Fractional distillation is a separation technique that relies on the differences in boiling points of components in a mixture. The theory behind it hinges on the principle that different substances have different vapor pressures at a given temperature.
Understanding Vapor Pressure
Vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase. Liquids with higher vapor pressures evaporate more readily, meaning they have a tendency to become gases at lower temperatures.
How Fractional Distillation Works
- Heating the Mixture: The mixture is heated, causing the components with lower boiling points to vaporize first.
- Vapor Movement: The vapors rise through a fractionating column, a tall, cylindrical structure packed with materials like glass beads or metal rings.
- Condensation: As the vapors ascend, they cool and condense on the packing material. The condensed liquid then drips back down the column, encountering more hot vapors.
- Re-Vaporization and Re-Condensation: This continuous process of vaporization and condensation leads to a gradual separation of the components. Components with lower boiling points travel higher in the column and are collected separately. Components with higher boiling points remain lower in the column and are collected separately.
- Collection: The separated components are collected as fractions, each representing a specific boiling point range.
Practical Applications
Fractional distillation is widely used in various industries:
- Petroleum Refining: Separating crude oil into different fractions like gasoline, kerosene, and diesel fuel.
- Chemical Industry: Separating mixtures of organic compounds.
- Liquor Production: Separating ethanol from fermented beverages.
- Air Separation: Separating air into its components like nitrogen and oxygen.
Examples
- Separating Ethanol and Water: When a mixture of ethanol and water is heated, ethanol vaporizes first due to its lower boiling point. The ethanol vapor rises, condenses, and is collected separately, leaving behind water in the original container.
- Separating Crude Oil: Crude oil is a complex mixture of hydrocarbons. Fractional distillation separates it into different fractions based on their boiling points. For example, gasoline is collected at a lower temperature than diesel fuel.
