218
The Chemistry and Technology of Petroleum
The actual observed boiling points during this distillation are, of course, much lower. Despite
the low pressure, the reboiler may have to be heated as high as 370°C (700°F) for such highboiling distillates.
From the actual boiling points obtained at reduced pressure, the so-called atmospheric equivalent temperatures (AETs) are calculated at which the material would boil under atmospheric pressure if it was stable and would not decompose. This provides a common basis for the categorization
and direct comparison of petroleum components across the entire volatility range accessible by
atmospheric and vacuum distillation.
Within the distillates, the physical and chemical properties are known to change only gradually
with the boiling point, allowing interpolations and even extrapolations with reasonable certainty.
Whenever the range of distillates is expanded, first by vacuum distillation, then by short-path distillation, the new distillate portions of the previously non-distillable residua follow the same patterns
as the previous distillates. The relationship of carbon-number or molecular weight, or of sulfur
and nitrogen concentrations, could be extended smoothly into the new regions. Such continuity is
no surprise but highly likely considering the nature of the petroleum precursors and the maturation chemistry. Thus, the data can be extrapolated across the dividing line between distillates and
residua, even into those ranges that are still beyond exact measurements.
As an alternative to empty column or spinning band distillations or to cut even deeper into the
stock, high-vacuum short-path distillation can be used. Other names for this technique are molecular distillation and/or wiped-film distillation although the latter is often performed with insufficient
vacuum and belongs then to a different category.
The most important feature of the short-path still is a very high vacuum of at least 10 −3 mmHg
that ensures that the mean-free-path length of the molecules in the gas phase is approximately
2–3 cm, which is the distance between the evaporator and the condenser surfaces. These conditions afford much lower distillation temperatures than possible in regular open columns at the same
surface temperature. The sample is spread into a very thin film on the evaporating surface for quick
evaporation and a short residence time. The combination of high vacuum, short distance, and short
residence time allows very deep distillation without decomposition. Modern versions of these shortpath stills can fractionate oils up to atmospheric equivalent boiling points of 700°C (1300°F) with
fairly high throughput rates. The small DISTACT laboratory short-path still, for example, has a rate
of about 100–800 mL/h with a residence time of less than a minute. Large production plants can
operate with throughputs of up to 300 L/m 2 /h.
9.2.3 AzeotroPIC And extrACtIve dIstIllAtIon
Chemical bonding between the components of the mixture creates properties unique to the mixture. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem
arises—the azeotropic composition limits the separation, and for a better separation this azeotrope
must be bypassed in some way. At the azeotropic point, the mixture contains the given component
in the same proportion as the vapor, so that evaporation does not change the purity, and distillation
does not affect separation. For example, ethyl alcohol and water form an azeotrope (azeotropic
mixture) at 78.2°C.
If the separation of individual components from petroleum itself or from petroleum products is
required, there are means by which this can be accomplished. For example, when a constant-boiling
mixture of hydrocarbons contains components whose vapor pressure is affected differently by the
addition of, say, a non-hydrocarbon compound, distillation of the hydrocarbon mixture in the presence of non-hydrocarbon additive may facilitate separation of the hydrocarbon components.
In general, the non-hydrocarbon additive is a polar organic compound and should also have
the ability to form a binary minimum constant-boiling (or azeotropic) mixture with each of the
hydrocarbons. Thus it is often possible to separate compounds that have very close boiling points
by means of azeotropic distillation.
The Chemistry and Technology of Petroleum
The actual observed boiling points during this distillation are, of course, much lower. Despite
the low pressure, the reboiler may have to be heated as high as 370°C (700°F) for such highboiling distillates.
From the actual boiling points obtained at reduced pressure, the so-called atmospheric equivalent temperatures (AETs) are calculated at which the material would boil under atmospheric pressure if it was stable and would not decompose. This provides a common basis for the categorization
and direct comparison of petroleum components across the entire volatility range accessible by
atmospheric and vacuum distillation.
Within the distillates, the physical and chemical properties are known to change only gradually
with the boiling point, allowing interpolations and even extrapolations with reasonable certainty.
Whenever the range of distillates is expanded, first by vacuum distillation, then by short-path distillation, the new distillate portions of the previously non-distillable residua follow the same patterns
as the previous distillates. The relationship of carbon-number or molecular weight, or of sulfur
and nitrogen concentrations, could be extended smoothly into the new regions. Such continuity is
no surprise but highly likely considering the nature of the petroleum precursors and the maturation chemistry. Thus, the data can be extrapolated across the dividing line between distillates and
residua, even into those ranges that are still beyond exact measurements.
As an alternative to empty column or spinning band distillations or to cut even deeper into the
stock, high-vacuum short-path distillation can be used. Other names for this technique are molecular distillation and/or wiped-film distillation although the latter is often performed with insufficient
vacuum and belongs then to a different category.
The most important feature of the short-path still is a very high vacuum of at least 10 −3 mmHg
that ensures that the mean-free-path length of the molecules in the gas phase is approximately
2–3 cm, which is the distance between the evaporator and the condenser surfaces. These conditions afford much lower distillation temperatures than possible in regular open columns at the same
surface temperature. The sample is spread into a very thin film on the evaporating surface for quick
evaporation and a short residence time. The combination of high vacuum, short distance, and short
residence time allows very deep distillation without decomposition. Modern versions of these shortpath stills can fractionate oils up to atmospheric equivalent boiling points of 700°C (1300°F) with
fairly high throughput rates. The small DISTACT laboratory short-path still, for example, has a rate
of about 100–800 mL/h with a residence time of less than a minute. Large production plants can
operate with throughputs of up to 300 L/m 2 /h.
9.2.3 AzeotroPIC And extrACtIve dIstIllAtIon
Chemical bonding between the components of the mixture creates properties unique to the mixture. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem
arises—the azeotropic composition limits the separation, and for a better separation this azeotrope
must be bypassed in some way. At the azeotropic point, the mixture contains the given component
in the same proportion as the vapor, so that evaporation does not change the purity, and distillation
does not affect separation. For example, ethyl alcohol and water form an azeotrope (azeotropic
mixture) at 78.2°C.
If the separation of individual components from petroleum itself or from petroleum products is
required, there are means by which this can be accomplished. For example, when a constant-boiling
mixture of hydrocarbons contains components whose vapor pressure is affected differently by the
addition of, say, a non-hydrocarbon compound, distillation of the hydrocarbon mixture in the presence of non-hydrocarbon additive may facilitate separation of the hydrocarbon components.
In general, the non-hydrocarbon additive is a polar organic compound and should also have
the ability to form a binary minimum constant-boiling (or azeotropic) mixture with each of the
hydrocarbons. Thus it is often possible to separate compounds that have very close boiling points
by means of azeotropic distillation.