Encyclopedia of Environmental Science and Engineering, Volume I and II

206 DESALINATION

temperature gradient between stages, to maintain the ther-

modynamic cycle. However, there are practical limits in the

excessive increase of the number of stages. The additional

investment to provide further stages to the plant should be

reasonable with respect to the savings of heat obtained by the

same stages. In large multi-stage flash evaporators for desalt-

ing of seawater, the cost of the heat transfer tubes is a very

important part of the total construction cost. The tempera-

ture drop in each stage and the difference between brine-inlet

temperature at the first stage and the discharge temperature at

the last stage are the main controlling parameters of the MSF

distillation process. In a cascading flashing stream of an MSF

evaporator, the combined heat capacity of the flashing brine

and distillate streams equals that of the recirculating brine

and the temperature rise of the recirculating brine equals the

temperature drop of the flashing stream.

The number of stages in an MSF distillation plant

is related to the performance of ratio. An increase in the

number of heat-recovery stages will generally result in a

higher performance ratio for a given product-water output

and in a decrease of steam consumption at the brine heater.

The performance ratio R can be correlated to the number

of stages by the following relation:

R

no recovery stages

no reject stages

TT

TT

kg

2326 kJ

max f

oF







..

(4)

Vapor bubbles almost explode to carry entrained brine to the

product. Demisters D (Figure 5) reduce this but give additional

temperature losses up to 0.11C (0.2F) per stage. Low flash

temperatures per stage reduces flash violence and entrainment

but this increases the number of stages to 40 or more and also

increases equipment costs and inefficiency, which increase

capacity and material costs.

FIGURE 7 Flow diagram of a typical 4-stage, mechanical vapor compression plant. Incoming seawater feed is preheated in the heat-

exchanger H, by the produced freshwater and the blowdown brine. The vapors released in the first stage are flashing to the second stage and

so go on up to the 4th (or to the nth stage). The vapor from the last stage is compressed in the compressor C. Electric power is generated in

a turbine to drive the compressor C. Compressed steam is circulated through the tubes of the condenser B, where it condenses, giving the

heat to the evaporating seawater. Released vapors are used as heating medium in the second stage, etc. The condensate, i.e., the produced

freshwater, leaving the flashing chambers F is collected after further cooling in the heat-exchanger H and brine is rejected.

brine blow

down

pre-heated

distilled

water

seawater

feed

Tf

Tf

seawater

distilled water

E E E

B B

D D D

B B

H

C

F F F F

Te

Te

Td

Td

Tk

Tk

Tmax = T 4

T 4

T 3

T 3

T 2

T 2

T 1

T 1

Tc

Tc

Tb

Tb

Ta

Ta

C004_001_r03.indd 206C004_001_r03.indd 206 11/18/2005 10:18:55 AM11/18/2005 10:18:55 AM