MULTISTAGEFLASH

Inthefirstyearsofthe1950decade,theMSF

systemwasdevelopedbytheU.S.A.navy.

ItshowedamuchbetteroperationconditionsthanMED,speciallyinthescalingbehavior,andwasrapidlyadoptedasthestandardevaporationmethodduringthefiftiesandsixties.

Agroupofstagesinseries,workingatdescendingpressuresandtemperatures.

Evaporationandcondensationhappenineverystage,thus,onesinglestagecouldoperateindependently.

Everystagehastwosections:Evaporation(Wherevaporisproduced)andcondensation(wherethevaporcondensatesafteritscontactwith coldertubes).

� Condenser� Tubes� Brineheater� Condensate�  Flashchambers� Demister� Waterboxes� Distillate

Defini&ons

STEAM

CONDENSATE BACK TO BOILER

BRINE

SEAWATER

PRODUCT

COOLING WATER VAPOR

EVAPORATION

DEMISTER

Each stage consists of a flash chamber and a heat exchanger/condenser, in which vapour flashed off in the flash chamber is condensed. The flash chamber is separated from the condenser by a demister (to remove entrained brine droplets from the flashing vapour) and a distillate trough (to select the condensate from the condenser above).

Heat exchanger/Condenser

Flash-Chamber

Mul&StageFlash

Mul&Stageflash�  Steamisusedtoheattubesofsalinewater�  Heatedwaterflowsinto“stages”thatareatlowerpressure�  Waterboilsrapidlyand“flashes”intosteam

Theamountofvaporproducedisproportionaltothedifferenceoftemperaturebetweentwochambers.

Theamountofwaterproduceddecreasesaftereachstage,becausethedifferenceoftemperaturebetweenchambersdecreasesaswell.

Thevaporisoutsidethecondensationtubesandthewaterflowsinside.

Thetemperatureofthecondensationwaterincreasesstagebystageandthetemperatureoftheevaporationwaterdecreases.

Thetwochambersofthestageareseparatedbyafilternameddemister,toavoidthatwaterdropsarrivetothecondensationchamber,increasingtheconductivityoftheproduct.

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Process description: How did it begin? • It had long been known that water could be heated above its nor- mal boiling point in a pressurized system. • If the pressure was released, a portion of the water would boil off or “flash”. The remaining liquid water would be cooled as the is- suing vapor took with it its heat of vaporization. • Since evaporation occurred from the bulk fluid rather than at a hot heat exchange surface, opportunities for scaling would be reduced.

What flashing looks like • Hot brine from the previous stage enters through slot at lower temperature and pressure stage • It senses the new lower pressure environment, and • Flashes!

Inthissystem,vaporflowsoutsidethetubes.

Refrigerationseawaterflowsinsidethetubes.Fullactionhappensinonlyonechamber.

It’sjustthecontraryasMED.

The function of the rejection section is to reject thermal energy from the plant and to allow to the product water and brine to exit the plant at the lowest possible temperature. The feed water is mixed with the large mass of water, which is recirculated round the plant, known as the “brine recirculation” flow. Then the feed water passes through a number of heat exchangers (stages), raising its temperature.

In the recovery section the released by condensation of vapour is used to heat the recirculating brine flow, which is recirculated from the bottom stage of the rejection section.

After passing through the last stage of the recovery section the water is heated up to its terminal Temperature in the brine heater. The flow then passes through a restriction into the top flash stage where the reduction of pressure causes a small fraction to flash off as vapour, which then passes up through a demister into the condenser where the vapour is condensed. The distillate condensed in each of the condensers is collected in a distillate train. Vapour produced in the flashing chambers is then condensed on the tube surface, thus transferring the Latent heat to the preheating re-circulated brine and the distillate produced is dripped into a collector.

MSF Description

The vapor pressure in each of these stages is controlled so that the heated brine enters each chamber at the proper temperature and pressure to cause instantaneous and violent boiling/evaporation. The process is repeated stage by stage, with decreasing pressures and temperatures and increasing brine salt concentration. As the process continues right down to the bottom stage of the plant in the rejection section, a part is rejected as “blowdown” and the rest is mixed with the incoming make up (feed water) and then recycled Once again via the brine recirculation pump.

The distillate condensed in each of the condensers is collected in a distillate train. The brine recirculation

flow rate in MSF is about nine times the production flow.

� Theearetwokindsofarrangements:

� Oncethrough(Verysimpledesign)

� Brinerecirculation(Themostused)

Mul&Stageflashtypes

Once through Multi Stage Flash Distillation (MSF)

In once through MSF there is no specific heat rejection section. The feed directly enters the heat recovery section – is pre-heated, passing up through the condensers - is heated finally in the brine heater and then passes down through the flash chambers. At the bottom stage the total brine flow is rejected.

Pros:Lowscalingpotential.(Waterinsidecondensingtubesisseawater,notbrine)Higheroperationtemperatures(Brineislessconcentrated)Cons:Allfeedwatermustbepretreated(Biggerdegasifieranddecarbonator,higherchemicalsconsumption)Biggervacuumsystem

Brine recirculation Multi Stage Flash Distillation (MSF)

Source: SIDEM

The advantages of the “brine recirculation” configuration are that the seawater pre-treated is in the order of only one third of the once-through design, the majority of the tube bundles work with deaerated brine water with lower corrosion and the incondesable gases released are reduced thus achieving higher efficiency of the stages.

HEAT REJECTION SECTION HEAT RECOVERY

SECTION

Pros:SmallervacuumsystemSmallerpretreatmentHigheroperationalflexibilityCons:HigherpumpinginvestmentHigherpumpingenergyHigherthermalenergyHigherscalingrisk

Oncethroughvs.Brinerecircula&on

Item Oncethrough Brinerecirculation

Max.TemperatureºC

90 110

Recovery%

10---15 10---20

EfficiencyKg/mJ

3.4–4.3 3.4–5.2

Brinesalinityppm 58,000 62,500

MSF PLANT LAS PALMAS II

Each of the Once through and Brine circulation MSF processes can be also arranged as a “long tube” or “cross tube” design.

In the long tube design, tubing is parallel to the concentrate flow in the vessel. Tubing is perpendicular

to the concentrate flow in the cross tube design Most large modern MSF distillers are of the cross-tube design.

MSF Process Arrangements

Multi Stage Flash

Cross Tube (Shuweihat, 76500m3/day) Long Tube (Gela, 4x14400m3/day)

Source: Unipa, Italy

www.prodes-project.org

Source: Unipa, Italy

Multi stage flash

Courtesy of CPass 1

stage Pass 2

Multi-stageflashdis&lla&onAdvantages•Produc&on ofwater with goodquality (5 to 50ppmtds)•ROproduces10to500ppmtds

•MostexperienceisfromMiddleEastwhereenergyischeap.

Disadvantages•Requireslargeenergyinput•Highmaintenancerequirements•LargerspaceneededthatROplant

•Largeamountofwaternecessaryforproduc&onandcooling

EVAPORATION CHAMBER

Distillate tray, demister supports and interstage walls - .....-..

..

Tube bundle tube supports roof plates and incondensable extraction pipes

Details of tube bundle and tube support

MSF Process Characteristics

Source: Bureau of Reclamation

*

MSF Materials of Fabrication

Source: Bureau of Reclamation

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Section Material of Construction

Brine Heater Shell Carbon steel (all plants)

Tubes Either 70/30 o,90/10 Cu-Ni or modified 66/30/2/2 Cu/Ni/Fe/Mn except Al-Jubail I (Titanium)

Heat Recovery Section

Flash Chamber

•  First high temperature stages Al-Jubail, Al-Khafji and the first two modules of Jeddah IV cladded with stainless steel

•  Al-Khobar II completely cladded with 90/10 Cu/Ni •  Al-Shuqaiq 1completely claded with stainless steel

Tubes All plants except Yanbu and Al-Jubail I: 90/10 Cu Ni Jubail I: Titanuim Yanbu 70/30 (1 to 10 stages) 90/10 (11 to 21 stages)

Heat Rejection Tubes All plants except Jeddah & Shoaiba : Titanium Jeddah II, III, IV 90/10 Cu/Ni Shoaiba 70/30 Cu Ni

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Projects which were recently built use the following materials of construction for the major components

Flash chamber of both recovery and heat rejection sections

Carbon steel lined with stainless steel (floor lined with 317L, walls with 316L and roof with either 316L or

304. Water boxes Carbon steel lined with 90/10

Copper-Nickel

Tubes Brine heater tubes modified 66/30/2/2 Cu/Ni/Fe/Mn ; heat recovery tubes:

Copper/Nickel (first four stages 70/30 and remaining stages 90/10)

Heat rejection tubes Titanium & modified 66/30/2/2 Cu/Ni/Fe/Mn

back-pressure turbine arrangement

Condensate Pump

MSF Distillers

Deaerator

Heater # 2 Heater # 1

G

Boiler

Fuel

Back Pressure Turbine

Ejector Moisture Separator

Extraction / Condensing Turbine

Condenser

Condensate Pump Heater # 1

Deaerator

MSF

Boiler

Fuel

To Ejectors

Extraction- condensing turbine arrangement

Power to water ratio 12 to 15 MW/MIGD

Jeddah II,III,IV Al Jubail I Yanbu I Alkhobar II

Scaled tubes in a distillation plant

Energy consumption in large desalination processes

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Process Thermal energy kWh/m³

Electrical energy kWh/m³

Total energy kWh/m³

MSF 7.5 - 12 2.5 – 3.5 10 – 15.5

MED 4 - 7 1.5 - 2 5.5 - 9

SWRO - 3 - 4 3 - 4

BWRO - 0.5 - 2.5 0.5 - 2.5

Energy effect

In fact, as it can be seen from the energy flow diagram below, the great part of the heat input to the MSF system is returned back to the sea with the seawater drain stream.

Reasons Constant Reduction of Investment per MIGD • optimized use of material of construction. •Reduction of redundant equipment. •Optimized mechanical design of evaporator vessel. •Optimized thermo-dynamic design parameters.

8

6 5 4

14 12

12 10

4 2

2010 0

6

8

1985 1990 1995 year

2000 2004

$ / I

GD

Price Trend for turn-key complete MSF plants

Parameter Value

Production 100 MIGD (18,940 m3/h)

Performance Ratio (2326KJ/Kg) >11

Sea water temperature (°C) 35

Sea water salinity (ppm) 45,000

LP steam P/T (bar a)/(°C) 2,8 / 140°C

Maximum TBT for MSF (°C) 110

Max TT for MED (°C) 66

Auxiliary power cost (c$/kWh) 3.27

Thermal energy cost ($/GJ) 9.4

Plant Location Overall capacity m3/d Unit capacity (MIGD)

Year commissioned

Jebel Ali K2 (UAE) 181,800 13.33 2001

Ras Laffan A (Qatar) 214,000 11.2 2003

Fujairah 1 (UAE) 284,000 12.5 2004

Shuweihat S1 (UAE) 454,000 16.7 2004

Shoaiba 3 (KSA) 880,000 16.1 2008

Jebel Ali L (UAE) 568,000 14 2005-2007

Ras Laffan B (Qatar) 272,700 15 2008

New Taweelah B (UAE) 314,000 17.2 2008

Ras Abu Fontas B2 (Qatar) 137,000 15 2007

Jebel Ali M (UAE) 626,400 17.5 2010

Shoaiba North (Kuwait) 204,500 15 2010

Ras Az Zour (KSA) 730,000 20 2013