Sea Water Treatment by MED-Desalination

Multiple Effect Desalination (MED)

In MED-Desalination units steam is condensed on one side of a tube wall(inside the tubes) while saline water is evaporated on the other side (outside the tubes) the energy used for evaporation is the heat of condensation of the steam. Usually there is a series of condensation-evaporation processes taking place (each called an "effect"). The saline water is usually applied to the tubes in the form of a thin film so that it will evaporate easily.


MED-Distillation Future

MED-desalination is no longer considered as only suitable for small and medium water production plants. With advanced technologies in Scale control and Material selection, MED isnow playing a major role in larger seawater desalination plants.MED is inherently superior in economy, low specific electricity consumption, and embraces the possibilities of using lower grade materials to better effect.

MED-Desalination Technology

An MED evaporator consists of several consecutive cells (effects) maintained at decreasing levels of pressure (and temperature) from thefirst (hot) cell to the last one (cold).

Each cell mainly consists ina tubes bundle. The top of the bundle is sprayed with the sea water make-up that then flows down from tube to tube by gravity.

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 effect (MED) (see sketch with 4 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 introducedin the system. Such number is called the Gain Output Ratio (GOR).

1. Very low electrical consumption (less than 1.0 kWh/m3)
2. Operate at low temperature (< 70°C) and at low concentration (< 1.5)
3. Produce steadily high purity distillate
4. Do not need complex pre-treatment of sea water and are tolerant to variations of sea water conditions
5. Are highly reliable and simple to operate
6. Reduce civil works cost
7. Are simple to install
8. Have a low maintenance cost
9. Operate 24 hours a day with minimum supervision
10. Ideal for coupling with power plants
11. Can be adapted to any heat source
12. Allow very high thermal efficiencies and savings in fuel costs
13. Range up to 15 MIGD (68 000 m3/day) per unit