problem, corrosion of flash-evaporation units is of
General. Distillation/condensation is the most
common desalination process. More than 70 percent of
all desalination facilities in use today employ some
variation of the distillation/condensation process.
condensation facilities that operate at temperatures less
than 205 degrees Fahrenheit are low-temperature units.
temperature distillation faciilties that operate at
In situations where waste heat is plentiful, low-
temperatures greater than 205 degrees Fahrenheit are
the most prevalent desalination facilities in the world
used. A waste-heat-recovery unit is shown in figure 6-4.
There are three methods of vaporizaiton:
For onshore application, low-pressure waste steam from
submerged tube vaporization; flash vaporization; and
power generation facilities can provide the necessary
thin-film vaporization. These methods are illustrated in
thermal energy for desalination systems. The most
figure 6-1. Submerged tube vaporization is the least
efficient vaporization technique, but it allows for easy
technology involve the use of waste heat or lowpressure
maintenance. This type of vaporization system is most
often used in exhaust gas waste heat recovery
compression system. Multiple stages then derive the
distillation systems. The flash vaporization technique is
maximum vapor and product water production from the
presently the most common technique in existing
distillation units. The impact of sprayed hot brine within
the evaporator unit causes both erosion and corrosion of
Using a thin-film sprayvaporization
The use of
process, the raw water is introduced at slightly less than
mechanical methods for vapor production and heat
atmospheric pressure through an orifice onto heat
transfer can result in a highly efficient desalination
exchanger tubes for immediate vaporization.
system. These systems operate at temperatures less
corrosive environment is reduced from the flash
than atmospheric boiling point and use a variety of
vaporization system, but scaling can occur on the heat
methods to vaporize raw waters. These mechanical
transfer surfaces. These vaporization techniques are
processes commonly use multiple effects to maximize
used in the two major high-temperature distillation
the efficiency of the applied mechanical energy.
processes, multiple-effect (ME) evaporation, and
a. Vapor compression. The technique of vapor
multistage flash (MSF) evaporation.
compression uses a mechanical energy source, such as
a. Multiple-effect evaporation units. To maximize
an engine of electric motor, to power a compression
turbine. This turbine draws vapor from the distillation
distillation/condensation system, several units or effects
vessel and compresses it, which raises the temperature
of the exhaust vapor. The vapor is then passed over a
effect is used to supply vaporization heat for the
heat exchanging condenser, where it returns to the
following effect. The next effect is a slightly lowered
liquid state as product water. The heat removed during
pressure and temperature. This gradual reduction by
heat transfer results in a much greater yield of product
production of more vapor. The more recent vapor-
water from a given quantity of thermal energy. A typical
multiple-effect evaporation unit is shown in figure 6-2.
concentrated brine byproduct that has had its excess
b. Multistage flash-evaporation units. Distillation
heat reduced by the multiple effects.
technology was advanced through the development of
Adding waste heat to vapor
b. Waste heat.
multistage flash evaporation units. Stages of flash
compression systems results in a highly efficient
Pressure is reduced gradually in each
successive stage to continue flash operation at
successively lower temperatures and pressures.
Because scaling is not a serious problem, this design
has become the most prevalent distillation process. A
typical multistage flash-evaporation unit is shown in
figure 6-3. Although internal scaling is not a great