WO1980000833A1

WO1980000833A1 - Desalination plant

Info

Publication number: WO1980000833A1
Application number: PCT/SE1979/000206
Authority: WO
Inventors: B Gustafsson
Original assignee: Projectus Ind Produkter Ab
Priority date: 1978-10-18
Filing date: 1979-10-12
Publication date: 1980-05-01
Also published as: SE7810875L; SE414302B; EP0020510A1; JPS55500800A

Abstract

A plant for seawater desalination comprises a vapour separation chamber (2) situated at an altitude of 6 to 10, preferably 8 to 10 meters above seawater level (Y) and connected to a vacuum pump (8) for evacuating non-condensable gases from the chamber. A delivery pipe (1a, 1b) extends from below the sea surface to the chamber from whence a return pipe (4a, 4b) extends back to below the sea surface. A solar collector (3) is coupled into the delivery pipe to supply necessary evaporation heat to the liquid flow and to control the liquid flows in the plant. Water vapour separated in the chamber (2) is condensed by heat exchange with cold seawater, possibly by means of a heat exchanger (6) in the delivery pipe. A shunt pipe (9) can be coupled between the delivery and return pipes for recirculating a part of the return flow from the chamber. A second solar collector (10) can be connected to the delivery pipe upstream of the shunt pipe connection to the delivery pipe (1a, 1b), to control recirculation of water from the return pipe (4a, 4b) in response to the salt content of the return water so that the plant automatically limits the salt content of the water in the plant.

Description

TITLE OF INVENTION:

DESALINATION PLANT

TECHNICAL FIELD

The invention relates to a plant for seawater desalination, comprising a first pipeline for delivering seawater to a vapour separation chamber, a heat source in the first pipeline, a second pipeline for removing seawater from the chamber, a third pipeline for taking water vapour from the chamber to a condenser and to a condensate collection vessel, and a vacuum pump connected to the vapour-carrying parts of the plant, arranged for providing a pressure in the chamber which is equal to or lower than the vapourpressure of the salt water coming into the chamber.

BACKGROUND ART:

In known plants of the kind described above, (see the U.S. Patent 3,515,645, for example) the salt water flow has to be controlled carefully so that the salt water level does not affect or disturb the vacuum source and so that unnecessary energy losses do not occur. Such control requires liquid pumps, control equipment and monitoring.

OBJECT OF THE INVENTION

One object of the invention is to provide a plant of the kind described in the introduction, which does not require special pump or control equipment and which is substantially self-regulating, even for varying temperature of the sea water flowing into the vapour separation chamber.

DISCLOSURE OF INVENTION

According to the invention, said object is obtained in a plant of the kind described in the introduction, substantially in that the vapour separation chamber is placed at a height in the order of magnitude 6-8 meters, preferably about 8-10 meters, above the seawater surface, the first and second pipelines opening out under the seawater surface and the chamber being arranged to provide a hydraulic connection between said pipelines.

By means of the invention there is thus provided a plant which provides self-circulation of the seawater through it. Circulation is controlled by heat flow from the heat source to the sea water, and the vapour separation occurring in the vapour separation chamber. The driving forces acting on the liquid in the plant are a) density-decreasing heating of the water in the first pipeline and the vapour formation therein, and b) the density increase due to increased salt concentration in the water departing through the second pipeline after having been subjected to the distillation process in the vapour separation chamber. It will therefore be appreciated that the heat source can consist to advantage of a solar collector, whereby the relative production of water vapour, i.e. the salt content in the water in the second pipeline, determines water circulation through the plant. Since the separation chamber is located at the height stated, only minimum control of the vacuum pump is required, so that the system is kept free from air and other non-condensable gases.

The altitude of the vapour separation chamber is adjusted to the expected temperature of the heated water. By the provision that the system is kept clear of air, the water will boil in the upper part of the head of water lifted by the vacuum, if the head of water has the possibility of being lifted freely upwards. The height of the head of water thus substantially depends on the temperature of the water. With a heat source in the form of a simple solar collector, the water temperature can be increased to 40°C, for example, corresponding to a height of about 9 m. However, utilization of a concentrating solar collector can very well be conceived, and this would increase the temperature to 75ºC, for example, corresponding to a height of about 6 m.

To advantage, the condenser can be arranged to transfer at least a part of the condensation heat of the water vapour to the salt water flow in the first pipeline at a point between the sea surface and the solar collector.

A fourth pipeline, with one end connected to the second pipeline, and with its other end connected to the first pipeline at a point between the heat source and the inlet of the first pipeline, is a suitable further expedient. This fourth pipeline thus forms a shunt pipe extending parallel to the hydraulic communication of the chamber between the first and the second pipeline. In the case where the condenser is arranged to transfer the condensation heat to the salt water flow of the first pipeline, the fourth pipe is suitably connected to the first pipeline between the condenser and solar collector. Furthermore, a second solar collector can be arranged in the first pipeline between the condenser and the fourth pipeline. The object here is to ensure that the proportioned density of the head of water, in the second pipeline under the fourth pipeline is higher than the proportioned density in the first pipeline, the object being to provide recirculation by the fourth pipeline to the first pipeline of a part of the salt water flow deparing from the chamber through the second pipeline. This also means returning heat from the second pipeline to the first pipeline, whereby the criterion is that the salt content must not be too high in the plant as a result of the vapour separation in the chamber. It can thus be said that the arrangement automatically separates salt water with a too high salt content from the plant. If it were therefore desirable to select a high limiting value for the salt content of the water flowing through the second pipeline, there is a possibility of being able to dispense with the second solar collector, if the condenser has a higher efficiency with respect to returning the heat to the first pipeline, and the water which flows into the first pipeline has a salt content which notably falls below the salt content in the outlet section of the second pipeline. For stabilizing the plant, a non-return valve should suitably be arranged in the outlet of the second pipeline, as well as in the inlet of the first, pipeline

The height of the liquid pillar in the vapour separator will vary in response to the temperature, salt content and vapour content in the liquid flow coming into the vapour separation chamber from the first pipeline, and the chamber should therefore be made with a relatively large height, e.g. 2 meters (the altitude of the chamber being defined as the expected water level in the chamber) to enable effective vapour separation, also for varying liquid levels therein.

The chamber should contain effective vapour separators, since variations in the heat supply result in variations in boiling intensity. Such separators can comprise vertical baffles, against which liquid drops and scum impinge, subsequently to flow down by gravitation to the liquid mass. Conventional labyrinths can also be arranged in the upper part of the chamber for further separating salt water drops and scum from the vapour flow.

To advantage, the vacuum pump can be connected to the upper part of the enclosed condensate collection vessel. If so required, the condensate can be continuously removed from the vessel by means of a liquid pump. The first solar collector is suitably made with a large vertical liquid passage height so that the energy increment to the liquid flow is continuously stepp d up during its flow towards the vapour separation chamber. It will be appreciated that the liquid will be subjected to a certain amount of overheating and vapour formation in the first solar collector, as a result of the liquid therein being under a certain pressure represented by the height and density of the outwardly situated head of water in the first pipeline and/or the first solar collector. This overheating/vapour formation in the first pipeline or the first collector favours the plant vapour production.

The invention is defined in the accompanying patent claims.

BRIEF DESCRIPTION OF THE DRAWING:

The invention will now be described in the following in the form of an embodiment example while referring to the accompanying drawing, which schematically shows a partly cut elevational view of an embodiment example of the inventive plant.

BEST MODES OF CARRYING OUT THE INVENTION:

On the drawing, a sea surface is denoted by the letter Y. A delivery pipe 1a, 1B containing a non-return valve 12 is provided to take the sea water up to a vapour separation chamber 2 situated at an altitude H of 8-10 meters above the sea surface Y. A return pipe 4a,

4b connects to the chamber 2 and opens out under the sea surface Y. The portion 4b of the return pipe contains a non-return valve 11. The portion 1b of the delivery pipe has a heat source 3 in the form of a vertically elongate solar collector which can be formed by a vertically orientated tube array connected in parallel. A second heat source 10 in the form of a corresponding solar collector is arranged in the portion la of the delivery pipe. A shunt pipe 9 extends from the return pipe 4a, 4b to the delivery pipe 1a, 1b between the solar collectors 3 and 10. A vapour pipe 5 extends from the upper part of the vapour separator via a heat exchanger 6 to a condensate collection vessel 7. The heat exchanger 6 is connected to the lower portion 1a of the delivery pipe under the solar collector 10 for preheating sea water flowing into the pipe 1a, and for condensing water vapour flowing out of the chamber 2. A vacuum pump 8 is connected to the condensate collection vessel 7 to evacuate the vapour separation chamber 2. A pipe 15 with a water pump 16 can be connected to the lower portion of the condensate collection vessel 7 for removing generated fresh water. The lower portion 4b of the return pipe passes a heat exchanger 14 arranged between the second solar collector 10 and the condenser 6 for returning heat to the water flowing through the delivery pipe. The plant shown has the following starting sequence, assuming that it is initially filled with air at atmospheric pressure. The vacuum pump 8 is started and lowers the pressure, With the aid of the non-return valves 11 and 12, sea water will be drawn up into the delivery pipe 1a and fill the return pipe 4b, 4a via the shunt pipe 9. On continued operation of the vacuum pump, the water will rise by degrees to a level where the level difference corresponds to the atmospheric pressure minus the vapour pressure of the water at the prevailing temperature, i.e. somewhere between 8.5 and 9.8 meters (the height H) so that hydraulic communication is provided between the delivery pipe 1b and return pipe 4a via the vapour separation chamber 2.

When the temperature in the liquid in the delivery pipe 1a, 1b increases as a result of the absorption of solar energy in the solar collectors 10, 3, the liquid density in the said pipe 1a, 1b is reduced and liquid circulation initiated. If the water temperature is higher in the delivery pipe 1b than the temperature corresponding to the vapour pressure in the vapour separation chamber 2, vapour formation will take place from a certain height H - h (where the total pressure in the liquid corresponds to the liquid temperature of the vapour pressure) and thereover. When vapour formation is thus initiated, the liquid flow in the delivery pipe 1b increases so that water and vapour enter the vapour separation chamber 2 at a relatively high rate.

The separator 2 automatically alters its separating capacity to suit the prevailing operational condition: at a high temperature of the liquid leaving the delivery pipe 1b, the liquid level in the chamber 2 will sink and a greater through-flow area will be liberated for the vapour phase. The vapour separator can thus be formed so that approximately the same vapour speed is maintained independent of of load variations (varying heat supply to the solar collectors 3, 10). The chamber 2 suitably includes a plurality of vertical baffles perpendicular to the liquid inlet, so that water drops and scum impinge against the baffles and the wall of chamber 2 so that they can be effectively separated by gravity.

The heat exchanger 14, condenser 6 and the second solar collector 10, which can be made correspondingly to the first solar collector, are arranged to give the water in the delivery pipe la a lower density than the water in the return pipe portion 4b, so that a hot return water flow from the delivery pipe 4a can be recirculated via the shunt pipe 9 to the delivery pipe portion 1b when the salt content in the return flow through the return pipe 4a is lower than the determined value required by the practically permissible boiling point - raising effect, which the increase in salt contents causes.

The heat exchanger 14 and condenser 6 possibly provide a heat increment in the delivery pipe portion 1a such that, taking into account the differences in salt content in the delivery pipe portion 1a and the return pipe portion 4b, it is possible to dispense with the second solar collector 10. However, for practical/economical reasons it may be suitable to dispense with the heat exchanger 14, whereby the need of heat supply in the delivery pipe portion la increases, probably to a level such that the second solar collector 10 is required to maintain the desired flow.

The point P in the delivery pipe portion 1b can assumed to be one in which the total pressure in the liquid corresponds to the vapour pressure for the liquid temperature .The temperature T_p at the point P is equal to the evaporation temperature at the liquid surface of the chamber 2 plus a small temperature increment, which is dependent on the height h of the head of liquid between the point P and the liquid surface in the chamber 2, and the density of this head of liquid. Above the point P there is thus initiated vapour formation serving to increase the flow rate in the delivery pipe portion 1b, thus increasing the vapour separation effect in the chamber 2.

The vacuum pump 8 can be environmentally driven e.g. by an electric motor supplied with current from an accumulator charged by a wind generator, a wave-driven generator or a solar cell. Alternatively, the pump can be mechanically driven by a windmill or wave power.

It will be appreciated that the inventive plant does not require any pumps or other special flow- regulating apparatus to control the liquid flows in spite of varying heat supply: This signifies an extremely simplified or automated flow control in the plant, which furthermore permits the possibility of utilizing solar collectors in a simple way for supplying evaporation energy and for providing desired control of the flows . Although the vacuum pump 8 only has the task of emptying the system of air orrother condensable gases, it will be appreciated that the need of control will be a minimum even for this pump. Furthermore, as mentioned, the pump can be driven by sun, wind or wave power, whereby there would be absolutely no need of any further outside energy supply to the plant.

Claims

C L A I M S

1. A plant for sea water desalination comprising a first pipeline (1a, 1b) for delivering salt water to a vapour separation chamber (2), a heat source (3) at the first pipeline, a second pipeline (4a, 4b) for taking away salt water from the chamber (2), a third pipeline (5) for leading water vapour from the chamber (2) to a condenser (6) and to a condensate collection vessel (7) and a vacuum pump (8) connected to the vapour-carrying parts (2, 5, 6, 7) of the plant, and which is arranged to establish a pressure in the chamber (2) which is equal to, or less than the vapour pressure for the sea water coming into said chamber, characterized in that the vapour separation chamber (2) is situated at an altitude (H) of between 6 and 20 meters, preferably between 8 and 10 meters above the sea water surface, and that the first and second pipelines (1a, 1b; 4a, 4b) open out under the surface of the water and that the vapour separation chamber is arranged to establish hydraulic communication between the first pipeline (1a, 1b) and the second pipeline (4a, 4b).

2. A plant as claimed in claim 1, characterized in that the heat source (3) comprises a first solar collector.

3. A plant as claimed in claim 2, characterized in that the condenser (6) is arranged to transfer at least a portion of the condensation heat to the first pipeline at a point between the sea surface (Y) and the first solar collector (3) .

4. A plant as claimed in claim 3, characterized in that a fourth pipeline (9) extends from the second pipeline (4a, 4b) and connects to the first pipeline (1a, 1b) at a point between the heat source (3) and the inlet of the first pipeline (1a, 1b).

5. A plant as claimed in claim 4, characterized in that the fourth pipeline (9) connects to the first pipeline (1a, 1b) between the condenser (6) and the heat source (3).

6. A plant as claimed in claim 2 and 5, characterized in that a second solar collector (10) is arranged at the first pipeline (1a, 1b) between the condenser (6) and the fourth pipeline (9) .

7. A plant as claimed in claim 1, characterized in that a non-return valve (11) is arranged at the outlet under the sea surface (Y) of the second pipeline (4a, 4b).

8. A plant as claimed in claim 1, characterized in that a non-return valve (12) is arranged at the inlet of the first pipeline (1a, 1b).