As a raw product, crude oil is of limited use. Refineries must separate and process the mix of hydrocarbons which make up crude oil before they can be transformed into hundreds of useful products such as gasoline, diesel and jet fuels.
The first and most important step is to separate it into various component or fractions. This takes place in a fractionating column, also known as an atmospheric distillation tower.
This is a tall steel tower with perforated trays. Since each fraction has a different boiling range, a distillation tower is able to separate the various fractions using heat and cooling.
Heavier hydrocarbons boil at much higher temperatures than lighter hydrocarbons.
They settle in trays at the bottom of the tower closest to furnace. The lighter fractions collect at the top. Distillation is a continuous process which begins by heating crude oil in a furnace. Then it turns into a vapor. The vapor rises through perforations in the trays that are fitted with bubble caps. These caps force the vapor to pass through a previously liquefied fraction in the tray. This cools the vapor enough for it to shed that fraction. The remaining vapor repeats this process as it continues upwards. As each fraction reaches the tray where the temperature is just below its own boiling point, it condenses, liquefies and is drawn off the tray by pipes. A number of trays are needed to collect the liquids from each fraction.
The products of distillation can be divided into four categories:
1 Gases and light gasoline The gases (methane, ethane, propane and butane) are commonly used to fuel refinery furnaces while the light gasoline is routed to gasoline blending.
2 Light distillates (naphta, kerosene) Naphta is used in the production of gasoline and petrochemicals. Kerosene is used as a jet fuel and stove oil.
3 Middle distillates (light and heavy gas oils) Light gas oils are made into jet, diesel and furnace fuels. Heavy gas oils undergo further chemical processing such as cracking to produce naphta and other products.
4 Residual productsResidual products are further processed to produce refinery fuels, heavy fuel oil, waxes, greases and asphalt
The next step is conversion. During this process fractions from distillation towers are transformed into streams (intermediate components) that eventually become finished products. 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, 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.
Hydrocracking applies the same principles but uses a different catalyst, slightly lower temperatures, much greater pressure and hydrogen to obtain chemical reactions..
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. Reforming uses heat, moderate pressure and catalysts to turn naphtha, a light, relatively low-value fraction, into highoctane gasoline components..