JP2688751B2 - Method for producing hollow carbon membrane fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention particularly relates to a membrane wall capable of selectively separating a gas composed of mixed molecules having similar molecular weights such as oxygen and nitrogen, oxygen and argon, and the like. The present invention relates to a method for producing hollow carbon membrane fibers having ultra-micro pores.

<Prior Art> As membranes for gas separation membranes, synthetic polymer membranes such as polysulfone membranes, silicon membranes, polyamide membranes and Teflon membranes are known. Each has its own gas separation characteristics,
In general, those having high separability tend to have low permeability, and those having high separability tend to have poor separability. These are mostly non-porous membranes. Porous membranes are attracting attention as a means for achieving high separability and high permeability.

On the other hand, a method of separating nitrogen from air by pressure swing adsorption (PSA) using molecular sieve activated carbon (MSC) as an adsorbent has been developed. In this method, when air is introduced into an adsorption tower filled with molecular sieve activated carbon, oxygen molecules having a fast adsorption rate are first adsorbed on the activated carbon, and then nitrogen is slowly adsorbed. When the adsorption is stopped in a short time of about 1 minute, concentrated nitrogen gas is obtained. Next, nitrogen gas is obtained by desorbing under reduced pressure to remove oxygen and adsorbing again for a short time, but a complicated process and several large adsorption towers are required.

Granular molecular sieve activated carbon (MSC) is a method of pyrolytic carbonizing polyvinylidene chloride etc.
A method of heat shrinking the pores by treating at a high temperature of 1800 ° C, heating the porous carbon material to 400 to 900 ° C and bringing it into contact with an inert gas containing hydrocarbons such as ethylene, benzene and toluene to carbonize it. It is produced by a method such as vapor deposition of pyrolytic carbon from hydrogen on the walls of pores, but an example applied to the production of fibrous activated carbon is not known.

Further, fibrous activated carbon and hollow fibrous activated carbon are also known, but usually the average diameter of their pores is in the range of 10 to 50Å, and the separation performance for the separation of a mixed gas having a close molecular weight as intended by the present invention is high. Is inferior.

<Problems to be solved by the invention> Pressure swing adsorption method (PSA method) using molecular sieve activated carbon
Repeats adsorption and desorption at intervals of about 1 minute, requiring a large-scale device and complicated steps.

According to the method for producing a hollow carbon membrane fiber of the present invention, it is possible to produce a hollow carbon membrane fiber having high molecular sieve characteristics, and by mixing a large number of the obtained hollow carbon membrane fibers into a module, mixing The gas mixture can be efficiently separated by either pressing the gas supply side or depressurizing the membrane permeation side, which enables downsizing of the device and energy saving process. is there.

<Means for Solving the Problem> In the separation of the mixed gas using the porous membrane, the separation mechanism differs depending on the size of the pores. That is, where the diameter of the pores is D and the mean free path of the gas molecules is λ, the value of λ / D is called the Knudsen number (Kn), and the Knudsen number is greater than 1 (K>
1) When, that is, when the pore size of the membrane is several tens to several hundreds Å,
The velocity of gas molecules that pass through the pores at a constant temperature is proportional to the reciprocal of the square root of the molecular weight, and the smaller the molecular weight, the larger. Therefore, it is known to be suitable for separating mixed gases having different molecular weight ratios. The ideal separation factor for nitrogen and oxygen calculated in this case is 0.94, indicating that separation is not possible.

When the pore size of the membrane is in the range of 10 to several tens of liters, simultaneously with Knudsen diffusion, gas molecules are adsorbed on the inner walls of the pores, and surface diffusion that moves due to the concentration gradient of the adsorption phase also occurs, causing the gas molecules and inner walls of the pores to move. Depending on the affinity, it may be possible to separate even those having similar molecular weights. Further, when capillary aggregation occurs, the separation ratio increases due to capillary aggregation diffusion.

When the pore diameter of the hollow membrane wall is a few Å ultra-micropores as in the present invention, the molecules diffuse while being affected by the potential of the pore wall, so the diffusion coefficient depends on the type of adsorption molecule and the adsorption temperature. To be affected.

There is no significant difference in the adsorption equilibrium constant between oxygen and nitrogen, but the diffusion coefficient is greatly affected by even a slight difference in the molecules. If the ultra-micropores can be controlled to 3 to 6Å, the difference in diffusion rate between the two becomes large, and the separation effect can be further enhanced.

The present invention enables the production of hollow carbon membrane fibers having such a pore size of 3 to 6Å, and the gist thereof is
The hollow carbon fiber is preliminarily activated in an atmosphere containing water vapor to form a hollow carbon fiber having a large number of pores having an average pore diameter of 10Å or more in the hollow carbon fiber membrane, and then heat treated in an inert gas. As a result, a large number of micropores with a carbon content of 75% or more and the size of the micropores measured by the molecular probe method is 3 to 6Å are present in the membrane part, and molecules with a size of 3Å or less at room temperature Adsorption amount is 0.1 cm ³ / g or more,
A method for producing a hollow carbon membrane fiber is characterized in that the hollow carbon membrane fiber has a characteristic that the adsorption amount of molecules having a size of 6Å or more is 0.1 cm ³ / g or less.

The method for producing the hollow carbon membrane fiber will be specifically described below. The polymer used in the production of the hollow carbon membrane fiber of the present invention includes cellulose, cellulose ester, polyvinyl alcohol, polyvinyl chloride, polyvinylidene, polyacrylonitrile, pitch, etc. The polyacrylonitrile-based polymer is preferable from the viewpoint of high film strength and morphological stability.

In the present invention, each of the above polymers is shaped into a hollow fiber under the optimum conditions. The fiber is treated to obtain a heat resistant structure (crosslinking, oxidation) and then 600 to 1000 in an inert atmosphere.
It is carbonized at a temperature of ° C. Further, when the activation treatment is performed in an atmosphere containing water vapor, hollow carbon membrane fibers having a porous structure with a pore size of 10 to 50 Å can be obtained.

The first means for obtaining the ultramicropore structure of the present invention is obtained by heat treatment at a temperature higher than that of the activation treatment. At a temperature lower than the activation treatment temperature, thermal shrinkage of the pores hardly occurs. If the treatment temperature is higher than 1200 ° C, the activation ability is reduced due to the development of carbon structure, which is not preferable. Therefore, the temperature is 900 to 1200 in an inert atmosphere.
The pores are thermally shrunk by heat treatment at a temperature of 1 ° C. for 1 minute or more. Attention here is the air present in the pores. If the air present in the pores is not sufficiently replaced by the inert gas, the pores will become even larger. In some cases, the fibers themselves are oxidatively burned and disappear. Therefore, in order to replace the air existing in the pores with nitrogen prior to the temperature rise, it is preferable to immerse in the liquid nitrogen for several tens of seconds or longer, preferably for 1 minute or longer, and then raise the temperature.

The second means for obtaining the ultra-micropores of the present invention is to immerse the hollow carbon membrane fiber having a porous structure having a pore size of 10 to 50Å in a liquid hydrocarbon compound having 5 or more carbon atoms,
After filling the pores with the compound, 40 in an inert atmosphere
It is characterized in that the treatment is carried out at a temperature in the range of 0 to 1200 ° C. to narrow the inside of the pores by carbon particles produced by incomplete combustion of the hydrocarbon compound or thermal decomposition.

Examples of the thermally decomposable carbon compound are as follows.

Paraffinic, olefinic, aromatic hydrocarbons and the like having 5 to 20 carbon atoms have a boiling point of 300 ° C or lower, and when incompletely burned at a temperature of 400 ° C or higher, they are carbonized via the gas phase.

The paraffinic hydrocarbon having 5 or more carbon atoms used in the treatment of the present invention is represented by C _n H _{2n + 2} , and pentane, hexane,
Examples include petroleum ether, naphtha, gasoline, kerosene, light oil, etc., which are composed of heptane, octadecane, eicosane and the like and mixtures thereof.

Examples of cyclic paraffins include cyclopentane, cyclohexane, cycloheptane, cyclooctyne and the like.

Ethylene hydrocarbons are represented by C _n H _2n , pentene,
Hexene, heptene, octene, cetene and the like.

The acetylene-based hydrocarbon having a C number of 5 or more represented by C _n H _2n-2 includes pentin, hexyne, octadecine and the like.

Furthermore, aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, and anthracene are also used as the thermally decomposable carbon compound.

In addition, fatty acids as derivatives from hydrocarbons, and animals containing a large amount of these, vegetable oil, cycloalcohol,
Alcohols such as higher alcohols, and halogen compounds such as dichloroethylene and tetrachlorocarbon, which have a low carbon number, are effective compounds because they generate a large amount of carbon at relatively low temperatures.

The thermally decomposable carbon compound used in the present invention does not invade into pores having a diameter of several Å, and a carbon compound having a carbon number of about 10 or more that invades only into pores having a diameter of 10 Å or more to be carbonized. It is more preferable to use.

The entry of hydrocarbons into the hollow membrane wall is usually performed at room temperature. If necessary, it can be heated before use. It is not clear whether the immersion time is due to the capillary ascending force, but it may be a short time. The hydrocarbons that have penetrated into the pores generate carbon by incomplete combustion or thermal decomposition at a temperature of 400 to 1200 ° C. in an inert atmosphere and are deposited on the inner wall of the pores to narrow the pores.

The ultramicropore structure produced by the method of the present invention is measured by the molecular probe method. That is, methanol (3 x 4.6 Å), benzene (3.2 x
6.6Å), n-pentane (4 × 4.8Å), chloroform (4.7Å), cyclohexane (5.1 × 6.6Å), isooctane (5.3 × 6.5Å), carbon tetrachloride (5.8 × 6.5Å), etc. The amount of adsorption at a temperature of 95 ° C and a relative pressure of 95% was determined by a gravimetric method, and the volume converted into liquid was calculated.

Here, in parentheses is Dc × Dz, Dc is the size of the molecule that can be adsorbed to the minimum length of the gap, considering the gap between the parallel pore walls that sandwich the molecule to be adsorbed, and Dz is the ink bottle Is a size for adsorbing molecules having a diameter smaller than the minimum diameter of the pores. Generally, a diatomic molecule has Dc = Dz,
Plate-like molecules such as benzene are said to have Dc <Dz. In the case of molecular sieve carbon, the value of Dc is often used as the molecular sieve diameter.

<Example> Hereinafter, the present invention will be described specifically with reference to examples.

The “specific surface area” of hollow carbon fiber is 150 to 1000m ² / g
And was obtained by applying the BET infinity equation to the adsorption isotherm of methanol vapor at a temperature of 20 ° C.

"Gas permeability" modularizes hollow carbon membrane fiber,
Using Yanagimoto transmission system GTR-10, temperature 30 ℃, internal pressure 760mmH
g, measured at an external pressure of 1 mmHg. The unit of gas permeability is cm ³ (ST
P) · cm / cm ² · cm · Hg · sec (hereinafter, this unit is abbreviated as PU).

The "strength" of the single fiber uses Tensilon UTM-II type,
The test length was measured at 100 mm.

"Elemental analysis" was measured by a differential thermal conductivity cell using Yanagimoto CHN Coder MT-II type (expressed as carbon content C% and nitrogen content N%).

 "Density" was measured by the JIS R-7601 density gradient tube method.

Example 1 Acrylonitrile 98 mol%, methacrylic acid 2 mol%
The acrylonitrile-based hollow-membrane fiber having an inner diameter of 200 μm and a thickness of 30 μm is treated in a flame-proofing furnace in an air atmosphere having a temperature gradient of 210 ° C. to 255 ° C. for a residence time of 50 minutes to obtain a density of 1.4 g / cm 2. ³ flame-resistant fibers were obtained. Then, it was treated in a carbonization furnace having a temperature gradient of 300 to 800 ° C. and a nitrogen atmosphere for a residence time of 1 minute. 50/50 nitrogen gas / steam
Porous treatment was performed at a temperature of 800 ° C. in a mixed gas containing vol%. The yield was 30% (weight ratio of treated fiber to raw fiber).

The fiber is again in a nitrogen gas atmosphere at 900 ° C, 1000 ° C, 1
The heat shrinkage treatment of the pores was performed at each temperature of 200 ° C. for 10 minutes. Table 1 shows various properties of the obtained hollow carbon membrane fiber.

The pore diameter in No. 1 (Comparative Example) is an average diameter obtained from the methanol isotherm curve by the method of Cranston assuming a cylindrical model in Kelvin's equation.

The pore diameters of the hollow carbon membrane fibers of Nos. 2 to 4 were determined by the molecular probe method. The inequality sign (<) of the pore size indicates the following.

Example 2 An acrylonitrile-based hollow membrane fiber having a composition of 96 mol% of acrylonitrile, 3 mol% of methyl acrylate and 1 mol% of itaconic acid was stayed in a flameproofing furnace with a temperature gradient of 225 ° C. to 242 ° C. and an air atmosphere. Processed to a density of 1.42 g / cm ³
Flame resistant fiber was obtained. Then, it was treated at 800 ° C. for 30 minutes in a mixed gas of nitrogen 60 vol% and steam 40 vol%. After immersing the hollow carbon membrane fiber in kerosene at room temperature for several minutes, 700 ° C
In the nitrogen atmosphere of 10 minutes, kerosene was incompletely combusted to deposit carbon particles in the pores in the membrane wall to narrow the pore diameter, thereby producing a hollow carbon membrane fiber. The properties of the fiber are shown in Table 2.

The pore diameter in No. 5 (Comparative Example) was determined by the method of Cranston et al., Assuming a cylindrical model in the Kelvin equation from the methanol isotherm adsorption curve.

The pore diameter of the No. 6 hollow carbon membrane fiber was determined by the molecular probe method.

Example 3 The No. 5 hollow carbon membrane fiber before pore narrowing obtained in Example 2 was immersed in carbon tetrachloride or benzene, and then subjected to the narrowing treatment of the present invention at 800 ° C.

 The properties of the resulting pore-narrowed fibers are shown in Table 3.

[Brief description of the drawings]

FIG. 1 shows that the hollow carbon membrane fibers produced by the heat-shrinking method of Example 1 were treated with organic gas having different molecular diameters at a relative pressure (P / Ps) of 0.95.
The adsorbed amount when adsorbed in 1. was measured by a gravimetric method and shown in 20 ° C. liquid equivalent volume. FIG. 2 is a result of measuring the organic gas adsorption capacity of the hollow carbon membrane fiber produced by the decomposition carbon method of Example 2. FIG. 3 is a result of measuring the organic gas adsorption capacity of the hollow carbon membrane fiber produced by the method of Example 3.

Claims (3)

(57) [Claims] 1. A hollow carbon fiber is preliminarily activated in an atmosphere containing water vapor, and the average pore diameter of the hollow carbon fiber membrane is 10
Å Hollow carbon membrane fiber with a large number of pores above, then heat treated in an inert gas to obtain a carbon content
There are a large number of micropores consisting of 75% or more, and the size of the micropores measured by the molecular probe method is 3 to 6Å, and the amount of adsorption of molecules of 3Å or less at room temperature is 0.1. cm ³
/ g or more and the adsorption amount of molecules with a size of 6Å or more is 0.1c
A method for producing a hollow carbon membrane fiber, which is a hollow carbon membrane fiber having a characteristic of m ³ / g or less. 2. A heat treatment condition of 1 at a temperature of 900 ° C. or higher.
The method for producing a hollow carbon membrane fiber according to claim 1, wherein a condition of not less than a minute is used. 3. An activated hollow carbon membrane fiber is immersed in a thermally decomposable carbon compound, the pores are filled with the compound, and then heat treated at a temperature of 400 to 1000 ° C., 2. The method for producing a hollow carbon membrane fiber according to claim 1, wherein the inside of the pores inside the hollow carbon membrane fiber are narrowed by the pyrolytic carbon particles.