MULTI STAGE FLASH
In the first years of the 1950 decade, the MSF system was developed by the U.S.A. navy. It showed a much better operation conditions than MED, specially in the scaling behavior, and was rapidly adopted as the standard evaporation method during the fifties and sixties.
A group of stages in series, working at descending pressures and temperatures. Evaporation and condensation happen in every stage, thus, one single stage could operate independently. Every stage has two sections: Evaporation (Where vapor is produced) and condensation (where the vapor condensates after its contact with colder tubes).
Defini&ons Condenser Tubes Brine heater Condensate Flash chambers Demister Water boxes Distillate
VAPOR COOLING WATER DEMISTER STEAM PRODUCT EVAPORATION SEAWATER BRINE CONDENSATE BACK TO BOILER
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& Stage Flash
Mul& Stage flash Steam is used to heat tubes of saline water Heated water flows into stages that are at lower pressure Water boils rapidly and flashes into steam
The amount of vapor produced is proportional to the difference of temperature between two chambers. The amount of water produced decreases after each stage, because the difference of temperature between chambers decreases as well. The vapor is outside the condensation tubes and the water flows inside.
The temperature of the condensation water increases stage by stage and the temperature of the evaporation water decreases. The two chambers of the stage are separated by a filter named demister, to avoid that water drops arrive to the condensation chamber, increasing the conductivity of the product.
5 Process description: How did it begin? It had long been known that water could be heated above its normal 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 issuing 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!
In this system, vapor flows outside the tubes. Refrigeration seawater flows inside the tubes. Full action happens in only one chamber. It s just the contrary as MED.
MSF Description 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.
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.
Mul& Stage flash types Thee are two kinds of arrangements: Once through ( Very simple design) Brine recirculation ( The most used)
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: Low scaling potential. (Water inside condensing tubes is seawater, not brine) Higher operation temperatures (Brine is less concentrated) Cons: All feed water must be pre treated (Bigger degasifier and decarbonator, higher chemicals consumption) Bigger vacuum system
Brine recirculation Multi Stage Flash Distillation (MSF) HEAT RECOVERY SECTION HEAT REJECTION SECTION 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.
Pros: Smaller vacuum system Smaller pre treatment Higher operational flexibility Cons: Higher pumping investment Higher pumping energy Higher thermal energy Higher scaling risk
Once through vs. Brine recircula&on Item Once through Brine recirculation Max. Temperature ºC Recovery % Efficiency Kg/mJ 90 110 10---15 10---20 3.4 4.3 3.4 5.2 Brine salinity ppm 58,000 62,500
MSF PLANT LAS PALMAS II
MSF Process Arrangements 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.
Multi Stage Flash Cross Tube (Shuweihat, 76500m3/day) Long Tube (Gela, 4x14400m3/day) Source: Unipa, Italy Source: Unipa, Italy www.prodes-project.org
Multi stage flash Courtesy of CPass 1 stage Pass 2
Multi-stage flash dis&lla&on Advantages Produc&on of water with good quality (5 to 50 ppm tds) RO produces 10 to 500 ppm tds Disadvantages Requires large energy input High maintenance requirements Larger space needed that RO plant Most experience is from Middle East where energy is cheap. Large amount of water necessary for produc&on and cooling
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
Section Material of Construction Brine Heater Shell Carbon steel (all plants) 20 Heat Recovery Section Tubes Flash Chamber Tubes Heat Rejection Tubes Either 70/30 o,90/10 Cu-Ni or modified 66/30/2/2 Cu/ Ni/Fe/Mn except Al-Jubail I (Titanium) 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 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) All plants except Jeddah & Shoaiba : Titanium Jeddah II, III, IV 90/10 Cu/Ni Shoaiba 70/30 Cu Ni
Projects which were recently built use the following materials of construction for the major components Flash chamber of both recovery Carbon steel lined with stainless steel and heat rejection sections (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 21
back-pressure turbine arrangement Back Pressure Turbine G MSF Distillers Boiler Ejector Moisture Separator Fuel Deaerator Heater # 2 Heater # 1 Condensate Pump
Extraction- condensing turbine arrangement Boiler Extraction / Condensing Turbine To Ejectors MSF Condenser Fuel Deaerator Heater # 1 Condensate Pump 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 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 9
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.
Price Trend for turn-key complete MSF plants 14 12 12 10 $ / IGD 8 6 8 6 5 4 4 2 0 1985 1990 1995 year 2000 2004 2010 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.
Production Parameter Value 100 MIGD (18,940 m 3 /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 m 3 /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