Evaporation is the transition of a liquid into 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 fluctuates over a wide range, deviating greatly in both directions from its average value. Some molecules that have a sufficiently high 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 spent on overcoming the interaction forces between molecules and the work of expansion (increase in volume) when the liquid transforms into vapor.
Evaporation is an endothermic process. If heat is not supplied to the liquid from the outside, it cools as a result of evaporation. The rate of evaporation is determined by the amount of vapor formed per unit time per unit surface of the liquid. This must be taken into account in industries involving the use, production or processing of flammable liquids. Increasing the rate of evaporation with increasing temperature results in the more rapid formation of explosive concentrations of vapors. The maximum evaporation rate is observed when evaporating into a vacuum and into 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 condensation rate, therefore, the lower the total evaporation rate. At the interface between the liquid and its saturated vapor, the evaporation rate (total) is close to zero. A liquid in a closed container evaporates and forms saturated steam. Vapor that is in dynamic equilibrium with the liquid is called saturated. Dynamic equilibrium at a given temperature occurs when the number of evaporating liquid molecules is equal to the number of condensing molecules. Saturated steam, leaving an open vessel into the air, is diluted by it and becomes unsaturated. Therefore, in the air
In rooms where containers with hot liquids are located, there is unsaturated vapor of these liquids.
Saturated and unsaturated vapors exert pressure on the walls of blood vessels. Saturated vapor pressure is the pressure of steam 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, or the shape of the vessel, but depends only on the temperature and nature of the liquid. With increasing temperature, the saturated 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 saturated vapor pressure of mixtures of liquids (oil, gasoline, kerosene, etc.) at the same temperature depends on the composition of the mixture. It increases with increasing content of low-boiling products in the liquid.
For most liquids, the saturated vapor pressure at various temperatures is known. The values of saturated vapor pressure of some liquids at various 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 from the table.
5.1 the saturated vapor pressure of a liquid is an integral part of the total pressure of the vapor-air mixture.
Let us assume that the mixture of vapor with air formed above the surface of carbon disulfide in a vessel at 263 K has a pressure of 101080 Pa. Then the saturated 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, air pressure decreases. The total pressure remains constant.
The part of the total pressure attributable to a given gas or vapor is called partial. In this case, the vapor pressure of carbon disulfide (10773 Pa) can be called partial pressure. Thus, the total pressure of the steam-air mixture is the sum of the partial pressures of carbon disulfide, oxygen and nitrogen vapors: 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 concentrations of liquid vapors in the air from the known total pressure of the mixture and the vapor pressure.
The vapor pressure of liquids is determined by the number of molecules striking the walls of the container or the concentration of vapor above the surface of the liquid. The higher the concentration of saturated steam, the greater its pressure will be. The relationship between the concentration of saturated steam and its partial pressure can be found as follows.
Let us assume that it would be possible to separate steam from air, and the pressure in both parts would remain equal to the total pressure Ptot. Then the volumes occupied by steam and air would correspondingly decrease. According to the Boyle-Mariotte law, the product of gas pressure and its volume at a constant temperature is a constant value, i.e. for our hypothetical case we get:
.
METHOD FOR CALCULATING PARAMETERS OF EVAPORATION OF FLAMMABLE UNHEATED LIQUIDS AND LIQUEFIED HYDROCARBON GASES
I.1 Evaporation rate W, kg/(s m 2), determined from 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 = 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 above 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 speed in the room, m/s | The value of coefficient h at temperature t, ° C, 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 (LPG), in the absence of data, it is allowed to calculate the specific gravity of vapors of evaporated LPG m LPG, kg/m 2, according to formula 1)
, (AND 2)
1) The formula is applicable at temperatures of the underlying surface from minus 50 to plus 40 °C.
Where M - molar mass of LPG, kg/mol;
L isp - molar heat of evaporation of LPG at the initial temperature of LPG T l, J/mol;
T 0 - initial temperature of the material on the surface of which LPG is poured, corresponding to the design temperature t p , K;
Tf - initial temperature of LPG, K;
l TV - thermal conductivity coefficient of the material on the surface of which LPG is poured, W/(m K);
a is the effective coefficient of thermal diffusivity of the material on the surface of which LPG is poured, 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 speed, 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 of flammable unheated liquids and liquefied hydrocarbon gases
1 Determine the mass of acetone vapor entering 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 with a maximum volume of V ap = 3 m 3 is installed. Acetone enters the apparatus by gravity through a pipeline with a diameter of d= 0.05 m with flow q, equal to 2 · 10 -3 m 3 /s. Length of the pressure pipeline section from the tank to the manual valve l 1 = 2 m. Length of the outlet pipeline section with diameter d = 0.05 m from the container to the manual valve L 2 is equal to 1 m. The air flow speed in the room with general ventilation running is 0.2 m/s. The air temperature in the room is tp = 20 ° C. The density r of acetone at this 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 (for manual shutdown).
Volume of acetone released from the outlet pipe V from is
The volume of acetone entering the room
V a = V ap + V n.t + V from = 3 + 6.04 · 10 -1 + 1.96 · 10 -3 = 6.600 m 3.
Based on the fact that 1 liter of acetone is poured onto 1 m2 of floor area, the calculated evaporation area S p = 3600 m2 of acetone will exceed the floor area of the room. Therefore, the floor area of the room is taken as the area of acetone evaporation equal to 50 m2.
The evaporation rate is:
W use = 10 -6 · 3.5 · 24.54 = 0.655 · 10 -3 kg/(s m 2).
The mass of acetone vapors formed during emergency depressurization of the apparatus T, kg, will be equal
t = 0.655 10 -3 50 3600 = 117.9 kg.
2 Determine the mass of gaseous ethylene formed during the evaporation of a spill of liquefied ethylene under conditions of emergency depressurization of the tank.
Data for calculation
An isothermal tank of liquefied ethylene with a volume V i.r.e = 10,000 m 3 is installed in a concrete embankment with a free area S ob = 5184 m 2 and a flanging height H ob = 2.2 m. The degree of filling of the tank is a = 0.95.
The liquefied ethylene supply pipeline enters the tank from the top, and the outlet pipeline exits from the bottom.
The diameter of the outlet pipeline d tp = 0.25 m. The length of the pipeline section from the tank to the automatic valve, the probability of failure of which exceeds 10 -6 per year and the redundancy of its elements is not ensured, L= 1 m. Maximum consumption of liquefied ethylene in the dispensing mode G liquid e = 3.1944 kg/s. Density of liquefied ethylene r l.e. at operating temperature T ek= 169.5 K is equal to 568 kg/m3. Density of ethylene gas r g.e at T ek equal to 2.0204 kg/m3. Molar mass of liquefied ethylene M zh.e = 28 · 10 -3 kg/mol. Molar heat of vaporization of liquefied ethylene L иcn at T eq is equal to 1.344 · 10 4 J/mol. The temperature of concrete is equal to the maximum possible air temperature in the corresponding climatic zone T b = 309 K. The thermal conductivity coefficient of concrete l b = 1.5 W/(m K). Thermal diffusivity coefficient of concrete A= 8.4 · 10 -8 m 2 /s. The minimum air flow speed is u min = 0 m/s, and the maximum for a given climatic zone is u max = 5 m/s. The kinematic viscosity of air n in at the design air temperature for a given climatic zone t р = 36 ° C is equal to 1.64 · 10 -5 m 2 /s. The thermal conductivity coefficient of air l in at t p is equal to 2.74 · 10 -2 W/(m · K).
If the isothermal tank is destroyed, the volume of liquefied ethylene will be
Free dike volume V about = 5184 · 2.2 = 11404.8 m3.
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 i.e. from the area of the strait at an air flow speed u = 5 m/s is calculated using formula (I.2)
The mass m i.e. at u = 0 m/s will be 528039 kg.
In practice, numerous solutions are widely used, consisting of two or more liquids that are readily soluble in each other. The simplest are mixtures (solutions) consisting of two liquids - binary mixtures. The patterns found for such mixtures can be used for more complex ones. Such binary mixtures include: benzene-toluene, alcohol-ether, acetone-water, alcohol-water, etc. In this case, both components are contained in the vapor phase. The saturated vapor pressure of the mixture will be the sum of the partial pressures of the components. Since the transition of a solvent from a mixture to a vapor state, expressed by its partial pressure, is more significant, the higher the content of its molecules in the solution, Raoult found that “the partial pressure of the saturated vapor of the solvent above the solution is equal to the product of the saturated vapor pressure above the pure solvent at the same temperature by its mole fraction in solution":
Where 
- saturated vapor pressure of the solvent above the mixture; 
- saturated vapor pressure above a pure solvent; N – mole fraction of solvent in the mixture.
Equation (8.6) is a mathematical expression of Raoult's law. To describe the behavior of a volatile solute (the second component of a binary system), the same expression is used:
.
(8.7)
The total saturated vapor pressure above the solution will be equal to (Dalton's law):
The dependence of the partial and total vapor pressure of the mixture on its composition is shown in Fig. 8.3, where the ordinate axis shows the saturated vapor pressure, and the abscissa axis shows the composition of the solution in mole fractions. In this case, along the x-axis, the content of one substance (A) decreases from left to right from 1.0 to 0 mole fractions, and the content of the second component (B) simultaneously increases in the same direction from 0 to 1.0. For each specific composition, the total saturated vapor pressure is equal to the sum of the partial pressures. The total pressure of the mixture varies from the saturated vapor pressure of one individual liquid 
to the saturated vapor pressure of the second pure liquid 
.
Raoult's and Dalton's laws are often used to assess the fire hazard of mixtures of liquids.
Mixture composition, mole fractions
Rice. 8.3 Diagram of solution composition - saturated vapor pressure
Typically, the composition of the vapor phase does not coincide with the composition of the liquid phase and the vapor phase is enriched in a more volatile component. This difference can also be depicted graphically (the graph looks similar to the graph in Fig. 8.4, only the ordinate is not temperature, but pressure).
In diagrams representing the dependence of boiling points on composition (diagram composition - boiling point rice. 8.4), it is usually customary to construct two curves, one of which relates these temperatures to the composition of the liquid phase, and the other to the composition of the vapor. The lower curve refers to liquid compositions (liquid curve) and the upper curve relates to vapor compositions (vapor curve).
The field contained between the two curves corresponds to a two-phase system. Any point located in this field corresponds to the equilibrium of two phases - solution and saturated vapor. The composition of the equilibrium phases is determined by the coordinates of the points lying at the intersection of the isotherm passing through the curves and the given point.
At temperature t 1 (at a given pressure), a liquid solution of composition x 1 will boil (point a 1 on the liquid curve), steam in equilibrium with this solution has composition x 2 (point b 1 on the steam curve).
Those. liquid of composition x 1 will correspond to vapor of composition x 2.
Based on the expressions: 
,
,
,
,
the relationship between the composition of the liquid and vapor phases can be expressed by the relationship:
.
(8.9)
Rice. 8.4. Composition-boiling point diagram of binary mixtures.
The real saturated vapor pressure of an individual liquid at a given temperature is a characteristic value. There are practically no liquids that have the same saturated vapor pressure at the same temperature. That's why 
always more or less 
. If 
>
, That 
>
, i.e. the composition of the vapor phase is enriched with component A. While studying solutions, D.P. Konovalov (1881) made a generalization called Konovalov’s first law.
In a binary system, vapor, compared to the liquid in equilibrium with it, is relatively richer in that component, the addition of which to the system increases the total vapor pressure, i.e. lowers the boiling point of a mixture at a given pressure.
Konovalov's first law is the theoretical basis for separating liquid solutions into their original components by fractional distillation. For example, a system characterized by point K consists of two equilibrium phases, the composition of which is determined by points a and b: point a characterizes the composition of saturated vapor, point b characterizes the composition of the solution.
Using the graph, it is possible to compare the compositions of vapor and liquid phases for any point contained in the plane between the curves.
Real solutions. Raoult's law does not hold for real solutions. There are two types of deviation from Raoult's law:
the partial pressure of solutions is greater than the pressure or volatility of vapors of ideal solutions.
The total vapor pressure is greater than the additive value. Such deviations are called positive, for example, for mixtures (Fig. 8.5 a, b) CH 3 COCH 3 -C 2 H 5 OH, CH 3 COCH 3 -CS 2, C 6 H 6 - CH 3 COCH 3, H 2 O- CH 3 OH, C 2 H 5 OH-CH 3 OCH 3, CCl 4 -C 6 H 6, etc.;
b
Rice. 8.5. Dependence of total and partial steam pressure on composition:
a – for mixtures with a positive deviation from Raoult’s law;
b – for mixtures with a negative deviation from Raoult’s law.
The partial pressure of solutions is less than the vapor pressure of ideal solutions. The total vapor pressure is less than the additive value. Such deviations are called negative. For example, for a mixture: H 2 O-HNO 3;
H 2 O-HCl;
CHCl 3 -(CH 3) 2 CO;
CHCl 3 -C 6 H 6 etc.
Positive deviations are observed in solutions in which dissimilar molecules interact with less force than homogeneous ones.
This facilitates the transition of molecules from solution to the vapor phase. Solutions with a positive deviation are formed with the absorption of heat, i.e. the heat of mixing of pure components will be positive, an increase in volume occurs, and a decrease in association.
Negative deviations from Raoult's law occur in solutions in which there is an increase in the interaction of dissimilar molecules, solvation, the formation of hydrogen bonds, and the formation of chemical compounds. This makes it difficult for molecules to move from solution to the gas phase. What is acetone? The formula of this ketone is discussed in a school chemistry course. 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
A dose of 60 ml is considered lethal. If 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, whose formula is CH3CHOCH3, 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 application 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, the chemical industry, and construction. Acetone is increasingly used to degrease fur and wool and remove wax from lubricating oils. It is this organic substance that painters and plasterers use in their professional activities.
How to store acetone, the formula of which is CH3COCH3? In order to protect this volatile substance from the negative effects of ultraviolet rays, it is placed in plastic, glass, and metal bottles away from UV.
The room where a significant amount of acetone is to be placed must be systematically ventilated and high-quality ventilation installed.
Features of chemical properties
This compound gets its name from the Latin word “acetum”, which means “vinegar”. The fact is that the chemical formula of 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, the chemical composition of which was deciphered only in the 30s of the 19th century.
Acetone, whose formula is CH3COCH3, was obtained by coking wood until the beginning of the 20th century. Following the increased demand for this organic compound during World War I, new methods of synthesis began to emerge.
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.
When pyrolyzed, ketene is obtained from it. The reaction with hydrogen cyanide produces acetonecyanidanhydrin. Propanone is characterized by the replacement of hydrogen atoms with halogens, which occurs at elevated temperatures (or in the presence of a catalyst).
Methods of obtaining
Currently, the bulk 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 alkylation 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. Metallic silver, copper, platinum, and nickel act as process accelerators.
Among the classical technologies for the production of acetone, the reaction of direct oxidation of propene is of particular interest. This process is carried out at elevated pressure and the presence of divalent 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 producing acetone, we note the 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, and explosives. It is acetone that is included in film adhesive, is a means for cleaning surfaces from polyurethane foam and superglue, a means of washing injection engines and a way to increase octane number fuel, etc.