Chemistry - Hydrocarbons and Fuels

Most of the compounds found in crude oil are hydrocarbons. They are so-called because they only contain hydrogen and carbon atoms, which are joined together by covalent chemical bonds. There are different families of hydrocarbon, each containing a variety of related chemicals. Most of the ones in crude oil are from the alkane family.

Alkanes are saturated hydrocarbons with one or more carbon atoms. Where there is more than one carbon atom, they are joined in chains, for example, butane. Butane has four carbon atoms in a chain. The carbon atoms at the end of each chain have three hydrogen atoms attached, the ones in the middle of the chain have only two attached. Saturated means that their carbon atoms are joined to each other only by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

Hydrocarbon compounds have different boiling points, and can be either solid, liquid or gas at room temperature:

• Small hydrocarbons with only a few carbon atoms have low melting and boiling points and are therefore gasses at room temperature.
• Hydrocarbons that have five to about 18 carbon atoms have moderate melting and boiling points are usually liquids at room temperature.
• Large hydrocarbons with many carbon atoms have high melting and boiling points and are solids.

Because they have different boiling points, the hydrocarbons in crude oil can be separated using fractional distillation. The crude oil is evaporated and injected into a fractionating column. The fractionating column is actually just a condenser which allows the vapourised crude oil to condense at different temperatures at different heights, thus enabling the fractions to be collected. Each fraction contains hydrocarbon molecules with a similar, but not identical, number of carbon atoms.