▶Thermal technologies, as the name implies, involve the heating of saline water and collecting the condensed vapor (distillate) to
produce pure water.? Thermal technologies have rarely been used for brackish water desalination, because of the high costs
involved. They have however been used for seawater desalination and can be sub-divided into three groups:? Multi-Stage Flash
Distillation (MSF), Multi-Effect Distillation(MED), and Vapor Compression Distillation (VCD).
♦ Multi-Stage Flash Distillation (MSF)
• This process involves the use of distillation through several (multi-stage) chambers.? In the MSF process, each successive
stage of the plant operates at progressively lower pressures.?The feed water is first heated under high pressure, and is led
into the first ‘flash chamber’, where the pressure is released, causing the water to boil rapidly resulting in sudden evaporation
This ‘flashing’ of a portion of the feed continues in each successive stage, because the pressure at each stage is lower than in
the previous stage. The vapor generated by the flashing is converted into fresh water by being condensed on heat exchanger
tubing that run through each stage. The tubes are cooled by the incoming cooler feed water. Generally, only a small percentage
of the feed water is converted into vapor and condensed.
• Multi-stage flash distillation plants have been built since the late 1950s. Some MSF plants can contain from 15 to 25 stages,
but are usually no larger than 15 mgd in capacity. MSF distillation plants can have either a ‘once-through’ or ‘recycled’
process. In the ‘once-through’ design, the feed water is passed through the heater and flash chambers just once and disposed
of, while in the recycled design, the feed water for cooling is recycled. Each of these processes can be structured as a ‘long
tube’ or ‘cross tube’ design. In the long tube design (built at Freeport in 1961), tubing is parallel to the concentrate flow, while
in the cross
tube design, tubing is perpendicular to the concentrate flow.
• MSF plants are subject to corrosion unless stainless steel is used extensively. In addition to corrosion, MSF plants are also
subject to erosion and impingement attack (U.S. Bureau of Reclamation, 2003). Erosion is caused by the turbulence of the feed
water in the flash chamber, when the feed water passes from one stage to another.
• Distillation processes produce about 3.4 billion gpd globally, which is about 50 percent of the worldwide desalination capacity.
MSF plants provide about 84 percent of that capacity.Most of those plants have been built overseas, primarily in the Middle East,
where energy resources have been plentiful and inexpensive.
♦ Multi-Effect Distillation (MED)
• The MED process has been used since the late 1950s and early 1960s. Multi-effect distillation occurs in a series of vessels
(effects) and uses the principles of evaporation and condensation at reduced ambient pressure. In MED, a series of evaporator
effects produce water at progressively lower pressures.? Water boils at lower temperatures as pressure decreases, so the water
vapor of the first vessel or effect serves as the heating medium for the second, and so on. The more vessels or effects there are,
the higher the performance ratio. Depending upon the arrangement of the heat exchanger tubing, MED units could be classified
as horizontal tube, vertical tube or vertically stacked tube bundles
♦ Vapor Compression Distillation
• The vapor compression distillation (VCD) process is used either in combination with other processes such as the MED, or by
itself. The heat for evaporating the water comes from the compression of vapor, rather than the direct exchange of heat from
steam produced in a boiler (Buros, 2000). Vapor compression (VC) units have been built in a variety of configurations.Usually,
a mechanical compressor is used to generate the heat for evaporation.The VC units are generally small in capacity, and are
often used at hotels, resorts and in industrial applications.