361
Structure of Petroleum
constituents depend on their molecular weights. The model also assumes that the asphaltenes are
partly dissolved and partly in the colloidal state, thereby accounting for both the solubility and
colloidal effect of high-molecular weight organic constituents in the lower molecular weight constituents. The proposed models can provide the tool for making satisfactory prediction of the phase
behavior of the deposition of high–molecular weight materials.
Because the issue of the deposition of asphaltenic materials problem is complex, it is necessary to
attempt an understanding of the deposition mechanism before an accurate and representative model
can be formulated. Utilization of kinetic theory of fractal aggregation has enabled the development of the fractal aggregation model. This model allows researchers to describe several situations,
such as phase behavior of heavy organic deposition, the mechanism of the association of the high
molecular weight constituents, the geometrical aspects of aggregates, the size distribution of the
sediments, and the solubility of the high molecular weight constituents in the solution under the
influence of a miscible solvent.
Another potential model involves use of the solubility parameter of the asphaltenes and the surrounding medium. It is known that the solubility of asphaltenes varies with the solubility parameter
of the surrounding liquid medium (Mitchell and Speight, 1973), and calculation or estimation of
the solubility parameters of various liquids is known. From these data, it is possible to estimate the
point at which asphaltenic material is precipitated when the composition of the oil is changed by the
addition of a hydrocarbon liquid (Mitchell and Speight, 1973). In fact, the solubility parameter has
been used successfully to determine the character of heavy oils and to investigate the separation of
asphaltenic material (Wiehe, 1996). The solubility parameter concept also recognized the gradation
of polarity of the asphaltenes as selective precipitation occurs during the addition of a nonsolvent
when the most polar constituents are precipitated first (Andersen and Speight, 1992).
The models that apply the solubility parameter concept calculate the interaction through
an assumption of the total crude oil as formed by asphaltenes and deasphaltened oil; hence, the
system is regarded as a two-component system. The changes in phase equilibrium is caused by
the changes in the solubility parameter of either of the two pseudo-components, which may happen either by dissolution of gas or alkane in the deasphaltened oil phase or by changes in temperatures. The amount of precipitated asphaltenes is calculated as the differences in asphaltenes
present in the oil and the solubility of asphaltenes at the saturation point. In the different models,
the change in the composition of the deasphaltened oil is taken as significant for the phase
equilibrium, and various methods, that is, cubic equations of state, are applied to determine the
properties of this fraction.
Further development of this concept (Speight, 1994) has led to the graphical representation of the
solubility parameters of polynuclear aromatic systems and estimation of the solubility parameter of
the asphaltenes based on hydrogen/carbon ratios (Figure 13.2). Further development of this knowledge can allow the progress of asphaltene deposition to be followed when changes on the solubility
parameters of the asphaltene constituents and/or the solubility parameter of the liquid medium
occur (Figure 13.3; Speight, 1994) and the region of sediment formation (or the region of instability
and incompatibility) is estimated using a simplified phase diagram.
In general, these models are, to a degree, applicable to the prediction of heavy organic deposition (asphaltene, paraffin/wax, resin) from petroleum due to changes in pressure, temperature, and
composition. However, the use of assumptions that do not reflect, or recognize, what might be the
actual chemical and physical structure of petroleum can lead to errors in the data. Further modeling must involve recognition of the more modern concepts of the structure of petroleum as well as
application of the models to the predictability of the location and amount of the deposition of the
sediments inside the producing wells and oil-transport pipelines.
Petroleum is a complex system that depends upon the relationship of the constituent fractions to
each other, and the relationships are dictated by molecular interactions. Thus, recovery chemistry
can only be generalized because of the intricate and complex nature of the molecular species that
make up the crude oil leading to difficulties in analyzing not only the recovered material but also
Structure of Petroleum
constituents depend on their molecular weights. The model also assumes that the asphaltenes are
partly dissolved and partly in the colloidal state, thereby accounting for both the solubility and
colloidal effect of high-molecular weight organic constituents in the lower molecular weight constituents. The proposed models can provide the tool for making satisfactory prediction of the phase
behavior of the deposition of high–molecular weight materials.
Because the issue of the deposition of asphaltenic materials problem is complex, it is necessary to
attempt an understanding of the deposition mechanism before an accurate and representative model
can be formulated. Utilization of kinetic theory of fractal aggregation has enabled the development of the fractal aggregation model. This model allows researchers to describe several situations,
such as phase behavior of heavy organic deposition, the mechanism of the association of the high
molecular weight constituents, the geometrical aspects of aggregates, the size distribution of the
sediments, and the solubility of the high molecular weight constituents in the solution under the
influence of a miscible solvent.
Another potential model involves use of the solubility parameter of the asphaltenes and the surrounding medium. It is known that the solubility of asphaltenes varies with the solubility parameter
of the surrounding liquid medium (Mitchell and Speight, 1973), and calculation or estimation of
the solubility parameters of various liquids is known. From these data, it is possible to estimate the
point at which asphaltenic material is precipitated when the composition of the oil is changed by the
addition of a hydrocarbon liquid (Mitchell and Speight, 1973). In fact, the solubility parameter has
been used successfully to determine the character of heavy oils and to investigate the separation of
asphaltenic material (Wiehe, 1996). The solubility parameter concept also recognized the gradation
of polarity of the asphaltenes as selective precipitation occurs during the addition of a nonsolvent
when the most polar constituents are precipitated first (Andersen and Speight, 1992).
The models that apply the solubility parameter concept calculate the interaction through
an assumption of the total crude oil as formed by asphaltenes and deasphaltened oil; hence, the
system is regarded as a two-component system. The changes in phase equilibrium is caused by
the changes in the solubility parameter of either of the two pseudo-components, which may happen either by dissolution of gas or alkane in the deasphaltened oil phase or by changes in temperatures. The amount of precipitated asphaltenes is calculated as the differences in asphaltenes
present in the oil and the solubility of asphaltenes at the saturation point. In the different models,
the change in the composition of the deasphaltened oil is taken as significant for the phase
equilibrium, and various methods, that is, cubic equations of state, are applied to determine the
properties of this fraction.
Further development of this concept (Speight, 1994) has led to the graphical representation of the
solubility parameters of polynuclear aromatic systems and estimation of the solubility parameter of
the asphaltenes based on hydrogen/carbon ratios (Figure 13.2). Further development of this knowledge can allow the progress of asphaltene deposition to be followed when changes on the solubility
parameters of the asphaltene constituents and/or the solubility parameter of the liquid medium
occur (Figure 13.3; Speight, 1994) and the region of sediment formation (or the region of instability
and incompatibility) is estimated using a simplified phase diagram.
In general, these models are, to a degree, applicable to the prediction of heavy organic deposition (asphaltene, paraffin/wax, resin) from petroleum due to changes in pressure, temperature, and
composition. However, the use of assumptions that do not reflect, or recognize, what might be the
actual chemical and physical structure of petroleum can lead to errors in the data. Further modeling must involve recognition of the more modern concepts of the structure of petroleum as well as
application of the models to the predictability of the location and amount of the deposition of the
sediments inside the producing wells and oil-transport pipelines.
Petroleum is a complex system that depends upon the relationship of the constituent fractions to
each other, and the relationships are dictated by molecular interactions. Thus, recovery chemistry
can only be generalized because of the intricate and complex nature of the molecular species that
make up the crude oil leading to difficulties in analyzing not only the recovered material but also