C6.2j To be able
to describe the separation of crude oil by fractional distillation and to name
the fractions produced
Crude Oil Fractionation
Crude oil
contains a mixture of hydrocarbons.
Hydrocarbons
are molecules formed from just two elements hydrogen and carbon.
When crude
oil is heated to a high temperature say over 400oC the oil vaporises,
all the hydrocarbons enter the gaseous phase.
If this hot
mixture is gradually allowed to cool as it rises up a long stainless steel column
the different hydrocarbons will start to condense.
These hydrocarbons
start to condense because the further they are along the column from the source
of heat the cooler it is.
These
columns, which can be many meters high, are called fractionating towers or
fractionating columns. They are the
characteristic sight you see when looking at an oil refinery.
The
arrangement of the fractionating tower means that at a given distance from the
heat source the column will be at a specific temperature lower than 400oC.
At this
particular temperature say it is 120oC. a particular group of
hydrocarbons will condense because the temperature is not high enough for them
to stay in the vapour state but the rest of the hydrocarbon molecules (smaller
molecules) will remain in the vapour state and pass along and up the column.
At the 120oC
point on the column, a bubble plate is placed to accelerate separation of the
more volatile smaller molecules of hydrocarbon.
In fact at
several points or temperatures along the fractionating column are these bubble
plates to facilitate the separation of the different molecules or fractions of
the crude oil.
A bubble plate is a tray of bubble caps.
The picture
below shows the different primary fractions from the initial fractionation of
crude oil.
Please note
that fractions are NOT pure compounds but mixtures of hydrocarbons molecules of
different lengths.
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