Petroleum refining is a complex business, involving chemistry, physics, mathematics and several engineering disciplines.
Crude oil is not a single substance, but a mixture of hundreds of compounds made up mostly of hydrogen and carbon, along with sulfur, nitrogen, and many other elements in very small quantities. These "hydrocarbons" range in density from materials as light as natural gas to those as heavy as road asphalt.
In crude oil, these substances are thoroughly intermingled. Each of these compounds, however, boils at a different temperature, and that is the secret to separating them.
Distillation -- boiling a liquid, condensing the vapors, and then collecting the resultant separate liquids -- is the first step in transforming crude oil into petroleum products. Distilling separates different parts of the crude oil that are sent into various portions of the refinery, which are then sent on for future processing.
The majority of the Crude barrel is used for making fuels (gasoline, jet fuel, and diesel). The remaining portion is processed to make asphalt and petroleum coke. There are several major units at the refinery that accomplish this process. The refinery has two (cokers), a catalytic cracking unit, a hydrocracking unit, a crude unit, and an alkylation unit among others. All play a major role in the refining process. The cat cracker is a major gasoline producing unit, while the hydrocracker produces diesel.
In these processes, additional lighter products are created - large hydrocarbon molecules are broken into smaller ones - and the resulting liquids are sent on to further processing. What remains is virtually pure carbon, or "coke." The coke produced by the Delayed Coker is sold as a fuel for power generation and industrial uses, whereas the coke manufactured by the Flexicoker is converted to a gas and is used to meet much of the Refinery's internal fuel needs.
The petroleum fractions emerging from initial distillation processes and from the Flexicoker and Delayed Coker are, for the most part, still far too heavy (that is, they are composed of hydrocarbon molecules that are too large) for use in gasoline, jet fuel, and diesel. However, under a variety of special conditions, these large molecules can be "cracked" - literally broken into smaller, more usable, varieties.
One of the Shell Martinez Refinery's major plants is the Catalytic Cracking Unit, or "cat cracker," as it is called. Heavy oils are mixed with a flowing sand-like catalyst at high temperatures and in a few seconds, are cracked into gasoline components and lighter gases.
Another catalytic process is hydrocracking, which takes intermediate oils, mixes them with hydrogen and passes this mixture over a catalyst at high pressure and moderate temperature. This converts them to gasoline, jet fuel, and diesel components.
Catalytic reforming and isomerization are processes that change the molecular structure of certain gasoline boiling range fractions. With the aid of a catalyst, low-octane gasoline fractions are chemically changed into a high-octane gasoline component suitable for blending into finished gasoline.
While many of the Martinez Refinery's big processing plants change heavy hydrocarbon molecules into lighter ones, units such as the Alkylation Plant and the Dimerization Plant do just the opposite. These plants take the lightest parts of crude oil and combine them to form gasoline components.
A careful concern for the environment -- both at the Refinery and where the products are consumed -- is an important part of Shell's operation. Several processes -- hydrotreating and sulfur recovery among them -- safely remove sulfur and other unwanted ingredients of the original crude oil, thus; these elements won't be released to the atmosphere.
In addition to all of this processing there is significant blending involved to make these products. Automotive gasoline, for example, is a mixture of many components and additives that meet octane and other specifications. This is aided by computer, with instruments in modern control rooms telling operators what's happening at all times. Shell even has special engines on site to test gasolines for the proper octane specifications.
As you can see, there are many aspects to the refining process, all of which are administered with great care.