Separation

Chevron's Burnaby Refinery

Heavy on the Bottom, Light on the Top

Modern separation - which is not terribly different from the "cooking" methods used at the Pico Canyon stills - involves piping oil through hot furnaces. The resulting liquids and vapours are discharged into distillation towers, the tall, narrow columns that give refineries their distinctive skylines.

Inside the towers, the liquids and vapours separate into components or fractions according to weight and boiling point. The lightest fractions, including gasoline and liquid petroleum gas (LPG), vapourize and rise to the top of the tower, where they condense back to liquids. 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. These tarlike fractions, called residuum, are literally the “bottom of the barrel.”

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.

The Three Basic Steps to Refining:

• Separation: Heavy on the Bottom, Light on the Top
• Conversion: Cracking and Rearranging Molecules to Add Value
• Treatment: the Finishing Touch