What is Crude Oil?
Crude oil is not a single compound like water. It is a mixture of hydrocarbon molecules, some large and some small. The temperature of water heated in a laboratory beaker rises until it reaches 212°F, then the water starts to boil. It keeps boiling at 212°F until all of the water boils away. It does this because every molecule is the same – H2O.
Crude oil in a laboratory flask will begin boiling at about room temperature (around 70°F); the temperature of the vapor above the liquid will rise as the crude oil continues to boil. It does this because as the temperature increases, various hydrocarbon molecules in the crude oil vaporize, or boil away. The remaining mixture requires even higher temperatures to boil. Hydrocarbon molecules boil at different temperatures, from -44°F for propane all the way to 1500°F for asphalt.
Light & Heavy Crude Oil
Depending on the mixture of hydrocarbon molecules, crude oil varies in color, composition and consistency. Different oil-producing areas yield significantly different varieties of crude oil.
The words "light" and "heavy" describe a crude oil’s density and its resistance to flow (viscosity). Some, which are low in metals and sulfur content, light in color and consistency, and flow easily, are known as "light." Less expensive, low-grade crude oils, which are higher in metals and sulfur content, and must be heated to become fluid, are known as "heavy." The term "sweet" is used to describe crude oil that is low in malodorous sulfur compounds such as hydrogen sulfide and mercaptans, and the term "sour" is used to describe crude oil containing high malodorous sulfur compounds.
High Value Products from Low Grade Crude Oil
Crude oil processing equipment varies from one refinery to the next. Generally, the more complex a refinery’s design and equipment, the better its ability to upgrade crude oil into high-value products.
Pascagoula's Complex Capabilities
The Pascagoula Refinery is Chevron Products Company's largest and most complex refinery. In 1983, the refinery expanded its crude oil processing capabilities to process and treat low-grade heavier, sour, foreign crude oil. The $1.3 billion Pascagoula Residuum Conversion Project expansion allows the refinery to make gasoline and other light products from the heaviest asphalt-like portion of crude oil.
The expansion added a Residuum Desulfurization (RDS) Unit, a Delayed Coker Unit, an additional hydrogen plant, upgraded the Crude Units to feed the heavy crude oil, and added additional Sulfur Units to handle the increased amount of sulfur recovered from sour crude oil. During the refinery’s 2003 Clean Fuels Project, the Residuum Desulfurization (RDS) Unit was converted from its original service of de-sulfurizing residuum. The unit is now a gas oil hydrotreater that produces ultra low-sulfur process feed for the FCC – an integral step in producing low-sulfur gasoline.
Due to its efficient operations and complex capabilities to treat less expensive sour, heavy crude oil, the Pascagoula Refinery's total conversion of crude oil to light products is about 85 percent, a significantly high conversion rate for low-grade crude oil.