seawater desalination in a direct contact heat exchanger

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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
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Page 5 and 6: which gives the decay of r o during
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