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The Chemistry and Technology of Petroleum
pressure range or where a separating agent that forms a pressure-sensitive azeotrope is
added to separate a pressure-insensitive azeotrope.
5. Reactive distillation where the separating agent reacts preferentially and reversibly with
one of the azeotropic constitutes. The reaction product is then distilled from the nonreacting components and the reaction is reversed to recover the initial component.
In simple distillation, a multi-component liquid mixture is slowly boiled in a heated zone and the
vapors are continuously removed as they form and, at any instant in time, the vapor is in equilibrium with the liquid remaining on the still. Because the vapor is always richer in the more volatile
components than the liquid, the liquid composition changes continuously with time, becoming more
and more concentrated in the least volatile species. A simple distillation residue curve is a means
by which the changes in the composition of the liquid residue curves on the pot changes over time
changes with time. A residue curve map is a collection of the liquid residue curves originating from
different initial compositions. Residue curve maps contain the same information as phase diagrams
but represent this information in a way that is more useful for understanding how to synthesize a
distillation sequence to separate a mixture.
All of the residue curves originate at the light (lowest boiling) pure component in a region, move
toward the intermediate boiling component, and end at the heavy (highest boiling) pure component
in the same region. The lowest temperature nodes are termed as unstable nodes, as all trajectories
leave from them; the highest temperature points in the region are termed stable nodes, as all trajectories ultimately reach them. The point that the trajectories approach from one direction and end
in a different direction (as always is the point of intermediate boiling component) is termed saddle
point. Residue curves that divide the composition space into different distillation regions are called
distillation boundaries.
Many different residue curve maps are possible when azeotropes are present. Ternary mixtures
containing only one azeotrope may exhibit six possible residue curve maps that differ by the binary
pair forming the azeotrope and by whether the azeotrope is minimum or maximum boiling. By
identifying the limiting separation achievable by distillation, residue curve maps are also useful in
synthesizing separation sequences combining distillation with other methods.
However, the separation of components of similar volatility may become economical if an
entrainer can be found that effectively changes the relative volatility. It is also desirable that the
entrainer be reasonably cheap, stable, nontoxic, and readily recoverable from the components. In
practice, it is probably this last criterion that severely limits the application of extractive and azeotropic distillation. The majority of successful processes, in fact, are those in which the entrainer and
one of the components separate into two liquid phases on cooling if direct recovery by distillation
is not feasible.
A further restriction in the selection of an azeotropic entrainer is that the boiling point of the
entrainer be in the range 10°C–40°C (18°F–72°F) below that of the components. Thus, although the
entrainer is more volatile than the components and distills off in the overhead product, it is present
in a sufficiently high concentration in the rectification section of the column.
17.5.6 extrACtIve dIstIllAtIon
Extractive distillation is the use of a third component to separate two close-boiling components
in which one of the original components in the mixture is extracted by the third component and
retained in the liquid phase to facilitate separation by distillation.
Using acetone–water as an extractive solvent for butanes and butenes, butane is removed as overhead from the extractive distillation column with acetone–water charged at a point close to the top
of the column. The bottoms product of butenes and the extractive solvent are fed to a second column
where the butenes are removed as overhead. The acetone–water solvent from the base of this column
is recycled to the first column. Extractive distillation may also be used for the continuous recovery
The Chemistry and Technology of Petroleum
pressure range or where a separating agent that forms a pressure-sensitive azeotrope is
added to separate a pressure-insensitive azeotrope.
5. Reactive distillation where the separating agent reacts preferentially and reversibly with
one of the azeotropic constitutes. The reaction product is then distilled from the nonreacting components and the reaction is reversed to recover the initial component.
In simple distillation, a multi-component liquid mixture is slowly boiled in a heated zone and the
vapors are continuously removed as they form and, at any instant in time, the vapor is in equilibrium with the liquid remaining on the still. Because the vapor is always richer in the more volatile
components than the liquid, the liquid composition changes continuously with time, becoming more
and more concentrated in the least volatile species. A simple distillation residue curve is a means
by which the changes in the composition of the liquid residue curves on the pot changes over time
changes with time. A residue curve map is a collection of the liquid residue curves originating from
different initial compositions. Residue curve maps contain the same information as phase diagrams
but represent this information in a way that is more useful for understanding how to synthesize a
distillation sequence to separate a mixture.
All of the residue curves originate at the light (lowest boiling) pure component in a region, move
toward the intermediate boiling component, and end at the heavy (highest boiling) pure component
in the same region. The lowest temperature nodes are termed as unstable nodes, as all trajectories
leave from them; the highest temperature points in the region are termed stable nodes, as all trajectories ultimately reach them. The point that the trajectories approach from one direction and end
in a different direction (as always is the point of intermediate boiling component) is termed saddle
point. Residue curves that divide the composition space into different distillation regions are called
distillation boundaries.
Many different residue curve maps are possible when azeotropes are present. Ternary mixtures
containing only one azeotrope may exhibit six possible residue curve maps that differ by the binary
pair forming the azeotrope and by whether the azeotrope is minimum or maximum boiling. By
identifying the limiting separation achievable by distillation, residue curve maps are also useful in
synthesizing separation sequences combining distillation with other methods.
However, the separation of components of similar volatility may become economical if an
entrainer can be found that effectively changes the relative volatility. It is also desirable that the
entrainer be reasonably cheap, stable, nontoxic, and readily recoverable from the components. In
practice, it is probably this last criterion that severely limits the application of extractive and azeotropic distillation. The majority of successful processes, in fact, are those in which the entrainer and
one of the components separate into two liquid phases on cooling if direct recovery by distillation
is not feasible.
A further restriction in the selection of an azeotropic entrainer is that the boiling point of the
entrainer be in the range 10°C–40°C (18°F–72°F) below that of the components. Thus, although the
entrainer is more volatile than the components and distills off in the overhead product, it is present
in a sufficiently high concentration in the rectification section of the column.
17.5.6 extrACtIve dIstIllAtIon
Extractive distillation is the use of a third component to separate two close-boiling components
in which one of the original components in the mixture is extracted by the third component and
retained in the liquid phase to facilitate separation by distillation.
Using acetone–water as an extractive solvent for butanes and butenes, butane is removed as overhead from the extractive distillation column with acetone–water charged at a point close to the top
of the column. The bottoms product of butenes and the extractive solvent are fed to a second column
where the butenes are removed as overhead. The acetone–water solvent from the base of this column
is recycled to the first column. Extractive distillation may also be used for the continuous recovery