Petroleum and cracking

Last time

Before we start: last time, in Esters, condensation polymers and proteins, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Black gold

Crude oil comes out of the ground as a thick black liquid. It is useless as it is. But separated and processed, it gives us petrol for cars, kerosene for planes, bitumen for roads and the starting chemicals for plastics. Two processes do the work: fractional distillation, and cracking.

What petroleum is

Petroleum, or crude oil, is a mixture of many hydrocarbons, mostly alkanes. It formed from dead sea creatures buried for millions of years. It is a fossil fuel and a non-renewable resource. The hydrocarbons have different chain lengths, so they have different boiling points. That is the key to separating them.

Fractional distillation

The petroleum is heated until it vaporises, and the vapour goes into the bottom of a tall fractionating column. The column is hot at the bottom and cooler at the top. Each hydrocarbon rises until it reaches a part of the column cool enough for it to condense. Then it is collected. The fractions are groups of hydrocarbons with similar boiling points, not pure substances.

The fractions

At the top: refinery gas, used for heating and cooking. Then gasoline, which is petrol for cars. Then naphtha, which is a feedstock for the chemical industry. Next, kerosene, which is jet fuel. Then diesel oil, for lorries and trains. Lower still, fuel oil for ships and power stations. Then lubricating oil, for lubricants, waxes and polishes. At the bottom, bitumen for roads.