Fractional Distillation
Definition and meaning of Fractional Distillation in chemistry.
Fractional distillation is a technique that separates a mixture of miscible liquids into individual components using differences in their boiling points. A fractionating column provides repeated cycles of vaporization and condensation to sharpen the separation.
In more detail
As vapor rises up through the fractionating column, it repeatedly condenses on packing material or trays inside the column, then re-vaporizes as more heat arrives from below. Each condensation and re-vaporization cycle enriches the rising vapor a little more in the lower-boiling, more volatile component.
This effectively packs many simple distillations into one tall apparatus, so fractional distillation can resolve mixtures whose boiling points sit too close together for a single simple distillation to separate cleanly, typically anything closer than about 25 degrees Celsius apart. Whole industries depend on this technique to isolate multiple useful components from one complex liquid mixture.
One important limitation exists. Azeotropes are mixtures that boil at a fixed, unchanging composition, so their vapor has the same makeup as the liquid. Fractional distillation alone cannot separate an azeotrope any further, no matter how many plates the column contains.
Chemistry students often use this same technique at a much smaller scale in the teaching lab, separating mixtures like ethanol and water using a compact fractionating column attached to a round-bottom flask.
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 |
| Limitation | Cannot fully separate an azeotrope |
Petroleum refineries pump crude oil into tall fractionating columns and heat it until it vaporizes. As the vapor rises, different fractions such as gasoline, kerosene, and diesel condense at different heights in the column, since each fraction has its own characteristic boiling range. Lighter fractions with lower boiling points rise highest before condensing near the cooler top of the tall column, while heavier fractions condense closer to the hotter bottom section.
Frequently asked questions
How is fractional distillation different from simple distillation?
Simple distillation uses a single vaporization-condensation step and works well only when boiling points differ by roughly 25 degrees Celsius or more. Fractional distillation adds a column that provides many repeated cycles, allowing separation of 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.
Why does crude oil separate into different products in a refinery column?
Crude oil is a mixture of hydrocarbons with a wide range of boiling points. Lighter fractions like gasoline rise higher in the column before condensing, while heavier fractions like diesel condense lower down, closer to the heat source.