Design of a MSF Desalination Plant to be supplied by a New Specific 42 MW Power Plant Located in Iran

Transcription

1 Design of a MSF Desalination Plant to be supplied by a New Specific 4 MW Power Plant Located in Iran ouzbe Safagat, Hoda Safagat, Fateme Ganbari, Pouya Sirous ezaei and oolla Espanani Abstract Nowadays, desalination of salt water is considered an important industrial process. In many parts of te world, particularly in te gulf countries, te multi-stage flas (MSF) water desalination as an essential contribution in te production of fres water. In tis study, a simple matematical model is defined to design a MSF desalination system and te feasibility of using te MSF desalination process in proximity of a 4 MW power plant is inestigated. Tis power plant can just proide 10 ton/ supereated steam from low pressure (LP) section of eat recoery steam generator (HSG) for termal desalting system. Te designed MSF system wit gained output ratio (GO) of 10.3 as 4 flasing stages and can produce 480 ton/d of fres water. Te expected performance caracteristics of te designed MSF desalination plant are determined. In addition, te effect of motie water pressure on te amount of non-condensable gases remoed by water jet acuum pumps is inestigated. Keywords Design, Dual-purpose power plant, Matematical model, MSF desalination B I. INTODUCTION Y increase in population, te demand for potable water as been significantly increased. Tus, using economically admissible metods to produce drinking water is considered as a igly essential issue. In tis regard, desalination of salt water as become a necessary industrial actiity across te world. Nowadays, most of te Middle East countries use te multi-stage flas (MSF) desalination process to produce drinking water. In Iran wic is a dry country, water production is an important callenge, and using MSF desalination metod as recently become an important subject of researc. Nowadays, termal desalination processes are popular tecniques for production of drinking water in te Middle East countries. Termal desalination is based on distillation process troug wic brackis water or seawater is conerted to potable water for use in domestic and industrial actiities. Termal desalination system can be constructed in proximity of a power plant. Ten, te termal energy for distillation would be proided by steam coming from turbine or eat recoery steam generator (HSG) sections of te power plant [1], []. Vapor compression (VC), multi-effect distillation (MED) and multistage flas distillation (MSF) are some conentional termal desalination metods wic te MSF desalination system is more common tan te oter systems [3], [4]. 65 percent of te world's desalination plants use MSF desalination tecnology and more tan 80 percent of seawater desalination in Gulf countries is performed by MSF desalination metod [5], [6]. Kuwait, Qatar, Oman, United Arab Emirates and Saudi Arabia are te Middle East countries tat considerably use MSF desalination [], [7]-[9]. Some of te soutern cities of Iran suc as Sirik, Gorze and Jask wic are located in proximity of Persian Gulf ae te potential of using termal desalting metods like multistage flas desalination. Considering te proximity to Persian Gulf and te fact tat some new 4 MW power plants are going to be built in tese cities, it would be igly aluable to design and construct some termal desalination plants in tis area. Te power plants are combined cycle power plants consisting of gas and steam turbines (Fig. 1). As sown in Fig. 1, required steam for desalination plant is prepared from LP section of te HSG. Te focus of tis study is to introduce an efficient MSF desalination plant supplied by a 4 MW power plant tat proides just 10 ton/ steam, wit respect to te allowable salinity of blowdown (80000 ppm) [7]. Tus, practical correlations are defined to design te MSF desalination system and ten operating parameters are determined. ouzbe Safagat is corresponding autor, Assistant Professor, Scool of Mecanical Engineering, Babol Nosirani Uniersity of Tecnology, Babol, Iran. (fax: +98 (111) 31099; rsafagat@nit.ac.ir) Hoda Safagat, Fateme Ganbari and Pouya Sirous ezaei are wit te Noin Fannaaran Firouz Tecnical Company, Babol, Iran. oolla Espanani is wit te Iran Power Deelopment Company, Teran, Iran. 515

2 Fig. 1 Scematic diagram of te 4 MW combined cycle power plant. P: bar, T: C, M: kg/s, LHV: Low Heat Value II. MATEIAL AND METHODS A. Process Description Te scematic diagram of te MSF process wit brine recirculation is sown in Fig.. Te process contains 4 flasing stages wic ae similar layout. Stages 1-1 comprise eat recoery section and stages -4 are considered as te eat rejection section. Eac stage usually contains brine orifice, condenser/preeater tubes, water boxes, distillate tray, enting line, demister and partition walls [10]. In te brine eater (eat input section), te preeated brine is eated to reac te maximum allowable brine temperature. Te brine eater operates as a sell and tube eat excanger. Furtermore, a deaerator is connected to stage 4 for preparation of te make-up flow. Te flas camber sell and te brine eater sell are fabricated from stainless steel, wile te condenser tubes in te cambers and te tubes of te brine eater are made of Cu-Ni As sown in Fig., tree water jet acuum pumps are implemented for producing required acuum in te stages. Te two of tem are considered to operate in eat recoery section and te tird one operates in eat rejection section and deaerator unit. Fig. Scematic diagram of te MSF desalination plant wit 4 flasing stages 516

3 As mentioned before, te 4 MW power plant prepares 10 ton/ of steam for termal desalination system. So, 6.5 ton/ of steam is used in te brine eater and te rest is consumed in te deaerator. B. Design of te system Usually, te MSF process modeling includes mass and energy balance, eat transfer equations, pysical property correlations and te temperature losses due to boiling point ariation. Te system design metodology is presented at tis section. Te seawater temperature, te difference in te temperature of inlet and outlet recycle streams of te condensers and te temperature difference between te inlet and outlet brine streams in flasing cambers are considered to be 6 C,.39 C and.39 C, respectiely. Te top brine temperature (TBT) and te blowdown temperature are assumed 94 C and 34 C, respectiely. Te pressure alues inside te aporization cambers are equal to te saturation apor pressure. Te specific eat of seawater at constant pressure depends on temperature and water salinity, and is defined as (1) : 3 3 C p [ A + BT + CT + DT ] 10 (1) were T is temperature ( C) and A B S S (3) 4 6 C S S (4) D S S (5) S S () 4 were S is water salinity (g/kg). Te amount of apor ( mɺ ) produced in eac stage depends s on te latent eat of eaporation at saturation temperature, recycle flow rate, specific eat of seawater and temperature ariation of recycle flow ( (6) ). mɺ C t mɺ λ (6) pi si si were i is te number of stages. Te salinity of brine aries by passing from te first stage to te last one. Te amount of increase in brine salinity depends on te brine flow of preious stage and its salinity ( (7) ): ɺ bi 1Si 1 m Si mɺ bi Te stage widt of te MSF system (W) can be determined by (8). Te brine sell load (SL) is te flasing brine flow rate per unit widt of te flas camber. Tis design parameter (sell load) affects bot of te widt of te stage and te nonequilibrium losses in te stage. In tis study, it is assumed tat te entire produced apor condenses to water. (7) mɺ (8) W SL SL D (9) Te lengt of te MSF flasing stage (L) is determined from distilled water (D), steam density (ρ steam ), maximum allowable apor release elocity (V m ) and te stage widt (W). Te maximum allowable release elocity aries troug te first stage to te last one and sould not exceed 10 m/s in te last stage of te rejection section [7]. In tis study, te maximum release elocities for te first and last stages are considered 1 and 7.7 m/s, respectiely. Te lengt of te stage is obtained by te (10) : D (10) L V W ρ steam m By obtained widt and lengt of te stages, te area of te flas camber and apor mass flux can be determined from (11) and (1) : A W L (11) mɺ s (1) φ A Stage eigt is te summation of te brine dept, eigt between brine leel and demister, demister eigt and te eigt aboe demister. Te brine dept in eac flasing camber is adjusted to seal te orifice. Te orifice controls bubble formation and apor release rate by controlling te brine flow rate. Te brine dept is always iger tan te gate eigt (H G ) by about m and sould be adjusted below 60 cm. Optimum eigt between brine surface and demister must be selected to keep te camber at its minimum eigt. Te demister eigt (H d ) depends on te steam density and steam mass flux by a correlation as eigt constant (K ). Te demister eigt can be obtained by te empirical equation (15) [11]. Te eigt aboe te demister includes te condenser diameter and space between demister and condenser. Condenser diameter depends on te number of tubes fixing on support plates. mɺ 0.5 (13) HG (ρ P) CdW K H d e (14) K 3.81φ (15) ρl ρ 1 ρ were C d is te orifice discarge coefficient and can be in te range of In tis study, C d is assumed 0.5. Te unit of pressure difference between consecutie stages ( P) is pascal. All te tubes in camber condensers are made of Cu-Ni wit inside and outside diameters of and m, 517

4 respectiely. As te condenser section in all cambers operates like a eat excanger, calculation of total eat transfer coefficient based on outside diameter (U o ) is accomplised by (16) (16) + + rt + rfi + rfo U d o i o i do 1 1 (17) rfo o Ao oπd ol 1 1 (18) rfi i Ai iπd il Ln( do / di) (19) rt πkl (0) gρ LkL o 0.7g Ntµ ( Tc Tsu ) Nusselt number for turbulent stream inside te sleek tubes is defined as (1). Using tis equation, te eat transfer coefficient inside te tube ( i ) can be determined. id i Nu 0.03e k d 0.8 Pr 0.4 (1) Te brine eater is a sell and tube eat excanger wic consists of 109 tubes made of Cu-Ni wit diameter alues like condenser tubes. Heat transfer surface area of brine eater is estimated from (). mɺ CP ( TBT t A 1) (), Ntπd ol U ( LMTD) Te pressure alue inside te brine eater and te apor saturation temperature are 1.7 bar and 115 C, respectiely. t1 TBT (3) ( LMTD) Ts TBT Ln( ) Ts t1 Te mentioned matematical models can be used to design a new MSF desalination plant. According to tese models, te effect of different operating parameters on performance of te plant can be inestigated. III. ESULTS AND DISCUSSION Te performance caracteristics of te MSF desalination plant were predicted using te design model mentioned in Material and Metods, and results are demonstrated in tis part. To calculate te matematical equations, te alues of some parameters are assumed and gien in Table I. Since tis study was conducted wit te aim of designing a MSF desalination plant appropriate for soutern cities of Iran, te salinity of seawater was considered te same as te salinity of Persian Gulf water (43300 ppm). TABLE I ASSUMED PAAMETES TO DESIGN THE MSF DESALINATION PLANT Parameter Value Seawater feed temperature, C 6. Seawater feed concentration, ppm Top brine temperature, C 94 Brine elocity inside te condenser tubes, m/s Table II presents some of te manufacturing caracteristics of te designed MSF desalination plant. According to te accomplised calculations, te widt of te stages in eac section is almost constant wile te lengts of te stages are different. Table III gies calculated lengt alue of flasing stages. Specification TABLE II SPECIFICATIONS OF MSF DESALINATION PLANT Heat rejection Heat recoery Brine eater section section Number of stages Number of tubes Tube lengt, m Tube outside diameter, m Tube inside diameter, m Stage widt, m TABLE III THE LENGTH OF THE STAGES Stage number Stage lengt, m As mentioned before, total eigt of eac camber is diided into four segments. Te results of calculated gate eigt, brine dept and demister eigt are presented in Table IV. Te distance between brine leel and demister and te space eigt aboe te demister are assumed to be 1.6 and m, respectiely. Tus, stage eigt for eat recoery section and eat rejection section is considered 3.9 and 4 m, respectiely. 518

5 TABLE IV THE VALUES OF GATE HEIGHT, BINE DEPTH AND DEMISTE HEIGHT Stage number Gate eigt, m Brine dept, m Demister eigt, m alues for all te streams inside te aporization cambers are gien in Table V. As indicated, temperature ariation for all streams between two consecutie stages is assumed to be.39 C. TABLE V TEMPEATUE VALUES ( C) FO THE DIFFEENT STEAMS WITHIN THE CHAMBES Stage number of intake recycle brine of effluent recycle brine of intake brine of effluent brine Te design pressure of te sell as a function of saturation temperature is processed and maintained by te acuum system along te stages. Table VI sows te pressure ariations inside te stages. Te first and te last stages ae maximum and minimum sell pressures wit te alues of about and 0.05 bar abs., respectiely. Table VI sows te predicted alue of brine salinity in eac stage, and indicates tat te maximum brine salinity is obtained in te last stage ( ppm). TABLE VI CALCULATED PESSUE AND BINE SALINITY FO ALL FLASHING STAGES Stage number Pressure, bar Brine salinity, ppm Oter calculated parameters of te MSF desalination plant like different stream flow rates are illustrated in Table VII. As sown, total distillate capacity of te MSF system is 480 ton/d. In addition, calculated eat transfer area for te tree sections of te MSF plant sows tat te brine eater section as iger eat transfer area compared to te oter two sections. Also, te eat transfer area of eat rejection section is iger tan te eat recoery section. TABLE VII THE VALUES OF SOME OF THE CALCULATED PEFOMANCE PAAMETES parameter alue Distillate capacity, ton/d 480 GO 10.3 Intake seawater flow rate, ton/ Blowdown flow rate, ton/ ecycle brine flow rate, ton/ Make-up flow rate, ton/ ejected coolant flow rate, ton/ 70.8 Distillate flow rate, ton/ Steam flow rate to brine eater, ton/ 6.5 Steam temperature to brine eater, C 10 Steam flow rate to deaerator, ton/ 3.5 Steam temperature to deaerator, C 30.8 Blowdown concentration, ppm Heat transfer area for eat rejection section, m 09.7 Heat transfer area for eat recoery section, m Heat transfer area for brine eater, m Pursuant to te exibited matematical model, termopysical properties of eac stage were calculated and te results are presented in Table VIII. By increasing in number of stages, te temperature decreased and te brine salinity increased. Tese ariations lead to partial decrease of specific eat capacity. On te oter and, te latent eat of aporization gradually rises by increasing te number of stages. 519

6 TABLE VIII THEMO-PHYSICAL CHAACTEISTICS OF THE MSF DESALINATION PLANT Stage Specific eat capacity, number Latent eat of aporization water, kj/kg kj/kg. C Te obtained alues of termal resistance of te tube material, termal resistance of te scale on te inside and outside of te tubes and te eat transfer coefficients are presented in Table IX. Te calculated oerall eat transfer coefficient based on te outside surface area for all flasing stages is also sown in Table IX. Passing te stages consecutiely leads to decrease in te alue of oerall eat transfer coefficient due to increase in te concentration of noncondensable gases. TABLE IX CALCULATED HEAT TANSFE COEFFICIENTS AND THEMAL ESISTANCES Stage number i o r fi 10-5 r fo 10-5 r t 10-6 U o Aerage alue of oerall eat transfer coefficients in eat rejection section, eat recoery section and brine eater is about 960.5, and W/m C, respectiely. elatie suction flows of water jet acuum pumps for different motie water pressures (4, 5 and 6 bar) are sown in Table X. By increase in te motie water pressure, te ratio of exausted non-condensable gases to inlet motie water increases because ig elocities of flow in te mixing zone of te jet pumps proides ideal condition for eat and mass transfer. Furtermore, te amount of remoed noncondensable gases decreases wen te stage number increases. TABLE X EFFECT OF MOTIVE WATE PESSUE ON THE VALUE OF PUMPING NONCONDENSABLE GASES Motie water pressure (bar) Stage number kg/ of pumped gas per m 3 / of motie water IV. CONCLUSION A MSF desalination system is designed to be supplied by a 4 MW power plant proiding only 10 ton/ steam. 65% of te total steam prepared by LP section of HSG in te 4 MW power plant is considered to be consumed in te MSF desalination plant. Te designed MSF desalination system as 4 flasing cambers wit te same widt (1 m) and produces 480 ton/d potable water (1056 ton/d can be used as drinking water). Heigt alues of eat recoery section and eat rejection section are estimated to be 3.9 and 4 m, respectiely. By operating tis desalination system, te alue of seawater salinity rises from ppm to te maximum allowable alue of about ppm. Nomenclature A Stage area, m A i Heat transfer area based on inner tube diameter, m A o Heat transfer area based on outer tube diameter, m A Heat transfer area of brine eater, m C p Specific eat at constant pressure, kj/kg C C d Orifice discarge coefficient D Distillate, MIGD d i Tube inside diameter, m d o Tube outside diameter, m g Graitational acceleration, m/s H d Heigt of te demister, m 50

7 H G i o Heigt of te gate in te flasing camber, m Heat transfer coefficient based on inner tube diameter, W/m C Heat transfer coefficient based on outer tube diameter, W/m C k Termal conductiity, kw/m C K Heigt constant of demister L Lengt of stage, m (LMTD) Logaritmic mean temperature difference, C l Lengt of tube, m mɺ Flow rate, kg/s N t Total number of tubes Nu d Nusselt number for stream inside tubes P Pressure, bar Pr Prandtle number e eynolds number r fi Termal resistance of te scale on te inside of te tubes, m K/W r fo Termal resistance of te scale on te outside of te tubes, m K/W r t Termal resistance of te tube material, m K/W S Salinity, ppm SL Sell load, ton/m T, C TBT Top brine temperature, C T c Condensation temperature, C T su Surface temperature, C t of recycle flow inside te condenser tubes, C U o Oerall eat transfer coefficient, W/m C U Oerall eat transfer coefficient at brine eater, W/m C V m Maximum allowable apor release elocity, m/s W Widt of stage, m ρ Density, kg/m 3 µ Viscosity λ Latent eat of eaporation, kj/kg φ Vapor mass flux, kg/ m [7] A. M. Helal, Uprating of Umm Al Nar East 4-6 MSF desalination plant, Desalination, ol. 159, pp , 003. [8] M.. S. Okela and I. A. Tag, Performance ealuation of a MSF desalination plant in Qatar, Engineering Journal of Qatar Uniersity, ol. 5, pp , 199. [9] N. M. Wade, (001) Distillation plant deelopment and cost update, Desalination, ol. 136, pp. 3-1, 001. [10] H. El-Dessouky,H. Ettouney, F. Al-Juwayel and H. Al-Fulaij, Analysis of multistage flas desalination flasing cambers, Cemical Engineering esearc and Design, ol. 8(A8), pp , 004. [11] O. A. Hamed, K. Ba-Mardouf, H. Al-Wasmi, K. Al-Sail, H. Abdulla and A. Al-Wadie, Assessment of te major design features of SWCC MSF desalination plants, 4 t SWCC Acquired Experience Conference, 004. Subscripts b i L S Brine Stage number Liquid ecycle Steam Vapor EFEENCES [1] A. Gambier, M. Fertig and E. Badreddin, Hybrid modeling for superisory control purposes for te brine eater of a multi stage flas desalination plant, Proceedings of te American Control Conference Ancorage, 00, pp [] N. Wade and K. Callister on, Desalination: te state of te art, Meeting of te Institutions Sout Eastern Branc, 1996, pp [3] K. A. Al-Sayji, Modeling, simulation, and optimization of large-scale commercial desalination plants, Dissertation of Doctor of Pilosopy, [4] E. A. Hawaidi and I. M. Mujtaba, Sensitiity of brine eater fouling on optimization of operation parameters of MSF desalination process using POMS, 0t European Symposium on Computer Aided Process Engineering- ESCAPE0, 010. [5] H. I. El-Dessouky, H. Saban and H. Al-amadan, Steady state analysis of multi stage flas desalination process, Desalination, ol. 103, pp , [6] U. J. H. Gibbons, Using desalination tecnologies for water treatment, ecommended by U.S. Congress, Office of Tecnology Assessment, OTA-BP-O-46 (Wasington, DC: U.S. Goernment Printing Office),