E L S E V I E R Desalination 116 (1998) 11-24

DESALINATION

Exergy losses in a multiple-effect stack seawater desalination plant

Ali M. E1-Nashar, Atef A. A1-Baghdadi Water and Electricity Department, Abu Dhabi, UAE

TeL~Fax +971 (2) 434906

Received 5 June 1997 ; accepted 30 October 1997

Abstract

The use of the Second Law of Thermodynamics to describe and quantify the exergy losses involved in the different processes taking place in desalination plants is of considerable importance both for the design and operation of these plants. Although the First Law of Thermodynamics is adequate in giving the overall plant performance indices, it does not show the actual irreversibilities in the different parts of the plant. An estimation of the exergy destruction involved in each part of the plant gives a quantitative measure of these irreversibilities which, if reduced during design or operation, can result in a reduction of energy consumption and increase plant performance. Based on actual measured data from a multiple-effect stack seawater desalination plant now in operation in the solar plant near Abu Dhabi, the exergy destruction was calculated for each source of irreversibility. The major exergy destruction was found to be caused by irreversibilities in the different pumps with the vacuum pump representing the main source of destruction. Major exergy losses are associated with the effluent streams of distillate, brine blow-down and seawater. Exergy destruction due to heat transfer and pressure drop in the different effects, in the preheaters and in the final condenser and in the flashing of the brine and distillate between the successive effects represents an important contribution to the total amount of exergy destruction in the evaporator.

Keywords: Exergy; Multiple effect; Seawater desalination

1. In troduct ion

Seawater distillation processes are very energy intensive since they consume a large amount o f energy resources to produce fresh water. The

*Corresponding author.

exergy method is particularly useful in optimizing the design and operation o f such processes with the aim o f reducing energy consumption.

This paper deals with the application o fexe rgy analysis to a multiple-effect stack (MES) distil- lation plant which is currently in operation at the solar desalination plant in Abu Dhabi, UAE. The

0011-9164/98/$ - see front matter © 1998 Elsevier Science B.V. All fights reserved PII S0011-9164(98)00053-8

12 A.M. El-Nashar, A.A. A1-Baghdadi / Desalination 116 (1998) 11-24

objective of such an analysis is to fully understand where energy inefficiencies and real energy losses are located and where modifications should be implemented to improve the overall utilization of energy in the plant. First, a description of the process and the block diagram are presented. After that, the calculating procedure to obtain the exergy balance, the exergy losses for each plant component is given.

The fundamentals of the exergy method of analysis are well covered in the texts by Moran [1] and Kotas [2] and in many papers, for example, the papers by Tribus et al.[3], EI-Sayed [4] and Gaggioli [5]. It is not the aim of this paper to add a new text to literature that is already available but rather to present a practical application of the exergy method. Detailed information about funda- mentals and basic calculating procedures can be found in the literature cited.

2. Multiple-effect stack evaporator

2.1. Description o f the MES evaporator

A flow diagram of the seawater MES evapo- rator is shown in Fig. 1. The evaporator has a rated capacity of 120m3/d and consists of a tower having 18 effects stacked vertically in a double-tier arrangement with the highest temperature effect (No. 1) located at the top of the tower and the bottom temperature effect (No. 18) located at the bottom. All even effects are located in one tier and the odd effects are located in the second tier. The double-tier arrangement is incorporated in one evaporator vessel. In addition to the 18 effects, the evaporator incorporates 17 evaporative-type pre- heaters and a final condenser.

Preheated seawater is sprayed on the evaporat- ing tube bundle of the first effect where part of it is evaporated, and the remaining brine descends to the next effect where more vapor is generated; this process is repeated from one effect to the next until the brine finally reaches the bottom effect where it is discharged as brine blow-down.

Heating water is supplied to the first effect as the external heating agent where it cools down as it flows inside the tubes. Vapor generated in each effect is used as the heating medium of the next effect where it flows and condenses inside the tubes and transfers its heat of condensation to the brine flowing as a film outside the tubes. The condensed vapor in each effect forms part of the final distillate production. The vapor generated in the bottom (18th) effect is condensed in the final condenser. The vapor generated in each effect passes through a mist separator (demister) to remove any entrained brine droplets before flow- ing to the next effect with the exception of the vapor produced in the bottom effect which is condensed in the final condenser. A small amount of the vapor produced in effects 1-17 is condensed in a preheater by exchanging heat with the feedwater.

The evaporator operates under vacuum which is maintained by a water-ring vacuum pump. The absolute pressure to be maintained in the final condenser is designed to be 50mmHg with the pressure in each effect varying from slightly below atmospheric in the first effect to about 50 mmHg in the bottom effect.

2.2. Testing and measurements

In order to evaluate the performance of the evaporator, the temperatures, pressures and flow rates of the main streams entering and leaving the evaporator have to be monitored on a 24-h basis with the data directed to a data acquisition system (DAS) for data recording and processing. Data signals from the DAS are transmitted to an on-line computer for further data analysis and display of plant condition.

The following fluid temperatures were measured: • seawater inlet to condenser • seawater outlet from condenser • heating water inlet • heating water outlet

A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24 13

Effect E2

Preheater P1 Effect E1

T~

heating water in

heating water out

Effect E4

P3 Effect E3

Ti

Effect

Effect EN TNi

Feedwater

Seawater in I Condenser

P

Seawater out

b

Seawater to outfall

Fig. 1. Flow diagram of the MES evaporator.

14 A.M. El-Nashar, A.A. A l-Baghdadi / Desalination 116 (1998) 11-24

feedwater temperature at the outlet of the first (top) preheater first (top) effect temperature last (bottom) effect temperature.

A constant-current system was adopted for temperature measurement using three-wire platinum-resistant thermometers which have an accuracy of±2% of full scale.

Circular contact pressure gauges and pressure switches were used for producing on-off electrical signals associated with a preset pressure level. These pressure gauges have an indication accuracy of+1.5%.

Flow rates of the heating water, feedwater and distillate were measured with vortex flow meters (Yokogawa Electric Co.) with an output of 4- 20mA DC output with an accuracy of+ l .0% of reading plus ±0.1% of full scale. Flow rates of sea- water and brine blow-down were measured with magnetic flow meters (Tokyo Keiso Co.) having an accuracy of±3%.

The DAS used is a Thermodac 32 (Eto Denki Co.) high precision data logger having 32 input channels capable of processing data from various types of sensors having analog and digital signals.

(kJ/kg) consisting of a mixture of substances

n

e : h- roS -Z i=1

i=1

where e~m is the thermo-mechanical component of flow exergy and e~ is the chemical component of flow exergy, xi is the mass fraction of substance i in the mixture, ~ti0 is the chemical potential of substance i in the stream at To and P0 and ~t ° is the chemical potential of substance i in the environ- ment. The exergy of a flow stream (kW) can be obtained by multiplying the specific exergy by the mass flow rate, thus obtaining

E,: EJ n

i=1

(2)

3. Analysis of exergy destruction and exergy losses

Exergy is an extensive thermodynamic function defined as the maximum amount of work obtain- able when a stream of substance is brought from its initial state to the environmental state. Unlike energy which is conserved in any process according to the First Law of Thermodynamics, exergy is actually destroyed (annihilated) due to irreversibilities taking place in any process which manifest itself in entropy creation or entropy increase. The function of immediate interest is the potential work of a flow stream, also called specific exergy or specific available energy, and is defined by Eq. (1) per unit mass of a flow stream

If the processes in a system do not involve relevant composition changes, there is no need to consider the chemical exergy component, and the only component which is relevant is the thermo- mechanical one.

Exergy destruction, also known as irreversi- bilities, exergy annihilations, or exergy consump- tion, is generated by entropy production within the process performed in each component. In addition to the irreversible exergy losses, there are some components in which exergy is rejected to the atmosphere due to hot stream effluents. These are effluent exergy losses.

Once the process flow diagram for the MES process has been set up , conventional mass, enthalpy and entropy balances for each component

A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24 15

or subsystem are calculated. The calculation of the total exergy for each stream should consider the physical exergy (thermal and mechanical) defined by its temperature and pressure relative to the environmental state at the temperature and pressure of the surrounding medium, and the chemical exergy, when needed, which depends on the chemical composition of the stream and that of the environment.

The irreversible exergy losses in each component can be obtained from the difference between exergy input and exergy output, or if the entropy values are used, the irreversible exergy losses can be obtained from the entropy difference multiplied by the temperature of the environment. In this way the effect of the selection of a particu- lar environmental model is canceled.

In the steady state, the difference between the exergy entering a subsystem and that leaving is lost work. The exergy balance can thus be written as

e- 2e =D (3) in out

where D is exergy destruction in the subsystem and E is the total exergy which consisting of flow exergy E f , exergy due to heat transfer E q and exergy due to work transfer E TM

E = E f + E q +E w

E w = w

(4)

In the MES evaporator exergy destruction takes place due to heat transfer, pressure drop, flashing and fluid pumping. The method of estimating each of these destruction is described in the following section.

3.1. Exergy destruction in heat transfer processes

3.1.1. Destruction in the preheaters, D 1

Each of the 17 preheaters in the MES plant consists of a four-pass heat exchanger with sea- water flowing inside the tubes and steam condens- ing on the outside. As shown by Koot [6] and Darwish et al. [7], the exergy destruction due to heat transfer in the preheaters can be expressed as

17 f D1 = E TolilfCPf n T, T , - Th(,_l~ (5)

where preheater 1 is the top preheater and preheater 17 is the bottom one; Tv, is the temperature of vapor supplied to preheater i, T m is the temperature of seawater leaving preheater i and Tph(,_l) is the corresponding temperature of pre- heater (i- 1).

3.1.2. Destruction in the evaporators, D 2

In the first effect evaporator, heating water flows inside the tubes and is cooled from Thw ~ to Thwo while seawater is sprayed on the outside and boils at a temperature T 1. The exergy loss due to heat transfer for this heat exchanger can be estimated from the following equation (see Bejan [8]).

D2,1 =lilhw%hwT 0 n Thwi) T1

(6)

In evaporators 2 through 18, condensing vapor flows inside the tubes while boiling brine is on the outside. The exergy loss due to heat transfer for these evaporators can be estimated from the following equation:

18

D2,2-18 = E Q, i-2

1 1 / (7)

16 A.M. El-Nashar, A.A. A L Baghdadi / Desalination 116 (1998) 11-24

where Q~ is the heat load of evaporator i, Te, is the brine temperature in the evaporator and Tv(i_l) iS the temperature of the vapor generated in evapo- rator (i- 1) and supplied to evaporator i. The heat load of evaporator i can be obtained by realizing that part of the vapor generated in evaporator (i- 1) is condensed in preheater (i-1) by heating the feedwater passing through this preheater. Thus, assuming equal temperature rise for each preheater, we can express the heat load in evapo- rator i as

Q, = rhd(,_l) L-rhfw Cpf~ Th' - T''~° (8) 17

The exergy loss due to heat transfer in all 18 evaporators can thus be obtained as

D 2 = rhh~ C h , T O In Thwi) T,

18

+E i=2

tha(,_ 1) L _rhyw Cp~ Tphli-_7Tw° ] (9)

1 1

3.1.4. Destruction due to heat transfer to the surroundings, D 4

Despite the fact that the MES evaporator is well insulated, heat loss from the evaporator to the surrounding takes place, and this is accompanied by exergy loss which depends on the temperature level at which the evaporator operates. I f we assume that the heat loss is Qc and that its average temperature is Tav =(T 1 + T18)118, the exergy loss associated with QL can be estimated from the equation

(11)

It is assumed that the heat loss to the surroundings amounts to 2% of the heat input in the first effect (according to Koot [6] and Darwish [7]).

3.1.5. Destruction due to heat transfer through intereffect walls, D 5

A small amount of heat is transferred across the intereffect walls due to the temperature difference between the successive stages. The exergy destruction associated with the heat across the intereffect wall of stage i can be estimated from

3.1.3. Destruction in the condenser, D 3

In the condenser, seawater enters the condenser tubes at T~, and leaves at T~.wo while vapor at Tv08) is condensed on the shell-side. The exergy loss due to heat transfer can be estimated from the equation

1 -Ew,] D 3 : r h w C . w T 0 n T'~° "" Zswi ~v(18)

(lO)

O5, i = T O 1 ~)U**A~,6T (12) r,-6r

where U,s is the intereffect overall heat transfer coefficient and Ais is the area of the intereffect wall. The exergy destruction across all intereffect walls can be obtained by summation:

18 ( i l) D5 = To U,A, 8V

,=, L- T T, " (13)

A.M. El-Nashar, A.A. A l-Baghdadi / Desalination l 16 (1998) I 1-24 17

3.2. Exergy destruction due to pressure drop

3.2.1. Destruction due to f lashing brine, D 6

Flashing of the brine takes place as the brine cascades down the successive effects. Flashing is an irreversible process and is accompanied by exergy destruction which was estimated by Koot [6] for flashing between effects i and i+1 by the equation

D6, i = mb, iC p ToS T 2

Thus, total exergy destruction due to brine flashing can be calculated from

17 To S T 2 D6 = E rob, i%

i=1

where Mj,~ is the accumulated distillate generated in effects 1,2,3.., i,

i

Ma. , = ~ ma, j (17) j=l

3.2.3. Destruction due to mist separators, i)8

The vapor released in each effect travels across a mist separator in order to remove any brine droplets entrained with the vapor before entering condensing in the evaporator tube bundle of the next effect. During its passage through the separator, the vapor experiences a pressure drop and also a drop in temperature with consequent exergy destruction.

The exergy loss through the mist separator of effect i can be expressed by the equation

APd, i D8,i = ma, iToR - - (18)

Pi

3.2.2. Destruction due to f lashing distillate, D 7

In the MES evaporator, the distillate produced from any effect is mixed with the combined distillate produced from all the preceding effects, and all this distillate is allowed to flash to the next effect which is at slightly lower pressure and temperature, thus experiencing exergy loss. The total exergy destruction due to distillate flashing can be estimated from an equation similar to the one above:

where APa, is the pressure drop in the mist separator of effect i , P, is the pressure in the same effect and R is the gas constant for steam. Thus the exergy destruction in the 18 effects is

18 AP a i D 8 : ~_, md, i ToR

i:l Pi (19)

The pressure drop in the mist separator is calculated from the experimental correlation given by Hanbury et al. [9],

17 To 6 T 2

,:l 2 / / / /+ 6

/ ) ' AP~i(bar ) = 3.29× 10-9phtv ma'i (20)

A.)

where t is the thickness of the separator (m), Pb is the density of brine droplets entrained in the vapor

18

(kg/m3), v is the vapor specific volume (m3/kg), A a is the area of the separator (m 2) and ma,, is the vapor flow rate in the ith effect (kg/s).

3.2.4. Destruction due to pressure drop in preheaters, D 9

A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24

operating data are used:

rhf = 17.5 x 103 kg/h

T O = 3 0 o c

T = 3 3 ° C

The feedwater flowing in the preheaters experiences frictional pressure drop in flowing through the tubes, U-turns and water boxes. The exergy destruction in the preheaters is calculated from

v aGh,, v0 17

D9 = Z Th"+ Th'('÷l) (21) i=1 2

v = 0.001 m3/kg

A P --:3x10 SPa

3.2.6. Destruction due to pressure drop in condenser, D H

The exergy destruction in the condenser can be estimated in a similar way by using the equation

where v is the specific volume of feedwater and APp~,i is the pressure drop for preheater i. 111 calculating APp,., ,it is assumed that all preheaters have the same pressure drop and that the specific volume of feedwater is constant in all preheaters.

3.2.5. Destruction due to pressure drop in feedwater control valve, D lo

The feedwater control valve maintains the feedwater flow at a constant level during opera- tion. A pressure drop of approx. 3 bar exists across this valve which corresponds to an exergy destruction of

vAPor DI° = rhfT° T

_ (17.5× 103 ) (303) (0.001)(3×105)

3600 (33 +273)(1000)

= l . 4 4 k W

(22)

In the above equation, the following typical plant

yA P c D11 = rh,.~ T o - -

T

_ (36× 103) (303) (0.001)(0.3 × 105) 3600 ( -~ S 2 - - ~ 1-0"-0--~

=0 .3kW

(23)

In the above estimate, the condenser pressure drop was taken as 0.3 bar and the average seawater temperature 33 °C.

3.3. Destruction in the pumps, DI2

Five main pumps are used in the MES evaporator: the mechanical water seal vacuum pump, the heating water pump, the seawater feed pump, the brine blow-down pump and the product water pump. All the pumps are motor-driven.

There are three sources ofexergy losses in any motor-driven pump: (1) exergy loss in the pump motor due to heat generated by its electrical resistance that is lost to the surrounding, (2) exergy loss in the motor and pump due to frictional effects which is totally converted into heat and dissipated

A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24 19

to the surrounding, and (3) exergy loss due to frictional effects in the pumped fluid which is converted into heat supplied to the fluid.

If W is the electrical energy supplied to the pump motor, lqmec h is the mechanical efficiency of the motor-pump set and rh, is the internal efficiency of the pump, the energy supplied to the fluid is "lqmechW and frictional energy generated (as heat) in the fluid is (1- rli,) rlm¢chW. This amount of heat is transferred to the fluid at an essentially constant temperature, T, since the temperature rise of the fluid during its passage through the pump is negligibly small. The exergy loss associated with this amount of heat generated is

of exergy. The specific chemical exergy for the brine

blowdown and distillate were calculated from the following equations from E1-Sayed [4]:

• Brine blow-down:

e c : e b ~ T o (x_x0) l n - - - d P ~ x 0 °x°

(26) • Distillate:

To (1 -Tlin)lqmec h m (24) T

Thus, the total exergy loss of a pump is

Di2 = (1 - lqmech)W+ ~ (1 - lqin)Tlmec h W (25)

In addition to the five main pumps, there are a number of small chemical dosing pumps which are neglected in this study because of their insignifi- cant exergy losses compared to the main pumps.

3.4. Exergy losses in the effluent streams

3.4.1. Losses due to distillate flow, brine blow- down and seawater discharge, L1

The exergy of the effluent streams can be calculated by multiplying their specific flow exergy by their mass flow rate. As was explained before, both thermo-mechanical and chemical components of exergy have to be accounted for if the chemical composition of the stream is different from seawater which is considered as the base composition. Thus warm seawater effluent will only contain thermo-mechanical exergy while both distillate and brine blow-down possess both types

Toxo

where e is the fugacity coefficient (=2.0), R is the universal gas constant (=8.31 kJ/kmole K), x is the mole fraction of the brine blow-down, x0 is the mole fraction of seawater and MW is the molecu- lar weight of water (= 18). The total dissolved salts (TDS) of seawater at the plant site is approx. 45,000 ppm.

3.4.2. Losses due to discharge o f non- condensable gases and vapor, L 2

A water ring vacuum pump is used to create the vacuum inside the evaporator and remove noncondensable gases released by seawater. The pump has a maximum capacity of 4.5m3/min at a suction pressure of 50Torr and is run by an electrical motor rated at 15 kW. A mixture of gas and vapor is drawn from the evaporator with their estimated quantities being rhg = 1.5 kg/h for gas and rhv=l 1 kg/h for vapor [10]

It is possible to calculate the exergy value of the ejected mixture of gas and vapor once their exit condition (pressure and temperature) and their mass flow rates are known,

20 A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24

Z2,g=li lg[(hg-hgo)-Zo(sg-Sgo)]

v : [(hv - hvo)- o(Sv (27)

Table 1 MES evaporator average flow rates and temperatures prevailing during 6:00-7:00 am on July 30, 1995

L 2 = Dl4,g + Dl4,v Effect Inlet Outlet Outlet Distillate no. brine flow, brine flow, brine produced,

m3/h m3/h temp., m3/h

°C

4. Results

In order to calculate typical values o f tile exergy destruction and exergy losses in the MES evaporator, operational data for one hour (6:00- 7:00 am) during July 30, 1995, were selected. The main operating parameters and measured data prevailing during this period were extracted from the plant data archive and are given below:

Heating water flow rate Heating water inlet temperature Heating water outlet temperature Feedwater flow Seawater flow Seawater temperature Seawater outlet temp. from condenser Distillate flow rate Specific heat consumption Feedwater pump discharge pressure Distillate pump discharge pressure Brine blow-down pump discharge

pressure Pressure drop across feed control valve

13.4 m3/h 75.9 °C 64.6°C

17.7 m3/h

36.0 m3/h 31.9°C 37.7°C

3.1 m3/h

kJ/kg dist. 8.1 bar 2.7 bar

2.4 bar 3.0 bar

The f low rates and temperatures in each o f the 18 effects are given in Table 1.

The power consumption and efficiencies o f the main pumps at their operating points during the test period were obtained from the characteristic curve o f each pump and are given in Table 2. The motor efficiency was assumed to be 0.94.

The exergy balance o f the evaporator is shown in Fig. 2 and demonstrates that 23.6kW of exergy is destroyed inside the evaporator by virtue o f the

l 17.7 17.4 64.7 0.3 2 17.4 17.3 63.3 0.1 3 17.3 17.1 61.8 0.2 4 17.1 16.9 60.3 0.2 5 16.9 16.7 58.8 0.2 6 16.7 16.6 57.3 0.1 7 16.6 16.4 55.9 0.2 8 16.4 16.2 54.4 0.2 9 16.2 16.1 52.9 0.1 10 16.1 15.9 51.4 0.2 11 15.9 15.7 49.9 0.2 12 15.7 15.6 49.4 0.1 13 15.6 15.4 47.0 0.2 14 15.4 15.3 45.5 0.1 15 15.3 15.1 44.0 0.2 16 15.1 15.0 42.5 0.1 17 15.0 14.8 41.0 0.2 18 14.8 14.6 39.5 0.2

Total distillate 3.1

Table 2 Power consumption and efficiency of main pumps at the operating period

Pumpname Power Pump consumption, efficiency kW

Water ring vacuum pump 11.5 Heating water pump 0.5 Seawater feed pump 5.7 Brine blow-down pump 1.9 Distillate (product) pump 1.2

0.4 0.51 0.44 0.47 0.17

A.M. El-Nashar, A.A. Al-Baghdadi / Desalination 116 (1998) 11-24 21

heat loss to ambient 0.22 kW

heating water in 62.2 kW

vent gases 0.1 kW

M E S E v a p o r a t o r heating water out 43.2 kW

total exergy destruct ion Z D = 30.38 kW

Fig. 2. Exergy balance of the MES evaporator.

i y brine blowdown 3.6 kW

I Lk r - seawater out 1.1 kW

h distillate 4.4 kW

different irreversibilities taking place during opera- tion. The details of these exergy destruction are given in the next section.

4.1. Exergy destruction and exergy losses

We differentiate between exergy destruction and exergy losses by noting that exergy destruction is normally the result of irreversibilities taking place due to energy conversion processes such as heat transfer and pressure drop while exergy loss is normally associated with an effluent stream which has a finite amount ofexergy due to the fact that its properties (pressure, temperature and concentration) are different from that of the environment. We refer to exergy destruction by the symbol "D" whereas exergy loss is given the symbol "L".

Table 3 gives values of the exergy destruction in the different plant components as well as the exergy losses associated with the effluent streams. It can be seen that the major exergy destruction takes place in the pumps and this accounts for 49.3% of the total whereas the effluents streams of distillate, brine blow-down and seawater discharge account for 34.9% of the total. The estimated exergy destruction in each of the main pumps is given in Table 4 which shows that the major portion of exergy destruction takes place in the water-ring vacuum pump which is responsible for

about 60% of the exergy destruction in all pumps. This pump has a rated capacity of 4.5 m3/min at a suction pressure of 50 Torr and its motor is rated at 15 kW and uses an air ejector to draw the vent gases from the evaporator instead of drawing the gases directly. The estimated quantities of vent gas consists of 1.5 kg/h of noncondensables (mostly air) and 2.6kg/h of entrained vapor with volu- metric flow rate calculated as follows:

• Noncondensable quantity 1.5 kg/h x 22.7 m3/kg =

• Vapor quantity 2.6 kg/h x 37.47 m3/kg =

• Total

34 m3/h

97 m3/h 131 m3/h 2.18 m3/min

Thus the estimated amount of vent gas (2.18 m3/ rain) is substantially lower than the pump capacity (4.5 m3/min) because of the motive air drawn through the air ejector. By drawing the vent gas directly from the evaporator, without the need of an air ejector, it is possible to reduce the pump capacity and pumping power by almost one half.

The excessive pump exergy destruction noted above is also due to the low pump efflciencies which are the result of running the pumps their best efficiency point at the test period and also due to the fact that these pumps have small capacity with generally lower efficiency than larger pumps.

22 A.M. El-Nashar, A.A. A1-Baghdadi / Desalination 116 (1998) 11-24

Table 3 Exergy destruction and losses in the MES evaporator

Source of destruction or loss Symbol Amount of exergy destruction/loss, kW Percentage of total

Preheaters Dt 1.50 5.7 Evaporators D 2 2.81 l 0.8 Condenser D3 3.0 l 1.5 Heat transfer to surrounding D 4 0.22 0.8 Heat transfer through intereffect walls D5 0.14 0.5 Brine flashing D 6 0.92 3.5 Distillate flashing D7 0. l 0 0.4 Mist separator D8 2.58E-5 Negligible AP in preheaters D9 2.04 7.8 AP in control valve D~0 1.44 5.5 AP in condenser D~ 0.54 2.1 Pumps D~2 12.88 49.3

Distillate, brine blow-down and L~ 9.10 34.9 seawater discharge

Noncondensables and vapor L2 0.10 0.4

Total 26.1 100

Table 4 Exergy destruction in the main pumps

Pump name Exergy destruction

kW %

Water ring vacuum pump 7.18 55.9 Heating water pump 0.26 2.0 Seawater feed pump 3.34 26.0 Brine blow-down pump 1.06 8.3 Distillate (product) pump 1.01 7.8

Total 12.85 100

For example, the brine blow-down pump has a best efficiency of only 50% at its rated capacity.

The exergy loss associated with the warm effluent streams (distillate, brine blow-down and seawater) amounts to 9.1 kW. The exergy of the distillate is 4.4 kW out of which about 96% repre- sents chemical exergy, and the rest (4% is thermo- mechanical. The brine blow-down has an exergy of

3.6kW, out of which 55% is chemical. The seawater effluent has an exergy of 1.1 kW which is totally thermo-mechanical. Although the portion of chemical exergy associated with the distillate and brine blow-down cannot be reduced, the thermo- mechanical portion of all effluent streams can be reduced by designing an evaporator with a relatively low bottom-effect brine temperature. This option has to be considered within a thermo- economic design analysis of the evaporator.

The exergy loss due to heat transfer in the evaporators (D2=2.81 kW) is mostly consumed in the first effect where 2.79 kW is destroyed there, with the remaining amount (0.02kW) destroyed in the other 17 evaporators. This is the result of the high average temperature difference prevailing in the first effect compared with the other effects due to the fact that the heating medium in this effect is water and not steam. If steam is used as the heating medium, a considerable amount ofexergy could be saved.

A.M. El-Nashar, A.A. AI-Baghdadi / Desalination 116 (1998) 11-24 2 3

An excessive amount of exergy is destroyed in the control valve in the feedwater line (Dl0 = 1.44 kW) which appears to be due to oversizing the capacity of the feedwater pump which made it necessary to run this valve at a 50% open position, resulting in a great exergy loss.

5 . C o n c l u s i o n s

The exergy destruction and exergy losses occurring in an operating MES evaporator has been estimated for a typical period of 1 h. It has been shown that the major exergy destruction takes place in the pumps, with the vacuum pump responsible for the largest portion of this exergy destruction. It was found that the exergy destruction of this pump can be reduced to about one half by connecting the vacuum pump directly to the evaporator vent line without using an air ejector as is currently being practiced. Large pump exergy destruction was also found to be due to low pump operating efficiency because pumps are not running at their best efficiency point.

The first effect was found to be responsible for a large amount of exergy destruction due to a high heat transfer, AT. The exergy of the other effects is much smaller due to their small AT. If steam is used as the heating agent, the first effect exergy destruction would be appreciably reduced.

It was found that an excessive amount of exergy is destroyed in the feedwater control valve due to oversizing of the feedwater pump.

A large amount of exergy is lost with the effluent steams of distillate, brine blow-down and seawater discharge, but a large portion of this lost exergy is actually in the form of chemical exergy that cannot be recovered. The thermo-mechanical portion of this lost exergy can be reduced by designing the evaporator with a lower bottom effect temperature.

6 . S y m b o l s

A - - area, m 2 Cp - - specific heat, kJ/kg K

D _ _

E - - h - - L - - n i - p - - Q - -

R - -

S - -

T - - t - - U - - V - -

W - - X - -

exergy destruction, kW exergy, kW specific enthalpy, kJ/kg exergy loss, kW mass flow rate, kg/s pressure, Pa heat load, kJ/s gas constant, kJ/kg K universal gas constant, kJ/kg mole K specific entropy, kJ/kg K temperature, K thickness of mist separator, m overall heat transfer coefficient, kW/m 2 K specific volume of vapor, ma/kg electrical or shaft work, kW mass or mole fraction fraction

G r e e k

g - -

p - -

op--

specific exergy, kJ/kg chemical potential, kJ/kg density, kg/m 3 efficiency fugacity coefficient

Subscr ip t s

1 , 2 , 3 . . - -

C

CV

d

f g h w

i

in

io

is

L

m e c h

o

p h

s w

tm v

index of exergy destruction/loss, also index of effect number

- - chemical, also condenser - - control valve - - distillate - - feedwater - - gas - - heating water - - species, also index for effect number - - intemal - - species i at environmental condition - - intereffect - - heat loss - - mechanical - - refers to environment condition - - preheater - - seawater - - thermo-mechanical - - vapor

24 A.M. El-Nashar, A.A. ALBaghdadi / Desalination 116 (1998) 11-24

Superscripts

f - - f low o - - substance in environment q - - heat w - - work

Acknowledgements

The authors wish to acknowledge the continued help and support o f Dr. Darwish M. K. Al Qubaisi.

References

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