GB2195908A - Desalination plant - Google Patents

Abstract

The invention concerns a multi-effect, vertical tube evaporator desalination plant having means for passing brine through the vertical tube evaporators (13) in an upward direction with flash tanks (15) located between the effects (12) so that brine passing from one effect to the next effect is flashed before flowing to the next effect as a substantially single phase liquid in quiescent form. This allows the plant to operate in a more stable and controllable manner. <IMAGE>

Description

SPECIFICATION Desalination plant BACKGROUND OF THE INVENTION This invention relates to a desalination plant and particularly to a multi-effect, flashdown, vertical tube evaporator desalination plant with upflow of the brine through the evaporator tubes and to a method of operating the same.

Desalination plants using multistage flashdown rather than straight boiling techniques for producing pure water from sea water (brine) were developed in the late 1960s and are the most common plants now in operation. In these plants the brine passes through a flash chamber at the base of each effect and passes in a substantially horizontal flow from one flash chamber substantially directly to the flash chamber at the base of the next effect.

All multi-effect systems using 2-phase boiling in the effects require that a temperature difference be created and maintained between the vapour produced and the spillover brine both of which go to the next effect. Due to the BPE the temperature of the vapour produced from boiling or flashing brine in any distillation process is inevitably lower than that of the brine from which it is released. Yet this same vapour must produce the heat input to the subsequent effect. Therefore the first essential is to lower the temperature of the brine outflow from an effect so that it can once again be boiled to provide fresh vapour for the next effect. This temperature drop is produced by flashing. A later stage in the development of desalination plants concerned the use of vertical tube evaporators with downflow of brine through the tubes.The brine was distributed to the tubes through intereffect flash control orifices at the top of each tube which were arranged to distribute the brine round the inside wall of each tube in an even falling film liquid annulus. Under ideal conditions, this thin film combined with the essential flashdown at entry, rapidly initiates and develops two phase boiling with much higher heat transfer coefficients than those obtained in multistage flash plants where the brine travels along the bottom of each effect.

However, ideal tube side thin film distribution eventually breaks down due to inevitable partial blockages of distributor nozzles, causing dry areas upon which scale builds up. The only sure way to remove this scale is by mechanical cleaning and the plant downtime for such cleaning is excessive.

U.S. Patent specification No.3830704 to Frank discloses a desalination plant with downflow (or alternatively upflow) of brine in the tube bundles but without flashing of the brine.

Another well tried desalination plant involves a single effect upflow vertical tube evaporator unit with the product vapour being compressed to increase its temperature to heat the brine. However, these are small scale units with the high performance stemming from the vapour compression. A further proposal has been made for a multi-effect vertical tube evaporator plant working on upflow of the brine through the tube bundles of each effect without vapour compression. Only a single large scale plant has been built on this principle in which the brine spilt over from the top of one effect is fed to the bottom of the next effect and the vapour from the one effect is fed to the next effect to heat the brine travelling up the tube bundles in that next effect to cause further boiling/evaporation.The brine is fed from one effect to the base of the tubes of the next effect through a flash plate containing a flash orifice for each tube. The consequent flashdown of the incoming brine temperature provides the essential T between the incoming heating vapour from the previous effect and the brine inside the tubes. However, this flashing results in considerable energy release to the brine which tends to pulsate.

Ideally, the flow at the base of the upflow tubes after feed through the nozzles provides a foamy 2-phase (liquid/vapour) consistency at the inlet ends of the tubes and preferably a flow at the output ends characterised by a foamy liquid annulus on the tube wall and a foam free vapour core. This ideal flow would give a dramatic increase in overall heat transfer coefficient but it has been found in practice that with this system instability occurs with tube flooding, followed by drying out so that this plant has never operated satisfactorily.

SUMMARY OF THE INVENTION An object of the present invention is to provide a multi-effect, vertical tube evaporator desalination plant which will operate with upflow of the brine in a more stable manner.

Accordingly the invention provides a multieffect vertical tube evaporator desalination plant arranged to feed brine through the vertical tube evaporators in an upward direction and in which flash tanks are provided between the effects so that the brine entering each subsequent effect is substantially single phase liquid in quiescent form.

By supplying the brine to the tubes in quiescent form a stable operation can be maintained. As well as remaining stable at maximum capacity operation, the plant can be run stably at down to 20% of maximum capacity.

In a preferred form the invention provides a multi-effect vertical tube evaporator desalination plant comprising a plurality of effects each having a plurality of vertical tubes, means for supplying brine to the first effect and heating the brine to cause evaporation, means for collecting vapour and condensed evaporate from each effect and feeding it to the next effect to heat the upflowing brine therein, between each successive pair of effects a flashtank arranged to receive the spillover brine from the top of the preceding effect and cause a lowering of pressure and temperature and thus produce evaporation, means for collecting the vapour (and condensed evaporate) from the flash tank and feeding it to the next but one successive effect and means for passing brine at the lower temperature and pressure from the flash tank to the bottom of the next effect and feeding it to the plurality of tubes as a substantially quiescest single phase liquid without a further flashing at entry. This provides a more stable operation and allows greater control of the process.

Preferably each flash tank includes a surge control means to produce greater control of the flow of brine to the next effect.

With advantage each flash tank is positioned at a height substantially corresponding to that of the tops of the tubes of the preceding effect.

Preferably, part of the vapour and condensate from each flash tank is fed direct to the next effect and part is fed first through preheaters for the incoming sea water feed supply.

Preferably, vent connections ensure that the pressure in each flash tank is the same as that at the top of the successive effect.

BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of desalination plant will now be described by way of example only, with reference to the accompanying diagrammatic sketches of which: Fig. 1 shows an overall view of a desalination plant, and Fig. 2 shows three adjacent effects of such a plant in more detail.

DETAILED DESCRIPTION OF ONE EMBODI MENT In the drawings the plant is shown with ten effects 12 but it-will be appreciated that any desirable number of effects may be provided up to a practicable limit of perhaps 30 effects.

Each effect comprises a plurality of vertical tubes, this bundle of tubes being indicated diagrammatically at 13 for carrying brine upwards and a casing or jacket 14 round the tubes, through which casing vapour from the previous effect is passed to heat the brine in the tubes and produce boiling/evaporation in the tubes. Between the- top of the tubes of each effect and the bottom of the tubes of the successive effect is a first connecting means including a flash tank 15.

Each flash tank 15 is located at a level at or only just below the brine outlet 19 of the previous effect. Each flash tank comprises an inlet comptment 19A and a main compartment 19B separated by a weir 20.The main compartment is maintained at a predetermined reduced pressure so that flashing occurs as brine flows over the weir. Connected to the base of the main compartment is a surge and level control tank 21.

In each effect, brine enters through a brine inlet 16, is distributed to the lower inlet ends of the tubes 13, without flashing, via a bottom tube sheet 17, and travels up the tubes and is heated therein causing boiling and evaporation. The brine from the upper outlet ends of the tubes spills over a top tube sheet 18 from which it flows through a brine outlet 19 to the following flash tank 15, of known form, maintained at a lower pressure so that as the brine flows over the weir 20 its pressure and thus temperature is reduced, releasing energy and causing vapourisation. The brine is collected from the base of the flash tank 15 in a surge tank 21 from where it is passed in a controlled flow, under gravity, in the form of a substantially quiescent liquid to the inlet 16 of the next effect. One or more brine pumps may be included in the brine flow line.

The vapour from the top of each effect passes through a mist eliminator pad 22 and at least some of it is passed through a heat exchanger 23 where some is condensed. The condensate/ vapour is fed via the heat exchanger 23 through second connecting means to a vapour inlet 24 in the jacket of the next effect to heat the brine passing up the tubes therein. The vapour from each flash tank passes through a mist eliminator pad 26 and is fed via the top of the next effect and the second connecting means to the vapour inlet 24 of the next but one effect. The pressure at the top of each flash tank is maintained at the same pressure as that at the top of the successive effect, for example by a connection 27, so that the vapour from that flash tank and the vapour from the top of the effect are joined together at the same temperature and pressure to feed to the inlet 24 of the following effect.The condensate/distillate 28 is fed from the bottom of each effect to the next through pipes indicated diagrammatically at 29. These pipes may extend through the bottom tube sheets 17 and may include one or more intereffect distillate pumps.

The concentrated brine leaving the last effect passes through a brine cooler 30 and can be returned to the sea on line 31. The distillate leaving the last effect is passed through a distillate cooler 32 and leaves on line 33 to provide a supply of fresh water. Sea water initially enters the plant on line 34 and passes through the distillate cooler and brine cooler where it is heated. Excess sea water used for this cooling only may be discharged to the sea with the rest forming the sea water supply. Additives may be added to the sea water supply at 36, for example descaling chemicals or surfactants. The supply then passes through the plurality of heat exchanges 23 each associated with one of the effects(the supply is thus heated by at least some of the vapour leaving each effect),through a final trim heater 37 and into the inlet 16 of the first effect. A boiler 38 produces steam which heats the final trim heater and is supplied to the inlet 24 of the first effect, extracted at 39 and recirculated through the boiler. A vapour compressor could provide a heat source as an alternative to yhe boiler.

A vacuum and non condensible gas venting and pressure control system 40 initiates and maintains the vapour pressures in the effects in normal manner. These pressures determine the vapour temperatures TV1,TV2,TV3---,in each stage.

Claims (10)

1. A multi-effect, vertical tube evaporator, desalination plant having means for passing brine through the vertical tube evaporators in an upward direction, including flash tanks located between the effects so that brine passing from one effect to the next effect is flashed before flowing to the next effect as a substantially single phase liquid in quiescent form.

2. A desalination plant comprising: a plurality of effects connected in series and comprising a first effect, at least one intermediate effect and and a last effect, Each of the effects comprising an upright tube bundle surrrounded by a jacket, the tubes of the tube bundle having upper outlet ends and lower inlet ends, a brine feed supply for supplying hot brine to the lower inlet ends of the tubes of the first effect and means for heating the brine in that effect to cause boiling/evaporation, the first and intermediate effects each having first connecting means connecting the upper outlet ends of the tubes to the lower inlet ends of the tubes of the next succesive effect to convey brine thereto, the first and intermediate effects each having second connecting means connecting it to the jacket of the next succeeding effect to pass vapour thereto, means for removing distillate from the plant, and each first connecting means including a flash tank remote from the following effect for flashing the brine passing therethrough, so that brine passing from the flash tank to the following effect will be at a lower temperature than the temperature at which it leaves the previous effect and will enter the next effect as a substantially single phase liquid in quiescent form.

3. A plant according to Claim 2 in which each flash tank is located substantially at the level of the upper ends of the tube bundle of the preceeding effect.

4. A plant according to Claim 2 or Claim 3 in which each first connecting means includes a surge control tank located downstream of the flash tank.

5. A plant according to Claim 2, 3 or 4 in which the brine feed supply includes a plurality of pre-heaters for the brine and the second connecting means includes means for passing at least part of the vapour from each effect through one of the pre-heaters.

6. A plant according to any of claims 2 to 5 including third connecting means connecting an upper vapour carrying portion of each flash tank to the second connecting means of the following effect.

7. A plant according to any of claims 2 to 6 including vent connections between the top of each flash tank and the top of the successive effect to equalise the pressures therein.

8. A plant according to any of claims 2 to 7 including pressure control means arranged to regulate the pressures in the effects and/or flash tanks.

9. A desalination plant substantially as described herein with reference to or as illustrated in the accompanying drawings.

10. A method of operating a desalination plant substantially as described herein.