JPS5959212A - Cellulose ether gas separation membrane - Google Patents

Abstract

PURPOSE:To obtain a cellulose ether hollow yarn dry membrane extremely high in gas separation capacity and permeability, by providing a dense active layer having gas separation capacity to the outer surface of a hollow yarn while a sponge like structure is provided under said active layer. CONSTITUTION:Cellulose ether with an alkoxyl substitution degree of 2.0-2.7 is dissolved in a mixed solution of a solvent such as dimethylformamide and a non-solvent such as triethylene glycol and the resulting solution is extruded into a gaseous atmosphere from a spinneret to be immersed into a coagulation bath containing water and a solvent and the formed extrudate is washed by water to form a wet hollow yarn. Thereafter, the hollow yarn is subjected to heat treatment by heating the same at 60-95 deg.C in water and, after drying, the treated yarn is further subjected to dry heat treatment at 50-100 deg.C for 5min or more. By the above-mentioned method, a cellulose ether hollow yarn dry membrane having dense active layer exhibiting a gas separation characteristic on the outer surface thereof and having a sponge like structure with a pore size of 0.01-3mum formed under said active layer is obtained.

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 (container/sea) in a certain homogeneous membrane of a certain gas (A) and a certain gas (E) is expressed as: Here, DA and DB are the permeability coefficients of A gas and B gas, respectively (cjt * c!1/sec, cwt, c
s+Hg) sA is the membrane area in contact with the feed gas (ej
)・Pl and P are the partial pressures (in Hg) of the gas on the supply side and the permeate side to the membrane.

To apply the membrane gas separation method, the target gas A
It is important to select a material with a large permeability coefficient and a sufficiently large or small separation coefficient PA/PB, and to make the membrane as large as possible and as thin as possible. The gas permeability of general polymer membranes varies depending on the type of gas, and the order of the permeability coefficient is hydrogen, helium >0
0. >O,, Ar>00・OH4, N2, it is easy to separate mixed gases with a large difference in permeability coefficients, such as hydrogen gas and nitrogen gas, but mixtures with small differences, such as carbon oxide. Separation of methane and methane is difficult. Once the type of mixed gas to be separated is determined, the difference in characteristics depending on the type of polymer separation membrane is that membranes with a large gas permeability coefficient generally tend to have a small 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 leaf 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, the oxygen permeability coefficient of the bo-1-no-ethylene terephthalate membrane is much smaller than that of bo-1-no-dimethylsiloxane by 1, but the separation coefficient is large, and an oxygen concentration of 50% can be obtained in one pass. Therefore, the membrane material is selected depending on the purpose. Furthermore, from a practical point of view, it is important to note that even if the permeability coefficient is large, effective gas permeation cannot be obtained unless the film thickness T is small. O
-'(cj/cwt, sea) CIIH9) Practical use is possible only when the above transmittance is obtained. However, no matter how thin a film can be processed into a film, it must have thermal and mechanical properties that can withstand the usage conditions, and this point is also important when selecting film materials. This is a factor to keep in mind. Furthermore, when putting separation membranes into practical use, membranes are assembled into modules housed in a certain pressure vessel in order to use them 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 viewpoints, the present inventors have worked diligently to develop 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.

That is, the hollow fiber has an active layer having a dense gas separation performance on the outer surface, a sponge-like structure with an average pore diameter of 0.01 to 3 μm in the lower layer, and an alkoxyl group substitution degree of 2.0 μm.
This is a cellulose ether-based medium thread drying membrane with a rating of 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 force permeability coefficient ranges from 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 optimal material by considering the separation performance, mechanical properties of the membrane, and processability for thinning the membrane from among these considerations. 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, it is of utmost importance that the gas separation performance be at a sufficiently high level for the practicality of the membrane, and that the gas permeability be sufficiently large, but cellulose ether is ranked in the middle among the above synthetic polymers. Although it is a semi-synthetic polymer material, under certain conditions it has extremely high processability into thin films and is easier to process into thin films than ordinary polymer materials.

The cellulose ether mentioned here is methyl cellulose.
These include ethyl cellulose, hydroxyethyl cellulose, and cyanoethyl cellulose.

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 forming a porous membrane or porous membrane. 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. especially,
Hollow fiber gas separation membranes made of cellulose ether with a degree of alkoxyl group substitution 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 objective here. When air is separated at a gauge pressure of 30% or more, the permeation rate of oxygen gas is 5
O"-' cc/cd, sea N 0IH9 or more, which was found to be practical.

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

Cellulose ether with an alkoxyl group substitution degree of 2.0 to 267 is dissolved in a mixed solution of an organic solvent and a non-solvent, extruded through a co-spinneret into a gas atmosphere, then coagulated in a coagulation bath, washed with water, heat treated, and then dried. Furthermore, by dry heat treatment, a hollow fiber membrane having particularly good oxygen and carbon gas separation performance and permeation performance can be obtained.

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, and N-methyl-2-pyrrolidone are the most preferred. Significant reciprocal effects appear.

The type of nonsolvent affects the permeation performance of the hollow fiber membrane. Therefore, in order to improve gas separation membrane performance, the most appropriate combination of solvent and nonsolvent is one of the most important technical elements. In the present invention, a nonsolvent of the following general formula is used.

R10(0,H40)nR.

(In the formula, R1 and R3 are each a hydrocarbon group having 1 to 6 true carbon atoms, -0, H4R' or -00R[, and R1 is -○IJ) -0001, -0ONH, or -OH
, M.W.

, Ri and %Id- each independently represents hydrogen or a hydrocarbon group having 1 to 6 carbon atoms, and On represents 2 to 10, such as triethylene glycol, tetraethylene glycol, polyethylene glycol, methylcarpitol, 1 dimethylcarpitol, Examples include methoxy triglycol, triethylene glycol monoethyl ether, acetylated polyethylene glycol, and aminoethylated polyethylene glycol, 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
, a mixture of solvent and polyether is used in a proportion of 50 to 70% by weight.

The coagulating liquid used in the present invention is preferably a mixed solvent of water and the solvent used for 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 releasing air or inert gas from a spinneret. Extrude into the atmosphere. Note that the spinneret used is an arc type, 0 type, or a double type spinneret in which a gas introduction tube is provided in the spinning hole.

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

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

Heat treatment temperature is 60-95°C, preferably 75-90°C
It is. If the heat treatment temperature is lower than 60'C, shrinkage during drying is large and stable membrane 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 carried out at 50 + l OO'C for 5 minutes or more to further densify the membrane and improve membrane separation performance.

By the method detailed above, cellulose ether hollow fibers having an active layer exhibiting dense gas separation performance on the outer surface of the hollow fibers and a spongy structure with an average pore diameter of 0.01 to 3μ in the lower layer are dried. It is possible to obtain a membrane.

The cellulose ether hollow fiber dry membrane obtained by these methods has an oxygen permeation rate of 5 X I 0 = CC
/ej, sea, 18 parts or more, the permeation rate ratio of oxygen and nitrogen as a gas separation coefficient is 3 or more, and the permeation rate of carbon dioxide is 4.
c\σH is 9 or more, and the carbon dioxide gas to nitrogen permeation rate ratio is 15 or more.

Note that the modular device is actually broken down using a general method and used for various purposes.

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

Example L 40 parts of ethyl cellulose (conversion degree of ethoxyl group if 2.5) was added to a mixed solution consisting of 50 parts of dimethylacetamide (DMAO) and 10 parts of ethylene glycol (NG), and the mixture was stirred and dissolved at 120°C.

This spinning stock solution was filtered and degassed, then extruded into the air using a double-tube spinneret, and then passed through a space time of 0.12 seconds, followed by a coagulation bath (tissue DMA020%, IG05%, water 75% by weight).
% by weight) at a temperature of 50'C, washed with water using a Nelson roller, and then rolled up at a speed of 20W1.

Next, the hollow fibers were wound with a spinner and heated under tension in hot water at 80° C. for 120 minutes in a bundled state.

The obtained wet hollow fibers were dried at 40℃ for 30 minutes using a hot air dryer.
It was dried for a minute 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 found to be 0.03 to 3.0 μm. 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.

Note 1: This hollow fiber was made into a strand with a length of 1 mm and a number of turns of 100, one end of which was opened, and after being glued with epoxy, it was attached to a pressure vessel and exposed to oxygen at pressures of 2 kg/lG and 5 kg/lG. The permeation rates of nitrogen and carbon dioxide gas were measured and the permeation rate ratio was determined.The results were as follows.

PO,: 8.3 X 1o-s part/-1BeC1"
H9PM,: 2,6 X 1O-1Boo,
: 6.1 x 10-'Os/Is
: 3-19 00, /N, : 23°46 Example N- Ethyl cellulose (ethoxyl group ft [c 2°2 )] was added to a mixed solution consisting of 56 parts of methyl-2-pyrrolidone and 600 parts of polyethylene glycol. 37 parts were added and dissolved with stirring 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 further heated to 90°C.
The pore diameter of the dry heat-treated sample was measured using a mercury porosimeter and was found to be 0003 to 2.6 μm. 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.

Fog: 7.7 x 10-' cc/C
i-, sec %-H9P1'J, : 2.5
X 10""POO,: 5.2 10-4 0, /N,: 3.0B 00, /N, : 20.8 Comparative Example Ethyl cellulose with an ethoxyl group substitution degree of 1.7 was used, and the other examples were as follows. A hollow fiber gas separation membrane was produced in the same manner as in Example 1. When the pore size was measured with a mercury porosimeter, the results were comparable to those of Example 1, but when the oxygen permeation rate was measured, it was 1.5Xlo'''6E/d・sec.
・It was so low that it could not be a practical gas separation membrane.

Also, those having a degree of ethoxyl group substitution of 2.9 were water-soluble and could not be spun.

Patent applicant: Toyobo Co., Ltd.

Claims (1)

[Claims] It has an active layer on the outer surface of the hollow fiber that exhibits precise gas separation performance, has a spongy structure with an average pore diameter of 0.01 to 3μ in the lower layer, and has an alkoxyl group substitution degree of 2.0 to 2.0.
2.7 cellulose ether-based gas separation membrane.