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                    The most used process in the world to desalinate sea water for the production of fresh water are distillation (thermal energy) and reverse osmosis (electrical energy).

                   In the distillation process, also called evaporative, heat is used to boil sea water at low temperatures, under vacuum conditions.

                  In the osmosis process, sea water at high pressure is forced to pass through a set of membranes, where only water molecules pass in their pores, retaining the salts.

                 The distillation process has three most famous methods: MED (Multi Effect Distillation), MSF (Multi Stage Flash) and MVC (Mechanical Vapor Compression).

                     In MED, the water vapors generated in one stage, are the heat source of the next stage. In this way, it is only necessary to supply thermal energy for the first stage. Therefore, the greater the number of stages, the greater the production and the lower the specific energy expended per gallon of water produced. In the other hand, the higher the capital costs for the construction because the smaller temperature difference between stages, and the larger the size of the evaporators, making the project increasingly capital-intensive.


                      On the MSF method, sea water is heated above 100ºC and directed to the first stage, that releases vapor due to the vacuum environment and then, flows to the second stage with a higher vacuum pressure and releases more vapor, and so on until it reaches the condenser after the last stage. In each of these stages, the vapor released is cooled in the condenser with the seawater that will feed the machine. Thus, at each stage the sea water is heated, and at the same time condensing the steam generated, producing more distilled fresh water.


                    The method of mechanical vapor compression is very interesting and also widely used in desalination plants. The steam generated in the evaporator is compressed and becomes the heat source that evaporates the sea water in a closed and continuous cycle. The energy expended is normally electrical power for the compressor driving motor. This method starts with some heat source, until the beginning of the boil, when the compressor is activated.


              Reverse osmosis process can operate with up to three stages, and requires high pressures to overcame the seawater osmosis pressure. Therefore, it is intensive in electrical power, whereas in distillation, the large consumption is thermal energy. Energy recovery devices can reduce significantly the power energy consumption.



              The decision of which method to use, as well as the number of stages, depends on a technical economic study, which takes into account the type and costs of the available energies (electrical or thermal), and the volume of fresh water required.

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