Fractional Distillation Definition and Examples

What You Need to Know About Fractional Distillation

Illustration of fractional distillation, the separation of mixture into component parts, or fractions by heating to temperature at which several fractions of compound evaporate
Dorling Kindersley / Getty Images

Fractional Distillation Definition

Fractional distillation is a process by which components in a chemical mixture are separated into different parts (called fractions) according to their different boiling points. Fractional distillation is used to purify chemicals and also to separate mixtures to obtain their components.

It's used as a lab technique and in industry, where the process has vast commercial significance.

The chemical and petroleum industry rely on fractional distillation. 

How Fractional Distillation Works

Vapors from a boiling solution are passed along a tall column, called a fractionating column. The column is packed with plastic or glass beads to improve the separation by providing more surface area for condensation and evaporation. The temperature of the column gradually decreases along its length. Components with a higher boiling point condense on the column and return to the solution; components with a lower boiling point (more volatile) pass through the column and are collected near the top. Theoretically, having more beads or plates improves the separation, but adding plates also increases the time and energy required to complete a distillation.

Fractional Distillation of Crude Oil

Gasoline and many other chemicals are produced from crude oil using fractional distillation. Crude oil is heated until it evaporates.

Different fractions condense at certain temperature ranges. The chemicals in a certain fraction are hydrocarbons with comparable numbers of carbon atoms. From hot to cold (largest hydrocarbons to smallest), the fractions may be residue (used to make bitumen), fuel oil, diesel, kerosene, naphtha, gasoline, and refinery gas.

Fractional Distillation of Ethanol

Fractional distillation cannot completely separate the components of a mixture of ethanol and water, despite the different boiling points of the two chemicals. Water boils at 100 °C while ethanol boils at 78.4 °C. If an alcohol-water mixture is boiled, the ethanol will concentrate in the vapor, but only up to a point because alcohol and water form an azeotrope. Once the mixture reaches the point where it consists of 96% ethanol and 4% water, the mixture is more volatile (boils at 78.2 °C) than the ethanol.

Simple vs Fractional Distillation

Fractional distillation differs from simple distillation because the fractionating column naturally separates compounds based on boiling point. It's possible to isolate chemicals using simple distillation, but it requires careful control of the temperature since only one "fraction" can be isolated at a time.

How do you know whether to use simple distillation or fractional distillation to separate a mixture? Simple distillation is faster, simpler, and uses less energy, but it's really only useful when there is a large difference between the boiling points of the desired fractions (more than 70 degrees Celsius). If there is only a small temperature difference between the fraction, fractional distillation is your best bet.

 Simple DistillationFractional Distillation
UsesUsed for separating relatively pure liquids that have large boiling point differences. Also useful for separating liquids from solid impurities.Used to isolate components of complex mixtures with small boiling point differences.
  • faster
  • requires less energy input
  • simpler, less expensive equipment
  • results in better separation of liquids
  • better at purifying liquids containing many different components
  • only useful for relatively pure liquids
  • requires a large boiling point difference between components
  • doesn't separate fractions as cleanly
  • slower
  • requires more energy
  • more complicated and expensive set-up