379
Instability and Incompatibility
oils because of higher solvency of naphthenes and aromatics over paraffinic constituents. This
phenomenon also acts in favor of the dissolution of any sludge that may form, thereby tending to
decrease sludge deposition. However, an increase in crude oil often accompanies sludge dissolution.
The higher the asphaltene content, the greater the tendency of the crude oil to form sludge,
especially when blended with other noncompatible stocks.
14.3.6 Pour PoInt
The pour point (Chapter 10) defines the cold properties of crude oils and petroleum and petroleum
products, that is, the minimal temperature at which they still retain their fluidity (ASTM D97).
Therefore, pour point also indicates the characteristics of crude oils: the higher the pour point, the
more paraffinic is the oil and vice versa. Higher pour point crude oils are waxy, and therefore they
tend to form wax-like materials that enhance sludge formation.
To determine the pour point (ASTM D97; ASTM D5327; ASTM D5853; ASTM D5949; ASTM
D5950; ASTM D5985; IP 15; IP 219; IP 441), the sample is contained in a glass test tube fitted with
a thermometer and immersed in one of three baths containing coolants. The sample is dehydrated
and filtered at a temperature 25°C (45°F) higher than the earlier anticipated cloud point. It is then
placed in a test tube and cooled progressively in coolants held at −1°C to +2°C (30°F to 35°F), −18°C
to −20°C (−4°F to 0°F) and −32°C to −35°C (−26°F to −31°F), respectively. The sample is inspected
for cloudiness at temperature intervals of 1°C (2°F). If conditions or oil properties are such that
reduced temperatures are required to determine the pour point, alternate tests are available that
accommodate the various types of samples.
14.3.7 ACIdIty
The acidity of petroleum or petroleum products is usually measured in terms of the acid number,
which is defined as the number of milliequivalents per gram of alkali required to neutralize the
acidity of the petroleum sample (ASTM D664; ASTM D974; ASTM D3242).
Acidity due to the presence of inorganic constituents is not expected to be present in crude oils,
but organic acidity might be found. Acidic character is composed of contributions from strong
organic acids and other organic acids. Values above 0.15 mg potassium hydroxide per gram are considered to be significantly high. Crude oils of higher acidities may exhibit a tendency of instability.
The acid imparting agents in crude oils are naphthenic acids and hydrosulfides (thiols, mercaptans, R–SH). These are sometimes present in the crude oil originally in small and varying concentrations. Normally, the total acidity of crude oils is in the range of 0.1–0.5 mg potassium hydroxide
per gram, although higher values are not exceptional.
Free hydrogen sulfide is often present in crude oils, a concentration of up to 10 ppm being acceptable in spite of its toxic nature. However, higher hydrogen sulfide concentrations are sometimes
present, 20 ppm posing serious safety hazards. Additional amounts of hydrogen sulfide can form
during the crude oil processing, when hydrogen reacts with some organic sulfur compounds converting them to hydrogen sulfide. In this case, it is referred to as potential hydrogen sulfide, contrary
to free hydrogen sulfide.
Acidity can also form by bacterial action insofar as some species of aerobic bacteria can produce
organic acids from organic nutrients. On the other hand, anaerobic sulfate–reducing bacteria can
generate hydrogen sulfide, which, in turn, can be converted to sulfuric acid (by bacterial action).
14.3.8 metAls (AsH) Content
The majority of crude oils contain metallic constituents that are often determined as combustion ash (ASTM D482). This is particularly so for the heavier feedstocks. These constituents, of
Instability and Incompatibility
oils because of higher solvency of naphthenes and aromatics over paraffinic constituents. This
phenomenon also acts in favor of the dissolution of any sludge that may form, thereby tending to
decrease sludge deposition. However, an increase in crude oil often accompanies sludge dissolution.
The higher the asphaltene content, the greater the tendency of the crude oil to form sludge,
especially when blended with other noncompatible stocks.
14.3.6 Pour PoInt
The pour point (Chapter 10) defines the cold properties of crude oils and petroleum and petroleum
products, that is, the minimal temperature at which they still retain their fluidity (ASTM D97).
Therefore, pour point also indicates the characteristics of crude oils: the higher the pour point, the
more paraffinic is the oil and vice versa. Higher pour point crude oils are waxy, and therefore they
tend to form wax-like materials that enhance sludge formation.
To determine the pour point (ASTM D97; ASTM D5327; ASTM D5853; ASTM D5949; ASTM
D5950; ASTM D5985; IP 15; IP 219; IP 441), the sample is contained in a glass test tube fitted with
a thermometer and immersed in one of three baths containing coolants. The sample is dehydrated
and filtered at a temperature 25°C (45°F) higher than the earlier anticipated cloud point. It is then
placed in a test tube and cooled progressively in coolants held at −1°C to +2°C (30°F to 35°F), −18°C
to −20°C (−4°F to 0°F) and −32°C to −35°C (−26°F to −31°F), respectively. The sample is inspected
for cloudiness at temperature intervals of 1°C (2°F). If conditions or oil properties are such that
reduced temperatures are required to determine the pour point, alternate tests are available that
accommodate the various types of samples.
14.3.7 ACIdIty
The acidity of petroleum or petroleum products is usually measured in terms of the acid number,
which is defined as the number of milliequivalents per gram of alkali required to neutralize the
acidity of the petroleum sample (ASTM D664; ASTM D974; ASTM D3242).
Acidity due to the presence of inorganic constituents is not expected to be present in crude oils,
but organic acidity might be found. Acidic character is composed of contributions from strong
organic acids and other organic acids. Values above 0.15 mg potassium hydroxide per gram are considered to be significantly high. Crude oils of higher acidities may exhibit a tendency of instability.
The acid imparting agents in crude oils are naphthenic acids and hydrosulfides (thiols, mercaptans, R–SH). These are sometimes present in the crude oil originally in small and varying concentrations. Normally, the total acidity of crude oils is in the range of 0.1–0.5 mg potassium hydroxide
per gram, although higher values are not exceptional.
Free hydrogen sulfide is often present in crude oils, a concentration of up to 10 ppm being acceptable in spite of its toxic nature. However, higher hydrogen sulfide concentrations are sometimes
present, 20 ppm posing serious safety hazards. Additional amounts of hydrogen sulfide can form
during the crude oil processing, when hydrogen reacts with some organic sulfur compounds converting them to hydrogen sulfide. In this case, it is referred to as potential hydrogen sulfide, contrary
to free hydrogen sulfide.
Acidity can also form by bacterial action insofar as some species of aerobic bacteria can produce
organic acids from organic nutrients. On the other hand, anaerobic sulfate–reducing bacteria can
generate hydrogen sulfide, which, in turn, can be converted to sulfuric acid (by bacterial action).
14.3.8 metAls (AsH) Content
The majority of crude oils contain metallic constituents that are often determined as combustion ash (ASTM D482). This is particularly so for the heavier feedstocks. These constituents, of