Multiple-Effect Distillation (also Multi Effect Distillation or MED) is a thermal distillation technology based on evaporation and condensation processes in multiple stages, called effects, that is mainly used for desalination. In most MED plants, the seawater enters all the effects in parallel and is raised to the boiling point after being preheated on tubes. The seawater is either sprayed or otherwise distributed onto the surface of the evaporator tubes in a thin film to further rapid boiling and evaporation. The tubes are heated by steam from a boiler or some other source, which is condensed on the opposite side of the tubes (inside). The condensate from the boiler steam is recycled to the boiler for reuse. Only a portion of the seawater applied to the tubes in the effects evaporates. The remaining feed water is collected and fed to the last effect, from where it is removed by a brine pump. The tubes in the various effects are heated in turn by the vapours arose from the previous effect. This vapour is condensed to a fresh water product, while giving off heat to evaporate a portion of the seawater feed in the effects. This continues for several effects, with 4 or 16 effects being found in a typical large plant. The remaining seawater of each effect flows to the next effect through pipes by gravity. Generally, these plants are powered by low temperature heat leading to Top Brine Temperatures (TBT) of 55-70°C and are combined with mechanical vapour compressors or thermal vapour compressors. The number of effects directly correlating with the Performance Ratio (PR). In contrast to this, the PR is not significantly influenced by TBT. Depending on the TBT, pre-treatment of the feed and usage of anti-scalants is necessary. Furthermore, corrosion problems limit the usage of cheap construction materials.
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