JP7734523B2 - Raw material liquid concentration system and concentration device - Google Patents

Description

The present invention relates to a raw material liquid concentration system and concentration device. Specifically, the present invention relates to a raw material liquid concentration system and concentration device that can efficiently concentrate a raw material liquid by concentrating the raw material liquid by separating a portion of the solvent from a raw material aqueous solution containing an organic solvent and a solute using a membrane distillation method, thereby suppressing the deterioration or loss of components in the raw material liquid.

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.
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.
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 separates components by sieving, and because it is a separation method that does not involve phase change, it can reduce energy consumption. Furthermore, membrane-based methods have the advantage of being easier to scale up than methods such as distillation (Non-Patent Document 1).

Another method for concentrating raw liquids is known as membrane distillation (MD), which separates the solvent in the raw liquid by utilizing the difference in vapor pressure between the liquids. One well-known membrane distillation method is direct contact MD (DCMD), in which the raw liquid is brought into contact with cooling water that is at a lower temperature than the raw liquid via a membrane, and vapor that does not contain the active substance in the raw liquid is transferred from the raw liquid to the cooling water, thereby concentrating the raw liquid (see, for example, Patent Document 1).

International Publication No. 2016/006670

Journal of Membrane Science 2011, 381, 21-33 Journal of Membrane Science 1999, 163, 333-348

However, with the OSN method described in Non-Patent Document 1, valuable materials with small molecular sizes permeate the membrane, making it difficult to recover them without loss. Furthermore, because the raw material liquid must be pressurized, solutes contained in the raw material liquid tend to adhere to the membrane surface, resulting in a reduced recovery rate.

On the other hand, according to the membrane distillation method of Patent Document 1, when the membrane distillation membrane becomes wet, the raw material liquid flows into the cooling water side, making it impossible to concentrate. Furthermore, according to Non-Patent Document 2, a porous membrane with a small pore size is selected as the membrane distillation membrane, which poses a problem of low vapor permeability. Furthermore, to increase the vapor permeability, it is necessary to increase the temperature of the raw material liquid. In this case, the raw material liquid is generally heated to 60°C or higher, and there have been few reports of applying membrane distillation to the concentration of raw material liquids containing solutes such as enzymes and peptides that are susceptible to thermal denaturation.
The present invention aims to provide a concentration system and a concentration device for raw material liquids in pharmaceutical and chemical manufacturing processes that suppress wetting of membrane distillation membranes, which is a problem in membrane distillation methods, and has an industrially feasible processing speed even at room temperature.

The present invention has been made to achieve the above object.
That is, the inventors discovered that when concentrating a raw material liquid by membrane distillation, by using a porous membrane that is highly vapor permeable and highly hydrophobic as the membrane for membrane distillation, wetting of the membrane distillation membrane by the raw material liquid can be suppressed, and vapor can be efficiently extracted even at room temperature. As a result, efficient concentration can be achieved while preventing denaturation of the active ingredients in the raw material liquid, and this led to the present invention.
That is, the present invention is as follows.

<<Aspect 1>> 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 a membrane distillation membrane, and the solvent in the raw material liquid passes through the membrane distillation membrane in a vapor state and moves to the cooling water side,
As the membrane for membrane distillation, a porous membrane having a surface water contact angle of 90° or more, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used,
The membrane distillation membrane is a hollow fiber membrane having a hydrophobic polymer attached to at least a portion thereof,
The porosity of the inside of the hollow fiber membrane is greater than the porosity of the outside.
Raw material liquid concentration system.
<<Aspect 2>> 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 a membrane distillation membrane, and the solvent in the raw material liquid is passed through the membrane distillation membrane in a vapor state and moved to the cooling water side,
As the membrane for membrane distillation, a porous membrane having a surface water contact angle of 90° or more, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used,
The membrane distillation membrane is a hollow fiber membrane having a hydrophobic polymer attached to at least a portion thereof,
The liquid penetration pressure of the hollow fiber membrane with respect to an aqueous ethanol solution having a concentration of 20 weight percent is 0.1 MPa or more and 0.5 MPa or less.
Raw material liquid concentration system.
<<Aspect 3>> 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 a membrane distillation membrane, and the solvent in the raw material liquid passes through the membrane distillation membrane in a vapor state and moves to the cooling water side,
As the membrane for membrane distillation, a porous membrane having a surface water contact angle of 90° or more, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used,
The membrane distillation membrane is a hollow fiber membrane having a hydrophobic polymer attached to at least a portion thereof,
The hydrophobic polymer is attached by contacting a solution having a polymer concentration of 0.2 to 5.0 mass % with the surface of the hollow fiber membrane and then drying the solution.
Raw material liquid concentration system.
<Aspect 4> The hydrophobic polymer is a polymer having a fluorine atom-containing group in a side chain,
the fluorine atom-containing group is at least one selected from a (per)fluoroalkyl group, a (per)fluoropolyether group, an alkylsilyl group, and a fluorosilyl group;
A raw material liquid concentrating system according to any one of aspects 1 to 3.
Aspect 5: The raw material liquid concentration system according to any one of Aspects 1 to 4, wherein the material of the membrane distillation membrane comprises at least one resin selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, and polychlorotrifluoroethylene.
Aspect 6: The raw material liquid concentrating system according to any one of Aspects 1 to 5, wherein the solvent comprises at least one selected from the group consisting of water, acetonitrile, methanol, ethanol, and isopropanol.
<Embodiment 7> The number average molecular weight of the solute is 100 to 50,000,
the solute is a compound having a surface active effect;
The raw material liquid concentrating system according to any one of claims 1 to 6.
Aspect 8: The raw material liquid concentrating system according to any one of Aspects 1 to 7, wherein the temperature of the raw material liquid is adjusted to a range of 5°C or higher and 50°C or lower.
<Aspect 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 a membrane distillation membrane, and the solvent in the raw material liquid is passed through the membrane distillation membrane in a vapor state and moved to the cooling water side,
determining the vapor pressures of all solvents contained in the raw material liquid and all solvents contained in the cooling water;
Calculating the rate at which the solvent passes through the membrane distillation membrane in a vapor state from a vapor pressure difference obtained by subtracting the vapor pressure of the solvent in the cooling water from the vapor pressure of each solvent in the raw material liquid,
The vapor pressure of the solvent is calculated by multiplying the vapor partial pressure calculated from the molar concentration of the solvent and the temperature by the activity coefficient calculated from the molar concentration of the solvent and the temperature.
A method for predicting the performance of a feedstock liquid concentration system.
<<Aspect 10>> A concentrating apparatus for concentrating a raw material liquid by a membrane distillation method, comprising: a membrane for membrane distillation;
As the membrane for membrane distillation, a membrane having a water contact angle on the membrane surface of 90° or more, an average pore size in the range of 0.02 μm to 0.5 μm, and a porosity of 60% to 90% is used.
Concentrator.

The concentration system of the present invention can concentrate a raw liquid containing a solute having an amino acid sequence, for example, by a membrane distillation process at around room temperature, without causing wetting and while substantially maintaining the solute composition of the raw liquid.
If the concentration system of the present invention is applied to the concentration of raw material liquids containing valuable substances in the manufacturing processes of pharmaceuticals, chemicals, etc., valuable substances that are sensitive to heat and pressure can be concentrated at a high recovery rate without denaturing them.

FIG. 1 is a conceptual diagram illustrating an example of an embodiment of a membrane module applied to a raw material liquid concentration system of the present invention. 1 is a conceptual diagram illustrating an example of an embodiment of a raw material liquid concentrating system of the present invention. FIG. 1 is a conceptual diagram for explaining an example of an embodiment when a hydrophobic polymer is attached to a membrane applied to the raw material liquid concentration system of the present invention. FIG. 1 is a conceptual diagram for explaining an example of an embodiment relating to measurement of liquid intrusion pressure applied to the raw material liquid concentration system of the present invention. 1 is an electron microscope photograph of a hollow fiber membrane in the raw material liquid concentration system obtained in Example 1. 1 is a graph showing the relationship between the inlet temperature of the raw material liquid and the flux of the raw material liquid (total flux of all solvents) in Examples 1 to 9. 1 is a graph showing the vapor pressure (calculated values) of ethanol, acetonitrile, and water at each temperature. 1 is a graph showing the relationship between the inlet temperature of the raw material liquid and the vapor pressure difference (calculated value) of ethanol, acetonitrile, and water at the start of operation in Examples 1 to 9. 1 is a graph showing the relationship between the organic solvent concentration in the raw material liquid and the flux of the raw material liquid (total flux of all solvents) at the start of operation in Examples 4, 7, and 10 to 13. 1 is a graph showing the relationship between the organic solvent concentration in the raw material solution and the flux of each solvent at the start of operation in Examples 4, 7, and 10 to 13. FIG. 1 is a schematic cross-sectional view showing an example of the distribution of a hydrophobic polymer in a hollow fiber membrane for membrane distillation. FIG. 12 is a schematic cross-sectional view showing the state of wetting with ethanol when a hollow fiber membrane for membrane distillation to which a hydrophobic polymer is attached in the distribution shown in FIG. 11 is immersed in ethanol. 1 is a graph showing the relationship between the vapor pressure difference and the flux of each solvent at the start of operation in Examples 4 to 13.

Preferred embodiments of the present invention will now be described in detail as non-limiting examples.
<<Raw material liquid concentration system>>
The raw material liquid concentration system of the present invention 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 contacted 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 moved to the cooling water side.
The raw material liquid concentrating system of the present invention will be described below with reference to the drawings.

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.
The membrane distillation membrane module of Figure 1 contains a plurality of hollow fiber membranes in a housing, both ends of which are adhesively fixed with an adhesive resin. Here, both ends of the membrane distillation membrane are open and not closed.
The side of the housing has a first housing side pipe for introducing cooling water and a second housing side pipe for discharging cooling water, and these pipes allow cooling water to circulate in the space outside the membrane distillation membrane. In Figure 1, the first and second housing side pipes are referred to as "Cooling Water inlet/outlet".
At both ends of the housing in the axial direction (the vertical direction in FIG. 1 ), there are a lower opening for introducing the raw material liquid and an upper opening for discharging the raw material liquid, which allow the raw material liquid to flow through the hollow part of the membrane distillation membrane. In FIG. 1 , the lower opening and the upper opening are referred to as "feed inlet/outlet."
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.

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.
In the present invention, a porous membrane having a water contact angle of 90° or more on the membrane surface, an average pore size in the range of 0.02 μm to 0.5 μm, and a porosity of 60% to 90% is used as the membrane for membrane distillation.

To concentrate a raw liquid using membrane distillation, the raw liquid is passed through one side of the membrane distillation membrane (in Figure 1, the hollow space of the hollow fiber membrane distillation membrane), while cooling water, which is at a lower temperature than the raw liquid, is passed through the other side (in Figure 1, the external space of the hollow fiber membrane distillation membrane). The pores connecting the two spaces inside the membrane wall then 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.

FIG. 2 shows an example of a concentration system of the present invention.
In the concentration system of FIG. 2, a means (piping system) for circulating the feed solution is provided at both axial ends of the membrane distillation membrane module (MD Membrane) shown in FIG. 1. The piping system for circulating the feed solution includes a feed solution storage tank, a pump for circulating the feed solution, and a temperature regulator for maintaining the temperature of the feed solution at a set temperature. This concentration system may, as necessary, be equipped with a flow meter (not shown) that indicates the circulation flow rate, a flow rate regulator (not shown) that adjusts the circulation flow rate, and a pressure gauge that displays the liquid pressure when supplying the feed solution to the membrane distillation membrane module. In addition, by providing a weight scale, a liquid level gauge, etc. in the feed solution storage tank, the membrane performance and concentration rate can be estimated based on the degree of decrease in liquid level, weight, etc.

Meanwhile, the housing side pipe of the membrane distillation membrane module is equipped with a means (piping system) for circulating cooling water. This piping system is equipped with a cooling water storage tank, a pump for circulating the cooling water, a flow meter that indicates the circulation flow rate, a flow rate regulator (not shown) that adjusts the circulation flow rate, and a temperature regulator 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 disposed of at this time, an overflow port can be installed in the cooling water storage tank to maintain a constant volume of cooling water.

In a raw material liquid concentration system for manufacturing pharmaceuticals, chemicals, etc., the raw material liquid must be kept at a low temperature (for example, 50°C or below) in order to handle raw material liquids containing substances that can be decomposed by heating, such as peptides and proteins. In addition, liquids with low surface tension, such as acetonitrile, methanol, ethanol, and isopropanol, are used as solvents for the raw material liquid.
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 membrane distillation membrane used in the present invention 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 porous membrane with an average pore size in the range of 0.02 μm to 0.5 μm and a porosity of 60% to 90%.
These requirements are discussed below.

<<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.
<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.
Examples of raw material liquids applicable to the present invention include pharmaceuticals, pharmaceutical raw materials, pharmaceutical active ingredients, pharmaceutical intermediates, etc. (hereinafter collectively referred to as "pharmaceutical active ingredients, etc."), solutions or dispersions containing chemicals; food; seawater; and produced water discharged from gas fields, oil fields, etc.
The liquid raw material concentration system of the present invention can obtain a concentrated solution from which the solvent has been removed while maintaining the composition of the liquid raw material A. Therefore, when the liquid raw material concentration system of the present invention is applied to the concentration of a solution or dispersion containing a pharmaceutical product or chemical as a solute, it becomes possible to concentrate the solution while maintaining the properties of the solute.

[Solute of raw material liquid]
As the pharmaceutical raw materials, 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 having surface active effects, and vitamins, and that this solute is dissolved or dispersed in an appropriate solvent.

An amino acid is a compound having an amino acid backbone 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.
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.

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.
Examples of unnatural amino acids that are preferred as pharmaceutical ingredients include labeled amino acids and functionalized amino acids.
A labeled amino acid is a non-natural amino acid in which an amino acid backbone is bound to a labeling compound, 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.
Examples of functionalized amino acids include photoresponsive amino acids, photoswitch amino acids, fluorescent probe amino acids, and fluorescently labeled amino acids.

A peptide refers to a compound in which 2 to less than 70 amino acid residues are bonded, and may be linear or cyclic. Examples of peptides include L-alanyl-L-glutamine, β-alanyl-L-histidine cyclosporine, and glutathione.
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.
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 peptide, blood coagulation factor VIII, somatomedin, glucagon, growth hormone-releasing factor, serum albumin, and calcitonin.

Examples of sugars include monosaccharides, disaccharides, sugar chains (excluding disaccharides), sugar chain derivatives, and the like.
Examples of monosaccharides include glucose, fructose, galactose, mannose, ribose, deoxyribose, etc. Examples of disaccharides include maltose, sucrose, lactose, etc.
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.

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.

Examples of vitamins include vitamin A, vitamin B, and vitamin C, and also include their derivatives and salts. Examples of vitamin B include vitamin B6 and vitamin B12.

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 .

The number average molecular weight of the solute contained in the raw material solution 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.
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.

[Solvent in the raw material liquid]
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.
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.

<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.

<Membrane for membrane distillation>
As the membrane for membrane distillation used in the present invention, it is preferable to use a porous membrane.
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.

The membrane distillation membrane used in the present invention has a water contact angle on at least one surface of 90° or more, preferably greater than 90°, more preferably 110° or more, and even more preferably 120° or more, from the viewpoint of avoiding wetting of the membrane. There is no upper limit to the water contact angle of the membrane in relation to the effects of the present invention, but in reality it may be 150° or less.
The water contact angle in this specification is a value measured by the sessile drop method in accordance with JIS R 3257. Specifically, 2 μL of pure water is dropped onto the surface of the object to be measured, and the angle formed between the object to be measured and the droplet is quantified by analyzing a projected image.
In the membrane distillation membrane used in the present invention, it is preferable that substantially the entire area of one surface exhibits a water contact angle within the above range.
The shape of the membrane for membrane distillation used in the present invention will be described later. When the membrane is in the form of a hollow fiber, it is preferable that the outer surface of the hollow fiber exhibits a water contact angle within the above range.

The average pore size of the membrane for membrane distillation is in the range of 0.02 μm or more and 0.5 μm or less, preferably in the range of 0.03 μm or more and 0.3 μm or less. When the average pore size is 0.02 μm or more, the vapor permeation resistance is not too large, and the concentration rate of the raw material liquid is increased. When the average pore size is 0.5 μm or less, the effect of suppressing membrane wetting is good. The average pore size is a value measured by the half-dry method in accordance with ASTM: F316-86.
From the viewpoint of achieving both vapor permeability and wetting suppression, the pore size distribution of the membrane is preferably narrow. Specifically, the pore size distribution, which is the ratio of the maximum pore size to the average pore size, is preferably in the range of 1.2 to 2.5, more preferably in the range of 1.2 to 2.0. The maximum pore size is a value measured using the bubble point method.

The porosity of the membrane distillation membrane is in the range of 60% to 90% in order to achieve both high vapor permeability and long-term durability. To achieve high vapor permeability, the porosity of the membrane distillation membrane is 60% or more, preferably 70% or more. To maintain the strength of the membrane itself well and to prevent problems such as breakage during long-term use, the porosity of the membrane distillation membrane is 90% or less, preferably 85% or less.
The surface opening ratio of the membrane distillation membrane is preferably 15% or more, more preferably 18% or more, and even more preferably 20% or more for each surface from the viewpoint of obtaining an efficient concentration rate, and is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less from the viewpoint of maintaining good strength of the membrane itself and making it less likely to cause problems such as breakage during long-term use. The surface opening ratio is a value determined by detecting pores using image analysis software in an image of the membrane surface observed with a scanning electron microscope (SEM).

The porosity of the membrane for membrane distillation may vary along the thickness of the membrane. By increasing the porosity on the side in contact with the raw liquid and decreasing the porosity on the side in contact with the cooling water, it is possible to improve long-term durability while maintaining vapor permeability.
When the membrane is in the form of a hollow fiber, since the raw material liquid is often passed through the inside of the hollow fiber, it is preferable that the porosity of the inside of the hollow fiber membrane is higher than that of the outside. Specifically, the porosity of the inner 5% of the hollow fiber in the cross-sectional direction of the membrane is preferably more than 1.0 times and not more than 3.0 times, more preferably 1.1 times or more and not more than 2.0 times, the porosity of the outer 5%.

Materials for constructing membranes for membrane distillation include materials containing at least one resin selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, and polychlorotrifluoroethylene. From the perspective of being able to produce membranes with excellent hydrophobicity, mechanical durability, and thermal durability with high membrane-forming properties, polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, or polychlorotrifluoroethylene are preferred.

In one embodiment of the present invention, a hydrophobic polymer may be attached to at least a portion of the membrane distillation membrane to improve 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, thereby imparting water repellency to the membrane or improving the water repellency of the membrane.
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.
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. 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.

Examples of hydrophobic polymers include polymers having siloxane bonds and fluorine atom-containing polymers, and more specific examples include the following:
(a) Examples of polymers having siloxane bonds include dimethylsilicone gel, methylphenylsilicone gel, reactive modified silicone gels having organic functional groups (amino groups, fluoroalkyl groups, etc.), silicone polymers that form crosslinked structures upon reaction with silane coupling agents, and polymer gels that are crosslinked products of these. (b) Examples of fluorine atom-containing polymers include polymers having fluorine atom-containing groups in their side chains, where the fluorine atom-containing groups are (per)fluoroalkyl groups, (per)fluoropolyether groups, alkylsilyl groups, fluorosilyl groups, etc.
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.

The hydrophobic polymer may be attached to the entire pores of the membrane distillation membrane, but from the viewpoint of preventing liquid from entering the pores and ensuring vapor permeability, it is preferable that the amount of attached hydrophobic polymer has a distribution in the thickness direction of the membrane distillation membrane, with more polymer attached to the surface layer of the membrane that comes into contact with the liquid and less polymer attached inside the thickness direction of the membrane, so that the pore structure is maintained.
From this viewpoint, it is preferable that the amount of hydrophobic polymer attached on the surface layer of at least one side of the membrane distillation membrane is greater than the amount attached inside, and it is more preferable that the amount of hydrophobic polymer attached decreases from the surface layer of one side of the membrane distillation membrane to the surface layer of the opposite side.
The "surface layer of the membrane" here refers to the part of the membrane that comes into contact with the liquid and its vicinity. Quantitatively, it refers to a range of about 10 μm from the outermost layer of the membrane toward the inside in the membrane thickness direction. On the other hand, the "inside of the membrane" refers to the part of the membrane that does not come into contact with the liquid and through which only vapor passes, and refers to the part other than the surface layer of the membrane.
In this specification, when comparing the amount of hydrophobic polymer between the surface layer (inside and outside of the film) and the inside in TOF-SIMS analysis or the like, the measurement data for the inside is taken as the data measured at a position approximately ±5% from the exact center of the film thickness portion.

Figure 11 shows an example of the distribution of hydrophobic polymers when the membrane distillation membrane is in the form of a hollow fiber. Figure 11(a) shows the distribution of hydrophobic polymers in a cross section perpendicular to the longitudinal direction of the hollow fiber, and Figure 11(b) shows the distribution of hydrophobic polymers in a cross section in the longitudinal direction of the hollow fiber. In Figures 11(a) and (b), the darker the gray, the higher the concentration of hydrophobic polymers present.
11(a) and (b), in the case of this hollow fiber membrane for membrane distillation, a large amount of hydrophobic polymer adheres to the vicinity of the inner surface of the hollow fiber membrane, and the amount of hydrophobic polymer is small near the outer surface. In this case, it is preferable to pass the raw material liquid (a) through the hollow part in contact with the inner surface where the amount of hydrophobic polymer adheres is large from the viewpoint of preventing wetting of the membrane.
The distribution of the hydrophobic polymer may be high near the outer surface of the hollow fiber and low near the inner surface, in which case it is preferable to pass the raw material liquid (a) through the outer space of the hollow fiber that is in contact with the outer surface where the amount of attached hydrophobic polymer is high.

The amount of hydrophobic polymer attached to a membrane distillation membrane can be determined directly as a mass by extracting the membrane distillation membrane to which the hydrophobic polymer has been attached with a good solvent for the hydrophobic polymer, and then removing the solvent from the extract.
Furthermore, a suitable surface analysis device can be used to analyze a membrane distillation membrane to which a hydrophobic polymer has been attached, and the amount of attachment can be determined from the signal intensity ratio between the constituent material of the membrane distillation membrane and the hydrophobic polymer. Since a surface analysis device can analyze any part of the membrane distillation membrane, the distribution of attached hydrophobic polymer can be determined by comparing the signal intensity ratios for each part of the membrane. Examples of surface analysis devices in this case include an IR (infrared spectroscopy) device, an XPS (X-ray photoelectron spectroscopy) device, and a TOF-SIMS (time-of-flight secondary ion spectroscopy) device.
In the present invention, the ratio of the amount of hydrophobic polymer attached on the inside to the amount attached on the outside is important for the adhesion of the hydrophobic polymer. If the intensity ratio is 1.2 times or more, it is determined that there is a difference between the state of the inside and outside of the present invention, and if the difference is 3.0 times or more, it is determined that there is a very large difference.

Hydrophobic polymers can be attached to membrane distillation membranes by applying a coating solution in which the hydrophobic polymer is dissolved in a suitable solvent to the membrane and drying it. The desired hydrophobic polymer distribution can be achieved by appropriately adjusting the coating location, the volatility (boiling point) of the coating solution solvent, the concentration of the hydrophobic polymer in the coating solution, and the drying conditions after coating. For example, when coating is performed by the immersion method, the lower the volatility (higher the boiling point) of the solvent in the coating solution and the milder the drying conditions after coating, the more easily the coating solution containing the hydrophobic polymer can migrate toward the membrane surface during the drying process, thereby generating a distribution along the membrane thickness.

When the hydrophobic polymer is attached in a distribution that decreases from the surface layer on one side of the membrane distillation membrane to the surface layer on the opposite side, the surface on which the amount of attached hydrophobic polymer is less may be subjected to a hydrophilization treatment.
The surface on which the hydrophobic polymer is less adhered has low hydrophobicity and high affinity with highly hydrophilic solvents, whereas the surface on which the hydrophobic polymer is more adhered has high hydrophobicity and low affinity with highly hydrophilic solvents. Utilizing this, only the surface on which the hydrophobic polymer is less adhered can be wetted with a highly hydrophilic solvent to be hydrophilized.
For example, when a membrane for membrane distillation in which the amount of attached hydrophobic polymer decreases from the surface layer on one side to the surface layer on the other side is immersed in, for example, ethanol, only the portion with a small amount of attached hydrophobic polymer becomes wet with ethanol, as shown in Figure 12. When a membrane for membrane distillation in this state is used for concentration by membrane distillation, the distance through which vapor passes in the thickness direction of the membrane is substantially shortened, and the amount of vapor passing per unit time can be increased.
When using such a hydrophilically treated membrane for membrane distillation, it is preferable to bring the hydrophilically treated side into contact with cooling water and the opposite side, on which a larger amount of hydrophobic polymer is attached, into contact with the raw material liquid.

The concentration of the hydrophobic polymer in the hydrophobic polymer solution used in the hydrophobic polymer deposition step is preferably within a certain range. Specifically, the polymer concentration is preferably 0.2 to 5.0 mass %, more preferably 0.4 to 4.0 mass %, and even more preferably 0.6 to 3.0 mass %.
When the polymer concentration is 0.2% by mass or more, the polymer is not insufficient even after being distributed in the membrane, and the polymer does not adhere to any part, so that the membrane can be sufficiently hydrophobized.In addition, when the polymer concentration is 5.0% by mass or less, the void part of the membrane is not filled with the polymer, so that the porosity of the membrane after drying can be maintained, and the vapor transmission rate can be prevented from decreasing.

The hydrophobicity of membrane distillation membranes can generally be determined by measuring the liquid entry 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. Specifically, the liquid entry pressure against a 20 weight percent aqueous ethanol solution is preferably 0.1 MPa to 0.5 MPa, and more preferably 0.2 MPa to 0.4 MPa. If this liquid entry pressure is high, the membrane will not easily wet even when handling raw materials containing organic solvents. On the other hand, achieving a high liquid entry pressure requires a small maximum pore size of the membrane; therefore, achieving a liquid entry pressure that is too high requires the use of a membrane with a low porosity. Therefore, to maintain a certain level of vapor permeability, it is preferable that the liquid entry pressure not be too high.

[Shape and size of membrane for membrane distillation]
The membrane for membrane distillation may have 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 for membrane distillation 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.
From the viewpoint of achieving both vapor permeability and mechanical strength of the membrane, the thickness of the membrane for membrane distillation of the present invention is preferably 10 μm to 1,000 μm, more preferably 20 μm to 500 μm. If the membrane thickness is 1,000 μm or less, high vapor permeability can be obtained, while if the membrane thickness is 10 μm or more, the membrane can be used without deformation.
When the membrane for membrane distillation is a hollow fiber membrane, the outer diameter is 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.

<Membrane distillation module>
In the present invention, when the membrane distillation membrane is in the form of a hollow fiber, the membrane may be used in the form of a membrane distillation membrane module in which a membrane bundle formed by bundling a plurality of hollow fiber membrane distillation membranes is packed in an appropriate module.
The shape of the module may be a cylinder, a polygonal prism, or other polyhedron, and there is no limitation on the shape.
Preferably, the hollow fiber membrane bundle is housed in a cylindrical or polygonal prism-shaped 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 that the hollow fiber bundle is fixed with the adhesive resin in a liquid-tight manner so that the flow paths inside and outside the hollow fiber membranes do not mix.

The adhesive resin is desired to have high mechanical strength and heat resistance at 100°C. Examples of adhesive resins include thermosetting epoxy resins and thermosetting urethane resins. From the viewpoint of heat resistance, epoxy resins are preferred. From the viewpoint of handling, urethane resins are preferred.
The adhesive fixing method may be any known adhesive method related to the production of membrane modules for membrane distillation.
The construction of the housing is selected primarily from the viewpoint of chemical resistance, which is necessary to prevent 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. From the above viewpoints, it is preferable to select the material 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.

<Performance prediction of raw material liquid concentration system>
In the raw material liquid concentration system of the present invention,
Calculate the vapor pressure of all solvents contained in the raw material liquid and the vapor pressure of all solvents contained in the cooling water,
The rate at which the solvent passes through the membrane distillation membrane in a vapor state can be calculated from the vapor pressure difference obtained by subtracting the vapor pressure of the solvent in the cooling water from the vapor pressure of the solvent in the raw material liquid.
Here, the vapor pressure of the solvent is determined by multiplying the vapor partial pressure determined from the molar concentration and temperature of the solvent by the activity coefficient determined from the molar concentration and temperature of the solvent.

In membrane distillation, the vapor transmission rate (flux) from the raw material liquid to the cooling water can be calculated by the following formula (1).
J is the flux of the membrane distillation membrane, and indicates the amount of vapor transferred (kg) per 1 ^m2 of membrane area and 1 h of operation time. α (kg/ ^m2 ·h) is the vapor permeability coefficient (kg/ ^m2 ·h·kPa) of the membrane distillation membrane, and indicates the flux per 1 kPa of vapor pressure difference. The vapor pressure difference (kPa) between the feed liquid and the permeation side can be calculated from the difference between the vapor pressure of the feed liquid ( _Pfeed : kPa) and the vapor pressure of the permeation side (cooling water) ( _Ppermeate : kPa).
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).
P ⁰ _i represents the vapor pressure (kPa) of only component i, χ _i represents the molar fraction of component i in the mixed solvent, and P _total represents the total pressure (kPa) of the mixed solvent.

However, in many cases, the components in the mixed solvent are not in an ideal state. For example, when the mixed solvent contains an azeotropic component, Pi is multiplied by the activity coefficient _γi as shown in the following formula (4).
Therefore, the flux J _i of each solvent (component i) can be expressed by the following 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 feed liquid and cooling water, respectively, and γ _ifeed and γ _ipermeate are the activity coefficients of component i in the feed liquid and cooling water, respectively.

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, for example, by Wilson's law using the following formulas (6) and (7).

Λ _ij and Λ _ji are called Wilson parameters, and can be calculated by the following equation (8).
T is the absolute temperature (K), and _aij and _bij are coefficients. _aij and _bij can be determined experimentally, and according to Aspenplus (registered trademark), the coefficients of each component in an ethanol aqueous solution and an acetonitrile aqueous solution are as shown in Table 1 below.

The P ⁰ _i can be calculated by the Antone equation shown in the following equation (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, acetonitrile, and water are as shown in Table 2 below.
When the numbers shown in Table 2 are used, the unit of P _i obtained from the above formula (9) is bar.

As described above, the flux of each component in the raw material liquid can be calculated from the composition and temperature of the raw material liquid and the cooling water. Knowing the flux of each component makes it possible to predict the performance of the concentration system, such as the solvent composition and concentration rate during raw material liquid concentration.
In other words, once the configuration of the raw liquid concentration system and the temperature of membrane distillation are determined, the speed at which a particular solvent passes through the membrane distillation membrane in vapor form can be determined from the vapor pressure difference obtained by subtracting the vapor pressure of the solvent in the cooling water from the vapor pressure of the solvent in the raw liquid.
This information can be used to predict, for example, the solvent composition after concentration. If the solvent composition after concentration is unfavorable for the valuable materials contained in the raw material liquid (if precipitation, deterioration, etc. is predicted), measures such as changing the cooling water composition, changing the membrane distillation temperature, or performing membrane distillation while adjusting the solvent composition of the concentrate can be taken to minimize loss of the valuable materials in the raw material liquid and enable efficient concentration of the raw material liquid.
Furthermore, the time required to reach a predetermined concentration ratio can be determined from the above information. Therefore, by comparing the time required to achieve a desired concentration ratio by changing, for example, the cooling water composition or the membrane distillation temperature, it is possible to optimize the raw material liquid concentration system to obtain a concentrated solution of the desired concentration ratio in the shortest time.

<Component retention of the raw material liquid concentration system of the present invention>
According to the above-described system for concentrating a raw material liquid of the present invention, a highly concentrated product can be efficiently obtained in which the composition of the solutes contained in the raw material liquid is substantially maintained.
Therefore, the raw material liquid concentrating system of the present invention is suitable for use in, for example, manufacturing processes for pharmaceuticals, chemicals, and the like.
The method for analyzing the solutes in the obtained concentrate may be appropriately selected depending on the types of components contained in the raw material liquid and the concentrate, and may be any of various known analytical methods, such as inductively coupled plasma mass spectrometry (ICP-MS), nuclear magnetic resonance spectroscopy (NMR), gas chromatography/mass spectrometry (GC/MS), colorimetry, fluorometry, and high performance liquid chromatography (HPLC).

<Concentration device of the present invention>
The concentrating device of the present invention includes the membrane for membrane distillation of the present invention as described above.
Therefore, the concentrating device of the present invention:
A concentrating apparatus equipped with a membrane distillation membrane and for concentrating a raw material liquid by a membrane distillation method,
As the membrane for membrane distillation, a membrane having a water contact angle of 90° or more on the membrane surface, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used.
It is a concentration device.
A hydrophobic polymer may be attached to at least a portion of the membrane distillation membrane, or the amount of hydrophobic polymer attached may be distributed in the thickness direction of the membrane distillation membrane, and the amount attached on the surface layer of at least one side of the membrane distillation membrane may be greater than the amount attached inside.
For other aspects of the concentrating device of the present invention, please refer to the description of the concentrating system of the present invention.

The following provides further explanation of examples that specifically demonstrate the configuration and effects of the present invention, but the present invention is not limited to these examples in any way.

<Physical properties of hollow fiber membranes for membrane distillation>
In this example, various physical properties of the hollow fiber membrane for membrane distillation were determined by the various measurement methods described below.

[Water contact angle on the outer surface of the hollow fiber membrane]
The water contact angle of the hollow fiber membrane for membrane distillation was measured by the sessile drop method in accordance with JIS R 3257.
A 2 μL droplet of pure water was dropped onto the outer surface of the hollow fiber membrane sample at 23° C. and 50% relative humidity, and the contact angle was calculated by image analysis to determine the angle between the droplet and the outer surface of the hollow fiber membrane. The measurement was performed five times, and the number average value was used as the water contact angle.

[Outer diameter, inner diameter, and membrane thickness of hollow fiber membrane]
The outer diameter and inner diameter of the hollow fiber membrane distillation membrane were determined by microscopic observation. Specifically, the hollow fiber membrane was thinly sliced in a direction perpendicular to the longitudinal direction with a razor or the like, a microscopic image of the cross section was obtained, and the outer diameter and inner diameter of the cross section were measured. The membrane thickness of the hollow fiber membrane was calculated using the following formula (10):

[Average pore diameter of hollow fiber membrane]
The average pore diameter of the hollow fiber membrane for membrane distillation was measured by the method for measuring average pore diameter (also known as the half-dry method) described in ASTM: F316-86.
The measurement was carried out on a hollow fiber membrane approximately 10 cm long, using ethanol as the liquid, under standard measurement conditions of 25° C. and a pressure increase rate of 0.01 atm/sec.
The average pore diameter was calculated by the following formula (11):
Average pore diameter [μm] = 2,860 × (s [dyne/cm]) / (p [Pa]) (11)
{In the above formula, s is the surface tension of the liquid used, and p is the half-dry air pressure.}
Here, the surface tension s of the liquid ethanol used at 25°C was 21.97 dyne/cm, so 2,860 × s = 62,834 was used, and equation (11) was transformed to the following equation (12). The value of the half-dry air pressure p was substituted into equation (12) to determine the average pore diameter of the hollow fiber membrane.
Average pore diameter [μm] = 62,834/(p[Pa]) ... (12)

[Maximum pore diameter of hollow fiber membrane]
The maximum pore size of the hollow fiber membrane for membrane distillation was measured by the bubble point method using ethanol as the immersion liquid.
One end of an 8 cm long hollow fiber membrane was blocked, and the other end was connected to a nitrogen gas supply line via a pressure gauge. Nitrogen gas was supplied in this state to replace the inside of the line with nitrogen, and then the hollow fiber membrane was immersed in ethanol. At this time, the hollow fiber membrane was immersed in ethanol while being slightly pressurized with nitrogen to prevent ethanol from flowing back into the line. With the hollow fiber membrane immersed in ethanol, the nitrogen gas pressure was slowly increased, and the pressure P (kg/cm ² ) at which nitrogen gas bubbles began to steadily emerge from the hollow fiber membrane was recorded. This P was calculated using the following formula (13):
d=C1γ/P (13)
{In equation (13), d is the maximum pore diameter of the hollow fiber, C1 is a constant, γ is the surface tension of the immersion liquid, and P is pressure.} The maximum pore diameter d of the hollow fiber membrane was calculated by substituting the above equation. In this case, the value of C1γ when ethanol was used as the immersion liquid was set to 0.632 (kg/cm), and the maximum pore diameter d (μm) was obtained.

[Porosity of hollow fiber membrane]
The porosity of the hollow fiber membrane for membrane distillation was calculated from the mass of the hollow fiber membrane and the density (true density) of the material constituting the hollow fiber membrane.
That is, the hollow fiber membrane is cut to a certain length, its mass is measured, and the mass is calculated by the following formula (14):
The void ratio of the hollow fiber was calculated using the following formula: {In the above formula, d is the true density of the raw polymer of the hollow fiber membrane, and π is the constant of the circumference of a circle.}
The porosity of the inner 5% and outer 5% of the hollow fiber membrane was determined by processing an electron microscope photograph of the cross section of the hollow fiber membrane using image analysis software. ImageJ (free software) was used for image analysis. The captured image was enhanced and filtered so that the pores were black and the non-pores were white. The pores were then counted. When the underlying polymer chains were visible inside the pores, the polymer chains were considered to be pores and counted. The surface pore ratio was calculated using the following formula (15):
Surface porosity [%] = 100 × (total area of each pore) / (area of measurement range) (15)
The area of the measurement range is the sum of the areas of each hole plus the sum of the areas of each non-hole. Holes on the boundary of the measurement range were not excluded but were included in the calculation.

[Amount of Hydrophobic Polymer Attached]
(1) IR and ATR Methods Comparison of the amount of hydrophobic polymer attached to hollow fiber membranes for membrane distillation was carried out by IR spectrum analysis and ATR method (total reflection method, internal reflection method) using a ZnSe crystal as a prism.
The measurement device used was a Spectrum One manufactured by PerkinElmer, and the pressing pressure on the crystal was measured with a pressure contour value of approximately 30. From the obtained IR spectrum, the ratio of the peak intensity derived from the hydrophobic polymer to the peak intensity derived from the constituent material of the hollow fiber membrane was calculated to calculate the amount of hydrophobic polymer attached to the membrane surface.
The hollow fiber membrane samples were cut from the module. For the analysis of the outer surface of the hollow fiber membrane, the hollow fiber membrane was cut at 1 cm intervals in a cross section perpendicular to the longitudinal direction, and for the analysis of the inner surface, the hollow fiber membrane was cut in the longitudinal direction.
In the examples, the hollow fiber membrane was made of polyvinylidene fluoride (PVDF) and the hydrophobic polymer was an acrylate polymer with a perfluoroalkyl group in the side chain. Therefore, the peak intensity ratio of v(C=O) at 1,734 cm ^-1 to (v(C-F) + v(C-O)) at around 1,180 cm- ¹ , v(C=O)/(v(C-F) + v(C-O)), was calculated and used as an index of the amount of hydrophobic polymer attached.
This peak intensity ratio was measured for each site of the hollow fiber membrane, and when the minimum value was taken as 1.0, sites showing a value of 1.2 or more were judged to have a large amount of hydrophobic polymer attached.

(2) TOF-SIMS Analysis The amount of hydrophobic polymer attached to the hollow fiber membrane for membrane distillation was compared by observing the cross section of the hollow fiber membrane by TOF-SIMS in addition to the above-mentioned IR and ATR methods.
As the observation sample, a sample obtained by slicing a hollow fiber membrane into thin pieces was used.
The equipment used was a nano-TOF manufactured by ULVAC-PHI, Inc., with Bi ₃ ⁺⁺ as the primary ion, an acceleration voltage of 30 kV, a current of 0.1 nA (as DC), an analysis area of 350 μm × 350 μm, an accumulation time of 120 minutes, and negative ions as the detected ions, and neutralization was performed with an electron gun. Mapping was performed using m/z values with characteristically strong intensities for each material of the hollow fiber membrane constituent material and the hydrophobic polymer, and the abundance ratio of the hydrophobic polymer in each part was determined by comparing the obtained intensity ratios. Under these conditions, an accumulation time of 30 minutes is sufficient, but in this example, an accumulation time of 120 minutes was used.
In the examples, the hollow fiber membrane was made of polyvinylidene fluoride (PVDF) and the hydrophobic polymer was an acrylate polymer having a perfluoroalkyl group in the side chain. Therefore, the intensity ratio of m/z=67, where the intensity of PVDF is particularly strong, and m/z=293, where the intensity of the perfluorinated acrylate polymer is particularly strong, was calculated.
Here, when the intensity ratio is two or more, it can be determined that there is a significant difference in the abundance rate of the hydrophobic polymer.

Example 1
(1) Preparation of hollow fiber membrane for membrane distillation In Example 1, a hollow fiber porous membrane was used as the membrane distillation membrane, and after a hydrophobic polymer was attached, the raw material liquid was concentrated by membrane distillation. This membrane distillation membrane was packed into a membrane distillation membrane module having the configuration shown in Figure 1 and used. The hollow fiber porous membrane was produced by the following method.
23 parts by mass of hydrophobic silica (AEROSIL-R972, manufactured by Nippon Aerosil Co., Ltd.) with an average primary particle size of 0.016 μm and a specific surface area of 110 m ² /g, 31 parts by mass of DOP (di-2-ethylhexyl phthalate, dioctyl phthalate), and 6 parts by mass of DBP (dibutyl phthalate) were mixed in a Henschel mixer, to which 40 parts by mass of polyvinylidene fluoride (Solef (registered trademark) 6010, manufactured by SOLVAY) with a weight average molecular weight of 310,000 was added and mixed again in the Henschel mixer. This mixture was mixed in a twin-screw kneading extruder and then pelletized.
The resulting pellets were melt-kneaded (240°C) in a twin-screw extruder, and the resulting molten mixture was extruded through a circular hole for extruding the molten material on the extrusion surface of a hollow fiber spindle attached to the extrusion port in the head at the tip of the extruder. Simultaneously, nitrogen gas was discharged as a hollow-forming fluid from a circular hole for discharging the hollow-forming fluid inside the circular hole for extruding the molten mixture and injected into the hollow portion of the hollow fiber-shaped extrudate. The hollow fiber-shaped extrudate was introduced into a water bath (40°C) with an idle running distance of 20 cm and wound up at a speed of 20 m/min.
The obtained hollow fiber was immersed in methylene chloride to extract and remove DOP and DBP from the hollow fiber, and then dried. Next, it was immersed in a 50% by mass aqueous solution of ethyl alcohol, and then in a 5% by mass aqueous solution of sodium hydroxide at 40°C for 1 hour to extract and remove silica from the hollow fiber. It was then washed with water and dried to obtain a porous hollow fiber membrane made of polyvinylidene fluoride.

(2) Preparation of membrane distillation membrane module In Example 1, a hollow fiber porous membrane was used as the membrane distillation membrane, and the raw material liquid was concentrated by membrane distillation without any hydrophobic polymer attached. This membrane distillation membrane was packed into a membrane distillation membrane module having the configuration shown in Figure 1.
The membrane for membrane distillation was a porous hollow fiber membrane made of PVDF obtained by the above-mentioned membrane production method, having an inner diameter (ID) of 0.68 mm, an outer diameter (OD) of 1.25 mm, a thickness (Thickness) of 0.28 mm, an average pore size of 0.21 μm and a maximum pore size of 0.29 μm as determined in accordance with ASTM-F316-86, and a porosity of 72%, cut into a length of 15 cm.
The water contact angle of this porous hollow fiber membrane was measured by the above-mentioned method and was found to be 103°.
A plurality of the above membrane distillation membranes were bundled together to form a membrane bundle, which was then housed in a housing. The membrane bundle was then adhered and fixed to the housing by centrifugal adhesion using a thermosetting epoxy resin as the adhesive resin.
By the above operations, ^three membrane distillation membrane modules containing hollow fiber membrane distillation membrane bundles were produced, each having an effective length (the length of the portion not embedded in the adhesive resin) of 8 cm and a total membrane area of the inner surface of the membrane distillation membranes of 120 cm2.

(3) Hydrophobization Treatment of Membrane Distillation Module The membrane distillation membrane module obtained by the above-described operation was subjected to the following hydrophobization treatment.
As the hydrophobic polymer, a fluororesin-based water repellent agent "FS-392B" (polymer concentration: 0.15% by mass) manufactured by FluoroTechnologies Corporation was used. The solvent in the water repellent agent was distilled off to concentrate the agent three times, and the polymer concentration was concentrated to 0.45% by mass before use.
In the hydrophobic treatment, the water repellent agent was injected into the inside of the hollow fiber membrane through the inlet of the membrane module using a syringe. The injection was continued until the entire membrane was wetted with the water repellent agent and the water repellent agent injected into the inside of the hollow fiber membrane seeped out to the outside of the hollow fiber membrane (see Figure 3). The excess water repellent agent was then removed from the membrane module. Dry air at 25°C was then circulated inside the hollow fiber membrane to dry it overnight, yielding a membrane distillation membrane module in which the entire membrane had been hydrophobized.

Two of the obtained membrane distillation modules were subjected to measurement of the liquid intrusion pressure before and after the hydrophobic treatment, and then disassembled to measure the physical properties of the hollow fiber membranes.
The liquid entry pressure was measured using the apparatus shown in Figure 4. First, 500 mL of a 20% 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 aqueous ethanol solution. Next, the outside of the hollow fiber membrane of the membrane distillation membrane module was filled with the aqueous ethanol solution. Then, the pressure applied to the tank was gradually increased until the water level outside the hollow fiber membrane of the module rose (the aqueous solution passed through the membrane). The applied pressure when the water level outside the hollow fiber membrane of the module began to rise was defined as the liquid entry pressure (LEP).
The measurement results before and after the hydrophobic treatment are shown in Table 3.

The remaining membrane distillation module was used to concentrate the raw material liquid.

A scanning electron microscope (SEM) image of the hollow fiber membrane after the hydrophobic treatment is shown in FIG.
Figure 5a) is an image of the cross section of a hollow fiber membrane, with the upper left of the image being the outside of the hollow fiber and the lower right being the inside. Figure 5b) is an enlarged image of the vicinity of the inner surface of the hollow fiber membrane in Figure 5a), and Figure 5c) is an enlarged image of the vicinity of the outer surface of the hollow fiber membrane in Figure 5a. Figure 5d) is an SEM image of the inner surface of the hollow fiber membrane, and Figure 5e) is an SEM image of the outer surface of the hollow fiber membrane. Figure 5f) is an SEM image showing the entire cross section of the hollow fiber membrane.

(2) Implementation of Membrane Distillation Using the membrane module for membrane distillation prepared above, a membrane distillation apparatus having the configuration shown in FIG. 2 was prepared, and membrane distillation was performed.
As a simulated liquid of a raw material liquid containing valuable materials, a solution having the following composition was used.
Solvent: Water Solute: NaCl 1,000 ppm
Liquid volume: 1,000g
NaCl was added to the feedstock solution as a substitute for a valuable substance and as a means of checking if the membrane for membrane distillation was wetting.
The above raw material liquid was filled into a raw material liquid storage tank.
The raw material liquid was circulated through the hollow fiber membranes of the membrane distillation module at a flow rate of 300 ml/min using a pump, and the temperature of the raw material liquid was adjusted using a temperature controller so that it was maintained at 25° C. at the inlet side of the membrane distillation module.

A cooling water storage tank holding 1,000 g of cooling water was installed, and a pump was used to flow the water at a flow rate of 300 ml/min around the outside of the hollow fiber membrane of the membrane distillation membrane module. The temperature of the cooling water was adjusted using a temperature controller to maintain it at 10°C. The conductivity of the cooling water leaving the membrane distillation membrane module was also constantly measured. If the conductivity of the cooling water rose to 500 μS/cm or higher, this was likely due to NaCl added to the feed solution being mixed into the cooling water. Therefore, in this case, it was determined that the membrane distillation membrane had become wet. The cooling water was allowed to flow into the module in a direction opposite to the feed solution, as shown in Figure 2.

The flux Jp (kg/m ² ·h) of the membrane distillation membrane was calculated using the following formula (15).
Wp is the weight of the permeated steam (kg), which is equal to the weight of the cooling water _Wc (kg) before and after the passage of a certain time minus the initial weight of the cooling water _Wc0 . A is the membrane area of the membrane for membrane distillation ( ^m2 ), and t is the operating time (h).

The permeation rate J _s (g/m ² ·h) of valuable materials was calculated using the following formula (16).
Δm _s is the change in the amount (kg) of solute contained in the cooling water, which can be calculated from the weights W _c and W _c0 of the cooling water before and after operation and the solute concentrations C _c and C _c0 in the cooling water.
The concentration factor F (times) of the solute in the raw liquid was calculated by the following formula (17) using the solute concentrations C _f and C _f0 in the raw liquid before and after the operation.

The rejection rate β (%) of the solute in the feed solution was calculated using the weights of solute in the feed solution before and after operation, m _f (kg) and m _f0 (kg), the weights of the feed solution, W _f (kg) and W _f0 (kg), and the solute concentrations in the feed solution, C _f (%) and C _f0 (%), according to the following formula (18).

The average flux during 2 hours of operation was 0.8 (kg/m ² ·h), the NaCl permeation was 0.01 (g/m ² ·h) or less, the concentration ratio was 1.02 times, and the NaCl rejection rate was 99.9% or more.

Example 2
Membrane distillation was carried out under the same conditions as in Example 1, except that the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module was 35°C.
The average flux during 2 hours of operation was 1.7 (kg/m ² ·h), NaCl permeation was 0.01 (g/m ² ·h) or less, the concentration ratio was 1.04 times, and the NaCl rejection rate was 99.9% or more.

Example 3
Membrane distillation was carried out under the same conditions as in Example 1, except that the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module was 45°C.
The average flux during 2 hours of operation was 3.1 (kg/m ² ·h), NaCl permeation was 0.01 (g/m ² ·h) or less, the concentration ratio was 1.08 times, and the NaCl rejection rate was 99.9% or more.

Example 4
Membrane distillation was carried out under the same conditions as in Example 1, except that a 15% by mass aqueous ethanol solution was used as the raw material liquid and the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module (100) was 24°C.
The flux of the organic solvent was calculated using the amount of permeated organic solvent Wos (kg), the weights of cooling water before and after operation _Wc (kg) and _Wc0 (kg), and the concentrations of the organic solvent in the cooling water _Cos (%) and _Cos0 (%), according to the following formula (19):
The solvent concentrations in the raw material liquid and the cooling water were determined by measuring the refractive index of the raw material liquid and the cooling water using a refractometer manufactured by ATAGO.
After 2 hours of operation, the ethanol concentration in the feed solution was 12.9% by mass. The average overall flux during the 2 hours of operation was 1.6 (kg/ ^m² ·h), the water flux was 0.6 (kg/ ^m² ·h), the ethanol flux was 1.1 (kg/ ^m² ·h), the NaCl permeation was less than 0.01 (g/ ^m² ·h), the concentration ratio was 1.04, and the NaCl rejection was more than 99.9%.
The activity coefficient of water obtained by Wilson's equation was 1.0, and the activity coefficient of ethanol was 3.5. The vapor pressure difference between the raw material liquid and the cooling water was 1.7 kPa for ethanol and 1.7 kPa for water.

Example 5
Membrane distillation was carried out under the same conditions as in Example 4, except that the temperature of the raw material liquid at the inlet side of the membrane module for membrane distillation was 34°C.
After 2 hours of operation, the ethanol concentration in the feed solution was 12.2% by mass. The average overall flux during the 2-hour operation was 3.2 (kg/ ^m² ·h), the water flux was 1.6 (kg/ ^m² ·h), the ethanol flux was 1.6 (kg/ ^m² ·h), the NaCl permeation was less than 0.01 (g/ ^m² ·h), the concentration ratio was 1.08, and the NaCl rejection was more than 99.9%.
The activity coefficient of water obtained by Wilson's equation was 1.0, and the activity coefficient of ethanol was 3.6. The vapor pressure difference between the raw material liquid and the cooling water was 3.1 kPa for ethanol and 3.9 kPa for water.

Example 6
Membrane distillation was carried out under the same conditions as in Example 3, except that the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module was 45°C.
After 2 hours of operation, the ethanol concentration in the feed solution was 10.7% by mass. The average overall flux during the 2 hours of operation was 5.4 (kg/ ^m² ·h), the water flux was 3.0 (kg/ ^m² ·h), the ethanol flux was 2.4 (kg/ ^m² ·h), the NaCl permeation was less than 0.01 (g/ ^m² ·h), the concentration ratio was 1.15, and the NaCl rejection was more than 99.9%.
The activity coefficient of water calculated by the Wilson equation was 1.0, and the activity coefficient of ethanol was 3.6. The vapor pressure difference between the raw material liquid and the cooling water was 5.4 kPa for ethanol and 7.9 for water.

Example 7
Membrane distillation was carried out under the same conditions as in Example 4, except that a 15% by mass aqueous solution of acetonitrile was used as the raw material liquid.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 10.4% by mass. The average overall flux during the 2-hour operation was 2.8 (kg/ ^m² ·h), the water flux was 0.6 (kg/ ^m² ·h), the acetonitrile flux was 2.2 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.07, and the NaCl rejection was 99.9% or more.
The activity coefficient of water calculated by Wilson's equation was 1.0, and the activity coefficient of acetonitrile was 7.9. The vapor pressure difference between the raw material liquid and the cooling water was 6.7 kPa for acetonitrile and 1.7 kPa for water.

Example 8
Membrane distillation was carried out under the same conditions as in Example 7, except that the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module was 35°C.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 9.0% by mass. The average overall flux during the 2-hour operation was 4.5 (kg/ ^m² ·h), the water flux was 1.6 (kg/ ^m² ·h), the acetonitrile flux was 2.9 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.12, and the NaCl rejection was 99.9% or more.
The activity coefficient of water calculated by Wilson's equation was 1.0, and the activity coefficient of acetonitrile was 7.6. The vapor pressure difference between the raw material liquid and the cooling water was 10.1 kPa for acetonitrile and 4.0 kPa for water.

Example 9
Membrane distillation was carried out under the same conditions as in Example 8, except that the temperature of the raw material liquid at the inlet side of the membrane distillation membrane module was 45°C.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 8.3% by mass. The average overall flux during the 2-hour operation was 6.5 (kg/ ^m² ·h), the water flux was 3.1 (kg/ ^m² ·h), the acetonitrile flux was 3.3 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.18 times, and the NaCl rejection rate was 99.9% or more.
The activity coefficient of water calculated by Wilson's equation was 1.0, and the activity coefficient of acetonitrile was 7.3. The vapor pressure difference between the raw material liquid and the cooling water was 14.9 kPa for acetonitrile and 7.9 kPa for water.

Example 10
Membrane distillation was carried out under the same conditions as in Example 4, except that a 30% by mass aqueous ethanol solution was used as the raw material liquid.
After 2 hours of operation, the ethanol concentration in the feed solution was 26.9% by mass. The average overall flux during the 2 hours of operation was 2.4 (kg/ ^m² ·h), the water flux was 0.5 (kg/ ^m² ·h), the ethanol flux was 1.9 (kg/ ^m² ·h), the NaCl permeation was less than 0.01 (g/ ^m² ·h), the concentration ratio was 1.06, and the NaCl rejection was more than 99.9%.
The activity coefficient of water calculated by Wilson's equation was 1.0, and the activity coefficient of ethanol was 2.6. The vapor pressure difference between the raw material liquid and the cooling water was 2.7 kPa for ethanol and 1.5 kPa for water.

Example 11
Membrane distillation was carried out under the same conditions as in Example 4, except that a 50% by mass aqueous ethanol solution was used as the raw material liquid.
After 2 hours of operation, the ethanol concentration in the feed solution was 47.6% by mass. The average overall flux during the 2 hours of operation was 3.4 (kg/ ^m² ·h), the water flux was 0.8 (kg/ ^m² ·h), the ethanol flux was 2.6 (kg/ ^m² ·h), the NaCl permeation was less than 0.01 (g/ ^m² ·h), the concentration ratio was 1.09, and the NaCl rejection was more than 99.9%.
The activity coefficient of water obtained by Wilson's equation was 1.2, and the activity coefficient of ethanol was 1.8. The vapor pressure difference between the raw material liquid and the cooling water was 3.8 kPa for ethanol and 1.3 kPa for water.

Example 12
Membrane distillation was carried out under the same conditions as in Example 4, except that a 30% by mass aqueous solution of acetonitrile was used as the raw material liquid.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 24.2% by mass. The average overall flux during the 2-hour operation was 4.7 (kg/ ^m² ·h), the water flux was 1.2 (kg/ ^m² ·h), the acetonitrile flux was 3.6 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.13, and the NaCl rejection was 99.9% or more.
The activity coefficient of water calculated by Wilson's equation was 1.1, and the activity coefficient of acetonitrile was 4.5. The vapor pressure difference between the raw material liquid and the cooling water was 8.1 kPa for acetonitrile and 1.6 kPa for water.

Example 13
Membrane distillation was carried out under the same conditions as in Example 4, except that a 50% by mass aqueous solution of acetonitrile was used as the raw material liquid.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 45.6% by mass. The average overall flux during the 2-hour operation was 5.3 (kg/ ^m² ·h), the water flux was 1.0 (kg/ ^m² ·h), the acetonitrile flux was 4.2 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.14, and the NaCl rejection was 99.9% or more.
The activity coefficient of water calculated by Wilson's equation was 1.3, and the activity coefficient of acetonitrile was 2.5. The vapor pressure difference between the raw material liquid and the cooling water was 8.9 kPa for acetonitrile and 1.6 kPa for water.

Example 14
Membrane distillation was carried out under the same conditions as in Example 7, except that 1000 ppm of sodium dodecyl sulfate (SDS) was used as the solute in the raw material liquid and a 50% by mass aqueous acetonitrile solution was used as the solvent.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 10.1% by mass. The average overall flux during the 2-hour operation was 3.2 (kg/ ^m² ·h), the water flux was 0.8 (kg/ ^m² ·h), the acetonitrile flux was 2.4 (kg/ ^m² ·h), the SDS permeation rate was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.08, and the SDS rejection rate was 99.9% or more.

Comparative Example 1
Membrane distillation was carried out under the same conditions as in Example 7, except that a hollow fiber membrane for membrane distillation was used, which had a porosity of 40% on the inside of the hollow fiber membrane after hydrophobization treatment, a porosity of 70% on the outside of the hollow fiber membrane, and a porosity of 60% throughout the hollow fiber membrane.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 14.1% by mass. The average overall flux during the 2-hour operation was 0.57 (kg/ ^m² ·h), the water flux was 0.12 (kg/ ^m² ·h), the acetonitrile flux was 0.44 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h), the concentration ratio was 1.01, and the NaCl rejection was 99.9% or more.

Comparative Example 2
Membrane distillation was carried out under the same conditions as in Example 7, except that a membrane before the hydrophobic treatment was used as the hollow fiber membrane for membrane distillation.
Ten minutes after the start of operation, membrane wetting occurred and the conductivity of the cooling water exceeded 500 μS/cm, so operation was terminated.

Comparative Example 3
Membrane distillation was carried out under the same conditions as in Example 7, except that a water repellent agent having a hydrophobic polymer concentration of 5.5% by mass was used for the hydrophobic treatment of the hollow fiber membrane for membrane distillation.
After 2 hours of operation, the acetonitrile concentration in the feed solution was 13.7% by mass. The average overall flux during the 2-hour operation was 0.82 (kg/ ^m² ·h), the water flux was 0.15 (kg/ ^m² ·h), the acetonitrile flux was 0.67 (kg/ ^m² ·h), the NaCl permeation was 0.01 (g/ ^m² ·h) or less, the concentration ratio was 1.02, and the NaCl rejection was 99.9% or more.

The above results are summarized in Tables 4 to 10.

The abbreviations in the table have the following meanings:
<Hollow fiber membrane constituent materials>
PVDF: Polyvinylidene fluoride (hydrophobic polymer)
FS-392B: Fluoropolymer water repellent "FS-392B" manufactured by Fluoro Technology Co., Ltd.
<Solvent (b)>
AcCN: acetonitrile IPA: isopropanol

Also,
FIG. 6 is a graph showing the relationship between the inlet temperature of the raw material liquid and the flux of the raw material liquid (total flux of all solvents) in Examples 1 to 9;
Figure 7 shows a graph showing the vapor pressure (calculated values) of ethanol, acetonitrile, and water at each temperature;
FIG. 8 is a graph showing the inlet temperature of the raw material liquid and the vapor pressure difference (calculated value) of ethanol, acetonitrile, and water at the start of operation in Examples 1 to 9;
FIG. 9 is a graph showing the relationship between the organic solvent concentration in the feed solution and the flux of the feed solution (total flux of all solvents) at the start of operation in Examples 4, 7, and 10 to 13;
FIG. 10 is a graph showing the relationship between the organic solvent concentration in the feed solution and the flux of each solvent at the start of operation in Examples 4, 7, and 10 to 13;
FIG. 13 is a graph showing the relationship between the vapor pressure difference and the flux of each solvent at the start of operation in Examples 4 to 13;
Each is shown.

Claims (8)

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 a membrane distillation membrane, and the solvent in the raw material liquid passes through the membrane distillation membrane in a vapor state and moves to the cooling water side,
As the membrane for membrane distillation, a porous membrane having a surface water contact angle of 90° or more, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used,
The membrane distillation membrane is a hollow fiber membrane having a hydrophobic polymer attached to at least a portion thereof,
the porosity of the inner 5% of the hollow fiber membrane in the cross-sectional direction of the membrane is more than 1.0 times and not more than 3.0 times the porosity of the outer 5% ;
The liquid penetration pressure of the hollow fiber membrane with respect to an aqueous ethanol solution having a concentration of 20 weight percent is 0.1 MPa or more and 0.5 MPa or less.
Raw material liquid concentration system. 2. The raw material liquid concentrating system according to claim 1 , wherein the hydrophobic polymer is attached by immersing the hollow fiber membrane in a solution having a polymer concentration of 0.2 to 5.0 mass % and then drying the solution. the hydrophobic polymer is a polymer having a fluorine atom-containing group in a side chain,
the fluorine atom-containing group is at least one selected from a (per)fluoroalkyl group, a (per)fluoropolyether group, an alkylsilyl group, and a fluorosilyl group;
The raw material liquid concentrating system according to claim 1 or 2 . 4. The raw material liquid concentration system according to claim 1, wherein the material of the membrane distillation membrane comprises at least one resin selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, ethylene- tetrafluoroethylene copolymer, and polychlorotrifluoroethylene. 5. The system for concentrating a raw material liquid according to claim 1 , wherein the solvent comprises at least one selected from the group consisting of water, acetonitrile, methanol, ethanol, and isopropanol. The number average molecular weight of the solute is 100 to 50,000;
the solute is a compound having a surface active effect;
The raw material liquid concentrating system according to any one of claims 1 to 5 . The raw material liquid concentrating system according to any one of claims 1 to 6 , wherein the temperature of the raw material liquid is adjusted to a range of 5°C or higher and 50°C or lower. A concentrating apparatus equipped with a membrane distillation membrane and for concentrating a raw material liquid by a membrane distillation method,
As the membrane for membrane distillation, a membrane having a water contact angle of 90° or more on the membrane surface, an average pore size in the range of 0.02 μm or more and 0.5 μm or less, and a porosity of 60% or more and 90% or less is used ,
The membrane distillation membrane is a hollow fiber membrane having a hydrophobic polymer attached to at least a portion thereof,
the porosity of the inner 5% of the hollow fiber membrane in the cross-sectional direction of the membrane is more than 1.0 times and not more than 3.0 times the porosity of the outer 5%;
The liquid penetration pressure of the hollow fiber membrane with respect to an aqueous ethanol solution having a concentration of 20 weight percent is 0.1 MPa or more and 0.5 MPa or less.
Concentrator.