M.A. Darwish Department of Mechanical and Industrial Engineering, Kuwait University, Kuwait

Transcription

1 MSF ENGINEERING M.A. Darwish Department of Mehanial and Industrial Engineering, Kuwait University, Kuwait Keywords : MSF System, Flash System, Reirulation, Distillate Ratio, Deaeration, Demisters Contents. Introdution 2. One-through MSF System 2.. Single-stage Flash System 2.2. Two-stage Flash System 2.3. Multistage (n-stage) systems 3. Reirulation MSF System 3.. Reirulation arrangements 3.2. Thermal Analysis of the Reirulation MSF System 3.3. The Reirulation to Distillate Ratio R/D 3.4. Terminal Temperature Differene 4. Performane Ratio 5. Heat Transfer Area of the Stages Condenser 6. Flow Sheet Development 7. Maintenane Design Features 7.. Operating Temperature 7.2. Flashing Brine Flow and Vapor Release Veloity 7.3. Gain Ratio or Performane ratio 7.4. Rated Capaity 7.5. Number of Stages n 7.6. Temperature Profile 7.7. Stage Effiieny η i 7.8. Deaeration 7.9. Dearators 7.0. Venting System 7.. Brine Level 7.2. Conentration Ratio 7.3. Plant Control and Instrumentation 8. Component Design and Material 8.. Evaporator Stage 8.2. Interstage Control Devies 8.3. Condensers Design 8.4. Demisters 8.5. Produt Water Troughs 8.6. Pumps 9. Rational Basis for Energy Comparison 9.. Speifi Mehanial Work Consumption 0. Cost Estimation 0.. Energy Requirements for an MSF System

2 0.2. Diret Boiler Driven MSF System. MSF Steam Supply to an MSF Desalting Plant from Co-generation Turbine.. The Rating of Dual-purpose Power/Desalting Plants 2. Referene Dual-purpose Power/Desalination Plant 2.. Energy Charged to Desalted Water by Different Alloation 3. Conlusion Glossary Bibliography and Suggestions for Further Study Proess Desription. Introdution In a multieffet (ME) desalination system the vapor forming the produt is generated by (a) boiling in the effets and (b) flashing between the effets due to the pressure drops between these effets. The perentages of vapor produed in a typial ME system by boiling (m b ) and by flashing (m f ) are 80 per ent and 20 per ent, respetively. The inrease of feed to distillate ratio F/D inreases the flashing to boiling ratio m f /m b and this requires an inrease of feed pre-heaters and dereases the effets of boiling heat transfer areas. The limit to this proedure is to inrease F/D to the extent that the required vapor is produed by flashing only, the boiling heat transfer surfae in eah effet is ompletely eliminated, and the feed heaters type ondenser surfae areas are inreased to ondense all vapor released. The first large desalting unit using this limiting ase, alled a multistage flash (MSF) system, was built in Kuwait by Westinghouse Company in 957 to desalt 0.5 mgal day - with four effets (alled stages here) and 3.3 performane ratio (PR). The design of the unit was based on ME system analysis (presented earlier) with a PR lose to but smaller than the number of effets (stages here). Silver filed a patent (British Patent Appliation No , September 957) for a MSF proess with different analysis than the ME analysis in some respets. The PR does not expliitly depend on the number of stages n. The number of stages n in an integer number greater than at least twie the required PR. (In pratie, the number of stages n is in the range of three times the PR). In 958 Silver designed the first two units based on his patent. One unit was installed in Kuwait to produe 4550 m 3 day - ( mgal day - ) and has 24 stages and a PR of 5.8. The seond unit has 40 stages produing 2775 m 3 day - (0.5 mgal day - ) and has a PR of 0.5. Both units were ommissioned with suess in 960. The dramati inrease in PR (ompared to ME systems at that time) and the simpliity of the struture over the ME system have made MSF desalting systems the most used method for desalting seawater in large quantities sine 960. Sine that time, individual units and plant apaities have ontinued to inrease. Examples of the units and plant apaities in Kuwait are given in Table. In 985, more than 80 per ent of the desalted seawater was produed by MSF plants.

3 Brine heater Heat reover stage First and seond All other stages Heat rejetion stage Clean heat transfer oeffiient (kw m -2 C - ) Design heat transfer Fouling fator (kw m -2 C - ) LMTD When lean At design point Veloity (m s - ) Material n (effiieny) Area (m 2 per stage) Number in stage Tube dimensions OD (mm) length (m) /30 Cu-Ni /30 Cu-Ni Al-brass Table. Data on typial 6 mgal day - plants operating at T O = C /30 Cu-Ni There are two possible types of MSF system: the one-through and reirulation MSF systems. The reirulation MSF system is the most widely used and appears superior to the one-through system in energy onsumption. The one through system loses too muh heat with brine rejetion. The next setions onsider both MSF systems. 2. One-through MSF system 2.. Single-stage Flash System The one-through MSF represents the limiting ase of the ME system when the feed flow rate is inreased to the extent that the vapor is generated by flashing only and the boiling heat transfer surfae is entirely omitted. The flash vapor is ondensed on the feed heater-type ondensers. In a single-stage flash desalting system (Figure ), the feed F, after pre-treatment, is pre-heated from t to t in the ondenser tubes. It is then heated in the brine heater (alled heat input setion, HIS) to temperature T o by ondensing the supply steam S (main energy soure). The feed, leaving the brine heater at temperature T o and pressure P o, enters the first flashing hamber (or stage) kept at P of saturation temperature T < T o. The feed beomes unstable and part of it is spontaneously vaporized (i.e. flashed in order to reah stability by dropping its temperature to T ). The vapor released by flashing flows upward through demisters (to separate entrained liquid brine) to the

4 ondenser tubes in the upper portion of the hamber. The vapor ondenses there and forms the produt. The heat released by vapor ondensation is gained (or reovered) by the heated feed. The temperature of the flashed vapor (from brine at T ) suffers some losses ΔTL due to non-equilibrium, boiling point elevation, and pressure drops through the demister and ondenser tubes. The following energy balanes an be arried out by assuming the following. Figure. Single-stage flashing desalting system. (a) Constant and average speifi heat C for the feed F, distillate D, and brine B, streams and onstant and average latent heat value L for ondensing vapor and supply steam. (b) A ompletely insulated stage. (i) For the ondenser tubes FC( t t ) = DL () (ii) For flashing brine FC( T T ) = DL (2) o (iii) For the brine heater

5 Q= SL= FC( T t ) (3) o where Q is the heat input rate (kj s - ) and S is the steam flow rate assumed to be supplied at saturated vapor ondition and leaves at saturated liquid ondition. For the whole desalting unit FCt + SL BCT + DCT (4) V Sine F = B+ D and TV = T ΔTL then SL FC( T t ) Equations (), (2), and (4) give t t = T T o D t t S T t = < 2330D PR = < (6) SL F L L = = D C( T T) C( t t ) o A typial example of a single-stage MSF system where T o = 90 C (top brine temperature, TBT), t = 30 C (seawater temperature), and temperature approah of the ondenser T - t = 4.2 C would give T o - t = 60 C, t = 57.9 C, T = 62. C, D/S = 0.875, and F/D = 9.9 (L is assumed equal to 2330 kj kg - ). The value of F/D (=9.9) is large ompared to F/D (=2.5) for a single-effet or ME system when the feed total dissolved solids (TDS) = p.p.m. and brine blowdown TDS = p.p.m. The areas of heat transfer surfae for both the brine heater and ondenser heat exhangers an be determined by equating the thermal load of eah exhanger with the overall heat transfer oeffiient U, multiplied by the surfae area multiplied by the effetive temperature (LMTD). For the brine heater (with subsript b for the parameters involved) (5) (7) SL = UbAb(LMTD) b (8) where (LMTD) b ( Ts t) ( Ts To) = nt ( t) ( T T) s s o (9) For the ondenser (with subsript )

6 DL = U (LMTD) A (0) ( Tv t) ( Tv t) (LMTD) = Tv t n T t v () Realling D/S = (t - t )/(T - t ) and T v = T - ΔTL, the total speifi area A t /D = (A b + A )/D is expressed by At T t L Ts t L T TL t = n + n Δ (2) D t t U ( T t ) T T U ( t t ) T ΔT t b o s o L The typial example mentioned earlier, with T s = 00 C, T o = 90 C, T = 62 C, t = 30 C, t = 58 C, ΔTL =.2 C and by assuming U b = 3 kw m -2 C - and U = 2.5 kw m -2 C - has a speifi total area A t /D = m 2 kg - s TO ACCESS ALL THE 94 PAGES OF THIS CHAPTER, Visit: Bibliography and Suggestions for Further Study AI-Zubaidi A A (987) Seawater desalination in Kuwait - a report on 33 years experiene. Desalination 63, -55. Akili D. Khawaji, Ibrahim K. Kutubkhanah, Jong-Mihn Wie, (2008), Advanes in seawater desalination tehnologies, Desalination 22, Elsevier, pp Ali Al-Odwani, Essam E.F. El-Sayed, Mohamed Al Tabtabaei, Mohamed Safar, (2006), Corrosion resistane and performane of opper-nikel and titanium alloys in MSF distillation plants, Desalination 20, Elsevier, pp Barba D, Bogazzi D, German A and Tagliferri (980) Analysis of large desalination plants. Desalination 33, -0. Birkett and Newton E H (98) Survey of Servie Behavior of Large Evaporative Desalting Plants, PB Washington, DC: Offie of Water Researh and Tehnology. C. Sommariva, V.S.N. Syambabu, (200), Inrease in water prodution in UAE, Desalination 38, Elsevier, pp Darwish M A (986) On the thermodynamis of dual purpose power-desalting plants, part I: using steam turbines. Desalination 64, Darwish M A (987) Critial omparison between energy onsumption in large apaity reverse osmosis (RO) and multistage flash (MSF) sea water desalting plants. Desalination 63, Darwish M A (988) Cogeneration power-desalination plants. Desalination 69,

7 Darwish M A (99) Thermal analysis in MSF desalting systems. Desalination 85, Darwish M A (995) Fuel ost harged to desalters in ogeneration power-desalination plants. Heat Reovery Systems and CHP 5(4), Darwish M A et al. (987) Energy onsumption and ost of different desalting systems. Desalination 64, Emad Ali, (2002), Understanding the operation of industrial MSF plants Part II: Optimization and dynami analysis, Desalination 43, Elsevier pp Emad Ali, (2002), Understanding the operation of industrial MSF plants Part I: Stability and steady-state analysis, Desalination 43, Elsevier pp Genthner K, Wangnik K, Bodendik F and Al Gobaisi D (997) The next size generation of MSF evaporators, 00,000 m 3 /d L. (Thermal and Hydrauli Design Reommendation IDA Conferene, Madrid, 997). Hanbury T (980) A Short Course on Desalination Tehnology. Jeddab, King Abdul Aziz University. Hisham El-Dessouky, S. Bingula, (995), A Stage-by-Stage Algorithm for Solving the Steady State Model of Multi-Stage Flash Desalination Plants, IDA 4, Volume IV, pp Joahim Gebel, Süleyman Yüe, (2008), A new approah to meet the growing demand of professional training for the operating and management staff of desalination plants, Desalination 220, Elsevier, pp Khan A H (986) Desalination proesses and multistage flash distillation pratie. Amsterdam: Elsevier. M.A. Darwish, Iain MGregor, (2005), Five days Intensive Course on - Thermal Desalination Proesses Fundamentals and Pratie, MEDRC & Water Researh Center Sultan Qaboos University, Oman M.A. Darwish, Ammar Alsairafi, (2004), Tehnial omparison between TVC/MED and MSF, Desalination 70, Elsevier, pp M.A. Darwish, Hassan K. Abdulrahim, (2008), Feed water arrangements in a multi-effet desalting system, Desalination 228, Elsevier, pp M.A. Darwish, N. Al-Najem, N. Lior, (2006), Towards Sustainable Energy in Seawater Desalting in the Gulf Area, Tenth International Water Tehnology Conferene, Alexandria, Egypt, pp M.A. Darwish, S. Alotaibi, S. Alfahad, (2008), On the redution energy and its ost in Kuwait, Desalination 220, Elsevier, pp Mohamed A. Dawoud, (2005), The role of desalination in augmentation of water supply in GCC ountries, Desalination 86, Elsevier, pp Mohamed Al-bahou, Zamzam Al-Rakaf, Hassan Zaki, Hisham Ettouney, (2007), Desalination experiene in Kuwait, Desalination 204, Elsevier, pp Nabil M. Abdel-Jabbar, Hazim Mohameed Qiblawey, Farouq S. Mjalli, Hisham Ettouney, (2007), Simulation of large apaity MSF brine irulation plants, Desalination 204, Elsevier, pp Nafiz Kahraman, Yunus A. Cengel, (2005), Exergy análisis of a MSF distillation plant, Energy Conversion and Management 46, Elsevier, pp Offie of Saline Water report (OSW), 966 OSW N0.24, Washington DC Offie of Saline Water report (OSW), 967 OSW N0.35, Washington DC Offie of Saline Water report (OSW), 980 OSW N0.08, Washington DC Roberton Borsani, Silvio Rebagliati (2005), Fundamentals and osting of MSF desalination plants and omparison with other tehnologies, Desalination 82, Elsevier, pp Sabine Lattermann, Thomas Höpner, (2008), Environmental impat and impat assessment of seawater desalination, Desalination 220, Elsevier, pp. -5.

8 Y.M. El-Sayed, (200), Designing desalination systems for higher produtivity, Desalination 34, Elsevier, pp