Hydrocarbons are the most important component of any oil. The concentration of natural hydrocarbons in different types of oil is not the same: from 100 (gas condensate) to 30%. On average, hydrocarbons make up 70% of the mass of this fuel.
Hydrocarbons in oil
About 700 hydrocarbons of a peculiar structure have been identified in the composition of oils. All of them are diverse in composition and structure, but at the same time they store information about the composition and structure of substances that form the basis of lipids of ancient bacteria, algae and higher plants.
The hydrocarbon composition of oil includes:
- Paraffins.
- Naphthenes (cycloalkanes).
- Aromatic hydrocarbons (arenes).
Alkanes (aliphatic saturated hydrocarbons)
Alkanes are the most important and well-studied hydrocarbons of any oil. The composition of oil includes hydrocarbons alkanes from C1 to C100. Their number ranges from 20 to 60% and depends on the type of oil. As the molecularmass fraction, the concentration of alkanes is reduced in all types.
If cyclic hydrocarbons of different structures are equally common in oil, then structures of a certain structure usually predominate among alkanes. Moreover, the structure, as a rule, does not depend on molecular weight. This means that in different types of oil there are certain homologous series of alkanes: alkanes of a normal structure, monomethyl-substituted with different positions of the methyl group, less often - di- and trimethyl-substituted alkanes, as well as tetramethylalkanes of the isoprenoid type. Alkanes of a characteristic structure make up almost 90% of the total mass of oil alkanes. This fact allows a good study of alkanes in various oil fractions, including high-boiling ones.
Alkanes of different fractions
At temperatures from 50 to 150 °C, fraction I is released, which includes alkanes with the number of carbon atoms from 5 to 11. Alkanes have isomers:
- pentane - 3;
- hexane – 5;
- heptane – 9;
- octane - 18;
- nonan - 35;
- Dean – 75;
- undecan – 159.
Therefore, fraction I can theoretically include about 300 hydrocarbons. Of course, not all isomers are present in oil, but their number is large.
The figure shows a chromatogram of alkanes C5 – C11 of oil from the Surgut field, where each peak corresponds to a certain substance.
At a temperature of 200-430 °С, alkanes of fraction II of composition С12 – С27 are isolated. The figure showschromatogram of alkanes of fraction II. The chromatogram shows the peaks of normal and monomethyl-substituted alkanes. The numbers indicate the position of the substituents.
At a temperature of >430°C, alkanes of fraction III of composition С28 – С40.
Isoprenoid alkanes
Isoprenoid alkanes include branched hydrocarbons with regular alternation of methyl groups. For example, 2, 6, 10, 14-tetramethylpentadecane or 2, 6, 10-trimethylhexadecane. Isoprenoid alkanes and straight chain alkanes make up the majority of the biological petroleum feedstock. Of course, there are many more options for isoprenoid hydrocarbons.
Isoprenoids are characterized by homology and disequilibrium, that is, different oils have their own set of these compounds. Homology is a consequence of the destruction of higher molecular weight sources. In isoprenoid alkanes, "gaps" in the concentrations of any homologues can be detected. This is a consequence of the impossibility of breaking their chain (the formation of this homologue) in the place where the methyl substituents are located. This feature is used to determine the sources of isoprenoid formation.
Cycloalkanes (naphthenes)
Naphthenes are saturated cyclic hydrocarbons of oil. In many oils, they predominate over other classes of hydrocarbons. Their content can vary from 25 to 75%. Found in all factions. As the fraction becomes heavier, their content increases. Naphthenes are distinguished by the quantitycycles in a molecule. Naphthenes are divided into two groups: mono- and polycyclic. Monocyclic are five- and six-membered. Polycyclic rings can include both five- and six-membered rings.
Low-boiling fractions contain predominantly alkyl derivatives of cyclohexane and cyclopentane, with methyl derivatives predominating in gasoline fractions.
Polycyclic naphthenes are found mainly in oil fractions that boil away at temperatures above 300 °C, and their content in fractions of 400-550 °C reaches 70-80%.
Aromatic hydrocarbons (arenes)
They are divided into two groups:
- Alkylaromatic hydrocarbons containing only aromatic rings and alkyl substituents. These include alkylbenzenes, alkylnaphthalenes, alkylphenanthrenes, alkylchrysepes, and alkylpicenes.
- Hydrocarbons of mixed structure type, containing both aromatic (unsaturated) and naphthenic (limiting) rings. Among them are distinguished:
- monoaromatic hydrocarbons - indanes, di-, tri- and tetranaphthenobenzenes;
- diaromatic hydrocarbons - mono- and dinaphthenonaphthalenes;
- hydrocarbons with three or more aromatic rings - naphthenophenanthrenes.
Technical significance of the hydrocarbon composition of oil
The composition of substances significantly affects the quality of oil.
1. Paraffins:
- Normal paraffins (unbranched) have a low octane number and high pour points. Therefore, inin the process of processing they are converted into hydrocarbons of other groups.
- Isoparaffins (branched) have a high octane number, i.e. high antiknock properties (isooctane is a reference compound with an octane number of 100), as well as low pour points compared to normal paraffins.
2. Naphthenes (cycloparaffins) along with isoparaffins have a positive effect on the quality of diesel fuel and lubricating oils. Their high content in the heavy gasoline fraction leads to high yield and high octane number of products.
3. Aromatic hydrocarbons worsen the environmental properties of the fuel, but have a high octane number. Therefore, during oil refining, other groups of hydrocarbons are converted into aromatic ones, but their amount, primarily benzene, in the fuel is strictly regulated.
Methods for studying the hydrocarbon composition of oil
For technical purposes, it is sufficient to establish the composition of oil by the content of certain classes of hydrocarbons in it. The fractional composition of oil is important for choosing the direction of oil refining.
In order to determine the group composition of oil, various methods are used:
- Chemical means carrying out a reaction (nitration or sulfonation) of the interaction of a reagent with a certain class of hydrocarbons (alkenes or arenes). By changing the volume or the amount of the resulting reaction products, the content of the determined class of hydrocarbons is judged.
- Physico-chemical include extraction and adsorption. This is how arenes are extractedsulfur dioxide, aniline or dimethyl sulfate, followed by adsorption of these hydrocarbons on silica gel.
- Physical includes the determination of optical properties.
- Combined - the most accurate and most common. Combine any two methods. For example, the removal of arenes by chemical or physico-chemical methods and the measurement of the physical properties of oil before and after their removal.
For scientific purposes, it is important to determine exactly which hydrocarbons are present or predominant in oil.
To identify individual molecules of hydrocarbons, gas-liquid chromatography is used using capillary columns and temperature control, chromatography-mass spectrometry with computer processing and chromatogram building for individual characteristic fragment ions (mass fragmentography or mass chromatography). NMR spectra on nuclei 13C.
are also used
Modern schemes for analyzing the composition of oil hydrocarbons include preliminary separation into two or three fractions with different boiling points. After that, each of the fractions is separated into saturated (paraffin-naphthenic) and aromatic hydrocarbons using liquid chromatography on silica gel. Further, aromatic hydrocarbons should be separated into mono-, bi- and polyaromatic using liquid chromatography using aluminum oxide.
Sources of hydrocarbons
Natural sources of oil and gas hydrocarbons are bioorganic molecules of various compounds, mainly their lipid components. Imimay be:
- higher plant lipids,
- algae,
- phytoplankton,
- zooplankton,
- bacteria, especially cell membrane lipids.
The lipid components of plants are very similar in chemical composition, however, certain variations of the molecules make it possible to determine the predominant participation of certain substances in the formation of this oil.
All plant lipids are divided into two classes:
- compounds consisting of molecules with a straight (or slightly branched) chain;
- compounds based on isoprenoid units of alicyclic and aliphatic series.
There are compounds consisting of elements belonging to both classes, such as wax. Wax molecules are esters of higher saturated or unsaturated fatty acids and cyclic isoprenoid alcohols - sterols.
Typical representatives of lipid natural sources of petroleum hydrocarbons are the following compounds:
- Saturated and unsaturated fatty acids of composition C12-C26 and hydroxy acids. Fatty acids are made up of an even number of carbon atoms, since they are synthesized from C2-acetate components. They are part of triglycerides.
- Natural wax - unlike fats, it does not contain glycerol, but higher fatty alcohols or sterols.
- Weakly branched acids having methyl substituents at the end of the chain opposite from the carboxyl group, for example, iso- and anteisoacids.
- Interesting substances are suberin and cutin, which are included indifferent parts of plants. They are formed by polymerized bound fatty acids and alcohols. These compounds are resistant to enzymatic and microbial attack, which protects aliphatic chains from biological oxidation.
Relic and converted hydrocarbons
All oil hydrocarbons are divided into two groups:
- Transformed - having lost the structural features characteristic of the original bioorganic molecules.
- Relic, or chemofossils - those hydrocarbons that have retained the characteristic features of the structure of the original molecules, regardless of whether these hydrocarbons were in the original biomass or were formed later from other substances.
Relic hydrocarbons that make up oil are divided into two groups:
- isoprenoid type - alicyclic and aliphatic structure, with up to five cycles in one molecule;
- non-isoprenoid - mostly aliphatic compounds having n-alkyl or lightly branched chains.
Relics of isoprenoid structure are much more numerous than non-isoprenoid ones.
Over 500 relic oil hydrocarbons have been isolated, and their number is increasing every year.
