PHYSICAL, CHEMICAL AND BIOLOGICAL ASPECTS OF WATER - Thermal Desalination Processes - Asghar Husain, Adel Al Radif, Ali El-Nashar and Klaus Wagnick

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1 THERMAL DESALINATION PROCESSES Asghar Husain, Ael International Center for Water an Energy Systems, Abu Dhabi, UAE Keywors: Boiling point elevation, Conensation, Evaporation, Flashing, Fouling factor, Vapor compression Contents 1. Introuction 1.1. Single-effect Boiling 1.2. Single-stage Flashing 1.3. Dual-effect Boiling 2. Multiple Effect Boiling (MEB) Plants 2.1. MEB Plant Analysis 3. Multistage Flash (MSF) Plants 3.1. Temperature Profiles 3.2. MSF Plant Analysis 3.3. Energy Consumption 3.4. Stage Temperature Loss ( T L ) 3.5. Non-equilibration Loss ( TE) 3.6. BPE 3.7. Demister Pressure Loss 3.8. Conenser Pressure Loss 3.9. Conenser Terminal Temperature Difference Fouling Factor Effect of Scaling Design Consieration Precise Analysis 4. Vapor Compression 4.1. Mechanical Compression 4.2. Thermal Compression Glossary Bibliography an Suggestions for further stuy Summary Desalination processes which involve the evaporation an conensation of water are escribe. These inclue multiple-effect boiling, multistage flashing, an vapor compression both mechanical an thermal. 1. Introuction In istillation seawater is boile (evaporate) to give off water vapor, which on conensation yiels salt-free liqui water. Two wiely use esalination processes which make use of this principle are (a) multistage flash istillation an (b) multieffect

2 boiling. The energy require to rive the process is supplie in the form of thermal energy. The term "multi" implies that multiple reuse of the thermal energy of steam involving the successive processes of evaporation an conensation. Another process, also base on this concept, is vapor compression istillation, which, however, is not as popular as the other two. Here, a compressor or a steam jet ejector supplies the energy to the process Single-effect Boiling The simplest conceptual scheme for esalination is shown in Figure 1, in which seawater is boile in an evaporator by conensing steam. The vapor so generate is conense by heat transfer to a stream of cooling seawater, part of which forms fee to the evaporator. The vapor conenses to pure water, which is the prouct of esalination. Figure 1. Single-effect boiling. An overall energy balance of the above process gives the heat input by steam Q: Q= m C ( T T ) + m C ( T T ) + m C ( T T ) + m ( Δ H), (1) where (Δ H ) To c o c f f o b b v o f T o is the isothermal enthalpy change ue to composition changes per unit of fee to the evaporator. The energy balance of the conenser is written as mc( T T) + mλ = ( m + m) C ( T T). (2) v v c Tc f c f g o Substituting for m c C f (T g -T o ) from Eq. (2) into Eq. (1),

3 Q= m C ( T T ) + m C ( T T ) m C ( T T ) + m λ + mc ( T T ) + m ( ΔH) f c o v v c f f g o Tc b b v o To (3) If the term m f (Δ H ) T o is neglecte in Eq. (3) an a mean specific heat C is use for all the streams incluing that of vapor, then it can be approximate to Q m f q = λt + C ( T ), c v Tg m m where q is the heat input per unit mass of prouct. Furthermore (T v - T g ) can be consiere as the sum of the boiling point elevation (BPE) an terminal ifference ΔT t. Therefore, Eq. (4) can be written as m f q λ T + C ( BPE + ΔTt ). c m Ultimately, the above equation can be approximate in terms of the performance ratio R (kilograms of prouct per 2326 kj) incorporating the reference value of the latent heat of vaporization λ r : λr R m f λt + C (BPE T) c +Δ t m In Eq. (5), if the magnitues of ifferent terms are taken as C = 4.2 kj kg -1 C; BPE = 1 C an ΔT t = 5 C an m f /m = 2. (4) (5) Because in evaporation practice the final concentration is normally not allowe to be more than twice that of the seawater; λ Tc λ r = 2326 kj kg -1, then the value of R cannot be more than 1 kg 2326 kj -1 in the single-effect evaporation as shown in Figure 1. The major source of irreversibility an, thus, entropy creation in

4 the single-effect evaporation process is in the wastage of available energy in the leaving streams. This is the motivation for the evelopment of multieffect systems to be escribe later, in which the available energy is utilize to prouce more istillate Single-stage Flashing An alternative process is shown in Figure 2, in which the fee m f is further heate by conensing steam in the brine heater. Here, the fee is prevente from boiling by keeping its pressure above the saturation pressure corresponing to its temperature. The fee is then throttle by passing through a valve into the flash chamber which is at a lower pressure. The vapor generate by flashing is conense, as usual, by heat transfer to a stream of cooling seawater, part of which forms the fee to the brine heater. The conense vapor (m ), being pure water, is the prouct of esalination. Figure 2. Single-stage flashing. For the single-stage flashing process, the overall energy balance an the energy balance for the conenser will be the same as given by Eqs (1) an (2), respectively. Hence, Eq. (3) will also express the heat output in this process. However, with flashing there is a basic ifference, i.e. the vapor release is now epenent on the fee rate. An energy balance for the flash chamber gives m f C( Th Tc ) = mc( Tv Tc ) + m λ Tc + mbc( Tv Tc ) or m f C[( Th Tv ) + ( Tv Tc )] = m f C( Tv Tc ) + m λtc Finally,

5 m m f λt = c ( CΔT ) f (6) where ΔT f = T h -T v is the temperature rop in the flashing process. If in eq. (2), T v - T c = BPE is neglecte in Eq. (2), then substituting Eq. (6) into it gives ΔT f mc = mf 1 T g T o (7) Equation (7) shows that, unless ΔT f > T g - T o, no aitional cooling water will be require in the conenser. An energy balance on the heater provies heat input per unit prouct as follows: mf mf q = C( Th Tg) C( Tf Tt BPE) m = m Δ +Δ + (8) Substituting Eq. (6) into Eq. (8), Δ + = + Tt BPE q λ Tc 1 ΔT f or λr R = Δ Tt + BPE λtc 1+ T Δ f Here again it is clear from Eq. (9) that in a single stage it is not possible to have a performance ratio of R bigger than one. The highest value of R is obtaine when the fee is flashe over the total available temperature range (T s - T o ) after allowing for ΔTt, BPE, an the seawater temperature rise in the conenser for a finite rate of cooling water. When the available energy from the outgoing streams is to be utilize, ΔTf has to be small, resulting in R 0.5 from Eq. (9). It may be seen from Eq. (5) that single-effect evaporation by boiling can still provie R Dual-effect Boiling (9) Compare to a single effect as shown in Figure 1, a further gain in the performance ratio can be obtaine by increasing the number of effects in series. A typical ual-effect istillation is shown in Figure 3, in which the vapor generate in the first evaporator is use as a heating meium in another evaporator operating at a lower temperature an pressure. The make-up enters in a backwar fee arrangement, i.e. first entering in the effect working uner lower pressure an the blow own is ischarge from the effect

6 uner higher pressure, having the following flow rates: M for steam, M f for make-up fee, an M 1 an M 2 for vapor generate in the first an secon effects, respectively. Figure 3. Simple ual-effect istillation. Energy input into the first effect (conensation of steam) = M s λ Energy input into the secon effect (conensation of vapor from first effect) = M 1 λ Assume the intereffect temperature ifferences T 1 - T 3 = T 3 - T 2 = ΔT. Furthermore, assume that the specific heat capacity of the liqui C P an latent heat of vaporization λ o not vary in the range of operation. Heat balance of the first effect: M λ = M λ + ( M M ) C Δ T (10) s 1 f 2 p Heat balance of the secon effect: M λ = M λ + M C Δ T (11) 1 2 f p Total evaporation: 1 2 M = M + M (12) Solving Eqs (10) an (11) for M 1 an M 2 an inserting these values in Eq. (12): 2 2 Msλ 2M f CpΔT 2M f ( CpΔT ) / λ M f CpΔT M =. λ CpΔT λ If, in the above equation, the term (MfCpΔT)/λ is ae an subtracte, then M 2M sλ 4M fcpδt M fcpδt = + λ C ΔT λ p (13)

7 Defining the following ratios, performanc e ratio (PR) M R = M s an recovery ratio (RR) M R c = M f, then M f Ms = M f / M Ms / M R =. R c Figure 4. Performance ratios of a ual-effect MEB plant. Diviing Eq. (13) by M s, we obtain

8 2 R = 3 ( CpΔT) CpΔT Rc ( Rcλ) λ (14) Equation (14) is plotte in Figure 4 for fixe values of C p an λ for four ifferent values of ΔT as parameters. It is clear that as ΔT is reuce, R approaches a value of TO ACCESS ALL THE 37 PAGES OF THIS CHAPTER, Visit: Bibliography an Suggestions for further stuy Billet R (1989) Evaporation Technology. Weinheim, Germany: VCH Publishers. Hanbury W T an Hogkiess T (1995) Desalination Technology 95. An Intensive Workshop an Course. Lenzie, Glasgow: Porthan Lt Easter Auchinloch. K.S. Speigler Y.M. El-Saye (1994 ) A Desalination Primer,Balaban Publishers M.A.Darwish,Iain McGregor (2005),Thermal esalination Processes,Funamentals an Practice,Five ays Intensive Course,MEDRC,Oman Morin O J (1995) Desalination state of the art Part I. Desalination an Water Reuse 4/3. Spiegler K S an Laire A D K (es) (1980) Principles of esalination, Part A. New York: Acaemic Press.