Rectification is an application of distillation and its uses include fractionation of crude oil. If the distillate obtained during distillation is distilled again, a new distillate is obtained with an even higher concentration of volatile components. As the procedure is repeated, the concentration of volatile components in the distillate increases on each occasion.
In practice, this multi-stage distillation process is carried out in the form of countercurrent distillation (rectification) in a column. The liquid mixture to be separated (feed) is fed to the bottom of the column, where it is brought to boiling point. The vapor produced moves upwards inside the column, exits it at the top and is condensed. Part of the condensate is carried away as top product. The remainder flows back into the column and move downwards as liquid opposite phase.