477
Distillation
of individual aromatics, such as benzene, toluene, or xylene(s), from the appropriate petroleum fractions. Prefractionation concentrates a single aromatic cut into a close-boiling cut, after which the
aromatic concentrate is distilled with a solvent (usually phenol) for benzene or toluene recovery.
Mixed cresylic acids (cresols and methylphenols) are used as the solvent for xylene recovery.
Extractive distillation is successful, because the solvent is specially chosen to interact differently
with the components of the original mixture, thereby altering their relative volatilities. Because
these interactions occur predominantly in the liquid phase, the solvent is continuously added near
the top of the extractive distillation column so that an appreciable amount is present in the liquid
phase on all of the trays below. The mixture to be separated is added through second feed point
further down the column. In the extractive column, the component having the greater volatility,
not necessarily the component having the lowest boiling point, is taken overhead as a relatively
pure distillate. The other component leaves with the solvent via the column bottoms. The solvent is
separated from the remaining components in a second distillation column and then recycled back
to the first column.
One of the most important steps in developing a successful (economical) extractive distillation
sequence is selecting a good solvent. In general, selection criteria for the solvent include the following: (1) the solvent should enhance significantly the natural relative volatility of the key component,
(2) the solvent should not require an excessive ratio of solvent to non-solvent (because of cost of
handling in the column and auxiliary equipment, (3) the solvent should remain soluble in the feed
components and should not lead to the formation of two phases, (4) the solvent should be easily
separable from the bottom product, (5) the solvent should be inexpensive and readily available,
(6) the solvent should be stable at the temperature of the distillation and solvent separation, (7) the
solvent should be non-reactive with the components in the feed mixture, and (8) the solvent should
be noncorrosive and nontoxic.
No single solvent or solvent mixture satisfies all of the criteria for use in extractive distillation.
However, the following solvent selection criteria assist in choosing the best possible solvent:
1. Screen by functional group or chemical family.
a. Select candidate solvent from the high-boiling homologous series of both light and
heavy key components.
b. Select candidate solvents from groups that tend to give positive (or no) deviations from
Raoult’s law for the key component desire in the distillate and negative (or no) deviations for the other key.
c. Select solvents that are likely to cause the formation of hydrogen bonds with the key component to be removed in the bottoms, or disruption of hydrogen bonds with the key to be
removed in the distillate. Formation and disruption of hydrogen bonds are often associated with strong negative and positive deviations, respectively, from Raoult’s law.
d. Select candidate solvents from chemical groups that tend to show higher polarity than
one key component or lower polarity than the other key.
2. Identify the individual candidate solvents.
a. Select only candidate solvents that boil at least 30°C–40°C above the key components
to ensure that the solvent is relatively nonvolatile and remains largely in the liquid
phase. With this boiling point difference, the solvent should also not form azeotropes
with the other components.
b. Rank the candidate solvents according to their selectivity at infinite dilution.
c. Rank the candidate solvents by the increase in relative volatility caused by the addition
of the solvent.
Residue curve maps are of limited use at the preliminary screening stage, because there is usually
insufficient information available to sketch the them, but they are valuable and should be sketched
or calculated as part of the second stage of the solvent selection.
Distillation
of individual aromatics, such as benzene, toluene, or xylene(s), from the appropriate petroleum fractions. Prefractionation concentrates a single aromatic cut into a close-boiling cut, after which the
aromatic concentrate is distilled with a solvent (usually phenol) for benzene or toluene recovery.
Mixed cresylic acids (cresols and methylphenols) are used as the solvent for xylene recovery.
Extractive distillation is successful, because the solvent is specially chosen to interact differently
with the components of the original mixture, thereby altering their relative volatilities. Because
these interactions occur predominantly in the liquid phase, the solvent is continuously added near
the top of the extractive distillation column so that an appreciable amount is present in the liquid
phase on all of the trays below. The mixture to be separated is added through second feed point
further down the column. In the extractive column, the component having the greater volatility,
not necessarily the component having the lowest boiling point, is taken overhead as a relatively
pure distillate. The other component leaves with the solvent via the column bottoms. The solvent is
separated from the remaining components in a second distillation column and then recycled back
to the first column.
One of the most important steps in developing a successful (economical) extractive distillation
sequence is selecting a good solvent. In general, selection criteria for the solvent include the following: (1) the solvent should enhance significantly the natural relative volatility of the key component,
(2) the solvent should not require an excessive ratio of solvent to non-solvent (because of cost of
handling in the column and auxiliary equipment, (3) the solvent should remain soluble in the feed
components and should not lead to the formation of two phases, (4) the solvent should be easily
separable from the bottom product, (5) the solvent should be inexpensive and readily available,
(6) the solvent should be stable at the temperature of the distillation and solvent separation, (7) the
solvent should be non-reactive with the components in the feed mixture, and (8) the solvent should
be noncorrosive and nontoxic.
No single solvent or solvent mixture satisfies all of the criteria for use in extractive distillation.
However, the following solvent selection criteria assist in choosing the best possible solvent:
1. Screen by functional group or chemical family.
a. Select candidate solvent from the high-boiling homologous series of both light and
heavy key components.
b. Select candidate solvents from groups that tend to give positive (or no) deviations from
Raoult’s law for the key component desire in the distillate and negative (or no) deviations for the other key.
c. Select solvents that are likely to cause the formation of hydrogen bonds with the key component to be removed in the bottoms, or disruption of hydrogen bonds with the key to be
removed in the distillate. Formation and disruption of hydrogen bonds are often associated with strong negative and positive deviations, respectively, from Raoult’s law.
d. Select candidate solvents from chemical groups that tend to show higher polarity than
one key component or lower polarity than the other key.
2. Identify the individual candidate solvents.
a. Select only candidate solvents that boil at least 30°C–40°C above the key components
to ensure that the solvent is relatively nonvolatile and remains largely in the liquid
phase. With this boiling point difference, the solvent should also not form azeotropes
with the other components.
b. Rank the candidate solvents according to their selectivity at infinite dilution.
c. Rank the candidate solvents by the increase in relative volatility caused by the addition
of the solvent.
Residue curve maps are of limited use at the preliminary screening stage, because there is usually
insufficient information available to sketch the them, but they are valuable and should be sketched
or calculated as part of the second stage of the solvent selection.