KR102042043B1 - A Draw Solution for forward osmosis using salt of organic acid and use thereof - Google Patents

Abstract

The present invention provides an induction solution for forward osmosis, characterized in that it contains an organic acid salt having a number average molecular weight of 1,000 or less as the induction osmotic solute. In addition, the present invention is a method for preparing a fluid purified by the osmotic pressure of the forward osmotic induction solution containing the forward osmotic induction solute, the fluid in the stock solution through the first membrane toward the forward osmosis induction solution by osmotic pressure A first step of transmitting; And a second step of permeating the fluid passing through the first membrane through a second membrane through which the forward osmosis inducing solute does not pass, and using the forward osmosis induction solution according to the present invention. Provide a method.
Induction solution for forward osmosis of the present invention by using an organic acid salt containing one or more salt forms in one molecule as the inducing solute, it provides an excellent osmotic pressure and can easily recover the inducing solute. Therefore, the water treatment device and the water treatment method using the same save energy in the separation and recovery of the induced solute, and the water treatment effect is excellent.

Description

A draw solution for forward osmosis using salt of organic acid and use

The present invention relates to an induction solution for forward osmosis using an organic acid salt and its use.

Forward osmosis (FO) is a technology that uses osmotic pressure caused by concentration differences. When a draw solution containing a low concentration salt is applied on one side and a high concentration salt (induced solute) is applied on the other side with a semipermeable membrane that selectively permeates only water, the salt concentration difference between the semipermeable membranes Osmotic pressure occurs, and the osmotic pressure causes the water in the influent containing the low concentration of salt to move to a high concentration of induction solution. At this time, it is important to specifically draw solutes that induce osmotic pressure, it is also important to maintain the osmotic pressure through the recovery of the induced solutes. That is, when water selectively penetrates from the lower osmotic pressure to the higher osmotic pressure by the osmotic pressure difference between the semi-permeable membranes, the higher osmotic pressure (induction solution) is diluted, so that the osmotic pressure is gradually lowered. In this case, only water can be selectively permeated in the diluted induction solution to maintain osmotic pressure.

This forward osmosis process is applied to sewage treatment, water treatment and seawater desalination and concentration processes, for example, because the energy required is relatively low compared to the distillation and reverse osmosis processes used for seawater desalination. In other words, the forward osmosis process is economical because it requires very little energy to permeate water in seawater. However, in the forward osmosis process, there are problems such as difficulty in recovering and reusing the induced solute and despreading, and there is still insufficient development of a suitable inducing solute to solve the problem.

Typically, in the case of inorganic salts, such as NaCl solution, which is most commonly used as a conventional induction solute, there is a problem that its recovery and separation are difficult. Therefore, it is limited to the process that does not require the recovery and recycling of the derived solution in the area where seawater and freshwater meet.

Regarding the recovery and reuse of the inducing solutes, there may be cases in which the inducing solutes (NH ₄ ) ₂ CO ₃ or NH ₄ HCO ₃ are used separately or together. (NH ₄ ) ₂ CO ₃ and NH ₄ HCO ₃ are decomposed into ammonia and carbon dioxide gas when heated to about 60 ℃ and can be easily removed, but ammonia is solubility in water that exists in recovered water depending on where it is used. There is a problem to remove ammonia. Therefore, a high temperature vacuum degassing process is needed. As described above, the induction solution using ammonium hydrogen carbonate requires a heating step in the separation process even though recovery and separation are possible, and it is difficult to prevent the loss due to water evaporation. Therefore, these characteristics lower the efficiency of the forward osmosis process is a factor that hinders the utilization of the process.

Other new induction solution materials include induction solutions using nano magnetic particles to which hydrophilic peptides are attached, and attempts to recover nanoparticles by applying a magnetic field. However, there are no specific recovery reports, and induce high osmotic pressure. There is a problem that is difficult.

In addition, osmotic pressure using ionic liquid is too expensive and the proper recovery is not easy.

The role is very important because the permeation performance and the presence or absence of the problem depends on the specific characteristics and concentration of the induction solution in the forward osmosis process as described above, such induction solution has high solubility, high osmotic pressure, low solute back diffusion, easy recovery and Conditions must be in place to be nontoxic.

Under these circumstances, the present inventors solved problems in the conventional forward osmosis process such as recovery of induction solution, difficulty of reusing and reverse diffusion as a result of using the organic acid salt as the forward solute osmotic induction, and at the same time excellent osmotic pressure It has been found that induction solutions for inducing solutes and forward osmosis can be provided.

A first aspect of the present invention provides a forward osmosis inducing solution containing an organic acid salt having a number average molecular weight of 1,000 or less.

A second aspect of the present invention provides ethylenediaminetetraacetic acid (EDTA) salts, lactates, succinates, citrates, acrylates, polyacrylates, sulfonates, polysulfates, phosphates, polyphosphates and It provides an induction osmotic solute containing a mixture thereof.

According to a third aspect of the present invention, there is provided a method for producing a purified fluid by osmotic pressure of an induction solution for forward osmosis containing an induction osmotic solute for forward osmosis, wherein the fluid in a fluid source through a first membrane The first step of permeating to the osmotic induction solution by osmotic pressure; And optionally, a second step of permeating a fluid that has permeated through the first membrane through a second membrane through which the forward osmosis inducing solute does not pass, wherein the forward osmosis inducing solution is used in accordance with the first aspect. It provides a manufacturing method characterized in that the osmotic induction solution.

In a fourth aspect of the present invention, there is provided an apparatus for purifying a fluid from a stock solution by osmotic pressure of an induction solution for forward osmosis containing an induction osmotic solution for forward osmosis, comprising: an induction solution for forward osmosis according to the first aspect; And a first membrane for distinguishing the stock solution from the induction solution for forward osmosis.

Hereinafter, the present invention will be described in detail.

Osmotic pressure (osmotic pressure) is fixed to a semi-permeable membrane that allows a solvent to pass but does not pass a solute, and when a solution and a pure solvent are separately placed on both sides, a certain amount of solvent penetrates into the solution to reach an equilibrium, where both sides of the semi-permeable membrane The temperature is the same, but the pressure difference causes a difference in pressure. Such osmotic pressure-inducing substances are called draw solutes.

In the present invention, the number average molecular weight is an average molecular weight obtained by averaging the molecular weights of the component molecular species of a general compound to a high molecular compound having a molecular weight distribution in a number fraction or a mole fraction, which can usually be obtained by a membrane osmotic method. have.

The present invention is characterized by using an organic acid salt having a number average molecular weight of 1,000 or less as an induction osmotic solute. When the organic acid salt is applied to the forward osmosis process, while providing an excellent osmotic pressure, it is easy to recover the inducing solute / solution and reuse thereof. Furthermore, by adjusting the pH and concentration of the organic acid salt solution it can change its forward osmosis characteristics. Also, the more organic acid groups (eg, carboxyl groups) in one molecule of the organic acid, the more than one organic acid salt in one molecule can be produced, thereby increasing the osmotic pressure under the same conditions.

When purifying a fluid such as water from a fluid source by the osmotic pressure of an induction solution for forward osmosis containing an induction osmotic solution for forward osmosis, the osmotic pressure is relatively smaller as the molecular weight of the inducing solute is higher. However, the present inventors compared EDTA chloride, which is a high molecular weight organic acid salt, with NaCl having a low molecular weight as an inducing solute having the same concentration (% by weight). In contrast, it was confirmed that the back diffusion of the solute was significantly reduced (Table 2 and Table 3).

The organic acid salt is an organic compound exhibiting acidity in an aqueous solution, and may include, but is not limited to, an organic acid group consisting of a carboxyl group, a sulfone group, a sulfin group, or a phosphoric acid group in a molecule. Furthermore, the organic acid may include 1 to 10 organic acid groups consisting of a carboxyl group, sulfone group, sulfin group or phosphoric acid group in one molecule. Preferably, the organic acid may be a polyvalent organic acid having two or more organic acid groups in one molecule. The organic acid groups in one molecule may each independently be a carboxyl group, a sulfone group, a sulfin group or a phosphoric acid group, and thus, different kinds of organic acid groups may be included in one molecule.

The organic acid salt may be a salt derived from a basic solution by treating the organic acid with a basic solution in a form in which the organic acid is present in an aqueous solution. The pH of the organic acid aqueous solution is low in acidity, but the addition of the basic solution produces an organic acid salt and gradually increases the pH. As a result, the induction solution for forward osmosis containing the organic acid salt of the present invention as an inducing solute may be provided by mixing the organic acid aqueous solution and the basic solution.

Non-limiting examples of the organic acid salt is the group consisting of ethylenediaminetetraacetic acid (EDTA) salt, lactate, succinate, citrate, acrylate, polyacrylate, sulfonate, polysulfonate, phosphate, polyphosphate and mixtures thereof There is a selection from. At this time, the more the organic acid salt is present in one molecule, the higher the osmotic pressure and permeation flow rate, so that a polyvalent organic acid salt (eg, EDTA salt) containing two or more organic acid groups in one molecule may be preferable. In the present invention, the organic acid salt may be converted to the form of a salt of several organic acid groups present in the molecule, it may include several organic acid salt form in one molecule. Thus, the osmotic effect can be better than that of the chloride forming only one salt in one molecule.

As used herein, the term "basic solution" refers to a solution having a basic pH of more than 7. A non-limiting example of the basic solution is an aqueous NaOH solution, an aqueous KOH solution, an aqueous Ca (OH) ₂ solution, NH ₄ OH Aqueous solution, Mg (OH) ₂ aqueous solution, Ba (OH) ₂ aqueous solution, Ba (OH) ₃ aqueous solution, Al (OH) ₃ aqueous solution and mixtures thereof, and preferably sodium hydroxide (NaOH) aqueous solution.

In one embodiment of the present invention, when using EDTA as the organic acid and NaOH solution as the basic solution, the EDTA salt according to the present invention can be formed in the form as shown in Scheme 1.

Scheme 1

As another example, when citric acid is used as the organic acid and NaOH solution as the basic solution, the citrate according to the present invention may be formed in the form as shown in Scheme 2.

Scheme 2

In the present invention, the induction solution may have a pH of 8 to 13, and the higher the pH is closer to 13 for higher forward osmosis flow rate (Table 1). The pH can be achieved by titrating a basic solution in an organic acid aqueous solution as described above.

The content of the organic acid salt in the induction solution may be 5 to 30% by weight.

Organic acid salts of the prior art induction solute and a large molecular weight compounds to the polymer as compared to inorganic salt and further a plurality of salt form in a molecule (e. G., -COO ^-) exhibits a high negative charge to form a, the osmotic efficiency It is excellent and easy to recover by simple membrane filtration or centrifugation.

The induction osmotic solute according to the second aspect of the present invention is an ethylenediaminetetraacetic acid (EDTA) salt, lactate, succinate, citrate, acrylate, polyacrylate, sulfonate, polysulfonic acid having a number average molecular weight of 1,000 or less. Salts, phosphates, polyphosphates and mixtures thereof.

According to the osmotic pressure of the induction solution for forward osmosis according to the third aspect of the present invention, a method for producing a purified fluid includes a first step of permeating a fluid in the stock solution to the forward osmosis induction solution by osmotic pressure through a first membrane; And optionally, a second step of permeating a fluid that has permeated through the first membrane through a second membrane through which the forward osmosis inducing solute does not pass, using an induction solution for forward osmosis according to the first aspect. Is characteristic. In this case, the second step may be omitted.

On the other hand, the device for purifying fluid from the stock solution by the osmotic pressure of the forward osmotic induction solution containing the forward osmotic induction solution according to the fourth aspect of the present invention, the forward osmosis induction solution according to the first aspect; And a first membrane for distinguishing the stock solution and the forward osmosis inducing solution.

The fluid may be water or drinking water, but is not limited thereto.

The induction solution for forward osmosis may select an organic acid having a suitable number of organic acid groups, or adjust the molecular weight, concentration and pH of the organic acid to achieve a desired forward osmosis flow rate.

The first membrane is preferably a semi-permeable forward osmosis membrane that is impermeable to a material other than the fluid to be permeated, and is preferably a water-permeable semi-permeable membrane when the fluid is water.

The second membrane is intended to allow the fluid to pass through the first membrane to pass through while the inductive osmotic solute does not pass through. Non-limiting examples of the second membrane include an ultra filtration membrane (UF), a nano filtration membrane (NF) or a reverse osmotic membrane (RO), preferably a nano filtration membrane (NF: nano). filtration membrane). Further, the second membrane may be a nanofiltration membrane having a fractional molecular weight of 200 to 2000. If the fractional molecular weight is less than 200, recovery of the induced solute / solution may not be easy. If the fractional molecular weight exceeds 2000, the recovery of the induced solute / solution may be considerably reduced.

Since the organic acid salts have a negative charge, it is preferable that the second membrane has a negative charge so that the organic acid salts do not pass through the second membrane by the electrostatic force.

In the present invention, the second step may be a step of separating and recovering the forward osmosis inducing solute from the forward osmosis inducing solution by membrane filtration through the second membrane. At this time, the recovered forward osmosis inducing solute may be used again by injecting into the forward osmosis inducing solution.

Recovery of the induced solute for forward osmosis from the induction solution for forward osmosis may be performed using a membrane such as an ultrafiltration membrane, a nanofiltration membrane or a reverse osmosis membrane, but may also be possible through a centrifuge.

1 illustrates a purification apparatus for treating water by osmotic pressure of an induction solution for forward osmosis containing an induction solution for forward osmosis according to one embodiment of the present invention, and one embodiment of the present invention will be described with reference to FIG. 1. do.

The operation mechanism of the forward osmosis purification apparatus moves water in the stock solution to be treated in the forward osmosis system 1 through the first membrane 11 to the induction solution for forward osmosis using a high osmotic pressure, and in the stock solution The induction solution for forward osmosis containing water may be transferred to the recovery system 2 to separate and remove the inducing solute and discharged as a purified fluid. The separated inducing solute may be reused by re-injecting into the induction solution for forward osmosis contacted with the stock solution and the first membrane 11 interposed therebetween.

Separation and recovery of the forward osmosis inducing solute in the recovery system 2 indicates that the organic acid salt (e.g., EDTA salt) that is the inducing solute has a large molecular weight and includes several salt forms in the molecule. Features can be used. That is, since it has a large particle size and charge, it can be easily separated from the purified fluid by filtration through the second membrane 21. For example, since the EDTA salt can be separated by filtration with a nanofiltration membrane having a molecular weight of 200 to 2000, this recovery system 2 can greatly reduce operating energy and at the same time facilitate filtration. Furthermore, since the organic acid salt has a negative charge, it may be preferable that the second membrane 21 has the same negative charge so that the organic acid salt does not pass through the second membrane 21 by the electrostatic force.

The addition and reuse of the organic acid salt, which is the separated and recovered inducing solute, again to the osmotic inducing solution in contact with the stock solution can be achieved through the connecting means (3).

The connecting means 3 may include a pipe through which the separated and recovered induced solute can flow, an electrical conductivity meter on the pipe, and a flow meter to maintain a constant concentration of the forward osmosis induction solution, but is not limited thereto. no. Through the electrical conductivity meter and flow meter, it is possible to monitor whether or not the reference concentration of the induced solute separated and recovered and the flow rate.

Means for discharging the remainder of the solute separated by the recovery system as a purified fluid (4) may include a pipe through which the purified fluid can be discharged, an electrical conductivity meter and a flow meter installed on the pipe, but is not limited thereto. It doesn't happen. Through the electrical conductivity meter and flow meter, it is possible to monitor whether or not exceeding the reference value of impurities and the flow rate as desired.

The stock solution may be sea water, brackish water, ground water, waste water, or the like. For example, drinking water that is purified fluid can be obtained by purifying seawater using the forward osmosis purification apparatus.

In addition, the forward osmosis system (1) is a fluid of the stock solution is passed through the first membrane 11 to the forward osmosis induction solution, it is possible to remove the solvent from the stock solution, thereby concentrating the stock solution It may be.

The induction solution for forward osmosis of the present invention provides an excellent osmotic pressure by using the organic acid salt as the inducing solute and can easily recover the inducing solute. Therefore, the water treatment apparatus and the water treatment method using the same can save energy in the separation and recovery of the induced solute, and the water treatment effect is excellent.

Figure 1 is a schematic diagram of the forward osmosis purification apparatus according to an embodiment of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, these examples are for illustrative purposes only and the scope of the present invention is not limited to these examples.

Example  1: Preparation and Characterization of Induction Solution

Sodium hydroxide (NaOH) solution was used to form a salt of carboxylic acid in 5 wt% aqueous solution of ethylenediaminetetraacetic acid (EDTA, Sigma Aldrich), and the pH was adjusted from 7 to pH 14 by adjusting the concentration of aqueous sodium hydroxide solution. Induction solution was prepared.

In relation to the possibility of applying the forward osmosis process of the prepared forward osmosis induction solution, the characteristics such as permeability, reverse diffusion and recovery were investigated in the following manner.

As a semipermeable membrane for forward osmosis, a polyamide forward osmosis membrane was prepared and used, pure water was used as a feed solution, and the prepared induction solution for forward osmosis was used as an induction solution.

Contacting pure water (conductivity 2 to 3 μS / cm) with an induction solution with a semipermeable membrane in the cell, and measuring the change in weight of the induction solution while circulating the feed and the induction solution at a rate of 2 L / min, the transmittance (permeate flow rate, flux ) Was determined. Furthermore, after 1 hour, the amount of dediffusion of the inducer of the induction solution to the pure water was measured using a conductivity meter. The results are shown in Table 1 below.

pH 8 9 10 11 12 13 Flux
(L / m ² hr) 6.97 10.52 12.17 14.68 14.96 16.12 back diffusion
(μS / cm) 11.71 26.9 17.88 28.5 7.14 7.77

Of EDTA in accordance with the increase of the pH of the induction solution of carboxylic acid (-COOH) is gradually salt form (-COO ^-) by the osmotic pressure is increased and the flux is increased, while the amount of the de-spreading is induced solute diffusion towards the feed bakkwimyeonseo There was no big change. This is because the forward osmosis membrane prevents the reverse diffusion of EDTA salt, so that the reverse diffusion rarely occurs, and thus the osmotic pressure may increase gradually. As a result, when the pH of the induction solution was 13, it showed the highest permeability, and furthermore, it was confirmed that the EDTA salt solution according to Example 1 may be suitable as an induction solution for forward osmosis.

Example  2: Characteristics of Induction Solution According to Concentration of Inducing Solute

The pH of the induction solution was fixed to 13, except that the concentration of the EDTA salt was changed to 5, 10, 20% by weight, the induction solution was prepared and tested in the same manner as in Example 1. The results are shown in Table 2 below.

EDTA salt concentration
(weight%) 5 10 20 Flux
(L / m ² hr) 16.12 21.30 32.19 back diffusion
(μS / cm) 7.77 6.66 6.98

As shown in Table 2, it can be seen that the highest transmittance (Flux) is shown when the concentration of the EDTA salt is 20% by weight, which is generally a trend of improving the transmittance as the osmotic pressure increases as the concentration of the induction solution is increased. It was confirmed to follow.

Comparative example  One: NaCl  Characteristics of Induction Solution According to Concentration

Induction solution was prepared by dissolving NaCl in water at 5, 10, 20% by weight instead of EDTA salt. Except for using the prepared induction solution, it was tested in the same manner as in Example 1. The results are shown in Table 3 below.

NaCl concentration
(weight%) 5 10 20 Flux
(L / m ² hr) 7.6 13.15 19.23 back diffusion
(μS / cm) 83 112 141

As shown in Table 3, it was confirmed that the permeation flow rate at the same concentration is low as well as the reverse diffusion compared to when using the EDTA salt.

Example  3: preparation of induction solution using different organic acids and Forward osmosis  characteristic

An induction solution was prepared and tested in the same manner as in Example 1, except that EDTA salt, lactate, succinate or citrate was used as the inducing solute and the pH of the induction solution was fixed to 13. The results are shown in Table 4 below.

Organic acid salts EDTA salt Lactic acid salt Succinate Citrate Number of organic acid groups in one molecule 4 One 2 3 Flux
(L / m ² hr) 16.12 5.13 8.99 15.68 back diffusion
(μS / cm) 7.77 8.96 7.50 6.95

As shown in Table 4, there was no significant difference in the amount of reverse diffusion depending on the type of organic acid salt, but it can be seen that the permeate flow rate was different depending on the number of organic acid groups, that is, the number of organic acid salts in one molecule. As a result, when the organic acid salt is contained in one molecule, it can be used as an inducing solute. Furthermore, as more organic acid salts are included in one molecule, the permeate flow rate is increased.

Example  4: recovery rate of induction solution

Using the EDTA salt or citrate derived solution prepared by the method according to Example 3, the recovery rate of the organic acid salt as the inducing solute was investigated in the following manner.

For recovery through reverse osmosis, commercially available nanofiltration membranes (NE 40, NE 90, SWRO) were used as separators. Permeate flow rate and salt removal rate (recovery rate) were measured while the pure water and the induction solution were contacted with the nanofiltration membrane in the cell interposed therebetween. The results are shown in Table 5 (EDTA salt) and Table 6 (citrate).

EDTA salt NE 40 NE 90 SWRO Flux
(L / m ² hr) 43.44 18.18 6.28 Rejection (%) 97.18 98.69 99.73

Citrate NE 40 NE 90 SWRO Flux
(L / m ² hr) 52.75 23.89 12.59 Rejection (%) 69 91.5 98

As shown in Table 5 and Table 6, the smaller the permeate flux (Flux), the more difficult the movement of the solution was confirmed that the high salt removal rate. Overall, EDTA salt showed higher recovery than citrate, but both showed excellent recovery through nanofiltration membrane.

Description of the main parts of the drawing
1: forward osmosis system
2: recovery system
3: inducing solute reintroduction means
4: purified fluid discharge means
11: first membrane
21: second membrane

Claims (21)

In the method for producing a purified fluid by the osmotic pressure of the forward osmotic induction solution containing the forward osmotic induction solute,
A first step of permeating the fluid in the fluid source through the first membrane toward the osmotic induction solution by osmotic pressure; And
Optionally, a second step of permeating fluid passing through the first membrane through a second membrane, wherein the forward osmotic inducing solute does not pass and has a negative charge;
The forward osmosis induction solution is characterized in that it contains an organic acid salt having a number average molecular weight of 1,000 or less as the induction osmosis for forward osmosis.
The method according to claim 1, wherein the organic acid salt includes 1 to 10 organic acid groups in one molecule, and the organic acid groups are each independently a carboxyl group, a sulfone group, a sulfin group, or a phosphoric acid group.
The method of claim 1, wherein the organic acid salt is an ethylenediaminetetraacetic acid (EDTA) salt, lactate, succinate, citrate, acrylate, polyacrylate, sulfonate, polysulfonate, phosphate, polyphosphate and mixtures thereof. Characterized in that selected from the group consisting of, manufacturing method.
The method of claim 1, wherein the organic acid salt is derived from a basic solution.
The method of claim 4, wherein the basic solution is NaOH aqueous solution, KOH aqueous solution, Ca (OH) ₂ aqueous solution, NH ₄ OH aqueous solution, Mg (OH) ₂ aqueous solution, Ba (OH) ₂ aqueous solution, Ba (OH) ₃ aqueous solution, Al (OH) ₃ aqueous solution and mixtures thereof, characterized in that selected from the group consisting of.
The method of claim 1, wherein the induction solution is pH 8-13.
The method according to claim 1, wherein the content of the organic acid salt in the induction solution is 5 to 30% by weight.
delete delete The method according to claim 1, wherein the second step is a step of separating and recovering the forward osmosis inducing solute from the forward osmosis inducing solution by membrane filtration through the second membrane.
The method according to claim 1, further comprising the step of re-injecting the forward osmosis inducing solute recovered in the second step into the forward osmosis inducing solution.
delete The method of claim 1, wherein the second membrane is a nano filtration membrane (NF).
The method of claim 1, wherein the second membrane is a nanofiltration membrane having a molecular weight of 200 to 2000.
The method of claim 1, wherein the fluid is water or drinking water.
An apparatus for purifying a fluid from a stock solution by osmotic pressure of an induction solution for forward osmosis containing an induction osmotic solution for forward osmosis,
Forward osmosis induction solution containing an organic acid salt having a number average molecular weight of 1,000 or less as an induction osmotic solute;
A first membrane for distinguishing the stock solution and the induction solution for forward osmosis; And
And a second membrane characterized by passing a fluid and carrying a (-) charge without passing the inducing solute for forward osmosis.
delete The purification apparatus according to claim 16, further comprising means for re-injecting the forward osmosis inducing solute separated and recovered from the forward osmosis inducing solution through the second membrane to the forward osmosis inducing solution.
delete The purification apparatus of claim 16, wherein the second membrane is a nanofiltration membrane.
17. The purification apparatus of claim 16, wherein the purification apparatus is used to concentrate the stock solution by removing a fluid that is a solvent from the stock solution.

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