Petroleum refinery plant, c 1950s.
© Science Museum / Science & Society Picture Library
Detail of model showing fractional distillation section showing crude oil as a complex mixture of hydrocarbons - long chains of carbon and hydrogen atoms of differing lengths. These different chain lengths, and hence different oil grades, are separated by fractional distillation (each type of oil boils at a specific temperature and collected). With the advent of the motor car, demand for the lower boiling point (bp) fractions (eg petrol) rose sharply, so a method was developed to 'crack' (break down) the higher bp fractions.