Inside the towers, the liquids and vapors separate into components or fractions according to their density and boiling point. Medium-weight liquids, including kerosene and diesel oil, stay in the middle. Heavier liquids, called gas oils, separate lower down. The heaviest fractions with the highest boiling points settle at the bottom. The various fractions are then piped to different stations or plants within the refinery.
Some fractions require relatively little additional processing to become road asphalt or jet fuel.
However, fractions destined to become high-value products, such as gasoline, typically require much more additional processing. This also is where a refinery makes money, because only through conversion can most low-value fractions become gasoline. Hydrocracking applies the same principles but uses a different catalyst, slightly lower temperatures, much greater pressure and hydrogen to obtain chemical reactions. The company's patented hydrocracking process, which takes place in the Isocracker unit, produces mostly gasoline and jet fuel. Some refineries also have cokers, which use heat and moderate pressure to turn residuum into lighter products and a hard, coallike substance that is used as an industrial fuel.