CA2308805A1

CA2308805A1 - Desalination process/equipment

Info

Publication number: CA2308805A1
Application number: CA002308805A
Authority: CA
Inventors: Asha Suppiah; Kamalamma Suppiah; Sellathurai Suppiah
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-05-12
Filing date: 2000-05-12
Publication date: 2001-11-12

Abstract

A desalination container is enclosed to contain water vapour. Salt water is provided to a fibrous hydrophilic support, preferably of woven cotton, placed adjacent a heated evaporator surface of the container. Water vapour condenses on a cooled condensation surface located adjacent to the evaporator surface and fibrous support. Fresh water is collected from the condensing surface as its condenses.

Description

y' TITLE: DESALINATION PROCESS/EQUIPMENT
FIELD OF THE INVENTION
This invention relates to the desalination of salt water. More particularly it provides an apparatus and method for producing fresh water from salt water.
BACKGROUND TO THE INVENTION
In many parts of the world there is an extreme shortage of drinking water. Fresh water which can be used by man is only a minute fraction of the total amount of water present as sea water. It is becoming important to find inexpensive ways to provide fresh water for drinking and agriculture for the increasing population of the world.
Nearly 65~ of the population of the world live within 60 km radius from an ocean. Many countries that experience a shortage of drinking water have an abundant amount of sunshine which could be converted to heat.
Currently about two-thirds of the world's sea water desalination is done by distillation. Reverse osmosis technology ("RO") is becoming more popular because it is currently more economical than distillation. Worldwide, there are 10,000 sea water desalination plants, producing about 20 Mm3/d of fresh water. Recently, more efficient and cheaper membranes have become available to allow treatment of sea water which has high salt content (3.5 to 4.6 wt~).
Mechanical vapour compression (MVC) is another method becoming popular.
In these processes one of the major costs is in the use of high pressure pumps to achieve the high pressure (~6 MPa) necessary to overcome the osmotic pressure of sea water.
This drives about 50$ of the water through the membrane as permeate, leaving a waste brine stream that is sent back to the sea. Better economies are achieved by recovering some of the pressure energy from the waste brine.
The total dissolved solid (TDS) content of water from RO is around 300 mg/L. Though this meets drinking water standards, it often does not suffice for industrial use.
Further treatment may be necessary. The osmotic pressure is dependent on temperature and the salt content and this will affect the RO operation.
In some situations, multistage flash (MSF) evaporation method may be better suited. In this method the pressure of the water is suddenly reduced below its equilibrium vapor pressure causing "explosive" boiling. This pressure reduction is achieved by introducing the sea water

2 into a chamber through an orifice. An external heat source is used.
In yet another method of multieffect distillation (MED), preheated sea water is sprayed onto the heat transfer area of a single-effect evaporator and the resulting vapour is transferred to the second and further stages by operating at progressively lower temperatures.
Generally, power requirements are higher for distillation methods than for R0. RO, MVC, thermocompression, MED and MSF require about 22, 38, 8, 8 and 16 kWh/1000 US
gal, respectively. MSF, MED and thermocompression also require steam llb/7-12 lb of water. Improvements are made to lower steam usage through serrated-profile tubes in the evaporator.
Use of solar energy for water distillation is well known, particularly in arid, coastal regions. In simple terms, sea water is evaporated by solar energy and then condensed to provide the fresh water. Since solar energy is free, the cost of the desalination process mainly depends on the cost of the materials used for construction of the desalinator. Since salt water is extremely corrosive, the right choice of materials is very important.

3 Techniques of increasing the rate of evaporation by increasing the surface area of evaporation are well known and many inventions, such as U. S. Patents 6, 001, 222; 2, 213, 894 and 3,801,474 exploit this idea.
Many countries often lack the technology and or the power requirements of the various methods outlined above.
Because of the corrosive nature of sea water, costs connected with materials of construction can be very high. Capital investments also become very important with some of these processes. Other factors that affect the choice of a process include waste production (amount of concentrated brine) and the area requirements for a given installation.
The current invention provides a simple method that is extremely low energy consuming and that does not require specialized materials or high capital investments. When properly designed, it would produce very little waste and it would require little maintenance.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more

4 specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
SUMMARY OF THE INVENTION
According to the present invention, in one aspect a surface is provided for the water portion of salt water to evaporate when heated with solar energy or any other form of heat. A cooler surface is provided adjacent to the evaporator surface for the vapour to condense and produce fresh water.
In order for the water to be evaporated more readily it is spread thinly over a support surface and simultaneously heated. This encourages the water component to evaporate readily, provided that the vapour pressure in the evaporating environment can be maintained below the equilibrium value at the temperature of the water present therein.
Accordingly, the salt water is preferably carried by a hydrophilic fibrous support, such as cotton, in order for it to evaporate more readily. Woven cotton spreads water very well due to the wicking effect caused by the surface tension of water. Though cotton can soak-up a large amount of water, it can also release it very easily when heated to relatively low temperatures in air whose relative humidity is less than

5 100 at these temperatures. In our daily lives we do this when we dry our clothes in an electric drier or in the sun.
When drying in the sun, it takes a short time when the humidity of the air is low and a longer time when the humidity is high.
Although cotton is a preferred support for evaporating water, any hydrophilic fiber may be employed, such a sisal, hemp and other similar materials.
In applying the present invention to desalinate sea water, sea water is spread on a preferably woven layer of cotton while it is heated, as by solar energy or any other forms of energy, in order to evaporate it. Since cotton is suited to be positioned next to a heated surface, or made to move as in a conveyor belt through a heated zone, the heat can be applied to the cotton in many convenient ways.
According to one variant of the invention, a layer of cotton is positioned inside a flat plate-type solar heat collector presented at a suitable angle to capture solar energy. The vapour produced by the heat absorbed is then condensed on an adjacent cooler surface and collected as fresh water.
Salt water may be dripped onto woven cotton while the cotton is held taut within the interior of a desalinator

6 container while heat is applied through the collector surface to the cotton layer. It is preferable to hold the cotton taut to minimize local accumulation of water and avoid dripping.
Water vapour is produced due to heat that is provided. At the same time water is condensed on a cooler condensing surface located adjacent to the fibrous support, across a gap within the desalinator container. The evaporator cotton used may or may not touch the heat collection surface, so long as it is exposed to heat.
The solar collector side of the desalinator may be made according to the current knowledge on solar energy collectors including the use of a simple glass plate. A
stainless steel sheet may be alternately employed as the heat collection surface, with the cotton preferably placed in intimate contact with such surface.
An advantage of using a hydrophilic fiber such as cotton for the evaporator is that the temperature rise in the desalinator container will be limited by the absorption of heat through vaporization of water. This will reduce the cooling load that must be accommodated by the collection surface.
There are various methods available to increase the capture of energy through the use of black bodies, including

7 cotton dyed black and the use of a glass painted black. The angle of the energy capture surface and with it the cotton layer can be easily adjusted from vertical to almost horizontal to maximize the capture of the heat energy.
At controlled flow rates of water through the evaporator, cotton of a density suitable for desalination, will hold the salt water sufficiently well so that no dripping from the cotton onto the condensing side of the desalinator occurs even when the heat capture side is almost horizontal.
The vapour condensing surface can be cooled by one of many means available. Cold salt water may be made to flow over external portion of the condensing surface via a cotton layer placed on the outside of the desalinator container. In this case the cotton is used advantageously to do even more for the desalinator. The layer of cotton placed on the external side of the condensing surface of the desalinator can carry a significant amount of water to effectively cool the surface for vapour condensation to occur on the inside surface. Alternately, a jacket may be provided on the external side for cooling water to flow directly against the cooled wall of the desalinator. As a further alternative a water pump may used to spray cooling water on the cooling

8 surface. Hence, a large number of choices is available for cooling the condensing side of the desalinator.
The condensing surface may be made of a number of materials that can be cooled easily including glass. Since the condensed water that comes in contact with the cooling surface is fresh water free of salt, problems related to corrosion are reduced on this side of the desalinator. If the external cooling is not carried out with salt water then a large choice of materials such as aluminum and steel is available for construction of this surface.
On the other hand, the condensing surface may be made of glass or stainless steel which would avoid all corrosion related problems. Even without external cooling, the condensing surface will be cooler since heat is applied to the evaporator cotton even if some of that heat is absorbed by the water evaporating from it. Therefore, in some cases external cooling may not have to be provided at all for the vapour to condense.
The escape of vapour from the desalinator container's interior should be minimized by at least closing its upper end, and preferably by closing the lower end as well. The objective is to contain convective air flow. Thus the narrow ends of the desalinator are preferably covered to

9 avoid water vapour escaping the desalinator. These covers may be made of a suitable material such as wood, plastic or metal.
The fresh water produced from the condensing vapour can be removed by a number of methods that make sure that a minimum amount of vapour within the desalinator is allowed to escape. Condensing water may be allowed to drip from the desalinator container through a narrow slot. The condensing water may also be removed from the desalinator using a short length of cotton layer wedged between the condensing surface and the cover plate to serve as a wick. The bottom edge of this cotton layer may then be cut to form one or more V-notches so that the condensed water rolling down the cotton will drip from the V-notches in an uniform, directed fashion into collection containers. Within the desalinator, this wicking layer of cotton need only extend a few millimetres into the interior of the desalinator container so that the area of the available cooling surface is not reduced.
The flow of sea water through the desalinator may be controlled depending on the type of operation desired. For example, the flow may be sufficiently small such that no brine exits the bottom of the desalinator. In this case, the water is completely evaporated leaving salt to accumulate on the cotton. On the other hand, a very small flow of water may be allowed to exit the evaporator in which case all the cotton inside the evaporator remains wet at the temperature of evaporation. In any case, the flow of water through the evaporator is preferably kept low so that the water is evaporated efficiently without wasting the injected heat through the exiting of heated water from the desalinator.
Periodically the cotton will have to be removed from the desalinator and the accumulated salt removed by washing the cotton with sea water, or by simply shaking it to let the salt fall off the cotton.
In this invention the fresh water production rate depends on the rate of flow of water through the evaporator cotton and the rate of heat capture through the heat collector surface. As mentioned above, wastage of heat can be avoided if excess water is not flowed through the evaporator cotton.
Excess water flowing this way will carry the heat out of the desalinator rendering the efficiency lower than achievable.
As a further variant of the invention, the woven evaporator cotton may be in the form of a circulating band that passes adjacent to the heat collecting surface.
Similarly the condensing surface may be cooled by a circulating band of moist cotton fabric. Preferably the rotations of these two bands are in opposite directions.

As another variant, the evaporating cotton may be fixed to a rotating wheel that carries moisture up from a reservoir and passes it closely to the heat collecting surface. A similar cooling wheel of cotton may carry cooling water up to the external side of the condensing surface.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side view of the desalinator showing the stationery cotton layer used for evaporation and the condensing surface;
Figure 2 is a side view showing the desalinator with circulating wetted cotton used for both evaporation and for cooling; and Figure 3 is a side view showing desalination using a wheel of cotton for evaporation.
Figure 4 is a face view of Figure 3.

Figure 5 is a pictorial depiction of an alternate embodiment wherein condensation occurs outside of the desalinator container.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the installation of Figure 1 a heat collector surface 1 made of a glass plate or metal is provided. A
cotton layer 2 is positioned adjacent the inner side of this heat collector surface 1. The inside surface of glass may be coated with black paint to increase absorption of the heat energy. Alternately, the cotton layer 2 may be dyed black.
The cotton 2 is preferably in intimate contact with this heat collecting surface 1 so that the heat is transferred efficiently to the water 3 being introduced therein from a salt water source.
The evaporating water 4 is condensed on the condensing surface 6 located on the other internal side of the desalinator container 5 to provide condensed fresh water 7.
This condensing surface 6 can be maintained at a cooler temperature simply by circulating cool salt water 8 through a cotton-drenched cooling layer 9 placed against its exterior surface. If sea water is used for this cooling process, it may be necessary to use glass or stainless steel on this side so that corrosion-related problems are avoided.
A significant amount of the heat is recovered from the condensing water vapour 4 developed within the desalinator. This recovered heat is present within the cooling water 8 on the cooling layer 9 placed on the exterior side of the condensing surface 6. This partially heated water 8 can be diverted as feed water for the evaporator side of the desalinator, taking advantage of the heat of condensation that it has absorbed.
The invention outlined above using cotton overcomes a number of deficiencies of the solar distillation apparatus marketed as the Rosendahl system (www.pege.org). In the Rosendahl system, sea water is evaporated by flowing it over a surface made of a special material ("soakage filter") and then condensed on the inside surface of the same glass plate-covering that lets the heat waves in. Since heat is received through this glass covering, the condensation process is made inefficient. In the present invention the vapour 4 is condensed on a significantly cooler surface condensating surface 6 since it is on the other side of the desalination container from cotton-evaporator layer 2.

Allowing the vapour 4 to condense on the "non-heated" side of the desalinator container 5 in the current invention is more efficient since the condensed water 7 is not heated unnecessarily by the heat entering through the heat collector surface 1.
According to the invention, the ability of the cotton 2 to spread the water 3 on its surface and hold a significant amount avoids the need for specialized materials for evaporation. The fact that the water 3 can be held by cotton 2 without dripping enables the provision of a condensing surface 6 that is located below the evaporating surface.
Another method of desalination using the above idea is shown in Figure 2. In this apparatus an endless evaporator cotton band 20 is circulated through the desalinator container 5 and a salt water reservoir 21 so that the salt water 22 picked up in the reservoir 21 by the cotton band 20 is evaporated fully or substantially within the desalinator container 5. The use of cotton to transport the water 22 into the desalinator in this fashion renders it possible for the water 22 to be heated conveniently by solar energy compared to other known arrangements. Here again, the current knowledge available to capture the sun energy can be used to its fullest capacity.
If the cotton is mostly dry as it exits the desalinator, provisions can be made to remove the salt residue on it by means of continuous vibration or scraping applied to the moving belt. In this case, salt can be a by-product of the process.
A similar endless cotton band 23 may be used to cool the condensing surface. Preferably this band 23 rotates in the opposite direction to band 20.
In case the heat collector is made of a material that is prone to corrosion, then the circulating cotton band may be made to pass adjacent to the interior side of the heating surface 1 without coming into contact with the actual 15 heat collector surface 1. If it is required to squeeze excess water out of cotton band 20 to avoid dripping within the desalinator, wiper blades 24 made of rubber materials that are resistant to salt water may be positioned at the entrance to the desalinator container 5.
20 If a need arises to place a separating layer between the circulating, heated, evaporator cotton band 20 and the condensation side, Teflon coated screens (not shown) may be used. Aluminum or stainless mesh screens may be coated with Teflon so that salt water 22 will not come in contact with the condensing surface 6. These screens placed between the evaporator cotton band 20 and the condensing surface 6 can be perforated so that the vapour 4 can flow freely through the screen without any resistance.
Since the only power requirement for this variant of the desalinator is that required to circulate the cotton belts 20,23, use of a low hp motor with a minimal overall power requirement is possible.
For a given heat collector with a specific area and heat absorption characteristics, the rate of fresh water 7 production in this apparatus can be controlled by adjusting the evaporator cotton belt 20 recirculation rate. The rate of circulation of cotton belt 20 may be programmed within a controller 26 to be a function of the heat available for absorption which would depend on the time of the day (when solar energy is used for evaporation).
The use of recirculating cotton 20 allows flexibility to produce very little waste brine 10 from the process since the rate of circulation can be adjusted to evaporate all most all the salt water 22 entering the desalinator container 5. The salt residue on the cotton belt 20 can be removed by a simple scraping or by vibration, or it can be redissolved in the sea water 22 in the pond or reservoir 21.
Another configuration for the desalinator is shown in Figure 3. This is similar to dehumidification wheels used for building humidity control. A wheel 30 made of cotton, almost half immersed in a sea water pond 31 rotates slowly, thereby picking up the water 32 from the pond and losing it in the top section where solar energy is used to evaporate the water 32. As the side view shows, the top half or so of the wheel 30 is contained within the desalinator chamber 5, adjacent a heat collecting surface 1 and adjacent a condensing surface 6. The condensing surface 6 can be cooled by one of many methods including another cotton wheel 33 located in intimate contact with the external side of the condensation surface 6.
A pump (not shown) may alternately be used instead to simply spray the pond water over the external side of the condensing surface 6 to maintain a lower temperature.
The structure supporting the cotton wheel 30 may be made of plastic materials for both the heated evaporator cotton and the cooling cotton wheel 33. Seals 34 between the rotating wheels 30,33 and the desalinator chamber 5 may be maintained with rubber wiper blade-type seals to provide maximum containment of water vapour 4.
In another configuration of the desalinator shown in Figure 5, multiple layers of cotton 40 may be used to evaporate the water in a common chamber 41 heated by solar energy 42 or other forms of heat. Sea water 43 can be gently sprayed on the layers of cotton 40 placed in the chamber 41 at suitable angles to capture the heat. The vapour 44 produced may be transferred continuously by applying a slight vacuum provided by a vacuum pump 45 into a separate cooling chamber 46 where it is condensed to form fresh water 47.
Example 1 A test was carried out using the set-up shown in Figure 1 with the evaporator cotton vertical and in contact with the heat collector surface. A 250 w infra red heat lamp was used to simulate solar power. It was placed 50 mm away from the heat collection surface of the desalinator.
Approximately 144 mL/h of water were dripped onto the top end of the evaporator cotton and allowed to flow down the cotton.
Sufficient water was made to flow over the condensation side cooling cotton to maintain it in a moist state.
Salt-free condensed water soaked the small piece of cotton at the bottom on the fresh water surface of the desalinator and dripped from it into a container within 15 min of starting the experiment.
In a 90 min long test, approximately 45 mL of fresh water was collected. The density of this fresh water was found to be 0.999 g/mL showing that there was no salt in the water. The material of the heat collection surface was stainless steel foil and the cotton layer was 100 mm wide by 300 mm long with a thickness of 0.3 mm. The gap between the cotton layer and the condensing surface was around 20mm. The salt water used for the test had a density of 1.033 g/mL which translates into 4.1$ salt in water and the brine collected had a density of 1.083 g/mL which represents about 11.5 salt in water. The temperature of vapour within the desalinator was within the range of 40 to 50°C and this temperature was reached within 5 min of starting the experiment. The exterior ambient temperature was 20oC.
Based on theoretical assessments, it is believed that a heat collector surface of 150 mm by 300 mm dimensions exposed to a solar flux of 256 cal/s/m2 can produce 50 gallons of water per hour as a maximum. The gap between the evaporator cotton layer and the condensing surface should be as short as possible. A gap of lOmm is believed to be practical if provision is made to minimize sagging of the cotton layer. In larger units a 20mm or even a 30-40mm gap may be suitable.
Based on the foregoing, an efficient, low cost system may be provided to produce fresh water from salt water.
CONCLUSION
The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow.
These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A desalination system comprising:
(1) a desalinator container with a heated evaporator surface;
(2) a cooler, condensing surface positioned adjacent to the evaporator surface upon which water vapour may condense and produce fresh water;
(3) a fibrous hydrophilic water support, positioned adjacent to the evaporator surface within the desalination container;
(4) feed means to supply salt water to the fibrous support;
(5) cooling means to cool the condensating surface; and (6) fresh water collection means to collect fresh water condensing on the condensing surface.

2. A system as in claim 1 wherein the container is closed at the top and open at the bottom to contain water vapour therein.

3. A system as in claim 1 wherein the container is substantially closed at the top and bottom to contain water vapour therein.

4. A system as in claims 1, 2 or 3 wherein the fibrous support is cotton.

5. A system as in claim 4 wherein the fibrous support is woven cotton.

6. A system as in claim 5 wherein the evaporator surface is coated to absorb solar energy.

7. A system as in claim 5 wherein the evaporator surface is transparent to pass solar energy to the woven cotton.

8. A system as in claim 7 wherein the cotton is darkened to absorb solar energy.

9. A system as claimed in claims 1, 2 or 3 comprising a jacket provided on the external side of the condensing surface whereby the vapour condensing surface may be cooled by water directed to flow over external portions of the condensing surface.

10. A system as in claims l, 2 or 3 comprising a fibrous hydrophilic layer placed on the outside of the condensing surface of the desalinator container to contain cooling water.

11. A system as claimed in claims l, 2 or 3 comprising a water pump positioned to spray cooling water on the condensing surface.

12. A system as in claims 1, 2 or 3 wherein the desalinator container is provided with a narrow slot along its lower edge whereby condensing water may be removed from the desalinator and further comprising a short length of hydrophilic fibrous material wedged within the slot to serve as a wick for removal of condensed water from the container.

13. A system as in claim 5 wherein the woven cotton is in the form of a circulating band that moves adjacent to the evaporator surface.

14. A system as in claim 13 wherein the cooling means comprises a water-impregnatable, circulating band of woven cotton that moves adjacent to the condensing surface.

15. A system as in claim 4 wherein the cotton is in the form of a rotating wheel that moves adjacent to the evaporator surface.

16. A system as in claim 15 wherein the cooling means comprises a rotating wheel for carrying cooling water that moves adjacent to the condensing surface.