The various components of crude oil have different sizes,
weights and boiling temperatures; so, the first step is to
separate these components. Because they have different
boiling temperatures, they can be separated easily by a
process called fractional distillation.
The steps for fractional distillation are discussed below.
Heat the mixture of two or more substances (liquids) with
different boiling points to a high temperature. Heating is
usually done with high pressure steam to temperatures of
about 1112 degrees Fahrenheit / 600 degrees Celsius. The
mixture boils, forming vapor (gases); most substances go into
the vapor phase.
The vapor enters the bottom of a long column (fractional
distillation column) that is filled with trays or plates. The trays
have many holes or bubble caps (like a loosened cap on a soda
bottle) in them to allow the vapor to pass through. They
increase the contact time between the vapor and the liquids in
the column and help to collect liquids that form at various
heights in the column. There is a temperature difference
across the column (hot at the bottom, cool at the top).
The Vapor Rises
As the vapor rises through the trays in the column, it
cools. When a substance in the vapor reaches a height
where the temperature of the column is equal to that
substance's boiling point, it will condense to form a
liquid. (The substance with the lowest boiling point will
condense at the highest point in the column; substances
with higher boiling points will condense lower in the
column.). The trays collect the various liquid fractions.
The collected liquid fractions may pass to condensers,
which cool them further, and then go to storage tanks, or
they may go to other areas for further chemical
The oil refining process starts with a fractional distillation
column. On the right, you can see several
chemical processors that are described in the next section.
Very few of the components come out of the fractional
distillation column ready for market. Many of them must be
chemically processed to make other fractions. For example, only
40% of distilled crude oil is gasoline; however, gasoline is one of
the major products made by oil companies. Rather than
continually distilling large quantities of crude oil, oil companies
chemically process some other fractions from the distillation
column to make gasoline.
The top distillates
The top of the fractionating column gives rise to gases and liquids that
have short carbon chains in their composition. While these products are
often used as fuels, many have industrial and chemical uses too.
Petroleum gases : Butane and propane and other petroleum gases are
formed right at the top of the distillation tower, where it is coolest, a very
mild 25°C: the temperature range that forms these gases is between 25°C
and 50°C. These gases are the lightest products formed in crude oil
distillation and are flammable gases.
Petrol These gases, being the lightest products formed and flammable
gases too, are then processed into Liquified Petroleum Gas (LPG), which is
usually a mixture of propane and butane. LPG is used for heating
applications and also hot air balloons in the case of propane.
Naphtha The petroleum gases have four or five hydrocarbons in their
chain. For these distillates, hydrocarbon chains are made up of three
carbon atoms in the case of propane (C3H8), and four carbon atoms in the
case of butane (C4H10).
The middle distillate fuel oils
Oils below this point have a flashpoint of higher than 42°C and are
classified as fuel oils. These are the middle distillates as they form in
the mid-section of the tower.
Paraffin: Used for heating and burning in domestic applications is
paraffin and paraffin wax, which forms in the column at a range of
100°C to 175°C. Paraffin has between twenty and forty carbon
atoms. Paraffin wax is further processed to be used in industry to
create candles, petroleum jelly that protects skin and many other
Kerosene: While paraffin can also refer to kerosene, it comes as a
result of paraffin’s history as a domestic heating fuel, which
kerosene is now the default.
Diesel: Paraffin comes in both liquid and solid form and is used
throughout the UK in a wide range of industries. It’s an extremely
versatile product, and so has a wide range of uses including heating
oil, beauty products, candles and medicines.
At the bottom extremity of the fractioning tower, the lower distillates form.
These have high densities, higher boiling points and are not used as fuels,
but more as grease for lubrication. These form at temperatures of 350°C to
Bunker fuels and heavy fuel oil
Marine gas oil (MGO)
Heavy fuel oil (HFO)
Marine fuel blends
Marine diesel oil (MDO)
Intermediate fuel oil (IFO)
Marine fuel oil (MFO)
In between MGO and HFO are three additional maritime oils, MDO, IFO and
MFO. These are produced by blending MGO with HFO in varied
Bituminous coal is a soft, dense, black coal. Bituminous coal
often has bands of bright and dull material
in it. Bituminous coal is the most common coal and has a
moisture content less than 20 %. Bituminous
coal is used for generating electricity, making coke, and
Bituminous coal has calorific values ranging from 6.8 - 9
Often referred to as hard coal, anthracite is hard, black and
lustrous. Anthracite is low in sulphur and high
in carbon. It is the highest rank of coal. Moisture content
generally is less than 15 %.
Anthracite has calorific values of around 9 kW/kG or above
When it comes to petroleum products that are produced to
ultimately be burned, it’s essential that waste is absolutely
minimised, so at the bottom of the distillation tower, where
temperatures reach 500°C to 600°C, the residue of crude oil is
retrieved and put to use.
Residue formed at the bottom of the fractionating column includes
bitumen and asphalt.
Bitumen: Refined bitumen is made from the residue of petroleum
refining, where the column is around 600°C. You’ll have come across
it every day as it is an essential component of construction – used in
tarring roads, sealing roofs and many other applications.
Asphalt: Bitumen also forms naturally – known as crude bitumen.