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Crude oil is a complicated mixture of hydrocarbons, with a
varying composition depending on its source. The hydrocarbons in crude oil
have different boiling points, according to the number of carbon atoms
their molecules contain and how they are arranged. Fractional distillation
uses the difference in boiling point to separate the hydrocarbons in crude
oil. The fractionating column is cooler at the top than the bottom, so the vapours cool as they rise.
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Vapours condense onto a tray when they reach the part of
the column which is cooler than their boiling point. The bubble caps
(figure 17) help to slow down the vapours has
they rise through the column. As the last gases from the bottom of the
column pass through the holes in a tray, any lighter hydrocarbons still in
the condensed liquid are boiled off, and rise through the column.
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