Composition of Crude Oil
Crude oils are from different sources, generated from different raw materials and source rocks. They are stored in different reservoir rocks. As a result of these the chemical composition of crude oil varies from crude oil field to field and even from one well to another well in the same field.
The composition of crude oil however determines the economic value, in particular the demand for it and the selling price. This article is therefore developed to explain the various general components of crude oil which is not necessarily a uniform composition.
Hydrocarbons
Crude oil is considered the major source of hydrocarbon in nature because the principal components of crude oils are hydrocarbon compounds.
Crude oil contains alkane of all types but not alkenes and alkynes except during processing (like thermal cracking) where alkenes are produced. This section will give descriptions of the various hydrocarbons found in crude oil.
Alkane (Paraffin)
Alkane is a class of straight and branched chain saturated hydrocarbons. It has the general formula CnH2n+2. Alkane is the major component of crude oil. Its simplest form is when n=1 which is CH4: methane.
This methane is the principal component of natural gas. Other alkanes, ethane (n=2), propane (n=3), and butane (n=4) are also gaseous at atmospheric temperatures and pressure.
Crude oil contains straight-chain saturated hydrocarbons (i.e. no branches) are often referred to as normal alkanes (n-alkanes, n-paraffin). Branched alkanes on the other hand are saturated hydrocarbons with an alkyl substituent or a side branch from the main chain.
Alkane in crude oil is the basis of the application of its fraction in fuel such as liquefied natural gas (LNG), petroleum motor spirit (PMS) which is gasoline among others.
Cycloalkanes (Naphthenes)
Another class of hydrocarbons that are commonly found in crude oil is the saturated cyclic hydrocarbons normally known as naphthenes. This class is also an important part of the hydrocarbon constituents of crude oils.
Their ratio is used to define the type of crude from a well. The type of crude oil can be said to be paraffinic (i.e. containing more of saturated straight and branched chain hydrocarbon) or naphthenenic type (containing more of saturated cyclic hydrocarbon).
The lower members of naphthenes are cyclopentane, cyclohexane, and their mono- substituted compounds.
These low members of naphthenes are prominent in the light and the heavy naphtha fractions and are precursors for aromatic hydrocarbons. Methylcyclopentane and cyclohexane can be used to produce benzene while Methylcyclohexane can be dehydrogenated into toluene.
Heavier petroleum fractions such as kerosene and gas oil may contain two or more fused cyclohexane rings and other bicyclic hydrocarbons up to C19 to C22 range.
Aromatics
The last class of hydrocarbon present in crude oil is the aromatics. Aromatic compounds such as the simple mononuclear aromatic compounds: benzene (C6H6), toluene (C7H8) and xylene (C8H10) are present in crude oils but with varying amounts based on crude oil type.
These low members of aromatic compounds are in small amounts in both crude oils and light petroleum fractions.
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These aromatic components of crude oil – benzene, toluene, and xylenes (BTX) are valuable intermediates for petrochemical productions and are also valuable components of gasoline fraction because they improve its octane rating.
Binuclear, Trinuclear and polynuclear aromatic hydrocarbons are also found in crude oil and are present in the heavier fractions of the crude oil distillation process.
Other aromatic compounds are the heterocyclic compounds (to be discussed in the next section under non-hydrocarbons). They are found more in the heavy crudes and crude residues. Asphaltenes are terms used for complex mixtures of aromatic and heterocyclic compounds.
Non-hydrocarbon Compounds
There are four key types of non-hydrocarbon compounds that occur in crude oils and in the refinery streams after distillation. They are the organic sulphur, nitrogen, oxygen and traces of metallic compounds.
The presence or absence of these compounds has impact on its economic value and sales. Crude with less sulphur compounds are more valuable in the oil market. This is because impurities poison the catalysts used in the catalytic processes for crude oil conversions.
Moreover, production of fuels having high sulphur and nitrogen contents raise environment pollution concern. These compounds also have corrosion promotion tendencies which can negatively affect the pipelines and the refinery facilities.
Sulphur Compounds
The presence of sulphur in crude oils is not in elemental form but mainly as organosulphur compounds. It may however also be present as hydrogen sulphide (H2S) which is the only inorganic sulphur compound found in crude oil.
The presence of both H2S and organosulphur compounds is harmful because of their corrosive nature. Organosulphur compounds present in crude oil can either be in form of acidic and non- acidic compounds.
Acidic sulphur compounds are the one that have sulphur-hydrogen bonds (-SH) that can donate proton such as thiols (mercaptans).
The non-acidic sulphur compounds found in crude fractions are in forms of thiophene, sulphides, and disulfides.
Crude oil can be sour or sweet depending on the sulphur content. Sour crudes contain hydrogen sulphide in high percentage. This classification of crude oil sales based on the sulphur content affect its sales in the global oil market.
This is possible because many organic sulphur compounds are heat sensitive thereby generating hydrogen sulphide in the cause of crude processing.
Crudes with high-sulphur are less desirable because treating the different refinery streams for acidic hydrogen sulphide increases production costs.
Hydrogen sulphide is often removed by either absorption in a suitable absorbent or oxidation of H2S to recover the sulphur as solid material.
Nitrogen Compounds
A number of organic nitrogen compounds occur in crude oils. They are mostly found in simple heterocyclic form as pyrrole (C4H5N) and pyridine (C5H5N). Another significant nitrogen based compound is a complex structure of porphyrin.
Generally, the nitrogen content in most crude oils is lower than 0.1 wt%. However, in heavy crudes, the percentage content of the nitrogen based compounds may be up to 0.9 wt %.
Nitrogen and sulphur compounds are not desirable in crude because of the poisoning effect of catalysts during crude oil processing.
They should be removed during processing using hydrotreatment of petroleum fractions, nitrogen compounds are hydrodenitrogenated to ammonia and the corresponding hydrocarbon. This need for removal impacts significantly on the cost of the petroleum.
Nitrogen compounds present in petroleum may be grouped into basic nitrogen compounds and non-basic nitrogen compounds.
The basic compounds are mainly those with pyridine ring, and the non-basic compounds are with pyrrole structure. Examples of the two classes of organic nitrogen compounds are presented below:
Basic nitrogen compounds
Pyridine
Quinoline
Isoquinoline
Acridine
Porphyrins
Porphyrins are another important nitrogen based compounds. They are complex derivatives of the basic material, porphine. These compounds are used to characterise crude oil and support the biogenic theory of crude oil origin.
They are recognized as the degradation products of the chlorophyll and of the hemes (haems) and hematins (haematins), the respiratory pigments of both plants and animals which are in the deposit that form petroleum.
Oxygen Compounds
Another very important component of crude oil is the oxygen based compounds. They are more complex than the sulphur and nitrogen types but do not constitute poison to crude oil processing catalysts.
They do not therefore impact negatively on the price of petroleum. Oxygen compounds found in crude oils may be weakly acidic such as carboxylic acids, cresylic acid, phenol, and naphthenic acid or non-acidic like ethers, esters, carbonyls, amides among others.
The non-acidic oxygen compounds are often less abundant than acidic compounds in most crude oil well and are often of no commercial value compared to the acidic oxygen compounds.
Metallic Compounds
In the section of nitrogen based compounds, we discussed the porphyrin material in crude oils.
These porphyrin rings are mostly completed with a metal. The most important of these metals is vanadium, followed by nickel. Other metals like iron and copper may also be present.
Out of all the metals in the periodic table, only V and Ni have been proven to definitely exist as chelates in significant amounts in a large number of fossil fuel such as crude oils and tar sand bitumen.
In general, the organic detritus of sediments from which crude oil is formed varies with the environment since aquatic organisms are different in their chemical makeup. This contributes to the variation in petroleum types: gross differences and structural variations of petroleum constituents.
The understanding of the differences is very important in source apportionment of petroleum. Thus, Ni/V ratio is used to investigate variations in the source materials and also variations in the conditions (geophysical) under which the petroleum was formed (Yakubov etal.,2016).
Many other metals occur in crude oils but not in the chelate organic forms. Sodium, calcium, magnesium, aluminium, iron, are present as inorganic salts, such as sodium and magnesium chlorides.
Although these metals in crudes are found in trace amounts, their presence is not desirable in petroleum and should be removed. Calcium and magnesium can form salts or soaps with carboxylic acids and act as emulsifiers making crude oil separation from water difficult.
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Sodium and magnesium chlorides generate hydrochloric acid which will corrode the crude oil processing facilities. Desalting of crude oils to remove sodium and magnesium chlorides is a necessary.
Vanadium and nickel can poison many of the catalysts used in crude oil conversion processes. Most of the vanadium and nickel compounds are concentrated in the heavy residues.
Overall, crude oil is made up of
| Carbon | – 83.0% to 87.0% |
| Hydrogen | – 10.0% to 14.0% |
| Nitrogen | – 0.1% to 2.0% |
| Oxygen | – 0.05% to 1.5% |
| Sulphur | – 0 .05% to 6.0% |
| Metals (Ni and V) | – <1000 ppm |
