Distillation is a process used to purify liquids. The type of distillation needed depends on the type of mixture which needs to be separated. If a liquid is contaminated with non-volatile components, then a simple distillation will work well. An example of this type of process would be what's needed in a desalination plant to purify water which contains dissolved salts. If we have a mixture of liquids, however, the distillation process is more complicated and a fractional distillation is required. An example of a fractional distillation is what's used in an oil refinery. In the distillation towers, crude oil is separated into its various liquid components based on differences in their boiling points.