To be refined, an oil product is first introduced to a fractioning tower where it vaporizes and then re-condenses into liquids of different densities and viscosities. Afterward, the oil is often subsequently broken down further and distilled. Following distillation, the finished product is frequently refined even further before designations such as octane rating can be assigned.
After the oil is heated to 350 degrees Celsius, it is placed inside the fractioning tower for vaporization. The tower itself is equipped with layers of trays, all designed with holes in them, to collect the oil as it re-liquefies and falls from the top. Lighter liquids products, such as kerosene and butane, collect in trays at the top of the tower, whereas denser ones collect near the bottom. These latter products are often used for such items as waxes and lubricants.
While some simpler refineries stop after this separation, many others, particularly in the United States, opt to process the substance further. For example, a 21st century industry standard requires a process called cracking, whereby larger hydrocarbon molecules are broken down into much smaller ones, producing a runnier liquid closer to that associated with motor vehicle fuels. Other refining equipment possibly employed in this stage include cokers and reformers. After cracking, many refineries engage in distillation and additional purification before transportation via pipeline, ship or truck to the consumer.