Porous carbon fiber, composite membrane for gas separation, and module for gas separation membrane

Abstract

The present invention addresses the problem of providing a porous carbon fiber that is suitable for a support body for various gas separation membranes, with which it is possible to suppress adhesion between the separation membranes, the porous carbon fiber being characterized in that the fiber diameter varies in the length direction, and the CV% calculated based on the variation of the fiber diameter is 1-10%, inclusive.

Description

Technical Field

[0001] The present invention relates to porous carbon fibers and composite membranes for gas separation using the same.

Background Art

[0002] Porous carbon materials have long been used as adsorbents and reaction fields supporting catalysts. Furthermore, in recent years, porous carbon fibers having through-holes have been reported, and their utilization in fluid separation membranes (carbon membranes) using through-holes or composite membranes for gas separation in which a separation functional layer is formed on the porous carbon fibers is expected (see, for example, Patent Document 1).

[0003] Since fluid separation by a separation membrane uses a pressure difference, a concentration difference, or a mass difference as a driving force, it has attracted attention as an energy-saving and compact method because of its low running cost, low equipment cost, and small required volume compared with other separation methods. In actual use, for the purpose of increasing the membrane area per unit volume, it is common to bundle a plurality of separation membranes, store them in a vessel, and then use them as a module. At this time, if the separation membranes are overly adhered to each other, there is a problem that the membrane efficiency decreases because the area where the fluid contacts the separation functional layer is limited. Furthermore, there has been a problem that capillary suction of the potting agent occurs between the bundles of the separation membranes during module formation, resulting in a decrease in the effective membrane area.

[0004] Therefore, as a means for creating an appropriate interval between the separation membranes, a method of winding a fibrous material around the outer periphery of the separation membrane and a method of producing a separation membrane in which the fiber diameter periodically changes in the longitudinal direction have been proposed (see, for example, Patent Documents 2 and 3).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] Although Patent Document 2 discloses an example of a hollow fiber membrane in which fibrous materials are spirally wound around the outer peripheral surface, in order to take such a form, there is a problem that the process of winding the fibrous materials around the separation membrane increases and the cost increases. Further, in a separation membrane with small surface irregularities, in addition to the difficulty of winding the fibrous materials with a constant pitch due to slippage, displacement of the fibrous materials occurs during modularization, so it may not be effective as a means for suppressing adhesion between the separation membranes.

[0007] In addition, Patent Document 3 discloses an example of a separation membrane in which the fiber diameter periodically changes in the longitudinal direction by controlling the spinning conditions. However, when the separation target is a gas, the separation membrane forms pores on the order of angstroms and requires precise control. Therefore, with this method, spots are likely to occur on the gas separation membrane, and there is a problem that the membrane performance is deactivated. Further, depending on the type of gas separation membrane, it is difficult to apply, and there is also a problem that it cannot be applied to various gas separation membranes.

[0008] Therefore, an object of the present invention is to provide porous carbon fibers that can suppress adhesion between separation membranes and can be applied to supports of various gas separation membranes.

Means for Solving the Problems

[0009] The present invention for solving the above problems is as follows.

[0010] A porous carbon fiber, characterized in that the fiber diameter varies in the longitudinal direction and the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less. wherein the fiber diameter varies periodically in the longitudinal direction

Effects of the Invention

[0011] According to the porous carbon fibers of the present invention, it is possible to suppress the adhesion between separation membranes and provide porous carbon fibers that can be applied to supports for various gas separation membranes.

Embodiments for Carrying Out the Invention

[0012] The porous carbon fibers of the present invention are characterized in that the fiber diameter varies in the longitudinal direction and the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less. Hereinafter, such porous carbon fibers will be described.

[0013] 〔Fiber diameter variation in the longitudinal direction〕 The porous carbon fibers of the present invention are such that the fiber diameter varies in the longitudinal direction, and it is important that the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less. Here, the fiber diameter refers to the diameter of a circle obtained by determining the cross-sectional area of an arbitrary cross-section perpendicular to the fiber axis of the porous carbon fiber and converting this into a circle. At this time, when the cross-section includes voids such as a hollow cross-section, the cross-sectional area calculated based on the outer periphery of the cross-section including the voids is used.

[0014] Also, CV% refers to the value obtained by dividing the standard deviation of the fiber diameter by the average fiber diameter and expressing it as a percentage. The values of the standard deviation of the fiber diameter and the average fiber diameter are determined by taking the cross-sections of at least 300 points of porous carbon fibers among the porous carbon fibers, obtaining the fiber diameter at each cross-section, dividing the total value of the fiber diameters by the measurement points to obtain the average fiber diameter, and defining the non-negative square root of the variance calculated from the fiber diameter at each measurement point and the obtained average fiber diameter as the standard deviation of the fiber diameter.

[0015] The porous carbon fibers of the present invention may be used as they are, but it is preferable to use them as a separation membrane in a state where a separation functional layer having a function of separating at least a part of the gas is formed.

[0016] In the composite membrane for gas separation of the present invention, it is preferable to appropriately select a substance constituting the separation functional layer having a function of separating gas from those conventionally known according to the properties of the gas to be separated. At this time, among the substances to be separated, the smaller the absolute value of the difference between the solubility parameter of the component permeating through the separation membrane and the solubility parameter of the substance constituting the separation functional layer, the smoother the permeation of the component permeating through the separation membrane. Therefore, it is preferable because a device capable of efficiently separating can be obtained. The absolute value of the difference in solubility parameter is preferably 2.0 or less, and more preferably 1.5 or less.

[0017] On the other hand, the larger the CV% of the porous carbon fiber of the present invention, the larger the change in the fiber diameter of the porous carbon fiber. Therefore, when the separation membranes are bundled, a gap is formed between the separation membranes, ensuring a gas flow path and reducing the pressure drop due to pressure loss, enabling efficient membrane separation. Furthermore, during module formation, the capillary suction of the potting agent is suppressed, preventing the surface of the separation membrane from being blocked by the potting agent and suppressing a decrease in the effective membrane area.

[0018] On the other hand, the smaller the CV% of the porous carbon fiber, the smaller and more uniform the change in the cross-sectional area of the fiber. Therefore, breakage due to stress concentration can be suppressed, the mechanical strength of the porous carbon fiber is high, and it has the characteristic of improved handleability. In addition, when a gas separation functional layer is formed on the porous carbon fiber to form a composite membrane for gas separation, the thickness of the gas separation functional layer can be uniformly laminated, suppressing defects in the gas separation functional layer.

[0019] From the above viewpoints, when the CV% calculated from the variation in the diameter of the porous carbon fibers is less than 1%, when the porous carbon fibers are bundled, the distance between the porous carbon fibers is small, and the reduction in the pressure loss of the gas flow path and the effect of suppressing the suction of the potting agent cannot be sufficiently obtained. Further, when the CV% exceeds 10%, breakage is likely to occur during the handling of the porous carbon fibers, and when a gas separation functional layer is formed on the porous carbon fibers to form a composite membrane for gas separation, there is a high possibility that defects will occur in the gas separation functional layer. Therefore, the CV% calculated from the variation in the diameter of the porous carbon fibers is 1% or more and 10% or less, and preferably 1.5% or more and 7% or less.

[0020] Further, the porous carbon fibers of the present invention preferably have a variation of 3% or more and 10% or less in the fiber diameter in a section of 0.1 m in the longitudinal direction. Here, the variation in the fiber diameter is obtained by equally dividing a 0.1 m long porous carbon fiber into 10 parts at equal intervals, measuring the fiber diameters at the 11 divided points, and calculating the rate of change from the minimum diameter to the maximum diameter based on the minimum diameter among the 11 measurement data as (maximum diameter - minimum diameter) / minimum diameter × 100 in percentage, and performing the same operation on 30 porous carbon fibers, and obtaining it from the average value of the 30 obtained change rates.

[0021] The larger the variation in the fiber diameter, the more appropriately the distance between the bundled separation membranes per unit length increases, so that a gas flow path is ensured and the pressure drop due to the pressure loss is alleviated. Therefore, when the porous carbon fibers are applied to a gas separation membrane, efficient membrane separation is possible. Further, during module formation, the suction of the potting agent due to capillary action is suppressed, so that the surface of the separation membrane is prevented from being blocked by the potting agent, and a decrease in the effective membrane area can be suppressed. On the other hand, the smaller the variation in the fiber diameter, the less extreme the change in the fiber shape, so the mechanical strength is high and the handleability is improved.

[0022] From the above viewpoints, the variation in the fiber diameter in a section of 0.1 m in the longitudinal direction is more preferably 3% or more and 7% or less.

[0023] Furthermore, it is preferable that the fiber diameter of the porous carbon fiber of the present invention varies periodically in the longitudinal direction. The periodicity of the variation in the fiber diameter can be determined by creating a correlogram from the fiber diameters of the porous carbon fibers measured in the longitudinal direction at 10 mm intervals. If the autocorrelation coefficient is 0.2 or more in the range where the lag shifted in the longitudinal direction is 50 mm or more, it can be determined that there is periodicity. The period can also be determined from the value of the lag at which the autocorrelation coefficient shows 0.2 or more. Compared with the disordered fiber diameter change, when the fiber diameter changes periodically, the distance between the bundled fibers is efficiently increased by the fiber diameter having periodicity, a gas flow path is secured, and the pressure drop due to the pressure loss is alleviated. Therefore, when the porous carbon fiber is applied to a gas separation membrane, it is preferable because efficient membrane separation is enabled.

[0024] The smaller the period of the periodic variation of the fiber diameter in the longitudinal direction, the more the bundled fibers are separated by an average distance at the thick part of the fiber diameter, so the interval increases. On the other hand, the larger the period, the less the change in the fiber diameter, and it is possible to prevent the decrease in mechanical strength due to stress concentration. Therefore, the mechanical strength is high and the handleability is improved.

[0025] When the porous carbon fiber of the present invention is applied to a separation membrane, from the viewpoint of increasing the interval between the bundled separation membranes and providing a separation membrane having excellent mechanical strength, the period of the periodic variation of the fiber diameter in the longitudinal direction is preferably 20 mm or more and 10000 mm or less, and more preferably 20 mm or more and 3000 mm or less.

[0026] 〔Porous carbon fiber〕 The porous carbon fibers of the present invention refer to carbon fibers containing many pores. These pores may be open to the outside or may exist as internal spaces without opening. The porous carbon fibers of the present invention preferably have a co-continuous structure at least in part. The co-continuous structure may cause the outer surface to open by connecting to the outer surface of the porous carbon fibers, or conversely, the outer surface may be blocked by the co-continuous structure being interrupted up to the outer surface of the porous carbon fibers. The co-continuous structure is a structure in which the branched portions and pore portions (void portions) of the carbon skeleton are three-dimensionally intertwined while being continuous respectively. Specifically, when observing the surface of a cross-section obtained by cutting a sample sufficiently cooled in liquid nitrogen with tweezers or the like using a scanning electron microscope, it can be confirmed by observing that the branched portions and void portions of the carbon skeleton are intertwined while being continuous respectively. Also, the fact that the porous carbon fibers have a co-continuous structure means that such a co-continuous structure is observed in any cross-section of the porous carbon fibers.

[0027] When the porous carbon fibers have a co-continuous structure, since the carbon skeleton is three-dimensionally continuous, an effect occurs in which the carbon skeleton supports the entire structure, enabling stress to be dispersed throughout the porous carbon fibers, and having great resistance to external forces such as compression and bending, that is, having great compressive strength and specific compressive strength. Also, since the voids are three-dimensionally connected, the voids can function as gas flow paths.

[0028] Examples of the mode of the co-continuous structure include a lattice shape and a monolithic shape, and are not particularly limited. However, in terms of being able to exhibit the above effects, a monolithic shape is preferable because the compressive strength in the fiber cross-section direction tends to improve. The monolithic shape refers to a form in which the carbon skeleton is three-dimensionally uniformly continuous in the co-continuous structure, and is distinguished from irregular structures such as structures formed by aggregation and connection of individual particles, or structures formed by voids generated by removing aggregated and connected template particles and the surrounding skeleton, or structures in which cell walls derived from organisms are continuous.

[0029] The structural period of the co-continuous structure of the porous carbon fiber is preferably 0.002 μm or more and 20 μm or less. The fact that the porous carbon fiber has a structural period of the co-continuous structure indicates that the uniformity of the co-continuous structure is high, which means that the thickness of the branches of the carbon skeleton and the pore size are uniform. As a result, the effect of improving the compressive strength of the gas separation membrane can be obtained. When the structural period of the co-continuous structure is 20 μm or less, the carbon skeleton and pores form a fine structure, and the compressive strength is improved. Therefore, the structural period is more preferably 10 μm or less, and even more preferably 5 μm or less. On the other hand, when the structural period of the co-continuous structure is 0.002 μm or more, the pressure loss when flowing gas through the voids decreases, and the gas permeation rate is improved. Further, when the pressure loss decreases, the effect of enabling separation and purification with less energy consumption is achieved. Therefore, the structural period is more preferably 0.02 μm or more, and even more preferably 0.1 μm or more.

[0030] The structural period of the co-continuous structure is calculated by the following formula from the scattering angle 2θ at the peak top of the scattering intensity obtained by irradiating the porous carbon fiber with X-rays and scattering at a small angle.

[0031] L = λ / (2sinθ) L: Structural period, λ: Wavelength of incident X-rays However, there may be cases where the structural period is large and X-ray scattering at a small angle cannot be observed. In such cases, the structural period is obtained by X-ray computed tomography (X-ray CT). Specifically, after performing Fourier transform on the three-dimensional image taken by X-ray CT, the circular average of the two-dimensional spectrum is taken to obtain a one-dimensional spectrum. The characteristic wavelength corresponding to the position of the peak top in the one-dimensional spectrum is obtained, and the structural period is calculated as the reciprocal thereof. At this time, those in which a plurality of peaks are observed are not suitable for calculating the structural period of the co-continuous structure of the present application. Generally, when a plurality of peaks are observed, it is a case having a very highly crystalline structure. Examples include microphase separation and those using mesoporous silica as a template, but the co-continuous structure of the present application is clearly different from this.

[0032] When the average diameter of all the pores forming the co - continuous structure of the porous carbon fibers is too small, the pressure loss increases and the gas permeability decreases. Therefore, it is preferably 30 nm or more, more preferably 100 nm or more. On the other hand, when the average diameter of all the pores is too large, the effect of the carbon branches supporting the entire structure decreases and the compressive strength decreases. Therefore, it is preferably 5,000 nm or less, more preferably 2,500 nm or less.

[0033] The average diameter of all the pores is a measured value obtained by measuring the pore size distribution by the mercury intrusion method. In the mercury intrusion method, pressure is applied to the pores of the co - continuous structure to allow mercury to penetrate, and the pore volume and specific surface area are determined from the pressure and the amount of mercury penetrated. Then, the pore diameter obtained from the relationship between the pore volume and the specific surface area when the pores are assumed to be cylinders is calculated. In the mercury intrusion method, a pore diameter distribution curve of 5 nm to 500 μm can be obtained.

[0034] When forming a separation functional layer on the porous carbon fibers of the present invention, it is preferable that the outer surface of the porous carbon fibers, that is, the interface with the separation functional layer of the porous carbon fibers is open - pored. When the pore part is open - pored at the interface with the separation functional layer, the pressure loss when gas permeates from the separation functional layer to the porous carbon fibers or vice versa decreases. Therefore, the gas permeation rate in the gas separation membrane can be improved. In addition, since irregularities are generated on the outer surface of the porous carbon fibers, the adhesiveness with the separation functional layer is improved by the anchor effect, and a gas separation membrane excellent in durability that suppresses peeling during use can be obtained.

[0035] The larger the opening diameter of the pore part at the interface between the porous carbon fibers and the separation functional layer, the more the gas permeation rate of the gas separation membrane is improved. Therefore, the average opening diameter is preferably 2 nm or more, more preferably 10 nm or more, and even more preferably 50 nm or more. On the other hand, when the opening diameter is too large, when forming the separation functional layer, the inorganic material may penetrate deep into the porous carbon fibers and cannot be uniformly laminated on the surface. Therefore, the average opening diameter is preferably 500 nm or less, more preferably 400 nm or less, and even more preferably 300 nm or less.

[0036] Here, the term "pores are open at the interface with the separation functional layer" means a state where, when an arbitrary cross-section of the porous carbon fiber is precisely fabricated using an ion milling device or the like and observed with an electron microscope, a portion where the pores in the porous carbon fiber are in direct contact with the interface is observed. The average pore opening diameter is calculated by measuring, at an arbitrary 10 locations, the length along the interface from one contact point between one carbon of the interface portion where the voids, which are the pore portions of the porous carbon fiber, are in direct contact with the interface to the other contact point, and taking the average value thereof.

[0037] The fiber diameter of the porous carbon fiber is preferably thinner because it is more flexible, more resistant to breakage, and can withstand high pressure. On the other hand, a thicker fiber diameter is preferable because it has excellent mechanical strength and can reduce the gas pressure loss. From these points, the average value of the fiber diameter of the porous carbon fiber is preferably in the range of 20 μm or more and 5,000 μm or less.

[0038] When the porous carbon fiber is a fiber having a hollow portion, that is, a hollow fiber, a lower hollow ratio is preferable because it can enhance the pressure resistance, and a higher hollow ratio is preferable because it can reduce the gas pressure loss. From these points, the hollow ratio is preferably in the range of 1% or more and 90% or less, and more preferably in the range of 5% or more and 60% or less.

[0039] <Method for producing porous carbon fiber, composite membrane for gas separation using the same, and gas separation module> The porous carbon fiber of the present invention, as an example, a step of dissolving a carbonizable resin and a sacrificial resin to form a resin mixture (step 1); a step of phase-separating the resin mixture in a compatible state and forming it into a fibrous shape (step 2); a step of removing the sacrificial resin from the phase-separated resin mixture (step 3); a step of obtaining a porous carbon fiber by carbonization treatment by heating (step 4); can be produced by a production method having the above steps.

[0040] The composite membrane for gas separation of the present invention further includes, as an example, a step of forming a separation functional layer on a porous carbon fiber to obtain a composite membrane for gas separation (Step 5); It can be manufactured by a manufacturing method having the same.

[0041] The gas separation module of the present invention further includes, as an example, a step of housing the composite membrane for gas separation to obtain a gas separation module (Step 6); It can be manufactured by a manufacturing method having the same.

[0042] [Step 1] Compatibilizing and mixing a carbonizable resin and a sacrificial resin Step 1 is a step of compatibilizing a carbonizable resin and a sacrificial resin to form a resin mixture. Here, the carbonizable resin is a resin that carbonizes upon heating and remains as a branch portion (carbon skeleton), and both thermoplastic resins and thermosetting resins can be used.

[0043] In the case of a thermoplastic resin, it is preferable to select a resin capable of performing an infusibilization treatment by a simple process such as heating or high-energy ray irradiation. In the case of a thermosetting resin, the infusibilization treatment is often unnecessary, and this is also cited as a suitable material.

[0044] Examples of thermoplastic resins include polyphenylene ether, polyvinyl alcohol, polyacrylonitrile, phenol resin, wholly aromatic polyester, polyimide resin, cellulose acetate, and polyetherimide. Examples of thermosetting resins include unsaturated polyester resin, alkyd resin, melamine resin, urea resin, polyimide resin, diallyl phthalate resin, lignin resin, urethane resin, polyfurfuryl alcohol resin, etc. These may be used alone or in a mixed state, but it is also preferable to mix them with each thermoplastic resin or thermosetting resin for ease of molding.

[0045] Among them, from the viewpoints of carbonization yield, spinnability, and economy, it is preferable to use a thermoplastic resin, and polyphenylene ether, polyvinyl alcohol, polyacrylonitrile, and wholly aromatic polyester are more preferably used.

[0046] The molecular weight of the carbonizable resin is preferably 10,000 or more in terms of weight average molecular weight. When the weight average molecular weight is 10,000 or more, the number of thread breaks is reduced in the process of forming into a spun yarn. On the other hand, the upper limit of the weight average molecular weight is not particularly limited, but it is preferably 1,000,000 or less from the viewpoints of spinnability and moldability and ease of resin extrusion.

[0047] The disappearing resin is a resin that can be removed at any stage after the formation of the phase separation structure in Step 2 described later.

[0048] The method for removing the disappearing resin is not particularly limited, and methods such as chemically removing it by depolymerization using a chemical, adding a solvent that dissolves the disappearing resin and removing it by dissolution, and heating to reduce the molecular weight of the disappearing resin by thermal decomposition and removing it are preferably used. These methods can be carried out alone or in combination, and when carried out in combination, they can be carried out simultaneously or separately.

[0049] As a method for chemically removing it, a method of hydrolyzing using an acid or an alkali is preferable from the viewpoints of economy and handleability. Examples of resins that are easily hydrolyzed by an acid or an alkali include polyester, polycarbonate, and polyamide.

[0050] As a method of adding a solvent that dissolves the disappearing resin and removing it, examples of preferable methods include a method of continuously supplying a solvent to the mixed carbonizable resin and disappearing resin to dissolve and remove the disappearing resin, and a method of mixing in a batch type to dissolve and remove the disappearing resin.

[0051] Specific examples of the disappearing resin suitable for the method of adding and removing a solvent include polyolefins such as polyethylene, polypropylene, and polystyrene, acrylic resins, methacrylic resins, polyvinylpyrrolidone, aliphatic polyesters, polycarbonates, and the like. Among them, an amorphous resin is more preferable from the viewpoint of solubility in a solvent, and examples thereof include polystyrene, methacrylic resin, and polycarbonate.

[0052] As a method of removing the disappearing resin by reducing its molecular weight by thermal decomposition, there are a method of batchwise heating and thermally decomposing the mixed carbonizable resin and disappearing resin, and a method of continuously supplying the mixed carbonizable resin and disappearing resin into a heat source and heating and thermally decomposing them.

[0053] The disappearing resin is preferably a resin that disappears by thermal decomposition when the carbonizable resin is carbonized by heating in Step 4 described below, does not cause a large chemical change during the insolubilization treatment described below, and is a thermoplastic resin having a carbonization yield of less than 10% after heating.

[0054] Specific examples of such disappearing resins include polyolefins such as polyethylene, polypropylene, and polystyrene, acrylic resins, methacrylic resins, polyacetals, polyvinylpyrrolidone, aliphatic polyesters, aromatic polyesters, aliphatic polyamides, polycarbonates, etc., and these may be used alone or in a mixed state.

[0055] In Step 1, the carbonizable resin and the disappearing resin are made compatible to form a resin mixture (polymer alloy). The term "made compatible" as used herein means creating a state in which a phase-separated structure of the carbonizable resin and the disappearing resin is not observed under an optical microscope by appropriately selecting temperature and / or solvent conditions.

[0056] The carbonizable resin and the disappearing resin may be made compatible by mixing only the resins, or may be made compatible by further adding a solvent.

[0057] Examples of systems in which multiple resins are compatible include systems that exhibit an upper critical solution temperature (UCST) type phase diagram where the system is in a phase-separated state at low temperatures but becomes a single phase at high temperatures, and conversely, systems that exhibit a lower critical solution temperature (LCST) type phase diagram where the system is in a phase-separated state at high temperatures but becomes a single phase at low temperatures.

[0058] Also, particularly in the case of a system in which at least one of the carbonizable resin and the disappearing resin is dissolved in a solvent, those in which phase separation described later is induced by the penetration of a non-solvent are also preferable examples.

[0059] The solvent to be added is not particularly limited, but the absolute value of the difference from the average value of the solubility parameters (SP values) of the carbonizable resin and the disappearing resin, which is an index of solubility, is preferably within 5.0.

[0060] Since it is known that the smaller the absolute value of the difference from the average value of the SP value, the higher the solubility, it is preferable that there is no difference. Also, the larger the absolute value of the difference from the average value of the SP value, the lower the solubility, and it becomes difficult for the carbonizable resin and the disappearing resin to be in a compatible state. Therefore, the absolute value of the difference from the average value of the SP value is preferably 3.0 or less, and more preferably 2.0 or less.

[0061] Examples of specific combinations of the carbonizable resin and the disappearing resin in a compatible system include, in the case of a solvent-free system, polyphenylene ether / polystyrene, polyphenylene ether / styrene-acrylonitrile copolymer, wholly aromatic polyester / polyethylene terephthalate, wholly aromatic polyester / polyethylene naphthalate, wholly aromatic polyester / polycarbonate, etc. Also, when it is difficult to be compatible from the viewpoints of degree of polymerization and stereoregularity, it is also preferable to appropriately perform copolymerization or chemical modification to make the combination of the two compatible.

[0062] Specific examples of combinations of systems containing solvents include polyacrylonitrile / polyvinyl alcohol, polyacrylonitrile / polyvinyl phenol, polyacrylonitrile / polyvinyl pyrrolidone, polyacrylonitrile / polylactic acid, polyvinyl alcohol / vinyl acetate-vinyl alcohol copolymer, polyvinyl alcohol / polyethylene glycol, polyvinyl alcohol / polypropylene glycol, polyvinyl alcohol / starch, etc. Also, when it is difficult to be compatible from the viewpoints of degree of polymerization and stereoregularity, it is also preferable to appropriately carry out copolymerization or chemical modification to make the combination of the two compatible.

[0063] The method of mixing the carbonizable resin and the disappearing resin is not limited, and various known mixing methods can be adopted as long as they can be uniformly mixed. Specific examples include a rotary mixer having a stirring blade, a kneading extruder with a screw, etc.

[0064] Also, it is preferable that the temperature (mixing temperature) when mixing the carbonizable resin and the disappearing resin is equal to or higher than the temperature at which both the carbonizable resin and the disappearing resin soften. Here, for the softening temperature, if the carbonizable resin or the disappearing resin is a crystalline polymer, the melting point may be appropriately selected, and if it is an amorphous resin, the glass transition point temperature may be appropriately selected.

[0065] By setting the mixing temperature to be equal to or higher than the temperature at which both the carbonizable resin and the disappearing resin soften, the viscosities of both can be lowered, enabling more efficient stirring and mixing. The upper limit of the mixing temperature is not particularly limited, but from the viewpoint of preventing deterioration of the resin due to thermal decomposition and obtaining porous carbon fibers with excellent quality, it is preferably 400 °C or lower.

[0066] Also, in Step 1, it is preferable to mix 90 to 10% by weight of the disappearing resin with respect to 10 to 90% by weight of the carbonizable resin. If the carbonizable resin is 10% by weight or more, it is possible to maintain the porous carbon fibers after carbonization, and it is also preferable because the yield is improved. Also, if the carbonizable resin is 90% by weight or less, it is preferable because the disappearing resin can efficiently form voids.

[0067] Regarding the mixing ratio of the carbonizable resin and the sacrificial resin, it can be arbitrarily selected in consideration of the compatibility of each resin. Specifically, generally, the compatibility between resins deteriorates as the composition ratio approaches 1:1. Therefore, when a system with not very high compatibility is selected as the raw material, it is also a preferred embodiment to improve the compatibility by approaching a so-called deviated composition, such as increasing or decreasing the amount of the carbonizable resin.

[0068] In addition, it is also preferable to add a solvent when mixing the carbonizable resin and the sacrificial resin. By adding a solvent, the viscosities of the carbonizable resin and the sacrificial resin are lowered, making molding easier, and also making it easier to compatibilize the carbonizable resin and the sacrificial resin.

[0069] The solvent mentioned here is not particularly limited, and any liquid at room temperature that can dissolve and swell at least one of the carbonizable resin and the sacrificial resin is acceptable. If it can dissolve both the carbonizable resin and the sacrificial resin, it is more preferable because it can improve the compatibility between the two.

[0070] From the viewpoint of improving the compatibility between the carbonizable resin and the sacrificial resin, lowering the viscosity and improving the fluidity, the addition amount of the solvent is preferably 20% by weight or more based on the total weight of the carbonizable resin and the sacrificial resin. On the other hand, from the viewpoint of the cost associated with the recovery and reuse of the solvent, it is preferably 90% by weight or less based on the total weight of the carbonizable resin and the sacrificial resin.

[0071] 〔Step 2〕Phase separation and molding Step 2 is a step of phase-separating the resin mixture in a compatible state in Step 1 and molding a precursor fiber of a porous carbon fiber with a fiber diameter that varies in the longitudinal direction.

[0072] The method of forming the resin mixture in a compatible state into fibers is not particularly limited, and a spinning method adapted to the phase separation method described below can be appropriately selected. If the resin mixture is a combination of thermoplastic resins, melt spinning can be performed after heating to a temperature equal to or higher than the softening temperature of the resin. Further, when the resin mixture contains a solvent, dry spinning, dry-wet spinning, wet spinning, etc. can be appropriately selected as solution spinning.

[0073] Melt spinning is a method of fiberizing by extruding a resin mixture heated and melted (in a fluid state) using a kneading extruder or the like from a die and winding it up while cooling. The process speed is faster than that of solution spinning, and it is excellent in productivity. Further, since the volatilization of the solvent does not occur, the cost for safety measures during the process can be suppressed, so it is preferable because it enables low-cost production.

[0074] Solution spinning is a method of fiberizing by metering a spinning dope composed of a resin mixture and a solvent prepared in advance and extruding it from a die, and it is possible to precisely control the phase separation state.

[0075] In particular, dry-wet spinning and wet spinning using a coagulation bath are more preferable embodiments because the phase separation state of the precursor fiber can be precisely controlled by appropriately combining heat-induced phase separation, non-solvent-induced phase separation, etc. described below.

[0076] The method of forming into fibers and separating the carbonizable resin and the disappearing resin is not particularly limited. For example, a heat-induced phase separation method that induces phase separation by a temperature change and a non-solvent-induced phase separation method that induces phase separation by adding a non-solvent can be mentioned.

[0077] These phase separation methods can be applied alone or in combination. Specific methods for combined application include, for example, a method of causing non-solvent-induced phase separation through a coagulation bath and then heating to cause heat-induced phase separation, a method of controlling the temperature of the coagulation bath to cause non-solvent-induced phase separation and heat-induced phase separation simultaneously, a method of cooling the resin discharged from the die to cause heat-induced phase separation and then bringing it into contact with a non-solvent, etc.

[0078] At this time, the phase separation conditions can be arbitrarily selected from the size of the structural period of the obtained porous carbon fiber, but it is preferable to appropriately select the temperature and the composition of the resin mixture in the compatible state. Although it is also related to the step of fixing the phase separation structure described later, the closer the temperature and the composition of the resin mixture in the compatible state are to the conditions for achieving the compatible state, the less likely the phase separation is to proceed, and the farther away, the more likely the phase separation is to proceed. By appropriately selecting these conditions, a precursor for obtaining a desired porous carbon fiber can be manufactured.

[0079] Furthermore, after passing through the coagulation bath and then washing with water and drying, a phase separation structure can be formed and fixed, and fibers that become precursors of porous carbon fibers can be obtained. Here, the coagulating liquid is not particularly limited, and examples include water, ethanol, an aqueous salt solution, and a mixed solvent of these and the solvent used in Step 1.

[0080] The method for obtaining the porous carbon fiber of the present invention in which the fiber diameter varies in the longitudinal direction and the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less is not particularly limited, but a method for obtaining precursor fibers in which the fiber diameter varies in the longitudinal direction is preferably used. And the forming method of the precursor fiber whose fiber diameter varies in the longitudinal direction is not particularly limited, and examples include a method of changing the discharge amount from the die, a method of changing the roll speed of the process, a method of stretching with hot water, and a method of utilizing the draw resonance phenomenon. These molding methods may be applied alone or in combination of a plurality. In particular, for these molding methods, it is preferable to adjust the conditions for periodically generating variations in the fiber diameter.

[0081] The method for obtaining the porous carbon fiber of the present invention in which the fiber diameter varies periodically in the longitudinal direction, or the porous carbon fiber of the present invention in which the period of the periodic variation of the fiber diameter in the longitudinal direction is 20 mm or more and 10,000 mm or less is not particularly limited, but a method for obtaining a precursor fiber in which the fiber diameter varies periodically in the longitudinal direction, or a precursor fiber in which the period of the periodic variation of the fiber diameter in the longitudinal direction is 20 mm or more and 10,000 mm or less is preferably used. The conditions for periodically generating the variation in the fiber diameter of the precursor fiber are not particularly limited, but by periodically changing the discharge amount from the die or the roll speed of the process using an inverter motor or an eccentric roll, etc., a precursor fiber in which the fiber diameter varies periodically can be obtained. Similarly, in the method utilizing the draw resonance phenomenon, the period can be controlled by adjusting the draft ratio, and a precursor fiber in which the fiber diameter varies periodically to some extent can be obtained.

[0082] These methods can be applied alone or in combination. The process of forming the precursor fiber in which the fiber diameter varies in the longitudinal direction was described as step 2, but it is not necessarily required to be performed in step 2, and it may be performed simultaneously with the non-melting treatment (step 3) and the carbonization treatment (step 4) described later.

[0083] 〔Step 3〕Removal of the sacrificial resin The precursor fiber of the porous carbon fiber obtained in step 2 is subjected to the removal treatment of the sacrificial resin (step 3) at least at one point before, simultaneously with, or after the carbonization step (step 4) before being subjected to the carbonization step (step 4). That is, for the sake of convenience of explanation, the removal treatment of the sacrificial resin is described as "step 3", but actually, step 3 does not necessarily have to be performed after step 2 and before step 4, and it may be performed simultaneously with step 4 or after step 4. Also, it may be performed simultaneously with the non-melting treatment described later.

[0084] The method for removing the sacrificial resin is not particularly limited. Specifically, there are methods of chemically decomposing and reducing the molecular weight of the sacrificial resin using acids, alkalis, enzymes, and oxygen and then removing it, methods of dissolving and removing the sacrificial resin with a solvent that dissolves the sacrificial resin, methods of decomposing and removing the sacrificial resin using radiation such as electron beams, gamma rays, ultraviolet rays, and infrared rays, or heat, and the like.

[0085] In particular, when the sacrificial resin can be removed by thermal decomposition, a heat treatment can be performed at a temperature at which 80% by weight or more of the sacrificial resin disappears before the carbonization treatment (step 4), or the sacrificial resin can be thermally decomposed and gasified and removed in the carbonization treatment (step 4) or the infusibilization treatment described later. It is preferable to thermally decompose and gasify the sacrificial resin and remove it simultaneously with the heat treatment in the carbonization treatment (step 4) or the infusibilization treatment described later because the productivity is increased.

[0086] 〔Infusibilization treatment〕 The precursor fibers of the porous carbon fibers are preferably subjected to an infusibilization treatment at any stage after the phase separation (step 2) and before being subjected to the carbonization treatment (step 4).

[0087] The method of the infusibilization treatment is not particularly limited, and known methods can be used. Specific methods include a method of causing oxidative crosslinking by heating in the presence of oxygen, a method of forming a crosslinked structure by irradiating with high-energy rays such as electron beams and gamma rays, and a method of impregnating and mixing a substance having a reactive group to form a crosslinked structure. Among these, the method of causing oxidative crosslinking by heating in the presence of oxygen is preferable because the process is simple and the manufacturing cost can be kept low. These techniques may be used alone or in combination, and each may be used simultaneously.

[0088] The heating temperature in the method of causing oxidative crosslinking by heating in the presence of oxygen is preferably 150°C or higher from the viewpoint of efficiently promoting the crosslinking reaction, and preferably 350°C or lower from the viewpoint of preventing deterioration of the yield due to thermal decomposition, combustion, etc. of the carbonizable resin and weight loss.

[0089] Moreover, although the oxygen concentration during the infusibilization treatment is not particularly limited, it is preferable to supply a gas having an oxygen concentration of 18% by volume or more because it is possible to keep the manufacturing cost low. Although the method of supplying the gas is not particularly limited, examples thereof include a method of directly supplying air into the heating device and a method of supplying pure oxygen into the heating device using a cylinder or the like.

[0090] As a method of forming a crosslinked structure by irradiating high-energy rays such as electron beams and gamma rays, there is a method of irradiating carbonizable resin with electron beams, gamma rays, etc. using a commercially available electron beam generator, gamma ray generator, etc. to induce crosslinking.

[0091] From the viewpoint of efficiently introducing the crosslinked structure by irradiation, the lower limit of the irradiation intensity is preferably 1 kGy or more, and from the viewpoint of preventing the strength of the precursor fiber of the porous carbon fiber from decreasing due to the decrease in molecular weight caused by the cleavage of the main chain, it is preferably 1,000 kGy or less.

[0092] As a method of forming a crosslinked structure by impregnating and mixing a substance having a reactive group, there are a method of impregnating a resin mixture with a low molecular weight compound having a reactive group and advancing the crosslinking reaction by heating or irradiating with high-energy rays, and a method of previously mixing a low molecular weight compound having a reactive group and advancing the crosslinking reaction by heating or irradiating with high-energy rays.

[0093] Step 4: Carbonization treatment Step 4 is a step of obtaining porous carbon fibers by carbonizing the precursor fibers of the porous carbon fibers obtained in Step 2 or the precursor fibers subjected to the removal of the sacrificial resin and / or the infusibilization treatment as necessary by heating.

[0094] In order to carbonize the precursor fibers of the porous carbon fibers, the carbonization treatment in this step is preferably performed by heating by heat conduction or microwave heating in an inert gas atmosphere.

[0095] Here, the inert gas refers to a gas that is chemically inert during heating. Specific examples include helium, neon, nitrogen, argon, krypton, xenon, carbon dioxide, etc. Among these, it is preferable to use nitrogen or argon from an economic perspective.

[0096] The flow rate of the inert gas only needs to be an amount that can sufficiently reduce the oxygen concentration in the heating device, and it is preferable to appropriately select an optimal value according to the size of the heating device, the supply amount of the raw material, the heating temperature, etc.

[0097] Although the upper limit of the flow rate is not particularly limited, from the perspectives of economy and reducing temperature changes in the heating device, it is preferable to appropriately set it according to the temperature distribution and the design of the heating device.

[0098] Also, if the gas generated during carbonization can be sufficiently discharged outside the system, it is more preferable because porous carbon fibers with excellent quality can be obtained. From this, it is preferable to determine the flow rate of the inert gas so that the concentration of the generated gas in the system becomes 3,000 ppm or less.

[0099] When heating is performed by heat conduction, the heating temperature is preferably 300 °C or higher, more preferably 400 °C or higher. Also, although the upper limit of the heating temperature is not limited, if it is 1,500 °C or lower, no special processing of the equipment is required, so it is preferable from an economic perspective. When simultaneously performing the removal of the sacrificial resin (step 3) described above, it is preferable to heat to a temperature exceeding the temperature at which the sacrificial resin thermally decomposes.

[0100] In this step, the precursor fibers of the porous carbon fibers can be cut and heated batchwise in a heating device, but it is more preferable to continuously heat without cutting the precursor fibers. The method of continuous heating is not particularly limited, but examples include heating by heat conduction or microwave heating. These heating methods are preferably methods of continuously supplying and taking out the precursor fibers of the porous carbon fibers into the heating device using rollers, conveyors, etc., because it is possible to increase productivity.

[0101] When performing batch processing in a heating device, the heating rate and cooling rate are not limited, and productivity can be increased by shortening the time required for heating and cooling. Therefore, a rate of 1 °C / min or higher is preferred. Also, the upper limit of the heating rate and cooling rate is not particularly limited and can be appropriately set within a range where defects such as cracks do not occur.

[0102] 〔Step 5〕Composite membrane for gas separation The composite membrane for gas separation of the present invention is a membrane having the porous carbon fiber and the separation functional layer of the present invention. Therefore, Step 5 for obtaining the composite membrane for gas separation is a step of forming a separation functional layer on the porous carbon fiber obtained in Step 4 to obtain a composite membrane for gas separation. Note that when the porous carbon fiber is not used as the composite membrane for gas separation, Step 5 can be omitted.

[0103] The type of the separation functional layer is not particularly limited, and examples include polymer membranes, zeolite membranes, silica membranes, carbon membranes, and the like.

[0104] As the method for forming the separation functional layer, a known method can be adopted. As general forming methods, for a high separation membrane, resin coating; for a zeolite membrane, hydrothermal synthesis; for a silica membrane, sol-gel method, counter-diffusion method; for a carbon membrane, heat treatment of non-melting and carbonization after resin coating can be mentioned. Examples of the coating method on the porous carbon fiber include dip coating method, nozzle coating method, spray method, vapor deposition method, cast coating method, and the like. From the ease of the manufacturing method, the dip coating method or the nozzle coating method is preferred.

[0105] 〔Step 6〕Module for gas separation The module for gas separation of the present invention is a module that houses the composite membrane for gas separation of the present invention. Therefore, Step 6 for obtaining the module for gas separation is a step of bundling the composite membranes for gas separation obtained in Step 5 and housing them in a vessel. Note that when the composite membrane for gas separation is not used as the module for gas separation, Step 6 can be omitted.

[0106] The composite membranes for gas separation obtained in Step 5 are bundled, housed in an element casing (hereinafter referred to as the casing), fixed with a potting agent, and both ends of the casing are sealed. As the potting method, for example, a centrifugal potting method in which the potting agent is permeated into the composite membrane for gas separation by using centrifugal force, a static potting method in which the potting material in a flowing state is fed by a metering pump or a head and permeated into the composite membrane for gas separation, etc. can be mentioned.

[0107] It is preferable to cut the potted composite membrane for gas separation at the potting site and open the ends of the composite membrane for gas separation. One or more casings equipped with the obtained composite membrane for gas separation can be housed in a vessel to produce a gas separation module.

Examples

[0108] Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto. The evaluations in each Example and Comparative Example were carried out by the following methods.

[0109] (CV% of fiber diameter) After determining the cross-sections of 300 porous carbon fibers among the porous carbon fibers, the fiber diameter was determined at each cross-section using an electron microscope (Model S 5500; Hitachi High-Technologies Corporation), and the CV% was calculated from the average fiber diameter and the standard deviation of the obtained data.

[0110] (Variation of fiber diameter) A 0.1 m long porous carbon fiber was equally divided into 10 parts at equal intervals, and the fiber diameters at the 11 divided points were measured. The change rate from the minimum diameter to the maximum diameter was calculated as (maximum diameter - minimum diameter) / minimum diameter × 100 as a percentage based on the minimum diameter among the 11 measurement data. The same operation was performed on 30 porous carbon fibers, and the variation of the fiber diameter was determined from the average value of the 30 change rates obtained respectively.

[0111] (Periodic variation of fiber diameter) The periodicity was evaluated by creating a correlogram from data obtained by measuring the fiber diameter of the porous carbon fibers at intervals of 10 mm along the fiber length, and using the autocorrelation coefficient within lags where the data was shifted by 50 mm or more. When the autocorrelation coefficient was 0.2 or more, it was determined that there was a periodic variation in the longitudinal direction, and otherwise it was determined that there was no periodic variation in the longitudinal direction. Also, the period was determined from the lag value at which the autocorrelation coefficient was 0.2 or more.

[0112] (Structure period of the co-continuous structure) Five porous carbon fibers were sandwiched between sample plates, and the positions of the light source, sample, and two-dimensional detector were adjusted so that information with a scattering angle of less than 10 degrees could be obtained from the X-ray source obtained from a CuKα line light source. From the image data (luminance information) obtained from the two-dimensional detector, the central portion affected by the beam stopper was excluded, a radial distance was provided from the beam center, and the luminance values at 1° intervals for 360° were summed to obtain a scattering intensity distribution curve. From the scattering angle θ at the position having a peak in the obtained curve, the structure period of the continuous structure portion was calculated by the following formula, and the average value of the five porous carbon fibers was determined as the structure period of the co-continuous structure.

[0113] Also, when the structure period was 1 μm or more and no peak of X-ray scattering was observed, a continuous rotation image was taken in the range of 180° or more in 0.3° steps with an X-ray microscope to obtain a CT image. Fourier transform was performed on the obtained CT image to obtain a graph of the scattering angle θ and the scattering intensity, and the structure period was obtained by the following formula in the same manner as described above.

[0114] L = λ / (2sinθ) L: Structure period, λ: Wavelength of incident X-ray (Evaluation of membrane performance) Twenty 30-cm-long composite membranes for gas separation were bundled and placed in a stainless steel casing with an outer diameter of φ6 mm and a wall thickness of 1 mm. The ends of the bundled composite membranes for gas separation were fixed to the inner surface of the casing with an epoxy resin-based adhesive, and both ends of the casing were sealed. Subsequently, the potted composite membranes for gas separation were cut at the potting site, and the ends of the composite membranes for gas separation were opened. A module was fabricated by housing the casing equipped with twenty composite membranes for gas separation in a stainless steel vessel, and the gas permeation rate was measured. The gases used for measurement were carbon dioxide and methane. In accordance with the pressure sensor method of JIS K7126-1 (2006), the pressure change on the permeate side per unit time of carbon dioxide and methane was measured at a measurement temperature of 25°C in an external pressure mode. Here, the pressure difference between the supply side and the permeate side was set to 0.11 MPa (82.5 cmHg).

[0115] Subsequently, the permeation rate Q of the permeated gas was calculated by the following formula, and the separation factor α was calculated as the ratio of the permeation rates of the gases of each component. Note that STP means standard conditions. Also, the membrane area was calculated from the outer diameter and length of the composite membrane for gas separation in the region contributing to gas permeation.

[0116] Q = [Gas permeation flow rate (×10 -6 cm 3 ·STP)] / [Membrane area (cm 2 ) × Time (s) × Pressure difference (cmHg) When the separation factor α was greater than 1.0, the separability was determined to be "present", and otherwise it was determined to be "absent".

[0117] (Sucking up of potting material) A composite membrane for gas separation having a porous carbon fiber and a separation functional layer was bundled into 100 bundles each and suspended, and a potting material (epoxy resin) was injected so that the lower end of the bundle of the composite membrane for gas separation was immersed up to 1 cm. After standing in a constant temperature bath at 50 °C for 12 hours to cure the potting material, the bundle was unwound from the upper end side, and the unwound portion (the portion where all the separation membranes were adhered due to the suction of the potting material) was defined as the reaching point of the potting material. The distance between the cured surface of the potting material and the reaching point was measured and defined as the suction height of the potting material.

[0118] [Example 1] Polyacrylonitrile (MW 150,000), polyvinylpyrrolidone (MW 40,000), and dimethyl sulfoxide (DMSO) as a solvent were charged into a separable flask, and mixed so that the ratio of polyacrylonitrile to polyvinylpyrrolidone was 1:1 and the polymer concentration was 20% by weight, and a uniform and transparent solution was prepared while stirring and refluxing.

[0119] The obtained polymer solution was discharged from the outer tube of a core-sheath type double nozzle, and an aqueous DMSO solution was simultaneously discharged from the inner tube, and then led to a mixed bath of water and DMSO. After passing through a mixed bath roll whose speed changes in a 10 s cycle, winding was performed to obtain hollow fiber-shaped PAN-based precursor fibers. The obtained PAN-based precursor fibers were washed with water and then dried. Subsequently, an infusibilization treatment was performed in an air atmosphere to produce infusibilized fibers.

[0120] Subsequently, the PAN-based precursor fibers were carbonized at a reaching temperature of 700 °C to produce porous carbon fibers.

[0121] Subsequently, the PAN-based precursor fibers were carbonized at a reaching temperature of 700 °C to produce porous carbon fibers.

[0122] The obtained porous carbon fibers were immersed in a polyacrylonitrile / DMSO solution (10 wt% polymer), then pulled up, immersed in water to remove the solvent, and dried at 100 °C for 24 hours to produce a laminate having a resin layer of polyacrylonitrile formed on the porous carbon fibers. Subsequently, the laminate was subjected to an infusibilization treatment in an air atmosphere. Subsequently, the infusibilized yarn was carbonized to produce a hollow fiber-shaped composite membrane for gas separation. The evaluation results of the porous carbon fibers and the composite membrane for gas separation are shown in Table 1.

[0123] [Example 2] Porous carbon fibers and a composite membrane for gas separation were produced in the same manner as in Example 1, except that draw resonance was expressed by adjusting the die shape, discharge rate, and mixed bath roll rotation speed to obtain hollow fiber-shaped PAN-based precursor fibers. The evaluation results of the obtained porous carbon fibers and the composite membrane for gas separation are shown in Table 1.

[0124] [Comparative Example 1] Porous carbon fibers and a composite membrane for gas separation were produced in the same manner as in Example 1, except that hollow fiber-shaped PAN-based precursor fibers were obtained under the condition that the mixed bath roll rotated at a constant speed. The evaluation results of the obtained porous carbon fibers and the composite membrane for gas separation are shown in Table 1.

[0125] [Comparative Example 2] Porous carbon fibers and a composite membrane for gas separation were produced in the same manner as in Example 1, except that hollow fiber-shaped composite fibers were obtained under the condition that the amplitude of the speed variation of the mixed bath roll with a speed change of 10 s period was increased. The evaluation results are shown in Table 1. The obtained composite membrane for gas separation had defects in the separation functional layer.

[0126]

Table 1

[0127] In the table, "average fiber diameter" means the average value of the fiber diameters.

[0128] In the table, "diameter variation" means the variation per 0.1 m in the longitudinal direction of the fiber diameter.

[0129] In the table, "periodic variation" means the periodic variation in the longitudinal direction of the fiber diameter.

[0130] In the table, "period" means the period of the periodic variation in the longitudinal direction of the fiber diameter.

[0131] In the table, "structural period" means the structural period of the co - continuous structure.

Claims

1. A porous carbon fiber, wherein the fiber diameter varies in the longitudinal direction, the CV% calculated from the variation of the fiber diameter is 1% or more and 10% or less, and the fiber diameter varies periodically in the longitudinal direction.

2. The porous carbon fiber according to Claim 1, wherein the average value of the fiber diameter is 20 μm or more and 5000 μm or less.

3. The porous carbon fiber according to Claim 1 or 2, having a co-continuous structure in at least a part thereof.

4. The porous carbon fiber according to Claim 3, wherein the structural period of the co-continuous structure is 0.002 μm or more and 20 μm or less.

5. The porous carbon fiber according to any one of Claims 1 to 4, wherein the fiber diameter varies by 3% or more and 10% or less in a 0.1 m section in the longitudinal direction.

6. The porous carbon fiber according to any one of Claims 1 to 5, wherein the period of the periodic variation is 20 mm or more and 10000 mm or less.

7. A composite membrane for gas separation, comprising the porous carbon fiber according to any one of Claims 1 to 6 and having a separation functional layer.

8. A gas separation membrane module in which the composite membrane for gas separation according to Claim 7 is housed.

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