Conversion: cracking and rearranging molecules to add value

This is where refining fanciest footwork takes place—where fractions from the dis­tillation towers are transformed into streams (intermediate components) that eventually become finished products. This also is where a refinery makes money, because only through conversion can most low-value fractions become gasoline.

The most widely used conversion method is called cracking because it uses heat and pressure to "crack" heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, bullet-shaped reactors and a network of fur­naces, heat exchangers and other vessels.

Fluid catalytic cracking, or "cat cracking," is the basic gasoline-making process. Using intense heat (about 1,000 degrees Fahrenheit), low pressure and a powdered cata­lyst (a substance that accelerates chemical reactions), the cat cracker can convert most relatively heavy fractions into smaller gasoline molecules.

Hydrocrackingapplies the same principles but uses a different catalyst, slightly lower temperatures, much greater pressure and hydrogen to obtain chemical reactions. Although not all refineries employ hydrocracking, Chevron is an industry leader in us­ing this technology to cost-effectively convert medium- to heavyweight gas oils into high-value streams. 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. Cokers are among the more peculiar-looking refinery structures. They resemble a series of giant drums with metal derricks on top.

Cracking and coking are not the only forms of conversion. Other refinery process­es, instead of splitting molecules, rearrange them to add value. Alkylation, for example, makes gasoline components by combining some of the gaseous byproducts of cracking. The process, which essentially is cracking in reverse, takes place in a series of large, horizontal vessels and tall, skinny towers that loom above other refinery structures.

Reforminguses heat, moderate pressure and catalysts to turn naphtha, a light, rela­tively low-value fraction, into high-octane gasoline components. Chevron's patented reforming process is called Rheniforming for the rheniumplatmum catalyst used.