Chemistry of oil. Flash, ignition and self-ignition temperatures

Vladimir Khomutko

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What is the flash point of petroleum products?

The flash point of petroleum products (FFL) is the value at which a substance heated under standard conditions releases an amount of vapor sufficient to form a combustible mixture in the air surrounding it, which flares up on contact with fire.

TVNP and the boiling point of petroleum products, which characterizes the degree of their evaporation, are closely related. In other words, the lighter the oil fraction, the higher its volatility, which means that this important indicator is lower.

For example, the TNR of gasoline oil fractions is in the negative range of values ​​(up to minus 40 degrees Celsius). Kerosenes form combustible air mixtures in the range from 28 to 60 degrees, and various types of diesel fuel - from 50 to 80 degrees. Heavy oil fractions flash in the range from 130 to 325 °C. If we talk about the crude oil itself, then the days of various types of oils TBOR can be both negative and positive.

Also, TVNR is highly dependent on the presence of moisture in a particular product, the presence of which reduces it. Therefore, in order to accurately determine the TBNR in the conditions of a measuring laboratory, the test substance is preliminarily dehydrated.

Currently, two main methods for determining TVNP that have state standards are used:

• in an open crucible (according to GOST-u 4333-87);
• in a closed crucible (according to GOST 6356-75).

The difference in the results obtained by these methods can be from 20 to 30 degrees. This is due to the fact that in an open crucible, part of the vapors emitted by the product escapes into the atmosphere, so the accumulation of their amount, sufficient for the formation of a combustible mixture, takes a little longer than when using a closed crucible. Accordingly, the TBNR obtained using an open crucible will be higher than when using a closed crucible.

Basically, an open crucible is used to determine this value for those oil fractions that are classified as high-boiling. These products include various types of petroleum oils and fuel oils. TBNP is considered to be such that the first blue flame appears on the surface of the test substance - and immediately disappears.

According to the value of this parameter, all petroleum products are divided into two categories:

• flammable;
• combustible.

The first category includes all petroleum substances in which this TVNP is less than 61 degrees Celsius when tested in a closed crucible, and not more than 66 - in an open one. Combustible substances are those whose TVNP is more than 61 and 66 degrees, respectively, according to the research method.

TVNP is the most important indicator by which explosiveness is determined (in other words, under what conditions a vapor of an oil substance forms an explosive mixture with atmospheric air).

Explosiveness has two indicators - the lower limit and the upper limit.

Their essence lies in the fact that if the concentration of vapors emitted by the product in the vapor-air mixture is lower than the lower limit, or higher than the upper limit, there will be no explosion. In the first case, this is due to the fact that the released heat is absorbed by excess air, which prevents the remaining parts of the fuel from igniting. In the second case, there is simply not enough oxygen in the vapor-air mixture for an explosion.

Other indicators important for petroleum products

These indicators include ignition, auto-ignition and solidification temperatures.

Ignition temperature of oil product

This temperature of oil products is always higher than that described in the first part of the article. If to determine the value of the flash of the appearance of the first flame with its subsequent attenuation, then this indicator requires such heating at which the substance will burn constantly. The difference between these two characteristics during measurement can be from 30 to 50 degrees.

The ignition temperature is taken to be the minimum at which the flash of the substance does not lead to an instantaneous extinction of the flame, but to the process of constant combustion of the product under study.

If the heating of the studied oil substance is continued, avoiding its contact with atmospheric air, and when high temperature values ​​are reached, such contact is created, then the substance can spontaneously ignite. The minimum readings of the device at which this occurs are the temperature of its self-ignition.

Pensky-Martens Flash Point Analyzer PMA 5

It is directly dependent on the chemical composition of the oil product. The highest values ​​of this indicator are characteristic of aromatic hydrocarbons, followed by naphthenic and paraffinic substances.

The dependence is simple - the lighter the oil fraction, the higher the self-ignition t value. For example, self-ignition of gasoline fractions can occur in the range from 400 to 450 degrees, and for gas oils - from 320 to 360.

Knowing this value is very important, since spontaneous ignition is a fairly common cause of fires in oil refineries, when any leakage in heat exchangers, pipelines or distillation columns (for example, due to depressurization of flange connections) leads to spontaneous combustion.

It should be remembered that if an oil product gets on the insulating material, it must be replaced as soon as possible, since the catalytic action of the product can cause spontaneous combustion at lower t than the autoignition temperature.

Determination of the pour point is necessary to ensure normal transportation through pipelines, as well as when using petroleum derivatives in conditions of severe frost (for example, in aviation, where the use of quickly solidifying fuel is not possible). In these areas, such a characteristic as the mobility of petroleum products is extremely important, on which the degree of their pumpability depends.

TVO-LAB-11 Automatic apparatus for determining the flash point in an open crucible

The pour point is the point at which a substance, tested under standard conditions, loses its mobility.

Decreased mobility and its complete loss can be explained by the following factors:

To create NKPP vapor above the surface of a liquid, it is sufficient to heat to a temperature equal to NTPRP, not the entire mass of the liquid, but only its surface layer.

In the presence of IS, such a mixture will be capable of ignition. In practice, the concepts of flash point and ignition point are most often used.

Under flash point understand the lowest temperature of a liquid at which, under the conditions of special tests, a concentration of liquid vapor is formed above its surface, capable of igniting from IZ, but the rate of their formation is insufficient for subsequent combustion. Thus, both at the flash point and at the lower temperature limit of ignition above the surface of the liquid, a lower concentration limit of ignition is formed, however, in the latter case, HKPRP is created by saturated vapors. Therefore, the flash point is always slightly higher than NTPRP. Although at the flash point there is a short-term ignition of vapors in the air, which is not capable of turning into a stable combustion of a liquid, nevertheless, under certain conditions, an outbreak of liquid vapors can be a source of fire.

The flash point is taken as the basis for the classification of liquids into flammable (flammable liquids) and combustible liquids (FL). Flammable liquids include liquids with a flash point in a closed crucible of 61 0 C or in an open crucible of 65 0 C and below, GZH - with a flash point in a closed crucible of more than 61 0 C or in an open crucible of 65 0 C.

I category - especially dangerous flammable liquids, these include flammable liquids with a flash point of -18 0 C and below in a closed crucible or from -13 0 C and below in an open crucible;

II category - permanently dangerous flammable liquids, these include flammable liquids with a flash point above -18 0 C to 23 0 C in a closed crucible or from -13 to 27 0 C in an open crucible;

Category III - flammable liquids, dangerous at elevated air temperatures, these include flammable liquids with a flash point of 23 to 61 0 C in a closed crucible or from 27 to 66 0 C in an open crucible.

Depending on the flash point, safe methods for storing, transporting and using liquids for various purposes are established. The flash point of liquids belonging to the same class naturally changes with changes in the physical properties of the members of the homologous series (Table 4.1).

Table 4.1.

Physical properties of alcohols

The flash point increases with increasing molecular weight, boiling point and density. These patterns in the homological series indicate that the flash point is related to the physical properties of substances and is itself a physical parameter. It should be noted that the pattern of changes in the flash point in the homologous series cannot be extended to liquids belonging to different classes of organic compounds.

When mixing flammable liquids with water or carbon tetrachloride, the pressure of flammable vapors at that the same temperature decreases, which leads to an increase in the flash point. Can be diluted with fuel liquid to such an extent that the resulting mixture will not have a flash point (see table. 4.2).

Fire extinguishing practice shows that the combustion of liquids that are highly soluble in water stops when the concentration of the combustible liquid reaches 10-25%.

Table 4.2.

For binary mixtures of combustible liquids that are highly soluble in each other, the flash point is between the flash points of pure liquids and approaches the flash point of one of them, depending on the composition of the mixture.

FROM rise in temperature of the liquid evaporation rate increases and at a certain temperature reaches such a value that, once ignited, the mixture continues to burn after the ignition source is removed. This liquid temperature is called flash point. For flammable liquids, it differs by 1-5 0 С from the flash point, and for GZh - by 30-35 0 С. At the ignition temperature of liquids, a constant (stationary) combustion process is established.

There is a correlation between the flash point in a closed crucible and the lower ignition temperature limit, which is described by the formula:

T sun - T n.p. \u003d 0.125T sun + 2. (4.4)

This relation is valid for T sun< 433 К (160 0 С).

The significant dependence of the flash and ignition temperatures on the experimental conditions causes certain difficulties in creating a calculation method for estimating their values. One of the most common of them is the semi-empirical method proposed by V. I. Blinov:

, (4.5)

where T sun - flash point, (ignition), K;

p sun - partial pressure of saturated vapor of the liquid at the flash point (ignition), Pa;

D 0 - diffusion coefficient of liquid vapor, m 2 / s;

n is the number of oxygen molecules required for the complete oxidation of one fuel molecule;

temperatureoutbreaks called the minimum temperature at which a vapor of an oil product forms a mixture with air capable of short-term formation of a flame when an external source is ignited (flame, electric spark, etc.) is introduced into it.

A flash is a weak explosion, which is possible within strictly defined concentration limits in a mixture of hydrocarbons with air.

Distinguish upper and lower concentration limit of flame propagation. The upper limit is characterized by the maximum concentration of organic matter vapor in a mixture with air, above which ignition and combustion when an external source of ignition is introduced is impossible due to a lack of oxygen. The lower limit is at the minimum concentration of organic matter in the air, below which the amount of heat released at the site of local ignition is insufficient for the reaction to proceed in the entire volume.

temperatureignition called the minimum temperature at which the vapors of the test product, when an external source of ignition is introduced, form a stable undamped flame. The ignition temperature is always higher than the flash point, often quite significantly - by several tens of degrees.

temperatureself-ignition What is the minimum temperature at which a mixture of petroleum products with air can ignite without an external source of ignition? The pa6ota of diesel internal combustion engines is based on this property of petroleum products. The auto-ignition temperature is several hundred degrees higher than the flash point. The flash point of kerosenes, diesel fuels, lubricating oils, fuel oils and other heavy petroleum products characterizes the lower explosive limit. The flash point of gasolines, whose vapor pressure at room temperature is significant, usually characterizes the upper explosive limit. In the first case, the determination is carried out during heating in the second - during cooling.

Like any conditional characteristic, the flash point depends on the design of the device and the conditions of determination. In addition, its value is influenced by external conditions - atmospheric pressure and air humidity. The flash point increases with increasing atmospheric pressure.

The flash point is related to the boiling point of the test substance. For individual hydrocarbons, this dependence, according to Ormandy and Krevin, is expressed by the equality:

T vsp \u003d K T ​​kip, (4.23)

where T flash - flash point, K; K - coefficient equal to 0.736; T boil - boiling point, K.

The flash point is a non-additive quantity. Its experimental value is always lower than the arithmetic mean value of the flash points of the components included in the mixture, calculated according to the rules of additivity. This is because the flash point depends mainly on the vapor pressure of the low-boiling component, while the high-boiling component serves as a heat transmitter. As an example, it can be pointed out that the ingress of even 1% gasoline into lubricating oil reduces the flash point from 200 to 170 ° C, and 6% gasoline reduces it by almost half. .

There are two methods for determining the flash point - in devices of a closed and open type. The values ​​of the flash point of the same oil product, determined in devices of different types, differ markedly. For highly viscous products this difference reaches 50, for less viscous products 3-8°C. Depending on the composition of the fuel, the conditions for its self-ignition change significantly. These conditions, in turn, are associated with the motor properties of fuels, in particular, detonation resistance.

The concept of flash point

flash point is the temperature at which an oil product heated under standard conditions emits such an amount of vapor that it forms a combustible mixture with the surrounding air, which flares up when a flame is brought to it.

For individual hydrocarbons, there is a certain quantitative relationship between the flash point and the boiling point, expressed by the ratio:

For petroleum products boiling over a wide temperature range, such a dependence cannot be established. In this case, the flash point of petroleum products is related to their average boiling point, i.e., with evaporation. The lighter the oil fraction, the lower its flash point. Thus, gasoline fractions have negative (up to minus 40°С) flash points, kerosene 28-60°С, oil 130-325°С. The presence of moisture, decomposition products in an oil product significantly affects the value of its flash point. This is used in production conditions to conclude on the purity of the kerosene and diesel fractions obtained during the distillation. For oil fractions, the flash point indicates the presence of volatile hydrocarbons. Of the oil fractions of various hydrocarbon compositions, oils from paraffinic low-sulfur oils have the highest flash point. Oils of the same viscosity from resinous aromatic naphthenic oils have a lower flash point.

Methods for determining the flash point

Two methods have been standardized for determining the flash point of petroleum products in open (GOST 4333-87) and closed (GOST 6356-75) crucibles. The difference between the flash points of the same petroleum products when determined in open and closed crucibles is very large. In the latter case, the required amount of oil vapor accumulates earlier than in open-type devices. In addition, in an open crucible, the resulting vapors freely diffuse into the air. The specified difference is greater, the higher the flash point of the oil product. The admixture of gasoline or other low-boiling fractions in heavier fractions (with fuzzy rectification) sharply increases the difference in their flash points in open and closed crucibles.

When determining the flash point in an open crucible, the oil product is first dehydrated with sodium chloride, sulfate or calcium chloride, then poured into the crucible to a certain level, depending on the type of oil product. The heating of the crucible is carried out at a certain rate, and at a temperature of 10°C below the expected flash point, it is slowly carried out along the edge of the crucible above the surface of the oil product with the flame of a burner or other incendiary device. This operation is repeated every 2°C. The flash point is the temperature at which a blue flame appears above the surface of the oil product. When determining the flash point in a closed crucible, the oil product is poured to a certain mark and, in contrast to the method described above, it is heated with continuous stirring. When the crucible lid is opened in this device, the flame is automatically brought to the surface of the oil product.

The determination of the flash point starts 10°C before the expected flash point - if it is below 50°C, and 17°C - if it is above 50°C. The determination is carried out through each degree, and at the time of determination, stirring is stopped.

All substances having a closed cup flash point below 61°C are flammable liquids(LVZH), which, in turn, are divided into:

• especially dangerous ( T ref below minus 18°С);
• permanently dangerous T ref from minus 18°С to 23°С);
• dangerous at elevated temperatures ( T ref from 23°C to 61°C).

Explosive limits

The flash point of an oil product characterizes the ability of this oil product to form an explosive mixture with air. A mixture of vapors with air becomes explosive when the concentration of fuel vapors in it reaches certain values. Accordingly, there are lower and upper explosive limit mixtures of oil vapors with air. If the concentration of oil vapors is less than the lower explosive limit, no explosion occurs, since the existing excess air absorbs the heat released at the starting point of the explosion and thus prevents the remaining parts of the fuel from igniting. When the concentration of fuel vapor in the air is above the upper limit of the explosion does not occur due to the lack of oxygen in the mixture. The lower and upper explosive limits of hydrocarbons can be determined, respectively, by the formulas:

In the homologous series of paraffinic hydrocarbons, with increasing molecular weight, both the lower and upper explosive limits decrease, and the explosive interval narrows from 5-15% (vol.) for methane to 1.2-7.5% (vol.) for hexane. Acetylene, carbon monoxide and hydrogen have the widest explosive ranges and are therefore the most explosive.

As the temperature of the mixture increases, the range of its explosiveness narrows slightly. So, at 17°C, the explosive range of pentane is 1.4-7.8% (vol.), and at 100°C it is 1.44-4.75% (vol.). The presence in the mixture of inert gases (nitrogen, merododioxide, etc.) also narrows the explosive range. An increase in pressure leads to an increase in the upper explosive limit.

The explosive limits of vapors of binary and more complex mixtures of hydrocarbons can be determined by the formula:

Flash point- this is the temperature at which an oil product heated under standard conditions emits such an amount of vapor that it forms a combustible mixture with the surrounding air, which flares up when a flame is brought to it.

This indicator is closely related to the boiling point, i.e. with evaporation. The lighter the oil product, the better it evaporates, the lower its flash point. For example, gasoline fractions have negative flash points (up to -40°C), kerosene fractions have flash points in the range of 28-60°C, diesel fuel fractions - 50-80°C, heavier oil fractions - 130-325°C . The flash points of various oils can be either positive or negative.

The presence of moisture in oil products leads to a decrease in the flash point. Therefore, when determining it in laboratory conditions, the oil product must be freed from water. There are two standard methods for determining the flash point: in an open (GOST 4333-87) and closed (GOST 6356-75) crucible. The difference in determining the flash point between them is 20-30°C. When determining a flash in an open crucible, part of the formed vapors flies into the air, and the required amount of them, necessary for a flash, accumulates later than in a closed crucible.

Therefore, the flash point of the same oil product, determined in an open crucible, will be higher than in a closed crucible. As a rule, the flash point in an open crucible is determined for high-boiling oil fractions (oils, fuel oils). The flash point is taken to be the temperature at which the first blue flame appears on the surface of the oil product and immediately goes out. The flash point is used to judge the explosive properties of an oil product, i.e. about the possibility of the formation of explosive mixtures of its vapors with air. There are lower and upper explosive limits.

If the concentration of oil vapor in a mixture with air is below the lower limit, an explosion will not occur, since the existing excess air absorbs the heat released at the point of explosion and thus prevents the ignition of other parts of the fuel.

When the concentration of oil vapors in a mixture with air above the upper limit of the explosion does not occur due to lack of oxygen in the mixture.

Ignition temperature. When determining the flash point, a phenomenon is observed when an oil product flares up and immediately goes out. If the oil product is heated even higher (by 30-50°C) and the source of fire is again brought to the surface of the oil product, then it will not only flare up, but will also burn quietly. The minimum temperature at which an oil product flares up and begins to burn is called the ignition temperature.

Auto ignition temperature. If the oil product is heated to a high temperature without contact with air, and then such contact is ensured, then the oil product may ignite spontaneously.

The minimum temperature corresponding to this phenomenon is called the autoignition temperature. It depends on the chemical composition. Aromatic hydrocarbons and petroleum products rich in them have the highest self-ignition temperatures, followed by naphthenes and paraffins.

The lighter the oil product, the higher its autoignition temperature. So, for gasoline it is in the range of 400-450°C, for gas oils - 320-360°C.

Self-ignition of petroleum products is often the cause of fires in factories. Any depressurization of flange connections in columns, heat exchangers, pipelines, etc. may cause a fire.

The insulating material doused with oil must be removed, since its catalytic action can cause self-ignition of the oil at much lower temperatures.

pour point. When transporting petroleum products through pipelines and using them in the low-temperature region in aviation, their mobility and good pumpability under these conditions are of great importance. The temperature at which the oil loses its mobility under standard test conditions is called the pour point.

The loss of mobility of the oil product can occur due to two factors: either an increase in the viscosity of the oil product, or due to the formation of paraffin crystals and thickening of the entire mass of the oil product.