Flash Distillation.



  • When a hot mixture is allowed to enter from a high-pressure zone into a low pressure zone, the entire liquid mixture is suddenly vaporized. This process is known as flash vaporization

  • During this process, the chamber is cooled. 

  • The less volatile fraction is condensed and the more volatile component remains in the vapor phase

  • This process requires time, hence liquid and vapor are kept in intimate contact until equilibrium is achieved.

  • Flash distillation is also called equilibrium distillation because separation of two liquids takes place when liquid and vapor phases are at equilibrium.

Equipment used for Flash Distillation:


  • It consists of a pump, which is connected to a feed reservoir. 

  • Pumps help in pumping the feed into the heating chamber. 

  • The heating chamber is heat supplied by steam. 

  • The other end of the pipe is directly introduced into the vapor-liquid separator through a reducing valve. 

  • The vapor outlet is provided at the top of the separator and liquid outlet is provided at the bottom.


  • The feed is pumped through a heater at a certain pressure. 

  • The temperature of the liquid is raised in the heating chamber but the liquid does not boil under high pressure (because boiling point increases). 

  • When the liquid enters into the vapor-liquid separator, due to drop in pressure, the liquid reaches the boiling point under that reduced pressure and the liquid suddenly boils. 

  • The vapor flashes out from the hot liquid. 

  • Since the vapor takes the latent heat, the liquid gets cooled down. 

  • The less volatile component of the vapor is condensed and the more volatile component remains in the vapor phase.  

  • The mixture is allowed for sufficient time so that vapor and liquid come to equilibrium. 

  • The vapor is separated through an outlet provided at the top and the liquid is collected at the bottom.

Commonly Asked Questions.

  1. Define distillation and write in detail about the flash distillation.

  2. With help of a well labelled diagram describe the principle and working of the apparatus used for flash distillation.

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