Introduction:

The correct nature of crude oil differs, quite broadly, from oil field to oil field however, on average; it consists of a hydrogen content of around 12% by weight and a carbon content approaching 88% by weight. The remainder is mostly sulphur and nitrogen.  In most of the cases there are hundreds of various chemical compounds in a sample of crude oil; however they almost all encompass the common property of having only carbon and hydrogen.

Composition of Crude Oil:

The crude oil mixture is comprised of the given:

A) Hydrocarbon compounds (that is, compounds made of only carbon and hydrogen).

B) Non-hydrocarbon compounds.

C) Organometallic compounds and inorganic salts (metallic compounds).

Hydrocarbon Compounds:

The main constituents of crude oil are hydrocarbon compounds. All hydrocarbon classes are present in the crude mixture, apart from alkenes and alkynes. This might point out that crude oil originated under a reducing atmosphere. The following is a short explanation of various hydrocarbon classes found in all the crude oil.

1) Alkanes (paraffins):

Alkanes are the saturated hydrocarbons having general formula C_nH_2n+2. The simplest alkane, methane (CH₄), is the main constituent of natural gas. Methane, ethane, propane and butane are gaseous hydrocarbons at ambient temperatures and atmospheric pressure. They are generally found related with crude oil in a dissolved state. Normal alkanes (that is, n-alkanes, n-paraffins) are straight-chain hydrocarbons having no branches. The branched alkanes are saturated hydrocarbons having an alkyl substituent or a side branch from the main chain. A branched alkane having the similar number of carbons and hydrogen as an n-alkane is termed as an isomer. For illustration, butane (C₄H₁₀) comprise of two isomers, n-butane and 2-methyl propane (isobutane). As the molecular weight of hydrocarbon increases, the number of isomers as well increases. Pentane (C₅H₁₂) consists of three isomers; hexane (C₆H₁₄) have five. The given illustrates the isomers of hexane:

Fig: Isomers of hexane

The isoparaffin is an isomer containing a methyl group branching from carbon number 2 of the main chain. Crude oil includes many short, medium, and long-chain normal and branched paraffins. Naphtha fraction (obtained as a light liquid stream from the crude fractionation) having a narrow boiling range might have a limited however still large number of isomers.

2) Cycloparaffins (Naphthenic):

Saturated cyclic hydrocarbons, generally termed as naphthenes, are as well part of the hydrocarbon constituents of the crude oil. Their ratio, although, based on the crude type. The lower members of naphthenes are cyclopentane, cyclohexane and their mono-substituted compounds. They are generally present in the light and the heavy naphtha fractions. Cyclohexane, replaced cyclopentane and substituted cyclohexane are significant precursors for the aromatic hydrocarbons.

The illustrations shown here are for three naphthenes of special significance. If a naphtha fraction includes these compounds, the first two can be transformed to benzene, and the last compound can dehydrogenate to toluene throughout processing. Dimethylcyclohexanes are as well significant precursors for xylenes. Heavier petroleum fractions like kerosene and gas oil might have two or more cyclohexane rings fused via two vicinal carbons.

Fig: Cycloparaffins

3) Aromatic Compounds:

The lower members of aromatic compounds are present in small amounts in crude oil and light petroleum fractions. The simplest mononuclear aromatic compound is benzene (C₆H₆). Toluene (C₇H₈) and xylene (C8H10) are as well mononuclear aromatic compounds found in the variable amounts in crude oil. Benzene, toluene and xylenes (BTX) are significant petrochemical intermediates and also valuable gasoline components. Separating BTX aromatics from crude oil distillates is not feasible as they are present in the low concentrations. Enriching a naphtha fraction by these aromatics is possible via a catalytic reforming procedure. Binuclear aromatic hydrocarbons are found out in heavier fractions as compare to naphtha. Trinuclear and polynuclear aromatic hydrocarbons, in combination with the heterocyclic compounds, are main constituents of heavy crude and crude residues. Asphaltenes are a complex mixture of aromatic and heterocyclic compounds. The nature and structure of some of such compounds have been investigated. The given are representative illustrations of a few aromatic compounds found in the crude oil:

Fig: Aromatic compounds found in crude oil

4) Non-Hydrocarbon Compounds:

Different kinds of non-hydrocarbon compounds take place in crude oil and refinery streams. The most significant are the organic sulphur, nitrogen and oxygen compounds. The traces of metallic compounds are as well found in crude oil. The presence of such impurities is injurious and might cause problems to some catalytic processes. Fuels having high sulphur and nitrogen levels cause pollution problems in addition to the corrosive nature of their oxidization products.

5) Sulphur Compounds:

Sulphur in the crude oil is mostly present in the form of organic sulphur compounds. Hydrogen sulphide is the mere significant inorganic sulphur compound found in the crude oil. Its presence, though, is injurious due to its corrosive nature. The organic sulphur compounds might usually be categorized as acidic and non-acidic. Acidic sulphur compounds are the thiols (mercaptans). Thiophene, sulphides and disulphides are illustrations of non-acidic sulphur compounds found in the crude fractions. Extensive research has been taken out to recognize a few sulphur compounds in a narrow light petroleum fraction. Illustrations of a few sulphur compounds from the two kinds are:

Fig: Examples of sulphur compounds

Sour crude oil consists of a high percentage of hydrogen sulphide. As numerous organic sulphur compounds are not thermally stable, hydrogen sulphide is often generated throughout crude processing. High-sulphur crudes are less desirable as treating the various refinery streams for acidic hydrogen sulphide increases the production costs. Most of the sulphur compounds can be eliminated from petroleum streams via hydro treatment methods, where hydrogen sulphide is produced and the corresponding hydrocarbon discharged. Hydrogen sulphide is then absorbed in an appropriate absorbent and recovered as sulphur.

6) Nitrogen Compounds:

The organic nitrogen compounds occur in crude oil either in the simple heterocyclic form as in pyridine (C₅H₅N) and pyrrole (C₄H₅N), or in a complex structure as in the porphyrin. The nitrogen content in most crude oil is very low and doesn't exceed 0.1 weight percent. In some heavy crude, though, the nitrogen content might reach up to 0.9 weight percent. Nitrogen compounds are more thermally stable than sulphur compounds and accordingly are concentrated in heavier petroleum fractions and residues. Light petroleum streams might have trace amounts of nitrogen compounds that must be eliminated as they poison numerous processing catalysts. Throughout hydro treatment of petroleum fractions, nitrogen compounds are hydrogenated to ammonia and the corresponding hydrocarbon. For illustration, pyridine is de-nitrogenated to ammonia and pentane:

Fig: Nitrogen compounds

Nitrogen compounds in crude might usually be categorized into basic and non-basic categories. Basic nitrogen compounds are mostly those having a pyridine ring, and the non-basic compounds encompass a pyrrole structure. Both the pyridine and pyrrole are stable compounds because of their aromatic nature. The given are illustrations of organic nitrogen compounds.

Fig: Basic nitrogen compounds

Porphyrins are non-basic nitrogen compounds.  The porphyrin ring system is comprised of four pyrrole rings joined by =CH-groups. The whole ring system is aromatic. Most of the metal ions can substitute the pyrrole hydrogen and form chelates. The chelate is planar around the metal ion and resonance yields in four equivalent bonds from the nitrogen atoms to the metal. Nearly all crude oil and bitumen have detectable amounts of vanadyl and nickel porphyrins.

Fig: Non-basic nitrogen compounds

Separation of nitrogen compounds is hard, and the compounds are susceptible to modification and loss throughout handling. Though, the basic low molecular weight compounds might be extracted by dilute mineral acids.

7) Oxygen Compounds:

Oxygen compounds in crude oil are more complicated than the sulphur compounds. Though, their presence in petroleum streams is not poisonous to processing catalysts. Most of the oxygen compounds found in crude oil are weakly acidic. They are carboxylic acids, cresylic acid, phenol and naphthenic acid. Naphthenic acids are mostly cyclopentane and cyclohexane derivatives having a carboxyalkyl side chain.  Naphthenic acids in the naphtha fraction encompass a special commercial significance and can be extracted via using dilute caustic solutions. The total acid content of most crude is usually low, however might reach as much as 3%, as in some California crude. Non-acidic oxygen compounds like esters, ketones and amides are less abundant than the acidic compounds. They are of no commercial value. The given exhibits some of the oxygen compounds generally found in the crude oil:

Acidic oxygen compounds: An aliphatic acid, aromatic acid, Cyclohexane carboxylic acid, Phenol, Para cresylic acid and ortho cresylic acid.

Non-Acidic Oxygen Compounds: Esters, Amides, ketone, Furan and Benzofuran

8) Metallic Compounds:

Most of the metals occur in crude oil. Some of the richer are sodium, calcium, magnesium, aluminium, iron, vanadium and nickel. They are present either as inorganic salts, like sodium and magnesium chlorides, or in the form of Organometallic compounds, like those of nickel and vanadium (as in porphyrins). Calcium and magnesium can make salts or soaps having carboxylic acids. Such compounds act as emulsifiers, and their presence is undesirable. However metals in crude are found in trace amounts, their presence is injurious and must be removed. Whenever crude oil is processed, sodium and magnesium chlorides generate hydrochloric acid that is very corrosive. Desalting crude oil is an essential step to decrease these salts. Vanadium and nickel are poisons to most of the catalysts and must be reduced to extremely low levels. Most of the vanadium and nickel compounds are concentrated in heavy residues. Solvent extraction processes are employed to decrease the concentration of heavy metals in the petroleum residues.

Properties of Crude Oil:

Crude oil distinct appreciably in their properties according to origin and the ratio of the various components in the mixture. Lighter crude oil usually outcomes more valuable light and middle distillates and are sold at higher prices. Crude oil having a high percent of impurities, like sulphur compounds, are less desirable than low-sulphur crude oil due to their corrosive properties and the additional treating cost. Corrosive properties of the crude oil is a function of numerous parameters among which are the kind of sulphur compounds and their decomposition   temperatures, the total acid number, the kind of carboxylic and naphthenic acids in the crude oil and their decomposition temperatures. It was found that naphthenic acids start to decompose at 600°F. Refinery experience has illustrated that above 750°F, there is no naphthenic acid corrosion. For a refiner, it is essential to establish some criteria to associate one crude oil to the other to be capable to assess its quality and select the best processing scheme. The given are some of the significant tests used to find out the properties of crude oil.

Density, Specific Gravity and API Gravity:

Density is stated as the mass of unit volume of a material at a particular temperature. A more helpful unit employed by the petroleum industry is specific gravity that is the ratio of the weight of a given volume of a material to the weight of the similar volume of water measured at the similar temperature. Specific gravity is employed to compute the mass of crude oil and its products. Generally, crude oil and their liquid products are first measured on the volume basis, and then changed to the corresponding masses by using the specific gravity. The API   (American Petroleum Institute) gravity is the other way to deduce the relative masses of crude oil. The API gravity could be computed mathematically by using the given equation:

O_API = - (141.5/Sp. gr. 60/60^o) - (131.5)

A low API gravity points out a heavier crude oil or a petroleum product, whereas a higher API gravity signifies a lighter crude oil or product. Specific gravities of crude oil approximately range from 0.82 for lighter crude to over 1.0 for heavier crude.

Salt Content:

The salt content expressed in milligrams of sodium chloride per liter oil (or in pounds/barrel) points out the amount of salt dissolved in water. Water in the crude oil is mostly present in an emulsified form. The high salt content in a crude oil presents serious corrosion problems throughout the refining method. In addition, high salt content is a main cause of plugging heat exchangers and heater pipes. A salt content more than 10 lb/1,000 barrels (stated as NaCl) needs desalting.

Sulphur Content:

Determination of the sulphur content in crude oil is significant as the quantity of sulphur present points out the kind of treatment needed for the distillates. To find out sulphur content, a weighed crude sample (or fraction) is burned in an air stream. All sulphur compounds are oxidized to sulphur dioxide, which is further oxidized to sulphur trioxide and lastly titrated by a standard alkali. Recognizing sulphur compounds in crude oil and their products is of little use to a refiner since all the sulphur compounds can simply be desulphurised via hydrogen to hydrogen sulphide and the corresponding hydrocarbon. The sulphur content of crude, though, is significant and is generally considered when finding out commercial values.

Pour Point:

The pour point of a crude oil or product is the lowest temperature at which the oil is examined to flow under the conditions of test. Pour point data points out the amount of long-chain paraffins (that is, petroleum wax) found in the crude oil. Paraffinic crude generally encompass higher wax content than the other crude kinds. Handling and transporting crude oil and heavy fuels is hard   at temperatures beneath their pour points often, chemical additives termed as pour point depressants are employed to enhance the flow properties of the fuel. Long-chain n-paraffins ranging from 16 to 60 carbon atoms in specific are responsible for near-ambient temperature precipitation. In the middle distillates, less than 1% wax can be enough to cause the solidification of the fuel.

Ash Content:

This test points out the quantity of metallic constituents in the crude oil. The ash left after fully burning an oil sample generally comprises of stable metallic salts, metal oxides and silicon oxide. The ash could be further analyzed for individual elements by employing spectroscopic methods.

Crude Oil Classification:

Different crude oil categorizations have been proposed via geochemists and petroleum refiners.  The interests of geochemists and petroleum refiners are different. Moreover, the physical and/or chemical parameters employed in the categorizations are different.

Petroleum refiners are mainly interested in the amount of the successive distillation fraction example - gasoline, naphtha, gas, kerosene, oil, lubricating distillate and the chemical composition or physical properties of these fractions viz-a-viz viscosity, cloud test and so on.

Geochemist and geologist are more interested in recognizing and characterizing the crude oil in order to relate them to the source rocks and to measure their grade of evolution. As a result they rely on the chemical and structural information of crude oil constituents, particularly on the molecules that are assume to convey the genetic information.

Thus, molecules at relatively low concentrations, similar to high molecular weight n-alkanes, steroids and terpenes, might be of great interest to geologist and geochemist.

Appreciable property differences appear between the crude oil as an outcome of the variable ratios of the crude oil components. For a refiner dealing having crudes of various origins, a simple criterion might be established to group crudes having similar features. Crude oil can be arbitrarily categorized into three or six groups based on the relative ratio of the hydrocarbon groups that predominate in the mixture. The given illustrates six kinds of crudes:

• Paraffinic.
• Paraffinic-naphthenic
• Naphthenic.
• Aromatic-Intermediate.
• Aromatic-asphaltic
• Aromatic-naphthenic.

Paraffinic Class:

This group of petroleum is produced in delta or coastal sediments of the continental margins, or in non-marine source bed and is comprised of light crude oil, some being fluid and some high-wax, high-pour point crude oil. The viscosity of these high-pour point oils at room temperature is high, due to a high content of n-alkanes (>20). At slightly elevated temperature of between 35-50^oC, though, the viscosity becomes normal. Moreover, specific gravity is generally below 0.85. The amount of resins plus asphaltenes is beneath 10%. Viscosity is usually low except whenever n-alkanes of high molecular weight are rich. The Aromatic content is subordinate and mostly comprised of mono and di-aromatics, frequently comprising mono-aromatic steroids. Benzothiophenes are extremely scarce; sulphur content is low to extremely low.

The paraffinic class includes some oils from North Africa, United States, South America, a few tertiary oils from West Africa, Libya, Indonesia and Central Europe.

Paraffinic-Naphthenic Class:

The Paraffinic-Naphthenic oil is as well produced in deltaic or coastal sediments of the continental margins or in non-marine source bed, it consists of a moderate resins plus-asphaltenes content, generally 5 to 15% and low sulphur content (0-1%). Aromatics amount to 25 to 40% of the hydrocarbons. Benzo- and di-benzothiophenes are fairly abundant.

Density and viscosity are generally higher than in the paraffinic oil however remain moderate.  The Paraffinic-Naphthenic class comprises numerous crude oil from Colorado, Paris, West Texas, North Aquitaine and North Sea basins.

Naphthenic Class:

There are some few crude oil in this class and this comprises some immature oils from the Jurassic and Cretaceous of South America. Though, the class comprises mostly degraded oil that generally includes less than 20% n+isoalkanes. They originate from the biochemical modification of paraffinic or paraffinic- naphthenic oil and they generally encompass low sulphur content. Illustrations are found in the Gulf Coast area, in the North Sea and in the Russia.

Aromatic Intermediate:

This class or group of crude oil is often produced in the marine sediments, deposited in a reducing atmosphere. They are comprised of crude oil that is often heavy. Resins and asphaltenes content is around 10 to 30% it might at time be higher whereas the sulphur content is above 1%.

Aromatics amount between 40 and 70% of hydrocarbons, the content of mono-aromatics and particularly those of steroid kind, is relatively low. Thiophene derivatives (benzo- and dibenzothiophene) are rich (25- 30% of the aromatics and more). The specific gravity is generally high (that is, more than 0.85).  This class comprises crude oil from Saudi Arabia, Quatar, Iraq, Kuwait, Syria, Turkey, and a few oils of West Africa, Venezuela, California and the Mediterranean (Spain, Sicily and Greece).

Aromatic-Asphaltic and Aromatic Naphthenic:

These are generally modified crude oil. Throughout the biodegradation, alkanes are first eliminated from crude oil. This yields in shift of the crude oil away from the alkane pole. Later, a more advanced degradation might comprise elimination of mono-cycloalkanes and oxidation. Then the position of oil is as well shifted toward the aromatic pole.  

Thus, most of the aromatic-naphthenic and aromatic-asphaltic oils are heavy, viscous oils resultant originally from degradation of paraffinic, paraffinic-naphthenic or aromatic-intermediate oils. The resin-plus- asphaltene content is generally above 25% and might reach 60%. Though, the relative content of resins and asphaltenes, and the quantity of sulphur, might differ according to the kind of the original crude oil.

The six classes of crude oil illustrated above are very unevenly populated. Most of the normal (non-degradable) crude oil belongs to only three classes. They are: aromatic-intermediate, paraffinic-naphthenic and paraffinic oil. This assessment is mainly based on various oil investigated. Though, if we now consider the net amount of known production and reserves, the relative significance of the classes is modified. By far the most significant classes with respect to quantity are the aromatic- naphthenic, then the aromatic-asphaltic and aromatic intermediate.

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