Porous hollow fiber membrane and method for producing same

Abstract

It is an object of the present disclosure to provide a porous hollow fiber membrane for membrane distillation that has excellent chemical resistance, mechanical strength and membrane distillation performance, as well as a membrane module for membrane distillation and a membrane distillation method using the same.SOLUTION: A total of areas of plastic parts having an area of 1 μ m2 or less included in each field of view is 70% or more with respect to the total area of the plastic parts, a total of areas of plastic parts having an area of 10 μ m2 or more is 15% or less with respect to the total area of the plastic parts, and a total of areas of plastic parts having an area of 1 μ m2 or less is 70% or more with respect to the total area of the plastic parts. And the total of the areas of the plastic parts having an area of 10 μ m2 or more is 15% or less with respect to the total area of the plastic parts.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a porous hollow fiber membrane, and a membrane distillation module, a membrane distillation apparatus, and a hydrophobic porous membrane that use the same. [Background technology]

[0002] There are many raw material liquids in the industry that require concentration and contain water and an organic solvent as the solvent.

[0003] One example is the pharmaceutical manufacturing process. In recent years, the development of pharmaceuticals with polymer structures of molecular weights of 10,000 or more, such as medium-molecular-weight peptides with molecular weights of 1,000 or more, has progressed in the field of pharmaceutical manufacturing. These pharmaceuticals are often synthesized and purified in organic solvents or mixed solvents containing water and organic solvents. However, in order to commercialize the resulting synthesized products or products, the organic solvent must be removed.

[0004] Removal of organic solvents from raw material solutions containing valuable substances such as pharmaceuticals as solutes has conventionally been carried out by vacuum distillation, thin-film distillation, etc. For example, Patent Document 1 describes a process in which a target product is obtained by purifying a raw material, such as a naturally-derived oil, by a method in which vacuum distillation and thin-film distillation are carried out in sequence.

[0005] In recent years, membrane distillation has attracted attention as an alternative method for removing organic solvents from a feed solution containing water and an organic solvent as solvents. In membrane distillation, the feed solution is brought into contact with an absorption liquid (cooling liquid) through a hydrophobic porous membrane that does not allow water to pass through. Then, driven by the difference in the vapor pressure of the organic solvent between the feed solution and the absorption liquid, only the vapor of the organic solvent in the feed solution passes through the hydrophobic porous membrane and moves to the absorption liquid side. This principle can be applied to remove (only) the organic solvent from the feed solution. For example, Patent Document 2 proposes removing organic solvents from a feed solution using a membrane distillation module, which is constructed by applying a water-repellent agent to the inside of a membrane module containing hollow fiber porous membranes to hydrophobize the porous membranes within the module.

[0006] Membrane distillation is also a method of obtaining distilled water by using a hydrophobic porous membrane that allows only water vapor to pass through the treated water. The water vapor that passes through the hydrophobic porous membrane due to the difference in saturated water vapor pressure is condensed from heated raw water (high-temperature water). Compared to reverse osmosis, which obtains purified water by applying pressure to raw water and filtering it through a reverse osmosis membrane, membrane distillation does not require high pressure and can reduce power energy. Furthermore, membrane distillation has extremely high separation performance for non-volatile solutes such as salt, making it possible to obtain highly pure water. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-181535 [Patent Document 2] International Publication No. 2021 / 070955 [Patent Document 3] International Publication No. 2023 / 249115 Summary of the Invention [Problem to be solved by the invention]

[0008] The vacuum distillation and thin-film distillation described in Patent Document 1 require the raw material liquid to be heated to a high temperature, which may result in deterioration or deactivation of the valuable material (solute) contained in the raw material liquid if the valuable material has low heat resistance.

[0009] On the other hand, in the membrane distillation method described in Patent Document 2, when the method is operated for a long period of time, the surface of the hydrophobic porous membrane may become wet with the raw material liquid, causing a "wetting" phenomenon in which the raw material liquid itself, rather than the vapor of the organic solvent, passes through the membrane, which is a problem.

[0010] Furthermore, Patent Document 3 describes the production of a membrane for membrane distillation in which a water-repellent agent is applied to a hollow fiber membrane made of PVDF to increase its hydrophobicity, with the aim of efficiently and stably removing organic solvents from a raw material solution containing water and organic solvents as solvents for a long period of time without causing deterioration or deactivation of valuable materials.

[0011] In view of the above circumstances, an object of the present invention is to provide a porous hollow fiber membrane for membrane distillation that is excellent in chemical resistance, mechanical strength, and membrane distillation performance, as well as a membrane module for membrane distillation and a membrane distillation method that use the same. [Means for solving the problem]

[0012] An example of the present invention that solves the above problems is as follows. <Aspect 1> A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section of the inside of the film, 2 A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure in which the area ratio of resin parts having the following area is 70% or more of the total area of ​​all resin parts included in each of the fields of view. <Aspect 2> A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section inside the film, 10 μm included in each field of view 2 A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure in which the area ratio of resin parts having an area of ​​15% or less to the total area of ​​all resin parts included in each of the fields of view. <Aspect 3> A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section of the inside of the film, 2 The area ratio of the resin parts having an area of ​​10 μm or less is 70% or more of the total area of ​​all resin parts included in each of the visual fields, and in all of the visual fields, 2A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure in which the area ratio of resin parts having an area of ​​15% or less to the total area of ​​all resin parts included in each of the fields of view. <Aspect 4> 4. The porous hollow fiber membrane for membrane distillation according to any one of aspects 1 to 3, wherein the porous hollow fiber membrane for membrane distillation is used alone without being coated with a water repellent agent. <Aspect 5> A membrane distillation membrane module using the porous hollow fiber membrane for membrane distillation according to any one of aspects 1 to 4, The membrane module for membrane distillation is configured by adhesively fixing both ends of the porous hollow fiber membrane for membrane distillation in a housing, A membrane module for membrane distillation, wherein a hydrophobic adhesive is used to fix both ends of the membrane module. <Aspect 6> A membrane distillation membrane module according to Aspect 5, wherein the hydrophobic adhesive is made of a silicone resin or a fluororesin. <Aspect 7> A membrane distillation method for performing a membrane distillation operation using the porous hollow fiber membrane for membrane distillation according to any one of aspects 1 to 4. <Aspect 8> A membrane distillation method for performing a membrane distillation operation using the membrane module for membrane distillation according to aspect 6. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a porous hollow fiber membrane that has excellent chemical resistance, mechanical strength, and membrane distillation performance and contains an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, and it is also possible to provide a membrane production method using a non-solvent, as well as a membrane distillation membrane module and a membrane distillation method that use a porous hollow fiber membrane that has open pores, interconnectivity, chemical resistance, mechanical strength, and membrane distillation performance. . [Brief explanation of the drawings]

[0014] [Figure 1]This shows an electron microscope photograph of the hollow fiber membrane in Example 1 after black and white binarization (porous parts are white and non-porous parts are black), and is a cross-sectional photograph of four locations at equal intervals from field of view (a) closest to the inner surface of the hollow fiber membrane to field of view (d) closest to the outer surface. [Figure 2] 1 shows a schematic diagram of a membrane module for membrane distillation of the present invention. [Figure 3] 1 shows a conceptual diagram of a fresh water generator of the present invention. [Figure 4] 1 is a conceptual diagram illustrating an example of an embodiment of a raw material liquid concentrating system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below with reference to the following embodiments, but the present invention is not limited to the following embodiments.

[0016] <Porous hollow fiber membrane> The porous hollow fiber membrane of the present invention will be described below. The porous hollow fiber membrane used in membrane distillation of this embodiment (hereinafter also referred to as "porous hollow fiber membrane for membrane distillation") has a cross section of the porous hollow fiber membrane in the membrane thickness direction perpendicular to the inner surface of the porous hollow fiber membrane, and a total of four fields of view, including a field of view including the inner surface, a field of view including the outer surface of the porous hollow fiber membrane, and two fields of view photographed at equal intervals between these fields of view, with a thickness of 1 μm or less. 2 The total area of ​​the resin parts having the following area is 70% or more of the total area of ​​the resin parts; 2 The total area of ​​the resin parts having the above area is 15% or less of the total area of ​​the resin parts; 2 The total area of ​​the resin parts having the following area is 70% or more of the total area of ​​the resin parts, and 10 μm 2 the sum of the areas of the resin parts having an area of ​​1 μm or more is 15% or less of the total area of ​​the resin parts. 2 The total area of ​​the resin parts having the following area is 70% or more of the total area of ​​the resin parts, and is 1 μm 2Ultra 10μm 2 The total area of ​​the resin parts having an area of ​​less than 15% of the total area of ​​the resin parts, and 2 The total area of ​​the resin parts having the above dimensions is 15% or less of the total area of ​​the resin parts.

[0017] 1(a) to 1(d) are images obtained by binarizing electron microscope photographs of one field of view in four equally spaced cross sections of the porous hollow fiber membrane used in this embodiment, from field of view (a) closest to the inner surface to field of view (d) closest to the outer surface. The cross section shown in Fig. 1(a) is a cross section of one field of view in a cross section perpendicular to the longitudinal direction of the porous hollow fiber membrane, and is an image obtained by binarizing an SEM image obtained by photographing the field of view closest to the inner surface of the porous hollow fiber membrane among the above-mentioned four fields of view.

[0018] Furthermore, within each of the above regions, the difference in the distribution of resin portions, i.e., the anisotropy of pore interconnectivity, can be virtually ignored between the membrane cross section in the membrane thickness direction perpendicular to the inner surface of the porous hollow fiber membrane and the cross section parallel to the inner surface. In this specification, the term "resin portion" refers to the dendritic skeleton portion of a three-dimensional network structure composed of resin, which forms numerous pores in the porous membrane. In Figure 1, the black portions are resin portions, and the white portions are pores.

[0019] The porous hollow fiber membrane has interconnected pores that are bent from the inside to the outside of the membrane. In the SEM image of the cross section of the membrane in the thickness direction perpendicular to the inner surface of the porous hollow fiber membrane, a field including the inner surface, a field including the outer surface of the membrane, and two fields of view taken at equal intervals between these fields, a total of four fields, were measured. 2 If the total area of ​​the resin parts having the following areas is 70% or more of the total area of ​​the resin parts, the interconnectivity of the pores will be high (i.e., the proportion of interconnected pores inside the membrane will be high), and the steam flux (permeation rate) and mechanical strength indexed by the tensile breaking elongation will also be high. However, if the total area of ​​the resin parts having the following areas is 70% or more of the total area of ​​the resin parts, the interconnectivity of the pores will be high (i.e., the proportion of interconnected pores inside the membrane will be high), and the mechanical strength indexed by the steam flux (permeation rate) and the tensile breaking elongation will also be high. 2If the ratio of the total area of ​​the resin portion having an area of ​​1 μm or less is too high, the dendritic skeleton of the three-dimensional network structure made of resin, which forms many pores in the porous membrane, will be too thin. 2 The total area of ​​the resin parts having the following area is maintained at 70% or more of the total area of ​​the resin parts, 2 The total area of ​​the resin parts having an area of ​​more than 10 μm is preferably 2% or more and 30% or less of the total area of ​​the resin parts. 2 More preferably, the total area of ​​the resin portions having an area of ​​1 μm or more is 15% or less of the total area of ​​the resin portions. 2 Ultra 10μm 2 The total area of ​​the resin parts having an area of ​​less than 10 μm is 15% or less of the total area of ​​the resin parts, and 2 It is more preferable that the total area of ​​the resin portions having an area of ​​1 μm or more is 2% or more and 15% or less of the total area of ​​the resin portions. 2 If the total area of ​​the resin portions having an area of ​​more than 2% to 30% of the total area of ​​the resin portions, the dendritic skeleton portion of the three-dimensional network structure composed of the resin will not be too thin, and the strength and tensile breaking elongation of the porous membrane can be appropriately maintained. Furthermore, the porous hollow fiber membrane for membrane distillation having the above-described three-dimensional network structure is preferably used alone in a membrane distillation module or membrane distillation operation without being coated with a water repellent agent.

[0020] In this embodiment, the cross-sectional view of each field of view of the porous hollow fiber membrane as shown in FIG. 1 is subjected to image analysis, and the results of measuring the area distribution of the resin portion can be obtained as a histogram.

[0021] In the cross-sectional view of the porous hollow fiber membrane shown in Figure 1, the resin portions appear as granules. Specifically, the area of ​​each of these granular resin portions is measured, and the area ratio of each granular resin portion to the total area of ​​all resin portions in the cross-sectional view can be shown as a histogram.

[0022] The method for measuring the area distribution of the resin portion in the cross-sectional view of each field of the porous hollow fiber membrane will be explained later. The porous hollow fiber membrane according to this embodiment contains an ethylene-tetrafluoroethylene copolymer and / or an ethylene-chlorotrifluoroethylene copolymer as a thermoplastic resin. FIG. 2 is a schematic diagram of a membrane module for membrane distillation according to this embodiment. The outer surface of the porous hollow fiber membrane for membrane distillation 20 shown in FIG. 2 does not have a spherulite structure but a three-dimensional mesh structure. The porous hollow fiber membrane for membrane distillation according to this embodiment has a three-dimensional mesh structure, which increases the interconnectivity of the pores.

[0023] The porous hollow fiber membrane 20 may contain up to about 5% by mass of components (impurities, etc.) other than the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer. For example, the porous hollow fiber membrane contains an organic liquid used during production. As described below, the porous hollow fiber membrane 20 contains at least a non-solvent (first solvent) used as the organic liquid during production, and may also contain a solvent or poor solvent (second solvent). These organic liquids can be detected by pyrolysis GC-MS (gas chromatography-mass spectrometry).

[0024] The first solvent is preferably a non-solvent that does not allow the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer to dissolve uniformly in the first organic liquid even when the temperature of the first mixed liquid, in which the ratio of the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer to the first organic liquid is 20:80, is raised to the boiling point of the first organic liquid.

[0025] The second solvent is preferably a solvent in which, in a second mixed liquid in which the ratio of ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer to the second solvent is 20:80, the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer dissolves uniformly in the second solvent at any temperature in the range of more than 25°C to the boiling point of the second solvent.

[0026] The method for determining whether a solvent is a non-solvent, a solvent, or a poor solvent in the present invention will be explained later in the description of the method for producing a porous hollow fiber membrane.

[0027] (Physical properties of porous hollow fiber membranes) Next, the physical properties of the porous hollow fiber membrane according to this embodiment will be described. The initial tensile elongation at break of the porous hollow fiber membrane is preferably 60% or more, more preferably 80% or more, even more preferably 100% or more, and particularly preferably 120% or more. The tensile elongation at break can be measured by the measurement method described in the Examples below.

[0028] From a practical standpoint, the compressive strength of the porous hollow fiber membrane 20 is 0.2 MPa or more, preferably 0.3 to 1.0 MPa, and more preferably 0.4 to 1.0 MPa.

[0029] The surface opening rate (surface opening rate) of the porous hollow fiber membrane 20 is preferably 10 to 60%, more preferably 10 to 50%, and even more preferably 10 to 45%. By using a membrane with an opening rate of 10% or more on the surface that comes into contact with the liquid to be treated as a membrane distillation membrane, vapor permeability can be improved. However, even if the opening rate is high, if the pore size is too large, wetting may occur. Therefore, the pore size on the outer surface of the porous hollow fiber membrane according to this embodiment is preferably 1,000 nm or less, more preferably 10 to 800 nm, and even more preferably 100 to 700 nm. A pore size of 1,000 nm or less can block desired components contained in the liquid to be treated, while a pore size of 10 nm or more can ensure sufficiently high vapor permeability.

[0030] The thickness of the porous hollow fiber membrane 20 is preferably 80 to 1,000 μm, and more preferably 100 to 300 μm. When the thickness of the porous hollow fiber membrane is 80 μm or more, the strength is high, while when it is 1,000 μm or less, the pressure loss due to membrane resistance is small.

[0031] The porosity of the porous hollow fiber membrane 20 is preferably 50 to 80%, and more preferably 55 to 65%. If the porosity is 50% or more, the vapor permeability is high, while if it is 80% or less, the mechanical strength is high.

[0032] The shape of the porous hollow fiber membrane 20 can be a circular single-layer membrane, or a multi-layer membrane in which the separation layer and the support layer supporting the separation layer have different pore sizes. The outer and inner surfaces of the porous hollow fiber membrane may have a modified cross-sectional structure, such as having protrusions.

[0033] FIG. 2 schematically shows an example of a membrane distillation membrane module that is preferably used when the concentration system according to this embodiment is operated using a membrane distillation membrane.

[0034] The membrane distillation membrane module 100 in Figure 2 contains a plurality of hollow fiber membranes 20 housed in a housing 10, and both ends of the membranes are bonded and fixed with an adhesive resin 30, such as a hydrophobic thermoplastic resin or adhesive. Both ends of the membrane distillation membrane are open and not blocked. The hydrophobic adhesive is preferably made of a silicone resin and / or a fluororesin, and more preferably made of a silicone resin or a fluororesin. The hydrophobic thermoplastic resin is preferably made of polyethylene or polypropylene, which is melted and bonded.

[0035] The side of the housing 10 has a first housing side pipe 11 for introducing cooling water CW and a second housing side pipe 12 for discharging cooling water CW, which allow cooling water CW to circulate in the external space of the membrane distillation membrane.

[0036] At both ends of the axial direction of the housing 10 (left and right direction in Figure 2), there are a lower opening for introducing raw material liquid a and an upper opening for discharging raw material liquid, which allow raw material liquid a to flow through the hollow part of the membrane distillation membrane.

[0037] The interior of the membrane distillation module is divided by the membrane distillation membrane into a space on the hollow side of the membrane distillation membrane and a space on the external space side of the membrane distillation membrane. These two spaces are fluidically isolated except that a specific solvent can pass through the outer wall of the membrane distillation membrane.

[0038] The membrane for membrane distillation is, for example, a hollow fiber, and its outer wall must be highly hydrophobic and porous, yet must not allow liquids to penetrate inside, and must allow only gases to pass through the outer wall. Furthermore, since high vapor permeability is required even at room temperature, the membrane must have a high porosity and an appropriate average pore size.

[0039] <Water production system> The fresh water generator of this embodiment is shown in Figure 3. The fresh water generator comprises an evaporator having a liquid phase section 1 through which treated water flows, a gas phase section 1, and a hydrophobic porous membrane separating the liquid phase section 1 from the gas phase section 1; a condenser having a liquid phase section 2 through which cooling water flows, a gas phase section 2, and a cooler separating the liquid phase section 2 from the gas phase section 2; and a gas phase section 3 (air gap) connecting the gas phase section 1 of the evaporator section with the gas phase section 2 of the condenser section, and the pressure in the gas phase sections 1 to 3 is preferably 1 kPa or more and less than the saturated vapor pressure of water at the temperature of the treated water.

[0040] The fresh water generator of this embodiment includes an evaporator section, a condenser section, and a third gas phase section (Air Gap). As described above, the evaporation section according to this embodiment has a hydrophobic porous membrane. The hydrophobic porous membrane separates the liquid phase section 1 and the gas phase section 1 in the evaporation section, and therefore has a membrane surface in contact with the liquid phase section 1 and a membrane surface in contact with the gas phase section 1. Because the treated water flows through the liquid phase section 1, the membrane surface of the hydrophobic porous membrane in contact with the liquid phase section 1 is the membrane surface in contact with the treated water, and the membrane surface of the hydrophobic porous membrane in contact with the gas phase section 1 is the other membrane surface of the inner surface in contact with the treated water.

[0041] The shape of the hydrophobic porous membrane may be, for example, a flat membrane type, a tubular type, a hollow fiber type, a spiral type, etc. From the viewpoint of making the membrane module compact, a hollow fiber membrane is preferred, which allows a large membrane area per unit volume.

[0042] In the freshwater production system of this embodiment, treated water refers to water that needs to be purified or concentrated for some purpose, and examples thereof include tap water, industrial water, river water, well water, lake water, seawater, industrial wastewater (wastewater from factories such as food factories, chemical factories, electronics factories, pharmaceutical factories, and wastewater treatment plants), and produced water discharged during the production of petroleum or natural gas.

[0043] From the viewpoint of water permeability, the treated water preferably has a water temperature (treated water temperature) of 50°C or higher, more preferably 80°C or higher.

[0044] The temperature of the treated water (treated water temperature) may be controlled by utilizing a heat source such as a heat exchanger or heater, but it is more preferable to control it by utilizing solar heat or waste heat from industrial processes, etc., because this eliminates or reduces the thermal energy costs required for heating.

[0045] In the fresh water generator of this embodiment, the condenser section is formed, for example, by housing the cooling body in a cylindrical resin or metal container, filling the gaps between the cooling bodies at the ends and the gap between the cooling body and the container with a fixing resin (potting resin), and fixing the cooling body to the container. The ends of the cooling body are open, and head sections with water passage ports are attached to both the top and bottom ends of the container. A connecting port is provided on the side of the container for connection to the evaporator section. The number of connecting ports is not particularly limited, and there may be one or more.

[0046] The cooling body may be hollow or flat, but a hollow tube is preferably used.

[0047] The cooling body is provided in the condenser, and the internal region of the cooling body becomes the liquid phase portion 2 through which the cooling water flows. The external region of the cooling body becomes the gas phase portion 2 within the container that constitutes the condenser.

[0048] The treated water passed through the liquid phase section 1 passes through the membrane wall of the hydrophobic porous membrane as water vapor and moves to the gas phase section 1, where it is cooled by a cooling body and becomes distilled water.

[0049] The condenser, which has a cooling body, is connected to a water collection container by a pipe, and the distilled water is discharged from the condenser and collected in the water collection container.

[0050] In the freshwater generation / dehydration system of this embodiment, the cooling water is not particularly limited as long as it is a liquid that flows through the liquid phase section 2, which is the internal space of the cooling body, and can cool water vapor, and examples thereof include tap water, industrial water, river water, well water, lake water, seawater, industrial wastewater (wastewater from factories such as food factories, chemical factories, electronics factories, pharmaceutical factories, and waste incineration plants), and produced water discharged during the production of petroleum or natural gas. In this embodiment, water used as treated water may also be used as cooling water.

[0051] From the viewpoint of condensation efficiency, the temperature of the cooling water is preferably 30°C or less, and more preferably 20°C or less. It is more preferable that:

[0052] The temperature of the cooling water may be controlled by utilizing a heat source such as a heat exchanger or a heater.

[0053] <Raw material liquid concentration system> The raw material liquid concentration system according to this embodiment is a raw material liquid concentration system that uses a membrane distillation method in which a raw material liquid containing a solvent and a solute is brought into contact with cooling water via a membrane distillation membrane, and the solvent in the raw material liquid is passed through the membrane distillation membrane in a vapor state and transferred to the cooling water side. The raw material liquid concentrating system according to this embodiment will be described below with reference to FIGS.

[0054] To concentrate a raw liquid using membrane distillation, the raw liquid is passed through one side of the membrane distillation membrane (in Figure 2, the hollow space of the hollow fiber membrane distillation membrane), and cooling water, which is at a lower temperature than the raw liquid, is passed through the other side (in Figure 2, the external space side of the hollow fiber membrane distillation membrane). The pores that connect the two spaces inside the membrane wall come into contact with the raw liquid and cooling water through the membrane wall. As a result, the vapor generated from the raw liquid, which has a high vapor pressure, passes through the membrane wall and moves to the cooling water, which has a low vapor pressure, where it is cooled and liquefied, concentrating the raw liquid.

[0055] FIG. 4 shows an example of a raw material liquid concentration system according to this embodiment. In the concentration system of Figure 4, a means (piping system) for circulating a feed solution a is provided at both axial ends of the membrane distillation membrane module (MD Membrane) shown in Figure 2. The piping system for circulating the feed solution a includes a feed solution storage tank 200, a pump P for circulating the feed solution, and a temperature controller TC for maintaining the temperature of the feed solution at a set temperature. This concentration system may also be equipped, as necessary, with a flow meter FM for indicating the circulation flow rate, a flow rate regulator (not shown) for adjusting the circulation flow rate, and a pressure gauge PG for indicating the liquid pressure when the feed solution is supplied to the membrane distillation membrane module. In addition, by providing a weight scale, a liquid level gauge LG, etc. in the feed solution storage tank, the membrane performance and concentration rate can be estimated from the degree of decrease in the liquid level, weight, etc.

[0056] Meanwhile, a means (piping system) for circulating cooling water CW is provided in the housing side pipe of the membrane distillation membrane module. This piping system is equipped with a cooling water storage tank 300, a pump P for circulating the cooling water, a flow meter FM that indicates the circulation flow rate, a flow rate regulator (not shown) that adjusts the circulation flow rate, and a temperature regulator TC that maintains the cooling water temperature at a set temperature. As the solvent moves from the raw material liquid to the cooling water during membrane distillation, the volume of the cooling water increases over time. Therefore, the storage volume of the cooling water storage tank 300 increases as membrane distillation continues. If the cooling water can be discarded at this time, an overflow port can be installed in the cooling water storage tank to maintain a constant volume of cooling water.

[0057] In raw material liquid concentration systems for manufacturing pharmaceuticals, chemicals, etc., raw material liquids containing substances that can be decomposed by heating, such as peptides and proteins, must be kept at low temperatures (for example, below 50°C). In addition, liquids with low surface tension, such as acetonitrile, methanol, ethanol, and isopropanol, are used as solvents for the raw material liquids.

[0058] For these reasons, membranes for membrane distillation are required to have strong hydrophobicity so that they do not get wet even with liquids with low surface tension, and high vapor permeability so that solvent vapor can be efficiently extracted from the raw material liquid at room temperature. Therefore, the hollow fiber membrane for membrane distillation used in this embodiment must be highly hydrophobic, with a water contact angle of 90° or more. It is also necessary to ensure high vapor permeability, and the membrane must be a highly porous membrane with high interconnectivity.

[0059] <<Each element of the raw material liquid concentration system>> The above has provided an overview of the concentration of a raw material liquid using the raw material liquid concentration system of the present invention. Next, each element constituting the raw material liquid concentration system of the present invention will be described in detail below.

[0060] <Raw material liquid> The source liquid is a fluid containing a solute and a solvent, and is intended to be concentrated by the system of the present invention. This source liquid may be an emulsion, so long as it is a fluid.

[0061] Examples of raw material liquids applicable to this embodiment include pharmaceuticals, pharmaceutical raw materials, pharmaceutical active ingredients, pharmaceutical intermediates, etc. (hereinafter, these are collectively referred to as "pharmaceutical active ingredients, etc."), solutions or dispersions containing chemicals, food products, seawater, and produced water discharged from gas fields, oil fields, etc.

[0062] In the raw material liquid concentration system according to this embodiment, a concentrated solution is obtained in which solvent b has been removed while the composition of raw material liquid a is maintained almost unchanged, as shown in Figure 2 or Figure 4. Therefore, when the raw material liquid concentration system according to this embodiment is applied to the concentration of a solution or dispersion containing a pharmaceutical product, chemical product, or the like as a solute, it becomes possible to concentrate the solution while maintaining the properties of the solute.

[0063] [Solute of raw material solution] As the pharmaceutical ingredients, chemicals, etc., it is preferable that the solute is a useful substance selected from the group consisting of amino acids, peptides, proteins, sugars, vaccines, nucleic acids, antibiotics, antibody-drug conjugates (ADCs), compounds with surfactant effects, and vitamins, and that this solute is dissolved or dispersed in an appropriate solvent.

[0064] An amino acid is a compound having an amino acid skeleton consisting of a carboxyl group, an amino group, and a moiety connecting them. In this specification, the term "amino acid" encompasses essential amino acids, non-essential amino acids, and unnatural amino acids.

[0065] Examples of essential amino acids include tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, isoleucine, etc. Examples of non-essential amino acids include arginine, glycine, alanine, serine, tyrosine, cysteine, asparagine, glutamine, proline, aspartic acid, glutamic acid, etc.

[0066] Unnatural amino acids refer to any artificial compound that does not exist in nature and has an amino acid backbone. However, unnatural amino acids used as pharmaceutical ingredients include those obtained by binding a desired labeling compound to the amino acid backbone. Examples of labeling compounds include dyes, fluorescent substances, luminescent substances, enzyme substrates, coenzymes, antigenic substances, and protein-binding substances.

[0067] Examples of unnatural amino acids that are preferred as pharmaceutical ingredients include labeled amino acids and functionalized amino acids.

[0068] A labeled amino acid is a non-natural amino acid in which a labeling compound is bound to an amino acid backbone, and specific examples thereof include amino acids in which a labeling compound is bound to an amino acid backbone containing an aromatic ring in the side chain.

[0069] Examples of functionalized amino acids include photoresponsive amino acids, photoswitch amino acids, fluorescent probe amino acids, and fluorescently labeled amino acids.

[0070] A peptide refers to a compound consisting of 2 to less than 70 amino acid residues bound together, and may be linear or cyclic. Examples of peptides include L-alanyl-L-glutamine, β-alanyl-L-histidine cyclosporine, and glutathione.

[0071] A protein generally refers to a compound in which amino acid residues are bound, and which has a longer chain than a peptide. As used herein, the protein is preferably one that is used as, for example, a protein drug.

[0072] Examples of protein preparations include interferon α, interferon β, interleukins 1 to 12, growth hormone, erythropoietin, insulin, granulocyte-colony stimulating factor (G-CSF), tissue plasminogen activator (TPA), natriuretic peptides, blood coagulation factor VIII, somatomedin, glucagon, growth hormone-releasing factor, serum albumin, and calcitonin.

[0073] Examples of sugars include monosaccharides, disaccharides, sugar chains (excluding disaccharides), sugar chain derivatives, and the like.

[0074] Examples of monosaccharides include glucose, fructose, galactose, mannose, ribose, deoxyribose, etc. Examples of disaccharides include maltose, sucrose, lactose, etc.

[0075] The term "sugar chain" as used herein refers to a concept excluding disaccharides, and examples thereof include glucose, galactose, mannose, fucose, xylose, glucuronic acid, iduronic acid, etc. Examples of sugar chain derivatives include saccharide derivatives such as N-acetylglucosamine, N-acetylgalactosamine, and N-acetylneuraminic acid.

[0076] Examples of vaccines include hepatitis A vaccine, hepatitis B vaccine, and hepatitis C vaccine; Examples of nucleic acids include oligonucleotides, RNA, aptamers, decoys, etc. Antibiotics include, for example, streptomycin and vancomycin; Each of them can be mentioned.

[0077] Examples of vitamins include vitamin A, vitamin B, vitamin C, etc., and also include derivatives, salts, etc. Examples of vitamin B include vitamin B6, vitamin B12, etc.

[0078] Examples of compounds having a surface active effect include polysaccharides, phospholipids, peptides, anionic surfactants such as sodium lauryl sulfate, cationic surfactants such as benzalkonium chloride, or nonionic surfactants such as amphoteric surfactants and polyethylene glycol.

[0079] The number average molecular weight of the solute contained in the raw material liquid may be about 100 to 75,000, preferably about 100 to 50,000, more preferably about 100 to 10,000, and is particularly preferably a low molecular weight compound of 100 to 6,000.

[0080] If the molecular weight of the solute is too small, it may pass through the membrane for membrane distillation, whereas if the molecular weight is too large, it may cause adhesion of the solute to the membrane surface, which is undesirable.

[0081] [Solvent in raw material solution] The solvent in the raw material liquid can be any inorganic or organic solvent as long as it is liquid and can dissolve or disperse the solute in the raw material liquid.

[0082] The solvent is preferably water, acetonitrile, methanol, ethanol, or isopropanol, and one or more selected from these may be used as appropriate. The solvent may also be, for example, a mixture containing two or more selected from water, acetonitrile, methanol, ethanol, and isopropanol.

[0083] <Concentrate> The concentrate obtained by concentrating the raw material liquid is obtained by preserving the solutes in the raw material liquid almost intact while preferentially separating the solvent. The raw material liquid concentration system of the present invention allows for arbitrary control of the type and amount of solvent separated from the raw material liquid.

[0084] <Method for manufacturing porous hollow fiber membrane> A method for producing a porous hollow fiber membrane will now be described. The method for producing a porous hollow fiber membrane comprises the steps of (a) preparing a molten mixture, (b) supplying the molten mixture to a multi-layer spinning nozzle and extruding the molten mixture from the spinning nozzle to obtain a hollow fiber membrane, and (c) extracting the organic liquid from the hollow fiber membrane. If the molten mixture contains an additive, the method for producing a porous hollow fiber membrane may further comprise the step of (d) extracting the additive from the hollow fiber membrane after step (c).

[0085] The concentration of the thermoplastic resin in the melt-kneaded product is preferably 20 to 60% by mass, more preferably 25 to 45% by mass, and even more preferably 30 to 45% by mass. If the concentration of the thermoplastic resin in the melt-kneaded product is 20% by mass or more, the mechanical strength is high, while if it is 60% by mass or more, the interconnectivity is likely to be poor. The melt-kneaded product may contain additives.

[0086] The melt-kneaded product may be composed of two components, a thermoplastic resin and an organic liquid, or may be composed of three components, a thermoplastic resin, an additive, and an organic liquid. The organic liquid preferably contains at least a non-solvent, as described below.

[0087] The extractant used in step (c) is preferably a liquid that does not dissolve the thermoplastic resin but has a high affinity for the organic liquid, such as methylene chloride or various alcohols.

[0088] When a melt-kneaded product containing no additives is used, the hollow fiber membrane obtained through step (c) may be used as the porous hollow fiber membrane. When a melt-kneaded product containing additives is used to produce a porous hollow fiber membrane, the method for producing a porous hollow fiber membrane for membrane distillation according to this embodiment preferably further comprises, after step (c), a step (d) of extracting and removing the additive from the hollow fiber membrane to obtain a porous hollow fiber membrane. As the extractant in step (d), it is preferable to use hot water or a liquid such as an acid or alkali that can dissolve the additives used but not the thermoplastic resin.

[0089] An inorganic substance may be used as an additive. The inorganic substance is preferably an inorganic fine powder. The primary particle size of the inorganic fine powder contained in the melt-kneaded mixture is preferably 50 nm or less, more preferably 5 nm or more and less than 30 nm. Specific examples of inorganic fine powders include silica, finely powdered silica, titanium oxide, lithium chloride, calcium chloride, and organic clay. Of these, finely powdered silica is preferred from the viewpoint of cost. The "primary particle size of the inorganic fine powder" mentioned above refers to a value determined by analyzing an electron microscope photograph. That is, a group of inorganic fine powders is first pretreated according to the method of ASTM D3849. Then, the particle diameters of 3,000 to 5,000 particles captured in a transmission electron microscope photograph are measured, and the primary particle size of the inorganic fine powder is calculated by arithmetically averaging these values.

[0090] The material of the inorganic fine powder in the porous hollow fiber membrane can be determined by identifying the elements present using fluorescent X-rays or the like.

[0091] Next, the details of the step (a) of preparing the molten mixture in the method for producing a porous hollow fiber membrane, that is, the production method of the present invention, will be described.

[0092] The method for producing a porous hollow fiber membrane according to the present embodiment is a method for producing a porous hollow fiber membrane containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, comprising: mixing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer with an organic liquid comprising at least a first organic liquid and a second organic liquid; and a step of melt-kneading a mixture containing an organic liquid and an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, the first organic liquid is at least one selected from the group consisting of sebacic acid esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleic acid esters, palmitic acid esters, stearic acid esters, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oils, and is a non-solvent in which, in a first mixed liquid containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer and the first solvent in a ratio of 20:80, the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer does not dissolve uniformly in the first solvent even when the temperature of the first mixed liquid is raised to the boiling point of the first solvent; The second organic liquid is at least one selected from sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearates, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oils, and is a solvent that, in a second mixed liquid in which the ratio of ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer to the second solvent is 20:80, causes the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer to dissolve uniformly in the second mixed liquid at any temperature higher than 25°C and lower than the boiling point of the second solvent.

[0093] The second organic liquid may be a poor solvent in which, in a second mixed liquid containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer and the second solvent in a ratio of 20:80, the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer does not dissolve uniformly in the second solvent when the temperature of the second mixed liquid is 25°C, but the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer dissolves uniformly in the second solvent when the temperature of the second mixed liquid is any temperature higher than 100°C and lower than the boiling point of the second solvent.

[0094] The method for producing a porous hollow fiber membrane of the present invention uses a nonsolvent for ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer as a raw material. Using a nonsolvent as a raw material for the membrane results in a porous hollow fiber membrane with a three-dimensional network structure. While the mechanism of action is not entirely clear, it is believed that mixing a nonsolvent with an organic liquid to reduce solubility inhibits polymer crystallization to a moderate extent, making it easier to form a three-dimensional network structure. For example, the nonsolvent, poor solvent, and solvent are preferably selected from sebacic acid esters, citric acid esters, acetyl citrate esters, adipate esters, trimellitic acid esters, oleic acid esters, palmitic acid esters, stearic acid esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils, and more preferably at least one selected from sebacic acid esters, citric acid esters, acetyl citrate esters, oleic acid esters, palmitic acid esters, stearic acid esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

[0095] A substance that can dissolve an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer at room temperature is called a solvent, an organic liquid that cannot dissolve the ethylene-tetrafluoroethylene copolymer at room temperature but can dissolve it at elevated temperatures is called a poor solvent for the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer, and an organic liquid that cannot dissolve the ethylene-tetrafluoroethylene copolymer even at elevated temperatures is called a non-solvent. In the present invention, poor solvents and non-solvents can be determined as follows.

[0096] The non-solvent is a first mixed liquid in which the ratio of ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer to the first solvent is 20:80, and the temperature of the first mixed liquid is raised to the boiling point of the first solvent, so that the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer does not dissolve uniformly in the first solvent.

[0097] The solvent is a solvent in which, in a second mixed liquid in which the ratio of the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer to the second solvent is 20:80, the ethylene-tetrafluoroethylene copolymer or the ethylene-chlorotrifluoroethylene copolymer dissolves uniformly in the second solvent when the temperature of the second mixed liquid is higher than 25°C and lower than the boiling point of the second solvent.

[0098] Furthermore, a poor solvent is one in which, in a second mixed liquid in which the ratio of ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer to the second solvent is 20:80, the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer does not dissolve uniformly in the second solvent when the temperature of the second mixed liquid is 25°C, but the ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer dissolves uniformly in the second solvent when the temperature of the second mixed liquid is any temperature higher than 100°C and lower than the boiling point of the second solvent.

[0099] To determine whether a substance is a solvent, poor solvent, or non-solvent, specifically, approximately 2 g of ethylene-tetrafluoroethylene copolymer or ethylene-chlorotrifluoroethylene copolymer and approximately 8 g of solvent are placed in a test tube, heated to the boiling point of the solvent in approximately 10°C increments using a test tube block heater, and the contents of the test tube are mixed with a spatula or the like to determine the solubility within the temperature range described above.

[0100] Specific examples of the above esters and their boiling points are as follows: The boiling points of acetyl tributyl citrate are 343°C, dibutyl adipate is 305°C, diisobutyl adipate is 293°C, bis 2-ethylhexyl adipate is 335°C, diisononyl adipate is above 250°C, and diethyl adipate is 251°C.

[0101] For example, if diethyl adipate is used as the solvent, ethylene-tetrafluoroethylene copolymers will mix uniformly and dissolve at around 200°C. On the other hand, if bis(2-ethylhexyl) adipate is used as the solvent, ethylene-tetrafluoroethylene copolymers will not dissolve.

[0102] By carrying out membrane distillation of the liquid to be treated using the porous hollow fiber membrane according to this embodiment, it is possible to carry out water production / dehydration and concentration operations with high efficiency. [Example]

[0103] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0104] [Example 1] The melt-kneaded mixture was extruded using a spinning nozzle with a double-pipe structure to obtain a porous hollow fiber membrane of Example 1. The melt-kneaded mixture was prepared using 40 mass% ethylene-tetrafluoroethylene copolymer (ETFE) resin (TL-081, manufactured by Asahi Glass Co., Ltd.) as a thermoplastic resin, 23 mass% finely powdered silica (primary particle size: 16 nm), 32.9 mass% bis-2-ethylhexyl adipate (DOA, boiling point 335°C) as a non-solvent, and 4.1 mass% diisobutyl adipate (DIBA, boiling point 293°C) as a poor solvent.

[0105] Under the membrane production conditions shown in Table 1, the extruded hollow fiber extrusions were passed through a free running distance of 120 mm and then solidified in water at 30°C. A porous hollow fiber membrane was produced by thermally induced phase separation. The extrusions were taken up at a speed of 5 m / min and wound into a hank. The resulting two-layer hollow fiber extrusions were immersed in isopropyl alcohol to extract and remove the solvent, and then immersed in water for 30 minutes to exchange the hollow fiber membrane for water. The hollow fiber membrane was then immersed in a 20% by mass aqueous solution of NaOH at 70°C for 1 hour and repeatedly washed with water to extract and remove the finely divided silica from the hollow fiber membrane. The membrane structure of the porous hollow fiber membrane of Example 1 exhibited a three-dimensional network structure as shown in Figures 1(a) to 1(d) when photographed using an electron microscope (described below).

[0106] [Example 2] A melt-kneaded mixture was prepared using 40% by mass of ethylene-chlorotrifluoroethylene copolymer (ECTFE) resin (Halar 901, manufactured by Solvay) as the thermoplastic resin, 23% by mass of finely powdered silica (primary particle size: 16 nm), 32.9% by mass of triphenyl phosphate (TPP, boiling point 370°C) as the non-solvent, and 4.1% by mass of the poor solvent bis-2-ethylhexyl adipate (DOA, boiling point 335°C). A porous hollow fiber membrane was produced in the same manner as in Example 1, except that the obtained melt-kneaded mixture was used.

[0107] The membrane structure of the porous hollow fiber membrane of Example 2 was shown to be a three-dimensional mesh structure when photographed using an electron microscope as described below.

[0108] [Comparative Example 1] A melt-kneaded mixture was prepared using polyvinylidene fluoride (PVDF) resin (KF-1000, manufactured by Kureha Corporation) as the thermoplastic resin, 32.9 mass% bis(2-ethylhexyl) adipate (DOA, boiling point 335°C) as the non-solvent, and 4.1 mass% acetyl tributyl citrate (ATBC, boiling point 343°C) as the poor solvent. A porous hollow fiber membrane was produced in the same manner as in Example 1, except that the discharge temperature was set to 240°C.

[0109] The membrane structure of the porous hollow fiber membrane of Comparative Example 1 was shown to have a three-dimensional mesh structure when photographed using an electron microscope as described below.

[0110] Comparative Example 2 Except for using only DIBA as the solvent and adjusting the amount of finely powdered silica, a porous hollow fiber membrane of Comparative Example 2 was obtained in the same manner as in Example 1. The membrane structure exhibited a spherulite structure.

[0111] The physical properties in the examples and comparative examples were determined by the following methods.

[0112] (1) Outer and inner diameters of the membrane The hollow fiber membrane was thinly sliced ​​with a razor blade, and the outer and inner diameters were measured using a 100x magnifying glass. Measurements were taken at 60 locations at 30mm intervals for each sample. The standard deviation and average value were calculated, and the coefficient of variation was determined as (standard deviation) / (average value).

[0113] (2) Electron microscope photography The porous hollow fiber membrane was cut into circular rings perpendicular to its length, stained with 10% phosphotungstic acid and osmium tetroxide, and embedded in epoxy resin. After trimming, the cross-sections were subjected to broad ion beam (BIB) processing to create smooth cross-sections, and then conductive treatment was performed to prepare microscopic specimens. Scanning electron microscope (SEM) images of the membrane cross-sections were taken at 5,000–10,000x magnification using a Hitachi SU8000 series electron microscope at an accelerating voltage of 1 kV. Four equally spaced locations were taken from the inner to outer surfaces of the membrane thickness (thick section) cross-section. Specifically, images were taken at designated fields within each of four fields (represented by circles 1–4 in Table 1): one including the inner surface of the membrane thickness (thick section) cross-section, one including the outer surface of the membrane, and two equally spaced fields between these two. The magnification can be changed depending on the average pore size, specifically, 5000x when the average pore size is 0.1 μm or more, 10,000x when the average pore size is 0.05 μm or more but less than 0.1 μm, and 30,000x when the average pore size is less than 0.05 μm. The field of view size was 2560 × 1920 pixels. Image processing was performed using ImageJ, and the captured SEM images were subjected to threshold processing (Image-Adjust-Threshold: Otsu method (Otsu selected)) to binarize the hole portion and the resin portion.

[0114] Surface open area ratio: The surface open area ratio was measured by calculating the ratio of resin parts to pore parts in the binarized image.

[0115] Resin area distribution: The size of the binarized granular resin parts included in the cross-sectional SEM image was measured using the "Analyze Particle" command in ImageJ (Analyze Particle: Size 0.10-Infinity). The total area of ​​all resin parts included in the SEM image was taken as ΣS, and the area was calculated as 1 μm 2 The area of ​​the following resin part is ΣS(≦1μm 2 ), then ΣS(≦1μm 2 ) / ΣS, 1μm 2 The area ratio of the resin part having the following area was calculated.2 The area of ​​the resin part above is ΣS(≧10μm 2 ), then ΣS(≧10μm 2 ) / ΣS, 10μm 2 The area ratio of the resin portion having the above area was calculated.

[0116] In addition, noise removal during binarization is performed at 0.1 μm 2 Resin parts with an area of ​​less than 0.1 μm are removed as noise. 2 The resin portion with the above area was analyzed. Noise removal was performed by applying median filter processing (Process-Filters-Median:Radius:3.0 pixels).

[0117] The granular resin parts cut off at the edges of the SEM image were also included in the measurement. No "include holes" processing was performed. No processing was performed to correct the shape from a "snowman" shape to a "flat" shape, etc.

[0118] Average pore size (pore diameter): Measured using the "Plugins-Bone J-Thickness" command in ImageJ. The spatial size was defined as the maximum circle size that could fit into the void.

[0119] (3) Tensile elongation at break (%) The load and displacement at tensile break were measured under the following conditions. According to the method of JIS K7161, the hollow fiber membrane was used as it was as a sample. Measuring equipment: Instron type tensile testing machine (Shimadzu AGS-5D) Distance between chucks: 5cm Pulling speed: 20 cm / min From the obtained results, the tensile elongation at break was calculated in accordance with JIS K7161.

[0120] <<Fabrication of membrane modules for membrane distillation>> For the membrane distillation membrane module, several hollow fiber membranes obtained by the above-mentioned membrane production method were cut into lengths of 15 cm and bundled together to form a membrane bundle, which was then housed in a housing. The membrane bundle for membrane distillation was adhered and fixed in the housing by centrifugal adhesion using a thermosetting epoxy resin (hardener: Sanmide 328 manufactured by Evonik, base resin: DEN431 manufactured by Olin) as the adhesive resin.

[0121] By the above operation, the effective length (the length of the part not buried in the adhesive resin) was 8 cm, and the total membrane area of ​​the inner surface of the membrane for membrane distillation was 30 cm. 2 A membrane distillation membrane module containing a hollow fiber membrane distillation membrane bundle was fabricated. Thereafter, a silicone resin (Silgel 612 manufactured by Wacker Asahi Kasei Silicone Co., Ltd.) was further laminated to a thickness of 5 mm on the adhesive fixing parts at both ends.

[0122] <Membrane distillation water production operation> The evaporation section and condensation section were connected so that the shortest distance (air gap) between the outer surface of the hydrophobic porous hollow fiber membrane in the evaporation section and the outer surface of the stainless steel tube in the condensation section was 30 mm, as shown in Figure 3. The outlet of the condensation section was connected to a water sampling container via piping, and as shown in Figure 3, a vacuum pump and vacuum control device were installed as the pressure reducing device and pressure regulator in Figure 3 from the water sampling container to adjust the pressure within the system.

[0123] Simulated seawater (3.5% by mass sodium chloride aqueous solution) at 65°C was passed through the hollow lumen of the hydrophobic porous hollow fiber membrane in the evaporation section at a flow rate of 600 mL / min, and cooling water at 30°C was passed through the lumen of the stainless steel tube in the condensation section at a flow rate of 600 mL / min to cool it. The pressure inside the module system was adjusted to 10 kPa with a vacuum pump, and membrane distillation was performed. Eight hours after the start of the experiment, the water collected in the water collection container was collected and the flux (kg / (m 2 After the operation was completed, the membrane was visually inspected to see if it was wet.

[0124] [Table 1]

[0125] In Table 1, circles 1 to 4 indicate the numbers of the four fields of view photographed sequentially at equal intervals from the inner surface to the outer surface of the membrane to be measured. Specifically, for example, circle 1 for the membrane of Example 1 represents a histogram of a cross-section of the innermost field of view of the porous hollow fiber membrane, and circle 4 for the membrane of Example 1 represents a histogram of a cross-section of the outermost field of view of the porous hollow fiber membrane. As shown in Table 1, in Examples 1 and 2, by mixing ETFE or ECTFE resin and a non-solvent into the membrane-forming solution during membrane production, a porous hollow fiber membrane with good pore openness, high connectivity from the inner surface to the outer surface of the membrane, and high membrane distillation performance can be provided.

[0126] On the other hand, the membrane distillation membrane of Comparative Example 1 was made of polyvinylidene fluoride, and was observed after 8 hours of operation and found to be wetting. The pore structure of Comparative Example 2 was a spherulite structure, and both the pore openness and interconnectivity were poor, indicating poor membrane distillation performance. [Industrial Applicability]

[0127] According to the present invention, a porous hollow fiber membrane is produced using a non-solvent, and therefore a porous hollow fiber membrane containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer is provided, which has good pore opening, chemical resistance, and mechanical strength. [Explanation of symbols]

[0128] 10. Housing 11 First housing side pipe 12 Second housing side pipe 20 Hollow fiber membrane for membrane distillation 30 Adhesive resin 100 Membrane distillation membrane module 200 Raw material liquid storage tank 300 Cooling water storage tank a Raw material liquid b. Solvent c Concentrate CW cooling water FM flow meter LG liquid level gauge P pump PG pressure gauge TC temperature controller

Claims

1. A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section of the inside of the film, ２ A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure in which the area ratio of resin portions having the following area is 70% or more of the total area of ​​all resin portions included in each of the fields of view.

2. A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section of the inside of the film, 10 μm included in each field of view ２ A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure, in which the area ratio of resin portions having an area of ​​15% or less to the total area of ​​all resin portions included in each of the fields of view.

3. A porous hollow fiber membrane for membrane distillation containing an ethylene-tetrafluoroethylene copolymer or an ethylene-chlorotrifluoroethylene copolymer, In all fields of view in the cross section of the inside of the film, ２ The area ratio of the resin portion having an area of ​​10 μm or less is 70% or more of the total area of ​​all resin portions included in each of the visual fields, and ２ A porous hollow fiber membrane for membrane distillation having a three-dimensional mesh structure, in which the area ratio of resin portions having an area of ​​15% or less to the total area of ​​all resin portions included in each of the fields of view.

4. The porous hollow fiber membrane for membrane distillation according to any one of claims 1 to 3, wherein the porous hollow fiber membrane for membrane distillation is used alone without being coated with a water repellent agent.

5. A membrane distillation membrane module using the porous hollow fiber membrane for membrane distillation according to any one of claims 1 to 3, The membrane module for membrane distillation is configured by adhesively fixing both ends of the porous hollow fiber membrane for membrane distillation in a housing, A membrane module for membrane distillation, wherein a hydrophobic adhesive is used to fix both ends of the membrane module.

6. The membrane module for membrane distillation according to claim 5 , wherein the hydrophobic adhesive is made of a silicone resin or a fluororesin.

7. A membrane distillation method for performing a membrane distillation operation using the porous hollow fiber membrane for membrane distillation according to any one of claims 1 to 3.

8. A membrane distillation method for performing a membrane distillation operation using the membrane module for membrane distillation according to claim 6.

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