16.8 Hydrocarbons and Petroleum Products
16.8.1 Hydrocarbons
Each year, huge amounts (about 3 10
9 tons) of hydrocarbons,
mainly from petroleum, are consumed. Because crude oil is
a mixture of hydrocarbons, the industrial core refining process starts with a simple distillation. This basic operation
consists to separate the crude oil into lighter, middle, and
heavy fractions. These latter heavy products are recovered,
sometimes at temperatures over 500
C.
The simplest refineries stop at this point. Other refineries
reprocess the heavier fractions into lighter products to maximize the output of the most desirable products, conducting to
changes in the molecular structure of the input with various
chemical reactions, some of them in the presence of a catalyst and some with thermal reactions (cracking or reforming
processes).
A catalytic cracker, for instance, uses the gasoil (heavy
distillate) output from crude distillation as its feedstock and
produces additional finished distillates (heating oil and diesel) and gasoline. A reforming unit produces higher-octane
components for gasoline from lower-octane feedstock that
was recovered in the distillation process. Sulfur removal is
accomplished in a hydrotreater. Some of the obtained
products are then recombined in order to correspond to
technical or safety guidelines.
From lighter to heaviest, petroleum fractions correspond
to liquid petroleum gases (LPG) (mainly propane and
butane), naphtha or ligroin (distillate intermediate further
processed to make gasoline), gasoline, “white spirit”
(containing less than 5 % benzene), lamp oil, kerosene, gas
oil or diesel distillate, lubricating oil, heavy fuel oil, and
residuals (coke, asphalt, tar, waxes, etc.).
Some chemical bases (olefins, aromatics, etc.) are also
produced and used as the initial reactants for the petrochemical industry, particularly to produce polymers.
As a consequence of the hydrocarbon exploitation
and intensive use, almost all ecosystems are polluted.
Microorganisms are major players in their natural elimination and their metabolic capacities are also made use of for
the remediation of polluted sites.
16.8.1.1 Different Kinds of Hydrocarbons
Hydrocarbons are, strictly speaking, composed exclusively
of carbon and hydrogen and can be classified into saturated,
unsaturated, and aromatic compounds (Fig. 16.33).
Saturated Hydrocarbons
They are divided into three structural groups: linear alkanes,
branched alkanes, and cycloalkanes. Normal n-alkanes
have a straight carbon chain: their molecular formula is
C n H 2n +2 and their carbon atom number ranges between
5 and 60. Branched alkanes have a skeleton with one
or several offshoots, usually a methyl group (ÀCH 3 );
pristane (2,6,10,14-tetramethylpentadecane) and phytane
(2,6,10,14-tetramethylhexadecane) (acyclic isoprenoids)
are the most abundant branched alkanes and are products
of the chlorophyll side chain. Cycloalkanes (also called
alicyclic hydrocarbons or naphthenes) have a saturated ring
and are essentially made of cyclopentane or cyclohexane
derivatives. Polycyclic derivatives (bi-, tri-, tetra-,
pentacyclic) are also present: drimanes, tricyclicterpane
(or sterane), and hopanes.
Unsaturated Hydrocarbons
Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms.
Those with at least one double bond are called alkenes and
those with at least one triple bond are called alkynes.
For instance, phytadienes are found in zooplankton, lamellibranch mollusks, and fishes. Cyanobacteria contain linear
alkenes (n-alkenes, C 19 –C 29 ).
Aromatic Hydrocarbons
They have 1 aromatic ring (mono-aromatic compounds) or
several aromatic rings (polyaromatic compounds or PAH).
Benzene and alkylbenzene(toluene, ethylbenzene, and the 3
isomers ortho-, meta-, and para-xylene that make the BTEX
group) contain only one benzene ring with methyl- and
ethyl-substituents. Naphthalene, phenanthrene, and benzo
(a)pyrene contain 2, 3, and 5 rings, respectively. Some
aromatic structures (or polyaromatics) are associated to
saturated rings (rings with 5 or 6 carbons). Such compounds
(e.g., steroids), are qualified as naphtenoaromatics.
16.8.1.2 Hydrocarbon Origin
Hydrocarbons found in the environment may have a natural
or anthropogenic origin (Fig. 16.34).
Hydrocarbons from Natural Origin
Natural sources are divided in four categories:
(i) Compounds synthesized or produced by living organisms
and released in the environment. They are ubiquitous
compounds whose chemical structure (alkanes, alkenes,
isoprenoids, etc.) often varies with the organisms that
synthesized them. For instance, regarding n-alkanes,
compounds with a long chain and an odd carbon atom
number (n-C 23 to n-C 33 ) are typical of higher plants
(Magnoliophytes). The analysis of n-alkanes isolated
from marine phytoplankton usually points out the dominance of n-C 15 , n-C 17 , and n-C 21 compounds. Benthic
and pelagic macro-algae essentially synthesize n-14 to n32 hydrocarbons with an observed maximum for n-C 15 or
n-C 17 . Hydrocarbons with odd carbon atom chains, n-C 21
to n-C 29 , are dominant with some benthic macro-algae.
The simplest alkane, methane, is synthesized by
716
J.-C. Bertrand et al.
16.8.1 Hydrocarbons
Each year, huge amounts (about 3 10
9 tons) of hydrocarbons,
mainly from petroleum, are consumed. Because crude oil is
a mixture of hydrocarbons, the industrial core refining process starts with a simple distillation. This basic operation
consists to separate the crude oil into lighter, middle, and
heavy fractions. These latter heavy products are recovered,
sometimes at temperatures over 500
C.
The simplest refineries stop at this point. Other refineries
reprocess the heavier fractions into lighter products to maximize the output of the most desirable products, conducting to
changes in the molecular structure of the input with various
chemical reactions, some of them in the presence of a catalyst and some with thermal reactions (cracking or reforming
processes).
A catalytic cracker, for instance, uses the gasoil (heavy
distillate) output from crude distillation as its feedstock and
produces additional finished distillates (heating oil and diesel) and gasoline. A reforming unit produces higher-octane
components for gasoline from lower-octane feedstock that
was recovered in the distillation process. Sulfur removal is
accomplished in a hydrotreater. Some of the obtained
products are then recombined in order to correspond to
technical or safety guidelines.
From lighter to heaviest, petroleum fractions correspond
to liquid petroleum gases (LPG) (mainly propane and
butane), naphtha or ligroin (distillate intermediate further
processed to make gasoline), gasoline, “white spirit”
(containing less than 5 % benzene), lamp oil, kerosene, gas
oil or diesel distillate, lubricating oil, heavy fuel oil, and
residuals (coke, asphalt, tar, waxes, etc.).
Some chemical bases (olefins, aromatics, etc.) are also
produced and used as the initial reactants for the petrochemical industry, particularly to produce polymers.
As a consequence of the hydrocarbon exploitation
and intensive use, almost all ecosystems are polluted.
Microorganisms are major players in their natural elimination and their metabolic capacities are also made use of for
the remediation of polluted sites.
16.8.1.1 Different Kinds of Hydrocarbons
Hydrocarbons are, strictly speaking, composed exclusively
of carbon and hydrogen and can be classified into saturated,
unsaturated, and aromatic compounds (Fig. 16.33).
Saturated Hydrocarbons
They are divided into three structural groups: linear alkanes,
branched alkanes, and cycloalkanes. Normal n-alkanes
have a straight carbon chain: their molecular formula is
C n H 2n +2 and their carbon atom number ranges between
5 and 60. Branched alkanes have a skeleton with one
or several offshoots, usually a methyl group (ÀCH 3 );
pristane (2,6,10,14-tetramethylpentadecane) and phytane
(2,6,10,14-tetramethylhexadecane) (acyclic isoprenoids)
are the most abundant branched alkanes and are products
of the chlorophyll side chain. Cycloalkanes (also called
alicyclic hydrocarbons or naphthenes) have a saturated ring
and are essentially made of cyclopentane or cyclohexane
derivatives. Polycyclic derivatives (bi-, tri-, tetra-,
pentacyclic) are also present: drimanes, tricyclicterpane
(or sterane), and hopanes.
Unsaturated Hydrocarbons
Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms.
Those with at least one double bond are called alkenes and
those with at least one triple bond are called alkynes.
For instance, phytadienes are found in zooplankton, lamellibranch mollusks, and fishes. Cyanobacteria contain linear
alkenes (n-alkenes, C 19 –C 29 ).
Aromatic Hydrocarbons
They have 1 aromatic ring (mono-aromatic compounds) or
several aromatic rings (polyaromatic compounds or PAH).
Benzene and alkylbenzene(toluene, ethylbenzene, and the 3
isomers ortho-, meta-, and para-xylene that make the BTEX
group) contain only one benzene ring with methyl- and
ethyl-substituents. Naphthalene, phenanthrene, and benzo
(a)pyrene contain 2, 3, and 5 rings, respectively. Some
aromatic structures (or polyaromatics) are associated to
saturated rings (rings with 5 or 6 carbons). Such compounds
(e.g., steroids), are qualified as naphtenoaromatics.
16.8.1.2 Hydrocarbon Origin
Hydrocarbons found in the environment may have a natural
or anthropogenic origin (Fig. 16.34).
Hydrocarbons from Natural Origin
Natural sources are divided in four categories:
(i) Compounds synthesized or produced by living organisms
and released in the environment. They are ubiquitous
compounds whose chemical structure (alkanes, alkenes,
isoprenoids, etc.) often varies with the organisms that
synthesized them. For instance, regarding n-alkanes,
compounds with a long chain and an odd carbon atom
number (n-C 23 to n-C 33 ) are typical of higher plants
(Magnoliophytes). The analysis of n-alkanes isolated
from marine phytoplankton usually points out the dominance of n-C 15 , n-C 17 , and n-C 21 compounds. Benthic
and pelagic macro-algae essentially synthesize n-14 to n32 hydrocarbons with an observed maximum for n-C 15 or
n-C 17 . Hydrocarbons with odd carbon atom chains, n-C 21
to n-C 29 , are dominant with some benthic macro-algae.
The simplest alkane, methane, is synthesized by
716
J.-C. Bertrand et al.