JPH0376973B2 - - Google Patents

Description

Detailed Description of the Invention The present invention is a hollow fiber membrane made of a cellulose ester material, which has gas selectivity given a structure exhibiting a specific pore size distribution and pore volume, and particularly excellent gas permeation performance. The present invention relates to a gas separation membrane having the following characteristics.

Research on fluid separation using membranes has been underway for quite some time, and there are already advances in the field of liquid separation.
Seawater desalination and purified water production using reverse osmosis membranes, abbreviated as RO, and artificial kidneys using dialysis membranes have already been put into practical use.

On the other hand, it has also been known for quite some time that polymer membranes generally have gas permeability, and their gas permeability varies depending on the type of gas, so they also have selectivity. Although many gas separation membranes and separation devices using them have been proposed, very few have actually been put into practical use.

In order for gas separation membranes to be put into practical use, it is necessary to develop membranes that have both high gas permeability and high selectivity at the same time. In other words, if the permeability is low, the membrane area required to treat a certain amount of gas will be large, and if the selectivity is low, multi-stage treatment will be required to obtain a certain concentration. However, equipment costs and running costs are high, and this method is not advantageous over conventional cryogenic separation methods and adsorption methods.

It is said that the reason why their practical application has not progressed is that it has been difficult to develop membranes that have both gas permeability and selectivity and are sufficiently competitive with conventional gas separation methods. Ta.

However, the gas separation method using membranes is a method of separating or concentrating gases based on the difference in gas permeation rate due to the difference in partial pressure of each component gas. However, there is a limit to the concentration achieved by one stage of membrane separation, and in order to apply it to highly concentrated areas, operations such as multi-stage cascades are still required, which makes it difficult to achieve a high concentration compared to conventional cryogenic separation methods and adsorption methods. You will be at a disadvantage. For this reason, it is increasingly recognized that gas separation methods using such membranes should be applied to areas with relatively low concentrations, such as gas concentration adjustment and recovery of various gases. In this region of low concentration, the gas separation method using membranes is very advantageous because it saves energy due to no inherent phase change and is easy to operate due to the simple mechanism and process.

By the way, the performance of a gas separation membrane is expressed by gas permeability and gas selectivity, as described above. Gas permeability is determined by the gas permeability coefficient P [cc (STP) cm/cm ² sec cmHg] of the material itself and/or the gas permeation rate K [cc (STP)/cm ² sec・cmHg]. Based on these relationships, even if the material has a small gas permeability coefficient, a membrane with a relatively high gas permeation rate can be obtained by reducing the thickness of the membrane.

In addition, the selectivity for gases is determined by the permeability coefficient ratio P ₁ /P ₂ or permeation rate ratio for each gas.
It is expressed as K ₁ /K ₂ and is called gas separation coefficient αk. The membrane performance is defined by the gas permeation rate K and the gas separation coefficient αk.

Now, the performance of these membranes differs depending on the various gas systems, and for example, taking the cellulose ester in the present invention, the separation coefficient αk is 2 to 5 for an oxygen/nitrogen system and several dozen or more for a hydrogen/methane system. becomes.

As mentioned above, we investigated the membrane separation characteristics for various membrane performances in the relatively low concentration region that is suitable for the membrane method among gas separations.When performing gas separation and concentration under certain conditions, In systems where αk is relatively small, such as the oxygen/nitrogen system, where αk is 10 or less, the concentration of the concentrated gas obtained varies greatly depending on the difference in αk. In systems where the value is 10 or more, the difference in αk does not make much of a difference in the concentration of concentrated gas obtained. That is,
For a system in which αk is large to a certain extent, a slight difference in αk does not matter much; rather, from a practical standpoint, it becomes important to increase the gas permeation rate K.

From this viewpoint, as a desirable membrane performance, we have investigated gas separation membranes with particularly excellent permeability, and as a result, we have arrived at the present invention.

That is, the present invention is a cellulose ester-based hollow fiber membrane in which the sum of the volumes of pores with a pore radius of 40 to 60 Å is 0.15 cm ² /g or more, and the sum of the pore volumes is 0.15 cm 2 /g or more. A gas separation membrane having a structure in which pores with a radius of 30 Å or more occupy 70% or more of the total pore volume, and the pore radius distribution curve has a maximum value in the pore radius range of 40 to 60 Å. It has practical gas selectivity, and is characterized by a particularly high hydrogen permeation rate. The pore size distribution used here is measured using a nitrogen adsorption method that applies capillary condensation.
BJH method (see J.Am.Chem.Soc.73 373 (1951))
by.

It is especially important for the hollow fiber separation membrane of the present invention to have the specific pore size distribution and pore volume described above. For example, if the pore size distribution shifts to the smaller pore size, gas Although it may be expected that the separation active layer necessary for separation can be made more dense and the gas separation coefficient αk can be improved,
On the other hand, the resistance to gas permeation increases, the gas permeation rate decreases, and the practicality of the membrane as a gas separation membrane decreases. On the other hand, if the pore size distribution shifts to the larger pore size, the entire membrane structure becomes coarser, making it difficult to maintain a dense layer, resulting in a decrease in the gas separation coefficient, which also impairs the practicality of the membrane as a gas separation membrane. cause problems.

In addition, the volume of pores represents a type of porosity,
In order to obtain a particularly high gas permeation rate, it is necessary to have the above-mentioned pore size distribution as well as a pore volume above a certain level.

Next, the gas separation membrane of the present invention can be produced, for example, by the method described below, but is not particularly limited thereto. Any method may be used as long as the cellulose ester hollow fiber membrane to which the cellulose ester is applied can be obtained.

A spinning stock solution is prepared by dissolving the cellulose ester polymer in a mixed solvent consisting of the solvent and a non-solvent that is not soluble in the polymer, and this is extruded from a spinneret into a gas atmosphere and then passed through a coagulation bath. The membrane is solidified and washed with water to form a wet hollow fiber membrane of cellulose ester polymer. The hollow fiber membrane thus obtained is subjected to multi-stage heat treatment and then dried. For drying, a solvent replacement method is used in which the solution is sequentially immersed in a water-miscible organic solvent and a water-immiscible organic solvent, but especially in the case of immersion in a water-miscible organic solvent, the solution is immersed in a water-miscible organic solvent at a low temperature of 0 to 10 degrees Celsius. Perform stepwise replacement using . After replacing the solvent, the contained solvent is freeze-dried. By combining these hollow fiber spinning methods, heat treatment methods, and drying methods, a hollow fiber gas separation membrane having the above-described pore size distribution and pore volume can be obtained. The above steps will be explained in more detail.

Cellulose ester polymers used in the present invention include cellulose acetate, cellulose triacetate, other cellulose esters, and derivatives thereof, and cellulose acetate is particularly preferred.

Further, the solvent used for dissolving the polymer is selected from dimethylformamide, dimethylacetamide, N-methyl 2-pyrrolidone, and the like. At the same time, a polyether represented by the following general formula is used as a non-solvent for the polymer used in the spinning dope.

R ₁ O−(C ₂ H ₄ O) _o −R ₂ (wherein, R ₁ and R ₂ are hydrogen, each has 1 carbon number
~6 hydrocarbon radicals, _-C2H4.R ' or _{-COR1 '} ', where R' denotes -CN, -COOR2 _' ', _{-CONH2 or -CH2NH2} , and R ₁ '' and _R2 '' each represent hydrogen or a hydrocarbon group having 1 to 6 carbon atoms. In addition, n in the said formula is an integer of 2-10. ) Examples of such polyethers include triethylene glycol, tetraethylene glycol,
polyethylene glycol, methyl carbitol,
Examples include dimethyl carbitol, methoxy triglycol, triethylene glycol monoethyl ether, acetylated polyethylene glycol, and aminoethylated polyethylene glycol, among which one type or a mixture of two or more types can be used.

The above cellulose ester polymers, solvents,
A spinning stock solution is prepared by mixing a non-solvent, and the mixing ratio is related to the structure of the obtained wet hollow fiber membrane and the performance of the final gas separation membrane. In the present invention, cellulose ester polymer 26
~39% by weight mixtures of solvent and non-solvent 61-74
% concentration range. The polymer, solvent, and non-solvent are mixed, heated and dissolved to form a spinning dope.

The spinning dope is filtered and degassed, and then extruded from the spinneret into a gaseous atmosphere such as air or inert gas. The spinneret used is of arc type, C type, or double tube type.

The spinning solution extruded into the gas atmosphere becomes a hollow fiber, is introduced into a coagulation bath, solidified, and then washed with water to form a wet hollow fiber membrane. The coagulating liquid used in the coagulating bath is a mixed solution of water and the solvent/non-solvent used in preparing the spinning stock solution, but in order to obtain the membrane structure of the present invention, the mixing ratio of the solvent and non-solvent must be adjusted. is selected from a range of 5 to 28% by weight based on the total amount of coagulation liquid.
In addition, the coagulation bath temperature is 10 to 35℃, preferably
Select from the range of 26-30℃.

The hollow fiber membrane obtained by such spinning is then subjected to multi-stage heat treatment to densify and stabilize the structure. Such heat treatment is based on a multi-stage heat treatment method in which the hollow fiber membrane is immersed in hot water under no tension. In order to obtain a gas separation membrane with a well-balanced gas permeation rate and selectivity, the mode of heat treatment is important, and it is essential to perform multistage heat treatment in different temperature ranges.
That is, the first stage heat treatment is carried out at a temperature of 91 to 96°C, and the second stage is carried out at a temperature of 91 to 96°C, which is higher than the temperature of the first stage. Further, the heat treatment time is 10 to 15 minutes.

The thus obtained wet hollow fiber membrane has a dense and thin skin layer, especially due to the coagulation and heat treatment conditions, and the core layer is also dense, which needs to be dried to form a gas separation membrane. In order to obtain an excellent gas separation membrane without destroying the membrane structure, it is important to carry out the following drying method.

First, a wet hollow fiber membrane is immersed in a low concentration water-miscible organic solvent aqueous solution at a low temperature to replace a portion of the water contained in the membrane with the organic solvent. After that, it was immersed in solutions with increasing concentrations of organic solvents, and finally
Replace with 100% organic solvent solution. These stepwise solvent replacement operations are preferably carried out at a low temperature of 0 to 10°C, and the concentration of the aqueous solvent solution used for initial immersion is preferably 5 to 25%. Further, the number of stages of concentration is preferably 3 to 5 stages.

Examples of water-miscible organic solvents include alcohols in general, such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol,
Sec-butyl alcohol, tert-butyl alcohol, etc. are used, and isopropyl alcohol is particularly preferred. The hollow fiber membrane displaced with a water-miscible organic solvent is further immersed in a water-immiscible, non-polar solvent for displacement. Hydrocarbons such as pentane, cyclopentane, n-hexane, cyclohexane, n-heptane, cycloheptane, naphtha, benzene, toluene, and xylene, or mixtures thereof, can be used as this non-polar solvent, but among these, hydrocarbons such as In this regard, cyclohexane and paraxylene are preferred.

The hollow fiber membrane substituted with a nonpolar solvent is frozen at a low temperature and then dried under reduced pressure, ie, so-called freeze-drying.

Cyclohexane and paraxylene have freezing points of 6-7°C and 13°C, respectively, which are very convenient for carrying out such drying processes.

After completing the freeze-drying of the nonpolar solvent, a dry hollow fiber gas separation membrane is obtained.

The cellulose ester hollow fiber gas separation membrane thus obtained has a relatively sharp pore size distribution. In other words, the sum of the volumes of pores with a pore radius of 40 to 60 Å is 0.15 cm ³ /g or more, and this is 70% of the total pore volume of pores with a pore radius of 30 Å or more.
In addition, the pore radius distribution curve
Since it has a structure having a maximum value in the range of 40 to 60 Å, gas permeation rate and gas selectivity are balanced.

The gas separation membrane obtained by the method exemplified here has a particularly high gas permeation rate for hydrogen and good separation performance from other gases, so it is useful not only for recovering and concentrating hydrogen gas, but also for other systems such as It can also be used for permeation, concentration, and separation of carbon dioxide, helium, and water vapor.

As mentioned above, the present invention will be further explained in detail using examples, but the present invention is not limited in any way by these examples.

Example 1 46 parts of N-methyl 2-pyrrolidone (NMP) and molecular weight
36 parts of cellulose acetate was added to a mixed solution consisting of 18 parts of methoxypolyethylene glycol (MPEG) of 350% and dissolved with thorough stirring at 100°C to obtain a spinning stock solution. After defoaming in an overvacuum, this was extruded into an air atmosphere through a double-tube spinneret, brought into contact with an air flow for about 0.1 seconds, and then introduced into a coagulating liquid and coagulated. The liquid composition of this coagulation bath is
NMP 19% by weight, MPEG 6% by weight, water 75%. The hollow fibers coagulated in the coagulation bath were washed continuously with water using a Nelson roller method and then wound at a speed of 18 m/s.

Then, the obtained wet hollow fiber membrane was first heated under no tension.
After being immersed in hot water at 91℃ and heat treated for 15 minutes,
A second heat treatment was performed at 92°C for 15 minutes.

The heat-treated hollow fiber membrane was shaped into a skein with a length of 50 cm and 100 turns, and was immersed in a 10% isopropyl alcohol solution for 30 minutes, and then in 25, 50, and 75% isopropyl alcohol aqueous solutions for 30 minutes each. It was immersed in 100% isopropyl alcohol for 3 hours. Each immersion solution was adjusted to 6°C. Thereafter, it was immersed in cyclohexane at 10°C for 5 hours. Note that the amount of liquid used for dipping in these solvents was about 20 times or more the amount of liquid contained in the wet hollow fiber membrane. The hollow fiber membrane substituted with cyclohexane was cooled to below 5° C., and after freezing the cyclohexane, drying under reduced pressure was carried out for more than 16 hours to obtain a dried gas separation membrane.

When the pore size distribution of this hollow fiber membrane was measured, the results shown in FIG. 1 were obtained. In addition, the volume of pores with a pore radius in the range of 40 to 60 Å is approximately 0.294 cm ³ /g
This is approximately 78% of the total volume of pores with a pore radius of 30 Å or more.

These measurements of pore size distribution were carried out using the Carlo Erba
The calculation was carried out using a Sorpto matic device, and the calculations were performed according to the BJH method described above.

Next, the gas permeation rates (K _H2 , K _CH4 ) of hydrogen and methane through this hollow fiber membrane were measured, and the separation coefficient (αk
(H ₂ /CH ₄ )), the pressure was 10 Kg/cm ² G,
At a temperature of 20°C, it was as follows.

K _H2 = 11.5×10 ^-5 cc(STP)/cm ²・sec・cmHg αk(H ₂ /CH ₄ )=43 [−] The hollow fiber membrane obtained as described above can be understood by comparing it with the comparative example described below. As such, gas permeation rate (K _H2 ) and gas selectivity (αk) are balanced. This is due to the integrity of the membrane manufacturing requirements, particularly the coagulation bath composition and multi-stage heat treatment conditions.

Comparative Example 1 In Example 1, the first heat treatment was conducted at 85℃ and the second step heat treatment was conducted at 90℃.
A gas separation membrane was obtained in the same manner as in Example 1 except that the temperature was changed to .degree. When the pore size distribution of this separation membrane was measured, it was as shown in FIG. In addition, the volume of pores with a pore radius in the range of 40 to 60 Å is approximately 0.196 cm ³ /
g, and about 48% of the total volume of pores with a pore radius of 30 Å or more.

The performance of this membrane was as follows.

K _H2 = 13.0×10 ^-5 cc(STP)/cm ³・sec・cmHg αk(H ₂ /CH ₄ )=26 [−] Comparative Example 2 In Example 1, the first heat treatment was performed at 97°C and the second stage heat treatment was performed at 98°C.
A gas separation membrane was obtained in the same manner as in Example 1 except that the temperature was changed to .degree. When the pore size distribution of this separation membrane was measured, it was as shown in FIG. The volume of pores with a pore radius in the range of 40-60 Å was about 0.107 cm ³ /g, and about 49% of the total volume of pores with a pore radius of 30 Å or more.

The performance of this membrane was as follows.

K _H2 = 5.1×10 ^-5 cc(STP)/cm ²・sec・cmHg αk(H ₂ /CH ₄ )=50[−]

[Brief explanation of drawings]

FIG. 1 shows a pore size distribution curve of a gas separation membrane according to the present invention, and FIGS. 2 and 3 show pore size distribution curves of separation membranes other than the present invention.

Claims (1)

[Claims] 1 A cellulose ester-based hollow fiber membrane in which the sum of the volumes of pores with a pore radius of 40 to 60 Å is 0.15
cm ³ /g or more, and the sum of its volumes has a pore radius of 30 Å.
A gas separation membrane characterized in that the pores occupy 70% or more of the total pore volume and have a maximum pore radius in the range of 40 to 60 Å.