193
Chemical Composition
Normal paraffin hydrocarbons (n-paraffins and straight-chain paraffins) occur in varying proportions in most crude oils. In fact, paraffinic petroleum may contain up to 20%–50% by weight
n-paraffins in the gas oil fraction. However, naphthenic or asphaltic crude oils sometimes contain
only very small amounts of normal paraffins.
Considerable quantities of iso-paraffins have been noted to be present in the straight-run
gasoline fraction of petroleum. The 2- and 3-methyl derivatives are the most abundant, and
the 4-methyl derivative is present in small amounts. If at all, and it is generally accepted that
the slightly branched paraffins predominate over the highly branched materials. It seems that the
iso-paraffins occur throughout the boiling range of petroleum fractions. The proportion tends to
decrease with increasing boiling point; it appears that if the iso-paraffins are present in lubricating oils, their amount is too small to have any significant influence on the physical properties of
the lubricating oils.
As the molecular weight (or boiling point) of the petroleum fraction increases, there is a concomitant decrease in the amount of free paraffins in the fraction. In certain types of crude oil, there
may be no paraffins at all in the vacuum gas oil (VGO) fraction. For example, in the paraffinic crude
oils, free paraffins will separate as a part of the asphaltene fraction but in the naphthenic crude oils,
free paraffins are not expected in the gas oil and asphaltene fractions. The vestiges of paraffins in
the asphaltenes fractions occur as alkyl side chains on aromatic and naphthenic systems. And, these
alkyl chains can contain 20 or more carbon atoms (Chapters 9 and 12).
8.3.1.2 Cycloparaffin Hydrocarbons (Naphthenes)
Although only a small number of representatives have been isolated so far, cyclohexane derivatives,
cyclopentane derivatives, and decahydronaphthalene (decalin) derivatives are largely represented in
oil fractions. Petroleum also contains polycyclic naphthenes, such as terpenes, and such molecules
(often designated bridge-ring hydrocarbons) occur even in the heavy gasoline fractions (boiling
point 150°C–200°C, 300°F–390°F). Naphthene rings may be built up of a varying number of carbon
atoms, and among the synthesized hydrocarbons there are individual constituents with rings of the
three-, four-, five-, six-, seven-, and eight-carbon atoms. It is now generally believed that crude oil
fractions contain chiefly five- and six-carbon rings. Only naphthenes with five- and six-membered
rings have been isolated from the lower boiling fractions. Thermodynamic studies show that naphthene rings with five and six carbon atoms are the most stable. The naphthenic acids contain chiefly
cycle pentane as well as cyclohexane rings.
Cycloparaffins (naphthenes) are represented in all fractions in which the constituent molecules
contain more than five carbon atoms. Several series of cycloparaffins, usually containing five- or
six-membered rings or their combinations, occur as polycyclic structures. The content of cycloparaffins in petroleum varies up to 60% of the total hydrocarbons. However, the cycloparaffin content
of different boiling range fractions of a crude oil may not vary considerably and generally remains
within rather close limits. Nevertheless, the structure of these constituents may change within the
same crude oil, as a function of the molecular weight or boiling range of the individual fractions as
well as from one crude oil to another.
The principal structural variation of naphthenes is the number of rings present in the molecule.
The mono- and bicyclic naphthenes are generally the major types of cycloparaffins in the lower
boiling fractions of petroleum, with boiling point or molecular weight increased by the presence of
alkyl chains. The higher boiling point fractions, such as the lubricating oils, may contain two to six
rings per molecule.
As the molecular weight (or boiling point) of the petroleum fraction increases, there is a concomitant increase in the amount of cycloparaffinic (naphthenic) species in the fraction. In the asphaltic (naphthenic) crude oils, the gas oil fraction can contain considerable amounts of naphthenic
ring systems that increase even more in consideration of the molecular types in the asphaltenes.
However, as the molecular weight of the fraction increases, the occurrence of condensed naphthene
ring systems and alkyl-substituted naphthene ring systems increases.
Chemical Composition
Normal paraffin hydrocarbons (n-paraffins and straight-chain paraffins) occur in varying proportions in most crude oils. In fact, paraffinic petroleum may contain up to 20%–50% by weight
n-paraffins in the gas oil fraction. However, naphthenic or asphaltic crude oils sometimes contain
only very small amounts of normal paraffins.
Considerable quantities of iso-paraffins have been noted to be present in the straight-run
gasoline fraction of petroleum. The 2- and 3-methyl derivatives are the most abundant, and
the 4-methyl derivative is present in small amounts. If at all, and it is generally accepted that
the slightly branched paraffins predominate over the highly branched materials. It seems that the
iso-paraffins occur throughout the boiling range of petroleum fractions. The proportion tends to
decrease with increasing boiling point; it appears that if the iso-paraffins are present in lubricating oils, their amount is too small to have any significant influence on the physical properties of
the lubricating oils.
As the molecular weight (or boiling point) of the petroleum fraction increases, there is a concomitant decrease in the amount of free paraffins in the fraction. In certain types of crude oil, there
may be no paraffins at all in the vacuum gas oil (VGO) fraction. For example, in the paraffinic crude
oils, free paraffins will separate as a part of the asphaltene fraction but in the naphthenic crude oils,
free paraffins are not expected in the gas oil and asphaltene fractions. The vestiges of paraffins in
the asphaltenes fractions occur as alkyl side chains on aromatic and naphthenic systems. And, these
alkyl chains can contain 20 or more carbon atoms (Chapters 9 and 12).
8.3.1.2 Cycloparaffin Hydrocarbons (Naphthenes)
Although only a small number of representatives have been isolated so far, cyclohexane derivatives,
cyclopentane derivatives, and decahydronaphthalene (decalin) derivatives are largely represented in
oil fractions. Petroleum also contains polycyclic naphthenes, such as terpenes, and such molecules
(often designated bridge-ring hydrocarbons) occur even in the heavy gasoline fractions (boiling
point 150°C–200°C, 300°F–390°F). Naphthene rings may be built up of a varying number of carbon
atoms, and among the synthesized hydrocarbons there are individual constituents with rings of the
three-, four-, five-, six-, seven-, and eight-carbon atoms. It is now generally believed that crude oil
fractions contain chiefly five- and six-carbon rings. Only naphthenes with five- and six-membered
rings have been isolated from the lower boiling fractions. Thermodynamic studies show that naphthene rings with five and six carbon atoms are the most stable. The naphthenic acids contain chiefly
cycle pentane as well as cyclohexane rings.
Cycloparaffins (naphthenes) are represented in all fractions in which the constituent molecules
contain more than five carbon atoms. Several series of cycloparaffins, usually containing five- or
six-membered rings or their combinations, occur as polycyclic structures. The content of cycloparaffins in petroleum varies up to 60% of the total hydrocarbons. However, the cycloparaffin content
of different boiling range fractions of a crude oil may not vary considerably and generally remains
within rather close limits. Nevertheless, the structure of these constituents may change within the
same crude oil, as a function of the molecular weight or boiling range of the individual fractions as
well as from one crude oil to another.
The principal structural variation of naphthenes is the number of rings present in the molecule.
The mono- and bicyclic naphthenes are generally the major types of cycloparaffins in the lower
boiling fractions of petroleum, with boiling point or molecular weight increased by the presence of
alkyl chains. The higher boiling point fractions, such as the lubricating oils, may contain two to six
rings per molecule.
As the molecular weight (or boiling point) of the petroleum fraction increases, there is a concomitant increase in the amount of cycloparaffinic (naphthenic) species in the fraction. In the asphaltic (naphthenic) crude oils, the gas oil fraction can contain considerable amounts of naphthenic
ring systems that increase even more in consideration of the molecular types in the asphaltenes.
However, as the molecular weight of the fraction increases, the occurrence of condensed naphthene
ring systems and alkyl-substituted naphthene ring systems increases.