Thermal distillation technologies are widely used in the Middle East, primarily because the region’s petroleum reserves keep energy costs low. The three major, large-scale thermal processes are multistage flash distillation, multi-effect distillation, and vapor compression distillation. Another thermal method, solar distillation, is typically used for very small production rates.

• Multi-stage flash distillation is a process that sends the saline feed water through multiple chambers. In these chambers, the water is heated and compressed to a high temperature and high pressure. As the water progressively passes through the chambers, the pressure is reduced, causing the water to rapidly boil. The vapor, which is fresh water, is produced in each chamber from boiling and then is condensed and collected.
• Multi-effect distillation (MED) employs the same principals as the multi-stage flash distillation process except that instead of using multiple chambers of a single vessel, MED uses successive vessels. The water vapor that is formed when the water boils is condensed and collected. The multiple vessels make the MED process more efficient.
• Vapor compression distillation (VCD) can function independently or be used in combination with another thermal distillation process. VCD uses heat from the compression of vapor to evaporate the feed water. VCD units are commonly used to produce fresh water for small- to medium-scale purposes such as resorts, industries, and petroleum drilling sites.
• Solar desalination is generally used for smallscale operations. Although the designs of solar distillation units vary greatly, the basic principals are the same. The sun provides the energy to evaporate the saline water. The water vapor formed from the evaporation process then condenses on the clear glass or plastic covering and is collected as fresh water in the condensate trough. The covering is used to both transmit radiant energy and allow water vapor to condense on its interior surface. The salt and un-evaporated water left behind in the still basin forms the brine solution that must be discarded appropriately. This practice is often used in arid regions where safe fresh water is not available.

(Mechell & Lesikar 2010)