35
Classification
Petroleum is a naturally occurring hydrocarbon insofar as it contains compounds that are composed of carbon and hydrogen only (Hobson and Pohl, 1973; Considine and Considine, 1984), which
do not contain any heteroatoms (nitrogen, oxygen, and sulfur as well as compounds containing
metallic constituents, particularly vanadium, nickel, iron, and copper). The hydrocarbons found in
petroleum are classified into the following types:
1. Paraffins, that is, saturated hydrocarbons with straight or branched chains, but without any
ring structure
2. Cycloparaffins (naphthenes), that is, saturated hydrocarbons containing one or more
rings, each of which may have one or more paraffin side-chains (more correctly known as
alicyclic hydrocarbons)
3. Aromatics, that is, hydrocarbons containing one or more aromatic nuclei such as benzene,
naphthalene, and phenanthrene ring systems that may be linked up with (substituted) naphthalene rings and/or paraffin side-chains
On this basis, petroleum may have some value in the crude state but, when refined, provides fuel
gas, petrochemical gas (methane, ethane, propane, and butane), transportation fuel (gasoline, diesel
fuel, and aviation fuel), solvents, lubricants, asphalt, and many other products.
In addition to the hydrocarbon constituents, petroleum does contain heteroatomic (nonhydrocarbon) species but they are in the minority compared to the macromolecular heteroatom-containing
kerogen (Costantinides and Arich, 1967; Tissot and Welte, 1978).
A widely used classification of petroleum distinguishes between crude oils either on
a paraffin base or on an asphalt base and arose because paraffin wax separates from some
crude oils on cooling but other oils show no separation of paraffin wax on cooling. The terms
paraffin base and asphalt base were introduced and have remained in common use (van Nes and
van Westen, 1951).
The presence of paraffin wax is usually reflected in the paraffin nature of the constituent fractions, and a high asphaltic content corresponds with the so-called naphthene properties of the
fractions. As a result, the misconception that paraffin-base crude oils consist mainly of paraffins
and asphalt-base crude oils mainly of cyclic (or naphthene) hydrocarbons has arisen. In addition to
paraffin- and asphalt-base oils, a mixed base had to be introduced for those oils that leave a mixture
of bitumen and paraffin wax as a residue by nondestructive distillation.
In practice, a distinction is often made between light and heavy crude oils (indicating the proportion of low boiling material present), which, in combination with the preceding distinction (paraffin,
asphaltic, and so on), doubles the number of possible classes.
2.2.3 CorrelAtIon Index
An early attempt to give the classification system a quantitative basis suggested that a crude should
be called asphaltic if the distillation residue contained less than 2% wax and paraffin if it contained more than 5%. A division according to the chemical composition of the 250°C–300°C
(480°F–570°F) fraction has also been used (Table 2.1), but the difficulty arising in using such a classification is that in the fractions boiling above 200°C (390°F) the molecules can no longer be placed
in one group because most of them are of a typically mixed nature. Purely naphthene or aromatic
molecules occur very seldom; cyclic compounds generally contain paraffin side chains and often
even aromatic and naphthene rings side by side. More direct chemical information is often desirable
and can be supplied by means of the correlation index (CI).
The correlation index, developed by the US Bureau of Mines, is based on the plot of specific gravity versus the reciprocal of the boiling point in degrees Kelvin (°K = °C + 273). For
pure hydrocarbons, the line described by the constants of the individual members of the normal
Classification
Petroleum is a naturally occurring hydrocarbon insofar as it contains compounds that are composed of carbon and hydrogen only (Hobson and Pohl, 1973; Considine and Considine, 1984), which
do not contain any heteroatoms (nitrogen, oxygen, and sulfur as well as compounds containing
metallic constituents, particularly vanadium, nickel, iron, and copper). The hydrocarbons found in
petroleum are classified into the following types:
1. Paraffins, that is, saturated hydrocarbons with straight or branched chains, but without any
ring structure
2. Cycloparaffins (naphthenes), that is, saturated hydrocarbons containing one or more
rings, each of which may have one or more paraffin side-chains (more correctly known as
alicyclic hydrocarbons)
3. Aromatics, that is, hydrocarbons containing one or more aromatic nuclei such as benzene,
naphthalene, and phenanthrene ring systems that may be linked up with (substituted) naphthalene rings and/or paraffin side-chains
On this basis, petroleum may have some value in the crude state but, when refined, provides fuel
gas, petrochemical gas (methane, ethane, propane, and butane), transportation fuel (gasoline, diesel
fuel, and aviation fuel), solvents, lubricants, asphalt, and many other products.
In addition to the hydrocarbon constituents, petroleum does contain heteroatomic (nonhydrocarbon) species but they are in the minority compared to the macromolecular heteroatom-containing
kerogen (Costantinides and Arich, 1967; Tissot and Welte, 1978).
A widely used classification of petroleum distinguishes between crude oils either on
a paraffin base or on an asphalt base and arose because paraffin wax separates from some
crude oils on cooling but other oils show no separation of paraffin wax on cooling. The terms
paraffin base and asphalt base were introduced and have remained in common use (van Nes and
van Westen, 1951).
The presence of paraffin wax is usually reflected in the paraffin nature of the constituent fractions, and a high asphaltic content corresponds with the so-called naphthene properties of the
fractions. As a result, the misconception that paraffin-base crude oils consist mainly of paraffins
and asphalt-base crude oils mainly of cyclic (or naphthene) hydrocarbons has arisen. In addition to
paraffin- and asphalt-base oils, a mixed base had to be introduced for those oils that leave a mixture
of bitumen and paraffin wax as a residue by nondestructive distillation.
In practice, a distinction is often made between light and heavy crude oils (indicating the proportion of low boiling material present), which, in combination with the preceding distinction (paraffin,
asphaltic, and so on), doubles the number of possible classes.
2.2.3 CorrelAtIon Index
An early attempt to give the classification system a quantitative basis suggested that a crude should
be called asphaltic if the distillation residue contained less than 2% wax and paraffin if it contained more than 5%. A division according to the chemical composition of the 250°C–300°C
(480°F–570°F) fraction has also been used (Table 2.1), but the difficulty arising in using such a classification is that in the fractions boiling above 200°C (390°F) the molecules can no longer be placed
in one group because most of them are of a typically mixed nature. Purely naphthene or aromatic
molecules occur very seldom; cyclic compounds generally contain paraffin side chains and often
even aromatic and naphthene rings side by side. More direct chemical information is often desirable
and can be supplied by means of the correlation index (CI).
The correlation index, developed by the US Bureau of Mines, is based on the plot of specific gravity versus the reciprocal of the boiling point in degrees Kelvin (°K = °C + 273). For
pure hydrocarbons, the line described by the constants of the individual members of the normal