360
The Chemistry and Technology of Petroleum
considers asphaltenes to be dissolved in oil in a true liquid state. In this case, asphaltene precipitation
is considered to depend on thermodynamic conditions of temperature, pressure, and composition.
This particular approach recognizes asphaltene precipitation as a thermodynamically reversible
process. The second approach considers asphaltenes to be solid particles that are suspended colloidally in the crude oil and are stabilized by resin molecules; consequently, the deposition process
is considered to be irreversible. If the material precipitated is a reacted derivative of the asphaltenes
(i.e., a sediment) this may be true, but if the precipitated material is unreacted asphaltene, then this
assumption may need a correction. One question of major interest during recovery operations is the
timing of sediment formation/deposition and the amount of the organic material that will separate
from the oil under specific conditions.
Two different models have been proposed to explain the behavior of petroleum and the potential for
solids deposition during recovery operations (Kawanaka et al., 1989; Islam, 1994; Park et al., 1994).
The continuous thermodynamic model utilizes the theory of heterogeneous polymer solutions
for the predictions of the onset point and amount of organic deposits from petroleum crude. A steric
colloidal model that is capable of predicting the beginning of organic deposition has also been
developed. A combination of these two models results in a fractal aggregation model. These efforts
have generally been adequate to predict the asphaltene–oil interaction problems (phase behavior
and/or flocculation), wherever it may occur during oil production and processing.
In the continuous thermodynamic model, the degree of dispersion of the high molecular weight
organic constituents in petroleum depends upon the chemical composition of the petroleum.
Precipitation of the high molecular weight material can be explained by a change in the molecular
equilibria that exist in petroleum through a change in the balance of oil composition. Moreover, the
precipitation process is considered to be reversible. Indeed, the reconstitution of petroleum after
fractionation has been demonstrated (Koots and Speight, 1975) and lends support to this model.
The ratio of polar to nonpolar molecules and the ratio of high- to low-molecular-weight molecules
in a complex mixture such as petroleum are the two factors primarily responsible for maintaining
mutual solubility. The stability of the system is altered by the addition of miscible solvents causing
the high–molecular weight and/or the polar molecules to separate from the mixture either in the
form of another liquid phase or as a solid precipitate. Hydrogen bonding and the sulfur-containing
and/or nitrogen-containing segments of the separated molecules could start to aggregate (for polymerize) and, as a result, produce a solid phase which separates from the oil.
In the steric colloidal model, the high–molecular weight materials in petroleum are considered
to be solid particles of different sizes suspended colloidally in the oil and stabilized by other petroleum constituents (i.e., resins) adsorbed on their surface.
The original hypothesis of the physical structure of petroleum (Pfeiffer and Saal, 1940) in which
resins played a role in the stabilization of the asphaltenes and later confirmation of the role of the
resins in petroleum (Swanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975) is
supportive of this model.
In the fractal aggregation model, it is assumed that Π–Π interactions are the principal means
by which asphaltenes associate. This assumption may not be completely valid because of the
evidence that favors hydrogen bonding between the molecular species and the observation that
asphaltene–resin interactions may predominate over asphaltene–asphaltene interactions in petroleum. The concept that asphaltene–asphaltene interactions may be the predominant interactions is
true for solutions of asphaltenes. This is reflected in the molecular weight data (Speight et al., 1985).
However, there is no guarantee that these interactions are predominant in petroleum (especially
with evidence that indicates the high potential for other interactions) (Speight, 1994).
Inasmuch as the high molecular weight constituents of petroleum have a wide range of polarity
and molecular weight distribution, such compounds have been considered to act as heterogeneous
polydisperse polymers. However, knowing with a new understanding of the nature of asphaltenes
(Speight, 1994), there may be some difficulties with this assumption. Nevertheless, in order to predict the phase behavior of these constituents, it has also been assumed that the properties of these
The Chemistry and Technology of Petroleum
considers asphaltenes to be dissolved in oil in a true liquid state. In this case, asphaltene precipitation
is considered to depend on thermodynamic conditions of temperature, pressure, and composition.
This particular approach recognizes asphaltene precipitation as a thermodynamically reversible
process. The second approach considers asphaltenes to be solid particles that are suspended colloidally in the crude oil and are stabilized by resin molecules; consequently, the deposition process
is considered to be irreversible. If the material precipitated is a reacted derivative of the asphaltenes
(i.e., a sediment) this may be true, but if the precipitated material is unreacted asphaltene, then this
assumption may need a correction. One question of major interest during recovery operations is the
timing of sediment formation/deposition and the amount of the organic material that will separate
from the oil under specific conditions.
Two different models have been proposed to explain the behavior of petroleum and the potential for
solids deposition during recovery operations (Kawanaka et al., 1989; Islam, 1994; Park et al., 1994).
The continuous thermodynamic model utilizes the theory of heterogeneous polymer solutions
for the predictions of the onset point and amount of organic deposits from petroleum crude. A steric
colloidal model that is capable of predicting the beginning of organic deposition has also been
developed. A combination of these two models results in a fractal aggregation model. These efforts
have generally been adequate to predict the asphaltene–oil interaction problems (phase behavior
and/or flocculation), wherever it may occur during oil production and processing.
In the continuous thermodynamic model, the degree of dispersion of the high molecular weight
organic constituents in petroleum depends upon the chemical composition of the petroleum.
Precipitation of the high molecular weight material can be explained by a change in the molecular
equilibria that exist in petroleum through a change in the balance of oil composition. Moreover, the
precipitation process is considered to be reversible. Indeed, the reconstitution of petroleum after
fractionation has been demonstrated (Koots and Speight, 1975) and lends support to this model.
The ratio of polar to nonpolar molecules and the ratio of high- to low-molecular-weight molecules
in a complex mixture such as petroleum are the two factors primarily responsible for maintaining
mutual solubility. The stability of the system is altered by the addition of miscible solvents causing
the high–molecular weight and/or the polar molecules to separate from the mixture either in the
form of another liquid phase or as a solid precipitate. Hydrogen bonding and the sulfur-containing
and/or nitrogen-containing segments of the separated molecules could start to aggregate (for polymerize) and, as a result, produce a solid phase which separates from the oil.
In the steric colloidal model, the high–molecular weight materials in petroleum are considered
to be solid particles of different sizes suspended colloidally in the oil and stabilized by other petroleum constituents (i.e., resins) adsorbed on their surface.
The original hypothesis of the physical structure of petroleum (Pfeiffer and Saal, 1940) in which
resins played a role in the stabilization of the asphaltenes and later confirmation of the role of the
resins in petroleum (Swanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975) is
supportive of this model.
In the fractal aggregation model, it is assumed that Π–Π interactions are the principal means
by which asphaltenes associate. This assumption may not be completely valid because of the
evidence that favors hydrogen bonding between the molecular species and the observation that
asphaltene–resin interactions may predominate over asphaltene–asphaltene interactions in petroleum. The concept that asphaltene–asphaltene interactions may be the predominant interactions is
true for solutions of asphaltenes. This is reflected in the molecular weight data (Speight et al., 1985).
However, there is no guarantee that these interactions are predominant in petroleum (especially
with evidence that indicates the high potential for other interactions) (Speight, 1994).
Inasmuch as the high molecular weight constituents of petroleum have a wide range of polarity
and molecular weight distribution, such compounds have been considered to act as heterogeneous
polydisperse polymers. However, knowing with a new understanding of the nature of asphaltenes
(Speight, 1994), there may be some difficulties with this assumption. Nevertheless, in order to predict the phase behavior of these constituents, it has also been assumed that the properties of these