38
The Chemistry and Technology of Petroleum
API gravity equal to 9.9 and one material having an API gravity equal to 10.1. Nor does the line of
demarcation make allowance for the limitations of the accuracy of the analytical method. Cleary
the use of one physical parameter be it API gravity, or any other physical property for that matter,
is inadequate to the task of classifying conventional petroleum, heavy oil, and tar sand bitumen.
2.2.6 vIsCosIty
At the same time, and in concert with the use of API gravity, the line of demarcation between
petroleum and heavy oil vis-à-vis tar sand bitumen has been drawn at 10,000 cP. Briefly, materials
having viscosity less than 10,000 cP are conventional petroleum and heavy oil while tar sand bitumen has a viscosity greater than 10,000 cP. Use of such a scale requires a fine line of demarcation
between the various crude oils, heavy oils, and bitumen to the point where it would be confusing to
have to differentiate between a material having a viscosity of 9,950 cP and one having a viscosity
of 10,050 cP. Furthermore, the inaccuracies (i.e., the limits of experimental error) of the method of
measuring viscosity also increase the potential for misclassification.
In the author’s experience, the viscosity of tar sand bitumen is usually in excess of 50,000 cP and
higher than 100,000 cP. But even using a higher line of demarcation does not circumvent the use of
one physical property and the difference between a material having viscosity equal to 49,900 and
50,100 cP (or 99,900 and 100,100 cP). Cleary the use of one physical parameter be it API gravity
or viscosity is inadequate to the task of classifying conventional petroleum, heavy oil, and tar sand
bitumen.
2.2.7 CArBon dIstrIButIon
A method for the classification of crude oils can only be efficient, first, if it indicates the distribution
of components according to volatility, and second, if it indicates the characteristic properties of the
various distillate fractions. The distribution according to volatility has been considered the main
property of petroleum, and any fractionating column with a sufficient number of theoretical plates
may be used for recording a curve in which the boiling point of each fraction is plotted against the
percentage by weight.
However, for the characterization of the various fractions of petroleum, the use of the n-d-M
method (n = refractive index, d = density, M = molecular weight (Chapter 11)) is suggested. This
method enables determination of the carbon distribution and thus indicates the percentage of carbon in aromatic structure (%C A ), the percentage of carbon in naphthene structure (%C N ), and the
percentage of carbon in paraffin structure (%C P ). The yields over the various boiling ranges can
also be estimated; for example, in the lubricating oil fractions, the percentage of carbon in paraffin
structure can be divided into two parts, giving the percentage of carbon in normal paraffins (%C nP )
and the percentage of carbon in paraffin side chains. The percentage of normal paraffins present in
lubricating oil fractions can be calculated from the percentage of normal paraffin carbon (%C nP ) by
multiplication by a factor that depends on the hydrogen content of the fractions.
It is also possible to extrapolate the carbon distribution to the gasoline range, on the one hand,
and to the residue, on the other hand. A high value of %C A at 500°C (930°F) boiling point usually indicates a high content of asphaltenes in the residue, whereas a high value of %C nP at 500°C
(930°F) boiling point usually indicates a waxy residue.
2.2.8 vIsCosIty-grAvIty ConstAnt
This parameter, along with the Universal Oil Products characterization factor, has been used to
some extent as a means of classifying crude oils. Both parameters are usually employed to give an
indication of the paraffin character of the crude oil, and both have been used, if a subtle differentiation can be made, as a means of petroleum characterization rather than for petroleum classification.
The Chemistry and Technology of Petroleum
API gravity equal to 9.9 and one material having an API gravity equal to 10.1. Nor does the line of
demarcation make allowance for the limitations of the accuracy of the analytical method. Cleary
the use of one physical parameter be it API gravity, or any other physical property for that matter,
is inadequate to the task of classifying conventional petroleum, heavy oil, and tar sand bitumen.
2.2.6 vIsCosIty
At the same time, and in concert with the use of API gravity, the line of demarcation between
petroleum and heavy oil vis-à-vis tar sand bitumen has been drawn at 10,000 cP. Briefly, materials
having viscosity less than 10,000 cP are conventional petroleum and heavy oil while tar sand bitumen has a viscosity greater than 10,000 cP. Use of such a scale requires a fine line of demarcation
between the various crude oils, heavy oils, and bitumen to the point where it would be confusing to
have to differentiate between a material having a viscosity of 9,950 cP and one having a viscosity
of 10,050 cP. Furthermore, the inaccuracies (i.e., the limits of experimental error) of the method of
measuring viscosity also increase the potential for misclassification.
In the author’s experience, the viscosity of tar sand bitumen is usually in excess of 50,000 cP and
higher than 100,000 cP. But even using a higher line of demarcation does not circumvent the use of
one physical property and the difference between a material having viscosity equal to 49,900 and
50,100 cP (or 99,900 and 100,100 cP). Cleary the use of one physical parameter be it API gravity
or viscosity is inadequate to the task of classifying conventional petroleum, heavy oil, and tar sand
bitumen.
2.2.7 CArBon dIstrIButIon
A method for the classification of crude oils can only be efficient, first, if it indicates the distribution
of components according to volatility, and second, if it indicates the characteristic properties of the
various distillate fractions. The distribution according to volatility has been considered the main
property of petroleum, and any fractionating column with a sufficient number of theoretical plates
may be used for recording a curve in which the boiling point of each fraction is plotted against the
percentage by weight.
However, for the characterization of the various fractions of petroleum, the use of the n-d-M
method (n = refractive index, d = density, M = molecular weight (Chapter 11)) is suggested. This
method enables determination of the carbon distribution and thus indicates the percentage of carbon in aromatic structure (%C A ), the percentage of carbon in naphthene structure (%C N ), and the
percentage of carbon in paraffin structure (%C P ). The yields over the various boiling ranges can
also be estimated; for example, in the lubricating oil fractions, the percentage of carbon in paraffin
structure can be divided into two parts, giving the percentage of carbon in normal paraffins (%C nP )
and the percentage of carbon in paraffin side chains. The percentage of normal paraffins present in
lubricating oil fractions can be calculated from the percentage of normal paraffin carbon (%C nP ) by
multiplication by a factor that depends on the hydrogen content of the fractions.
It is also possible to extrapolate the carbon distribution to the gasoline range, on the one hand,
and to the residue, on the other hand. A high value of %C A at 500°C (930°F) boiling point usually indicates a high content of asphaltenes in the residue, whereas a high value of %C nP at 500°C
(930°F) boiling point usually indicates a waxy residue.
2.2.8 vIsCosIty-grAvIty ConstAnt
This parameter, along with the Universal Oil Products characterization factor, has been used to
some extent as a means of classifying crude oils. Both parameters are usually employed to give an
indication of the paraffin character of the crude oil, and both have been used, if a subtle differentiation can be made, as a means of petroleum characterization rather than for petroleum classification.