Encyclopedia of Environmental Science and Engineering, Volume I and II

208 DESALINATION

surfaces. Three simultaneous factors are required for the for-

mation of the scale:

1. Local supersaturation of the solution.


2. Nucleation, which when formed includes the rate
   of further scale deposition.


3. Sufficient contact time of the solution and the
   nucleus.

Under certain conditions a soft, amorphous material,

called sludge, may be deposited or remain suspended in the

brine and is generally more easily removed than hard scale.

If the ions contained in seawater are combined in the

form in which they usually deposit, the resulting compounds

will be approximately:

CaCO 3 109 mg/L

CaSO 4 · 2H 2 O 1,548 mg/L

MgCl 2 3,214 mg/L

MgSO 4 2,233 mg/L

NaCl 26,780 mg/L

assuming that hydrogen carbonate decomposes to carbon-

ate before precipitation occurs. Alkaline scale, CaCO 3 and

Mg(OH) 2 , results from the decomposition of the hydrogen

carbonate ion. On heating seawater up to 82C (180F), the

hydrogen carbonate ion decomposes and calcium carbonate

is formed.

A second type of scale, called acid scale, is due to three

forms of calcium sulfate: the anhydrite CaSO 4 , the hemihy-

drate and the dihydrate CaSO 4 · 2H 2 O, or gypsum. While the

precipitation of CaCO 3 and Mg(OH) 2 is mainly affected by

CO 3 2– concentration, pH and temperature affect the solubility of

calcium sulfates in addition to the concentrations of other ions

present. CaSO 4 has as well decreasing solubility in the tem-

perature ranges of interest. The solubility increases in chloride

solutions, as the concentration approaches 4 to 5% chloride and

then decreases to values comparable to those in chloride free

water as the chloride concentration becomes 10 to 15%.

Maximum brine temperature provided in the design, maxi-

mum allowable brine concentration and brine recirculation rate

are also affected by the formation of scale. These operating

variables and the plant availability are closely tied to the eco-

nomics of the process as the production rate is generally low-

ered. Periodic plant shutdowns for descaling would be required

either by an acid clean or, in extreme cases, by mechanical

cleaning of the tubes. Incrustation allowances to reduce the

frequency of shut-downs are made in designing evaporators,

which are provided with a sufficient larger heat-exchange sur-

face in order to maintain the design capacity. The term fouling

is often extended to this type of admissible scaling.

VVC DISTILLER ALTERNATIVE BACK-UP HEAT SOURCES

ELECTRIC HEATER WASTE HEAT CONDENSER

SCALE

INHIBITOR

FEEDWATER

FEED

PREHEATER

BRINE

PRODUCT

WATER

EVAPORATION

VACUUM PUMP

VENT VAPOUR BLOWER

BRINE

ELECTRIC HEATER WASTE HEATEXCHANGER

EXTERNAL HEAT SOURCE

CONDENSATE

STEAM

BRINE

FIGURE 9 Flow diagram of a vacuum vapor compression (VVC) unit. The vacuum vapor compression distillers are small capacity units,

and can produce fresh water from any kind of water, as dirty waters, with low energy consumption. It operates without acid treatment at

70 C (158F) and uses, as heat source, electricity or can be combined with hot gas or hot water 80C. The salt water feed is preheated and

sprayed to the top of the evaporator A, where it is distributed as a thin film over the outside of the tubes T. The thin film boils at the tube

surface due to condensation of the hotter vapor inside the tubes. The vapor thus created is compressed in the blower B and its temperature

is raised, before it passes to the inside of the tubes T, where it condenses to form fresh water. Most of the heat is thus effectively recycled.

C, D, E, represent the auxilliary parts of heating which can be adapted to the main distillation unit U, according to their availability.

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

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