253
Petroleum Analysis
to a critical value beyond which further additions cause little change; the critical value corresponds
closely with that required for a monomolecular layer on the exposed surface, where it is adsorbed
and accounts for the lowering.
A high proportion of the complex phenomena shown by emulsions and foams can be traced to
these induced surface tension effects. Dissolved gases, even hydrocarbon gases, lower the surface
tension of oils, but the effects are less dramatic and the changes probably result from dilution. The
matter is presumably of some importance in petroleum production engineering in which the viscosity and surface tension of the reservoir fluid may govern the amount of oil recovered under certain
conditions.
On the other hand, although petroleum products show little variation in surface tension, within
a narrow range the interfacial tension of petroleum, especially of petroleum products, against
aqueous solutions provides valuable information (ASTM D971). Thus, the interfacial tension of
petroleum is subject to the same constraints as surface tension, that is, differences in composition,
molecular weight, and so on. When oil-water systems are involved, the pH of the aqueous phase
influences the tension at the interface; the change is small for highly refined oils, but increasing pH
causes a rapid decrease for poorly refined, contaminated, or slightly oxidized oils.
A change in interfacial tension between oil and alkaline water has been proposed as an index for
following the refining or deterioration of certain products, such as turbine and insulating oils. When
surface or interfacial tensions are lowered by the presence of solutes, which tend to concentrate on
the surface, some time is required to obtain the final concentration and hence the final value of the
tension. In such systems dynamic and static tension must be distinguished; the first concerns the
freshly exposed surface having nearly the same composition as the body of the liquid; it usually
has a value only slightly less than that of the pure solvent. The static tension is that existing after
equilibrium concentrations have been reached at the surface.
The interfacial tension between oil and distilled water provides an indication of compounds in
the oil that have an affinity for water. The measurement of interfacial tension has received special
attention because of its possible use in predicting when an oil in constant use will reach the limit of
its serviceability. This interest is based on the fact that oxidation decreases the interfacial tension
of the oil. Furthermore, the interfacial tension of turbine oil against water is lowered by the presence of oxidation products, impurities from the air or rust particles, and certain antirust compounds
intentionally blended in the oil. Thus, a depletion of the antirust additive may cause an increase in
interfacial tension, whereas the formation of oxidation products or contamination with dust and rust
lowers the interfacial tension.
In following the performance of oil in service, a decrease in interfacial tension indicates oxidation, if it is known that antirust additives and contamination with dust and rust are absent. In
the absence of contamination and oxidation products, an increase in interfacial tension indicates
a depletion trend in the antirust additive. Very minor changes over appreciable periods of time
signify satisfactory operating conditions. The addition of makeup oil to a system introduces
further complications in following the effects of service on the interfacial tension of a particular
charge of oil.
10.3.5 metAl Content
Heteroatoms (nitrogen, oxygen, sulfur, and metals) are found in every crude oil and the concentrations have to be reduced to convert the oil to transportation fuel. The reason is that if nitrogen and
sulfur are present in the final fuel during combustion, nitrogen oxides (NO x ) and sulfur oxides (SO x )
form, respectively. In addition, metals affect many upgrading processes adversely, poisoning catalysts in refining and causing deposits in combustion.
Heteroatoms do affect every aspect of refining. Sulfur is usually the most concentrated and is
fairly easy to remove; many commercial catalysts are available that routinely remove 90% of the
sulfur. Nitrogen is more difficult to remove than sulfur, and there are fewer catalysts that are specific
Petroleum Analysis
to a critical value beyond which further additions cause little change; the critical value corresponds
closely with that required for a monomolecular layer on the exposed surface, where it is adsorbed
and accounts for the lowering.
A high proportion of the complex phenomena shown by emulsions and foams can be traced to
these induced surface tension effects. Dissolved gases, even hydrocarbon gases, lower the surface
tension of oils, but the effects are less dramatic and the changes probably result from dilution. The
matter is presumably of some importance in petroleum production engineering in which the viscosity and surface tension of the reservoir fluid may govern the amount of oil recovered under certain
conditions.
On the other hand, although petroleum products show little variation in surface tension, within
a narrow range the interfacial tension of petroleum, especially of petroleum products, against
aqueous solutions provides valuable information (ASTM D971). Thus, the interfacial tension of
petroleum is subject to the same constraints as surface tension, that is, differences in composition,
molecular weight, and so on. When oil-water systems are involved, the pH of the aqueous phase
influences the tension at the interface; the change is small for highly refined oils, but increasing pH
causes a rapid decrease for poorly refined, contaminated, or slightly oxidized oils.
A change in interfacial tension between oil and alkaline water has been proposed as an index for
following the refining or deterioration of certain products, such as turbine and insulating oils. When
surface or interfacial tensions are lowered by the presence of solutes, which tend to concentrate on
the surface, some time is required to obtain the final concentration and hence the final value of the
tension. In such systems dynamic and static tension must be distinguished; the first concerns the
freshly exposed surface having nearly the same composition as the body of the liquid; it usually
has a value only slightly less than that of the pure solvent. The static tension is that existing after
equilibrium concentrations have been reached at the surface.
The interfacial tension between oil and distilled water provides an indication of compounds in
the oil that have an affinity for water. The measurement of interfacial tension has received special
attention because of its possible use in predicting when an oil in constant use will reach the limit of
its serviceability. This interest is based on the fact that oxidation decreases the interfacial tension
of the oil. Furthermore, the interfacial tension of turbine oil against water is lowered by the presence of oxidation products, impurities from the air or rust particles, and certain antirust compounds
intentionally blended in the oil. Thus, a depletion of the antirust additive may cause an increase in
interfacial tension, whereas the formation of oxidation products or contamination with dust and rust
lowers the interfacial tension.
In following the performance of oil in service, a decrease in interfacial tension indicates oxidation, if it is known that antirust additives and contamination with dust and rust are absent. In
the absence of contamination and oxidation products, an increase in interfacial tension indicates
a depletion trend in the antirust additive. Very minor changes over appreciable periods of time
signify satisfactory operating conditions. The addition of makeup oil to a system introduces
further complications in following the effects of service on the interfacial tension of a particular
charge of oil.
10.3.5 metAl Content
Heteroatoms (nitrogen, oxygen, sulfur, and metals) are found in every crude oil and the concentrations have to be reduced to convert the oil to transportation fuel. The reason is that if nitrogen and
sulfur are present in the final fuel during combustion, nitrogen oxides (NO x ) and sulfur oxides (SO x )
form, respectively. In addition, metals affect many upgrading processes adversely, poisoning catalysts in refining and causing deposits in combustion.
Heteroatoms do affect every aspect of refining. Sulfur is usually the most concentrated and is
fairly easy to remove; many commercial catalysts are available that routinely remove 90% of the
sulfur. Nitrogen is more difficult to remove than sulfur, and there are fewer catalysts that are specific