seawater desalination in a direct contact heat exchanger

Isı Bilimi ve Tekniği Dergisi, 29, 2, 01-09, 2009J. of Thermal Science and Technology©2009 TIBTD Printed in TurkeyISSN 1300-3615SEAWATER DESALINATION IN A DIRECT CONTACT HEAT EXCHANGER:GLOBAL AND LOCAL MODEL RESULTSMurat TEKELİOĞLUMechanical Engineering Department, University of Nevada, Reno, NV 89557 USA,tekelioglu_murat@yahoo.com(Geliş Tarihi: 23. 12. 2008, Kabul Tarihi: 19. 03. 2009)Abstract: As the world population and the industrial and residential demands increase, it becomes important tocompensate for the potable water shortage. To overcome the aforementioned crisis, several desalination techniqueshave been used for many years. In this work, we develop and implement global and local models to manifest theeffectiveness of direct contact heat exchange in desalination of seawater. Direct contact processes offer manyadvantages such as low cost, absences of scale build-up and resistance of the heat transmitting material. The globalmodel results show that about 65% of thermal seawater evaporation is possible. Local model, based on evaporation ofone single droplet, was used to establish sizing parameters on the heat exchanger, e.g., evaporator length, crosssectionarea, and volume. It was shown of the seawater to have a short evaporation length.Keywords: Direct contact heat transfer, Seawater, Desalination, Evaporation.DENİZ SUYUNUN BİR DİREK TEMAS ISI DEĞİŞTİRİCİSİNDE ARITILMASI:GLOBAL VE LOKAL MODEL SONUÇLARIÖzet: Dünyadaki nüfus ve endüstriyel ve konutlardaki talep artışları arttıkça, içme suyu ihtiyacının karşılanmasıönemli olmaktadır. Bu problemin etkilerini azaltmak amacı ile, çeşitli tuzlu su arıtma projeleri yıllardan beriuygulanmaktadır. Bu çalışmada, direk temas ısı değiştirme etkinliğini deniz suyu arıtılmasında göstermek amacıyla,global ve lokal modeller geliştirip uygulamaktayız. Direk temas ısı transfer prosesleri bir çok avantajlar sunar: düşükmaliyet, ısı transfer yüzeyinde tortu birikiminin olmaması ve ısı transferi direncinin bulunmaması gibi. Genel modelsonuçları, proseste, yaklaşık %65 deniz suyu thermal buharlaşmasının mümkün olduğunu göstermektedir. Bir tekdamlacığın buharlaşmasını temel alan lokal model, ısı değiştiricisi boyutlarının (uzunluk, kesit alanı, ve hacim)belirlenmesinde kullanılmıştır. Deniz suyunun, kısa bir buharlaşma mesafesi olduğu gösterilmiştir.Anahtar kelimeler: Direk temas ısı transferi, Deniz suyu, Tuzlu su arıtılması, Buharlaşma.NOMENCLATURELetter2A Cross-section area [ m ]2a Effective heat transfer surface area [ m ]g Gravitational acceleration [ N / kg ]h Enthalpy [ kJ / kg ]h Convective heat transfer coefficient[ W /( m2 K ) ]L Vapor layer thickness [ mm ]LMTD Logarithmic mean temperature difference[ Δ K or Δ o C ]M & Mass flux [ kg /( s ⋅ m2) ]m& Mass flow rate [ kg / s ]2p Pressure [ N / m ]r Ellipsoidal half-width [ mm ]T Temperature [ K ]t Time [ s ]U Velocity [ m / s ]U Heat transfer coefficient [ W /( m2 ⋅ K ) ]u X- component of the velocity [ m / s ]u Mean (average) velocity [ m / s ]V Volume [ liter ]v Y- component of the velocity [ m / s ]v Superficial velocity, m& /( ρ ⋅ A)[ m / s ]w Z- component of the velocity [ m / s ]x Length [ m ]Greekε Void fraction [ % ]μ Dynamic viscosity [ Pa ⋅ s ]φ Salinity [ % mass]ρ Density [ kg / m3]1

SubscriptatmbbccpAtmosphericBrineBeneathContinuous phaseCombustion productsC 3H 8 PropaneCO 2 Carbon dioxided Dispersed phasee Evaporatorf LiquidfgLiquid-to-gasH 2O(g)Water vapor (combustion products)m Mean (average)N 2 Nitrogeno Outero Initialp Productsp Constant pressureR References Surfaces Saturatedsw Seawatert, wv Total water vaporv Vaporv Volumetricwv Water vaporINTRODUCTIONIn contrast to the rising problem of potable watershortage, most of the world surface is covered by water.However, the world’s 94% of available water is saltyand only 6% is fresh (International DesalinationAssociation (IDA), 2000). Further, 72% of the latter isunderground and 27% is in glaciers. Therefore, to solvethe problem of potable quality water shortage, it issuitable to implement desalination methods to turn thesaline water into a potable water source. This processalso benefits the agriculture.Installed capacity of the desalination plants, from mostto-the-least,were given as multi-stage flashing (44%),reverse osmosis (42%), electro dialysis (6%), multieffectdistillation (4%), and vapor-compression (4%)(IDA, 2000). It was noted that the total productioncapacity of the multi-stage flashing (MSF) and reverseosmosis (RO) systems were competitive with9.8 ⋅ 106m3/ day versus 9.6 ⋅ 106m3/ day as of 1997.A research overview on the processes of MSF and ROsystems was presented (Khawaji et al., 2008). Thermaland membrane systems and other alternative methodswere discussed under the economics of such plants. Thefouling of membrane in membrane distillation (MD)systems was studied (Hsu et al., 2002). The aim was todistill the raw seawater, a NaCl solution, and a pre-treatwater in MD and compare the permeate fluxes( kg /( m2 h ) ) with presence of fouling at the membranesurface. They observed with the raw seawater, nearlyhalf permeate flux (more fouling) compared to NaClsolution. They applied ultrasound cleaning technique tothe membrane to overcome the fouling. A similar MDprocess was investigated (Termpiyakul et al., 2005). Amembrane distillation coefficient ( C , kg /( m2⋅ h ⋅ Pa))was found on pure water and the other C values wereestimated for high salt concentrations (up to 35,000mg / L ). In the other study, theoretical and experimentalvalues of condensation and evaporation coefficientswere included (Marek and Straub, 2001). They pointedout the real gas effects in the processes and found thatthe condensation coefficient for water was greater thanits evaporation coefficient. A theoretical model for theevaporation of dispersed volatile drops in direct contactof a continuous immiscible liquid was described(Kendoush, 2004). The author developed heat transfercoefficients for n-pentane and n-butane coolant drops.Direct contact MD was given (Yun et al., 2006) forhigh-concentration NaCl solutions from theoretical andexperimental aspects.In this paper, we investigate the effectiveness of directcontact heat exchange process in seawater desalination.A spray- column was used. With two types of fuels(Hydrogen and Propane) tested on two models (globaland local), the outcomes aimed to determine the amountof water vapor out of seawater evaporation andevaporator dimensions such as length, cross-sectionarea, and volumeGLOBAL MODELA schematic direct-contact heat-exchanger (DCHX) isshown in Fig. 1. The combustion products enter in thespray-column and rise toward the falling-off seawaterdroplets from a sprinkler system and give their heat tothe seawater. The evaporated seawater (water vapor) isreceived from the top and the much denser brine (in saltcontent) is delivered from the bottom of the system. Thesensible cooling of the combustion products is the onsetfor the seawater mass transfer (latent heat). The watervapor received from the top of the heat exchanger iscondensed in a condenser. A pre-heater can also replacethe condenser, which preheats, e.g., the incomingseawater up to its temperature of saturation.Applying the first law of thermodynamics to the systemshown in Fig. 1, we getm & swhsw+ m&cphcp= m&php + m&bhb(1)where m& sw is the seawater mass flow rate ( kg / s ),h sw is the seawater enthalpy ( kJ / kg ), m& cp is themass flow rate of the combustion products ( kg / s ),h cp is the enthalpy of the combustion products2

( kJ / kg ), m& p is the mass flow rate of the products( kg / s ), h p is the enthalpy of the products ( kJ / kg ),m& b is the brine mass flow rate ( kg / s ), and h b is thebrine enthalpy ( kJ / kg ). The two mass balanceequations givem & sw + m&cp = m&b + m&p(2)Seawater.m swEvaporatorBrine.m bProducts .m pCombustionproducts.m cpFigure 1. Schematic of the direct contact heat exchanger withinput and output.where m& p , m & p = m&cp + m&wv is comprised of the bothcombustion products and water vapor of seawaterevaporationm & sw = m&b + m&wv(3)The salt concentration balance equationφ* sw m & sw = φ b m&b + φ p m&p(4)where φ sw , φ b , and φ p are the seawater, brine, andproducts salinities, respectively, and m& * p is taken asH 2O(g)(sum of the water vapor of products ofcombustion, m & H 2O(g),cp and seawater evaporation,m& wv ). We solved Eqs. (1)-(4) for m& sw , m& wv , m& p ,and m& b , then calculated m & t, wv = m&wv + m&H 2O(g),cp .In the calculations, φ sw = 3.53%, φ b =10%,φ p = 0.025% , h sw = 405.8kJ/ kg , h b = 381kJ/ kg ,h cp (at T cp = 1197K, T cp = 1263K, T cp = 423K)were used and m& cp values were estimatedexperimentally from the combustion of C 3H8with air.As the fuel mass flow rates were relatively small in ourpreliminary experiments, m& C3H8values were taken 10times as greater of those, or as−32.1⋅ 10 , 3.0⋅10−3kg / s−30.3⋅ 10 ,−31.2⋅ 10 ,Figures 2 and 3 show m& wv and m & t , wv changes withm& cp of C 3H8and H 2 fuels. m& cp ’s were calculated,respectively, for the C3H8-air combustion and H 2 -aircombustion with no dissociation or association in theproducts fromm & cp = m&CO2 + m&H 2O(g)+ m&N 2(5)m & cp = m&H 2O(g)+ m&N 2(6)where the right hand side represents the products gasesout of combustionsWater vapor (10 -3 kg/s)6005004003002001000WV, Tcp = 1197 K (Propane-Air)TWV, Tcp = 1197 KWV, Tcp = 1263 K (Hydrogen-Air)TWV, Tcp = 1263 KWV, Tcp = 423 K (Hydrogen-Air)TWV, Tcp = 423 K0 0.5 1 1.5 2 2.5 3 3.5Hydrogen or Propane mass flow rate (10 -3 kg/s)Figure 2. Water vapor mass flow rate with fuels mass flowrate (equal mass flow rate).Water vapor (10 -3 kg/s)2520151050WV, Tcp = 1263 K (Hydrogen-Air)TWV, Tcp = 1263 K0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16Hydrogen mass flow rate (10 -3 kg/s)Figure 3. Water vapor mass flow rate with Hydrogen massflow rate (equal mole flow rate).LOCAL MODELIn order to assess sizing parameters on the evaporator, alocal evaporation model was used, which assumed100% droplet evaporation (thus, m&b = 0g/ s , andm & sw = m&wv ). Fig. 4 shows the ellipsoidal droplet used3

in the current model.r o = 250μm.r o= 250μmand t e was given forL2dpu = −(11)12μ v dxIn the column, the dispersed phase denotes the seawaterand the continuous phase denotes the combustionproducts. The model we used herein was similar to theone earlier given by (Elshaik, 1990) except in that thedroplet shape we considered was ellipsoidal.Considering the simplifying assumptions in Cartesian(x, y, z-) coordinates∂uv ≅ 0 , w ≅ 0 , ≅ 0∂x(7)Navier-Stokes equations in x-direction (on layer L ) canbe given as2dp d u0 = − + μ v(8)dx 2dywhere μ v is the dispersed phase (seawater) vapordynamic viscosity ( Pa ⋅ s ). We assumed laminar flowregime inside the layer and μ v was taken as constant.Integrating this equation under boundary conditionsu ( 0) = 0 and u ( L)= 0 , we gety dpu = ( y − L)(9)2 μ v dxEvaporation mass flow rate on the ring element in Fig. 4can be written asu(2πxL)ρ M ( x2v = & π )(12)where M & is the mass flux ( kg /( s ⋅ m2) ), ρ v is the3dispersed phase density ( kg / m ). After combining thelast two equationsdp 6μMx= −v&(13)dx 3ρvLand taking the integration forflow, u = 0 ), we getx = r , p = 0 (no vapor3μ v M 2 2p(x)=& ( r − x )(14)3ρvLwhere r = r(t)and 0 ≤ x ≤ r in the course ofevaporation. Writing a force balance on the droplettaking into account its weightr( ρ l − ρv) Vg =∫p(2πx)dx(15)0Substituting p (x), we findrroH3 μ M&( ρ gvl − ρv) = r(16)8 3ρvLyLFigure 4. Ellipsoidal droplet and vapor layer on the ringelement beneath, r = r(t), L = L(t), and H = 3ro. The thirddimension is also r = r(t)and r oThe mean velocity for the water vapor isL1u =∫udyL0with Eq. (9)x(10)where ρ l is the dispersed phase liquid density( kg / m3), g is the gravitational acceleration( g = 9.81N/ kg ), r ( = r(t)) is the mean droplet width4π( m ), and V 3r33= is the droplet volume ( m ). Rate3of evaporation was given on single droplet asdV 2ρl= −πrM&(17)dtPluggingr4π3V = 3rand integrating between r o and3M&r = ro− t(18)12ρl4

which gives the decay of r o during evaporation. Inaddition, heat given to the droplets by the combustionproductsQ & = hcAs( Tcp−Ts)(19)and the required heat for the droplets evaporation isQ & = MA &b h fg(20)in which h c is the convective heat transfer coefficientfor the combustion products ( W /( m2 K ) ), A s is thesurface area of the ellipsoidal droplet from the KnudThomsen formula ( p = 1. 6075 , with a relative error ≤1.061%) ( m2), T cp is the combustion productstemperature ( K ), T s is the saturation temperature forthe dispersed phase at p = 101. 325kPa( T s = 373.15K+ BPR(0.28K)) where BPR is theseawater boiling point rise (Billet, 1989; Wark, 1995),2A b is the area beneath the seawater droplet ( m ), andh fg is the latent heat of vaporization for the dispersedphase ( kJ / kg ). Equating last two equations0.622⎛ 2 31.60751⎞4h⎜ ⋅ + ⎟c( Tcp− Ts)⎜ 3 ⎟M& =⎝⎠(21)h fgand substituting into Eq. (18), we find0.622⎛ 2 31.60751⎞h ⎜ ⋅ + ⎟c( Tcp− Ts)⎜ 3 ⎟r = r⎠o −⎝t (22)3ρlh fgIn order to determine the decay of droplet volume, wesubstitute Eqs. (21) and (22) into Eq. (17) and integratebetween V ( 0) = Voand V ( t)= V to get0.622⎛ ⎞⎜ 2⋅31.6075+ 14π h⎟c( T −⎜⎟ cp Ts)3⎝⎠V −Vo= −×ρlh fgt ⎛⎜ hc∫ ⎜ro−⎜ 3ρlh fg0 ⎝20.622⎛ 1.6075 ⎞⎞⎜ 2⋅3+ 1⎟( Tcp−Ts) t dt⎜ 3 ⎟⎟ ⎟⎟ ⎝⎠⎠(23)Evaporation times calculated via Eqs. (22) and (23)were identical. The seawater mass flow rate wascalculated due to the sensible cooling of the combustionproductsm & swc p, cp ( Tcp− Ts)=m&cp h fg(24)where c p , cp is the specific heat for the combustionproducts ( kJ /(kgK)): c p , cp was evaluated at1Δ T m = ( Tcp+ Ts) and m& cp s were calculated as in2Eqs. (5) and (6) with no dissociation or association inreactionsThe cross-section area of the evaporator was givenbased on the properties of the combustion productsm&cpAe=ρcp( 1−ε d ) vcp(25)where ρ cp is the density of the combustion products( kg / m3), v cp is the velocity of the combustionproducts ( m / s), and ε d is the void fraction defined asthe ratio of the volume of dispersed phase to that of totalevaporator volume (-). Taking into account the fact thatthe evaporator is occupied by the dispersed andcontinuous phases throughout1ε d =ρ swm&cp1+ρcpm&swwithρ f + ρvρ sw = ,2(26)pρ =atmcp(27)⎛ T + ⎞⎜cp TsRcp⎟⎝ 2 ⎠In order to find the evaporation length,Δ xeΔ x e = U Rte(28)wherevdvUcR = −ε d 1− ε d(29)and vd= m&sw /( ρswAe) and vc= m&cp /( ρcpAe)inside the evaporator column. Hence, through Eqs. (25)and (28), the evaporator volume was estimatedVe≅ AeΔxe(30)5

Lastly, the volumetric heat transfer coefficient ( U v )was calculated fromUa m&swhfgU v = =VeVe⋅ LMTDor,(31)m& swhfg = U vVe⋅ LMTD(32)2a being the effective heat transfer surface area ( m )and U ≅ hc. The fact ( U ≅ hc) is due to the twofactors: First is the absence of the heat transferresistance of the heat transmitting material and secondis the negligible heat transfer resistance of the dispersedphase compared to the combustion products (Kreith andBoehm, 1988). Logarithmic mean temperaturedifference ( LMTD ) was defined as( Tcp− Tsw)− ( Tp− Tsw)LMTD =(33)Tcp− TswlnTp− TswIn using LMTD , the products were assumed to exit theevaporator as superheated at T p = 375KWater vapor (10 -3 kg/s)9080706050403020100WV, Tcp = 1197 K (Propane-Air)TWV, Tcp = 1197 KWV, Tcp = 1263 K (Hydrogen-Air)TWV, Tcp = 1263 KWV, Tcp = 423 K (Hydrogen-Air)TWV, Tcp = 423 K0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Hydrogen or Propane mass flow rate (10 -3 kg/s)Figure 5. Water vapor mass flow rate with fuels mass flowrate (equal mass flow rate).We present local model resultsm& wv , m & t , wv , A e , t e ,Δ x e , and V e in Figs. (5)- (16). In Figs. (5) and (6),m& wv and m & t , wv increase with m& H 2 or m& C3H8due tothe total enthalpy carried away by the combustionproducts, or m& cp . The effect of void fraction ( ε d ) onthe cross- section area of the evaporator is shown in Fig.(7). ε d values were less than 0.1% for the Propane-aircombustion ( T cp = 1197K), Hydrogen-air combustion( T cp = 1263K), and Hydrogen-air combustion( T cp = 1263K, equal mole flow rate) and less than0.01% for the Hydrogen-air combustion ( T cp = 423K).These results indicate that indeed the dispersed phaseoccupies only a small fraction of the total evaporatorvolume.In Figs. (8)-(11),values was nearly the same forΔ xewas seen to decrease with h c ; t eT cp = 1197KandT cp = 1263K which substantially differed forT cp = 423K (See Fig. 12). As h c increased, V e wasseen to decrease. This was shown in Figs. (13)-(16).Results in Figs. (8)- (16) were included for uniform(constant) h c values. In the course of evaporation, h cchanges were reported for a bubble column design(Kendoush, 2004) and the references therein.Water vapor (10 -3 kg/s)4.543.532.521.510.50WV, Tcp = 1263 K (Hydrogen-Air)TWV, Tcp = 1263 K0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16Hydrogen mass flow rate (10 -3 kg/s)Figure 6. Water vapor mass flow rate with fuel mass flow rate(equal mole flow rate).Ae (m 2 )7.06.05.04.03.02.01.00.0Propane-Air (Tcp = 1197 K)Hydrogen-Air (Tcp = 1263 K)Hydrogen-Air (Tcp = 423 K)Hydrogen-Air (Tcp = 1263 K) Eq Mole0 0.5 1 1.5 2 2.5 3 3.5Hydrogen or Propane mass flow rate (10 -3 kg/s)Figure 7. Evaporator cross-section area versus fuels massflow rate.Larger values for m& wv in Figs. (2) and (3) versus Figs.(5) and (6) was due to the properties of the combustionproducts evaluated at T cp (global model) versus T m(local model). Heat of vaporization ( h fg ) valuesdirectly plugged into the local model resulted in smallvalues for m& wv6

Δx e (10 -2 m)2.01.81.61.41.21.00.80.60.40.20.0Propane: 0.3/(1000) kg/sPropane: 1.2/(1000) kg/sPropane: 2.1/(1000) kg/sPropane: 3.0/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 8. Evaporation length versus heat transfer coefficient,Propane-air combustion ( T cp = 1197K).Δxe (10 -2 m)2.01.81.61.41.21.00.80.60.40.20.0Hydrogen: 0.014/(1000) kg/sHydrogen: 0.055/(1000) kg/sHydrogen: 0.096/(1000) kg/sHydrogen: 0.137/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 11. Evaporation length versus heat transfer coefficient,Hydrogen-air combustion, equal mole flow rate withT cp = 1263K .Δx e (10 -2 m)2.01.81.61.41.21.00.80.60.40.20.0Hydrogen: 0.3/(1000) kg/sHydrogen: 1.2/(1000) kg/sHydrogen: 2.1/(1000) kg/sHydrogen: 3.0/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 9. Evaporation length versus heat transfer coefficient,Hydrogen-air combustion ( T cp = 1263K).Δx e (10 -2 m)35302520151050Hydrogen: 0.3/(1000) kg/sHydrogen: 1.2/(1000) kg/sHydrogen: 2.1/(1000) kg/sHydrogen: 3.0/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 10. Evaporation length versus heat transfer coefficient,Hydrogen-air combustion ( T cp = 423K).te (s)302520151050Propane-Air (Tcp = 1197 K)Hydrogen-Air (Tcp = 1263 K)Hydrogen-Air (Tcp = 423 K)Hydrogen-Air (Tcp = 1263 K) Eq Mole0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 12. Evaporation time versus heat transfer coefficientfor the fuels.Ve (liter)454035302520151050Propane: 0.3/(1000) kg/sPropane: 1.2/(1000) kg/sPropane: 2.1/(1000) kg/sPropane: 3.0/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 13. Evaporator volume versus heat transfer coefficientfor Propane-air combustion ( T cp = 1197K).7

V e (liter)120100806040Hydrogen: 0.3/(1000) kg/sHydrogen: 1.2/(1000) kg/sHydrogen: 2.1/(1000) kg/sHydrogen: 3.0/(1000) kg/swas focused on. The spray-column evaporation showsunique fluid dynamics characteristics compared to abubble column design and the process inside the presentspray-column somewhat resembles a swamp cooler.From our global model centering on first law ofthermodynamics, we showed that about 65% of theincoming seawater evaporation was possible. Thisdefined the recovery ratio ( m & wv / m&sw ) of the seawaterevaporation process.2000 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 14. Evaporator volume versus heat transfer coefficientfor Hydrogen-air combustion ( T cp = 1263K).V e (liter)10008006004002000Hydrogen: 0.3/(1000) kg/sHydrogen: 1.2/(1000) kg/sHydrogen: 2.1/(1000) kg/sHydrogen: 3.0/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 15. Evaporator volume versus heat transfer coefficientfor Hydrogen-air combustion ( T cp = 423K).V e (liter)5.04.54.03.53.02.52.01.51.00.50.0Hydrogen: 0.014/(1000) kg/sHydrogen: 0.055/(1000) kg/sHydrogen: 0.096/(1000) kg/sHydrogen: 0.137/(1000) kg/s0 1000 2000 3000 4000 5000 6000h c (W/(m 2 K))Figure 16. Evaporator volume versus heat transfer coefficientfor Hydrogen-air combustion, equal mole flow rate withT cp = 1263K .CONCLUSIONSWe developed two heat transfer models and evaluatedthe effectiveness of a direct-contact heat exchangeprocess in desalination of seawater. A spray- columnLocal model centering on Navier- Stokes Eqs. was usedto estimate the sizing on the heat exchanger includingevaporator length, cross-section area, and volume. Itwas found that for a given enthalpy of the combustionproducts, only certain amount of seawater canevaporate, thereby, be desalinated. Increasing T cp wasfound to turn out lower Δ xe, t e , and V e but increasem& sw , m& wv , and U v .The other similar methods of thermal distillation, withlower yields compared to MSF and RO processes,include distillation via freezing and solar humidificationAn efficient direct- contact heat transfer process wasproposed in desalination of seawater. Experimentaltesting is crucial to validate the process, which mayinitially substitute a cheaper fuel, e.g., a hydrocarbon.ACKNOWLEDGEMENTSThe author thanks to Dr. Byard D. Wood through theU.S. Bureau of Reclamation and U.S. Department ofEnergy for their support on this project.REFERENCESBillet, R., Evaporation Technology, VCH Publishers,New York, 1989.Elshaik N. M., Direct Contact Heat Transfer to aDispersed Droplet with Change of Phase, NorthCarolina State University, Dissertation thesis, 1990.Hsu, S. T., Cheng, K. T., and Chiou, J. S., SeawaterDesalination by Direct Contact Membrane Distillation,Desalination 143, 279-287, 2002.International Desalination Association (IDA), 2000.Available at http://www.idadesal.org/Kendoush, A. A., Theory of Convective DropEvaporation in Direct Contact with an ImmiscibleLiquid, Desalination 169, 33–41, 2004.Khawaji, A. D., Kutubkhanah, I. K., and Wie, J-M.,Advances in Seawater Desalination Technologies,Desalination 221, 47–69, 2008.Kreith, F. and Boehm, R. F., Direct Contact HeatTransfer, Hemisphere Pub Corp, Washington, 1988.8

Marek, R. and Straub, J., Analysis of the EvaporationCoefficient and the Condensation Coefficient of Water,Int. J. of Heat and Mass Transfer 44, 39-53, 2001.Termpiyakul, P., Jiraratananon, R., and Srisurichan, S.,Heat and Mass Transfer Characteristics of a DirectContact Membrane Distillation Process forDesalination, Desalination 177, 133-141, 2005.Yun, Y., Ma, R., Zhang, W., Fane, A. G., and Li, J.,Direct Contact Membrane Distillation Mechanism forHigh Concentration NaCl Solutions, Desalination 188,251-262, 2006.Wark, K., Advanced Thermodynamics for Engineers,McGraw-Hill, New York, 1995.Murat TEKELİOĞLU received his B.Sc. degree in Mechanical Engineering from theIstanbul Technical University in 1998. He holds M.Sc. and Ph.D. degrees in MechanicalEngineering from the University of Nevada, Reno, earned in 2000 and 2005, respectively.His research interests include thermal, electrical, and optical systems design andsimulation. He is an Assistant Professor of Mechanical Engineering Department inKarabük University and currently teaching at Gazi University in Ankara.9