Refining

Crude oils only have value because they can be turned into refined products that consumers need. Each refined product has its own separate market, and each crude oil grade yields a different mix of products. The technology and process of refining has become highly elaborate since the late 19th century, when it was focused almost exclusively on kerosene production.

The refining process begins with "fractional distillation", which separates the hydrocarbons in crude oil on the basis of a physical characteristic: differing boiling points. Horizontal trays divide the column at intervals. As the oil boils, it vaporises. Each hydrocarbon rises to a tray at a temperature just below its own boiling point. There, it cools and turns back into a liquid.

The lightest fractions rise the highest: liquefied petroleum gases, petrol, and naphtha. Next come kerosene (used as jet fuel), heating oil, and diesel fuel for trucks, buses, trains, and ships. The heaviest fractions stay at the bottom of the column.

After passing through a fractionating column that operates in a partial vacuum, these fractions become lubricating oils and waxes. The remains, which include asphalt, are known as "residual" products. Through chemical reactions, refineries can convert fractions in limited demand to fractions in greater demand petrol, for example. Naphtha and other fractions used as petrochemical "feedstocks" also undergo additional processing.
With different markets requiring distinct mixes of products from different combinations of crude oil feedstocks, the variety of refinery configurations is significant.

They are most commonly categorised by their complexity: the amount of upgrading they have, and therefore a measure of their ability to produce higher-value, light products such as petrol.

There are no exact specifications for these categories. However, the oil industry usually thinks in terms of three basic levels of complexity.
A simple refinery involves atmospheric distillation, plus some common secondary processes -- usually reforming and hydrotreating -- to raise the quality of the product output to current, commercial standards. It therefore produces large volumes of residual fuel oil, especially from heavier grades of crude oil.
A complex refinery goes one step further. It makes more of the desirable light products, such as petrol and gas oil, by using the heavy gas oils and gases from some refining processes as feedstocks for others, such as cracking and alkylation.
A highly complex refinery simply adds more sophistication to this additional secondary processing, eating up even more of the low value, heavy products through residue destruction technologies such as coking. Some highly complex refineries are integrated with sophisticated petrochemical plants or lubricants plants that enhance the value of a crude oil even more significantly.