How is diesel fuel made?

Crude oil that is pumped out of the ground is composed of thousands of different hydrocarbon compounds. The carbon atoms link together in chains of different lengths, shapes and sizes. Each individual chain length or molecular size has different properties including progressively higher boiling points, so they can be separated by boiling point or distillation.

Distillation is the first major process at an oil refinery. As the crude oil is heated, different hydrocarbon compounds are separated by their boiling temperatures. The lightest components such as ethane and propane come off the top of the distillation tower. The next components, called naphtha, are used for making gasoline.

The next heavier portion of crude oil coming from the distillation tower is used to make diesel fuel. Diesel fuel contains larger hydrocarbon molecules, with more carbon atoms than gasoline. Simple distillation does not produce enough gasoline and diesel fuel, so heavier fractions of crude oil are broken into smaller compounds by thermal or catalytic cracking, or hydrocracking, to produce higher volumes of gasoline and diesel fuel. To lower the level of sulphur in the fuel, some of these fractions may be hydrotreated. Various component streams are blended to meet the required diesel fuel specifications.