Download File - Garbally Chemistry

Chapter21
Petrol
Crude oil
• Crude oil is a fossil fuel.
• It was formed from the bodies of tiny sea
creatures which died millions of years ago.
• Bacteria turned the bodies of these creatures
into crude oil and natural gas.
Petrol
Petrol is formed by the fractional distillation of crude oil.
Some of the different fractions formed are:
 Petroleum Gas - Metacarpans added for smell.
 Petrol
 Naphta - Petrochemical Industry
 Kerosene - Aviation Fuel.
 Diesel.
 Lubricating Oil.
Octane Number
Knocking - early explosion due to petrol-air mixture. The petrol explodes
as it is compressed instead of being caused by a spark. (Auto-ignition).
Octane Number - Measure of the tendency of fuels to resist knocking.
The octane number of a fuel can be measured by comparing the
performance of 2, 2-4 tri-methyl pentane (iso-octane) which has an
octane number of 100 and Heptane which is not efficient and has an
octane number of 0.
The shorter the alkane the higher the octane number.
The more branched the chain the higher the octane number.
Cyclic compounds have a higher octane number than straight chain
compounds.
Octane Number
Making Petrol
Tetra-ethyl lead -Reduces knocking. However it caused health problems
and is poisonous.
4 ways to increase octane number.
1. Isomerisation. Branching caused by alkanes heated in the presence of a
suitable catalyst.
2. Catalytic Cracking. Long chained hydrocarbons broken down into short
chain molecules for which there is great demand.Large alkanes form
alkanes and alkenes.
3. Reforming (DeHydrocyclisation.) Reforming involves the use of catalysts
to form ring compounds.-Hydrogen given off.
4. Adding Oxygenates. Three oxygen containing molecules, methanol,
ethanol, and MTBE are added to petrol to increase its octane number.
MTBE stands for methyl tertiary butyl ether. (2-methoxy-2methylpropane)