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 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.
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 density and Prandtl number decrease with increasing temperature of gaseous benzene. 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/m3, which is much lower than that of liquid benzene (see).
Note: Be careful! Thermal conductivity in the table is indicated to the power of 10 3. Remember to divide by 1000.
Thermal conductivity of benzene vapor
The table shows the thermal conductivity of benzene vapor at atmospheric pressure depending on temperature in the range from 325 to 450 K.
Note: Be careful! Thermal conductivity in the table is indicated to the power of 10 4. Don't forget to divide by 10000.
The table shows the values of the saturated vapor pressure of benzene in the temperature range from 280 to 560 K. Obviously, when benzene is heated, its saturated vapor pressure increases.
Sources:
1.
2.
3. Volkov A.I., Zharsky I.M. Large chemical reference book. - M: Soviet School, 2005. - 608 p.
n16.doc
Chapter 7. VAPOR PRESSURE, PHASE TEMPERATURESTRANSITIONS, SURFACE TENSION
Information on the vapor pressure of pure liquids and solutions, their boiling and solidification (melting) temperatures, as well as surface tension are necessary for calculations of various technological processes: evaporation and condensation, evaporation and drying, distillation and rectification, etc.
7.1. Vapor pressure
One of the simplest equations for determining the saturated vapor pressure of a pure liquid depending on temperature 
is Antoine's equation:
, (7.1)
Where A, IN, WITH– constants, characteristic of individual substances. The constant values for some substances are given in table. 7.1.
If two boiling temperatures are known at corresponding pressures, then, taking WITH= 230, constants can be determined A And IN by jointly solving the following equations:
; (7.2)
. (7.3)
Equation (7.1) corresponds quite satisfactorily to experimental data in a wide temperature range between the melting temperature and 
= 0.85 (i.e. 
= 0.85). This equation provides the greatest accuracy in cases where all three constants can be calculated on the basis of experimental data. The accuracy of calculations using equations (7.2) and (7.3) is significantly reduced already at 
250 K, and for highly polar compounds at 0.65.
The change in vapor pressure of a substance depending on temperature can be determined by the comparison method (according to the linearity rule), based on the known pressures of the reference liquid. If two temperatures of a liquid substance are known at corresponding saturated vapor pressures, we can use the equation
, (7.4)
Where 
And 
– saturated vapor pressure of two liquids A And IN at the same temperature 
; 
And 
– saturated vapor pressure of these liquids at temperature 
; WITH– constant.
Table 7.1. Vapor pressure of some substances depending on
on temperature
The table shows the values of the constants A, IN And WITH Antoine's equation: , where is the saturated vapor pressure, mmHg. (1 mm Hg = 133.3 Pa); T– temperature, K.
| Substance name | Chemical formula | Temperature range, o C | A | IN | WITH |
|
| from | before |
|||||
| Nitrogen | N 2 | –221 | –210,1 | 7,65894 | 359,093 | 0 |
| Nitrogen dioxide | N 2 O 4 (NO 2) | –71,7 | –11,2 | 12,65 | 2750 | 0 |
| –11,2 | 103 | 8,82 | 1746 | 0 |
||
| Nitrogen oxide | NO | –200 | –161 | 10,048 | 851,8 | 0 |
| –164 | –148 | 8,440 | 681,1 | 0 |
||
| Acrylamide | C 3 H 5 ON | 7 | 77 | 12,34 | 4321 | 0 |
| 77 | 137 | 9,341 | 3250 | 0 |
||
| Acrolein | C 3 H 4 O | –3 | 140 | 7,655 | 1558 | 0 |
| Ammonia | NH 3 | –97 | –78 | 10,0059 | 1630,7 | 0 |
| Aniline | C6H5NH2 | 15 | 90 | 7,63851 | 1913,8 | –53,15 |
| 90 | 250 | 7,24179 | 1675,3 | –73,15 |
||
| Argon | Ar | –208 | –189,4 | 7,5344 | 403,91 | 0 |
| –189,2 | –183 | 6,9605 | 356,52 | 0 |
||
| Acetylene | C2H2 | –180 | –81,8 | 8,7371 | 1084,9 | –4,3 |
| –81,8 | 35,3 | 7,5716 | 925,59 | 9,9 |
||
| Acetone | C3H6O | –59,4 | 56,5 | 8,20 | 1750 | 0 |
| Benzene | C6H6 | –20 | 5,5 | 6,48898 | 902,28 | –95,05 |
| 5,5 | 160 | 6,91210 | 1214,64 | –51,95 |
||
| Bromine | BR 2 | 8,6 | 110 | 7,175 | 1233 | –43,15 |
| Hydrogen bromide | HBr | –99 | –87,5 | 8,306 | 1103 | 0 |
| –87,5 | –67 | 7,517 | 956,5 | 0 |
||
Continuation of the table. 7.1
| Substance name | Chemical formula | Temperature range, o C | A | IN | WITH |
|
| from | before |
|||||
| 1,3-Butadiene | C4H6 | –66 | 46 | 6,85941 | 935,53 | –33,6 |
| 46 | 152 | 7,2971 | 1202,54 | 4,65 |
||
| n-Butane | C4H10 | –60 | 45 | 6,83029 | 945,9 | –33,15 |
| 45 | 152 | 7,39949 | 1299 | 15,95 |
||
| Butyl alcohol | C4H10O | 75 | 117,5 | 9,136 | 2443 | 0 |
| Vinyl acetate | CH 3 COOCH=CH 2 | 0 | 72,5 | 8,091 | 1797,44 | 0 |
| Vinyl chloride | CH 2 =CHСl | –100 | 20 | 6,49712 | 783,4 | –43,15 |
| –52,3 | 100 | 6,9459 | 926,215 | –31,55 |
||
| 50 | 156,5 | 10,7175 | 4927,2 | 378,85 |
||
| Water | H 2 O | 0 | 100 | 8,07353 | 1733,3 | –39,31 |
| Hexane | C 6 H 1 4 | –60 | 110 | 6,87776 | 1171,53 | –48,78 |
| 110 | 234,7 | 7,31938 | 1483,1 | –7,25 |
||
| Heptane | C 7 H 1 6 | –60 | 130 | 6,90027 | 1266,87 | –56,39 |
| 130 | 267 | 7,3270 | 1581,7 | –15,55 |
||
| Dean | C 10 H 22 | 25 | 75 | 7,33883 | 1719,86 | –59,35 |
| 75 | 210 | 6,95367 | 1501,27 | –78,67 |
||
| Diisopropyl ether | C6H14O | 8 | 90 | 7,821 | 1791,2 | 0 |
| N,N-Dimethylacetamide | C 4 H 9 ON | 0 | 44 | 7,71813 | 1745,8 | –38,15 |
| 44 | 170 | 7,1603 | 1447,7 | –63,15 |
||
| 1,4-Dioxane | C4H8O2 | 10 | 105 | 7,8642 | 1866,7 | 0 |
| 1,1-Dichloroethane | C2H4Cl2 | 0 | 30 | 7,909 | 1656 | 0 |
| 1,2-Dichloroethane | C2H4Cl2 | 6 | 161 | 7,18431 | 1358,5 | –41,15 |
| 161 | 288 | 7,6284 | 1730 | 9,85 |
||
| Diethyl ether | (C 2 H 5) 2 O | –74 | 35 | 8,15 | 1619 | 0 |
| Isobutyric acid | C4H8O2 | 30 | 155 | 8,819 | 2533 | 0 |
| Isoprene | C 5 H 8 | –50 | 84 | 6,90334 | 1081,0 | –38,48 |
| 84 | 202 | 7,33735 | 1374,92 | 2,19 |
||
| Isopropyl alcohol | C3H8O | –26,1 | 82,5 | 9,43 | 2325 | 0 |
| Hydrogen iodide | HI | –50 | –34 | 7,630 | 1127 | 0 |
| Krypton | Kr | –207 | –158 | 7,330 | 7103 | 0 |
| Xenon | Heh | –189 | –111 | 8,00 | 841,7 | 0 |
| n-Xylene | C 8 H 10 | 25 | 45 | 7,32611 | 1635,74 | –41,75 |
| 45 | 190 | 6,99052 | 1453,43 | –57,84 |
||
| O-Xylene | C 8 H 10 | 25 | 50 | 7,35638 | 1671,8 | –42,15 |
| 50 | 200 | 6,99891 | 1474,68 | –59,46 |
||
Continuation of the table. 7.1
| Substance name | Chemical formula | Temperature range, o C | A | IN | WITH |
|
| from | before |
|||||
| Butyric acid | C4H8O2 | 80 | 165 | 9,010 | 2669 | 0 |
| Methane | CH 4 | –161 | –118 | 6,81554 | 437,08 | –0,49 |
| –118 | –82,1 | 7,31603 | 600,17 | 25,27 |
||
| Methylene chloride (dichloromethane) | CH2Cl2 | –28 | 121 | 7,07138 | 1134,6 | –42,15 |
| 127 | 237 | 7,50819 | 1462,59 | 5,45 |
||
| Methyl alcohol | CH 4 O | 7 | 153 | 8,349 | 1835 | 0 |
| -Methylstyrene | C 9 H 10 | 15 | 70 | 7,26679 | 1680,13 | –53,55 |
| 70 | 220 | 6,92366 | 1486,88 | –71,15 |
||
| Methyl chloride | CH3Cl | –80 | 40 | 6,99445 | 902,45 | –29,55 |
| 40 | 143,1 | 7,81148 | 1433,6 | 44,35 |
||
| Methyl ethyl ketone | C4H8O | –15 | 85 | 7,764 | 1725,0 | 0 |
| Formic acid | CH2O2 | –5 | 8,2 | 12,486 | 3160 | 0 |
| 8,2 | 110 | 7,884 | 1860 | 0 |
||
| Neon | Ne | –268 | –253 | 7,0424 | 111,76 | 0 |
| Nitrobenzene | C 6 H 5 O 2 N | 15 | 108 | 7,55755 | 2026 | –48,15 |
| 108 | 300 | 7,08283 | 1722,2 | –74,15 |
||
| Nitromethane | CH 3 O 2 N | 55 | 136 | 7,28050 | 1446,19 | –45,63 |
| Octane | C 8 H 18 | 15 | 40 | 7,47176 | 1641,52 | –38,65 |
| 40 | 155 | 6,92377 | 1355,23 | –63,63 |
||
| Pentane | C5H12 | –30 | 120 | 6,87372 | 1075,82 | –39,79 |
| 120 | 196,6 | 7,47480 | 1520,66 | 23,94 |
||
| Propane | C 3 H 8 | –130 | 5 | 6,82973 | 813,2 | –25,15 |
| 5 | 96,8 | 7,67290 | 1096,9 | 47,39 |
||
| Propylene (propene) | C3H6 | –47,7 | 0,0 | 6,64808 | 712,19 | –36,35 |
| 0,0 | 91,4 | 7,57958 | 1220,33 | 36,65 |
||
| Propylene oxide | C3H6O | –74 | 35 | 6,96997 | 1065,27 | –46,87 |
| Propylene glycol | C 3 H 8 O 2 | 80 | 130 | 9,5157 | 3039,0 | 0 |
| Propyl alcohol | C3H8O | –45 | –10 | 9,5180 | 2469,1 | 0 |
| Propionic acid | C 3 H 6 O 2 | 20 | 140 | 8,715 | 2410 | 0 |
| Hydrogen sulfide | H2S | –110 | –83 | 7,880 | 1080,6 | 0 |
| Carbon disulfide | CS 2 | –74 | 46 | 7,66 | 1522 | 0 |
| Sulfur dioxide | SO 2 | –112 | –75,5 | 10,45 | 1850 | 0 |
| Sulfur trioxide () | SO 3 | –58 | 17 | 11,44 | 2680 | 0 |
| Sulfur trioxide () | SO 3 | –52,5 | 13,9 | 11,96 | 2860 | 0 |
| Tetrachlorethylene | C 2 Cl 4 | 34 | 187 | 7,02003 | 1415,5 | –52,15 |
End of table. 7.1
| Substance name | Chemical formula | Temperature range, o C | A | IN | WITH |
|
| from | before |
|||||
| Thiophenol | C6H6S | 25 | 70 | 7,11854 | 1657,1 | –49,15 |
| 70 | 205 | 6,78419 | 1466,5 | –66,15 |
||
| Toluene | C 6 H 5 CH 3 | 20 | 200 | 6,95334 | 1343,94 | –53,77 |
| Trichlorethylene | C2HCl3 | 7 | 155 | 7,02808 | 1315,0 | –43,15 |
| Carbon dioxide | CO 2 | –35 | –56,7 | 9,9082 | 1367,3 | 0 |
| Carbon oxide | CO | –218 | –211,7 | 8,3509 | 424,94 | 0 |
| Acetic acid | C 2 H 4 O 2 | 16,4 | 118 | 7,55716 | 1642,5 | –39,76 |
| Acetic anhydride | C 4 H 6 O 3 | 2 | 139 | 7,12165 | 1427,77 | –75,11 |
| Phenol | C6H6O | 0 | 40 | 11,5638 | 3586,36 | 0 |
| 41 | 93 | 7,86819 | 2011,4 | –51,15 |
||
| Fluorine | F 2 | –221,3 | –186,9 | 8,23 | 430,1 | 0 |
| Chlorine | Cl2 | –154 | –103 | 9,950 | 1530 | 0 |
| Chlorobenzene | C 6 H 5 Cl | 0 | 40 | 7,49823 | 1654 | –40,85 |
| 40 | 200 | 6,94504 | 1413,12 | –57,15 |
||
| Hydrogen chloride | HCl | –158 | –110 | 8,4430 | 1023,1 | 0 |
| Chloroform | CHCl 3 | –15 | 135 | 6,90328 | 1163,0 | –46,15 |
| 135 | 263 | 7,3362 | 1458,0 | 2,85 |
||
| Cyclohexane | C6H12 | –20 | 142 | 6,84498 | 1203,5 | –50,29 |
| 142 | 281 | 7,32217 | 1577,4 | 2,65 |
||
| Tetrachloride carbon | CCl 4 | –15 | 138 | 6,93390 | 1242,4 | –43,15 |
| 138 | 283 | 7,3703 | 1584 | 3,85 |
||
| Ethane | C2H6 | –142 | –44 | 6,80266 | 636,4 | –17,15 |
| –44 | 32,3 | 7,6729 | 1096,9 | 47,39 |
||
| Ethylbenzene | C 8 H 10 | 20 | 45 | 7,32525 | 1628,0 | –42,45 |
| 45 | 190 | 6,95719 | 1424,26 | –59,94 |
||
| Ethylene | C2H4 | –103,7 | –70 | 6,87477 | 624,24 | –13,14 |
| –70 | 9,5 | 7,2058 | 768,26 | 9,28 |
||
| Ethylene oxide | C2H4O | –91 | 10,5 | 7,2610 | 1115,10 | –29,01 |
| Ethylene glycol | C 2 H 6 O 2 | 25 | 90 | 8,863 | 2694,7 | 0 |
| 90 | 130 | 9,7423 | 3193,6 | 0 |
||
| Ethanol | C2H6O | –20 | 120 | 6,2660 | 2196,5 | 0 |
| Ethyl chloride | C 2 H 5 Cl | –50 | 70 | 6,94914 | 1012,77 | –36,48 |
When determining the saturated vapor pressure of water-soluble substances using the linearity rule, water is used as a reference liquid, and in the case of organic compounds insoluble in water, hexane is usually taken. The values of saturated vapor pressure of water depending on temperature are given in table. P.11. The dependence of saturated vapor pressure on hexane temperature is shown in Fig. 7.1.
Rice. 7.1. Dependence of saturated vapor pressure of hexane on temperature
(1 mm Hg = 133.3 Pa)
Based on relationship (7.4), a nomogram was constructed to determine the saturated vapor pressure depending on temperature (see Fig. 7.2 and Table 7.2).
Above solutions, the saturated vapor pressure of the solvent is less than above a pure solvent. Moreover, the higher the concentration of the dissolved substance in the solution, the greater the decrease in vapor pressure.
Allen
6
1,2-Dichloroethane
26
Propylene
4
Ammonia
49
Diethyl ether
15
Propionic
56
Aniline
40
Isoprene
14
acid
Acetylene
2
Iodobenzene
39
Mercury
61
Acetone
51
m-Cresol
44
Tetralin
42
Benzene
24
O-Cresol
41
Toluene
30
Bromobenzene
35
m-Xylene
34
Acetic acid
55
Ethyl bromide
18
iso-Oil
57
Fluorobenzene
27
-Bromonaphthalene
46
acid
Chlorobenzene
33
1,3-Butadiene
10
Methylamine
50
Vinyl chloride
8
Butane
11
Methylmonosilane
3
Methyl chloride
7
-Butylene
9
Methyl alcohol
52
Chloride
19
-Butylene
12
Methyl formate
16
methylene
Butylene glycol
58
Naphthalene
43
Ethyl chloride
13
Water
54
-Naphthol
47
Chloroform
21
Hexane
22
-Naphthol
48
Tetrachloride
23
Heptane
28
Nitrobenzene
37
carbon
Glycerol
60
Octane
31*
Ethane
1
Decalin
38
32*
Ethyl acetate
25
Dean
36
Pentane
17
Ethylene glycol
59
Dioxane
29
Propane
5
Ethanol
53
Diphenyl
45
Ethyl formate
20
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:
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