383
Instability and Incompatibility
Bottle tests constitute the predominant test method, and the test conditions have varied in volume,
type of glass or metal, vented and unvented containers, and type of bottle closure. Other procedures
have involved stirred reactor vessels under both air pressure and oxygen pressure, and small volumes
of fuel employing a cover slip for solid deposition (ASTM D4625). All of these procedures are gravimetric in nature.
There are several accelerated fuel stability tests that can be represented as a time–temperature
matrix (Goetzinger et al., 1983; Hazlett, 1992). A graphical representation shows that the majority of
the stability tests depicted fall close to the solid line, which represents a doubling of test time for each
10°C (18°F) change in temperature. The line extrapolates to approximately 1 year of storage under
ambient conditions. Temperatures at 100°C (212°F) or higher present special chemical problems.
The heteroatom content of the deposits formed in stability studies varied as the source or type of
dopant and fuel liquid source varied. This would be an anticipated result. The color changes of both
the fuel and the deposits formed are more difficult to interpret.
It is also worthy of note that fractionation of petroleum and its products may also give some
indication of instability. There are many schemes by which petroleum and related materials might
be fractionated (Chapter 9). It is not the intent to repeat the details of these in this chapter. However,
a brief overview is necessary since fractional composition can play a role in stability and incompatibility phenomena.
Petroleum can be fractionated into four broad fractions by a variety of techniques (Chapters 9
and 10), although the most common procedure involves precipitation of the asphaltene fraction and
the use of adsorbents to fractionate the deasphalted (deasphaltened) oil. The fractions are named for
convenience and the assumption that fractionation occurs by specific compound type is not quite true.
In general terms, studies of the composition of the incompatible materials often involves determination of the distribution of the organic functional groups by selective fractionation that is
analogous to the deasphalting procedure and subsequent fractionation of the maltene constituents
(Chapters 9 and 10):
1. Heptane-soluble materials: often called maltenes or petrolenes in petroleum work
2. Heptane-insoluble material, benzene (or toluene)-soluble material: often referred to as
asphaltenes
3. Benzene (or toluene)-insoluble material: referred to as carbenes and carboids when the
fraction is a thermal product
4. Pyridine-soluble material (carbenes) and pyridine insoluble (carboids)
Carbon disulfide and tetrahydrofuran have been used in place of pyridine. The former (carbon
disulfide), although having an obnoxious odor and therefore not much different from pyridine, is
easier to remove because of the higher volatility. The latter (tetrahydrofuran) is not as well established in petroleum science as it is in the coal-liquid-related research. Thus, it is more than likely that
the petroleum researcher will use carbon disulfide or, pyridine, or some suitable alternate solvent. It
may also be necessary to substitute cyclohexane as an additional step for treatment of the heptaneinsoluble materials prior to treatment with benzene (or toluene). The use of quinoline has been suggested in place of pyridine, but this solvent presents issues associated with its high boiling point.
Whichever solvent separation scheme is employed, there should be ample description of the procedure so that the work can be repeated not only in the same laboratory by a different researcher but also
in different laboratories by various researchers. Thus, for any particular feedstock and solvent separation scheme, the work should be repeatable and reproducible within the limits of experimental error.
Fractionation procedures allow a before-and-after inspection of any feedstock or product and can
give an indication of the means by which refining or use changes the composition of the feedstock.
In addition, fractionation also allows studies to be made of the interrelations between the various
fractions. For example, the most interesting phenomenon (in the present context) to evolve from the
fractionation studies is the relationship between the asphaltenes and the resins.
Instability and Incompatibility
Bottle tests constitute the predominant test method, and the test conditions have varied in volume,
type of glass or metal, vented and unvented containers, and type of bottle closure. Other procedures
have involved stirred reactor vessels under both air pressure and oxygen pressure, and small volumes
of fuel employing a cover slip for solid deposition (ASTM D4625). All of these procedures are gravimetric in nature.
There are several accelerated fuel stability tests that can be represented as a time–temperature
matrix (Goetzinger et al., 1983; Hazlett, 1992). A graphical representation shows that the majority of
the stability tests depicted fall close to the solid line, which represents a doubling of test time for each
10°C (18°F) change in temperature. The line extrapolates to approximately 1 year of storage under
ambient conditions. Temperatures at 100°C (212°F) or higher present special chemical problems.
The heteroatom content of the deposits formed in stability studies varied as the source or type of
dopant and fuel liquid source varied. This would be an anticipated result. The color changes of both
the fuel and the deposits formed are more difficult to interpret.
It is also worthy of note that fractionation of petroleum and its products may also give some
indication of instability. There are many schemes by which petroleum and related materials might
be fractionated (Chapter 9). It is not the intent to repeat the details of these in this chapter. However,
a brief overview is necessary since fractional composition can play a role in stability and incompatibility phenomena.
Petroleum can be fractionated into four broad fractions by a variety of techniques (Chapters 9
and 10), although the most common procedure involves precipitation of the asphaltene fraction and
the use of adsorbents to fractionate the deasphalted (deasphaltened) oil. The fractions are named for
convenience and the assumption that fractionation occurs by specific compound type is not quite true.
In general terms, studies of the composition of the incompatible materials often involves determination of the distribution of the organic functional groups by selective fractionation that is
analogous to the deasphalting procedure and subsequent fractionation of the maltene constituents
(Chapters 9 and 10):
1. Heptane-soluble materials: often called maltenes or petrolenes in petroleum work
2. Heptane-insoluble material, benzene (or toluene)-soluble material: often referred to as
asphaltenes
3. Benzene (or toluene)-insoluble material: referred to as carbenes and carboids when the
fraction is a thermal product
4. Pyridine-soluble material (carbenes) and pyridine insoluble (carboids)
Carbon disulfide and tetrahydrofuran have been used in place of pyridine. The former (carbon
disulfide), although having an obnoxious odor and therefore not much different from pyridine, is
easier to remove because of the higher volatility. The latter (tetrahydrofuran) is not as well established in petroleum science as it is in the coal-liquid-related research. Thus, it is more than likely that
the petroleum researcher will use carbon disulfide or, pyridine, or some suitable alternate solvent. It
may also be necessary to substitute cyclohexane as an additional step for treatment of the heptaneinsoluble materials prior to treatment with benzene (or toluene). The use of quinoline has been suggested in place of pyridine, but this solvent presents issues associated with its high boiling point.
Whichever solvent separation scheme is employed, there should be ample description of the procedure so that the work can be repeated not only in the same laboratory by a different researcher but also
in different laboratories by various researchers. Thus, for any particular feedstock and solvent separation scheme, the work should be repeatable and reproducible within the limits of experimental error.
Fractionation procedures allow a before-and-after inspection of any feedstock or product and can
give an indication of the means by which refining or use changes the composition of the feedstock.
In addition, fractionation also allows studies to be made of the interrelations between the various
fractions. For example, the most interesting phenomenon (in the present context) to evolve from the
fractionation studies is the relationship between the asphaltenes and the resins.