BP Educational Service
Science at Work
Fuels from crude oil
What is crude oil?
What is a fuel?
BP uses crude oil to make petrol, diesel
and other products.
Crude oil is formed from the remains of tiny marine
creatures. These creatures captured the Sun’s
energy through photosynthesis. The energy was
stored in their remains as chemical energy. Over
millions of years, these remains were transformed
into oil by heat and pressure.
Nearly every molecule in crude oil is made up of
just two elements: hydrogen and carbon. These
elements combine in chains to create molecules
called hydrocarbons. Because their chains are
different lengths, each type of hydrocarbon has
different properties.
Crude oil is not very useful in its raw form. It does
not release its energy very efficiently. Oil refineries
convert crude oil into more useful forms. They
separate the crude oil into its main parts and
process these into different products. Each product
needs the right mix of hydrocarbons. Some of the
most important products are fuels, including petrol
and diesel.
Fuels are substances that contain stored chemical
energy. Burning the fuel releases this energy.
The energy can be used to do useful work. Drivers
can use petrol as a fuel because it contains the
chemical energy stored for millions of years in
crude oil.
Filling up with petrol at the pump
1. Cars use the energy from petrol or diesel.
What form of energy does the fuel contain?
What other forms of energy is it
converted into?
1
Fractional distillation
When a liquid is heated, it evaporates. Some
liquids evaporate at different temperatures from
others: they have different boiling points.
Distillation is evaporation followed by
condensation. This process can separate a mixture
of liquids that have different boiling points.
Refineries use fractional distillation to separate
crude oil into its main groups of hydrocarbons,
or fractions. Each fraction contains molecules
with similar boiling points.
Fractions with lower boiling points condense
in trays near the top of the fractionating tower.
Fractions with higher boiling points condense
in trays nearer the bottom.
Each fraction is collected separately and processed
into finished products.
2. Why must the temperature in the
fractionating tower be hotter at the bottom
and cooler at the top?
A fractionating tower
2
Fractional distillation
Uses
<40°C
Liquified
petroleum gas
(LPG)
3
• Camping gas
Vapours rise
Bubble cap
Gas
• Industrial gas
40–210°C
Gasoline
• Petrol
40–190°C
Naphtha
• Chemical feedstock
190–270°C
Kerosene
• Jet fuel
• Heating oil
270–360°C
Gas oil
• Diesel oil
1
Pre-heated
crude oil
2
360–540°C
5
4
Heavy oils
and waxes
• Lubricating oils
• Waxes
• Industrial fuels
Liquids fall
>540°C
Residue
• Fuel oil
• Bitumen
Magnified view
of a bubble cap
A fractionating tower and its products
3
1
The crude oil is heated to about 350°C to
vaporise it.
2
It is pumped into the fractionating tower,
near the base.
3
The vaporised oil rises through the tower, which
is kept hot at the bottom and cooler at the top.
4
As the fractions cool, they condense
back to liquids.
5
These liquids collect in trays and are
piped away.
Products of
fractional distillation
Fractions contain molecules with different chain
lengths. The length of the chains determines how
the molecules behave. This gives each fraction
particular properties.
3. Why do you think bitumen cannot
be used as a fuel for cars?
The fractions are processed into products that use
these properties. Some of the most important
products are shown in the table.
Name
Boiling point
range
Average chain
length*
Description
Uses
Liquefied
petroleum gas
(LPG)
<40°C
3
A flammable gas; heavier than air
Heat and power in the refinery;
bottled gas for camping
and industry
Gasoline
40–210°C
8
Volatile, flammable liquid
Fuel in cars
Naphtha
40–190°C
10
Liquid
Chemical ‘feedstock’ (raw material)
used to make plastics, medicines
and other petrochemical products
Kerosene
190–270°C
12
Liquid
Jet fuel, heating oil
Gas oil
270–360°C
20
Heavier, thicker liquid;
less volatile
Fuel in cars, lorries, trains
and boats
Heavy oils
and waxes
360–540°C
35
Thick, viscous oils and
solid waxes
Lubricating oils; fuels for power
stations, industrial boilers and
large ships; waxes in candles and
waterproof food cartons
Residue
>540°C
40
Dark, very viscous liquid or solid;
soft and sticky when heated
Heavy fuel oils; bitumen –
a black solid at room temperature
that flows when heated and is
used for road surfaces and in
roofing materials
*Typical number of carbon atoms. Some hydrocarbon
molecules can also be ring-shaped.
4
Matching supply to demand
BP extracts oil in nearly 30 countries around the
world. Oil from different reservoirs was formed
under different conditions. As a result, the oil in
each reservoir is a unique mixture of hydrocarbons.
The nature of these mixtures determines how
much of each fraction the crude oil can provide.
C12H26
C8H18 + 2C2H4
kerosene
octane + ethene
o
heat (400–700 C) + catalyst
(powdered aluminium oxide)
BP customers tend to demand a different mix of
products from what is present in an oil reservoir.
For example, only a small part of North Sea oil can
be separated into the fraction that produces petrol.
However, this is one of BP’s most important
products in the UK.
Refineries match supply to demand in two ways:
1. They blend crude oils together. The final mixture
of oils contains more of the fractions that are
most in demand. These are usually the lighter
fractions.
2. They turn some of the heavier fractions into
lighter ones using a process called cracking.
This process breaks down longer hydrocarbon
chains into shorter ones.
Cracking of kerosene
(This diagram uses just one of the types of molecule found
in kerosene.)
Cracking is a thermal decomposition reaction. Long
hydrocarbon chains are broken down into shorter
chains by heat. A catalyst speeds up the process.
For example, kerosene is a mixture of molecules
that can be cracked into octane and ethane.
4. Why do you think cracking is called
a decomposition process?
Cracking allows refineries to make the most of
crude oil’s potential value. For example, the diesel
fraction in North American crude oil can be cracked
to produce petrol and ethene, which are much
more in demand than diesel itself.
‘We’re the problem-solvers,
the ones that answer the
phone calls from refineries
terminals and marketers
about fuels. Problem-solving
is like detective work.’
5. Why does cracking help to increase the
value of crude oil?
US manager of quality and
technical service, BP Global
Fuels Technology
5
Fuels and the environment
Fuels provide essential benefits to societies
across the world. Without fuels, people’s lives
would be very different. How would you travel?
How would food and other products get to the
supermarket shelves?
However, fuels can release a number of
substances into the environment when they
are used.
Carbon dioxide is a greenhouse gas and
contributes to climate change.
Sulphur dioxide is formed from sulphur impurities
in fuels. It dissolves in water high in the
atmosphere, which falls as acid rain.
Nitrogen oxides are formed because of the high
temperatures in car engines. These oxides can
contribute to acid rain, and to the smog that affects
some cities in hot weather.
BP Ultimate fuel delivery
Particulates are tiny particles of soot. They may
cause breathing problems in areas with high traffic
levels, and may contribute to smog.
6. How would your life change if we stopped
using fuels today?
Fuels provide unique benefits. But they are also a
unique resource: at the moment, there is no single
technology that could completely replace them and
prevent the emissions described above.
Scientists at BP and other organisations are
developing cleaner fuels that create fewer
emissions. One example is BP’s Ultimate fuels.
These fuels can remove sooty deposits from inside
car engines, which helps the engine to burn fuel
more efficiently. This reduces harmful
exhaust emissions.
Another way to reduce the impact of fuels on the
environment is to use them more efficiently. New
technologies, better designs and changes in our
everyday behaviour can all help.
6
Glossary
Acid rain – rainfall made so acidic by atmospheric
pollution that it can harm forests, lakes and rivers.
Combustion – the chemical reaction between a
fuel and oxygen that creates oxides and releases
energy.
Condense – when a substance changes from a
gas to a liquid.
Cracking – a thermal decomposition reaction in
which long hydrocarbon chains are broken down
into shorter chains by heat; a catalyst speeds up
the process.
Distillation – the separation of a mixture of liquids
with different boiling points, using evaporation and
condensation.
Energy – the ability to do work; has many forms,
including chemical, kinetic, potential, thermal and
light energy.
Evaporate – when a substance changes from a
liquid to a gas.
Fuel – a substance or material that can be burned
(combusted) to release energy.
BP Educational Service
PO Box 105,
Rochester, Kent, ME2 4BE
Tel: +44 (0) 871 472 3020
Fax: +44 (0) 871 472 3021
Email: [email protected]
Website: www.bp.com/bpes
© BP International Ltd 2008