JPH0453572B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cellulose ester gas separation hollow fiber membrane and a method for producing the same. In particular, the present invention relates to a hollow fiber membrane that has extremely high hydrogen gas separation performance and permeation performance.

Research on gas separation using membranes has been going on for a long time, including the recovery of hydrogen gas from natural gas etc. using polyethylene terephthalate hollow fiber membranes, the recovery of helium gas from helium mixed exhaust gas, and the recovery of helium gas from air using silicone rubber films. Concentration of oxygen gas has been reported, but it has rarely been put into practical use due to insufficient gas permeability and gas separation properties.

The gas permeation rate Q (cc/sec) of a certain type of gas A and gas B in a certain type of uniform membrane is Q _A = D _A・A・(P ₁ − P ₂ )/T Q _B = D _B・A・It is expressed as (P ₁ −P ₂ )/T. Here, D _A and D _B are the permeability coefficients of A gas and B gas (cm ² cm/sec.cm ² .cmHg), respectively.
A is the membrane area (cm ² ) in contact with the feed gas, P ₁ and
P ₂ is the partial pressure (cmHg) of the gas on the feed and permeate sides of the membrane.

In order to apply the gas separation method using a membrane, the permeability coefficient for the target gas A must be large, and the separation coefficient must be
It is important to select a material with sufficiently large or small P _A /P _B and to create a film with as large a film area as possible and as small a film thickness as possible. Generally, the gas permeability of a certain polymer membrane varies depending on the type of gas.
In descending order of permeability coefficient: hydrogen, helium > CO ₂ >
O ₂ , Ar > CO, CH ₄ , N ₂ , and it is easy to separate mixed gases with large differences in permeability coefficients, such as hydrogen gas and nitrogen gas, but mixtures with small differences such as carbon monoxide. It is difficult to separate methane and methane. Also, once the type of a certain mixed gas to be separated is determined, the difference in characteristics depending on the type of polymer separation membrane is that the membrane with a larger gas permeation coefficient generally has a smaller separation coefficient, and if both the permeation coefficient and separation coefficient are There is no material with great performance, and it is extremely important to select a material that achieves the purpose. For example, when trying to selectively concentrate oxygen over air, a polydimethylsiloxane membrane has an extremely high permeability coefficient for oxygen gas, but a small separation coefficient, making it difficult to obtain an oxygen concentration of 35% or more in one pass. On the other hand, polyethylene terephthalate membrane has a much smaller oxygen permeability coefficient than that of polydimethylsiloxane, but has a large separation coefficient, and can obtain an oxygen concentration of 50% in one pass. Therefore, the membrane material is selected depending on the purpose. What is also important from a practical point of view is that even if the permeability coefficient is large, effective gas permeation cannot be obtained unless the film thickness T is _small . Only when it grows larger can it be put into practical use. However, no matter how thin the film is processed, the film must have thermal and mechanical properties that can withstand the conditions of use, and this point must also be carefully considered when selecting the film material. It is a necessary factor. Furthermore, in order to put separation membranes into practical use, they are assembled into modules housed in a certain pressure vessel in order to use the membranes efficiently, but it is necessary to increase the membrane area per module and reduce the number of modules per processing amount. This will result in a reduction in brand equipment costs. Such membrane module configurations include hollow fiber type, flat membrane spiral type, plate and frame type, and tubular type, but the hollow fiber type has the highest packing density in the module and is advantageous in terms of module cost. be. From these viewpoints, the inventors have worked hard to develop and research gas separation membranes, and have succeeded in developing a practical membrane that has unprecedented gas separation performance, particularly hydrogen gas separation performance and permeation performance.

That is, a cellulose ester-based hollow fiber membrane is partially hydrolyzed and dried to obtain a hollow fiber membrane for gas separation, and the hollow fiber membrane has a degree of ablation in the following range: 0.67Y≦x≦ 0.98Y However, x is the degree of dilution Y of the hollow fiber after hydrolysis, which is the degree of dilution Y of the hollow fiber before hydrolysis, and is 52 to 62.

Various polymer materials can be considered as gas separation membrane materials, such as polyvinyl alcohol, polyamide, polyester, polyvinyl chloride, polypropylene, polyethylene, polycarbonate, or polysiloxane, but gas permeability varies widely depending on these materials. For example, the oxygen gas permeability coefficient is
They exist in the order of 10 ^-13 to 10 ^-3 , and the separation performance varies widely depending on the type of polar group in the polymer. It is desirable to select the most suitable material from these materials by considering the separation performance, mechanical properties of the membrane, and processability to form a thin film. The inventors have been studying various materials from this point of view for many years, and have developed a practical membrane with high gas separation performance by using cellulose ester polymer and partially hydrolyzing it. I have reached a conclusion that can be reached. In other words, for practical purposes, the most important characteristics of a membrane are that the gas separation performance is at a certain high level and the gas permeability is sufficiently large. Although it is a semi-synthetic polymer material that is ranked in terms of its properties and has a medium separation performance, it has extremely high processability into thin films, so it is easier to make thin films than ordinary polymer materials by devising a film-forming method, so it is used in various separation membranes. It has been put into practical use as
The cellulose ester referred to here is an ester derived from cotton linters, pulp, etc. with organic acid chloride, organic acid anhydride, etc.
As RCO, R is an alkyl group or an alkylaryl group having 1 to 17 carbon atoms, and the degree of substitution is up to 3. Specific examples include triacetylcellulose, diacetylcellulose, dibutylcellulose, and dibenzoylcellulose.

In the present invention, if the degree of oxidation x of the hollow fiber after hydrolysis is less than 67% compared to that Y before hydrolysis, the shrinkage rate during moist heat treatment, solvent replacement, and drying of the hollow fiber will increase, resulting in hydrogen The balance between separation performance and permeation performance is lost. That is, although the separation performance is good, the permeation performance is deteriorated, which is not preferable. On the other hand x
If is larger than 0.98Y, the separation performance will deteriorate, which is not preferable.

The method for producing hollow fiber membranes of the present invention involves dissolving cellulose ester in a mixed solution of an organic solvent and a non-solvent, extruding this spinning dope through a spinneret into a gas atmosphere, and subsequently introducing it into a coagulation bath to remove the adhered coagulation liquid. After washing with water
1 second with 0.05% to 10% by weight alkaline aqueous solution
Hydrolysis treatment is carried out for 30 seconds, the adhering alkaline aqueous solution is further neutralized with an acidic aqueous solution, and a partially hydrolyzed wet hollow fiber is obtained by subsequent washing with water. Next, in order to give the desired properties, a wet heat treatment is performed, the water contained in the hollow fiber is replaced with an organic solvent, and the water contained in the hollow fiber is replaced with an organic solvent. The objective is to obtain a hollow fiber asymmetric membrane with a large permeation rate ratio.

To explain the manufacturing method in more detail, the hollow fibers after coagulation and moist heat treatment without partial hydrolysis with an alkaline aqueous solution after washing with water are 0.05% to 10% by weight.
A method of treating with an aqueous alkali solution, neutralizing with an aqueous acid solution, washing with water, replacing with an organic solvent, and drying is also possible. In addition, the hollow fiber after the moist heat treatment in both treatment methods is made into a skein fiber, one end of which is opened, fixed with epoxy resin, and this hollow fiber is attached to a pressure vessel.
Using 0.2% sodium chloride (NaCl) aqueous solution as feed water, the raw water is circulated at a pressure of 30Kg/cm ² and the amount of permeated water is determined.
NaCl rejection rate was measured. The performance in this case is permeated water amount 100/m ² days to 300/m ² days NaCl removal rate 90
% or more is preferable.

When the amount of permeated water is less than 100/ ^m2 days, the hydrogen permeation rate decreases, and when it is more than 300/m2 ^days
NaCl rejection rate becomes smaller. If the NaCl rejection rate is less than 90%, the permeation rate ratio of hydrogen and methane will be small.

The selection of the solvent used in the method of the present invention is important because the gas selective permeation performance of the hollow fiber membrane varies greatly depending on the type of solvent. From this point of view, many solvents for cellulose acetate polymers have been proposed, but dimethylformamide, dimethylacetamide, and N-methyl-2-pyrrolidone are the most preferred, and they have a synergistic effect on permeation performance when used in combination with a non-solvent. appears greatly.

The type of non-solvent affects the permeation performance of the hollow fiber membrane. Therefore, in order to improve the performance of gas separation membranes, the most appropriate combination of solvent and non-solvent is the most important technical element.

In the present invention, those of the following general formula are used.

R ₁ O(C ₂ H ₄ O) _o R ₂ (In the formula, R ₁ and R ₂ are each hydrogen, carbon number 1
to 6 hydrocarbon groups, -C ₂ H ₄ R' or -COR1'', where R' represents -CN, -COOR2'', -CONH ₂ or -CH ₂ NH ₂ , and R1'' and R2'' are Each represents hydrogen or a hydrocarbon group having 1 to 6 carbon atoms. n is 2~
Examples of polyethers represented by the above general formula include triethylene glycol, tetraethylene glycol, polyethylene glycol, methyl carbitol, dimethyl carbitol, methoxy triglycol, triethylene glycol monoethyl ether, and acetylated polyethylene. Glycol, aminoethylated polyethylene glycol, etc. may be mentioned, and these may be used alone or in combination of two or more.

The cellulose ester concentration in the spinning stock solution is closely related to the spinnability and performance of the hollow fiber membrane. In the method of the present invention, 30 to 45% by weight of cellulose ester,
A mixture of solvent and polyether is used in a proportion of 70-55% by weight.

As the alkali for hydrolysis, sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. are used. The alkali concentration is between 0.05% and 10% by weight. Temperature is 5℃~50℃, preferably 10℃~
It is 30℃. Processing time for alkaline aqueous solution is 1 second ~
The time is 30 seconds, preferably 3 seconds to 15 seconds.

The coagulating liquid used in the present invention is preferably a mixed solvent of water and the solvent used in the stock solution.The coagulating temperature is preferably 0 to 25°C.

The method for spinning hollow fiber membranes of the present invention requires cellulose ester to be dissolved in a mixed solution consisting of a solvent and polyether, heated, stirred, and degassed. Extrude the gas into the gaseous atmosphere.

Note that the spinneret used is an arc type, C type, or double tube type in which a gas introduction tube is provided in the spinning hole. The extrusion-spun hollow fibers are coagulated by immersion in a water bath containing a solvent and polyether through a gas atmosphere, and then washed with water, followed by partial hydrolysis with an alkaline aqueous solution, acid neutralization, and water washing to form wet hollow fibers. obtain. It is then subjected to a moist heat treatment to impart the desired properties. The moist heat treatment is carried out by immersing the hollow fibers in a heating medium that is inert to the hollow fibers. The heat treatment medium may be any inert medium, but water is preferred from the viewpoint of handling issues and economy. The heat treatment temperature is 85-99°C. If the heat treatment temperature is lower than 85°C, the shrinkage during solvent displacement drying will be large, making it impossible to obtain a membrane with stable performance. Furthermore, if the heat treatment temperature is 99° C. or higher, the film will shrink significantly during the heat treatment, making the film performance impractical.

The method for drying the wet hollow fibers obtained in this way includes, as a first step, replacing the water contained in the membrane with a water-miscible organic solvent, and as a second step, replacing the water contained in the membrane with a water-miscible organic solvent. A method is adopted in which the organic solvent is replaced with a non-polar organic solvent and then the membrane is dried.

The water-miscible organic solvents used include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, acetone, etc., and the non-polar organic solvents include n-hexane, cyclohexane, n-bentane, oftha, toluene, and benzene. etc. can be used. Immersion displacement temperature is 0
~25°C is preferred.

Next, it is dried in a gas atmosphere at a temperature below 40°C. Dry hollow fiber membranes are tested at 40% to give the desired performance
It is preferable to carry out dry heat treatment at a temperature of 150°C to 150°C.

As described in detail above, the present invention relates to a partially hydrolyzed cellulose ester hollow fiber membrane that has particularly high hydrogen gas separation performance and permeability. The degree of oxidation x is controlled as shown in the following equation.

0.67Y≦x≦0.98Y Hereinafter, the present invention will be explained in more detail with reference to specific examples, but the present invention is not limited to this range.

Example 1 38 parts of cellulose acetate with an axification degree of 54.8%, 44 parts of dimethylformamide (DMF) and 18 parts of methoxypolyethylene glycol (MPEG) with a molecular weight of 350.
The mixture was stirred and dissolved at 105°C for 2 hours to prepare a spinning stock solution.

This spinning dope was filtered and degassed, and then spun using a double-tube spinneret with an inner tube having an outer diameter of 0.8 mm, an inner tube having an inner diameter of 0.5 mm, and an outer tube having a diameter of 1.0 mm. The spinning stock solution is supplied from the outer tube, while 30 parts of DMF is added as the core solution.
30 parts of MPEG and 40 parts of water were supplied to the inner tube.
The hollow spinning dope that has come out of the double-tube spinneret is 6 cm thick.
Run in the air, then add 21 parts of DMF,
9 parts of MPEG, 70 parts of water, coagulated in a coagulation bath at a temperature of 10°C, washed with water, run for 5 seconds in a 3% by weight sodium hydroxide aqueous solution bath, then an acetic acid aqueous solution bath,
I went through a water bath. The obtained hollow fibers were wound with a winding machine and heat-treated in hot water at 90° C. for 20 minutes in a bundled state. The outer diameter of this hollow fiber was 230μ, the inner diameter was 110μ, and the average degree of axification was 52.3%. After opening one end of this hollow fiber and gluing it with epoxy resin, this hollow fiber was installed in a pressure vessel, and raw water was circulated at a pressure of 30 kg/cm ² using 0.2% sodium chloride (NaCl) aqueous solution as feed water. The amount of permeated water and NaCl rejection rate were measured. The performance in this case is permeated water amount 150/m ² days,
The NaCl exclusion rate was 96.5%. After moist heat treatment, the strand-shaped hollow fibers were treated with a 50% by weight aqueous isopropyl alcohol solution and 100% isopropyl alcohol for 10 minutes each, dehydrated, and then immersed in n-hexane for 8 hours at 30°C.
dried in a vacuum dryer.

One end of this hollow fiber was fixed with epoxy resin in the same way as above, and it was installed in the pressure vessel of a gas separation test device, and hydrogen and methane gases were charged at 10 kg/h each.
As a result of measuring the gas permeation rate at a pressure of cm ² , the hydrogen permeability was 11.5×10 ^-5 cm ² /cm ² . The permeation rate ratio of hydrogen and methane was 98 at sec.cmHg.

Example 2 Cellulose triacetate 39 with an axification degree of 61.5%
43 parts of N-methylpyrrolidone (NMP) and 18 parts of ethylene glycol (EG) were dissolved at 180°C to prepare a spinning stock solution. Spinning was carried out in the same manner as in Example 1. Using EG as the core liquid, run it through 5 cm of air in the same manner as in Example-1, and then continue
20 parts of NMP, 5 parts of EG, and 75 parts of water were coagulated in a coagulation bath at a temperature of 10°C, and after washing with water, a 5% by weight sodium hydroxide aqueous solution was run for 6 seconds, and then passed through an acetic acid aqueous solution bath and a water washing bath to obtain a hollow fiber. . The obtained hollow fibers were heat treated with hot water at 91° C. for 20 minutes in the same manner as in Example 1.
This hollow fiber had an outer diameter of 210 μm and an inner diameter of 100 μm, and the average degree of acetylation was 55.7%. This hollow fiber was used in Example 1.
As a result of measuring the permeated water amount and NaCl rejection rate under the same conditions as above, the permeated water amount was 140/m ² days, NaCl rejection rate was 97.8%.
It was hot. The solvent was replaced and dried using the same method and conditions as in Example 1, and the permeation rate of hydrogen and methane was measured. As a result, the permeation rate of hydrogen was 12.5×10 ^-5 cm ² /cm ² . sec.cmHg
The permeation rate ratio of hydrogen and methane was 91.

Example 3 The spinning stock solution of Example 1 was spun using the same method and conditions to obtain hollow fibers that were not treated with sodium hydroxide or acetic acid.

This hollow fiber was heat treated in the same manner as in Example 1, and the amount of permeated water and NaCl rejection rate were measured. As a result, the amount of permeated water was 250
/m ² ·day, the NaCl exclusion rate was 98.2%. Next, this hollow fiber was run for 7 seconds in a 5% by weight sodium hydroxide washing bath, and passed through an acetic acid aqueous solution bath and a water washing bath. The average degree of acetylation of this hollow fiber was 51.5%.
After solvent replacement and drying using the same method and conditions as Example 1,
As a result of measuring the permeation rate of hydrogen and methane, the permeation rate of hydrogen was 10.8×10 ^-5 cm ² /cm ² . The permeation rate ratio of hydrogen and methane was 35 at sec.cmHg.

Comparative Example 1 The hollow fibers of Example 3 after the hot water treatment were immersed in a 5% by weight aqueous sodium hydroxide solution for 3 minutes, neutralized with an acetic acid aqueous solution, and washed with water. The average degree of acetylation of this hollow fiber was 34.1%. Solvent replacement as in Example 3,
After drying, the hydrogen and methane permeation rates were measured and the hydrogen permeability was 1.4×10 ^-5 cm ² /cm ² . The permeation rate ratio of hydrogen and methane was 54 at sec.cmHg.

Comparative Example 2 The hollow fibers of Example 2 after coagulation were washed with water and heat treated in the same manner as in Example 2 without being treated with sodium hydroxide. The NaCl removal rate of this hollow fiber was 98.9%, and the amount of permeated water was 200/m ² days. After solvent replacement and drying in the same manner as in Example 2, hydrogen and methane permeation rates were measured, and the hydrogen permeation rate was 8.5×10 ^-5 cm ² /cm ² . sec.cm
The permeation rate ratio of hydrogen and methane in Hg was 31.

Claims (1)

[Claims] 1. A dry membrane for gas separation in which the degree of acetylation x after hydrolysis of cellulose ester hollow fibers is within the following range. 0.67Y≦x≦0.98Y (where Y is the degree of acetylation of the hollow fiber before hydrolysis, 52
~62) 2 Hollow fibers are formed from cellulose ester, and then partial hydrolysis is carried out in an alkaline aqueous solution satisfying the range of the following formula, after which water contained in the hollow fibers is replaced with an organic solvent and dried. A method for producing a dry membrane for gas separation. 0.67Y≦x≦0.98Y (where x is the degree of acetylation of the hollow fiber after hydrolysis, Y
is the acetylation degree of the hollow fiber before hydrolysis, which is 52 to 62. )