SIMULATION OF MULTISTAGE FLASH DESALINATION PROCESS1

Osman A. Hamed, Mohammad AK. Al-Sofi, Monazir Imam, G. M. Mustafa

Khalid Bamardouf and Hamad Al-Washmi

Saline Water Conversion Corporation P.O.Box 8328, Al-Jubail -31951, Saudi Arabia Tel: + 966-3-343 0012, Fax: + 966-3-343 1615

Email: rdc@swcc.gov.sa ABSTRACT

The majority of large-scale desalination plants in the Arab Gulf area employ the

multistage flash (MSF) processes. MSF processes are energy intensive and it is,

therefore, essential to search for design and operating conditions which lead to

reduction of energy dissipation and consequently lower water production cost. This

paper reports a simulation study which was carried out to examine to what extent the

thermal irreversibility of an MSF process is influenced by variation of the most

important design and operating parameters.

The impact of variation of top brine temperature, number of stages and terminal

temperature approach on the distiller performance ratio and irreversibility were

explored and presented graphically in the term of thermal grids. The simulation study

revealed that within the selected range of number of stages (16-40) and top brine

temperature (90 120oC), the MSF distiller irreversibility is highly influenced by the

number of stages and to a lesser extent by the variation of the top brine temperature.

The simulated results are compared with one year operating data obtained from an

MSF distiller. The distiller is a cross-tube configuration, which consists of 22 stages

and operating at a top brine temperature ranging between 90 to 98oC. Using a steady

state simulation program, an envelop of possible operating conditions is constructed

for the distiller to interrelate performance ratio, production, recycle flow and top brine

temperature. Stage-wise simulation of individual heat transfer coefficients and fouling

factors showed that both clean overall heat transfer coefficients (Uc) and fouling

factors (FF) are stage dependent and conversely the operating overall heat transfer

1 Presented in the International Conference on Seawater Desalination Technologies on the Threshold on the New Millennium, Kuwait, Nov. 4-7, 2000. 2. Published in Desalination Journal 134, (2001) 195-203.

coefficient (UD) is to a great extent less dependent. The dependence of the distiller

irreversibility on the process conditions is reported. The distiller exergy losses varied

between 52 and 62 kJ/kg of distillate. Process details, which are responsible for

distiller irreversibility are pinpointed and opportunities for better utilization of

available energy are discussed. INTRODUCTION The majority of desalination plants in the Arab Gulf Region are currently employing

the multistage flash process (MSF) which is producing around 67% of the total world

capacity of land-based desalting projects which are capable of producing 100 m3/day

[1]. The success and popularity of the process is due to its simplicity, inherent

robustness and vast amount of acquired experience which resulted in reducing material

and operating costs and increasing life expectancy. The basic technology of modern

large scale MSF is similar to the early units and apart from the development of on-line

ball cleaning system and scale control techniques, use of corrosion resistant materials

and increase of unit capacity to 12 MIGD [2], the development of the process can be

considered as evolutionary rather than revolutionary [3]. A brief history of the

development of the MSF process as well as specific design features of a modern MSF

plant are presented by Darwish, et al. [4].

Although, coupling of MSF process with power generation system has greatly

contributed in reducing energy requirement by about 50% or more compared to single

purpose desalination plants using the same kind of fuel [5], but it is still considered as

an energy intensive process [6]. The need for detailed thermodynamic analysis is thus

quite evident in order to get a detailed understanding of the process and search for

optimal process irreversibility. Theoretical analysis of the MSF process has been

published [7,8] where the relationships among the various designs and operating

parameters were examined. Characteristics of a number of simplified configurations

including single stage flashing, once through and recycle MSF units were reported [9].

The exergy method is particularly useful for optimizing the design and operation of the

MSF process with the aim of reducing energy consumption. Concepts of first and

second laws of thermodynamics are both used rather than just the first law, to identify

process irreversibility and evaluate the thermodynamic losses of the process.

Applications of the exergy or availability method to distillation processes were reported

[10-15].

In this paper the operational performance of an MSF distiller representing Al-Jubail

Phase II plant is analyzed and simulated . A computer program is used for the

simulation study and its overall logic and algorithm has been previously reported by

Hamed et al. [10]. The simulation program has got the capability to perform

energy/exergy performance calculation of the MSF system. A detailed parametric

analysis is also intended to be carried out.

AL JUBAIL PHASE II PLANT Al Jubail Phase II power and desalination plant is the largest in the world. It consists of

ten Boiler-Turbo-Generator ( BTG ) sets of about 130 MW output each . Each one of

the ten back-pressure turbine feeds low grade steam to four MSF distillers ( i.e. forty

distillers in total are installed). Thirty distillers have 19 stages in the recovery section

and 3 stages in the rejection section each. The remaining 10 distillers have 17 stages in

the heat recovery and 2 stages in the heat rejection section each. The evaporators are

designed for operation at a brine top temperature of 90.6oC with production of 5.2

MIGD and a top temperature of 112.8oC with a production of 6.29 MIGD [16,17].

One distiller from the group having 22 stages was selected for this simulation study

and its design and operating characteristics are shown in Table-1.

OPERATION SIMULATION The simulation program was firstly used to verify that the distiller would operate at the

ce area requirements.

Comparison between design and simulated values are shown on Table 2. When

performance ratios are compared a good agreement was obtained between design and

simulated values and the difference between them is -0.23 and .14 percent for LTO and

HTO respectively. It also shows that the surface area requirements for LTO and HTO

are 9.93 and 5.2 percent less than the available area.

Steady state simulation are normally used to generate operating envelopes to describe

the possible operating conditions of the MSF distiller [18,19]. An envelope of possible

operating conditions of the selected distiller is shown on Figure 1 for winter conditions.

It illustrates the operation as performance ratio vs recycle to distillate mass ratio with

the following boundary conditions :

Nodes along line CF constant recycle concentration ratio of 1.4 and TBT

decreased from 112.8 to 90oC Nodes along line FED constant TBT of 90oC and recycle concentration ratio

decreased from 1.4 to 1.25 Nodes along line DA constant recycle ratio of 1.25 and TBT increasing from

90 to 112.8oC Nodes along ABC″ constant TBT of 112.8oC and recycle concentration ratio

increased from 1.2 5 to 1.4 Simulated Stage-Wise Heat Transfer Coefficients and Fouling Factors Very little information is published on the stagewise heat transfer coefficients and

fouling factors [20] and in most cases an average value for the whole recovery section

is determined. The simulation program has been used to calculate the overall heat

transfer coefficients and fouling factors of each individual stage and simulation results

are shown in Figure 2. The clean overall heat transfer coefficient (Uc) of each stage

which is calculated from the individual heat transfer coefficients of recycle brine and

condensing vapors inside and outside the tubes respectively, is highly dependent on the

number of recovery stage. The highest value of Uc is obtained in the first stage and is

progressively decreasing towards the low temperature stages. This is because the

individual heat transfer coefficients of the recycle brine and the condensing vapors are

dependent on their physical properties which are highly influenced by stage operating

pressure and temperature. Although, the high temperature stages of the recovery section

are having higher clean overall heat transfer coefficients compared to the low

temperature stages, the operating overall heat transfer coefficients (UD) of the different

stages remain virtually constant. This is due to the fact that stages of higher

temperature are experiencing high fouling factors which is offsetting the advantages

gained by the clean heat transfer coefficients. The simulated fouling factors of all stages

are lower than the design values and the gap between the two values widens towards

the low temperature stages. OPERATIONAL PERFORMANCE Operational data for the assigned distiller were collected for a period of one year. The

distiller was operating a TBT of 90oC for the first 200 days as shown in Figure 3. It

was then acid cleaned for the first time after 16 years of operation and the TBT was

raised to 98oC. Comparison between the observed and simulated performance ratio

was shown in Fig. 3(b) and very close agreement between the two values was observed.

Figures 3 (c) and (d) show the variation of the overall heat transfer coefficients (OHTC)

and fouling factors (FF) with time. It can be noticed that the impact of acid cleaning

was more significant on the thermal performance of the recovery section due to the

increase of its OHTC and reduction of FF.

Second law thermal analysis of the distiller is shown in Figure 3(a). the overall exergy

destruction during the pre-acid period was ranging between 58 and 62 kJ/kg and then

dropped to 55-52 kJ/kg after acid cleaning. It also shows the break down of exergy

losses among the major subsystems which included heat recovery and rejection section,

brine heater, ejector system and wasted in leaving streams. The exergy losses in the

recovery section after acid cleaning are less than that in the pre-acid period, which is

accordingly reflected in the reduction of the overall exergy losses. The large exergy

losses in the recovery section are the result of the exergy losses in feed heaters which

are having condensing area several times higher than that of the brine heater and heat

rejection section and those caused by flashing of brine and distillate in each recovery

stage.

PARAMATERIC ANALYSIS A comprehensive simulation study was carried out to examine the interrelationships

between the top brine temperature, number of stages, performance ratio, exergy losses

and the terminal temperature approach. The terminal temperature approach is the

difference in temperature between the condensing vapor and the recirculating stream

leaving the condenser and is an important design parameter depending on the heat

transfer area and overall heat transfer coefficient. The interrelationships between the

different parameters are presented in the form of grid network which are shown in

Figures 4 and 5 for TTD of 2 and 4 oC, respectively. Both figures reveal that increasing

number of stages while keeping top brine temperature constant such as lines AC & BD,

results in an increase of both performance ratio and specific condensing area and a

decrease of exergy losses. Increase of number of stages will decrease temperature drop

per stage which will reduce the irreversibility of the system due to the reduction in

condenser and flash exergy losses and results in an improved thermal performance.

Increase of performance ratio will reduce steam consumption and consequently

minimize operating cost while increase in surface area results in an increase of capital

expenditure. Therefore, optimum performance ratio (i.e. that of lowest production cost

of water which is site specific) has to be determined by making cost tradeoffs between

the cost of process energy and capital cost of the process.

Conversely, increasing top brine temperature while keeping number of stages constant

(lines such as AB and CD) will increase the performance ratio and decrease the

specific condensing area but its net effect on exergy destruction is not significant.

Within the selected range of number of stages and top brine temperature, the simulation

analysis revealed the dependence of exergy losses on number of stages is more

appreciable than that due to variation of top brine temperature.

CONCLUSIONS 1. Performance and operation of a 22-stage MSF distiller is simulated. An operating

envelope with specific boundary conditions is generated and presented to describe

the distiller possible operating conditions.

2. A one year operation data is used to carry out an energy/exergy thermal analysis

of the whole distiller as well as the major subsystems. Dependence of

performance ratio, heat transfer coefficients and exergy losses on the most

important operation parameters, is discussed.

3. A detailed parametric study is carried out. The impact of number of stages and

top brine temperature on performance ratio, exergy losses and specific condensing

area is presented in the form of grid network.

Table 1 : Design Characteristics of the Selected Distiller (winter conditions) at low (L) and high temperature operation (HTO).

Parameter LTO (90.6

C ) HTO (112.8 C)

Product water flow m3/Hr 987 1184 Recycle brine flow m3/Hr 10817 9556

Makeup flow m3/Hr 2923.55 3504.5 Blow down flow m3/Hr 1936.55 2320.5

Sea water to rejection m3/Hr 6996 7130 Tube side velocity m/s 1.98 1.58 Brine heater heat transfer coefficient W/m2 oC 2257 2182 Fouling allowance m2 oC/W Heat recovery and brine heater

0.000176 0.000176

Performance ratio kg/1000 kJ 3.44 4.08 Table 2 : Comparison of design and simulated parameters of the selected distiller (winter conditions) at low (L) and high temperature operation (HTO).

Parameter LTO (90.6 oC) HTO (112.8 oC)

Design Simulated Diff. Design Simulated Diff.

Set product (t/hr) 987 987 0.0% 1184 1184 0.0% Flow (t/hr) 10817 10733.6 0.77% 9556 9348.1 2.17% Recycle Conc. Ppm 63700 63461 0.37% 61400 61508 0.17% Steam to brine heater (t/Hr )

126.77 126.64 0.1% 131.417 132,09 -0.51%

PR ( kg /1000 kJ ) 3.44 3.448 -0.23 % 4.08 4.074 0.14% Total surface Area requirements m2

88459.6 (available)

79672.2 9.93% 88459.6 (available)

83798.6 5.2%

ACKNOWLEDGEMENT The authors gratefully acknowledge the help and useful discussion provided by Mr.

Abdul Salam Al-Mobayed and Anwar Ehsan of Al-Jubail Plants.

Figure 1 :Operating Analysis of a 22-stage MSF distiller.

7.8

8

8.2

8.4

8.6

8.8

9

9.2

9.4

9.6

7.5 8 8.5 9 9.5 10 10.5 11 11.5

Recycle to Distillate Ratio

Per

form

ance

Rat

io (

kg /

2326

kJ)

1.4

1.25

1.325

98 C

102 C

100 C

106 C

TBT=112.8 C

96 C

109 C

94 C

92 C90 C

112.8 C

112.8 C

109 C

106 C

102 C

100 C

98 C

96 C

94 C

92 C

90 C

90 C

Brine Conc. Ratio

A

B

C"

D

E

F

Figure 2 : Variation of Heat Transfer Coefficient and Fouling Factor with Recovery Stage Number

1000

2000

3000

4000

5000

0 2 4 6 8 10 12 14 16 18 20

Ove

rall

Hea

t T

ran

sfer

C

oef

f.(k

W/m

2K

)

Simulated UD Simulated Uc

0

0.04

0.08

0.12

0.16

0.2

0 2 4 6 8 10 12 14 16 18 20

Recovery Stage Number

Fo

uli

ng

Fac

tor(

m2 K

/kW

)

Simulated FF FF(design)

Figure 3: Operation Performance of MSF Distiller

20

40

60

80

100

120

Te

mp

era

ture

(oC

) T B T F lash Range

0

10

20

30

40

50

60

70

0

20 40 60 80 100

120

140

160

180

200

220

240

260

280

300

320

340

360

D a y s

Ex

erg

y D

es

tru

cti

on

(kJ

/kg

)

Dest. In B/H Dest in Recov.

Dest . In Rejec. Wasted in leav ing Stream

W e s t e d i n E j e c t o r Tota l Losses

7

8

9

10

PR

(kg

/23

26

kJ

)

Simula ted P l a n t

0

1

2

3

4

5

6

Bri

ne

He

ate

r

OH

TC

(kW

/m2 K

)

0

0.2

0.4

0.6

0.8

Fo

uli

ng

Fa

cto

r(m

2K

/kW

)Fouled HTC of Br ine Heater Foul ing F. o f Br ine Heater

0

1

2

3

4

5

6

He

at

Re

c.

OH

TC

(kW

/m2K

)

0

0.2

0.4

0.6

0.8

1

Fo

uli

ng

Fa

cto

r

(m2 K

/kW

)

Av. HTC of H.Recovery Sec. Av. Fou l ing F. o f H.Rec. Sec.

a

b

d

e

c

Acid Cleaning

2428

3236

40

Figure 4: Dependence of Performance Ratio, Exergy Losses and Specific Condensing Area on TBT and Number of Stages

20

TBT, oC

Figure 5: Dependence of Performance Ratio, Exergy Losses and Specific Condensing Area on TBT and Number of Stages

TBT, oC

B

T B T oC

50

60

70

80

90

1 0 0

1 1 0

6 7 8 9 10 11 12 13 14

P e r f o r m a n c e R a t i o , k g / 2326 kJ

Ex

erg

y L

os

se

s,

kJ

/kg

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

Co

nd

en

sin

g A

rea

, m

2 /(k

g/s

)

9 0

1 0 0

1 1 2

1 2 0

N u m b e r o f S t a g e s

1 6

A

B

CD

16

20

24

28

36

40

32

N u m b e r o f S t a g e s

1 620

A

2428

32 36 40 C

2024

28

3236

D40

40

C

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4

P e r f o r m a n c e R a t i o , k g / 2 3 2 6 k J

Ex

erg

y L

os

se

s,

kJ

/kg

9 0

1 1 0

1 3 0

1 5 0

1 7 0

1 9 0

2 1 0

2 3 0

Co

nd

en

sin

g A

rea

, m

2 /(k

g/s

)

901 0 01 1 21 2 0

16

N u m b e r o f S t a g e s

32

20

24

32

36 40

28

Number of Stages 16

24

36

20

D

C

A 24

28

36

20

3240

40

28

DC

BA

Stages 16

20

24

32 36

40

28

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