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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 products Residual 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 high-
octane gasoline components.

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