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Petroleum, meaning literally
“rock oil,” is the term used to
describe a myriad of
hydrocarbon-rich fluids that
have accumulated in subterranean
reservoirs. (also called crude
oil) varies dramatically in
color, odor, and flow properties
that reflect the diversity of
its origin.
Petroleum products are any petroleum-based products that can be
obtained by refining and comprise
refinery gas, ethane, liquefied
petroleum gas (LPG), naphtha, gasoline,
aviation fuel, marine fuel, kerosene,
diesel fuel, distillate fuel oil,
residual fuel oil, gas oil, lubricants,
white oil, grease, wax, asphalt, as well
as coke.
Crude oils are complex mixtures
of these hydrocarbons. Oils
containing primarily paraffin
hydrocarbons are called
paraffin-based or paraffinic.
Traditional examples are
Pennsylvania grade crude oils.
Naphthenic-based crudes contain
a large percentage of
cycloparaffins in the heavy
components. Examples of this
type of crude come from the
United States midcontinent
region. Highly aromatic crudes
are less common but are still
found around the world. |
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Crude oils tend to be a mixture of
paraffins, naphthenes, aromatics, with
paraffins and naphthenes the predominant
species. Resins and asphaltenes may also
be present in crude oil. Resins and
asphaltenes are the colored and black
components found in oil and are made up
of relatively high-molecular weight,
polar, polycyclic, aromatic ring
compounds. Pure asphaltenes are
nonvolatile, dry, solid, black powders,
while resins are heavy liquids or sticky
solids with the same volatility as
similarly sized hydrocarbons.
High-molecular-weight resins tend to be
red in color, while lighter resins are
less colored. Asphaltenes do not
dissolve in crude oil but exist as a
colloidal suspension. They are soluble
in aromatic compounds such as xylene,
but will precipitate in the presence of
light paraffinic compounds such as
pentane. Resins, on the other hand, are
readily soluble in oil.
Petroleum products are highly complex chemicals, and considerable
effort is required to characterize their
chemical and physical properties with
a high degree of precision and accuracy.
Indeed, the analysis of petroleum
products is necessary to determine the
properties that can assist in resolving
a process problem as well as the
properties that indicate the function
and performance of the product in
service.
Crude petroleum and the products obtained there from contain a
variety of compounds, usually but not
always hydrocarbons. As the number of
carbon atoms in, for example, the
paraffin series increases, the
complexity of petroleum mixtures also
rapidly increases. Consequently,
detailed analysis
of the individual constituents of the
higher boiling fractions becomes
increasingly difficult, if not
impossible.
Additionally, classes (or types) of hydrocarbons were, and
still are, determined based on the
capability to isolate them by separation
techniques. The
four fractional types into which
petroleum is subdivided are paraffins,
olefins, naphthenes, and aromatics
(PONA). Paraffinic hydrocarbons
include both normal and branched
alkanes, whereas olefins refer to normal
and branched alkenes that contain one or
more double or triple carbon-carbon
bonds. Naphthene (not to be confused
with naphthalene) is a term specific to
the petroleum industry that refers to
the saturated cyclic hydrocarbons
(cycloalkanes). Finally, the term
aromatics includes all hydrocarbons
containing one or more rings of the
benzenoid structure.
Although
not directly derived from composition,
the terms light and heavy or sweet and
sour provide convenient terms for use in
descriptions. For example, light
petroleum (often referred to as
conventional petroleum) is usually rich
in low-boiling constituents and waxy
molecules whereas heavy petroleum
contains greater proportions of
higher-boiling, more aromatic, and
heteroatom-containing (N-, O-, S-, and
metal containing) constituents. Heavy
oil is more viscous than conventional
petroleum and
requires enhanced methods for recovery.
Bitumen is near solid or solid and
cannot be recovered by enhanced oil
recovery methods.
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Conventional (light) petroleum
is composed of hydrocarbons
together with smaller amounts of
organic compounds of nitrogen,
oxygen, and sulfur and still
smaller amounts of compounds
containing metallic
constituents, particularly
vanadium, nickel, iron, and
copper. The processes by which
petroleum was formed dictate
that petroleum composition will
vary and be site specific, thus
leading to a wide variety of
compositional differences.
The term site specific is
intended to convey that
petroleum composition will be
dependent on regional and local
variations in the proportion of
the various precursors that went
into the formation of the
protopetroleum as well as
variations in temperature and
pressure to which the precursors
were subjected. |
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Thus the purely hydrocarbon content may
be higher than 90% by weight for
paraffinic petroleum and 50% by weight
for heavy crude oil and much lower for
tar sand bitumen. The nonhydrocarbon
constituents are usually concentrated in
the higher-boiling portions of the crude
oil. The carbon and hydrogen content is
approximately constant from crude oil to
crude oil even though the amounts of the
various hydrocarbon types and of the
individual isomers may vary widely. Thus
the carbon content of various types of
petroleum is usually between 83% and 87%
by weight and the hydrogen content is in
the range of 11–14% by weight.
General aspects of petroleum quality (as
a refinery feedstock) are assessed by
measurement of physical properties such
as relative density
(specific gravity - which affects on
Oil Price), refractive index, or
viscosity, or by empirical tests such as
pour point or oxidation stability that
are intended to relate to behavior in
service. In some cases the evaluation
may include tests in mechanical rigs and
engines either in the laboratory or
under actual operating conditions.
Measurements of bulk properties are
generally easy to perform and,
therefore, quick and economical. Several
properties may correlate well with
certain compositional characteristics
and are widely used as a quick and
inexpensive means to determine those
characteristics. The most important
properties of a whole crude oil are its
boiling-point distribution, its density
(or API gravity), and its viscosity. The
boiling-point distribution, boiling
profile, or distillation assay gives the
yield of the various distillation cuts,
and selected properties of the fractions
are usually determined.
It is a prime property in its own right
that indicates how much gasoline and
other transportation fuels can be made
from petroleum without conversion.
Density and viscosity are measured for
secondary reasons.
The
former helps to estimate the paraffinic
character of the oil, and the latter
permits the assessment of its
undesirable residual material that
causes resistance to
flow. Boiling-point distribution,
density, and viscosity are easily
measured and give a quick first
evaluation of petroleum oil. Sulfur
content, another
crucial and primary property of a crude
oil, is also readily determined. Certain
composite characterization values,
calculated from density and
mid-boiling point, correlate better with
molecular composition than density
alone.
The acceptance of heavy oil and bitumen
as refinery feedstocks has meant that
the analytical techniques used for the
lighter feedstocks have
had to evolve to produce meaningful data
that can be employed to assist in
defining refinery scenarios for
processing the feedstocks. In addition,
selection of the most appropriate
analytical procedures will aid in the
predictability of feedstock behavior
during refining. This same rationale can
also be applied to feedstock behavior
during recovery operations.
Indeed,bitumen, a source of synthetic
crude oil, is so different from
petroleum that many of the test
methods designed for petroleum may need
modification.
References:
1. Petroleum & Gas Field Processing, H K. Abdel-Alal and Mohamed
Aggour, King Fahd University of
Petroleum & Minerals
2. Petroleum Engineering Handbook, L.W.Lake, Vol.1 "General Engineering" |
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