Multi-Stage Flash

Introduction to Multi-Stage Flash

Multi-stage flash (MSF) is a thermal desalination process.  It distills water by boiling it under vacuum (also known as flashing), then condensing the pure water vapour.  It uses multiple shell and tube heat exchangers called stages.

MSF was the market leader in terms of sea water desalination for many years.  Around 2009 reverse osmosis overtook MSF as the most popular in terms of installed sea water desalination capacity.

How MSF works

  1. Heated sea water is run through the bottom of the shell side (outside) of each stage.
  2. A vacuum is applied to the shell side, causing the sea water to flash.
  3. Flashing causes the upper shell side to fill with pure water vapour, leaving more salty liquid brine at the bottom.
  4. Relatively cold seawater is passed through the tube side of each stage.
  5. The cooling of the tubes causes the water vapour to condense into distilled water.
  6. The distilled water is then caught in drip trays where it is withdrawn from the stage.
  7. The brine from one stage is passed onto the next, which is at a lower pressure, hence it flashes again and the process is repeated.
  8. The seawater initially enters the tube side of the final stage, which is the coolest and lowest pressure stage.  It is then passed thought the tubes of each increasingly hot stage until it reaches the end, relatively warm.
  9. The warmed seawater then goes into another heat exchanger where it is contacted with steam (normally waste steam from a power plant) and raised to around 90-120 °C.
  10. The hot seawater then enters the bottom of the first stage (see step 1)
  11. The process is normally split, into a heat gain and heat rejection sections.  The seawater is pumped from the tube side of the heat rejection system to the tube side of the heat gain section.  This allows greater flows of seawater to pass through the heat rejection section when needed.
  12. At the same point, in MSF with brine recycling, some of the final stage brine is added to the seawater.  This allows anti-scalant (which is only required in the hottest stages) to be recycled.