239
Fractional Composition
poisons are nonvolatile and, hence, the asphaltene fraction contains most of the coke-forming constituents and catalyst poisons that were originally present in heavy oil or in a residuum.
One of the early findings of composition studies was that the behavior and properties of any
material are dictated by composition (Speight, 2000). Although the early studies were primarily
focused on the composition and behavior of asphalt, the techniques developed for those investigations have provided an excellent means of studying heavy feedstocks (Tissot, 1984). Later studies
have focused not only on the composition of petroleum and its major operational fractions but on
further fractionation, which allows different feedstocks to be compared on a relative basis to provide
a very simple but convenient feedstock map.
Such a map does not give any indication of the complex interrelationships of the various fractions
(Koots and Speight, 1975), although predictions of feedstock behavior are possible using such data.
It is necessary to take the composition studies one step further using sub-fractionation of the major
fractions to obtain a more representative indication of petroleum composition.
Thus by careful selection of an appropriate technique it is possible to obtain an overview of
petroleum composition that can be used for behavioral predictions. By taking the approach one
step further and by assiduous collection of various sub-fractions it becomes possible to develop
the petroleum map and add an extra dimension to compositional studies (Long and Speight, 1989).
Petroleum and heavy feedstocks then appear more as a continuum than as four specific fractions.
Such a concept has also been applied to the asphaltene fraction of petroleum (Long, 1981) in
which asphaltenes are considered a complex state of matter based on molecular weight and polarity
(Long, 1981). The advantage of such a concept is that it can be used to explain differences in asphaltene yield with different hydrocarbons (pentane and heptane) and also differences in the character
of asphaltenes from petroleum and from coal liquids.
Furthermore, petroleum can be viewed as consisting of two continuous distributions, one of
molecular weight and the other of molecular type. Using data from molecular weight studies and
elemental analyses, the number of nitrogen and sulfur atoms in the aromatic and polar aromatic
fractions was also exhibited. These data showed that not only can every molecule in the resins and
asphaltenes have more than one sulfur or nitrogen but also some molecules probably contain both
sulfur and nitrogen. As the molecular weight of the aromatic fraction decreases, the sulfur and nitrogen contents of the fractions also decrease. In contrast to the sulfur-containing molecules, which
appear in both the naphthene aromatics and the polar aromatics fractions, the oxygen compounds
present in the heavy fractions of petroleum are normally found in the polar aromatics fraction.
Other work (Long and Speight, 1989) involved the development of a different type of compositional map using the molecular weight distribution and the molecular type distribution as
coordinates. The separation involved the use of an adsorbent such as clay, and the fractions were
characterized by solubility parameter as a measure of the polarity of the molecular types. The
molecular weight distribution can be determined by gel permeation chromatography. Using these
two distributions, a map of composition can be prepared using molecular weight and solubility
parameter as the coordinates for plotting the two distributions. Such a composition map can provide
insights into many separation and conversion processes used in petroleum refining.
The molecular type was characterized by the polarity of the molecules, as measured by the
increasing adsorption strength of an adsorbent. At the time of the original concept, it was unclear
how to characterize the continuum in molecular type or polarity. For this reason, the molecular type
coordinate of their first maps was the yield of the molecular types ranked in order of increasing
polarity. However, this type of map can be somewhat misleading because the areas are not related
to the amounts of material in a given type. The horizontal distance on the plot is a measure of the
yield and there is not a continuous variation in polarity for the horizontal coordinate. It was suggested that the solubility parameter of the different fractions could be used to characterize of both
polarity and adsorption strength.
In order to attempt to remove some of these potential ambiguities, more recent developments of
this concept have focused on the solubility parameter. The simplest map that can be derived using
Fractional Composition
poisons are nonvolatile and, hence, the asphaltene fraction contains most of the coke-forming constituents and catalyst poisons that were originally present in heavy oil or in a residuum.
One of the early findings of composition studies was that the behavior and properties of any
material are dictated by composition (Speight, 2000). Although the early studies were primarily
focused on the composition and behavior of asphalt, the techniques developed for those investigations have provided an excellent means of studying heavy feedstocks (Tissot, 1984). Later studies
have focused not only on the composition of petroleum and its major operational fractions but on
further fractionation, which allows different feedstocks to be compared on a relative basis to provide
a very simple but convenient feedstock map.
Such a map does not give any indication of the complex interrelationships of the various fractions
(Koots and Speight, 1975), although predictions of feedstock behavior are possible using such data.
It is necessary to take the composition studies one step further using sub-fractionation of the major
fractions to obtain a more representative indication of petroleum composition.
Thus by careful selection of an appropriate technique it is possible to obtain an overview of
petroleum composition that can be used for behavioral predictions. By taking the approach one
step further and by assiduous collection of various sub-fractions it becomes possible to develop
the petroleum map and add an extra dimension to compositional studies (Long and Speight, 1989).
Petroleum and heavy feedstocks then appear more as a continuum than as four specific fractions.
Such a concept has also been applied to the asphaltene fraction of petroleum (Long, 1981) in
which asphaltenes are considered a complex state of matter based on molecular weight and polarity
(Long, 1981). The advantage of such a concept is that it can be used to explain differences in asphaltene yield with different hydrocarbons (pentane and heptane) and also differences in the character
of asphaltenes from petroleum and from coal liquids.
Furthermore, petroleum can be viewed as consisting of two continuous distributions, one of
molecular weight and the other of molecular type. Using data from molecular weight studies and
elemental analyses, the number of nitrogen and sulfur atoms in the aromatic and polar aromatic
fractions was also exhibited. These data showed that not only can every molecule in the resins and
asphaltenes have more than one sulfur or nitrogen but also some molecules probably contain both
sulfur and nitrogen. As the molecular weight of the aromatic fraction decreases, the sulfur and nitrogen contents of the fractions also decrease. In contrast to the sulfur-containing molecules, which
appear in both the naphthene aromatics and the polar aromatics fractions, the oxygen compounds
present in the heavy fractions of petroleum are normally found in the polar aromatics fraction.
Other work (Long and Speight, 1989) involved the development of a different type of compositional map using the molecular weight distribution and the molecular type distribution as
coordinates. The separation involved the use of an adsorbent such as clay, and the fractions were
characterized by solubility parameter as a measure of the polarity of the molecular types. The
molecular weight distribution can be determined by gel permeation chromatography. Using these
two distributions, a map of composition can be prepared using molecular weight and solubility
parameter as the coordinates for plotting the two distributions. Such a composition map can provide
insights into many separation and conversion processes used in petroleum refining.
The molecular type was characterized by the polarity of the molecules, as measured by the
increasing adsorption strength of an adsorbent. At the time of the original concept, it was unclear
how to characterize the continuum in molecular type or polarity. For this reason, the molecular type
coordinate of their first maps was the yield of the molecular types ranked in order of increasing
polarity. However, this type of map can be somewhat misleading because the areas are not related
to the amounts of material in a given type. The horizontal distance on the plot is a measure of the
yield and there is not a continuous variation in polarity for the horizontal coordinate. It was suggested that the solubility parameter of the different fractions could be used to characterize of both
polarity and adsorption strength.
In order to attempt to remove some of these potential ambiguities, more recent developments of
this concept have focused on the solubility parameter. The simplest map that can be derived using