Multiple Effect Distillation

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MED or Multiple Effect Distillation are used for various industrial applications for treating water. The whole process is carried out in vessels that are arranged in series. In Multiple Effect Distillation (MED), steam that circulates in the submerged tube is used for heating the water. To promote rapid boiling and evaporation the sea water is either sprayed or otherwise distributed onto the surface of evaporator tubes in a thin film. The steam from the boiler is used to heat the tubes and is condensed on the opposite sides of the tubes. The condensed water is again recycled to be used for generating steam.

Water is produced at lower pressure in MED, using a series of evaporator. MED occurs in series of vessels and the water vapor of the first vessel or effect serves as the heating medium for the second, and so on. Performance ratio is directly proportional to the number of vessels. If more vessels or effects are used the performance ratio will rise.

Only that portion of the seawater is evaporated that is applied to the tubes in the first effect. For the second effect remaining fed water is used which is again applied to a tube bundle. The steam produced in the first effect is used for heating these tube bundles. The vapor or steam produced by the effect is condensed to obtain fresh water output whereas the heat generated in this effect is used to evaporate a portion of the remaining sea water feed in the next effect.

There is difference between the boiling temperature and the pressure in each evaporator. In order to maintain the difference between the boiling points of pure and salt water the pressure in the vapor space of the first evaporator is reduced. Moreover the temperature of the sea water must be several degrees lower than that of the condensing steam to maintain reasonable heat exchange between the pipes containing the condensing steam and the boiling sea water.

Water should be delivered by the pump at atmospheric pressure because system pressure is less than atmospheric pressure. They also help in exhausting the steam space of the evaporators. Air and other non condensing gas is also removed by employing steam ejector which would accumulate and eventually stop the boiling process.