A simple distillation usually gives a crude distillation where the initial distillate has to be redistilled to reach optimum purity. The separation and purification of two or more liquids into various fractions by distillation is called fractional distillation. Fractional distillation is really the systematic redistillation of the distillates within a column with the fractions increasing in purity as the fractions move up the column. This is similar to repeatedly collecting and redistilling the various fractions. The column is filled with pieces of glass, glass beads, chips, metal wire, etc. These beads act as small condensers.
The vapour leaves the surface of the liquid and travels upwards through the packed column. It condenses on the cooler beads and falls back as a liquid. The initial vapours transfer heat to the lower beads. The recondensed vapour then vaporizes again but this time move a little higher in the column and the process repeats. The less volatile vapours reach the beads which were heated by the more volatile vapour. Since the temperature of the beads are less than the boiling point of the less volatile vapour, they also recondense and fall back. But this now heats the lower beads and the process continues. The more volatile vapours pass through the column condensing and recondensing. But as the higher boiling vapours heat the lower beads, these beads revaporize the higher volatile vapours within the column. Thus the liquids do not return to the distillation flask as they move upward.
Each repeated distillation causes a greater concentration of the more volatile liquid in the rising vapour. The less volatile liquids also are in turn enriched as they pass upward through the column.