Porous membrane for membrane distillation and membrane module for membrane distillation

Abstract

Provided is a porous membrane for membrane distillation in which: in all visual fields of a cross section of the inside of the porous membrane, the area proportion of resin portions having an area of 0.1-1 µm2 included in the respective visual fields is not less than 70% with respect to the total area of all the resin portions included in the visual fields; the total of the areas of resin portions having an area of more than 1 µm2 to less than 10 µm2 is 2-30% with respect to the total area of the resin portions; the area proportion of resin portions having an area of not less than 10 µm2 included in the visual fields is not more than 15% with respect to the total area of all the resin portions included in the visual fields; the porous membrane is formed of a resin that is a polymer of a thermoplastic resin or a mixture having said polymer; and a water repellent polymer that is different from the resin is adhered to the surface of the porous membrane.

Description

Porous membrane for membrane distillation and membrane module for membrane distillation

[0001] The present invention relates to a porous membrane for membrane distillation and a membrane module for membrane distillation including the same. More specifically, the present invention relates to a membrane module for membrane distillation capable of concentrating a raw material liquid containing a high concentration of an organic solvent.

[0002] There are many industrial processes, such as pharmaceutical and chemical manufacturing processes, where a raw material liquid that needs to be concentrated contains both water and an organic solvent as a solvent.

[0003] In particular, substances with amino acid sequences, such as peptides and proteins, are widely used as diagnostic and testing agents and pharmaceuticals. Because they are very expensive, it is important to recover them in high yields without denaturing them during the manufacturing process.

[0004] In recent years, research has focused on the organic solvent nanofiltration (OSN) method as a method for recovering solutes from solutions containing organic solvents without heating. The OSN method is a technique for separating components by sieving, and because it is a separation method that does not involve phase change, it is possible to reduce the energy load. Furthermore, methods that use membranes have the advantage of being easier to scale up than methods such as distillation (Non-Patent Document 1).

[0005] Known representative methods for concentrating and removing specific components include vacuum distillation, reverse osmosis (RO), and membrane distillation (MD).

[0006] In the vacuum distillation method, the solvent is removed by reducing the pressure of the raw material solution. However, this method requires heating the raw material solution to about 50°C, which may cause problems such as changes in the quality of the active ingredient in the raw material solution.

[0007] The RO method uses a membrane that allows solvents to permeate at the molecular level. The RO method concentrates a raw liquid by pressurizing the raw liquid to a predetermined high pressure, supplying it to a reverse osmosis (RO) membrane module, and allowing the raw liquid to permeate through the RO membrane to remove the solvent (typically water) in the raw liquid. However, because the RO method requires pressurizing the raw liquid, clogging of the RO membrane is likely to occur when applied to raw liquids containing a large amount of high molecular weight substances, etc. Therefore, the RO method may be unsuitable for application to raw liquids for, for example, pharmaceuticals, foods, beverages, etc. Furthermore, in the RO method, the osmotic pressure of the solvent (filtered solvent) in the concentrated raw liquid does not exceed the pressure of the high-pressure pump used for pressurization. Therefore, the concentration rate of the raw liquid using the RO method is limited by the capacity of the pump.

[0008] Membrane distillation (MD) is a technique for separating and concentrating a solvent from a raw material liquid using a vapor pressure difference as a driving force. In membrane distillation, the solvent vapor in the raw material liquid evaporated on the surface of a hydrophobic membrane diffuses and permeates through the membrane and condenses on the other membrane surface, thereby separating and concentrating the solvent from the raw material liquid. As a membrane distillation method, for example, the DCMD method (Direct Contact MD) is well known, in which the raw material liquid is brought into contact with cooling water having a lower temperature than the raw material liquid through a membrane, and the solvent vapor in the raw material liquid moves from the raw material liquid to the cooling water, thereby concentrating the raw material liquid.

[0009] Membrane distillation does not require heating or pressurization, so it can prevent the solutes in the raw liquid from deteriorating. Furthermore, because only vapor passes through the membrane, scaling, which is a concern with RO membranes, is also prevented, and it is expected that the desired concentration effect can be maintained for a long period of time.

[0010] Membrane distillation has attracted attention as a method for obtaining high-purity water because of its extremely high separation performance for non-volatile solutes such as salts.

[0011] In recent years, membrane distillation has been attracting attention not only for its use in fresh water production, but also as a method for concentrating specific components (solutes) present in raw liquid without heating or pressure, and as a method for removing specific components (volatile components) from raw liquid without heating or pressure.

[0012] In the pharmaceutical and food industries, organic components are often contained in raw materials, making it desirable to use membrane distillation to purify and concentrate organic-containing water. However, when the surface tension of the raw material is reduced by the organic components, the pores of the porous membrane used in membrane distillation become wet with the raw material. As a result, the raw material passes through the pores of the porous membrane and leaks from the surface in contact with the raw material to the other surface, a phenomenon known as wetting. The adhesion of water-insoluble components such as oils can also cause wetting. When wetting occurs, the raw material flows out of the membrane, preventing membrane distillation from functioning.

[0013] Patent Document 1 describes that the flux increases significantly when a hydrophobic porous membrane is used whose surface porosity and air permeability coefficient are equal to or greater than a certain value. However, when the membrane comes into contact with a raw material liquid having a low surface tension, a raw material liquid containing a surfactant, a raw material liquid containing a component that hydrophilizes the membrane, a raw material liquid containing a large amount of oil, or a raw material liquid containing an organic substance, wetting may occur.

[0014] Patent Document 2 describes that wetting can be suppressed by coating a hydrophobic porous hollow fiber membrane, in which the average pore size of the hydrophobic porous hollow fiber is 0.02 μm or more and 0.5 μm or less and the porosity is 60% or more and 90% or less, with a water repellent agent. However, hollow fiber membranes produced by the above method are prone to unevenness in the water repellent agent coating on the hollow fiber membrane surface, and wetting can occur in areas where the water repellent agent coating is insufficient when the hollow fiber membrane comes into contact with a raw material liquid containing a high concentration of organic components.

[0015] International Publication No. 2016 / 006670 International Publication No. 2021 / 070955

[0016] Journal of Membrane Science 2011, 381, 21-33

[0017] As described above, there is a demand for a method for stably removing organic components from a raw material solution containing high concentrations of organic components, but no method has yet been established that can concentrate the solute components in the raw material solution while maintaining their quality.

[0018] The present invention aims to provide a membrane module for membrane distillation that suppresses wetting of membrane distillation membranes, which is a problem in membrane distillation, and has an industrially feasible processing speed for raw material liquids containing high concentrations of organic components, even at room temperature.

[0019] The present invention has been made to achieve the above-mentioned object. The present inventors have discovered that a porous membrane having high flux and high hydrophobicity can be produced as a membrane distillation membrane, and further, by adjusting the concentration of a solvent contained in the porous membrane, a membrane distillation membrane having both high flux and wetting resistance can be obtained, and have thus achieved the present invention.

[0020] The present invention is as follows: <<Aspect 1>> A porous membrane for membrane distillation, wherein in all visual fields in a cross section of the inside of the porous membrane for membrane distillation, a 0.1 μm area included in each visual field ２ 1 μm or more ２ The area ratio of the resin portion having an area of ​​1 μm or less is 70% or more of the total area of ​​all resin portions included in each of the fields of view, and ２ Super 10μm ２ the area ratio of the resin portion having an area of ​​less than 10 μm contained in each of the fields of view is 2% or more and 30% or less of the total area of ​​all the resin portions contained in each of the fields of view; ２A membrane distillation porous membrane, wherein the area ratio of resin portions having an area of ​​at least 10 μg / g is 15% or less of the total area of ​​all resin portions included in each field of view, and the membrane distillation porous membrane is composed of a resin that is a thermoplastic polymer or a mixture of a plurality of such polymers, and a water repellent polymer different from the resin is adhered to the surface of the membrane distillation porous membrane. Aspect 2: The membrane distillation porous membrane according to Aspect 1, wherein the content of a solvent contained in the membrane distillation porous membrane is 10 μg / g or less. Aspect 3: The membrane distillation porous membrane according to Aspect 2, wherein the solvent contained in the membrane distillation porous membrane is at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearic acid esters, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oil. Aspect 4: The porous membrane for membrane distillation according to any one of Aspects 1 to 3, wherein the water repellent polymer is a polymer having a fluorine atom-containing group in a side chain, and the side chain contains at least one selected from a fluoroalkyl group or a perfluoroalkyl group, a fluoropolyether group or a perfluoropolyether group, a fluoroalkylsilyl group, and a fluorosilyl group. Aspect 5: The porous membrane for membrane distillation according to any one of Aspects 1 to 4, wherein the porous membrane for membrane distillation is a porous hollow fiber membrane for membrane distillation. Aspect 6: A membrane distillation membrane module comprising the porous membrane for membrane distillation according to any one of Aspects 1 to 5, wherein both ends of the porous membrane for membrane distillation are adhesively fixed with a thermosetting resin. Aspect 7: A method for producing a membrane distillation membrane module according to Aspect 6, comprising a step of immersing the porous membrane for membrane distillation or the membrane distillation membrane module in a solution containing an alcohol. Aspect 8: The method for producing a membrane distillation membrane module according to Aspect 7, wherein the alcohol contains at least one selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol.Aspect 9: A feedstock liquid concentration system using a membrane distillation method in which a feedstock liquid containing a solvent and a solute is contacted with cooling water via the porous membrane for membrane distillation according to any one of Aspects 1 to 5, and the solvent in the feedstock liquid is passed through the porous membrane for membrane distillation in a vapor state to transfer to the cooling water side. Aspect 10: A feedstock liquid concentration system using a membrane distillation method in which a feedstock liquid containing a solvent and a solute is contacted with cooling water via the membrane distillation membrane module according to Aspect 6, and the solvent in the feedstock liquid is passed through the porous membrane for membrane distillation in a vapor state to transfer to the cooling water side. Aspect 11: A method for concentrating a feedstock liquid using the membrane distillation membrane module according to Aspect 6. Aspect 12: A method for concentrating a feedstock liquid using the feedstock liquid concentration system according to Aspect 9 or 10. Aspect 13: A method for removing an organic solvent from a feedstock liquid containing an organic solvent, using the membrane module for membrane distillation according to Aspect 6.

[0021] According to the present invention, it is possible to provide a membrane module for membrane distillation that combines high flux and wetting resistance, and thus it is possible to treat a raw material liquid containing a high concentration of organic components at an industrially feasible rate even at room temperature.

[0022] FIG. 1 is a schematic diagram showing an example of a membrane module for membrane distillation according to one embodiment of the present invention. FIG. 2 is a histogram showing the measurement results of the area distribution of the resin portion in the cross section of the porous hollow fiber membrane of Example 1. FIG. 3 is an example of a cross-sectional SEM image of the porous hollow fiber membrane used in one embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of a method for immersing a porous hollow fiber membrane in alcohol. FIG. 5 is a schematic diagram showing an example of a method for passing alcohol (single pass). FIG. 6 is a schematic diagram showing an example of a method for passing alcohol (circulation). FIG. 7 is a schematic diagram showing an apparatus for evaluating the leak rate of 50% by mass ethanol (EtOH). FIG. 8 is a schematic diagram showing a steam flux measurement apparatus.

[0023] Preferred embodiments of the present invention will now be described in detail by way of non-limiting examples.

[0024] The feedstock liquid concentration system of this embodiment is a feedstock liquid concentration system that uses a membrane distillation method in which a feedstock liquid containing a solvent and a solute is contacted with cooling water via a membrane distillation membrane (e.g., a porous membrane distillation membrane) or a membrane module including the same (e.g., a membrane distillation membrane module), and the solvent in the feedstock liquid is passed through the membrane distillation membrane in a vapor state and transferred to the cooling water side. In addition, a method for concentrating a feedstock liquid using the feedstock liquid concentration system is also an aspect of the present disclosure.

[0025] The raw material liquid concentration system of this embodiment will be described below with reference to FIG.

[0026] FIG. 1 shows an example of a membrane module for membrane distillation that is preferably used when concentrating a raw material liquid by membrane distillation in the concentration system of the present invention.

[0027] The membrane distillation membrane module 100 shown in Fig. 1 contains a plurality of hollow fiber membranes 20 for membrane distillation housed in a housing 10, and both ends of the membranes are bonded and fixed with an adhesive resin 30. Both ends of the hollow fiber membranes 20 for membrane distillation are open and not closed. Each membrane is preferably hollow fiber-shaped.

[0028] 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 the cooling water CW, which allow the cooling water CW to circulate in the external space of the hollow fiber membrane for membrane distillation 20. In Figure 1, the first and second housing side pipes (11, 12) are referred to as "cooling water inlet / outlet".

[0029] The housing 10 has a left opening (a) for introducing the raw material liquid and a right opening (b) for discharging the raw material liquid at both ends in the axial direction (left-right direction in FIG. 1 ), which allow the raw material liquid to flow through the hollow part of the membrane distillation hollow fiber membrane 20. In FIG. 1 , the left opening and the right opening are referred to as “raw material liquid inlet / outlet.”

[0030] The interior of the membrane distillation membrane 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 predetermined solvent can pass through the outer wall of the membrane distillation membrane.

[0031] <Membrane distillation membrane> The membrane distillation membrane is, for example, in the form of a hollow fiber, and its surface must be highly hydrophobic, porous, yet not allow liquid to penetrate inside, and only allow gas to pass through the membrane thickness. In addition, since high vapor permeability is required even at room temperature, it is necessary to adjust the surface porosity and average pore size within appropriate ranges.

[0032] To concentrate a raw material liquid by membrane distillation, the raw material liquid is passed through one side of the membrane distillation membrane (in FIG. 1, the hollow space of the hollow fiber membrane distillation membrane 20), while cooling water, which is at a lower temperature than the raw material liquid, is passed through the other side (in FIG. 1, the external space side of the hollow fiber membrane distillation membrane 20). The pores communicating with both spaces inside the membrane wall come into contact with the raw material liquid and cooling water through the membrane wall. As a result, the vapor generated on the raw material liquid side, which has a high vapor pressure, passes through the pores in the membrane wall and moves to the cooling water, which has a low vapor pressure, where it is cooled and liquefied, thereby concentrating the raw material liquid.

[0033] As the membrane for membrane distillation used in this embodiment, it is preferable to use a porous membrane.

[0034] Porous membranes have pores that are interconnected through the membrane from one surface to the other. The pores may be voids in the network of the membrane material (e.g., polymer) and may be branched or continuous. The pores may be permeable to vapor but not liquid.

[0035] <Porous membrane> The shape of the porous membrane for membrane distillation used in this embodiment will be described later. For example, when observing and photographing the cross section of the membrane using a scanning electron microscope (SEM) or the like, the following relationship is satisfied in all visual fields in the cross section inside the porous membrane for membrane distillation: (i) 0.1 μm ２ 1 μm or more ２(ii) The area ratio of the resin portion having an area of ​​1 μm or less in each of the fields of view is 70% or more with respect to the total area of ​​all the resin portions included in each of the fields of view; ２ Super 10μm ２ (iii) the total area of ​​the resin portions having an area of ​​less than 10 μm is 2% or more and 30% or less of the total area of ​​all resin portions included in each of the fields of view; and ２ The area ratio of the resin portion having an area equal to or greater than 15% of the total area of ​​all resin portions included in each of the fields of view is 15% or less.

[0036] Without wishing to be bound by theory, it is possible that the porous membrane may have interconnected pores formed inside it that are bent from the inside to the outside of the membrane. In each of the four fields of view, which are a field including the inner surface of the membrane, a field including the outer surface of the membrane, and two fields of view photographed at equal intervals between these fields, the distance between the inner surface and the outer surface of the membrane is 1 μm so as to satisfy the relationship (i) above. ２ If the sum of the areas of the resin portions having the following areas is 70% or more of the total area of ​​the resin portions, the pore interconnectivity is high (i.e., the proportion of interconnected pores within the film is high). The higher the pore interconnectivity, the higher the vapor flux of the raw material liquid. Furthermore, a film with high pore interconnectivity is less likely to cause aggregation in the thick film portion, and therefore has high wetting resistance.

[0037] On the other hand, if the pores have low interconnectivity, they may become blocked midway through the thick part, and depending on the operating conditions, steam may condense in the blocked part, preventing sufficient flux from being obtained. Also, wetting may occur from the part where steam condenses in the blocked part. However, ２ If the ratio of the total area of ​​the resin portions having an area of ​​1 μm or less is too high, the dendritic skeleton portion of the three-dimensional network structure made of the resin may become too thin. ２ The sum of the areas of the resin portions having the following areas is 70% or more of the total area of ​​the resin portions, and the above relationships (ii) and (iii) are satisfied, and 1 μm ２The total area of ​​the resin parts having an area of ​​more than 10 μm is 2% or more and 30% or less of the total area of ​​the resin parts included in each of the above fields of view, ２ From this viewpoint, the total area of ​​the resin portions having an area of ​​1 μm or less is 15% or less of the total area of ​​the resin portions. ２ Super 10μm ２ It is preferable that 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 included in each of the above fields of view, and / or ２ It is 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 included in each of the above fields of view. ２ If the total area of ​​the resin parts having an area of ​​more than 2% and less than 30% of the total area of ​​the resin parts, the dendritic skeleton part of the three-dimensional mesh structure composed of the resin will not be too thin, and the strength of the porous membrane can be appropriately maintained.

[0038] The method for measuring the area distribution of the resin portion in the cross section of each field of view of the porous membrane for membrane distillation is as described in the examples below, and may be the same as the method for measuring the area distribution of the resin portion in the cross section of each field of view of the porous hollow fiber membrane described below.

[0039] <Porous hollow fiber membrane for membrane distillation> When the porous membrane for membrane distillation used in this embodiment is a hollow fiber membrane, in an SEM image of a membrane cross section in the thickness direction perpendicular to the inner surface of the hollow fiber membrane, in each region of a total of four visual fields, including a visual field including the inner surface, a visual field including the outer surface of the porous hollow fiber membrane, and two visual fields photographed at equal intervals between these visual fields, the thickness is within the following range: 1 μm ２ The sum of the areas of the resin parts having the following areas 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 areas is 70% or more of the total area of ​​the resin parts, and ２The 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 ２ Super 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 The total area of ​​the resin portions having the above area is 15% or less of the total area of ​​the resin portions.

[0040] The porous hollow fiber membrane has a structure in which interconnected pores are formed inside the membrane, which are bent from the inside to the outside. In an SEM image of a 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 photographed at equal intervals between these fields, a total of four fields, are shown. ２ If the sum of the areas of the resin portions having the following areas is 70% or more of the total area of ​​the resin portions, the pore interconnectivity is high (i.e., the proportion of interconnected pores within the film is high). The higher the pore interconnectivity, the higher the vapor flux of the raw material liquid. Furthermore, a film with high pore interconnectivity is less likely to cause aggregation in the thick film portion, and therefore has high wetting resistance.

[0041] On the other hand, if the pores of the porous hollow fiber membrane have low interconnectivity, the pores may be blocked midway through the membrane thickness, and depending on the operating conditions, steam may condense in the blocked areas, resulting in insufficient flux. Furthermore, wetting may occur from the blocked areas where steam has condensed. However, if the pores of the porous hollow fiber membrane have low interconnectivity, the pores may be blocked midway through the membrane thickness, resulting in insufficient flux. ２ If the ratio of the total area of ​​the resin portion having an area of ​​1 μm or less is too high, the dendritic skeleton portion of the three-dimensional network structure composed of the resin will be too thin. ２ The total area of ​​the resin parts having the following areas is maintained at 70% or more of the total area of ​​the resin parts, ２ The total area of ​​the resin portions having an area of ​​more than 10 μm is preferably 2% or more and 30% or less of the total area of ​​the resin portions,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 Super 10μm ２ The total area of ​​the resin portions having an area of ​​less than 10 μm is 15% or less of the total area of ​​the resin portions, and ２ It is more preferable that the total area of ​​the resin portions having the above area is 2% or more and 15% or less of the total area of ​​the resin portions. ２ If the total area of ​​the resin parts having an area of ​​more than 2% and less than 30% of the total area of ​​the resin parts, the dendritic skeleton part of the three-dimensional mesh structure composed of the resin will not be too thin, and the strength of the porous hollow fiber membrane can be appropriately maintained.

[0042] Fig. 2 is a histogram showing the results of measuring the area distribution of the resin portion by image analysis of cross-sectional views of each field of view of the porous hollow fiber membrane shown in Fig. 3. Fig. 2 shows the measurement results of the porous hollow fiber membrane of Example 1 described later.

[0043] In the cross-sectional view of the porous hollow fiber membrane photographed as shown in Figure 3, the resin portions appear as granules. Figure 2 shows a histogram in which the area of ​​each of these granular resin portions was measured and the area ratio of each granular resin portion to the total area of ​​all resin portions in the cross-sectional view was plotted. Circles 1 to 4 in Figure 2 indicate the numbers of the four fields of view photographed sequentially at equal intervals from the inner surface to the outer surface of the porous hollow fiber membrane. Specifically, for example, circle 1 in Example 1 represents a histogram of a cross-sectional view photographed from the field of view closest to the inner surface of the porous hollow fiber membrane of Example 1, and circle 4 in Example 1 represents a histogram of a cross-sectional view photographed from the field of view closest to the outer surface of the porous hollow fiber membrane of Example 1.

[0044] The method for measuring the area distribution of the resin portion in the cross section of each field of view of the porous hollow fiber membrane will be explained in the examples below.

[0045] The surface porosity (surface porosity) of the porous hollow fiber membrane of the membrane distillation membrane is preferably 25 to 60%, more preferably 25 to 50%, and even more preferably 25 to 45%. By using a membrane having an opening ratio of 25% or more on the surface that comes into contact with the liquid to be treated for membrane distillation, a membrane distillation membrane with high flux can be obtained. On the other hand, from the viewpoint of achieving both anti-wetting properties, the average pore size of the pores on the outer surface of the porous hollow fiber membrane according to this embodiment is preferably 10 nm to 700 nm, more preferably 20 nm to 600 nm. Methods for measuring the surface porosity and the average pore size will be described in the examples below.

[0046] The thickness of the porous hollow fiber membrane for membrane distillation is preferably 80 to 1,000 μm, more preferably 100 to 300 μm. When the thickness of the porous hollow fiber membrane for membrane distillation is 80 μm or more, the strength is increased, and when the thickness is 1,000 μm or less, the pressure loss due to membrane resistance is reduced.

[0047] The porosity of the porous hollow fiber membrane for membrane distillation is preferably 50 to 80%, more preferably 55 to 65%. A porosity of 50% or more increases the vapor flux, while a porosity of 80% or less increases the mechanical strength.

[0048] Furthermore, the porous hollow fiber membrane used in the membrane distillation of this embodiment preferably has a three-dimensional mesh structure rather than a spherulite structure, which can improve the interconnectivity of pores formed from the inner surface to the outer surface of the porous hollow fiber membrane.

[0049] <Material (quality) of porous membrane> The resin constituting the porous membrane is not limited, but from the viewpoint of achieving both high flux and wetting resistance, it is preferably a thermoplastic resin (in one embodiment, a thermoplastic polymer), and more preferably is composed of a thermoplastic polymer or a resin that is a mixture of multiple types of such polymers.

[0050] Examples of thermoplastic resins include fluororesins, polyolefins, copolymers of olefins and halogenated olefins, halogenated polyolefins, and mixtures thereof. Examples of fluororesins include those selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures of these resins. Other thermoplastic resins include polyethylene, polypropylene, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride (which may contain hexafluoropropylene domains), and mixtures thereof.

[0051] These materials are excellent as membrane materials because they are thermoplastic and therefore easy to handle and strong. Among them, vinylidene fluoride resin, tetrafluoroethylene resin, hexafluoropropylene resin, or mixtures thereof, homopolymers and copolymers of ethylene, tetrafluoroethylene, and chlorotrifluoroethylene, or mixtures of the above homopolymers or copolymers are preferred because they have excellent mechanical strength, chemical strength (chemical resistance), and good moldability. More specific examples include fluororesins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer.

[0052] The membrane distillation membrane of this embodiment may have a hydrophobic polymer attached to at least a portion of the membrane distillation membrane to improve its hydrophobicity. The hydrophobic polymer can form a hydrophobic coating on at least one surface of the membrane distillation membrane or on the inside of the membrane to improve its hydrophobicity.

[0053] The hydrophobic polymer can be applied to the membrane distillation membrane in the form of a solution dissolved in a suitable solvent such as water or a fluorinated solvent, and then the solvent is evaporated to adhere the hydrophobic polymer to the surface or the interior, or both, of the membrane. The application of the hydrophobic polymer solution may be carried out by an appropriate method such as spraying or immersion.

[0054] As used herein, the term "hydrophobic polymer" refers to a polymer that has low affinity for water, and may be, for example, a polymer having a hydrophobic structure. The hydrophobic polymer may be different from the thermoplastic resin described above. Examples of the hydrophobic structure include a non-polar group or a low-polarity group, a non-polar skeleton or a low-polarity skeleton, etc. Examples of the non-polar group or low-polarity group include a hydrocarbon group, a fluorine-containing group, etc., and examples of the non-polar skeleton or low-polarity skeleton include a hydrocarbon main chain, a siloxane main chain, etc.

[0055] The hydrophobic polymer is preferably a water repellent polymer, and examples thereof include polymers having a siloxane bond and fluorine atom-containing polymers, and more specifically, examples thereof include the following: (a) As polymers having a siloxane bond, for example, dimethyl silicone gel, methylphenyl silicone gel, reactive modified silicone gels having organic functional groups (amino groups, fluoroalkyl groups, etc.), silicone polymers that form crosslinked structures by reacting with silane coupling agents, and polymer gels that are crosslinked products thereof; (b) As fluorine atom-containing polymers, polymers having fluorine atom-containing groups in their side chains, where the fluorine atom-containing groups are (per)fluoroalkyl groups, (per)fluoropolyether groups, fluoroalkylsilyl groups, fluorosilyl groups, etc.

[0056] In particular, the hydrophobic polymer is preferably a polymer of a (meth)acrylate monomer and / or a vinyl monomer having a (per)fluoroalkyl group and / or a (per)fluoropolyether group having 1 to 12 carbon atoms.

[0057] The hydrophobicity of a membrane distillation membrane can generally be determined by measuring the liquid intrusion pressure of water. However, when handling raw materials containing organic solvents, surfactants, etc., membrane distillation membranes are required to have high hydrophobicity against solutions containing organic solvents. Therefore, the liquid intrusion pressure of water alone cannot accurately evaluate wetting resistance to organic solvents. Therefore, the membrane distillation membrane module of this embodiment evaluates wetting resistance to organic solvents by measuring the leakage rate of an aqueous solution containing an organic solvent when pressurized at a predetermined pressure using the aqueous solution. Specifically, a 50% by mass aqueous ethanol solution is sealed inside the hollow fiber membrane of the membrane distillation membrane module, and the leakage rate of the aqueous ethanol solution leaking to the outside of the membrane is measured when pressurized from the inside of the hollow fiber membrane at 60 kPa. The leakage rate is preferably 0.001 LMH to 1.5 LMH or 0.001 LMH to 1 LMH, more preferably 0.005 LMH to 1.0 LMH or 0.005 LMH to 0.9 LMH, and even more preferably 0.005 LMH to 0.8 LMH. The smaller the leakage rate, the higher the resistance to wetting of the raw material liquid containing an organic solvent. On the other hand, to reduce the leakage rate, it is necessary to use a membrane with a small maximum pore size and low porosity. However, membranes with small maximum pore size and low porosity have low vapor flux and therefore take a long time to concentrate. Therefore, in order to maintain a constant vapor flux, it is preferable that the leakage rate of the ethanol aqueous solution is not too small. A specific evaluation device and evaluation method for the leakage rate of a 50% by mass ethanol aqueous solution from a membrane distillation membrane module will be described in the Examples below with reference to FIG. 7.

[0058] Examples of solvents that can be contained in the porous membrane for membrane distillation according to this embodiment include various esters such as phthalate esters, sebacate esters, citrate esters, acetyl citrate esters (in one embodiment, acetyl tributyl citrate, abbreviated as ATBC), adipate esters (in one embodiment, bis 2-ethylhexyl adipate, abbreviated as DOA), trimellitate esters, oleate esters, palmitate esters, stearic acid esters, and phosphate esters; fatty acids having 6 to 30 carbon atoms; and epoxidized vegetable oils.

[0059] For example, in the case of a porous hollow fiber membrane, the solvent used during production is included, and as described below, the porous hollow fiber membrane includes a first solvent (hereinafter also referred to as a non-solvent) or a second solvent (hereinafter also referred to as a good solvent or a poor solvent) used as a solvent during production, or both.

[0060] The first solvent is at least one selected from the group consisting of sebacic acid esters, citrate esters, acetyl citrate esters (e.g., acetyl tributyl citrate, abbreviated as ATBC), adipate esters (e.g., bis(2-ethylhexyl) adipate, abbreviated as DOA), trimellitate esters, oleic acid esters, palmitic acid esters, stearic acid esters, phosphate esters, phthalic acid esters (e.g., bis(2-ethylhexyl) phthalate, abbreviated as DOP and dibutyl phthalate, abbreviated as DBP), fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

[0061] The second solvent is at least one selected from sebacic acid esters, citrate esters, acetyl citrate esters (e.g., acetyl tributyl citrate, abbreviated as ATBC), adipic acid esters (e.g., bis(2-ethylhexyl) adipate, abbreviated as DOA), trimellitic acid esters, oleic acid esters, palmitic acid esters, stearic acid esters, phosphate esters, phthalic acid esters (e.g., bis(2-ethylhexyl) phthalate, abbreviated as DOP and dibutyl phthalate, abbreviated as DBP), fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils, and is a different solvent from the first solvent. Examples of fatty acids having 6 to 30 carbon atoms include capric acid, lauric acid, and oleic acid. Examples of epoxidized vegetable oils include epoxy soybean oil and epoxidized linseed oil.

[0062] The content of the solvent contained in the porous membrane for membrane distillation is calculated by the analytical method described in the Examples. The solvent weight calculated by the analytical method described below is defined as the content of the solvent contained in the porous membrane for membrane distillation according to this embodiment. It should be noted that the porous membrane for membrane distillation may contain components (impurities, etc.) other than the thermoplastic resin described above. Examples of components such as impurities include solvents. From the viewpoint of achieving the desired effect, the content of the solvent in the porous membrane for membrane distillation according to this embodiment is preferably adjusted to 100 μg / g or less, more preferably adjusted to 50 μg / g or less, even more preferably adjusted to 10 μg / g or less, particularly preferably adjusted to 9 μg / g or less, extremely preferably adjusted to 8 μg / g or less, and most preferably adjusted to 5 μg / g or less. The lower limit of the content of the solvent in the porous membrane for membrane distillation according to this embodiment may be adjusted to, for example, 0.01 μg / g or more.

[0063] The content of the first solvent or the second solvent contained in the porous hollow fiber membrane is preferably as low as possible from the viewpoint of hydrophobicity. If the content of the first solvent or the second solvent contained in the porous hollow fiber membrane is high, the hydrophobicity of the membrane decreases, and the anti-wetting properties decrease. Therefore, an effective means for adjusting the content of the first solvent or the second solvent in the membrane is to immerse the porous hollow fiber membrane in a solvent in which the first solvent or the second solvent is soluble before applying the water repellent polymer.

[0064] The solvent in which the porous hollow fiber membrane is immersed is not particularly limited, but is preferably a solvent in which the first solvent and the second solvent are soluble and in which the porous hollow fiber membrane is insoluble.

[0065] Examples of solvents that satisfy the above conditions include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-2-propanol, ethylene glycol, and glycerin. Among these, it is preferable to use at least one selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol. The alcohols may be used alone or in combination.

[0066] The method for immersing a porous membrane for membrane distillation, such as a porous hollow fiber membrane, in alcohol is not particularly limited, and examples thereof include a method in which the porous hollow fiber membrane is immersed in a container containing alcohol as shown in FIG. 4, a method in which alcohol is passed through a porous hollow fiber membrane or a membrane module in a single pass as shown in FIG. 5, and a method in which alcohol is circulated and passed through a porous hollow fiber membrane or a membrane module as shown in FIG. 6.

[0067] If the content of the first solvent or the second solvent contained in the membrane distillation membrane used in this embodiment is high, the water repellency of the membrane distillation membrane will decrease and the anti-wetting properties will decrease. Therefore, by reducing the content of the solvent in the membrane, the water repellency of the membrane distillation membrane can be further improved. The content of the first solvent or the second solvent contained in the membrane distillation membrane is preferably 100 μg / g or less, more preferably 50 μg / g or less, even more preferably 10 μg / g or less, even more preferably 9 μg / g or less, particularly preferably 8 μg / g or less, and most preferably 5 μg / g or less. The content of the first solvent or the second solvent contained in the membrane distillation membrane may be zero.

[0068] On the other hand, if the content of the first solvent or the second solvent contained in the porous membrane for membrane distillation is too small, the water repellent coating on the membrane distillation membrane is likely to be uneven, resulting in a decrease in wetting resistance. Therefore, the content of the first solvent or the second solvent contained in the porous membrane for membrane distillation is preferably 0.01 μg / g or more, more preferably 0.015 μg / g or more.

[0069] <Shape and size of membrane distillation membrane> The shape of the membrane distillation membrane may be any shape, such as a flat membrane, a hollow fiber membrane, a tubular membrane, etc. When a flat membrane is used, the shape of the membrane module can be selected from spiral, pleated, laminated, etc. When the membrane distillation membrane is flat, the membrane is sealed in a bag shape and a spacer or the like is inserted as appropriate so that the raw material liquid and the cooling liquid do not mix and a flow path is secured.

[0070] When the membrane for membrane distillation is a hollow fiber membrane, the outer diameter is, for example, 300 μm or more and 5,000 μm or less, preferably 350 μm or more and 4,000 μm or less, and the inner diameter of the hollow fiber membrane is, for example, 200 μm or more and 4,000 μm or less, preferably 250 μm or more and 3,000 μm or less.

[0071] <Membrane distillation membrane module and its manufacturing method> In this embodiment, when the membrane distillation membrane is hollow fiber shaped, a membrane bundle formed by bundling a plurality of hollow fiber membrane distillation membranes may be used in the form of a membrane distillation membrane module packed in an appropriate module.

[0072] The module may have any shape, such as a cylinder, a polygonal prism, or other polyhedron. Preferably, the module has a structure in which a hollow fiber membrane bundle is housed in a cylindrical or polygonal prism housing so that the longitudinal direction of the hollow fibers coincides with the axial direction of the housing, and both ends of the hollow fiber bundle are fixed to the inside of the housing with an appropriate adhesive resin. In this case, it is preferable to fix the hollow fiber bundle with the adhesive resin in a liquid-tight manner so that the flow paths inside and outside the hollow fiber membranes do not mix.

[0073] The adhesive resin is preferably a resin having high mechanical strength and high resistance to organic solvents. Examples of the adhesive resin include thermosetting epoxy resins and thermosetting urethane resins. Epoxy resins are preferred from the viewpoint of organic solvent resistance.

[0074] The adhesive fixing method may be any known adhesive method related to the production of membrane modules for membrane distillation.

[0075] The construction of the housing is selected primarily from the viewpoint of organic solvent resistance, which prevents performance degradation due to solutes and solvents contained in the raw material liquid and cooling water, as well as pressure resistance, heat resistance, impact resistance, weather resistance, etc. For example, resin, metal, etc. can be used for the housing. From the above viewpoints, the construction material of the housing is preferably selected from synthetic resins such as polypropylene, polysulfone, polyethersulfone, polyvinylidene fluoride, ABS resin, fiber-reinforced plastic, and polyvinyl chloride resin, and metals such as stainless steel, brass, and titanium.

[0076] The method for producing a membrane distillation membrane module according to this embodiment is not limited to the following steps, but may include, for example, the following steps: forming a porous membrane for membrane distillation from a solvent other than the thermoplastic resin and thermosetting resin described above; forming a membrane bundle from a plurality of porous membranes for membrane distillation; storing the membrane bundle in the housing described above and bonding and fixing it with a thermosetting resin to form a membrane module; optionally, immersing the porous membrane for membrane distillation, membrane bundle, or membrane module in a solution containing the alcohol described above; attaching the hydrophobic polymer or water repellent polymer described above to the porous membrane for membrane distillation, membrane bundle, or membrane module; optionally, drying the hydrophobic polymer or water repellent polymer; The order of these steps can be arbitrarily changed. To obtain a desired distribution of the hydrophobic polymer or water repellent polymer and a desired solvent content or residual solvent amount, the coating location, the volatility (boiling point) of the solvent in the coating liquid, the concentration of the hydrophobic polymer or water repellent polymer in the coating liquid, the drying conditions after coating, and the alcohol immersion conditions can be appropriately adjusted. The alcohol immersion conditions for the porous hollow fiber membrane are the same as those described above.

[0077] In membrane distillation, the vapor flux from the raw material liquid to the cooling water can be determined by the method described in the Examples.

[0078] Furthermore, a method for concentrating a feedstock liquid using the membrane distillation membrane module described above is also an aspect of the present disclosure. Furthermore, in another embodiment of the present disclosure, a method for removing an organic solvent from a feedstock liquid containing an organic solvent is provided using the membrane distillation membrane module described above.

[0079] In this example, various physical properties of the hollow fiber membrane for membrane distillation were determined by the various measurement methods described below.

[0080] [Outer and inner diameters of hollow fiber membranes] The porous hollow fiber membranes were thinly sliced ​​using a razor, and the outer and inner diameters were measured using a 100x magnification. Measurements were taken at 60 points at 30 mm intervals for each sample, and the average values ​​were used as the outer and inner diameters of the membrane.

[0081] [Electron Microscopy] 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-section of the sample was subjected to broad ion beam (BIB) processing to create a smooth cross-section, and then subjected to a conductive treatment to prepare a microscopic specimen. Scanning electron microscope (SEM) images of the prepared microscopic specimen were taken at 5,000 to 10,000 magnifications at an accelerating voltage of 1 kV from the inner surface to the outer surface of the membrane thickness (thick portion) cross-section at four equally spaced locations. Specifically, images were taken at predetermined fields within each of four fields: one including the inner surface of the membrane thickness (thick portion) cross-section, one including the outer surface of the membrane, and two fields of view equally spaced between these two fields. Measurements can be performed at different magnifications depending on the average pore size. Specifically, when the average pore size is 0.1 μm or more, the magnification is 5000x; when the average pore size is 0.05 μm or more but less than 0.1 μm, the magnification is 10,000x; and when the average pore size is less than 0.05 μm, the magnification is 30,000x. The field of view size was 2560 x 1920 pixels. Image processing was performed using ImageJ, and the captured SEM images were subjected to threshold processing (Image-Adjust-Threshold: Otsu method (select Otsu)) to binarize the pore portion and the resin portion.

[0082] (Surface porosity of hollow fiber membrane) Surface porosity: The surface porosity was measured by calculating the ratio of resin parts to pore parts in the binarized image taken with the electron microscope. The detailed procedure is described below. An example of a cross-sectional view of a porous hollow fiber membrane used in one embodiment of the present invention is shown in Figure 3.

[0083] (Area Distribution of Resin Portions) Area distribution of resin portions: Using the "Analyze Particle" command of ImageJ (Analyze Particle: Size 0.10-Infinity), the size of each binarized granular resin portion contained in the SEM image of the cross section was measured.

[0084] 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. ２Resin portions with an area of ​​0.1 μm or more were analyzed. Noise removal was performed by median filtering (Process-Filters-Median: Radius: 3.0 pixels). Therefore, when the total area of ​​all resin portions included in the SEM image is ΣS, for example, 0.1 μm ２ 1 μm or more ２ The area ratio of the resin portion having the following area is ΣS (0.1<S≦1 μm ２ ) / ΣS; 1 μm ２ Super 10μm ２ The area ratio of the resin portion having an area of ​​less than ΣS (1 < S < 10 μm ２ ) / ΣS; and 10 μm ２ The area ratio of the resin portion having an area of ​​10 μm or more is defined as ΣS (≧10 μm ２ ) / ΣS, respectively.

[0085] The granular resin parts cut off at the edges of the SEM image were also measured. No "Include Holes" processing was performed. No processing was performed to correct the shape from a "snowman" shape to a "flat" shape, etc.

[0086] (Average pore size) Average pore size (pore size): The average pore size (pore size) was measured using the "Plugins-Bone J-Thickness" command of ImageJ. The space size was defined as the maximum circle size that could fit into the gap.

[0087] [Solvent Content in Hollow Fiber Membrane] The solvent content in the hollow fiber membrane of the membrane module obtained in the Examples and Comparative Examples was calculated using the following method. A hollow fiber membrane was cut out from the membrane module, and approximately 1 g was freeze-pulverized and extracted for 20 hours with 100 mL of a mixture of (ethanol: 1-propanol: 2-propanol = 85.5% by mass: 9.6% by mass: 4.9% by mass). The extract was filtered through a glass filter, and the filtrate was evaporated under reduced pressure and dried in vacuo. The filtration residue was then redissolved in the mixture to prepare a sample. The solvent in the sample was quantified using an LC-MS system. The LC system used was an ACQUITY UPLC I-Class manufactured by Nihon Waters K.K., and the MS system used was a microTOF QIII manufactured by Bruker Japan Co., Ltd. The measurement conditions were a photodiode array detector (scan range 195 nm to 600 nm) and a mass spectrometer. The column used was an ACQUITY UPLC BEH-Phenyl (internal diameter 2.1 mm, length 100 mm, particle size 1.7 μm, manufactured by Nihon Waters K.K.). The column temperature was kept constant at around 50°C, and the mobile phase consisted of 0.1% formic acid aqueous solution and 0.1% formic acid acetonitrile solution. The mobile phase was delivered at 0.25 mL / min, with the mobile phase mixture ratio changing from 95% to 0% for 0.1% formic acid aqueous solution and from 5% to 100% for 0.1% formic acid acetonitrile solution over 20 minutes. The mass spectrometer used electrospray ionization, and measurements were performed in positive ion mode. DataAnalysis software (manufactured by Bruker Japan Co., Ltd.) was used for analysis. Quantitation of the solvent in the extract was performed using the external standard method. LC / MS measurements were performed using solvent standards, a calibration curve was created using the solvent peak area values, and the amount of solvent in the sample was quantified (μg / mL) to determine the amount of solvent in the extract. Quantitative analysis was performed for each solvent using the above method, and the quantified values ​​for each solvent were added together to determine the total amount of solvent in the extract. This was then divided by the weight of the hollow fiber membrane used for extraction to calculate the solvent content (μg / g) in the hollow fiber membrane.

[0088] [Leakage Rate of Ethanol Aqueous Solution] The liquid leakage rate of the membrane modules obtained in the examples and comparative examples was measured using the device shown in Figure 7. First, 500 mL of a 50% by mass aqueous ethanol solution was placed in a tank and pressurized to fill the inside of the hollow fibers of the membrane distillation membrane module with the ethanol aqueous solution. Next, the outside of the hollow fiber membrane of the membrane distillation membrane module was filled with the ethanol aqueous solution. A pressure of 60 kPa was then applied to the tank, and the leakage rate of the 50% by mass aqueous ethanol solution was calculated from the rate of increase in the water level outside the hollow fibers of the module. A leakage rate of less than 0.20 LMH was evaluated as extremely good (◎), a leakage rate of 0.20 LMH or more but less than 1.0 LMH was evaluated as good (◯), a leakage rate of 1.0 LMH or more but less than 2.0 LMH was evaluated as fair (Δ), and a leakage rate of 2.0 LMH or more, or a leakage rate of 2.0 LMH or more even at a pressure of less than 60 kPa, was evaluated as poor (×).

[0089] [Steam Flux] The steam flux of the membrane modules obtained in the Examples and Comparative Examples was measured using the apparatus shown in FIG.

[0090] A 20% by mass aqueous ethanol solution (a) was circulated inside the hollow fiber membrane of the membrane module 100, and cooling water CW was circulated to the outside of the hollow fiber membrane through the membrane. The circulation was carried out for 30 minutes. From the amount of ethanol that moved as vapor from the inside of the hollow fiber membrane to the outside of the hollow fiber membrane, the amount of ethanol transferred per hour was calculated, and the calculated amount was applied to the formula described below to obtain the vapor flux (kg / (m)) of the membrane module. ２ × h) was calculated. ２ ×h)) or more is good (◯), and 0.40 (kg / (m ２ When the resistance was less than ×h), it was evaluated as poor (×).

[0091] In membrane distillation, the vapor flux from the raw material liquid to the cooling water can be calculated by the following formula (1). In equation (1), J is the flux of the membrane for membrane distillation, and the membrane area is 1 m ２ , the steam transfer rate (kg) per hour of operation. ２ h) is the vapor permeability coefficient of the membrane for membrane distillation (kg / m２ The vapor pressure difference (kPa) between the feed liquid and the permeation side is expressed as the vapor pressure of the feed liquid (P ｆｅｅｄ : kPa), vapor pressure of the permeation side (cooling water) (P ｐｅｒｍｅａｔｅ : kPa).

[0092] According to Raoult's law and Dalton's law, the vapor partial pressure Pi of each solvent (component i) in an ideal mixed solvent can be expressed by the following formulas (2) and (3). In formula (2) or (3), P ｉ ０ is the vapor pressure of component i alone (kPa), χ ｉ is the mole fraction of component i in the mixed solvent, P ｔｏｔａｌ indicates the total pressure (kPa) of the mixed solvent.

[0093] However, in many cases, the components in the mixed solvent are not in an ideal state. For example, when there is an azeotropic component in the mixed solvent, ｉ is expressed by the activity coefficient γ ｉ The value is multiplied by .

[0094] Therefore, the FluxJ of each solvent (component i) ｉ can be expressed by the following formula (5). In formula (5), α ｉ is the vapor permeability coefficient of component i, χ ｉｆｅｅｄ and χ ｉｐｅｒｍｅａｔｅ are the molar fractions of component i in the raw material liquid and cooling water, respectively, and γ ｉｆｅｅｄ and γ ｉｐｅｒｍｅａｔｅ and denote the activity coefficient of component i in the raw material liquid and cooling water, respectively.

[0095] When the mixed solvent is a mixed solvent of two azeotropic components i and j, the activity coefficients γ ｉ and γ ｊ can be calculated using, for example, Wilson's law using the following formulas (6) and (7).

[0096] Λ ｉｊ and Λ ｊｉare called Wilson parameters, and can be calculated by the following equation (8). In equation (8), T is the absolute temperature (K), a ｉｊ and b ｉｊ indicate the coefficients. ｉｊ and b ｉｊ can be determined experimentally, and according to Aspenplus (registered trademark), the coefficients for an ethanol aqueous solution are as shown in Table 1 below.

[0097]

[0098] P in the above formulas (2) to (4) ｉ ０ can be calculated by the Antone equation shown in the following formula (9). In formula (9), A ｉ , B ｉ , and C ｉ are coefficients specific to component i, respectively. According to the National Institute of Standards and Technology (NIST), the coefficients for ethanol and water are as shown in Table 2 below.

[0099]

[0100] When the numbers shown in Table 2 are used, P obtained by the above formula (9) ｉ The unit is bar.

[0101] The configuration and effects of the present invention will be further described below with reference to examples, but the present invention is not limited to the following examples.

[0102] [Example 1] (1) Preparation of hollow fiber membrane A spinning nozzle with a double-tube structure was used to obtain the hollow fiber membrane of Example 1. A melt-kneaded mixture was prepared from 40% by mass of PVDF resin (KF-W#1000, manufactured by Kureha Corporation) as a thermoplastic resin, 23% by mass of finely powdered silica, 32.9% by mass of bis-2-ethylhexyl adipate (DOA) as a non-solvent, and 4.1% by mass of acetyl tributyl citrate (ATBC, boiling point 343°C) as a poor solvent.

[0103] The extruded hollow fiber extrusions were passed through an idle running distance of 120 mm and then solidified in water at 30°C. A porous hollow fiber membrane was then 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 bis(2-ethylhexyl) adipate and tributyl acetylcitrate.

[0104] The hollow fiber membrane was then immersed in water for 30 minutes to exchange the water, and then immersed in a 20% by mass aqueous solution of NaOH at 70°C for 1 hour, and then repeatedly washed with water to extract and remove the finely powdered silica.

[0105] (2) Immersion of hollow fiber membrane in alcohol 1,000 pieces of the hollow fiber membrane were cut out to a length of 60 cm, tied together with an insulok, and then inserted into a housing with a total length of 70 cm. 2 L of an aqueous solution of 99.5% by mass of ethanol was passed through the housing by circulating the solution using a tube pump.

[0106] After the 5-minute liquid flow, the ethanol was completely discharged, and 2 L of water was passed through the housing for 5 minutes to replace the ethanol with water. After the water replacement, the housing was dried under pressure.

[0107] (3) Hydrophobization Treatment of Hollow Fiber Membrane A fluororesin-based water repellent agent "FS-1610TH73-8.0" (polymer concentration 8.0 mass%) manufactured by FluoroTechnology was used as the hydrophobic polymer having at least one side chain selected from a (per)fluoroalkyl group, a (per)fluoropolyether group, an alkylsilyl group, and a fluorosilyl group. The fluororesin-based water repellent agent was diluted with a fluorine-based diluent "TH73" manufactured by FluoroTechnology to a concentration of 0.85 mass% before use.

[0108] The water repellent agent prepared as above at 0.85% by mass was passed through the housing having a total length of 70 cm and containing the hollow fiber membrane, to adhere the fluororesin polymer to the hollow fiber membrane.

[0109] After the water repellent agent was passed through, excess water repellent agent was collected and dried by passing compressed air through the housing.

[0110] (4) Preparation of membrane module for membrane distillation The hollow fiber membranes of (3) above were cut into lengths of 15 cm, and 70 of these were bundled together to form a membrane bundle. This was then housed in a housing, and the membrane bundle was adhered and fixed in the housing by centrifugal adhesion using a thermosetting epoxy resin as the adhesive resin.

[0111] 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 hollow fiber membrane was 120 cm ２ A membrane module for membrane distillation containing a hollow fiber membrane bundle was fabricated.

[0112] (5) Immersion of Membrane Module for Membrane Distillation in Alcohol The membrane module for membrane distillation obtained in (4) above was subjected to the following alcohol immersion treatment.

[0113] Using a syringe, an aqueous solution of ethanol with a concentration of 99.5% by mass was injected into the inside of the hollow fiber membrane through the inlet of the membrane module. After the injection so that the entire membrane was wet with the aqueous solution of ethanol and the aqueous solution injected into the inside of the hollow fiber membrane seeped out to the outside of the hollow fiber membrane, the membrane was held in a state where both the inside and outside of the membrane were immersed in the aqueous solution of ethanol for 5 minutes. Thereafter, the aqueous solution of ethanol was discharged from the membrane module, and the membrane was dried under pressure.

[0114] (6) Hydrophobization treatment of membrane distillation membrane module The membrane distillation membrane module obtained above was subjected to the following hydrophobization treatment. As the hydrophobic polymer, a fluororesin-based water repellent agent "FS-1610TH73-8.0" (polymer concentration 8.0 mass%) manufactured by FluoroTechnologies was used. The fluororesin-based water repellent agent was diluted with a diluent "TH73" manufactured by FluoroTechnologies to adjust the concentration to 0.85 mass% before use.

[0115] In the hydrophobic treatment, first, silicone caps were attached to one inlet of the housing connected to the inside of the hollow fiber membrane of the membrane module and the other inlet connected to the outside of the hollow fiber membrane. Using a syringe, a water repellent agent adjusted to a concentration of 0.85% by mass was injected into the inside of the hollow fiber membrane from the other inlet on the inside of the hollow fiber of the membrane module. After continuing the injection so that the entire membrane was wetted with the water repellent agent and the water repellent agent injected on the inside of the hollow fiber membrane seeped out to the outside of the hollow fiber membrane, silicone caps were attached to the other inlet on the inside of the hollow fiber and the other on the outside of the hollow fiber membrane, and the mixture was shaken for 2 minutes. The silicone caps were then removed, and excess water repellent agent was removed from the membrane module. Dry air was then flowed into the inside of the hollow fiber membrane at 1.0 mL / min and the outside of the hollow fiber membrane at 0.15 mL / min, allowing the membrane to dry overnight, obtaining a membrane distillation membrane module in which the entire membrane was hydrophobized.

[0116] Table 3 shows the blending composition of the hollow fiber membrane of the obtained membrane module, the manufacturing conditions of the membrane module, and various performances of the membrane module of Example 1. In addition, a histogram of the measurement results of the area distribution of the resin portion in the cross section of the porous hollow fiber membrane of Example 1 is shown in Figure 2.

[0117] It was confirmed that the film structure of Example 1 was a three-dimensional network structure with high interconnectivity. It was also found to have a high vapor flux and high wetting resistance.

[0118] [Examples 2 and 3] In Example 2, the alcohol used in the alcohol immersion was changed to Solmix AP-7 manufactured by Japan Alcohol Sales Co., Ltd., and "FS-1610TH-8.0" was used as the water repellent containing a hydrophobic polymer having at least one side chain selected from a (per)fluoroalkyl group, a (per)fluoropolyether group, an alkylsilyl group, and a fluorosilyl group, and the water repellent was diluted with diluent TH to a concentration of 1.0 mass % before use, except that the procedure was carried out in the same manner as in Example 1.

[0119] In Example 3, the non-solvent was changed to 32.9% by mass of bis(2-ethylhexyl) phthalate (DOP), the poor solvent was changed to 4.1% by mass of dibutyl phthalate (DBP), and the alcohol used in the alcohol immersion was changed to Solmix AP-7. Other than this, the same procedures as in Example 1 were carried out.

[0120] Example 4 Example 4 was carried out in the same manner as Example 2, except that the alcohol immersion was not performed.

[0121] Example 5 Example 5 was carried out in the same manner as Example 3, except that the alcohol immersion was not performed.

[0122] Comparative Example 1 In Comparative Example 1, the hollow fiber membrane prepared in Example 1 was used, but the hollow fiber membrane and the membrane distillation membrane module were not immersed in alcohol, and the hollow fiber membrane and the membrane distillation membrane module were not subjected to hydrophobic treatment when the module was prepared.

[0123] Comparative Example 2 Comparative Example 2 was carried out in the same manner as in Example 1, except that the hollow fiber membrane prepared in Example 3 was used and the module was prepared without being subjected to a hydrophobic treatment.

[0124] Comparative Example 3 was carried out in the same manner as in Example 1, except that a non-solvent was not used and the concentration of the poor solvent ATBC was changed from 4.1% by mass to 37.0% by mass to produce a hollow fiber membrane. The membrane structure of Comparative Example 3 exhibited a spherulite structure. The membrane also had a very low vapor flux.

[0125] Table 3 shows the composition of the hollow fiber membrane, the processing conditions during module production, and various performance characteristics. The membrane structures of the membrane distillation membranes in the examples all exhibited a three-dimensional mesh structure. In addition, it was confirmed that the solvent content in the membrane was reduced to 10 μg / g or less in the membrane module that had been immersed in alcohol, and it was found that the membrane had a high steam flux and high wetting resistance. Note that although Example 4 is a membrane module that was not immersed in alcohol, the solvent content in the membrane was reduced to 10 μg / g or less. On the other hand, in Example 4, the leakage rate of a 50% by mass aqueous ethanol solution was 1.0 LMH, resulting in inferior wetting resistance compared to the membrane module that had been immersed in alcohol.

[0126]

[0127] REFERENCE SIGNS LIST 10 Housing (container) 11 First housing side pipe 12 Second housing side pipe 20 Hollow fiber membrane for membrane distillation 30 Adhesive resin a Raw material liquid b Raw material liquid after removal of organic solvent 100 Membrane module for membrane distillation 200 Raw material liquid storage tank 300 Absorption water storage tank CW Cooling water LG Liquid level gauge P Pump PG Pressure gauge

Claims

1. A porous membrane for membrane distillation, wherein in all visual fields in a cross section inside the porous membrane for membrane distillation, 0.1 μm included in each visual field ２ 1 μm or more ２ The area ratio of the resin portion having an area of ​​1 μm or less is 70% or more of the total area of ​​all resin portions included in each of the fields of view, and ２ Super 10μm ２ the area ratio of the resin portion having an area of ​​less than 10 μm contained in each of the fields of view is 2% or more and 30% or less of the total area of ​​all the resin portions contained in each of the fields of view; ２ a porous membrane for membrane distillation, wherein the area ratio of the resin portion having an area of ​​100% or more is 15% or less of the total area of ​​all resin portions included in each of the fields of view, and the porous membrane for membrane distillation is made of a resin that is a thermoplastic polymer or a mixture of multiple types of such polymers, and a water repellent polymer different from the resin is adhered to the surface of the porous membrane for membrane distillation.

2. The porous membrane for membrane distillation according to claim 1, wherein the content of the solvent contained in the porous membrane for membrane distillation is 10 μg / g or less.

3. The porous membrane for membrane distillation according to claim 2, wherein the solvent contained in the porous membrane for membrane distillation is at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearic acid esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

4. The porous membrane for membrane distillation according to claim 1 or 2, wherein the water repellent polymer is a polymer having a fluorine atom-containing group in a side chain, and the side chain contains at least one selected from a fluoroalkyl group or a perfluoroalkyl group, a fluoropolyether group or a perfluoropolyether group, a fluoroalkylsilyl group, and a fluorosilyl group.

5. The porous membrane for membrane distillation according to claim 1 or 2, wherein the porous membrane for membrane distillation is a porous hollow fiber membrane for membrane distillation.

6. A membrane module for membrane distillation comprising the porous membrane for membrane distillation according to claim 5, wherein both ends of the porous hollow fiber membrane for membrane distillation are adhesively fixed with a thermosetting resin.

7. A method for producing a membrane distillation membrane module according to claim 6, comprising a step of immersing the porous membrane for membrane distillation or the membrane distillation membrane module in a solution containing alcohol.

8. The method for producing a membrane module for membrane distillation according to claim 7, wherein the alcohol includes at least one selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol.

9. A raw material liquid concentration system using a membrane distillation method in which a raw material liquid containing a solvent and a solute is contacted with cooling water via the porous membrane for membrane distillation described in claim 5, and the solvent in the raw material liquid is passed through the porous hollow fiber membrane for membrane distillation in a vapor state and moved to the cooling water side.

10. A raw material liquid concentration system using a membrane distillation method in which a raw material liquid containing a solvent and a solute is contacted with cooling water via the membrane distillation membrane module described in claim 6, and the solvent in the raw material liquid is passed through the porous hollow fiber membrane for membrane distillation in a vapor state and moved to the cooling water side.

11. A method for concentrating a raw material liquid using the membrane module for membrane distillation according to claim 6.

12. A method for concentrating a raw material liquid using the raw material liquid concentration system according to claim 9.

13. A method for removing an organic solvent from a raw material liquid containing the organic solvent, using the membrane module for membrane distillation according to claim 6.

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