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 are 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 the lipids of ancient bacteria, algae and higher plants.
The hydrocarbon composition of oil includes:
- Paraffins.
- Naphthenes (cycloalkanes).
- Aromatic hydrocarbons (arenas).
Alkanes (aliphatic saturated hydrocarbons)
Alkanes are the most important and well-studied hydrocarbons of any oil. The composition of oil includes alkanes hydrocarbons from C 1 to C 100 . Their number ranges from 20 to 60% and depends on the type of oil. As the molecular weight of the fraction increases, the concentration of alkanes decreases in all types.
If cyclic hydrocarbons of different structures are found equally frequently in oil, structures of a certain structure usually prevail among alkanes. Moreover, the structure, as a rule, does not depend on molecular weight. This means that certain homologous series of alkanes are present in different types of oil: alkanes of normal structure, monomethyl substituted with different positions of the methyl group, less often di- and trimethyl substituted alkanes, as well as isoprenoid tetramethyl alkanes. Alkanes of characteristic structure make up almost 90% of the total mass of oil alkanes. This fact allows us to study alkanes in various, including high-boiling, oil fractions.
Alkanes of different fractions
At a temperature of 50 to 150 ° C, fraction I is released, which contains 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 theoretically may include about 300 hydrocarbons. Of course, not all isomers are present in oil, but their number is large.
The figure shows a chromatogram of C 5 - C 11 alkanes from the oil of the Surgut field, in which each peak corresponds to a specific substance.
At a temperature of 200-430 ° C, alkanes of fraction II of composition C 12 - C 27 are released . The figure shows the chromatogram of fraction II alkanes. The chromatogram shows peaks of normal and monomethyl substituted alkanes. The numbers indicate the position of the substituents.
At temperatures> 430 ° C, alkanes of fraction III of composition C 28 - C 40 are released .
Isoprenoid alkanes
Branched hydrocarbons with the correct alternation of methyl groups are referred to isoprenoid alkanes. For example, 2,6,10,14-tetramethylpentadecane or 2,6,10-trimethylhexadecane. Unbranched chain isoprenoid alkanes and alkanes make up the predominant mass of the biological source material of oil. Of course, there are many more options for isoprenoid hydrocarbons.
Isoprenoids are characterized by homology and disequilibrium, that is, different oils are characterized by their own set of these compounds. Homology is a consequence of the destruction of higher molecular sources. In isoprenoid alkanes, “gaps” in the concentrations of any homologs can be detected. This is a consequence of the impossibility of breaking their chain (the formation of this homologue) in the place where methyl substituents are located. This feature is used to determine the sources of isoprenoid formation.
Cycloalkanes (naphthenes)
Naphthenes are the ultimate cyclic oil hydrocarbons. In many oils, they prevail over other classes of hydrocarbons. Their content can range from 25 to 75%. Present in all fractions. As the fraction becomes heavier, their content increases. Naphthenes are distinguished by the number of cycles in the molecule. Naphthenes are divided into two groups: mono-and polycyclic. Monocyclic are five- and six-membered. Polycyclic may include five- and six-membered rings.
The low boiling fractions contain predominantly alkyl derivatives of cyclohexane and cyclopentane, with methyl derivatives prevailing in gasoline fractions.
Polycyclic naphthenes are mainly contained in oil fractions, boiling off at a temperature of more than 300 ° C, and their content in fractions of 400-550 ° C reaches 70-80%.
Aromatic hydrocarbons (arenas)
They are divided into two groups:
- Alkylaromatic hydrocarbons, which include only aromatic rings and alkyl substituents. These include alkylbenzenes, alkylnaphthalenes, alkylphenanthrenes, alkylchrysepes and alkylpicenes.
- Hydrocarbons of a mixed type of structure containing both aromatic (unsaturated) and naphthenic (limit) 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 indicators of oil.
1. Paraffins:
- Normal paraffins (unbranched) have a low octane number and high pour points. Therefore, in the process of processing they turn into hydrocarbons of other groups.
- Isoparaffins (branched) have a high octane number, that is, high antiknock properties (isooctane is a reference compound with an octane number of 100), as well as low solidification temperatures 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 a high yield and high octane number of products.
3. Aromatic hydrocarbons degrade the environmental properties of the fuel, but have a high octane rating. Therefore, in the processing of oil, other groups of hydrocarbons are converted into aromatic ones, but the amount of them, primarily benzene, in fuel is strictly regulated.
Methods of 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 ones involve a reaction (nitration or sulfonation) of the interaction of a reagent with a specific class of hydrocarbons (alkenes or arenes). By changing the volume or quantity of the resulting reaction products, the content of the determined class of hydrocarbons is judged.
- Physicochemicals include extraction and adsorption. Thus, the arenas are extracted with sulfur dioxide, aniline or dimethyl sulfate, followed by adsorption of these hydrocarbons on silica gel.
- Physical include the determination of optical properties.
- Combined - the most accurate and most common. Combine two of any methods. For example, the removal of arenas 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 in oil are contained or predominate.
To identify individual hydrocarbon molecules, gas-liquid chromatography using capillary columns and temperature determination, gas chromatography-mass spectrometry with computer processing and the construction of chromatograms for individual characteristic fragment ions (mass fragmentography or mass chromatography) are used. NMR spectra on 13 C nuclei 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. Next, aromatic hydrocarbons should be divided into mono-, bi- and polyaromatic using liquid chromatography using alumina.
Hydrocarbon sources
Natural sources of oil and gas hydrocarbons are bioorganic molecules of various compounds, mainly their lipid components. They can be:
- higher plant lipids,
- seaweed,
- phytoplankton,
- zooplankton,
- bacteria, especially lipids of cell membranes.
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 unbranched (or weakly branched) chain molecules;
- compounds based on isoprenoid units of the alicyclic and aliphatic series.
There are compounds consisting of elements belonging to both classes, for example, wax. Wax molecules are esters of higher saturated or unsaturated fatty acids and cyclic isoprenoid alcohols - sterols.
Typical representatives of natural lipid sources of petroleum hydrocarbons are the following compounds:
- Saturated and unsaturated fatty acids of composition C 12 -C 26 and hydroxy acids. Fatty acids consist of an even number of carbon atoms, as they are synthesized from C 2 -acetate components. They are part of triglycerides.
- Natural wax - unlike fats, does not contain glycerin, but higher fatty alcohols or sterols.
- Weakly branched acids having methyl substituents at the opposite end of the chain from the carboxyl group, for example iso- and anteiso acids.
- Interesting substances are suberin and cutin, which are found in different parts of plants. They are formed by polymerized bound fatty acids and alcohols. These compounds are resistant to enzymatic and microbial effects, which protects aliphatic chains from biological oxidation.
Relic and transformed hydrocarbons
All oil hydrocarbons are divided into two groups:
- Converted - lost the structural features characteristic of the original bioorganic molecules.
- Relict, or chemofossilia, are those hydrocarbons that retained the characteristic structural features 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;
- nonisoprenoid - mainly aliphatic compounds having n-alkyl or weakly branched chains.
There are much more relics of the isoprenoid structure than nonisoprenoid ones.
Over 500 relict oil hydrocarbons have been allocated, and their number is increasing every year.