Fractional Distillation: Industrially • Oil is pre-heated • then passed into column. • The fractions condense at different heights • The temperature of column decreases upwards • The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the van der waals forces • Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures • and big molecules condense at the bottom at higher temperatures. Key points to learn This is a physical process involving the splitting of weak van der waals forces between molecules Vacuum distillation unit • Heavy residues from the fractionating column are distilled again under a vacuum. • Lowering the pressure over a liquid will lower its boiling point. Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down. Petroleum is a mixture consisting mainly of alkane hydrocarbons Petroleum fraction: mixture of hydrocarbons with a similar chain length and boiling point range N Goalby chemrevise.org 1 naptha (chemicals) fuel oil bitumen lubricating oils diesel oil kerosene (jet fuel) fuel gas (bottled) petrol/gasoline 20° C 40° C
22.214.171.124 Fractional distillation of crude oil
Alkanes are saturated hydrocarbons.
Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.