MULTI STAGE FLASH - Desalación · Once through Multi Stage Flash Distillation (MSF) In once...
Transcript of MULTI STAGE FLASH - Desalación · Once through Multi Stage Flash Distillation (MSF) In once...
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.
5
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
20
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
21
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
9
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