Oil Refining + Octane Numbers
Fractional Distillation of Crude Oil
• How to become a millionaire .......... Find a
crude oil well and know how to fraction it!!
• Crude oil found in large quantities in Iran ,
USA, Nigeria, Russia, North Sea
Fossils
• Crude oil was formed from the bodies of tiny
sea creatures that died millions of years ago
• Crude oil is pumped from underneath the
ground as is a thick black substance with an
unpleasant smell
• It does not burn easily and must be undergo
fractional distillation to separate out all the
useful components
• Fractional Distillation
• The crude oil is heated in a furnace and starts to
evaporate, the longer carbon chains are heavier
and do not rise as easily as shorter (lighter)
carbon chains
• This means that the longer chains carbon
fractions are collected first while the shorter
chains get collected higher up the column
• Remember larger hydrocarbons have higher
boiling points while smaller hydrocarbons have
lower boiling points
Fractionating
Column
NB. You do not need
to memorise the
temperatures but you
do need to learn the
fractions and the uses
of the products of
each fraction
Fractionating Column at Whitegate Oil
Refinery
Refinery Gas
• Top of the column,
methane, ethane, propane
+ butane are gases at 25⁰C
• Some used as fuel mostly
bottled for sale
• Since these are odourless
sulfur compounds called
Metacarptants are added
for safety
• Bottled gas is mainly a mix
of propane + butane which
are liquefied under high
pressure
Petrol (Light Gasoline)
• Used as motor fuel
• A mixture of at least
100 compounds mostly
hydrocarbons of 5-10
carbons
Naphta
• Very useful to
petrochemical industry
• It is a source for a huge
number of useful
chemicals eg,
medicines, plastics,
synthetic fibres,
detergents, solvents
etc.
• 7-10 carbons in length
Kerosene/Parrafin
• Used as aviation fuel
and in certain lamps
• 10-14 carbon length
• NB as the number of
carbon atoms increases
so does the boiling
point as more heat is
required to break the
larger number of bonds
Diesel Oil/GasOil
• More difficult to
vaporise than petrol
• Therefore diesel engine
has a different design
• Trucks, buses, trains
and some cars use
diesel
• 14-19 carbon length
Lubricating Oil
• 19-35 carbons in length
• Cannot be vaporised
easily so cannot be
used as a fuel
• Used as a lubricant to
reduce wear and tear
Fuel Oil
• 30-40 carbons in length
• Used in ships, power
stations, heating plants,
oil heating in homes
Bitumen
• More than 35 carbon
atoms in length
• Very high boiling point
• Used in Tar to resurface
roads
When an oil well is discovered..
• Refinery Chemists analyse crude oil to
determine the quality of the oil and assess the
quantity of each product in the oil
Octane Number
• Petrol obtained from the fractionating column
is not a very efficient fuel
• In the internal combustion engine in a car the
power to turn the wheels is produced by the
explosive reaction between air and petrol
Knocking
• Smooth running of an engine depends on this
explosion occurring at exactly the right time
• If the explosion occurs too early the pistons
vibrate and a metallic noise is heard from the
engine
• Early explosion can occur if the petrol + air
explode when they are compressed instead of
waiting for the spark
• This early explosion is referred to as knocking
or auto-ignition
• Straight chain alkanes such as nonane, octane
and heptane ignite very easily and explode too
soon
• Branched cahin alkanes such as 2,2,4trimethylpentane (iso-octane) do not tend to
auto-ignite
• Petrol mixtures with large amounts of
branched chain alkanes are more efficient
than those which contain straight chain
molecules
• To indicate the efficiency of a particular type
of petrol a number called the Octane Number
is assigned to it
Octane Number
• The octane number of a fuel is a measure of the
tendency of the fuel to resist knocking
• Since 2,2,4-trimethylpentane is a very efficient
fuel it is assigned an octane number of 100
• Heptane is not an efficient fuel and is assigned an
octane number of 0
• The octane number of a fuel is calculated by
comparing it’s efficiency in an engine to 2,2,4trimethylpentane and heptane
What should my petrol be??
• Chemical analysis can also indicate the octane
number of a fuel
• Good quality petrol should have an octane
number of 97
• The shorter the alkane chain the higher the
octane number
• Which do you think would have a higher
octane number hexane or pentane?
• The more branched the chain the higher the
octane number
• Which do you think would be the best fuel
heptane,
3-methylhexane,
2,3,-dimethylpentane
• Cyclic compounds have a higher octane
number than straight chain compounds
• Which is a better fuel? Hexane or cyclohexane??
Making Petrol
• 1920’s it was found that
adding small amounts of
lead compounds to petrol
helped reduce the amount
of knocking
• Lead pollution from car
exhausts had potential
damage to health and was
phased out since 2000
• Lead also damaged
catalytic converter in car
exhausts
• Now have unleaded petrol
Increasing octane number of fuel
• This is now achieved in one of the following 4
ways;
1. Isomerisation
2. Catalytic Cracking
3. Reforming
4. Adding Oxygenates
Isomerisation
• This involves changing straight chain alkanes into
their isomers
• Alkanes are heated in the presence of a suitable
catalyst and the chains break
• The chains are allowed to reconnect but they are
more likely to reform in branched-chains than in
straight-chains
• This is commonly done on a large industrial scale
with pentane and hexane (NB fig 21.29)
Catalytic Cracking
• Catalytic cracking is the breaking down of long
chain hydrocarbon molecules into short chain
molecules for which there is greater demand
• In oil refineries the heavier fractions ( such as fuel
oil, diesel oil and kerosene) are heated in the
presence of a catalyst
• The short chain alkanes produced tend to be
highly branched and hence have a high octane
number
Reforming (Dehydrocyclisation)
• Reforming involves the use of catalysts to form ring
compounds
• Straight chain alkanes are converted to cycloalkanes
and these are converted to aromatic compounds
• Aromatic compounds have high octane numbers and
petrol contains 3-4% benzene, because benzene is a
carcinogen this is a cause for concern
• Hydrogen may also be produced in reforming reaction,
this is a useful substance and may be piped away for
various purposes
Adding Oxygenates
• An oxygenate is any fuel that contains oxygen in its
molecules
• 3 oxygen containing compounds, methanol, ethanol
and MTBE are commonly added to petrol to increase
its octane number
• Another advantage to adding oxygenates to fuel is that
they cause very little pollution when they burn and are
cleaner fuels