JPH11227087A - Porous polytetrafluoroethylene membrane for gas dissolution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the liquid pressure resistance, the diffusion properties of gas into the interior of a membrane, the absorption properties of a gas into a liquid to be treated or the like by providing an asymmetric membrane consisting of a densely formed layer with small dia. pores arranged on the side coming into contact with the liquid to be treated and a support layer with larger dia. pores than the pores of the densely formed layer. SOLUTION: A densely formed layer 2 with small dia. pores are arranged on the side coming into contact with a liquid to be treated and a support layer 3 with larger dia. pores than the pores of the densely formed layer is arranged on the side coming into contact with a liquid to be treated to form a tubular asymmetric membrane 4 of a composite structure. Plural pieces of tubular asymmetric membrane 4 are stored, in a bundle form, inside a case 5, which is, in turn, sealed with a sealing part 6. Further, an integrally formed inner cylinder 5 as a support is introduced into an outer cylinder 7 and a cap 9 is attached to the outer cylinder 7 through a gasket 8. The liquid to be treated is made to flow into the inside of the tubular asymmetric membrane from a liquid inlet 10 and is drained from a liquid outlet 11. The gas such as an ozone gas is introduced through a gas inlet 12 and is made to circulate to the outside of the tubular asymmetric membrane to be discharged from a gas outlet 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous polytetrafluoroethylene membrane for dissolving a gas, and more particularly, to a dense layer having a small pore diameter, excellent hydraulic resistance, and a large gas-liquid contact area. And a gas comprising a composite structure asymmetric membrane having a relatively large pore diameter, a suitable porosity and thickness, a good gas diffusion property, and a support layer having excellent mechanical strength. The present invention relates to a porous polytetrafluoroethylene membrane for dissolution.

[0002]

2. Description of the Related Art Liquids such as water in which various gases such as ozone and carbon dioxide are dissolved are used as cleaning liquids. For example, in a semiconductor manufacturing process, ozone-added ultrapure water is used in a wet cleaning process. Ozone-added ultrapure water is ultrapure water to which a very small amount (on the order of ppm) of ozone has been added. Ozone dissolved in ultrapure water acts as a clean and powerful oxidizing agent, decomposes and removes residual organic substances such as a surfactant on a silicon wafer, and forms a uniform and flat oxide film. Also, in the liquid crystal display manufacturing process, ultra-pure water that is appropriately heated is used for cleaning the glass substrate, cleaning after the etching process, cleaning after the rubbing process, etc., but uses ozone-added ultra-pure water. By doing
It has been proposed to improve the removal efficiency of organic substances and resist residues, and some of them have been put to practical use.

[0003] However, it has been difficult to efficiently dissolve a gas such as ozone in a liquid such as ultrapure water while controlling the amount of dissolution. Conventionally, as a method of dissolving a gas in a liquid, a method of mechanically mixing a gas and a liquid, a method of bubbling a gas in a liquid, and a porous PTFE tube made of polytetrafluoroethylene (hereinafter abbreviated as PTFE) are used. Methods of use are known. Among these methods, the mechanical mixing method has a problem that it is difficult to efficiently dissolve a certain amount of gas in a liquid, and there is a problem that a small amount of metal ions flowing out of the mixing device or parts are mixed. Liquids mixed with metal ions cannot be used for semiconductor and liquid crystal related applications that require high purity. In the bubbling method, gas dissolution efficiency is poor, it is difficult to control the amount of gas dissolved, and moreover, gas that has not been dissolved during bubbling is likely to be released out of the system.
Inappropriate for use in dissolving harmful gases such as ozone. Also in the bubbling method, there is a problem that a small amount of metal ions are mixed in from a device or a component to be used.

On the other hand, in the method using a porous PTFE tube, a gas and a liquid to be treated are brought into contact with each other through a tubular porous PTFE membrane to form a porous PTFE tube.
Since the gas is dissolved in the liquid to be treated by utilizing the hydrophobicity and gas permeability of the E film, the problem of mixing of metal ions can be avoided. In addition, the porous PTFE tube can be made into a module by bundling a large number of the tubes, so that the membrane area per unit volume can be increased, and the permeation through the membrane can be achieved by adjusting the partial pressure of the gas. The gas can be efficiently dissolved according to Henry's law of gas dissolution while preventing bubbling of the gas into the liquid. Therefore, Japanese Patent Application Laid-Open No. Hei 3-188988 proposes using a porous PTFE tube obtained by stretching an extruded PTFE tube as a membrane of an ozone dissolving module.

However, the single-layer porous P
It is difficult for TFE tubes to balance liquid pressure resistance with gas permeability and dissolution efficiency. In general, a gas-dissolving porous membrane has (1) that the inside of the membrane has liquid pressure resistance enough not to be wet with the liquid to be treated, (2) that gas such as ozone diffuses quickly inside the membrane, 3) It is required to have various characteristics such as a high gas absorption rate for the liquid to be treated. However, conventional porous PT
The FE tube was inferior to these properties.

That is, when a porous PTFE tube is used as a porous membrane for dissolving a gas, it is generally 1 to 3.
It is required to have a liquid pressure resistance enough to withstand a liquid pressure of about kg / cm ² , but for that purpose, it is necessary to reduce the stretching ratio and the pore diameter. However, since the porous PTFE tube is obtained by stretching the extruded tube in the longitudinal direction, if the stretching ratio is reduced and the pore diameter is reduced, the porosity becomes extremely small. As the porosity of the porous PTFE tube decreases, in addition to the decrease in gas diffusivity and permeability, the pore area of the membrane surface where the gas is absorbed by contact with the liquid to be treated also decreases, The gas absorption rate (dissolution efficiency) decreases. On the other hand, in a porous PTFE tube, when the porosity is increased by increasing the stretching ratio, the pore diameter also increases, and as a result, the liquid pressure resistance decreases, and the liquid to be treated enters the inside of the membrane (into the pores) at a low pressure. In some cases, the dissolution efficiency of the gas is reduced, or in some cases, the liquid to be treated is leaked, and the liquid cannot be used. Further, the porous PTFE tube has a limitation in the tube size at the stage of extrusion molding, so that it is difficult to reduce the wall thickness and increase the gas permeability. Furthermore, since the porous PTFE tube has a variation in pore diameter, it is difficult to make the gas permeation amount constant, and the removal of impurities contained in the gas is not sufficient.

In order to improve the problem of the conventional porous PTFE tube, Japanese Patent Application Laid-Open No. 7-21880 discloses
(A) a tubular film formed by laminating a porous PTFE film obtained by stretching a PTFE sheet around the outer periphery of a porous PTFE tube; and (b) a tubular film formed by winding a porous PTFE film on an ozone. It has been proposed for use in lysis modules. PTFE
Porous PTF obtained by uniaxially or biaxially stretching a sheet
The E film can not only be an extremely thin film, but also can make the pore diameter uniform and arbitrarily control the porosity (porosity). Therefore, the publication states that the porous PTFE film layer
It is possible to make the permeation amount of ozone uniform, to provide an excellent filter performance for filtering impurities in ozone, and to obtain a tubular membrane having high strength, excellent pressure resistance, and excellent compression resistance. Described. Examples 1 and 2 of the publication disclose examples in which a porous PTFE film is wrapped around the outer periphery of the porous PTFE tube and then fired to form a tubular film, which is used in an ozone dissolving module. Have been. Further, in Example 3 of the publication, a porous PTFE film was wound around a stainless steel pipe five times in a roll form, fired, then the stainless steel pipe was pulled out to obtain a tubular membrane, which was used for an ozone dissolving module. It is shown. In these ozone dissolving modules, ultrapure water is flowed inside the tubular membrane, and ozone gas is ventilated outside.

However, it cannot be said that the tubular membrane described in Japanese Patent Application Laid-Open No. Hei 7-21880 fully utilizes the advantages of the porous PTFE film. More specifically, in the tubular membrane of (a), a porous PTFE tube layer having a large pore diameter is arranged inside a tube through which a liquid to be treated flows, and a porous membrane having a pore diameter arranged outside in contact with ozone gas. The small porous PTFE film layer
Does not contribute to gas-liquid contact. That is, in the tubular film of (a), the porous PTFE tube layer having poor performance is arranged on the side where the gas comes into contact with the liquid to be treated and the gas is absorbed (dissolved in the liquid). . Therefore, the tubular membrane has insufficient reliability with respect to the working pressure of the fluid to be treated, and partially or entirely allows the fluid to be treated to enter the inside of the membrane. In that case, the liquid to be treated that has entered the inside of the film is difficult to flow and becomes stagnant, so that the ozone concentration locally increases, and the amount of dissolved ozone gas in the liquid to be treated at the gas-liquid contact interface becomes extremely large. drop down. As a result, the ozone gas does not constantly reach the liquid to be treated, so that the solubility of the ozone gas in the liquid to be treated flowing out of the ozone dissolving module at a predetermined flow rate is extremely reduced. In addition, the tubular membrane has a porous P
A part of the TFE tube layer is swollen, and the porous PTFE film layer wound around the outside is likely to peel off.

The tubular film formed by winding the porous PTFE film of (b) is laminated many times (for example, five times in Example 3) in order to secure mechanical strength. In order to ensure interlayer adhesion, an adhesive such as PFA or FEP is interposed between the layers, so that the effective number of pores is reduced, and the gas permeability including the collapse of the film is greatly reduced. However, the original characteristics of the porous PTFE film cannot be utilized.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a gas-dissolving porous material which is excellent in liquid pressure resistance, gas diffusion into a membrane, gas absorption into a liquid to be treated, and mechanical strength. To provide a porous polytetrafluoroethylene membrane. The present inventors have conducted intensive studies to overcome the problems of the prior art, and as a result, on the side that comes into contact with the liquid to be treated, a small hole diameter, excellent liquid pressure resistance, and a large gas-liquid contact area. It is possible to arrange a dense layer that can have a relatively large pore diameter, moderate porosity and thickness on the side that comes into contact with the gas,
Porous P for gas dissolution consisting of an asymmetric membrane having a composite structure in which a support layer having excellent gas permeability and excellent mechanical strength is disposed.
They came to the TFE film. The gas-dissolving porous PTFE membrane of the present invention can sufficiently achieve the above object. The present invention has been completed based on these findings.

[0011]

Thus, according to the present invention, a gas and a liquid to be treated are brought into contact with each other through a porous polytetrafluoroethylene membrane, and the gas is used to dissolve the gas in the liquid to be treated. In the porous polytetrafluoroethylene membrane for gas dissolution, on the side in contact with the liquid to be treated,
An asymmetric membrane having a composite structure in which a dense layer (A) having a small pore diameter is disposed and a support layer (B) having a pore diameter larger than the pore diameter of the dense layer is disposed on a side that contacts the gas. There is provided a porous polytetrafluoroethylene membrane for gas dissolution characterized by the following.

According to the present invention, the following inventions are provided. 1. The gas-dissolving porous polytetrafluoroethylene film is a tubular film, and a dense layer (A) is disposed on the inner surface side in contact with the liquid to be treated, and the support is provided on the outer surface side in contact with the gas. The above-mentioned porous polytetrafluoroethylene membrane for gas dissolution, which is a tubular asymmetric membrane having a composite structure in which the layer (B) is arranged. 2. A composite in which a dense layer (A) formed of a porous polytetrafluoroethylene sheet in which a tubular asymmetric membrane is stretched at least in a uniaxial direction and a support layer (B) formed of a porous polytetrafluoroethylene tube are laminated. The above-mentioned porous polytetrafluoroethylene membrane for dissolving gas, which has a structure. 3. In the gas-dissolving porous membrane module accommodating a plurality of gas-dissolving porous tetrafluoroethylene membranes, the gas dissolving using the gas-dissolving porous tetrafluoroethylene membrane as the gas dissolving porous tetrafluoroethylene membrane is performed. For porous membrane module.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION (Asymmetric Membrane of Composite Structure) The porous PTFE film for dissolving gas of the present invention is obtained by asymmetricalizing the porous membrane structure by a substantially two-layer composite structure having different functions. The dense layer is disposed on the side that comes into contact with the liquid to be treated, and the support layer having a large pore diameter is disposed on the side that comes into contact with the gas. More specifically, the gas-dissolving porous PTFE membrane of the present invention is a porous membrane suitably used for a gas-dissolving porous membrane module, the structure of which is substantially double-layered on both sides. On the side in contact with the liquid to be treated, a dense layer with a small pore size and excellent liquid pressure resistance, and a large open area with a large gas-liquid contact area is arranged on the side in contact with the liquid to be treated. Has a suitable thickness, a pore diameter, a porosity, and a support layer having a suitable strength.

The porous PTFE membrane for dissolving gas of the present invention comprises:
It may be a flat membrane or a tubular membrane. The dense layer can be formed of a porous PTFE sheet (film) having a small pore diameter and preferably a large porosity. When the gas-dissolving porous PTFE film is a tubular film, the inner dense layer may be formed by a porous PTFE tube having a small pore size, but the porosity is increased to increase gas diffusivity and gas-liquid. To increase the contact area and further reduce the film thickness, it is necessary to use a porous PT having a small pore size and a large porosity.
It is preferable to arrange the FE sheet formed in a tubular shape in a dense layer. The support layer can be formed from a porous PTFE sheet or tube having a large pore size, preferably a large porosity.

Since the dense layer has a small pore size, it is excellent in liquid pressure resistance, has no wetting (penetration into the inside of the membrane) by the liquid to be treated, and when a porous PTFE sheet is used for the dense layer,
Since the porosity can be increased to increase the gas-liquid contact area, the rate of gas absorption into the liquid to be treated (dissolution efficiency)
Can be increased. In the case of a tubular film, since the dense layer is inside, it is possible to secure sufficient liquid pressure resistance to prevent the liquid to be treated from entering the inside of the film due to the pressure of the liquid to be treated. There is no problem such as swelling. on the other hand,
Support layer (in the case of a tubular membrane, usually porous PTFE
The tube layer) has a large pore size and a large porosity, so it has excellent gas diffusivity inside the membrane and has sufficient mechanical strength to withstand the pressure of the liquid to be treated. Can be.

The porous PTFE membrane for dissolving gas of the present invention comprises:
Generally, it is used in a modular form. In the gas-dissolving porous PTFE membrane of the present invention, when dissolving a gas such as ozone in a liquid to be treated, it has a liquid pressure resistance and a gas absorption (gas-absorbing property).
A conventional porous PTFE tube by dividing the functions such as liquid contact area), gas diffusivity, and mechanical strength into a composite asymmetric membrane in which porous membranes suitable for each function are appropriately arranged. Excellent performances not found in the film-like film can be exhibited. As described above, the asymmetric membrane having a composite structure of the present invention may be a flat membrane (which is often modularized with a support) or a tubular membrane. In any case, the dense layer having a small pore size and high liquid pressure and a support layer having a large pore size and porosity and easy gas diffusion are provided. Flow to the support layer side. The porous PTFE film for gas dissolution of the present invention is particularly preferably a tubular film in practical use, from the viewpoint that it is easy to make a module in which the film is densely packed and is easily modularized.

(Dense layer of asymmetric membrane) The pore size (average pore size) of the dense layer is usually 0.01 μm or more and less than 0.5 μm, preferably 0.01 to 0.2 μm. If the pore size of the dense layer is too large, the liquid pressure resistance is poor, and under normal use conditions,
It becomes easy to get wet by the liquid to be treated. If the pore size of the dense layer is too small, it is difficult to produce a porous membrane,
It becomes difficult to increase the porosity. The porosity of the dense layer is usually 25 to 95%, preferably 50 to 90%. The higher the porosity, the higher the gas diffusivity inside the dense layer and the more the gas-liquid on the film surface. Can increase the contact area.

The porous PTFE membrane for liquid dissolution of the present invention comprises:
An asymmetric membrane with a composite structure that can provide mechanical strength such as pressure resistance and compression resistance mainly by the support layer, and has hydraulic resistance (the ability to prevent the inside of the film from getting wet with the liquid to be treated). The thickness of the dense layer can be reduced as much as possible because it is maintained at a very surface portion in the thickness direction of the dense layer. If the dense layer is made thin, gas permeability can be increased. However, the thickness of the dense layer needs to consider film strength enough to withstand long-term use, adhesion of dirt, and the like. As a result of comprehensive examination of these, it was found that the thickness of the dense layer was preferably in the range of 5 to 300 μm, more preferably 10 to 150 μm.

The dense layer is preferably formed of a porous PTFE sheet. The porous PTFE sheet is, for example,
JP-B-42-13560, JP-B-51-1899
No. 1, JP-B-56-17216, and the like. For example,
A liquid lubricant is mixed with the unsintered PTFE powder and formed into a sheet by extrusion or rolling. The liquid lubricant is dried and removed from the obtained sheet-like molded product, or is stretched at least in a uniaxial direction (normally, a uniaxial direction or a biaxial direction) without removing the liquid lubricant. By this method, a porous PTFE sheet having a fine fibrous structure composed of fine fibers (fibrils) and nodes connected to each other by the fibers is generally obtained. According to this method, a porous PTFE sheet having a variety of pore sizes, porosity, pore structure, and film thickness can be obtained as desired.

It is necessary that the dense layer is not wetted by the liquid pressure of the liquid to be treated. Therefore, a film having a small hole diameter, high liquid pressure resistance, and a structure having as large an opening area as possible is arranged. As the porous film constituting the dense layer, a material obtained by extruding a mixture of an unsintered PTFE powder and a liquid lubricant, rolling, molding into a sheet, and then stretching at least uniaxially is preferable. In general, this type of sheet-like molded product is thin and highly molecularly oriented by rolling after extrusion, and can be stretched at a high stretching ratio. Therefore, compared to a porous PTFE tube that cannot be rolled,
A thin, high-porosity sheet having a small pore diameter can be obtained. The porous PTFE sheet has a higher specific surface area because the entanglement of the fibers is larger in the biaxially stretched sheet than in the uniaxially stretched sheet, despite the higher porosity and the smaller film thickness. It is small and has high hydraulic resistance.
Generally, porous PTFE obtained by biaxial stretching
Since the sheet is highly fiberized as a result of biaxial stretching, the porosity is high and the pore area on the membrane surface is large.
That is, the biaxially stretched porous PTFE sheet has a greater chance of gas-liquid contact, which is advantageous for gas dissolution. The ratio of the fiber portion and the non-fiber portion (opening portion) on the surface of the dense layer (the side in contact with the liquid to be treated) is better as the ratio of the non-fiber portion is higher, and the ratio of the non-fiber portion is usually 50%. Above, preferably 7
Desirably, it is 0% or more. At the time of heat compounding process,
Since some fiber shrinkage occurs and the non-fiber part (opening part) tends to be small, it is desirable to use a porous PTFE sheet that shrinks less during heating. Although the strength of the porous sheet can be improved by sintering and fixing the stretched structure, the unsintered porous sheet may be sintered after being laminated with the support layer.

(Support Layer of Asymmetric Membrane) The support layer has a pore size larger than the pore size of the dense layer, and is given such a strength as to maintain the shape with respect to the liquid pressure of the liquid to be treated. , Pore size, film thickness, porosity, etc. are designed. The pore size of the support layer is usually more than 0.2 μm and 10 μm or less, preferably 0.5 to 2 μm. However, the pore size of the support layer needs to be relatively smaller than the pore size of the dense layer to be composited. If the pore size of the support layer is too small, the gas diffusivity decreases, and the gas permeability of the gas and the gas absorption (dissolution efficiency) of the liquid to be treated decrease. If the pore size of the support layer is too large, the pressure resistance, workability, and the like are reduced. The porosity of the support layer is usually 25 to 95%, preferably 50 to 95%.
~ 90%. If the porosity of the support layer is too low, the diffusivity of the gas decreases. The porosity of the support layer is preferably as large as possible in consideration of the balance between the pressure resistance and the workability. The thickness of the support layer is usually in the range of 0.1 to 5 mm, preferably 0.3 to 2 mm, from the viewpoint of the balance between pressure resistance and gas diffusivity.

The support layer can be formed from a porous PTFE sheet in the same manner as described above. In this case, a biaxially stretched one having a high porosity is preferable from the viewpoint of gas diffusivity. However, when a tubular membrane is used, the support layer is preferably formed of a porous PTFE tube. That is, when the gas-dissolving porous PTFE membrane of the present invention is a tubular membrane, a dense layer is disposed on the inner surface side in contact with the liquid to be treated, and a support layer is formed on the outer surface side in contact with the gas. It is preferably a tube-shaped asymmetric membrane of a composite structure arranged. In this case, it is preferable to use a porous PTFE tube having a large pore diameter and a high porosity as the support layer from the viewpoint of pressure resistance and workability.

The porous PTFE tube is made of unsintered PT
It can be prepared by mixing a liquid lubricant with the FE powder, molding the mixture into a tube by extrusion or the like, and then drying or removing the liquid lubricant without stretching, or stretching the molded product in at least one axial direction. it can. The strength of the porous PTFE tube can be improved by sintering and fixing the stretched structure, but the unsintered PTFE tube may be sintered after being laminated with the dense layer. With a porous PTFE tube, if the pore size is 0.2 μm or less, only a very small porosity can be obtained, but if the pore size is increased, the porosity can be increased, and the mechanical strength can be increased. it can. The porous PTFE tube expanded not only in the length direction but also in the radial direction has a large heat shrinkage in the radial direction, and a large shrinkage force at the time of heat fusion acts to achieve a dense structure. This is preferable because it can be more firmly fused with the layer. In this case, the expansion coefficient is usually 10 to 100.
%, Preferably 20 to 70%. As the PTFE used in the present invention, not only a homopolymer of tetrafluoroethylene but also a modified PT
It may be a copolymer containing a small amount of the second and third components, such as a so-called FE. When the modified PTFE is used, the heat fusion property is improved.

(Tube-like asymmetric membrane) The porous PTFE membrane for dissolving gas of the present invention is preferably a tubular asymmetric membrane in that the membrane is densely packed and modularization is easy. Such a tubular asymmetric membrane has a composite structure in which a dense layer formed of a porous PTFE sheet stretched at least in a uniaxial direction, and a support layer made of a porous PTFE tube are laminated. It is particularly preferable in terms of the balance between the mechanical strength and the diffusivity / absorption of gas. In order to produce a tubular asymmetric membrane having such a structure, for example, as shown in FIG. 1, a single or double or more porous PTFE sheet 2 is provided on the outer peripheral surface of a rod-shaped support (for example, a stainless steel rod) 1. Wrapped around
A porous PTFE tube is put on the PTFE tube, and then the porous PTFE tube is heat-fused to a temperature not lower than the melting point of PTFE (about 327 ° C.) in a state where heat shrinkage in the tube axis direction is prevented. And a method of extracting the rod-shaped support. At the time of heat fusion, the porous PTFE sheets are thermally fused together and between the porous PTFE sheet and the porous PTFE tube. As a result, as shown in FIG. 2, a tubular dense layer 2 formed of a porous PTFE sheet is formed.
And a support layer 3 composed of a porous PTFE tube 3 to form a tubular asymmetric membrane having a composite structure. In addition, at the time of heat fusion, the porous PTFE sheet and the porous P
If the TFE tube is unsintered or semi-sintered, it is sintered at the same time.

The method for winding the porous PTFE sheet around the outer peripheral surface of the rod-shaped support may be a rolled shape or a spiral shape. By this winding, the porous P
It is sufficient that a tubular article having an inner surface made of a TFE sheet is formed. However, when spirally wound, unevenness can be provided on the inner surface of the tube due to the step due to the thickness of the porous PTFE sheet itself, thereby causing a turbulent flow in the liquid to be treated and further improving the gas dissolving efficiency. Can be done. The number of windings of the porous PTFE sheet is preferably adjusted so that the total thickness is in the range of preferably 5 to 300 μm, more preferably 10 to 150 μm, so that the number of windings is not unnecessarily increased. When the adhesion between the porous PTFE sheets or between the porous PTFE sheet and the porous PTFE tube is insufficient, FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene) may be used as the adhesive resin. For example, a powder or a sheet of a heat-fusible fluororesin such as an ethylene-perfluoroalkylvinylether copolymer) may be provided in a linear or dot shape. In addition, FEP, PFA are used for PTFE to be used in advance.
Alternatively, a sheet or tube may be formed from PTFE powder in which the molecular chain ends are partially modified, and the adhesiveness may be improved using this.

As the rod-shaped support, it is preferable to use a cylinder or a cylinder made of metal such as stainless steel from the viewpoint of heat resistance and handleability. For the rod-shaped support, for example, it is possible to finish the surface smoothly, such as by centerless polishing,
This is preferable for extracting the formed tubular asymmetric membrane without breaking. Further, the rod-shaped support that has been thermally expanded in the above-mentioned heat fusion step contracts by cooling and returns to its original size, so that the integrated composite tubular membrane can be easily removed from the mold. . In this method, the porous PT
FE sheets, porous PTFE tube and porous PT
In order to increase the adhesive strength between the FE sheets, it is preferable to use one that is not sintered or has a low degree of surface sintering.

When the tubular asymmetric membrane of the present invention is used, a liquid flows inside and an internal pressure is applied, so that the dense layer constantly suppresses the support layer, and a force is applied in a direction to be integrated. Therefore, this tubular asymmetric membrane can maintain a stable shape for a long period of time and can be used for a long period of time. The tubular asymmetric membrane of the present invention generally has an inner diameter of 0.5 to 20 mm, a pore diameter of 0.01 to 10 μm, and a porosity of 25 to 95%. Considering the pressure loss, etc., the inner diameter is 0.5
８8 mm, the pore diameter of the dense layer is preferably 0.01-0.2 μm, and the higher the porosity, the better.

(Porous Membrane Module for Dissolving Gas) The porous PTFE membrane for dissolving gas of the present invention is usually used in the form of a module. Modularization consists of multiple porous PTFE
The membrane is housed in a container so that the gas and the liquid to be treated do not come into direct contact with each other but through the porous membrane, and in that case, the gas is in the support layer, and the liquid to be treated is in the dense layer. , So as to make contact with each other.
FIG. 3 shows a cross-sectional view of an example of a module using a tubular asymmetric membrane. A large number of tubular asymmetric membranes 4 are bundled and stored in a case (inner cylinder) 5, and at both ends, between the tubular asymmetric membranes is sealed with a sealing resin or elastomer to form a sealing portion 6. I do. After that, the one obtained by integrating the split inner cylinder 5 as a support is inserted into the outer cylinder 7,
The cap 9 is attached via the gasket 8. The liquid to be processed flows from the liquid inlet 10 to the inside of the tubular asymmetric membrane, and is discharged from the liquid outlet 11. A gas such as ozone gas is introduced from the gas inlet 12, flows outside the tubular asymmetric membrane, and is discharged from the gas outlet 13. During this time, gas is absorbed and dissolved in the liquid to be treated via the tubular asymmetric membrane. The thickness, diameter, length, number, etc. of the tubular asymmetric membrane are appropriately designed according to the use of the liquid to be treated, the required treatment amount, and the like. When the asymmetric membrane of the composite structure is a flat membrane, the asymmetric membrane is held by a suitable support to be modularized. If the inner cylinder, the outer cylinder, the gasket, the cap, and the like are all made of resin such as PTFE, it is possible to prevent a trace amount of metal from being mixed in the apparatus and parts. If necessary, the gas can be circulated in a closed system.

The liquid to be treated for dissolving a gas such as ozone gas is usually an aqueous liquid such as water, pure water or ultrapure water.
In that case, the gas can be dissolved in the liquid to be treated by utilizing the hydrophobicity and gas permeability of the porous PTFE membrane. When the liquid to be treated is one in which a surfactant is mixed in an aqueous liquid, or when it is an organic solvent or the like, the composite asymmetric porous PTFE membrane particularly has a CF ₃ group in a dense layer. If an oil repellent is coated, gas can be dissolved in these liquids to be treated due to oil repellency and gas permeability. As gas, ozone gas, oxygen gas,
Examples include carbon dioxide gas and ammonia gas.

[0030]

The present invention will be described more specifically below with reference to examples and comparative examples.

Example 1 For a dense layer, a porosity of 85 was used.
%, A pore size of 0.2 μm and a thickness of 60 μm, a biaxially stretched unfired porous PTFE sheet was prepared, and as shown in FIG. 1, this porous PTFE sheet 2 was attached to the outer peripheral surface of a stainless steel rod 1 having a diameter of 2.0 mm. Half-wrapped into two layers. On this, an inner diameter of 2.5 mm and an outer diameter of 3.3 m are used for a support layer.
A porous PTFE tube 3 having an inner diameter of 1 μm, a pore diameter of 1 μm, and a porosity of 75%, whose inner surface was not completely sintered, was covered, and both ends were tied with wires to prevent the tube from shrinking in the longitudinal direction. This was heated at 360 ° C. for 15 minutes, then cooled, the wires at both ends were removed, and the porous PTFE layer was extracted to obtain a tubular asymmetric membrane as shown in FIG. The resulting tubular asymmetric membrane had an inner diameter of 2.0 mm and an outer diameter of 2.8.
mm, and the water pressure resistance was 2.9 kg / cm ² .

Using this tubular asymmetric membrane, a module having 40 tubular asymmetric membranes was prepared as shown in FIG. Forty tubular asymmetric membranes 4 are made of PTF
The PTFE gasket 8, P
A TFE cap 9, a liquid-to-be-processed 10, a liquid-to-be-processed 11, a gas inlet 12, a gas outlet 13 and the like are provided, and these have a diameter of 50 mm and a length of 500 mm.
Is housed in a cylindrical PTFE case body 7.
Using this module, pure water (oxygen concentration 0 ppm) flows at a pressure of 1.1 kg / cm ² and a flow rate of 1 L / min from the liquid inlet 10 to be treated, while ozone gas having an ozone concentration of 140 g / Nm ³ is supplied from the gas inlet 12. Ventilation was performed at a pressure of 0.5 kg / cm ² . The liquid temperature was 25 ° C. The ozone concentration of pure water in which ozone collected from the pure water outlet 11 is dissolved is 10
ppm.

Example 2 A biaxially stretched unfired film having a porosity of 65%, a pore diameter of 0.1 μm, and a thickness of 40 μm was prepared and used as a porous PTFE sheet.
A tubular asymmetric membrane was produced in the same manner as in Example 1.
The resulting tubular asymmetric membrane had an inner diameter of 2.0 mm, an outer diameter of 2.8 mm, and a water pressure resistance of 4.1 kg / cm ² . Using this tubular asymmetric membrane, a module was produced in the same manner as in Example 1. Using this module, pure water (oxygen concentration: 0 ppm) is supplied from the liquid inlet 10 to be treated at a pressure of 2.
A flow of 5 kg / cm ² and a flow rate of 1 L / min was performed, while 100% pure oxygen gas was passed through the gas inlet 12 at a pressure of ² kg / cm ² . The liquid temperature was 25 ° C. The dissolved oxygen concentration of the pure water in which the oxygen collected from the pure water outlet 11 is dissolved is 13 p
pm.

Comparative Example 1 An inner diameter of 2.0 mm, a porosity of 40%, a pore diameter of 0.2 μm, and a thickness of 0.1 mm were obtained by an extrusion / stretching method.
A 5 mm porous PTFE tube was prepared. The water resistant pressure of this porous PTFE tube was 2.5 kg / cm ² . A module was produced from the body using the porous PTFE tube in the same manner as in Example 1. Using this module, pure water (oxygen concentration: 0 ppm) was supplied from the inlet of the liquid to be treated 10 at a pressure of 2.5 kg / cm ² and a flow rate of 1 L.
/ Min, while 100% pure oxygen from gas inlet 12
Gas was vented at a pressure of ² kg / cm ² . Liquid temperature is 25 ℃
Met. The dissolved oxygen concentration of pure water in which oxygen collected from the liquid to be treated outlet 11 was dissolved showed an initial concentration of 8 ppm. However, when the operation was further continued, the dissolved concentration gradually decreased. When the module was disassembled for investigating the cause, it was found that water had partially entered the inside of the membrane.

[0035]

According to the present invention, there is provided a porous PTFE film for gas dissolution which is excellent in liquid pressure resistance, gas diffusion into the inside of the film, gas absorption into the liquid to be treated, and mechanical strength. Provided. The porous PTFE membrane of the present invention is an asymmetric membrane having a composite structure, and is configured so that the liquid to be treated is brought into contact with the dense layer.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a step of forming a tubular dense layer using a porous PTFE sheet.

FIG. 2 is a schematic diagram showing an example of a tubular asymmetric membrane of the present invention in which a porous PTFE tube is put on a tubular dense layer formed by using a porous PTFE sheet, and is composited. is there.

FIG. 3 is a cross-sectional view of a module using the tubular asymmetric membrane of the present invention.

[Explanation of symbols]

 1: rod-shaped support 2: porous PTFE sheet 3: porous PTFE tube 4: tubular asymmetric membrane 5: inner cylinder 6: sealing portion 7: outer cylinder 8: gasket 9: cap 10: inlet for liquid to be treated 11: Liquid to be treated outlet 12: Gas inlet 13: Gas outlet

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Claims (4)

[Claims] 1. A gas-dissolving porous polytetrafluoroethylene membrane used for bringing a gas into contact with a liquid to be treated through a porous polytetrafluoroethylene membrane and dissolving the gas in the liquid to be treated. On the side that contacts the liquid to be treated, a dense layer (A) having a small pore diameter is disposed, and on the side that contacts the gas, a support layer (B) having a pore diameter larger than the pore diameter of the dense layer is disposed. A porous polytetrafluoroethylene membrane for dissolving gas, characterized in that it is an asymmetric membrane having a composite structure. 2. A gas-dissolving porous polytetrafluoroethylene membrane is a tubular membrane, and a dense layer (A) is disposed on an inner surface side in contact with a liquid to be treated, and an outer surface in contact with a gas. The porous polytetrafluoroethylene membrane for gas dissolution according to claim 1, wherein the support is a tubular asymmetric membrane having a composite structure on which a support layer (B) is disposed. 3. A dense layer (A) in which a tubular asymmetric membrane is formed by a porous polytetrafluoroethylene sheet stretched at least in a uniaxial direction, and a support layer (B) made of a porous polytetrafluoroethylene tube. 3. The porous polytetrafluoroethylene film for gas dissolution according to claim 2, wherein the porous polytetrafluoroethylene film has a composite structure in which is laminated. 4. The gas-dissolving porous membrane module accommodating a plurality of gas-dissolving porous tetrafluoroethylene membranes according to claim 1, wherein the gas-dissolving porous tetrafluoroethylene membrane is used as the gas-dissolving porous tetrafluoroethylene membrane. A gas-dissolving porous membrane module using the gas-dissolving porous tetrafluoroethylene membrane described above.