The non-equilibrium factor and the flashing evaporation rate inside the flash chamber of a multi-stage flash desalination plant

DESALINATION Desalination 114 (1997) 277-287

ELSEVIER

The non-equilibrium factor and the flashing evaporation rate inside the flash chamber of a multi-stage flash desalination plant Hassan E.S. Fath Mechanical Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Fax: (002-03) 597-1853 Received 25 October 1997; accepted 27 November 1997

Abstract

A study was undertaken to measure the non-equilibrium factor and to correlate the flashing evaporation rate inside the flash chamber of a multi-stage flash (MSF) desalination plant. A computer code was developed to quantitatively simulate the MSF desalination plant operation and solve the mass, heat and salt balance equations. The simulator was tested against an MSF pilot plant with a four-stage heat recovery section and a two-stage heat rejection section, and then used to technically evaluate the flash chamber performance. For a constant top brine temperature of 112°C, the results indicated that (1) the non-equilibrium factor (1-~) varies between 0.24 to 0.66 (low values of flash chamber effectiveness, 13).In order to reach thermodynamic equilibrium, the flashing evaporation process should be enhanced through increasing the brine superheat, flashing surface area, number of active nucleation sites, and brine residence time inside the flash chamber; (2) the average flashing heat flux ranges from 100 to 200kW/m 2 and increases with the brine superheat and the stage pressure. Similar to the surface nucleate boiling, the flashing heat flux could be correlated as: Q"= 0.055 (Tb(~v)-Tsar) 3.

Keywords: Desalination; MSF; Flashing

1. I n t r o d u c t i o n

Multi-stage flash (MSF) distillation plants are widely used in saline water desalination; they produce most of the desalinated water in the world today. Seawater brine evaporation is carried out by the progression of the brine through a series of connected flash stages at decreasing pressures and temperatures. At each

stage part of the flashing brine is evaporated and condensed through a cooling tube bundle where recirculated brine is driven by recirculating pumps. Three main processes take place at each stage: (1) flashing of the superheated brine; (2) separation of the vapor from the brine-vapor interface; (3) vapor condensation. The last two processes are rather well known. With regard to

0011-9164/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH SO011-9164(98)0001 8-6

278

H.E.S. Fath / Desalination 114 (1997) 277-287

the fh-st process--flashing (known also as bulk or volume boiling)--there is not sufficient published data, particularly for the pressure range below atmospheric. Thermodynamically flashing occurs when a liquid is exposed to a sudden pressure drop below the saturation vapor pressure (for pure liquids) or the equilibrium vapor pressure (for solutions) corresponding to the liquid temperature. Under adiabatic conditions part of the liquid vaporizes to regain equilibrium and draws its latent heat of vaporization from the remaining liquid. The liquid temperature drops towards the saturation temperature (for pure liquids) or the equilibrium temperature (for solutions) corresponding to the lower pressure. The flashing process in MSF plants is usually considered to be in thermodynamic equilibrium, i.e., the temperature and pressure in any section correspond to the saturation (or equilibrium) curve(s). It is known, however, that there are considerable differences between thermodynamic equilibrium and the actual processes when flashing is developing continuously. Brine needs more time to reach the thermodynamic equilibrium than what takes place in the MSF flash chamber. Therefore, the brine coming out of the flash chamber always retains some residual superheat which considerably influences both the technical and economical characteristics of the MSF distillation plant design. Flashing in MSF distillation plants may take place in two ways. Firstly, it may occur by free evaporation at the free surface of the liquid; secondly, in the form of bubbles within the bulk of the liquid. The evaporation due to both free surface evaporation and ebullition is integrally linked with the thermofluids processes of fluid dynamics, heat transfer, mass transfer, and thermodynamics. The complexity of the flashing phenomena still hinders detailed modeling and innovative improvements of the process. Even the correlations developed for characterizing the process provide results which widely vary from each other, depending upon many factors

including the flashing technique. Miyatake [1,2], for example, indicated that the value of the flash evaporation rate of flashing liquid jet attains more than ten times as much as those of superheated pool liquid exposed to a sudden pressure drop in a container and those of superheated flowing liquid of an MSF evaporator. Understanding the flashing process in MSF distillation plants is essential, therefore, for any comprehensive attempt to improve the flashing process and to enhance both heat and mass transfer rates. The problem is difficult to solve, either theoretically or experimentally. This fact is evidenced with the relatively small amount of published data, as well as by the fair amount of conflicting conclusions. This paper investigates the characteristics of the flashing process in MSF plants, and in particular: (1) the approach to thermodynamic equilibrium of the flashing stream, and (2) the flashing evaporation rate. The analysis will be based on the experimental results obtained from AI-Sofi et al. [3] for a 20m3/d (nominal capacity at a TBT of 90°C) MSF test plant (described below).

2. Test plant and measurements

The simulated test plant comprised of four stages of heat recovery and two stages of heat rejection is shown in Fig. 1 [3]. After passing through the condenser of the heat rejection section as cooling water, part of the seawater is extracted, deaerated and then supplied to the heat recovery section as make-up water. The rest of the seawater is discharged from the system as the heat rejection flow. Part of the brine is discharged as blowdown from stage 6, while most of the brine is extracted and diluted with the make-up water and then recirculates through the condensers of the heat recovery stages one by one as condenser cooling water. After passing through the condenser's tube bundle of the heat recovery section, the brine is heated up to its TBT in the brine heater and fed to the first-stage flash

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H.E.S. Fath / Desalination 114 (1997) 277-287

p

Prg.l,ilu~ ~L

Reducing Station

Mgdium Pressure Steam

ow Pressure Steam Sea Water Out (Reject flow)

Duuper

/ 2 stages Heat Rejection I

4 stages Heat Recovery Section

|

Sea Water In,/ neearbo~tator

Boi|er Brine Make-up Pump ] Condensate Pump

/

Blow Down ¢

I

neaera~l o r IL

f.j~..~

Recirculation Pump Product Water ' (Distilate)

Distilate PumpkD

Tank

Fig. 1. MSF test plant, 20 t/d [3]. chamber (low pressure steam from a fire tube boiler is used to heat the brine in the brine heater). Flash evaporation of the brine continues in the flash chamber of each stage as brine flows from the first stage toward the last. Once again, part o f this brine is discharged from stage 6 as brine blowdown, and the bulk after being mixed with make-up flows into the brine recirculating pump, and the process is repeated. Flashed vapor is condensed into distillate, flows from stage 1 to stage 6, and is then collected and delivered to the distillate tanks by the distillate (product water) pump through a flash tank.

3. Simulation model

The prediction of the MSF distillation system performance was made by using the thermodynamic relationships expressed in heat, mass, and salt balance equations. The formulation of the equations connecting these quantities and relationships will be presented below based on

average values of fluid simulation assumptions are:

properties.

Other

1. Mass and heat losses in the vacuum system and heat losses from the flash chamber walls are relatively small (less than one order of magnitude less than the other parameters) and are therefore neglected. 2. Brine heater heat load, QSH, and TBT are constant. 3. The brine concentration increases with evaporation. Boiling point elevation (BPE) is calculated at a given brine concentration factor (Co) according to the relations: BPE = A x C + B x C 2 where A = 0.2009 + 0.002876 x T+ 0.0000002 T 2 B = 0.0257 ÷ 0.000193 x T+ 0.00000001 T 2

280

H.ES. Fath / Desalination 114 (1997) 277-287 Sea W a t e r To Condenser m =l 5000 T ~i39.5

m = 18,300 T,,~ = 39.5

Life Steam

.........-._..->

m,t =313

I Brine Heater

I

Sea W a t e r In

ii 1

m,,j = 23000 Ti, I ~ !

i ._~istdlate P ; T,M! i Tsar2 ! T,tt3 ! Tin4 iFlashingi B r i n e i

Distillate

............ .I.

[

T~_~__. T~.:_~ T~.~

T

T~i '

i

i

k

6530 T ~ '= 39.5

~b7

m~

!

.....

Make Up

densate

JT,,p=39.5 Blow Down

[ m = 3250 .............

¢

TI~ = 39.5, mbd= 2500

m = Mass

Flow Rate (kg / hr) T = Temperature ( C ) Product W a t e r mp = 750, Tp = 37,2

0

1

Fig. 2. MSF plant heat and mass balance. For the plant shown in Fig. 2, the overall mass and heat balance equations can be summarized as follows:

i=N

Product = ~ m:O

(5)

i=l

where mseain = mseaout-mmkup ,

mmkup = m p + m b d

Q~,: Q./, -- (Q~.oo,- Q,~.~) + Q. + Q,~

(1)

(:)

Detailed modeling should also satisfy the conservation equations for each flash chamber which are influenced by other stages. For a typical stage (i) shown in Fig. 3, the mass and heat balance equations can be summarized as:

Qst = m a x Ahst

(6)

Qs~oo,:(m~.oo,×C,~.×~'~.oo,)

(7)

Q=.-- (m,o..×C~.× r~o)

(8)

Q. : m × % × r ,

(9)

mt,(~+1)= mb(o- me(o , me(o = mp(0

(3)

Qbd = mbdXCbdXTbd

(10)

Qe(i)= Qt,(o-Qb(,+1)=(Qc(o-Qc(,+l))+Qp(0

(4)

Qb(,)= mb(,)xCb(o xTb(,)

(11)

281

H.E.S. Fath / Desalination 114 (1997) 277-287

Recirculated Brine In me O)

Recirculated Brine Out ~ mc 03 .~ T, o)

'

I "

............. I ! I

*'-.. Brine In mbi Tbt

-

Flashing *: Brine

.

m~,.T.

Brine O u t mb(~D Tbo+t) Fig. 3. Heat and mass balance of a flashing chamber.

Q,(o = m
(12)

Qc(0 = me(0 x cc( 0 x 7"(0

(13)

The flashing chamber effectiveness, defined as 13, is expressed as the ratio of brine temperature decrease due to adiabatic vaporization to the total initial superheat. The extent of the vaporization process completion or of the approach to thermodynamic equilibrium is known as the non-equilibrium factor (1-13). The non-equilibrium factor is, therefore, defined as T* O

Distilate Out

m,(~,Ts,to

Stage i

1 -13 -

T, o,l)

(14)

A computer model was developed to simulate the operation of the 20 t/d MSF test plant shown in Figs. 1 and 2. In this simulator the conservation equations are satisfied in each stage as well as the plant as a whole. The governing equations supplied with the required thermodynamic relations, Eqs. (1)-(13), are numeric-

ally solved using an explicit iterative technique. The calculation procedure is initiated at the hot side of the plant for a given TBT. Then it proceeds iteratively to the other stages with the flashing brine until steady values of flow and temperature of the recirculation streams are obtained.

4. R e s u l t s a n d d i s c u s s i o n

Typical results of the simulated six-stage MSF test plant are given in Table 1. The total distillate production shown is relatively low in summer and high in winter mainly due to the difference in the plant terminal temperature difference (TBT-seawater inlet temperature). Fig. 4a shows the temperature distribution results inside the MSF test plant for the nine tests given in Table 1. The temperature level inside each stage drops from the brine temperature (dashed lines) through the saturation temperature (solid horizontal lines) and down to the recirculate brine temperature (semi-dashed lines). The brine superheat (Tb(av)-Tsat) was found to vary between 10 oC and 17 oC, as shown

282

H.E.S. Fath / Desalination 114 (1997) 277-287

Table 1 Typical results Stage no. Parameter

Total Ave.

Test no. 1

2

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m 2

106.6 12.64 145.4 170.4

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m 2

4

5

106.4 106.4 10.43 12.43 1 5 1 . 3 150.9 176.9 176.9

106.9 13.89 139.6 163.8

107.0 106.9 13.22 12.24 136.4 137.7 1 5 9 . 8 161.2

1 0 6 . 3 106.6 15.04 14.54 153.9 146.2 180.9 171.7

106.3 15.89 154.9 182.3

95.65 15.65 143.3 170.4

95.46 9.46 138.6 163.8

96.13 12.93 145.9 173.0

96.44 12.74 144.9 171.7

94.56 13.76 156.7 186.2

94.02 14.62 167.6 199.4

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m 2

84.10 15.12 141.7 170.4

84.61 84.15 84.63 84.89 84.40 82.11 82.51 80.53 14.60 1 4 . 1 5 1 4 . 0 3 14.29 13.60 1 4 . 5 1 1 4 . 4 1 15.33 130.9 130.9 141.9 1 4 4 . 1 1 4 6 . 3 1 5 1 . 3 154.7 164.1 1 5 7 . 3 1 5 7 . 3 170.4 173.0 175.6 1 8 2 . 3 186.2 198.1

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m2

73.99 12.99 118.7 144.2

72.98 72.99 73.18 73.22 14.99 14.49 1 4 . 8 8 15.42 1 4 0 . 1 1 4 0 . 1 1 2 8 . 2 131.4 170.4 170.4 1 5 5 . 8 159.8

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m 2

60.78 17.78 148.9 145.2

59.63 59.26 61.35 60.96 60.54 54.86 1 7 . 6 3 16.26 1 7 . 6 5 17.46 18.24 15.76 1 5 2 . 1 1 7 0 . 1 1 3 4 . 1 1 4 0 . 1 1 2 7 . 1 130.4 148.4 165.9 130.6 1 3 6 . 5 123.9 127.6

57.42 19.52 121.2 118.7

53.71 20.31 123.6 121.7

Brine ave. temp.,°C Brine superheat, °C Distillate, kg/h Heat flux, kW/m 2

50.89 9.89 59.5 58.07

49.48 10.98 60.6 59.35

48.96 10.96 46.2 45.24

52.05 50.17 1 3 . 3 5 12.67 63.7 87.8 62.32 86.06

49.67 13.18 99.3 97.40

43.03 9.63 111.5 109.8

47.21 15.41 90.3 89.03

41.04 13.84 131.9 130.6

Distillate, kg/h Mass flux, kg/h.m 2

757.5 1322

773.6 1349

771.7 1345

753.4 1318

789.9 1374

867.5 1513

815.2 1427

896.3 1560

in Table 1. Fig. 4b is an enlargement o f the low pressure end o f the first test. For the purpose o f developing the flashing process understanding, two aspects will be addressed in some detail, namely (1) the flash chamber non-equilibrium factor, and (2) the flashing evaporation rate.

4.1. Flash chamber non-equilibrium factor Fig. 5 shows the variations o f the flash chamber effectiveness, 13, for the six stages o f

3

95.23 10.23 144.0 170.4

784.7 1364

6

96.18 12.68 146.0 172.9

7

8

95.12 14.72 157.8 187.5

72.42 68.44 69.62 1 4 . 8 1 1 4 . 3 3 15.62 133.6 164.4 144.9 1 6 2 . 5 200.8 176.9

9

66.65 16.15 154.1 188.9

the test plant (all superimposed data are eliminated for clarity o f the figure). The value o f the non-equilibrium factor (1-13) varies between 0.66 and 0.26 (13 varies between 0.34 and 0.74), depending on the operating condition. The results indicate that the flash chambers provide insufficient residence time (and other influencing parameters) for the flashing brine to achieve the full temperature flash down or 13value o f 1.0, i.e., the non-equilibrium factor (1-13) equal zero. Lior and Greif [4] indicated in their experimental study in a single-stage flash chamber, that 13

283

t~E.S. Fath / Desalination 114 (1997) 277-287

120

120

tO0

•

"~

%-----'1

"" "-.

X

80

\ - ~ . "-. "1;~. ~-.. "..~o "-.

60

1

~ce

_ " ~_1 "" ~ Figure (4.b) " "r--"--i .~

80 o.

e

100

_

~.

40

L

\!

"

120 1011

..............

20

60 IZO "

. •~

iO0

"'-

'~L ~.

'

~

4°

i

l

20 80-

i-.,

"-!

," ~ . ~ .

20

I

I

I

1

I

I

l

I

I

I

I

I

J

I

1I

III --

IV

V

I

VI

I

I

t

f

I

i

I

,I

I

I

I

I

1

n

S t a l e No.

III ~

IV

V

I

Vl

Stage No.

I

1

L

1I

ll/

I

i

Iv

I

V

l

Vl

. . . - - . - - 4 . S l a l e No.

Fig. 4a. Temperature profile of the MSF plant at constant TBT and at different seawater inlet temperatures•

70

!

-

k

T..,,

\'\

6e --

\\ \\

"\

\\

T.' 50 Tb7 --.------4t-

\.

1\-

Make-up

T~

Sea Water Out, (Reject Flow)

Rcirculatien Brine

Blow down

T..ts Writ 6

"~, 4

30 --

Sea Water In

20

II

IT!

IV

Stage No.

V

VI

Fig. 4b. Detailed temperature profile near heat rejection section•

284

H.E.S. Fath / Desalination 114 H997) 277-287

%

[] o

4O

2O Stage I I

10

100

I

90

i

Stage

Stage

Stage

Stage

Stage

n

m

iv

v

vl

I 80 I 90

, 80

I 80 I 70

70 I

i 60 I

,

70

60 •.

I 50 F SO

40 '

I 30 I

-

40

30

Saturation Temperature (C)

increases with the brine superheat. Their results agree with the present study for the high pressure stages (stages 1 and 2) of enhanced flashing. Lower pressure stages behave differently, however, due to the limited interstage pressure differential and the lack of sufficient nucleation sites. The increase in the flash chamber non-equilibrium factor causes an increase in residual superheating, thus leading to greater heat consumption, resulting in the rise of the MSF unit capital expenditure. Nonequilibrium represent for designers an important irreversibility which directly affects the heat transfer surface requirements. Thermodynamically, it is desirable to bring all of the superheated brine close to the equilibrium state determined by the saturation conditions in the stage, and from this standpoint this should be accomplished within a flash stage of minimal cost. Approaching thermodynamic equilibrium could be achieved through enhancing the flashing process. Artificial nucleation enhances flash evaporation of superheated liquid, thereby reducing the non-equilibrium loss, even in the case of lower superheat. Miytake et al. [5] showed that the best method of enhancement of flashing may be by injection of nucleated liquid into the low pressure vapor zone. Lee and Seui [6] showed also that with a lower liquid level,

2o

Fig. 5. Non-equilibrium factor (flashing chamber effectiveness) for different stages.

most of the bubbles grow in the vicinity of the sluice gate, and higher evaporation performance is achieved (due to the presence of more active nucleation sites). On the contrary, with a higher liquid level (higher hydrostatic pressure), the bubble growth is delayed and the bubly region extends downward; and the number of bubble nuclei passing through the flash stage increases. This yields the higher outlet temperature and the larger values of the non-equilibrium factor. On the other hand, Miyatake et al. [1] showed that the non-equilibrium temperature difference, NETD = Tb(i+l) -mean temperature of emanating vapor, can significantly be reduced as the temperature of the liquid is increased and as entering hot brine is propelled to the flashing zone near the free surface (through a baffle) and is reduced with the fluid flow rate (a shorter residence time for the brine inside the flash chamber). It has to be stressed here that the extent of cascade staging the MSF distiller contributes greatly to the value of the non-equilibrium factor (1-13). In this respect, it has to be emphasized that the above analysis for the non-equilibrium factor (1-13) needs to be carried out further into larger commercial units since the studied data pertain to a limited cascade staging MSF unit with six stages. Nevertheless, the conceptual

285

H.E.S. Fath / Desalination 114 (1997) 277-287

approach of this work can be an initial step for much broader work whereby larger and longer cascaded MSF units can be studied.

4.2. Flashing evaporation rate It is worth mentioning here that both surface nucleate boiling and flashing processes (known also as "bulk boiling") behave in a similar way. Consequently, the flashing evaporation rate and flashing heat flux depend on the same parameters similar to surface nucleate boiling. The brine superheat is a basic parameter that significantly influences the flashing process and flashing heat flux. The measured brine superheat (ranging between 10-17 °C) is sufficiently high to provide the energy required for evaporation and maintains bubble generation. The variation of the flashing heat flux with brine superheat is shown in Fig. 6. The relatively higher heat fluxes obtained at the higher pressure stages of the heat recovery section are due to the presence of more active nucleation sites (vapor bubbles, noncondensable gases and suspended scale particles) as compared to the lower pressure stages of the heat rejection section. At these later stages most of the free and dissolved gases are released and ejected out of the flash chambers through the vacuum system, and the interstage pressure differential may not be sufficient to cause dynamic vapor bubbling, in addition to the precipitation of much of the suspended scales in higher temperature stages. The flashing heat flux, as shown in Table 1, was found to increase also with increasing the interstage pressure differential, in agreement with Lior and Greif [4]. Fig. 7 shows the variation in the flashing heat flux for the six stages at different saturation temperatures. For each stage the figure shows that the flashing heat flux increases with decreasing stage saturation temperature (it is more apparent at low pressure stages). Decreasing the flash chamber pressure (saturation temperature) increases the brine superheat and also releases more dissolved non-

300

G

~ 200

/

=1

l,o 100

Stage i

i

i

,

.

l

I

.

,

StageI V

1

!

.

i

.

i

I

I

I

,

I

~

I

I

t

,

.

_

i

.

60

200

100

1,o

Stqe V

Stage .

/

6O

$tige I

I

r

,

,

Q

Brine

•

--

i

.

.

i 20

,

,~

.

m I

,

|

20

tO

i

o°/

2OO

[,o

,

Superheazt

I

.

30 (C)

I

I 10

,

Brine

,

,

30

Sup~beat( C )

Fig. 6. Correlating flashing heat flux with brine superheat for different stages.

condensable gases. The latter will act as active nucleation sites for bubble formation and enhances, therefore, the flashing process and heat flux. At higher pressure stages, however, the release of dissolved gases are relatively high enough to initiate bubble nucleation and enhance the flashing heat flux. Increasing the flashing surface area significantly enhances the flashing evaporation rate. This was confirmed by Miyatake [2] who indicated that the most flash evaporation occurs when a superheated liquid is ejected because the liquid is shattered into a spray by the bubble growth within it. This phenomenon is termed the "spray flash evaporation". The flash evaporation ratio in spray flash attains more than ten times as much as other flashing techniques. The authors also indicated that by means of injection of the

286

N.E.S. Fath / Desalination 114 (1997) 277-287 S~g+ No.

I

II

300

m

\

\

V

iv

\

I

200

\

++

\

m

~ go 6O

60

Jl I I 1 80 100

60

SO tO0

t

i

50

i i

I

70 -90

40

I . i

i i 60 gO 30

40

5O

25 3Q

40

Saturation Temperature (C)

Fig. 7. Variation of flashing heat flux with saturation temperature.

,oo.

nucleated liquid through some nozzle configuration into a low-pressure vapor zone. Fig. 8. shows the variation in the average flashing heat flux with the brine superheat for the data of the six stages (pressure from 0.80 bar to 0.08 bar) with superimposed data omitted. Similar to surface nucleate boiling, the flashing heat flux can be expressed as

/

200

.100

a " = C(Tb-

6C

40 ~

(ATr~)~

20 ,

10

I

I

6

I I II

I0

....

- - ExperimentalResults O Stage I Stage lI t9 Stage III I D Stage IV 1~ Stage V • Stage IV

J,,,l,,,l

20

, I,I,

40

60

J" Brine Superheat DTsm(C)

Fig. 8. Average flashing heat flux versus brine superheat. bubble nuclei into the flashing liquid, the flash evaporation rate is improved. Thus a method of enhancement of flashing may be the ejection of

T

sat)n

(15)

where c and n were found to be 0.055 and 3, respectively. The above empirical correlation may require further work in order to reveal its true scientific as well as practical value and, therefore, a better understanding of the flashing process in the flash chamber. For the purpose of developing a compact and efficient flash evaporator, and to improve the operation and economy of MSF systems, the flash evaporation rate should, therefore, be enhanced. From the above analysis, it is recommended to enhance flashing through: •

increasing brine superheat, either by increasing TBT or by brine supplementary

H.E.S. Fath / Desa#nation 114 (1997) 277-287 heating (such as steam bubbling or surface heating); see also Miyatake [7] increasing the flashing surface area (using, for example, multi-trays and/or brine dripping or spraying) increase the number of active nucleation sites which may be introduced artificially; see also Miyatake [2] allow for more brine residence time (for bubble nucleation, growth, transfer to and release from the liquid-vapor interface) by increasing the brine flow path within the flash chamber The existing proven MSF distillation system has been modified (based on the above parameters and others) and recently patented by the author [9]. The modified system increases the MSF unit capacity, yielding lower water production cost and improved plant operating performance. The patented system is available for

287

Subscripts av b bd BH c e fg

--------

i

--

mkup p sat sea st

------

Average Brine Blowdown Brine heater Condenser Evaporation Vapor-liquid Stage number (I) Make-up Product Saturation Seawater Steam

References

marketing.

[1] O. Miyatake, T. Hashimoto and N. Lior, Desalination, 91 (1993) 51. [2] O. Miyatake, Desalination, 96 (1994) 163.

5.

[3]

Symbols

C

Co h m

N P Q Q,, T TBT

Specific heat, kJ/kg.C Salt concentration Specific enthalpy, kJ/kg - - Mass flow rate, kg/s - - Number of stages - - Pressure, bar - - Heat transfer rate, kW - - Flashing heat flux, kW/m 2 - - Temperature, °C - - Top brine temperature, °C

-

-

-

-

-

-

Greek (1-[3)

--

Non-equilibrium factor [see Eq. (14)]

M . A . K . A I - S o f i et al., H e a t transfer m e a s u r e m e n t s as a criterion for p e r f o r m a n c e e v a l u a t i o n o f s c a l e inhibitor in MSF plants, 7th International C o n f . ,

IDA, UAE, 1995. [4] N. Lior and R Greif, Proc., Fifth International Symposium of Fresh Water from the Sea, 2 (1976) 95. [5] O. Miyatake, T. Tomimura and Y. Ide, J. Solar Energy Eng., 107 (1985) 176. [6] K.W. Seul and S.Y. Lee, Desalination, 85 (1992) 161. [7] O. Miyatake, T. Tomimura, Y. Ide, M. Yuda and T. Fuji, Desalination, 37 (1981) 351. [8] S. Gopalakrishna, V.M. Purushothamam and N. Lior, Third World Congress on Desalination and Water Reuse, Cannes, 2 (1987) 139. [9] H.E.S. Fath, Combined MSF/ME distillation system, Patent No. 2,190,299, 1996.