What Is Distillation? Chemistry Definition

Understand the Principles of Distillation

distillation
This is an example of a simple setup for distillation to separate components of a chemical mixture. Dorling Kindersley / Getty Images

Distillation is an important separation process in chemistry, industry, and food science. Here is the definition of distillation and a look at the types of distillation and its uses.

Key Takeaways: Distillation

  • Distillation is the process of separating components of a mixture based on different boiling points.
  • Examples of uses of distillation include purification of alcohol, desalination, crude oil refining, and making liquefied gases from air.
  • Humans have been using distillation since at least 3000 BC in the Indus valley.

Distillation Definition

Distillation is a widely used method for separating mixtures based on differences in the conditions required to change the phase of components of the mixture. To separate a mixture of liquids, the liquid can be heated to force components, which have different boiling points, into the gas phase. The gas is then condensed back into liquid form and collected. Repeating the process on the collected liquid to improve the purity of the product is called double distillation. Although the term is most commonly applied to liquids, the reverse process can be used to separate gases by liquefying components using changes in temperature and/or pressure.

A plant that performs distillation is called a distillery. The apparatus used to perform distillation is called a still.

History

The earliest known evidence of distillation comes from a terracotta distillation apparatus dating to 3000 BC in the Indus valley of Pakistan. Distillation was known to be used by the Babylonians of Mesopotamia. Initially, distillation is believed to have been used to make perfumes. Distillation of beverages occurred much later. The Arab chemist Al-Kindi distilled alcohol in 9th century Irag. Distillation of alcoholic beverages appears common in Italy and China starting in the 12th century.

Uses of Distillation

Distillation is used for many commercial processes, such as the production of gasoline, distilled water, xylene, alcohol, paraffin, kerosene, and many other liquids. Gas may be liquefied and separate. For example: nitrogen, oxygen, and argon are distilled from air.

Types of Distillation

Types of distillation include simple distillation, fractional distillation (different volatile 'fractions' are collected as they are produced), and destructive distillation (usually, a material is heated so that it decomposes into compounds for collection).

Simple Distillation

Simple distillation may be used when the boiling points of two liquids are significantly different from each other or to separate liquids from solids or nonvolatile components. In simple distillation, a mixture is heated to change the most volatile component from a liquid into vapor. The vapor rises and passes into a condenser. Usually, the condenser is cooled (e.g., by running cold water around it) to promote condensation of the vapor, which is collected.

Steam Distillation

Steam distillation is used to separate heat-sensitive components. Steam is added to the mixture, causing some of it to vaporize. This vapor is cooled and condensed into two liquid fractions. Sometimes the fractions are collected separately, or they may have different density values, so they separate on their own. An example is steam distillation of flowers to yield essential oil and a water-based distillate.

Fractional Distillation

Fractional distillation is used when the boiling points of the components of a mixture are close to each other, as determined using Raoult's law. A fractionating column is used to separate the components used a series of distillations called rectification. In fractional distillation, a mixture is heated so vapor rises and enters the fractionating column. As the vapor cools, it condenses on the packing material of the column. The heat of rising vapor causes this liquid to vaporize again, moving it along the column and eventually yielding a higher purity sample of the more volatile component of the mixture.

Vacuum Distillation

Vacuum distillation is used to separate components that have high boiling points. Lowering the pressure of the apparatus also lowers boiling points. Otherwise, the process is similar to other forms of distillation. Vacuum distillation is particularly useful when the normal boiling point exceeds the decomposition temperature of a compound.

Sources

  • Allchin, F. R. (1979). "India: The Ancient Home of Distillation?". Man. 14 (1): 55–63. doi:
  • Forbes, R. J. (1970). A Short History of the Art of Distillation from the Beginnings up to the Death of Cellier Blumenthal. BRILL. ISBN 978-90-04-00617-1.
  • Harwood, Laurence M.; Moody, Christopher J. (1989). Experimental organic chemistry: Principles and Practice (Illustrated ed.). Oxford: Blackwell Scientific Publications. ISBN 978-0-632-02017-1.