WO2002032813A1

WO2002032813A1 - Process and plant for multi-stage flash desalination of water

Info

Publication number: WO2002032813A1
Application number: PCT/GB2001/003234
Authority: WO
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: 2002-04-25
Anticipated expiration: 2003-04-21
Also published as: JO2223B1; AU2001270867A1; EG22839A; EA004968B1; GB2369783A; EA200300493A1; MA25954A1; GB0117455D0; KR100783686B1; KR20030041854A; DZ3474A1; GB2369783B

Abstract

The invention relates to a process for the desalination of salt water and to plant suitable for operating such a process. The process comprises: providing a brine heater; providing at least one desalination zone comprising a condenser and means for collecting condensate from the condenser; providing a heat exchanger; supplying a feed stream comprising salt water as a coolant to the condenser to pre-heat the feed stream; supplying the pre-heated feed stream to the brine heater; supplying a first heating stream comprising steam to the brine heater further to heat the pre-heated feed stream; 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 a evaporate comprising water vapor and condensing the evaporate in the desalination zone.

Description

DESCRIPTION

PROCESS AND PLANT FOR MULTI-STAGE FLASH DESALINATION OF..WATER

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 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.

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;

j . 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

recycle stream; and

1. supplying the heated thermal recycle stream to the at least one 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 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 to the brine heater further to heat the

pre-heated feed stream;

h. optionally recovering at least a portion of the first heating stream from

the brine heater;

i. supplying a second heating stream, optionally comprising at least a

recycle stream;

j . 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.

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

sfream 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 non-condensible

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 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 scale-forming 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 determines 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 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 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 of toxic boiler treatment chemicals

in the second heating stream by ensuring isolation of the 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 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 sfream 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 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 sfream, 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 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; 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

sfream is recirculated.

Figure 11 illustrates the arrangement for return of hot recirculated feed sfream

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 sfream 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 sfream 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 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 and 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 of the 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 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

sfream 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 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 the

desalination stage of the MSF units. Feed stream is extracted 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 sfream 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 sfream 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 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 11 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 1

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. 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

inline 107 is heated through heat rejection stage 105 to atemperature of42.68°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 110. Extracted air

is returned in line 111, 1 12 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 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 atemperature of 42.057°C

and a salinity of 6.28%.

The combined make up and recycle stream in line 117 will now be called the

feed stream.

The feed stream in line 117 passes on as coolant to the condenser tubes 118

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 3.

The feed sfream passing through condenser tubes 118 passes on into the 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 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 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 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 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 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.

Table 1

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). 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 atemperature of 42.68 °C

and a portion of this (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 115' into line 116' and is joined in line 117' by a recycle

sfream 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 of42.057°C

and a salinity of 6.28%. The combined make up and recycle stream in line 117' will now be called the

feed stream.

The feed sfream in line 117' passes on as coolant to the condenser tubes 118'

of a desalination zone 119'. Desalination zone 119' is the last in a series of

120', the second desalination zone in the series is shown as 121' and split lines 122'

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 124'

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 allow rate of 76.597 kg/s, atemperature 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 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 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 a portion is then re-circulated via 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 sfream 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 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

each stage.

Table 2

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). 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 118'.

The thermal recycle sfream is taken in line 144" (shown as a dotted line in Figure 8

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 sfream is supplied to line

146a' and then preceeds 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 sfream 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

each stage. Table 3

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

of product water per day (approximately 60 million litres of water per day).

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 sfream 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 sfream 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 of the recovered first heating sfream, to the heat exchanger to

heat the thermal recycle sfream; 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 sfream;

(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

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 depleted

feed sfream to the heat exchanger;

(k.) supplying a second heating stream, optionally comprising at

least a portion of the recovered first heating sfream, to the heat exchanger to

heat the thermal recycle stream; and

(1.) supplying the heated thermal recycle sfream 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 sfream;

(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 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 sfream;

(j .) supplying the heated feed stream from the brine heater and the

heated thermal recycle sfream 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 sfream 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.

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.

11. 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 11 , 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 sfream 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 means for operating a

process according to any one of claims 1 to 15.