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Introduction
Saturates: The saturates (aliphatics) are non-polar hydrocarbons including straight
and branched chain alkanes, as well as cycloalkanes (naphthenes). Cycloalkanes con-
tain one or more rings, which may also have several alkyl side chains. Saturates are
generally are the lightest fraction of the crude oil. The saturate consist primarily of
straight-chain alkanes, mainly ranging from C20 to C30 [18].
Aromatics: The unsaturated aromatic fraction is common to all crude oils, yet
rarely amount to more than 15% of the total oil content. They tend to be more
concentrated in the heavy fractions of petroleum, such as gas oil, lubricating oil
and the residuum (see Figure 1.2). The most common aromatics found in crude oil
are simple derivatives of benzene such as toluene and xylene [19]. Aromatics may
also contain alkyl chains and cycloalkane rings, along with additional aromatic rings
which are termed Polycyclic Aromatic Hydrocarbons or PAH’s. Not all PAH’s are
classed within the Aromatic fraction. High molecular weight PAH’s such as chrysene
and benzoperylene fall into the polar or asphaltene fractions respectively.
Polars/Resins:This fraction is comprised of polar molecules containing heteroatoms
such as nitrogen, oxygen or sulphur. This fraction is defined by solubility, and one
common definition is that it is the fraction soluble in light alkanes such as pentane
and heptane, but insoluble in liquid propane [20]. Resins have a higher H:C ratio
than asphaltenes, 1.2-1.7 compared to 0.9-1.2 for the asphaltenes [20]. Although
resins are structurally similar to asphaltenes, they have lower molecular weights.
Naphthenic acids are commonly regarded as a part of the resin fraction.
Asphaltenes: The asphaltene fraction, is usually defined as the fraction of the
crude oil which is insoluble in light alkanes such as pentane, hexane, or heptane [21].
Asphaltenes are soluble in aromatic solvents like toluene and benzene. Pure as-
phaltenes are non-volatile black solids, and contain the largest percentage of het-
These acids are classified as monobasic carboxylic acids of the general formula RCOOH, where
R represents a cycloalkane structure. The term ‘naphthenic acid’ is often used to account for
all carboxylic acids present in crude oil, including acyclic and aromatic acids. Nearly all crude
oils contain some naphthenic acids. These are extremely complicated mixtures and have been
of interest due to the serious production problems related to their occurrence, causing corrosion
problems at refineries when processed.
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