MULTI-STAGE FLASH DISTILLATION (MSF) is a water desalination process
that distills sea water by flashing a portion of the water into steam
in multiple stages of what are essentially countercurrent heat
Multi-stage flash distillation
* 1 Principle * 2 See also * 3 References * 4 External links
Schematic of a 'once-through' multi-stage flash desalinator
A - Steam in
B - Seawater in
C - Potable water out
D - Waste out
E - Steam out
F - Heat exchange
The plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector. The sequence has a cold end and a hot end while intermediate stages have intermediate temperatures. The stages have different pressures corresponding to the boiling points of water at the stage temperatures. After the hot end there is a container called the brine heater.
When the plant is operating in steady state , feed water at the cold inlet temperature flows, or is pumped, through the heat exchangers in the stages and warms up. When it reaches the brine heater it already has nearly the maximum temperature. In the heater, an amount of additional heat is added. After the heater, the water flows through valves back into the stages that have ever lower pressure and temperature. As it flows back through the stages the water is now called brine, to distinguish it from the inlet water. In each stage, as the brine enters, its temperature is above the boiling point at the pressure of the stage, and a small fraction of the brine water boils ("flashes") to steam thereby reducing the temperature until an equilibrium is reached. The resulting steam is a little hotter than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger tubes, thereby heating the feed water as described earlier.
The total evaporation in all the stages is up to approximately 15% of the water flowing through the system, depending on the range of temperatures used. With increasing temperature there are growing difficulties of scale formation and corrosion. 120 °C appears to be a maximum, although scale avoidance may require temperatures below 70 °C.
The feed water carries away the latent heat of the condensed steam, maintaining the low temperature of the stage. The pressure in the chamber remains constant as equal amounts of steam is formed when new warm brine enters the stage and steam is removed as it condenses on the tubes of the heat exchanger. The equilibrium is stable, because if at some point more vapor forms, the pressure increases and that reduces evaporation and increases condensation.
In the final stage the brine and the condensate has a temperature near the inlet temperature. Then the brine and condensate are pumped out from the low pressure in the stage to the ambient pressure. The brine and condensate still carry a small amount of heat that is lost from the system when they are discharged. The heat that was added in the heater makes up for this loss.
The heat added in the brine heater usually comes in the form of hot steam from an industrial process co-located with the desalination plant. The steam is allowed to condense against tubes carrying the brine (similar to the stages).
The energy that makes possible the evaporation is all present in the brine as it le