•chemrevise.org
•19/08/2013
Fractional Distillation and Cracking
N Goalby
Chemrevise.org
Fractional Distillation
Petroleum is a mixture
consisting mainly of alkane
hydrocarbons
20° C
40° C
110° C
Crude
oil
Different components
(fractions) of this
mixture can be
drawn off at different
levels in a
fractionating column
because of the
temperature
gradient.
180° C
250° C
fuel gas (bottled)
petrol/gasoline
naptha (chemicals)
kerosene (jet fuel)
diesel oil
300° C
fuel oil (ship boilers)
340° C
Furnace
350° C
lubricating oils
bitumen
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Key points to remember about fractional
distillation
• Oil is pre-heated
• then passed into column.
• The fractions condense at different heights
•
•
•
•
•
The temperature of column decreases upwards
The separation depends on boiling point.
Boiling point depends on size of molecules.
Similar molecules (size, bp, mass) condense together
Small molecules condense at the top at lower
temperatures
• and big molecules condense at the bottom at higher
temperatures.
Vacuum Distillation
fuel gas (bottled)
Vacuum distillation unit
Lowering the pressure
over a liquid will lower
its boiling point. Heavy Crude
oil
residues from the
fractionating column
are distilled again
under a vacuum.
This means that the heavier
fractions can be further
separated without high
temperatures which would
break them down.
•
petrol/gasoline
naptha (chemicals)
kerosene (jet fuel)
diesel oil
Furnace
Click here for animation of distillation
fuel oil
lubricating
oils
Furnace
bitumen
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CRACKING
Involves the breaking of C-C bonds in alkanes
Economics
• The demand for some fractions exceeds supply from crude
oil distillation
• Alkenes (C2H4) and smaller alkanes have a high demand
but relatively low abundance
• The larger alkanes (C10H22) have a low demand but high
abundance
• Cracking turns the larger hydrocarbons into higher value
and more useful smaller hydocarbons
• cracking also saves having to extract more crude oil and
so preserves crude oil stocks.
Ethene is used to make polymers and ethanol
Smaller alkanes (octane) are used for petrol or fuels
THERMAL CRACKING
Conditions:
HIGH PRESSURE ... 7000 kPa
HIGH TEMPERATURE ... 400°C
to 900°C
Bonds can be broken anywhere in the molecule by C-C bond
fission and C-H bond fission.
Example
Equations
C8H18
C12H26
C6H14 + C2H4
C10H22 + C2H4
PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making
polymers and ethanol
PRODUCES HYDROGEN ... used in the Haber Process and in
margarine manufacture
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CATALYTIC CRACKING
Conditions:
SLIGHT PRESSURE
HIGH TEMPERATURE ... 450°C
ZEOLITE CATALYST
ZEOLITES are crystalline aluminosilicates; clay like substances
PRODUCES BRANCHED, and CYCLIC ALKANES, and
AROMATIC HYDROCARBONS
USED FOR PRODUCING MOTOR FUELS
Branched and cyclic hydrocarbons burn more cleanly and are used to
give fuels a higher octane number
Cheaper than thermal cracking because it saves energy as lower
temperatures and pressures are used
•
Click here for animation of a catalytic cracker
Cracking
Cracking involves breaking up large alkane molecules into smaller alkanes
and alkenes. A mixture of products is formed.
Catalyst of aluminium oxide Al2O3, and
a high temperature needed.
ceramic fibre
soaked in
paraffin
mixture of gaseous
products
Bunsen valve
(prevents suck back
aluminium oxide
granules
The gaseous
alkenes formed
will react with
bromine water.
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Examination Question
1. The petroleum and petrochemicals industries use a small
number of processes to produce a large number of final
products. Three of these processes are fractional distillation,
vacuum distillation and cracking.
(a)
Describe briefly how fractional distillation can be used to
convert crude oil into a small number of fractions.
(4)
(b)
The residue from the fractional distillation process is
usually vacuum distilled. This process allows the compounds
in the residue to boil at temperatures much lower than their
normal boiling points. State whether the sizes of the
molecules of the compounds in the residue are smaller or
larger than those in the other fractions and suggest why the
process of vacuum distillation is used.
(2)
Examination Question
(c) Cracking involves free radical reactions. It is carried out by
heating long-chain hydrocarbons, often in the presence of
steam and a catalyst. Steam cracking is used to produce a
mixture of alkanes and alkenes in reactions such as the
following:
C9H20
C6H14 + C3H6
(i) Give a name for each compound produced in this reaction. (2)
(ii)Nonane, C9H20, can be cracked to give other products. One
molecule of nonane can give two molecules of ethene, one
molecule of ethane and one molecule of another substance.
What is the molecular formula of the other substance?
(1)
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Examination Question
2. The petrochemicals industry produces a vast range of organic compounds
derived from crude oil. Some of the most important processes involve
simple molecules containing two carbon atoms.
(a) Give the name of the first stage in the refining of crude oil and state why this
process is used.
(2)
(b) Ethene can be produced by cracking alkanes. State two conditions used in
cracking processes and write an equation for a reaction in which ethene is
formed from butane.
(3)
3. Petrol is obtained, not only by fractional distillation of crude oil, but also by
cracking of hydrocarbons from heavy fractions.
State why hydrocarbons from heavy fractions are cracked and explain why
these hydrocarbons are less easy to ignite than those in petrol.
(4)
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