JP2016041404A - Forward osmosis fresh water generation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forward osmosis fresh water generation system capable of suppressing minimally contamination of a solute into produced fresh water, while improving efficiency for generating fresh water from raw water, which is a fresh water generator using a mannosyl erythritol lipid derivative, a metal salt of the derivative or the like as the solute.SOLUTION: A forward osmosis fresh water generation system includes a forward osmosis device 11 having a forward osmosis membrane, a membrane separation unit 21 having a membrane filter, a dilution absorption liquid line 31 and a high-concentration absorption liquid line 32 for connecting the forward osmosis device to the membrane separation unit.SELECTED DRAWING: Figure 1

Description

  Embodiments of the present invention relate to a forward osmosis water making system using forward osmosis (FO) membranes.

  When two types of aqueous solutions having different solute concentrations are brought into contact with each other through the semipermeable membrane, an osmotic pressure difference is generated between the solutions. In fresh water generation technology using the forward osmosis method, the pressure difference is reduced (the direction in which the difference in solute concentration is eliminated), that is, the phenomenon that water moves from the lower solute concentration to the higher one, and the fresh water is converted from the raw water. Can be obtained.

  In the forward osmosis method, raw water (feed solution), for example, using an absorption solution (draw solution) in which a solute (water-soluble compound) is dissolved so that its concentration (osmotic pressure) is higher than the salt concentration of seawater, power (pressure) Without applying water, move water from the raw water side to the absorbent. Thereafter, fresh water is obtained from the raw water by separating the water and the solute in the absorbent. Further, the separated solute is again dissolved in the absorbing solution, regenerated into a high concentration solution, and circulated. In the forward osmosis method, fresh water can also be obtained from raw water such as sewage, waste water, and industrial waste water in which various components are dissolved.

  As the solute of the absorbing liquid, for example, a volatile solute or a solute having a large change in solubility due to temperature is used. However, when these solutes are used, a heat source is required to heat and cool the absorbing liquid, and it is difficult to completely separate the solutes.

  In addition, the use of so-called biosurfactants as the solute of the absorbing solution has been studied. Biosurfactants are self-organized in water and become colloidal molecules with a large structure. Therefore, it is expected that biosurfactants can be separated from water without a heat source to heat and cool the absorbing solution.

JP2014-50777 JP2014877751A

  An object of the present embodiment is to provide a forward osmosis freshwater generation system capable of minimizing the mixing of solutes into production freshwater while improving the efficiency of producing freshwater from raw water.

According to the embodiment, a forward osmosis device having a forward osmosis membrane that partitions a supply liquid flow path to which raw water is supplied and a first absorbent liquid flow path to which an absorption liquid containing a solute is supplied; A membrane separation device having a filtration membrane that divides a second absorption liquid channel to which the absorption liquid discharged from the absorption liquid channel is supplied and a fresh water channel from which fresh water is discharged; and the first absorption liquid flow A dilute absorbent liquid line connecting a passage and the second absorbent liquid flow path, connecting the second absorbent liquid flow path and the first absorbent liquid flow path, and connecting the second absorbent liquid flow path to the first absorbent liquid flow path. There is provided a forward osmosis freshwater generation system comprising a high-concentration absorption liquid line provided with a pump for returning the absorption liquid to one absorption liquid flow path. The solute has the following general formula

(One of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ and R ₄ Are one of an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen, A is a carboxylic acid or hydrogen, and at least one of A is a carboxylic acid.)
Or a metal salt of the compound.

It is the schematic which shows the forward osmosis fresh water generation system which concerns on 1st Embodiment. It is the schematic which shows the forward osmosis fresh water generation system which concerns on 2nd Embodiment.

  Hereinafter, the forward osmosis freshwater generation system according to the embodiment will be described with reference to the drawings. In addition, embodiment is not limited to the following description.

(First embodiment)
The forward osmosis freshwater generation system according to the first embodiment will be described with reference to FIG.

  The forward osmosis fresh water generation system 1 includes a forward osmosis device 11 having a forward osmosis membrane (semipermeable membrane) 12, a membrane separation device 21 having a filtration membrane 22, and a dilution for connecting the forward osmosis device 11 and the membrane separation device 21. It has the absorption liquid line 31, the high concentration absorption liquid line 32 which connects the forward osmosis device 11 and the membrane separation apparatus 21, and the circulation pump 41 installed on the high concentration absorption liquid line 32.

  The forward osmosis device 11 includes a supply liquid flow path 13 and a first absorption liquid flow path 14 partitioned by a forward osmosis membrane (semipermeable membrane) 12. The raw water line 15 is connected to the upstream side of the supply liquid flow path 13 of the forward osmosis device 11. The drain line 16 is connected to the downstream side of the supply liquid flow path 13 of the forward osmosis device 11. The absorption liquid flowing in the first absorption liquid flow path 14 of the forward osmosis device 11 contains a solute at a higher concentration than the raw water supplied to the supply liquid flow path 13.

  The membrane separation device 21 has a second absorption liquid channel 23 and a fresh water channel 24 partitioned by a filtration membrane 22. The fresh water line 25 is connected to the fresh water flow path 24.

  One end of the diluted absorbent liquid line 31 is connected to the downstream side of the first absorbent liquid flow path 14 of the forward osmosis device 11, and the other end is connected to the upstream side of the second absorbent liquid flow path 23 of the membrane separation device 21. It is connected. The absorption liquid discharged from the first absorption liquid flow path 14, that is, the absorption liquid in the diluted absorption liquid line 31, is called a diluted absorption liquid because the concentration of the solute is low. The diluted absorbent discharged from the first absorbent liquid channel 14 of the forward osmosis device 11 is supplied to the second absorbent liquid channel 23 of the membrane separation device 21 through the diluted absorbent liquid line 31.

  One end of the high-concentration absorption liquid line 32 is connected to the downstream side of the second absorption liquid flow path 23 of the membrane separation device 21, and the other end is upstream of the first absorption liquid flow path 14 of the forward osmosis device 11. Connected to the side. The absorption liquid discharged from the second absorption liquid flow path 23 of the membrane separation device 21, that is, the absorption liquid in the high concentration absorption liquid line 32 is called a high concentration absorption liquid because the concentration of the solute is high. The high concentration absorption liquid discharged from the second absorption liquid channel 23 of the membrane separation device 21 is supplied to the first absorption liquid channel 14 of the forward osmosis device 11 through the high concentration absorption liquid line 32 by the circulation pump 41. The That is, the absorption liquid is circulated between the first absorption liquid flow path 14 of the forward osmosis device 11 and the second absorption liquid flow path 23 of the membrane separation device 21 through the diluted absorption liquid line 31 and the high concentration absorption liquid line 32. The

  As raw water, seawater, sewage, drainage, and industrial wastewater can be used. The raw water is supplied to the forward osmosis device 11 through the raw water line 15 after pretreatment such as turbidity is performed if necessary.

  The forward osmosis membrane (semipermeable membrane) 12 is not particularly limited in terms of material, shape, size, structure, etc., and may be any membrane that selectively permeates water. For example, a reverse osmosis (RO) membrane may be used. it can. The material of the membrane is appropriately selected according to the raw water quality and the type of the absorbing solution, and examples thereof include cellulose acetate, aromatic polyamide, aromatic sulfone, polybenzimidazole, and graphene. Examples of the shape include a flat membrane, a spiral type module using a flat membrane, a hollow fiber type module, and a cylindrical type module.

  The absorbing solution contains, as a solute, a surfactant that self-assembles in water and becomes, for example, a micelle structure, vesicle structure, ribbon structure, or string-like micelle structure. By selecting the pore size of the filtration membrane 22 of the membrane separation device 21 to an appropriate size (for example, 0.01 to 1 μm) with respect to the self-assembled surfactant, the surfactant as a solute in the filtration membrane 22 And water can be easily separated.

  Surfactants include those produced by chemical synthesis using petroleum as raw materials, yeasts (single cell fungi that have cell walls and vegetatively proliferate for a certain period of the life cycle), bacteria (single cell microorganisms and nuclear membranes) And so-called biosurfactants produced from organic substrates by the action of a group of prokaryotes that do not have any). In particular, the latter biosurfactant is preferable because it has high biodegradability, is hypoallergenic, is environmentally friendly, and can secure a production amount. Furthermore, since biosurfactant has a hydroxyl group, a carboxy group, an amino group, and an asymmetric carbon in a well-balanced state, it not only becomes a micelle structure or ribbon structure in water, but also becomes a colloidal molecule having a large structure by self-organization.

  Biosurfactants are classified into sugar type, amino acid type, organic acid type, and polymer type from the structure of hydrophilic group. Specific biosurfactants include glycoforms of sophorolipid, mannosyl erythritol lipid, mannosyl mannitol lipid, mannosyl arabitol lipid, mannosyl ribitol lipid, rhamnolipid, trehalo lipid, cerviolipid and derivatives thereof. An acyl peptide-based ornithine lipid or surfactin can also be used.

In this embodiment, a mannosyl erythritol lipid derivative represented by the following general formula or a metal salt of this derivative is used as the solute of the absorbing solution.

(One of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ and R ₄ One is an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen, A is hydrogen or a carboxylic acid, and at least one of A is a carboxylic acid. A may be a carboxylic acid.)
Hereinafter, the above derivatives are referred to as mono-, di- or tricarboxylated mannosyl erythritol lipids. The effects obtained by using mono-, di- or tricarboxylated mannosyl erythritol lipids as solutes are described in detail below.

Mannosyl erythritol lipid which is a biosurfactant is represented by the structural formula shown below. Among the derivatives of mannosyl erythritol lipids, those in which both R _{3 and} R ₄ are hydrogen and one of R _{1 and} R ₂ is a hydrogen or a hydrophilic group such as a sugar chain exhibit high solubility. Suitable for solutes.

(Here, one of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ And one of R ₄ is an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen.)
By modifying the above mannosyl erythritol lipid derivative with a compound having a carboxy group, a surfactant in which a part or all of the OH groups of the erythritol moiety are substituted with carboxylic acid can be obtained. For example, when succinic acid or succinic anhydride, which is a dicarboxylic acid, is reacted with mannosyl erythritol lipid in the presence of a lipase catalyst, one of the OH group of the erythritol moiety and one of the carboxy groups of succinic acid (or succinic anhydride) is produced. An ester bond is made and the OH group is modified with the other carboxy group. The compound that modifies the OH group of the erythritol moiety is not limited to succinic acid, and a dicarboxylic acid having two carboxy groups or an anhydride thereof can be used.

  It is not always necessary to modify all of the OH groups in the erythritol part of the mannosyl erythritol lipid with carboxy groups. By adjusting the amount of lipase catalyst and the amount of dicarboxylic acid added, 1 to 3 OH groups of the erythritol moiety can be modified with a carboxy group.

As an example, the chemical formula of tricarboxylated mannosyl erythritol lipid in which all of A in the general formula is modified with a carboxy group is shown below.

(One of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ and R ₄ Is an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen, and R ₅ is an alkyl chain having 2 or more carbon atoms.)
Such mono-, di- or tricarboxylated mannosyl erythritol lipids have improved solubility in water as carboxy groups become carboxylate ions compared to those in which the OH group of the erythritol moiety is not modified with a carboxy group. . Mono-, di- or tricarboxylated mannosyl erythritol lipids can be used as the solute of the absorption liquid of the forward osmotic freshwater system.

  In addition, a sodium salt obtained by reacting mono-, di- or tricarboxylated mannosyl erythritol lipids with sodium hydroxide can be used as the solute of the absorption liquid of the forward osmosis water making system. When a sodium salt of mono-, di- or tricarboxylated mannosyl erythritol lipid is used, the osmotic pressure in the forward osmosis device 11 can be improved and the amount of permeated water (flux) of the forward osmosis membrane 12 can be increased.

As an example, the chemical formula of the sodium salt of tricarboxylated mannosyl erythritol lipid is shown below.

(One of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ and R ₄ Is an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen, and R ₅ is an alkyl chain having 2 or more carbon atoms.)
Here, the compound used for modification of the carboxy group to the mannosyl erythritol lipid derivative may be any carboxylic acid as described above, but is a dicarboxylic acid having a carbon chain of 2 carbon atoms between two carboxy groups. It is preferable to use a certain succinic acid or succinic anhydride which is an anhydride thereof. By using succinic acid or succinic anhydride, the increase in the molecular weight of the mannosyl erythritol lipid derivative accompanying carboxylation can be minimized. As a result, the number of moles of solute contained in the absorbing solution at the time of saturation is large even when the solubility is the same as compared with the case where a dicarboxylic acid having a large molecular weight is modified. Therefore, the osmotic pressure (Π) represented by the following formula increases, and the amount of permeated water (flux) of the forward osmosis membrane 12 in the forward osmosis device 11 increases.

Π = CRT
Here, Π: osmotic pressure [atm], C: molar concentration [mol / dm ^3] , R: gas constant [atm · dm ³ / K · mol], and T: temperature [K].

  The filtration membrane 22 of the membrane separation device 21 preferably has a pore diameter as large as possible as long as it does not permeate a surfactant that is self-assembled in water, that is, a solute molecular aggregate. The pore diameter is preferably 0.01 to 1 μm, for example. For example, polyethylene, polyolefin, polyethersulfone, polyvinylpyrrolidone, or ceramic can be used as the material of the filtration membrane. The shape of the filtration membrane can be, for example, a flat membrane, a spiral-type module using a flat membrane, a hollow fiber type module, or a cylindrical module. As the filtration membrane, for example, a microfiltration membrane (MF) for water treatment can also be used. Further, an ultrafiltration membrane (UF) having a fractional molecular weight of 1000 to 10,000 Da can also be used.

  Next, the effect | action of the forward osmosis fresh water generation system which concerns on this embodiment is demonstrated.

  In the forward osmosis device 11, raw water is supplied from the raw water line 15 as a supply liquid to the supply liquid flow path 13, and a high concentration absorption liquid is supplied from the high concentration absorption liquid supply line 32 to the first absorption liquid flow path 14. While the raw water flows in the supply liquid flow path 13, it comes into contact with the absorption liquid flowing in the first absorption liquid flow path 14 via the forward osmosis membrane 12, and an osmotic pressure difference is generated between the raw water and the high concentration absorption liquid. Using the osmotic pressure difference as a driving force, water moves through the semipermeable membrane from the lower solute concentration to the higher solute concentration. That is, the water in the raw water moves from the supply liquid channel 13 side to the first absorption liquid channel 14 side by forward osmosis. At this time, the supply liquid is concentrated by the movement of the water toward the high-concentration absorbing liquid, and is drained through the drain line 16 as concentrated drainage.

  The absorption liquid diluted by absorbing water from the raw water is supplied to the second absorption liquid flow path 23 of the membrane separation device 21 through the diluted absorption liquid line 31. At this time, the surfactant, which is a solute in the diluted absorbent, is self-assembled, and is a molecular assembly having a size larger than the pore size of the filtration membrane 22 of the membrane separation device 21. Moreover, since the pressure is applied to the diluted absorption liquid by the absorption liquid osmotic pressure in the forward osmosis device 11, from the diluted absorption liquid flowing through the second absorption liquid channel 23, for example, a pore diameter of 0.01 to 1 μm. Only fresh water is separated as filtered water by the filtration membrane 22 and moves to the fresh water flow path 24. The fresh water that has moved to the fresh water flow path 22 is supplied to the user as drinking water or the like through the fresh water line 25.

  On the other hand, the diluted absorption liquid becomes a high-concentration absorption liquid by concentrating the solute as a part of water moves to the fresh water side in the second absorption liquid channel 23. The high concentration absorption liquid is discharged to the high concentration absorption liquid line 32 and returned to the first absorption liquid flow path 14 of the forward osmosis device 11 by the operation of the circulation pump 41. The circulation amount of the absorbing liquid can be adjusted as appropriate by arbitrarily setting the flow rate of the circulation pump 41. Since the pressure for filtering the diluted absorbent by the membrane separator 21 uses the pressure energy generated as the osmotic pressure by the forward osmosis device 11, the energy necessary for the operation of the circulation pump 41 circulates the absorbent, In addition, the power consumption can be kept low because it is sufficient to prevent the backflow of the high-concentration absorbent from the first absorbent liquid flow path 14 of the forward osmosis device 11.

  In the circulation of the absorbing solution between the forward osmosis device 11 and the membrane separation device 21 in the forward osmosis freshwater generator 1, a sodium salt of di- or tricarboxylated mannosyl erythritol lipid was used as the solute of the absorbing solution. Examples are detailed below.

  The diluted absorbent whose concentration has been reduced by absorbing the water in the raw water by the normal osmosis device 11 passes through the diluted absorbent line 31 from the first absorbent fluid channel 14 and the second absorbent fluid channel 23 of the membrane separator 21. To be supplied. Here, when an acid derivative of mannosyl erythritol lipid is used as a solute, this solute self-assembles (such as micelles or vesicles) to generate molecular aggregates (such as micelles or vesicles) and becomes enormous. When the diluted absorbent is supplied to the second absorbent flow path 23 of the membrane separation device 21, the giant solute molecular aggregate in the diluted absorbent has a pore size (for example, 0.01 to 1 μm) of the filtration membrane 22. ), It can be easily separated into solute and fresh water by the filtration membrane 22.

  In particular, when the solute is a mono-, di- or tricarboxylated mannosyl erythritol lipid, the solute is ionized into a carboxylate ion in water to improve the solubility, resulting in a high concentration absorbent. As a result, the solute molar concentration in the absorption liquid increases, the osmotic pressure in the forward osmosis device 11 improves, and the amount of permeated water (flux) of the forward osmosis membrane 12 of the forward osmosis device 11 increases.

  Furthermore, when the sodium salt of mono, di or tricarboxylated mannosyl erythritol lipid is used as the solute, the solute dissociates into sodium ion and carboxylate ion in water, and the carboxylic acid of mono, di or tricarboxylated mannosyl erythritol lipid is dissociated. One mole or more of sodium ions is produced per mole of acid ions. As a result, the solute molar concentration in the absorption liquid increases, the osmotic pressure (Π) expressed by the following formula improves, and the amount of permeated water (flux) of the forward osmosis membrane 12 of the forward osmosis device 11 increases.

Π = CRT
Here, Π: osmotic pressure [atm], C: molar concentration [mol / dm ^3] , R: gas constant [atm · dm ³ / K · mol], and T: temperature [K].

  When the sodium salt of monocarboxylated mannosyl erythritol lipid is dissociated, a maximum of 1 mole of sodium ion is produced per mole of carboxylate ion. When the sodium salt of dicarboxylated mannosyl erythritol lipid is dissociated, a maximum of 2 moles of sodium ions are produced per mole of carboxylate ions. When the sodium salt of tricarboxylated mannosyl erythritol lipid is dissociated, a maximum of 3 moles of sodium ions are produced per mole of carboxylate ions. The higher the molar concentration of sodium ions in the absorbing solution, the greater the amount of permeated water (flux) of the forward osmosis membrane 12 of the forward osmosis device 11. Therefore, the dicarboxylated mannosyl erythritol lipid is more divalent than the monocarboxylated mannosyl erythritol lipid sodium salt. The sodium salt is preferred. Furthermore, the sodium salt of tricarboxylated mannosyl erythritol lipid is preferred to the sodium salt of dicarboxylated mannosyl erythritol lipid.

  According to the present embodiment, in the circulation of the absorption liquid between the forward osmosis device 11 and the membrane separation device 21, the solute molar concentration in the absorption liquid is changed to the impurity concentration (such as salinity) in the raw water by the forward osmosis device 11. On the other hand, the diluted absorption liquid can be easily separated into a mannosylerythritol lipid molecular aggregate (solute) and water (fresh water) by the filtration membrane 22 of the membrane separation device 21.

(Second Embodiment)
A forward osmosis freshwater generation system according to a second embodiment will be described with reference to FIG. However, the same members as those in FIG.

  The forward osmosis fresh water generation system 1 shown in FIG. 2 includes a first pH adjustment device 51 provided on a diluted absorption liquid line 31 that connects the forward osmosis device 11 and the membrane separation device 21, and a high concentration absorption liquid line 32. It has the 2nd pH adjustment apparatus 52 provided on the top. The first pH adjuster 51 injects acid such as hydrochloric acid or sulfuric acid as a pH adjuster into the diluted absorbent discharged from the forward osmosis device 11, and the pH of the diluted absorbent is adjusted to the acidic side, preferably pH 3-5. By adjusting, the sodium salt of mono-, di- or tricarboxylated mannosyl erythritol lipid in the diluted absorbent is changed to carboxylic acid. Since mono-, di- or tricarboxylated mannosyl erythritol lipids are acidic and insoluble, they are precipitated in a dilute absorption solution and become larger molecular aggregates by self-assembly. As a result, the diluted absorbent can be easily separated into solute and fresh water by the filtration membrane 22 of the membrane separator 21.

  On the other hand, in the second pH adjusting device 52, caustic soda (sodium hydroxide) or the like as an alkaline pH adjusting agent is injected into the high concentration absorbing liquid discharged from the membrane separation device 22 (or the circulation pump 41) to absorb the high concentration. Adjusting the pH of the solution to neutral or alkaline, preferably pH 7-8, changes the solute from mono, di or tricarboxylated mannosyl erythritol lipid to sodium salt of mono, di or tricarboxylated mannosyl erythritol lipid Let Since this sodium salt dissociates into the carboxylate ion and sodium ion of mono-, di- or tricarboxylated mannosyl erythritol lipid, the solute in the high-concentration absorbing solution dissolves.

  As described above, when the pH adjusters 51 and 52 are used, the self-organization of the solute into the molecular assembly is more efficiently advanced in the membrane separator 21 than when the pH adjusters 51 and 52 are not used. The solute molar concentration in the high concentration absorbent introduced into the forward osmosis device 11 can be further increased. Thereby, in the circulation of the absorption liquid between the forward osmosis device 11 and the membrane separation device 21, the solute molar concentration in the absorption liquid in the forward osmosis device 11 and the concentration of impurities (such as salinity) in the raw water. The difference, that is, the osmotic pressure difference between the absorption liquid and the raw water can be made higher, and on the other hand, the diluted absorption liquid can be more easily separated into the solute and the fresh water by the filtration membrane 22 of the membrane separation device 21, so that it is efficient from the raw water. Fresh water can be produced.

  As described above, according to the present embodiment, the following effects can be obtained.

  The absorption liquid contains a surfactant that self-assembles in water as a solute and forms a molecular assembly larger than the pore diameter of the filtration membrane of the membrane separator. For this reason, the size of the molecular assembly of the surfactant can be enlarged while the diluted absorbent is circulated through the diluted absorbent line 31 that connects the forward osmosis device 11 and the membrane separation device 21. Can be separated.

  In the present embodiment, a mannosyl erythritol lipid derivative having high water solubility is modified with a compound having a carboxy group, and a part or all of the OH groups of the erythritol part are substituted with carboxylic acid. Mono-, di- or tricarboxylated mannosyl erythritol lipids with improved properties are used as solutes. Also, by reacting mono, di or tricarboxylated mannosyl erythritol lipid with sodium hydroxide to form sodium salt, it dissociates into sodium ion and carboxylate ion of mono, di or tricarboxylated mannosyl erythritol lipid in water. Solute is obtained. As a result, the osmotic pressure (Π) is improved by increasing the solute molar concentration in the absorbent.

  By using the absorption liquid containing this solute, the diluted absorption liquid is separated into a molecular aggregate (solute) of fresh water and a surfactant in the filtration membrane 22 of the membrane separator 21 having a pore diameter of, for example, 0.01 to 1 μm. Can be easily separated, and the osmotic pressure can be improved to increase the amount of permeated water (flux) of the forward osmosis membrane 12 of the forward osmosis device 11.

  In the present embodiment, the sodium salt of mono-, di- or tricarboxylated mannosyl erythritol lipid is adjusted by adjusting the pH of the diluted absorbent discharged from the forward osmosis device 11 to the acidic side by the first pH adjusting device 51. The carboxylic acid is insoluble in water and precipitates in the absorbing solution. On the other hand, by adjusting the pH of the high concentration absorbent discharged from the membrane separation device 22 (or the circulation pump 41) from the second pH adjusting device 52 to the neutral or alkaline side, the mono, di or tri The carboxylated mannosyl erythritol lipid changes to a soluble sodium salt and dissociates to yield the carboxylate and sodium ions of the mono, di or tricarboxylated mannosyl erythritol lipid. Therefore, the absorbing solution is returned to the high concentration absorbing solution in which these ions are dissolved.

  As a result, in the circulation of the absorbing solution between the forward osmosis device 11 and the membrane separation device 21, the solute molar concentration in the absorbing solution is set in the raw water by the forward osmosis device 11 than when the pH adjusting devices 51 and 52 are not provided. On the other hand, the diluted absorbent can be easily separated into solute and fresh water by the filtration membrane 22 of the membrane separator 21.

  According to at least one embodiment described above, mixing of solutes in production fresh water can be minimized while improving the efficiency of producing fresh water from raw water.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Forward osmosis water generation system, 11 ... Forward osmosis device, 12 ... Forward osmosis membrane, 15 ... Raw water line, 21 ... Membrane separator, 22 ... Filtration membrane, 31 ... Dilution absorption liquid line, 32 ... High concentration absorption liquid line 41 ... circulation pump, 51 ... first pH adjusting device, 52 ... second pH adjusting device.

Claims (4)

A forward osmosis device having a forward osmosis membrane that partitions a supply liquid flow path to which raw water is supplied and a first absorption liquid flow path to which an absorption liquid containing a solute is supplied;
A membrane separation device having a filtration membrane that partitions a second absorption liquid channel to which the absorption liquid discharged from the first absorption liquid channel is supplied and a fresh water channel from which fresh water is discharged;
A diluted absorbent liquid line connecting the first absorbent liquid flow path and the second absorbent liquid flow path;
A high concentration absorbent having a pump connected to the second absorbent channel and the first absorbent channel and returning the absorbent from the second absorbent channel to the first absorbent channel Line,
The solute has the following general formula

(One of R ₁ and R ₂ is a fatty acid residue having 2 to 16 carbon atoms, the other of R ₁ and R ₂ is hydrogen or a hydrophilic group, both R ₃ and R ₄ are acetyl groups, R ₃ and R ₄ Are one of an acetyl group and the other of R ₃ and R ₄ is hydrogen, or both R ₃ and R ₄ are hydrogen, A is a carboxylic acid or hydrogen, and at least one of A is a carboxylic acid.)
Or a metal salt of the compound. The solute is a compound in which the carboxylic acid is —CO (CH ₂ ) _n COOH, or a sodium salt of the compound, wherein n is an integer of 2 or more. Forward osmosis freshwater generation system. The forward osmotic freshwater generation system according to claim 1, wherein the solute is a compound in which the carboxylic acid is —CO (CH ₂ ) ₂ COOH, or a sodium salt of the compound.   A first pH adjusting device for injecting acid into the absorbing solution of the diluted absorbing solution line; and a second pH adjusting device for injecting alkali into the absorbing solution of the high concentration absorbing solution line. The forward osmosis freshwater generation system according to claim 1.

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