CHARACTERISTICS OF CRUDE OIL AND PROPERTIES OF PETROLEUM PRODUCTS
Petroleum or crude oil is a viscous brown-to-black liquid mixture. Historically, the word petroleum
comes from two Latin words: petra, meaning ‘‘stone/rock,” and oleum, meaning ‘‘oil”
The petroleum industry generally classifies crude oil by three criteria: the geographic location where it
is produced (e.g. West Texas Intermediate, Brent, or Oman), its API gravity (an oil industry measure
of density), and its sulfur content. This classification is important because it affects both the
transportation costs to the refinery and the refining costs to meet sulfur standards imposed on fuels in
the consuming countries. The largest volume products of the industry, on the other hand, are fuel oil
and gasoline.
The composition of crude oils, their qualities, and the major factors included in determining their values
are highlighted. Crude oil classification systems are covered as well. The major types of refined
petroleum products produced and utilized, and their economic importance are described.
Crude Oil and Product Composition
The composite mixture forming crude oils and its products is a complex one. However, the compounds
found in crude oils and its products generally belong to three broad classes: hydrocarbon compounds,
non-hydrocarbon compounds, and metallic compounds, discussed below.
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Hydrocarbons
The major constituents of most crude oils and their products are hydrocarbon compounds made of
hydrogen and carbon only. These compounds belong to one of the following subclasses: alkanes,
cycloalkanes, alkenes, and aromatics.
i)
Alkanes (Paraffins)
Alkanes are saturated compounds having the general formula (CnH2n+2). The simplest hydrocarbon
compound, methane, may be present in small amounts dissolved in the crude or may be produced during
the refining process. Alkanes are relatively unreactive compounds in comparison to alkenes and
aromatics. However, they are the main components of feeds used in olefin production. Alkanes may
either be straight-chain or branched compounds. Branched hydrocarbons in the naphtha range are more
valuable than the nonbranched (straight) isomers since they are useful for gasoline production because
of their higher-octane rating.
ii)
Cycloalkanes (Cycloparaffins, Naphthene)
Cyclo- and bi-cycloalkanes are normally present in crude oils and its fractions in variable proportions.
Cyclohexane, substituted cyclohexane, and substituted cyclopentanes found in the naphtha range are
important precursors for aromatic production via isomerization and dehydrogenation reactions. The
presence of large amounts of these cyclic compounds in the naphtha range has its economic merits since
the rate of aromatization of these compounds is much faster than from alkanes and branched alkanes.
Catalytic reforming is a process for enriching the naphtha with aromatics and isoparaffins.
iii)
Alkenes (Olefins)
Alkenes are unsaturated hydrocarbon compounds with the general formula (CnH2n). These compounds
are quite active and react by addition to many simple reagents such as chlorine, hydrochloric acid, and
water. The simplest alkene, ethylene, is an important monomer in petrochemical production. Light
olefinic hydrocarbons are generally used to produce many chemicals and polymers. Alkenes are
typically not present in crude oils, but they are produced during the processing of crude oils at high
temperatures. Catalytic cracking is the main refining process that produces alkenes. However, due to
the need for large amounts of light olefins for petrochemical use, they are produced by a noncatalytic
steam cracking of ethane, propane, naphtha, gas oil, or residues. Light olefins are then separated and
purified for chemical use. Butadiene is a by-product of hydrocarbon steam cracking. Butadiene is an
active conjugated diolefin and is considered the most important monomer for synthetic rubber
production. Olefins and diolefins generally react by addition.
Light olefins and diolefins may react and produce high-molecular-weight commercial polymers. For
example, polyethylene is the most important thermoplastic, and polybutadiene is the most widely used
synthetic rubber. High-molecular-weight olefins may be present in heavy petroleum fractions from
cracking processes. The presence of large amounts of olefins in these fractions may be unfavourable
because of their instability and tendency to polymerize and to get oxidized.
iv)
Aromatic Compounds
Aromatic compounds are normally present in crude oils and their products. However, only mononuclear
aromatics in the range of C6-C8 have gained commercial importance. The simplest aromatic compound,
benzene (C6H6), is very reactive and is one of the basic raw materials for petrochemical production.
Aromatics in this range are not only important petrochemical feedstocks but are also valuable motor
fuels. The main process for producing aromatics is catalytic reforming of naphtha. The product
reformate is highly rich in C6–C8 aromatics which increase the octane rating of the reformate.
Asphaltenes, which are concentrated in heavy fuel oils and asphalt, are polynuclear aromatics of a
complex structure.
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Non-Hydrocarbon Compounds
Many types of non-hydrocarbon compounds occur in crude oil and refinery streams. The most important
are sulfur, nitrogen, and oxygen compounds. Metals are also present in trace amounts, mainly in the
form of organometallics.
High sulfur heavy-petroleum feedstocks to catalytic cracking units should be hydro desulfurized before
being cracked to avoid poisoning of the catalyst. Naphtha feed to catalytic reformers is hydrotreated to
reduce sulfur compounds to very low levels to ensure a long lifecycle for the expensive platinum
catalyst.
Nitrogen compounds in crude oils are usually low and are thermally more stable than sulfur compounds.
Only trace amounts of nitrogen compounds are found in light streams. Nitrogen compounds in crudes
are normally in the form of heterocyclic compounds such as pyridine and pyrrole. They may have a
complex structure, as in porphyrins, which are usually found in heavy fuel oils and residues. Nitrogen
compounds in petroleum have not yet proved to have any special commercial value. During
hydrotreatment (hydrodesulfurization) of petroleum streams, hydrodenitrogenation takes place and the
nitrogen content is reduced to acceptable levels in the feeds to catalytic processes.
Oxygen compounds in crude oils are more complex than sulfur compounds. However, oxygen
compounds are not poisonous to processing catalysts. Most oxygen compounds are weakly acidic, such
as phenol, cresylic acid, and naphthenic acids.
Many of these oxygen compounds, however, are concentrated in the heavier portion of the crude. Some
of the oxygen compounds in the naphtha and kerosene fractions are of commercial value, such as
naphthenic acids and cresylic acid. They are extracted using an aqueous sodium hydroxide solution.
However, hydrotreatment of these fractions reduces these weak acids to low levels to the extent that
their extraction would not be feasible.
Metallic Compounds: many metals are found in crude oils. Some of the more abundant are sodium,
calcium, magnesium, aluminium, iron, vanadium, and nickel. These normally occur in the form of
inorganic salts soluble in water, as in the case of sodium chloride, or in the form of organometallic
compounds, as in the case of iron, vanadium, and nickel compounds, or in the form of salts of carboxylic
acids (soaps), as in the case of calcium and magnesium. The organometallic compounds are usually
concentrated in the heavier fractions and in crude oil residues. However, during processing crude oils,
some of the volatile organometallic compounds are found in the lighter fractions. The presence of high
concentrations of vanadium compounds in naphtha used in catalytic-reforming feeds produces
permanent poisons. These feeds should be hydrotreated not only to reduce the metallic poisons but also
to desulfurize and denitrogenate the sulfur and nitrogen compounds. Hydrotreatment may also be used
to reduce the metal content in heavy feeds to catalytic cracking. A lot of research is currently being
invested to reduce the metal content in heavy products and residues.
Crude Oil Properties
Properties of crude oils vary appreciably and depend mainly on the origin of the crude. No two crudes
would have the same characteristics even if they are from the same family or field. However, many
physical and chemical tests have been developed to help in establishing some general criteria to relate
the crudes to one another and to help the refiner to select the best sequence of refining for maximum
profitability as well as the energy users in the boilers. Other tests have also been developed to test for
the quality of the products in relation to their utilization. Some of these tests are also important in
controlling some of the harmful compounds that pollute the environment.
The following are some of the most important tests used for the purpose of classifying the crude quality,
along with a brief explanation about each specification.
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Specific Gravity and API Gravity
The specific gravity of crude oils is sometimes used as a rough indication of the quality of the crude. A
high specific gravity of a crude oil. A high specific gravity of a crude oil would normally mean a lower
percentage of the valuable light and middle fractions. Specific gravity is also used to calculate the mass
or weight of crude oils and its products. Usually crudes and products are first measured on a volume
basis and then changed to the corresponding masses using the specific gravity.
Higher API gravity indicates a lighter crude or product, while a low API gravity would mean heavy
crude or product.
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Ash Content
Ash content of a crude oil or a fuel oil is an indication of metals and salts present in the test sample.
The ash is usually in the form of metal oxides, stable salts, and silicon oxides. The crude sample is
usually burned in an atmosphere of air, and the ash is the material left unburned.
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Salt Content
The salt content, expressed as sodium chloride, indicates the amount of salt dissolved in water. The
water in crudes is found in variable amounts (normally small) in emulsion form. Salt in crudes and in
heavier products may create serious corrosion problems, especially in the top-tower zone and the
overhead condenser.
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Carbon Residue
Carbon residue is a rough indication of the asphaltic compounds and the materials that do not evaporate
under conditions of the test, such as metals and silicon oxides. Carbon residue is more important for
diesel fuels, lubricating oils, and heavy fuel oils since it may affect engine performance if high carbon
deposition takes place.
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Sulfur Content
Total sulfur content in crudes and heavy products is determined by burning a sample in a stream of air.
The produced sulfur dioxide is further oxidized to sulfuric acid which is titrated with a standard alkali.
Identification of individual sulfur compounds in crude oils or its products is not necessary. The sulfur
content of crude oils may be taken in consideration in addition to the specific gravity in determining
their commercial value. It has been observed that denser crudes also have higher sulfur content.
Crude Oil Classification
Although there is no specific method for classifying crude oils, it is useful for a refiner to establish some
simple criteria by which the crude can be classified. A broad classification of crudes has been developed
based on some simple physical and chemical properties. Crude oils are generally classified into three
types depending on the relative amount of the hydrocarbon class that predominates in the mixture:
1. Paraffinic constituents are predominantly paraffinic hydrocarbons with a relatively lower
percentage of aromatics and naphthenes.
2. Naphthenics contain a relatively higher ratio of cycloparaffins and a higher amount of asphalt
than in paraffinic crudes.
3. Asphaltics contain a relatively large amount of fused aromatic rings and a high percentage of
asphalt.
The price of petroleum crude oil is basically determined by four factors:
i)
ii)
iii)
iv)
American Petroleum Institute (API) gravity
Sulfur content
Viscosity
Capillarity
i)
API Gravity
API gravity is the density, or specific gravity, of crude oil as measured against a common denominator,
in this case an equal amount of water at 60°F.
ii)
Sulfur content
Sulfur content is described as the percentage of sulfur impurity in a sample of petroleum crude oil.
iii)
Viscosity
Viscosity is a measure of the fluidity or resistance-to-flow characteristics of crude oil. Viscosity, like
API gravity, is measured against water as a standard reference.
iv)
Capillarity
Capillarity is a measure of the adherence properties of crude oil.
Of these four factors that affect crude oil prices, only API gravity and sulfur content are of concern to
the refiner. API gravity essentially tells the refiner how much crude oil he is getting for his money. The
higher the API gravity, the greater the potential value of the crude oil. Therefore, the price of crude oil
is adjusted through a differential to equalize the quality in density to the price. Sulfur content tells the
refiner the amount of basic impurity that is in the crude oil. Even though the API gravity figure might
be attractively high, this might reflect a high sulfur content.
Crude Oil Products
Products from crude oils are diversified and may be produced either directly by distilling the oil in an
atmospheric distillation unit where physical separation of different fractions takes place or by further
processing one or more of these fractions or the residue from the atmospheric distillation in a differing
and more complex processing unit. All time economics will play the major determining element in
running the refinery and will determine where a specific stream will be utilized, processed, or blended.
For example, the residual fuel oil from the atmospheric distillation may be utilized directly as a burner
fuel or may be used as feed to a vacuum distillation unit, which is an optimum option, for producing
more gas oil and lubricating oil base stocks, or alternatively may be introduced to a catalytic cracking
unit for maximizing gasoline and middle distillates production, which will substantially improve the
refinery economics and place it as a complex refinery configuration. Fractions obtained from an
atmospheric distillation unit are gases, naphtha, kerosene, or jet fuel and gas oil. The last three products
are known as mid-distillates. The bottom product is residual fuel oil. For a refinery, maximizing fuel
production is the main objective to provide a positive margin. The trend has changed toward integrating
a fuel refinery with a chemical plant as one complex for maximum profit. An integrated fuel-chemical
refinery may include, in addition to atmospheric and vacuum distillation units, a catalytic reforming
unit with an aromatic extraction process, a catalytic cracking or hydrocracking unit to upgrade heavy
fractions and residues, and possibly a steam cracking unit for olefin production. Some properties of
petroleum products in relation to their end uses are described below.
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Refinery Gases
The lightest materials from the crude distillation tower are light hydrocarbon gas mixtures from methane
to butanes and some pentanes. These are further processed for separation of the propane-butane mixture
for use as LPG. Propane LPG is used as a prime portable fuel in homes, transportation, and agriculture.
It may be used or marketed alternatively as a feed to cracking units for olefin production. Recently, a
new catalytic process based on LPG to aromatics in high selectivity has been proposed. However, the
economic ramifications of this process versus catalytic reforming have yet to be evaluated. Examples
of the refinery gases include hydrogen and hydrogen sulphide.
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Liquid Products
Liquid products from conventional refining processes include light fractions such as naphtha and jet
fuel, gasoline, kerosene and middle distillates such as gas oil and lube oil base stocks. Heavier liquid
products are residual fuel oils.
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Solid Products
Solid products from conventional refining processes include asphalt, petroleum coke and carbon black.