Page 95 - Environmental Control in Petroleum Engineering

82 Environmental Control in Petroleum Engineering

with common organic chemistry symbolism used for more complex
molecules, only the carbon-to-carbon bonds are shown in this figure.
The carbon atoms are found at the intersections of the bonds, and the
hydrogen atoms are inferred around the carbon atoms such that the
four carbon bond sites are all occupied. For naphthenes, two hydrogen
atoms are found with each carbon atom. Some hydrocarbons can have
multiple rings, with shared carbon atoms. An example of one of these
condensed rings is decaline.
The fifth family of hydrocarbons is the aromatics. These compounds
are also ring structures, but each carbon has only one hydrogen atom
and the remaining bond sites are shared among the adjacent carbon
atoms. This results in very stable carbon-to-carbon bonds. These bonds
are conveniently written as an alternating double-single bond, as shown
in Figure 3-5, although each carbon-to-carbon bond is identical. Ben-
zene is the simplest of the aromatic hydrocarbons. Other aromatics can
be created by replacing one of the hydrogen atoms with a carbon
chain, as shown in Figure 3-5. Three isomers of xylene are also
possible, with only one isomer shown in the figure.
A sixth family of hydrocarbons is the polyaromatic hydrocarbons.
Condensed aromatics are also known as polycyclic aromatics, poly-
aromatic hydrocarbons (PAH), or polynuclear aromatics (PNA). These
condensed ring structures have aromatic rings sharing carbon atoms
with other rings. Two examples are shown in Figure 3-6. The poly-
aromatic fraction of crude oil ranges between about 0.2% and 7,4%.
Other families of hydrocarbons contain atoms other than carbon and
hydrogen. Alcohols are formed by replacing a hydrogen atom with an
oxygen-hydrogen atom pair. Organic acids are formed by replacing the
three hydrogen atoms at the end of a hydrocarbon chain with a double
bonded oxygen atom and an oxygen-hydrogen atom pair. Amines are
formed by replacing a hydrogen atom with a nitrogen atom having two
hydrogen atoms bonded to it. The chemical structures of several such
compounds are shown in Figure 3-7.
Other families of hydrocarbons can be created if a carbon atom in
the carbon chain or ring is replaced by other elements. Sulfur and
nitrogen are commonly found as a carbon substitute. Heavy metals are
found in complex compounds called porphyrins.
A final family of hydrocarbons is the asphaltenes. These are large
polyaromatic hydrocarbons that contain sulfur, oxygen, or nitrogen.
They contain typically three to ten ring structures. Pure asphaltenes