ABSTRACT
Petroleum is by far the most commonly used source of energy, especially as the source of
liquid fuels (Figure 3.1), and its use is projected to continue at least at the current levels for at
least two decades (Figure 3.2). However, there is petroleum and there is petroleum and all are
not equal. In recent years, the average quality of crude oil has worsened. This is reflected in a
progressive decrease in API gravity (i.e., increase in density) and a rise in sulfur content
(Swain, 1991, 1993, 1997, 2000). However, it is now believed that there has been a recent
tendency for the quality of crude oil feedstock to stabilize. Be that as it may, the nature of
crude oil refining has changed considerably.
