Chemistry
323
-675°C (-1250°F) under vacuum. The residuum amounts to a small percentage of a
very light crude oil and up to 30-40 wt% of a heavy crude. Its major constituents are
resins, asphaltenes, and some high molecular weight oils and waxes. The residuum accounts
for most of the total
NSO
content and the heavy metals. The resins and asphaltenes
precipitate out when the residuum (or crude oil) is treated with liquid propane below
70°F. Additional treatment of this precipitate with n-pentane separates the soluble resins
from the insoluble asphaltenes. The amount of resins always exceeds the asphaltene
content of a crude oil. Resins are light to dark colored and range from thick viscous
materials to amorphous solids. Asphaltenes appear as dark brown to black, amorphous
solids. Together, they may possess nearly 50% of the total nitrogen and sulfur in the
crude oil, predominantly in the form of heterocyclic condensed ring structures containing
aromatic and cycloparaffinic rings. Asphaltenes may account for as much as 25 wt%
of the residuum (up to 12% of the crude oil). Colorless oils are the most paraffinic,
while asphaltenes are the most aromatic. Dark oils and resins show similar degrees of
paraffinicity and aromaticity. Up to 40 wt% of saturated hydrocarbons may be present
in the residuum; however, this comprises only
1-3
wt% of the total crude. The rest
are aromatic and N/O-containing compounds. In the nonasphaltene fraction of the
residuum, the typical aromatic structure is a highly substituted, condensed polynuclear
aromatic molecule, with an average formula, C,,,H,,,S. The substituents are fused
naphthenic rings, which in turn are substituted with long (C,5-C2,,) alkyl side chains,
having intermittent methyl branches. The average structure for a
N/O
compound is
similar in features excepting for slightly higher aromaticity and shorter (C,,,--C,J alkyl
side chains. Other types of
NSO
compounds described previously may also be present.
In the asphaltene fraction, pure hydrocarbons become rare at molecular weights
above
800,
and polar functional groups become very common. Asphaltenes exist as a
dispersion of colloidal particles in an oily medium. Their molecular weight distribution
can extend up to 200,000
or
more. A typical asphaltene molecule has 10-20 condensed
aromatic and naphthenic rings, with both alkyl and cycloalkyl substituted side
chains. Some of these structures contain free-radical sites that allow complex forma-
tion with vanadium, nickel, etc. The observed polar functional groups in the high
molecular weight compounds include carboxylic acids, amides, phenols, carbazoles
and pyridine benzologs.
Metals.
The metals present in crude oils usually exist as complexes of cyclic organic
molecules called porphyrins. The parent structure of the porphyrins is tetramethene-
tetrapyrrole (C2,,H4N4), also known as porphin. In a metalloporphyrin, the transition
metal atom is held at the center of the porphyrin ring by coordination with the four
pyrrole
N
atoms. Inorganic compounds
of
metals are probably not related to the
genesis
of
petroleum. Nickel and vanadium are present in petroleum in concentrations
of less than
1
ppb to -1000 ppm, in combined form with porphyrins. Some of the
lighter metalloporphyrins are volatile, while the high molecular weight porphyrins
appear in the nonvolatile residuum. When oil deposits occur together with saline
water, the produced oil-water emulsions contain soluble salts of sodium, calcium,
and magnesium. Other metals may be present in the inorganic state as suspended
solids
in the produced fluids along with any clays
or
mineral matter derived from the
rock matrix and piping.
Figure
2-77
shows how the weight distributions
of
the different molecular types
vary during the fractional distillation of a naphthenic crude oil. Saturated aliphatic
hydrocarbons (i.e., paraffins and naphthenes) are the predominant constituents in
the light gasoline fraction.
As
the boilingpoint is raised, the paraffin content decreases,
and the
NSO
content increases continuously. About 75 wt%
of
the residuum is
composed of aromatics and
NSO
compounds.