Working Principle of Multi Effect Desalination Packages

Multi Effect Desalination (MED) is an advanced type of thermo-compression desalination, known as TCD, which is used to reduce total dissolved solids (TDS) of water. Desalinated water has low conductivity which makes it suitable to be used as boiler feed water. MEDs produce extremely pure water by means of multiple distillations. MED packages can be found at power generation and utility units where there is a need for vast amount of desalinated water. These packages can decrease water conductivity from 50,000 \mu S/cm (seawater) to about 10 \mu S/cm. MEDs consist of multi stages in series. Low-pressure steam (LPS) is used as heat source for initial vaporization. Inlet seawater is used as heat sink in final condenser. Main advantage of MEDs is that they can provide their own heat of vaporization by decreasing the pressure of shell-side for each consecutive stage (except from the first stage which receives the heat from external LPS).

After pre-heating in condenser’s tubes, inlet seawater is split into multiple parts and sprayed on the exchanger’s tubes of all stages. Each stage has a lower shell-side pressure than the previous stage.

As for a two stages MED, LPS enters first stage’s tubes and condenses at exit. The sprayed seawater on first stage evaporates to some extent. The vaporized portion from the first stage is directed into the tubes of second stage. Because the sprayed seawater on second stage’s tubes is in lower pressure, it evaporates, removing heat from tube-side steam. Thus makes it condense. The vaporized portion from second stage is ejected to condenser’s shell and condenses.

The hogging ejectors provide partially vacuum condition in condenser. Therefore, non-condensable components are released from condenser area. Moreover, the vacuum condition, directs the steam into the condenser. There is also an ejector, which compresses the low pressure steam of the last stage by means of LPS (as motive). Thus, providing a negative pressure for the steam to circulate between the stages. At the end, all the unevaporated portion of seawater in each stage’s sump are collected and rejected to sea as brine. On the other hand, all the condensed steam from each stage plus condenser are mixed to form the final desalinated product.

An antiscalant chemical is injected into seawater prior to vaporization. Usually, MED packages have their own low pressure (about 5 barg) boilers to supply the required LPS. These boilers are fed with a portion of desalinated water produced by the package itself.

Since both condenser and first stage exchanger are in contact with seawater with high amount of chloride ions, stainless steel cannot be used as material of construction. As a result, these exchangers are normally made of titanium.

Flow diagram of multi effect desalination package
(click for full scale image)

Multi effect desalination packages are used to produce pure water with low amount of minerals. They are available in all steam generation units such as utilities in oil and gas industry as well as energy section. Their working principle is based on distillation. MEDs have lower energy consumption than regular TCDs due to self-heating vaporization. They are capable of operating at low temperature (< 70°C) to reduce fouling problem.

As boilers require extremely pure water with minimum TDS, it is recommended for boiler feed water to have a maximum conductivity of 1 \mu S/cm. Therefore, there shall be a further purification section downstream of MED packages. This further purification is done via ion-exchange mixed bed prior to deaerators.

Reverse osmosis (RO) is another method in order to desalinate water. It offers easier operation with lower CAPEX. Although reverse osmosis packages have gained huge acceptance among engineers, MEDs are still popular and manufactured in various plants.