WO2018156756A1

WO2018156756A1 - Hybrid forward osmosis and freezing desalination system and method

Info

Publication number: WO2018156756A1
Application number: PCT/US2018/019215
Authority: WO
Inventors: Abdulkarem Ibrahim AMHAMED; Adel Obaid SHARIF
Original assignee: Qatar Foundation
Current assignee: Qatar Foundation
Priority date: 2017-02-23
Filing date: 2018-02-22
Publication date: 2018-08-30
Anticipated expiration: 2019-08-23

Abstract

The hybrid forward osmosis and freezing desalination system (10) includes a forward osmosis chamber (12) and a freezer or chiller (22) in fluid communication with a draw solution outlet (18) of a permeate side (PS) of the forward osmosis chamber (12). The freezer (22) freezes a mixture of a draw solution and desalinated water to produce frozen crystals of draw solution suspended in desalinated water. A separator (24) receives the frozen crystals of draw solution suspended in desalinated water and outputs the desalinated water as a final fresh water product. A melter (28) receives the frozen crystals of the draw solution and melts the frozen crystals of the draw solution to produce recovered draw solution. A recirculation pump recirculates the recovered draw solution back through the permeate side (PS) of the forward osmosis chamber (12).

Description

HYBRID FORWARD OSMOSIS AND FREEZING DESALINATION SYSTEM

AND METHOD

TECHNICAL FIELD

The disclosure of the present patent application relates generally to water desalination, and particularly to a hybrid forward osmosis and freezing desalination system and method that combines indirect freezing desalination with forward osmosis filtration to produce potable water.

BACKGROUND ART

Conventional freezing desalination systems and methods take advantage of the fact that crystals of water ice are mostly pure water. When salt water or the like is frozen, the pure water freezes into solid crystals with dissolved salts and other contaminants being left separate, allowing for the separation of the mostly pure water ice and a concentrated brine. Fig. 2 illustrates a conventional freezing desalination system 100. Salt water is input to a conventional freezer 102 where the salt water is frozen to produce pure water ice in a concentrated brine slurry. Liquid brine may be tapped from the freezer 102, and the remainder of the combined water ice and brine slurry is delivered to a separator and washer unit 104. The separator and washer unit 104 separates the pure water ice from the brine slurry and outputs the brine, where it may be combined with the brine tapped from the freezer 102. The separator and washer unit 104 further uses pure wash water to wash any remaining brine and/or brine residue from the pure water ice crystals. The washed ice crystals are delivered to a melter 106, where they are melted into pure water. A portion of the pure water is drawn off as the final purified water product, and a second portion of the pure water is fed back to the separator and washer 104 as the wash water. For purposes of efficiency, the melter 106 may use at least a portion of the heat produced by the freezer 102 during the freezing process.

There are numerous different desalination processes that are based on the basic freezing desalination process, as described above with reference to Fig. 2. In direct contact freezing, a liquid hydrocarbon refrigerant that is not water soluble, such as propane, is brought into direct contact with a source of salt water (e.g., seawater) and is vaporized. In the freezer, the liquid hydrocarbon, which is initially maintained at a high pressure, is injected into the seawater by jet sprays or nozzles. The hydrocarbon then evaporates due to the lower pressure in the freezer. Expansion and evaporation of the hydrocarbon cools the seawater to below its freezing point, thus forming the crystals of pure water ice. Although efficient, the direct contact freezing process requires the handling of hydrocarbon refrigerants outside of the small-scale sealed circuits in which such refrigerants are typically used. In addition to the environmental and health risks of such hydrocarbon refrigerants, the equipment required to apply the refrigerant, as well as ancillary safety equipment, can be costly and difficult to use.

In the indirect freezing process, unlike the direct freezing process, the seawater is not brought into direct contact with refrigerant. Similar to that described above with reference to Fig. 2, ice is formed on the surface of the seawater through a conventional refrigeration process. However, the feed seawater is first pumped through a heat exchanger to reduce its temperature, and the freezer applies further cooling. As in the system of Fig. 2, the ice and brine slurry is pumped to a wash column where the ice and brine are separated. The ice is then transported to the melter, where the heat released from condensation of the compressed refrigerant (i.e., heat recycled from the freezer) melts the ice. A small part of the product freshwater is bypassed to the wash column and is used to wash the ice crystals, and the larger part is passed through the heat exchanger to cool the feed seawater and then be discharged for storage. This process, however, has a number of disadvantages, including a large energy expenditure, a relatively large scale (i.e., a large metallic heat transfer area is required for both the freezing and melting stages), and the need for equipment that is both complex and expensive.

Further, in the vacuum freezing desalination process, evaporation and freezing take place concurrently in the freezer, with seawater being added as ice, brine slurry and water vapor are withdrawn. To vaporize a portion of the water, a vacuum is applied, thus lowering the temperature of the seawater to below the point of freezing (for the pure water) as water on the surface evaporates. The application of the vacuum, as well as the specialized pumps and conduits for fluid transfer, make this process relatively costly, both in terms of energy expenditure, as well as the need for expensive specialized equipment. Thus, a hybrid forward osmosis and freezing desalination system and method solving the aforementioned problems is desired.

DISCLOSURE OF INVENTION The hybrid forward osmosis and freezing desalination system combines indirect freezing desalination with forward osmosis filtration to desalinate salt water, such as seawater. The hybrid forward osmosis and freezing desalination system includes a forward osmosis chamber having a semipermeable membrane, which defines a feed side of the chamber and a permeate side of the chamber. As in conventional forward osmosis, the draw solution has a relatively high osmotic pressure such that desalinated water is drawn through the semipermeable membrane, resulting in brine on the feed side and a mixture of the draw solution and desalinated water on the permeate side. The brine may be drawn off through a brine outlet on the feed side. Prior to introduction into the feed side, the seawater may be pre-treated in any desired manner, such as by additional filtration and/or chemical processes.

A freezer is in fluid communication with a draw solution outlet of the permeate side of the forward osmosis chamber. The freezer freezes the mixture of the draw solution and the desalinated water to produce frozen crystals of the draw solution suspended in the desalinated water. The draw solution is selected such that any draw solutions have a higher freezing temperature than the freezing temperature of the desalinated water (i.e., 0°C at ambient pressure). An exemplary draw solution is dimethyl sulfoxide (DMSO). It should be understood that any suitable type of draw solution having a freezing point higher than 0°C may be used, such as glycerin, for example.

A separator is in fluid communication with the freezer for receiving the frozen crystals of the draw solution suspended in the desalinated water. The separator separates the frozen crystals of the draw solution from the desalinated water, and the desalinated water is output from the separator through an outlet port as the final fresh water product. A melter is in fluid communication with the separator for receiving the frozen crystals of the draw solution. The melter melts the frozen crystals of the draw solution to produce recovered draw solution, which is then collected in a draw solution reservoir, which is in fluid communication with the melter. The draw solution reservoir outputs the recovered draw solution to a recirculation pump, which recirculates the recovered draw solution back through the permeate side of the forward osmosis chamber to produce the osmotic pressure gradient between the feed side and the permeate side.

These and other features of the present disclosure will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a hybrid forward osmosis and freezing desalination system.

Fig. 2 is a schematic diagram of a conventional prior art freezing desalination system. Similar reference characters denote corresponding features consistently throughout the attached drawings.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Referring to Fig. 1, the hybrid forward osmosis and freezing desalination system 10 combines indirect freezing desalination with forward osmosis filtration to desalinate a source of salt water, such as a seawater feed. The hybrid forward osmosis and freezing desalination system 10 includes a forward osmosis chamber 12 having a semipermeable membrane 14, which defines a feed side FS of the forward osmosis chamber 12 and a permeate side PS of the forward osmosis chamber 12. As in conventional forward osmosis, the draw solution (in the permeate side PS) has a relatively high osmotic pressure such that desalinated water is drawn through the semipermeable membrane 14, resulting in brine on the feed side FS and a mixture of the draw solution and desalinated water on the permeate side PS. The brine may be drawn off through a brine outlet 20 on the feed side FS. Prior to introduction into the feed side FS, the seawater may be pre-treated in any desired manner, such as by additional filtration and/or chemical processes (shown at 34 in Fig. 1).

A freezer 22 is in fluid communication with a draw solution outlet 18 of the permeate side PS of the forward osmosis chamber 12. The freezer 22 freezes the mixture of the draw solution and the desalinated water to produce frozen crystals of the draw solution suspended in the desalinated water. The draw solution is selected such that the draw solution has a higher freezing temperature than the freezing temperature of the desalinated water (i.e., 0°C at ambient pressure). An exemplary draw solution is dimethyl sulfoxide (DMSO).

A separator 24 is in fluid communication with the freezer 22 for receiving the frozen crystals of the draw solution suspended in the desalinated water. The separator 24 separates the frozen crystals of the draw solution from the desalinated water, and the desalinated water is output from the separator 24 through an outlet port 26 as the final fresh water product. A melter 28 is in fluid communication with the separator 24 for receiving the frozen crystals of the draw solution. The melter 28 melts the frozen crystals of the draw solution to produce recovered draw solution, which is then collected in a draw solution reservoir 30, which is in fluid communication with the melter 28. The draw solution reservoir 30 outputs the recovered draw solution to a recirculation pump 32, which may be any suitable type of pump, that recirculates the recovered draw solution back through the permeate side PS of the forward osmosis chamber 12 to produce the osmotic pressure gradient across membrane 14 and between the feed side FS and the permeate side PS.

In the above, it should be understood that any suitable type of separator may be used, including a separator which is integrated with freezer 22 as a single unit. Further, it should be understood that the draw solution leaving melter 28 may be diluted with a portion of unseparated pure water, thus an additional separation system be utilized to remove this water from the draw solution, such as a density-based separation process or the like.

As noted above, the draw solution may be dimethyl sulfoxide (DMSO), for example. DMSO has anti-corrosion properties, thus removing or reducing the incidence of corrosion, which is typical in conventional desalination systems. Further, DMSO requires only one- seventh of the latent heat necessary for phase change, compared against that of water. Although DMSO may be used as the draw solution, it should be understood that any suitable type of draw solution may be used in the hybrid forward osmosis and freezing desalination system, such as, for example, glycerin. Such a draw solution is preferably selected to have a relatively high freezing point (ideally around 0°C or above), to be non-toxic and non- corrosive, to have a relatively low molecular weight (to form a high osmotic pressure solution with water), to have high solubility in water, and to have a reasonable cost and be readily available. The draw solution may be organic or inorganic salts, or a combination thereof, meeting these preferred criteria. Returning to the exemplary choice of DMSO, DMSO has a high boiling point (189°C), a high freezing point (18.5°C), is a non-toxic solvent, a colorless liquid, and is also a polar aprotic solvent that dissolves both polar and nonpolar compounds. DMSO is also miscible in a wide range of organic solvents, as well as water.

It is to be understood that the hybrid forward osmosis and freezing desalination system and method is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.

Claims

1. A hybrid forward osmosis and freezing desalination system, comprising:

a forward osmosis chamber having a semipermeable membrane defining a feed side of the forward osmosis chamber and a permeate side of the forward osmosis chamber, the feed side having a seawater inlet for receiving seawater, the permeate side having a draw solution inlet for receiving draw solution and a draw solution outlet, whereby the forward osmosis chamber is adapted for producing brine on the feed side and a mixture of the draw solution and desalinated water on the permeate side in normal operation;

a freezer in fluid communication with the draw solution outlet of the permeate side of the forward osmosis chamber, the freezer being adapted for freezing the mixture of the draw solution and desalinated water to produce frozen crystals of the draw solution suspended in the desalinated water, the draw solution having a higher freezing temperature than the desalinated water;

a separator in fluid communication with the freezer for receiving the frozen crystals of draw solution suspended in desalinated water, the separator having an outlet port, the separator being adapted for separating the frozen crystals of draw solution from the desalinated water and outputting the desalinated water from the separator through the outlet port;

a melter in fluid communication with the separator for receiving the frozen crystals of the draw solution, the melter being adapted for melting the frozen crystals of draw solution to produce recovered draw solution; and

a recirculation pump in fluid communication with the melter for recirculating the recovered draw solution to the permeate side of the forward osmosis chamber to produce an osmotic pressure gradient between the feed side and the permeate side.

2. The hybrid forward osmosis and freezing desalination system as recited in claim 1, further comprising a draw solution reservoir in fluid communication with the melter and the recirculation pump for collecting the recovered draw solution and outputting the recovered draw solution to the recirculation pump.

3. The hybrid forward osmosis and freezing desalination system as recited in claim 1, wherein the feed side of the forward osmosis chamber has a brine outlet for outputting brine.

4. The hybrid forward osmosis and freezing desalination system as recited in claim 1, wherein the draw solution comprises dimethyl sulfoxide.

5. The hybrid forward osmosis and freezing desalination system as recited in claim 1, wherein the draw solution comprises glycerin.

6. A hybrid forward osmosis and freezing desalination system, comprising:

a forward osmosis chamber having a semipermeable membrane defining a feed side of the forward osmosis chamber and a permeate side of the forward osmosis chamber, the feed side having a seawater inlet for receiving seawater, the permeate side having a draw solution inlet for receiving draw solution and a draw solution outlet, whereby the forward osmosis chamber is adapted for producing brine on the feed side and a mixture of draw solution and desalinated water on the permeate side;

a freezer in fluid communication with the draw solution outlet of the permeate side of the forward osmosis chamber, the freezer being adapted for freezing the mixture of draw solution and desalinated water to produce frozen crystals of draw solution suspended in desalinated water;

the draw solution, the draw solution being dimethyl sulfoxide and having a higher freezing temperature than desalinated water;

a melter in fluid communication with the separator for receiving the frozen crystals of draw solution, the melter being adapted for melting the frozen crystals of the draw solution to produce recovered draw solution; and

7. The hybrid forward osmosis and freezing desalination system as recited in claim 6, further comprising a draw solution reservoir in fluid communication with the melter and with the recirculation pump for collecting the recovered draw solution and outputting the recovered draw solution to the recirculation pump.

8. The hybrid forward osmosis and freezing desalination system as recited in claim 6, wherein the feed side of the forward osmosis chamber has a brine outlet for outputting brine.

9. A hybrid forward osmosis and freezing desalination method, comprising the steps of: inputting seawater into a feed side of a forward osmosis chamber;

introducing a draw solution into a permeate side of the forward osmosis chamber to produce an osmotic pressure gradient across a semipermeable membrane dividing the feed side from the permeate side;

outputting brine from the feed side of the forward osmosis chamber;

outputting a mixture of the draw solution and desalinated water from the permeate side of the forward osmosis chamber;

freezing the mixture of the draw solution and the desalinated water to produce frozen crystals of the draw solution suspended in the desalinated water;

separating the desalinated water from the frozen crystals of the draw solution; and melting the frozen crystals of the draw solution to recirculate the draw solution to the permeate side of the forward osmosis chamber.

10. The hybrid forward osmosis and freezing desalination method as recited in claim 9, wherein the draw solution comprises dimethyl sulfoxide.

11. The hybrid forward osmosis and freezing desalination method as recited in claim 9, wherein the draw solution comprises glycerin.