Trends in Properties of Fractions

Hydrocarbons and their Properties

  • Crude oil is a complex mixture of hydrocarbons. Hydrocarbons are compounds made up of hydrogen and carbon only, and they can be found in gases or liquids.

  • The hydrocarbons in crude oil vary greatly in their size, structure and boiling points, which allow them to be separated into different fractions via a process known as fractional distillation.

Fractional Distillation of Crude Oil

  • Fractional distillation is a physical separation technique that separates components of a mixture based on differences in their boiling points.

  • Crude oil is heated until it vaporises, and the vapour is then fed into a fractionating column. The column is cooler at the top and hotter at the bottom.

  • Fractions that have lower boiling points (usually the smaller molecules) rise to the top of the column, condense and are collected.

  • Fractions with higher boiling points (usually the larger molecules) condense lower down in the column or remain uncondensed at the bottom.

Trends in Properties of Fractions

  • The hydrocarbons in various fractions of crude oil have different properties. These properties can be related to the size of the hydrocarbon molecules.

  • The smaller the hydrocarbon molecule, the lower the boiling point. Consequently, fractions with smaller hydrocarbons are often gases at room temperature.

  • Larger hydrocarbon molecules have higher boiling points. These fractions condense at the bottom of the fractionating column and are typically thicker liquids or solids.

  • The larger the molecule, the more viscous (thicker) the fraction is. So, fractions with large molecules like bitumen are thick and slow-moving.

  • Larger hydrocarbons are generally less flammable than smaller ones. Gaseous fractions burn very easily while larger fractions are not easily ignited.

  • A key point to remember is that different fractions have different uses depending on their properties. For example, the gaseous fraction is used as fuel in homes while bitumen is used for road surfacing.