Petrochemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 51

Among the C8 fraction, ethylbenzene (136.2 ∘ C), p-xylene (138.3 ∘ C),
m-xylene (139.1 ∘ C) and o-xylene (144.4 ∘ C), have similar boiling points.
Other separation processes are often used [1].
One of these is azeotropic distillation. In an azeotrope, the composition of
the liquid and the vapor is the same. If a liquid is at the azeotropic composi-
tion, it cannot be purified by distillation. In an ideal mixture, the tendency for
a component to vaporize is the same in the mixture as in the pure component.
However in non-ideal mixtures, the intermolecular forces between the
components are different than in the pure liquid. This happens with ethanol
and water. If you were to take a 50% (by mass) ethanol mixture and boil it, the
composition of the vapor would be enriched in the lower boiling ethanol. Con-
densation of the vapor would result in a composition greater than 50% ethanol.
Redistillation might increase the ethanol composition further. However, when
we reach 95.6% ethanol, the composition of the vapor is the same as the com-
position of the liquid. Therefore, distillation of 95.6% ethanol does not result
in an increase in the percentage of the lower boiling ethanol in the condensed
vapor. Rather, the condensed vapor is 95.6% ethanol. This mixture is known
as a constant boiling mixture or an azeotropic mixture. The boiling point of
this particular mixture is lower than that of either pure water or pure ethanol,
so this is called a low boiling azeotrope. For other mixtures, we can have
high boiling azeotropes, where the boiling point of the mixture is higher than
either of the components. To this point, we have discussed two-component
or binary azeotropes, but recognize that they can be more complicated and
can have three (ternary azeotropes) or even more components.
One separation process takes advantage of azeotropes. An auxiliary agent
such as acetone or methanol is added to a mixture of alkanes, cycloalkanes,
and predominantly aromatics. The agent forms a low boiling azeotrope with
the alkanes and cycloalkanes thereby enabling their removal from the aro-
matics. The remaining agent is then often extracted from the aromatic high
boiling stream and recycled.
Another process that is sometimes used is extractive distillation [2, 3].
The concept of extractive distillation is that a non-volatile polar solvent such
as sulfolane is added, which has a different effect on the volatility of the
components of the hydrocarbon mixture. Typically, the solvent decreases
the volatility of the aromatic compounds, making them easier to separate
from the aliphatics. The solvent is continuously added near the top of the
extractive distillation column. Aliphatics are removed overhead and solvent
and aromatics from the bottom of the column. The aromatics can be separated
from the extractive solvent by distillation in a solvent recovery column. The
boiling point of sulfolane (285 ∘ C) is significantly higher than even o-xylene
(144.4 ∘ C) enabling the isolation of the aromatic stream. Optionally, this can
be further purified by a water wash.