WO2013025012A1

WO2013025012A1 - Forward osmotic desalination device using membrane distillation method in which part of draw solution is fed directly into forward osmostic type separator

Info

Publication number: WO2013025012A1
Application number: PCT/KR2012/006373
Authority: WO
Inventors: Sung Mo Koo; Sang Jin Lee; Sung Min Shim
Original assignee: Stx Heavy Industries Co., Ltd.
Priority date: 2011-08-12
Filing date: 2012-08-10
Publication date: 2013-02-21
Also published as: KR20130017933A; KR101298724B1

Abstract

The present invention relates to a freshwater separator employing a membrane distillation method, and a forward osmosis desalination system comprising the freshwater separator. More particularly, the present invention relates to a desalination system in which only a portion of a diluted draw solution is separated into gas and freshwater in a freshwater separator and the other portion of the diluted draw solution is fed directly into a forward osmosis separator, so that the efficiencies of the seawater separator and the desalination system comprising same are increased and the energy consumption is reduced.

Description

FORWARD OSMOTIC DESALINATION DEVICE USING MEMBRANE DISTILLATION METHOD IN WHICH PART OF DRAW SOLUTION IS FED DIRECTLY INTO FORWARD OSMOSTIC TYPE SEPARATOR

The present invention relates to a freshwater separator using a membrane distillation method, and a forward osmosis desalination system comprising the freshwater separator. More particularly, the present invention relates to a desalination system in which only a portion of a diluted draw solution is separated into gas and freshwater in a freshwater separator and the other portion of the diluted draw solution is fed directly into a forward osmosis separator after initialization of draw solution concentration, so that the efficiencies of the seawater separator and the desalination system comprising the same are increased and the energy consumption is reduced.

Various processes for removing contaminants from raw water and producing freshwater have been studied. Particularly, if the raw water is seawater, a system for carrying out this process is designated as a seawater desalination system and carries out the process of removing not only Cl and Na, but also a number of inorganic salts.

Many desalination method such as evaporation, reverse osmosis (RO), crystallization, electrodialysis, forward osmosis (FO), etc. have been used to desalinate seawater into fresh water. However, the process of forward osmosis has only rarely been used to example to produce a small amount of emergency water, rather than to desalinate a large amount of seawater.

In recent years, studies on forward osmosis processes have been actively conducted, and patents related thereto include US 7,560,029, US 7,566,402 and the like. FIG. 1 shows a schematic view of a prior art separator 100 for desalinating seawater using forward osmosis, as described in the aforementioned patent documents.

Osmosis is a physical phenomenon in which water naturally moves through the membrane from the lower concentration side to the higher concentration side to maintain the equilibrium concentration. Osmosis takes place when different concentrations of solutions are separated from each other by a membrane 110 having selective permeability. Pressure occurring when a relatively large amount of water moves to the higher concentration side is defined as osmotic pressure.

Forward osmosis is an osmotic process that uses a semi-permeable membrane to separate water from a low-concentration aqueous solution, but the driving force for this separation is an osmotic pressure gradient, unlike a reverse osmosis process that uses hydraulic pressure. In the forward osmosis process, a draw solution having a concentration (about 5-10 times) higher than feed water is used to induce a net flow such that only water contained in the solution water passes through the membrane.

Using the draw solution, osmosis in the membrane 110 occurs so that only water in seawater is permeated to the higher concentration of the draw solution. Seawater is discharged as brine, and the draw solution is diluted and passes through a separate draw solution separator 120. In the draw solution separator 120, freshwater and draw solutes are separated from the diluted draw solution, and the separated draw solutes are concentrated and fed to the forward osmosis process. This process is repeated in the system so that freshwater can be continuously produced.

Generally, in seawater desalination systems, the production of freshwater relative to the energy input or the chemical input is problematic. Particularly, in the case of forward osmosis desalination systems, the recovery rate of the draw solution is directly related to the efficiency of the seawater desalination system.

US 2009/0297431 discloses a method for increasing the recovery rate of a draw solution. In this method, the draw solution is recovered using MSF (Multiple-stage Flash Distillation) or MED (Multi-Effect Distillation). Cases such as these have the problems of having to use a large number of chambers in order to achieve the desired recovery rate, the method is difficult to apply in practice, separate pressure control is required to make the process complex, and energy consumption is high.

Technologies capable of solving such problems include Korean Patent Registration No. 1020316 owned by the present inventor (see FIG. 2).

In the case of Korean Patent Registration No. 1020316, when a diluted draw solution is fed from a forward osmosis separator 100 into a freshwater separator 1000, the diluted draw solution is separated into gas and freshwater in the freshwater separator 1000, after which the separated gas and freshwater are concentrated again to make a concentrated draw solution which should be fed again into the forward osmosis separator 100. Specifically, the process for separating and re-concentrating the draw solution is required, during which unnecessary energy consumption can occur.

In addition, a separate heating member is adopted to remove solid-state ammonium. This process of removing solid-state ammonium also causes unnecessary energy consumption, thus reducing the efficiency of the desalination system.

Accordingly, the present invention has been made keeping in mind of the above problems occurring in the prior art, and an object of the present invention is to increase the recovery rate of a draw solution, thereby improving the efficiency of the desalination system.

Particularly, an object of the present invention is to avoid the unnecessary energy consumption which occurs when the process for separating and re-concentrating the draw solution is repeated due to the characteristics of the forward osmosis separator disclosed in Korean Patent Registration No. 1020316 as well as the energy consumption required to remove solid-state ammonium, thereby increasing the efficiency of the desalination system.

In order to accomplish the above objects, one aspect of the present invention provides a freshwater separator including: a first membrane contactor into which a portion of a diluted draw solution is fed and in which gas and freshwater are separated from the fed diluted draw solution; a second membrane contactor into which the separated gas is fed so that it may dissolve in a fluid flowing therein; and a vacuum pump connected with the first membrane contactor and the second membrane contactor, wherein the fluid containing the gas dissolved therein from the second membrane contactor is mixed with the other portion of the diluted draw solution. Preferably, the freshwater separator further includes a recycling tank in which the fluid containing the gas dissolved therein from the second membrane contactor is mixed with the other portion of the diluted draw solution.

Preferably, the freshwater separator further includes: a buffer tank into which the diluted draw solution is fed; and a diluted draw solution chamber which receives a portion of the diluted draw solution from the buffer tank and feeds the received portion into the first membrane contactor, wherein the buffer tank feeds the other portion of the diluted draw solution into the recycling tank.

Preferably, water vapor separated in the first membrane contactor passes through a condenser and is fed as water into the diluted draw solution chamber.

Preferably, the freshwater separator further includes a concentration control tank which receives the fluid containing the gas dissolved therein from the second membrane contactor and which is in fluid communication with the recycling tank.

In this case, the freshwater separator more preferably further includes a solid solute capture tank in which solid solutes of the gas separated in the first membrane contactor are collected, wherein the solid solutes collected in the solid solute capture tank are fed into the concentration control tank.

More preferably, a portion of the diluted draw solution is fed into the diluted draw solution chamber, and the other portion of the diluted draw solution is fed via the concentration control tank into the recycling tank.

Preferably, the first and second membrane contactors include: a distribution tube which is located within the first and second membrane contactors so that fluid may flow therein, the distribution tube including a plurality of openings; and a plurality of hollow fiber cartridges surrounding the distribution tube.

Another aspect of the present invention provides a desalination system including: a forward osmosis separator including a membrane; and a freshwater separator which is in fluid communication with the forward osmosis separator, wherein the forward osmosis separator uses a forward osmosis method in which raw water may be fed to one side of the membrane and discharged as brine, and a concentrated draw solution may be fed to the other side of the membrane and discharged as a diluted draw solution and wherein the freshwater separator includes: a buffer tank into which the diluted draw solution is fed; a diluted draw solution chamber into which a portion of the diluted draw solution is fed from the buffer tank; a first membrane contactor into which a portion of a diluted draw solution is fed and in which gas and freshwater are separated from the fed diluted draw solution; a second membrane contactor into which the separated gas is fed so that it may dissolve in a fluid flowing therein to form the concentrated draw solution; and a vacuum pump connected with the first membrane contactor and the second membrane contactor, wherein the other portion of the diluted draw solution fed into the buffer tank is fed into the recycling tank.

Preferably, NH₄HCO₃(solid) is used as draw solute in the draw solution in which a large amount of NH₃ ⁺ and HCO₃ ^- is solved. In this case, more preferably, a portion of the diluted draw solution which is fed into the diluted draw solution chamber makes up 10-40% of the total of the diluted draw solution which is fed into the freshwater separator.

Preferably, water vapor separated in the first membrane contactor is condensed in a condenser and is fed into the diluted draw solution chamber.

Preferably, the freshwater separator further includes: a concentration control tank which is in fluid communication with the second membrane contactor; and a solid solute capture tank in which solid solutes of the gas separated in the first membrane contactor are collected, wherein the concentration control tank is fed with one or more of the other portion of the diluted draw solution fed into the buffer tank, the solid solutes collected in the solid solute capture tank, and the concentrated draw solution formed in the second membrane contactor, and the draw solution is controlled to be a predetermined concentration in the concentration control tank and is fed through the recycling tank to the other side of the membrane of the forward osmosis separator.

The present invention has several advantages. First, according to the present invention, the fresh water production is increased because the transmembrane vapor is returned to the diluted drow solution chamber to produce additional fresh water. Second, in case the diluted draw solute flowing from FO to MD is excessively changed to fresh water, the consumed energy is increased. Therefore, according to the present invention, unnecessary energy consumption is reduced because only necessary amount of fresh water is produced, thereby increasing the energy efficiency of the entire system. Third, according to the present invention, the recovery rate of the draw solute is increased because solid-state ammonium is directly returned to the solid solute capture tank.

Accordingly, high freshwater production can be achieved with low equipment investment costs and low maintenance and management costs.

FIG. 1 is a schematic view showing a forward osmosis desalination system according to the prior art.

FIG. 2 is a schematic view showing another example of a forward osmosis desalination system according to the prior art.

FIG. 3 is a desalination system according to one embodiment of the present invention.

FIG. 4 is a desalination system according to another embodiment of the present invention.

FIG. 5 is a partial cross-sectional perspective view showing a membrane contactor according to the present invention.

In the accompanying drawings of the specification, valves, pressure meters, thermometers and the like which can be located in pipelines, tanks, chambers and the like are not illustrated. Such valves, pressure meters and thermometers may be those known in the prior art and may be used at suitable positions depending on the choice of the user.

As described below, a system of the present invention may also consist of a freshwater separator 1000 without a forward osmosis separator.

Raw water which may be introduced to one side of a membrane of the forward osmosis separator 100 may be seawater, brackish water, wastewater, contaminated water, or other solutions.

A desalination system according to Korean Patent Registration No. 1020316 which is prior art will now be described with reference to FIG. 2.

The desalination system comprises a forward osmosis separator 100 and a freshwater separator 1000. The forward osmosis separator 100 comprises a membrane 110, in which, as shown in FIG. 1, at one side of the membrane, raw water is introduced and brine is discharged, and at the other side of the membrane, a concentrated draw solution is introduced and a diluted draw solution is discharged.

The diluted draw solution discharged from the forward osmosis separator 100 is introduced through a buffer chamber 200 into a diluted draw solution chamber 300 provided with a heater 310. The heater 310 serves to maintain an optimum temperature at which gas can be separated from the draw solution.

The diluted draw solution is introduced through a filter 320 into membrane contactors 400 (400a and 400b), and for this purpose, a feed pump 360 is located in the pipeline. In the membrane contactors 400, gas is separated from the introduced draw solution.

The separated gas passes through

concentrators

453a, 453b, 454a and 454b while vapor is separated from the separated gas and vapor mixure, and the gas remaining after the separation of the vapor is fed by vacuum pumps 450, 450a and 450b into a membrane contactor 600.

Condensers

453a, 453b, 454a and 454b are connected to a cooler 750 by cooling

water circulating pipelines

753a, 753b, 754a and 754b so that it is maintained at the optimum temperature.

The gas separated in the membrane contactors 400 is fed into the membrane connector 600, and a process that is the reverse of that carried out in the membrane contactors 400 is carried out so that the gas dissolves in the introduced water to reproduce a concentrated draw solution. The concentrated draw solution is introduced again into a concentrated draw solution chamber 700, and a portion thereof is introduced by a feed pump 750 into the membrane contactor 600.

The concentrated draw solution chamber 700 is connected to a storage chamber 800, and freshwater may be introduced through a separate pipeline 520 into the storage chamber to control the concentration of the draw solution. The concentration of the concentrated draw solution is controlled to be a preferred concentration in this manner and is introduced by a feed pump 860 into the forward osmosis separator 100 in which the forward osmosis desalination process is repeated.

In the prior art shown in FIG. 2, the diluted draw solution should be introduced into the freshwater separator 1000, and the concentrated draw solution should be introduced again into the forward osmosis separator 100. Thus, in the membrane contactors 400, the reaction separating all of the fed diluted draw solution into gas and freshwater is carried out, and in the membrane contactor 600, a reaction that is the reverse of that of the membrane contractors 400 is carried out.

Specifically, because the draw solution is separated into gas and freshwater in the membrane contactors 400, and the draw solution is formed by the reverse reaction in the membrane contactor 600, the extra energy could be minimized in the re-concentration process of draw solute.

A method and system according to the present invention will now be described in further detail with reference to FIG. 3 and FIG. 4 in which like reference numerals are used for elements that are the same as those shown in FIG. 2. The description of the same elements and principles will be omitted in this embodiment.

The main idea of the present invention is that only a portion of the diluted draw solution fed from the forward osmosis separator 100 into the freshwater separator 1000 is separated into gas and freshwater and that the other portion of the diluted draw solution is returned to the forward osmosis separator 100.

The basis of this method lies in the fact that the concentrated draw solution should be fed again into the membrane due to the characteristics of the forward osmosis process. Specifically, when all of the diluted draw solution fed from the forward osmosis separator 100 into the freshwater separator 1000 is separated into gas and freshwater, an extremely large amount of gas should dissolve again during the process of preparing the draw solution with an initial concentration to suck same amount of water per hour from feed solution in the forward osmosis separator after the production of freshwater. Moreover, a large portion of production fresh water has to be used to make an initial amount of draw solution again. For this reason, energy is unnecessarily wasted. This energy waste results in a low efficiency of the freshwater separator and/or the forward osmosis separator.

Thus, according to the present invention, only a portion of the diluted draw solution fed into the freshwater separater 1000 is fed into the diluted draw solution chamber 440 by a pump P2, and the other portion of the diluted draw solution is sent into the concentration control tank 850 using a pump P1 and fed into a recycling tank 350 and then fed into the forward osmosis tank 100.

A portion of the diluted draw solution fed into the diluted draw solution chamber 440 is fed by a pump P3 into the membrane contactors 400 in which it is separated into gas and freshwater. The freshwater is stored in a freshwater tank 500.

Vapor can pass through membrane pours with gas depending on operating conditions. An increase in the production rate of vapor means a decrease in the production of freshwater. Thus, the condensed water from water vapor can be recovered and pass again through the membrane contactors 400, whereby effective freshwater production can be achieved even under various operating conditions.

For this purpose, the gas and vapor mixture passed through a condenser 453 so that only vapor contained is condensed in the condenser. The condensed water is getting increased and then fed again into the diluted draw solution 440 using a pump P4 and subjected to the separation process in the membrane contactors 400.

Additional membrane contactors (not shown) may be located between the condenser 453 and the freshwater tank 500 depending on the choice of the manufacturer or user. This can control the quality in the given operating conditions.

Gases (e.g., NH₃(g) and CO₂(g)) other than the vapor are fed into the membrane contactor 600 by a vacuum pump VP.

Vapor can be mostly removed from the gas and vapor mixture using, for example, the above-described condenser 453, but in some cases, some vapor can remain still in the mixture. In this process, if the surrounding temperature decreases, solid solutes can be produced from the mixture. For example, in the case in which some vapor together with NH₃(g) and CO₂(g) remains in the gas, solid-state ammonium will be produced in vacuum pump and pipeline if the surrounding temperature decreases to about 60℃.

Such solid solutes must necessarily be removed, because they can clog pipelines or damage vacuum pump and membranes. Under the current circumstances, in order to remove such solid solutes, a large amount of energy could be consumed so that the surrounding temperature is maintained at a predetermined temperature or higher. In the prior art shown in FIG. 2, a separate heating member is adopted.

Accordingly, a solid solute capture tank 650 is preferably located in advance of the membrane contactor 600 so that solid solutes such as solid-state ammonium are naturally collected without consuming much energy.

Adopting the solid solute capture tank 650 in place of the heating member can reduce energy consumption, and the solid solutes thus collected may also be fed as an aqueous solution into the concentration control tank 850 using a pump P6, whereby these solid solutes may be used to control the concentrated draw solution which is fed into the forward osmosis separator 100.

In the membrane contactor 600, gas and freshwater are fed and a reaction that is the reverse reaction carried out in the membrane contactor 400 is carried out to produce a draw solution which is fed by pumps P5 and P7 through feedback pipelines into a concentrated draw solution chamber 700, thus producing a concentrated draw solution of the preferred concentration.

The concentrated draw solution produced in the concentrated draw solution chamber 700 is fed into the concentration control tank 850 using a pump P8 and combined with the diluted draw solution (not separated into gas and freshwater) and the solid solutes from the solid solute capture tank 650 so that it is controlled to be the preferred concentration.

The concentrated draw solution thus controlled is fed by a pump P9 into the recycling tank 350 and then fed into the forward osmosis separator 100.

The membrane contactor 400 and the separation process will now be described in detail with reference to FIG. 5.

FIG. 5 illustrates only a hollow type membrane contactor which is an example of the membrane contactor 400, but it is to be understood that the membrane contactor is not limited thereto and a flat type membrane contactor may also be applied. Namely, it is to be understood that the membrane contactor having the function described below may be any type.

The same configuration may be applied to different membrane contactors 600 which are used in embodiments of the present invention. Particularly, in the membrane contactor 600, a reaction that is the reverse reaction carried out in the membrane contactors 400 is carried out, and the detailed description thereof will be omitted. Also, it is noted that the membrane contactor 400 in which gas is separated and the membrane contactor 600 in which the gas dissolves can be designated as a first membrane contactor and a second membrane contactor, respectively, so that they can be distinguished.

The membrane contactor 400 comprises: a housing 410; an inlet 411 through which a draw solution is fed; an outlet 412 through which freshwater is discharged after gas was discharged; and

gas outlets

413 and 414 through which gas is discharged.

The housing 410 includes a distribution tube 430 and a cartridge 420 surrounding the distribution tube. The distribution tube 430 has a plurality of openings 431 through which only gas can pass and not liquid, because the membrane of the distribution tube is a hydrophobic membrane. In the distribution tube 430, a draw solution introduced from the inlet 411 can flow, and according to the Henry's law, the gas or vapor separated from the draw solution can flow from the distribution tube 430 through the openings 431 into the cartridge 420, and then can be discharged to the outside through the

gas outlets

413 and 414.

The cartridge 420 consists of a plurality of hollow fiber membranes 421.

More specifically, a vacuum can be formed in the cartridge 420 by a vacuum pump 450 (see FIGS. 2 to 4). A vacuum pump is not the only tool for forming a vacuum. A vacuum can be more easily formed by using an ejector 460. An ejector 460 is a kind of pump that sucks a fluid of low energy by kinetic energy of a fluid which is sprayed through a nozzle at a high speed. If NH₃(g) and CO₂(g) gases are sucked and are then ejected along with a liquid which is sprayed at a high speed through an ejector, NH₃(g) and CO₂(g) gases are naturally mixed and concentrated with the liquid which is sprayed at a high speed. Therefore, the re-concentration process of NH₃(g) and CO₂(g) gases of the present invention can be more simplified.

When the draw solution introduced through the inlet 411 passes through the distribution tube 430 in this environment, gas is separated from the draw solution by the Henry's law. The separated gas exits the draw solution, passes through the openings and the hollow fiber membrane 421, and finally is discharged to the outside of the membrane contactor 400 through the

gas outlets

413 and 414.

Gas is discharged from the draw solution so that the concentration of gas in the draw solution markedly decreases, and the partial pressure of dissolved gas can be controlled using the temperature, the mass flow rate and/or the degree of vacuum, whereby almost all gas present in the draw solution can be separated from the draw solution, thereby desalinating the draw solution.

Freshwater is discharged to the outside through the outlet 412.

In one embodiment of the present invention, NH₄HCO₃(liquid) may be used as the draw solution. It is to be understood, however, that any other solution may be used as the draw solution.

If NH₄HCO₃(liquid) is used as the draw solution, NH₃(gas) and CO₂(gas) are separated from the NH₄HCO₃(liquid) solution in the membrane contactor 400. Herein, a suitable temperature at which NH4HCO₃is separated into NH₃, CO₂and H₂O is about 30 to 60℃. which could be changed according to the degree of vacuum. Specifically, in the gas phase, solid-state ammonium starts to be generated at about 60℃ or lower.

The generation of solid-state ammonium can reduce the recovery rate of the draw solute into draw solution during the re-concentration process, clog pipelines and cause great damage to the vacuum pump and membrane. To solve this problem, according to the present invention, the solid solute capture tank 650 is adopted as described above. So the generated solid solute can be easily captured and recycled to the concentration control tank 850.

According to the same principle that is applicable to the reverse reaction, the temperature of the concentrated draw solution chamber 700 which is cooled by the cooler 750 is preferably 5~ 20℃.

As described above, the main idea of the present invention is that only a portion of the diluted draw solution fed from the forward osmosis separator 100 into the freshwater separator 1000 is separated into gas and freshwater and that the other portion of the diluted draw solution is directly returned to the forward osmosis separator 100.

Herein, the issue is how the draw solution is returned to the forward osmosis separator 100 is discussed, in which the amount of draw solution returned may vary depending on the performance of the forward osmosis separator 100. If an excessively large amount of the draw solution is returned to the forward osmosis separator 100, the production of freshwater will decrease so that the total system efficiency will decrease, and if an excessively small amount of the draw solution is returned, the load of the membrane distillation/separation system will increase. Thus, it is required to find a suitable ratio at which the draw solution is returned. The present inventor has performed a number of experiments and, as a result, has found that a portion of the diluted draw solution which is separated into gas and freshwater is most preferably 10-40% of the total of the diluted draw solution in terms of system efficiency.

Another exemplary embodiment of the present invention will be explained with reference to FIG. 4.

In the first exemplary embodiment illustrated in FIG. 3, freshwater stored in the freshwater tank 500 is supplied to the concentrated draw solution chamber 700, but in another exemplary embodiment illustrated in FIG. 4, a diluted draw solution stored in the buffer tank is supplied to the concentrated draw solution chamber 700. Futher, it is possible for a diluted draw solution separated from the forward osmosis separator 100 to be directly supplied to the concentrated draw solution chamber 700 without a separate buffer tank 200. Therefore, the second exeplary embodiment has an advantage that the final freshwater output stored the freshwater tank 500 increases.

Unlike the first exemplary embodiment, the second exemplary embodiment has a disadvantage that the concentration of the concentrated draw solution chamber 700 increases because a diduted draw solution, not freshwater, is supplied to the concentrated draw solution chamber 700. That is, compared with the first exeplary embodiment, in the second exeplary embodiment, the freshwater output increases, but the re-concentration efficiency decreases.

However, because continuous circular re-concentration is conducted according to the feature of the present invention, the reduction rate of the re-concentration efficiency is insignificant.

FIG. 3 shows three

first membrane contactors

400a, 400b and 400c and one second membrane contactor 600, but the numbers of the first and second membrane contactors are not limited.

Also, the plurality of membrane contactors may be connected in series, in parallel, or in a combination thereof.

Moreover, it should be understood that, in view of membrane capacity and the like, one vacuum pump may be used or two or more vacuum pumps may be used, or similar vacuum devices and thus the number of vacuum pumps or similar vacuum devices are not limited.

In addition, as described above, an additional membrane contactor (not shown) may be provided downstream of the condenser 453 to increase the degree of desalination.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

A freshwater separator comprising:

a first membrane contactor into which a portion of a diluted draw solution is fed and in which gas and freshwater are separated from the fed diluted draw solution;

a second membrane contactor into which the separated gas is fed so that it dissolves in a fluid flowing therein; and

vacuum pumps connected with the first membrane contactor and the second membrane contactor,

wherein the fluid containing the gas dissolved therein from the second membrane contactor is mixed with the other portion of the diluted draw solution.
The freshwater separator of claim 1, further comprising a recycling tank in which the fluid containing the gas dissolved therein from the second membrane contactor is mixed with the other portion of the diluted draw solution.
The freshwater separator of claim 2, further comprising:

a buffer tank into which the diluted draw solution is fed; and

a diluted draw solution chamber which receives a portion of the diluted draw solution from the buffer tank and feeds the received portion into the first membrane contactor,

wherein the buffer tank feeds the other portion of the diluted draw solution into the concentration control tank.
The freshwater separator of claim 3, wherein water vapor separated from draw solution in the first membrane contactor passes through a condenser so that it is condensed and fed into the diluted draw solution chamber.
The freshwater separator of claim 3 or 4, further comprising a concentration control tank which receives the fluid containing the gas dissolved therein from the second membrane contactor and is in fluid communication with the recycling tank.
The freshwater separator of claim 5, further comprising a solid solute capture tank in which solid solutes of the gas separated in the first membrane contactor are collected.
The freshwater separator of claim 6, wherein the solid solutes collected in the solid solute capture tank flow into the concentration control tank.
The freshwater separator of claim 5, wherein a portion of the diluted draw solution is fed into the diluted draw solution chamber, and the other portion of the diluted draw solution is fed via the concentration control tank into the recycling tank.
The freshwater separator of claim 3, wherein the first and second membrane contactors comprises:

a distribution tube which is located within each of the first and second membrane contactors so that a fluid flows therein, the distribution tube including a plurality of openings; and

a plurality of hollow fiber cartridges surrounding the distribution tube.
The freshwater separator of claim 1, wherein the freshwater separator further comprises:

a concentrated draw solution chamber, where the concentrated draw solution containing melted gas is stored in the second membrane contactor, which is in fluid communication with the recycling tank,

wherein freshwater separated from the first membrane contactor is supplied to the concentrated draw solution chamber.
A desalination system comprising:

a forward osmosis separator including a membrane; and

a freshwater separator which is in fluid communication with the forward osmosis separator,

wherein the forward osmosis separator uses a forward osmosis method in which raw water is fed to one side of the membrane and discharged as brine, and a concentrated draw solution may be fed to the other side of the membrane and discharged as a diluted draw solution, and wherein the freshwater separator comprises:

a buffer tank into which the diluted draw solution is fed;

a diluted draw solution chamber into which a portion of the diluted draw solution is fed from the buffer tank;

a first membrane contactor into which a portion of a diluted draw solution is fed and in which gas and freshwater are separated from the fed diluted draw solution;

a second membrane contactor into which the separated gas is fed so that it dissolves in a fluid flowing therein to form the concentrated draw solution;

a tank which receives the concentrated draw solution from the membrane contactor; and

a vacuum pump connected with the first membrane contactor and the second membrane contactor,

wherein the other portion of the diluted draw solution fed into the buffer tank is fed into the tank.
The desalination system of claim 11, wherein the draw solution is NH₄HCO₃(liquid), and the gas includes NH₃(gas) and CO₂(gas).
The desalination system of claim 12, wherein the portion of the diluted draw solution which is fed into the diluted draw solution chamber makes up 10-40% of the total of the diluted draw solution which is fed into the freshwater separator.
The desalination system of claim 11, wherein water vapor separated from draw solution in the first membrane contactor passes through a condenser so that it is condensed and fed vapor into the diluted draw solution chamber.
The desalination system of claim 14, wherein the freshwater separator further comprises:

a concentration control tank which is in fluid communication with the second membrane contactor; and

a solid solute capture tank in which solid solutes of the gas separated in the first membrane contactor are collected,

wherein the concentration control tank is fed with one or more of the other portion of the diluted draw solution fed into the buffer tank, the solid solutes collected in the solid solute capture tank, and the concentrated draw solution formed in the second membrane contactor, and the draw solution is controlled to a predetermined concentration in the concentration control tank and is fed through the recycling tank to the other side of the membrane of the forward osmosis separator.
The desalination system of claim 11, wherein the freshwater separator further comprises:

a concentrated draw solution chamber, where the concentrated draw solution from the second membrane contactor is stored, which supplies the concentrated draw solution to the tank,

wherein freshwater separated from the first membrane contactor is supplied to the concentrated draw solution chamber.
The desalination system of claim 11, wherein the freshwater separator further comprises:

a concentrated draw solution chamber, where the concentrated draw solution from the second membrane contactor is stored, which supplies the concentrated draw solution to the tank,

wherein another part of the diluted draw solution supplied to the buffer tank is supplied to the concentrated draw solution chamber.