Distillation is a phase
separation method whereby saline water is heated to produce
water vapor, which is then condensed to produce freshwater.
The various distillation processes used to produce potable
water generally operate on the principle of reducing the vapor
pressure of water within the unit to permit boiling to occur
at lower temperatures, without the use of additional heat.
· Multistage Flash (MSF)
The incoming seawater passes through the heating stage(s)
and is heated further in the heat recovery sections of each
subsequent stage. After passing through the last heat recovery
section, and before entering the first stage where
flash-boiling (or flashing) occurs, the feedwater is further
heated in the brine heater using externally supplied steam.
This raises the feedwater to its highest temperature, after
which it is passed through the various stages where flashing
takes place. The vapor pressure in each of these stages is
controlled so that the heated brine enters each chamber at the
proper temperature and pressure (each lower than the preceding
stage) to cause instantaneous and violent boiling/evaporation.
The freshwater is formed by condensation of the water vapor,
which is collected at each stage and passed on from stage to
stage in parallel with the brine. At each stage, the product
water is also flash-boiled so that it can be cooled and the
surplus heat recovered for preheating the feedwater.
Because of the large amount of flashing brine required in
an MSF plant, a portion (50% to 75%) of the brine from the
last stage is often mixed with the incoming feedwater,
recirculated through the heat recovery sections of the brine
heater, and flashed again through all of the subsequent
stages. A facility of this type is often referred to as a
"brine recycle" plant. This mode of operation reduces the
amount of water-conditioning chemicals that must be added, and
can significantly affect operating costs. On the other hand,
it increases the salinity of the brine at the product end of
the plant, raises the boiling point, and increases the danger
of corrosion and scaling in the plant. In order to maintain a
proper brine density in the system, a portion of the
concentrated brine from the last stage is discharged to the
ocean. The discharge flow rate is controlled by the brine
concentration at the last stage.