WO1993002964A1

WO1993002964A1 - Desalination plant, especially for use in marine and off-shore installations

Info

Publication number: WO1993002964A1
Application number: PCT/DK1991/000223
Authority: WO
Inventors: Bo Juul Andersen; Einar Wassini
Original assignee: Alfa-Laval Desalt A/S
Priority date: 1991-08-09
Filing date: 1991-08-09
Publication date: 1993-02-18
Also published as: JPH07500526A; KR0169155B1; ES2087819B1; ES2087819A1; PL169577B1; EP0600876A1

Abstract

In a desalination plant (1) comprising an evaporator (2), a separator (3) for separating droplets of brine from the mixture of steam and such droplets produced by the boiling process in the evaporator (2), and a condenser (4) for condensing the steam into freshwater leaving the plant at an outlet (18), the novel feature is that the separator (3) is of a type with 'distributed effect', e.g. consisting of a number of cells bounded by zig-zag baffles (24), each such cell performing a separating function. With this arrangement, the flow cross-sectional area and hence the horizontal space requirement of the separator (3) does not have to exceed that of the evaporator (2), in contrast to Prior-Art plants, in which the separator comprised a large inverted dish requiring much space to operate, as the centrifugal separating effect solely took place at its periphery.

Description

Desalination plant, especially for use in marine and off-shore installations.

TECHNICAL FIELD. The present invention relates to a desalination plant of the kind set forth in the preamble of claim 1.

BACKGROUND ART.

In previously known desalination plants, the separator comprises a large inverted dish, forcing the flow of the mixture of steam and brine droplets from the evaporator to be diverted towards and around its peripheral edge, where a relatively sudden change of direction produces a centrifugal effect, thus separating the brine droplets from the steam. Since the centrifugal effect only takes place at the edge of the inverted dish separator, the dish has to be relatively large with a diameter of the order of 1.2 times the diameter of the outlet part of the evaporator. This means, of course, partly that the housing has to be large in order to acco- modate the inverted dish separator, partly that the central region of the separator constitutes a "dead space", in which no effective separation takes place.

DISCLOSURE OF THE INVENTION. It is the object of the present invention to provide a desalination plant of the type referred to initially, in which the above-mentioned "dead space" is eliminated, and in which the space requirement of the separator is conside¬ rably reduced, and this object is achieved with a desalina- tion plant, according to the present invention also exhibi¬ ting the features set forth in the characterizing clause of claim 1.

With this arrangement, separation takes place over substantially the full flow cross-sectional area of the separator, for which reason the latter need not be larger than the outlet portion of the evaporator. Advantageous embodiments of the desalination plant according to the invention, the effects of which will be evident from the following detailed portion of the present specification, are set forth in claims 2-5.

BRIEF DESCRIPTION OF THE DRAWINGS.

In the following specification, the present invention will be explained in more detail with reference to the drawings, in which Figure 1 is a partly diagrammatic representation of an exemplary embodiment of a desalination plant according to the present invention, seen in vertical cross-section,

Figure 2 is a perspective view of a fragment of the separator used in the plant shown in Figur 1, and Figure 3 shows a practical exemplary embodiment of the plant shown in Figure 1, seen in side view and with cer¬ tain parts removed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS. The desalination plant 1 shown diagrammatically in

Figure l comprises three main components known in principle from previously known desalination plants, viz.

- an evaporator 2, adapted to heat seawater and keep it boiling so as to produce a mixture of steam (i.e. water vapour) and droplets containing sea salts in a considerably higher concentration than in the original seawater,

- a separator 3, adapted to separate droplets and steam coming from the evaporator 2 in such a manner that the droplets fall down and are collected as a small stream of brine, whereas the steam is allowed to continue upwards in the plant, and

- a condenser 4, adapted to cool the steam coming from the separator 3 so as to make this steam condense and form pure water. Seawater is delivered to the plant at an increased pressure from a pump (not shown) through a seawater inlet 5, from which the relatively cool seawater flows into the tubes 6 of the condenser 4, thus providing the requisite cooling for the condensation of the steam being produced by the evaporator 2. During this process of heat exchange in the condenser 4 the seawater will necessarily be heated somewhat, thus leaving the condenser through a cooling-water outlet 7.

From the cooling-water outlet 7, the seawater, now in a preheated state, flows through a pipe 8, the lower end of which has a comparatively narrow branch in the form of a feed-water inlet tube 9, the main flow through the pipe 8 leading to the drive-nozzle in an ejector 10.

The portion of the seawater flowing through the feed- water inlet tube 9 enters the lower part of the evaporator 2 and flows upwardly through the evaporator tubes 13, the latter being heated by hot water, in the present case con¬ stituting cooling water from a diesel engine, such as the ship's propulsion engine, and flowing in through a jacket- water inlet 11 and out through a jacket-water outlet 12. The supply of seawater through the feed-water inlet tube 9, the supply of hot water through the jacket-water inlet 11, and other operating conditions of the evaporator 2 are adjusted in a manner to provide so-called "rising- film evaporation" in order to provide optimum operating conditions and avoid or minimize the formation of scale.

The means for obtaining this adjustment may comprise a re¬ stricted orifice 14 placed within the feed-water inlet tube 9.

As mentioned before, the steam produced in the eva- porator 2 continues upwardly to the separator 3, carrying with it a great number of the droplets formed during the process of boiling the water in the evaporator 2. Droplets in this manner reaching the separator 3 will be caught in same and agglomerated into larger drops or small streams, returning under the force of gravity towards the evaporator 2, on top of which they are intercepted by a brine-collecting pan 15 and made to flow through a brine-outlet tube 16.

The pure water formed in the condenser 4 by the con- ^■ densation of the steam formed in the evaporator 2 is col¬ lected by a condensate-collecting pan 17, from which it leaves the plant through a freshwater outlet 18. For reasons to become evident, this outlet has to deliver the freshwater through some means, such as a pump, preventing entry of air into the space inside the condenser 4.

During normal operation, the pressure in the inter- communicating spaces in the evaporator 2 , the separator 3 and the condenser 4 is kept at a level considerably below normal atmospheric pressure. The purpose of this is to lower the boiling point of the sea water in the evaporator 2 to a point, where it can be made to boil easily by supplying so- called low-grade heat, in the present example heat from the cooling jacket of a diesel engine. It is the purpose of the ejector 10 to establish and maintain this lowered pressure. To this end, the ejector 10 comprises two suction inlets, viz. a liquid-suction inlet 19 and a gas-suction inlet 20. The drive nozzle 21 of the ejector 10 is driven by the main stream of preheated sea water flowing through the pipe 8 from the cooling-water outlet 7 in the condenser 4.

In operation then, the seawater being pumped in through the sea water inlet 5 and flowing through the con- denser 4, in which it is preheated while providing the re¬ quisite cooling effect for the condensation, will - except for the small proportion branched off into the feed-water inlet tube 9 - drive the ejector 10, the latter aspirating brine from the brine-outlet tube 16 through the liquid-suc- tion inlet 19, at the same aspirating air and other uncon- densable gases liberated in the evaporator 2 from the con¬ denser 4 through a gas-suction tube 22 indicated in broken lines.

In this manner, the ejector 10 provides three func- tions at the same time, viz.

- the removal of brine resulting from the operation of the evaporator 2,

- maintaining the desired low pressure to keep the boiling point down, and

- the removal of uncondensable gases that would otherwise collect in the condenser 4 and prevent it from operating.

As will be evident from Figure 1, there is only one seawater inlet, viz. the inlet 5, and one seawater outlet, viz. the seawater outlet 23 from the ejector 10 (also car¬ rying with it brine and uncondensable gases) . This feature makes the plant shown in Figure 1 considerably easier to install than plants having a greater number of seawater inlets and/or outlets.

From what has been stated above with regard to the pressure in the interconnected spaces in the evaporator 2, the separator 3 and the condenser 4, it will now be under¬ stood why the freshwater outlet 18 requires some form of means to prevent outside air from entering this space. Such means may as mentioned before be constituted by a suitable pump (not shown) . As already indicated in Figure 1, the separator 3 is effective over substantially the whole of its flow cross- sectional area in separating the droplets of brine from the steam in the mixture produced by the boiling process in the evaporator 2. This means, of course, that the flow cross- sectional area of the separator 3 can be of the same order of magnitude as, preferably equal to the flow cross-sectional area of at least the adjacent part of the evaporator 2.

Figure 2 shows a fragment of an example of a separator 3 capable of such a "distributed effect". To this end, the separator 3 consists of a number of cells arranged in rows, the major part of the walls of each cell being constituted by zig-zag-baffles 24, each having a number of protruding relief chevrons 25 producing such flow conditions in each cell, that the droplets of brine are deposited on the baffles 24 and trickle downwards to the lowermost, input side of the separator 3, from where they fall down onto the brine-col- lecting pan 15 on top of the evaporator. The fragment shown in Figure 2 is in fact a fragment of a mist eliminator type T271 from Gesellschaft fur Verfahrenstechnik Dinslaken mbH & Co. KG, DW-4220 Dinslaken, Germany, said mist eliminator having proved suitable for the present purpose. It will, however, be obvious that any type of separator being effec¬ tive over substantially the full flow cross-sectional area may be used, provided, of course, that its operating para¬ meters are otherwise suitable for this purpose. Figure 3 shows a practical embodiment of a desalina¬ tion plant of the type shown diagrammatically in Figure 4, but does not show the ejector 10 and the external intercon¬ nections between the main components of the plant.

Thus, Figure 3 shows the evaporator 2 with its dished bottom wall 26 resting, preferably secured to, a base 27 adapted to be mounted on a floor or a deck. On top of the evaporator 2 there is the separator 3 with its zig-zag baff¬ les 24, and on top of this again is the condenser 4, the latter extending crosswise beyond the width of the cylin- drical housing constituted by the outer walls of the com¬ ponents 2, 3 and 4, the protruding portions being housed in protruding housing parts 28. The top of the condenser 4 is closed by a dished top wall 29, so that the whole plant ressembles a vertical boiler. The active part of the separator 3, i.e. the part comprising the zig-zag baffles 24, is releasably secured inside the outer wall of the separator 3 by means of an inwardly protruding peripheral rim 30 formed in said outer wall and a bezel ring 31 releasably inserted into a peri- pheral groove 32 also formed in said outer wall.

When for some reason or other it is necessary to remove the active part of the separator 3, this may be done by dividing the housing by dismantling a flange joint 33, thus gaining access to the bezel ring 31 and the active part of the separator 3 constituted by the zig-zag baffles 24. Parts shown in Figure 3 having reference numbers not mentioned in the above description of Figure 3 have already been described in connection with Figure 1, to which refe¬ rence is made.

List of Parts

1. desalination plant

2. evaporator

3. separator 4. condenser

5_. seawater inlet

6. condenser tube

7. cooling-water outlet

8. pipe 9. feed-water inlet tube

10. ej ector

11. jacket-water inlet

12. jacket -water outlet

13. evaporator tube 14. restricted orifice

15. brine-collecting pan

16. brine-outlet tube

17. condensate-collecting pan

18. freshwater outlet 19. liquid-suction inlet

20. gas-suction inlet

21. drive nozzle

22. gas-suction tube

23. seawater outlet 24. zig-zag-baffler

25. protruding relief chevron

26. dished bottom wall

27. base

28. protruding housing part 29. dished top wall

30. inwardly protruding peripheral rim

31. bezel ring

32. peripheral groove

33. flange joint

Claims

C L A I M S :

1. Desalination plant (1) of the type comprising a) an evaporator (2) adapted to convert water to be desali¬ nated to a mixture of water vapour and entrained therein droplets containing dissolved solids in a raised con¬ centration, b) a separator (3) placed above said evaporator (2) and adap¬ ted to separate said droplets from said water vapour, al¬ lowing the droplets to be collected and form a flow of brine leaving the plant through suitable outlet means (16) and the vapour to flow upwards into c) a condenser (4) placed above said separator (3) and adap¬ ted to condense said water vapour so as to form freshwater and to allow same to leave the plant through suitable outlet means (18) , and d) means to supply water to be desalinated, heat for the evaporator and cooling effect for the condenser, as well as to remove uncondensable gases and maintain a suitable pressure within the intercommunicating spaces in the evaporator and the condenser, characterized in e) that said separator (3) is of a type with distributed effect, i.e. that at substantially all points on its full flow cross-sectional area it is capable of receiving said mixture to be separated on its inlet side facing the evaporator (2) , of performing said separation between its inlet and outlet sides, of allowing the droplets separated out to fall down below it, and of allowing the water vapour to flow upwards to said condenser (4) .

2. Desalination plant according to claim 1, charac¬ terized in that said separator (3) is of a type constituted by an assembly of separator cells each being adapted to perform the functions mentioned in claim 1, item e.

3. Desalination plant according to claim 1 or 2, charac- terized in a) that at least that part of its housing containing said separator (3) is generally circular-cylindrical, b) that said separator (3) has an outside wall extending between its inlet and outlet sides and of generally cir¬ cular-cylindrical shape so as to fit snugly within said housing part, c) that said housing is adapted to be opened by being divided at a location (33) between said evaporator (2) and said condenser (4) , and d) that said housing part comprises means (30, 31, 32) for releasably securing the separator (3) in its operating position, said location (33) being so placed as to allow the separator (3) to be removed from the housing when opened.

4. Desalination plant according to claim 3, .charac- terized in that the whole housing, possibly with the excep¬ tion of protruding parts (28) containing portions of the condenser (4) , is generally circular-cylindrical with sub¬ stantially the same diameter as said housing part and com¬ prises dished botton (26) and top (29) walls, of which the bottom wall (26) is secured to or integral with a base (27) for mounting the plant on a floor or a deck.

5. Desalination plant according to claim 3 or 4, characterized in that said means (30, 31, 32) for releasably securing the separator in its operating position comprise a) an inwardly protruding peripheral rim (30) in the wall of said housing part, b) a peripheral groove (32) in the wall of said housing part at a distance below said rim (30) corresponding at least to the height of the separator (3) , and c) a bezel ring (31) adapted to fit into said groove.