WO1987007250A1

WO1987007250A1 - Freezing process

Info

Publication number: WO1987007250A1
Application number: PCT/AU1987/000152
Authority: WO
Inventors: Robert Blackmore Collins
Original assignee: Robert Blackmore Collins
Priority date: 1986-05-23
Filing date: 1987-05-25
Publication date: 1987-12-03

Abstract

An apparatus and process for the production of ice and/or the concentration of a fluid. An insulated container (1) containing a solution (2) such as brine has the concentration of the brine increasing with depth, the lowermost layers being at a lower temperature than the higher layers. The brine is cooled (4), ice crystals forming at a distance from the cooling means, the ice floating to the surface. In a further embodiment, the density varies with depth, and in a still further embodiment, a liquid (7) immiscible with the solution (2) and of lower density floats on the solution (2) from which ice crystals are recovered.

Description

FREEZING PROCESS"

This invention relates to an improved freezing process, more particularly for a process utilizing density gradients in conjunction with the freezing of water for useful purposes

BACKGROUND OF THE INVENTION

Most brine solutions have the property that the freezing temperature of the solution is lowered at higher solution densities. Also it is known that brine solutions can have a salt density gradient layer (or halocline) in which the density of the salt solution increases with depth there being virtually fresh water at the upper surface, the salt density increasing with depth. This property is exploited in the present invention to freeze water "at a distance"

In known processes for the production of ice, particularly in the freezing type desalination processes two mam problems occur, the ice growth on the refrigerated surfaces, and the separation and/or washing of the ice from the brine solution.

It is an object of this invention to provide a freezing or desalination process that is simple, low in capital and running costs, and virtually eliminates corrosion and scaling problems

BRIEF STATEMENT OF THE INVENTION

Thus there is provided according to the invention, a process for the production of fresh water ice, or fresh water from a brine solution of increasing density levels with depth, the process providing means to lower the temperature of the solution so that the bottom salinity layer is at a temperature below 0° C, the temperature increasing upwardly in the solution so that the ice crystals form in the solution at a level in the solution below the means for cooling the solution, the ice crystals either being removed or melted at the top of the solution or in a further low density liquid for the production of fresh water. Also there is provided according to the invention, an apparatus for producing fresh water ice or fresh water from a brine solution, including means for providing increasing density levels in the brine with depth, means for cooling the lower level of the brine to below 0° C, the brine solution increasing in temperature towards the top thereof, whereby ice crystals form in the solution at a level below the means for cooling, means for removing the ice from the surface of the brine solution or from a further low density liquid by either ice removal, or by melting the ice on the surface for removing the fresh water

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully describe trie invention reference will now be made to the accompanying drawings in which

FIG 1 illustrates a process for salt gradient freezing desalination, FIG. 2 shows a further embodiment, FIG 3 shows a still further embodiment,

FIG. 4 shows another embodiment using a further low density liquid, and

FIG. 5 shows a typical salt balance example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a container containing a brine solution, the density of the brine solution increases with depth from virtually fresh water at the top to the higher density brine solution at the bottom.

If temperature at the bottom is lowered to be below freezing point, for example to -6° C to -10° C by a refrigeration unit with the temperature at the top of the brine solution being at 0° C, and with cooling coils positioned in the brine solution depending upon the density gradient, the ice crystals will form in the solution below the cooling coils, these ice crystals forming and floating to the surface where they may be removed. In any density gradient there is a diffusion of the solute from the more concentrated regions to the less concentrated regions. This rate of diffusion depends on the gradient and the temperature and is very low for most salt solutions near freezing, For sodium chloride solutions near freezing it is less than 0.2 grams per hour per metre squared. The freezing process can be used to maintain the density gradient against the diffusion which is tending to equalize the solution and eliminate the density gradient when solutions, such as seawater freeze, the ice is nearly pure water as the salt is excluded during the crystallization process Freezing can thus be used to extract fresh water from a region of the density gradient and transported upward to the surface This provides the mechanism for counteracting diffusion and maintaining a density gradient over time. The whole system can be controlled by the rate of heat extraction of the refrigeration equipment and/or the rate of feed water addition.

There are several advantages of freezing processes over distillation processes for applications such as the desalination of water . The main one is the much lower energy consumption. It only requires 348 kilojoules to freeze one kilogram of water compared to 2,320 kilojoules to vapourize one kilogram of water Another significant advantage over other desalination techniques is the higher resistance to corrosion and scaling. Corrosion is expoentially reduced with decreasing temperature. This means low cost materials can be used in the plant construction, and little or no pretreatment of feed water is provided to prevent scaling or corrosion.

FIG. 1 illustrates one form of the invention. An insulated container 1 holds the brine solution 2 in which a density gradient has been established A mechanical refrigeration unit 3 having an evaporator 4 and condenser 5 with expansion valve 6 maintains a temperature gradient with the coldest temperature at the bottom. The temperature is maintained so that the freezing of water occurs at the desired level below the evaporator 4 in the density gradient. The ice crystals float to the surface without disturbing the density gradient where they are melted by the heat from the refrigerant condenser 5. A dynamic equilibrium is set up with the temperature and density gradients remaining stationary while fresh water is removed from the surface by pipe 7, feed water through pipe 8 is added at or preferably below the level as the refrigerant evaporator 4 and heavier waste brine is removed from the bottom through outlet pipe 9. The pipes, 7, 8 and 9 pass through a heat exchanger 10 to conserve the energy requirements of the system.

Turning now to FIG. 2, there is illustrated a further embodiment with the brine solution 2 in the insulated container 1. Instead of the brine solution being cooled within the chamber 1 , the brine solution is circulated outside of the container 1 , by a pump 1 1 passing the brine solution through a heat exchanger 12 to chill the brine solution, the brine flowing through outlet 13 and being injected through inlet 14. The fresh water is removed through outlet 7 at the top of the brine solution 2, with the feed water again being fed into the bottom of the container through the inlet 8.

FIG. 3 illustrates a further development of the embodiment of FIG. 2 In this embodiment the feed water is fed into the recirculated brine by feed pipe 15 into injector 16 in the inlet pipe 14. In this way the feed water will immediately freeze into pure water ice particles which will float to the surface.

In a further embodiment, there is shown in FIG. 4 a freeze separation process using immiscible fluids with different freezing temperatures. Above the brine solution 2 in container 1 , there is provided a layer of a liquid of lower specific gravity than the brine, and having a lower freezing temperature in this example there is provided a layer of silicon oil 17 having a specific gravity of 0.95, the water below having a specific gravity of 1.00. A cooling coil 18 which may be the evaporator of a refrigeration plant is situated in the silicon oil 17, and in operation ice crystals will form at the interface of the two immiscible liquids, and float into the silicon oil where they can be mechanically separated. The oil can be at a temperature of -5° C. with the water at 0° C. In this instance the water can be a brine solution, or can be pure water if desired.

While the invention has been particularly described in relation to brine and water for the production of ice and desalination, it is to be realized that the invention is not limited thereto but includes fluids immiscible with each other and which have different freezing points, or using fluids with different salt, sugar or other dissolved solids concentrations which have different freezing points. Thus as described fluids with different freezing points in contact with each other produce ice. One fluid can be Injected into the other, or the colder can float on top.

Thus the application of the invention includes any situation where ice or other crystals need to be separated from a fluid solution. Commercial applications include ice production for off-peak energy storage, concentration of liquids in the processing industry, concentration of waste water and desalination.

Turning now to FIG 5 this illustrates the salt balance. The salt content of the product water can be controlled by the rate of ice production. This ice production rate is many thousands of times greater than the salt diffusion rate and insures that the product water is of whatever quality desired

It is well known that the salt density gradient layer, or halocline is easily maintained in a brine solution and this has been applied in what are called solar ponds However, in the present invention, the thermal density gradient is opposite that of a solar pond, and hence the thermal density gradient is of no adverse effect, and in fact the haloclme and thermal density gradients reinforce each other The only loss is that the salt concentration tends to rise slowly to the top because of diffusion, and the present invention provides means to maintain the linear density gradient in the halocline

While the invention has been particularly described in relation to a desalination process or for cooling energy storage, there are also applications in other food or chemical processing industries where this technique could be more effective in concentrating products than conventional distillation techniques. Thus, the area of application for this invention could be in cooling energy storage and ice production. Ice produced at night using off-peak power could be used the next day to meet cooling loads in commercial buildings. This invention makes the production of a large volume of ice with a very small heat exchange at the bottom rather than cooling coils throughout the whole volume. Simple ice production can also be accomplished with this concept, without the mechanical complexities of conventional ice making equipment, thus in other freezing type equipment the cooling coils are often covered in ice which acts as an insulator, and in known units this problem is usually overcome by having a large number of coils or surface area.

While the invention has been particularly described in relation to use of a saline solution, for example seawater, borewater or the like, the invention is particularly adaptable also to other solutions which have the effect of lowering the freezing point of water, and in which a concentration gradient can be established, for example in sugar solutions. Also while reference is made to sodium chloride solutions, other saline solutions can also be used.

Although the invention has been described in one form it is to be realized that the invention is not to be limited thereto but can include various modifications falling withm the spirit and scope of the invention

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

1 . A process for the production of pure frozen crystals of a liquid in a solution containing substances dissolved in the liquid, the process including means for lowering the temperature of the solution to a temperature below the freezing point of the liquid whereby the crystals of the liquid being of lesser density than the solution float to the surface for removal of the crystals from the surface of the solution.

2. A process as defined in claim 1. wherein the solution has greater concentrations of the substance with depth.

3 A process as defined in claim 2, wherein the solution is brine or saline water and the crystals are pure water

4. A process as defined in claim 1 , wherein a further liquid immiscible with the solution and of lower specific gravity floats on tne solution from which the crystals are collected.

5. A process as defined in claim 4, wherein the further liquid is a silicon on ana the solution is brine or saline water

6 A process as defined in claim 1 , wherein by the production of crystals from the solution, the solution is concentrated, and removal of the concentrated solution

7. A process for the production of pure ice crystals or pure water from a brine solution, said process including the steps of providing the brine solution with increasing salinity with depth thus lowering the freezing point with depth, cooling the brine solution with a cooling coil at the lower portion of the solution whereby ice crystals form at a lower level in the solution than the cooling coil and float to the surface of the solution, and removing the ice crystals from the surface.

8. A process as defined in claim 7, wherein the cooling coil is the evaporator of a refrigeration system, the condenser of the refrigeration system being situated at the top of the solution to raise the temperature thereof to melt the ice crystals so that pure water is withdrawn from the top thereof.

9. Apparatus for the production of frozen crystals from a liquid in a solution containing at least one substance dissolved in the liquid and to concentrate said solution, said apparatus comprising an insulated container, means for cooling said solution whereby crystals of the liquid float to the surface of the solution and means for removing said crystals from the surface of the solution, and means for adding further solution to the container.

10 Apparatus as defined in claim 9, wherein said means for cooling said solution comprise a cooling coil immersed in said solution.

1 1 Apparatus as defined in claim 10, wherein said solution is brine, the concentration of salt in the brine increasing with depth, said cooling coil being immersed in the area of greater concentration to cool the solution below freezing point, the temperature of the solution decreasing with depth, ice crystals forming above said cooling coil at a region of lesser concentration and thus higher temperature.

12. Apparatus as defined in claim 1 1 , wherein the cooling coil is a refrigeration coil.

13. Apparatus as defined in claim 9, wherein said means for cooling comprises means circulating said solution outside said insulated container through a heat exchanger.

14. Apparatus as defined in claim 9, wherein said solution is brine, and a further liquid immiscible with said solution floats on said solution, said crystals being collected from said further liquid.

15. Apparatus as defined in claim 14, wherein said further liquid is a silicon oil.

16. Apparatus as defined in claim 1 1 , wherein the cooling coil is an evaporator coil of a refrigeration system, and the condenser coil of the refrigeration system is situated at the top of the solution to raise the temperature thereof to melt the ice, and means for withdrawing the pure water from the top of the solution.

17. Apparatus as defined in claim 13, wherein the means for adding further solution comprises a feed line connected to a venturi in the means circulating the solution outside the container, the crystals forming on injection into the solution.

18 Apparatus for the production of pure ice crystals or pure water from a brine solution substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.