CRUDE OIL. Definition and Formation of Crude Oil Fractional Distillation of Crude Oil Cracking and Reforming of Crude Oil Fractions Impact of the Oil Industry on the Environment . Definition and Formation of Crude Oil. Crude oil is a complex mixture of naturally
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occurring hydrocarbons found within the earth.
It consists mainly of a variety of alkanes,
cycloalkanes and aromatic hydrocarbons.
microscopic plants and animals that were buried
and preserved in rocks millions of years ago.
deeper within layers of rocks and with time and
temperature they were converted to crude oil.
depends on its source. Most crude oil looks
like thin, brown treacle (molasses), while some
are almost colourless volatile(easily evaporated at
normal temp.) liquids and others are thick black
and selectively modified before it can be useful.
This process is called refining and occurs in an
oil refinery. The primary process for separating the
components of crude oil is fractional distillation.
point ranges are shown in the table below.
Cracking is a process in which large hydrocarbon
molecules are broken down into smaller molecules.
For example, dodecane could break into decane and ethene:
C₁₂H₂₆ C₁₀H₂₂ + C₂H₄
or into nonane and propene:
C₁₂H₂₆ C₉H₂₀ + C₃H₆
Cracking is carried out either using heat (thermal cracking)
or a catalyst (catalytic cracking).
Thermal cracking involves rapidly heating the
hydrocarbon to temperatures of about 800⁰C and
then cooling it. This process occurs within a second.
The high temperatures can cause the C - C bond
to undergo homolytic fission, leaving each carbon
with a single unpaired electron. For example,
octane could break down into a hexyl radical and
an ethyl radical:
The raised dot ( ̇) indicates an unpaired electron.
These free radicals can then undergo further reactions.
For example, they can lose a hydrogen atom to form
a stable molecule. The ethyl radical becomes ethene
in such a reaction:
C₂H₅ ̇ C₂H₄ + H ̇
with the decyl radical to form decane.
H˙ + C₁₀H₂₁˙ C₁₀H₂₂
Or two hydrogen atoms can combine together to form
H ̇ + H ̇ H₂
Thermal cracking is generally used for cracking the
residue fractionof crude oil.
Catalytic cracking (cat-cracking) involves the use of
a catalyst at lower temperatures to break the bonds
of the hydrocarbon molecules. The reaction is
usually catalyzed by a powdered mixture of
alumina and silica (Al₂O₃/SiO₂) at about 500⁰C.
The C-C bond undergoes heterolytic fission which
results in a mechanism involving carbocations.
fractions such as diesel oil and kerosene into
smaller molecules. The cracked compounds are
important to produce more valuable fuel as well
a chemical feedstock for the petrochemical industry.
1.) a). Briefly describe the principles involved in
i). fractional distillation (3 marks)
ii). cracking. (2 marks)
b). Give a balanced equation to illustrate cracking.
c). Explain the importance of EITHER fractional
distillation OR cracking. (1 mark)
d). Suggest ONE adverse effect associated with the extraction
of crude oil and comment on the environmental problems
that result. (3 marks)