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Petroleum fraction

A complex mixture consisting of various types of hydrocarbon and non hydrocarbon compounds

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Petroleum fraction refers to various types of hydrocarbons and Non hydrocarbon compounds A complex mixture. The molecular weight of petroleum fractions ranges from tens to thousands, so the boiling point range is very wide, from normal temperature to over 500 ℃.

Chinese name: Petroleum fraction
Foreign name: Petroleum cut
Interpretation: Hydrocarbon and non hydrocarbon compounds

Type: Petrochemistry
processing method: fractionation
Discipline: petroleum refining engineering

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Basic Introduction

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Petroleum is a multi-component complex mixture with a wide boiling point range from normal temperature to above 500 ℃. Therefore, it is necessary to fractionate oil whether it is used for research or processing. Fractionation is to "cut" oil into several "fractions" according to the difference of boiling points of components, such as<200 ℃ fraction, 200~350 ℃ fraction, etc. The boiling point range of each fraction is referred to as distillation range or boiling range for short.

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Distillates are often called gasoline, kerosene, diesel oil, lubricating oil and other petroleum products, but distillates are not petroleum products. To meet the requirements of oil product specifications, petroleum products need to add one more fraction to become petroleum products. Various petroleum products often overlap in the range of fractions. For example, there is an overlap between the distillate ranges of jet fuel and light diesel oil. In order to unify the name, crude oil is generally treated at atmospheric pressure distillation The light fraction from the initial distillation temperature (initial distillation point) to 200 ℃ (or 180 ℃) is called gasoline fraction (also called light oil or naphtha fraction), and the middle fraction between 200 ℃ (or 180 ℃) and 350 ℃ is called coal diesel fraction, or Atmospheric gas oil (AGO for short).

Since crude oil has obvious decomposition since 350 ℃, distillate fractions with boiling point higher than 350 ℃ under reduced pressure, and then convert the boiling point of distillate fractions under reduced pressure into normal pressure boiling point. Generally, the high boiling point fraction equivalent to 350~500 ℃ under normal pressure is called vacuum fraction or lubricating oil fraction, or Vacuum gas oil (VGO for short); The residual oil after vacuum distillation>500 ℃ is called vacuum residue (VR); At the same time, the oil with atmospheric distillation temperature greater than 350 ℃ is also called atmospheric residue or atmospheric heavy oil (AR for short). The following table shows the fraction composition of some domestic and foreign crude oils.

Fraction composition of some domestic and foreign crude oils
Name of crude oil	Fraction composition (mass fraction),%
Name of crude oil	Initial boiling point~200 ℃	200℃~350℃	350℃~500℃	>500℃
Daqing	eleven point five	nineteen point seven	twenty-six	forty-two point eight
victory	seven point six	seventeen point five	twenty-seven point five	forty-seven point four
isolated island	six point one	fourteen point nine	twenty-seven point two	fifty-one point eight
Liaohe River	nine point four	twenty-one point five	twenty-nine point two	thirty-nine point nine
North China	six point one	nineteen point nine	thirty-four point nine	thirty-nine point one
central plains	nineteen point four	twenty-five point one	twenty-three point two	thirty-two point three
Xinjiang (oil pipeline)	fifteen point four	twenty-six	twenty-nine point nine	twenty-nine point seven
Xinjiang (Korla)	nineteen point six	thirty-one point one	twenty-six point one	twenty-three point two
Xinjiang (nine districts)	two point three	eighteen point nine	twenty-eight point nine	forty-nine point nine
Shanjia Temple	one point two	twelve point two	eighteen point three	sixty-eight point three
Saudi Arabia (light)	twenty-three point three	twenty-six point three	twenty-five point one	twenty-five point three
Saudi Arabia (mixed light and heavy)	twenty point seven	twenty-four point five	twenty-three point two	thirty-one point six
United Arab Emirates (Meripo)	thirty-one point five	thirty point six	twenty-three point two	fourteen point seven
UK (North Sea)	twenty-nine	twenty-seven point six	twenty-five point four	eighteen
Indonesia (Minas)	eleven point nine	thirty point two	twenty-four point eight	thirty-three point one

Compared with foreign crude oils, the content of vacuum residue above 500 ℃ in the crude oils of major oil regions in China is higher, and the content of gasoline fractions below 200 ℃ is less. The low content of gasoline fraction and high content of residue in crude oil is a characteristic of the composition of crude oil fractions in China.

The fractions obtained from direct fractionation of petroleum are called straight run fractions, which basically retain the original properties of petroleum, for example, they basically do not contain unsaturated hydrocarbons. After secondary processing (such as catalytic cracking) of petroleum straight run fractions, the chemical composition of the obtained fractions is different from that of the corresponding straight run fractions. For example, the chemical composition of catalytic cracking products contains unsaturated hydrocarbons (not all secondary processing products contain unsaturated hydrocarbons).^[1]

Hydrocarbons

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In terms of chemical composition, petroleum mainly contains hydrocarbon and non hydrocarbon substances. Hydrocarbons and non hydrocarbons exist in each fraction of petroleum, but the relative content of hydrocarbons and non hydrocarbons varies greatly due to different origins and types of petroleum. Some petroleum (light petroleum) can contain more than 90% hydrocarbons, but some petroleum (heavy petroleum) can contain even less than 50% hydrocarbons. In the same crude oil, with the increase of boiling range of fractions, the hydrocarbon content decreases but the non hydrocarbon content gradually increases. In the lightest light oil fraction, the content of non hydrocarbons is very small, and hydrocarbons account for the majority. Even the gasoline fraction obtained from sour crude oil has a hydrocarbon content of 98%~99%. On the contrary, the content of hydrocarbons in high boiling petroleum fractions, especially in vacuum residue, decreases significantly.

In order to understand the hydrocarbon composition of petroleum, we must first understand the representation method of hydrocarbon composition. The element composition of petroleum, which is the simplest representation of hydrocarbon composition, and the atomic ratio of hydrogen to carbon is also an important parameter to characterize the average chemical structure of petroleum. However, it is not enough to understand oil only from the element composition, and it often cannot meet the requirements of production and scientific research. In order to further understand the hydrocarbon composition in petroleum, there are three other methods of expression.

1. Composition of single hydrocarbon

The composition of monomer hydrocarbon indicates the content of each monomer compound in petroleum and its fractions. There are many monomer compounds in petroleum and its fractions, and with the increase of boiling range (or relative molecular weight) of petroleum fractions, the number of monomer compounds increases sharply. Due to the limited means of analysis and separation, at present, the representation of the composition of single hydrocarbon is only used to describe the composition of petroleum gas and petroleum low boiling fraction. For example, hundreds of monomer compounds in gasoline fractions can be analyzed and identified by gas chromatography.

2. Family composition

The representation method of monomer hydrocarbon composition is too elaborate, and the group composition representation method is often used when it is not necessary or impossible to analyze monomer compounds in practical applications. The so-called "family" is a class of compounds with similar chemical structures. The families to be divided into depend on the analysis method and practical application needs. Generally, the analysis of gasoline fractions is expressed by the content of alkanes, cycloalkanes and aromatic hydrocarbons. If the cracked gasoline is to be analyzed, the analysis of unsaturated hydrocarbon shall be added because it contains unsaturated hydrocarbon. If more detailed analysis is required for gasoline fractions, alkanes can be further divided into normal alkanes and iso alkanes, cycloalkanes into cyclohexane series and cyclopentane series, and aromatic hydrocarbons into benzene and other aromatic hydrocarbons.

Kerosene, diesel oil and vacuum fraction have different analysis items due to different analysis methods. For example, if liquid-solid chromatography is used, the group composition is usually saturated hydrocarbons (alkanes and cycloalkanes), light aromatics (monocyclic aromatics), medium aromatics (bicyclic aromatics), and heavy aromatics（ Polycyclic aromatic hydrocarbon ）And the content of non hydrocarbon components. If used Mass spectrometry The group composition can be expressed by the content of alkanes (normal alkanes, isoparaffins), cycloalkanes (monocyclic, bicyclic and polycyclic cycloalkanes), aromatic hydrocarbons (monocyclic, bicyclic and polycyclic aromatic hydrocarbons) and non hydrocarbon compounds.

For vacuum residue, solvent treatment and liquid chromatography are generally used to separate the vacuum residue into four components: saturated component, aromatic component, resin and asphaltene. If necessary, the aromatic component and resin can be further separated into light, medium and heavy aromatics and light, medium and heavy resins.

3. Representation method of structure family composition

Due to the large number of various types of molecules in high boiling fraction and residue relative molecular mass The molecular structure is complex due to the increase in size, which often contains aromatic rings, naphthenic rings and alkyl side chains of considerable length and number in one molecule.^[1]

Non hydrocarbon

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Petroleum contains a considerable amount of non hydrocarbon compounds, especially in heavy petroleum fractions and vacuum oil. As mentioned earlier, hydrocarbons are the main part of petroleum. The main elements of petroleum are carbon and hydrogen, and the total amount of sulfur, nitrogen, oxygen and other miscellaneous elements generally accounts for 1%~5%. However, it must not be considered that the content is insignificant, because it refers to elements. In petroleum, sulfur, nitrogen and oxygen mainly exist not in the form of elements but in the form of compounds. Therefore, from the perspective of non hydrocarbon compounds, their locations in petroleum are considerable.

The existence of non hydrocarbon compounds has a great impact on the petroleum processing technology and the performance of petroleum products. For example, most refining processes and catalyst poisoning problems in petroleum processing, environmental pollution problems in petrochemical plants, storage and use of petroleum products and many other problems are closely related to non hydrocarbon compounds.

In order to better solve some problems in petroleum processing and product application, as well as to reasonably utilize the petroleum resources of non hydrocarbon compounds, it is necessary to understand the chemical composition, existing forms and distribution laws of non hydrocarbon compounds in petroleum.

Non hydrocarbon compounds in petroleum mainly include sulfur, nitrogen Oxygenates And gelatinous asphaltic substances.^[1]

Sulfur compounds in petroleum

1. Distribution of sulfur in petroleum and its fractions

Sulfur is one of the components of petroleum. The sulfur content of different oils varies greatly, from a few ten thousandths to a few percent. For example, the sulfur content of Karamay crude oil in China is only 0.04%~0.09% (mass fraction), while that of Venezuela crude oil is as high as 5.5% (mass fraction). Since sulfur has a great impact on oil addition, oil product application and environmental protection, sulfur content is often used as an important indicator for evaluating oil.

Generally, the oil with sulfur content higher than 2.0% (mass fraction) is called high sulfur oil, the oil with sulfur content lower than 0.5% (mass fraction) is called low sulfur oil, and the oil with sulfur content between 0.5% and 2.0% (mass fraction) is called sour oil. Most of the crude oil in China belongs to low sulfur oil (such as Daqing crude oil) and sour oil (such as Gudao crude oil).

The distribution of sulfur in petroleum fractions generally increases with the increase of boiling range of petroleum fractions, and most sulfur is concentrated in heavy fractions and residue. The following table shows the distribution of sulfur in each fraction of major crude oils in China. The data shows that the sulfur content of gasoline fraction is the lowest, and that of vacuum residue is the highest. Except for Tuha and Lunyilian crude oils, about 70% of the sulfur in most crude oils in China is concentrated in vacuum residue.^[2]

Distribution of sulfur in crude oil fractions in china
Distillate (boiling range) ℃	Sulfur content, μ g/g
Distillate (boiling range) ℃	Daqing	victory	isolated island	Liaohe River	central plains	Jianghan	Tuha	Wheel couplet
crude oil	one thousand	eight thousand	twenty thousand and nine hundred	two thousand and four hundred	five thousand and two hundred	eighteen thousand and three hundred	three hundred	eight thousand five hundred and ninety-eight
	one hundred and eight	two hundred	one thousand and six hundred	sixty	two hundred	six hundred	twenty	thirty
200~250	one hundred and forty-two	one thousand and nine hundred	five thousand and two hundred	one hundred and thirty	one thousand and three hundred	four thousand and four hundred	one hundred and ten	two hundred and fifty
250~300	two hundred and eight	three thousand and nine hundred	eight thousand and eight hundred	four hundred and sixty	two thousand and two hundred	five thousand and nine hundred	two hundred	nine hundred and eighty
300~350	four hundred and fifty-seven	four thousand and six hundred	twelve thousand and three hundred	eight hundred and eighty	two thousand and eight hundred	six thousand and three hundred	three hundred	three thousand and twenty
350~400	five hundred and thirty-seven	four thousand and six hundred	fourteen thousand and two hundred	one thousand one hundred and ninety	three thousand and four hundred	ten thousand and four hundred	three hundred and fifty	five thousand five hundred and forty
400~450	six hundred and twenty-seven	six thousand and three hundred	eleven thousand and twenty	one thousand and one hundred	three thousand and four hundred	fifteen thousand and four hundred	four hundred and forty	six thousand six hundred and forty
450~500	eight hundred and two	five thousand and seven hundred	thirteen thousand and three hundred	one thousand four hundred and sixty	four thousand and three hundred	sixteen thousand	six hundred and eighty	eight thousand five hundred and seventy
>500 (residual oil)	one thousand and seven hundred	thirteen thousand and five hundred	twenty-nine thousand and three hundred	three thousand and six hundred	nine thousand and four hundred	twenty-three thousand and five hundred	nine hundred and forty	sixteen thousand and seven hundred
Sulfur in residue/sulfur in crude oil,%	seventy-four point seven	seventy-three point three	seventy-five	seventy	sixty-eight	seventy-two point two	thirty point one	thirty-eight point one

The cut temperature of Jianghan crude oil is slightly different.

It must be pointed out that some sulfur compounds are thermally unstable Crude distillation It is easy to decompose into sulfide with small molecule in the process, so the determination of sulfur content in distillation products often can not correctly reflect the true distribution of sulfur in the original petroleum fractions.

2. Existing forms of sulfur in petroleum and its fractions

The existing forms of sulfur in petroleum have been determined as elemental sulfur, hydrogen sulfide, mercaptan, thioether Disulfide , thiophene and other types of organic sulfur compounds, in addition to a small number of other types of sulfur compounds. These sulfur compounds can be divided into two categories according to their properties: active sulfide and inactive sulfide. Active sulfide mainly includes elemental sulfur, hydrogen sulfide, mercaptan, etc. Their common feature is that they have a strong corrosive effect on metal equipment; Inactive sulfides mainly include sulfide, disulfide, thiophene and other sulfides that have no corrosive effect on metal equipment. Some inactive sulfides will be converted into active sulfides after thermal decomposition.

In addition to elemental sulfur and hydrogen sulfide, sulfide in petroleum exists in the form of organic sulfide.^[1]

Nitrogen containing compounds in petroleum

1. Nitrogen content and distribution in petroleum

The nitrogen content in the oil is generally lower than the sulfur content, usually within the range of 0.05%~0.5% (mass fraction), and only about 4% of the crude oil has a nitrogen content of more than 0.6% (mass fraction). The distribution of nitrogen in petroleum also increases rapidly with the increase of boiling range of fractions, and about 80% of nitrogen is concentrated in heavy oil above 400 ℃. The nitrogen content of crude oil in China is relatively high, and about 90% of nitrogen is concentrated in the residue of most crude oils in China.

Nitrogen compounds in petroleum have adverse effects on the catalytic processing of petroleum and the performance of products Catalyst poisoning Inactivation, or instability of petroleum products, easy to generate colloidal precipitation Engine fuel Nitrogenous compounds in are generated during combustion Nitrogen oxides It is harmful to human health and pollutes the environment, so it must be removed as far as possible.

2. Types of nitrogen compounds in petroleum

Nitrogen containing compounds in petroleum are generally divided into two categories according to their acid-base properties: basic nitrogen containing compounds and non basic nitrogen containing compounds. Basic nitrogenous compounds refer to the nitrogenous compounds that can be titrated by perchloric acid in the sample solution of glacial acetic acid (50% by volume) and benzene (50% by volume). The nitrogenous compounds that cannot be titrated by perchloric acid are called non basic nitrogenous compounds.

The basic nitrogen-containing compounds in petroleum and its fractions mainly include pyridine, quinoline, isoquinoline and acridine, and the content of aniline derivatives in petroleum is very low. The ring number of basic nitrogen-containing compounds increased with the increase of boiling range of fractions.^[1]

Petroleum Oxygenates

The oxygen content in oil is generally in the range of a few thousandths. Only a few oils have high oxygen content, up to 2%~3%. However, if the oil is exposed to the air for a long time before or after processing, its oxygen content will greatly increase. The oxygen content in petroleum is mostly obtained from element analysis by subtraction method (that is, the content of carbon, hydrogen, sulfur and nitrogen is subtracted by 100%), which actually includes all the analysis errors, so the data is not very reliable. Although the oxygen content in petroleum is very low, the amount of oxygenated compounds in petroleum is still considerable.

The oxygen element in petroleum exists in the form of organic oxygenates. There are roughly two types of these oxygenates: acidic oxygenates and neutral oxygenates. Acidic oxygenates in petroleum include naphthenic acids, aromatic acids, fatty acids and phenols, which are collectively called petroleum acids. Neutral oxygenates in petroleum include ketones, aldehydes and acetic acids, which contain very little in petroleum, so the oxygenates in petroleum are mainly acidic oxygenates.

1. Content and acidity (or acid value) of acidic oxygenates in petroleum

The content of acidic oxygenates in petroleum is generally expressed indirectly by acidity (or acid value). Acidity refers to the amount of acid compound required to neutralize 100mL oil sample Potassium hydroxide Milligrams (mg KOH/100mL), generally applicable to light oil products; Acid value refers to the milligram of potassium hydroxide (mg KOH/g) required to neutralize acidic compounds in 1g oil sample, which is generally applicable to heavy oil products. It should be pointed out that acidity (or acid value) and acid content are not the same concept. The content of acid compounds in the oil sample is not only related to its acidity (or acid value), but also related to Average relative molecular weight of When the acidity (or acid value) of the oil sample is the same, relative molecular mass The larger the content, the higher the content of acidic compounds. If the relative molecular weight of the sample is the same, the higher the acidity (or acid value), the higher the content of acidic compounds.

2. Acidity in petroleum Oxygenates

(1) Naphthenic acids in petroleum

It is generally believed that most of the alkenyl acids in petroleum with less than eight carbon atoms are fatty acids, but the content of fatty acids in petroleum is very small,

It is mainly naphthenic acid, which accounts for about 90% of the acid oxygenates in petroleum. The content of naphthenic acid varies according to the origin of petroleum and the type of crude oil. Paraffin based petroleum has less naphthenic acid content, while intermediate base and naphthenic cornerstone have more naphthenic acid content. Naphthenic acid generally has the highest content in the middle fraction (boiling range is about 250~400 ℃).

(2) Petroleum phenols

Among the acidic oxygenates of petroleum, there are fatty acids and phenols besides naphthenic acids, whose content usually does not exceed 10% of the total amount of acidic oxygenates.

Phenols Most of them are found in the thermal conversion of petroleum and catalytic cracking oil products. Most of the phenols in the low boiling fraction are the thermal decomposition products of macromolecular phenols with poor thermal stability in heavy oil. They are mainly cresol, xylenol, and also contain cresol and naphthol.

The structural feature of phenols is that there is one or several hydroxyl functional groups in the molecule connected with the aromatic ring, which is acidic and can react with the base to form salts and dissolve in the alkaline solution.

The iron salt of phenol is a strong stain, such as phenol and Ferric trichloride The reaction can give a strong purple color. This method is a qualitative method to detect the presence of phenols.

Because the light radicals in the phenolic structure are directly connected to the benzene ring, they have a strong impact on the chemical properties of the benzene ring, enabling the phenol to undergo condensation and oxidation reactions, and even the oxygen in the air can make the phenol oxidized and blackened.

3. Neutrality in oil Oxygenates

Since there are very few neutral oxygenates in petroleum and it is a group of very complex mixtures (including alcohols, esters, aldehydes, ketones, benzofurans, etc.), there has been little research so far.

Alcohols in petroleum are relatively stable compounds, which can only be oxidized under certain conditions. Petroleum Hydroxy compound (aldehydes and ketones) have strong reaction ability and are easy to be oxidized to form acids. The oil also contains esters, which are mainly found in the fraction above 350 ℃ and residual oil. In addition, ethers, often cyclic ethers, are also found in petroleum. Neutral oxygen containing substances such as benzofuran, dibenzofuran and naphthenic furan are also found in petroleum Heterocyclic compound 。^[1]

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