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Products > Technical principles of drying
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TU
PU
TG
PG |
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Capillary action
Diffusion
Evaporation, vapourisation
Providing heat
Ambient temperature
Ambient pressure
Temperature of product
Pressure in the product
PU < PG |
From the practical viewpoint: in an open pot, heated water boils when its vapour pressure rises above the ambient air pressure.
As the temperature increases, the vapour pressure also increases, because more molecules in the liquid are brought to the boil, and hence more vapour is produced.
To dry a substance, the liquid must therefore be converted into the vapour state, and as heat energy is consumed continuously during vaporisation of a liquid, this energy must also be supplied continuously.
Liquid pure substances have a constant but pressure-dependent boiling point:
At a constant ambient pressure of 1013mbar, diethyl ether boils at 34.6°C, ethanol at 78.4°C and water at 100°C.
Continuous vaporisation (boiling) however, is possible only when the vapour is discharged, i.e., the atmosphere in the drying oven must be replaced continuously.
Because drying is not possible (evaporation and vaporisation) if the liquid content in the atmosphere is equal to the vapour pressure, or even exceeds it.
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relatively dry (unsaturated) air
relatively moist (saturated) air
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The higher the temperature and the greater the temperature differences between the ambient atmosphere and the vapour pressure of a substance to be vaporised, the faster the drying.
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The drying can be started or accelerated by:
- Increasing the temperature - Increasing the pressure difference (vacuum chamber)
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Technical fundamentals for drying under vacuum
Technical terms (related to vacuum):
Unit of pressure:
In vacuum technology, the most commonly used unit of pressure is the millibar (mbar)
Converting units of pressure:
1 Pa = 1 N/m²
1 bar = 100.000 Pa = 105 Pa
1 mbar = 100 Pa = 10² Pa 1 h Pa
1 Torr = 133.32 Pa
1 mm HG = 1 Torr = 133.32 Pa
1 mm WS = 9.80665 Pa
Total pressure:
The total pressure in a container is the sum of all partial pressures of gases and vapours contained inside.
For instance, air is composed of different gases such as nitrogen, oxygen, trace gases and water vapour, see below.
Composition of atmospheric air:
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Gas type |
% weight |
% volume |
Partial pressure, mbar |
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Nitrogen, N2 |
75.51 |
78.1 |
792 |
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Oxygen, O2 |
23.01 |
20.93 |
212 |
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Argon, Ar |
1.29 |
0.93 |
9.47 |
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Carbon dioxide, CO2 |
0.04 |
0.03 |
0.31 |
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Neon, Ne |
1.2 x 10-3 |
1.8 x 10-3 |
1.9 x 10-2 |
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Helium, He |
7 x 10-5 |
7 x 10-5 |
5.3 x 10-3 |
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Methane, CH4 |
2 x 10-4 |
2 x 10-4 |
2 x 10-3 |
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Krypton, Kr |
3 x 10-4 |
1.1 x 10-4 |
1.1 x 10-3 |
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Laughing gas (nitric oxide), N2O |
6 x 10-5 |
5 x 10-5 |
5 x 10-4 |
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Hydrogen, H2 |
5 x 10-6 |
5 x 10-5 |
5 x 10-4 |
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Xenon, Xe |
4 x 10-5 |
8.7 x 10-6 |
9 x 10-5 |
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Ozone, O3 |
9 x 10-6 |
7 x 10-6 |
7 x 10-5 |
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Total |
100 % |
100 % |
1013 mbar |
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Water vapour (50% RH at 20 °C) |
1.6 % |
1.15 % |
11.7 mbar |
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Air pressure at 50% RH / 20 °C |
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1024 mbar |
Partial pressure:
Partial pressure of a certain gas or vapour is that pressure which the gas or vapour would have, if it was the sole substance in a container.
Vapour:
Vapour is a gas that occurs in liquid and gaseous phase side by side at a particular temperature.
Gas:
Along with solid and liquid states, gas is one of the three states of aggregation. Gas is formed from vaporisation or sublimation, i.e. if the temperature is so high or the pressure is so low that all the liquid evaporates, then it is called gas.
Saturation vapour pressure:
Saturation vapour pressure is the pressure of the vapour phase of a substance, i.e. when the liquid and vapour phases are in equilibrium.
The saturation vapour pressure for a given pure substance is a function of the vapour temperature.
Further evaporation is not possible if the partial pressure reaches the saturation vapour pressure.
Boiling point:
Boiling point is the point at which a liquid changes from liquid to gaseous form. This is also referred to as vaporisation or boiling. On the other hand, it is called evaporation when the change from liquid to gaseous form takes place below the boiling point. Evaporation takes place at the surface of the liquid.
The boiling point of a liquid depends on temperature and pressure. The boiling point is a pair of variates in the phase diagram, consisting of saturation temperature (boiling temperature) and the saturation vapour pressure (boiling pressure).
Water boils, for example, at a total pressure of 1000 mbar and a temperature of 100 °C. At a total pressure of 100 mbar, water boils at about 46 °C and at 10 mbar at 7 °C.
Boiling and melting points (freezing points) of some solvents at 1000 mbar:
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Solvent |
Melting point, °C |
Boiling point, °C |
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Ethyl alcohol Benzol Chloroform Diethyl ether Isopropanol Methanol Nitromethane Water |
-114.5 5,49 -63.5 -116.4 -89.5 -97.9 -29.2 0.0 |
78 >100 61 34.6 82.4 64.7 101.8 100 |
For pure elements, the melting point is the same as the freezing point. In contrast to boiling temperature, melting temperature is affected very little by pressure.
Vacuum:
Vacuum is defined as that state which if formed due to the rarefaction of gases and vapours at pressures between the atmospheric pressure (1013 mbar) and 0 mbar by pumping out the contents of a container, i.e. the pressure or the density is less than the surrounding atmosphere.
Classification of vacuum:
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Pressure range |
Pressure in mbar |
Number density per cm ³ |
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Rough vacuum |
103 to 100 |
2.65 x 1019 to 2.65 x 1016 |
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Fine vacuum |
100 to 10-3 |
2.65 x 1016 to 2.65 x 1013 |
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High vacuum |
10-3 to 10-7 |
2.65 x 1013 to 2.65 x 109 |
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Ultra high vacuum |
10-7 to 10-12 (∞) |
2.65 x 109 to 2.65 x 104 |
How does drying take place in a vacuum drying chamber
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The drying process is explained using the example of water at a temperature of 20 °C:
As already mentioned and as can be seen in the saturation vapour pressure diagram, the saturation vapour pressure of water is 23.37 mbar at 20 °C. Or in other words: Water boils at a total pressure of less than 23.37 mbar already at 20 °C.
If the partial pressure of water is less than this value, the liquid will evaporate till the saturation vapour pressure is reached. Once this happens, further drying does not take place.
If the drying process is continued in a conventional drying oven (i.e. without vacuum), either the temperature must be increased > which also increases the value of the saturation vapour pressure< or the air, in which the partial vapour pressure has already reached the saturation pressure, should be replaced with unsaturated air.
In the vacuum drying chamber, the total pressure which is about 1000 mbar (1 bar) in the conventional drying chamber is reduced below the value of the saturation vapour pressure (23.37 mbar at 20 °C) using a vacuum pump. |
Water in liquid state immediately changes into gaseous state. It starts to boil. This again increases the total pressure in the vacuum drying chamber up to a maximum of saturation vapour pressure. If this pressure is reached, further vaporisation or evaporation does not take place.
By continually pumping out the water vapour (H2O gases) which is formed, it is ensured that the total pressure and thus the partial pressure of water vapour always remains below the saturation vapour pressure.
This process can be continued till all the liquid vaporises.
If other substances are dried using vacuum, only the values for the saturation vapour pressure changes at certain temperatures. The principle of the drying process is the same.
Since energy is required for changing the state from liquid to gaseous (for 1 litre of water at 20 °C, 2.454 x 106 J / kg = 0.68 kWh / kg = 860 kcal / kg), energy must be supplied in the form of heat in order to maintain a particular constant temperature in the substance to be dried.
The basis of drying is the vapour pressure of the liquid.
In the convection-based drying process, moisture in the drying oven is converted into vapour. Hence, it is mainly the vapour pressure of the liquid to be vaporised that is crucial.
If one increases the temperature to such an extent that the vapour pressure becomes equal to the atmospheric pressure, the liquid starts to boil. Vapour bubbles are formed inside the liquid against the external pressure, which leave the liquid. Continuous boiling is possible only when the vapour is discharged through the air vent.
Drying can take place only by virtue of the pressure difference between the ambient air pressure (atmosphere) and the vapour pressure of the liquid to be vaporised.
During drying the ambient air pressure is always below the vapour pressure, PU < PU
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TU
PU
TG
PG |
|
Capillary action
Diffusion
Evaporation, vapourisation
Providing heat
Ambient temperature
Ambient pressure
Temperature of product
Pressure in the product
PU < PG |
From the practical viewpoint: In an open pot, heated water boils when its vapour pressure rises above the ambient air pressure.
As the temperature increases, the vapour pressure also increases, because more molecules in the liquid are brought to the boil, and hence more vapour is produced.
To dry a substance, the liquid must therefore be converted into the vapour state, and as heat energy is consumed continuously during vaporisation of a liquid, this energy must also be supplied continuously.
Liquid pure substances have a constant but pressure-dependent boiling point:
At a constant ambient pressure of 1013mbar, diethyl ether boils at 34.6°C, ethanol at 78.4°C and water at 100°C.
Continuous vaporisation (boiling) however, is possible only when the vapour is discharged >by means of a vacuum pump<, i.e. the atmosphere in the drying oven must be replaced continuously.
Because drying is not possible (evaporation and vaporisation) if the liquid content in the atmosphere is equal to the vapour pressure, or even exceeds it.
The higher the temperature and the greater the temperature differences between the ambient atmosphere and the vapour pressure of a substance to be vaporised, the faster the drying.
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The drying can be started or accelerated by:
- Increasing the temperature - Increasing the pressure difference (vacuum chamber)
|
Water in the liquid state does convert immediately to a gaseous state. It begins to boil. Thus, the total pressure in the vacuum drying oven increases again up to the maximal possible pressure, i.e. up to the saturation vapor pressure. If this pressure is reached, no further vaporization or evaporation takes place more.