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International Water Technology Journal, IWTJ Vol. 5 –No.1, March 2015

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INVESTIGATION OF ENERGY EFFICIENCY AND PRODUCED WATER

IN DESALINATION DISTILLITION SYSTEMS

Kashi.A

K.N.Toosi University of Technology, Tehran, Iran,

Email: [email protected]

ABSTRACT

According to international crisis of water in future, increasing of using desalination

systems is necessary for most countries having dry climate. This paper tries to present a

comparison of energy efficiency and produced water in multi effect desalination systems.

In this study, the effects of some input parameters (such as motive steam temperature,

concentration, and input water temperature) on energy efficiency and produced water are

investigated, in addition an analysis of relations governing all types of desalination are

presented and conditions of their operation are discussed.The result of this analysis could

guide designer and researcher to choose the best model with considering the natural and

limiting conditions.

Keywords: MED desalination systems, produced water, energy efficiency.

Received 18, Jul2014.Accepted14, Feb 2015

1 INTRODUCTION

Liver and Wang in 2007 conducted astudy on the use of multi-stage desalination plants with

thermal vapor compression in the gas turbinesof power plants with steam injection.

Using several methods, they studied the economy ofthe mentioned system.The findings

indicated that out of the seven methods, three of them were concerned with thermo-economic

analysis.

Fiorini and Sciubba in 2007 conducted athermo-economic analysis on MED desalination

plants. They offered a model for studying thermodynamic and thermo-economic

simulationsofMED desalination plants with parallel flow regime.

Desalination plants consist of two MED desalination plants connected to a combined cyclein

the power plant whose needed steam is provided bytheback pressure turbine.

As researchers asserted, in these types of system,the totalprice of the water produced

ismostlyaffected by the capital cost rather than operational costs.In addition,the temperature of

the input steam into MED had been set between 348°kand368°k.

Moreover, they had conducted asensitivity analysis for temperature and rate of feed water

exergy to evaluate the dependence rate of thetotalprice of water.

Chacartegui and co-workers in 2009, provided modified the combined cycleina power plant.

They performed a parametric analysis and studied different systems for heat recovery from

steam turbines connected to condenser and they found that the process in these desalination

plants needs salty water with temperature between70°kand100°k,a range of temperature which

is lower than the saturation temperature of salty water and has fewer repair and maintenance

problems.

mailto:[email protected]


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Pre-treatment in these types of desalination plants is easier than other types. Since this type

has more flexibility incontinuous changes of water temperature. The produced water in these

units has high purity andcan be used for process considerations.

Luo and co-workers studied the gas turbine cycleusing a metal-steam reformer in order to

reduce pollution.Theyheld thatthe desalinated water is consumed moreby the modified cycle.

An MED desalination plant was usedfor providing the required water. The presentarticle is

an attempt to continuethe studies mentioned in.

Zhao and co-workers used MED desalination plant for distilling waste water ofa refinery in

China in 2011.Their thermo-dynamical model was based on material and energy balance

atdifferent stages of desalination plant.

In recent years, researches have carried out studies on MED, MED-TVC desalination plants

in Iran as well. These studiesare mainly about simulation of thermodynamic and thermo-

economic analysis of this type of desalination plant and usually in form ofcase studies for

particular locations or in form of theories.

However, it should be noted that all of these studiesare solelyconducted for the sake of

researchand for reviewing the process of this type of treatment. Kamaliet al in three articles

developed a model for simulation of thermodynamic MED-TVC desalination and then

optimized it and developeda model compared with experimental data ofthe desalination plant in

Kish island to evaluate its efficiency.

The developed model was based on design principles of shell and tube exchangers; however,

nothing has been mentioned about economic aspects of the study. The effect of suction steam

needed for the ejector on optimal performance of water treatment has also been assessedin the

study.

Ameriet al provided a thermodynamic model for MED with producing 2000 litters per day

and described the number of steps involved in desalinationon indexes of performance in

desalination device.

Sayadiand co-workers proposeda model of MED-TVC and optimized it using

thermodynamic analysis. The developed model was related to one of the desalinations in Kish

Island.

In 2012, Aghanajafi and Kashi analysed MED desalination and provided a complete analysis

in this field . This present article moves along Aghanajafi and Kashi’s study and completesthe

thermodynamic analyses of all types of MED.

Aghanajafiand co-workers also did extensive research on MED in addition to combine

MED+RO and gas turbine cycles .Inthesestudies, an integrated model was developed for

different parts of the cycle and thermodynamic and economic methods were used.These two

approaches were reviewed.

2 MEDPROCESS

MED process is the first processused for producing significant amounts of distilled water

from sea water. This process works in numerous steps and by using reduced pressure of

different bodies (different effects)it causes integration and regulation in the whole process.


3

After injectingthe feed waterintothe first stage (effect), it reaches the boiling point.Water

steam produced in the first effect provides the required heat forthe next effect and it is

condensed in evaporation tubes with horizontal arrays.

This operation is repeated for the next effects, too. For entering intothe effects,sea water is

sprayed or distributed into spray nozzles as a thin layer of water in order to aidthe boiling and

evaporation (as seem in Fig.1),the condensed steam exits from evaporator.

When thenumber of stages increases, the rate of thermal loss from the brine decreases.

Figure 1.single stage of desalination distillation MED

In this process, the steam in letto the first effect enters into the first effect. Sea water (feed

water)also enters the first effect and the entrance of steam into the heat exchanger of the first

effect causes sea water sprayed on the condenser to evaporate when the entering steam

condenses in the tubes of heat exchanger the distilled water flows out of the bottom of the

effect.

Steams resulted from sea water evaporation in the first stage, like a source of energy enters

he second stage and evaporates some amount of sea water in the second stage. This process

continues up to the final stage.

So, for evaporating sea water only in the first stage (first heat exchanger),the external energy

is put into the system and for the next stages the excess heat energy is not used .Thus, by

entering the heat obtained from the boiler once, the sea water evaporates in many effects, and

then the produced vapour in each effect moves to the next ones as the entrance energy.

Arequirement for this event is the drop of pressure in each stage so the pressure in each stage

is less than its previous one. In this system, a compressor is usually used for increasing the

efficiency and also creating vacuum, which can be in form of thermal (thermo-compressor)or

electrical compressor. The system is called MED-TVCif the first type is used.

3 MODELLING MED DESALINATION DISTILLATION

An attempt was made in the present study to present a complete thermodynamic

analysis of the most common types of MED desalination. In this modelling, some

components like the ejector have been eliminated from the thermodynamic analysis.

Generally, the multi stage desalination distillation consists of a condenser in the last effect

for condensingthe vapour produced in the last stage, and many evaporators in each effect. The


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main function of the last condenser is to exit excess heat from the system. First, the sea water

with a specific flow rate and temperature enters the condenser.

Then, by exchanging the heat between the steam inside the pipes and the entered water, the

water temperature raises and part of it exits as cooling water from the system and the remaining

water enters the desalination as the feed water. Another component called flash chamber is also

eliminated.

The condensed steam in the second evaporator tubes enters the flash chamber in the same

stage. By facing the working pressure of the flash chamber which is lower than the effect

pressure, some of the distilled water in the flash chamber evaporates and along with the steam

produced in the second evaporator enters the pipes of the third stage evaporator as hot steam

and process continues to the end of the cycle.

In what follows, it will be shown that some flashed steams in these stages are in significant

in comparison with the amount of condensed steam. This point has been neglected in some

articles and analysis.

For modelling and simulation of desalination, assumptions... (shown below, have been made:

(1) Desalinations work in stable conditions.

(2) Physical properties of different flows in average temperatures of input and output are

calculated.

(3) Steam formed in each stage is free from salt.

(4) Heat loss from desalination to environment is neglected.

(5) In order to consider environmental aspects, the percentage of salt in the total brines is

assumed maximum 70000 ppm.

(6) Initial temperature difference between stages is constant

∆T =TI−TN

N−1 (1)

T2 = TI − ∆T (2)

Ti+1 = Ti − ∆T i = 1... N (3)

T1: The temperature of first effect (first stage)

TN: The temperature of last effect (last stage)

𝑇vi : The temperature of steam formed in each stage

T`i: The temperature of flash chamber in stage i.

𝑇𝑣𝑖−1 = 𝑇𝑖 − 𝐵𝑃𝐸 𝑇,𝑋 (4)

𝑇`𝑖 = 𝑇𝑣𝑖−1 −𝑁𝐸𝐴𝑖 𝑇 (5)

𝐵𝑃𝐸 = 𝐴𝑋 + 𝐵𝑋2 + 𝐶𝑋3 (6)

A = 8.32 × 10−2 + 1.883 × 10−4 × T − 4.02 × 10−6T2

B = 7.625 × 10−4 + 9.02 × 10−5 × T − 5.2 × 10−7T2

C = 1.522 × 10−4 + 3 × 10−6 × T − 3 × 10−8T2 (7)

NEAi = 33 ×(Ti−1−Ti )0.55

Tvi (8)


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Tvi is the steam temperature formed in each stage, which is lower than the boiling point of

that stage by BPE rate (Eq.6). Also T'i is the flash chamber temperature in stage i, which by rate

of NEA is lower than the steam temperature formed in the lower stage,

The equationbelow (Eq.9)shows relation between saturation pressure and saturation

temperature:

Tsat = (42.6776 −3892.7

ln P

1000−9.48654

) (9)

3.1 Modelling of parallel feed water system

All of the governing equations (10) ... (18) in the parallel section have been obtained from

reliable resources

The construction of MED desalination with parallel feed water is shown in Fig.2.

Figure 2.The scheme of MED desalination with parallel feed water configuration[15].

The first effect

D1 = F1 − B1 (10)

D1 =1

λ1[Ms × λs − F1Cp(T1 − TF)] (11)

F1XF1 = B1XB1 (12)

The ith effect

Bi = Fi + Bi−1 − Di (13)

Di =1

λi (Di−1 × λi−1 − Fi × Cp × Ti − TFi + Bi−1 × Cp × (Ti−1 − Ti)] (14)

FiXFi + Bi−1XBi−1 = BiXBi (15)

The Last effect

BN = FN + BN−1 − DN (16)

DN =1

λN (DN−1 × λN−1 − FN × Cp × TN − TFN + BN−1 × Cp × (TN−1 − TN )] (17)

FN XFN + BN−1XBN−1 = BN XBN (18)

In equations above: (10) … (18):

𝐷𝑖 : Steam produced in each stage

𝐹𝑖 : Feed water flow rate entering each effect

𝐵𝑖 : Brine ofeach effect which is gathered in the end of effect

𝑋𝐵𝑖 : Brine concentration of each effect which exits from it.


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𝜆𝑖 : Enthalpy of the steam formed in each stage, which is BPE rate less than boiling

point of that stage.

𝐶𝑝: Specific heat capacity of salt water.

𝑀𝑠: Flow rate ofthe steam input tothe first effect (this steam flows outofthedesalination

system after condensation, which is also known as the motive steam).

3.2. Forward model:

The construction of MED desalination with forward feed water is shown in Fig.3.

Figure 3.The scheme of MED desalination with forward feed water configuration[15].

The first effect

F = D1 + B1 (19)

DI =1

λ1[Ms × λs − F1Cp T1 − TF1 ] (20)

F1XF1 = B1XB1 (21)

The ith effect:

Bi = Bi−1 − Di (22)

Di =1

λ i[Bi−1 × Cp × Ti−1 − Ti + Di−1λi−1] (23)

Bi−1XBi−1 = BiXBi (24)

The last effect

BN = BN−1 − DN (25)

DN =1

λN[DN−1λN−1 + BN−1Cp TN−1 − TN ] (26)

BN XBN = BN−1XBN−1 (27)

3.3. Backward model

The construction of MED desalination with backward feed water is shown in Fig.4.

Figure 4.The scheme of MED desalination with backward feed water configuration[15].


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The first effect:

B1 = B2 − D1 (28)

DI =1

λ I[Ms × λS − B2 T2 − T1 ] (29)

B1XB1 = B2XB1 (30)

The ith effect:

Bi = Bi+1 − Di (31)

Di =1

λ i[−Bi+1 × Cp × Ti − Ti + Di−1λi−1] (32)

Bi−1XBi = Bi+1XBi +1 (33)

The last effect

F = BN + DN (34)

DN =1

λN[DN−1λN−1 − F × Cp × (TN − TF)] (35)

FXF = BN XBN (36)

4 INTRODUCTION OF THE OUTPUT PARAMETERS

GOR

GOR (Gain Ratio) GOR = D

MS (37)

Qd

Qd= Specific heat consumption Qd= Ms .λs

D (38)

𝜼𝒒

ηq =Q loss from waste water

λs.Ms (39)

Q=heat loss through exit waste water

Q= m Cp (TB − Tout ) (40)

𝑇𝐵= Temperature of brine in last effect

𝑇𝑜𝑢𝑡 = Environment temperature

D= Amount of distilled water

5 DESIGN CONDITIONS OF THE PROJECTS

Environment conditions of this project have been adapted to the conditions of the

Persian Gulf. In addition,the salt concentration in the Persian Gulf and also sea water

temperature were both taken into account.

Salt concentration in different areasof the sea is different.In this project the range of

concentration is 35000<Xw<45000 ppm.

Water temperature is consideredto be15°c<Tw<35°c


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Temperature difference between the effects, has been set in the range of 2<ΔT<4°c. In

addition, the effect of the temperature difference on the exit parameters has been

studied.The temperature difference between the effects has been assumed as the motive

force of the mass and heat transfer. An increase in the number of effects in an MED

system causes ΔT to decrease when terms of temperature difference between first

effect and last effect is constant. This also causesa decrease in the motive force in heat

transfer between cold and hot currents.

Because of environment conditions, salt concentration in the brine must not more

exceed70000 ppm.Neglecting the effects of this point on the environment couldnot

becompensable

The last Effect temperature is usually considered to be between 38°cand48°c. The

minimum temperature in the last effect is usuallyconsidered to be about 48°cbecause of

environmental issues; the increase in the temperature of thebrinewith the passage of

time causesan increase inthe local salinity of the sea water, which may endanger

special spices or may cause excessive grow thin one spices andhence

beingfurtherdamages to the ecosystem of the region.Moreover, with an increaseinthe

temperature in the last effect and exit of the brine with high temperature,the heat loss

throughthebrinewouldalso increase.

The minimum temperature has also been considered about 38°c,because of vacuum

ejector inability, the first effect temperature has beenconsidered to beabout 61°ctoo.

The temperature of the input steam to the first effect or motive steam isset to be

between 60°cand70°c. This temperature limitation arises fromthefollowing: the

maximum temperature in the first effect is 71°cbecause ofthe sedimentationintubes

bundles of the heat exchanger in first effect that reducessystem efficiency and

performance and in some cases causes disorder in the functioningof the desalination

system .

BPE which is usuallyabout 0.8 has been considered .

The number of the effects for the three modelsof MED desalination is considered

variable and its numbers in the studies has been considered: 4, 6, 8.

6 RESULTS:

6.1. Gain Output Ratio

As can be seen inFig.5, with an increase inthe temperature ofthe entering water to the effects,

the rate ofthe producedwater increases. Since the input water temperature to the effects is less

than the stage temperature, some of the energy of the motive steam is spent on increasing the

input water temperature to each effectif the sea water temperature ishigh;a loweramount of this

energy will be spent for increasing the input water.

Also less energy will be spent forincreasingthe feed water temperatureto that effect.As a

result, more salty water is evaporated and consequently more distilled water is produced.


9

Figure 5.The effect ofthe sea water temperature on GOR.

As seen in Fig.5, in all types of desalination, increasing the input water causes an increase in

the produced distilled water (or GOR), but the rate of the increase in different models is

different.

The backward model has a lowerrate of change. It can be said that this model has less

sensitivity to input water temperature and it can deliver more distilled water in comparison with

other models.

The considerable point in this diagram is the low efficiency ofthedistilled water production

inthe forward model and one of the important reasons for the selection of water treatments is

taking this parameter into account.

In Fig.6 theeffect ofthe input saturated steam temperature or motive steam on the amount of

water production or GOR has been shown.

Figure 6.The effect of the motive steam temperature on GOR

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

25 27.5 30 32.5 35

GO

R

Temperature of sea water (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-6

Parallel-4

Parallel-8

2.5

3

3.5

4

4.5

5

5.5

6

6.5

60 62.5 65 67.5 70

GO

R

Temperature of motive steam (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


10

The motive steam, in fact, has the role of the input energy to the system and since with

increasing the temperature,theenthalpy of steam decreases; it can be mentioned thatwith

increasingthe temperature, the input energy to the system reduces and as a result, the rate of the

produced water also decreases,Itcanalsobe seen in this diagram that the backward model has the

highest rate of GOR and the forward model also has the leastrate.

In Fig.7,the changes inthe produced water compared withthe changes in the input salt

concentration have been shown.One of the limitations in desalination designing is the

concentration oftheoutletbrine, which because of the environmental reasons has limitations. The

brine concentration in effectsshould not usually exceed 10000 ppm.

Figure 7.Theeffect of salinity of sea water on GOR

Now if the sea water (feed water) concentration increases,the system capability for

producing and extracting potable waterwillreduce. This fact is true for all types of MED water

treatments. As illustrated in Fig.7, with an increase inwater concentration from 35000 to 45000

ppm, the amount ofthe produced water or GOR decreases.

The more increase in the concentration of input waterwith the process of successive

distillation which occurs in the effects of MED desalination, thelesssteam is produced and

finally less produced water is obtained.

Fig.8 describes how anincrease in the temperature difference between effects, changes the

produced water.

Figure 8.The effect of difference of temperature between effects on GOR

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

35000 37500 40000 42500 45000

GO

R

Salinity of sea water (ppm)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4

2.5

3.5

4.5

5.5

6.5

7.5

2 2.5 3 3.5

GO

R

Difference of Temperature between effects (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6


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In this analysis, the first effect temperature is kept constant and then with paying attention to

the temperature differences and their changes,the last effect temperature could be determined.

In the parallel and forward models, with an increase in thetemperature difference, the rate

ofthe produced water will increase, but the opposite takes placeinthe backward model. With

increasing the temperature difference between the stages, the last effect temperature decreasesin

theparallel and forwardmodels.

The exitingbrineflows outofthelast effect and the brine temperature is alsoequal to the last

effect temperature.So, with an increase inthis temperature difference,the lost heat energy from

the system decreases; however, in backward model, this brineexits from the last effect.

In the backward model,the feed water entersthelast effect, which by an increase in the

temperature difference, the temperature in this effect decreases and as a result,the temperature

difference between the input water and the last effect is reduced, and finally the less steam in

the last effect is produced which causes a reduction in the water produced in the whole system.

Fig.9 shows the rate of water production based on changes of the motive steam flow rate.

Figure 9.The effect of the motive steam on the produced water.

In all models,with an increase in the rate of the motive steam, the rate of the water

production in MED desalinationincreased. As pointed out,the motive steam plays the role of the

input energy to system whose flow rate increases. Thismeans more energy enters the system

andit will in turn result in an increase in the distilled water.

The backward desalination produces the highest amounts of produced waterincomparison

with other models. Other specifications are shown inthe diagram.

6.2. Specific heat consumption (Qd)

This parameter has been introduced for describing some properties of the desalination. This

parameter is obtainedthrough dividingtheenthalpy ofthe input steam flow rate by the amount

ofthe produced water.

20000

40000

60000

80000

100000

120000

140000

10000 12500 15000 17500 20000

pro

du

ced

wat

er

flow rate of motive steam

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


12

It concluded that the smaller this rate the more amount of distilled water desalination system

can produce with receiving less amount of energy fromthe input steam. Fig.10 shows the rate of

changes of (Qd) in terms of the changes of the sea water temperature.

Figure 10.The effect of temperature of sea water on specific heat consumption.

As seen in all models of MED desalination, with a rise inthe sea water temperature, which

serves as theinput water to the effects, the Qd rate or specific heat consumption reduces. A sit

was mentioned inthe previous sections, with a rise inthe sea water temperature, the less motive

steam is spent for the elevation and increasingthe water temperature tothe effect temperature. As

a result, more distilled water is produced and it can be said that Qd is reduced and it will have a

decliningtrend.

The interesting point in this diagram is that according to our expectation,the backward model

has a less amount of specific heat consumption and the forward model also has the highest

value.Fig.11 shows the relation between the specific heat consumption (Qd) and the motive

steam temperature.

Figure 11.The effect of temperature of the motive steam on the specific heat consumption.

300

400

500

600

700

800

900

22 27 32 37

Qd

(kj

/kg)

Temperature of sea water (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4

300

400

500

600

700

800

900

58 60 62 64 66 68 70 72

Qd

(kj

/kg)

Temperature of motive steam (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


13

Regarding Fig.11, it is seen that all lines are horizontal and there are no changes in the rate

of Qdin comparison to the motive steam temperature. According to what was presented in the

previous sections, Qd is a fraction whose nominator consists of the enthalpy of the input steam

and its denominator is the produced water.

The significant point is that the rate of the produced water is a function of the input water

enthalpy, but since the nominator and denominator of this equation (Eq.38),are function of the

steam enthalpy and intensity of the change inthe nominator and denominator is almost equal, so

slope of line Qd is zero and is completely horizontal.

Like previous diagram, forward model in this diagram has the highest value of Qd and the

backward model has the lowest amount. The relation betweenthe concentration changes of the

sea water and Qd is shown in Fig.12.

Figure 12.The effect of the salinity of sea water on the specific heat consumption

As illustrated in Fig.12, with an increase inthesalt concentration of thesea water (feed

water),Qdincreases .With an increase intheinlet salt concentration tothe effects, the capability of

the system for producing distilled water will decrease because the specific heat consumption

with distilled water production has a negative correlation.

Fig.13 shows the relationship between temperature difference between effects and Qd.

300

400

500

600

700

800

900

33000 35000 37000 39000 41000 43000 45000 47000

Qd

(kj

/kg)

Salinity of sea water (ppm)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


14

Figure 13.The effect of temperature difference on specific heat consumption.

Considering Fig.13, it can be seen that the backward desalination has a rising trend but the

other two models, forward and parallel, have declining trends. As described in the GOR

diagram, because the brine oftheforward model and parallel flowed out of the last effect but in

backward modelit exited from first effect and the input water entered last effect.

The changes(the increases) inthe temperature difference causes a decrease intheproduced

distilled water in this model and finally Qd in this system will increase. As it is seen,the forward

model has the highest amount and the backward model has the lowest amount of specific heat

consumption likethe previous diagrams.

Fig.14 has many similarities to Fig.11.In fact in this diagram shows the rate of the changes

of Qd in terms of the flow rate of the input motive steam to the system.

Figure 14.The effect of the motive steam on the specific heat consumption

300

400

500

600

700

800

900

1.5 2 2.5 3 3.5 4

Qd

(kj

/kg)

Difference of temperature between effects (°C)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4

300

400

500

600

700

800

900

9000 11000 13000 15000 17000 19000 21000

Qd

(kj

/kg)

Amount of motive steam (Ton/Day)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


15

As mentioned in the GOR discussion, the rate ofthe water production in the desalinations is

proportional tothe input motive steam flow rate (of course, it is assumed that the desalination

has a variable capacity) and as seen in thespecific heat consumption(Qd), the denominator of

this fraction is a coefficient of the motive steam and its denominator is the produced water,

which is further proportional to the motive steam flow rate.

So,the Qd slope is zero andthe changes in the steam flow rate have no effect onthe increase

or decrease in this parameter. In all diagrams,as the number of effects in a model decreases, the

consumption ofthe specific heat will increase and the forward model usually has a highervalue

in comparison withtheother models.

Fig.15 showsthe relation between the input motive steam flow rate andthe types of

desalinations and the number of effects with heat loss. As it was expected, with an increase

intheflow rate of the input of the motive steam, both the amount ofthe produced distilled water

and the rate of the brine produced increases and as a result, the heat loss through the waste water

also increases.

Figure 15.Theeffect ofthe motive steam onthe heat loss

The study aimed at completing each analysisof the effects of an MED desalination. The

study yielded important findings that would enable us to optimize this type of system more

effectively and efficiently.

CONCLUSION

In this paper, thermodynamic investigation of common MED desalination systemsis

presented. First all common types of distillation MED systems are introduced and simulated

2500

3500

4500

5500

6500

7500

8500

9500

10500

9000 11000 13000 15000 17000 19000 21000

Q (

he

at lo

ss)

(kj/

kg.K

)

Amount of motive steam (Ton/Day)

Backward-8

Bachward-6

Bachward-4

Forward-8

Forward-6

Forward-4

Parallel-8

Parallel-6

Parallel-4


16

then mass and energy balance are developed for all types. A detailed thermodynamic analysis is

performed and afterwards, in order to achieve a realistic design, some limiting conditions

considered. It seems that:

Increasing the input water causes an increase in the produced distilled water (or GOR),

but the rate of the increase in different models is different.

Increasing the temperature of motive steam, the input energy to the system reduces and

as a result, the rate of the produced water also decreases.

With an increase in water concentration, the amount of the produced water or GOR

decreases.

With an increase in this temperature difference, the lost heat energy from the system

decreases; however, in backward model, this brine exits from the last effect. The

backward desalination produces the highest amounts of produced water in comparison

with other models.

With a rise in the sea water temperature, the less motive steam is spent for the elevation

and increasing the water temperature to the effect temperature.

With an increase in the inlet salt concentration to the effects, the capability of the

system for producing distilled water will decrease.

with an increase in the flow rate of the input of the motive steam, both the amount of

the produced distilled water and the rate of the brine produced increases and as a result,

the heat loss through the waste water also increases.

APPENDIX

A) Studying specific heat capacity of salty water (Cp)

In most articles, Cp value is considered to be about 4.18. Although in this study this

approximation is acceptable, it is studied to obtain its exact value. The relationship which has

been provided for Cp in articles is shown as follows:

Cp = [A + B × T + C × T2 + (D × T3)] × 10−3 (a)

A = 4206.8 − 6.6197 X + 1.2288 × 10−2 X2

B = −1.1262 + 5.4178 × 10−2 X − 2.2719 × 10−4 X2

C = 1.2026 × 10−2 − 5.3566 × 10−4 X + 1.8906 × 10−6 X2

D = 6.8777 × 10−7 + 1.517 × 10−6 X − 4.4268 × 10−9 (X2)

It has to be noted that in the relation above T is in Centigrade’s degree and X is salt water

concentration which is equal to percentage of salinity: (ppm ×10-4

)


17

Figure .a. The effect of temperature of sea water and salinity on Specific heat capacity.

As it can be seen in the relations above, Cp is the function of salt water concentration and

temperature. Analyzing Cp function, it can be seen that, this parameter is mostly dependent on

concentration and with alterations of this parameter, its value changes.

It can be said that, in this study considering Cp value about 4.18 kj/kg.k is a favored

approximation which in most articles also a number in this range is considered.

B) The study of latent enthalpy (λ)

Showing the relationship of this project, the following equation has been used for

obtaining latent enthalpy [17].

𝜆 = 2501.897149 − 2.407064037 × 𝑇 + 1.192217 × 10−3 × 𝑇2 − 1.5863 × 10−5 ×𝑇3 (b)

Figure .b. The effect of temperature on Enthalpy


18

C) Flashed steam (Dì)

As described in previous sections, with the entrance of condensed steam (distilled water)

from each effect to the next effect, because of pressure difference between two effects, some

liquid evaporates which in this part it is referred to as flashed steam and is denoted by Dì.

Dì= (Di-1)(Cp) 𝑇𝑣𝑖−1−𝑇`𝑖

𝜆𝑖 (c)

REFRENCES

C. Luo, N. Lior, H. Lin. (2011). Proposal and analysis of a dual-purpose system

integrating a chemically recuperated gas turbine cycle with thermal seawater desalination.

Energy, 3791-3808.

Cand. Wi-Ing. Frank Münk. (2008). Ecological and economic analysis of seawater

desalination plants.UNIVERSITY OF KARLSRUHE, Institute for Hydromechanics.

Desalination for Safe Water Supply. (2007). Public Health and the Environment World

Health Organization, Geneva.

D. Zhao, J. Xue, S. Li, H. Sun. (2011). Theoretical analysis of thermal and economical

aspects of multi-effect distillation desalination dealing with high-salinity

wastewater.Desalination, 292-298.

F.N. Alasfour, M.A. Darwish, A.O. Bin Amer. (2005). Thermal analysis of ME-

TVC+MEE desalination systems.Desalination, 39-61.

H. Sayyaadi, A. Saffari. (2010). Thermo economic optimization of multi-effect

distillation desalination systems.Applied energy, 1122-1133.

H. Sayyaadi, A. Saffari. (2010). Various approach in optimization of multi-effects

distillation desalination systems using a hybrid meta-heuristic optimization tool.

Desalination, 138-148.

HoseynSayyaadi, ArashSaffari. (2010). Thermo economic optimization of multi effect

distillation desalination systems.Applied Energy, 1122-1133.

ImanJanghorbanEsfahani, AbtinAtaei, VidyaShetty K, TaeSuk Oh, Jae Hyung Park,

ChangKyooYoo. (2012). Modeling and genetic algorithm-based multi-objective

optimization of the MED-TVC desalination system. Desalination, 87–104.

Kashi.A, AghanajafiC. (2012). Thermo-Economic Analysis of a Desalination Device

from the Second law of Thermodynamics’ Point of View and Parameter

Investigation.International Journal of Scientific & Engineering Research, Volume 3, Issue

6.1-11.

M. Ameri, S. SeifMohammadi, M. Hosseini, M. Seifi. (2009). Effect of design

parameters on multi-effect desalination system specification.Desalination, 266-283.

M.A. Sharaf, A.S. Nafey, Lourdes García-Rodríguez. (2011). Exergy and thermo-

economic analyses of a combined solar organic cycle with multi effect distillation (MED)

desalination process. Desalination, 135–147.


19

M. Shakouri, H. Ghadamian, R. Sheikholeslami. (2010). Optimal model for multi effect

desalination system integrated with gas turbine. Desalination, 254-263.

P. Fiorini, E. Sciubba. (2007). Modular simulation and thermo economic analysis of a

multi-effect distillation desalination plant.Energy, 459-466.

Phil Dickie. (2007). MAKING WATER, WWF’s Global Freshwater Programmer.

R. Chacartegui, D. Sanchez, N. Di Gregorio, F. J. Jimenez-Espadafor. (2009).

Feasibility analysis of a MED desalination plant in a combined cycle based cogeneration

facility. Applied thermal engineering, 412-417.

R. K. Kamali, A. Abbassi, S. A. SadoughVanini, M. SaffarAvval. (2008).

Thermodynamic design and parametric study of MED-TVC.Desalination, 596-604.

R.K.Kamali, S. Mohebinia. (2008). Experience of design and optimization of multi-

effects desalination systems in iran. Desalination, 639-645.

R.K.Kamali, S. Mohebinia. (2009). A simulation model and parametric study of MED-

TVC process.Desalination, 340-351.

R.Kouhikamali, M. Sanaei, M. Mehdizadeh. (2011). Process investigation of different

locations of thermo-compressor suction in MED-TVC plants.Desalination, 134-138.

R.K. Kamali, A. Abbassi, S.A. SadoughVanini, M. SaffarAvval. (2008).

Thermodynamic design and parametric study of MED-TVC.Desalination, 596–604.

SeyedEhsanShakib, MajidAmidpour, Cyrus Aghanajafi. (2012). Simulation and

optimization of multi effect desalination coupled to a gas turbine plant with HRSG

consideration. Desalination, 366-376.

SeyedEhsanShakib, Seyed Reza Hosseini, MajidAmidpour, Cyrus Aghanajafi. (2012).

Multi-objective optimization of a cogeneration plant for supplying given amount of power

and fresh water.Desalination, 225-234.

Y. Wang, N. Lior. (2007). Fuel allocation in a combined steam- injected gas turbine and

thermal seawater desalination system. Desalination, 306-326.

Y. Wang, N. Lior. (2007). Performance analysis of combined humidified gas turbine

power generation and multi-effect thermal vapor compression desalination

systems.Desalination, 243-256.

Y. Wang, N. Lior. (2007). Performance analysis of combined humidified gas turbine

power generation and multi-effect thermal vapor compression desalination

systems.Desalination, 84-104

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