Properties of crude oil

When the number of carbon atoms is the same, aromatic hydrocarbons have the highest relative density, followed by cycloalkanes, with alkanes having the lowest density; the density of alkenes is slightly higher than that of alkanes； For homologous hydrocarbons, the relative density increases as their molecular mass increases ； The relative density of the various fractions in oil increases as their boiling range increases ； The order of relative density for fractions of the same boiling range in different crude oils is: naphthenic > intermediate > paraffinic.

General principle: The greater the molecular weight, the greater the density； The higher the number of carbon atoms in hydrocarbons, the greater their density ； The higher the boiling point, the greater the density

There are not only many polymers, but also numerous inorganic substances such as sulfur, nitrogen, oxygen, nickel, vanadium, and so on. The density of a substance is related to its molecular weight, but this varies depending on the circumstances. For substances with the same number of carbon atoms, the more branches there are, the lower the density; there are also isomers and so on. It is estimated that there will be comprehensive answers regarding organic and inorganic substances.

Oil is a mixture of complex organic compounds, which includes hydrocarbons composed of carbon and hydrogen, as well as non-hydrocarbons made up of carbon, hydrogen, and other elements. The structure and content of these hydrocarbons and non-hydrocarbons determine the properties of petroleum and its products. (1) Hydrocarbons: Hydrocarbons are the main components of petroleum, which primarily consist of alkanes, cycloalkanes, and aromatic hydrocarbons; alkenes are generally not present in petroleum. Olefins are only present in the secondary processed products of oil and in shale tar. (1) Alkanes: Alkanes are a major component of petroleum. Under normal temperature and pressure, n-alkanes with one to four carbon atoms are gases, those with five to fifteen carbon atoms are liquids, and those with sixteen or more carbon atoms are solids. Natural gas that contains a large amount of methane and small amounts of ethane and propane is called dry gas ； Natural gas that contains not only large amounts of methane and ethane but also small amounts of vapor of volatile liquid hydrocarbons (such as pentane, hexane, and octane) is called wet gas. It is usually classified based on the amount of gaseous gasoline, which are liquid hydrocarbons above butane in natural gas; if there is less than 100 grams of gaseous gasoline per cubic meter of natural gas, it is referred to as lean gas. Enriched gas generally contains more than 100 grams of gasoline, with some even reaching 700–800 grams. Alkanes with carbon chains ranging from C5 to C16 are primarily found in gasoline and kerosene. Smaller alkane molecules have low boiling points and are volatile, exerting a significant influence on the properties of these oils. Alkanes with a carbon chain length of C16 or more generally exist in petroleum in a dissolved state; when the temperature drops, they crystallize out in a solid form, which is known as wax. Waxes are usually found in diesel fractions, and the amount of wax present has a significant impact on the freezing point of the oil. Waxes are further divided into paraffin wax and ozokerite; paraffin wax is primarily composed of n-alkanes, while ozokerite is mainly made up of isoparaffins and cycloalkanes. Paraffin wax forms plate-like or strip-like crystals, whereas ozokerite forms needle-like crystals. Under normal conditions, alkanes have relatively stable chemical properties. However, under special conditions, it can also undergo oxidation, halogenation, nitration, and thermal decomposition reactions. (2) Naphthenes: Naphthenes are saturated cyclic compounds and are also a major component of petroleum. Petroleum mainly contains cycloalkanes and their homologs with five-membered and six-membered rings, among which there are more six-membered ring cycloalkanes than five-membered ring cycloalkanes; high-boiling petroleum fractions also contain bicyclic and polycyclic cycloalkanes. The content of naphthenes varies in petroleum fractions; their relative proportion increases as the boiling point of the fraction rises, while it decreases in heavier petroleum fractions due to the increased presence of aromatic hydrocarbons. Naphthenes have high anti-knock properties and low freezing points, as well as good lubrication properties and viscosity-temperature characteristics; they are excellent components for gasoline, jet fuel, and lubricants. Naphthenes have properties similar to alkanes, but their specific gravity, melting point, and boiling point are higher than those of the corresponding alkanes. Their chemical reactivity is slightly greater; they can undergo reactions such as oxidation, halogenation, nitration, and thermal decomposition under certain conditions. More importantly, naphthenes can be dehydrogenated to form aromatic hydrocarbons under certain conditions. (3) Aromatic hydrocarbons: Aromatic hydrocarbons refer to hydrocarbons that contain benzene rings, and they are also one of the main components of petroleum. Aromatic hydrocarbons are present in lower amounts in gasoline, while they are present in higher amounts in the high-boiling-point fractions. In addition to monocyclic aromatic hydrocarbons, oil also contains bicyclic and polycyclic aromatic hydrocarbons; some of these polycyclic aromatic hydrocarbons are fluorescent, which is why oil can emit various types of fluorescence. Aromatic hydrocarbons have good anti-knock properties and are excellent components for gasoline. However, when burned in lamp kerosene, it produces lamp soot that causes the wick to coking, thus becoming a harmful component; therefore, the aromatic hydrocarbon content in lamp kerosene needs to be restricted. Aromatic hydrocarbons with multiple rings and short side chains present in lubricating oils degrade their viscosity-temperature properties; they are prone to oxidation by air at high temperatures, leading to the formation of gum, and therefore must be removed from lubricating oils. Aromatic hydrocarbons have a higher specific gravity and refractive index than alkanes and cycloalkanes with the same number of carbon atoms. Concentrated sulfuric acid reacts with benzene and its derivatives to produce benzenesulfonic acid. It is commonly used to separate aromatic hydrocarbons from petroleum, and it can also be employed for analyzing the compositional profile of oils and petroleum fractions. When aromatic hydrocarbons react with nitric acid, depending on the conditions, one or several hydrogen atoms can be substituted in sequence. Under the action of catalysts such as aluminum trichloride and phosphoric acid, aromatic hydrocarbons are oxidized to aldehydes and acids, and further reactions of these oxides can yield gel-like substances. Aromatic hydrocarbons can be hydrogenated in the presence of catalysts such as nickel. (4) Olefins: Petroleum generally does not contain olefins. Olefins are mainly found in products derived from the secondary processing of petroleum, and their content is high in shale tar. Olefins can be further classified into monoolefins, diolefins, and cycloolefins based on the number and structure of their double bonds. At normal temperature and pressure, monoolefins with C2–C4 chains are gases, those with C5–C18 chains are liquids, and those with C19 or more chains are solids. The physical properties of olefins are similar to those of corresponding alkanes, with a higher specific gravity and refractive index compared to alkanes with the same number of carbon atoms. Olefin molecules contain double bonds; as a result, they have highly reactive chemical properties. Olefins can react with various substances, and under certain conditions they can undergo hydrogenation to form alkanes. Small olefin molecules can combine to form larger olefin molecules. In air, olefins are prone to oxidation to form acidic substances or gums; particularly diolefins and cyclic olefins are more susceptible to oxidation. Therefore, the stability of oils is related to the amount of olefins present. The copolymerization reaction of olefins is an important reaction for producing synthetic lubricants. The alkylation reactions of olefins with aromatic hydrocarbons, and of olefins with alkanes, are important methods for producing high-octane components. (II) Non-hydrocarbons: The content of non-hydrocarbon compounds in petroleum is not high; they are mainly organic compounds formed from elements such as sulfur, nitrogen, and oxygen, as well as carbon and hydrogen. These compounds account for 10-20% of petroleum on average, but they have a significant impact on the properties of the oil and on the methods used for its processing. These non-hydrocarbons are harmful to petroleum products and need to be removed, but if they are properly treated and utilized comprehensively, they can be turned from a hazard into an asset, enabling the production of important chemical products; for example, sulfur can be recovered while removing it from oil. 1. Oxides: The oxides present in petroleum include acidic oxides and neutral oxides. Acidic oxides include naphthenic acids, fatty acids, phenols, hydroxyacids, etc. Neutral oxides include aldehydes, ketones, esters, ethers, etc., in extremely small amounts. Among the oxygen-containing compounds, naphthenic acids and phenols are the most important. Naphthenic acids are primarily found in the middle fractions of petroleum. Their concentration increases gradually starting from the kerosene fraction, continuing through the diesel and light lubricating oil fractions, before decreasing again. The acidic oxygen-containing compounds in petroleum are collectively referred to as petroleum acids, with naphthenic acids accounting for about 95% of the total amount of petroleum acids. All oils contain naphthenic acids, in amounts of less than 1% of the oil’s composition, with variations depending on the source. Naphthenic acid is an oily liquid that is corrosive and has a distinctive odor. Its specific gravity ranges from 0.96 to 1.0%. It is soluble in petroleum but insoluble in water; it is also soluble in sulfuric acid. When it reacts with alkalis, salts are formed, and these salts are water-soluble. Therefore, alkalis can be used to refine and separate naphthenic acid from petroleum. Naphthenic acid can be recovered by acidification from the alkaline residues of straight-run diesel and cracked diesel obtained from victory processes. Naphthenic acids and their metal salts are valuable chemical raw materials; naphthenate salts are commonly used as insecticides and fungicides, agricultural growth promoters, detergents, pigments, etc. Among phenols are phenol, cresol, ethylphenol, naphthol, etc. Phenols have a strong odor and are corrosive. Phenols are acidic and can react with bases. Refinery wastewater usually contains phenols, and untreated wastewater containing phenols can severely poison crops and fish. Phenol can be used as a disinfectant, as well as a raw material for synthetic fibers, pharmaceuticals, dyes, and more. Other neutral oxygen-containing compounds undergo further oxidation, eventually forming gums that affect the performance of the oil. Oxidized compounds in oils must be removed during refining. When analyzing oils, acidity or acid value is commonly used to indicate the content of organic acids in the oil. Acidity is the number of milligrams of potassium hydroxide required to neutralize 100 milliliters of oil ； The acid value is the number of milligrams of potassium hydroxide required to neutralize 1 gram of oil. They are important indicators for controlling the degree of oil refining and determining when the oil becomes unusable. 2. Nitrogen compounds: The nitrogen content in petroleum is generally less than 0.1%. The more gel-like substances are present in oil, the higher its nitrogen content ； The nitrogen content also increases as the boiling point of the fraction rises. The nitrogen content in Russian oil is 0.08-0.18%. Nitrogen compounds in petroleum can be divided into basic nitrogen and neutral nitrogen. So-called basic nitrogen refers to nitrogen compounds that react with perchloric acid-acetic acid solutions, while those that do not react are neutral nitrogen. Basic nitrogen compounds include pyridine, quinoline, indole, hexahydracarbazole, and amines. Neutral nitrogen compounds include indole, pyrrole, and carbazole. When the content of basic nitrogen compounds is high, the oil will darken in color and develop an unpleasant odor after being stored for a longer period of time. This is because nitrogen compounds are unstable and oxidize to form gums when exposed to air for an extended period. Nitrogen compounds can poison certain catalysts. Therefore, nitrogen compounds in oil should be removed during refining. However, pyridine, quinoline, and the like are valuable chemical raw materials; nitrogen-containing compounds such as pyridine and quinoline can be recovered from the acid residues resulting from oil refining to be used as raw materials in the pharmaceutical industry or as corrosion inhibitors in the petroleum refining process. 3. Sulfur-containing compounds: Crude oil with a sulfur content of less than 0.5% is considered low-sulfur crude oil; those with a sulfur content of 0.5% or more are classified as sulfur-containing crude oil, while those with a sulfur content of 2% or more are considered high-sulfur crude oil. The sulfur content of imported Russian crude oil is 0.6-1.0%. Sulfur compounds are primarily found in heavy crude oils, and as the boiling point of the fractions increases, the sulfur content in those fractions also increases. 90% of the sulfur in crude oil is concentrated in atmospheric residue. Sulfur in petroleum exists mostly in the form of organic sulfur, with only a very small amount present as elemental sulfur. The composition of sulfur compounds in crude oil varies greatly depending on the origin and properties of the crude oil. Currently, the types of sulfur compounds identified in petroleum can be divided into three major categories. The first category consists of acidic sulfur-containing compounds, mainly hydrogen sulfide and thiols. The contents of hydrogen sulfide and thiols in oil are low; most of the hydrogen sulfide and thiols in oils are products of the decomposition of other sulfur compounds during oil processing. Thiols have a very unpleasant smell; it can be detected in the air at a concentration of just 0.00001 milligrams per liter. Hydrogen sulfide and thiols are present in the highest amounts in gasoline. Thiols and alkenes can condense to form gums, which affect the stability of gasoline. Thiols can decompose to form hydrogen sulfide at high temperatures. Thiols and hydrogen sulfide are both corrosive to metals, with hydrogen sulfide being particularly effective in causing metal corrosion. During oil refining, such sulfur compounds must be removed. The second category consists of neutral sulfur compounds, mainly thioethers and disulfides, with thioethers being more abundant among them. The amount of thioethers increases as the boiling point of the fractions rises; in fractions with high boiling points, sulfur in the form of thioethers can sometimes account for over 70% of the total sulfur content. The content of disulfides is low, and it is mostly concentrated in the high-boiling fraction. It is not corrosive in itself, but it can decompose into thiol and hydrogen sulfide at temperatures between 130–160°C; therefore, the decomposition products are corrosive. During oil refining, such sulfur compounds should also be removed. The third category consists of heat-stable sulfur compounds, mainly thiophene and **thiophenes, as well as phenylthiol. Thiophene has an aromatic odor, and its physical and chemical properties are similar to those of benzene and its derivatives. It is extremely stable to heat, but it is soluble in sulfuric acid; this property can be utilized to remove thiophene from oils. Thiophenol is a highly reactive substance that rapidly forms corresponding adducts with conjugated olefins, while simultaneously triggering the auto-oxidation of the olefins to produce large amounts of gum. Sulfur-containing oils also have another characteristic: as the sulfur content increases, the contents of nitrogen and metals generally increase as well. The impact of sulfur compounds on product quality: As the amount of sulfur compounds in oil increases, so do the levels of nitrogen, heavy metals, and asphaltenes. Using conventional refining methods, the quality of straight-run products, as well as secondary processed products such as those obtained through catalytic cracking and coking, is poor and does not meet the specified requirements. Gasoline contains sulfur compounds, which reduce its lead sensitivity, degrade its combustion properties, increase carbon buildup in the cylinders, and accelerate engine corrosion and wear. Sulfur compounds have a significant impact on the storage stability of oils; they not only cause unpleasant odors but also substantially promote the formation of gums, with p-**thiophenol and naphthylthiophenol having the most adverse effects. The harmful effects are more pronounced when sulfur compounds coexist with nitrogen compounds. Catalyst contamination occurs due to the use of sulfur-containing oil as the feedstock for secondary processing. It has a high ash content when used as fuel oil. Petcoke produced by delayed coking from residue oil containing 1% sulfur often has a sulfur content of over 1.5%, which does not meet the requirements for producing electrode coke; the use of such sulfur-containing residue oil in steelmaking results in steel of substandard quality. To obtain qualified products, hydrogenation technology or other desulfurization technologies are required. Corrosion of equipment: Within the temperature range of 260–480°C, sulfur-containing compounds cause severe corrosion of ordinary steel, and this temperature range coincides with the operating temperature range of typical oil refining plants. The high-temperature heavy oil sections and low-temperature light oil sections of refining units suffer from severe corrosion; as a result, some materials used in these refining equipment are required to be aluminum-impregnated steel, alloy steel, or alloy linings, which increases the investment costs for the units. During operation, alkali, ammonia, and corrosion inhibitors must be added to reduce corrosion, which increases operational costs. Environmental pollution: The combustion of sulfur-containing oils produces sulfur dioxide, which causes environmental pollution; a concentration of sulfur dioxide in the atmosphere that is harmful to humans is 0.11 ppm. To address the issue of air pollution, it is necessary to minimize the sulfur content in fuel oil and fuel gas, and to install various desulfurization devices. This helps to reduce environmental pollution while also allowing sulfur to be recovered as a raw material for the chemical industry. During the processing of sulfur-containing oils, it is also necessary to address the treatment of acidic wastewater, waste acid, and waste alkali. 4. Gelatinous substances: Petroleum often contains a type of dark brown or black, sticky substance. These are complex organic polymers containing nitrogen, oxygen, and sulfur, with unknown compositions; they are referred to as gelatinous substances. Their presence in petroleum is significant, ranging from 5% to 10% in lower concentrations to 30% to 40% in higher concentrations. They have low volatility, and during the distillation of petroleum, they remain mainly in the residue. Those with lower boiling points also end up in the fractionated oils, with their concentration increasing as the molecular weight and boiling point of the fractions rise. Based on their differences in chemical characteristics and physical properties, colloidal substances can be divided into three categories: The first category is neutral gums – these are colloidal substances in a viscous liquid or semi-solid state, with colors ranging from yellow to dark brown. Gums have a strong dyeing property; just 0.005% of such gums can turn colorless gasoline into a light yellow-to-red color. The specific gravity of gelatin is slightly greater than 1. The molecular weight is generally between 600 and 1000; it is soluble in petroleum ether, benzene, and diethyl ether, and exists as a true solution in petroleum. Most of the gums are concentrated in residue, while low-molecular-weight gums can also be distributed in petroleum fractions along with hydrocarbons; the gum content increases as the boiling point rises, starting from kerosene. Gums are easily adsorbed; therefore, after treating the oil with petroleum ether and then using silica gel for adsorption, it is possible to determine the amount of gums present in the oil. These gums are referred to as silica gel gums. Resin is an unstable compound; when adsorbed by adsorbents or treated with sulfuric acid, it readily forms asphaltenes, and this process becomes even easier when heated and vaporized. When treated with sulfuric acid, the gums not only condense into asphaltenes but also a portion of them dissolves in the sulfuric acid, while another portion forms sulfonic acids. Substances that react with sulfuric acid under certain conditions are generally referred to as sulfuric acid gels. Sulfated gums include gums, asphaltenes, and substances that can react with sulfuric acid or dissolve in it; therefore, the amount of sulfated gums in the same oil is greater than that of gelatinous gums. The second category is asphaltenes: like resins, asphaltenes are also neutral substances. They are dark brown or black solids that are brittle, have a density greater than 1, and a very high molecular weight of around 1300. Asphaltene is insoluble in petroleum ether and ethanol, but soluble in benzene, carbon disulfide, trichloromethane, carbon tetrachloride, aromatic hydrocarbons, and resins. Asphaltene is a heterocyclic compound formed by the condensation of various neutral resins, and its composition is more complex than that of resins. At temperatures above 300°C, asphaltenes decompose into coke and gases. The third category consists of asphaltenic acids and their anhydrides: Asphaltenic acids and their anhydrides are acidic colloidal substances that are present in small amounts in petroleum. They have a gel-like appearance, a density greater than 1, and exist as viscous liquids or black solids. Asphaltic acid and its anhydride are insoluble in benzene and petroleum ether, but soluble in ethanol and trichloromethane; when heated to above 200°C, they turn into unsaponifiable substances. Asphaltic acid differs from naphthenic acid in that it has a very high molecular weight, and its sodium salt is insoluble in water. Gummy substances in petroleum products often need to be removed. The cracking feed contains gums, which increases the coke production during the cracking process. Residues containing a large amount of colloidal substances can be used to produce asphalt, which is used for road paving and is an important building material.

Back upstairs: which has a higher boiling point, alkanes or aromatics? I think it’s the alkanes. But alkanes have a lower density than aromatics!

1. Density is related to molecular weight. 2. Density is related to composition. 3. Density is related to temperature and pressure conditions. It is common that the larger the structural molecule, the higher its molecular weight and density. . . :lol

The density of crude oil is primarily related to the relative molecular weight of its molecules as well as its molecular composition; a higher density indicates a higher proportion of heavier components

Well, boiling point and density do not have a one-to-one relationship

Relationship between relative density and chemical composition: 1. When the number of carbon atoms in the molecule is the same, the density of aromatic hydrocarbons is greater than that of cycloalkanes, which is in turn greater than that of alkanes. 2. For hydrocarbons in the same family, the greater the molecular weight, the greater the density. 3. Fractions of the same boiling range from different crude oils: naphthenic > intermediate > paraffinic. 4. The relative density of various fractions in petroleum increases as their boiling range increases; the higher the boiling range, the greater the relative density. This is partly due to the increase in relative molecular mass, but more importantly because heavier fractions generally contain a higher proportion of aromatic hydrocarbons. As for vacuum residue, it contains a high amount of aromatic hydrocarbons (especially polycyclic aromatic hydrocarbons), as well as significant amounts of resins and asphalts; therefore, its specific gravity is the highest, approaching or even exceeding 1.0. This post was last edited by sfyh on 2009-4-13 20:03.]