in any manner.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawing, where like structure is indicated with like reference numerals and wherein:

[0008] FIG. 1 schematically depicts an apparatus for separating a target chemical species from a secondary chemical species according to one or more embodiments disclosed herein;

[0009] FIG. 2 schematically depicts an apparatus for separating a target chemical species from a secondary chemical species according to one or more embodiments disclosed herein; and

[0010] FIG. 3 depicts a cross section of an asymmetric hollow carbon fiber, according to one or more embodiments of the present disclosure.

[0011] For the purpose of describing the simplified schematic illustrations and descriptions of the relevant figures, the numerous valves, temperature sensors, electronic controllers and the like that may be employed and well known to those of ordinary skill in the art of certain chemical processing operations are not included. It should be understood that these components are within the spirit and scope of the presently disclosed embodiments. However, operational components, such as those described in the present disclosure, may be added to the embodiments described in this disclosure.

[0012] It should be understood that features described in the various drawings may be used in combination with other aspects in different drawings. That is, the embodiment of FIG. 1 may utilize features of the embodiment of FIG. 2, as would be understood by those skilled in the art.

[0013] It should further be noted that arrows in the drawings refer to process streams. However, the arrows may equivalently refer to transfer lines that may serve to transfer process streams between two or more system components. Additionally, arrows that connect to system components define inlets or outlets in each given system component. The arrow direction corresponds generally with the major direction of movement of the materials of the stream contained within the physical transfer line signified by the arrow. Furthermore, arrows that do not connect two or more system components signify a product stream which exits the depicted system or a system inlet stream that enters the depicted system. Product streams may be further processed in accompanying chemical processing systems or may be commercialized as end products. System inlet streams may be streams transferred from accompanying chemical processing systems or may be non-processed feedstock streams. Some arrows may represent recycle streams, which are effluent streams of system components that are recycled back into the system. However, it should be understood that any represented recycle stream, in some embodiments, may be replaced by a system inlet stream of the same material, and that a portion of a recycle stream may exit the system as a system product.

[0014] Additionally, arrows in the drawings may schematically depict process steps of transporting a stream from one system component to another system component. For example, an arrow from one system component pointing to another system component may represent “passing” a system component effluent to another system component, which may include the contents of a process stream “exiting” or being “removed” from one system component and “introducing” the contents of that product stream to another system component.

[0015] It should be understood that according to the embodiments presented in the relevant figures, an arrow between two system components may signify that the stream is not processed between the two system components. In other embodiments, the stream signified by the arrow may have substantially the same composition throughout its transport between the two system components. Additionally, it should be understood that in one or more embodiments, an arrow may represent that at least 75 wt.%, at least 90 wt.%, at least 95 wt.%, at least 99 wt.%, at least 99.9 wt.%, or even 100 wt.% of the stream is transported between the system components. As such, in some embodiments, less than all of the streams signifiedby an arrow may be transported between the system components, such as if a slip stream is present.

[0016] It should be understood that two or more process streams are “mixed” or “combined” when two or more lines intersect in the schematic flow diagrams of the relevant figures. Mixing or combining may also include mixing by directly introducing both streams into a like reactor, separation device, or other system component. For example, it should be understood that when two streams are depicted as being combined directly prior to entering a separation unit or reactor, that in some embodiments the streams could equivalently be introduced into the separation unit or reactor and be mixed in the reactor.

[0017] Reference will now be made in greater detail to various embodiments, some embodiments of which are illustrated in the accompanying drawing. Whenever possible, the same reference numerals will be used throughout the drawing to refer to the same or similar parts.DETAILED DESCRIPTION

[0018] The present disclosure is directed to methods of separating a target chemical species from a secondary chemical species. Generally, the methods are described herein in the context of processing systems, such as those of FIGS. 1 and 2. However, it should be understood that the methods may be utilized independently of the processing systems described herein, and it is contemplated that other systems may be utilized to practice the presently described technology.

[0019] As used in the present disclosure, passing a stream or effluent from one unit “directly” to another unit may refer to passing the stream or effluent from the first unit to the second unit without passing the stream or effluent through an intervening reaction system or separation system that substantially changes the composition of the stream or effluent. Heat transfer devices, such as heat exchangers, preheaters, coolers, condensers, or other heat transfer equipment, and pressure devices, such as pumps, pressure regulators, compressors, or other pressure devices, are not considered to be intervening systems that change the composition of a stream or effluent. Combining two streams or effluents together also is not considered to comprise an intervening system that changes the composition of one or both of the streams or effluents being combined. Simply dividing a stream into two streams havingthe same composition is also not considered to comprise an intervening system that changes the composition of the stream.

[0020] As used throughout the present disclosure, the terms “upstream” and “downstream” may refer to the relative positioning of unit operations with respect to the direction of flow of the process streams. A first unit operation of a system may be considered “upstream” of a second unit operation if process streams flowing through the system encounter the first unit operation before encountering the second unit operation. Likewise, a second unit operation may be considered “downstream” of the first unit operation if the process streams flowing through the system encounter the first unit operation before encountering the second unit operation.

[0021] It should be understood that separation processes described in this disclosure may not completely separate all of one chemical constituent from all of another chemical constituent. It should be understood that the separation processes described in this disclosure “at least partially” separate different chemical components from one another, and that even if not explicitly stated, it should be understood that separation may include only partial separation. It should be understood that a “separation unit” is a separation unit used primarily for the separation of two or more gases.

[0022] As used in this disclosure, a “reverse-selective” membrane refers to a membrane that exhibits the property of allowing some larger molecules to permeate faster than smaller ones. Unlike traditional membranes that facilitate the movement of substances from an area of high concentration to low concentration, a reverse-selective membrane facilitates the transport of larger molecular species across the separation membrane faster than smaller molecular species.

[0023] As used in this disclosure, an “ultra-selective” membrane refers to a membrane that operates through specialized materials with nanoporous structures or functional groups that enable preferential gas permeation of the smaller species. In an ultra-selective membrane, gas molecules interact with the membrane's surface and permeate based on size exclusion, adsorption, diffusion, or chemical affinity.

[0024] As used in this disclosure, a “storage vessel” refers to a container in which one or more fluids may be stored. For example, a storage vessel may store liquid, gas, or acombination of both. The storage vessels are not limited by geometric shape and / or size, and can include tanks, drums, silos, pipelines, and the like.

[0025] As used in this disclosure, the term “retentate” refers to the portion of a gas feed that is retained by the membrane. In other words, the retentate does not pass through the membrane.

[0026] As used in this disclosure, the term “permeate” refers to the portion of a gas feed that diffuses through or penetrates the membrane.

[0027] As used in this disclosure, the term “carbon-containing compound” refers to any chemical species that contains a carbon atom. For example, the carbon-containing compound may refer to, but is not limited to, ethylene or carbon dioxide.

[0028] As used in this disclosure, the term “analyte” refers to a gaseous stream whose chemical constituents are being identified and measured.

[0029] Now referring to FIG. 1, a two-stage membrane separation apparatus 100 suitable for separating a target chemical species from a secondary chemical species is depicted. The two-stage membrane separation apparatus 100 may include a first membrane 110 and a second membrane 120. The membranes may be housed within a mechanical apparatus suitable for containing a gas, such as a storage vessel. According to embodiments, and as described herein, a target chemical species that is included in a mixed gas may be separated from a secondary chemical species within the mixed gas.

[0030] Still referring to FIG. 1, according to one or more embodiments, a mixed gas 101 may be introduced into the first membrane 110 through a gas inlet. The gas inlet may be a pipe or other like conduit. In one or more embodiments, the mixed gas comprises a target chemical species and at least a secondary chemical species. As described herein, the “target chemical species” refers to a chemical species that is selectively separated from the mixed gas at the first membrane as a first retentate. In embodiments, the target chemical species may be a carbon-containing compound or hydrogen. The secondary chemical species may be any chemical species that is not the target chemical species and permeates the first membrane. It is contemplated that more than a single chemical species may be selectively separated from the mixed gas in the processes described herein.

[0031] In some embodiments, the target chemical species may be hydrogen, and the secondary chemical species may be a carbon containing compound, such as ethylene or carbon dioxide. It is also contemplated that the target chemical species may be the carbon containing compound, such as ethylene or carbon dioxide, and the secondary chemical species may be hydrogen. The composition of the target chemical species is dependent upon the composition of the mixed gas. In one or more embodiments, the concentration of the target chemical species in the mixed gas is greater than the concentration of the secondary chemical species in the mixed gas. For example, when the concentration of hydrogen in the mixed gas is greater than the concentration of the carbon containing compound, the target chemical species may be hydrogen and the secondary chemical species may be the carbon containing compound. Conversely, when the concentration of the carbon containing compound is greater than or equal to the concentration of hydrogen, the target chemical species may be the carbon containing compound. The mixed gas containing the target chemical species and the secondary chemical species may be produced from a cracker plant, autothermal reactor, deethanizer, C2 splitter, or C3 splitter, but is not limited to such sources.

[0032] Still referring to FIG. 1, after introducing a mixed gas 101 into the first membrane 110, the mixed gas 101 is separated into a first retentate 103 and a first permeate 102 at the first membrane 110. The first retentate 103 may comprise an increased concentration of the target chemical species compared to the mixed gas. The first retentate 103 may be introduced to a second membrane 120 downstream of the first membrane 110. Notably, the retentate 103 may remain at relatively the same pressure as the mixed gas, reducing the need for interstage compression.

[0033] The concentration of the target chemical species and secondary chemical species within any stream may differ in embodiments, depending on the stage cut. As used herein, the term “stage cut” refers to the fraction of feed gas that permeates the membrane. In some embodiments, the stage cut may be from 1% to 50%, for example from 5% to 50%, from 10% to 50%, from 15% to 50%, from 20% to 50%, from 25% to 50%, from 30% to 50%, from 35% to 50%, from 40% to 50%, from 45% to 50%, from 1% to 40%, from 1% to 30%, from 1% to 20%, or even from 1% to 10%.

[0034] In some embodiments, the concentration of the target chemical species within the first retentate 103 may be from 50 mol.% to 95 mol.%, for example from 50 mol.% to 95mol.%, from 55 mol.% to 95 mol.%, from 60 mol.% to 95 mol.%, from 65 mol.% to 95 mol.%, from 70 mol.% to 95 mol.%, from 75 mol.% to 95 mol.%, from 80 mol.% to 95 mol.%, from 85 mol.% to 95 mol.%, from 90 mol.% to 95 mol.%, from 50 mol.% to 90 mol.%, from 50 mol.% to 80 mol.%, from 50 mol.% to 70 mol.%, or even from 50 mol.% to 60 mol.%.

[0035] In some embodiments, the concentration of the secondary chemical species within the first retentate 103 may be from 5 mol.% to 50 mol.%, for example from 10 mol.% to 50 mol.%, from 15 mol.% to 50 mol.%, from 20 mol.% to 50 mol.%, from 25 mol.% to 50 mol.%, from 30 mol.% to 50 mol.%, from 35 mol.% to 50 mol.%, from 40 mol.% to 50 mol.%, from 45 mol.% to 50 mol.%, from 5 mol.% to 40 mol.%, from 5 mol.% to 30 mol.%, from 5 mol.% to 20 mol.%, or even from 5 mol.% to 10 mol.%.

[0036] The first permeate 102 may comprise an increased concentration of the secondary chemical species compared to the mixed gas. The first permeate 102 may be introduced to an atmosphere outside of the mechanical apparatus where the first membrane 110 is housed. In some embodiments, the concentration of the secondary chemical species in the first permeate 102 may be from 85 mol.% to 99 mol.%, for example from 87 mol.% to 99 mol.%, from 89 mol.% to 99 mol.%, from 91 mol.% to 99 mol.%, from 93 mol.% to 99 mol.%, from 95 mol.% to 99 mol.%, from 97 mol.% to 99 mol.%, from 90 mol.% to 99 mol.%, from 92 mol.% to 99 mol.%, from 94 mol.% to 99 mol.%, from 96 mol.% to 99 mol.%, from 98 mol.% to 99 mol.%, from 90 mol.% to 98 mol.%, from 92 mol.% to 98 mol.%, from 94 mol.% to 98 mol.%, from 96 mol.% to 98 mol.%, from 90 mol.% to 97 mol.%, from 92 mol.% to 97 mol.%, from 94 mol.% to 97 mol.%, from 96 mol.% to 97 mol.%, from 90 mol.% to 96 mol.%, from 92 mol.% to 96 mol.%, or even from 94 mol.% to 96 mol.%,

[0037] In some embodiments, the concentration of the target chemical species in the first permeate 102 may be from 1 mol.% to 15 mol.%, for example from 3 mol.% to 15 mol.%, from 5 mol.% to 15 mol.%, from 13 mol.% to 15 mol.% from 7 mol.% to 15 mol.%, from 9 mol.% to 15 mol.%, from 11 mol.% to 15 mol.%, from 13 mol.% to 15 mol.%.

[0038] After introducing the first retentate 103 into the second membrane 120, the first retentate is separated into a second retentate 105 and a second permeate 104 at the second membrane 120. The second retentate 105 may comprise an increased concentration of thesecondary chemical species. The second retentate 105 may be introduced to an atmosphere outside of the mechanical apparatus where the second membrane 120 is housed. In some embodiments, the concentration of the secondary chemical species within the second retentate 105 may be from 5 mol.% to 50 mol.%, for example from 10 mol.% to 50 mol.%, from 15 mol.% to 50 mol.%, from 20 mol.% to 50 mol.%, from 25 mol.% to 50 mol.%, from 30 mol.% to 50 mol.%, from 35 mol.% to 50 mol.%, from 40 mol.% to 50 mol.%, from 45 mol.% to 50 mol.%, from 5 mol.% to 40 mol.%, from 5 mol.% to 30 mol.%, from 5 mol.% to 20 mol.%, or even from 5 mol.% to 10 mol.%.

[0039] In some embodiments, the concentration of the target chemical species within the second retentate 105 may be from 50 mol.% to 95 mol.%, for example from 50 mol.% to 95 mol.%, from 55 mol.% to 95 mol.%, from 60 mol.% to 95 mol.%, from 65 mol.% to 95 mol.%, from 70 mol.% to 95 mol.%, from 75 mol.% to 95 mol.%, from 80 mol.% to 95 mol.%, from 85 mol.% to 95 mol.%, from 90 mol.% to 95 mol.%, from 50 mol.% to 90 mol.%, from 50 mol.% to 80 mol.%, from 50 mol.% to 70 mol.%, or even from 50 mol.% to60 mol.%.

[0040] After introducing the first retentate 103 into the second membrane 120, the first retentate is separated into a second retentate 105 and a second permeate 104 at the second membrane 120. The second retentate 105 may comprise an increased concentration of the secondary chemical species. The second retentate 105 may be introduced to an atmosphere outside of the mechanical apparatus where the second membrane 120 is housed.

[0041] In some embodiments, the concentration of the secondary chemical species within the second retentate 105 may be from 5 mol.% to 50 mol.%, for example from 10 mol.% to50 mol.%, from 15 mol.% to 50 mol.%, from 20 mol.% to 50 mol.%, from 25 mol.% to 50 mol.%, from 30 mol.% to 50 mol.%, from 35 mol.% to 50 mol.%, from 40 mol.% to 50 mol.%, from 45 mol.% to 50 mol.%, from 5 mol.% to 40 mol.%, from 5 mol.% to 30 mol.%, from 5 mol.% to 20 mol.%, or even from 5 mol.% to 10 mol.%.

[0042] In some embodiments, the concentration of the target chemical species within the second retentate 105 may be from 50 mol.% to 95 mol.%, for example from 50 mol.% to 95 mol.%, from 55 mol.% to 95 mol.%, from 60 mol.% to 95 mol.%, from 65 mol.% to 95 mol.%, from 70 mol.% to 95 mol.%, from 75 mol.% to 95 mol.%, from 80 mol.% to 95mol.%, from 85 mol.% to 95 mol.%, from 90 mol.% to 95 mol.%, from 50 mol.% to 90 mol.%, from 50 mol.% to 80 mol.%, from 50 mol.% to 70 mol.%, or even from 50 mol.% to 60 mol.%.

[0043] The second permeate 104 may comprise an increased concentration of the target chemical species. The second permeate 104 may be introduced to an atmosphere outside of the mechanical apparatus where the second membrane 120 is housed. In some embodiments, the concentration of the target chemical species in the second permeate 104 may be from 95 mol.% to 99.9 mol.%, for example from 96 mol.% to 99.9 mol.%, from 97 mol.% to 99.9 mol.%, from 98 mol.% to 99.9 mol.%, or from 99 mol.% to 99.9 mol.%. In some embodiments, the concentration of the secondary chemical species in the second permeate 104 may be from 0.01 mol.% to 5 mol.%, for example from 1 mol.% to 5 mol.%, from 2 mol.% to 5 mol.%, from 3 mol.% to 5 mol.%, or from 4 mol.% to 5 mol.%.

[0044] In embodiments, the first membrane 110 and the second membrane 120 may comprise any membrane material sufficient to provide a separation mechanism for hydrogen and a carbon-containing compound, such as ethylene. For example, the membrane material of the first membrane 110 and / or the second membrane 120 may comprise, but are not limited to, a carbon molecular sieve, a polymer, zeolites, metal organic frameworks, facilitated transport membranes, and mixed matrix membranes with different transport properties.

[0045] CMS membranes are typically produced through thermal pyrolysis of polymer precursors. For example, it is known that defect-free hollow fiber CMS membranes can be produced by pyrolyzing cellulose hollow fibers. In addition, many other polymers have been used to produce CMS membranes in fiber and dense film form, among which polyimides have been favored. Polyimides have a high glass transition temperature, are easy to process, and perform better than most other polymeric membranes, even prior to pyrolysis.

[0046] In embodiments, the hollow fiber CMS membrane may be asymmetric. As used herein, the term “asymmetric” refers to a property of the hollow fiber CMS membrane in which the hollow fiber CMS membrane has at least one relatively more dense layer and at least one relatively less dense layer. For instance, in embodiments, one layer of the hollow fiber CMS membrane may be greater than or equal to 1 pm and less than or equal to 10 pm and be more dense than a second layer. The second layer may be thicker than the first layer,such as greater than or equal to 20 pm and less than or equal to 200 pm. In embodiments, the asymmetric membrane may be an entity composed of an extremely thin, dense skin over a thick porous substructure, which may be of the same or different material as that of the dense skin layer. In embodiments, the asymmetric membrane may be fabricated in a single step by phase inversion, or the thin layer may be coated on the pre-prepared porous support using a dip coating method. These layers in the asymmetric membranes may be created physically by coating or created by chemical modification. The asymmetric membrane may be in the form of a hollow fiber configuration or a film configuration. In embodiments, the asymmetric membrane may contain a third layer of the same or different material as needed to enhance the membrane performance.

[0047] The gas permeation properties of a hollow fiber CMS membrane may be determined by gas permeation experiments. Two intrinsic properties have utility in evaluating separation performance of a membrane material: its “permeability,” a measure of the hollow fiber CMS membrane’s intrinsic productivity; and its “selectivity,” a measure of the hollow fiber CMS membrane’s separation efficiency. One typically determines “permeability” in Barrer (1 Barrer=10'10[cm3(STP) cm] / [cm2s cmHg], calculated as the flux (?ij) divided by the partial pressure difference between the hollow fiber CMS membrane upstream and downstream ( Pi), and multiplied by the thickness of the hollow fiber CMS membrane (Z).

[0048] Another term, “permeance,” is defined herein as productivity of asymmetric hollow fiber membranes and is typically measured in Gas Permeation Units (GPU) (1 GPU = 106[cm3(STP)] / [cm2s cmHg]), determined by dividing permeability by effective membrane separation layer thickness.

[0049] Finally, “selectivity” is defined herein as the ability of one gas’s permeability through the hollow fiber CMS membrane or permeance relative to the same property of another gas. It is measured as a unitless ratio.

[0050] Now referring to FIG. 3, a cross section of an asymmetric hollow carbon fiber 308 according to embodiments is shown. The asymmetric hollow carbon fiber 308 may comprise an outer wall surrounding a hollow interior space 310. The outer wall may comprise a dense separating layer 314 and a porous support layer 312 between the dense separating layer 314 and the hollow interior space 310. The dense separating layer 314 may have a thickness of from about 2 microns to about 6 microns. The dense separating layer 314 and the porous support layer 312 may comprise a nanographitic structure with interplanar spacings less than 3.6 angstroms and a basal plane greater than 1.3 nm. The dense separating layer 314 may comprise micropores with radii greater than 4 angstroms. Methods for making such fibers and for making carbon molecular sieve membranes comprising such fibers are also disclosed in embodiments. Further, methods for using a carbon molecular sieve membrane comprising such fibers to separate ethylene from a gas feed comprising ethylene and ethane are also disclosed in embodiments.

[0051] Without being bound by theory it is believed that the combination of properties of the asymmetric hollow carbon fibers disclosed herein such as the thickness of the dense separating layer 314, interplanar spacing size, basal plane size, and micropore size are, taken together, associated with high gas selectivity without an accompanying decrease in gas permeance. It is believed that this combination of properties may act cooperatively to provide an improvement in gas selectivity and gas permeance when compared to hollow carbon fibers that do not have the disclosed combination of properties.

[0052] In embodiments, the hollow carbon fibers may be asymmetric. As used herein, the term “asymmetric” refers to a property of the hollow carbon fibers in which the hollow carbon fiber has at least one relatively more dense layer, which may be the dense separating layer 314 and at least one relatively less dense layer, which may be the porous support layer 312. For instance, in embodiments, one layer of the hollow carbon may be greater than or equal to 2 pm and less than or equal to 6 pm and be more dense than a second layer. The second layer may be thicker than the first layer, such as greater than or equal to 20 pm and less than or equal to 200 pm. In one or more embodiments, the dense separating layer 314 may have a density greater than the density of the porous support layer 120.

[0053] In one or more embodiments, the dense separating layer 314 may have a thickness of from about 2 microns to about 6 microns. For example, the dense separating layer 314 mayhave a thickness of from 2 microns to 6 microns, such as from 2 microns to 5 microns, from 2 microns to 4 microns, from 2 microns to 3 microns, from 3 microns to 6 microns, from 3 microns to 5 microns, from 3 microns to 4 microns, from 4 microns to 6 microns, from 4 microns to 5 microns, or from 5 microns to 6 microns. Without being bound by theory, it is believed that a dense separating layer 110 having a thickness from about 2 microns to about 6 microns, when part of the combination of asymmetric hollow carbon fiber properties as disclosed herein may improve the gas selectivity of the fiber when compared to asymmetric hollow carbon fibers that do not have a dense separating layer 110 having a thickness from about 2 microns to about 6 microns.

[0054] In one or more embodiments, the dense separating layer 314 and the porous support layer 312 may comprise a nanographitic structure. As used in the present disclosure the term “nanographitic” refers to a structure comprising graphite crystals less than 100 nm in size. In one or more embodiments the nanographitic structure may have interplanar spacings less than 3.6 angstroms (A). As used in the present disclosure the term “interplanar spacings” refers to the distance between the planes of carbon atoms that form graphite crystals. For example, the nanographitic structure may have interplanar spacings less than 3.5 A, less than 3.4 A, or even less than 3.35 A. In some embodiments, the nanographitic structure may have interplanar spacings from 3.3 A to 3.6 A. For example, the nanographitic structure may have interplanar spacings from 3.3 A to 3.6 A, from 3.3 A to 3.5 A, from 3.3 A to 3.4 A, from 3.4 A to 3.6 A, from 3.4 A to 3.5 A, or from 3.5 A to 3.6 A.

[0055] The basal plane of the nanographitic structure may, according to one or more embodiment be greater than 1.3 nm. As used in the present disclosure the term “basal plane” refers to the size of the planes of carbon atoms that form graphite crystals. For example, the basal plane of the nanographitic structure may be greater than 1.5 nm, greater than 5 nm, greater than 10 nm, greater than 20 nm, greater than 30 nm, greater than 40 nm, or even greater than 50 nm. In some embodiments the basal plane may be less than 100 nm. For example, the basal plane of the nanographitic structure may be from 1.3 nm to 100 nm, from 1.3 nm to 75 nm, from 1.3 nm to 50 nm, from 1.3 nm to 40 nm, from 1.3 nm to 30 nm, from 1.3 nm to 20 nm, from 1.3 nm to 10 nm, from 1.3 nm to 5 nm, from 1.3 nm to 1.5 nm, from 1.5 nm to 100 nm, from 1.5 nm to 75 nm, from 1.5 nm to 50 nm, from 1.5 nm to 40 nm, from 1.5 nm to 30 nm, from 1.5 nm to 20 nm, from 1.5 nm to 10 nm, from 1.5 nm to 5 nm, from 5 nm to 100 nm, from 5 nm to 75 nm, from 5 nm to 50 nm, from 5 nm to 40 nm, from 5 nm to30 nm, from 5 nm to 20 nm, from 5 nm to 10 nm, from 10 nm to 100 nm, from 10 nm to 75 nm, from 10 nm to 50 nm, from 10 nm to 40 nm, from 10 nm to 30 nm, from 10 nm to 20 nm, from 20 nm to 100 nm, from 20 nm to 75 nm, from 20 nm to 50 nm, from 20 nm to 40 nm, from 20 nm to 30 nm, from 30 nm to 100 nm, from 30 nm to 75 nm, from 30 nm to 50 nm, from 30 nm to 40 nm, from 40 nm to 100 nm, from 40 nm to 75 nm, from 40 nm to 50 nm, from 50 nm to 100 nm, from 50 nm to 75 nm, or from 75 nm to 100 nm.

[0056] Without being bound by theory it is believed that having a nanographitic structure with a basal plane greater than 1.3 nm and interplanar spacings less than 3.6 A may be associated with asymmetric hollow carbon fibers with improved gas selectivity and gas permeance when compared to asymmetric hollow carbon fibers without such a nanographitic structure.

[0057] As disclosed herein, in one or more embodiments, the dense separating layer 314 may comprise micropores. In one or more embodiments, the micropores may have radii greater than 4 A as determined by small angle x-ray scattering. For example, the micropores may have radii greater than 4.2 A, greater than 4.4 A, greater than 4.6 A, greater than 4.8 A, greater than 5.0 A, greater than 5.2 A, greater than 5.4 A, greater than 5.6 A, greater than 5.8 A, or even greater than 6.0 A. In one or more embodiments, the micropores may have radii less than 10 A as determined by small angle x-ray scattering. For example, the micropores may have radii from 4.2 A to 10 A, from 4.2 A to 9.5 A, from 4.2 A to 9 A, from 4.2 A to 8.5 A, from 4.2 A to 8 A, from 4.2 A to 7.5 A, from 4.2 A to 7 A, from 4.2 A to 6.5 A, from 4.2 A to 6 A, from 4.2 A to 5.5 A, from 4.2 A to 5 A, from 4.2 A to 4.5 A, from 4.5 A to 10 A, from 4.5 A to 9.5 A, from 4.5 A to 9 A, from 4.5 A to 8.5 A, from 4.5 A to 8 A, from 4.5 A to 7.5 A, from 4.5 A to 7 A, from 4.5 A to 6.5 A, from 4.5 A to 6 A, from 4.5 A to 5.5 A, from 4.5 A to 5 A, from 5 A to 10 A, from 5 A to 9.5 A, from 5 A to 9 A, from 5 A to 8.5 A, from 5 A to 8 A, from 5 A to 7.5 A, from 5 A to 7 A, from 5 A to 6.5 A, from 5 A to 6 A, from 5 A to 5.5 A, from 5.5 A to 10 A, from 5.5 A to 9.5 A, from 5.5 A to 9 A, from 5.5 A to 8.5 A, from 5.5 A to 8 A, from 5.5 A to 7.5 A, from 5.5 A to 7 A, from 5.5 A to 6.5 A, from 5.5 A to 6 A, from 6 A to 10 A, from 6 A to 9.5 A, from 6 A to 9 A, from 6 A to 8.5 A, from 6 A to 8 A, from 6 A to 7.5 A, from 6 A to 7 A, from 6 A to 6.5 A, from 6.5 A to 10 A, from 6.5 A to 9.5 A, from 6.5 A to 9 A, from 6.5 A to 8.5 A, from 6.5 A to 8 A, from 6.5 A to 7.5 A, from 6.5 A to 7 A, from 7 A to 10 A, from 7 A to 9.5 A, from 7 A to 9 A, from 7 A to 8.5 A, from 7 A to 8 A, from 7 A to 7.5 A, from 7.5 A to 10 A, from 7.5 A to 9.5 A, from7.5 A to 9 A, from 7.5 A to 8.5 A, from 7.5 A to 8 A, from 8 A to 10 A, from 8 A to 9.5 A, from 8 A to 9 A, from 8 A to 8.5 A, from 8.5 A to 10 A, from 8.5 A to 9.5 A, from 8.5 A to 9 A, from 9 A to 10 A, from 9 A to 9.5 A, or from 9.5 A to 10 A. Without being bound by theory it is believed that asymmetric hollow carbon fibers comprising a dense separating layer 110 comprising micropores with radii greater than 4 A as determined by small angle may improve the gas selectivity of asymmetric hollow carbon fibers when compared to asymmetric hollow fibers that do not comprise micropores with radii greater than 4 A.

[0058] In one or more embodiments, the asymmetric hollow carbon fibers may have a nanocrystalline graphite structure. As used in the present disclosure the term “nanocrystalline graphite” refers to a fiber that has transitioned from an amorphous or non-crystalline carbon structure to a crystalline graphite structure. The transition from amorphous carbon to crystalline graphite may be determined using Raman Spectroscopy. Raman spectroscopy uses light to measure molecular vibrations and these vibrations can be used to determine characteristics such as molecular structure and crystallinity. Without being bound by theory, it is believed that as a carbon fiber transitions from an amorphous structure to a nanocrystalline structure the intensity ratio of the Raman D peak to the Raman G peak initially increases followed by a decrease in the intensity ratio. It is believed that this “turnover” of the intensity ratio may be used to determine if an asymmetric hollow carbon fiber 100 has transitioned from an amorphous carbon structure to a nanocrystalline graphite structure.

[0059] In one or more embodiments, the asymmetric hollow carbon fiber 308 may comprise less than 50% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber. For example, the asymmetric hollow carbon fiber 308 may comprise less 45% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber, such as less than 40%, less than 35%, less than 30%, less than 25%, less than 20%, less than 15%, or even less than 10% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber. Without being bound by theory it is believed that a pyrrole based nitrogen percentage of less than 50 % of the total amount of nitrogen in the fiber may indicate that the fiber has been formed via pyrolysis at a higher temperature compared to a fiber that has a pyrrole based nitrogen percentage of greater than 50 % based on the total amount of nitrogen in the fiber.

[0060] In one or more embodiments, the dense separating layer 314 of the asymmetric hollow carbon fiber 308 may comprise oxygen in an amount greater than 5 atomic% of the total amount of atoms of the dense separating layer 314. In one or more embodiments, the dense separating layer 314 of the asymmetric hollow carbon fiber 100 may comprise oxygen in an amount less than 15 atomic% of the total amount of atoms of the dense separating layer 314. For example, the asymmetric hollow carbon fiber 100 may comprise oxygen in an amount of from 5 atomic% to 15 atomic% of the total atoms of the dense separating layer 110, such as from 5 atomic% to 12.5 atomic%, from 5 atomic% to 10 atomic%, from 5 atomic % to 7.5 atomic%, from 7.5 atomic% to 15 atomic%, from 7.5 atomic% to 12.5 atomic%, from 7.5 atomic% to 10 atomic%, from 10 atomic% to 15 atomic%, from 10 atomic% to 12.5 atomic%, or from 12.5 atomic% to 15 atomic%. Without being bound by theory, it is believed that an oxygen content of from 5 atomic% to 15 atomic% of the total atoms of the dense separating layer may be associated with improved gas permeance and selectivity of the fiber when compared to fibers with oxygen content less than 5 atomic% or greater than 15 atomic%.

[0061] In one or more embodiments, the asymmetric hollow carbon fiber 308 may have a D(002) peak of less than 4.0 A in transmission as measured by X-ray scattering at an energy level of 17 keV. For example, the asymmetric hollow carbon fiber 100 may have a D(002) peak of less than 3.9 A, less than 3.8 A, less than 3.7 A, less than 3.6 A, or even less than 3.5 A. In some embodiments the D(002) peak may be at least 3.3 A in transmission. For example, the asymmetric hollow carbon fiber 100 may have a D(002) peak of from 3.3 A to 4.0 A, from 3.3 A to 3.9 A, from 3.3 A to 3.8 A, from 3.3 A to 3.7 A, from 3.3 A to 3.6 A, from 3.3 A to 3.5 A, from 3.3 A to 3.4 A, from 3.4 A to 4.0 A, from 3.4 A to 3.9 A, from 3.4 A to 3.8 A, from 3.4 A to 3.7 A, from 3.4 A to 3.6 A, from 3.4 A to 3.5 A, from 3.5 A to 4.0 A, from 3.5 A to 3.9 A, from 3.5 A to 3.8 A, from 3.5 A to 3.7 A, from 3.5 A to 3.6 A, from 3.6 A to 4.0 A, from 3.6 A to 3.9 A, from 3.6 A to 3.8 A, from 3.6 A to 3.7 A, from 3.7 A to 4.0 A, from 3.7 A to 3.9 A, from 3.7 A to 3.8 A, from 3.8 A to 4.0 A, from 3.8 A to 3.9 A, or from 3.9 A to 4.0 A. Without being bound by theory, it is believed that a D(002) peak of less than 4.0 A as measured by X-ray scattering at an energy level of 17 keV indicates that the bulk of the fiber carbonized to form an asymmetric hollow carbon fiber 308.

[0062] In one or more embodiments, a plurality of asymmetric hollow carbon fibers may be combined to form a carbon molecular sieve membrane. In embodiments, the carbon molecularsieve membrane may comprise channels that form between the dense separating layers 314 of adjacent fibers. In embodiments, the carbon molecular sieve membrane may not comprise an external support.

[0063] In embodiments, the asymmetric membrane may be an entity composed of an extremely thin, dense skin over a thick porous substructure, which may be of the same or different material as that of the dense skin layer. In embodiments, the asymmetric membrane may be fabricated in a single step by phase inversion, or the thin layer may be coated on the pre-prepared porous support using a dip coating method. These layers in the asymmetric membranes may be created physically by coating or created by chemical modification. The asymmetric membrane may be in the form of a hollow fiber configuration. In embodiments, the asymmetric membrane may contain a third layer of the same or different material as needed to enhance the membrane performance.

[0064] Reverse-

[0065] In some embodiments, the first membrane 110 may be a reverse-selective, carbon molecular sieve (CMS) membrane. In other embodiments, the second membrane 120 may be a reverse-selective, carbon molecular sieve (CMS) membrane. In embodiments, the first membrane 110 or the second membrane 120 may be made by any of the methods disclosed in U.S. Application No. 2022 / 037,840, “Reverse Selective / Surface Flow Polyimide Derived CMS Membrane for Gas Separation,” and / or U.S. Application No. 2022 / 037,838, “Process of Making Reverse Selective / Surface Flow CMS Membrane for Gas Separation,” which are each incorporated by reference in their entirety herein.

[0066] In embodiments, the reverse-selective, hollow fiber CMS membrane, which has been pyrolyzed and oxidized, may have an ethylene / hydrogen selectivity of greater than or equal to 5, such greater than or equal to 5 and less than or equal to 50, greater than or equal to 10 and less than or equal to 50, greater than or equal to 15 and less than or equal to 50, greater than or equal to 20 and less than or equal to 50, greater than or equal to 25 and less than or equal to 50, greater than or equal to 30 and less than or equal to 50, greater than or equal to 35 and less than or equal to 50, greater than or equal to 40 and less than or equal to 50, greater than or equal to 45 and less than or equal to 50, greater than or equal to 5 and less than or equal to 40, such greater than or equal to 5 and less than or equal to 35, greater than or equalto 5 and less than or equal to 30, greater than or equal to 5 and less than or equal to 25, greater than or equal to 5 and less than or equal to 20, greater than or equal to 5 and less than or equal to 15, or even greater than or equal to 5 and less than or equal to 10. It is envisioned that the range of ethylene / hydrogen selectivity may be greater than or equal to any of the selectivities described herein and less than or equal to any of the selectivities described herein.

[0067] In embodiments, the reverse-selective, hollow fiber CMS membrane, which has been pyrolyzed and oxidized, may have a carbon dioxide / hydrogen selectivity of greater than or equal to 5, such greater than or equal to 5 and less than or equal to 50, greater than or equal to 10 and less than or equal to 50, greater than or equal to 15 and less than or equal to 50, greater than or equal to 20 and less than or equal to 50, greater than or equal to 25 and less than or equal to 50, greater than or equal to 30 and less than or equal to 50, greater than or equal to 35 and less than or equal to 50, greater than or equal to 40 and less than or equal to 50, greater than or equal to 45 and less than or equal to 50, greater than or equal to 5 and less than or equal to 40, such greater than or equal to 5 and less than or equal to 35, greater than or equal to 5 and less than or equal to 30, greater than or equal to 5 and less than or equal to 25, greater than or equal to 5 and less than or equal to 20, greater than or equal to 5 and less than or equal to 15, or even greater than or equal to 5 and less than or equal to 10. It is envisioned that the range of carbon dioxide / hydrogen selectivity may be greater than or equal to any of the selectivities described herein and less than or equal to any of the selectivities described herein.

[0068] In embodiments, the reverse-selective, hollow fiber CMS membrane, which has been pyrolyzed and oxidized, may have an ethylene permeability of greater than or equal to 400 GPU when measured while separating an analyte stream comprising ethylene in the permeate, and hydrogen in the retentate, at a temperature that is greater than or equal to 20 °C and less than or equal to 35 °C. For example, the ethylene permeability may be greater than or equal to 400 GPU and less than or equal to 800 GPU, greater than or equal to 400 GPU and less than or equal to 800 GPU, greater than or equal to 450 GPU and less than or equal to 800 GPU, greater than or equal to 500 GPU and less than or equal to 800 GPU, greater than or equal to 550 GPU and less than or equal to 800 GPU, greater than or equal to 600 GPU and less than or equal to 800 GPU, greater than or equal to 650 GPU and less than or equal to 800 GPU, greater than or equal to 700 GPU and less than or equal to 800 GPU, greater than or equal to 750 GPU and less than or equal to 800 GPU, greater than or equal to 400 GPUand less than or equal to 800 GPU, greater than or equal to 400 GPU and less than or equal to 750 GPU, greater than or equal to 400 GPU and less than or equal to 700 GPU, greater than or equal to 400 GPU and less than or equal to 650 GPU, greater than or equal to 400 GPU and less than or equal to 600 GPU, greater than or equal to 400 GPU and less than or equal to 550 GPU, greater than or equal to 400 GPU and less than or equal to 500 GPU, or even greater than or equal to 400 GPU and less than or equal to 450 GPU. It is envisioned that the range of ethylene permeance may be greater than or equal to any of the permeances described herein and less than or equal to any of the permeances described herein.

[0069] In embodiments, the reverse-selective, hollow fiber CMS membrane, which has been pyrolyzed and oxidized, may have a carbon dioxide permeability of greater than or equal to 1200 GPU when measured while separating an analyte stream comprising carbon dioxide in the permeate, and hydrogen in the retentate, at a temperature that is greater than or equal to 20 °C and less than or equal to 35 °C. For example, the carbon dioxide permeability may be greater than or equal to 1200 GPU and less than or equal to 1800 GPU, greater than or equal to 1250 GPU and less than or equal to 1800 GPU, greater than or equal to 1300 GPU and less than or equal to 1800 GPU, greater than or equal to 1350 GPU and less than or equal to 1800 GPU, greater than or equal to 1400 GPU and less than or equal to 1500 GPU, greater than or equal to 1450 GPU and less than or equal to 1800 GPU, greater than or equal to 1500 GPU and less than or equal to 1800 GPU, greater than or equal to 1550 GPU and less than or equal to 1800 GPU, greater than or equal to 1600 GPU and less than or equal to 1800 GPU, greater than or equal to 1650 GPU and less than or equal to 1800 GPU, greater than or equal to 1700 GPU and less than or equal to 1800 GPU, greater than or equal to 1750 GPU and less than or equal to 1800 GPU, greater than or equal to 1200 GPU and less than or equal to 1750 GPU, greater than or equal to 1200 GPU and less than or equal to 1700 GPU, greater than or equal to 1200 GPU and less than or equal to 1650 GPU, greater than or equal to 1200 GPU and less than or equal to 1600 GPU, greater than or equal to 1200 GPU and less than or equal to 1550 GPU, greater than or equal to 1200 GPU and less than or equal to 1500 GPU, greater than or equal to 1200 GPU and less than or equal to 1450 GPU, greater than or equal to 1200 GPU and less than or equal to 1400 GPU, greater than or equal to 1200 GPU and less than or equal to 1350 GPU, greater than or equal to 1200 GPU and less than or equal to 1300 GPU, greater than or equal to 120 GPU and less than or equal to 1300 GPU, or even greater than or equal to 1200 GPU and less than or equalto 1250 GPU. It is envisioned that the range of carbon dioxide permeance may be greater than or equal to any of the permeances described herein and less than or equal to any of the permeances described herein.

[0070] Ultra- Selective Membrane

[0071] In some embodiments, the first membrane 110 may be an ultra-selective, carbon molecular sieve (CMS) membrane. In other embodiments, the second membrane 120 may be an ultra-selective, carbon molecular sieve (CMS) membrane. In embodiments, the first membrane 110 or the second membrane 120 may be made by any of the methods disclosed in U.S. Application No. 2022 / 037,843, "Methods for Manufacturing Hollow Fiber Carbon Membranes", which is incorporated by reference in its entirety herein.

[0072] In embodiments, the ultra-selective, hollow fiber CMS membrane has a hydrogen / ethylene selectivity of greater than or equal to 5, such greater than or equal to 5 and less than or equal to 500, greater than or equal to 10 and less than or equal to 500, greater than or equal to 25 and less than or equal to 500, greater than or equal to 50 and less than or equal to 500, greater than or equal to 75 and less than or equal to 500, greater than or equal to 75 and less than or equal to 500, greater than or equal to 100 and less than or equal to 500, greater than or equal to 125 and less than or equal to 500, greater than or equal to 150 and less than or equal to 500, greater than or equal to 175 and less than or equal to 500, greater than or equal to 200 and less than or equal to 500, greater than or equal to 225 and less than or equal to 500, greater than or equal to 250 and less than or equal to 500, greater than or equal to 275 and less than or equal to 500, greater than or equal to 300 and less than or equal to 500, greater than or equal to 325 and less than or equal to 500, greater than or equal to 375 and less than or equal to 500, greater than or equal to 400 and less than or equal to 500, greater than or equal to 425 and less than or equal to 500, greater than or equal to 450 and less than or equal to 500, greater than or equal to 475 and less than or equal to 500, greater than or equal to 5 and less than or equal to 450, greater than or equal to 5 and less than or equal to 400, such greater than or equal to 5 and less than or equal to 350, greater than or equal to 5 and less than or equal to 300, greater than or equal to 5 and less than or equal to 250, greater than or equal to 5 and less than or equal to 200, greater than or equal to 5 and less than or equal to 150, greater than or equal to 5 and less than or equal to 100, or even greater than or equal to 5 and less than or equal to 50. It is envisioned that the range ofhydrogen / ethylene selectivity may be greater than or equal to any of the selectivities described herein and less than or equal to any of the selectivities described herein.

[0073] In embodiments, the ultra-selective, hollow fiber CMS membrane has a permeate permeability of greater than or equal to 50 GPU when measured while separating an analyte stream comprising a carbon containing compound in the permeate and hydrogen in the retentate, at a temperature that is greater than or equal to 20 °C and less than or equal to 35 °C. For example, the hydrogen permeability may be greater than or equal to 200 GPU and less than or equal to 800 GPU, greater than or equal to 200 GPU and less than or equal to 800 GPU, greater than or equal to 250 GPU and less than or equal to 800 GPU, greater than or equal to 300 GPU and less than or equal to 800 GPU, greater than or equal to 350 GPU and less than or equal to 800 GPU, greater than or equal to 400 GPU and less than or equal to 800 GPU, greater than or equal to 450 GPU and less than or equal to 800 GPU, greater than or equal to 500 GPU and less than or equal to 800 GPU, greater than or equal to 550 GPU and less than or equal to 800 GPU, greater than or equal to 600 GPU and less than or equal to 800 GPU, greater than or equal to 650 GPU and less than or equal to 800 GPU, greater than or equal to 700 GPU and less than or equal to 800 GPU, greater than or equal to 750 GPU and less than or equal to 800 GPU, greater than or equal to 400 GPU and less than or equal to 800 GPU, greater than or equal to 400 GPU and less than or equal to 750 GPU, greater than or equal to 400 GPU and less than or equal to 700 GPU, greater than or equal to 400 GPU and less than or equal to 650 GPU, greater than or equal to 400 GPU and less than or equal to 600 GPU, greater than or equal to 400 GPU and less than or equal to 550 GPU, greater than or equal to 400 GPU and less than or equal to 500 GPU, or even greater than or equal to 400 GPU and less than or equal to 450 GPU,. It is envisioned that the range of hydrogen permeance may be greater than or equal to any of the permeances described herein and less than or equal to any of the permeances described herein.

[0074] It is contemplated that the two-stage membrane separation apparatus 100 may be used in combination with other separation technologies such as a cryogenic distillation column as a hybrid process to achieve higher purity products. The use of the two-stage membrane separation apparatus would decrease the energy load required for cryogenic distillation with a purer starting feed.

[0075] Now referring to FIG. 2, a two-stage membrane separation apparatus 200 suitable for separating a target chemical species from a secondary chemical species is depicted may be substantially similar to the apparatus 100 depicted in FIG. 1. The difference between the apparatus 200 depicted in FIG. 2 and the apparatus 100 depicted in FIG. 1 relates to the introduction of a recycle stream.

[0076] Still referring to FIG. 2, according to one or more embodiments, a mixed gas 101 may be introduced into the first membrane 210 through a gas inlet. The mixed gas 101 may comprise the same target chemical species and secondary chemical species as disclosed in FIG. 1.

[0077] After introducing a mixed gas 101 into the first membrane 210 through a gas inlet, the mixed gas is separated into a first retentate 203 and a first permeate 202 at the first membrane 210. The first retentate 203 may comprise an increased concentration of the target chemical species. The first retentate 203 may be introduced to a second membrane 220.

[0078] In some embodiments, the concentration of the target chemical species within the first retentate 203 may be from 5 mol.% to 40 mol.% , for example from 10 mol.% to 40 mol.%, from 15 mol.% to 40 mol.%, from 20 mol.% to 40 mol.%, from 25 mol.% to 40 mol.%, from 30 mol.% to 40 mol.%, from 35 mol.% to 40 mol.%, from 5 mol.% to 30 mol.%, from 5 mol.% to 20 mol.%, or even from 5 mol.% to 10 mol.%.

[0079] In some embodiments, the concentration of the secondary chemical species within the first retentate 103 may be from 60 mol.% to 95 mol.%, for example from 65 mol.% to 95 mol.%, from 70 mol.% to 95 mol.%, from 75 mol.% to 95 mol.%, from 80 mol.% to 95 mol.%, from 85 mol.% to 95 mol.%, from 90 mol.% to 95 mol.%, from 60 mol.% to 90 mol.%, from 60 mol.% to 80 mol.%, or even from 60 mol.% to 70 mol.%.

[0080] The first permeate 202 may comprise an increased concentration of the secondary chemical species. The first permeate 202 may be introduced to an atmosphere outside of the mechanical apparatus where the first membrane 220 is housed.

[0081] In some embodiments, the concentration of the secondary chemical species in the first permeate 102 may be from 90 mol.% to 99 mol.%, for example from 92 mol.% to 99mol.%, from 94 mol.% to 99 mol.%, from 96 mol.% to 99 mol.%, or from 98 mol.% to 99 mol.%.

[0082] In some embodiments, the concentration of the target chemical species in the first permeate 202 may be from 1 mol.% to 10 mol.%, for example from 2 mol.% to 10 mol.%, from 4 mol.% to 10 mol.%, from 6 mol.% to 10 mol.%, or from 8 mol.% to 10 mol.%.

[0083] After introducing the first retentate 203 into the second membrane 120, the first retentate is separated into a second retentate 205 and a second permeate 204 at the second membrane 220. The second retentate 205 may comprise an increased concentration of the secondary chemical species. The second retentate 205 may be combined with the mixed gas 101 to form a recycled feedstream 201 upstream of the second retentate 205.

[0084] In some embodiments, the concentration of the secondary chemical species within the second retentate 205 may be from 50 mol.% to 95 mol.%, for example from 50 mol.% to 95 mol.%, from 55 mol.% to 95 mol.%, from 60 mol.% to 95 mol.%, from 65 mol.% to 95 mol.%, from 70 mol.% to 95 mol.%, from 75 mol.% to 95 mol.%, from 80 mol.% to 95 mol.%, from 85 mol.% to 95 mol.%, from 90 mol.% to 95 mol.%, from 50 mol.% to 90 mol.%, from 50 mol.% to 80 mol.%, from 50 mol.% to 70 mol.%, or even from 50 mol.% to 60 mol.%.

[0085] In some embodiments, the concentration of the target chemical species within the second retentate 205 may be from 5 mol.% to 50 mol.% for example from 10 mol.% to 50 mol.%, from 15 mol.% to 50 mol.%, from 20 mol.% to 50 mol.%, from 25 mol.% to 50 mol.%, from 30 mol.% to 50 mol.%, from 35 mol.% to 50 mol.%, from 40 mol.% to 50 mol.%, from 45 mol.% to 50 mol.%, from 5 mol.% to 40 mol.%, from 5 mol.% to 30 mol.%, from 5 mol.% to 20 mol.%, or even from 5 mol.% to 10 mol.%.

[0086] The second permeate 204 may comprise an increased concentration of the target chemical species. The second permeate 204 may be introduced to an atmosphere outside of the mechanical apparatus where the second membrane 220 is housed. In some embodiments, the concentration of the target chemical species in the second permeate 204 may be from 90 mol.% to 98 mol.%, for example from 92 mol.% to 98 mol.%, from 94 mol.% to 98 mol.%, or from 96 mol.% to 98 mol.%. In some embodiments, the concentration of the secondarychemical species in the second permeate 204 may be from 2 mol.% to 10 mol.%, for example from 4 mol.% to 10 mol.%, from 6 mol.% to 10 mol.%, or from 8 mol.% to 10 mol.%.

[0087] Although the membrane separation apparatus depicted in FIG. 1 and FIG. 2 contain two stages, it is contemplated that the system could contain any number of stages. For example, a membrane separation apparatus could contain three stages, where the second permeate 104 or 204 is introduced to a third membrane, where the second permeate 104 or 204 is further separated into a third permeate and a second permeate. The third membrane could be the same as the first membrane or the second membrane, and the third permeate may comprise an increased concentration of the target chemical species, and the third retentate comprises an increased concentration of the at least one other chemical species.EXAMPLES

[0088] Examples are provided herein which may disclose one or more embodiments of the present disclosure. However, the Examples should not be viewed as limiting on the claimed embodiments hereinafter provided.

[0089] Hydrogen and Ethylene

[0090] Table 1 summarizes the performance data for various combinations of reverse- selective and ultra-selective membranes, as described in embodiments herein. Advanced carbon molecular sieve membranes were utilized with reverse-selective and ultra-selective gas transport properties and may be manufactured according to any of the methods described or disclosed above. The mixed gas feed comprised 50% ethylene and 50% hydrogen. In each case, the feed pressure was 52 psig, and the first permeate pressure was 2 psig. The feed temperature was 35 °C at a flow rate of 150 seem. All concentrations are listed in mol.%.

[0091] Four process simulation examples were studied to understand the multi-stage membrane process proposed in this invention. Reverse-selective membranes showed an ethylene permeance of 611 GPU and a hydrogen permeance of 37 GPU with an ethylene / hydrogen selectivity of 16.67. Ultra-selective membranes showed an ethylene permeance of 2.7 GPU and a hydrogen permeance of 506 GPU with a hydrogen / ethylene selectivity of 186.Table 1

[0092] As seen above, the inventive examples 1-4 show that regardless of the order of the reverse-selective membrane and ultra-selective membrane, the system yields very similar results for the same mixed gas.

[0093] Inventive Example 1 utilizes a reverse-selective membrane followed by an ultra- selective membrane. The first permeate comprises 90.36 mol.% ethylene after separation of the mixed gas through the reverse-selective membrane. The second permeate comprises 98.97 mol.% hydrogen after separation of the first retentate through the ultra-selective membrane. Similar results are seen with Inventive Example 3, where the order of the reverse- selective and ultra-selective membranes are switched. The first permeate comprises 98.94 mol.% hydrogen after separation of the mixed gas through the ultra-selective membrane. The second permeate comprises 90.61 mol.% ethylene after separation of the first retentate through the reverse-selective membrane.

[0094] As demonstrated in Inventive Examples 1 and 3, each membrane captures either mostly hydrogen or mostly ethylene, regardless of the order of the membranes. Additionally, the membrane system achieves these compositions without the need for inter-stage compression. Thus, each component, hydrogen or ethylene, may be isolated and separated to the desired purity.

[0095] Inventive Example 2 utilizes a reverse-selective membrane followed by an ultra- selective membrane with a recycle. The first permeate comprises 96.16 mol.% ethylene after separation of the mixed gas through the reverse-selective membrane. The second permeate comprises 95.69 mol.% hydrogen after separation of the first retentate through the ultra- selective membrane. Similar results are seen with Inventive Example 4, where the order ofthe reverse-selective and ultra-selective membranes are switched and the recycle is still included. The first permeate comprises 95.56 mol.% hydrogen after separation of the mixed gas through the ultra-selective membrane. The second permeate comprises 96.02 mol.% ethylene after separation of the first retentate through the reverse-selective membrane.

[0096] Notably, adding the recycle stream, as seen in Inventive Examples 2 and 4, decreases the purity of hydrogen when compared to Inventive Examples 1 and 3, where hydrogen purity is 98.97 mol.% and 98.94 mol.%. However, the recycle stream in each of Inventive Examples 2 and 4 allows the system to achieve greater ethylene purity than Inventive Examples 1 and 3, where ethylene purity is 90.36 mol.% and 90.61 mol.%. Without being bound by any theory, it is believed that the recycle stream systems result in a lower purity of hydrogen because adding additional ethylene to the feedstream results in dilution of the hydrogen in the feedstream, which in turn results in a lower concentration hydrogen and a lower driving force. Alternatively, without being bound by any theory, it is believed that the recycle stream systems result in a lower purity of hydrogen in a trade-off for higher recovery of the same. As a result, the recycle systems could be utilized where a higher purity of ethylene is desired or needed.

[0097] The comparative examples Cl and C2 show similar achievements for singlecomposition purity. For example, Comparative Example Cl demonstrates that the utilizing of two reverse-selective membranes achieves about 90 mol.% purity for ethylene, while Comparative Example C2 demonstrates that the use of two ultra-selective membranes achieves about a 98 mol.% purity for hydrogen. However, without the combination of both a reverse-selective membrane and an ultra-selective membrane, only one species of relatively high purity can be achieved, and inter-stage compression is required, increasing the energy needs of the system.

[0098] Hydrogen and Carbon Dioxide

[0099] Table 2 summarizes the performance data for various combinations of reverse- selective and ultra-selective membranes, as described in embodiments herein. Advanced carbon molecular sieve membranes were utilized with reverse-selective and ultra-selective gas transport properties and may be manufactured according to any of the methods described or disclosed above. The mixed gas feed comprised 50% carbon dioxide and 50% hydrogen.In each case, the feed pressure was 52 psig, and the first permeate pressure was 2 psig. The feed temperature was 35 °C at a flow rate of 150 seem. All concentrations are listed in mol.%.

[0100] Four process simulation examples were studied to understand the multi-stage membrane process proposed in this invention. Reverse-selective membranes showed a carbon dioxide permeance of 1500 GPU and a hydrogen permeance of 75 GPU with a carbon dioxide / hydrogen selectivity of 20. Ultra-selective membranes showed a carbon dioxide permeance of 15 GPU and a hydrogen permeance of 500 GPU with a hydrogen / carbon dioxide selectivity of 33.Table 2

[0101] As seen above, the inventive examples 5-8 show that regardless of the order of the reverse-selective membrane and ultra-selective membrane, the system yields very similar results for the same mixed gas.

[0102] Inventive Example 5 utilizes a reverse-selective membrane followed by an ultra- selective membrane. The first permeate comprises 91.8 mol.% carbon dioxide after separation of the mixed gas through the reverse-selective membrane. The second permeate comprises 94.8 mol.% hydrogen after separation of the first retentate through the ultra- selective membrane. Similar results are seen with Inventive Example 7, where the order of the reverse-selective and ultra-selective membranes are switched. The first permeate comprises 94.7 mol.% hydrogen after separation of the mixed gas through the ultra-selective membrane. The second permeate comprises 92.0 mol.% carbon dioxide after separation of the first retentate through the reverse-selective membrane.

[0103] As demonstrated in Inventive Examples 5 and 7, each membrane captures either mostly hydrogen or mostly carbon dioxide, regardless of the order of the membranes.Additionally, the membrane system achieves these compositions without the need for interstage compression. Thus, each component, hydrogen or carbon dioxide, may be isolated and separated to the desired purity.

[0104] Inventive Example 6 utilizes a reverse-selective membrane followed by an ultra- selective membrane with a recycle. The first permeate comprises 91.2 mol.% carbon dioxide after separation of the mixed gas through the reverse-selective membrane. The second permeate comprises 95.2 mol.% hydrogen after separation of the first retentate through the ultra-selective membrane. Similar results are seen with Inventive Example 8, where the order of the reverse-selective and ultra-selective membranes are switched and the recycle is still included. The first permeate comprises 96.2 mol.% hydrogen after separation of the mixed gas through the ultra-selective membrane. The second permeate comprises 88.8 mol.% carbon dioxide after separation of the first retentate through the reverse-selective membrane.

[0105] Notably, adding the recycle stream, as seen in Inventive Examples 6 and 8, decreases the purity of carbon dioxide when compared to Inventive Examples 5 and 7, where carbon dioxide purity is 91.8 mol.% and 92.0 mol.%. However, the recycle stream in each of Inventive Examples 6 and 8 allows the system to achieve greater hydrogen purity than Inventive Examples 5 and 7, where hydrogen purity is 94.8 mol.% and 94.7 mol.%. As a result, the recycle systems could be utilized where a higher purity of hydrogen is desired or needed.

[0106] The comparative examples C3 and C4 show similar achievements for singlecomposition purity. For example, Comparative Example C3 demonstrates that the utilizing of two reverse-selective membranes achieves about 91 mol.% purity for carbon dioxide, while Comparative Example C4 demonstrates that the use of two ultra-selective membranes achieves about a 95 mol.% purity for hydrogen. However, without the combination of both a reverse-selective membrane and an ultra-selective membrane, only one species of relatively high purity can be achieved, and inter-stage compression is required, increasing the energy needs of the system.

[0107] It should now be understood that reverse-selective and ultra-selective membranes can be utilized interchangeably in the separation of ethylene and hydrogen. Regardless of the order of the membranes, very similar product purities are achieved. Additionally, thiscombination of reverse-selective and ultra-selective membranes results in achieving the purification of both ethylene and hydrogen in the same system without the need for interstage recompression.

[0108] The subject matter of the present disclosure has been described in detail and by reference to specific embodiments. It should be understood that any detailed description of a component or feature of an embodiment does not necessarily imply that the component or feature is essential to the particular embodiment or to any other embodiment. Further, it should be apparent to those skilled in the art that various modifications and variations can be made to the described embodiments without departing from the spirit and scope of the claimed subject matter.

[0109] It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present technology, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”

[0110] Any quantitative value expressed in the present application may be considered to include open-ended embodiments consistent with the transitional phrases “comprising” or “including.”[OHl] It should be understood that where a first component is described as “comprising” a second component, it is contemplated that, in some embodiments, the first component “consists” or “consists essentially of’ that second component. It should further be understood that where a first component is described as “comprising” a second component, it is contemplated that, in some embodiments, the first component comprises at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or even at least 99% that second component (where % is molar % unless otherwise specified).

[0112] It should be understood that any two quantitative values assigned to a property may constitute a range of that property, and all combinations of ranges formed from all stated quantitative values of a given property are contemplated in this disclosure. It should be appreciated that compositional ranges of a chemical constituent in a composition should beappreciated as containing, in some embodiments, a mixture of isomers of that constituent. In additional embodiments, the chemical compounds may be present in alternative forms such as derivatives, salts, hydroxides, etc.

[0113] It is also noted that recitations herein of “at least one” component, element, etc., should not be used to create an inference that the alternative use of the articles “a” or “an” should be limited to a single component, element, etc.

[0114] It should further be understood that streams may be named for the components of the stream, and the component for which the stream is named may be the major component of the stream (such as comprising from 50 weight percent (wt. %), from 70 wt. %, from 90 wt. %, from 95 wt. %, from 99 wt. %, from 99.5 wt. %, or even from 99.9 wt. % of the contents of the stream to 100 wt. % of the contents of the stream). It should also be understood that components of a stream are disclosed as passing from one system component to another when a stream comprising that component is disclosed as passing from that system component to another. For example, a disclosed “mixed gas” passing from a first system component to a second system component should be understood to equivalently disclose “hydrogen” passing from a first system component to a second system component, and the like.

Claims

CLAIMS1. A process for separating hydrogen (H2) and a carbon-containing compound, the process comprising: introducing a mixed gas to a first membrane, the mixed gas comprising a target chemical species and a secondary chemical species; separating a first retentate and a first permeate at the first membrane, wherein the first retentate comprises an increased concentration of the target chemical species and the first permeate comprises an increased concentration of the secondary chemical species; introducing the first retentate to a second membrane; and separating a second retentate and a second permeate at the second membrane, wherein the second retentate comprises an increased concentration of the secondary chemical species and the second permeate comprises an increased concentration of the target chemical species, wherein the target chemical species is H2 or the carbon-containing compound, the secondary chemical species is H2 or the carbon-containing compound, and the target chemical species is different than the secondary chemical species.

2. The process of claim 1, wherein the carbon-containing compound comprises ethylene (C2H4).

3. The process of claim 1 or 2, wherein: separating the first retentate and the first permeate at the first membrane comprises introducing the mixed gas to a reverse-selective membrane, the reverse- selective membrane comprising a carbon molecular sieve (CMS) membrane operable to produce the first retentate having an increased concentration of the target chemical species and the first permeate having an increased concentration of the secondary chemical species, wherein the target chemical species is H2.

4. The process of claim 3, wherein separating the second retentate and the second permeate at the second membrane comprises introducing the first retentate to an ultra-selective membrane, the ultra- selective membrane comprising a carbon molecular sieve (CMS) membrane operable to produce the second permeate having an increased concentration of the target chemical species and the second retentate having an increased concentration of the secondary chemical species.

5. The process of claim 1 or 2, wherein: separating the first retentate and the first permeate at the first membrane comprises introducing the mixed gas to an ultra-selective membrane, the ultra-selective membrane comprising a carbon molecular sieve (CMS) membrane operable to produce the first permeate having an increased concentration of the target chemical species and the first retentate having an increased concentration of the secondary chemical species, wherein the target chemical species is C2H4.

6. The process of claim 5, wherein: separating the second retentate and the second permeate at the second membrane comprises introducing the first retentate to a reverse- selective membrane, wherein the reverse- selective membrane is operable to produce the second retentate having an increased concentration of the target chemical species and the second permeate having an increased concentration of the secondary chemical species.

7. The process of any one of claims 3 to 6, wherein the reverse-selective membrane is a hollow fiber or film CMS membrane comprising a pyrolyzed and oxidized polyimide membrane.

8. The process of claim 7, wherein the reverse-selective membrane has a selectivity greater than or equal to 10.

9. The process of claim 7 or 8, wherein the reverse-selective membrane has a permeate permeability greater than or equal to 100 GPU when measured while separating an analyte stream comprising a permeate and a retentate at a temperature that is greater than or equal to 20 °C and less than or equal to 35 °C.

10. The process of any one of claims 3 to 6, wherein the ultra- selective membrane is a CMS membrane comprising a plurality of asymmetric hollow carbon fibers, the plurality of asymmetric hollow fibers comprising an outer wall surrounding a hollow interior space.

11. The process of claim 10, wherein:the outer wall comprises a dense separating layer having a thickness from 2 microns to 6 microns and a porous support layer between the dense separating layer and the hollow interior space; and channels form between the dense separating layers of adjacent asymmetric hollow carbon fibers.

12. The process of claim 10 or 11, wherein the dense separating layer and the porous support layer comprise: a nanographitic structure with interplanar spacings less than 3.6 angstroms; a basal plane greater than 1.3 nm; and the dense separating layer comprises micropores with radii greater than 4 angstroms.

13. The process of any previous claim, further comprising: introducing the second permeate to a third membrane; and separating a third retentate and a third permeate at the third membrane.

14. The process of claim 13, wherein: the third membrane is the same as the first membrane or the second membrane; and the third permeate comprises an increased concentration of the target chemical species and the third retentate comprises an increased concentration of the secondary chemical species.

15. The process of any previous claim, further comprising combining the second retentate with the mixed gas.

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