Petroleum goes through multiple operations intended to separate it into fractions, to improve product quality (e.g. by removing sulfur), and to enhance yields of the more valuable fractions relative to those for which there is less demand. Collectively these operations comprise the technology of oil refining. The objective of refining is to convert petroleum into useful, marketable products through processes that are both economically feasible and environmentally acceptable. Considering on the one hand the complexity and variability of petroleum, and on the other hand the demands imposed for meeting economic and environmental criteria, it is remarkable that petroleum products cost us, the consumers, as little as they do. The primary products of refining are liquid fuels mainly used in the transportation sector, and to a lesser extent for space heat, process heat, or raising steam for electricity generation. Secondary products include feedstocks for the chemical and polymer industries, and carbon materials. The refining situation faces a third constraint, in that the quality of the crude oil available to refineries is slowly but steadily decreasing. Over a century, the API gravity of crudes processed in refineries has dropped from 30–40°, typical of oils processed in the late nineteenth century, to 15–30° by the end of the twentieth century.

Generally, the desirable and valuable products that can be made from petroleum have H/C ratios higher than those of the petroleum itself. Two strategies provide routes to these products. One is to increase the hydrogen content by adding hydrogen from an external source. Examples include hydrogenation (e.g. conversion of aromatics to cycloalkanes) and hydrocracking, discussed in Chapter 15. The alternative involves removing carbon, known as carbon rejection. The folk saying “If you can't raise the bridge, lower the river” relates directly; the numerical value of any ratio can be increased by increasing the numerator or reducing the denominator. Many thermal processes discussed in Chapter 16 yield, in addition to the desired light products, highly aromatic tars or solid carbonaceous residues as the rejected carbon.