
Learn more about Multi-effect distillation
There is a different desalination technology. Distillation was the original method, employed thousands of years ago, and remained the most frequent until the 1990s.
The method entails evaporating water using heat or vacuum processes, leaving solids behind and then condensing the desalted vapor into liquid form.
Such procedures include multi-stage flash, multiple-effect distillation, vapor distillation, and vapor compression.
What is multi-effect distillation?
MED is a desalination technique that uses heat and energy to create drinkable water.
Multistage-flash evaporation, often known as multiple-effect distillation, is a variation of simple distillation.
The conversion of a liquid to vapor in this process, which is generally utilized by huge commercial desalting plants, does not need heating.
The liquid is simply transferred from a high-pressure container to a lower-pressure container.
The liquid vaporizes quickly due to the lower pressure, and the resultant vapor is subsequently condensed into the distillate.
One of the most prevalent MED procedures is steam condensation inside horizontal tubes and saltwater evaporation on the outside. A tube bundle is allowed to sink with seawater.
Due to the low resistance of the thin falling layers, heat transmission on both sides of the heat transfer region is regarded as very effective, allowing efficient operation with a minimum temperature differential across the tube walls.
It’s also restricted because high fluxes cause the film to boil, forming bubbles and producing dry areas that can lead to salt precipitation. Of course, this should be avoided.
Because of the minimal temperature differential between the heat transfer surfaces, a vast number of effects may be created between the temperature of the steam at the first stage and the temperature of the cooling saltwater on the other side.
More stages improve the performance ratio, or GOR – Gain Output Ratio, which is the amount of water generated per ton of starting steam while lowering the process’s energy consumption.
The GOR in MED can reach 15 depending on the starting steam temperature.
Components of multi-effect desalination
From the first (hot) cell to the last (cold), a multi-effect desalination (MED) evaporator consists of many sequential cells (effects) maintained at decreasing pressure (and temperature) (cold).

Each cell is mostly made up of a tube bundle.
The saltwater makeup is sprayed on the top of the bundle and then flows down by gravity from tube to tube.
Inside the tubes, hot steam is injected.
Because make-up flow cools the tubes from the outside, the steam condenses inside the tubes and becomes distillate (freshwater).
Two types of multi-effect distillation
Multiple effect distillation with thermal vapor compression (MED-TVC)
Due to its high heat transfer coefficient attained by thin film evaporation, Multi-Effect Desalination with Thermo-Vapor Compression (MED –TVC), a new-generation technology, is flexible and small.
It also has a modest scaling potential and operates at low temperatures.
Furthermore, TVC is employed in the MED plant to compress the vapor from the final effect, which is then recycled as heating media in the first effect, improving the steam economy and lowering the cooling saltwater demand.
The MED-TVC saltwater desalination facility uses seawater to generate distilled water.
Six effects are contained in a horizontal cylindrical tank in this evaporative procedure.
Recirculated saltwater is sprayed in six different patterns across the horizontal tubes of the MED evaporators, resulting in the evaporation of a percentage of the sprayed seawater’s water content.
In the following effect, the vapor formed is desisted and used for heating. In the evaporator, thin layer of evaporation of sprayed saltwater happens on the outside of horizontal tubes, while condensation of vapor occurs on the inside, resulting in a high heat transfer coefficient.
Each effect sends concentrated brine and condensed vapor (product distilled water) to the following effect, which is kept at a slightly lower pressure.
Multiple effect distillation with mechanical vapor compression (MED-MVC)
When steam is not available, the MED method can still be used by adding a Mechanical Vapor compressor (MED-MVC).
A centrifugal compressor powered by an electric engine recycles the vapor from the cold cell to the hot one in this type. With today’s compressor technology, a maximum capacity of 5,000 m3/day may be achieved.
Read more about desalination
Principles of operation
This sort of system has several chambers. The pipes carry hot steam into the chamber, while the seawater is repelled by the pipes.
The heated steam pipes deliver heat to the seawater, causing the steam to condense and the seawater to evaporate. Despite the large temperature differential, it is insufficient to evaporate all the water.
As a result, any residual water is transported to the next step.
Meanwhile, condensation causes the hot steam to lose some of its temperature and pressure.
The generated heated steam from step 1 is brought into contact with the residual water in the second stage.
The seawater can evaporate in each step while the steam condenses due to the changing pressures in the chambers.
This cycle is repeated several times, and the residual vapor is utilized to warm the supply water after the operation.
The generated steam condenses on a traditional shell-and-tube heat exchanger in the final cell.
The ‘distillate condenser,’ as it is known, is cooled by salt water.
Part of the warmed saltwater is utilized to make up the unit at the condenser’s output, while the rest is discarded to the sea.
Brine and distillate are gathered from cell to cell until they reach the last one, where centrifugal pumps remove them.
Advantages and disadvantages of multi-effect distillation
Major Advantages of Multi-effect distillation systems:
• Ability to make the most of available energy (steam economy)
• Using less energy to run the entire system.
• Condensing load reduction with added effects (less cooling water needed)
• Control of liquor flow.
• Convenient collection and handling of foul condensate.
• It reduces the boiling point of the second effect by lowering the vapor pressure of the second effect relative to the first effect.
• It can operate at a minimum temperature
• Water that extracts has high quality
While the Major disadvantages of multi-effect distillation systems
• High headroom is required.
• A falling film evaporator has a larger pressure drop across the tubes.
• The hydrostatic head at the bottom of the tubes may raise the temperature of the product, causing temperature sensitivity troubles.
• Expensive to construct.
• It requires high pumping power.
Reference
[1] Multiple Effect Distillation, 2018, [online] Available at: https://www.sciencedirect.com/topics/engineering/multiple-effect-distillation
[2] MED – Multi-Effect Desalination, [online] Available at: https://www.alfalaval.nl/nl/producten/proces-oplossingen/fresh-water-solutions/multi-effect-plate-desalination/med-multi-effect-desalination/
[3] Multiple Effect Distillation – MED MVC, [online] Available at: https://www.veoliawatercom/en/solutions/technologies/multiple-effect-distillation-med-mvc
[4] multi-effect distillation (MED), [online] Available at: https://www.eolss.net/sample-chapters/c07/e6-144-44-00.pdf
[5] What are the advantages of using a multi-effect evaporator over a single-effect evaporator? , [online] Available at: https://first-law-comic.com/what-are-the-advantages-of-using-multi-effect-evaporator-over-single-effect-evaporator/