Evaporation is the transition of a liquid to vapor from a free surface at temperatures below the boiling point of the liquid. Evaporation occurs as a result of the thermal movement of liquid molecules. The speed of movement of molecules varies widely, deviating strongly in both directions from its average value. Some of the molecules with a sufficiently large kinetic energy escape from the surface layer of the liquid into the gas (air) medium. The excess energy of the molecules lost by the liquid is expended on overcoming the forces of interaction between the molecules and the work of expansion (increase in volume) during the transition of the liquid into vapor.

Evaporation is an endothermic process. If heat is not supplied to the liquid from the outside, then as a result of evaporation, it cools. The evaporation rate is determined by the amount of vapor generated per unit of time per unit of liquid surface. This must be taken into account in industries related to the use, production or processing of flammable liquids. Increasing the evaporation rate with increasing temperature leads to a more rapid formation of explosive vapor concentrations. Max speed Evaporation is observed during evaporation in a vacuum and in an unlimited volume. This can be explained as follows. The observed rate of the evaporation process is the total rate of the process of transition of molecules from the liquid phase V 1 and condensation rate V 2 . The total process is equal to the difference between these two speeds: . At constant temperature V 1 does not change, but V 2 proportional to the vapor concentration. When evaporating into a vacuum in the limit V 2 = 0 , i.e. the total speed of the process is maximum.

The higher the vapor concentration, the higher the rate of condensation, hence the lower the total rate of evaporation. At the interface between the liquid and its saturated vapor, the evaporation rate (total) is close to zero. The liquid in a closed vessel, evaporating, forms a saturated vapor. A saturated vapor is a vapor that is in dynamic equilibrium with a liquid. Dynamic equilibrium at a given temperature occurs when the number of evaporating liquid molecules is equal to the number of condensing molecules. Saturated vapor, leaving an open vessel into the air, is diluted by it and becomes unsaturated. Therefore, in the air

In every room where containers with hot liquids are located, there is an unsaturated vapor of these liquids.

Saturated and unsaturated vapors exert pressure on vessel walls. Saturated vapor pressure is the pressure of vapor in equilibrium with a liquid at a given temperature. The pressure of saturated steam is always higher than that of unsaturated steam. It does not depend on the amount of liquid, the size of its surface, the shape of the vessel, but depends only on the temperature and nature of the liquid. As the temperature rises, the saturation vapor pressure of a liquid increases; At the boiling point, the vapor pressure is equal to atmospheric pressure. For each temperature value, the saturated vapor pressure of an individual (pure) liquid is constant. The saturation vapor pressure of mixtures of liquids (oil, gasoline, kerosene, etc.) at the same temperature depends on the composition of the mixture. It increases with an increase in the content of low-boiling products in the liquid.

For most liquids, the saturation vapor pressure at various temperatures is known. Pressure values saturated vapors some liquids at different temperatures are given in table. 5.1.

Table 5.1

Saturated vapor pressure of substances at different temperatures

Substance	Saturated vapor pressure, Pa, at temperature, K
Butyl acetate Baku aviation gasoline Methyl alcohol carbon disulfide Turpentine Ethanol Ethyl ether ethyl acetate

Found in Table.

5.1 The saturation vapor pressure of a liquid is a component of the total pressure of the mixture of vapors with air.

Let us assume that the mixture of vapors with air formed above the surface of carbon disulphide in a vessel at 263 K has a pressure of 101080 Pa. Then the saturation vapor pressure of carbon disulfide at this temperature is 10773 Pa. Therefore, the air in this mixture has a pressure of 101080 - 10773 = 90307 Pa. With increasing temperature of carbon disulfide

its saturated vapor pressure increases, the air pressure decreases. The total pressure remains constant.

The part of the total pressure attributable to a given gas or vapor is called partial pressure. In this case, the vapor pressure of carbon disulfide (10773 Pa) can be called partial pressure. Thus, the total pressure of the vapor-air mixture is the sum of the partial pressures of the vapors of carbon disulfide, oxygen and nitrogen: P steam + + = P total. Since the pressure of saturated vapors is part of the total pressure of their mixture with air, it becomes possible to determine the vapor concentrations of liquids in air from the known total pressure of the mixture and the vapor pressure.

The saturation vapor pressure of liquids is determined by the number of molecules hitting the walls of the vessel, or by the concentration of vapor above the surface of the liquid. The higher the concentration of saturated steam, the greater its pressure. The relationship between the concentration of saturated vapor and its partial pressure can be found as follows.

Let us assume that it would be possible to separate the vapor from the air, and the pressure in both parts would remain equal to the total pressure Ptot. Then the volumes occupied by steam and air would decrease accordingly. According to the Boyle-Mariotte law, the product of the gas pressure and its volume at a constant temperature is a constant value, i.e. for our hypothetical case, we get:

.

METHOD FOR CALCULATION OF EVAPORATION PARAMETERS OF COMBUSTIBLE UNHEATED LIQUIDS AND LIQUEFIED HYDROCARBON GASES

I.1 Evaporation rate W, kg / (s m 2), determined by reference and experimental data. For flammable liquids not heated above ambient temperature, in the absence of data, it is allowed to calculate W according to formula 1)

W \u003d 10 -6 h p n, (I.1)

where h - coefficient taken according to table I.1 depending on the speed and temperature of the air flow over the evaporation surface;

M - molar mass, g/mol;

p n - saturated vapor pressure at the calculated liquid temperature t p, determined from reference data, kPa.

Table I.1

Air flow rate in the room, m/s	The value of the coefficient h at temperature t, ° С, air in the room
	10	15	20	30	35
0,0	1,0	1,0	1,0	1,0	1,0
0,1	3,0	2,6	2,4	1,8	1,6
0,2	4,6	3,8	3,5	2,4	2,3
0,5	6,6	5,7	5,4	3,6	3,2
1,0	10,0	8,7	7,7	5,6	4,6

I.2 For liquefied hydrocarbon gases (LHG), in the absence of data, it is allowed to calculate the specific mass of vapors of evaporated LHG m LHG, kg/m 2, according to formula 1)

, (AND 2)

1) The formula is applicable at the temperature of the underlying surface from minus 50 to plus 40 °C.

where M - molar mass of LPG, kg/mol;

L is the molar heat of vaporization of LPG at the initial temperature of LPG T W, J/mol;

T 0 - the initial temperature of the material, on the surface of which LPG is spilled, corresponding to the calculated temperature t p , K;

T W - initial temperature of LPG, K;

l tv - coefficient of thermal conductivity of the material on the surface of which LPG is poured, W / (m K);

a - effective coefficient of thermal diffusivity of the material, on the surface of which LPG is spilled, equal to 8.4· 10 -8 m 2 / s;

t - current time, s, taken equal to the time of complete evaporation of LPG, but not more than 3600 s;

Reynolds number (n - air flow velocity, m/s; d- characteristic size of the LPG strait, m;

u in - kinematic viscosity of air at the design temperature t p, m 2 / s);

l in - coefficient of thermal conductivity of air at the design temperature t p, W / (m K).

Examples - Calculation of evaporation parameters for flammable unheated liquids and liquefied hydrocarbon gases

1 Determine the mass of acetone vapor entering the volume of the room as a result of emergency depressurization of the apparatus.

Data for calculation

In a room with a floor area of ​​50 m 2, an apparatus with acetone was installed with a maximum volume of V ap = 3 m 3. Acetone enters the apparatus by gravity through a pipeline with a diameter d= 0.05 m with flow q, equal to 2 10 -3 m 3 / s. The length of the pressure pipeline section from the tank to the manual valve l 1 = 2 m. The length of the section of the outlet pipeline with a diameter d= 0.05 m from the tank to the manual valve L 2 is equal to 1 m. The air flow rate in the room with general ventilation operating is 0.2 m / s. The air temperature in the room t p \u003d 20 ° C. The density r of acetone at a given temperature is 792 kg / m 3. The saturated vapor pressure of acetone p a at t p is 24.54 kPa.

The volume of acetone released from the pressure pipeline, V n.t is

where t is the estimated pipeline shutdown time, equal to 300 s (with manual shutdown).

The volume of acetone released from the discharge pipeline V from is

The volume of acetone entering the room

V a \u003d V an + V n.t + V from \u003d 3 + 6.04 10 -1 + 1.96 10 -3 \u003d 6.600 m 3.

Based on the fact that 1 liter of acetone is spilled on 1 m 2 of the floor area, the calculated evaporation area S p \u003d 3600 m 2 of acetone will exceed the floor area of ​​the room. Therefore, the floor area of ​​the room equal to 50 m 2 is taken as the area of ​​evaporation of acetone.

The evaporation rate is equal to:

W isp \u003d 10 -6 3.5 24.54 \u003d 0.655 10 -3 kg / (s m 2).

The mass of acetone vapors generated during emergency depressurization of the apparatus T, kg will be equal to

t \u003d 0.655 10 -3 50 3600 \u003d 117.9 kg.

2 Determine the mass of gaseous ethylene formed during the evaporation of the spill of liquefied ethylene in conditions of emergency depressurization of the tank.

Data for calculation

An isothermal liquefied ethylene tank with a volume of V i.r.e = 10000 m 3 is installed in a concrete dike with a free area S vol = 5184 m 2 and a flanging height H vol = 2.2 m. The degree of filling of the reservoir a = 0.95.

The input of the pipeline for supplying liquefied ethylene into the tank is made from above, and the output of the outlet pipeline is from below.

The diameter of the discharge pipeline d tp = 0.25 m. The length of the pipeline section from the reservoir to the automatic valve, the probability of failure of which exceeds 10 -6 per year and the redundancy of its elements is not provided, L= 1 m. The maximum flow rate of liquefied ethylene in the issuance mode G l.e = 3.1944 kg / s. Density of liquefied ethylene r l.e at operating temperature T eq\u003d 169.5 K is equal to 568 kg / m 3. Density of gaseous ethylene rg.e at T eq equal to 2.0204 kg / m 3. Molar mass of liquefied ethylene M zh.e = 28 10 -3 kg/mol. Molar heat of vaporization of liquefied ethylene L and cn at T eq it is equal to 1.344 10 4 J/mol. The temperature of the concrete is equal to the maximum possible air temperature in the corresponding climatic zone T b = 309 K. The coefficient of thermal conductivity of concrete l b = 1.5 W / (m K). Thermal diffusivity of concrete but\u003d 8.4 10 -8 m 2 / s. The minimum air flow velocity u min = 0 m/s, and the maximum for a given climatic zone u max = 5 m/s. The kinematic viscosity of air n at the calculated air temperature for a given climatic zone t p \u003d 36 ° C is 1.64 10 -5 m 2 / s. The coefficient of thermal conductivity of air l in at t p is 2.74 10 -2 W / (m K).

With the destruction of the isothermal tank, the volume of liquefied ethylene will be

Free dike volume V about = 5184 2.2 = 11404.8 m 3.

Due to the fact that V zh.e< V об примем за площадь испарения S исп свободную площадь обвалования S об, равную 5184 м 2 .

Then the mass of evaporated ethylene m.e. from the area of ​​the strait at an air flow velocity u = 5 m/s is calculated by the formula (I.2)

Mass m ee at u = 0 m/s will be 528039 kg.

What is acetone? The formula for this ketone is seen in school course chemistry. But not everyone has an idea of ​​how dangerous the smell of this compound is and what properties this organic substance has.

Features of acetone

Technical acetone is the most common solvent used in modern construction. Since this compound has a low level of toxicity, it is also used in the pharmaceutical and food industries.

Technical acetone is used as a chemical raw material in the production of numerous organic compounds.

Doctors consider it a narcotic substance. When inhaling concentrated acetone vapors, serious poisoning and damage to the central nervous system. This compound poses a serious threat to the younger generation. Drug users who use acetone vapor to induce a state of euphoria are at great risk. Doctors fear not only for the physical health of children, but also for their mental state.

A dose of 60 ml is considered lethal. When a significant amount of ketone enters the body, loss of consciousness occurs, and after 8-12 hours - death.

Physical Properties

Under normal conditions, this compound is in a liquid state, has no color, and has a specific odor. Acetone, the formula of which is CH3CHNOCH3, has hygroscopic properties. This compound is miscible in unlimited quantities with water, ethyl alcohol, methanol, chloroform. It has a low melting point.

Features of use

Currently, the scope of acetone is quite wide. It is rightfully considered one of the most popular products used in the creation and production of paints and varnishes, in finishing works, in the chemical industry, and in construction. Increasingly, acetone is used to degrease fur and wool, to remove wax from lubricating oils. This organic matter used by painters and plasterers in their professional activities.

How to save acetone, whose formula is CH3COCH3? In order to protect this volatile substance from negative impact ultraviolet rays, it is placed in plastic, glass, metal bottles away from UV.

The room where a significant amount of acetone is supposed to be placed must be systematically ventilated and high-quality ventilation installed.

Features of chemical properties

This compound got its name from the Latin word "acetum", meaning "vinegar" in translation. The fact is that chemical formula acetone C3H6O appeared much later than the substance itself was synthesized. It was obtained from acetates and then used to make glacial synthetic acetic acid.

Andreas Libavius ​​is considered the discoverer of the compound. At the end of the 16th century, by dry distillation of lead acetate, he managed to obtain a substance chemical composition which was deciphered only in the 30s of the XIX century.

Acetone, whose formula is CH3COCH3, was obtained by coking wood until the beginning of the 20th century. After the increase in demand during the First World War, it organic compound, new methods of synthesis began to appear.

Acetone (GOST 2768-84) is a technical liquid. In terms of chemical activity, this compound is one of the most reactive in the class of ketones. Under the influence of alkalis, adol condensation is observed, as a result of which diacetone alcohol is formed.

During pyrolysis, ketene is obtained from it. In reaction with hydrogen cyanide, acetone cyanidanhydrin is formed. Propanone is characterized by the substitution of hydrogen atoms for halogens, which occurs at elevated temperatures (or in the presence of a catalyst).

How to get

At present, the majority of the oxygen-containing compound is obtained from propene. Technical acetone (GOST 2768-84) must have certain physical and operational characteristics.

The cumene method consists of three stages and involves the production of acetone from benzene. First, cumene is obtained by alkylating it with propene, then the resulting product is oxidized to hydroperoxide and split under the influence of sulfuric acid to acetone and phenol.

In addition, this carbonyl compound is obtained by the catalytic oxidation of isopropanol at a temperature of about 600 degrees Celsius. The accelerators of the process are metallic silver, copper, platinum, nickel.

Among the classical technologies for the production of acetone, the direct oxidation of propene is of particular interest. This process is carried out at elevated pressure and the presence of bivalent palladium chloride as a catalyst.

You can also get acetone by fermenting starch under the influence of the bacteria Clostridium acetobutylicum. In addition to the ketone, butanol will be present among the reaction products. Among the disadvantages of this option for obtaining acetone, we note an insignificant percentage yield.

Conclusion

Propanone is a typical representative of carbonyl compounds. Consumers are familiar with it as a solvent and degreaser. It is indispensable in the manufacture of varnishes, medicines, explosives. It is acetone that is part of the glue for film, is a means for cleaning surfaces from mounting foam and superglue, a means for washing injection engines and a way to increase the octane number of fuel, etc.

Name component	Antoine equation coefficients
Butanol-1
Vinyl acetate
Methyl acetate
Morpholine
Formic acid
Acetic acid
pyrrolidine
benzyl alcohol
Ethanthiol
Chlorobenzene
Trichlorethylene *
Chloroform
Trimethylborate *
Methyl ethyl ketone
ethylene glycol
ethyl acetate
2-methyl-2-propanol
Dimethylformamide

Notes: 1)

* data.

Main literature

Serafimov L.A., Frolkova A.K. The fundamental principle of the redistribution of concentration fields between areas of separation as the basis for the creation of technological complexes. -Theor. basics of chem. technol., 1997–T. 31, no. 2. pp.184–192.

Timofeev V.S., Serafimov L.A. Principles of technology of basic organic and petrochemical synthesis. - M.: Chemistry, 1992. - 432 p.

Kogan V. B. Azeotropic and extractive distillation. - L.: Chemistry, 1971. - 432 p.

Sventoslavsky V.V. Azeotropy and polyazeotropy. - M.: Chemistry, 1968. -244 p.

Serafimov L.A., Frolkova A.K. General laws and classification of binary liquid solutions in terms of excess thermodynamic functions. Methodical instructions. – M.: A/O Rosvuznauka, 1992. 40 p.

Wales S. Phase equilibrium in chemical technology. T.1. - M.: Mir, 1989. - 304 p.

Thermodynamics of liquid-vapor equilibrium. / Edited by Morachevsky A.G. - L.: Chemistry, 1989. - 344 p.

Ogorodnikov S.K., Lesteva T.M., Kogan V.B. Azeotropic mixtures. Handbook. - L .: Chemistry, 1971. - 848 p.

Kogan V.B., Fridman V.M., Kafarov V.V. Equilibrium between liquid and vapor. Reference manual, in 2 volumes. - M.-L.: Nauka, 1966.

Lyudmirskaya G.S., Barsukova T.V., Bogomolny A.M. Equilibrium liquid-steam. Directory. -L.: Chemistry, 1987.-336 p.

Reid R., Prausnitz J., Sherwood T. Properties of gases and liquids. - L .: Chemistry, 1982. -592 p.

Belousov V.P., Morachevsky A.G. Heats of mixing of liquids. Handbook.  L .: Chemistry, 1970  256 p.

Belousov V.P., Morachevsky A.G., Panov M.Yu. Thermal properties of nonelectrolyte solutions. Directory. - L.: Chemistry, 1981. - 264 p.

The table shows the thermophysical properties of benzene vapor C 6 H 6 at atmospheric pressure.

The values ​​of the following properties are given: density, heat capacity, thermal conductivity coefficient, dynamic and kinematic viscosity, thermal diffusivity, Prandtl number depending on temperature. Properties are given in the temperature range from .

According to the table, it can be seen that the values ​​of the density and Prandtl number decrease with increasing temperature of gaseous benzene. The specific heat capacity, thermal conductivity, viscosity and thermal diffusivity increase their values ​​when benzene vapor is heated.

It should be noted that the vapor density of benzene at a temperature of 300 K (27 ° C) is 3.04 kg / m 3, which is much lower than that of liquid benzene (see).

Note: Be careful! The thermal conductivity in the table is given to the power of 10 3 Do not forget to divide by 1000.

Thermal conductivity of benzene vapor

The table gives the values ​​of the thermal conductivity of benzene vapor at atmospheric pressure depending on the temperature in the range from 325 to 450 K.
Note: Be careful! The thermal conductivity in the table is given to the power of 10 4 . Don't forget to divide by 10000.

The table shows the pressure of the saturated vapor of benzene in the temperature range from 280 to 560 K. Obviously, when benzene is heated, the pressure of its saturated vapor increases.

Sources:
1.
2.
3. Volkov A. I., Zharsky I. M. Big chemical reference book. - M: Soviet School, 2005. - 608 p.