Learning Experience 1.2
Oil is essential to our lives and lifestyles – not just for mobility and
heat but for thousands of products which we use or encounter every
day.
Crude oil is of little use when it first comes out of the ground or from
deep below the seabed. It has to be processed and turned into useful
products at an oil refinery.
Not all crude oils are the same. Some are thick and tarry, for example
some from South America, whilst others are lighter with lower density,
such as some from the North West Shelf of Australia.
How does an oil refinery turn crude oil into the
useful products we use every day?
Before we learn about the process of fractional distillation it is
important to look at what crude oil is actually composed of.
Crude oil is a mixture of many different hydrocarbons which vary in
length. The larger the hydrocarbon molecule:





the
the
the
the
the
more carbon atoms in its chain
higher its boiling point
less volatile it is
less easily it flows (viscous)
less easily it ignites (flammable)
In order to turn crude oil into useful products we need to separate the
different hydrocarbons into fractions. A particular fraction contains
hydrocarbons of similar size, with boiling points in a specific range.
For example, fractional distillation produces a ‘gasoline’ fraction which
contains hydrocarbons with 8 carbon atoms, and a kerosene fraction
which contains hydrocarbons with 15 carbon atoms.
ACTIVITY
Write definitions for the following terms:
hydrocarbon:
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fraction:
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boiling point:
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The process of fractional distillation at an oil
refinery
Because the various components of crude oil have different boiling
temperatures, they can be separated by a process called fractional
distillation. The steps of fractional distillation are as follows:
1. You heat the crude oil to a high temperature.
2. The mixture boils, forming vapour (gases); most substances go
into the vapor phase.
3. The vapour enters the bottom of a long column (fractional
distillation column) that is filled with trays or plates.
1. The trays have many holes or bubble caps (like a
loosened cap on a soda bottle) in them to allow the
vapor to pass through.
2. The trays increase the contact time between the vapor
and the liquids in the column.
3. The trays help to collect liquids that form at various
heights in the column.
4. There is a temperature difference across the column
(hot at the bottom, cool at the top).
2. The vapour rises in the column.
3. As the vapour rises through the trays in the column, it cools.
4. When a substance in the vapor reaches a height where the
temperature of the column is equal to that substance's boiling
point, it will condense to form a liquid. (The substance with the
lowest boiling point will condense at the highest point in the
column; substances with higher boiling points will condense lower
in the column.).
5. The trays collect the various liquid fractions.
Source: “How Stuff Works” http://science.howstuffworks.com/oil-refining4.htm
What must be done to the crude oil before it enters the fractioning
tower?
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Describe the fractioning tower in terms of temperature.
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Do larger or smaller hydrocarbons have a higher boiling point?
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What type of hydrocarbons turn back into liquids at the bottom of the
tower?
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What happens to the hydrocarbons that have lower boiling points?
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At what point do the very short chained hydrocarbons exit the
fractioning tower?
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