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The Chemistry and Technology of Petroleum
case of crude oils, sediments and deposits are closely related to sludge, at least as far as compositions
are concerned. The major difference appears to be in the character of the material.
There is also the suggestion (often, but not always, real) in that the sediments and deposits originate from the inorganic constituents of petroleum. They may be formed from the inherent components of the crude oil (i.e., the metalloporphyrin constituents) or from the ingestion of contaminants
by the crude oil during the initial processing operations. For example, crude oil is known to pick up
iron and other metal contaminants from contact with pipelines and pumps.
Sediments can also be formed from organic materials, but the usual inference is that these
materials are usually formed from inorganic materials. The inorganic materials can be salt, sand,
rust, and other contaminants that are insoluble in the crude oil and which settle at the bottom
of the storage vessel. For example, gum typically forms by way of a hydroperoxide intermediate
that induces polymerization of olefins. The intermediates are usually soluble in the liquid medium.
However, gum that has undergone extensive oxidation reactions tends to be higher in molecular
weight and much less soluble. In fact, the high-molecular-weight sediments that form in fuels are
usually the direct result of autoxidation reactions. Active oxygen species involved include both
molecular oxygen and hydroperoxides. These reactions proceed by a free-radical mechanism, and
the solids produced tend to have increased incorporation of heteroatom and are thus also more polar
so increasingly less soluble in the fuel.
The most significant and undesirable instability change in fuel liquids is the formation of solids, termed filterable sediment. Filterable sediments can plug nozzles, filters, coke heat exchanger
surfaces and otherwise degrade engine performance. These solids are the result of free-radical
autoxidation reactions. Although slight thermal degradation occurs in nonoxidizing atmospheres,
the presence of oxygen or active oxygen species, such as hydroperoxides, will greatly accelerate
oxidative degradation as well as significantly lower the temperature at which undesirable products
are formed. Solid deposits that form as the result of short-term high temperature reactions share
many similar chemical characteristics with filterable sediment that form in storage.
The soluble sludge/sediment precursors that form during processing or use may have a molecular
mass in the several hundred range. For this soluble precursor to reach a molecular weight sufficient
to precipitate (or to phase separate), one of two additional reactions must occur. Either the molecular
weight must increase drastically as a result of condensation reactions leading to the higher molecular weight species. Or, the polarity of the precursor must increase (without necessarily increasing
the molecular weight) by incorporation of additional oxygen, sulfur, or nitrogen functional groups.
Additionally, the polarity may increase because of the removal of nonpolar hydrocarbon moieties
from the polar core, as occurs during cracking reactions. In all three cases, insoluble material will
form and separate from the liquid medium.
Additives are chemical compounds intended to improve some specific properties of fuels or other
petroleum products. Different additives, even when added for identical purposes, may be incompatible with each other, for example, react and form new compounds. Consequently, a blend of two or
more fuels, containing different additives, may form a system in which the additives react with each
other and so deprive the blend of their beneficial effect.
The chemistry and physics of incompatibility can, to some extent, be elucidated (Wallace, 1969;
Por, 1992; Power and Mathys, 1992; Mushrush and Speight, 1995) but many unknowns remain.
In addition to the chemical aspects, there are also aspects such as the attractive force differences,
such as
1. Specific interactions between like or unlike molecules (e.g., hydrogen bonding and electron
donor–acceptor phenomena)
2. Field interactions such as dispersion forces and dipole–dipole interactions
3. Any effects imposed on the system by the size and shape of the interacting molecular
species
The Chemistry and Technology of Petroleum
case of crude oils, sediments and deposits are closely related to sludge, at least as far as compositions
are concerned. The major difference appears to be in the character of the material.
There is also the suggestion (often, but not always, real) in that the sediments and deposits originate from the inorganic constituents of petroleum. They may be formed from the inherent components of the crude oil (i.e., the metalloporphyrin constituents) or from the ingestion of contaminants
by the crude oil during the initial processing operations. For example, crude oil is known to pick up
iron and other metal contaminants from contact with pipelines and pumps.
Sediments can also be formed from organic materials, but the usual inference is that these
materials are usually formed from inorganic materials. The inorganic materials can be salt, sand,
rust, and other contaminants that are insoluble in the crude oil and which settle at the bottom
of the storage vessel. For example, gum typically forms by way of a hydroperoxide intermediate
that induces polymerization of olefins. The intermediates are usually soluble in the liquid medium.
However, gum that has undergone extensive oxidation reactions tends to be higher in molecular
weight and much less soluble. In fact, the high-molecular-weight sediments that form in fuels are
usually the direct result of autoxidation reactions. Active oxygen species involved include both
molecular oxygen and hydroperoxides. These reactions proceed by a free-radical mechanism, and
the solids produced tend to have increased incorporation of heteroatom and are thus also more polar
so increasingly less soluble in the fuel.
The most significant and undesirable instability change in fuel liquids is the formation of solids, termed filterable sediment. Filterable sediments can plug nozzles, filters, coke heat exchanger
surfaces and otherwise degrade engine performance. These solids are the result of free-radical
autoxidation reactions. Although slight thermal degradation occurs in nonoxidizing atmospheres,
the presence of oxygen or active oxygen species, such as hydroperoxides, will greatly accelerate
oxidative degradation as well as significantly lower the temperature at which undesirable products
are formed. Solid deposits that form as the result of short-term high temperature reactions share
many similar chemical characteristics with filterable sediment that form in storage.
The soluble sludge/sediment precursors that form during processing or use may have a molecular
mass in the several hundred range. For this soluble precursor to reach a molecular weight sufficient
to precipitate (or to phase separate), one of two additional reactions must occur. Either the molecular
weight must increase drastically as a result of condensation reactions leading to the higher molecular weight species. Or, the polarity of the precursor must increase (without necessarily increasing
the molecular weight) by incorporation of additional oxygen, sulfur, or nitrogen functional groups.
Additionally, the polarity may increase because of the removal of nonpolar hydrocarbon moieties
from the polar core, as occurs during cracking reactions. In all three cases, insoluble material will
form and separate from the liquid medium.
Additives are chemical compounds intended to improve some specific properties of fuels or other
petroleum products. Different additives, even when added for identical purposes, may be incompatible with each other, for example, react and form new compounds. Consequently, a blend of two or
more fuels, containing different additives, may form a system in which the additives react with each
other and so deprive the blend of their beneficial effect.
The chemistry and physics of incompatibility can, to some extent, be elucidated (Wallace, 1969;
Por, 1992; Power and Mathys, 1992; Mushrush and Speight, 1995) but many unknowns remain.
In addition to the chemical aspects, there are also aspects such as the attractive force differences,
such as
1. Specific interactions between like or unlike molecules (e.g., hydrogen bonding and electron
donor–acceptor phenomena)
2. Field interactions such as dispersion forces and dipole–dipole interactions
3. Any effects imposed on the system by the size and shape of the interacting molecular
species