ABSTRACT
Crude oil is a naturally occurring wide-boiling-range mixture of hydrocarbons acquired under the earth and sea that is often classified as light, medium, heavy, or extra-heavy, referring to its gravity as measured on the American Petroleum Institute (API) scale. In general, crude oils are commercially defined as follows: light crude oil has an API gravity higher than 31.1
°
, medium oil has an API gravity between 31.1
°
and 22.3
°
, heavy oil possesses an API gravity between 22.3
°
and 10
°
, and extra-heavy oil or bitumen has an API gravity less than 10
°
,
which cannot flow without being heated or diluted (Riazi, 2005). Crude oil produced by different petroleum fields differs importantly in viscosity and sulfur content. The more viscous and high-sulfur-content (>2.0 wt%) crudes are called heavier or sour, whereas low-sulfur-content (<0.5 wt%) crudes are called sweet (Gary and Handwerk, 1994). Most of the refineries are not capable of processing heavy or sour crudes due to their complex nature, which compels refiners to process the crude oil in some of the selective refineries that are able to upgrade them to more valuable products.
