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The Chemistry and Technology of Petroleum
Sulfur-containing compounds are the only heteroatom compounds to be found in this fraction
(Mair, 1964; Rall et al., 1972). Usually, the total amount of the sulfur in this fraction accounts for
<1% of the total sulfur in the crude oil. In naphtha from high-sulfur (sour) petroleum, 50%–70% of
the sulfur may be in the form of mercaptans (thiols). Over 40 individual thiols have been identified,
including all the isomeric C 1 to approximately six compounds plus some C 7 and C 8 isomers plus
thiophenol (Rall et al., 1972). In naphtha from low-sulfur (sweet) crude oil, the sulfur is distributed
between sulfides (thioethers) and thiophenes. In these cases, the sulfides may be in the form of
both linear (alkyl sulfides) and five- or six-ring cyclic (thiacyclane) structures. The sulfur structure distribution tends to follow the distribution hydrocarbons; that is, naphthenic oils with a high
cyclocyloalkane content tend to have a high thiacyclane content. Typical alkyl thiophene derivatives in naphtha have multiple short side chains or exist as naphthenothiophenes (Rall et al., 1972).
Methyl and ethyl disulfides have been confirmed to be present in some crude oils in analyses that
minimized their possible formation by oxidative coupling of thiols (Aksenov and Kayanov, 1980;
Freidlina and Skorova, 1980).
8.4.2 mIddle dIstIllAtes
Saturated species are the major component in the mid-distillate fraction of petroleum but aromatics,
which now include simple compounds with up to three aromatic rings, and heterocyclic compounds
are present and represent a larger portion of the total. Kerosene, jet fuel, and diesel fuel are all
derived from raw middle distillate which can also be obtained from cracked and hydroprocessed
refinery streams.
Within the saturated constituents, the concentration of n-paraffins decreases regularly from
C 11 to C 20 . Two isoprenoid species (pristane = 2,6,10,14-tetramethylpentadecane and phytane =
2,6,10,14-tetramethylhexadecane) are generally present in crude oils in sufficient concentration to
be seen as irregular peaks alongside the n-C 17 and n-C 18 peaks in a gas chromatogram. These isoprene derivatives, believed to arise as fragments of ancient precursors, have relevance as simple
biomarkers, to the genesis of petroleum. The distribution of pristane and phytane relative to their
neighboring n-C 17 and n-C 18 peaks has been used to aid the identification of crude oils and to detect
the onset of biodegradation. The ratio of pristane to phytane has also been used for the assessment of the oxidation and reduction environment in which ancient organisms were converted into
petroleum.
Mono- and dicycloparaffins with five or six carbons per ring constitute the bulk of the naphthenes in the middle distillate boiling range, decreasing in concentration as the carbon number
increases (Tissot and Welte, 1978) and the alkylated naphthenes may have a single long side chain
as well as one or more methyl or ethyl groups. Similarly, substituted three ring naphthenes have
been detected by gas chromatography and adamantane has been found in crude oil (Lee et al., 1981).
The most abundant aromatics in the mid-distillate boiling fractions are di- and tri-methyl naphthalenes. Other one- and two-ring aromatics are undoubtedly present in small quantities as either
naphtheno or alkyl homologues in the C 11 –C 20 range. In addition to these homologues of alkylbenzenes, tetralin, and naphthalenes, the mid distillate contains some fluorene derivatives and phenanthrene derivatives. The phenanthrene structure appears to be favored over that of anthracene
structure (Tissot and Welte, 1978) and this appears to continue through the higher boiling fractions
of petroleum (Speight, 1994b).
The five-membered heterocyclic constituents in the mid-distillate range are primarily the thiacyclane derivatives, benzothiophene derivatives, and dibenzothiophene derivatives with lesser amounts
dialkyl-, diaryl-, and aryl-alkyl sulfides (Aksenov and Kayanov, 1980; Freidlina and Skorova, 1980).
Alkylthiophenes are also present. As with the naphtha fractions, these sulfur species account for a
minimal fraction of the total sulfur in the crude.
Although only trace amounts (usually ppm levels) of nitrogen are found in the middle distillate
fractions, both neutral and basic nitrogen compounds have been isolated and identified in fractions
The Chemistry and Technology of Petroleum
Sulfur-containing compounds are the only heteroatom compounds to be found in this fraction
(Mair, 1964; Rall et al., 1972). Usually, the total amount of the sulfur in this fraction accounts for
<1% of the total sulfur in the crude oil. In naphtha from high-sulfur (sour) petroleum, 50%–70% of
the sulfur may be in the form of mercaptans (thiols). Over 40 individual thiols have been identified,
including all the isomeric C 1 to approximately six compounds plus some C 7 and C 8 isomers plus
thiophenol (Rall et al., 1972). In naphtha from low-sulfur (sweet) crude oil, the sulfur is distributed
between sulfides (thioethers) and thiophenes. In these cases, the sulfides may be in the form of
both linear (alkyl sulfides) and five- or six-ring cyclic (thiacyclane) structures. The sulfur structure distribution tends to follow the distribution hydrocarbons; that is, naphthenic oils with a high
cyclocyloalkane content tend to have a high thiacyclane content. Typical alkyl thiophene derivatives in naphtha have multiple short side chains or exist as naphthenothiophenes (Rall et al., 1972).
Methyl and ethyl disulfides have been confirmed to be present in some crude oils in analyses that
minimized their possible formation by oxidative coupling of thiols (Aksenov and Kayanov, 1980;
Freidlina and Skorova, 1980).
8.4.2 mIddle dIstIllAtes
Saturated species are the major component in the mid-distillate fraction of petroleum but aromatics,
which now include simple compounds with up to three aromatic rings, and heterocyclic compounds
are present and represent a larger portion of the total. Kerosene, jet fuel, and diesel fuel are all
derived from raw middle distillate which can also be obtained from cracked and hydroprocessed
refinery streams.
Within the saturated constituents, the concentration of n-paraffins decreases regularly from
C 11 to C 20 . Two isoprenoid species (pristane = 2,6,10,14-tetramethylpentadecane and phytane =
2,6,10,14-tetramethylhexadecane) are generally present in crude oils in sufficient concentration to
be seen as irregular peaks alongside the n-C 17 and n-C 18 peaks in a gas chromatogram. These isoprene derivatives, believed to arise as fragments of ancient precursors, have relevance as simple
biomarkers, to the genesis of petroleum. The distribution of pristane and phytane relative to their
neighboring n-C 17 and n-C 18 peaks has been used to aid the identification of crude oils and to detect
the onset of biodegradation. The ratio of pristane to phytane has also been used for the assessment of the oxidation and reduction environment in which ancient organisms were converted into
petroleum.
Mono- and dicycloparaffins with five or six carbons per ring constitute the bulk of the naphthenes in the middle distillate boiling range, decreasing in concentration as the carbon number
increases (Tissot and Welte, 1978) and the alkylated naphthenes may have a single long side chain
as well as one or more methyl or ethyl groups. Similarly, substituted three ring naphthenes have
been detected by gas chromatography and adamantane has been found in crude oil (Lee et al., 1981).
The most abundant aromatics in the mid-distillate boiling fractions are di- and tri-methyl naphthalenes. Other one- and two-ring aromatics are undoubtedly present in small quantities as either
naphtheno or alkyl homologues in the C 11 –C 20 range. In addition to these homologues of alkylbenzenes, tetralin, and naphthalenes, the mid distillate contains some fluorene derivatives and phenanthrene derivatives. The phenanthrene structure appears to be favored over that of anthracene
structure (Tissot and Welte, 1978) and this appears to continue through the higher boiling fractions
of petroleum (Speight, 1994b).
The five-membered heterocyclic constituents in the mid-distillate range are primarily the thiacyclane derivatives, benzothiophene derivatives, and dibenzothiophene derivatives with lesser amounts
dialkyl-, diaryl-, and aryl-alkyl sulfides (Aksenov and Kayanov, 1980; Freidlina and Skorova, 1980).
Alkylthiophenes are also present. As with the naphtha fractions, these sulfur species account for a
minimal fraction of the total sulfur in the crude.
Although only trace amounts (usually ppm levels) of nitrogen are found in the middle distillate
fractions, both neutral and basic nitrogen compounds have been isolated and identified in fractions