Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 207

Temperature differences between brine and vapor streams

leaving stages may be from 0.45 to 1.7C (
3/4 to 3F)

besides boiling point elevation.

In Figure 10 the flow diagram is given of a multi-stage-

flash evaporator and the temperature profile across the plant.

Figure 11 gives the two arrangements of the multi-stage-

flash chambers. Configuration No. 1, with long tubes is pre-

ferred by American construction companies. The cross type

No. 2, is usually preferred by European contractors.

Combined Distillation Plants

Significant economic advantages may be expected from

combining different distillation processes, especially where

desalting plants are designed with large capacities. Studies and

design experience indicate that the combined system possesses

substantial advantages in cost compared with the unit form of

multi-stage plant of the same capacity. The savings in cost are

due primarily to lower capital investment, and lower opera-

tional and maintenance costs. A further advantage of the com-

bined plant is its high operational stability at varying loads.

Many designs have been proposed and many combi-

nations have been tried, mainly for small capacity or pilot

size plants. Commercial application used the vertical tube-

multi-stage-flash (VTE/MSF) and the vertical tube-vapor

compressor processes. Tubes with fluted surfaces are used

in the vertical tube evaporator plants to obtain enhanced

heat transfer performance. The heat recovery section of the

multi-stage-flash plant is used as feed preheater of the verti-

cal tube plant, which is the main evaporator for the distil-

late production. The combined vapor compression-vertical

tube evaporator process uses the heat recovery section of

the multi-stage-flash as preheater for the vapor compression

plant as it is more efficient than the heat-exchangers in the

single vapor compression process.

Scale Formation and Its Prevention

Formation of scale deposits on and fouling of heat transfer

surfaces is one of the most serious problems of distillation

equipment operating with sea or brackish water. As the scale

deposits lower the efficiency of heat transfer surfaces and

increase the pressure drop, pretreatment of feed water is

necessary to prevent the deposition of scale.

As the salt concentration increases during progressive

evaporation, the critical point may be reached at which the

solubility limit of scale-forming compounds contained in

the feed water, is exceeded and formation of scale occurs.

The term scale is applied particularly to describe hard,

adherent, normally crystalline deposits on the heat transfer

FEED WATER

43.5°C

43°C

2nd EFFECT

PRODUCT WATER

ANTI-SCALE

CHEMICAL

INJECTION

SEAWATER SUPPLY

VENT EJECTOR

MAIN EJECTOR

STEAM SUPPLY (32°C)

BLOW DOWN BRINE

1st EFFECT

47°C

EVAPORATING TUBE EVAPORATING TUBE

C

T

A

C

T

FIGURE 8 A two effect reheat or thermal compression unit. The low pressure, low temperature vapors from the second effect are

sucked by the steam jet ejector A, driven by a small quantity of high pressure boiler steam, and delivers a hotter compressed mixture of

stream and vapors to the condenser tubes T of the first effect. The seawater feed is sprayed onto the outside of the horizontal condenser

tubes T. Part of the rejected brine from the last stage is used as feed to the first stage. Condensate, or product fresh water is collected in

the last stage chamber C and distributed through the pump P. (Courtesy Sasakura Engineering Co., Ltd., Japan.)

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