Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 209

heat recovery stages

heat rejection stages

cooling water

cooling water

vent chemicals

brine

blowdown

seawater

feed

seawater

feed

fresh water

fresh

water

brine

blow down

condensate

boiler

steam S

brine heating

temperature

Tf

Tf

TF

TB

TO

TC

TC

Tmax

Tmax

T

max

• T

C

FIGURE 10 Flow diagram of a multi-stage-flash evaporator with brine recirculation, and temperature profile across the plant. Cold

water feed is pre-heated inside the condenser tubes of the heat rejection stage A, then circulating through the condensers of the recovery

stages, is heated to temperature Tc. Finally is heated by steam and reaches the highest temperature in the process Tmax. By this temperature

is fed to the 1st stage of the recovery section. The recovery and the rejection sections are enclosed in a single long vessel. The rejection

stage removes excess heat from the flashing brine. Each stage is operating at a lower pressure than the preceding stage, with temperature

fall from stage to stage. The vapor generated in the flash chambers condenses on the tubes of the condenser giving its latent heat of con-

densation to the heated seawater into the tubes. Leaving the heat rejection stage, brine is blow-down. Usually 50% to 75% of the brine

recirculates after mixing with cold seawater. In the temperature profile, the stepped line shows the temperature fall at each stage of the heat

recovery and heat rejection sections. Tmax is the maximum temperature to which the seawater feed is heated. Tf is the discharge temperature

of the brine in the last stage and To is the outlet temperature from the recovery stage to the rejection stage. TF is the temperature of cold

seawater feed. These temperatures are correlated to the performance ratio R.

There are several techniques used to avoid deposition of

scale. Calcium carbonate and magnesium hydroxide forma-

tion can be controlled by acid injection and pH adjustment

or by the addition of polyphosphates. Sulphuric acid trans-

forms the carbonates to sulphates so that only one type of

scale-forming salt remains present. Phosphates precipitate

calcium and magnesium as sludge, minimizing the effects

on the heat transfer surfaces. Either of the chemicals is intro-

duced into seawater before its entry into the deaerator. In the

deaerator the dissolved gases, together with carbon dioxide

evolved during acidification, are eliminated. The pH of the

treated seawater is controlled by the addition of dilute caus-

tic soda, which binds the remaining carbon dioxide.

Calcium sulphate scale is more difficult to control.

When formed on heat transfer surfaces, removal is difficult,

if not impossible. To prevent deposits two methods may be

applied. Seed crystals are injected into the hot seawater to

promote precipitation of scale-forming compounds on these

seeds, which then form a sludge rather than a deposit on the

heat transfer surfaces. Ion exchange treatment is applied

to eliminate completely both calcium and magnesium ions

from the solution.

A technique of seawater pretreatment is the LMC or

lime-magnesium carbonate process. 70 to 80% of the cal-

cium originally present in seawater is removed and this

permits operation at higher temperatures and concentration

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