The separation of a mixture using fractional distillation
or fractional crystallization. In the latter, the mixture is dissolved in
a hot solvent. Then, as the solution gradually
cools, the components of the mixture gradually crystallize out one at a
time – the least soluble on first.
|Hot crude oil is fed into the side of a tall column
(up to 70m high) divided at 1 meter intervals by a series of trays.
As the oil fills a tray, it overflows to the tray below, cascading
slowly down the column. Hot steam entering at the base of the column
rises up through the bubble caps in each tray, vaporizing part of
the oil. On each tray the hot vapor from the tray below vaporizes
some of the lower boiling material which rushes up the column. As
it rises it cools, allowing part to condense back to liquid. In this
way each tray separates a mixture with a slightly lower boiling point
than the tray below. The mixtures required are run off from trays
at different levels and passed to smaller columns (strippers) for
In fractional distillation, a liquid mixture is boiled and the vapor allowed
to pass up a long vertical condenser, called a fractionating column.
The lowest-boiling (most volatile) components in the mixture pass up to
the top of the column, while the least volatile ones remain near the bottom.
The various components – called fractions –
can be drawn off at the appropriate levels. The technique is the key process
in petroleum refining.