Encyclopedia of Environmental Science and Engineering, Volume I and II

210 DESALINATION

factors, thereby increasing the water recovery ratio for

saline solutions and favourably affecting the economics

of the desalting process. Treatment of the seawater with

H 2 SO 4 , on the other hand, reduces the corrosiveness of the

water by eliminating dissolved oxygen and carbon dioxide

in the deaerator.

Acid treatment converts bicarbonate to carbon dioxide

gas, whereas caustic treatment yields the carbonate ion,

which combines with the calcium ion present in seawater

to precipitate calcium carbonate. Nucleation of scale on the

heat transfer surfaces is inhibited and precipitation of solids

is dispersed in suspension by the addition of small amounts

of certain commercial preparations. The sludge formed is

then removed in the blowdown.

The first commercially available scale control compound

was Hagevap, a mixture of sodium tripolyphosphate lignin

Condenser Tube

Condenser Tube

Mist Separator

Mist Separator

Fresh Water Produced

Partition Plate

Partition Plate

Brine

Tube Plate

Water Chamber

2

1

Brine

FIGURE 11 Drawing of a long tube multi-stage-flash (No. 1) and a cross tube multi-stage-flash chamber. The long tube configuration has

a condenser running through each stage and the tubes are parallel to the flow direction of the flashing brine. High technical skill is necessary

to seal the partition plates where the condenser runs through the partition plate. Long tube MSF plants are adopted for larger desalination

plants. The cross tube type desalination units have the tubes across the top of the flash chamber and at right angles to the flow direction of

the flashing brine. The cross tube configuration has condensers independent of each other and is mainly used for smaller desalination plants.

(Courtesy of Sasakura Engineering Co., Ltd., Japan).

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