Distillation is used to separate chemically similar or non-reactive compounds. Gradually increasing the temperature of a liquid mixture will result in the evaporation of compounds of different boiling temperatures. Following evaporation, the vapor is channeled toward a condenser which is being cooled by cold water. This causes the vapor to condense as a liquid, which then flows into a collecting vessel as distillate.
Distillation is used to purify a reactant by removing any unwanted, volatile impurity. Solvents with low boiling points, such as diethyl ether, can be removed using a water-cooled condenser. Compounds with higher boiling points do not require special cooling: Liquids that evaporate at more than 280 degrees Fahrenheit are condensed using an air condenser, which is a glass tube that allows air to flow around the condensate. Sometimes, the boiling point of an impurity will be close to the decomposition point of the desired product. In such cases, reduced pressure or steam distillation is used. Reduced pressure inside the apparatus causes the impurity or solvent to evaporate at a lower temperature, preventing decomposition of the product.
Although most of the world's surface is covered in water, seawater's very high salt content makes it unsuitable for drinking or irrigation. In an attempt to solve water shortage problems, many drought-affected countries, including Saudi Arabia, the United Arab Emirates and Australia, have resorted to using desalination of seawater. The multistage flash (MSF) process removes salt from seawater using several heating and condensation stages.
The two most common industrial applications of distillation are the separation of hydrocarbons from crude oils and constituent gases from air. Hydrocarbons are produced using a process called fractional distillation. Crude oil or petroleum is heated to boil off increasingly heavy hydrocarbon fractions. The lightest fractions, include propane and butane, used as domestic fuels. Medium-weight compounds include gasoline and kerosene used respectively, as automobile and aviation fuels. The next heaviest compounds are used as diesel fuels for trains and freight vehicles or as heavy duty heating oils. The heaviest compounds are used for waterproofing or road surfacing or are broken up into lighter compounds using a process called cracking.