Fractional Distillation Definition and Examples

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 to separate mixtures to obtain their components.

The technique is used in labs and in industry, where the process has vast commercial significance. The chemical and petroleum industry rely on fractional distillation.

How It 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.

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 might be residue (used to make bitumen), fuel oil, diesel, kerosene, naphtha, gasoline, and refinery gas.

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 degrees Celcius while ethanol boils at 78.4 degrees Celcius. 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 degrees Celcius) than the ethanol.

Simple vs. Fractional Distillation

Fractional distillation differs from simple distillation because the fractionating column naturally separates compounds based on their boiling points. It's possible to isolate chemicals using simple distillation, but it requires careful control of the temperature because 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 Celcius). If there is only a small temperature difference between the fractions, fractional distillation is your best bet.

Here's a breakdown of the differences between simple and fractional distillation: