As we have seen, cracking, isomerisation and reforming produces lots of
superior fuel molecules compared to those originally in crude oil - or
rather, there were insufficient suitable fuels molecules in the crude oil.
Petrol producers have to mix a variety of hydrocarbons with
the right properties to produce a suitable fuel for road vehicles.
There are two important properties to take into
consideration:
(i) The
octane rating of the fuel mixture
The octane rating is the fuels ability to resist
auto-ignition, that causes 'knocking'.
Octane number is explained in the next section and
the higher the octane number the better!
The branched alkanes and cyclic compounds
(alicyclic or aromatic) have the highest octane numbers,
compared straight chain unbranched alkanes.
(detailed discussion in the next section)
(ii)
The volatility of the
hydrocarbons
Every liquid exerts a vapour pressure in the atmosphere
above its surface. The maximum vapour pressure depends on the
temperature of the liquid and rises exponentially with increase in
temperature.
The diagram on the right shows typical saturated vapour pressure curves
of maximum pvap (mmHg) versus
temperature - in this case for tetrachloromethane CCl4, ethanol
C2H5OH,
benzene C6H6, water H2O
and ethanoic acid CH3COOH.
These are relatively volatile compounds, and this vapour pressure
behaviour is relevant to hydrocarbons which can exert similar vapour
pressures at room temperature.
The vapour pressure a liquid exerts is a measure of how volatile
a liquid is and this is relevant to a discussion about the design of
petrol mixture which must take into account a wide range of ambient
temperatures.
Relatively small branched alkanes have lower boiling
points than long unbranched (linear) alkanes.
This mean the former hydrocarbons are more volatile
and vaporize more easily.
For example, the highly
branched 2,2,4-trimethylpentane ('isooctane')
(CH3)3CCH2CH(CH3)2
has boiling point of 99oC,
whereas for the same molecular formula (C8H18),
the linear non-branched isomer octane ('n-octane')
CH3(CH2)6CH3
has a much higher boiling point of
126oC.
See isomerism page for
Explanation on why the intermolecular forces
are weaker in the branched isomer
Petrol must contain the appropriate mixture that
gives the appropriate volatility required.
The exact mixture ('blend') differs around the world
and also varies with the seasons.
The petrol blend must maintain, as far as is
practical, a reasonably constant volatility.
This means winter blends, at a lower temperature,
must be more volatile than summer blends - which would not be
volatile enough in winter, making the car engine more difficult to
start.
Conversely, a winter blend would vapourise to easily
in warmer summer temperatures causing vapour lock.
Vapour lock happens when the 'too volatile' fuel
boils in your carburetor or your fuel line.
The vaporized fuel creates back pressure in your
fuel system and prevents gas from getting to your engine, which
would stall.
So, to be an efficient fuel. a petrol mixture must have
the appropriate volatility and high octane number.
When petrol is injected into the cylinders of a combustion
engine, it should not ignite until a spark is produced with precise
timing appropriate to the rotary motion of the engine cycle.
If the fuel ignites prematurely, you can hear a
characteristic knocking sound, which is indicative of an inefficient
under performing engine. The knocking can also physically damage the
engine due to the extra vibration produced.
This effect is called auto-ignition and is caused by
the high temperature compression of the petrol vapour in the engine
cylinders.
The octane rating of a fuel molecule (or petrol mixture)
is a measure of how likely it is too auto-ignite.
The higher the octane rating, the less likely is the fuel
to auto-ignite and cause knocking.
Different hydrocarbons have different octane ratings.
Therefore, the different hydrocarbons are blended
together to give a petrol mixture the appropriate octane rating -
the mixture with the best antiknock performance.
Linear unbranched alkanes tend to have lower octane
ratings, a higher tendency to auto-ignite, than branched alkanes,
cyclic alkanes (alicyclic) and aromatic hydrocarbons like benzene.
Linear heptane (C7H16), far to readily
auto-ignites and has a defined
octane rating of 0 - not good!
Highly branched 2,2,4-trimethylpentane (C8H18) has
a defined octane rating of 100
(very good), on the arbitrarily defined octane rating number scale.
The octane rating of petrol in the UK is usually 95, but you
can pay a bit more for higher octane rated petrol.
If you take hydrocarbons you can do an interesting
comparison of the octane rating of several molecules,
I've deliberately quoted octane ratings for
structural isomers (carbon chain isomers) e.g.
(a)
pentane (C5H12) has an octane rating of 62
isomeric
2-methylbutane (C5H12) has an octane rating of
93
In (a) you can see introducing branching
considerably increases the octane rating.
(b)
linear (unbranched) hexane (C6H14), octane rating
25
isomeric
2-methypentane (C6H14) has an octane rating of
73
and
3-methypentane (C6H14) has an octane rating of
75/86? (data sources differ)
As in (a), in (b) you can see introducing branching
considerably increases the octane rating.
These branched isomers are produced in
reforming processes.
(c)
linear heptane (C7H16) has an octane number of 0.
isomeric
3-methylhexane has an octane rating of 65.
and
2,3-dimethylpentane (C7H16) has an octane
rating of 91
Again, in (c) you can see introducing branching
considerably increases the octane rating.
You can also see that the greater the branching of
the isomer, the higher the octane number.
These branched isomers are produced in
reforming processes.
(d)
cyclic
alkane (alicyclic) cyclohexane (C6H12), octane
rating 83
aromatic benzene (C6H6), octane rating 106
These hydrocarbons are produced in
reforming processes. from the linear alkane hexane.
(e)
methylcyclohexane (C7H14) has an octane rating
of 70
methylbenzene (C7H8) has an octane number
of 120
These hydrocarbons are produced in
reforming processes. from the linear alkane heptane.
From (c) (d) and (e) you can see that moving from a
linear alkane to cyclic alkane and then to a cyclic aromatic
hydrocarbon greatly increases the octane rating at each stage in the
reforming process.
Branched and cyclic alkane and aromatic hydrocarbon
compounds are important components in petrol mixtures designed to produce
the cleanest most efficient burning, with good antiknock properties,
particularly as lead tetraethyl is now banned.
You should now appreciate much more one important
consequence of cracking crude oil fractions.
