The resulting liquids and vapors are discharged into distillation towers, the tall, narrow columns that give refineries their distinctive skylines. Inside the towers, the liquids and vapors separate into components or fractions according to weight and boiling point. Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle. Heavier liquids, called gas oils, separate lower down, while the heaviest fractions with the highest boiling points settle at the bottom. The fractions now are ready for piping to the next station or plant within the refinery.
Some components require relatively little additional processing to become asphalt base or jet fuel.
However, most molecules that are destined to become high-value products require much more 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 Chevron 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.