373
Instability and Incompatibility
An analogous test, the thin-film oven test (TFOT) (ASTM D1754), and an aging test (IP 390) are
used to indicate the rate of change of various physical properties such as penetration (ASTM D5),
viscosity (ASTM D2170), and ductility (ASTM D113) after a film of asphalt or bitumen has been
heated in an oven for 5 h at 163°C (325°F) on a rotating plate. A similar test is available for the
stability of engine oil by thin-film oxygen uptake (TFOUT) (ASTM D4742).
This test establishes the effects of heat and air based on changes incurred in the aforementioned
physical properties measured before and after the oven test. The allowed rate of changes in the relevant bitumen properties after the exposure of the tested sample to the oven test is specified in the
relevant specifications (ASTM D3381).
Attractive as they may be, any tests that involve accelerated oxidation of the sample must
be used with caution and consideration of the chemistry. Depending on the constituents of the
sample, it is quite possible that the higher temperature and extreme conditions (oxygen under
pressure) may not be truly representative of the deterioration of the sample under storage conditions. The higher temperature and the oxygen under pressure might change the chemistry of the
system and produce products that would not be produced under ambient storage conditions. An
assessment of the composition of the fuel prior to storage and application of the test will assist in
this determination.
In general, fuel instability and fuel incompatibility can be related to the heteroatom-containing
compounds (i.e., nitrogen-, oxygen-, and sulfur-containing compounds) that are present. The degree
of unsaturation of the fuel (i.e., the level of olefinic species) also plays a role in determining instability/incompatibility. And, recent investigations have also implicated catalytic levels of various
oxidized intermediates and acids as especially deleterious for middle distillate fuels.
Fuel incompatibility can have many meanings. The most obvious example of incompatibility
(nonmiscible) is the inability of a hydrocarbon fuel and water to mix. In the present context, incompatibility usually refers to the presence of various polar functions (i.e., heteroatom function groups
containing nitrogen, or oxygen, or sulfur, and even various combinations of the heteroatoms) in the
crude oil.
Instability reactions are usually defined in terms of the formation of filterable and nonfilterable
sludge (sediments, deposits, and gums), an increased peroxide level, and the formation of color bodies. Color bodies in and of themselves do not predict instability. However, the reactions that initiate
color body formation can be closely linked to heteroatom-containing (i.e., nitrogen-, oxygen-, and
sulfur-containing) functional group chemistry.
Thus, petroleum constituents are incompatible when sludge, semisolid, or solid particles (for
convenience here, these are termed secondary products to distinguish them from the actual petroleum product) are formed during and after blending. This phenomenon usually occurs prior to
use. If the secondary products are marginally soluble in the blended petroleum product, use might
detract from solubility of the secondary products, and they will appear as sludge or sediment
that can be separated by filtration or by extraction (ASTM D4310). When the secondary products
are truly insoluble, they separate and settle out as a semisolid or solid phase floating in the fuel
or are deposited on the walls and floors of containers. In addition, secondary products usually
increase the viscosity of the petroleum product. Standing at low temperatures will also cause a viscosity change in certain fuels and lubricants (ASTM D2532). Usually the viscosity change might
be due to separation of paraffins as might occur when diesel fuel, and similar, engines are allowed
to cool and stand unused overnight in low-temperature climates.
In the current context, the meaning of the term incompatibility is found when it is applied to
petroleum recovery and petroleum refining. The application occurs when a product is formed in the
formation or well pipe (recovery) or in a reactor (refining), and the product is incompatible with
(immiscible with or insoluble in) the original petroleum or its products. An example of this is the
formation and deposition of wax and other solids during recovery or the formation of coke precursors
and even of coke during many thermal and catalytic operations. Coke formation is considered to be an
initial phase-separation of an insoluble, solid, coke precursor prior to coke formation proper. In the
Instability and Incompatibility
An analogous test, the thin-film oven test (TFOT) (ASTM D1754), and an aging test (IP 390) are
used to indicate the rate of change of various physical properties such as penetration (ASTM D5),
viscosity (ASTM D2170), and ductility (ASTM D113) after a film of asphalt or bitumen has been
heated in an oven for 5 h at 163°C (325°F) on a rotating plate. A similar test is available for the
stability of engine oil by thin-film oxygen uptake (TFOUT) (ASTM D4742).
This test establishes the effects of heat and air based on changes incurred in the aforementioned
physical properties measured before and after the oven test. The allowed rate of changes in the relevant bitumen properties after the exposure of the tested sample to the oven test is specified in the
relevant specifications (ASTM D3381).
Attractive as they may be, any tests that involve accelerated oxidation of the sample must
be used with caution and consideration of the chemistry. Depending on the constituents of the
sample, it is quite possible that the higher temperature and extreme conditions (oxygen under
pressure) may not be truly representative of the deterioration of the sample under storage conditions. The higher temperature and the oxygen under pressure might change the chemistry of the
system and produce products that would not be produced under ambient storage conditions. An
assessment of the composition of the fuel prior to storage and application of the test will assist in
this determination.
In general, fuel instability and fuel incompatibility can be related to the heteroatom-containing
compounds (i.e., nitrogen-, oxygen-, and sulfur-containing compounds) that are present. The degree
of unsaturation of the fuel (i.e., the level of olefinic species) also plays a role in determining instability/incompatibility. And, recent investigations have also implicated catalytic levels of various
oxidized intermediates and acids as especially deleterious for middle distillate fuels.
Fuel incompatibility can have many meanings. The most obvious example of incompatibility
(nonmiscible) is the inability of a hydrocarbon fuel and water to mix. In the present context, incompatibility usually refers to the presence of various polar functions (i.e., heteroatom function groups
containing nitrogen, or oxygen, or sulfur, and even various combinations of the heteroatoms) in the
crude oil.
Instability reactions are usually defined in terms of the formation of filterable and nonfilterable
sludge (sediments, deposits, and gums), an increased peroxide level, and the formation of color bodies. Color bodies in and of themselves do not predict instability. However, the reactions that initiate
color body formation can be closely linked to heteroatom-containing (i.e., nitrogen-, oxygen-, and
sulfur-containing) functional group chemistry.
Thus, petroleum constituents are incompatible when sludge, semisolid, or solid particles (for
convenience here, these are termed secondary products to distinguish them from the actual petroleum product) are formed during and after blending. This phenomenon usually occurs prior to
use. If the secondary products are marginally soluble in the blended petroleum product, use might
detract from solubility of the secondary products, and they will appear as sludge or sediment
that can be separated by filtration or by extraction (ASTM D4310). When the secondary products
are truly insoluble, they separate and settle out as a semisolid or solid phase floating in the fuel
or are deposited on the walls and floors of containers. In addition, secondary products usually
increase the viscosity of the petroleum product. Standing at low temperatures will also cause a viscosity change in certain fuels and lubricants (ASTM D2532). Usually the viscosity change might
be due to separation of paraffins as might occur when diesel fuel, and similar, engines are allowed
to cool and stand unused overnight in low-temperature climates.
In the current context, the meaning of the term incompatibility is found when it is applied to
petroleum recovery and petroleum refining. The application occurs when a product is formed in the
formation or well pipe (recovery) or in a reactor (refining), and the product is incompatible with
(immiscible with or insoluble in) the original petroleum or its products. An example of this is the
formation and deposition of wax and other solids during recovery or the formation of coke precursors
and even of coke during many thermal and catalytic operations. Coke formation is considered to be an
initial phase-separation of an insoluble, solid, coke precursor prior to coke formation proper. In the