GB2413321A - Process and plant for multi-stage flash desalination of water - Google Patents

Abstract

The present invention provides a process and a plant for the desalination of salt water in which at least three desalination zones connected in series are provided. In the process brine at elevated pressure is heated in heat exchanger 201 using heated fluid, e.g. steam in line 202, which is cooled and returned to line 203. The hot brine is released into the first desalination zone 205, where water vapour 206 flashes off as the brine is cooled by the reduction in pressure. The vapour is condensed on condensation conduits 207 cooled by the recirculated brine at a lower temperature flowing towards the heat exchanger 207. The condensed water is collected in product water channel 208. The cooled flashing brine and product water cascade into the next desalination zone. An evaporate 216 from an intermediate desalination zone 214, is withdrawn and is cooled and condensed in a heat pump 217 before being returned to the product water channel 218 of the intermediate desalination zone 214. A portion of the flashing brine stream is drawn from the intermediate desalination zone 214 is extracted by pump 219 and mixed with the brine in the condensation conduit 220 upstream of the intermediate desalination zone. The multiple desalination zones are maintained at successively lower pressures from the upstream end of the series to the downstream end thereof.

Description

DESCRIPTION

PROCESS AND PLANT FOR MULTI-STAG_

FLASH DESALINATION OF WATER

The present invention relates to a process and plant for the desalination of salt water, particularly seawater.

Conventional desalination plants operate according to a multi-stage flash (MSF) process. Flashing is the process whereby water vapour is evaporated from salt lo water and the resulting water vapour is then condensed and collected. Water may be caused to boil by, for example, a reduction in pressure. In an MSF process a salt water supply is sequentially fed to a number of flashing zones and a substantially salt-free condensate is collected in each zone.

WO-A-02/32813 discloses a process and plant for the desalination of salt water in which there is provided a heat exchanger for heating brine and at least one desalination zone, and also a heat exchanger which is used to heat a thermal recycled stream extracted from a product stream, a depleted feed stream, or a feed stream.

JP-A-60172386 discloses an MSF apparatus which is intended for the efficient manufacture of distilled water from seawater. This apparatus provides for the recycling of a part of the feed stream, which is then returned to the main feed stream before being passed through the heat exchanger for heating brine.

JP-A-58112082 discloses the use of a heat exchanger in connection with an engine cooling system.

United States Patent No. 3926739 discloses an MSF apparatus in which a portion of brine is recovered and heated isothermally and the nonevaporated portion resulting therefrom is returned to the desalination zone.

United States Patent No. 5133837 discloses an MSF unit in which dimpled plates are used to provide an effective condensation surface for the brine evaporate.

There is a current and growing need for effective salt water desalination technology in many areas of the world where supplies of fresh water are short.

The need for such technology is likely to increase substantially because of increased water shortages brought about by global warming and increasing demand for fresh water.

MSF desalination processes are currently in commercial use to supply fresh water in arid areas of the world where there is access to brackish water and/or to seawater. However, the capital and operating costs of such plant is high, largely because of the volume requirements of the process and the energy input required to evaporate large volumes of water vapour at a sufficiently high rate. In an attempt to minimize the energy requirement, MSF technology has been commercially applied in conjunction with power generating plant in order to make use of the available thermal energy.

Despite such improvements in the energy efficiency of desalination plant, there remains a need to provide an improved process and apparatus for desalination of salt water which improves energy efficiency and therefore lowers cost and damage to the environment in relation to conventional desalination plants. These applications of desalination require a low specific steam consumption for the desalination process in order to minimise the consumption of energy from fuel and to produce the power and water products at the lowest possible cost.

According to the present invention there is provided a process for the desalination of salt water comprising the steps of: a) providing a heat exchanger for heating brine; b) providing in series connection an upstream desalination zone, a downstream desalination zone, and an intermediate desalination zone, each desalination zone comprising a condensation conduit connecting the zones in series and a product water channel connecting the zones in series and arranged to collect condensate from the condensation conduit; c) providing a heat pump; d) supplying a feed stream comprising salt water as a coolant to the condensation conduit to pre-heat the feed stream; e) supplying the pre-heated feed stream to the heat exchanger further to heat the pre-heated feed stream; f) supplying the heated feed stream from the heat exchanger to the upstream desalination zone in the series, evaporating at least a portion of the heated feed stream in the upstream desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the upstream desalination zone; g) recovering from the upstream desalination zone a product water stream comprising the condensate and a depleted feed stream comprising salt water; h) supplying the product water stream to the product water channel of the intermediate desalination zone; i) supplying the depleted feed stream to the intermediate desalination zone, evaporating at least a portion of the depleted feed stream in the intermediate desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the intermediate desalination zone; I) withdrawing from the intermediate desalination zone a portion of the evaporate, and supplying said evaporate to the heat pump; k) cooling and condensing the withdrawn evaporate through the heat pump and returning the cooled, condensed evaporate to the product water channel of the intermediate desalination zone; 1) withdrawing from the intermediate desalination zone a recycle portion of the depleted feed stream and supplying said recycle portion to the condensation conduit upstream of the intermediate desalination zone; m) recovering from the intermediate desalination zone a product water stream comprising the condensate and a further depleted feed stream comprising salt water; n) supplying the product water stream from the intermediate desalination zone to the product water channel of the downstream desalination zone; o) supplying the further depleted feed stream from the intermediate desalination zone to the downstream desalination zone, evaporating at least a portion of the further depleted feed stream in the downstream desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the downstream desalination zone; and p) recovering a product stream from the product water channel in the downstream desalination zone. s

In the process of the invention, it is preferred that the desalination zones in the series are maintained at successively lower pressures from the upstream end of the series to the downstream end thereof. It is also preferable that successive desalination zones in the series are maintained at successively lower temperatures from the upstream end of the series to the downstream end thereof.

The upstream desalination zone may comprise a single stage, or there may be provided a plurality of stages, themselves connected in series and/or in parallel, in the upstream desalination zone.

The intermediate desalination zone may comprise a single stage, or there may be provided a plurality of stages, themselves connected in series andIor in parallel, in the intermediate desalination zone. When the intermediate desalination zone comprises a plurality of stages, it is envisaged that the evaporate may be withdrawn from one or more of the stages, provided that the evaporate withdrawn from one stage is, after cooling and condensing through the heat pump, returned to the product water channel of one of the intermediate desalination zone stages, preferably the same stage from which the evaporate was withdrawn. It is also envisaged that depleted feed stream may be withdrawn from one or more of the stages, provided that depleted feed stream withdrawn from one stage is returned to the condensate conduit upstream of that stage. The depleted feed stream may be withdrawn from the same stage from which evaporate is also withdrawn, or from a different intermediate stage, or from both.

The downstream desalination zone may comprise a single stage, or there may be provided a plurality of stages, themselves connected in series and/or in parallel, in the downstream desalination zone.

When a plurality of upstream desalination stages are provided in series, successively lower temperatures and/or pressures, may be maintained between one or more neighbouring pairs of upstream desalination stages.

When a plurality of intermediate desalination stages are provided in series, successively lower temperatures and/or pressures, may be maintained between one or more neighbouring pairs of intermediate desalination stages.

When a plurality of downstream desalination stages are provided in series, successively lower temperatures and/or pressures, may be maintained between one or more neighbouring pairs of downstream desalination stages.

The intermediate desalination zone is preferably maintained at a temperature and/or pressure intermediate that of the temperature and/or pressure of the upstream and downstream desalination zones.

The intermediate desalination zone is preferably configured for reduced material flow with respect to the upstream desalination zone in one or more of the condensation conduit, the product water channel and in the part of the zone beneath the product water channel where flashing occurs. In one particularly preferred process of the invention, the intermediate desalination zone is smaller than the upstream desalination zone.

The desalination process of the invention represents a significant improvement in the energy efficiency of conventional desalination plants, for example MSF plants. The desalination process of the invention operates by heating salt water and then allowing the heated salt water to flash in the desalination zones. The heated salt water flashes progressively through the series of desalination zones from the upstream end to the downstream end. The flashing steam in each desalination zone is condensed on the condensation conduit, which acts as a condenser, and the resulting condensate is collected in the product water channel. Preferably, the condensation conduit comprises at least one pipe carrying the incoming feed stream as a coolant, and the product water channel comprises a product water trough, in which the condensation conduit is nested.

The product water trough links each desalination zone and the product water cascades from stage to stage in parallel with the feed stream, flashing at each stage, with flashed steam being recovered as water in the product trough.

The heating stream supplied to the heat exchanger for heating brine preferably comprises steam from the heat pump which, in turn, may be supplied with heat from associated steam-raising plant. Preferably, the heat exchanger for heating brine is incorporated within the heat pump.

The process of the invention has the following significant advantages.

5. The heat consumption of the plant per unit of distilled water production is reduced by a significant amount compared with conventional desalination plants, for example by 30% to 40%.

Reduced heat consumption means that the plant arranged to operate in accordance with the invention generates less pollution per unit of distilled water production than does a conventional desalination plant.

Existing conventional MSF technology can conveniently be adapted to operate in accordance with the process of the invention and the capital cost associated with such modification of conventional plant is significantly lower than other types of modification which may be envisaged to improve the efficiency of the conventional plant.

In a new plant design, significant capital savings in the cost of the distiller can be made by reducing the size of the downstream desalination zone with respect to the upstream desalination zone.

Although it is primarily envisaged that new plant will be designed to operate the process of the invention, it is also possible that the process of the invention may be operated in a modified conventional plant (i.e. a conventional plant retro fitted in accordance with the invention).

Suitable heat pumps for use in the process of the invention include vapour compression heat pumps and absorption heat pumps. Absorption heat pumps are preferred, particularly when the plant is the recipient of exhaust steam from a turbine, for example.

One particularly preferred type of heat pump utilizes lithium bromide as the absorbent.

The process of the invention may utilize a single heat pump or, alternatively, a plurality of heat pumps may be used. in this case, the plurality of heat pumps may be connected in series, or in parallel, as desired.

The heat exchanger for heating brine may be provided separate from the heat pump (as in the Figure 2 plant described below), or may form part of the heat pump (as in the Figure 3 and 4 plant described below). In either case, heat from an external source may be provided to the heat exchanger either directly and/or via the heat pump.

The coolant for the condensation conduit (and condenser) is preferably unheated feed stream. The feed stream itself may comprise make-up feed from, say, seawater, and may further comprise a recycle stream from a desalination zone.

The condensation conduit (condensing tubes) in each desalination zone section are cooled by recirculating and/or make-up feed stream which flows through the

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zones in the opposite direction to the flashing feed stream. The feed stream is thus progressively preheated prior to the heat exchanger for heating brine. After passing through the heat exchanger for heating brine, the feed stream is released to flash in the first desalination zone through a control valve. The control valve preferably maintains the feed stream pressure above the steam pressure in the heat exchanger for heating brine to stop the feed stream boiling inside the heat exchanger for heating brine and also to avoid any possible steam leakage into the feed stream, since the steam from outside plant (used as the heating stream in the heat exchanger for heating brine) may be contaminated with traces of toxic chemicals used for boiling water treatment. These could otherwise contaminate the MSF unit product water in case other heat exchanger for heating brine tube leak.

In one preferred process of the invention, a plurality of intermediate desalination zones are provided and the cooled, condensed evaporate withdrawn from a first intermediate desalination zone is returned via the heat pump to the product water channel between the first intermediate desalination zone and a second intermediate desalination zone.

In one preferred process according to the invention, the downstream desalination zone is connected in series further downstream with a heat rejection section of the desalination plant. The heat rejection stage may be arranged similarly to the desalination zone, but the condenser tubes are cooled by seawater circulation, not by feed stream. Seawater circulation in the heat rejection stage rejects waste heat from the cycle and maximizes fresh water production. At least part of the cooled feed stream from flashing in the heat rejection stage, which is maintained at low pressure in relation to the upstream desalination zones, may be recirculated through the unit by large pumps. The product water is replaced by make-up from the seawater outlet from the hottest heat rejection stage which is de-aerated and returned to the lowest pressure stage. The re-circulating and/or make-up feed stream concentration is controlled by continuously blowing down a small amount of brine from the lowest pressure stage to the seawater discharge.

Each stage of flashing of the feed stream may result in some noncondensable gases being released. These may be extracted by a system of vents and vacuum ejectors to avoid stage condensing surfaces becoming ineffective through being blanketed by these gases, thereby reducing the performance of the MSF unit. Similar provisions for the heat exchanger for heating brine help to ensure that its heat transfer performance is maintained by proper extraction of non-condensable gases. The systems mean that the product water and the heating stream have a very low level of dissolved gases.

The design of the MSF desalination unit includes many alternative possibilities of configuring each stage of heat rejection and desalination, and of selection of the number of stages which form parts of the whole unit. The direction of flow of the flashing feed stream may be parallel to that of the re-circulating and/or make-up feed stream, the so called long tube design, or it may be perpendicular to it, the so called cross-flow design. The number of stages in the desalination are not limited. For example, the process of the invention may include the use of up to about 20 or more desalination zones, with the condensers of earlier zones in the series being cooled by feed stream which is sequentially heated in each zone as it moves towards the heat exchanger for heating brine, and the condensers of later zones being cooled by cold seawater to effect maximum condensation.

The heat exchange tubes may be of any suitable material, such as cupronickel, brass, titanium and various grades and specifications of stainless steel, in order to tolerate the chemically aggressive hot seawater flowing at the velocities necessary for optimum heat transfer. The heat transfer surface is preferably kept as free as possible of internal deposition of scale-forming insoluble materials from the hot seawater by chemical dosing and often by the circulation of soft rubber balls in the water flow. The choice of diameter of the tubes is limited by such mechanical cleaning measures, restricting optimization for heat transfer performance or cost. Chemical cleaning measures may also be used.

The invention will now be more particularly described with reference to the following drawings, in which: Figure 1 shows a flow diagram of a conventional multi-stage flash desalination plant; Figure 2 shows a flow diagram of a multi-stage flash desalination plant arranged to operate in accordance with the process of the invention; Figure 3 shows a flow diagram of a multi-stage flash desalination plant arranged to operate in accordance with a preferred process of the invention; and Figure 4 shows a flow diagram, with associated numerical parameters, of the internal operation of a heat pump and heat exchanger for heating brine suitable for use in the process of the invention.

Referring to Figure 1, brine at elevated pressure is heated in brine heater 101 by to means of heating stream 102, normally steam, which is cooled and returned in line 103. The hot brine is reduced in pressure through valve 104 and released into chamber 105 where water vapour 106 flashes off as the brine is cooled by the drop in pressure as the brine enters chamber 105. The vapour is condensed on tubes 107 cooled by recirculated brine at a lower temperature flowing towards IS brine heater 101. The condensed vapour is collected in product tray 108. Noncondensable gases are extracted from a condensing tube nest to flow as stream 109 to the next lower pressure chamber 110. The cooled flashing brine and product condensate cascade into the next chamber through restrictions 111 and 112 respectively, while recirculated brine flows into the condensing tubes from outlet 113 of the condensing tubes in the next chamber.

The multiple similar chambers are cascaded with similar interconnections with the brine being reduced in pressure and temperature progressively until the last such chamber 114. The recirculated brine is delivered to this chamber by a pump 115 from the lowest pressure chamber of heat rejection section 116. The heat rejection section may include a cascade of chambers as before but with the cooling brine flow being supplied by pump 117 from a seawater intake. The warmed brine from the condensing tubes of the heat rejection section is discharged to sea in line 118. A small part of the discharged flow is passed at reduced pressure through vessel 119, which acts as a de-aerator. The de aerated flow in line 120 is mixed with the inlet flow to re-circulation pump 115. To control the brine concentration, part of the flashed brine stream in the lowest pressure heat rejection chamber is extracted by pump 121 for discharge to the sea. The cascaded flow of condensate is extracted from the lowest pressure chamber by pump 122 for delivery as product fresh water.

Non-condensable gases collected in line 123 from de-aerator vessel 119, are combined with non-condensable gases from heat rejection section 116and are extracted by ejector system 124 with condensers cooled by seawater. The ejector system is supplied with high pressure steam in line 125.

Referring to Figure 2, brine at elevated pressure is heated in heat exchanger 201 using heating fluid in line 202, for example steam, which is cooled and returned in line 203. The hot brine is reduced in pressure through valve 204 and released into chamber 205 where water vapour 206 flashes off as the brine is cooled by the drop in pressure. The vapour is condensed on tubes 207 cooled by recirculated brine at a lower temperature flowing towards heat exchanger 201.

The condensed water vapour is collected in product tray 208. Noncondensable gases are extracted from the condensing tube nest to flow as stream 209 to the lower pressure chamber 210. The cooled flashing brine and product condensate cascade into the next chamber through restrictions 211 and 212 respectively, while recirculated brine flows into the condensing tubes from the outlet 213 of the condensing tubes in chamber 210.

The multiple similar chambers are cascaded with similar interconnections into chamber 214. This chamber (or series of chambers) operates at a temperature intermediate that of first chamber 205 and last similar chamber 215. Some of the vapour released in chamber 214 is extracted as stream 216 and is supplied to heat pump 217, where it is cooled and returned as a condensate stream in line 218, which is added to the flow of condensate leaving chamber 214. A part of the flashing brine flow leaving chamber 214 is extracted by pump 219 and mixed with the brine leaving chamber 214 in line 220. The remaining flashing brine, condensate flow and non-condensable gas flow are cascaded into further stages, in the Figure 2 plant, without steam or flashing brine extraction.

The multiple similar chambers are cascaded with similar interconnection, with the brine being reduced in pressure and temperature progressively until the last such chamber 215. The recirculated brine is delivered to this chamber by a pump 221 from the lowest pressure chamber of heat rejection section 222. The heat rejection section may include a cascade of chambers as before but with the cooling brine flow being supplied by a pump 223 from a seawater intake. The warmed brine from the condensing tube to the heat rejection section is discharged to sea in line 224. A small part of the discharge flow is passed at reduced pressure through a vessel 225, which acts as a de-aerator. The de- aerated flow 226 is mixed with the inlet flow to the re-circulation pump 221. To control the brine concentration, part of the flashing brine stream in the lowest pressure heat rejection chamber is extracted by pump 227 for discharge to the sea. The cascaded flow of condensate is extracted from the lowest pressure chamber by pump 228 for delivery as product fresh water.

The non-condensable gases 229 collected from heat rejection chamber 222, and from de-aerator vessel 225 are extracted by an ejector system 230 with condensers cooled by seawater. The ejector system is supplied with high pressure steam in line 231.

The heat pump 217 uses high temperature energy stream 232 to increase the lS temperature of the heat extracted from the vapour stream in line 216 to deliver higher temperature fluid or fluids 202 to heat the brine in the heat exchanger 201.

The lower temperature fluid or fluids 203 are recirculated to the heat pump while the cooled high temperature energy stream 233 is returned to the external heat source.

Referring to Figure 3, there is shown a schematic flow diagram depicting one preferred arrangement of plant for use in operating the process and plant of the invention. The plant shown in Figure 3 comprises an upstream series of MSF units 301 to 312, of which only 301 and 302 are shown in Figure 3, an intermediate series of MSF units 313, 314, and a downstream series of MSF units 315 to 319, of which only 319 is shown in Figure 3. Beyond downstream MSF unit 319 is a further series of heat rejection units 320 to 322, of which only 322 is shown in Figure 3.

MSF units 301 to 319 are connected in series, with condensing tubes 323 carrying brine from the downstream end towards the upstream end and into heat pump 324, which incorporates a heat exchanger for heating brine. The operation of heat pump 324 will be more particularly described in Figure 4. However, the feed brine stream in line 323 is heated to its target temperature in heat pump 324 and continues in line 325, through pressure let down valve 326 and into MSF unit 301, where the brine is flashed as indicated by arrows 327 and condensed on tubes 323. Condensed product water is collected in product water tray 328. A product water stream cascades in line 329 into the product water tray of l 5 neighboring MSF unit 302. Meanwhile, a depleted flashing brine stream cascades through restriction 330 into MSF unit 302, where a similar process of flashing and condensation takes place. The product water and depleted brine streams cascade sequentially through MSF units 303 to 312 (indicated by the dotted line in Figure 3 between MSF unit 302 and MSF unit 313). In the plant depicted in Figure 3, MSF units 301 through to 312 represent the so-called upstream desalination zones in accordance with the invention.

The intermediate desalination zone required in the process of the invention is represented in Figure 3 by two MSF units 313 and 314. In operation, unit 313 is \ similar to the upstream units 301 to 312 except that a portion of the flashed vapour in unit 313 is withdrawn in line 331 and supplied to heat pump 324, where it is cooled and returned in line 332 to the product water cascade in the intermediate desalinaqtion zone, in the case of the Figure 3 plant immediately downstream of unit 313. The operation of the heat pump in this respect will be explained in connection with Figure 4. A similar vapour extraction may be made from unit 314 and is shown in the Figure 3 plant in line 333, returning as a cooled product water stream in line 334 to the intermediate desalination zone, in the case of the Figure 3 plant immediately downstream of unit 314.

Part of the flashing brine stream is removed from unit 314 in line 335, through recycle pump 336 and rejoins the feed brine in condensing tubes 323 upstream of the intermediate desalination zone via line 337.

The downstream desalination zone comprises, in the Figure 3 plant, MSF units 315 to 319, of which only 319 is shown in Figure 3, the remaining units being indicated by the dotted line in Figure 3 between MSF units 314 and 319. Product water in MSF unit 319 cascades in line 338 to a heat rejection stage comprising three units, of which only the last, 322, is shown in Figure 3. The depleted brine stream cascades similarly in line 339. In operation the heat rejection stage is much as described already in connection with Figures 1 and 2. Make-up sea water is supplied in line 340 through make-up pump 341 and passes in line 342 to the cooling tubes of heat rejection unit 322. Part of the returning make-up stream is discharged through lines 343 and 344, and part is passed to de-aerator 345, a de-aerated make-up stream continuing in line 346, to be combined in line 323 with a recycle stream in line 347. Product water in unit 322 is collected in line 348, and non-condensables are passed in line 349 and combined with non- condensables from de-aerator 345 in steam ejector 350 supplied with steam in line 351.

Referring to Figure 4, there is shown a schematic flow diagram depicting asuitable arrangement of heat pump, incorporating a heat exchanger for heating brine, for use in the process and plant of the invention. Feed brine stream in line 401 is supplied from the condensing tubes of the highest temperature MSF unit (not shown in Figure 4) to heat exchanger 402, which is supplied with hot water in line 403. Cooled water is recovered from heat exchanger 402 in line 404 as water and is returned to the steam raising plant. The hot brine stream continues in line 405 to absorption unit 406 supplied in line 407 with a suitable absorbent and in line 408 with steam extracted from an intermediate MSF zone (not shown in Figure 4), which may be supplied via optional tube exchanger 409. Steam supplied from the MSF unit to the heat pump is returned as condensate to an appropriate zone of the MSF unit. Steam from line 408 is condensed in chiller 406 and its latent heat is used to heat the brine stream in line 405, which continues on, as a hotter stream, in line 410. The weak absorbent and solution are returned in line 411. The hot brine in line 410 cascades into second absorption chiller unit 412, supplied in line 413 with strong absorbent and in line 414 with steam extracted from an intermediate MSF zone (not shown in Figure 4), which may be supplied via optional tube exchanger415. Steam supplied from the MSF unit to the heat pump is returned as condensate to an appropriate zone of the MSF unit. Steam from line 414 is absorbed in absorber 412 and its latent heat is used to heat the brine stream in line 410, which continues on, as a hotter stream, in line 416. Absorbent is cascaded in line 407 to absorption unit 406. s

The hot brine stream in line 416 is supplied to heat exchanger for heating brine 417, supplied with heating steam in lines 418 and 419. Brine at its target temperature is recovered in line 420 and supplied as a flashing brine stream to the highest temperature stage of the MSF unit (not shown in Figure 4).

The recovered absorbent in line 411 is returned as a relatively weak solution and is heated through heat exchanger 421, supplied with heating water in line 422, which continues as the heating water stream in line 403. The heated absorbent stream continues on in line 423 and is supplied to the bottom section of evaporator unit 424, supplied with heating steam in line 425. The heating steam in line 425 may, for example, be supplied from associated steam raising plant, and continues as a heating stream in line 426 to join the heating stream in line 422. Evaporate from evaporator 424 is recovered in line 419 and is supplied to heat exchanger for heating brine 417 as a heating stream. An absorbent solution of increased strength is supplied to the bottom section of second evaporator unit 428, supplied with heating steam in line 429. The heating steam in line 429 may, for example, be supplied from associated steam raising plant, and continues as a heating stream in line 422. Evaporate from evaporator 428 is recovered in line 418 and is supplied to heat exchanger for heating brine 417 as a heating stream.

An absorbent solution of further increased strength is supplied in line 430, via heat exchanger 421, in line 413 to absorption 412.

Water is recovered from heat exchanger for heating brine 417 in line 431 and returned to heat exchangers 409 and 415.

The invention will now be more particularly described with reference to the following examples based on a plant operating substantially in accordance with Figures 3 and 4.

Examples

A plant designed in accordance with Figure 3 and configured in accordance with the stage geometry described in Table 1 and the stage detail described in Table 2 was modeled and the results are shown in Table 3. Stages 1 to 12 represent the upstream desalination zones, Stages 13 and 14 represent the intermediate desalination zones, Stages 15 to 19 represent the downstream desalination zones and Stages 20 to 22 represent the heat rejection stages.

TABLE 1

htc adJustment Recovery stages 19 factor1.015 _ tube friction Reject stages 3 factor0 03 Non-equlllbnum loss calc _ _1

STAGE _

GEOMETRY

Stage 1 234567 8 1011 number of tubes 2570 _257025702570257025702570 2570257025702570 tube length m 23 8 23 823.823 823 823 823.8 23.823.823 823 8

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tube od m 0.044 0 0440 0440 0440 0440 0440 044 0 0440.0440 0440 044 tube wall m 0 001 0.0010 0010 0010 0010 0010 001 0 0010.0010.0010 001 tube rows 60 606060606060 _60606060 stage width m __ 25 252525252525 25252525 stage length m 4 1 4 14 24.24 24.24 4 4.44.64.64.6 brine depth m O OOOOOO OOOO matenal conducbvty W/m2K 29 4 29 4 29 4 45 45 45 45 45 45 45 45 9 OOE- 9 OOE- 9 OOE- 9 OOE- 9.00E- 9 OOE- 9.00E- 9.00E- 9.00E- 9 OOE- 9. 00E fouling factor m2K/W 05 05 05 05 05 05 _ 05 05 05 05 05 Demister area m2 _ 32.00 27 00 27 00 29.00 29 00 29 00 _33.00 33 00 37.00 37 00 37. 00 demister thckness m 0 08 0 08_ 0 08 0 08 0.08 0 08 0 08 0.08 0 08 0. 08 0.08 demlster loss coeff. 0 001 0 001 0 001 0.001 0.001 0 001 0 001 0 001 0.001 0.001 0.001 tube bundle loss coeff 0 0003 0 0003 0.0003 0 0003 0 0003 0.0003 0 0003 0 0003 0.0003 0.0003 0 0003 Stage __ 12 13 14 15 16 17 18 19 20 21 1 22 number of tubes 2570 1600 1600 1600 1600 1600 1600 1600 1200 1200 1 1200 tube length m _ 23 8 23 8 23.8 23 8 23.8 23.8 23.8 23 8 23.8 23 8 1 23.8 tube od m 0 044 0 044 0.044 0.044 0 044 0.044 0.044 0 044 0.044 0 044 1 0 044 tube wall m 0 001 0 001 0 001 0 001 0.001 0 001 0 001 0 001 0.001 0.001 1 0 001 tube rows 60 60 60 60 60 60 60 60 60 60 1 60 stage wdth m 25 25 25 25 25 25 25 25 25 25 1 25 stage length m 4 9 4 9 4 9 5 1 5.1 5 3 5.3 5.4 4.6 4 8 1 _ bnne depth m O O O O O O O O matenal conductvity W/m2K 100 100 100 100 100 100 100 100 21 6 21.6 1 216 9 OOE- 9 OOE- 9 OOE- 9.00E- 9 OOE- 9.00E- 9.00E- 9 OOE- 9 75E- 9 75E- 1 9. 75E fouling factor __ mZK/W 05 05 05 05 05 05 05 05 05 05 1 05 Demister area m2 43.00 43.00 43 00 47 00 47 00 52 00 52.00 56.00 48.00 50.00 1 59 00 demister thickness m 0.08 0.08 0 08 0 08 0 08 0.08 0.08 0.08 0 08 0.08 1 0 08 demister loss coeff 0 001 0.001 0 001 0.001 0 001 0.001 0 001 0. 001 0 001 0 001 1 0 001 1 tube bundle loss I I coeff. 0.0003 0 0003 0. 0003 0.0003 0.0003 0 0003 0.0003_ 0 0003 0 0003 0.0003 1 0.000

TABLE 2

STAGE DETAIL

Stage 12345 678 1011 tube velocty m/s2.2952.2902 2852 2792.274 2 2692 2642 2602.2552 2512 247 _ __._ nternal htc W/m' K1167311534113931124911100 109501079710642104841032510163 External htc - W/m2 K 8955 8893 8827 8777 8704 8628 8548 8467 8382 8296 8207 overall htc W/m Z.K 3236 3217 3198 3301 3278 3255 3231 3206 3180 3153 3126 tube head loss m 4 5648 4 5432 4 5222 4.5016 4 4814 4 4619 4.4431 4.4249 4 4074 4.3905 4 3743

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tube end loss m 0 3052 0 3038 0 3024 0.3010 0 2997 0.2984 0 2971 0 2959 0. 2947 0.2936 0 2925 total tube loss bar 0 4806 0.4795 0.4784 0 4773 0 4762 0.4752 0.4742 0 4732 0.4723 0.4714 0 4705 condensing heat flow kW 98357 98077 97811 99885 99551 99187 98857 98436 98044 97556 97024 heat balance 3.61 E- 3 44E- 3.25E- 3 11 E- 2.92E- 2.75E- 2.56E- 2.44E- 2 32E- 2 31 E- 2 37E error % 06 06 06 06 06 06 06 06 06 06 06 1 12E- 1 81E- 2 35E- 2 75E3.00E- 3 14E- 3.16E- 3.10E- 2 96E- 2 77E- 2. 54E Pressure error bar 06 06 06 06 06 06 06 06 06 06 06 temperature 2.71E- 3.94E- 5.10E- 6 17E- 7 16E8 02E- 8 74E- 9.29E- 9.69E- 9 91E- 9 96E error C 05 05 05 05 05 05 05 05 05 05 05 bnne weir loading m31him 1185 1175 1166 1156 1147 1138 1129 1121 1112 1104 1096 steam release.

rate m/s 0 595 0 650 0 694 0 781 0 858 0.944 0 994 1.098 1.162 1 289 1 431 demster velocity m/s 1 905 2.468 2 700 2 827 3 106 3.418 3 313 3 659 3 613 4 005 4.448 tube wall shear stress N/mA2 4 129 4 298 4 473 4. 658 4.852 5 054 5 264 5.483 5 710 5 946 6.190 Stage 12 13 14 15 16 17 18 19 20 21 22 tube velocity m/s 2 243 1.974 1. 970 1 967 1 964 1.960 1.957 1 955 2 459 2.456 2.454 internal htc W/m2 K 9996 8903 8728 8547 8364 8177 7988 7795 9306 9113 8914 External htc W/m'K 8131 8004 7844 7720 7600 7477 7351 7222 7065 6931 6793 overall htc W/m' K 3218 3079 3036 2996 2956 2915 2873 2829 2646 2612 2577 tube head ioss m 4 3587 3 3762 3 3638 3.3519 3.3406 3 3300 3 3200 3 3108 5 2383 5.2279 5.2180 tube end ioss m 0.2915 0 2260 0 2252 0.2244 0.2237 0. 2229 0.2223 0.2217 0.3501 0.3494 0 3487 total tube loss bar 0.4697 0 3634 0 3627 0.3621 0 3615 0 3609 0 3604 0 3599 0 5633 0.5627 0 5622 condensng heat flow kW 98908 59214 60201 59985 59477 58951 58346 57711 43084 43412 43752 heat balance 2 48E- 2 37E- 2 88E- 3 58E- 4 37E- 5 34E- 6 63E- 8.13E- 7 27E- 8 59E- 1 OOE error % 06 06 06 06 06 06 06 06 06 06 05

_ _ __

2.28E- 2 01E- 1 72E- 1 39E- 1 10E- 8 46E- 6 24E- 4.37E- 3 36E- 2 38E- 1 54E Pressure error bar 06 06 _ 06 06 0607 07 07 07 07 07 temperature 1 - 9 84E- 9 56E- 9.02E- 8.16E- 7.20E-6.12E- 4.94E- 3 70E- 2.91E- 1.97E- 9. 91E error C 05 05 05 05 0505 05 05 05 05 06 bnne wer loadng m31him 1088 1080 1072 545 541537 533 529 525 523 520 steam release rate m/s 1 537 1 828 2 098 1 236 1 4091 551 1 775 1 997 1 963 2 106 2 088 demster velocty m/s 4.379 5.209 5.978 3 353 3 8233 952 4 523 4.815 4 703 5.056 4.867 tube wall shear stress N!m^2 _ 6.448 5.683 5.947 6 226 6. 516_ 6 819 7.132 7.458 10 561 10.938 11.332

TABLE 3

STAGE

SUMMARY

flSaas9hieng =12345678 10 11 pressure bar a1 23651 11080 99620 88940 79260 70500 62590 55460 49050 4330 0.3815 bnne temperature C_6 90103. 82100.7497.6094 4791 3488 2485.14_ 82.0678.99 75 94 Sabnity %6 886 916. 956.997.037.067 107 147 177 21 7.25 flashing InUet flashng outlet kg/s8222817881358092804980067964_ 792278817840 _7800 flow kg/s817881358092_8049800679647922788178407800 7761 producton kg/s43. 8943.3542 8343.3342.7842.2341.7041 1440.6140.04 39.47 tube Iniet temp C100 2897.2094 1290 9787.8484 7281.6078.5175.4272 35 69.29 temp C103. 38100 2897 2094.1290 9787 8484 7281 6078.5175 42 72.35 tube coolmg flow _ kg/s8222822282228222822282228222822282228222 8222 Demister ioss bar0 00020.00030 00030.00030 00040.00040.00030.00040.00030.0003 0.0004 Demister ioss C0 00490 00810.00960.01040 01240 01490.01380.01670.01610 0195 0.0237 bolilng point nse C1 1671 1541 1411.1281.1141.1001 0861 0721. 0581 044 1.029 ioss C0.00120 00150 00180 00220 00270 00330.00400 00490 00600.0073 0.0090 non equlllbnum loss C0.08590 09250.09960 10780.11620. 12530.13510 14570 15720.1696 0.1832 heat transfer coefflclent 3236321731983301327832553231320631803153 3126 flashng =12131415161718192021 22 pressure bar.a0.33440.29040.25100 21490. 18360.15640.13300 11280 10010.0879 0.0770 brine temperature C72.8369.5766 2862 8559 4556.0952 7749.4847 0044.53 42 05 Salinity %7.297 327.367 417 457 497 537.577 607.63 7 66 flasilng Intiet _ flow kg/s7761772176793916389438723851383138103795 3780 flashing outlet flow kg/s7721767976383894387238513831381037953780 3765 produchon kg/s39. 8822 3922.4621.9321 5021 1020.6820 2615 1314 97 14.99 tube Iniet tube ouUet C_6.1862 7159.2655 8352 4349 0545 71_42 4140 2537 63 35.00 temp C69 2966 1062 7159.2655 8352.4349 0545 7142 8440 25 37 63 tube cooling __ flow kg/s822245004500_ 45004500450045004500_42004200 4200 Demister ioss_ bar0 00030 00040 0005_0 00010 00010.00010 00010.00010.00010.0001 0 0001 DemisteNoss C0 02270 03160 04100.01270 01630 01710.02200 02460 02320. 0254 0.0242 rise C1 0140 9980.9820.9650.94720 92980.91230.89470.88220. 8685 0 8545 tube bundle non- bar0 01180.02100 02760.01190.01540 02010. 02630 03440 02740 0350 0.0450 equdlbrium ioss C0 19910.21870 23920 26460 29080 32030 35350.39090.29280.3554 0.4181 heat transfer coefflcent 3218307930362996295629152873282926462612 2577

Claims (17)

Fir ' /? - 7 (- CLAIMS

1. A process for the desalination of salt water comprising the steps of: a) providing a heat exchanger for heating brine; b) providing in series connection an upstream desalination zone, a downstream desalination zone, and an intermediate desalination zone, each desalination zone comprising a condensation conduit connecting the zones in series and a product water channel connecting the zones in series and arranged to collect condensate from the condensation conduit; c) providing a heat pump; d) supplying a feed stream comprising salt water as a coolant to the condensation conduit to pre-heat the feed stream; e) supplying the pre-heated feed stream to the heat exchanger further to heat the pre-heated feed stream; f) supplying the heated feed stream from the heat exchanger to the upstream desalination zone in the series, evaporating at least a portion of the heated feed stream in the upstream desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the upstream desalination zone; g) recovering from the upstream desalination zone a product water stream comprising the condensate and a depleted feed stream comprising salt water; h) supplying the product water stream to the product water channel of the intermediate desalination zone; i) supplying the depleted feed stream to the intermediate desalination zone, evaporating at least a portion of the depleted feed stream in the intermediate desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the intermediate desalination zone; I) withdrawing from the intermediate desalination zone a portion of the evaporate, and supplying said evaporate to the heat pump; k) cooling and condensing the withdrawn evaporate through the heat pump and returning the cooled, condensed evaporate to the product water channel of the intermediate desalination zone; 1) withdrawing from the intermediate desalination zone a recycle portion of the depleted feed stream and supplying said recycle portion to the condensation conduit upstream of the intermediate desalination zone; m) recovering from the intermediate desalination zone a product water stream comprising the condensate and a further depleted feed stream comprising salt water; n) supplying the product water stream from the intermediate desalination zone to the product water channel of the downstream desalination zone; o) supplying the further depleted feed stream from the intermediate desalination zone to the downstream desalination zone, evaporating at least a portion of the further depleted feed stream in the downstream desalination zone to provide an evaporate comprising water vapour, condensing the evaporate on the condensation conduit and collecting at least some of the condensate in the product water channel in the downstream desalination zone; and p) recovering a product stream from the product water channel in the downstream desalination zone.

2. A process according to claim 1 wherein successive desalination zones in the series are maintained at successively lower pressures from the upstream end of the series to the downstream end thereof.

3. A process according to claim 1 or claim 2 wherein successive desalination zones in the series are maintained at successively lower temperatures from the upstream end of the series to the downstream end thereof.

4. A process according to any one of claims 1 to 3 wherein the upstream desalination zone comprises a plurality of desalination stages, themselves connected in series and/or in parallel.

5. A process according to claim 4 wherein a plurality of upstream desalination stages are provided in series, and successively lower temperatures and/or pressures are maintained between one or more neighbouring pairs of upstream desalination stages.

6. A process according to any one of claims 1 to 5 wherein the intermediate desalination zone comprises a plurality of intermediate desalination stages, themselves connected in series and/or in parallel.

7. A process according to claim 6 wherein the evaporate is withdrawn from one or more intermediate desalination stages and is returned, after cooling and condensing through the heat pump, to the product water channel downstream of the stage from which it was withdrawn

8. A process according to claim 6 or claim 7 wherein the depleted feed stream is withdrawn from one or more intermediate desalination stages and is supplied to the condensate conduit upstream of the stage from which it was withdrawn

9. A process according to any one of claims 6 to 8 wherein the evaporate and the depleted feed stream are withdrawn from the same intermediate desalination stage.

10. A process according to any one of claims 6 to 9 wherein the evaporate and the depleted feed stream are withdrawn from different intermediate desalination stages.

11. A process according to any one of claims 6 to 10 wherein a plurality of intermediate desalination stages are provided in series, and successively lower temperatures and/or pressures, are maintained between one or more neighbouring pairs of intermediate desalination stages.

12. A process according to any one of claims 1 to 11 wherein the downstream desalination zone comprises a plurality of downstream desalination stages, themselves connected in series and/or in parallel.

13. A process according to claim 12 wherein a plurality of downstream desalination stages are provided in series, and successively lower temperatures and/or pressures, are maintained between one or more neighbouring pairs of downstream desalination stages.

14. A process according to any one of claims 1 to 13 wherein the intermediate desalination zone is maintained at a temperature and/or pressure to intermediate that of the temperature and/or pressure of the upstream and downstream desalination zones.

15. A process according to any one of claims 1 to 14 wherein the intermediate desalination zone is configured for reduced material flow with respect to the upstream desalination zone in one or more of the condensation IS conduit, the product water channel and in the part of the zone where flashing occurs.

16. A process according to claim 15 wherein the intermediate desalination zone is smaller than the upstream desalination zone.

17. A plant for the desalination of salt water comprising means for operating a process according to any one of claims 1 to 16.