Petroleum
Oil has been used for
lighting purposes for many thousand years. In areas where oil is found in
shallow reservoirs, seeps of crude oil or gas may naturally develop, and some
oil could simply be collected from seepage or tar ponds.
The Sumerians, who
lived in the Tigris-Euphrates valley 4000 B.C. and are thought to form the
cradle of civilizations, used asphalt widely in construction and ornamental
works. In the Bible, Noah is commanded to build an ark that also includes instructions
for caulking the vessel with pitch (Genesis 6:14). There is also reference to the
use of tar as a mortar when the Tower
of Babel
Approximately 2000 years
ago, Arabian scientists developed methods for the distillation of petroleum,
which were introduced into Europe by way of Spain . This represents a documented
use of the volatile derivatives of petroleum which led to a continued interest
in petroleum and its derivatives as medicinal materials and materials for
warfare, in addition to the usual construction materials.
The Egyptians were the first
to adopt the practice of embalming their dead rulers and wrapping the bodies in
cloth. Starting with 1000 B.C., asphalt was used in mummification. Historically,
we know of tales of eternal fires where oil and gas seeps would ignite and
burn. One example dating back to 1000 B.C. is the site where the famous oracle
of Delphi would be built, and 500 B.C. Chinese
were using natural gas to boil water.
Petroleum is perhaps
the most important substance consumed in modern society. It provides not
only raw materials for
the ubiquitous plastics and other products, but also fuel for energy, industry,
heating, and transportation.
The fuels
that are derived from petroleum
supply more than half of the world’s total supply of energy. Gasoline, kerosene,
and diesel oil provide fuel for automobiles, tractors, trucks, aircraft, and
ships. Fuel oil and natural gas are used to heat homes and commercial
buildings, as well as to generate
electricity. Petroleum products are the basic materials used for
the manufacture of synthetic fibers for clothing and in plastics,
paints, fertilizers, insecticides, soaps, and synthetic rubber. The uses of
petroleum as a source of raw material in manufacturing are central to the
functioning of modern industry.
Products we get out of
oil include ink, crayons, bubble gum, dishwashing liquids, deodorant, eyeglasses,
CDs and DVDs, tires, ammonia, heart valves, and many others.
Petroleum is scattered throughout
the earth’s crust, which is divided into natural groups or strata, categorized
in order of their antiquity.
The
word petroleum, derived from the
Latin petra
The
origin of petroleum is still disputable for the scientist of the entire world.
Among the existing theories about the origin of petroleum and gas the greatest
distribution was received by the
organic theory of the mixed origin, according
to which the animal and vegetative residuals participate in petroleum and gas
formation (natural hydrocarbon systems (NHCS)). Therefore all combustible
minerals are called caustobioliths (from Greek, caustos- combustible, bios
- life, lithos - stone). These residuals under the influence of complicated
chemical and biochemical processes turned into putrid silt (sapropel), into which residuals of
highly organized plants could be admixed.
Putrid
silt and humus substances loaded in salt-water pools, were subjected to further
modifications, gradually turning into gum-form substance (initial phase - diagenesis). Due to high temperatures
and pressure more deep chemical and biochemical transformations of these
substances happened which resulted in formation of complicated hydrocarbon
mixtures (main phase of formation - catagenesis).
There is an alternative
theory about the formation of oil and gas deposits that could change estimates
of potential future oil reserves. According to this theory, oil is not a fossil
fuel at all, but was formed deep in the Earth's crust from inorganic materials.
The theory was first proposed in the 1950s by Russian and Ukranian scientists. Geologists
argue that formation of oil deposits requires the high pressures only found in
the deep mantle and that the hydrocarbon contents in sediments do not exhibit
sufficient organic material to supply the enormous amounts of petroleum found
in supergiant oil fields. The idea of the theory is that hydrogen and carbon,
under high temperatures and pressures found in the mantle during the formation
of the Earth, form hydrocarbon molecules which have gradually leaked up to the
surface through cracks in rocks. The organic materials which are found in
petroleum deposits are easily explained by the metabolism of bacteria which
have been found in extreme environments similar to Earth's mantle.
In the crude state petroleum has minimal value,
but when refined it provides high-value liquid fuels, solvents, lubricants, and
many other products.
Petroleum
is a mixture of gaseous, liquid, and solid hydrocarbon compounds that occur in
sedimentary rock deposits throughout the world and also contains small
quantities of nitrogen-, oxygen-, and sulfur-containing compounds as well as
trace amounts of metallic constituents.
Petroleum
and the equivalent term crude oil
cover a wide assortment of materials consisting of mixtures of hydrocarbons and other compounds
containing variable amounts of sulfur, nitrogen, and oxygen, which may vary
widely in volatility, specific gravity, and viscosity. Metal-containing constituents, notably those
compounds that contain vanadium and nickel, usually occur in the more viscous
crude oils in amounts up to several thousand parts per million and can have
serious consequences during processing of these feedstocks. Because petroleum is a mixture of widely
varying constituents and proportions, its physical properties also vary widely
and the color is from colorless to black.
Petroleum occurs underground, at various
pressures depending on the depth. Because of the pressure, it contains
considerable natural gas in solution. Petroleum underground is much more fluid
than it is on the surface and is generally mobile under reservoir conditions
because the elevated temperatures (the geothermal gradient) in subterranean
formations decrease the viscosity.
Crude
petroleum is a mixture of compounds boiling at different temperatures that can
be separated into a variety of different generic fractions by distillation. And the terminology of these fractions has
been bound by utility and often bears little relationship to composition.
The
molecular boundaries of petroleum cover a wide range of boiling points and
carbon numbers of hydrocarbon compounds and other compounds containing
nitrogen, oxygen, and sulfur, as well as metallic (porphyrinic) constituents. However, the actual boundaries of such a
petroleum map can only be arbitrarily defined in terms of boiling point and
carbon number. In fact, petroleum is so diverse that materials from different
sources exhibit different boundary limits, and for this reason alone it is not
surprising that petroleum has been difficult to map in a precise manner.
Oil
chemical and physical (fractional) composition can vary not only with the
location and age of the oil field but also with the depth of the individual
well. Indeed, two adjacent
wells may produce petroleum with markedly different characteristics.
ULTIMATE
(ELEMENTAL) COMPOSITION
With few
exceptions, the proportions
of the elements
(carbon, hydrogen, nitrogen, oxygen, sulfur, and
metals) in petroleum
(whatever and wherever
the source) vary
over fairly narrow limits:
Carbon, 83.0%
to 87.0%
Hydrogen, 10.0%
to 14.0%
Nitrogen, 0.1%
to 2.0%
Oxygen, 0.05%
to 1.5%
Sulfur
, 0 .05%
to 6.0%
Metals
(Ni and V), <1000 ppm.
The narrow range of variation is quite
surprising when the variation of the precursors is considered and even more
surprising when one considers the wide variation in physical properties from
the lighter, more mobile crude oils at one extreme to the heavier asphaltic crude
oils at the other extreme. In addition, when the many localized or regional
variations in maturation conditions are assessed, it is perhaps surprising that
the ultimate compositions are so similar. Perhaps this observation, more than
any other observation, is indicative of the similarity in nature of the
precursors from one site to another.
CHEMICAL COMPONENTS
Petroleum
contains an extreme range of organic functionality and molecular size. In fact,
the variety is so great that it is unlikely that a complete
compound-by-compound description for even a single crude oil would be possible.
In
very general terms (and as observed from elemental analyses), petroleum, heavy
oil, bitumen, and residua are a complex composition of: (1) hydrocarbons; (2)
nitrogen compounds; (3) oxygen compounds; (4) sulfur compounds; and (5)
metallic constituents.
The hydrocarbon content of petroleum may be as high
as 97% by weight (e.g.,in the lighter paraffinic crude oils) or as low as 50%
by weight or less as illustrated by the heavy asphaltic crude oils.
Nevertheless, crude oils with as little as 50% hydrocarbon components are still
assumed to retain most of the essential characteristics of the hydrocarbons. It
is, nevertheless, the nonhydrocarbon (sulfur, oxygen, nitrogen, and metal)
constituents that play a large part in determining the processability of the
crude oil. But there is more to the composition of petroleum than the
hydrocarbon content. The inclusion of organic compounds of sulfur, nitrogen,
and oxygen serves only to present crude oils as even more complex mixtures, and
the appearance of appreciable amounts of these nonhydrocarbon compounds causes
some concern in the refining of crude oils. Even though the concentration of
nonhydrocarbon constituents (i.e., those organic compounds containing one or
more sulfur, oxygen, or nitrogen atoms) in certain fractions may be quite
small, they tend to concentrate in the higher boiling fractions of petroleum. Indeed,
their influence on the processability of the petroleum is important,
irrespective of their molecular size and the fraction in which they occur.
The presence of traces of nonhydrocarbon
compounds can impart objectionable characteristics to finished products,
leading to discoloration or lack of stability during storage. On the other
hand, catalyst poisoning and corrosion are the most noticeable effects during refining
sequences when these compounds are present. It is therefore not surprising that
considerable attention must be given to the nonhydrocarbon constituents of
petroleum as the trend in the refining industry, of late, has been to process
more heavy crude oil as well as residua that contain substantial proportions of
these nonhydrocarbon materials.
HYDROCARBON CONSTITUENTS
The isolation of pure compounds from petroleum
is an exceedingly difficult task, and the overwhelming complexity of the
hydrocarbon constituents of the higher molecular weight fractions as well as
the presence of compounds of sulfur, oxygen, and nitrogen, are the main causes
for the difficulties encountered. It is difficult on the basis of the data
obtained from synthesized hydrocarbons to determine the identity or even the
similarity of the synthetic hydrocarbons to those that constitute many of the
higher boiling fractions of petroleum. Nevertheless, it has been well
established that the hydrocarbon
components of petroleum are composed of paraffinic, naphthenic, and aromatic
groups. Olefin groups are not usually
found in crude
oils, and acethylenic
hydrocarbons are very
rare indeed. It
is therefore convenient to divide the
hydro carbon components of
petroleum into the
following three classes:
1.
Paraffins, which are
saturated hydrocarbons with
straight or branched
chains, but without any
ring structure
2.
Naphthenes, which are
saturated hydrocarbons containing
one or more
rings, each of which
may have one
or more paraffinic
side chains
3.
Aromatics, which are
hydrocarbons containing one
or more aromatic
nuclei, such as benzene,
naphthalene, and phenanthrene
ring systems, which
may be linked
up with (substituted) naphthene
rings or paraffinic
side chains.
The relationship between the various
hydrocarbon constituents of crude oils is one of hydrogen addition or
hydrogen loss (Figure
2.1). This is
an extremely important
aspect of petroleum composition and there is no reason to deny the
occurrence of these interconversion
schemes during the
formation, maturation, and in
situ alteration of petroleum.
Saturated
hydrocarbons can be linear, i.e. straight-chain (general formula CnH2n
+ 2) wherein the carbon atoms are joined in a
snake-like structure, branched (general formula CnH2n+2,
n > 3) wherein the carbon backbone splits off in one or more directions.
The
members of the series (in terms of number of carbon atoms) are named as
follows: methane, CH4; ethane, C2H6; propane,
C3H8; butane, C4H10; pentane, C5H12;
hexane, C6H14; heptane, C7H16;
octane, C8H18, and so on.
Generally,
the boiling point of hydrocarbons rises 20 - 30 °C for each carbon added
to the chain. A straight-chain hydrocarbon will have a boiling point higher
than a branched-chain one due
to the greater surface area in contact, thus the greater van der Waals forces,
between adjacent molecules.
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