475
Distillation
(propane and propylene) from the lighter gases is accomplished by further distillation in a fractional
distillation tower similar to those previously described and particularly designed to handle liquefied
gases. Further separation of hydrocarbon gases is required for petrochemical manufacture.
17.5.4 suPerFrACtIonAtIon
The term superfractionation is sometimes applied to a highly efficient fractionating tower used to
separate ordinary petroleum products. For example, to increase the yield of furnace fuel oil, heavy
naphtha may be redistilled in a tower that is capable of making a better separation of the naphtha
and the fuel oil components. The latter, obtained as a bottom product, is diverted to furnace fuel oil.
Fractional distillation as normally carried out in a refinery does not completely separate one
petroleum fraction from another. One product overlaps another, depending on the efficiency of the
fractionation, which in turn depends on the number of trays in the tower, the amount of reflux used,
and the rate of distillation. Kerosene, for example, normally contains a small percentage of hydrocarbons that (according to their boiling points) belong in the naphtha fraction and a small percentage that should be in the gas oil fraction. Complete separation is not required for the ordinary uses
of these materials, but certain materials, such as solvents for particular purposes (hexane, heptane,
and aromatics), are required as essentially pure compounds. Since they occur in mixtures of hydrocarbons they must be separated by distillation and with no overlap of one hydrocarbon with another.
This requires highly efficient fractional distillation towers specially designed for the purpose and
referred to as superfractionators. Several towers with 50–100 trays operated with a high reflux ratio
may be required to separate a single compound with the necessary purity.
17.5.5 AzeotroPIC dIstIllAtIon
Azeotropic distillation is the use of a third component to separate two close-boiling components
by means of the formation of an azeotropic mixture between one of the original components and
the third component to increase the difference in the boiling points and facilitates separation by
distillation.
All compounds have definite boiling temperatures, but a mixture of chemically dissimilar compounds sometimes causes one or both of the components to boil at a temperature other than that
expected. For example, benzene boils at 80°C (176°F), but if it is mixed with hexane, it distills at
69°C (156°F). A mixture that boils at a temperature lower than the boiling point of either of the
components is called an azeotropic mixture.
Two main types of azeotropes exist, that is, the homogeneous azeotrope, where a single liquid
phase is in the equilibrium with a vapor phase, and the heterogeneous azeotropes, where the overall
liquid composition that forms two liquid phases is identical to the vapor composition. Most methods
of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen
chemicals to facilitate the separation.
The five methods for separating azeotropic mixtures are the following:
1. Extractive distillation and homogeneous azeotropic distillation where the liquid separating agent is completely miscible.
2. Heterogeneous azeotropic distillation, or more commonly, azeotropic distillation, where
the liquid separating agent (the entrainer) forms one or more azeotropes with the other
components in the mixture and causes two liquid phases to exist over a wide range of compositions. This immiscibility is the key to making the distillation sequence work.
3. Distillation using ionic salts. The salts dissociate in the liquid mixture and alter the relative volatilities sufficiently that the separation become possible.
4. Pressure-swing distillation where a series of column operating at different pressures are
used to separate binary azeotropes that change appreciably in composition over a moderate
Distillation
(propane and propylene) from the lighter gases is accomplished by further distillation in a fractional
distillation tower similar to those previously described and particularly designed to handle liquefied
gases. Further separation of hydrocarbon gases is required for petrochemical manufacture.
17.5.4 suPerFrACtIonAtIon
The term superfractionation is sometimes applied to a highly efficient fractionating tower used to
separate ordinary petroleum products. For example, to increase the yield of furnace fuel oil, heavy
naphtha may be redistilled in a tower that is capable of making a better separation of the naphtha
and the fuel oil components. The latter, obtained as a bottom product, is diverted to furnace fuel oil.
Fractional distillation as normally carried out in a refinery does not completely separate one
petroleum fraction from another. One product overlaps another, depending on the efficiency of the
fractionation, which in turn depends on the number of trays in the tower, the amount of reflux used,
and the rate of distillation. Kerosene, for example, normally contains a small percentage of hydrocarbons that (according to their boiling points) belong in the naphtha fraction and a small percentage that should be in the gas oil fraction. Complete separation is not required for the ordinary uses
of these materials, but certain materials, such as solvents for particular purposes (hexane, heptane,
and aromatics), are required as essentially pure compounds. Since they occur in mixtures of hydrocarbons they must be separated by distillation and with no overlap of one hydrocarbon with another.
This requires highly efficient fractional distillation towers specially designed for the purpose and
referred to as superfractionators. Several towers with 50–100 trays operated with a high reflux ratio
may be required to separate a single compound with the necessary purity.
17.5.5 AzeotroPIC dIstIllAtIon
Azeotropic distillation is the use of a third component to separate two close-boiling components
by means of the formation of an azeotropic mixture between one of the original components and
the third component to increase the difference in the boiling points and facilitates separation by
distillation.
All compounds have definite boiling temperatures, but a mixture of chemically dissimilar compounds sometimes causes one or both of the components to boil at a temperature other than that
expected. For example, benzene boils at 80°C (176°F), but if it is mixed with hexane, it distills at
69°C (156°F). A mixture that boils at a temperature lower than the boiling point of either of the
components is called an azeotropic mixture.
Two main types of azeotropes exist, that is, the homogeneous azeotrope, where a single liquid
phase is in the equilibrium with a vapor phase, and the heterogeneous azeotropes, where the overall
liquid composition that forms two liquid phases is identical to the vapor composition. Most methods
of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen
chemicals to facilitate the separation.
The five methods for separating azeotropic mixtures are the following:
1. Extractive distillation and homogeneous azeotropic distillation where the liquid separating agent is completely miscible.
2. Heterogeneous azeotropic distillation, or more commonly, azeotropic distillation, where
the liquid separating agent (the entrainer) forms one or more azeotropes with the other
components in the mixture and causes two liquid phases to exist over a wide range of compositions. This immiscibility is the key to making the distillation sequence work.
3. Distillation using ionic salts. The salts dissociate in the liquid mixture and alter the relative volatilities sufficiently that the separation become possible.
4. Pressure-swing distillation where a series of column operating at different pressures are
used to separate binary azeotropes that change appreciably in composition over a moderate