Fractional Distillation
Definition and meaning of Fractional Distillation in chemistry.
Fractional distillation is a technique for separating a mixture of miscible liquids into its components by exploiting differences in their boiling points, using repeated vaporization-condensation cycles within a fractionating column.
In more detail
As vapor rises through the column, it repeatedly condenses on packing or trays and re-vaporizes, with each cycle enriching the vapor in the more volatile (lower-boiling) component. This effectively packs many simple distillations into one apparatus, so it can resolve mixtures whose boiling points are too close together (typically within about 25°C) for simple distillation to separate cleanly. The technique is central to industries that must isolate multiple components from a complex liquid mixture. A key limitation is that azeotropes, mixtures that boil at a fixed composition, cannot be fully separated this way.
Key facts
| Field | General Chemistry |
|---|---|
| Separation basis | Differences in boiling point / vapor pressure |
| Apparatus | Fractionating column packed with plates or beads |
| Major industrial use | Crude oil refining, ethanol purification |
Petroleum refineries use tall fractionating columns to separate crude oil into fractions such as gasoline, kerosene, and diesel, which condense at different heights in the column according to their boiling ranges.
Frequently asked questions
How is fractional distillation different from simple distillation?
Simple distillation involves a single vaporization-condensation step and works well only when boiling points differ by roughly 25°C or more. Fractional distillation adds a column that provides many successive vaporization-condensation cycles, allowing separation of liquids with much closer boiling points.
Can fractional distillation separate an azeotrope?
No. An azeotrope is a mixture whose vapor has the same composition as the liquid at a given pressure, so distillation alone cannot separate it further; other methods like azeotropic or extractive distillation are needed.