Azeotropic Distillation
In chemistry, azeotropic distillation is separation of two compounds (typically) with the addition of a third compound that causes the formation of a heterogeneous (two liquid phase) azeotrope with one of the two compounds to be separated. An example of this is the addtion of benzene (now no longer used) to separate ethanol and water.
In the above case, benzene is highly immiscible with water, and less so with ethanol. The benzene therefore causes the water to be more volatile than it would be in ethanol. This azeotrope is then the lightest boiler in the distillation column and is taken overhead with the water. The ethanol is the product out of the bottom of the distillation column. Note that the above discussion assumed continuous distillation.
