JPH0226529B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cellulose ether gas separation hollow fiber membrane. In particular, the present invention relates to a cellulose ether-based hollow fiber membrane that has good oxygen and carbon gas separation performance and permeation performance.

Research on gas separation using membranes has been going on for a long time. Polyethylene terephthalate hollow fiber membranes have been used to recover hydrogen gas from natural gas, helium gas has been recovered from helium mixed exhaust gas, and silicone rubber films have been used to recover hydrogen gas from air. Concentration of oxygen gas has been reported, but it is extremely rare that it has 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 It is expressed as _B・A・(P ₁ −P ₂ )/T. Here, D _A and D _B are the permeability coefficients of A gas and B gas, respectively (cm ² cm/sec, cm ² , cmHg),
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 PA/PB, and to make the film as large as possible and as thin 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 a large difference in permeability coefficients, such as hydrogen gas and nitrogen gas, but it is easy to separate gases with a small difference in permeability coefficients, such as monoxide gas. Separating carbon and methane is difficult. 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 the difference between the permeation coefficient and separation coefficient is There is no material that has great performance for both, and it is extremely important to select a material that achieves the purpose. For example, when attempting to selectively concentrate oxygen over air, a polydimethylsiloxane membrane has an extremely high permeability coefficient for oxygen gas, but a small separation coefficient of 35% per permeation.
It is difficult to obtain an oxygen concentration higher than that. 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.
can be processed into a sufficiently small thin film and D _A /T is 3×
Practical use is possible only when a transmittance of 10 ^-5 (cm ² /cm ² , sec, cmHg) or more is obtained. However, no matter how thin the film is, it must have thermal and mechanical properties that can withstand the usage conditions, and this point is also important when selecting the film material. This is a factor to keep in mind. 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 reduce plant 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 membrane packing density in the module and is advantageous in terms of module cost. It is. From these points of view, the present inventors have worked diligently on research and development of gas separation membranes, and have succeeded in developing a practical membrane with extremely high gas separation performance and permeation performance that has never been seen before.

In other words, the outer surface of the hollow fiber has an active layer with dense gas separation performance, and the lower layer has pores with an average pore diameter of 0.01.
This is a cellulose ether-based hollow fiber dry membrane having a sponge-like structure of ~3μ and a degree of alkoxyl group substitution of 2.0 to 2.7.

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 ^-8 , 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 present inventors have been studying various materials from this point of view for many years, and have reached the conclusion that a practical membrane with high gas separation performance can be created using a cellulose ether polymer. In other words, from the viewpoint of practicality of the membrane, it is most important that the gas separation performance is at a certain high level and that the gas permeability is sufficiently large, but cellulose ether ranks in the middle among the above synthetic polymers. It is a semi-synthetic polymer material that has been
Under certain conditions, it has extremely high thin film processability and is easier to process into thin films than ordinary polymeric materials.

The cellulose ether mentioned here includes methylcellulose, ethylcellulose, hydroxyethylcellulose, cyanoethylcellulose, and the like.

Membrane manufacturing methods include casting a solution of a polymer dissolved in an organic solvent onto a glass plate to form an asymmetric flat membrane, extruding it into hollow fibers and coagulating it to form an asymmetric hollow fiber, or porous membranes or porous membranes. Although a method of forming a composite membrane in which a thin film is coated on a hollow fiber is adopted, the method of forming a hollow fiber asymmetric membrane is the most advantageous in terms of productivity and area expansion. In particular, cellulose ether hollow fiber gas separation membranes with an alkoxyl group substitution degree of 2.0 to 2.7 are excellent for the separation of oxygen and nitrogen and separation of carbon dioxide and other nitrogen and methane gases, which are the objectives here. It has been found that when air is separated at a gauge pressure of ² , the permeation rate of oxygen gas is 5×10 ^-5 cc/cm ² , sec, cmHg or higher at an oxygen concentration of 30% or higher, which is practical.

Next, the film forming method will be described in detail.

Cellulose ether with an alkoxyl group substitution degree of 2.0 to 2.7 is dissolved in a mixed solution of an organic solvent and a non-solvent,
By extruding it through a spinneret into a gas atmosphere, then coagulating it in a coagulation bath, washing it with water, drying it after heat treatment, and then drying it, a hollow fiber membrane with particularly good oxygen and carbon gas separation performance and permeation performance is produced. can get.

The gas selection for the hollow fiber membrane is important depending on the type of solvent used in the present invention. Up to now, many solvents for cellulose ether polymers have been proposed, but from this point of view, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N
-Methyl-2-pyrrolidone is most preferred, and its combined use with a non-solvent has a significant synergistic effect on permeation performance.

The type of nonsolvent 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 one of the most important technical factors.

In the present invention, a non-solvent of the following general formula is used.

R ₁ O(C ₂ H ₄ O) _o R ₂ (In the formula, R ₁ and R ₂ each have a hydrogen carbon number of 1 to 6
hydrocarbon group, -C ₂ H ₄ R' or -COR ₂ '', where R' is -CN, -COOR ₂ '', -CONH ₂ or -
It represents CH ₂ NH ₂ , and R ₁ ″ and R ₂ ″ each independently represent hydrogen or a hydrocarbon group having 1 to 6 carbon atoms. (n is an integer of 2 to 10) Examples of the polyether represented by the above general formula include triethylene glycol, tetraethylene glycol, polyethylene glycol, methyl carbitol, dimethyl carbitol, methoxy triglycol, and triethylene glycol monoethyl ether. , acetylated polyethylene glycol, aminoethylated polyethylene glycol, etc., and these may be used alone or in combination of two or more.

The cellulose ether concentration in the spinning dope has a strong relationship with the spinnability and gas separation performance of the hollow fiber membrane.

In the present invention, 30 to 50% by weight of cellulose ether and 50 to 70% by weight of a mixture of solvent and polyether
used at a rate of

The coagulating liquid used in the present invention is preferably a mixed solvent of water and the solvent used in the stock solution, and the coagulating temperature used is preferably 20 to 70°C.

The method for spinning hollow fiber membranes of the present invention involves dissolving cellulose ether in a mixed solution consisting of a solvent and polyether by stirring and heating if necessary, performing filtration and defoaming, and expelling air or inert gas from a spinneret. Extrude into a gas atmosphere. The spinneret is arc type, C
A double type is used in which a gas introduction tube is provided in the mold or spinning hole.

The extrusion-spun hollow fibers are passed through a gas atmosphere and immersed in a water bath containing a solvent and polyether.
After solidification, the fibers are washed with water to obtain wet hollow fibers.

Next, the heat treatment for densifying the hollow fiber membranes is carried out by immersing the hollow fibers in a heating medium that is inert to the hollow fibers. Although any inert heat treatment medium may be used, it is preferable to use water from the viewpoint of handling problems and economic efficiency.

The heat treatment temperature is 60-95°C, preferably 75-90°C. If the heat treatment temperature is lower than 60°C, the shrinkage during drying will be large and a membrane with stable performance cannot be obtained. Furthermore, if the heat treatment temperature is 95° C. or higher, the film will shrink significantly during the heat treatment, making the film performance impractical.

Freeze drying or ordinary air drying can be used to dry the wet hollow fibers thus obtained. In the air drying method, the temperature is preferably 10 to 50°C. After drying, dry heat treatment is performed at 50 to 100°C for 5 minutes or more to further densify the membrane and improve membrane separation performance.

By the method detailed above, a cellulose ether-based hollow fiber dry membrane having an active layer exhibiting dense gas separation performance on the outer surface of the hollow fiber and a spongy structure with an average pore diameter of 0.01 to 3 μm in the lower layer is produced. Obtainable.

The cellulose ether hollow fiber dry membrane obtained by these methods has an oxygen permeation rate of 5×
10 ^-5 cc/cm ² , sec, cmHg or more, the gas separation coefficient has a permeation rate ratio of oxygen and nitrogen of 3 or more, and the carbon gas permeation rate is 4×10 ^-5 cc/cm ² , sec. ,
cmHg or more, and the carbon dioxide gas to nitrogen permeation rate ratio is 15 or more.

Note that the modular device is implemented by a general method and used for various purposes.

Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 Ethyl cellulose (degree of ethoxyl substitution 2.5) was added to a mixed solution of 50 parts of dimethylacetamide (DMAC) and 10 parts of ethylene glycol (EG).
1 part and stirred and dissolved at 120°C.

After filtering and degassing this spinning stock solution, it was extruded into the air using a double-tube spinneret, and passed for a space time of 0.12 seconds.
Subsequently, it was coagulated in a coagulation bath (tissue DMAC 20% by weight, EG 5% by weight, water 75% by weight) at a temperature of 50°C, washed with water using a Nelson roller method, and then rolled up at a speed of 20 m/min.

Next, the hollow fibers were wound with a winding machine and heat-treated in hot water at 80°C for 20 minutes under tension in a bundled state.

The obtained wet hollow fibers were dried at 40℃ using a hot air dryer.
It was dried for 30 minutes and then subjected to dry heat treatment at 90°C. The outer diameter of the thus obtained hollow fiber membrane was measured using a mercury porosimeter and was found to be 0.03 to 3.0μ. Furthermore, when the cross section of the hollow fiber was observed using a scanning electron microscope, it was found that the surface layer was dense and the layer underneath was spongy.

This hollow fiber was made into a skein yarn with a length of 1 m and a number of turns of 100, one end of which was opened, and after being bonded with epoxy, it was attached to a pressure vessel to produce 2Kg/cm ² G and 5Kg/cm ² G.
The permeation rates of oxygen, nitrogen, and carbon dioxide gas were measured at the pressure of , and the permeation rate ratio was determined. The results were as follows.

PO ₂ : 8.3×10 ^-5 cc/cm ² , sec, cmHg PN ₂ : 2.6×10 ^-5 〃 PCO ₂ : 6.1×10 ^-4 〃 O ₂ /N ₂ : 3.19 CO ₂ /N ₂ : 23.46 Example 2 37 parts of ethyl cellulose (degree of ethoxyl group substitution: 2.2) was added to a mixed solution consisting of 56 parts of N-methyl-2-pyrrolidone and 7 parts of polyethylene glycol #600, and the mixture was stirred and dissolved at 130°C.

This spinning stock solution was subjected to spinning in the same manner as in Example 1.

Next, the hollow fibers were heat treated with hot water at 87°C for 20 minutes.
This wet hollow fiber was dried in a freeze dryer, and then
When the pore size of the material subjected to dry heat treatment at ℃ was measured using a mercury porosimeter, it was 0.03 to 2.6μ. Furthermore, when the cross section of the hollow fiber was observed using a scanning electron microscope, it was found that the surface layer was dense and the underlying layer was spongy. Note that the permeation rate was measured in the same manner as in Example 1. The results were as follows.

PO ₂ : 7.7×10 ^-5 cc/cm ² , sec, cmHg PN ₂ : 2.5×10 ^-5 〃 PCO ₂ : 5.2×10 ^-4 〃 O ₂ /N ₂ : 3.08 CO ₂ /N ₂ : 20.8 Comparative example A hollow fiber gas separation membrane was produced in the same manner as in Example 1 except for using ethyl cellulose with a degree of ethoxyl substitution of 1.7. When the pore diameter was measured with a mercury porosimeter, the results were almost the same as those in Example 1, but when the oxygen permeation rate was measured,
The gas separation membrane had a low value of 1.5×10 ^-5 cc/cm ² ·sec·cmHg and could not be used as a practical gas separation membrane. Also, those with a degree of ethoxyl group substitution of 2.9 were water-soluble and could not be spun.

Claims (1)

[Claims] 1. Has an active layer on the outer surface of the hollow fiber that exhibits dense gas separation performance, and the lower layer has pores with an average pore diameter of 0.01~
A cellulose ether gas separation membrane having a 3μ sponge-like structure and a degree of alkoxyl group substitution of 2.0 to 2.7.