IL96503A - Apparatus and method for desalination of sea water - Google Patents

Description

APPARATUS AND METHOD FOR DESALINATION D'-»n ηρηηπ ΠΙΡΡΙ IRI II The present invention relates to an apparatus and to a method for desalinating sea water.

More particularly, it is concerned with an improved apparatus and method of the flash-evaporation type using solar energy, characterized by an improved method of removing precipitated salt from a heated tubular conduit.

The solar evaporation of sea water has already been carried out thousands of years ago, the object often then having been to produce salt. In modern times the object is more often the production of fresh water, an outstanding example being the large scale solar still which operated from 1872 for several decades at Las Salinas in Chile using flat plate collectors. Many different designs for the desalination of sea water are known, some being described in Israel Patents 16044, 45901 and 63921.

Commercial desalination installations must produce large quantities of water at a cost competitive with alternative sources, and solar energy plants can achieve this only by covering a large area with moderate cost solar energy collectors.

Improved efficiency of operation is possible using concentrating parabolic reflectors to obtain higher temperatures than are possible with solar energy absorbers not provided with concentration means.

For desalination purposes horizontal reflectors having an East-West oriented axis are adequate, and they can produce superheated steam even at moderate concentration ratios. Such reflectors are well known, one type being described in Spanish Patent 454,576. This type of reflector, using a high concentration ratio, has been commercially proved by the Luz Corporation in a large power-producing installation in the Mojave Desert, U.S.A. Such collectors are well suited to mass production and are easily installed, and need to track the sun in one axis only. Tracking devices are known, some types being described in Israel Patents 57766 and 69517. In order to reduce radiation losses from the heated conduit, selective coatings may advantageously be applied to the surface on which the solar energy is to be absorbed, several types being described in Israel Patents 58214, 60372 and 69507.

The solar water distiller proposed in U.S. Patent 4,921,580 uses a paraboloid or spherical reflector, referred to as a convex lens number 17, which may be suited to its use for a small apparatus for a residential building, but would be difficult to upscale for distilling large quantities of sea water. Furthermore, it requires sun tracking means in 2 axes, and no provision is made for the removal from the boiler of salt precipitates .

A ton of average sea water holds in solution approximately 35 kilograms of solids, mostly as salts such as sodium chloride.

Except in desalination plants using open evaporation basins, such salts must be removed periodically in order to avoid clogging the evaporation chamber being used. This necessity is provided for by the apparatus described in U.S. Patent 3,300,393, wherein salts are removed from the inside of the heated chamber by means of a scraper which operates when this chamber is revolved by an electric motor. The scraper will, however, wear against the chamber tube and is required to be resistant to the corrosive attack of sea water. Also, chamber tube bearings and leak-proof revolving-bearing joints are needed to support and connect the chamber, and the reliability of the system will be dependent on these components and also on the electric motor, its speed reducer and the drive gears revolving the chamber tube. Obviously, the capital and operating cost of these items will raise the cost of the desalinated water produced.

It is therefore one of the objects of the present invention to obviate the disadvantages of the prior art desalination apparatus and methods and to provide an apparatus and method which are durable, moderate in capital cost and will produce desalinated water at a cost lower than presently known systems.

It is a further object of the present invention to provide means for the collection of the salt extracted from sea water.

This the present invention achieves by providing an apparatus for desalinating sea water by flash evaporation using solar energy, comprising a tubular conduit through which sea water may be pumped and having an inlet valve and an outlet valve and a solar reflector mounted adjacent to said conduit for focusing the sun's rays directly onto an exterior surface of a longitudinal segment of said tubular conduit, characterized in that said conduit is in fluid communication with a valved condensation tank and with a valved salt flushing outlet, and said apparatus is further provided with switching means for sequentially closing said outlet valve and said inlet valve to effect a rise in pressure and in temperature in said tubular conduit, and thereafter for closing passage to said valved condensation tank and opening passage to said valved salt flushing outlet, whereby accumulated salt in said tubular conduit is flushed through said salt flushing outlet together with superheated pressurized liquid therefrom.

In a further preferred embodiment of the present invention, there is provided apparatus for desalinating sea water by flash evaporation with solar energy, further provided with a liquid-liquid heat exchanger, wherein part of the heat of the water leaving the cooling coils of said valved condensation tank is transferred to the sea water entering said tubular conduit.

Here it should be noted that the flash evaporation process has long been known, having originated as the Alberger process for salt manufacture. This process may operate continuously, on batches, or by some intermediate method. A sudden reduction of vapour pressure, usually in several stages, causes condensation in one or several cooled vessels. Incoming feedwater may be used for cooling the vessel and the feedwater is thereby preheated to achieve heat saving. Among the objects of the present invention is to utilize the flash evaporation process to achieve economic production of desalinated water.

The invention will now be described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is a perspective view of a preferred embodiment of the apparatus of the invention, the tubular conduit being shown fragmented; Fig. 2 is a cross-sectional view of the tubular conduit, showing the removal of accumulated salts; Fig. 3 is a schematic view of the apparatus, showing additional peripheral components, and Fig. 4 is a schematic view of a second embodiment of the tubular conduit and components for salt discharge.

There is seen in Fig. 1 a horizontally oriented tubular conduit 10, having a preferred inner diameter of about 100 to 130 millimeters and preferably having an exterior surface 12 coated with an electrolytic selective surface of high absorbance/ reflectance ratio and having long-term durability at 250° C. A non-return valve 13 is in fluid connection with the tubular conduit 10, being preceded by an inlet gate valve 14 arranged to allow sea water 16 to be pumped into the tubular conduit 10. The inlet gate valve 14 is preferably provided with means such as a solenoid 18 to allow for automatic actuation by remote control. An outlet valve 20 has an entry port 22 which is in fluid connection with the opposite side of the tubular conduit 10, this valve also being arranged for automatic operation, but being suitable for the control of steam at 180° C and pressures up to 12 bar, although the normal desalination action takes place at between 120° and 160° C.

A solar reflector 24 is mounted adjacent to the tubular conduit 10 for focusing the sun's rays 26 directly onto the exterior surface 12 of a longitudinal segment of the tubular conduit 10.

Preferably, the solar reflector 24 is of the horizontal parabolic trough type, sometimes referred to as cylindrical, having an East-West orientation, and is, of course, provided with means 28 for single-axis elevational tracking of the sun. The solar reflector 24 has a concentration ratio of approximately 1:3 to 1:6, this being adequate for producing the temperature and pressure required for desalination by producing steam 30 and for the salt-flushing action which will be described below.

The outlet valve 20 has an outlet port 32 which is in fluid communication with a valved condensation tank 34. When a pressure of 4 - 6 bar is reached inside the tubular conduit 10, the outlet valve 20 is opened and steam 36 is discharged into the condensation tank 34. The resulting drop in pressure inside the tubular conduit 10 sends a signal to the solenoid 18 to allow additional sea water 16 to pass through the inlet valve 14 and into the tubular conduit 10.

The steam 36 is condensed to form desalinated water 38 inside the condensation tank 34, while heat is removed therefrom. An array of fins 39 is provided on the inner surface of the condensation tank 34, the fins 39 facilitating the conversion of the steam 36 to water drops 48.

Switching means 40 are provided to operate the inlet valve 14, the outlet valve 20 and a three-way valve 42 whose ports are connected respectively to the tubular conduit 10, the outlet valve 20 and to a valved salt flushing outlet 44. The switching means 40 may take the form of a standard microprocessor-based programmable controller, having inputs connected to the various sensors and outputs to control all valves and pumps.

Periodically, normal operation is interrupted and a salt-removal procedure is initiated, which will now be described with reference also to Fig. 2. Precipitated salt 46 accumulates in the lower part of the tubular conduit 10 as a result of sea water evaporation taking place therein. Periodically, when the build-up of precipitated salt covers about one-third of the tubular conduit 10, switching means 40 are activated to close the inlet gate valve 14 and the outlet valve 20, thereby closing the passage 50 to the valved condensation tank 34 as well as to shut off the pump 76. Furthermore, the three-way valve 42 is operated to close off the salt flushing outlet 44 from the tubular conduit 10.

The continued solar radiation concentrated on the tubular conduit 10 will then effect a rise in pressure and temperature therein, typically to 8 - 10 bar and 180° C. Subsequently, the switching means 40 will operate the three-way valve 42 to effect a connection between the tubular conduit 10 and the salt flushing outlet 44, whereby precipitated salt 46 in the tubular conduit 10 is flushed through the salt flushing outlet 44 together with superheated, supersaturated pressurized liquid.

The increase in pressure and temperature in the closed tubular conduit 10 results in a redissolving of much of the precipitated salt into the superheated liquid to form a supersaturated solution, and the shock waves created by the sudden release of steam at 8 - 10 bar and 180° temperature and the resulting inrush of sea water 16 are sufficient to dislodge and transport the remaining accumulated precipitated salt 46 from the inner surfaces of the tubular conduit 10, this object being achieved without any mechanical parts being introduced therein, and without any need to revolve the tubular conduit 10.

On completion of the flushing action, the duration of which will take only a few seconds, depending on the length of the tubular conduit 10, the pump 76 is restarted, the inlet valve 14 is reopened and the three-way valve 42 is operated so as to close the connection to the salt flushing outlet 44. Sea water 16 will be pumped into the conduit tube 10, the inlet gate valve 14 will be closed and solar heating will cause normal evaporation action to resume.

Referring now to Fig. 3, there is seen a salt storage tank 54 connected to the salt flushing outlet 44. The salt storage tank 54 is provided with a vent 56 to allow the escape of steam and air, a drain 58 to allow the removal of water and some means 60 to allow the collection of produced salt 62. Provision of the salt storage tank 54 is all that is needed to enable the apparatus of the present invention to produce salt as a by-product.

A sensor 64 is associated with and detects the amount of salt in the tubular conduit 10. It is operationally connected to the switching means 40 and will cause the initiation of the previously-described salt removal operation when approximately one-third of the conduit tube 10 is internally covered with precipitated salt 46.

In order to effect an improvement in operational efficiency, some of the heat removed from the valved condensation tank 34 may be utilized for heating the incoming sea water 16 by use of a liquid-liquid heat exchanger 66. A suitable heat-transfer fluid 52 is circulated by a pump 68 in a closed circuit 69 from cooling coils 70 through one set of coils 72 in the heat exchanger 66. The inlet pipe 74 in the heat exchanger 66 is used for passage of the sea water 16, which is thus heated prior to entering the tubular conduit 10 due to the action of the pump 76. The pump 76 is controlled to operate when the level of the sea water 16 inside the tubular conduit 10 falls below a predetermined level.

The pump 76 is arranged to cease operation either when the required sea water level is reached or when the inlet valve 14 is closed.

A pressure sensor 78 in fluid connection with the tubular conduit 10 is provided and is operationally connected to the switching means 40, and during normal operation will cause periodic opening of the outlet valve 20 and the consequent release of steam 30 from the tubular conduit 10 into the valved condensation tank 34. After passage of the steam 30, the outlet valve 20 is again closed by the switching means 40.

A desalinated water tank 80 for storing the produced water 81 is connected to a pump 82, which in turn is connected to the condensation tank 34.

If electric power for the operation of the pumps and control elements is not conveniently available at the site where the apparatus is to be used, generation equipment will be included as part of the apparatus. Photovoltaic panels 84 are connected to charge electric storage batteries 86, which are sized to power all pumps, control equipment and other services, such as lighting. A small diesel engine generator 88 provides back-up power.

There is seen in Fig. 4 a tubular conduit 90 which is oriented at a shallow angle to the horizontal, typically between 5° and 7°. A parabolic solar reflector 92 is sloped at the same angle, so that the sun's rays 26 are concentrated to heat the tubular conduit 90, the latter being almost completely filled with sea water 16. An outlet valve 94 is in fluid communication with the higher extremity of the tubular conduit 90, and is controlled in response to a signal from the pressure sensor 78 to periodically release steam 30 to the passage 50 and to allow fresh sea water 16 to be pumped into the tubular conduit 90 after passing through the check valve 13 and the inlet gate valve 14. A salt flushing outlet 96 is adjacent to the lower extremity of the tubular conduit 90, and is periodically brought into fluid connection thereto by means of a three-way valve 98, thereby initiating the salt flushing action which has been previously explained. It is obvious that the slope of the tubular conduit 90 greatly facilitates the removal therefrom of precipitated salt 46.

Components for steam condensation, pumping, salt collection, desalinated water collection, power supply, and switching means used have been previously described with reference to Fig. 3.

The present invention also provides the following method for desalinating sea water: A. Pumping sea water through a tubular conduit 10 having an inlet valve 14 and an outlet valve 20; B. Focusing the sun's rays 26 by means of a solar reflector 24 onto an exterior surface of a longitudinal segment of the tubular conduit 10 to effect the evaporation of sea water 16 passing therethrough; C. Venting steam 30 generated in the tubular conduit 10 to a valved condensation tank 34; D. Cooling the condensation tank 34 to convert the steam 36 to desalinated water 38, and E. Periodically flushing accumulated salt 46 from the tubular conduit 10 by sequentially closing the outlet valve 20 and the inlet valve 14 to effect a rise in pressure and temperature in the tubular conduit 10 by closing passage to the valved condensation tank 34 and thereafter opening passage to a valved salt flushing outlet 44, whereby accumulated salt 46 in the tubular conduit 10 is flushed through the salt flushing outlet 44 together with superheated, supersaturated pressurized liquid therefrom.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (7)

WHAT IS CLAIMED IS:

1. Apparatus for desalinating sea water by flash evaporation using solar energy, comprising a tubular conduit through which sea water may be pumped and having an inlet valve and an outlet valve and a solar reflector mounted adjacent to said conduit for focusing the sun's rays directly onto an exterior surface of a longitudinal segment of said tubular conduit, characterized in that said conduit is in fluid communication with a valved condensation tank and with a valved salt flushing outlet, and said apparatus is further provided with switching means for sequentially closing said outlet valve and said inlet valve to effect a rise in pressure and in temperature in said tubular conduit, and thereafter for closing passage to said valved condensation tank and opening passage to said valved salt flushing outlet, whereby accumulated salt in said tubular conduit is flushed through said salt flushing outlet together with superheated pressurized liquid therefrom.

2. Apparatus for desalinating sea water by flash evaporation with solar energy as claimed in claim 1, wherein said salt flushing outlet is connected to a vented salt storage tank.

3. Apparatus for desalinating sea water by flash evaporation with solar energy as claimed in claim 1, wherein said solar reflector is a cylindrical parabolic reflector with a concentration ratio in the range of 1:3 to 1:6.

4. Apparatus for desalinating sea water by flash evaporation with solar energy as claimed in claim 1, wherein said switching means is operationally connected to a sensor for detecting the amount of salt deposit in said conduit.

5. Apparatus for desalinating sea water by flash evaporation with solar energy as claimed in claim 1, further provided with a liquid-liquid heat exchanger, wherein part of the heat of the water leaving the cooling coils of said valved condensation tank is transferred to the sea water entering said tubular conduit.

6. Apparatus for desalinating sea water by flash evaporation with solar energy as claimed in claim 1, wherein said tubular conduit and said solar reflector are mounted at a shallow angle to the horizontal, and wherein said salt flushing outlet is adjacent to the lower extremity of said tubular conduit.

7. A method for desalinating sea water, comprising: pumping sea water through a tubular conduit having an inlet valve and an outlet valve; focusing the sun's rays by means of a solar reflector onto an exterior surface of a longitudinal segment of said conduit to effect the evaporation of sea water passing therethrough; - In ¬ venting steam generated in said conduit to a valved condensation tank; cooling said condensation tank to convert said steam to water, and periodically flushing accumulated deposited salt from said conduit by sequentially closing said outlet valve and said inlet valve to effect a rise in pressure and temperature in said conduit by closing passage to said valved condensation tank and thereafter opening passage to a valved salt flushing outlet, whereby accumulated salt in said conduit is flushed through said salt flushing outlet together with superheated pressurized liquid therefrom. For the Applicant: WOLFF, BREGMAN AND GOLLER by: