It is a dark coloured oily liquid with offensive odour, found at various depths in many regions below the earth’s surface. It is also called rock oil, mineral oil or crude oil. It is covered by an atmosphere of a gaseous mixture known as natural gas.
It contains mainly alkanes, cycloalkanes, aromatic hydrocarbons, sulphur, nitrogen and oxygen compounds.
When subjected to fractional distillation, it gives different fractions at different temperatures.
LPG (Liquified Petroleum gas):
It is a mixture of butane and isobutane with a small amount of propane. A strong foul smelling substance, called ethyl mercaptan (C2H5SH) is added to LPG cylinders, to help in the detection of gas leakage.
CNG (Compressed Natural Gas):
It consists mainly of methane (95%), which is a relatively unreactive hydrocarbon and makes its nearly complete combustion possible.
Artificial Methods for Manufacturing Petrol:
From higher alkanes, petrol or gasoline is obtained by cracking or pyrolysis.
From coal, petrol can be synthesised by following two processes :
(i) Bergius process:
(ii) Fischer- Tropsch process:
The best catalyst for this process is a mixture of CO, thoria, magnesia and kieselguhr.
The overall yield in this process is slightly higher than Bergius process.
The quality of petrol is expressed in terms of octane number which is defined as the percentage of iso-octane by volume in a mixture of iso-octane and n-heptane which has the same antiknock properties as the fuel under test.
The octane number is 100 for iso-octane (2, 2, 4-trimethylpentane)
Natural gas has octane number 130.
TEL (tetraethyl lead) is used as antiknocking compound.
Octane number is increased by isomerisation, alkylation or aromatization.
Quality of diesel oils is measured in terms of cetane number which is defined as the percentage of cetane (hexadecane) by volume in a mixture of cetane and a-methyl naphthalene which has the same ignition property as fuel oil under similar experimental conditions.
It is 100 for cetane and 0 for α – methyl naphthalene.