The FRACTIONAL DISTILLATION OF OIL
and the USES of the FRACTIONS
Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes on
Oil, useful products, environmental problems, introduction to
2. Fractional distillation of crude oil in an oil refinery AND
the uses of fractions (related to their molecular
–This page describes the
separation of useful products from crude oil by the process of fractional
distillation, part of the oil refining process in the petrochemical industry. Crude oil provides the starting raw material for making lots of
different chemicals for a variety of uses. The uses of the fractions from
fractional distillation fuel gas, LPG, refinery gas, gasoline, petrol,
naphtha, paraffin, kerosene, diesel oil, gas oil, fuel oil, lubricating oils,
wax and bitumen fractions are tabulated and many are non–renewable fuels. The uses of a fraction is related to
its physical properties e.g. ease of vaporisation & boiling point or its
viscosity ('stickiness') and the dangers of flammability are pointed out too. There
is also a discussion on what makes a good fuel and reference to alternative
fuels. These notes on fractional distillation of oil and the uses of oil fractions are
designed to meet the highest standards of knowledge and understanding required
for students/pupils doing GCSE chemistry, IGCSE chemistry, O
Level chemistry and KS4 science courses. These revision notes on ??? should prove useful for the
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SEPARATION of the crude oil mixture into fractions
by fractional distillation
the USES of
oil is a finite resource found in rocks. Crude oil is the remains of an ancient
biomass consisting mainly of plankton that was buried in mud and subjected to
heat and pressure that slowly end up as a yellow to brown liquid which may form
pools or be absorbed into porous rocks like shale.
Crude oil is an important raw material, and the source of
many useful substances such as fuels and a chemical feedstock for the
petrochemical industry, from which endless products, including plastics and
drugs, are eventually manufactured, BUT, it is a finite resource, and won't last
Many useful materials on which modern life depends are
produced by the petrochemical industry, such as fuels, solvents, lubricants,
polymers, detergents and host of other specialised chemicals - even drugs
for medicinal formulations.
When initially pumped out of the ground crude oil is a
complex mixture of a very large number of compounds most of which are hydrocarbons, molecules composed of hydrogen and carbon atoms only and many of
them are hydrocarbons called alkanes (a particular series of organic
compounds, that compounds based on carbon - details in other sections).
The hydrocarbons may consist of molecules based on chains of
carbon atoms (mostly 1 to 40), sometimes linear ('straight') and sometimes
branched (a side-chain of C atoms) and others are based on rings of carbon
Straight chain or linear alkane hydrocarbons: ,
Branched alkane hydrocarbons:
Hydrocarbon ring compounds
Full explanation of the
structure of the homologous series of alkanes and don't worry about the
The complex mixture of hydrocarbons in crude oil can be separated into fractions by
the technique of fractional distillation.
Crude oil cannot be used directly but must be
refined before commercially useful products are produced by the
petrochemical industry (collectively called petrochemicals).
The oil refining process principally involves
fractional distillation into useful fractions i.e. products with
specific uses, but further processing may needed to diversify both the quantity
and nature of particular oil based products.
A fraction is a
mixture of liquids (in this case hydrocarbons) with a relatively narrow (restricted)
boiling point range of molecules, i.e. those with a similar number of carbon
atoms in the chain.
Within each fraction obtained from crude oil the
hydrocarbon molecules have a similar number of carbon atoms and
similar physical properties.
The uses of the fractions very much depends on their physical properties,
which in turn are dependant on the length of the molecule i.e. the carbon atom
chain in a hydrocarbon molecule.
molecules are only made of a chemical combination of carbon and hydrogen atoms.
They are compounds
because they consist of atoms of at least two different elements.
All the bonding is
covalent C–C or C–H bonds
What goes on in an oil refinery?
Crude oil is a complex mixture
of many compounds, but mainly
hydrocarbon compound molecules.
- A mixture consists of two or more
compounds which are NOT chemically combined.
- The chemical
properties of each substance in the mixture is unchanged as the there are
no chemical bonds between the hydrocarbon molecules.
- Therefore a mixture can be separated
quite easily by physical
means eg fractional distillation.
- See notes on
This means crude
oil can be separated by physical methods, in this case by fractional
distillation, because they have different boiling and condensation points.
- The liquids must also be completely
soluble in each other, that is they must all be miscible liquids.
- When the temperature is high enough, the
kinetic energy of a particular hydrocarbon molecule will be sufficient
for it to escape the intermolecular forces in the liquid and become a
- The intermolecular forces are much
weaker than the strong carbon - carbon bonds in the hydrocarbon
molecule, so it vaporises without decomposes.
At the bottom of the fractionating column the crude oil is heated to vapourise it
(evaporated or boiled) and the vapour passed into the fractionating
column – a large construction of many levels and pipes, see the 'simple'
- A fractionating column acts in the same
way as a fractional distillation apparatus in the school/college laboratory but on
an industrial scale!
- In an oil refinery the fractionating
columns are very tall with huge surface area to give the best chance of
separating the dozens of hydrocarbons in the crude oil (see diagram on
This is a continuous process (not a
batch process). The fractionating column works continuously with
heated–vapourised crude oil piped in at the bottom and the various
fractions condensed and constantly tapped off from various levels, each
with a different condensation temperature range.
Up the fractioning column the temperature
gradually decreases (temperature gradient), so the highest boiling
(least volatile) molecules tend to be at the bottom and the lowest
boiling (most volatile) hydrocarbons go to the top. The rest of the
hydrocarbon molecules then condense out in narrow temperature range i.e. the
different fractions condense out in a gradual way from top to bottom
depending on their boiling point.
In other words the
most volatile fraction, i.e. the molecules with the lowest boiling points
(shortest hydrocarbon molecules),
boil or evaporate off first and go higher up the column and condense out at
the higher levels in the fractionating column at the lowest temperature.
The higher the boiling point (the higher the
condensation point) the lower down the column the hydrocarbon condenses.
So all of the hydrocarbon molecules separate
out according to their boiling/condensation point so that the highest boiling fraction,
i.e. the less volatile molecules with higher boiling points
(longest hydrocarbon molecules), tend to condense
more easily lower down the column, albeit at the higher temperatures.
The process is perhaps more correctly called
fractional condensation but it is still referred to as fractional
The bigger the molecule, the greater the
intermolecular attractive forces between the molecules, so the higher the boiling
point or condensation point
(see physical property trends).
- This is an important rule to know since
the intermolecular forces (intermolecular bonding) affect the physical
properties including melting point and viscosity too, and this has a
bearing on how each fraction is used, see below.
Note: Covalent chemical bonds like C–C
or C–H are
not broken in the process, only the intermolecular force of attraction
is weakened to allow the initial evaporation or boiling and this.
The fractions are then further processed to produce
fuels and chemical feedstock for the petrochemical industry.
These include fuels such as liquified petroleum gas,
petrol, diesel oil, kerosene, heavy fuel oil which are all non-renewable
fossil fuels, as is methane from natural gas.
From the chemical feedstock and petrochemical industry
we produce many useful materials on which our modern life depends e.g.
solvents, lubricants, medicines, polymers, detergents etc.
The fractions are listed below with the approximate
boiling point ranges and approximate number of carbon atoms in the
THE FRACTIONAL DISTILLATION
OF CRUDE OIL
fractions at the different
condensation levels (% in crude
Number of C atoms in the hydrocarbon molecule fraction
The approximate boiling range in oC of
USES of the fraction
Many are useful fuels - alkane hydrocarbons, but they
non-renewable fossil fuels - specific use depends on physical
properties (see later)
A simplified diagram of a fractionating
column used in the fractional distillation of crude oil
The decrease and increase trends for
the hydrocarbon molecules are given on the left of fractionating column
Fuel Gas, LPG, refinery gas
1 to 4
mainly propane and butane gases
which can be compressed or liquified
CH4 (domestic heating), ethane another gaseous
fuel, C3–4 easily liquefied petroleum gas, portable energy source e.g. bottled gas for
heating and cooking
(butane), higher pressure cylinders (propane), feedstock for other organic
Gasoline – petrol
5 to 7
25 to 75oC
easily vaporised, highly flammable, easily ignited, car fuel – petrol
C 6 to 10
75 to 190oC
no good as a fuel, but valuable raw material source of organic chemicals to make
other things, cracked to make more petrol and alkenes
10 to 16
190 to 250oC
less volatile, less flammable than petrol, domestic central heating fuel, (paraffin) aircraft jet fuel
Diesel oil, gas oil
14 to 20
250 to 350oC
less volatile than petrol,
diesel fuel for some cars and larger vehicle like haulage trucks, trains, central heating fuel,
cracked to make more petrol and alkenes
Heavy fuel oil, heating oil, lubricating oil, greases
C >20 to ~30
not so easily evaporated, not as flammable, safe to store, liquid fuel oil
for power stations and ships, quite viscous (sticky) and can also be used
for lubricating oils (lubricants, 'mineral oils') and greases.
RESIDUE – fuel oil, lubricating oils, waxes
C >30, maybe up to several hundred
high boiling liquids or low melting solids, that boil over 350oC
bitumen components boil over a 500oC - 700oC
melting solids used as candle wax, clear
waxes and polishes (can be dyed) AND the biggest
bitumen/asphalt – low melting solid used on roads as it
forms a thick, black, tough and resistant adhesive surface on cooling, used as
a roofing waterproofing material (it sticks rock
chips on roofs or road surfaces)
2b. a mental picture of the
increasing length of hydrocarbon molecules will help you understand more about
how, and why, the physical properties of hydrocarbon molecules changes with
increasing length AND how their physical properties affect how each fraction is
used commercially after the fractional distillation of crude oil.
Note that the longer the hydrocarbon molecule,
the more flexible or wiggly it gets!
on relating the physical properties of the hydrocarbon fractions to
their uses and dangers
The important physical
properties of hydrocarbons like alkanes all depend on the forces between
the molecules - the intermolecular forces (intermolecular
bonding). This is NOT covalent bonding, covalent bonds are the much
stronger C-C and C-H bonds between the atoms in the molecule itself.
Quite simply, with important consequences (e.g. how they are used), the
bigger the molecule the bigger these weak electrical attractive
forces are, which I've done my best to illustrate in the two
diagrams (right and below). As the hydrocarbon molecule gets bigger, the
surface to surface contact area increases allowing more points for the
intermolecular bonding attractive forces to happen. Hence the increase
in viscosity, melting points and boiling points with increase in
molecular mass. The intermolecular bonds are much weaker than the
covalent bonds, so when the hydrocarbon molecules have enough kinetic
energy, it is the intermolecular forces which are overcome on
vapourisation and the molecules stays intact.
The different fractions
are a range of physical properties which
vary with molecular size. Down the list
above (and below) the
longer the carbon chain, the bigger the molecule gets ...
... the more viscous
the molecule (stickiness! less runny, more sticky)
as the intermolecular
attractive forces between molecules
increases the bigger the molecule
in a series of molecules of similar structure.
Note for advanced level
students: Intermolecular forces or intermolecular bonding, are non–polar weak electrical attractive forces,
often described as Van der Waals forces, and correctly described
as instantaneous dipole – induced dipole
... the molecule has a higher
melting point as more vibrational kinetic energy is
needed to overcome the increasing intermolecular attractive forces holding the
molecules together to form the crystals which increases with increase in
size of molecule.
... the molecule has
a higher boiling
point as more particle kinetic energy is
needed to overcome the increasing intermolecular forces between the liquid molecules.
3. follows from 2. ie the intermolecular forces increase between the
hydrocarbon molecules increases as they get bigger (longer carbon chain).
molecule is less
flammable as they become less volatile, again due to
increasing intermolecular forces with increasing size of molecule so for
example, petrol (small molecules) is much more flammable than lubricating oil
(much bigger molecules, much longer carbon chain).
This raises health and safety issues about handling,
distributing and storing flammable hydrocarbons. The smallest
molecules (natural gas to petrol) are the most volatile and
therefore the most easily ignited. Any naked flame or spark could
set off a fire and explosion and even.
We can now apply these ideas
to explain these hydrocarbons are used, usually as fuels.
- Further comments on the use of
the fractions related to the use of the hydrocarbons from crude oil
- The examples are discussed in
order of increasing molecular size – increase in carbon chain
- Note that all fuels are processed at
the oil refinery to reduce the concentration of sulfur/sulphur
compounds (desulfurisation/desulphurisation) to reduce the
air pollution effects on burning.
- It should be noted that liquid
fuels like petrol, diesel, central heating oil etc. are east to
store and distribute to wherever they are need in homes or
factories and they are so readily available, that change may be
necessary, but it will be slow.
- See notes on pollution, global
CH4 Methane natural gas, either
from gas fields (eg under North Sea) or from an oil refinery can be
piped to power electricity generation or domestic heating in the
very low boiling refinery gas fractions,
can be stored under
pressure as bottled gas, and because the gas readily flows
under the control of a simple valve, they can be conveniently pumped to burner systems.
Being gases are easily
ignited but explosive ! The intermolecular forces are very
weak in a gas, that's why they are gases at room temperature, and
they are fluid just like a liquid, so easily moved through pipes.
intermolecular forces are now great enough to raise the boiling
point of the hydrocarbons to above room temperature and make them
liquids at room temperature. Vehicle fuels like petrol
must be liquid
at room temperature for
compact and convenient storage but they must be easily vapourised
(low boiling point) to
mix with air in the engine prior to ignition. However, the ease of
vaporisation due to weak intermolecular forces, does however make them
highly flammable !
- C10H22 to C16H34
Paraffin and kerosene are bigger molecules, bigger intermolecular forces, so less
flammable. This makes safer to use in domestic heating and jet
aircraft fuel, but not as easily ignited.
- C14H30 to C20H42 Diesel
oil is not as volatile, flammable or as easily ignited as petrol
due to the higher boiling point and intermolecular forces. However
as a vehicle fuel it doesn't have to be vapourised first, the
diesel fuel is sprayed into the engine cylinder and mixed with air
and ignites under compression from the piston action.
- C14H30 to C20H42
Fuel oil molecules are getting quite big, higher boiling points,
higher intermolecular forces and more viscous, but not too viscous to pump
to a ships boiler or locomotive diesel engine. Fuel oil is not very volatile
and so not as flammable and dangerous to use as petrol or diesel etc.
(typically 36 C atoms, C36H74) Lubricating oil must be quite viscous
to stick onto surfaces and are virtually non-volatile due to the
even higher intermolecular forces. Smaller molecules might be more runny but
they would evaporate away! It is also water repellent and helps
reduce corrosion on moving metal parts from factory machines to cars
(typically 31 C atoms, C31H64) Candle wax is very convenient as a
solid for a humble lamp (especially in power cuts!), but via a wick,
the heat from the flame is sufficient to vaporise the hydrocarbons
to burn them and give a big enough luminous yellow flame to act as a
source of light. At this point in the fractions the intermolecular
forces are now sufficient to create solids at room temperature,
albeit low melting like candle wax.
C60H122 (many much bigger) Bitumen is a water repellent solid at
room temperature but is readily melted (sometimes too easily in hot
weather). Used as base for a road chipping top surface or sometimes
directly as the top layer in roads. Bitumen is also used to make waterproof roofing felt.
Bitumen consists of the largest molecules in crude oil with the
greatest intermolecular forces between them, so they are solid at
room temperature and very high boiling up to 700oC.
- For more products derived from
crude oil other than fuels see ...
2c. Energy resource evaluation
- What makes a good
It may seem a curious question these
days, but its only fair to consider what is the best fossil fuel?
some fossil fuels pollute more than others, and some leave a bigger
Factors that should
be taken into consideration - often factors overlap
Combustion - burning
characteristics (see also toxicity, pollution and climate
Ease of ignition
- how easily does it burn, gas most easy to ignite and control, less
so for petrol/diesel and coal the most difficult to ignite and
control. Examples described on previous section 2b. on uses.
kJ of heat energy released per kg, sometimes referred to as the
'energy density', the bigger the better.
Burning hydrocarbons doesn't produce must waste other than the
gaseous products, but burning coal produce smoke and large
quantities of ash.
Geographical convenience - is it imported?, fluctuations in oil production levels
and the market price.
The price of oil can vary
with market forces determined by the World's economy AND political
instability and wars, particularly in the major oil producing
Middle-East Arab Gulf states affect the price too.
alternative energy resources, we are at the mercy of forces beyond our
If stocks or production
rates fall, the price of crude oil rises and richer countries can afford
more costly oil and can stockpile it, developing countries will struggle
In order to preserve our
crude oil and gas supplies we sometimes compromise our ideals when
dealing with the politics of countries we may think unsuitable.
distribution: Important health
and safety issues to consider here e.g.
Coal is very dirty but safe
to store, difficult to ignite,
Natural gas (explosive
flammable gas) much more dangerous to store in large tanks under
pressure, but the gas is easy and
more convenient to distribute via pipes, but gas leaks are
potentially VERY dangerous, as you see sometimes on the news!
Petrol is quite volatile
and the flammable vapour easily ignited.
Diesel is not as
volatile and central heating oil even less so, but spills of any of
these fuels is potentially harmful to the environment or a fire
Again, some specific examples
were described in the previous section 2b.
Costs of exploration and
extraction can be high for oil
Coal mines are dangerous
to operate, good health and safety policies don't come cheaply, and
its the same for operating oilfields and petrochemical complexes -
Costs of transporting
AND even after
considering all of these factors, as already mentioned ...
and climate change:
Greenhouse effect - which
fuel produces the least or most carbon dioxide for the energy
released?, methane is one of the best fossil fuels in this respect.
The sulphur content of fuel (most removed before fuel used to minimise
sulphur dioxide and acid rain formation), some coals are very high
in sulfur, but can't be removed from it, though you can clean the
smoke by removing the sulfur dioxide using an alkali like calcium
The efficiency of combustion e.g.
minimum carbon monoxide and soot levels, again methane is one of the
most clean burning fuels.
Coal produces a lot of
smoke and larger hydrocarbon fuels like oil, tend to burn with a
Ease of use:
Transferred easily e.g. oil and gas readily piped around and readily
ignited for a quick start in power station. Coal is more trouble to
transport and does not ignite as easily.
- Each fossil fuel has a different cost, efficiency and cleanliness
- Generally speaking in 'cleanliness' the order is
(natural gas) > alkanes in petrol > heavy oil ...
- and from left
to right there is also an increase in C/H atom ratio in the molecule so
more CO2 is produced too.
2d. Alternative fuels to fossil fuels
- more on other energy resources
OIL IS A VALUABLE
CHEMICAL RESOURCE but non-renewable
Apart from the obvious value
of crude oil as an non-renewable energy source, should we
using this very valuable source of organic chemicals by merely burning
most of it?
AND how long will oil reserves
AND what happens if
the oil runs out?
Therefore isn't it in our
own interest to manage the finite oil reserves remaining and conserve
AND seek other sources of
energy to power our lives?
AND perhaps influence the
course of global warming?
BUT the trouble is, oil is
very convenient, readily available, and new reserves are still being
found and the oil and gas trapped in deep layers of shale are being
AND it will take time to
develop new technologies.
The world's population is
steadily increasing and countries like China and India have huge energy
demands for home and industry, and are pressured into building many
fossil fuel power stations, though China is a leading player in large
scale wind power projects.
It should be noted that
liquid fuels like petrol, diesel, central heating oil etc. are east to
store and distribute to wherever they are need in homes or factories and
they are so readily available, that change may be necessary, but
progress will be slow.
HYDROGEN - FUEL
of the FUTURE?:
can be used as fuel and a long-term possible alternative to fossil
It burns with
a pale blue flame in air reacting with oxygen to be oxidised to
It is a
non-polluting clean fuel since the only combustion product is
water and so its use would not lead to all environmental problems
associated with burning fossil fuels.
It is easily
distributed in pipes like natural gas, but there are health and safety
issues to do with storage and distribution since it is, like natural
gas, highly flammable and explosive.
It would be
ideal if it could be manufactured by electrolysis of water e.g.
using solar voltaic-cells or some kind of but the technology is in
be used to power
cells see the "Extra Electrochemistry" page
renewable fuels are discussed on other pages ...
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Useful products from
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pollutants etc ...
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Crude Oil and its Fractional distillation
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