The principle of fractional distillation is based on the establishment of a large number of theoretical vaporization-condensation cycles (theoretical plates): the apparatus of a simple distillation is modified by inserting a fractionating column between the distillation flask and the distillation head, Figure 4. The fractionating column provides a large surface area in which the initial distillate is redistilled and condensed again. This process continues as the vapors rise up the column until the vapors finally make it into the condenser. These vapors and the final distillate will contain a greater percentage of the lower boiling liquid. Continuous repetition of redistillation process in fractional distillation gives good separation of the volatile liquid components.
A simple distillation apparatus (Fig.3) is less efficient than a fractional distillation apparatus (Fig.4), but is used to purify materials containing only small amounts of impurities with much higher or lower boiling points