A MED evaporator consists of several consecutives cells (or effects) maintained at decreasing levels of pressure and temperature from the first hot cell to the last cold one.
Each cell mainly consists in a horizontal tubes bundle. The top of the bundle is sprayed with sea water make-up that flows down from tube to tube by gravity.
Basics on the MED process
Heating steam is introduced inside the tubes. Since tubes are cooled externally by make-up flow, steam condenses into distillate (fresh water) inside the tubes. At the same time sea water warms up and partly evaporates by recovering the condensation heat (latent heat). Due to evaporation sea water slightly concentrates when flowing down the bundle and gives brine at the bottom of the cell. The vapour raised by sea water evaporation is at a lower temperature than heating steam.However it can still be used as heating media for the next effect where the process is repeated. The decreasing pressure from one cell to the next one allows brine and distillate to be drawn to the next cell where they will flash and release additional amounts of vapour at the lower pressure. This additional vapour will condense into distillate inside the next cell.
This process is repeated in a series of effects (Multiple Effect Distillation) (see sketch 2 with 3 effects). In the last cell, the produced steam condenses on a conventional shell and tubes heat exchanger. This exchanger, called "distillate condenser" is cooled by sea-water. At the outlet of this condenser, part of the warmed sea-water is used as make-up of the unit, the other part is rejected to the sea. Brine and distillate are collected from cell to cell till the last one from where they are extracted by centrifugal pumps. The thermal efficiency of such evaporator can be quantified as the number of kilos of distillate produced per one kilo of steam introduced in the system. Such number is called the Gain Output Ratio (GOR).
The GOR of the evaporator in Sketch 2 can be enhanced by addition of a thermocompressor between one of the cells and the hot one.Using LP or MP steam this static compressor will take part of the vapour raised in one of the cells and recycle it into higher pressure vapour to be used as heating media for the first one. Sketch 3 shows the typical arrangement of an MED evaporator with thermocompression (MED-TVC).Whereas the GOR of the evaporator in sketch 2 would be in the range of 3 (using LLP steam) the GOR of sketch 3 would rather each 6 (using LP or MP steam).
When no steam is available, it is still possible to use the MED process with a Mechanical Vapour Compressor (MED-MVC). In such case the vapour is recycled from the cold cell to the hot one by means of a centrifugal compressor driveby an electrical engine (Sketch 4).The electrical consumption of such system in the range of 8 to 15 kWh/m3. Due to current limitation in compressors technology the maximum capacity of MED-MVC units is 5000 m3/day.