205
Chemical Composition
shall be present in resin constituents and asphaltene constituents rather than the derivatives of
kata-condensed polynuclear aromatic system (Figure 8.7).
8.4.1 gAses And nAPHtHA
Methane is the main hydrocarbon component of petroleum gases with lesser amounts of ethane,
propane, butane, isobutane, and some C 4
+ hydrocarbons. Other gases such as hydrogen, carbon
dioxide, hydrogen sulfide, and carbonyl sulfide are also present.
Saturated constituents with lesser amounts of mono- and diaromatics dominate the naphtha fraction. While naphtha covers the boiling range of gasoline, most of the raw petroleum naphtha molecules have low octane number. However, most raw naphtha is processed further and combined with
other process naphtha and additives to formulate commercial gasoline.
Within the saturated constituents in petroleum gases and naphtha, every possible paraffin from
methane (CH 4 ) hydrocarbon to n-decane (n-C 10 H 22 , normal decane) is present. Depending upon the
source, one of these low boiling paraffins may be the most abundant compound in a crude oil reaching several percent. The iso-paraffins begin at C4 with isobutane as the only isomer of n-butane.
The number of isomers grows rapidly with carbon number and there may be increased difficulty in
dealing with multiple isomers during analysis.
In addition to aliphatic molecules, the saturated constituents consist of cycloalkanes (naphthenes)
with predominantly five or six carbon rings. Methyl derivatives of cyclopentane and cyclohexane,
which are commonly found at higher levels than the parent unsubstituted structures, may be present
(Tissot and Welte, 1978). Fused ring dicycloalkanes such as cis-decahydronaphthalene (cis-decalin)
and trans-decahydronaphthalene (trans-decalin) and hexahydroindan are also common but bicylic
naphthenes separated by a single bond, such as cyclohexyl cyclohexane, are not.
The numerous aromatic constituents in petroleum naphtha begin with benzene, but its C 1 –C 3
alkylated derivatives are also present (Tissot and Welte, 1978). Each of the alkyl benzene homologues through the 20 isomeric C 4 alkyl benzenes have been isolated from crude oil along with
various C 5 derivatives. Benzene derivatives having fused cycloparaffin rings (naphtheno-aromatics)
such as indane and tetralin have been isolated along with a number of their methyl derivatives.
Naphthalene is included in this fraction while the 1- and 2-methyl naphthalenes and higher homologues of fused two-ring aromatics appear in the mid-distillate fraction.
(a)
(b)
(c)
FIGURE 8.6 Examples of peri-condensed aromatic systems: (a) chrysene, (b) picene, and (c)
1,2,5,6-dibenzanthracene.
(a)
(b)
(c)
FIGURE 8.7 Examples of kata-condensed (cata-condensed) aromatic systems: (a) ovalene, (b) pyrene, and
(c) coronene.
Chemical Composition
shall be present in resin constituents and asphaltene constituents rather than the derivatives of
kata-condensed polynuclear aromatic system (Figure 8.7).
8.4.1 gAses And nAPHtHA
Methane is the main hydrocarbon component of petroleum gases with lesser amounts of ethane,
propane, butane, isobutane, and some C 4
+ hydrocarbons. Other gases such as hydrogen, carbon
dioxide, hydrogen sulfide, and carbonyl sulfide are also present.
Saturated constituents with lesser amounts of mono- and diaromatics dominate the naphtha fraction. While naphtha covers the boiling range of gasoline, most of the raw petroleum naphtha molecules have low octane number. However, most raw naphtha is processed further and combined with
other process naphtha and additives to formulate commercial gasoline.
Within the saturated constituents in petroleum gases and naphtha, every possible paraffin from
methane (CH 4 ) hydrocarbon to n-decane (n-C 10 H 22 , normal decane) is present. Depending upon the
source, one of these low boiling paraffins may be the most abundant compound in a crude oil reaching several percent. The iso-paraffins begin at C4 with isobutane as the only isomer of n-butane.
The number of isomers grows rapidly with carbon number and there may be increased difficulty in
dealing with multiple isomers during analysis.
In addition to aliphatic molecules, the saturated constituents consist of cycloalkanes (naphthenes)
with predominantly five or six carbon rings. Methyl derivatives of cyclopentane and cyclohexane,
which are commonly found at higher levels than the parent unsubstituted structures, may be present
(Tissot and Welte, 1978). Fused ring dicycloalkanes such as cis-decahydronaphthalene (cis-decalin)
and trans-decahydronaphthalene (trans-decalin) and hexahydroindan are also common but bicylic
naphthenes separated by a single bond, such as cyclohexyl cyclohexane, are not.
The numerous aromatic constituents in petroleum naphtha begin with benzene, but its C 1 –C 3
alkylated derivatives are also present (Tissot and Welte, 1978). Each of the alkyl benzene homologues through the 20 isomeric C 4 alkyl benzenes have been isolated from crude oil along with
various C 5 derivatives. Benzene derivatives having fused cycloparaffin rings (naphtheno-aromatics)
such as indane and tetralin have been isolated along with a number of their methyl derivatives.
Naphthalene is included in this fraction while the 1- and 2-methyl naphthalenes and higher homologues of fused two-ring aromatics appear in the mid-distillate fraction.
(a)
(b)
(c)
FIGURE 8.6 Examples of peri-condensed aromatic systems: (a) chrysene, (b) picene, and (c)
1,2,5,6-dibenzanthracene.
(a)
(b)
(c)
FIGURE 8.7 Examples of kata-condensed (cata-condensed) aromatic systems: (a) ovalene, (b) pyrene, and
(c) coronene.