Since the 20th century, oil has become one of the most important minerals for human civilization. A number of unique properties make it not only the main component of a modern fuel and energy economy, but also a valuable chemical raw material. Below we will consider some of the features of oil associated with its physical properties and composition.
general characteristics
Oil is an oily liquid, combustible, characterized by low ash content, which is a complex combination of various hydrocarbons mixed with other compounds. Along with brown and hard coals, anthracite, schists, peat and sapropel, it belongs to caustobiolites - combustible minerals of organic origin, but it has some features that distinguish it from other representatives of this group of minerals.
The color of the oil can be different: from black, brown and dark red to greenish and light yellow. Sometimes oil is even colorless (the so-called white oil).
A distinctive feature of oil is a specific smell, which may vary slightly between oils, but it is always recognizable. The consistency varies from mobile, fluid to dense, resembling resin.
Differences in color and odor are due to the concentration of aromatic hydrocarbon and impurity components. The specific characteristics of oil are important in the exploration, production and processing of this mineral, and also determine the many operational qualities of various types of oil products.
Chemical elements contained in oil
One of the most important factors affecting the physicochemical properties of oil is its chemical composition. It consists of many components, but the main components of any oil are carbon (80–88% by weight) hydrogen (11–14%).
In addition, sulfur, oxygen and nitrogen are present in different concentrations in it. Their content can range from 0.5 to 8% by weight, which has a significant effect on quality. In small quantities, the elemental composition of oil includes many metals, such as vanadium, copper, nickel, calcium, and others, as well as iodine, boron, arsenic, and others.
Oil is such a complex multicomponent chemical system that it is almost impossible to completely determine its individual composition. It is known that various oils may contain more than fifty chemical elements, and the isolation of many of them is extremely difficult due to the complexity of not only the chemical composition, but also the structure of this liquid mineral.
Sulfur and oil properties
Sulfur is almost always present in oil both in the composition of sulfur compounds (in thiols, hydrogen sulfide, sulfides and others), and in free form. Its content can reach 5%. The presence of sulfur is of great importance. Firstly, it affects the boiling point of oil. Secondly, it increases its oxidizing properties, contributing to the corrosion of equipment, tanks and pipelines.
According to the sulfur content, oil is divided into the following groups:
- low sulfur (up to 0.5% sulfur in the composition);
- sulphurous (0.5–2%);
- sour (over 2%) - the most aggressive.
Group chemical composition of oil
Oil is a complex solution of some hydrocarbons in others. This solution forms a colloidal system with clots of insoluble high molecular weight compounds and other impurities. As the main components, oil contains hydrocarbons of three main groups:
- Paraffinic (alkanes) - saturated, or saturated, hydrocarbons, such as methane, ethane, and so on, containing the maximum possible number of hydrogen atoms. The general formula of alkanes is C n H 2 n + 2 . These compounds are most chemically stable. At 5-16 carbon atoms in the molecule, alkanes are liquids, with a larger number of them - solids. The alkane content in oil ranges from 25 to 75% by weight.
- Naphthenic (cyclanes) are saturated cyclic hydrocarbons with the general formula C n H 2 n , for example, cyclopentane or cyclohexane. They are characterized by higher melting and boiling points than alkanes. Thanks to naphthenes, various fuels and lubricating oils acquire high performance. The composition of oil may include from 25 to 80% of naphthenic hydrocarbons.
- Aromatic (arenas) are unsaturated cyclic hydrocarbons. These include benzene, naphthalene, anthracene and others. Arenas are characterized by a higher density, as well as the ability to replace hydrogen with other atoms. In gasolines and engine oils, arenas are also a valuable component, but they worsen the quality of kerosene and diesel fuel. The proportion of aromatic hydrocarbons is from 15 to 50%.
Depending on the predominance of a particular group of hydrocarbons, oil is divided into methane (paraffinic), naphthenic, aromatic and intermediate types.
In addition, the physicochemical properties of oil depend on various resins, asphaltenes and other heteroatomic substances, as well as on the presence and concentration of organometallic compounds, gases, water, mineral salts and other impurities.
Oil paraffin
Hydrocarbons of the alkane series with a molecular mass of 240 and higher, the molecules of which contain 17 or more carbon atoms, under normal conditions are solids - paraffins and ceresins. In reservoir oil, they are in a dissolved state, but upon opening the formation and rising to the surface with lowering temperature and pressure, paraffins in the oil are able to crystallize and precipitate. This solid sediment waxes the pores in the reservoir, parts and walls of elements of oil production equipment, which significantly complicates and increases the cost of production.
The content of paraffins emit such groups of oils as:
- low paraffin (up to 1.5%);
- paraffin (1.5–6.0%);
- highly paraffinous (over 6%).
The paraffin content also affects the temperature range of oil boiling and solidification.
Gas and water content
Under reservoir conditions, oil is part of a fluid — a mixture that also contains water and gas and fills the porous rock — the reservoir. To ensure the commercial quality of oil, it is subjected to dehydration. As for associated gas, its hydrocarbon components are a valuable product and are used in various industries.
The gas content is characterized by such a value as the gas factor. It shows how much gas released during oil recovery was dissolved in it under formation conditions. For most oils, the gas factor is between 30 and 100 m 3 per tonne of oil.
Gas is divided into dry, consisting of light hydrocarbons (methane, ethane), and fatty, containing a large percentage of higher hydrocarbons. The solubility of fatty gas is higher than dry. It can serve as a raw material for the production of liquefied gases, condensates, gas gasoline.
Fractional composition of oil
By distillation methods, oil is not divided into individual compounds, but into groups of substances, each of which boils in a certain temperature range. Such parts are called fractions (distillates). Different fractions of oil have the following boiling point temperatures:
- 40–120 ° C - gasoline fraction;
- 120–180 ° C - naphtha fraction (heavy oil);
- 180–245 ° C — kerosene fraction;
- 245–350 ° C - diesel (gas oil, solar) fraction.
These fractions are called light; at the same time, products distilled off at temperatures up to 200 ° C are light fractions, in the range from 200 to 300 ° C - medium and above 300 ° C - heavy (oil) fractions. The higher molecular weight hydrocarbon components the fraction contains, the heavier it is and requires higher temperatures for distillation.
After distillation of the light distillates, a dark fuel oil fraction remains, subjected to further - secondary - distillation in order to obtain various engine oils or fuels. A high boiling point (more than 500 ° C) fractionation residue containing heavy sulfur substances, resins and asphaltenes is called tar.
The fractional composition of oil depends on the ratio of the amount of hydrocarbons with different molecular weights, which, in turn, is largely affected by the conditions of formation, migration, and accumulation of oil in the reservoirs.
Density and viscosity
Density (specific gravity) is one of the main properties of oil, affecting its commodity characteristics. The more valuable light fractions are contained in black gold, the lower its density. The oil density in kg / m3 can vary from 730 to 1040. Several classes of oil are distinguished by this indicator:
- ultra-light with a density below 0.78 g / cm 3 or 780 kg / m 3 ;
- ultralight (0.78–0.82 g / cm 3 or 780–820 kg / m 3 );
- light (0.82–0.87 g / cm 3 or 820–870 kg / m 3 );
- average (0.87–0.92 g / cm 3 or 870–920 kg / m 3 );
- heavy (0.92–1.00 g / cm 3 or 920–1000 kg / m 3 );
- superheavy - the density in this case exceeds 1000 kg / m 3 , such oil is heavier than water.
In practice, they usually use the concept of relative density. This value reflects the ratio of the absolute density of oil in kg / m3 to the density of water.
Light oils are characterized by a predominant alkane content, while heavy oils are characterized by an increased concentration of cyclic hydrocarbons, high molecular weight resins and asphaltenes.
One more important characteristic of oil, viscosity, is related to density - viscosity. Light oils generally have a lower viscosity, that is, are more mobile. It should be borne in mind that the viscosity and temperature factors are also strongly affected by temperature and gas factors. Gas-saturated oil in the reservoir has a lower viscosity.
Thermal indicators of oil and oil products
Important physical and chemical properties of oil include indicators such as solidification and boiling points, flash and ignition.
There is a wide range from 30–40 to 550 ° C and even higher, within which various fractions boil. The range of the boiling point of oil can vary and also depends on the chemical composition. So, naphthenic and aromatic hydrocarbons, like heavy sulfur compounds, boil at a higher temperature.
The crystallization of substances that make up oil is an equally complex step-by-step process. The freezing point of oil ranges from –80 ° C to +30 ° C. Oil that does not change position in a vessel at an inclination of 45 ° is considered to be frozen. Naphthenes are characterized by a lower pour point than liquid alkanes. The presence of paraffins, in contrast, raises the pour point.
Indicators such as flash point and ignition temperature also depend on the composition of oil, more precisely, on the limits of distillation of the oil product. Light - gasoline - oil fractions already flare up at –35 ° C, kerosene - at 30–60 ° C, diesel - at 30–90 ° C. The ignition temperature of oil and oil products is always slightly higher than the flash point, and this difference increases significantly for heavier fractions.
Thermal properties
The specific heat of oil (that is, the amount of energy needed to heat 1 kilogram of substance per 1 degree Kelvin) ranges from 1.7 to 2.2 kJ / kg ∙ K at 20 ° C. The higher the density of oil, the lower its heat capacity. For comparison, the specific heat of water at the same temperature is about 4.18 kJ / kg ∙ K.
The thermal conductivity of oil depends on many factors, such as composition, temperature, pressure, phase state. Alkanes have the lowest thermal conductivity, and aromatic hydrocarbons have the highest (with the same number of carbon atoms).
One of the main properties of oil, which give it exceptional value as a raw material for the production of fuel, is the specific heat of combustion. This value characterizes the ratio of the thermal energy released during combustion to the mass of completely burned fuel.
In terms of specific heat of combustion, oil and petroleum products (as well as natural combustible gas) are superior to all other types of fuel. So, for crude oil, this parameter is 40–45 MJ / kg (for the best fossil fuels - 31 MJ / kg). The heat of combustion depends on the density and, to some extent, on the characteristics of the chemical composition, but varies quite narrowly, that is, this important property is inherent in all varieties of "liquid black gold." Light gasoline fractions have an even greater calorific value.
The famous and mysterious oil
Mankind has been using oil for a long time and is extremely active and widely used, but, unfortunately, does not always make it the most effective, economical and environmentally friendly methods. This is partly because we do not all know about oil.
For example, the complete chemical composition of its various species is unknown. Although at present the concept of the biogenic origin of oil, which has proved its predictive power, is considered the most justified, there is no agreement on the factors of oil formation. There is no complete understanding of the processes of occurrence of reservoir deposits, of their lithological and structural features.
Meanwhile, all these issues are related to the formation of the physicochemical properties of oil, which play a huge role in the exploration, production, processing and use of such a valuable non-renewable natural resource in various industries.