General considerations

In most cases, crude oil is unsuitable for direct use. It is converted into such useful products as gasoline, motor oil, and petrochemicals through the refining process. Petroleum refining began in the United States and Russia in the second half of the 19th century, following the discovery in 1859 of rock oil in Pennsylvania. In the earliest refineries simple stills separated crude oil into impure gasoline, kerosene, lubricating oil, and fuel oil fractions. Kerosene was the principal marketable product. To improve its odour and appearance the kerosene was treated chemically with caustic soda or sulfuric acid.

The earliest automobile fuel was composed of those fractions of crude oil that were too light to be included in kerosene. Before the invention of the automobile this fraction had been virtually unsalable; as demand for it rose, refiners increased production and improved quality. Methods for the continuous distillation of crude oil were introduced.

After World War I a major improvement in refining came with development of the cracking process, consisting of heating surplus heavier oils under pressure and thereby cracking, or splitting, their large molecules into the smaller ones that form the lighter, more valuable fractions. After appropriate chemical treatment, gasoline manufactured by cracking performed better in automobile engines than gasoline derived from straight distillation.

During the 1930s and World War II, sophisticated refining processes involving the use of catalysts led to further improvements in the quality of fuels and increased their supply. These improved processes, including catalytic cracking, polymerization, alkylation, and isomerization, permitted the petroleum industry to meet the high-performance demands of combat aircraft and, after the war, to supply the increasing quantities required by commercial aviation.

The 1950s and 60s brought a large-scale demand for jet fuel and for high-quality lubricating oils. Catalytic reforming was established as the leading process for up-grading automotive motor gasolines for use in the higher compression engines. Hydrocracking, accomplished by the addition of hydrogen during refining, also improved the crude-oil fractions.

Types of crude oils. The various hydrocarbon compounds that are mixed together in crude oil have different boiling points, but, apart from the lightest, the differences between the boiling points of neighbouring members in the rising scale of molecular weight are so small that they cannot be separated by ordinary distillation. Fortunately, separating is not usually necessary; most common petroleum products consist of mixtures of compounds whose boiling points fall within a specified range.

Typical boiling ranges are: motor gasoline 25° – 150 °C (75° – 300 °F), kerosene 150° – 230 °C (300° – 450 °F), diesel oils 230° – 340 °C (450° – 650 °F), and fuel oils above 340° C. In practice, however, there are many different grades of each product with different boiling-point specifications. Crude oils vary widely in boiling range and composition. The proportions of the products obtained by distillation of six typical crude oils ranges from West Venezuelan Bachaquero Heavy crude to the light Algerian crude Hassi-Messaoud. The percentage of lighter products (gasoline and naphtha) increases. Middle distillates (which include kerosene and gas oil) also increase, while fuel-oil content diminishes.

Composition of crude oil. Four main types of hydrocarbons are present in crude oil: normal paraffins, isoparaffins, cycloparaffins (also called naphthenes), and aromatics. Some crude oils, such as Pennsylvanian, consist mainly of paraffins. Others, such as the heavy Mexican and Venezuelan crudes, are predominantly naphthenic and are rich in asphalt (a high-boiling semisolid material). Wax is usually, but not always, associated with paraffin-base crudes. The U.S. Bureau of Mines classifies crude oils as paraffin base, naphthene base, or mixed base by determining the properties of key fractions distilled from the oil.

In addition to the hydrocarbons, compounds of sulfur, nitrogen, and oxygen are present in small amounts in crude oil. These are harmful unless removed. Also, there are usually traces of vanadium, nickel, chlorine, and arsenic.