GB2369783A

GB2369783A - Desalination process

Info

Publication number: GB2369783A
Application number: GB0117455A
Authority: GB
Inventors: Paul Michael Willson; George Andrew Atkinson
Original assignee: PB POWER Ltd; PBC International Inc
Current assignee: PB POWER Ltd; PBC International Inc
Priority date: 2000-10-21
Filing date: 2001-07-18
Publication date: 2002-06-12
Anticipated expiration: 2021-07-18
Also published as: GB0117455D0; GB2369783B; JO2223B1; DZ3474A1; AU2001270867A1; KR20030041854A; KR100783686B1; EG22839A; WO2002032813A1; MA25954A1; EA004968B1; EA200300493A1

Abstract

In a multi-stage desalination process, the pre-heated recirculated feed stream is heated in brine heater 1 and then released to flash in first desalination stage via valve 3. The feed stream is flashed through a series of desalination zones 4, the steam condensing on tubes 5 cooled by the recirculated feed stream. Product water is collected in water trough 6, and flashes down between stages to heat rejection stage(s) 7. Feed stream is recirculated by pumps 8 through desalination and heat rejection zones. Non-condensible gases are extracted by ejectors 9. Condensing sections of heat exchange rejection stages are cooled by sea water 11. Warm sea water is recycled <I>via</I> deaerator 12 and make up pump 13. The thermal recycle stream may be a portion of the product stream, a portion of the depleted feed stream or the feed stream before it is supplied to the first heat exchange zone.

Description

DESCRIPTION

PROCESS AND PLANT

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

Conventional desalination plants operate according to a multi-stage flash (MSF) process. Flashing is the process whereby water vapor is evaporated from salt water and the resulting water vapor 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.

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 sea water. 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 vapor 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 minimize the consumption of energy from fuel and to produce the power and water products at the lowest possible costs.

According to the present invention there is provided a process for the desalination of salt water comprising the steps of: a. providing a brine heater; b. providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; c. providing a heat exchanger; d. supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; e. supplying the pre-heated feed stream to the brine heater; f. supplying a first heating stream to the brine heater further to heat the pre-heated feed stream; g. optionally recovering at least a portion of the first heating stream from the brine heater; h. supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the

desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; i. Recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water; j. supplying as a thermal recycle stream a portion of the product stream to the heat exchanger; k. supplying a second heating stream, optionally comprising at least a portion ofthe recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; and 1. supplying the heated thermal recycle stream to the at least one desalination zone.

The process of the invention improves the thermal efficiency of the desalination processes compared to conventional desalination plants. Supplying the heated thermal recycle stream to the desalination zone is an effective means of returning heat (which may otherwise be wasted) to the desalination zone and thereby reducing the overall energy requirements of the plant.

In another process according to the invention, the thermal recycle can be provided by the depleted feed stream rather than the product stream. The feed stream for thermal recycle can be taken from the desalination zone. Thus, there is further provided in accordance with the invention a process for the desalination of salt water comprising the steps of: a. providing a brine heater,

b. providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; c. providing a heat exchanger; d. supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; e. supplying the pre-heated feed stream to the brine heater; f. supplying a first heating stream to the brine heater further to heat the pre-heated feed stream; g. optionally recovering at least a portion of the first heating stream from the brine heater; h. supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; i. recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water; i. supplying as a thermal recycle stream a portion of the depleted feed stream to the heat exchanger; k. supplying a second heating stream, optionally comprising at least a portion of the recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; and 1. supplying the heated thermal recycle stream to the at least one

- s - desalination zone.

Alternatively (or as well), the thermal recycle stream may be taken from the feed stream before it is supplied to the brine heater. Accordingly, the invention further provides a process for the desalination of salt water comprising the steps of: a. providing a brine heater; b. providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; c. providing a heat exchanger; d. supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; e. supplying a first portion ofthe feed stream as a pre-heated feed stream W tne brine heater; f. supplying as a thermal recycle stream a second portion of the teed strewn to the heat exchanger; g. supplying a first Realms stream lo me Drine neater Iurtner lo neat me in_.._v ted feed stream; h. optional!' 1 UVV 1111 aL 1 avL a U1 LlUll U1 Lll LlivL ll vv L111 vLliv ll LlV111 the brine heater; i. supplying a second heating stream, optionally comprising at least a r of the recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; j. S ^ J A. A 7 Ace_ ant._ ___ V _. _. ^ - _ - _ _ _ ____ ___ ____ __ __ ___ _ _ __ _ _ _ _

- 6 thermal recycle stream from the heat exchanger to the at least one desalination zone, evaporating at least a portion ofthe heated feed stream and the heated thermal recycle stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate in the desalination zone; k. recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water.

In step j if more than one desalination zone is present the desalination zone to which the heated feed stream and the heated thermal recycle stream are respectively supplied may be the same or different.

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 zone.

Preferably, a plurality of desalination zones are provided, arranged in series so that the heated salt water can flash down progressively through a series of desalination zones. Preferably, successive desalination zones in the series are maintained at successively lower pressures. The flashing steam in each desalination zone is condensed on the condenser and the condensate is collected. Preferably, the condenser comprises at least one pipe carrying a coolant and the collecting means comprises a product water trough in which the condenser tube is nested. In this case 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 the

flashed steam being recovered as water in the product trough.

Preferably, the first heating stream supplied to the brine heater comprises steam. This may be supplied from associated steam-raising plant.

The process of the invention may utilise a single heat exchanger to heat the thermal recycle stream. Alternatively, a plurality of heat exchangers may be used.

In this case, the plurality of the heat exchangers may be connected in series, or in parallel, as desired.

Preferably, the coolant for the condenser tubes is unheated feed stream. The feed stream itself may comprise make-up feed from, say, sea water, and a recycle from the desalination zone. In this case the condensing tubes in each desalination zone section are cooled by recirculating and/or make-up feed stream which flows through the zones in the opposite direction to the flashing feed stream. The recirculated feed stream is thus progressively preheated prior to the brine heater.

After passing through the brine heater, 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 brine heater to stop the feed stream boiling inside the heat exchange zone and also to avoid any possible steam leakage into the feed stream since the steam may be contaminated with traces of toxic chemicals used for boiler water treatment. These could otherwise contaminate the MSF unit product water in case of a brine heater tube leak.

In one preferred process the condensing tubes in the coldest stages of the MSF unit, the heat rejection section, are cooled by a sea water circulation to reject

the waste heat from the cycle and maximize fresh water production. The cool feed stream from flashing in the lowest pressure stage is recirculated through the unit by large pumps. The product water is replaced by make-up from the sea water outlet from the hottest heat rejection stage which is deaerated and returned to the lowest pressure stage. The recirculating 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 sea water discharge.

Each stage of flashing of the feed stream may result in some noncondensible 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 ofthe MSF unit. Similar provisions for the brine heater help to ensure that its heat transfer performance is maintained by proper extraction of non-condensible gases. These systems mean that the product water and the second heating stream have a very low level of dissolved gases. The design of the MSF desalination units includes many alternative possibilities of configuring each stage of heat exchange 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 recirculating and/or make-up feed stream, the so called long tube design, or may 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 processes 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 brine heater 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 cupro-

nickel, brass, titanium and various grades and specifications of stainless steel in order

to tolerate the chemically aggressive hot sea water flowing at the velocities necessary for optimum heat transfer. The heat transfer surface is preferably kept as free as possible of internal deposition of scaleforming insoluble minerals from the hot sea water 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.

In a conventional MSF plant, the amount of heat exchange tube surface in each stage and the number of stages detennines the amount of steam required per unit of product delivered by an MSF plant. Significant increases in heat transfer surface are required to produce a reduction in specific steam consumption, a 5% reduction in specific steam consumption typically requiring a 15% increase in heat transfer surface at significant additional cost.

Conventional plants include a brine heater which uses thermal energy from saturated steam at low pressure and temperature, typically with a saturation temperature below 115 C, to heat the feed stream prior to flashing in the first stage

- 10 of the MSF unit. The water condensed in the brine heater from the steam supply is returned to the steam-raising facility by a pump at a temperature close to the saturation temperature of the steam in the brine heater.

In the process of the invention, the first heating stream may comprise steam from a steam raising power plant and in this case the process of the invention may include the additional step of recovering the second heating stream from the heat exchanger and returning the recovered stream to the steam raising power plant. In this case, the temperature of the return condensate to the steam-raising plant determines the lowest temperature to which that plant can cool the boiler exhaust gases. In practical heat recovery steam generators a margin of 15 to 20 C between the stack gases and the water temperature of the returned condensate would be conventional. This results in a stack temperature for a power and water production plant of this type in excess of 130 C. Where gas is used as the fuel for such a power and water production plant a stack temperature below 100 C would be feasible but for the higher water return temperature. The result of the higher stack temperature is a reduced efficiency of thermal energy use by the whole process, representing several percent additional heat input being required, incurring considerable cost and consumption of fuel over the life of the plant.

The process of invention therefore provides a process for improving the energy consumption of a desalination plant and of a combined power and multi-stage desalination plant. The process of the invention provides a power and desalination facility with a reduced consumption of fuel energy for a relatively small increase in

- 11 overall cost. The invention reduces the steam demand of the MSF desalination unit for a given output of fresh water, reducing the capacity, size and cost of steam-raising plant and any associated steam turbines, steam pipe work, valves and support structures. The process of the invention enables an improvement in steam consumption by the MSF unit without increasing the number of stages in the MSF unit or changing heat transfer surface requirements within the MSF unit. The process of the invention achieves a reduction in the steam consumption at a lower cost than the alternatives of increasing the heat transfer surface or number of stages. The invention has only slight effects on the operation of the associated MSF unit whose design can conveniently be adapted to accommodate the connections for the thermal recycle stream and connection of the heat exchanger in the second heating stream outlet or outlet pipe work. The invention may thus be applied readily to existing MSF units with minimum modification. The heat exchanger in the second heating stream requires only modest surface area to achieve the desired benefits and, when the thermal recycle stream is product water, can be constructed of cheaper, easier to fabricate materials than for sea water since the water on both sides of the exchanger is fresh and contains very low levels of dissolved oxygen. The invention avoids any potential contamination of product water by traces oftoxic boiler treatment chemicals in the second heating stream by ensuring isolation ofthe condensate from the product water even after heat exchange tube failure by necessitating a pressure of product water in the heat exchanger above the pressure of the brine heater before any product water can be recirculated into the MSF unit. The invention introduces a thermal

- 12 recycle stream which does not affect operation of the associated MSF unit except in terms of steam consumption; break down or failure of any part of the additional water circuit only affects steam consumption and has no adverse effect on the continuity of output of the MSF unit, which is often a critical requirement.

The recovery of heat from the second heating stream in the heat exchanger may be maximized by the use of a heat exchanger of low log mean temperature difference (LMTD). Where the thermal recycle stream comprises product water the clean water involved in the heat exchange permits a plate and frame heat exchanger to be used to give high heat transfer performance with low LMTD at an economic price. Alternatively, a modestly sized tubed heat exchanger permits good heat recovery with a reasonable surface area. The optimum range of LMTD for the heat exchanger is between 3 and 20 C.

The heat exchanger may be located externally to the brine heater with any recovered first heating stream (which may be condensate) draining or being pumped into the heat exchanger as all or part of the second heating stream. The second heating stream may comprise hot water from other sources or from elsewhere in the process. Alternatively, the heat exchanger may be integrated with the brine heater with suitable shrouding. The internal mounting arrangement eliminates connections and the separate heat exchange casing whilst simplifying construction and installation works. The flow of the thermal recycle stream relative to the second heating stream affects the heat recovery from second heating stream. The ratio of these flows is

- 13 preferably in the range of from about 0.5 to about 1. 15, more preferably in the range of from about 0.7 to about 1. 15, most preferably in the range of from about 0.85 to about 1.15.

Where the thermal recycle stream is taken from the desalination zone, the stage of abstraction of the stream from the zone determines the lowest temperature of return of the second heating stream. When more than one desalination zone is used, as is the case in a preferred process according to the invention, the lowest point for abstraction is the lowest pressure desalination zone of the MSF unit. The highest point is the next stage lower in pressure than the stage to which the thermal recycle stream is returned. Taking the thermal recycle stream from a higher pressure stage increases the lowest return temperature of the second heating stream, which may be advantageous in optimizing the performance and cost of the combined power and water plant, although it reduces the benefit to the steam consumption of the MSF unit. In a preferred process according to the invention using a plurality of desalination zones, the zone to which the thermal recycle stream is returned to the MSF unit affects the effectiveness of the heat transfer to the desalination zone. The optimum stage operates at a pressure just below the saturation pressure of water corresponding to the temperature of the thermal recycle stream.

The prevention of the second heating stream, which may be contaminated with traces of toxic boiler treatment chemicals, from mixing with the thermal recycle stream is desirable. The heat exchanger ensures this segregation under normal

- 14- conditions. The segregation can be maintained even if there is a leakage in the heat exchanger if the pressure in the thermal recycle side is maintained above that in the second heating stream side. This can be assured by a pressure sustaining device set to the maximum brine heater pressure in the thermal recycle stream circuit between the heat exchanger outlet and the return port on the MSF unit. This device may be a control valve and associated measuring control means or may be a weir in a vessel vented to the return stage and located in the circulating water circuit at a height sufficient to ensure that the sum of the return stage pressure and the static head of water below the weir lip always exceeds the brine heater pressure.

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 first cross section of a conventional cross-flow MSF unit; Figure 2a shows a second cross-section through a-a of Figure 2; Figure 3 shows a flow diagram of a multi-stage flash desalination plant arranged to operate in accordance with a first process according to the invention; Figure 4 shows a cross-section illustrating one arrangement for extraction of product water as thermal recycle stream from the product trough; Figure 5 illustrates one external arrangement for the heat exchanger; Figure 6 illustrates an alternative heat exchanger arrangement within the brine heater shell;

- 15 Figure 7 illustrates the arrangement of the connection to return the recirculated hot thermal recycle stream to a higher temperature stage; Figure 8 shows a flow diagram of a multi-stage flash desalination plant arranged to operate in accordance with second and third processes of the invention; Figure 9 illustrates alternative connections for the abstraction of depleted feed stream for thermal recycle.

Figure 10 shows the external arrangement of the heat exchanger when feed stream is recirculated.

Figure 1 1 illustrates the arrangement for return of hot recirculated feed stream to a higher temperature stage.

The basic arrangement of the multi-stage desalination process is shown in Figure 1. The pre-heated recirculated feed stream is heated in the brine heater 1 before being released to flash in the first desalination stage via the pressure sustaining valve 3. The feed stream is flashed down through a series of desalination zones 4 with the steam being condensed on tubes 5 cooled by the recirculated and make-up feed stream. The product water is collected in the product water trough 6 and flashes down between stages successively to the heat rejection stage 7. There may be several such heat rejection stages cascaded as for the desalination zones. The feed stream is recirculated by pumps 8 through the desalination zones and the heat rejection stages.

Non-condensible gases are extracted and cascaded down the stages and extracted by ejectors 9 at the lowest pressure stage with sea water cooled condenser 10. The condensing sections of the heat rejection stages are cooled by sea water 11. The

- 16 warm sea water return is used to provide make up for the cycle via the deaerator 12 and the make up pump 13.

Figures 2 and 2a illustrate the physical arrangement of a desalination stage of a conventional MSF unit. The feed stream enters the stage via a weir 14. The steam flashes off end flows through demister pads 15 and is condensed on the tube bundle 16 cooled by the feed stream. The product water is collected in the product tray 17 and flows through into the product trough 18 from which it flows via a weir into the product trough of the next stage. Non-condensible gases released from the flashing feed stream are extracted via the vent line 19 to the ejector system.

Figure 3 shows how the MSF cycle is modified by the invention when product water is used as the thermal recycle stream. The condensate from the brine heater 1' flows through heat exchanger 147 before being returned to the steam raising plant. A partial recirculation of the product water is drawn from the product water trough of a lower pressure desalination stage of the MSF unit 3'. This product water is delivered by a pump 145 to the secondary side of heat exchanger 147. The flow from the heat exchanger outlet is maintained at pressure by a pressure sustaining valve 150 or weir device 151, where alternative piping connections are shown with broken lines, before being returned to the product trough of a higher pressure stage desalination zone 121 ofthe MSF unit.

Figure 4 illustrates the connection to the product water channel 18 by means of a hot well 20 from which the product water is extracted.

Figure 5 shows the arrangement of the brine heater 126 elevated above

- 17- ground level and adjacent to the desalination section of the MSF unit 120. The heat exchanger 147 is in the form of a tubed heat exchanger 21, and would be located at ground level adjacent the brine heater 126 and piped to the second heating stream hot well 22 with the second heating stream flowing through the heater exchanger shell before discharge to associated steam raising plant (not shown). The thermal recycle stream from a relatively lower pressure desalination zone is piped to end 146 of heat exchanger 147 and is heated through heat exchanger 147 and then piped in line 148 to the product trough of a relatively higher pressure desalination zone. A plate and frame heat exchanger would be arranged in a similar way.

Figure 6 illustrates the arrangement of the brine heater 126 enclosing the heat exchanger 147'. The tube bundle is within the outer shroud tube which guides the second heating stream from the heat exchanger inlet 23 to the discharge 24 where the second heating stream is piped through the brine heater shell. The recirculated thermal recycle stream is pumped into the heat exchanger at the second heating stream outlet end 146' while the hot thermal recycle is piped from the other end 148' to the return connection to the product trough.

Figure 7 shows the connection returning the hot water product to the feed stream trough 25 of a higher temperature stage of the MSF unit. The recirculated product water is delivered by the external connecting pipe 154 to a distribution box and weir 151 or sparge pipe arrangement from which it is discharged above the level of the product water level in the trough.

Figure 8 shows how the MSF cycle is modified by the invention when feed

- 18- stream is recirculated. The condensate from the brine heater 1', supplemented by a hot water from the thermal cycle 154', flows through the heat exchanger before being returned to the steam raising plant (not shown). A partial recirculation of the feed stream from the desalination stage 3' is drawn either from the feed stream channel of a lower pressure stage and pumped forward or alternatively drawn from the main recirculating and/or make-up feed stream flow between the condensing tube nests of the stages where alternative piping connections are shown in broken lines.

Whichever way the feed stream is drawn from the lower pressure stage it is piped to the secondary side of the heat exchanger. The flow from the heat exchanger secondary side outlet is maintained at pressure by a pressure sustaining valve 150' before being returned to the feed stream channel 149' of a higher pressure stage of the desalination section of the MSF unit.

Figure 9 shows the alternative arrangements to abstract feed stream from She desalination stage of the MSF units. Feed stream is exkacted from the feed stream channel 26 via a hot well 27. Alternatively the feed stream is extracted from the main recirculating and/or make-up feed stream loop pipe or connections to it 28. Figure 10 illustrates the arrangement of the brine heater 29 elevated

above ground level adjacent the desalination section of the MSF unit 30. The multi-pass tubed heat exchanger 31 Is located at ground level adjacent to the brine heater and pumped to the second heating stream hot well 32 with the condensate flowing in turn through the passes in each of the heat exchanger shells before discharge to the steam raising plant (not shown). The cool recirculated feed stream is piped to the tube side

- 19 of the first pass of the heat exchanger adjacent to the condensate outlet 33 and the outlet of the final pass of the tube side of the heat exchanger 34 is piped to the return connection to the feed stream channel.

Figure 1 I shows the return connection for the feed stream to the feed stream channel 35. A sparge pipe or distributor box 36 returns the hot feed stream to the channel, distributing it over a part of the width of the feed stream channel.

It will be appreciated that the configuration of plant, pipework, control valves, pumps, release valves, flow controllers and other items of standard equipment shown are illustrated by way of example only and that the process and plant of the invention are not limited to the configurations shown.

The invention will now be more particularly described by way of the following Examples. Example 1 is of a process in accordance with the invention in which a portion of the product water is recirculated as a thermal recycle stream.

Example

Referring to Figure 3, a sea water feed stream is supplied in line 100 at a rate of 6638.9 kg/s and temperature of 35 C. The sea water salinity is 4.45%. A portion of the sea water in line 100 is fed in line 101 as a coolant to an air ejector 102. Air ejector/condenser 102 is supplied in line 103 with a mixture of water vapor and non-

condensible gases (principally air) extracted by extractor 104 from heat rejection stage 105. Non-condensible gases are discharged in line 106a and any residual condensible materials are discharged from air ejector/condensor 102 in line 106. In this example, the flow rate of sea water in line 101 is 250 kg/s.

- 20 The remaining sea water from line 100 (at a flow rate of 6388.9 kg/s) proceeds in line 107 as a coolant in heat rejection stage 105. The coolant sea water in line 107 is treated through treat rejection stage 105 to a temperature of 42.68 C and a portion of this (at a flow rate of 1819.5 kg/s) is supplied in line 108 as make-up.

The remaining sea water in line 107 is discharged from the plant in line 109.

Make-up sea water in line 108 passes through de-aerator 1 10. Extracted air is returned in line 111, 112 and 113 to extractor 104 and then on in line 103 to air ejector/condensor 102.

De-aerated make-up sea water passes on from de-aerator 110 in line 114 and is pumped through pump 115 into line i i 6 and is joined in line 117 by a recycle stream of brine from line 143. In this example, the combined r ke-up and recycle StreaAmA in line 117 flows at a rate of 6194.4 kg/s and has a temperature of 42.057 C and a salinity of 6.28%.

The combined make up and recycle stream in line 1 17 W1A11 now be called the land stream.

The feed stream "n line 1 1 / passes on as cooing IO me cL,rl AcA^5cr lunch 1 1 o 1 a desalination zone 119. Desalination zone 119 is the last in a series of desalination zones. In Figure 3 the first desalination zone in the series is shown as l 20, the second desalination zone in the series is shown as 121 and split lines 122 Indicate the presence of further similar desalination zones not actually depicted in Figure 3.

[ 1- A- A _ 111 O BAAS _

- 2 1 -

condenser tubes 123 of desalination zone 121 and then into the condenser tubes 124 of desalination zone 120. The pre-heated feed stream passes on in line 125 at a temperature of 102.829 C to brine heater 126. Brine heater 126 is supplied in line 127 with steam at a flow rate of 76.356 kg/s, a temperature of 130 C and a pressure of 2 bar. The heated feed stream exits brine heater 126 via line 128 and flow controller 129 at a temperature of 110 C and a pressure of 2 bar. The heated feed stream in line 128 passes on to first desalination zone 120.

First desalination zone 120 comprises a bosom zone 130 for receiving the flashing brine, a demister 131 through which evaporate from the flashing brine passes before condensing on tubes 124 and being collected in product water Boughs 132.

The flashing brine and product water cascade down through the desalination zones in parallel in lines 133 and 134 respectively. After exiting the final desalination zone 119, the flashing brine passes in line 135 to heat rejection stage 105. The product water passes in lines 136 and 136a to the product water trough 137 of heat rejection stage 105.

As with the desalination zones, heat rejection stage 105 may comprise a series of similar zones arranged in series.

In heat rejection stage 105, sea water supplied in line 107 is used as the coolant and the final product collected in product trough 137 is supplied to storage in line 138. The remaining brine in heat rejection stage 105 is discharged in line 139 and a portion is then re-circulated via pump 140, line 141 and line 143 as a recycle stream to line 1 17. The remaining portion is discharged via pump 140, line 141 and

- 22 line 142.

From the last desalination zone 119, a thermal recycle stream is taken in accordance with the invention. A portion of the product water in line 136 (76 kg/per second in this example) is extracted in line 144 through recycle pump 145 and into line 146 at a temperature of 48.20 C and a salinity of 0%. The thermal recycle stream in line 146 passes into the tubes of heat exchanger 147 and passes on in lines 148 and 149 to the product water trough of desalination zone 121. The heated thermal recycle stream is maintained at pressure by a pressure sustaining valve 150 in this example. Alternatively, shown in dotted lines in Figure 3, a weir device 151 may be used to sustain the pressure of the heated thermal recycle stream.

Heat exchanger 147 is supplied with a heating stream from the bottom of brine heater 126 in lines 152 and 153. Alternatively, or as well, a heating stream from an external source (for example associated steam raising plant) may be supplied in the line 154. Steam or hot water is removed from the system in line 155 Table 1 shows a number of parameters of this Example 1 in each of 20 stages of a process according to the invention. In this Example, the 20 stages comprise the brine heater, 16 desalination zones and 3 heat rejection stages. The measured parameters are as follows: A is the feed stream temperature (in C) at the inlet to each stage B is the feed stream temperature (in C) at the outlet of each stage C is the feed stream flow though each stage (kg/s) D is the flow rate (in kg/s) of the flashing brine flowing out of each stage

- 23 P is the pressure (in bar absolute) in each stage m is the production rate (kg/s) of product water in each stage M is the additive production rate (kg/s) in total of product water at the end of each stage.

- 24 Table 1

brine temp brine temp Tubeside flashing stage stage total tube inlet tube outlet cooling brine out pressure production product stage flow of stage flow A B C D m M C C kg/s kg/s bar kg/s kg/s BH 102.83 110 00 6194.4 6194. 4 1.668 0

1 98.97 102.83 6194.4 6153.2 1.210 41.15 41.15

2 95.11 98.97 6194.4 6112.9 1.059 40.37 157.52

3 91.27 95.11 6194.4 6073.1 0.924 39.80 197.32

4 87.41 91.27 6194.4 6033.1 0.802 39.97 237.29

5 83.56 87.41 6194.4 5993.7 0.695 39.38 276.67

6 79.72 83.56 6194.4 5954.9 0.601 38.83 315.49

7 75.90 79.72 6194.4 5916.8 0.519 38.10 353.60

8 72.08 75.90 6194.4 5878.9 0.446 37.90 391.50

9 68.26 72.08 6194.4 5842.0 0.382 36.90 428.40

10 64.49 68.26 6194.4 5806.1 0.328 35.91 464.31

11 60.61 64.49 6194.4 5769.4 0.278 36.69 501.00

i2 56.72 60.6i 6i94.4 5733.i 0.235 36.32 537.32 13 52.94 56.72 6194.4 5698.4 0.198 34.70 572.02

14 49.32 52.94 6194.4 5665.5 0.168 32.87 604.89

15 45.60 49.32 6194.4 5630.3 0.140 35.19 640.08

16 42.06 45.60 6194.4 5597.8 0.117 32.55 672.63

39.94 42.68 6388.9 5571.5 O. ION 26.25 622.88

18 37.38 39.94 6388.9 5547.3 0.089 24.23 647.11

19 35.00 37.38 6388.9 5524.9 0.078 22.36 669.47

Overall in this Example the output ratio (kilograms of product water produced per kilogram of steam supplied to the system) is 8.795. A process plant operating in accordance with this Example is capable of producing 12.75 million imperial gallons of product water per day (approximately 60 million litres of water per day).

- 25 Example 2

Referring to Figure 8, a sea water feed stream is supplied in line 100' at a rate of 6638.9 kg/s and temperature of 35 C. The sea water salinity is 4.45%. A portion of the sea water in line 100' is fed in line 101' as a coolant to an air ejector 102'. Air ejector/condenser 102' is supplied in line 103' with a mixture of water vapor and non-

condensible gases (principally air) extracted by extractor 104' from heat rejection stage 105'. Non-condensible gases are discharged in line 106a' and any residual condensible materials are discharged from air ejector/condensor 102' in line 106'. In this example, the flow rate of sea water in line 101' is 250 kg/s.

The remaining sea water from line 100' (at a flow rate of 6388.9 kg/s) proceeds in line 107' as a coolant in heat rejection stage 105'. The coolant sea water in line 107' is heated through heat rejection stage 105' to a temperature of 42.68 C and a portion ofthis (at a flow rate of 1822 kg/s) is supplied in line 108' as make-up.

The remaining sea water in line 107' is discharged from the plant in line 109'.

Make-up sea water in line 108' passes through de-aerator 110'. Extracted air is returned in line 111', 112' and 113' to extractor 104' and then on in line 103' to air ejector/condensor 102'.

De-aerated make-up sea water passes on from de-aerator 110' in line 114' and is pumped through pump l l S' into line 116' and is joined in line 117' by a recycle stream of brine from line 143'. In this example, the combined make-up and recycle stream in line 117' flows at a rate of 6194. 4 kg/s and has a temperature of 42.057 C and a salinity of 6.28%.

- 26 The combined make up and recycle stream in line 117' will now be called the feed stream.

The feed stream in line 1 17' passes on as coolant to the condenser tubes 1 18' of a desalination zone 119'. Desalination zone 119' is the last in a series of desalination zones. In Figure 3 the first desalination zone in the series is shown as 120', the second desalination zone in the series is shown as 121' and split lines 122' indicate the presence of further similar desalination zones not actually depicted in Figure 8.

The feed stream passing through condenser tubes 118' passes on into the condenser tubes 123' of desalination zone 121' and then into the condenser tubes i24' of desalination zone 120'. The pre-heated feed stream passes on in line 125' at a temperature of 102.807 C to brine heater 126'. Brine heater 126' is supplied in line 127' with steam at a flow rate of 76.597 kgls, a temperature of i 30 w and a press-tire of 2 bar. The heated feed stream exits brine heater 126' via line 128' and flow controller 129' at a temperature of 110 C and a pressure of 2 bar. The heated feed stream in line 128' passes on to first desalination zone 120'.

First desalination zone 120' comprises a bottom zone 130' for receiving the flashing brine, a demister 131' through which evaporate from the flashing brine passes before condensing on tubes 124' and being collected in product water troughs 132'. The flashing brine and product water cascade down through the desalination zones in parallel in lines 133' and 134'. After exiting the final desalination zone 119', the flashing brine passes in line 135'to heat rejection stage 105'. The product water

- 27 passes in line 136' to the product water trough 137' of heat rejection stage 105'.

In heat rejection stage 105', sea water supplied in line 107' is used as the coolant and the final product collected in product trough 137' is supplied to storage in line 138'. The remaining brine in heat rejection stage 105' is discharged in line 139' and aportionisthenre-circulated vie pump 140', line 141' and line 143' as a recycle stream to line 117'. The remaining portion is discharged via pump 140', line 141'and line 142'.

From the last desalination zone 119', a thermal recycle stream is taken in accordance with the invention. A portion of the flashing brine in desalination zone 119' (85.49 kg/per second in this example) is extracted in line 144' (shown as a dotted line in Figure 8 because Figure 8 also depicts an alternative thermal recycle, as will be described in Example 3) through recycle pump 145' and into line 146' at a temperature of (49.89) C and a salinity of (6.94) %. The thermal recycle stream in line 146' passes through line 146a' into the tubes of heat exchanger 147' and passes on in lines 148' and 149' to the bottom of desalination zone 121'. The heated thermal recycle stream is maintained at pressure by a pressure sustaining valve 150' in this example.

Heat exchanger 147' is supplied with a heating stream from the bottom of brine heater 126' in lines 152' and 153'. Alternatively, or as well, a heating stream from an external source (for example associated steam raising plant) may be supplied in line 154'. Steam or hot water is removed from the system in lines 154' and 155'.

The following Table 2 shows a number of parameters of this Example 2 in

- 28 each of 20 stages of a process according to the invention. In this Example, the 20 stages comprise the brine heater, 16 desalination zones and 3 heat rejection stages.

The measured parameters are as follows: A is the feed stream temperature (in C) at the inlet to each stage B is the feed stream temperature (in C) at the outlet of each stage C is the feed stream flow though each stage (kg/s) D is the flow rate (in kg/s) of the flashing brine flowing out of each stage P is the pressure (in bar absolute) in each stage m is the production rate (kg/s) of product water in each stage M is the additive production rate (kg/s) in total of product water at the end of each stage.

- 29 Table 2

brine temp brine temp Tubeside flashing stage stage total | tube inlet tube outlet cooling brine out pressure production product stage flow of stage flow A B C D m M C C kg/s kg/s bar kg/s kg/s BH 102.81 110.00 6194. 4 6194.4 1.669 0 0

. 1 98.94 102.81 6194.4 6153.1 1.210 41.30 41.30

2 95.08 98.94 6194.4 6197.4 1.057 41.20 82.50

3 91.25 95.08 6194.4 6157.1 0.923 40.25 122.75

4 87.39 91.25 6194.4 6116.7 0.801 40.44 163.18

5 83.55 87.39 6194 4 6076.9 0.694 38.85 203.03

6 79.71 83.55 6194.4 6037.6 0.601 39.29 242.32

7 75.91 79.71 6194.4 5999.0 0.519 38.57 280.89

8 72.08 75.91 6194.4 5960.6 0.446 38.37 3 19.26

9 68.27 72.08 6194.4 5923.3 0.383 37.37 356.63

10 64.50 68.27 6194.4 5886.9 0.328 36.35 392.98

11 60.63 64.50 6194.4 5849.8 0.278 37.15 430.13

12 56.73 60.63 6194.4 5812.9 0.235 36.81 466.94

13 52.96 56.73 6194.4 5777.6 0.196 35.16 502.10

14 49.34 52.96 6194.4 5744.5 0.168 32.28 535.39

15 45.61 49.34 6194.4 5708.8 0.140 35.65 571.04

16 42.07 45.61 6194.4 5675.8 0.118 33.01 604.05

17 39.96 42.70 6389.9 5564.1 0.102 26.30 630.35

. 18 37.39 39.96 6388.9 5639.8 0.089 24.26 654.61

19 35.00 37.39 6388.9 5517.4 0.078 22.39 677.00

Overall in this Example the output ratio (kilograms of product water produced per kilogram of steam supplied to the system) is 8.865. A process plant operating in accordance with this example is capable of producing 12.90 million imperial gallons of product water per day (approximately 60 million litres of water per day).

- 30 Example 3

Example 3 was conducted similarly to Example 2. However, with reference to Figure 8 the thermal recycle stream from the desalination zone 119' is taken not from the flashing brine in said zone but from the feed stream brine in the condenser tubes 1 18'.

The thermal recycle stream is taken in line 144" (shown as a dotted line in Figure to indicate that it is an alternative to taking the recycle from the flashing brine in line 144', as described in Example 2). Thereafter, the recycle stream is supplied to line 1 46a' and then proceeds as in Example 2 Table 3 shows a number of parameters of this Example 3 in each of 20 stages of a process according to the invention. In this example, the 20 stages comprise the brine heater, 16 desalination zones and 3 heat rejection stages. The measured parameters are as follows: A is the feed stream temperature (irl C) at the inlet to each stage B is the feed stream temperature (in C) at the outlet of each stage C is the feed stream flow though each stage (kg/s) D is the flow rate (in kg/s) of the flashing brine flowing out of each stage P is the pressure (in bar absolute) in each stage m is the production rate (kg/s) of product water in each stage M is the additive production rate (kg/s) in total of product water at the end of each stage.

- 31 Table 3

brine temp brine temp Tubeside flashing stage stage total tube inlet tube outlet cooling brine out pressure production product stage flow of stage flow A B C D P m M C C kg/s kg/s bar kg/s kg/s BH 102.80 110.00 6121.45 6121.45 1.665 0

98.91 102.80 6121.45 6080.4 1.209 41.05 41.05

2 95.03 98.91 6121.45 6112.5 1.056 40.83 81.87

3 91.17 95.03 6121.45 6072.5 0.920 40.02 121.89

4 87.29 91.17 6121.45 6032.4 0.798 40.15 162.04

5 83.43 87.29 6121.45 5992.8 0.691 39.53 201.57

6 79.58 83.43 6121.45 5953.9 0.598 38.94 240.51

7 75.77 79.58 6121.45 5915.7 0.515 38.18 278.69

8 71.94 75.77 6121.45 5877.8 0.445 37.95 316.64

9 68.12 71.94 6121.45 5840.8 0.380 36.92 353.56

10 64.35 68.12 6121.45 5804.9 0.325 35.89 389.45

11 60.48 64.35 6121.45 5768.3 0.276 36.62 426.07

56.60 60.48 6121.45 5733.1 0.233 36.28 462.35

13 52.84 56.60 6121.45 5697.4 0.197 34.62 496.97

49.23 52.84 6121.45 5664.7 0.167 32.75 529.72

15 45.52 49.23 6121.45 5629.6 0.139 35.05 564.77

42.00 45.52 61ZI.45 5597.2 0.117 32.43 597.20

17 39.90 42.61 6388.9 5571.2 0.101 26.03 623.23

37.37 39.90 6388.9 5547.1 0.088 24.03 647.26

19 35.00 37.37 6388.9 5524.9 0.078 22.19 669.45

Overall in this Example the output ratio (kilograms of product water produced per kilogram of steam supplied to the system) is 8.868. A process plant operating in accordance with this example is capable of producing 12.75 million imperial gallons of product water per day (approximately 60 million litres of water per day).

Claims

t ', A': i & / - 32 CLAIMS

A process for the desalination of salt water comprising the steps of: (a.) providing a brine heater; (b.) providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c.) providing a heat exchanger; (d.) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e.) supplying the pre-heated feed stream to the brine heater; (I. ) supplying a first heating strewing comprising steam to the brine heater further to heat the pre-heated feed stream; (g.) optionally recovering at least a portion of the first heating stream from Me vine neater; (h.) supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; (i.) recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water; (j.) supplying as a thermal recycle stream a portion of the product stream to the heat exchanger; (k.) supplying a second heating stream, optionally comprising at

- 33 least a portion of the recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; and (1.) supplying the heated thermal recycle stream to the at least one desalination zone.
2. A process for the desalination of salt water comprising the steps of: (a.) providing a brine heater; (b.) providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c.) providing a heat exchanger; (d.) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e.) supplying the pre-heated feed stream to the brine heater; (f. ) supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; (g.) optionally recovering at least a portion of the first heating stream from the brine heater; (h.) supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; (i.) recovering from the desalination zone a product stream

- 34 comprising the condensate and a depleted feed stream comprising salt water; (I.) supplying as a thermal recycle stream a portion of the depleted feed stream to the heat exchanger; (k.) supplying a second heating stream, optionally comprising at least a portion of the recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; and (1.) supplying the heated thermal recycle stream to the at least one desalination zone.
3. A process for the desalination of salt water comprising the steps of: (a.) providing a brine heater, (b.) providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c.) providing a heat exchanger; (d.) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e.) supplying a first portion of the feed stream as a pre-heated feed stream to the brine heater; (f.) supplying as a thermal recycle stream a second portion of the feed stream to the heat exchanger; (g.) supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; (h.) optionally recovering at least a portion of the first heating

- 35 stream from the brine heater; (i.) supplying a second heating stream, optionally comprising at least a portion of the recovered first heating stream, to the heat exchanger to heat the thermal recycle stream; (I.) supplying the heated feed stream from the brine heater and the heated thermal recycle stream from the heat exchanger to the at least one desalination zone, evaporating at least a portion of the heated feed stream and the heated thermal recycle stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate in the desalination zone; (k.) recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water.
4. A process according to claim 1, wherein the heated thermal recycle stream is supplied to the condensate collecting means of the desalination zone.
5. A process according to claim 4, wherein the ratio of thermal recycle stream to condensate in the collecting means of the desalination zone is from about 0.85:1 to about 1.15:1.
6. A process according to any one of claims 1 to 5, wherein there is provided a plurality of desalination zones connected in series.
7. A process according to claim 6, wherein the thermal recycle stream is taken from a first desalination zone and the heated thermal recycle stream is supplied to a second desalination zone.

- 36
8. A process according to claim 7, wherein the second desalination zone is nearer in the series of desalination zones to the brine heater than is the first desalination zone.
9. A process according to claim 8, wherein the first desalination zone is maintained at a lower pressure than the second desalination zone.
10. A process according to any one of claims 1 to 9, wherein the thermal recycle stream is supplied to the heat exchanger at a pressure greater than that of the second heating stream supplied to the heat exchanger.
A process according to any one of claims 1 to 10, wherein the feed stream is supplied to the brine heater at a pressure greater than that of the first heating stream supplied to the brine heater.
12. A process according to any one of claims 1 to 1 1, wherein the temperature of the thermal recycle stream is not more than 5 C greater than the saturation temperature of water in the at least one desalination zone
13. A process according to any one of claims 1 to 12, comprising recovering at least a portion of the first heating stream from the brine heater.
14. A process according to claim 13, wherein the second heating stream comprises at least a portion of the recovered first heating stream.
15. A process according to any one of claims 1 to 14, wherein a plurality of heat exchangers for heating the thermal recycle stream are provided.
16. A plant for the desalination of salt water comprising: (a) a brine heater (b) at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c) a heat exchanger; (d) means for supplying a thermal recycle stream to the heat exchanger to heat the thermal recycle stream; and (e) means for supplying the heated thermal recycle stream to the desalination zone, wherein the plant is arranged for operating a process according to any one of claims lto 15.

16. A plant for the desalination of salt water comprising means for operating a process according to any one of claims 1 to 15.

Amendments to the claims have been filed as follows CLAIMS

1. A process for the desalination of salt water comprising the steps of: (a) providing a brine heater; (b) providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c) providing a heat exchanger; (d) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e) supplying the pre-heated feed stream to the brine heater; (f) supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; (g) supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; (h) recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water; (i) supplying as a thermal recycle stream a portion of the product stream to the heat exchanger; (I) supplying a second heating stream, to the heat exchanger to heat the thermal recycle stream, and (k) supplying the heated thermal recycle stream to the at least one

- - desalination zone.

2. A process for the desalination of salt water comprising the steps of: (a) providing a brine heater; (b) providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; (c) providing a heat exchanger; (d) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e) supplying the pre-heated feed stream to the brine heater; (f) supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; (g) supplying the heated feed stream from the brine heater to the at least one desalination zone, evaporating at least a portion of the heated feed stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate on the condenser in the desalination zone; (h) recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water; (i) supplying as a thermal recycle stream a portion of the depleted feed stream to the heat exchanger; (I) supplying a second heating stream, to the heat exchanger to heat the thermal recycle stream; and

- - (k) supplying the heated thermal recycle stream to the at least one desalination zone.

3. A process for the desalination of salt water comprising the steps of: (a) providing a brine heater; (b) providing at least one desalination zone comprising a condenser and means for collecting' condensate from the condenser; (c) providing a heat exchanger; (d) supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; (e) supplying a first portion ofthe feed stream as a pre-heated feed stream to the brine heater; (f) supplying as a thermal recycle stream a second portion of the feed stream to the heat exchanger; (g) supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; (h) supplying a second heating stream, to the heat exchanger to heat the thermal recycle stream; (i) supplying the heated feed stream from the brine heater and the heated thermal recycle stream from the heat exchanger to the at least one desalination zone, evaporating at least a portion of the heated feed stream and the heated thermal recycle stream in the desalination zone to provide an evaporate comprising water vapor and condensing the evaporate in the

4. desalination zone; (j) recovering from the desalination zone a product stream comprising the condensate and a depleted feed stream comprising salt water.

4. A process according to claim 1, wherein the heated thennal recycle stream is supplied to the condensate collecting means of the desalination zone.

5. A process according to claim 4, wherein the ratio of thermal recycle stream to condensate in the collecting means of the desalination zone is from about 0.85:1 to about 1.15:1.

6. A process according to any one of claims 1 to 5, wherein there is provided a plurality of desalination zones connected in series.

7. A process according to claim 6, wherein the thermal recycle stream is taken from a first desalination zone and the heated thermal recycle stream is supplied to a second desalination zone.

8. A process according to claim 7, wherein the second desalination zone is nearer in the series of desalination zones to the brine heater than is the first desalination zone.

9. A process according to claim 8, wherein the first desalination zone is maintained at a lower pressure than the second desalination zone.

10. A process according to any one of claims 1 to 9, wherein the thermal recycle stream is supplied to the heat exchanger at a pressure greater than that of the second heatir c, stream supplied to the heat exchanger.

A process according to any one of claims 1 to 10, wherein the feed

All stream is supplied to the brine heater at a pressure greater than that of the first heating stream supplied to the brine heater.

12. A process according to any one of claims I to 1 1, wherein the temperature of the thermal recycle stream is not more than 5 C greater than the saturation temperature of water in the at least one desalination zone 13. A process according to any one of claims 1 to 12, comprising recovering at least a portion of the first heating stream from the brine heater.

14. A process according to cla m 13, wherein the second heating stream comprises at least a portion of the recovered first heating stream.

15. A process according to any one of claims 1 to 14, wherein a plurality of heat exchangers for heating the thermal recycle stream are provided.