Encyclopedia of Environmental Science and Engineering, Volume I and II

202 DESALINATION

temperatures and pressures. Thus, water alone is vaporized,

which is the main product, and the dissolved solids remain

in the residual liquid, the brine.

In the chemical industry, when an evaporation process

is applied, the water vapors are usually discarded and the

emphasis is given to the recuperation of the dissolved solids.

In desalination the term “distillation” predominates over

the correct term “evaporation.” The process is performed in

evaporators, where heat is supplied to the solution, to change

phase.

The productivity is expressed either as the net evapo-

ration or “gain output ratio” (GOR), i.e., the kilos of pro-

duced distilled water per kilo of boiler steam used, (kg/kg)

or as performance ratio R. It is usually prefered, instead of

the GOR, to use the term performance ratio which defines

the mass of distillate produced per 2326 kJ (gal/1000 BTU)

of heat input to the brine heater in case of MSF distilla-

tion or to the first effect in case of multiple-effect evapo-

rators. The latter definition is thermodynamically more

accurate, as it refers to the enthalpy of the steam instead to

the mass.

Thermodynamic considerations lead to a common char-

acteristic of all distillation process, that the percentage of

evaporated water with respect to the circulating seawater is

as much larger as is the difference between the maximum

and minimum temperature of the saline solution. As the

minimum temperature is defined by the temperature of the

incoming seawater, enlarging of the temperature difference

can only be obtained by increasing the initial maximum

temperature of the salt water feed. Limitations due to the

appearance of phenomena like scale formation and corro-

sion, which are becoming more important at higher tempera-

tures, define an allowable maximum temperature for each

distillation process. An appropriate pretreatment of the salt

water is necessary to make an increase of the feed water tem-

perature possible.

The economics of the distillation process might be

affected by the following parameters:

• The chemical additives for feed water pre-
  treatment


• Scale formation which decreases performance


• Increase of maintenance costs due to corrosion

Corrosion may increase fixed changes, when more

expensive materials of construction must be used.

Multiple-Effect Distillation (ME)

Theoretically, in single-effect distillation 1 kg of distillate

will be produced for every kg of steam consumed and the

gain output ratio of the plant will be 1. In fact, despite pre-

heating of the feed, a large part of the enthalpy of the vapors,

FIGURE 4 The first historically known solar distillation equipment, according to Giovanni

Batista De La Porta. The sun evaporates the water inside the glass vessels and distilled water is

collected beneath the vessels. “De distillations,” Libri IX, Rome 1608.

C004_001_r03.indd 202C004_001_r03.indd 202 11/18/2005 10:18:52 AM11/18/2005 10:18:52 AM