Reverse selective carbon film and manufacturing method thereof

Abstract

A method of making a reverse selective asymmetric hollow fiber carbon membrane may include heating an asymmetric hollow fiber carbon membrane. The method may also include exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse selective asymmetric hollow fiber carbon membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere, the weight of the reverse selective asymmetric hollow fiber carbon membrane may be from-5% to 20% by weight less than the weight of the asymmetric hollow fiber carbon membrane.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application Serial No. 63 / 480,737, filed on January 20, 2023, the entire disclosure of which is hereby incorporated by reference herein. Technical Field

[0003] Embodiments of the present disclosure relate generally to hollow fiber carbon molecular sieve (CMS) membranes for gas separations, and particularly to methods for producing asymmetric hollow fiber CMS membranes with reverse selectivity. Background Art

[0004] Membranes are widely used for the separation of gases and liquids, including, for example, the separation of acidic gases, such as CO2 and H2S from natural gas, and the removal of O2 from air. Gas transport through such membranes is typically modeled by an adsorption-diffusion mechanism. Currently, polymeric membranes have been well studied and widely used for gas separation due to their ease of processing and low cost. However, CMS membranes have been shown to have attractive separation performance characteristics that exceed those of polymeric membranes.

[0005] CMS membranes are typically produced by pyrolysis of polymer precursors. For example, it is well known that defect-free hollow fiber CMS membranes can be produced by pyrolysis of cellulose hollow fibers. In addition, many other polymers have been used to produce CMS membranes in fiber and dense membrane form, with polyimides being favored. Polyimides have high glass transition temperatures, are easy to process, and even before pyrolysis, perform better than most other polymer membranes.

[0006] The separation characteristics of CMS membranes are primarily influenced by the following factors: (1) the pyrolysis precursor, (2) the pyrolysis temperature, (3) the heat soak time, and (4) the pyrolysis atmosphere. For example, increases in both temperature and heat soak time have been shown to increase selectivity but decrease permeability for CO₂ / CH₄ separations. Furthermore, precursor polymers with a rigid, tightly packed structure tend to result in CMS membranes with higher selectivity compared to less rigid precursor polymers.

[0007] In gas separation technology, it is desirable to produce reverse selective / surface flow membranes as reported in the review articles by D. Paul et al. (Progress in Polymer Science, 38, (2013) 740–766) and Hill et al. (Science, Vol. 296, April 19, 2002). Most of the development in this field has focused on polymer membranes. However, in the field of CMS membranes, there are not many reports in the literature on the formation of reverse selective asymmetric hollow filament CMS membranes. Reverse selective membranes are generally prepared by incorporating specific functional groups that enhance the solubility of one gas relative to another during the separation of gas molecules. Synthetically, the process of producing such chemical compositions is easier for polymer membranes than for CMS membranes because it is difficult to incorporate such functional groups into CMS membranes. For example, pyrolysis temperatures greater than 500°C convert most of the polymer structure into a carbon structure, and most of the functional groups that can enhance the solubility of gas molecules are therefore lost during pyrolysis. Therefore, it is difficult to enhance solubility-driven gas transport in CMS membranes, and the membrane separation performance is mainly controlled by size-based or diffusion-based separation mechanisms.

[0008] Therefore, there is a need to produce asymmetric hollow fiber carbon membranes that also function as reverse selective gas transport membranes. Summary of the Invention

[0009] One type of separation application in which CMS membranes can be used is gas separation. For example, CMS membranes that can be used to separate CO2 from H2, CO2 from N2, and ethylene from H2 are desirable.

[0010] According to one or more embodiments described herein, a method for manufacturing a reverse-selective asymmetric hollow fiber carbon membrane may include heating the asymmetric hollow fiber carbon membrane. The method may further include exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse-selective asymmetric hollow fiber carbon membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere, the weight of the reverse-selective asymmetric hollow fiber carbon membrane may be -5 wt% to 20 wt% less than the weight of the asymmetric hollow fiber carbon membrane.

[0011] It should be understood that both the foregoing summary and the following detailed description present embodiments of the present technology and are intended to provide an overview or framework for understanding the nature and features of the technology as claimed. The accompanying drawings are included to provide an additional understanding of the technology and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and, together with the description, serve to explain the principles and operation of the technology. Additionally, the drawings and description are meant to be illustrative only and are not intended to limit the scope of the claims in any way.

[0012] Additional features and advantages of the described embodiments will be set forth in the detailed description that follows. Additional features and advantages of the described embodiments will be apparent to those skilled in the art in part from this description, or may be learned by practicing the described embodiments (including the detailed description that follows), the accompanying drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The following detailed description of certain embodiments of the present disclosure may be best understood when taken in conjunction with the following drawings, in which like reference numerals indicate like structures, and in which:

[0014] FIG. 1A is a flow chart including steps for making a reverse-selective asymmetric hollow fiber carbon membrane according to one or more embodiments described herein;

[0015] Figure 1B is another flow chart including steps for making a reverse-selective asymmetric hollow fiber carbon membrane according to one or more embodiments described herein;

[0016] Figure 2 is a schematic diagram of a system for pyrolysis and oxidation of hollow fiber CMS membranes according to embodiments described herein;

[0017] Figure 3 is a graph of temperature on the x-axis and simulated O2 concentration on the y-axis for embodiments described herein;

[0018] Figure 4A is a graph depicting weight changes of films formed at different pyrolysis temperatures during oxidation of the films;

[0019] Figure 4B It is a depiction Figure 4A A graph showing the oxidation temperature of the oxidized film over time;

[0020] Figure 4C is a graph depicting the weight change of a film during oxidation of the film at different peak oxidation temperatures;

[0021] Figure 5A is a graph depicting the hydrogen permeability of comparative oxidized fibers;

[0022] Figure 5B is a graph depicting comparative carbon dioxide permeability of oxidized fibers;

[0023] Figure 5C is a graph depicting the methane permeability of comparative oxidized fibers;

[0024] Figure 5D is a graph comparing the hydrogen and carbon dioxide selectivities of oxidized fibers;

[0025] Figure 6A is a graph depicting hydrogen permeability over time for two membranes according to one or more embodiments described herein; and

[0026] Figure 6B is a graph depicting ethylene permeability over time for two membranes according to one or more embodiments described herein. DETAILED DESCRIPTION

[0027] The specific embodiments of the present application will now be described. However, the present disclosure can be implemented in different forms and should not be construed as being limited to the embodiments set forth in the present disclosure. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art.

[0028] Now refer to Figure 1A , depicts a process flow diagram of a method for manufacturing a reverse-selective asymmetric hollow fiber carbon membrane. According to one or more embodiments, the method depicted in flow diagram 100 may include at least step 110 of heating the asymmetric hollow fiber carbon membrane, step 120 of exposing the heated asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere, and step 130 of cooling the reverse-selective asymmetric hollow fiber carbon membrane.

[0029] As described herein, reverse selective asymmetric hollow fiber carbon membranes can be Figure 1A As used herein, the term "reverse selectivity" refers to a membrane that allows at least some larger molecules to permeate the membrane at a faster rate than at least some smaller molecules.

[0030] As used herein, the term "asymmetric" refers to a property of a hollow fiber carbon membrane in which the hollow fiber carbon membrane has at least one relatively dense layer and at least one relatively less dense layer. For example, in an embodiment, one layer of a hollow fiber CMS membrane may be greater than or equal to 1 μm and less than or equal to 10 μm and may be denser than a second layer. The second layer may be thicker than the first layer, such as greater than or equal to 20 μm and less than or equal to 200 μm.

[0031] The weight of the reverse selective asymmetric hollow fiber carbon membrane formed by the method described herein can be less than -5 wt % to 20 wt % of the weight of the asymmetric hollow fiber carbon membrane before oxidation. For example, the weight of the reverse selective asymmetric hollow fiber carbon membrane can be less than -5 wt % to 15 wt % of the weight of the asymmetric hollow fiber carbon membrane, such as less than -5 wt % to 10 wt %, -5 wt % to 5 wt %, -5 wt % to 0 wt %, 0 wt % to 20 wt %, 0 wt % to 15 wt %, 0 wt % to 10 wt %, 0 wt % to 5 wt %, 5 wt % to 20 wt %, 5 wt % to 15 wt %, 5 wt % to 10 wt %, 10 wt % to 20 wt %, 10 wt % to 15 wt %, 15 wt % to 20 wt % or any combination of these ranges. Without being bound by theory, it is believed that during the oxidation of the asymmetric hollow fiber carbon membrane, oxygen-containing compounds may be formed on the surface of the fiber. Further oxidation may then result in the formation of carbon dioxide from these oxygenates, which is released from the membrane as a gas. The formation of oxygenates may cause the weight of the fiber to increase during oxidation, and the formation of carbon dioxide may cause the weight loss of the fiber. It is believed that the formation of balanced oxygenates and carbon dioxide may beneficially affect the separation characteristics of the reverse selective asymmetric hollow fiber carbon membrane, such as membrane stability, membrane permeability, and gas selectivity, compared to the reverse selective membrane formed in the case of an unbalanced formation of oxygenates and carbon dioxide. The weight of the asymmetric hollow fiber carbon membrane changes from -5 wt % to 20 wt % during oxidation, which may indicate a preferred balance between the formation of oxygenates and the formation of carbon dioxide.

[0032] In addition, it is believed that the oxidation of the asymmetric hollow fiber carbon membrane is affected by the interaction of multiple factors, such as the temperature of the membrane during exposure to the oxygen-containing atmosphere, the oxygen concentration of the oxygen-containing atmosphere, the exposure time, and the type of oxidant used. It is believed that by measuring the weight change of the asymmetric hollow fiber carbon membrane during oxidation, it is possible to balance these factors to form a reverse selective asymmetric hollow fiber carbon membrane with beneficial membrane properties. For example, when compared to oxidation at a lower temperature, at a higher temperature, a shorter exposure time and / or a lower oxygen concentration can be used to achieve a target weight change. Basically, even when oxidation is performed under different conditions such as the type of oxidant or the oxidation temperature, the weight change of the membrane during oxidation can be used to determine the ideal oxidation conditions.

[0033] 1 , in one or more embodiments, a method of manufacturing a reverse selective asymmetric hollow fiber carbon membrane may include heating the asymmetric hollow fiber carbon membrane at a temperature of 200° C. to 1200° C. 110. For example, the asymmetric hollow fiber carbon membrane may be heated to the following temperatures: 200° C. to 1100° C., 200° C. to 1000° C., 200° C. to 900° C., 200° C. to 800° C., 200° C. to 700° C., 200° C. to 600° C., 200° C. to 500° C., 200° C. to 400° C., 200° C. to 300° C., 300° C. to 1200° C., 300° C. to 1100° C., 300° C. to 1000° C., 300° C. to 7 ... to 900℃, 300℃ to 800℃, 300℃ to 700℃, 300℃ to 600℃, 300℃ to 500℃, 300℃ to 400℃, 400℃ to 1200℃, 400℃ to 1100℃, 400℃ to 1000℃, 400℃ to 900℃, 400℃ to 800℃, 400℃ to 700℃, 400℃ to 600℃, 400℃ to 500℃, 500℃ to 1200℃, 500℃ to 1100℃, 500℃ to 1000℃, 500℃ to 900℃, 500℃ to 800℃, 500℃ to 700℃, 500℃ to 600℃, 600℃ to 1200℃, 600℃ to 1100℃, 600℃ to 1000℃, 600℃ to 900℃, 600℃ to 800℃, 600℃ to 700℃, 700℃ to 1200℃, 700℃ to 1100℃, 700 ℃ to 1000℃, 700℃ to 900℃, 700℃ to 800℃, 800℃ to 1200℃, 800℃ to 1100℃, 800℃ to 1000℃, 800℃ to 900℃, 900℃ to 1200℃, 900℃ to 1100℃, 900℃ to 1000℃, 1000℃ to 1200℃, 1000℃ to 1100℃, 1100℃ to 1200℃, or any combination of these ranges. In some embodiments, the asymmetric hollow fiber carbon membrane may be heated at a temperature of 900℃ to 1200℃. In some embodiments, the asymmetric hollow fiber carbon membrane may be heated to a temperature of 200℃ to 600℃. In other embodiments, the asymmetric hollow fiber carbon membrane may be heated to a temperature of 700℃ to 1000℃. Without wishing to be bound by theory, it is believed that heating the asymmetric hollow fiber carbon membrane to a temperature below 200° C. may not be sufficient to oxidize the asymmetric hollow fiber carbon membrane. As described herein above, it is believed that the temperature range to which the asymmetric hollow fiber is heated can be beneficially adjusted based on other oxidation conditions, such as, for example, the type of oxidant, to target a specific weight variation of the membrane.

[0034] In one or more embodiments, the heating of the asymmetric hollow fiber carbon membrane in step 110 can be performed in a furnace. In one or more embodiments, the heating of the asymmetric hollow fiber carbon membrane in step 110 can be performed in a furnace under an inert atmosphere. In some embodiments, the heating can utilize a controlled inert purge gas atmosphere. For example, an inert gas such as argon can be used as the purge gas atmosphere. Other suitable inert gases can include, but are not limited to, nitrogen, helium, or combinations thereof.

[0035] In one or more embodiments, the oxygen concentration in the furnace can be less than 10 ppm during the heating period of step 110. For example, the oxygen concentration in the furnace during the heating period can be less than 9 ppm, less than 8 ppm, less than 7 ppm, less than 6 ppm, less than 5 ppm, less than 4 ppm, less than 3 ppm, less than 2 ppm, or even less than 1 ppm. In some embodiments, the atmosphere of the furnace can be substantially free of oxygen during the heating period of step 110.

[0036] 1, step 120 generally includes exposing the heated asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere. In one or more embodiments, the oxygen-containing atmosphere can include one or more of air, oxygen, carbon dioxide, or steam. In embodiments, the oxygen-containing atmosphere may include one or more of air, oxygen, carbon dioxide, or steam in an amount from 100 ppm to 1,000,000 ppm of the oxygen-containing atmosphere, such as from 100 ppm to 1000 ppm, from 1000 ppm to 5000 ppm, from 5000 ppm to 10,000 ppm, from 10,000 ppm to 25,000 ppm, from 25,000 ppm to 50,000 ppm, from 50,000 ppm to 100,000 ppm, from 100,000 ppm to 250,000 ppm, from 250,000 ppm to 500,000 ppm, from 500,000 ppm to 750,000 ppm, from 750,000 ppm to 1,000,000 ppm, or any combination of these ranges.

[0037] In one or more embodiments, the oxygen-containing atmosphere can include oxygen in an amount from 100 ppm to 1000 ppm. For example, the oxygen-containing atmosphere may include 100 ppm to 900 ppm, 100 ppm to 800 ppm, 100 ppm to 700 ppm, 100 ppm to 600 ppm, 100 ppm to 500 ppm, 100 ppm to 400 ppm, 100 ppm to 300 ppm, 100 ppm to 200 ppm, 200 ppm to 1000 ppm, 200 ppm to 900 ppm, 200 ppm to 800 ppm, 200 ppm to 700 ppm, 200 ppm to 600 ppm, 200 ppm to 500 ppm, 200 ppm to 400 ppm, 200 ppm to 300 ppm, 300 ppm to 1000 ppm, 300 ppm to 900 ppm, 300 ppm to 800 ppm, 300 ppm to 700 ppm, 300 ppm to 600 ppm, 300 ppm to 500 ppm ppm, 400 ppm to 800 ppm, 400 ppm to 700 ppm, 400 ppm to 600 ppm, 400 ppm to 500 ppm, 500 ppm to 1000 ppm, 500 ppm to 900 ppm, 500 ppm to 800 ppm, 500 ppm to 700 ppm, 500 ppm to 600 ppm, 600 ppm to 1000 ppm, 600 ppm to 900 ppm, 600 ppm to 800 ppm, 600 ppm to 700 ppm, 700 ppm to 1000 ppm, 700 ppm to 900 ppm, 700 ppm to 800 ppm, 800 ppm to 1000 ppm, 800 ppm to 900 ppm, or 900 ppm to 1000 ppm.

[0038] In some embodiments, the oxygen-containing atmosphere may include oxygen in an amount greater than or equal to 1000 ppm, such as greater than or equal to 5000 ppm, greater than or equal to 10,000 ppm, greater than or equal to 15,000 ppm, greater than or equal to 20,000 ppm, greater than or equal to 25,000 ppm, greater than or equal to 30,000 ppm, greater than or equal to 40,000 ppm, greater than or equal to 50,000 ppm, greater than or equal to 75,000 ppm, greater than or equal to 100,000 ppm, greater than or equal to 150,000 ppm, or even greater than or equal to 200,000 ppm.

[0039] Still referring to the exposing step 120, in one or more embodiments, the asymmetric hollow fiber carbon membrane can be exposed to the oxygen-containing atmosphere for a period of 1 minute to 1000 minutes. For example, the asymmetric hollow fiber carbon membrane can be exposed to the oxygen-containing atmosphere for 1 minute to 900 minutes, 1 minute to 800 minutes, 1 minute to 700 minutes, 1 minute to 600 minutes, 1 minute to 500 minutes, 1 minute to 400 minutes, 1 minute to 300 minutes, 1 minute to 200 minutes, 1 minute to 100 minutes, 100 minutes to 1000 minutes, 100 minutes to 900 minutes, 100 minutes to 800 minutes, 100 minutes to 700 minutes, 100 minutes to 60 0 min, 100 min to 500 min, 100 min to 400 min, 100 min to 300 min, 100 min to 200 min, 200 min to 1000 min, 200 min to 900 min, 200 min to 800 min, 200 min to 700 min, 200 min to 600 min, 200 min to 500 min, 200 min to 400 min, 200 min to 300 min, 300 min to 1000 min, 300 min to 900 minutes, 300 minutes to 800 minutes, 300 minutes to 700 minutes, 300 minutes to 600 minutes, 300 minutes to 500 minutes, 300 minutes to 400 minutes, 400 minutes to 1000 minutes, 400 minutes to 900 minutes, 400 minutes to 800 minutes, 400 minutes to 700 minutes, 400 minutes to 600 minutes, 400 minutes to 500 minutes, 500 minutes to 1000 minutes, 500 minutes to 900 minutes, 50 0 min to 800 min, 500 min to 700 min, 500 min to 600 min, 600 min to 1000 min, 600 min to 900 min, 600 min to 800 min, 600 min to 700 min, 700 min to 1000 min, 700 min to 900 min, 700 min to 800 min, 800 min to 1000 min, 800 min to 900 min, 900 min to 1000 min, or any combination of these ranges.

[0040] In one or more embodiments, the asymmetric hollow fiber carbon membrane may be maintained at a specific temperature during exposure to an oxygen-containing atmosphere. In embodiments where the oxygen-containing atmosphere comprises air or oxygen, the asymmetric hollow fiber carbon membrane may be maintained at a temperature of 200°C to 600°C, such as 200°C to 550°C, 200°C to 500°C, 200°C to 450°C, 200°C to 400°C, 200°C to 350°C, 200°C to 300°C, 200°C to 250°C, 250°C to 600°C, 250°C to 550°C, 250°C to 500°C, 250°C to 450°C, 250°C to 400°C, 250°C to 350°C, 250°C to 300°C, 300°C to 600°C, 300°C to 550°C, ℃ to 500℃, 300℃ to 450℃, 300℃ to 400℃, 300℃ to 350℃, 350℃ to 600℃, 350℃ to 550℃, 350℃ to 500℃, 350℃ to 450℃, 350℃ to 400℃, 400℃ to 600℃, 400℃ to 550℃, 400℃ to 500℃, 400℃ to 450℃, 450℃ to 600℃, 450℃ to 550℃, 450℃ to 500℃, 500℃ to 600℃, 500℃ to 550℃, 550℃ to 600℃ or any combination of these ranges.

[0041] In an embodiment, the asymmetric hollow fiber carbon membrane may be maintained at a temperature of 700°C to 1000°C, such as 700°C to 950°C, 700°C to 900°C, 700°C to 850°C, 700°C to 800°C, 700°C to 750°C, 750°C to 1000°C, 750°C to 950°C, 750°C to 900°C, 750°C to 850°C, 750°C to 800°C, 800°C to 1000°C, 800°C to 950°C, 800°C to 900°C, 800°C to 850°C, 850°C to 1000°C, 850°C to 950°C, 850°C to 900°C, 900°C to 1000°C, 900°C to 950°C, 950°C to 1000°C, or any combination of these ranges, in the presence of an oxygen-containing atmosphere including carbon dioxide.

[0042] Still referring to the flowchart 100 of Figure 1, step 130 generally includes cooling the reverse selective asymmetric hollow fiber carbon membrane. In one or more embodiments, the membrane can be cooled to a temperature of less than 50°C, such as less than 40°C or even less than 30°C. In one or more embodiments, the asymmetric hollow fiber carbon membrane can be exposed to an oxygen-containing atmosphere before cooling.

[0043] Now refer to Figure 1BAs shown in flow chart 102, in one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may include an additional cooling step 140 prior to the exposing step 120. In one or more embodiments, the furnace may be cooled to a temperature greater than or equal to 500° C. before exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere. For example, the furnace may be cooled to a temperature greater than or equal to 550° C., greater than or equal to 600° C., greater than or equal to 650° C., greater than or equal to 700° C., greater than or equal to 750° C., greater than or equal to 800° C., greater than or equal to 850° C., greater than or equal to 900° C., greater than or equal to 950° C., greater than or equal to 1000° C., greater than or equal to 1050° C., greater than or equal to 1100° C., or even greater than or equal to 1150° C. In some embodiments, the furnace may be cooled to 500°C to 700°C, such as 500°C to 675°C, 500°C to 650°C, 500°C to 625°C, 500°C to 600°C, 500°C to 575°C, 500°C to 550°C, 500°C to 525°C, 525°C to 700°C, 525°C to 675°C, 525°C to 650°C, 525°C to 625°C, 525°C to 600°C, 525°C to 575°C, 525°C to 550°C, 550°C to 700°C, 550°C to 675°C, 550°C to 650°C. ℃, 550℃ to 625℃, 550℃ to 600℃, 550℃ to 575℃, 575℃ to 700℃, 575℃ to 675℃, 575℃ to 650℃, 575℃ to 625℃, 575℃ to 600℃, 600℃ to 700℃, 600℃ to 675℃, 600℃ to 650℃, 600℃ to 625℃, 625℃ to 700℃, 625℃ to 675℃, 625℃ to 650℃, 650℃ to 700℃, 650℃ to 675℃, 675℃ to 700℃, or a temperature of any combination of these ranges.

[0044] In one or more embodiments, the oxygen concentration in the oxygen-containing atmosphere may increase as the temperature of the furnace decreases during cooling of the furnace. In embodiments, the oxygen concentration in the furnace may increase by 2.0 ppm to 3.0 ppm for every 1°C decrease in the temperature of the furnace. For example, the oxygen concentration in the furnace may increase by 2.0 ppm to 2.9 ppm for every 1°C decrease in the temperature of the furnace, such as 2.0 ppm to 2.8 ppm, 2.0 ppm to 2.7 ppm, 2.0 ppm to 2.6 ppm, 2.0 ppm to 2.5 ppm, 2.0 ppm to 2.4 ppm, 2.0 ppm to 2.3 ppm, 2.0 ppm to 2.2 ppm, 2.0 ppm to 2.1 ppm, 2.1 ppm to 3.0 ppm, 2.1 ppm to 2.9 ppm, 2.1 ppm to 2.8 ppm, 2.2 ppm to 2.9 ppm, 2.1 ppm to 2.9 ppm, 2.2 ppm to 2.8 ppm, 2.2 ppm to 2.9 ... .1ppm to 2.7ppm, 2.1ppm to 2.6ppm, 2.1ppm to 2.5ppm, 2.1ppm to 2.4ppm, 2.1ppm to 2.3ppm, 2.1ppm to 2.2ppm, 2.2ppm to 3.0ppm, 2.2ppm to 2.9ppm, 2.2ppm to 2.8ppm, 2.2ppm to 2.7ppm, 2.2ppm to 2.6ppm, 2.2ppm to 2.5ppm, 2.2ppm to 2.4ppm, 2.2ppm to 2.3ppm, 2.3ppm to 3.0ppm, 2.3ppm to 2.9ppm, 2.3ppm to 2.8ppm, 2.3ppm to 2.7ppm, 2.3ppm to 2.6ppm, 2.3ppm to 2.5ppm, 2.3ppm to 2.4ppm, 2.4ppm to 3.0ppm, 2.4ppm to 2.9ppm, 2.4ppm to 2.8ppm, 2.4ppm to 2.7ppm, 2.4ppm to 2.6ppm, 2.4ppm to 2.5ppm, 2.5ppm to 3.0ppm In some embodiments, the oxygen concentration in the furnace may be increased by 2.3 ppm to 2.5 ppm for every 1° C. decrease in the temperature of the furnace. In some embodiments, the oxygen concentration in the furnace may be increased at a rate such that the maximum oxygen concentration in the furnace is reached when the furnace has cooled to a temperature of 100° C. to 200° C.

[0045] In one or more embodiments, the oxygen-containing atmosphere can enter the furnace via diffusion. In other embodiments, the oxygen-containing atmosphere can enter the furnace by injecting the gas into the furnace through a gas inlet.

[0046] In one or more embodiments, during exposure to the oxygen-containing atmosphere, a pressure differential can be created across the asymmetric hollow fiber carbon membrane such that the oxygen-containing atmosphere is drawn through the cross-section of the asymmetric hollow fiber carbon membrane. In some embodiments, the pressure differential can be created by adding an oxygen-containing gas at one end of the furnace and flowing the gas through the furnace to exit at the opposite end of the furnace.

[0047] Having described a method for making reverse selective asymmetric hollow fiber carbon membranes, a system 200 that can be utilized with the methods of the present disclosure is described in Figure 2 is provided in . The flow control device 10 is first set to the "on" configuration, which means that the fluid is caused to flow from the purge gas reservoir 14 through the gas conduit 12 to the furnace 16, which contains the hollow fiber carbon membrane (not shown). During the heating step 110, most or even all of the fluid entering the furnace 16 originates from the purge gas reservoir 14. After heating, the furnace 16 can be allowed to cool. When the temperature within the furnace 16 is reduced to the desired oxidation start temperature, the flow control device 10 reduces the flow of fluid from the purge gas reservoir 14. In an embodiment where the oxygen-containing atmosphere can be injected into the furnace through the gas inlet, the flow control device 18 can increase the flow of fluid from the oxidant reservoir 20 into the furnace 16 via the gas conduit 22. Figure 2 In the embodiment shown, both conduit 12 and conduit 22 feed into inlet 24. Oxidant reservoir 20 may contain a premixed volume of oxidant and carrier gas, such as, for example, air (oxygen as the oxidant and nitrogen as the carrier gas) or a mixture of oxygen (oxidant) and argon (carrier gas). In embodiments where the oxygen-containing atmosphere can enter the furnace via diffusion, flow control device 10 may be used to reduce or stop the flow of fluid from purge gas reservoir 14 into the furnace. The oxygen-containing atmosphere, such as air, may then enter the furnace via diffusion via conduit 28 and outlet 26.

[0048] Still refer to Figure 2 During heating and oxidation, gases from furnace 16 may pass through outlet 26 and may ultimately be exhausted via conduit 28. Conveniently, these gases may be analyzed using oxygen sensor 30 to allow for proper control of the process, including gas composition during pyrolysis. Although embodiments of the present disclosure have been described in the context of system 200, other systems suitable for use with the embodiments described herein are contemplated.

[0049] As used herein, the term "conduit" includes, but is not limited to, casing, liner, pipe, tube, coiled tubing, and mechanical structures having an internal void.

[0050] As used herein, the term "reservoir" includes any container of any size capable of holding a fluid, whether in liquid or gaseous form. Exemplary reservoirs include, but are not limited to, cylinders, storage tanks, bladders, inflatable membranes (such as balloons), drums, and bottles.

[0051] As used herein, the term "flow control device" includes, but is not limited to, ball valves, butterfly valves, choke valves, diaphragm valves, gate valves, globe valves, knife valves, needle valves, pinch valves, piston valves, plug valves, solenoid valves, and sliding valves.

[0052] In one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may further include cooling the reverse selective asymmetric hollow fiber carbon membrane to a temperature below 50° C. For example, the reverse selective asymmetric hollow fiber carbon membrane may be cooled to a temperature below 45° C., such as below 40° C., below 35° C., below 30° C., below 25° C., or even below 20° C.

[0053] In one or more embodiments, the method of making a reverse-selective asymmetric hollow fiber carbon membrane may further include forming the asymmetric hollow fiber carbon membrane. In an embodiment, forming the asymmetric hollow fiber carbon membrane may include providing a polymer precursor, heating the polymer precursor to a pyrolysis temperature of 800° C. to 1200° C., pyrolyzing the polymer precursor at the pyrolysis temperature to form the asymmetric hollow fiber carbon membrane, and cooling the asymmetric hollow fiber carbon membrane to a temperature below 50° C.

[0054] As described herein, in one or more embodiments, a method for making a reverse selective asymmetric hollow fiber carbon membrane may include providing a polymer precursor. For example, the polymer precursor may be any useful polymer for making a hollow fiber carbon membrane, such as a polyimide. When a polyimide is used, the polyimide may be a conventional or fluorinated polyimide. In an embodiment, the polymer precursor may include a monomer A X 、B Y and C Z A polymer wherein X, Y, and Z are the mole fractions of A, B, and C present in the polymer, respectively. In embodiments, X+Y+Z = 1. In other embodiments, X+Y+Z < 1, and other monomers are present in the polymer.

[0055] Each of A, B and C is a monomer selected from the group consisting of: 2,4,6-trimethyl-1,3-phenylenediamine (DAM); oxydiphenylamine (ODA); dimethyl-3,7-diaminodiphenyl-thiophene-5,5'-dioxide (DDBT); 3,5-diaminobenzoic acid (DABA); 2,3,5,6-tetramethyl-1,4-phenylenediamine (duene); m-phenylenediamine (m-PDA); 2,4-diaminotoluene (2,4-DAT); tetramethylmethylenedianiline (TMMDA); 4,4'-diamino-2,2'-biphenyldisulfonate Acid (BDSA); 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandione (6FDA); 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA); pyromellitic dianhydride (PMDA); 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA); 4,4'-oxydiphthalic anhydride (ODPA); 5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane (DAPI); and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). In an embodiment, the polyimide may contain at least two different moieties selected from the group consisting of: DAM; ODA; DDBT; DABA; durene; m-PDA; 2,4-DAT; TMMDA; BDSA; 6FDA; BPDA; PMDA; NTDA; and BTDA.

[0056] In an embodiment, A is a monomer selected from the group consisting of 6FDA, ODPA, and BTDA; B is DAM; and C is a monomer selected from the group consisting of BPDA and PMDA. In an embodiment, A is 6FDA; B is DAM; and Z is 0. In an embodiment, the polyimide may be MATRIMID TM 5218 (Huntsman Advanced Materials), a commercially available polyimide, wherein A is BTDA; B is DAPI; and Z is 0.

[0057] In an embodiment, the polyimide may comprise, consist essentially of, or consist of 6FDA / BPDA-DAM, as shown in formula (1), which can be synthesized via thermal or chemical methods from a combination of three commercially available monomers: DAM; 6FDA, and BPDA. In an embodiment of formula (1), X+Y may be from 0.1 to 0.9, and Z may be from 0.1 to 0.9. In an embodiment of formula (1), X+Y may be from 0.1 to 1, and Z may be from 0 to 0.9. In an embodiment of formula (1), X may be 0 and Y+Z may be 1. In an embodiment, X and Z may be from 0.25, 0.3, or 0.4 to 0.9, 0.8, or 0.75. In an embodiment, X+Y is 0.5, and Z is 0.5. Formula (2) below shows a representative structure of 6FDA / BPDA-DAM with the potential to adjust the ratio between X and Z to tune polymer properties. In embodiments, a 1:1 ratio of X to Y may also be abbreviated as 6FDA / BPDA (1:1)-DAM.

[0058]

[0059] In an embodiment, the polyimide may be formed by the reaction of a diamine and a dianhydride. In such an embodiment, at least one of A, B, and C is a diamine, and at least another of A, B, and C is a dianhydride. In an embodiment, the molar ratio of diamine to dianhydride of the total diamines and total dianhydrides may be greater than or equal to 49:51 to 51:49. In an embodiment, the molar ratio of diamine to dianhydride of the diamine and dianhydride may be about 50:50.

[0060] In embodiments, more than one dianhydride can be used together with a diamine. In such embodiments using two dianhydrides (dianhydride 1 and dianhydride 2), the mol ratio of dianhydride 1 to dianhydride 2 can be greater than or equal to 20:80 and less than or equal to 80:20. For example, the mol ratio can be greater than or equal to 25:75 and less than or equal to 75:25, greater than or equal to 30:70 and less than or equal to 70:30, greater than or equal to 35:65 and less than or equal to 65:35, greater than or equal to 40:60 and less than or equal to 60:40, or even greater than or equal to 45:55 and less than or equal to 55:45. In embodiments, the mol ratio of dianhydride 1 to dianhydride 2 can be about 50:50. In embodiments, a dianhydride can be used together with more than one diamine. In such embodiments using two diamines (diamine 1 and diamine 2), the mol ratio of diamine 1 to diamine 2 can be greater than or equal to 20:80 and less than or equal to 80:20. For example, the molar ratio can be greater than or equal to 25:75 and less than or equal to 75:25, greater than or equal to 30:70 and less than or equal to 70:30, greater than or equal to 35:65 and less than or equal to 65:35, greater than or equal to 40:60 and less than or equal to 60:40, or even greater than or equal to 45:55 and less than or equal to 55:45. In embodiments, the molar ratio of diamine 1 to diamine 2 can be about 50:50.

[0061] In embodiments, the polymer precursor membrane produced but not pyrolyzed is substantially defect-free. "Defect-free" means that the selectivity of a gas pair passing through the hollow fiber membrane is at least 90% of the selectivity of the same gas pair passing through a dense membrane prepared from the same composition as used to make the polymer precursor membrane. By way of illustration, the O2 / N2 selectivity (also referred to as "dense membrane selectivity") of the 6FDA / BPDA (1:1)-DAM polymer is 4.1.

[0062] In embodiments, the precursor polymer can be formed into a hollow fiber. Conventional processes for making these fibers or membranes can be used. For example, a coextrusion process comprising a dry-jet wet spinning process (wherein an air gap exists between the tip of the spinning head and the coagulation or quenching bath) or a wet spinning process (with zero air gap distance) can be used to make the hollow fiber.

[0063] In an embodiment, the asymmetric membrane can be an entity consisting of an extremely thin, dense skin on a thick porous substructure, and the thick porous substructure can be a material identical or different from the dense surface layer. In an embodiment, the asymmetric membrane can be manufactured in a single step by phase inversion, or a thin layer can be coated on a pre-prepared porous support using a dip coating method. These layers in the asymmetric membrane can be produced by physical coating or by chemical modification. The asymmetric membrane can be in the form of a hollow fiber configuration. In an embodiment, the asymmetric membrane can include a third layer of the same or different material as needed to enhance membrane performance.

[0064] Pyrolysis conditions affect the physical properties of the hollow fiber CMS membrane. Any suitable support for holding the hollow fiber CMS membrane during pyrolysis may be used, including sandwiching between two wire meshes or using a stainless steel mesh sheet in combination with stainless steel wire, and as described in U.S. Patent No. 8,709,133 at column 6, line 58 to column 7, line 4, which is incorporated by reference.

[0065] The precursor polymer can be pyrolyzed under various inert gas purges or vacuum conditions (e.g., a pressure of less than or equal to 0.1 millibar) to form a hollow fiber CMS membrane (i.e., the precursor polymer is carbonized). U.S. Patent No. 6,565,631 describes a heating method for pyrolyzing polymer fibers to form hollow fiber CMS membranes, and the U.S. Patent No. 6,565,631 is incorporated herein by reference. For polymer fibers, the pyrolysis temperature may be greater than or equal to 800°C and less than or equal to 1200°C. The pyrolysis temperature can be adjusted in combination with the pyrolysis atmosphere to adjust the performance characteristics of the resulting hollow fiber CMS membrane. For example, the pyrolysis temperature may be 1000°C or higher. Optionally, the pyrolysis temperature may be greater than or equal to 900°C and less than or equal to 1000°C. In embodiments, the pyrolysis temperature may be greater than or equal to 825°C and less than or equal to 1200°C, greater than or equal to 850°C and less than or equal to 1200°C, greater than or equal to 875°C and less than or equal to 1200°C, greater than or equal to 900°C and less than or equal to 1200°C, greater than or equal to 925°C and less than or equal to 1200°C, greater than or equal to 950°C and less than or equal to 1200°C, greater than or equal to 975°C and less than or equal to 1200°C, greater than or equal to 1000°C and less than or equal to 1200°C. , greater than or equal to 1025°C and less than or equal to 1200°C, greater than or equal to 1050°C and less than or equal to 1200°C, greater than or equal to 1075°C and less than or equal to 1200°C, greater than or equal to 1100°C and less than or equal to 1200°C, greater than or equal to 1125°C and less than or equal to 1200°C, greater than or equal to 1150°C and less than or equal to 1200°C, greater than or equal to 1175°C and less than or equal to 1200°C, greater than or equal to 800°C and less than or equal to 1175 ... 825°C and less than or equal to 1175°C, 850°C and less than or equal to 1175°C, 875°C and less than or equal to 1175°C, 900°C and less than or equal to 1175°C, 925°C and less than or equal to 1175°C, 950°C and less than or equal to 1175°C, 975°C and less than or equal to 1175°C, 1000°C and less than or equal to 1175°C, 1025°C and less than or equal to 1175°C, 1050°C or more and 1175°C or less, 1075°C or more and 1175°C or less, 1100°C or more and 1175°C or less, 1125°C or more and 1175°C or less, 1150°C or more and 1175°C or less, 800°C or more and 1150°C or less, 825°C or more and 1150°C or less, 850°C or more and 1150°C or less,875°C or more and 1150°C or less, 900°C or more and 1150°C or less, 925°C or more and 1150°C or less, 950°C or more and 1150°C or less, 975°C or more and 1150°C or less, 1000°C or more and 1150°C or less, 1025°C or more and 1150°C or less, 1050°C or more and 1150°C or less, 1075°C or more and 1150°C or less, 1100°C or more and 1150°C or less, 1125°C or more and 115 0°C, 800°C or more and 1125°C or less, 825°C or more and 1125°C or less, 850°C or more and 1125°C or less, 875°C or more and 1125°C or less, 900°C or more and 1125°C or less, 925°C or more and 1125°C or less, 950°C or more and 1125°C or less, 975°C or more and 1125°C or less, 1000°C or more and 1125°C or less, 1025°C or more and 1125°C or less, 1050°C or more and 1125°C or less ℃, 1075℃ or more and 1125℃ or less, 1100℃ or more and 1125℃ or less, 800℃ or more and 1100℃ or less, 825℃ or more and 1100℃ or less, 850℃ or more and 1100℃ or less, 875℃ or more and 1100℃ or less, 900℃ or more and 1100℃ or less, 925℃ or more and 1100℃ or less, 950℃ or more and 1100℃ or less, 975℃ or more and 1100℃ or less, 1000℃ or more and 1100℃ or less , greater than or equal to 1025°C and less than or equal to 1100°C, greater than or equal to 1050°C and less than or equal to 1100°C, greater than or equal to 1075°C and less than or equal to 1100°C, greater than or equal to 800°C and less than or equal to 1075°C, greater than or equal to 825°C and less than or equal to 1075°C, greater than or equal to 850°C and less than or equal to 1075°C, greater than or equal to 875°C and less than or equal to 1075°C, greater than or equal to 900°C and less than or equal to 1075°C, greater than or equal to 925°C and less than or equal to 1075°C, greater than or equal to 950°C and less than or equal to 1075°C, greater than or equal to 975°C and less than or equal to 1075°C,1000°C or more and 1075°C or less, 1025°C or more and 1075°C or less, 1050°C or more and 1075°C or less, 800°C or more and 1050°C or less, 825°C or more and 1050°C or less, 850°C or more and 1050°C or less, 875°C or more and 1050°C or less, 900°C or more and 1050°C or less, 925°C or more and 1050°C or less, 950°C or more and 1050°C or less, 975°C or more and 1050°C or less, 1000°C or more and 1050°C or less, 1025°C or more and 1050°C or less, 800°C or more and 1025°C or less, 825°C or more and 1025°C or less, 850°C or more and 1025°C or less, 875°C or more and 1025°C or less, 900°C or more and 1025°C or less, 925°C or more and 1025°C or less, 950°C or more and 1025°C or less, 975°C or more and 1025°C or less, 1000°C or more and 1025°C or less, 800°C or more and 1000°C or less, 825°C or more and 1000°C or less, 850°C or more and 1000°C or less, 875°C or more and 1000°C or less, 900°C or more and 1000°C or less, 925°C or more and 1000°C or less, 950°C or more and 1000°C or less, 975°C or more and 1000°C or less, 800°C or more and 975°C or less, 825°C or more and 975°C or less, 850°C or more and 975°C or less, 8 75°C and less than or equal to 975°C, 900°C and less than or equal to 975°C, 925°C and less than or equal to 975°C, 950°C and less than or equal to 975°C, 800°C and less than or equal to 950°C, 825°C and less than or equal to 950°C, 850°C and less than or equal to 950°C, 875°C and less than or equal to 950°C, 900°C and less than or equal to 950°C, 925°C and less than or equal to 950°C, 800°C and less than or equal to 925°C, 825°C and less than or equal to 925°C,850° C. or greater and 925° C. or less, 875° C. or greater and 925° C. or less, 900° C. or greater and 925° C. or less, 800° C. or greater and 900° C. or less, 825° C. or greater and 900° C. or less, 850° C. or greater and 900° C. or less, 875° C. or greater and 900° C. or less, 800° C. or greater and 875° C. or less, 825° C. or greater and 875° C. or less, 850° C. or greater and 875° C. or less, 800° C. or greater and 850° C. or less, 825° C. or greater and 850° C. or less, or 800° C. or greater and 825° C. or less. Without wishing to be bound by theory, it is believed that at pyrolysis temperatures below 800° C., desired gas selectivities may not be achieved. However, it is also believed that at temperatures above 1200°C, the structure of the CMS membrane may be damaged. It is envisioned that the acceptable pyrolysis temperature range may be greater than or equal to any of the temperatures described herein and less than or equal to any of the temperatures described herein.

[0066] The pyrolysis soak time (i.e., the duration at the pyrolysis temperature) may vary (and may not include the soak time), but may be, for example, greater than or equal to 1 hour and less than or equal to 10 hours, greater than or equal to 2 hours and less than or equal to 8 hours, greater than or equal to 4 hours and less than or equal to 6 hours. An exemplary heating schedule may include: (1) starting at a first set point of approximately 50°C; (2) heating at a rate of approximately 13.3°C per minute to a second set point of approximately 250°C; (3) heating at a rate of approximately 3.85°C per minute to a third set point of approximately 535°C; (4) heating at a rate of approximately 0.25°C per minute to a fourth set point of approximately 550°C to 700°C. The fourth set point may then be maintained for the determined soak time.

[0067] As mentioned above, the precursor polymer can be pyrolyzed under various inert gas purges or vacuum conditions. In an embodiment, the precursor polymer can be pyrolyzed under vacuum with low pressure (e.g., less than or equal to 0.1 mbar). In an embodiment, pyrolysis uses a controlled inert purge gas atmosphere. By way of example, an inert gas (e.g., argon) is used as a purge gas atmosphere. Other suitable inert gases include, but are not limited to, nitrogen, helium, or any combination thereof.

[0068] In one or more embodiments, the formed hollow fiber CMS membrane is cooled to a temperature close to room temperature, such as less than or equal to 50° C. The cooling can be at any useful rate, such as passive cooling (e.g., turning off the power to the furnace and allowing it to cool naturally). Alternatively, faster cooling may be required, such as by using known techniques to achieve faster cooling. Known techniques include, but are not limited to, cooling fans or using water-cooled jackets, purging with a gas having a temperature lower than that of the hollow fiber CMS membrane, or opening the furnace to the ambient environment.

[0069] In one or more embodiments, a method for separating ethylene from a gas feed comprising ethylene and hydrogen comprises passing the gas feed through a reverse selective asymmetric hollow fiber carbon membrane manufactured by the method described herein and separating the gas feed into a first stream having an increased ethylene concentration and a second stream having an increased hydrogen concentration. In some embodiments, the reverse selective asymmetric hollow fiber carbon membrane is desirably manufactured as a module comprising a sealable package comprising a plurality of carbon membranes comprising at least one reverse selective asymmetric hollow fiber carbon membrane manufactured using the method described herein contained within the sealable package. The sealable package has: an inlet for introducing a gas feed consisting of at least two different gas molecules; a first outlet for allowing the discharge of a permeate gas stream; and a second outlet for the discharge of a retentate gas stream.

[0070] In a first aspect of the present disclosure, a method for manufacturing a reverse-selective asymmetric hollow fiber carbon membrane may include heating the asymmetric hollow fiber carbon membrane. The method may also include exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form the reverse-selective asymmetric hollow fiber carbon membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere, the weight of the reverse-selective asymmetric hollow fiber carbon membrane may be -5 wt% to 20 wt% less than the weight of the asymmetric hollow fiber carbon membrane.

[0071] A second aspect of the present disclosure may include the first aspect, wherein the weight of the inverted asymmetric hollow fiber carbon membrane is -5 wt% to 5 wt% less than the weight of the asymmetric hollow fiber carbon membrane.

[0072] A third aspect of the present disclosure may include any preceding aspect or combination of aspects, wherein the oxygen-containing atmosphere includes one or more of air, oxygen, carbon dioxide, or steam.

[0073] A fourth aspect of the present disclosure may include any of the preceding aspects or combinations of aspects, wherein the method further includes heating the asymmetric hollow fiber carbon membrane in a furnace at a temperature of 600°C to 1200°C under an inert atmosphere, cooling the furnace to a temperature greater than or equal to 500°C, and exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere during cooling of the furnace to form a reverse selective asymmetric hollow fiber carbon membrane.

[0074] A fifth aspect of the present disclosure may include the fourth aspect, wherein the asymmetric hollow fiber carbon membrane is heated to a temperature of 900° C. to 1200° C. prior to exposure to the oxygen-containing atmosphere.

[0075] A sixth aspect of the present disclosure may include the fourth aspect or the fifth aspect, wherein the oxygen concentration in the oxygen-containing atmosphere increases as the temperature of the furnace decreases during cooling of the furnace.

[0076] A seventh aspect of the present disclosure may include the fourth to sixth aspects, wherein the oxygen concentration in the furnace increases by 2 ppm to 3 ppm per 1° C. decrease in the temperature of the furnace.

[0077] An eighth aspect of the present disclosure may include any preceding aspect or combination of aspects, wherein the oxygen-containing atmosphere enters the furnace via diffusion or wherein the oxygen-containing atmosphere is injected into the furnace through a gas inlet.

[0078] A ninth aspect of the present disclosure may include the first to third aspects, wherein the asymmetric hollow fiber carbon membrane is heated to a temperature greater than or equal to 550° C. and exposed to an oxygen-containing atmosphere for a period of 1 minute to 1000 minutes before being cooled.

[0079] A tenth aspect of the present disclosure may include any preceding aspect or combination of aspects, wherein the oxygen concentration in the oxygen-containing atmosphere is 100 ppm to 1000 ppm.

[0080] An eleventh aspect of the present disclosure may include the first to third aspects, wherein the oxygen-containing atmosphere comprises air, the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature of 200° C. to 600° C., and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

[0081] A twelfth aspect of the present disclosure may include the eleventh aspect, wherein the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature of 250° C. to 500° C., and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

[0082] A thirteenth aspect of the present disclosure may include the first to third aspects, wherein the oxygen-containing atmosphere includes carbon dioxide, the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature of 700° C. to 1000° C., and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

[0083] A fourteenth aspect of the present disclosure may include any preceding aspect or combination of aspects, wherein the method further comprises generating a pressure differential across the asymmetric hollow fiber carbon membrane during exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is drawn through a cross section of the asymmetric hollow fiber carbon membrane.

[0084] A fifteenth aspect of the present disclosure may include any of the preceding aspects or combinations of aspects, wherein the method further includes providing a polymer precursor, heating the polymer precursor to a pyrolysis temperature of 800°C to 1200°C, pyrolyzing the polymer precursor at the pyrolysis temperature to form an asymmetric hollow fiber carbon membrane, and cooling the asymmetric hollow fiber carbon membrane to a temperature below 50°C.

[0085] Test Method

[0086] Gas permeability and selectivity

[0087] The gas permeation characteristics of a membrane can be determined by gas permeation experiments. Two intrinsic properties are used to evaluate the separation performance of a membrane material: its "permeability" (a measure of the intrinsic production capacity of the membrane); and its "selectivity" (a measure of the separation efficiency of the membrane). "Permeability" is usually determined in units of barrer (1 bar = 10 -10 [cm 3 (STP)cm] / [cm 2 s cmHg]), which is based on the flux (n i ) divided by the partial pressure difference between the upstream and downstream sides of the membrane (Δp i ) and multiply it by the film thickness (l) to calculate.

[0088]

[0089] Another term "permeability" is defined herein as the productivity of an asymmetric hollow fiber membrane and is usually expressed in gas permeation units (GPU) (1 GPU = 10 -6 [cm 3 (STP)] / [cm 2 s cmHg]) measurement, which is determined by dividing the permeability by the effective membrane separation layer thickness.

[0090]

[0091] Finally, "selectivity" is defined herein as the ability of one gas to permeate through a membrane or the permeability of another gas of the same nature. It is measured as a unitless ratio.

[0092]

[0093] Example

[0094] Various embodiments of the present disclosure will be further illustrated by the following examples. The examples are illustrative in nature and should not be construed as limiting the subject matter of the present disclosure.

[0095] Example 1 - Oxidant Concentration Calibration

[0096] The relationship between the increase in average O2 concentration in the furnace and the decrease in temperature was determined using the ideal gas law: PV = nRT, where P is pressure, V is volume, n is the number of moles of O2, R is the ideal gas constant, and T is temperature. In the calculations, the volume was constant and the pyrolysis temperature was 925°C. Initially, 100% argon was used as the purge gas. After reaching the pyrolysis temperature, the furnace was allowed to cool naturally. When the temperature reached 575°C, the gas purge was stopped, and the O2 concentration induced by backdiffusion of the furnace air was determined.

[0097] Figure 3 A graph is provided with temperature on the x-axis and simulated O2 concentration on the y-axis. It is clear from the graph that the O2 concentration increases linearly with decreasing temperature. Furthermore, this relationship can be used to determine the expected O2 concentration when pyrolysis, reheating, and oxidation are performed.

[0098] Example 2 - Formation of asymmetric hollow fiber carbon membrane

[0099] The CMS membranes were made using 6FDA-BPDA-DAM polymer. 6FDA-BPDA-DAM was obtained from Akron Polymer Systems, Akron, OH. The polymer was dried at 110°C under vacuum for 24 hours and then formed into a stock solution. The stock solution was made by mixing 6FDA-BPDA-DAM polymer with the solvents and compounds in Table 1 and evaporating in a 1% teflon film coated with polytetrafluoroethylene (TEFLON). TM ) Qorpak with sealed lid TM Roller mixing was performed in a glass bottle at a roller speed of 5 revolutions per minute (rpm) for a period of about 3 weeks to form a homogeneous stock solution.

[0100] Table 1- Stock solution formula

[0101]

[0102] NMP = N-methyl-2-pyrrolidone; THF = tetrahydrofuran

[0103] The homogenous stock solution was charged to a 500 milliliter (mL) syringe pump and degassed overnight by heating the pump to a set point temperature of 50°C to 60°C using a heating tape.

[0104] The core liquid (85% by weight of NMP and 15% by weight of water by weight of the total core liquid) was loaded into a separate 100 mL syringe pump, and the stock solution and the core liquid were then co-extruded through a spinning head operated at a flow rate of 180 mL / hr for the stock solution; 60 mL / hr of the core liquid, and both the core liquid and the stock liquid in the line between the delivery pump and the spinning head were filtered using 40 μm and 2 μm metal filters. The temperature was controlled at a set point temperature of 70° C. using thermocouples and heating tapes placed on the spinning head, stock filter, and stock pump.

[0105] After passing through a 15 cm air gap, the nascent fibers formed from the spinning head were quenched in a water bath (50° C.) and the fibers were phase separated. A 0.32 meter (M) diameter polyethylene drum was used to collect the fibers through a TEFLON guide and operated at a take-up rate of 30 meters per minute (M / min).

[0106] The fibers were cut from the drum and rinsed at least four times in separate water baths over a 48-hour span. The rinsed fibers in a glass container were solvent exchanged three times with methanol for 20 minutes and then with hexane for 20 minutes before the fibers were recovered and dried under vacuum at a set point temperature of 110° C. for one hour or at 75° C. for 3 hours.

[0107] Prior to pyrolyzing the fibers, sample amounts of the above fibers (also referred to as "precursor fibers") were tested for skin integrity. One or more hollow precursor fibers were poured into 1 / 4 inch (0.64 cm) (outer diameter, OD) stainless steel tubing. Connect each tubing end to 1 / 4 inch (0.64 cm) stainless steel tee; and each tee is connected to 1 4 inch (0.64 cm) female and male NPT pipe fittings, which were epoxy-sealed to the NPT connectors. Mixed gas permeation tests were conducted in a constant pressure system maintained at 35°C and a pressure drop of 52 psig. The gases were fed to the shell side of the module. Helium was fed to the pore side of the fiber as a purge gas. Gases that permeated through the membrane along with the helium purge gas were analyzed using gas chromatography. As previously mentioned, gas permeation rates were calculated using permeate flow rate, permeate composition, membrane area, and transmembrane pressure drop. The selectivity for each gas pair was calculated as a ratio of the individual gas permeabilities.

[0108] The hollow fibers are pyrolyzed to form CMS membranes by placing precursor fibers on stainless steel wire mesh plates and binding them to the plates using stainless steel wire. The hollow fibers and mesh plate combination is placed in a quartz tube placed in a tube furnace. The fibers are pyrolyzed under an inert gas atmosphere (argon flows at a rate of 200 standard cubic centimeters per minute (sccm)). The precursor fibers are pyrolyzed in a pyrolysis chamber at room temperature with an oxygen content of less than 10 ppm. Argon is used as an inert purge gas. After pyrolysis, the pyrolysis chamber is cooled. The pyrolysis temperature is the highest temperature at which the fibers are also heated during pyrolysis.

[0109] Example 3 - Weight Change of Carbon Films from Continuous Oxidation in Air

[0110] In Example 3, the three asymmetric hollow fiber carbon membranes formed in Example 2 were oxidized by heating the membranes under a continuous air purge of 30 standard cubic centimeters per minute (sccm). Two of the membranes were oxidized at 400°C, one of the 400°C oxidized membranes was formed by pyrolysis at 550°C, and the other 400°C oxidized membrane was formed by pyrolysis at 925°C. The membranes were placed in a quartz tube furnace and the temperature was first increased to 150°C at a ramp rate of 10°C / minute (min) and held for 30 minutes to remove pre-adsorbed species, and then the temperature was increased to 400°C at a ramp rate of 1°C / min and held at 400°C for 8 hours. The temperature of the furnace varied with time during oxidation. Figure 4B Shown in.

[0111] Thermogravimetric analysis (TGA) was performed during the oxidation of the membranes and the results were given in Figure 4A As shown in Figure 4A As shown in Figure 2, the 925°C pyrolysis film had 81% residue after oxidation at 400°C for 8 hours, while the 550°C pyrolysis film had only 34% residue and was therefore mechanically weaker. It is believed that the different oxidation properties are caused by the different graphitization degrees of the 550°C and 925°C pyrolysis films. A higher pyrolysis temperature is desirable to obtain fibers with high mechanical strength after oxidation. In addition, as Figure 4A As shown, after reaching the final oxidation temperature, the weight loss of the membrane is almost linear, so it is possible to use shorter oxidation times to improve the weight loss of the fibers and, therefore, the mechanical strength of the membrane.

[0112] The second film formed by pyrolysis at 925°C was oxidized under a continuous air purge of 30 sccm and an oxidation temperature of 300°C. The film was placed in a quartz tube furnace and the temperature was first increased to 150°C at a heating rate of 10°C / min and held for 30 minutes to remove pre-adsorbed substances. The temperature was then increased to 300°C at a heating rate of 1°C / min and held at 300°C for 8 hours.

[0113] TGA was performed during the oxidation of the membrane and the results were Figure 4C Figure 2 compares the weight changes of two 925°C pyrolyzed films during oxidation at 300°C and 400°C. The 300°C oxidized film shows an initial slight weight gain followed by a slight weight loss. It is believed that the initial weight gain may be due to the formation of oxygen-containing compounds on the carbon film surface, and the subsequent weight loss may be due to the formation of carbon dioxide.

[0114] Example 4 - Permeation Performance of Membranes Oxidized Under Continuous Air

[0115] In Example 4, the seven fiber samples pyrolyzed at 925°C in Example 2 were oxidized in a quartz tube furnace at various temperatures under a constant 300 seem air flow. Comparative Samples A to D were oxidized by first increasing the temperature to 350°C at a ramp rate of 1°C / min and then holding at 350°C for 8 hours. Samples 1 and 2 were oxidized by first increasing the temperature to 300°C at a ramp rate of 1°C / min and then holding at 300°C for 8 hours. Comparative Sample E was oxidized by first increasing the temperature to 250°C at a ramp rate of 1°C / min and then holding at 250°C for 8 hours.

[0116] The permeability of seven oxidized fibers was tested by sealing the single fiber in a Sawagelok shell. The module was then connected to a permeation unit at 35°C. Two mixed gas tests were performed sequentially. First, the shell side of the hollow fiber was pressurized to 52 pounds per square inch gauge (psig) using an equimolar H2 / CO2 / CH4 feed at a total rate of 300 sccm. The hole side of the hollow fiber was continuously purged with 25 sccm of He. The permeate was carried to a gas chromatograph (GC) for composition analysis via the He purge. The H2 / CO2 / CH4 test was performed for approximately 3 to 15 hours. The shell side of the hollow fiber was then pressurized to 52 psig using an equimolar H2 / C2H4 feed at a total rate of 200 sccm. The hole side of the hollow fiber was continuously purged with 25 sccm of He. The permeate was carried to a GC for composition analysis via the He purge. The results of the permeation tests are reported in Table 2.

[0117] Table 2

[0118]

[0119] As shown in Table 2, Comparative Samples A through C were not stable enough to be tested for penetration and broke before results could be obtained. 5A to 5DAs shown, Comparative Sample D has a very rapid permeability drop in the first 2 to 3 hours of the test. Samples 1 and 2 show good reverse selectivity, as indicated by a C2H4 / H2 ratio greater than 5 for both samples. In addition, both Samples 1 and 2 have very high C2H4 permeabilities (greater than 1500 GPU). This indicates that an oxidation temperature of 300°C under these conditions achieves fibers with good reverse selectivity performance.

[0120] Example 5 - Permeability Performance of Membranes Oxidized During Cooling

[0121] In Example 5, the 925°C pyrolyzed film from Example 2 was reheated to a temperature of 925°C in a quartz tube furnace under an Ar gas purge flow. The furnace was then allowed to cool, and when the furnace cooled to a temperature of 575°C, the purge flow was stopped, and air was allowed to diffuse back into the furnace until fully cooled to produce Sample 3. The simulated rate of air diffusion can be found at Figure 3 Seen in. Figure 2 A schematic diagram of the furnace setup used to oxidize the samples in Example 5 is shown.

[0122] The pyrolyzed CMS hollow fibers and / or oxidized CMS hollow fibers are placed in a stainless steel housing to form a membrane assembly for further testing. The membrane module is placed in a temperature-controlled oven (Quincy Lab, Inc., Chicago, IL) in Chicago, Illinois. The test gas flow rate is controlled by a mass flow controller (Brooks Instrument, Hatfield, PA) and the pressure is monitored and controlled by a pressure sensor. In these experiments, the single-fiber CMS fiber optic module is maintained at a constant upstream pressure at 35 ° C. Argon is used as a purge gas to transport the permeate to a downstream flow meter and a gas chromatograph (GC). The composition of the permeate and sweep mixture was measured using a Maxum II process GC (Siemens, Munich, Germany), and the permeate flow rate was measured using a Mesalabs Bios DryCal flow meter (Mesa Labs, Inc., Butler, NJ). The volumetric flow rate from the Bios DryCal flow meter and the composition from the GC were used to analyze the permeability and selectivity of the fiber in the test gas system. The results of the testing are shown in Table 3.

[0123] Table 3

[0124]

[0125] As indicated in Table 3, Sample 3, which was reheated and exposed to air during the cooling process, had significantly higher permeabilities for CO2 and ethylene when compared to Comparative Sample F, which was neither reheated nor exposed to air. Table 3 further indicates that Sample 3 had significantly higher selectivity for ethylene over H2 when compared to Comparative Sample F. An ethylene over H2 selectivity greater than 1 indicates that ethylene permeates through the membrane more than H2, indicating that Sample 3 is a reverse selective membrane.

[0126] Example 6 - Permeability Performance of Membranes Oxidized by Oxygen Immersion

[0127] In Example 6, the 925°C pyrolyzed membrane from Example 2 was oxidized under a continuous oxygen concentration. The membrane was first reheated to 575°C in a quartz tube furnace. Once the furnace reached temperature, 905 ppm of oxygen was added to the furnace and maintained at temperature for 2 hours to produce Sample 4. Sample 4 was then tested using the permeation test method of Example 5, and the results are shown in Table 4.

[0128] Table 4

[0129]

[0130] As indicated in Table 4, Sample 4, which was reheated and exposed to 905 ppm O for 2 hours, had significantly higher permeabilities for CO and ethylene when compared to Comparative Sample F, which was neither reheated nor exposed to air. Table 4 further indicates that Sample 4 had significantly higher selectivity for ethylene over H than Comparative Sample F. An ethylene over H selectivity greater than 1 indicates that ethylene permeated through the membrane more than H, indicating that Sample 4 is a reverse selective membrane.

[0131] Example 7 - Permeability Performance of Membranes Oxidized with Carbon Dioxide

[0132] In Example 7, the 925°C pyrolyzed membrane from Example 2 was oxidized under carbon dioxide. The membrane was first reheated to 800°C in a quartz tube furnace. Once the furnace reached temperature, carbon dioxide gas was added to the furnace and the concentration was maintained at temperature for 8 hours to produce Sample 5. Sample 5 was then tested using the permeation test method of Example 5, and the results are shown in Table 5.

[0133] Table 5

[0134]

[0135] As indicated in Table 5, Sample 5, which was exposed to CO2 at 800°C for 8 hours, had significantly higher permeabilities for CO2 and ethylene when compared to Comparative Sample F, which was not exposed to CO2. Table 5 also indicates that Sample 5 had significantly higher selectivity for ethylene over H2 when compared to Comparative Sample F. An ethylene over H2 selectivity greater than 1 indicates that ethylene permeates through the membrane more than H2, indicating that Sample 5 is a reverse selective membrane.

[0136] It should be apparent to those skilled in the art that various modifications may be made to the described embodiments without departing from the spirit and scope of the claimed subject matter. Therefore, this specification is intended to cover modifications and variations of the described embodiments as long as such modifications and variations fall within the scope of the appended claims and their equivalents.

[0137] It should be noted that one or more of the following claims and detailed description utilize the term "wherein" or "wherein" as a transition phrase. For purposes of defining the present technology, it should be noted that this term is introduced in the claims as an open transition phrase that is used to introduce a recitation of a series of features of a structure and should be interpreted in a manner similar to the more commonly used open-ended term "comprising."

[0138] It should be understood that any two quantitative values assigned to a property may constitute a range for that property, and all combinations of ranges formed by all of the described quantitative values for a given property are contemplated in this disclosure. Where multiple ranges of quantitative values are provided, these ranges may be combined to form broader ranges, which is contemplated in the embodiments described herein.

Claims

1. A method for manufacturing a reverse selective asymmetric hollow fiber carbon membrane, the method comprising: Heating asymmetric hollow fiber carbon membranes; exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse-selective asymmetric hollow fiber carbon membrane; wherein after exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere, the weight of the reverse-selective asymmetric hollow fiber carbon membrane is -5 wt % to 20 wt % less than the weight of the asymmetric hollow fiber carbon membrane. 2 . The method according to claim 1 , wherein the weight of the inverse asymmetric hollow fiber carbon membrane is −5 wt % to 5 wt % less than the weight of the asymmetric hollow fiber carbon membrane.

3. The method according to claims 1 and 2, wherein the oxygen-containing atmosphere comprises one or more of air, oxygen, carbon dioxide or steam.

4. The method according to any preceding claim, further comprising heating the asymmetric hollow fiber carbon membrane in a furnace at a temperature of 600° C. to 1200° C. under an inert atmosphere; cooling the furnace to a temperature greater than or equal to 500° C.; and The asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere during the cooling of the furnace to form a reverse-selective asymmetric hollow fiber carbon membrane. 5 . The method of claim 4 , wherein the asymmetric hollow fiber carbon membrane is heated to a temperature of 900° C. to 1200° C. prior to exposure to the oxygen-containing atmosphere.

6. The method according to claims 4 and 5, wherein the oxygen concentration in the oxygen-containing atmosphere increases as the temperature of the furnace decreases during the cooling of the furnace.

7. The method according to any one of claims 4 to 6, wherein the oxygen concentration in the furnace increases by 2 ppm to 3 ppm for every 1°C decrease in the temperature of the furnace.

8. A method according to any preceding claim, wherein the oxygen-containing atmosphere enters the furnace via diffusion or wherein the oxygen-containing atmosphere is injected into the furnace through a gas inlet.

9. The method according to any one of claims 1 to 3, wherein: heating the asymmetric hollow fiber carbon membrane to a temperature greater than or equal to 550° C.; and The asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for a period of 1 minute to 1000 minutes before being allowed to cool.

10. A method according to any preceding claim, wherein the oxygen concentration in the oxygen-containing atmosphere is from 100 ppm to 1000 ppm.

11. The method according to any one of claims 1 to 3, wherein: The oxygen-containing atmosphere includes air; exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere at a temperature of 200° C. to 600° C.; and The asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

12. The method according to claim 11, wherein: exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere at a temperature of 250° C. to 500° C.; and The asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

13. The method according to any one of claims 1 to 3, wherein The oxygen-containing atmosphere includes carbon dioxide; exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere at a temperature of 700° C. to 1000° C.; and The asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for 1 minute to 1000 minutes.

14. The method of any preceding claim, further comprising creating a pressure differential across the asymmetric hollow fiber carbon membrane during exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is drawn through a cross-section of the asymmetric hollow fiber carbon membrane.

15. The method according to any preceding claim, further comprising: providing a polymer precursor; heating the polymer precursor to a pyrolysis temperature of 800° C. to 1200° C.; pyrolyzing the polymer precursor at the pyrolysis temperature to form an asymmetric hollow fiber carbon membrane; as well as The asymmetric hollow fiber carbon membrane was cooled to a temperature below 50°C.

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