JP2018122293A - Method for manufacturing carbon film for gas separation and infusible fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a gas permeation speed of a PAN carbon film for gas separation.SOLUTION: A method for manufacturing a carbon film for gas separation has an infusibilization process in which a precursor fiber containing a polyacrylonitrile is infusibilized, and a carbonization process in which an infusible fiber, that has passed the infusibilization process, is carbonized. In the infusibilization process, infusibilization is so performed that a nitrile consumption index K1 of an infusible fiber surface, which is measured by the following, is 0.5 or more, and a nitrile consumption index K2 at a position having a depth of 10 μm from the infusible fiber surface is 0.5 or more. Nitrile consumption index: "an absorbance of 2240 cm/an absorbance of 2940 cm" calculated from an absorbance of C-H vibration (2940 cm) and an absorbance of a nitrile group (2240 cm) by a microscopic ATR method with use of an infrared spectrometer.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a carbon membrane for gas separation.

  Gas separation by gas separation membranes uses pressure difference, concentration difference, and mass difference as driving force, so running costs and equipment costs are low, and the required volume is small, so it is an energy-saving method compared to other separation methods Attention has been paid. Among them, the carbon membrane has a feature that the applicable range of the separation environment is wide because it has higher heat resistance and chemical resistance than the organic membrane. For example, in a natural gas refining plant, there is a concern that the membrane may be deteriorated by hydrogen sulfide contained as impurities as well as separation of methane gas and carbon dioxide as main components. In addition, for the enrichment of uranium, a membrane separation method using Knudsen diffusion has been performed for a long time, but the deterioration of the membrane due to hydrogen fluoride generated during the separation has been a problem. In such a separation environment, the practical application of a chemical resistant carbon membrane is expected.

  Resin used as a raw material for the carbon membrane for gas separation is exemplified by polyphenylene oxide (PPO), polyimide (PI), polyacrylonitrile (PAN), etc. Among them, the PAN-based carbon membrane is easy to mold, It is also attracting attention from an economical viewpoint because it is inexpensive. For example, Patent Document 1 reports a PAN-based carbon film having a core layer having a co-continuous porous structure and a skin layer having substantially no co-continuous porous structure.

International Publication No. 2016/013676

  It has been reported that the carbon membrane for gas separation described in Patent Document 1 has a good gas permeation rate. However, further improvement in gas permeation rate has been required for the carbon membrane for gas separation. An object of the present invention is to provide a PAN-based carbon membrane for gas separation that further improves the gas permeation rate.

  In order to solve the above problems, the present invention provides a carbon membrane for gas separation having an infusibilization step for infusible precursor fibers containing polyacrylonitrile and a carbonization step for carbonizing the infusible fibers that have undergone the infusibilization step. In the infusibilization step, the nitrile consumption index K1 on the surface of the infusible fiber is 0.5 or more, and the nitrile consumption index K2 at a depth of 10 μm from the surface of the infusible fiber is 0.5 or more. It is the manufacturing method of the carbon membrane for gas separation which performs an infusible process so that it may become.

  With the production method of the present invention, the gas permeation rate can be further increased in the PAN-based carbon membrane for gas separation.

[Infusibilization process]
The present invention relates to a nitrile at a position where the nitrile consumption index K1 on the surface of the infusible fiber is 0.5 or more and a depth of 10 μm from the surface of the infusible fiber in the infusibilization step of infusibilizing the precursor fiber containing PAN. The infusibilization process is performed so that the consumption index K2 is 0.5 or more.

  The precursor fiber of the separation membrane containing PAN (hereinafter sometimes referred to as “PAN-based precursor fiber”) is polyacrylonitrile (PAN) or a copolymer thereof (hereinafter, PAN and its copolymer are referred to as “PAN”). Fiber which may be referred to as a “system resin”).

  Examples of the PAN resin include a polymer obtained by copolymerizing an acrylonitrile monomer and a copolymer component such as a monomer having a carboxylic acid group, a monomer having a carboxylic acid ester group, or an acrylamide monomer. Such a copolymer is preferable in that the cyclization in the infusibilization process easily proceeds.

  Examples of the monomer having a carboxylic acid group include acrylic acid, methacrylic acid, maleic acid, itaconic acid and the like.

  Examples of the monomer having a carboxylic acid ester group include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, and hydroxypropyl acrylate. Acrylic esters, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, lauryl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, Examples include methacrylic acid esters such as diethylaminoethyl methacrylate.

  Examples of acrylamide monomers include acrylamide, N-methylol acrylamide, and diacetone acrylamide.

  As the PAN-based resin, a resin obtained by copolymerizing a plurality of monomers selected from a monomer having a carboxylic acid group, a monomer having a carboxylic acid ester group, and an acrylamide-based monomer with an acrylonitrile monomer may be used.

  In addition to these, PAN resin of the copolymer includes unsaturated monomers such as styrene, vinyl toluene, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl bromide, vinylidene bromide, vinyl fluoride, and vinylidene fluoride. Further, p-sulfophenyl methallyl ether, methallyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid and alkali metal salts thereof may be contained.

  The copolymerization ratio of these copolymer components is preferably 0.1 to 10 mol%. By making it 0.1 mol% or more, cyclization can be promoted, and by making it 10 mol% or less, shrinkage of the fiber sheet during infusibilization can be suppressed. For these purposes, the copolymerization component is more preferably 0.2 to 5 mol%.

  The weight average molecular weight of the PAN-based resin is preferably 1 to 2 million from the viewpoint of fiber orientation control and infusibilization in a separate process, and is preferably 100,000 to 500,000 from the viewpoint of spinnability. preferable.

  The PAN-based precursor fiber is preferably made of a resin mixture in which a disappearance resin is mixed with a PAN-based resin. The disappearing resin is a resin that can be selectively removed by heating by subsequent infusibilization treatment or carbonization treatment, or by additional treatment at any stage before or after them. When the lost resin disappears and the carbonized PAN is carbonized, the lost resin portion remains as a void portion, and a porous carbon material is obtained. By having the porous structure, the gap portion can secure the gas flow path. Further, the carbonized PAN-derived branch portions support each other's structures, so that the material has a certain degree of resistance against deformation such as tension and compression. In the case of such a resin mixture, it is preferable to mix 90 to 10% by weight of the disappearing resin with respect to 10 to 90% by weight of the PAN-based resin.

  When producing a PAN-based precursor fiber using a resin mixture, it is preferable to further perform a step of forming a porous precursor structure by phase-separating and immobilizing the resin mixture. The method of phase separation of the mixed carbonizable resin and the disappearing resin is not particularly limited. For example, a thermally induced phase separation method in which phase separation is induced by a temperature change, non-solvent induction in which phase separation is induced by adding a non-solvent. Examples include a phase separation method.

These phase separation methods can be used alone or in combination. Specific methods for use in combination include, for example, a method in which non-solvent induced phase separation is caused through a coagulation bath and then heated to cause heat-induced phase separation, or a temperature in the coagulation bath is controlled to control a non-solvent induced phase. Examples thereof include a method of causing separation and thermally induced phase separation at the same time, a method of bringing the material discharged from the die into cooling and causing thermally induced phase separation, and then contacting with a non-solvent.
In particular, it is preferable to use a non-solvent induced phase separation method because the phase separation size of the surface layer can be controlled and a gas separation carbon membrane having a pore size suitable for the separation target can be easily formed.

  As a method for removing the lost resin, a method of removing the resin by reducing the molecular weight by thermal decomposition by heating during infusibilization treatment or carbonization treatment is preferable. In addition, you may perform the removal of the loss | disappearance resin by thermal decomposition by the process of heating a fiber separately from an infusibilization process or a carbonization process.

  When the disappearing resin is removed by thermal decomposition during carbonization, it is preferable to use a resin that is removed at the same time as the carbonization treatment at the highest temperature without causing a large chemical change in the infusible treatment. As such a resin, a resin having a carbonization yield of less than 10% is preferable. Specifically, polyolefins such as polyethylene, polypropylene, and polystyrene, acrylic resins, methacrylic resins, polyacetals, polyvinylpyrrolidones, aliphatic polyesters, aromatic polyesters. , Aliphatic polyamide, polycarbonate and the like. These resins can be used alone or as a mixed resin.

  Also, at any stage before or after the infusibilization treatment or carbonization treatment, the disappearing resin is chemically removed by using chemicals, or the solvent is dissolved and removed by adding a solvent that dissolves the disappearing resin. The disappearing resin may be removed. As a method of chemically removing, a method of hydrolyzing with an acid or an alkali is preferable from the viewpoints of economy and handleability. Examples of the resin that is susceptible to hydrolysis by acid or alkali include polyester, polycarbonate, and polyamide. As a method of adding and removing the solvent that dissolves the disappearing resin, the solvent is continuously supplied to the mixed PAN-based resin and the disappearing resin to dissolve and remove the disappearing resin, or by a batch method. Examples thereof include a method of mixing and dissolving and removing the disappearing resin. Specific examples of the disappearing resin suitable for the method of adding and removing the solvent include polyolefins such as polyethylene, polypropylene, and polystyrene, acrylic resins, methacrylic resins, polyvinyl pyrrolidone, aliphatic polyesters, and polycarbonates. Among them, an amorphous resin is more preferable because of its solubility in a solvent, and examples thereof include polystyrene, methacrylic resin, and polycarbonate.

  In addition, these methods may be used independently and may be used combining several methods, and when using combining, each may be implemented simultaneously or separately.

  The PAN-based precursor fiber can be obtained by spinning a PAN-based resin or the aforementioned resin mixture into a fiber shape. In addition, fibrous form shall mean the form whose average length is 100 times or more with respect to an average diameter.

  The spinning method is not particularly limited, and melt spinning, dry spinning, dry-wet spinning, wet spinning, electrospinning, etc. can be applied, but wet spinning and dry-wet spinning from the viewpoint of excellent spinnability and productivity of PAN-based resins. Is preferred. The solvent in wet spinning or dry wet spinning is preferably a solvent that dissolves the PAN-based resin and the disappearing resin, and examples thereof include dimethylformamide, dimethylacetamide, and dimethylsulfoxide.

  The cross-sectional shape of the PAN-based precursor fiber is not limited at all, and may be a round cross-section, a multi-leaf cross-section such as a triangular cross-section, a flat cross-section, or a hollow cross-section, and can have any shape.

  In the case of a hollow cross section, since the hollow portion functions as a gas flow path, it is not necessary for the PAN-based precursor fiber to have a porous precursor structure as described above, and use only a PAN-based resin. Can do. The shape of the hollow portion is not particularly limited, and may be any shape such as a multi-leaf cross-section such as a round cross-section or a triangular cross-section, a flat cross-section, or a shape having a plurality of hollow portions.

  The PAN-based precursor fiber preferably has an average fiber diameter of 20 to 5000 μm. The average fiber diameter is a diameter when the cross-sectional area of the PAN-based precursor fiber is converted into a circle having the same area. The larger the fiber diameter, the more pyrolysis gas is generated inside the fiber during carbonization, so the pores of the gas separation carbon membrane expand when the pyrolysis gas diffuses and permeates from the inside of the fiber to the outside. Since the pore diameter is enlarged, the gas permeation rate of the carbon membrane can be improved. On the other hand, as the fiber diameter is smaller, the bending rigidity is lowered. Therefore, it is possible to prevent breakage or breakage due to bending, and thus excellent process passability is achieved. Considering these balances, the average fiber diameter is preferably in the range of 40 to 3000 μm, more preferably in the range of 50 to 2000 μm.

  Moreover, when setting it as a hollow cross-section fiber, it is preferable that a film thickness shall be 10-500 micrometers. The film thickness of the hollow fiber is preferably as large as possible to form a separation membrane with excellent pressure resistance, and as the thickness is low, it is preferable in terms of improving the gas permeation rate and forming a separation membrane with few defects. Considering these balances, it is particularly preferable that the hollow fiber has a thickness of 20 to 200 μm. As a method for producing a hollow cross-section fiber, for example, a compatible resin mixture or a compatible resin solution added with a solvent is extruded from the outer tube of a hollow tube spinning nozzle having a double tube structure, and air, nitrogen, etc. are extracted from the inner tube of the spinning nozzle. And a method of extruding the same gas, the same solvent as the spinning dope, a non-solvent, or a mixture thereof.

  Moreover, a hollow cross-section fiber can also be produced using a core-sheath structure fiber containing a disappearing resin that forms a hollow part by disappearance in the core part as a precursor fiber. In this case, the precursor fiber is extruded by extruding a compatible resin mixture or a compatible resin solution to which a solvent is added from the outer tube of a hollow fiber spinning nozzle having a double tube structure, and extruding the lost resin solution from the inner tube of the spinning nozzle. It is preferable to produce it. In this embodiment, the PAN-based precursor fiber having a hollow cross section can be produced by removing the disappearing resin solidified after spinning. In addition, the kind of elimination | eradication resin and its removal method apply to the aspect mentioned above about the resin mixture which made the elimination resin compatible with PAN-type resin.

  Infusibilization is a process in which the crosslinked structure of PAN-based precursor fibers progresses. As a method for this, PAN-based precursor fibers are oxidized and crosslinked by heat treatment in an oxygen atmosphere (in a gas environment containing 1% or more of oxygen). A method of forming a crosslinked structure by irradiating high energy rays such as an electron beam and a gamma ray, a method of impregnating and mixing a substance having a reactive group to form a crosslinked structure, etc. The method of causing oxidative crosslinking by heating at a temperature is preferable because the process is simple and the production cost can be kept low. These techniques may be used singly or in combination, and each may be used simultaneously or separately.

  In the infusibilization step, the PAN-based precursor fiber is not adjusted so that the nitrile consumption index K1 on the surface is 0.5 or more and the nitrile consumption index K2 at a depth of 10 μm from the surface is 0.5 or more. Perform fusing treatment. In the present specification, fibers obtained by infusibilizing PAN-based precursor fibers are referred to as “infusible fibers”.

The nitrile consumption index is an index for confirming the degree of progress of the infusible reaction by the microscopic ATR method using an infrared spectroscopic device. Specifically, the absorbance of C-H vibration (2940 cm ^-1) and measured the absorbance of the nitrile group of (2240 cm ^-1), is calculated from these absorbance "Absorbance / 2940 cm absorbance ^-1 2240 cm ^-1" Is the nitrile consumption index. The nitrile consumption index is measured by the method described in Examples described later. The smaller the value of K1 in the infusible fiber, the more the chemical resistance is improved because the cyclization reaction on the surface of the separation membrane proceeds. On the other hand, as the value of K1 is larger, a coarse pore diameter is formed and the gas permeation rate is improved. From the viewpoint of producing a separation membrane having an excellent gas permeation rate while maintaining chemical resistance, the infusibilization treatment in the infusibilization step may be performed such that K1 of the infusible fiber is 0.6 or more and 0.9 or less. Preferably, it is more preferably performed so as to be 0.65 or more and 0.85 or less.

  In the infusibilization step, the infusibilization treatment is performed so that the nitrile consumption index K2 at a depth of 10 μm from the surface also becomes 0.5 or more.

  The larger the value of K2 of the infusible fiber, the more pyrolytic gas is generated in the carbonization process. As a result, the pore diameter is enlarged or the number of pores is increased, so that the gas permeation rate of the carbon membrane is improved. On the other hand, as K2 is smaller, the risk of thread breakage can be reduced because rapid thermal decomposition of the resin is suppressed in the carbonization step. From the viewpoint of continuous productivity of the separation membrane, the infusibilization treatment in the infusibilization step is preferably performed such that K2 of the infusible fiber is 0.55 or more and 0.9 or less, and is 0.6 or more and 0.85 or less. It is more preferable to carry out.

  In the present invention, by setting the values of K1 and K2 in such a range, a large amount of pyrolysis gas is generated in the subsequent carbonization process as compared with the conventional PAN-based infusible fiber, and is generated inside the fiber. It is considered that when the pyrolysis gas diffuses to the outside, the pore diameter is enlarged or the number of pores is increased, so that a separation membrane having an excellent gas permeation rate can be obtained.

  When the infusibilization treatment is performed by heat treatment, the oxygen concentration is not particularly limited. However, supplying a gas having an oxygen concentration of 1% or more, particularly air, as it is can reduce the manufacturing cost. In this case, the infusibilization temperature is preferably 150 ° C. or higher from the viewpoint of efficiently proceeding with the cross-linking reaction, and 350 ° C. or lower from the viewpoint of obtaining fibers in good yield without weight loss due to thermal decomposition or combustion of the resin. Is preferred.

  The infusibilization time is preferably 1 minute or more and less than 50 minutes. By setting the infusibilization time to 1 minute or longer, crosslinking is progressing and deformation at high temperatures is suppressed, which is preferable from the viewpoint of maintaining the cross-sectional shape and preventing yarn breakage. Further, by setting the infusibilization time to less than 50 minutes, it is preferable in that the bending rigidity of the PAN-based precursor fiber can be kept low, and the process permeability is excellent and the gas permeation rate of the separation membrane is improved. From this viewpoint, the infusibilization time is more preferably 5 to 45 minutes. In the present specification, the infusibilization time refers to the heating time excluding the temperature rise time and the temperature fall time. If the infusibilization step is within the above-mentioned temperature and infusibilization time range, for example, the first infusibilization treatment temperature is T1, the infusibilization time is S1, the second infusibilization treatment temperature is T2, When the melting time is S2, the infusibilization step may be performed a plurality of times so that 150 ° C. ≦ T1 <T2 ≦ 350 ° C. and 1 minute ≦ S1 + S2 <50 minutes. The lower limit of the rate of temperature increase and the rate of temperature decrease is not particularly limited, but it is preferable that the rate is 1 ° C./min or higher because productivity can be improved by shortening the time required for temperature increase and decrease.

[Carbonization process]
A carbon membrane for gas separation can be obtained by carbonizing the infusible fiber that has undergone the infusibilization process as described above. In order to sufficiently carbonize the infusible fiber, the heat treatment in the carbonization step is preferably performed by heating the infusible fiber to 400 ° C. or higher in an inert gas atmosphere. The inert gas refers to a substance that is chemically inert when heated, and specifically includes helium, neon, nitrogen, argon, krypton, xenon, carbon dioxide, and the like. Of these, nitrogen or argon is preferred from an economical viewpoint. The upper limit of the heating temperature during the heat treatment is not particularly limited, but it is more economical as it is lower, so 1500 ° C. or less is preferable.

  The carbon membrane for gas separation obtained in the carbonization process can be separated by the difference in the permeability of substances using the holes opened in the membrane. As the separation mechanism, molecular sieving, Knudsen diffusion, and the like are known, but the gas separation carbon membrane of the present invention is not particularly limited.

  The carbon membrane for gas separation of the present invention is particularly preferably used for carbon dioxide separation.

(Nitrile consumption index)
The infusible fiber is sliced parallel to the fiber axis, the length of the width in the direction perpendicular to the fiber axis of the sliced cross section is obtained, sliced parallel to the fiber axis at a depth of 10 μm from the surface of the infusible fiber, A sample was obtained in which the center line position in the longitudinal direction was 10 μm deep from the surface. For the IR measurement, an FT-IR apparatus (manufactured by JASCO Corporation, product name IRT-3000) was used, and measurement was performed as follows under the condition of 32 integrations. First, the background spectrum of the measurement atmosphere was measured. Subsequently, the infusible fiber that was not sliced on the ATR prism surface and the sample at a depth of 10 μm from the surface prepared as described above were brought into close contact with each other, and the spectrum measurement was performed five times. Reads each absorption spectrum when the respective measurement results and zero absorption spectrum of 2500 cm ^-1 absorbance (peak intensity) of (2240,2940cm ^-1), the ratio of the absorbance at 2240 cm ^-1 to the absorbance of 2940 cm ^-1 determined to calculate the "nitrile consumption indicator (K) = 2240 cm absorbance / 2940 cm absorbance ^{-1 -1",} and five average respectively K1, K2.

(Gas transmission rate)
The carbon membrane for gas separation is cut into a length of 10 cm, and 20 of these are bundled and accommodated in a stainless steel casing having an outer diameter of 6 mm and a wall thickness of 1 mm, and the ends of the bundled gas separation membrane are bonded with an epoxy resin adhesive. The gas separation membrane module was produced by fixing to the inner surface of the casing and sealing both ends of the casing. Nitrogen, carbon dioxide and methane are used as measurement gases, and the permeation side per unit time of nitrogen, carbon dioxide and methane is measured at an external pressure type at a measurement temperature of 25 ° C. according to the pressure sensor method of JIS K7126-1 (2006). The pressure change was measured. Here, the pressure difference between the supply side and the transmission side was set to 0.11 MPa (82.5 cmHg). Subsequently, the permeation rate Q of the permeated gas was calculated by the following formula, and the separation coefficient α was calculated as the ratio of the permeation rate of the gas of each component. STP means standard conditions. The membrane area was calculated from the outer diameter and length of the gas separation membrane in a region contributing to gas permeation.
Q = [gas permeation flow rate (× 10 ⁻⁶ cm ³ · STP)] / [membrane area (cm ² ) × time (s) × pressure difference (cmHg)
[Example 1]
70 g of polyacrylonitrile (MW 150,000) manufactured by Polyscience, 70 g of polyvinyl pyrrolidone (MW 40,000) manufactured by Sigma-Aldrich, and 400 g of dimethyl sulfoxide (DMSO) manufactured by Wakken as a solvent were placed in a separable flask. A uniform and clear solution was prepared while stirring and refluxing for 3 hours. At this time, the concentration of polyacrylonitrile and the concentration of polyvinylpyrrolidone were 15% by weight, respectively.

  The obtained polymer solution is discharged from the outer tube of the core-sheath type double cap, the polymer solution is discharged from the inner tube at the same time as a 90% by weight aqueous solution of DMSO, and then guided to a coagulation bath composed of 100% water. To obtain a hollow fiber-like PAN-based precursor fiber. The obtained PAN-based precursor fiber was translucent and caused phase separation. The obtained PAN-based precursor fiber was washed with water and then dried.

  Thereafter, infusibilization treatment was performed in an air atmosphere at a heating rate of 10 ° C./min, 240 ° C., and an infusibilization time of 30 minutes, to produce an infusible fiber.

Subsequently, the PAN-based precursor fiber was carbonized at an ultimate temperature of 600 ° C. for a treatment time of 5 minutes to produce a separation membrane. The produced gas separation carbon membrane was a carbon membrane for gas separation having a co-continuous porous structure in the inner layer and a dense layer having substantially no co-continuous porous structure in the surface layer.
[Example 2]
A separation membrane was prepared in the same manner as in Example 1 except that the infusibilization time was 5 minutes.
[Example 3]
A separation membrane was prepared in the same manner as in Example 1 except that the infusibilization time was 45 minutes.
[Example 4]
Example 1 after performing heat treatment at an infusibilization temperature of 240 ° C. and an infusibilization time of 20 minutes, and then performing a second heat treatment at an infusibilization temperature of 320 ° C. and an infusibilization time of 1 minute. A separation membrane was prepared by this method.

[Example 5]
A separation membrane was prepared in the same manner as in Example 1 except that the coagulation bath was changed to a mixed bath of water and DMSO and the phase separation size was expanded. The produced gas separation carbon membrane was a carbon membrane for gas separation having a bicontinuous porous structure as a whole.
[Comparative Example 1]
A separation membrane was prepared in the same manner as in Example 1 except that the infusibilization time was 60 minutes.
[Comparative Example 2]
A separation membrane was prepared in the same manner as in Example 5 except that the infusibilization time was 120 minutes.

  Table 1 shows the conditions of the infusible treatment, the physical properties of the infusible fibers, and the gas separation performance of the finally obtained carbon membrane for gas separation in each example and comparative example.

Claims (10)

A method for producing a carbon membrane for gas separation, comprising: an infusibilization step for infusifying a precursor fiber containing polyacrylonitrile; and a carbonization step for carbonizing the infusible fiber that has undergone the infusibilization step. The nitrile consumption index K1 measured by the following on the surface of the infusible fiber is 0.5 or more and the nitrile consumption index K2 at a depth of 10 μm from the surface of the infusible fiber is 0.5 or more. A method for producing a carbon membrane for gas separation that performs infusibilization.
Nitrile consumption index: infrared spectrometer the absorbance of C-H vibrations measured by microscopic ATR method using (2940 cm ^-1) and the absorbance of the nitrile group absorbance "2240 cm ^-1 calculated from (2240 cm ^-1) / 2940cm ⁻¹ absorbance ” 2. The method for producing a carbon membrane for gas separation according to claim 1, wherein in the infusibilization step, heat treatment is performed in the presence of oxygen at 150 ° C. or more and 350 ° C. or less for an infusibilization time of 1 minute or more and less than 50 minutes. The manufacturing method of the carbon membrane for gas separation of Claim 1 or 2 whose said precursor fiber is a fiber with an average fiber diameter of 20-5000 micrometers which has a porous precursor structure inside. The gas according to claim 1, wherein the precursor fiber is a hollow cross-section fiber having a thickness of 10 to 500 μm, or a core-sheath structure fiber including a disappearing resin that forms a hollow portion by disappearance in the core portion. A method for producing a carbon membrane for separation. In the infusibilization step, the nitrile consumption index K1 on the surface of the infusible fiber is 0.6 or more and 0.9 or less, and the nitrile consumption index K2 at a depth of 10 μm from the surface of the infusible fiber is 0.55 or more. The method for producing a carbon membrane for gas separation according to any one of claims 1 to 4, wherein the infusibilization treatment is performed so as to be 0.9 or less. In the infusibilization step, assuming that the first infusibilization temperature is T1, the infusibilization time is S1, the second infusibilization temperature is T2, and the infusibilization time is S2, 150 ° C. ≦ T1 <T2 ≦ 350 ° C. And the manufacturing method of the carbon membrane for gas separation in any one of Claims 1-5 which performs an infusible process in multiple times so that it may become 1 minute <= S1 + S2 <50 minutes. The method for producing a carbon membrane for gas separation according to any one of claims 1 to 6, wherein the precursor fiber is made of a resin mixture in which a disappearance resin is mixed with a polyacrylonitrile resin. The method for producing a carbon membrane for gas separation according to any one of claims 1 to 7, wherein the carbon membrane for gas separation is for carbon dioxide separation. A method for producing an infusible fiber, which is a precursor of a carbon membrane for gas separation, having an infusibilization step of infusifying a precursor fiber containing polyacrylonitrile, the nitrile on the surface of the infusible fiber in the infusibilization step A method for producing an infusible fiber, wherein the infusibilization treatment is performed such that the consumption index K1 is 0.5 or more and the nitrile consumption index K2 at a position 10 μm deep from the surface of the infusible fiber is 0.5 or more. It is an infusible fiber obtained by infusibilizing a precursor fiber made of polyacrylonitrile, and the nitrile consumption index K1 on the surface is 0.5 or more and the nitrile consumption index K2 at a depth of 10 μm from the surface is 0. An infusible fiber of 5 or more.