Distillation, the separation of one substance from another by evaporation and condensation. The product obtained by distillation is called the distillate. This process is used for purifying liquids having undesired substances in them, for separating different kinds of liquids from each other, and for producing chemical changes in solid substances. There are three main types of distillation: simple, fractional, and destructive.

Simple Distillation

Simple distillation is used to eliminate solid impurities from a liquid. Water can be freed of salt or other minerals through this process. Water is boiled in a closed container called a boiler. Vapor evaporating from the surface of the boiling water passes through a pipe leading from the boiler to a vessel called a condenser. The condenser, in turn, opens into a third vessel. The entire apparatus is called a still.

In its simplest form, the condenser is a tube surrounded by a larger tube filled with cold water. The water vapor from the boiler never touches the cold water, which cools the vapor as it passes through the inner tube. As the vapor loses its heat, it becomes liquid and flows into the collecting vessel. The process continues until all the water in the boiler has evaporated. Solid impurities that had been in the water remain on the bottom of the boiler.

In vacuum distillation, the atmospheric pressure inside the boiler is reduced, allowing the liquid to boil at a lower temperature. In steam distillation, steam is fed into the boiler and turns the substance to be distilled into vapor.

Fractional Distillation

This method is used to separate two or more liquids in the same mixture. The principle is the same as that used in simple distillation, except that the mixture is distilled two or more times.

Each liquid in a mixture boils at a different temperature. When the mixture in the boiler is heated to the lowest of these boiling points, all of the liquid having that boiling point will evaporate. But the resulting vapor will have a high proportion of the liquid that has the next lowest boiling point. This vapor is turned back into liquid in the condenser and flows into the retort.

At this point, a fraction of the total liquid originally in the boiler has been distilled. This fraction may be redistilled one or more times until the liquid with the lowest boiling point is pure. Then the temperature of the boiler is raised so that the liquid with the second lowest boiling point can be distilled and redistilled until pure. This process continues until every liquid in the mixture is separated.

In practice, the process is speeded up through the use of a fractionating tower. Vapors that will boil off at low temperatures are lighter and therefore rise higher in the tower than those that boil off at high temperatures. Pipes high in the tower carry light vapors to the condenser, while those low in the tower carry away heavy vapors. The various fractions can thus be distilled simultaneously and continuously rather than having a separate process for each fraction.

In the form of fractional distillation called rectification, part of the vapor is condensed and then mixed with new vapor. This process helps purify the product.

Fractional distillation is one method of producing alcohol (although it does not produce absolute, or pure, alcohol) and is an important process in petroleum refining.

Destructive, or Dry, Distillation

Destructive distillation is the process of heating a solid substance in order to decompose it. This process results in the complete destruction of the original substance and the formation of new ones. Usually, a portion of the substance being heated is converted into carbon, and for this reason the process is sometimes called carbonization.

A substance is placed in a vessel from which all the air is removed. The substance is then heated until it has turned into relatively pure carbon. (The lack of air prevents it from burning.) Other materials in the original substance turn to vapor and pass through a pipe that goes through a condenser to a retort.

Destructive distillation is used in converting wood into charcoal. The vapor collected as the wood heats is separated by fractional distillation into acetic acid, acetone, and wood alcohol. The same process is used to convert coal into coke. Other products obtained from the distilled coal are coal gas, coal tar, and ammonia.