IL31373A - Desalination system - Google Patents

Description

DESALINATION SYSTEM P.A. 31373 35 The Invention relates to a desalination system using a triple sea water transformation point wherein sea water is introduced into a cold vacuum region under conditions which cause part of the sea"1 water to evaporate, the heat withdrawn for evaporation freezing a portion of the sea water into ice crystals, while the remainder of the sea water is concentrated into brine. The vapors subsequently are compressed and condensed into fresh water and the ice crystals are melted into fresh water. The fresh water is withdrawn from the system.

It has been proposed heretofore to desalinate sea water by a vacuum freeze-vapor compression process.

In this prior process the incoming sea water is cooled by fresh water and brine heat exchangers, being forced through the heat exchangers by pumps. The cooled sea water then is introduced into a pool at the bottom of a first low pressure vessel. Above this pool a low pressure is maintained such that a triple sea water transformation point prevails, whereby vapor rises from the pool, ice crystals are formed in the pool and the sea wate in the pool is concentrated into brine.

The vapor rises in this first vessel to a compressor which increases the pressure of the vapor, although still keeping it at a subatmospheric pressure. The compressed vapor is cooled by an ammonia heat exchanger which causes come of the vapor to condense into fresh water. The balance of the vapor is lead to a second low pressure vessel. Brine and ice crystals from the pool in the first vessel are transferred to the second vessel where P.A. 31373/11 compreesed vapor so as to condense the same and melt the crystalβ to fresh water.

The Installation and maintenance of two such large vessels is costly. Furthermore, the use of pumps to force the incoming sea water through heat exchangers on its way to the evaporation region is likewise costly. Moreover, the use of ammonia to cool the compressed low pressure vapor represents a considerable expense. Still further, the ice crystals are not sufficiently cleansed of brine ;before melting.

The present Invention provides a system which overcomes the difficulties of the prior art by carrying out all of the lo pressure operations in a single large vessel rather than a pair of vessels,, by eliminating the use of pumps for feeding the sea water to the flash evaporation point by using a high level storage tank* by cooling the compressed vapor with an ice crystal heat exchanger rather than an ammonia heat exchanger, and by using a better device for cleansing the ice crystals* In general · the apparatus of m present invention passes sea water through a filter into a de-aerator* Then the sea water is raised, as by pumping, to a storage tank* Preferably* the water level in the storage tank is higher than the level at which the sea water is Introduced into the flash evaporation zone* so that the stored sea water will flow by gravity without the use of pumps* Moreover* the level of the sea water in the storage tank is maintained at a predetermined point in order to assist In controlling the rate of flow of sea water to th flash evaporation zone. , The height of the storage tank also is related to the degree of vacuum maintained in the flash evaporation zone (about 3.3mm of mercury) so as to obtain a desirable rate of flow of the sea water from the storage tank to the flash evaporation zone through various heat exchangers on the way to said zone* The first heat exchanger through which the sea water passes after it leaves the storage tank is a fresh water heat exchanger.

The coolin medium is cold roduct fresh water cooling medium here being the deeply cooled brine which leaves the flash evaporation zone and is pumped out of the low pressure vessel. The sea water* when it leaves the brine heat exchanger, it at a temperature of approximately -1.0°C.

Thereafter, the cooled sea water flows through a mechanical refrigerator, for example, a refrigerator of the compression-expansion type.

This refrigerato supercools the sea water to below its freezing point in order to create small nuclei of ice crystals. The supercooled sea water is introduced into a vacuum chamber in such a fashion that flash evaporation is encouraged as distinguished from merely flowing into a pool at the bottom of a vessel, as heretofore has bean the case. For this purpose, the water is introduced in the form of thin films or drops, e.g., jets* It is not neoessary to pump the cold sea water into the vacuum chamber because a high speed of inflow is created by the very low pressure within said chamber.

The chamber is maintained at the triple sea water transformation vacuum of about 3.3mm of As a esult of the mercury. -¾7he heat ofc evaporation of the are formed vaporizing sea water ·£»«>·.ice crystals /and concentrates the remaining sea water into brine. The rising vapor is at a temperature of . about__-1.5°C and the brine is about -2,9°C. The lceycrystaie^ and brine fall to the bottom of the low pressure vessel where they form a pool of mixed ice crystals and brine. Approximately one pound of vapor is crystals formed. The proportion of ice crystals to brine is about JfO# , to 60# by weight. The ice crystals and brine are constantly agitated in the pool* The ice crystals and brine are pumped out o the vacuum chambe to a covered vessel at atmospheric pressure where the ice crystals rise to the top of the brine, are mechanically removed, and are blasted with compressed cold air and cold product fresh water to remove the film of brine adhering to the ice crystals.

The cleansed ice cjjetala are used in a manner soon to be described* The water vapor rising from the flash evaporation zone Is cooled to a temperature slightly lower than -i.9°C by passage through a heat exchanger in the low pressure vessel. The cooling medium o this heat exchanger is the brine pumped out of the low pressure vessel which, as will be recalled, ie at a temperature of about -2.9°C. If desired, a mechanical refrigerator may be employed to lower th Moreover! 1 prevent the tips of the radial compressor blades from flying off by employing a reinforcing ring which circumscribes said tips and holds them in fixed position. Furthermore, increasing the inertial momen of the fail by providing a ring at the tip of the blades enables the fan to be turned at higher speedsi e vapo which now has been compressed to about 6mm of mercury, and which has had its temperature thereby correspondingly slightly elevated, is passed through a heat exchanger which, unlike the heat exchanger of the prior art that used ammonia as the cooling medium, employs ice crystals with a product fresh water lubricant as the cooling medium.' It will be observed that these ice crystals are held out of direct contact with the Compressed water vapor by the mechanical structure of the heat exchanger. At the given. ressure, to wit,' about 6mm of mercury, and th β temperature of the ice crystals,, the compressed water vapor will condense to form product fresh water and where it is joined to the product fresh water formed by melting the ice. crystals in the manner just described* Fig. 1 is a diagrammatic view of an apparatus fox* carrying out my new system; Pig. 2 is an enlarged vertical central croee-sectional view of a portion of the apparatus shown in Fig. 1 , the same constituting the top of the single large lox* pressure Vessel and the fan-type compressor; Fig.3 Is a sectional view taken substantially along the line 3—3 of Fig. 2j and Fig, and 5 are enlarged sectional views taken substantially along the lines k and 5«— 5 » respectively, of Fig. 3.

Referring now in detail to the drawings , the reference numeral 10 denotes a plant embodying my invention and for carrying out the method thereof. For the purpose of brovity, I will describe the apparatus and process simultaneously.

Salt water is drawn from the sea by an intake pump (not shown) and passes through a filter (not shown ) and, if desired, through an ion exchanger.

The sea water now enteree a pfcpe 12 that leads to a de-aerator 1 havin an air removal pump 16. A pump 18 draws sea water from the bottom of the de- aerator and feeds it through a pipe 20 to an elevated storage tank 22. This storage tank is high enough to cause the sea water to flow from the tank through varioud heat exchangers to the vacuum chamber v/1 thou t the use Of pumps. The storage tank is vented to the Suitable 31373-2 ^ means may constitute a thin film of oil or any other water immiscible non-toxic fluid floating on the upper surface o the water in the tank. As shown, said means constitutes a thin rubber membrane Zh having its periphery sealed to the tank. The membrane is sufficiently elastic to permit the volume of water stored in the tank to vary considerably.

A float 26 resting on the membrane controls a movable contact 28 arranged to cooperate with a stationary contact. 30, Lead wires 32 run from the contacts to a motor control 3^ fo** the motor of the pump 18, the control being such that when the contacts engage, the pump 18 is idled. Thereby* a substantially constant level of water i the storage tank will be maintained.

Sea water leaves the storage tank through a pipe 36. The pipe 36 runs to a product cold water external heat exchange 38 of any simple type, herein shown as constituting a chamber subdivided into various subchambers by baffle partitions. A cooling medium flows through the su chambers, said cooling medium constituting the product cold water about to leave the plant, The sea water is passed through the sub- chambers by means of convoluted pipes O, After leaving the product cold water external heat exchanger, the cooled sea water passes through a pipe k2 which leads to a second external heat exchanger kk wherein the cooling medium is cold brine about to leave the plant 10. The heat exchanger kk likewise is preferably of a simple type and may be of the same - -of the product fresh water* so that it may be prevented ( from becoming contaminated.

The sea water, when it leaves the brine heat exchanger, is at a temperature of approximately - 1°C„ A pipe k6 leads the deeply cooled sea water from the brine heat exchanger to a mechanical refrigeration cooling means k8 which, as shown, is of the compression expansion type* This refrigeration cooling means will reduce the temperature of the sea water to slightly below the freezing temperature thereof, i.e., supercools it by a small amount, whereby to induce the formation of small nuclei of ice crystals so as subsequently to encourage speedy ice crystal formation.

A valve 50 in the pipe k6 controls the rate of flow of the sea water on its way from the storage tank to the flash evaporation zone. Said valve is regulated by a level gauge 52 which is responsive to the level of the pool of concentrated brine and ice aystals at the bottom of the low pressure vessel.

When the level rises beyond a predetermined point, the valve 50 is partially closed to reduce the rate of intBl'ow.

From the mechanical refrigerator k8 the supercooled sea water is led by a pipe ¾ to a low pressure vessel 56 and specifically to a vacuum chamber 58 within said veeeel. The pressure within the vacuum chamber is maintained at about 3· 3mm of mercury absolute in a suitable manner. As shown, this is accomplished by an air removal vacuum pump 60 the intake of which is connected to another chamber, soon to be described, within the low pressure vessel 56· The supercooled sea water is introduced into the vacuum chamber 58 in such a manner as to assume a form presenting a very hy dropleta· One such arrangement which I have shown constitutes a helical manifold 62 connected to the pipe 5¾ and winding about the exterior of the low pressure vessel 56, Leading inwardly from the manifold through the walls of the low. pressure vessel are a, series, for example, four, of horizontal nozzles disposed about 90° apart, The nozzles may be constructed, as by utilization of a central, diversionary plug, to induce the formation of a mist as the sea water squirts into the. vacuum chamber, or, as illustrated,! the nozzles may be terminated inwardly of the vacuum chamber in horizontally disposed, thin oblong openings, so that the sea water leaving the nozzles assumes the form of thin horizontal films. These films cross one another Centrally of the vacuum chamber. In any ^vent, the sea water, as it enters the vacuum chamber. Is broken up into tiny drops, or other physical forms, of dimensions having a large surface area to overall mass ratio which encourages speedy flash . evaporation. .. - It is pointed out that the sea water is able to flow from the storage tank to the nozslee under a gravity head without the aid of pumps. However, the sea water is drawn rapidly into the vacuum pump so as to form the thin films by the suction action of the vacuum in the chamber 58 and also by the evacuating action of a brine-ice crystal pump soon to be des transformation point conditions which; are by themselves well known. In other words, the temperature and the pressure at which the evaporation takes place are suo!ti that vapor, sea water and ice crystals can exist side by side and under these conditions some of the sea water will turn into fresh water vapor and the heat required to thus vaporize the water is drawn from the balance of the sea water to in turn cause the formation of ice crystals and concentration of the remainder of the sea water into brine* It has been found thai about one pound of vapor is produced for each seven and one-half pounds of ice crystals that are formed.

The water vaporizes in the vacuum chamber and its processing will be described at a later point.

The ice crystals and the residua of brine vi ich has become concentrated due to the removal of some fresh water as vapor and some fresh water as ice crystals drops down along with the ice crystals into a pool 6¾ at the bottom of the low pressure vessel 56. The level of this pool is sensed by the level gauge 52 which, as noted aforesaid, regulates the valve 50, To prevent the ice crystals from caking into a asass and to increase the efficiency of formation of the ice crystals, I provide an agitating means such, for instance, as a propeller 66, near the bottom of the pool driven through a suitable gear train by a motor 68. by th© level gauge 52 , the speed of the pump being increased when the pool rises above a predetermined level and decreasing when the pool falls below a predetermined level.

The mixture of ice crystals and brine leaving the pump 72 is discharged through a pipe 7^ into a separating chamber 76, The bottom of the separating chamber is defined by a foratninous plate 78* thereby ensuring that the ice crystals remain in the upper part of the chamber where they will tend in any event to rise due to their lonror density. The brine in the chamber 76 descends into an accumulation chamber 80 through which it is withdrawn by a pump 82 controlled by a level gauge 83 which senses the level of the brine and ice in the separating chamber 76» The gauge do-energises the pump 82 when the level o the brino and ice approaches the top of the chamber 76, The circulation of the exiting brine will be discussed later. However, it may be pointed out hero that its temperature is about -2.9°C . One side of the chamber 76 , 80 is defined by a partition Qh, Said partition provides a boundary between the chambers 76 , 80 and an ice storage chamber 86.

Ice is removed from the brine in the separating compartment 76 in any suitable fashion. Th© equipment employed for this purpose can be of simple design, inasmuch as, unlike the prior art, the ice is separated not within a low pressure tank but at ambient atmospheric pressure, and access to the ice -¾ diameter and is located well within the separating chamber 76. The drum 92, which is the discharge drum, is located above the ice storage chamber 86 and above and adjacent the top edge of the partition 8k, Either one of the drums is motor driven in such a direction that the top flight of the screen belt moves from right to left, as shown in Pig.1.

The screen is provided with a plurality of scoops 9k so arranged that when the drums and screen belt are moved in the direction indicated by the arrow A, the scoops will pick; up ice as they move upwardly on the right-hand side of the drum 90.

At the point of discharge the ice is clear of the separating chamber 76 and the scoops are inverted, i.e., turned upside down, thereby discharging ice into the ice storage chamber 86 on the left-hand side of the partition 8k, The partition k defines the aone of domarka ion between the brine which is on the right of the partition and the prpduct fresh water wliic is on the loft of the partition, most of the water at this time atill being in the form of ice crystals.

The ice, as it is picked up from the sepax'titing chamber 76, still has a residual surface film or coating of brine thereon Which would contaminate the sweet ess of the product fresh vrater leaving the plant 10. To reduce this salinity, I provide a highly efficient defilmizing arrangement in the form of a mixed air and product sweet water discharge blast directed downwardly Anto the ice carried up on the screen belt 86. Specifically, Z provide a nozzle or manifold to which air surrounding "atmosphere, to a heat exchanger 100 located In a product fresh vater accumulator 102 next to the ice storage chamber 86. From the heat exchanger 100 cold compressed ai is led to the nozzles °6. In addition, product cold fresh water is led from the accumulator 102 to the manifold 6 by a pipe 10b. The nozzle, i.e., manifold, is elongated and disposed trassversely across the screen belt 88 with the discharge o the nozzles directed downwardly onto the screen.

The nozzle is of the Venturi type so that the air leaving it will aspirate fresh water from the accumulator 102 , the nozzle being so designed that under the pressures prevailing the ratio of compressed ai to water is about 3 to 1 parts by weight. Thus, a mixture of cold, compressed air and water will be discharged from the nozzle 96 onto the ice leaving the separating chamber. This •lischarge will eubstantlolly clean the surface of the ice crystals with minimum melting and mi imum waste of product sweet water. The cleaning water will fall down into the separating chamber 76 where it will slightl dilute the exiting brine.

This arrangement for cleaning the brine results in a particularly low salt contamination of the product fresh water to a point where the salt is only about 250 ppm of the product fresh water.

A valve controlled pipe 106 is located near the bottom of the partition between the chambers 86 , 102 and is set to permit a trickle of water to pass from A pipe 108 leads from the bottom of the melting chamber to a pump 110 which forces the slurry of ice and product fresh water through a pipe 112 that leads to a heat exchanger 11 Ipcated within the low pressure vessel 56. In the heat exchanger 11 the ice crystals are in large part millted for a purpose later to be described, and then are returned through a pipe 116 to the product fresh water accumulator 102,, Product fresh water leaves the accumulator 102 through a pipe 118 which leads to the product fresh water external heat exchanger 38. Thereafter! the product fresh water leaves the plant through a valve controlled pipe 120· In asmuch as the accumulator 102 is at a higher level than the heat exchanger 38, the outflow of the product fresh water is under gravity. The top of the heat exchanger 38 is closed to prevent leakage of the product fresh water. A level gauge 121 senses the level of the product fresh water in the accumulator 102 and regulates the valve 120 to maintain said level at a predetermined height shortly below the top of the partition 8h, The vapor rising from the triple sea water transformation zone in the vacuum chamber 58 is at a temperature of about -1.9°C. This vapor must be condensed into fresh water and recovered in order to maintain plant efficiency. The first step in condensation of the vapor is to further chill the same. This is accomplished by drawing the vapor through a heat exchanger 122 directly above the heat exchanger 122 conveniently is in the form of a bank of pipes through which a cooling medium is circulated. The cooling medium le the cold brine which hae left the pool 6k and has had the ice crystals removed therefrom. The cooling medium thus Is fed from t e pump 82 through a pipe 12 which leads it to the, aforesaid heat exchanger 122 , thereby lowering the temperature of; the vapor, but not eufficient to condense the same*' Optionally, the brine flowing through the pipe 12k is additionally cooled by a mechanical refrigeration unit 125 to further lower the temperature of the vapor exiting from the heat exchange 122; however, the additional cooling is n t enough to condenee the vapor at this point* The brine leaving the heat exchanger 122 flows through a pipe 126 to the -external brine heat exchanger kk where it cools the Incoming sea water. Thereafter, the brine is discharged through a pipe 128. The discharged brine may be led back to the · sea.

To aid in the condensation of the water vapor to the product fresh water it is necessary to compress the vapor cooled by the heat exchanger 122. This operation is performed by a compressor 131 disposed above the vacuum chamber 58 and the heat exchanger 122.. , The compressor 131 is of a large radial fan type$ hav ng a central intake, radial flow, and peripheral discharge. The space into which the fan exhausts is evacuated of air by the air removal pump 60 and the pressure of the i maintain the pressure of the region immediately above the pool* i.e. I the flash evaporation region, at an absolute pressure of about 3.3 mm of mercury.

The compressor is of a very powerful type inasmuch as it must handle a large volume of vapor and raise the pressure thereof from about 3.3 mm of mercury absolute to about 6 mm of mercury absolute. Said compressor constitutes a shaft 132 which is vertically oriented and is located at the top and centrally of the low pressure vessel 56. The shaft is driven by a motor 13^· The shaft is rotated at a very high speed, e.g., 30,000 rpm to 35*000 rpm, such speed being necessary to handle the amount of vapor which is present and to achieve the pressure differential jUst described. The sBaft is journalled in rotary and thrust bearings 136 which are enclosed in a housing 138 atop the pressure vessel. The bearings 36 are cooled by a heat exchanger 1¾0 that encircles the shaft 132 and is located close to the bearings. Any cooling medium can be used in the heat exchanger 1 0, for example, sea water or product cold water, or, if desired, mechanical refrigeration. Such cooling of the bearings is highly desirable since it permits the high speed of rotation of the compressor.

The shaft 132 mounts a set of radial compressor blades 14·2. The blades 1¾2 turn in a compressor casing which is formed by a lower horizontal flat annulus tkk and a horizontal top plate 1¾6. The peripheral space between the annulus and the blade constitutes the exhaust of the compressor and the opening in the annulus constitutes the intake of the ¾ exchanger 122 to the compressor 131.

A flat horizontal ring 1¾ ties together the tips of the radial compressor blades 142. Said ring stiffene the assembly of the bladesf exerts a compressive force at high speeds tending to reduce the bursting stress exerted on the tips of the blades at high speeds of rotation, and increases the moment of inertia of the compressor blade assembly, thus enabling the compressor to be rotated at higher speeds<> Attention is directed to the fact that the inner edge of the ring is tapered to increase efficiency of water vapor flow outwardly over the ring. It also will be observed that the blades 1^2 are cocked, so that their lower edges lead their upper edges, thereby raising the suction input efficiency of the compressor.

The water vapor which has now been raised to a pressure of about 6mm. of mercury absolute is discharged into an annular condensing chamber 15 located within the low pressure vessel 56 and surrounding the chimney l48. The water vapor has been slightly heated by the compression, b t its temperature is still quite low since it was chilled by the heat exchanger 122 prior to its compression. The annular condensing chamber 150 contains the heat exchanger 114 which lowers the temperature of the water vapor discharged from the compressor to a point where condensation will take place It will be recalled that the cooling medium for the heat exchanger 114 is the slurry of ice crystals and product fresh water withdrawn from the melting chamber 86. The condensed water vapor formed in the chamber fresh water Is withdrawn from said accumulator by a pump 15^ whioh feeds the water through a pipe 156 to the primary product fresh water accumulator 102.

All cold portions of the plant are covered with insulation to minimize heat losees* It will be understood, of course, that all of the components of the plant, such, for Instance, as pipes, the large low pressure vessel, pumps, compressors, etc* are constructed so as to be readily disassembled for cleaning purposes and that many valves are employed, which are not shown, in order to out o_^ff flow when the plant is idled and is partially disassembled for cleaning* If thus will be seen that I have provided a system which achieves the various objects of my invention and which is well adapted to meet the conditions of practical use* As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth;, it is to be understood that all matter herein described or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense*

Claims (9)

3/3 WHAT IS CLAIMED ISl

1. A plant for the desalination of sea vrater comprising! a single low pressure vessel, a vacuum chamber within said vessel, means conveying sea water into said vacuum chamber, heat exchangers cooling the sea water on its way to the vacuum chamber, means maintaining a vacuum of about 3· 3· mm. Hg in said vacuum chamber,,, said means including a large volume compressor within said low pressure vessel, the sea .water being converted into vapor, ice crystals and brine at a triple sea water transformation point in said vacuum chamber, means withdrawing the brine and ice crystals from the vacuum chamber to atmospheric pressure, means separatin the ice crystals from the brine, means cleaning the ice crystals of residual brine, means melting the ice crystals into product fresh water, said compressor compressing said vapor to a higher subatmo spheric pressure and directing the compressed vapor to a condensing chamber within the low pressure vessel,, heat exchanger means in the condensing chamber condensing the compressed vapor into product fresh water, said heat exchanger means including a cooling medium maintained separate from the compressed vapor, means removing air from the condensing chamber, and means withdrawing the product fresh water from the condensing chamber to atmospheric pressure.

2. A plant as set forth in Claim 1 wherein the ice crystals are employed as the cooling medium in the heat exchanger that condenses the compressed water vapor into product fresh water. P.A. 31373/11 o

3. A plant as set forth in Claim 1 wherein a heat exchanger is employed to cool the water vapor before the same is compressed*

4. ¾,· A plant as set forth in Claim 3svh©reln the cooling medium for the heat exchanger that cools the vapor before the same is compressed is the brine withdrawn from the vacuum chamber.

5. · A plant as set forth in Claim 1 including refrigerator means for supercooling the sea water before it is Introduced into the vacuum chamber.

6. A plant as set forth in Claim 1 which further includes a sea water storage tank, said tank being at a level higher than the level at which sea \*ater is conveyed into said vacuum chamber, whereby sea wate can flow by gravity along from the storage tank, to the vacuum chamber* there being no pumps in the path of flow of the sea water from the storage tank to the vacuum chamber, and further wherein heat exchangers are provided in said path of flow for reducing the temperature of the sea water.

7. * A plant as set forth in Claim 6 wherein a valve is included in said path of flow so as to aid in maintaining a predetermined level of brino and ice crystals in the vacuum chamber.

8. A plant as set forth in Claim 6 wherein means is included to maintain the sea water in the storage tanlfe at a substantially constant level.

9. * I a plant for the desalination of sea water in which at a certai point of operatio ice crystals float in a bath of brine and in which the ice crystals