Simulation of Multi-stage Flash（MSF）Desalination Process

doi:10.4236/ampc.2012.24B052

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Advances in Ma terials Ph ysics an d Ch e mistry, 2012, 2, 200-205

doi:10.4 236 / ampc .2012.24B052 Published Online December 20 12 (http ://www.SciRP. org/ j ou r nal/ ampc)

Simulation of Multi-stage Flash (MSF)

Desalination Process

Lian-ying Wu, Sheng -na n X iao, Cong-jie Gao

College of Chemi st ry and Chemical Engi neering, Oc ean University of China, Shandong, Qingdao 266 100, Chin a

Received 20 12

ABSTRACT

MSF seawater desalination has become an important technology to solve the scarce of fresh water resources in the world. But the

high energy cost is the bottle-neck of extendibility and application. In this paper, the principle of MSF is analyzed and the single

flash stage is divided into several elementary unit operations. The Aspen Plus is adopted to simulate MSF desalination process. The

effect factor of MSF system, such as the feed seawater temperature, the top brine temperature（TBT） and the stage number, is

investigated and the optimum operation condition is obtained.

Keywords: S eawater Desalination; MSF; Process Simulation

1. Introduction

The fresh water resource is one of the most important factors

which constrain the economic development, social progress and

human survival. Currently, there are more than 100 countries

and regions, in where the fresh water is shortage, about 1.5

billion people can not get the clean drinking water, 2.0 billion

people are living without safe water, and the consumption of

water is increasing at a rate of 4% per year [ 1,2]. China is one

of the countries that the United Nations recognized as the 13

most water-poor countries, per capita fresh water resources is

only 1/4 of the world average level, and the temporal and spa-

tial distribution of freshwater resources is uneven. And the

lacking is ch aracteristic with bo th at reso urces and water quali-

ty [3-5]. Water scarcity has become a major bottleneck that

constraints the world's sustainable economic development [6].

Desalination has become recognized as an effective measure

to solve the water shortage [1]. However, high energy con-

sumption is one of the major bottlenecks to limit its promotion

and application. Therefore, it is important significance to seek

methods to reduce the desalination energy consumption. This

paper is trying to use Aspen Plus software to simulate the MSF

desalination process and analyze the effects of the operating

parameters to the MSF desalination system. At the same time,

the method of reducing energy consumption is provided as

reference t o the design of MSF s yste m.

2. The Princip le of MSF D esal inat ion

The MSF s ystem (shows in Figure 1) consists of three sections:

heat-rejection, heat-recovery and brine heater. The heat-rejec-

tion and heat-recovery consist of a number of flash chambers

(stages) con n ected to one ano ther. Raw seawater fir st co me in to

the h eat-reco very sectio n to condense t he st eam produced in the

flash chamber, at the same time it is heated. Most of the water

coming fro m the heat -reject io n sectio n retu rn to th e sea , and t he

remaining part will be mixed with part of the brine rejected

from the l ast stage o f the heat-rej ection section; and then tran s-

ported by brine circulating pump into the condenser of the last

stage flash chamber o f the heat reco very section . The recycling

brine, which flowing along with the opposite direction of the

flash brine flow direction, is heated by the flash steam produc-

ing in every flash room, and the flash steam is condensed. The

heated circulating brine coming out from the condenser tube of

the first flash chamber is transported into the brine heater and is

further heated to a specified temperature, which is the top brine

temperature (TBT). And then it flows into the lower level of the

first flash chamber, and th e circulating brine begin s to progres-

sively flash along per flash stage. The flashed steam will be

condensed in the condenser tube, and then it flows into the

fresh water tank as product water.

3. The Simulation and Analysis of MSF

Desalin ation Proce ss

Performance Ratio (PR) and the rate of brine flow (RBF) are

two important parameters that have a major impact on the in-

vestment costs and the operating costs of the desalination sys-

tems [10]. This paper mainly studies the influence of the feed

seawater temperature, TBT and the number of flash stage on

PR and RBF. Then, the expecting optimal operating conditions

of the multi-stage flash desalination system is provided.

The realization of the MSF process in the Aspen Plus

MSF desalination system is composed of N-level flash units.

Figure 2(a) represents the actual structure of a flash unit; Fig-

ure 2(b) shows the structural decomposition of a flash unit. The

flash unit is split up into four compartments: the pre-heater, the

flash chamber, the fresh water room and the steam room. Then

it is very easy to be described with the Aspen Plus. The pre-

heater in top of the flash chamber could be simulated by the

HETRAN module of the Aspen Plus, the flash chamber could

be used FLASH module to be simulated and the fresh water

room and the steam room could be used MIXER module to be

simulated. After the MSF is be transformed, the single flash

unit will be constructed based on the actual logistics connection

L. -Y. WU ET AL.

201

Figure 1. A circulating-brine multistage flash (MSF ) desalination plant.

Figure 2. The structure o f a single flash stage .

of the stream. Subsequently, connecting the multiple flash units

sequential, the simulation of the process of MSF system is

achieved in the Aspen Plus.

4. Analysis of Simulation Results

4.1. The Influence of Feed Seawater Temperature

Here, the heat recovery section and the heat rejection section

are specified as 18 stages and 3 stages in th e MSF system. The

TBT is set to 110℃ and the freshwater production is set to

100t/h. The difference between feed seawater temperature and

the rejected brine temperature is 10℃. The feed seawater tem-

perature is varied from 10℃ to 3 5℃ with interval of 5℃.The

vary trend of PR and RBF long the feed sea water te mperat ure is

obtained. The results are shown in Figure 3, Figure 4.

Figure 3 gives the chart of feed seawater temperature vs. PR.

From the chart, the PR of the MSF system showed a gradually

rising trend when the feed seawater temperature is increased.

The PR of MSF system did not change significantly and over

the range of 10 ~ 20℃, but it was approximately straight up

between 20 ~30℃. When the feed seawater temperature goes

beyond to 30℃, the changed trend of PR becomes to be gently.

The chart of RBF vs. feed seawater temperature is shown as

Figure 4. The value of RBF almost changed and the line is

level when the feed seawater temperature is varied from 10℃

to 20℃. But it was rapidly increasing when the feed water

temperature passed 20℃.

While the freshwater production, the TBT and the number of

flash stage are specified as a constant, the lower of the feed

seawater temperature, the more of the steam required to heat

the brine to TBT. But when the feed seawater temperature

beyond to 25℃, the temperature of circulating brine, which

coming out from the first stage flash pre-heater and being

heated by the multi-stage pre-heater, slowly changed. And the

required amount of heating steam tends to change slowly, so

the PR became relatively gentle. At the same number of flash

stages, the temperature of rejected brine increased when the

feed seawater temperature is increased. Because the tempera-

ture difference of between the feed seawater and the rejected

brine is set to 10℃. However, the TBT is keep as an unvaried

L. -Y. WU ET AL.

202

value,

therefore the total difference in temperature and the stage tem-

perature difference will be reduced simultaneously. Conse-

quently, the producing fresh water of per unit flash stage will be

decreased . In or der to produ ce the sa me quan ti ty of fresh water,

the amount of circulating brine must be increased. Increasing

the amount of circulating brine flowrate will make huge oper-

ating costs. Summarizing, the appropriate feed seawater tem-

perature is about 25℃.

4.2. The Influence of TBT

Here, the heat recovery section and the heat rejection section

are specifi ed as 18 stages and 3 stages respecti vely in the MS F

system. The freshwater production is set to 100t/h. The feed

water temperature is set to 25℃ and the temperature of re-

jected b rine t emperature i s set to 3 5℃. The TBT is varied fro m

80℃ to 120℃ with interval of 10℃. The simulation results

are shown in Figure 5 and Figure 6.

10 15 20 25 30 35

sea water temperature/℃

Figure 3. Effect of feed seawater tempera ture on the PR.

10 15 20 25 30 35

720000

740000

760000

780000

800000

820000

840000

RBF/(kg/h)

sea water temperature/℃

L. -Y. WU ET AL.

203

Figure 4. Effect of feed seawater tempera ture on the RBF.

8090100 110 120

9.0

9.5

10.0

10.5

11.0

11.5

12.0

TBT/℃

Figure 5. Effect of top brine temperature on the PR.

8090100 110 120

600000

700000

800000

900000

1000000

1100000

1200000

RBF/(kg/h)

TBT/℃

Figure 6. Effect of top brine temperature on the RBF.

Figure 5 and Figure 6 give a PR chart o f MSF and the RBF

chart of MSF respectively when the TBT is changed. From

Figures 5 and 6, the PR increased firstly and where-after de-

creased step by step with the TBT is increased. And the maxi-

mum value of PR is obtained at 90℃. But the RBF chart of

MSF showed decreasing trend, and the reduced trend is wea-

kened gradu all y while t he TBT is in creased . As th e temperat ur e

of rejected brine is constant, the total flash temperature differ-

ence of s ystem and th e flash temp eratu re d ifference o f per s tage

were incr eased after th e TBT is incr eased. With t he same num-

ber of flash stage, the total amount of fresh water would be

increased due to the producing fresh water quantity of single

stage increased. So the RBF of MSF would be decreased to

guarantee the total production of fresh water to constant. On the

other hand, though the required amount of steam heating will

be large on account of hoisting the TBT. But the RBF was re-

duced more obviously at the range of 80 ~ 90℃, so the result is

that the PR increased rapidly and the required amount of steam

heating reduced markedly. While the TBT is higher than 90, the

amount of increasing heat steam took the more important role

in influence of PR than the amount of reducing RBF. Accor-

dingly, the PR diminished when the TBT is higher than 90.

L. -Y. WU ET AL.

204

Therefore, considering the performance ratio and operating

costs, the TBT of MSF system should be between 90 ~ 110℃

approp ri ate.

4.3. The Influence of F l ash Sta ge Number

In this section, the PR and the RBF influenced by the flash

stage number will be discussed. The given as fellow: the TBT

is 110℃, feed water temperature is 25℃, the temperature of

discharged brine is 35℃ and the fresh water production is

100t/h in the MSF system. The results are dep icted in F igure 7

and Figur e 8.

When the flash stage number is increased, the PR increased

gradual ly, and the more t he stage number is, t he greater the P R

is achieved. By contraries, the amount of the circulating brine

showed a decreasing t r end.

As the sa me total d if ferenc e in temp eratur e is fixed, t he differ -

ence in temperature of single stage was decreasing when the

number of flash stage was increased. Therefore the flashing of

brine would be more close to the reversible process, and the

energy loss would be reduced. At the same time, the brine

pre-heater could recovery more heat and the required quantity

of heating steam could be cut down. Consequently, the PR

should be increased. On the other hand, the closer the brine

evaporation process to the reversible process, the more the fresh

water produced in an single-stage evaporation unit. To holding

the same fresh water production, the number of flash stage is

bigger and the required amount of the circulating brine is little.

But the more stage number, the equipment will become more

complexity and the investment costs will be higher. Consider-

ing the investment costs and operating costs, the number of

flash stage should be more reasonable for the 20-25 level.

18 20 22 24 2628 30

stage number

Figure 7. Effect of flash stage number on the PR.

L. -Y. WU ET AL.

205

18 20 22 24 2628 30

745000

750000

755000

760000

765000

770000

RBF/(kg/h)

stage number

Figure 8. Effect of flash stage number on RBF.

5. Conclusions

In this paper, multi-stage flash desalination technology is ana-

lyzed. The single flash unit is divided into several basic opera-

tions, and the Aspen Plus software is used to simulate the mul-

ti-stage flash desalination process. Several influenced factor of

the MSF system are discussed, such as the feed sea water tem-

perature, the TBT and the number of flash stage. The results

shown: when the fresh water production, the TBT and the

number of flash stage are fixed, the lower the feed water tem-

perature, the greater the amount of required external heating

steam, the smaller the PR and the greater the amount of circu-

lating brine. Similarly, fasting the fresh water production, the

feed sea water te mperature an d the number of flash stage, when

the top brine temperature was increased, the PR increased at

first and then decreased, but the RBF was gradually reduced;

With the fresh water production, the TBT and the feed sea wa-

ter temperature co nstantly, the P R was increased graduall y and

the RBF was reduced gradually when the flash stage number

was increasing. Synthetically, the best operating conditions of

the MS F system sho uld be selected as follows: feed water te m-

perature is 25℃, the TBT is 90 ~ 110℃, and the number of

flash stage is 20~ 25.

6. Acknowledgements

This work was financially supported by National Natural

Science Foundation of China (20976173&21076202).

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