Encyclopedia of Environmental Science and Engineering, Volume I and II

204 DESALINATION

has a lower pressure than the saturation pressure of pure

water. It will, therefore, in losing the superheat, condense

at a lower temperature than the boiling point of the solu-

tion. If this vapor is compressed to a higher pressure, the

energy input results in a rise in temperature. With sufficient

rise in pressure and temperature, the recompressed vapor

might be used as a source of heat for evaporating the same

salt solution.

Heat needs to be supplied to the system only at the start-

up for elevating the temperature of the solution to the boiling

point. Once boiling has started, it is maintained by the exter-

nal supply of power and no more by the addition of heat as

the cycle is repeated.

Vapor compression distillation accounts only 3.7% of

total world wide desalination capacities and about 6.2% of

distillation processes, for units producing 100 m^3 /d or more

fresh water. Daily productivity is 686,500 m^3 /d.^5

The energy source may be mechanical or electrical

power to drive the compressors for mechanical vapor com-

pression. Thermal vapor compression, or “thermocompres-

sion,” uses high pressure steam to compress the vapors to

higher temperatures. Vacuum vapor compression uses elec-

tric or waste heat to reheat the vapors, circulating by the use

of a blower.

Mechanical Vapor Compression (MVC) The MVC process

uses compressors to reheat the vapors to higher tempera-

tures. High or low pressure compressors are used, depending

on the capacity of the system. As high capacity plants have

many stages, the necessary temperature is higher and they

are high pressure compressors. Low capacity plants use low

pressure compressors. The higher the compression pressure,

the smaller is the volume of the compressor and that of the

vapor. Due to high temperatures, high capacity MVC plants

are prone to scale formation.

Figure 7 presents the flow-sheet of a four-stage mechani-

cal vapor compression evaporator and the T-S diagram of the

thermodynamic operation.^20

Thermocompression Thermocompression uses high pres-

sure steam ejector to re-heat the vapors released from the

last stage. Figure 8 gives a typical two stage thermal vapor

compression diagram.

Another type of thermal vapor compression operates

under vacuum inside the evaporation chamber. The low pres-

sure vapors are circulated by a vapor blower and heated by

electric heater, hot water or hot gas, according to the avail-

able heat source. A suitable adaptor for the various heat

sources is necessary.

FRESH WATER

SEAWATER FEED

BRINE BLOW-

DOWN

DECARB-

ONATOR

COOLING WATER

OUT

CONDEN-

SER

COOLING

WATER IN

VENT

SEAWATER BRINE BRINE

BOILER

STEAM

CONDENSATE

PREHEATED FEED

WATER

B

B

A

A

AIR

VENT

CO 2 ,

D D D D

C C CC

F

1 2 N–1 N

FIGURE 5 Flow diagram of a multiple-effect-evaporator for seawater desalination, of the falling type. Seawater is preheated in the heaters

C and pumped on the top of the first evaporator No. 1, from where falls down inside the vertically oriented tubes B. A thin film of brine is

formed inside (detail A and B). In the first effect steam from the boiler forms a thin film of condensate outside the tubes. In the following

effects the vapors of each effect condenses outside the tubes of the next effect. The rest of the space of the evaporator is then filled with

water vapors. The brine accumulates in the bottom of the evaporator from where is fed to the next effect by the pumps D. A distributor cap

is fitted on each tube to ensure even distribution of the brine. The produced fresh water is used to preheat the seawater fed in the heaters C.

Part of the seawater is acid treated and a decarbonator F, is used for the removal of air and CO 2.

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