Membrane for membrane distillation

Abstract

Provided is a membrane for membrane distillation capable of achieving hydrophobicity, air permeability, and interlayer delamination resistance all at high levels when performing membrane distillation. This membrane for membrane distillation includes a fiber layer and a base material layer. The fiber layer contains a polyphenylene ether. The mean fiber diameter of fibers forming the fiber layer is in the range of 0.001-10 µm. The thickness of the fiber layer is 0.01-10 mm. The proportion of the thickness of the fiber layer with respect to the thickness of the membrane for membrane distillation overall is 1-99%.

Description

Membrane distillation membrane

[0001] The present invention relates to a membrane distillation membrane.

[0002] The membrane distillation method is a method in which only water vapor in the water to be treated is permeated through a porous membrane, and water vapor that has passed through the porous membrane due to the saturated water vapor pressure difference is cooled and condensed from the heated high-temperature water to be treated (hereinafter also referred to as raw water), and the condensed water is recovered as permeated water. That is, the membrane distillation method is a technique for separating solutes (salts) and water in the water to be treated using the vapor pressure difference generated through a porous membrane as a driving force, and can also be said to be a technique for distilling and purifying water through a porous membrane. In the membrane distillation method, since the separation driving force is the vapor pressure difference, compared with the reverse osmosis method in which pressure is applied to raw water and filtered through a reverse osmosis membrane to obtain purified water, high pressure is not required, power energy can be reduced, solutes do not permeate through the membrane, and only water vapor permeates through the membrane. Therefore, highly pure permeated water can be obtained, and in addition, since low-temperature waste heat (60 to 70 ° C) can be used, it has the characteristic of high heat utilization efficiency. In addition, in the reverse osmosis method, as the solute concentration of raw water increases, the pressure required for water production increases dramatically, so there is a limit to the solute concentration that can concentrate raw water. In the membrane distillation method, compared with the reverse osmosis method, water production from raw water with a high solute concentration is possible, so the solutes contained in the raw water can be highly concentrated. As described above, when the membrane distillation method is used, it is possible to obtain high-purity water and highly concentrate the solutes contained in the raw water. Therefore, the membrane distillation method can be used not only for the production of high-purity water, but also for reducing the volume of the wastewater to be treated and recovering valuable substances to which the waste treatment method is not applicable.

[0003] For example, in Citation 1, a porous membrane for membrane distillation composed of fibrous substances using polyolefin and having polytetrafluoroethylene supported on the surface of the fibrous substances is disclosed, and it is described that the liquid repellency and air permeability are improved and the long-term stability is excellent.

[0004] International Publication No. 2020 / 246550

[0005] However, while Patent Document 1 shows improvements in the liquid-repellent and permeable properties of porous membranes for membrane distillation, these are not sufficient, and further improvements were desired in the use of organofluorine compounds (PFAS) from the standpoint of bioaccumulation issues.

[0006] The present invention has been made in view of the above problems, and provides a porous membrane that can achieve a high level of compatibility between hydrophobicity, permeability, and delamination resistance when performing membrane distillation.

[0007] As a result of diligent research to solve the above problems, the inventors of the present invention have found that a membrane distillation diaphragm containing a fibrous material containing polyphenylene ether can solve the problems of the conventional technology described above, and have completed the present invention.

[0008] In other words, the present invention is as follows: [1] A membrane distillation diaphragm comprising a fiber layer and a base layer, wherein the fiber layer comprises polyphenylene ether, the average fiber diameter of the fibrous material constituting the fiber layer is in the range of 0.001 to 10 μm, the thickness of the fiber layer is in the range of 0.01 to 10 mm, and the ratio of the thickness of the fiber layer to the total thickness of the membrane distillation diaphragm is 1 to 99%. [2] The membrane distillation diaphragm according to [1], wherein the base layer is made of a material other than fluororesin. [3] The membrane distillation diaphragm according to [1] or [2], wherein the average fiber diameter of the fibrous material is in the range of 0.001 to 1 μm. [4] A membrane distillation module comprising the membrane distillation diaphragm according to any one of [1] to [3].

[0009] According to the present invention, a membrane distillation diaphragm containing polyphenylene ether is obtained that achieves a high level of compatibility between hydrophobicity, permeability, and delamination resistance.

[0010] This is a schematic diagram of the distillation apparatus used in the example.

[0011] The following describes in detail embodiments for carrying out the present invention (hereinafter referred to as "this embodiment"). This embodiment is an example for illustrating the present invention, and the present invention is not limited to this embodiment. The present invention can be appropriately modified and implemented within the scope of its gist.

[0012] (Definition of membrane distillation membrane) The membrane distillation membrane of this embodiment includes a fiber layer and a base layer, preferably consisting of a fiber layer and a base layer. The fiber layer is used in the membrane distillation method to obtain the purified liquid by allowing the vapor of the crude liquid to be purified to permeate through it, and the base layer is used to maintain mechanical strength.

[0013] The fiber layer of this embodiment contains polyphenylene ether. The polyphenylene ether is preferably in the form of a fiber.

[0014] Because the fibrous material made of polyphenylene ether exhibits excellent hydrophobicity, even during long-term membrane distillation, it is possible to suppress the hydrophilization of its surface by water vapor and the penetration of the feed liquid into the membrane. Only the vapor of the crude liquid to be purified permeates the fibrous material, achieving a high removal rate and obtaining a purified liquid.

[0015] From the viewpoint of the hydrophobicity, heat resistance, and long-term durability at high temperatures of the resulting membrane distillation diaphragm, it is preferable that the membrane distillation diaphragm of the present invention has at least one fiber layer, and that the fiber layer is arranged on the surface layer of one or both sides of the multilayer membrane.

[0016] Polyphenylene ether The polyphenylene ether of the present invention consists of a repeating unit structure of the following formula (1). (In general formula (1), O is an oxygen atom, and R １ ~R ４ Each of these independently represents one of the groups selected from the group consisting of a hydrogen atom, a halogen atom, an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted aromatic hydrocarbon group such as a phenyl group, a haloalkyl group, an aminoalkyl group, a hydrocarbon oxy group, and a halo hydrocarbon oxy group (where at least two carbon atoms separate the halogen atom and the oxygen atom).

[0017] Specific examples of polyphenylene ethers include, for example, poly(2,6-dimethyl-1,4-phenylene ether), poly(2-methyl-6-ethyl-1,4-phenylene ether), poly(2-methyl-6-phenyl-1,4-phenylene ether), and poly(2,6-dichloro-1,4-phenylene ether). Polyphenylene ether copolymers, such as copolymers of 2,6-dimethylphenol with other phenols (e.g., 2,3,6-trimethylphenol and 2-methyl-6-butylphenol), are also included. Among these, poly(2,6-dimethyl-1,4-phenylene ether) and copolymers of 2,6-dimethylphenol and 2,3,6-trimethylphenol are preferred, and poly(2,6-dimethyl-1,4-phenylene ether) is even more preferred.

[0018] The above-mentioned method for producing polyphenylene ether is not particularly limited, and conventionally known methods can be used. Specific examples of methods for producing polyphenylene ether include, for example, the method described in U.S. Patent No. 3,306,874, which involves producing 2,6-xylenol by oxidative polymerization using a complex of cuprous salt and an amine as a catalyst, as described in U.S. Patent No. 3,306,875, U.S. Patent No. 3,257,357, U.S. Patent No. 3,257,358, Japanese Patent Publication No. 52-17880, Japanese Unexamined Patent Publication No. 50-51197, Japanese Unexamined Patent Publication No. 63-152628, and others.

[0019] Polyphenylene ether may be used alone, mixed with polymers that are compatible with polyphenylene ether, or mixed with polymers, inorganic particles, glass fibers, etc. that are incompatible with polyphenylene ether. In addition, known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, flame retardants, and coloring pigments may be mixed and used as needed, within a range that does not impair film-forming properties and does not impair the requirements and effects of the present invention.

[0020] 《Fibrous Materials》 In this embodiment, fibrous materials include woven or knitted fabrics, nonwoven fabrics, cotton-like materials, etc., made of long or short fibers, as well as fibrous materials obtained from stretched films. Formed according to the required characteristics such as material, thickness, basis weight, and fiber diameter can be used.

[0021] Methods for obtaining nonwoven fabrics include methods for forming sheets from single-component fibers, composite fibers such as core-sheath fibers and side-by-side fibers, and short fibers such as split fibers by carding, airlaid, or wet papermaking; and methods for forming sheets from continuous fibers by spunbond, meltblown, electrospinning, or force spinning. Conventional known methods can be used. Furthermore, electrospinning methods are classified into solution-type electrospinning methods that use polymer solutions as raw materials and melt-type electrospinning methods that use molten polymers. In the present invention, adopting solution-type electrospinning as a method for producing membrane distillation diaphragms is preferable because it allows for the efficient and simple acquisition of fibrous materials made of polyphenylene ether with small fiber diameters, and is preferable from the viewpoint of hydrophobicity and permeability.

[0022] 《Electrospinning Method》 The basic apparatus configuration of the electrospinning method (hereinafter referred to as the solution-type electrospinning method) consists of a nozzle, a target, and a DC high-voltage power supply. When a high voltage is applied while supplying the spinning solution to the nozzle, an electrical repulsive force is generated in the spinning solution, causing the spinning solution to be ejected toward the target, and fibrous material can be obtained on the target when the spinning conditions are appropriate. The spinning conditions for obtaining fibrous material include the molecular weight of the polymer, the solvent, the concentration of the solution, the applied voltage, and the distance between the nozzle and the metal collector, and these are adjusted as appropriate. The polymer concentration of the solution varies depending on the type of polymer, molecular weight and solvent used, so it is not possible to say definitively, but if the polymer concentration is too low, productivity will decrease and it may be difficult to obtain uniform fibrous material, and if the polymer concentration is too high, the viscosity will increase and it will be difficult to eject, so it is generally preferable to adjust it to about 0.1 to 60 wt%.

[0023] The polymer concentration in the polyphenylene ether solution of the present invention depends on the structure and molecular weight of the polymer used, but when a polymer with a weight-average molecular weight of about 50,000 is used, it is in the range of 5 to 40 wt%. When a polymer with a weight-average molecular weight of about 30,000 is used, it is in the range of 10 to 60 wt%.

[0024] If the polymer concentration is less than 5 wt%, the solution viscosity is too low, making it difficult to form a fibrous structure, which is undesirable. Conversely, if the polymer concentration exceeds 40 wt%, the solution viscosity is too high, resulting in a larger fiber diameter in the resulting fibers and making the nozzle prone to clogging, which is also undesirable.

[0025] While there are no particular limitations on the molecular weight of the polyphenylene ether used, if the molecular weight is too low, the spinnability may decrease, and if the molecular weight is too high, the viscosity may increase, making spinning difficult. Even in such cases, electrospinning can be made possible by adjusting the solution viscosity by controlling the polymer concentration. It is also possible to adjust the surface tension and conductivity of the spinning solution by adding additives.

[0026] The solvent used is not particularly limited as long as it can dissolve polyphenylene ether at the above concentration. Examples include halogenated hydrocarbon solvents such as chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane, and 1,1,2,2-tetrachloroethane. Other examples include aromatic hydrocarbon solvents such as benzene, toluene, and o-xylene, cyclic ether solvents such as tetrahydrofuran and dioxane, phenolic solvents such as o-chlorophenol, and N-methyl-2-pyrrolidone (NMP). These solvents may be used individually or as a mixture of two or more solvents. A mixed solvent containing a halogenated hydrocarbon compound and a low-volatility solvent with a boiling point of 140°C or higher is preferred because it provides good solubility of polyphenylene ether, has a low volatility rate which reduces needle contamination and clogging, and allows for stable spinning over long periods. A mixed solvent consisting of an aromatic hydrocarbon solvent and at least one solvent selected from N-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide is preferable because the solvent has low toxicity, good solubility of polyphenylene ether, and a low volatility rate that reduces needle contamination and clogging, allowing for stable spinning over long periods. To increase the dielectric constant of the solvent, a small amount of poor solvent may be mixed in so as not to cause precipitation of polyphenylene ether. Examples of poor solvents include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and decalin.

[0027] In the present invention, to increase the electrical conductivity of the solution, an organic acid salt or an inorganic acid salt may be added to the solution of the electrospinning method. Examples of inorganic acid salts include monovalent inorganic salts such as lithium chloride, potassium chloride, sodium chloride, lithium bromide, potassium bromide, and potassium fluoride, and divalent inorganic salts such as calcium chloride, magnesium chloride, calcium bromide, and calcium fluoride. Examples of organic acid salts include ammonium salts such as tetrabutylammonium chloride, tetrabutylammonium bromide, and benzyltriethylammonium chloride. The amount added is in the range of 0.01 to 5 wt%.

[0028] The preferred range of applied voltage varies depending on the type and properties of the polymer used, so it cannot be stated definitively. However, generally, 0.1 kV or higher is preferred, 1 kV or higher is more preferred, 50 kV or lower is preferred, and 40 kV or lower is even more preferred.

[0029] The distance between the nozzle and the target cannot be generalized as it depends on the type and properties of the polymer used and the applied voltage, but if the distance is too short, discharge will occur, and if it is too long, the spray performance will deteriorate. Therefore, a distance of 10 mm or more is preferred, 30 mm or more is more preferred, 1000 mm or less is preferred, and 500 mm or less is even more preferred. The target is not particularly limited in shape, such as a flat plate, roll, edge, or grid.

[0030] The average fiber diameter of the fibrous material used is in the range of 0.001 μm to 10 μm. When the average fiber diameter is 0.001 μm or more, the resulting porous material for membrane distillation has excellent mechanical strength and high-temperature durability. When the average fiber diameter is 10 μm or less, the resulting porous material for membrane distillation has excellent air permeability and hydrophobicity due to the increased specific surface area. The average fiber diameter of the fibers is preferably 0.001 μm to 5 μm, more preferably 0.001 μm to 3 μm, and even more preferably 0.001 μm to 1 μm.

[0031] The thickness of the fibrous material is in the range of 0.01 mm to 10 mm. When the thickness is 0.01 mm or more, the strength of the membrane distillation diaphragm is excellent. When the thickness is 10 mm or less, the permeability of the membrane distillation diaphragm is excellent. The thickness of the fibrous material is more preferably 0.02 mm or more, even more preferably 0.05 mm or more, even more preferably 0.1 mm or more, even more preferably 5 mm or less, even more preferably 3 mm or less, and even more preferably 1 mm or less.

[0032] The ratio of the thickness of the fiber layer to the total thickness of the membrane distillation diaphragm is 1% or more and 99% or less, preferably 5% or more, more preferably 10% or more, preferably 70% or less, and more preferably 50% or less. When the ratio of the fiber layer thickness is 1% or more, the membrane distillation performance is improved, and when it is 99% or less, the strength of the membrane is improved by the substrate, and the durability can be improved.

[0033] <Base Layer> The fibrous material made of polyphenylene ether obtained by the present invention is used in combination with base layers of other materials and components to meet other requirements such as handling properties, strength, and heat resistance. For example, it is possible to create a membrane distillation diaphragm by depositing the fibrous material made of polyphenylene ether onto a base material such as a nonwoven fabric (such as spunbond nonwoven fabric or meltblown nonwoven fabric), film, woven fabric, knitted fabric, membrane, paper, or mesh. Furthermore, the cross-sectional shape of the base material is not limited to circular; irregular shapes such as rectangular, star-shaped, or clover-shaped, as well as core-sheath cross-sections, can also be used. Among these, nonwoven fabrics, woven fabrics, and meshes are preferred from the viewpoint of efficiency and durability during long-term operation of membrane distillation, and meshes are more preferred.

[0034] The thickness of the base layer is preferably 0.01 mm or more, more preferably 0.05 mm or more, even more preferably 0.1 mm or more, preferably 10 mm or less, more preferably 7 mm or less, and even more preferably 5 mm or less, from the viewpoint of the permeability, strength, and flexibility of the resulting membrane distillation diaphragm.

[0035] The material of the base layer is not particularly limited and includes, for example, polypropylene, polyethylene, polystyrene, polyethylene oxide, modified polyphenylene ether, polyphenylene sulfide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, poly-m-phenylene terephthalate, poly-p-phenylene isophthalate, polyvinyl chloride, polyvinylidene chloride-acrylate copolymer, polyacrylonitrile, polyacrylonitrile-methacrylate copolymer, polycarbonate, polyarylate, polyester carbonate, polyurethane, polyamide, nylon, aramid, polycaprolactone, polylactic acid, polyglycolic acid, collagen, polyhydroxybutyric acid, polyvinyl acetate, polypeptide, carbon fiber, glass fiber, glass, non-ferrous materials such as aluminum, iron, metal alloys, etc.

[0036] From the perspective of resistance to delamination when the resulting membrane distillation diaphragm is used for a long period at high temperatures, and from the perspective that in recent years the toxicity and environmental persistence of organofluorine compounds (PFAS) such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have become a concern, and the use of materials containing these compounds, as well as materials that may decompose and regenerate after being released into the environment, is being avoided, it is undesirable to use fluororesin as a substrate. Because fluororesin has a very low solubility parameter (SP value), which is an indicator of polarity, the difference in SP values ​​between the fluororesin used as a substrate and the polyphenylene ether used in the fibrous material becomes large, resulting in low interlayer strength, which is undesirable because it can lead to delamination when exposed to high temperatures.

[0037] 《Membrane Distillation Diaphragm and Module for Membrane Distillation》 The membrane distillation diaphragm made of polyphenylene ether obtained in the present invention can be used for the purification of crude liquids. The crude liquid may be an aqueous liquid or a liquid containing an organic solvent. Examples of such applications include seawater desalination, ultrapure water production (semiconductor factories, etc.), boiler water production (thermal power plants, etc.), water treatment in fuel cell systems, industrial wastewater treatment (food factories, chemical factories, electronics factories, pharmaceutical factories and incineration plants, etc.), water production for dialysis, water production for injection, associated water treatment (heavy oil, shale oil, shale gas and natural gas, etc.), and recovery of valuable substances from seawater. When the crude liquid is water containing water-soluble impurities, the membrane distillation diaphragm can be suitably used to produce water purified by membrane distillation.

[0038] The membrane distillation diaphragm is preferably used as a membrane distillation module. The membrane distillation module may include, for example, a high-temperature tank into which crude liquid is introduced, a receiving tank for recovering the purified liquid, and a low-temperature tank containing a refrigerant for cooling the vapor flowing into the receiving tank. The membrane distillation diaphragm may be installed so as to separate the high-temperature tank and the receiving tank. In this case, it is preferable that the hydrophilic main surface of the membrane distillation diaphragm of the membrane distillation module be installed so as to be in contact with the crude liquid. That is, it is preferable that the membrane distillation diaphragm of the membrane distillation module be installed so that the hydrophilic main surface of the membrane distillation diaphragm faces the high-temperature tank and the hydrophobic main surface of the membrane distillation diaphragm faces the receiving tank. The membrane distillation module may be equipped with a heating device for heating the crude liquid introduced into the high-temperature tank, or it may be equipped with a cooling device for lowering the internal temperature of the receiving tank. It is preferable to use the heating device or the cooling device to make the temperature difference between the temperature of the crude liquid and the internal temperature of the receiving tank 20°C or more. This allows for efficient condensation of the vapor that has permeated the membrane distillation diaphragm, improving the recovery efficiency of the purified liquid.

[0039] The embodiment will be described in more detail below based on the following examples, but this embodiment is not limited to the following examples.

[0040] "Production of Porous Membrane" [Example 1] 15 parts by weight of poly(2,6-dimethyl-1,4-phenylene ether) (manufactured by Asahi Kasei Corporation, Zylon S201A) as a polyphenylene ether, 85 parts by weight of chloroform / decalin = 8 / 2 as a solvent, and 1 part by weight of tetrabutylammonium bromide as an additive salt for the purpose of imparting conductivity were stirred until a uniform solution was obtained to prepare an electrospinning solution. A polyester spunbond (PET) non-woven fabric (manufactured by Unitika Ltd., Marlex, basis weight 70 g / m ２ , thickness 150 μm) was placed on a collector with a diameter of 100 mm, and electrospinning (manufactured by Mecc Co., Ltd., electrospinning apparatus NANON-04) was performed under the conditions of an applied voltage of 20 kV, a nozzle of 27G, a nozzle-collector distance of 150 mm, a discharge rate of 1.0 mL / h, a time of 2 hours, a cleaning interval of every 2 minutes, and a collector rotation speed of 100 rpm. After drying, a porous membrane (membrane distillation separator) was formed. The layer thickness of the fibrous material was 110 μm.

[0041] [Examples 2 - 5] Membrane distillation separators with varying layer thicknesses of the fibrous material were prepared as shown in Table 1 by changing the electrospinning time, and membrane distillation separators were obtained in the same manner as in Example 1 except for this.

[0042] [Example 6] A membrane distillation separator was obtained in the same manner as in Example 1 except that a PET mesh (mesh size 10 μm, film thickness 150 μm) was used as the base material. The layer thickness of the fibrous material was 110 μm.

[0043] [Comparative Example 1] A porous membrane was obtained in the same manner as in Example 1 except that 45 parts by weight of poly(2,6-dimethyl-1,4-phenylene ether) (manufactured by Asahi Kasei Corporation, Zylon S201A) and 55 parts by weight of chloroform / decalin = 8 / 2 as a solvent were used. The layer thickness of the fibrous material was 110 μm.

[0044] [Comparative Example 2] A porous membrane was obtained in the same manner as in Example 1 except that a PVDF membrane (manufactured by Merck, Durapore, pore size 0.45 μm, film thickness 125 μm) was used as the base material. The layer thickness of the fibrous material was 115 μm.

[0045] [Fiber Diameter and Thickness of Fiber-Like Substance Composed of Polyphenylene Ether] The layer composed of the fiber-like substance of the obtained porous membrane was photographed at a magnification at which dozens of fibers were observed in one field using a scanning electron microscope, and the fiber diameters of any 10 fibers were measured. The average value was taken as the average fiber diameter.

[0046] The fiber-like substance of the obtained porous membrane and the substrate were measured for the thickness of each layer using an optical microscope.

[0047] [Hydrophobicity] The obtained porous membrane was cut out into a size of 10 cm × 10 cm and placed on a stage where the tilt accuracy could be adjusted. In an environment of a temperature of 23°C and a humidity of 50%, with the layer of the fiber-like substance from which dust had been removed with an antistatic brush facing up, 0.05 mL of water was dropped onto its surface from a height of 1 cm using a micropipettor. The stage was tilted, and the angle at which the water droplet slid out was measured. This measurement was repeated 5 times, and the hydrophobicity was evaluated according to the following criteria. The smaller the angle, the more excellent the hydrophobicity of the porous membrane. Excellent: Less than 45° Poor: 45° or more

[0048] [Air Permeability] The obtained porous membrane was cut out into a size of 5 cm × 5 cm, and using a Gurley densometer (manufactured by Toyo Seiki), in accordance with JIS P 8117, the time for 100 mL of air to pass through the porous membrane was measured. The air permeability of the porous membrane was evaluated according to the following criteria. The shorter the air passage time, the faster the gas passage rate of the porous membrane and the more excellent the air permeability. Excellent: The air passage time for 100 mL of air is less than 30 seconds Poor: The air passage time for 100 mL of air is 30 seconds or more

[0049] [Interlayer Peel Resistance] The obtained porous membrane was cut out into a size of 5 cm × 5 cm, fixed by pinning at the four sides, and placed in hot water at 80°C and left for 4 weeks. The interlayer peel resistance of the porous membrane was evaluated according to the following criteria. Excellent: No interlayer peeling between the fiber-like substance and the substrate Poor: There is interlayer peeling between the fiber-like substance and the substrate [Membrane Distillation Property] Membrane distillation was carried out using the apparatus shown in Figure 1. An aqueous 4% NaCl solution warmed to 80°C was used as the feed solution and circulated onto the membrane using a pump at a flow rate of 40 mL / min.

[0050] <Membrane Distillation Suitability> The membrane distillation suitability of the obtained porous membrane was evaluated by measuring the NaCl concentration of the liquid that permeated the membrane after 10 minutes of operation using a conductivity-type digital salinity meter (ES-421, manufactured by Atago). A lower NaCl concentration indicates better membrane distillation suitability. Excellent: NaCl concentration of the liquid that permeated the membrane is less than 0.05% Poor: NaCl concentration of the liquid that permeated the membrane is 0.05% or higher

[0051] <Long-term operational capability (efficiency)> The long-term operational capability (efficiency) of membrane distillation of the obtained porous membrane was evaluated by measuring the amount of liquid collected with an NaCl concentration of less than 0.05% after 60 minutes of operation, according to the following criteria: A: Collected amount of 30 g or more B: Collected amount of 5 g or more but less than 30 g C: Collected amount of 1 g or more but less than 5 g D: Membrane distillation impossible (NaCl concentration of liquid permeated through the membrane is 0.05% or more) ND: Long-term operation impossible due to membrane delamination

[0052] <Long-term operational capability (durability)> The long-term operational capability (durability) of the obtained porous membrane for membrane distillation was evaluated by visually checking the condition of the membrane after 60 minutes of operation, according to the following criteria: A: No abnormalities on the surface, and it is firm when pulled by hand. B: There is some fuzzing on the surface, but it is firm when pulled by hand. C: There is a lot of fuzzing, and it tears when pulled by hand. D: Not suitable for membrane distillation.

[0053]

[0054] Table 1 shows that Examples 1 to 6 exhibited excellent hydrophobicity, permeability, delamination resistance, and film distillation properties, as well as a good balance of these properties. Comparative Examples 1 and 2 were found to be inferior to the examples in one or a balance of hydrophobicity, permeability, delamination resistance, film distillation properties, or any of these properties.

[0055] The membrane distillation diaphragm containing polyphenylene ether obtained in this invention has excellent hydrophobicity, permeability, and delamination resistance, and can therefore be used for the purification of crude liquids.

[0056] 1. Membrane distillation diaphragm 2. Funnel 3. Liquid collection section 4. Receiving tray 5. Flow path 6. Container 7. Stirrer 8. Weighing scale 9. Pump 10. Heating and stirring machine 11. Stock solution 12. Distillate

Claims

1. A membrane distillation diaphragm comprising a fiber layer and a base layer, wherein the fiber layer comprises polyphenylene ether, the average fiber diameter of the fibrous material constituting the fiber layer is in the range of 0.001 to 10 μm, the thickness of the fiber layer is 0.01 to 10 mm, and the ratio of the thickness of the fiber layer to the total thickness of the membrane distillation diaphragm is 1 to 99%.

2. The membrane distillation diaphragm according to claim 1, wherein the base layer is made of a material other than fluororesin.

3. The membrane distillation diaphragm according to claim 1, wherein the average fiber diameter of the fibrous material is in the range of 0.001 to 1 μm.

4. A membrane distillation module comprising a membrane distillation diaphragm according to any one of claims 1 to 3.

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