Fractional Distillation


Distillation is employed to separate two or more liquids that are miscible. The boiling points of these liquids should be fairly far apart.


  1. The process of fractional distillation has been used to separate crude oil in petroleum industry into various useful fractions such as gasoline, kerosene oil, diesel oil, lubricating oil etc.
  2. Fractional distillation of liquid air is used to separate gases of the air. Air is a homogeneous mixture of a number of gases such as nitrogen, oxygen, inert gases (argon is the major component along with small amounts of helium, neon, krypton and xenon), carbon dioxide etc. These can be separated from air by fractional distillation. Air is first compressed by increasing the pressure and then cooled by decreasing the temperature. The remaining gases of the air get condensed to form liquid air. The liquid air is then subjected to fractional distillation. The gas with lower boiling point gets distilled first.
  3. Fractional distillation has been used to separate a mixture of acetone (b.p. 329k) and methyl alcohol (b.p. 338k).

Limitations: The components of constant boiling mixture called azeotropes cannot be separated by fractional distillation. For example, rectified spirit consists of 95% alcohol (b.p. 780C or 351k) and 5% water (b.p. 1000C or 373k). Its components cannot be separated by fractional distillation because they form a constant boiling mixture (azeotrope) even though their boiling points differ by 22. C.

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