GB2341855A

GB2341855A - Combined cycle desalination unit

Info

Publication number: GB2341855A
Application number: GB9910189A
Authority: GB
Inventors: Ali Hussein Liban
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-06-10
Filing date: 1999-05-05
Publication date: 2000-03-29
Anticipated expiration: 2019-05-05
Also published as: GB9910189D0; GB9812415D0; GB2341855B

Abstract

A combined cycle desalination unit including an evaporation tank 1, a cooling tank 2, cooling tubes 3 mounted in the space separating the tanks, and external solar panels 14 coupled to a laser unit 16 which together generate a laser beam of suitable wavelength to be absorbed by an energy exchanger 21 mounted inside the evaporation tank. Sea water is taken into the unit via inlet 6 so that the energy exchanger and cooling tubes are immersed. The energy exchanger converts energy from the laser beam into heat and thus intensely heats the water layer in contact with the energy exchanger without the heat reaching the other layers. This layer is evaporated as dry steam and the salt previously contained therein left behind in the water layers below for removal from the unit through discharge line 10. Sea water entering the unit is evaporated by contact with this dry steam for further distillate production. The cooling tubes condense the steam for collection outside the unit as distillate.

Description

COMBINED CYCLE DESALINATION UNIT This invention relates to a combined

cycle desalination unit Desalination plants are a useful method of converting sea water to potable water in countries with arid and semi-arid climates. The said plants consist of sea water intake structures,intake pipes, pump stations, feeder pipes,de-aeratersbrine heaters,evaporator units and discharge channels. Various machinery and equipment are required forthe said conversion and are connected through pipes of different sizes and types to meet the temperature and pressure variations& Rowever,the said plants have a few disadvantages affecting their efficiency. Solar energy incident in the tropical regions of the world is estimated at an annual average of 8 KW MJ112. Existing desalination plants are not adapted to make use of this vast energy source in the said areas where the deziand for water Is often high. Solar energy is too scattered to bring about the levels of evaporation required for desalination purposes. Large amounts of sea water is heated in the said plants and brought to their bo-11-1- point using vacuum systems before evaporation ran take place. Only a fraction of a percent of the sea water is evaporated at a given time in the repeating multi-stage System ut-11-1 ed in most of these plants. As a result the -ximl- efficiency of the said plants varies between 25-SOZ. much energy is lost through the handling of the unused brine. Also the various stages of the process have to be kept separate for control purposes.encouraging pressure and 0 temperature build-up in the system.

-2Kultl--star,e flash evaporation or MSF desalination plants are at present the most commonly used awans for converting sea water to potable water. This. syste:a uses a combination of heat and pressure reduction to cause the sea water to evaporate or flash for distillate production. Sea water is first heated from 90P to 220F in a brine heater increasing its pressure to about 17.2 psi. Then it is fed into the evapotor where the pressure on the surface of the water is reduced using a vacuum system so that the water flashes at lower temperatures instead of its no bollinR point of 212F. The evaporated water is cooled when the resulting steam comes in contact with external. surface of tubes carrying sea water circulated in the system at around 9CF. The resulting condensate is collected in trays for further treatment. The unevaporated brine is recIrculated, for further heat exchange with the incoming sea water.

The said system has a number of disadvantages. It depends on the surface area of the c6o1Ing tftbep for.condensate production which cannot be increased indefinitely without increat ing wall thickness affecting heat exchange. Larger tube diameters reduces the space matrix separating the tube layers and affect efficiency of cooling. As a result only about 10% of the incoming sea water is converted to potable water and the renaIning 90Z mostly rejected as brine and returned to sea often at high temperatures at a considerable energy loss in the system. A typical MPS des-111n-ation plant way consist of 16-30 stages to increase production of potable water. Large amounts of sea water are therefore to unnecessarily handled and this increases the aniount of equipment requ- 1red.

Sea water contains dissolved salts which separate out of solution during heating and evaporation in the desalination process. These salts are deposited on the internal surfaces of components such as evaporators, condensing tubes and brine recirculation tubes. These salts form a scale of nouconduct' material which interferes with the heat exchange performance of these component. Scaling is therefore a major problems in the operation and maintenance of the said plants. Sea water contain dissolved gases such as oxygen and carbon dioxide which are liberated at higher temperatures in the desalination process. Conventional desaliantion plants handle large amounts of sea water at uni ormly high temperatures. This increases the amount of gases and dearators have to be used to remove these gases prior to brine heating. Dissolved gases are a major problem in conventional plants because of tbe- large volume they occupy when heated and the corrosive nat:ure of gases such asi oxygen. Moreover, conventional desaBmation plants depend on a vacuum system which decreases the pressure in the evaporation vessel to cause the water to flash below its normal boiling point of 212F. This is achieved by reducing the pressure on the water surface below the atmospheric value of 14.7 psi in a number of gradual stages. Air ejectors are used for this purpose. Lover sea water temperatures require higher pressure reductions for continuous evaporation and desalinated water production. Air ejectors are volumetric devices and the amount of vacuum generated depends on their internal dimensions.vhich cannot be increases optimally to increase the production of desalinated water. Air ejectors can reduce the said pressures down to 1 psi. Further pressure reduction would impose external atmospheric pressure stresses on the evaporators and other components and therefore cannot be used for Increasing production. This limits the utility of the vacuum system and the development of the said plants.

-5According to the present invention there is provided solar panels to convert solar energy to electricity which is used to operate a laser unit to further generatc-a laser beam of an appropriate wavelength. The said beam is then passed through an energy exchanger consisting of stainless steel filings to absorb the radiation in the said beam and convert it to heat energy which is in turn used for evaporation and desalination purposes. Because of its unique heating properties,the laser beam only heats the energy exchanger and the sea water in Immediate contact with it, evaporating it and simultaneously leaving beh-Ind the salt resulting from the said evaporation in the water layers below. This invention therefore overcomes the above mentioned difficulties by converting the scattered solar energy into a concentrated form and combines the various stages of the desalination processes of sea water intake,evaporation and discharge of brine. Cooling tubes are also provided for condensing the steam.

A specific embodiment of the Invention will now be described by way of example with reference to the attached drawings in which: Fig. 1 Shows a perspective of the proposed combined cycle desalination unit, Fig. 2 shows a section through the said unit showing the evaporation tank, cooling tank,cooling tubes,ler probe,sea water intake and brine discharge lines. Fig. 3 shows a detail of the laser probe consisting of fiber optic cables which change the direction of laser beam. Fig. 4 Shows a detail of the Lmer bean rotation and the method of their passing through the energy excIan er under the water level within the the said evaporation tank.

Referring to the drawings the- combined cycle desalination unit consists of an evaporation tank 1.a cooling tank 2,cooling tubes 3, sea water intake line 6,brine discharge line 10,intake valve 9, discharge valve 13, solar panels 14,Laser unit 16,1aser probe 17, a motor 18,a battery 19jiber optic cables 20 and an energy exchanger 21 Sea water contain dissolved ions of sodium chloride, calcium carbonate, calcium sulphate, magnesium sulphate and calcium bicarbonate in various proportions. The -1- ions are those of Cl- at 23,000 ppa follwded by those of Na + at 13,000 ppa so 4 ++ at 2700 ppa,Hg ++ at 1300 ppaca ++ at 400 PPR,K + at 400 ppa and HCO 3 + at 140 ppm giving a total of 41,040 ppa of dissolved solids. The desalination process consists of different methods of removing these salts so that the processed water can be used for human consumption after further suitable treatment. Evaporation of the sea water is the common method utilized for this purpose. The method of evaporation employed depends greatly an the portion of the heating curve of water used which then governs the type of equipment utilized. Graph 1 represents the said curve with the heat content of water (h) plotted again t it temperature (T). A given mass of water at W has a heat content of 28.0 Btullb at 600F. If the heat content is increased to 180 Btullb then the water temperature rises to 212OF which is the boiling point of water at atmospheric pressure of 14.7 psi. The heat content must be increased to Y before any evaporation can take place. This requires an additional energy of 970 Btullb. This is known as the latent of water needed to overcome the nolecular bonds holding the water structure together. We can see from this that much more energy is required to evaporate a given mass of water than to heat it to its boiling point. The steam absorbs more energy if its heat content is increased and the heating curve shifts towards towards Z at increasing temperature and pressure as super heated steam. Conventional desalination plants such those using the MSF systems utilize the lower end of the said curve as shown in Graph 1.

Large amounts of water are first heated from 90F to 212"F along the line WX. The pressure on the surface of the water is then reduced in a number of descending stages using a vacuum system, so that evaporation can take place at temperatures lower than the normal boiling point of water at 212F. Air ejectors are utilized for generating the required vacuum which depends on the amount of air they cain remove.and hence on their internal volume. Each air ejector therefore has a certain capacity which cannot be adjusted to increase the amount of vacuum for higher production. The alternative in the said plants is to increase the number of pressure reduction stages which increases their size and the equipment used for the desalination process. The increased size also extends the distance separating the heat rejection and the heat recovery sections in the said plants which further contributes to their complexity.

Graph Showing a Portion of the Heating Curve for Water 1200- DRY SATURATED STEAM y S z LINE 1000- :2:2 L U05 CP Q2 800 WET STEAM ZONE r. -0 0 - r_ cil -2 2 U r 0 m 0 E C> 600 400 X W WATER LINE 0 40 80 120 160 200 240 Temperature (T) 'F -9In this invention the evaporation tank 1 and the cooling tank 2 are concentric and the cooling tubes 3 are fitted in the space separating then as shown in Fig.1 and Fig. 2. The said tubes are connected to the said evaporation tank through the vapour inlet 22. The sea water intake llne 6, brine discharge line 10 and their branches are connected to the said tanks as shown in Fig.2. The solar panels 14 and the laser unit 16 are mounted on the evaporation tank. The laser probe 17 consisting of fiber optic cables 20 is -fitted between the said laser unit and the said evaporation tank descending downwards so that the bottom of end of the said cables are submerged under the water surface contained herein and the laser beam 24 aligns with the energy exchanger 21 as shown In Fig. 2 and Fig. 4. The said intake line 6 is provided with an intake valve 9 to control the amount of water coming into the said unit. The discharge line 10 is also provided with a discharge valve 13 ch controls the anKnmt of water going out of the said unit. A battery 19 Is provided to store electric power. Sea water is fed into the said unit through the said intake line. This is divided into a branch 7 supplying the said evaporation tank and another branch 8 supplying the cooling tank as shown in Fig. 2. The said solar panels convert solar energy to electric power to operate the said laser unit so that a laser beam 24 of an appropriate wavelength Is generated to produce the heat reqouired for sea water evaporation. Laser energy is intense and coherent with very high electron densities reaching upto 10 19 photons/sec. It is therefore better suited to heating, evaporation and distillate -10production than the- -- saldimethods utilized in conventioanl desalination plants particularly for the upper portion of Graph 1. Sea water cannot absorb much of the laser energy because of transparency. An energy exchanger consisting of stainless steel f ilings is therefore used as an energy absorber which converts the radiation energy in the laser beam to heat energy. More of this intense heat energy is tran f erred to the water layer in direct contact with the energy exchanger than to the other water towards the bottom of the evaporation tank. The topmost layer of the water is therefore selectively evaporated before enough heat reaches the other layers. Thus a given mass of water at W close to the energy exchanger absorbs the said heat energy and its heat content increased from 28 Btullb to 180 Btullb and its temperature increased from 60F to 212F at atmospheric pressure of 14.7 psi. Further heat absorption increases its heat content to 1150 Btullb at Y where evaporation takes place. It then absorbs wore heat to change into super heated steam along line YS. Sea water in the f ora of a spray enters the evaporation tank and comes in contact with the. dry steam in the upper portion of the said tank. The incoming water is at around 60F and is heated along the line WXYS chabging Imediately -Unto steam because of the intense heat and its nalt content lef t behind in the unevaporated water layers below. A motor 18 rotates the said probe so that the laser beam 24 continuously heats the said energy exchain er. The evaporated water is constantly replaced by another layer as water finds its level inside the evaporation tank after each evapotation. The resulting steam is fed into the cooling tubes via the inlet 22. Condensation takes place inside the cooling tubes externally cooled by the water circulating in the cooling tank.

-.7 - The distillate is then delivered outside then unit via the outlet 23 as shown in Fig. 2.

We can see from the above that the amount of sea water in the proposed unit is contimwasly renewed and the steam generated herein is continuosuly removed by condensation preventing large pressure and temperature build-up. The beat energy from the laser unit is localized because of its inherent heating characteristics. The different parts of the said unit are also open to each other giving it a in-built self- adjusting mechamism ladrin in the said plants.

The invention therefore combines the different stages of sea water I ntalre, evaporation, condensation and discharge of brine in one location for the purpose of simplifying the desalination process,in conventional pliants. Therefore it has the capacity for reducing the amount of equipment.as veU as complexities encountered in constraction. installations,controls.processing, operation and maintenance of the said plants.to overcome the above difficulties.

Claims

CLAIKS.

1. A combined cycle desalination unit including a sea water evaporation tank and a cooling tank externally fitted with a suitable energy source and internally with a suitable energy exchanger and cooling tubes and in which the heat reaching the said energy exchanger is intense enough to selectively evaporate the sea water layer in contact with it before the heat reaches the 6ther layers and in which the disttj-late resulting frow the said ev.3poration is condensed and delivered outside the said unit via the cooling tubes

2. A combined cycle desalinatian unit of claim 1 in which the said evaporation tank and cooling tank are concentric with the cooling tubes and fitted in the space separating them.

3. A combined cycle desalination unit of claim 1 in which the mean are provided to transmit the said energy to the said energy exthahger via a suitable conduit of a suitable shape and thus change the direction of the said energy so that it passes through the horizontally oriented energy exchanger.

4. A combine cycle desalination unit of claim 1 in which the mean are provided to rotate the said conduit and hence the said energy within the said evaporation tank and through the said energy exchanger.

5. A combined cycle desalination unit of claim 1 in which the inten ity of the said energy is suitably high enough to ef f ect immediate evaporation of the sea water layer in contact with said energy exchanger during the said rotation so that the salt resulting from the said evaporation is Instantly left behind in the unevaporated water for discharge outside the unit.

CLAIKS (CONTINUED)

6. A combined cycle desalination unit of claim 1 in which the mean are provided to control the amount of sea water coming into and the amount discharging out of the said unit to suit the steam generation,cooling and desalinated water production required.

7. A combined cycle desalination unit of claim 1 in which the said energy exchanger is located just below the water level within the said evaporation tank to facilitate steam generation.

8. A combined cycle desalination unit of claim 1 in which the said energy exchanger is made of a suitable energy absorbing material.

9. A combined cycle desalination unit of claim 1 in which the said cooling tubes are connected to the said evaporation tank and are externally cooled by the sea water circulating within the cooling tank so that the steam passing inside them is condensed and delivered outside the unit as distillate.

10. A combined cycle desalination unit substantially as herein described and illustrated in the accompanying drawIngs.

11. A combined cycle desal-In-ation unit as clal- d in any proceeding claim which is made of a suitable metal