Petroleum is the most important substance consumed in modern society. It provides not only fuel and energy for transportation but is also used in plastics, paint, fertilizer, insecticide, medicine, and elsewhere. The exact composition of petroleum varies widely from source to source, but the percentage of chemical elements changes over fairly narrow limits. Hydrogen and carbon are the major components, and sulfur, nitrogen, oxygen, and metals are present in relatively lower quantities (Table 1.1). Usually, petroleum or crude oil comes from deep underground, where the vestiges of plants and animals from millions of years ago have been heated and pressurized over time. It is blackish in color and has a characteristic odor that comes from the presence of small amounts of chemical compounds containing sulfur, nitrogen, and metals.

The change in crude oil quality around the world (e.g., heavy petroleum production has been increased in recent years) has obliged crude oil refiners to reconfigure current refineries and to design new refineries specifically to process heavier feedstocks (i.e., blends of various crude oils with elevated amount of heavy petroleum). These new feeds are characterized by high amounts of impurities (sulfur, metals, nitrogen, asphaltenes) and low distillate yields, which make them more difficult than light crude oils to process.

Comparisons of some properties of various crude oils are presented in Tables 1.2 and 1.3. It is clear that light and heavy crude oils have remarkable differences. Heavy petroleum is characterized by low API gravity, large amounts of impurities, and low distillates yields; light petroleum is of much better quality. In general, the lower the API gravity (i.e., the heavier the crude oil), the higher the impurities content and the lower the distillates yield. Such properties make processing of heavy petroleum different from that used for light crude oil refining. In other words, a refinery capable of processing light petroleum cannot, without changes in some units or even complete reconfiguration, be employed to process 100% heavy petroleum.

In general, light crude oil is rich in light distillates, and heavy crude oil, in residuum. However, the petroleum composition may vary with its API gravity and origin. Physical properties and exact chemical composition of crude oil also vary from one source to another. As a guide to chemical composition, Table 1.4 provides qualitative data on saturate, aromatic, resin and asphaltene (SARA) contents in the heavy fractions present in various crude oils. The most complex impurity of petroleum is asphaltene, which consists of condensed polynuclear aromatics containing small amounts of heteroatoms (S, N, O) and traces of nickel and vanadium. Asphaltenes are typically defined as brown and black powdery material produced by the treatment of petroleum, petroleum residua, or bituminous materials with a low-boiling liquid hydrocarbon (e.g., pentane or heptane); and soluble in benzene (and other aromatic solvents), carbon disulfide, and chloroform (or other chlorinated hydrocarbon solvents). Asphaltene molecules are grouped together in systems of up to five or six sheets, which are surrounded by the maltenes (all those structures different from asphaltenes that are soluble in n-heptane) and resin.

The properties of petroleum, such as viscosity, density, boiling point, and color, may vary widely, and the ultimate or elemental analysis varies over a narrow range for a large number of samples. Metals have a tendency to concentrate more in the heavier fraction (asphaltene) than in the saturated and aromatic fractions. The higher the asphaltene content in crude oil, the higher the metal content; however, the increase in vanadium concentration is not proportional to that of nickel. Nitrogen and sulfur can be present in traces in light petroleum, but with heavier or extra heavy crude oil, the sulfur and nitrogen contents also increase.