Evaporation

With an evaporation material from a liquid is transferred into the gaseous state without reaching the boiling point. The evaporation occurs when the gas phase is not saturated with vapor from the liquid.

Simplified description

A single substance is in a closed vessel, which may be gaseous or liquid at the given temperature depending on the pressure, so an equilibrium between the liquid and the gas above the liquid forms. The pressure which then prevails in the vessel, the vapor pressure of the liquid. Is pumped from the steam, so bothers you the balance between gas and liquid, so that more fluid passes into the gas phase. The liquid boils. By replacing the steam with another gas ( eg, air), as here, some of the liquid is evaporated until the equilibrium is re-established. However, the pressure at which this occurs is the attached gas considerably greater. The boiling temperature to be higher for this pressure. Therefore, the liquid does not boil, but it evaporates.

Classification of the phenomenon in the thermodynamics

The evaporation itself represents a phase transition and therefore derives also from the laws of thermodynamics, without which one can not understand this process. In accordance with the Maxwell - Boltzmann distribution, the particles have a speed distribution of a gas, but also in a similar shape of the particles a liquid to. Therefore, there are always at the same time for both slow and fast particles, these have a specific kinetic energy and the percentage increase in speed as well as the faster the particles with increasing temperature. As fast particles with a sufficiently high kinetic energy are in this case able to overcome the attractive forces which act by their neighboring particles on it, some of them always change from the liquid to the gaseous phase. However, there are always decelerated particles of the gaseous phase into the liquid phase is recycled, which is why a dynamic equilibrium is established with time, without any influence from the outside and without one of the phases is used up. In the Earth's atmosphere, such a balance is not always achieved, and if it is so disturbed that more particles escape from the liquid phase as enter into it, it is called an evaporation. The evaporation can also lead to the complete disappearance of the liquid phase, which is called dehydration.

The liquid phase cools down the evaporation process and thus leads to the so-called evaporative cooling, the environment, the evaporation heat is supplied in the form of Latent Heat.

Vs. evaporation. boil

In thermodynamic equilibrium, the partial pressure corresponding to the saturation vapor pressure of the gas phase of the evaporating substance. So Evaporative then occurs when the saturated vapor pressure is greater than the partial pressure. However, this process is slow, since the liquid phase is stable in itself, as long as the vapor pressure is lower than the total pressure. However Corresponds to the vapor pressure to the total pressure or exceeds this, the boiling point is reached and it boils the substance. Evaporation is therefore only possible if a foreign material gas is still present - usually air - this represents the residual pressure available. The reverse process - the condensation - occurs when the saturated vapor pressure is below the partial pressure.

Water evaporation

Water evaporates at room temperature, provided that the air is not saturated with water vapor, which would correspond to the equilibrium described above.

On the principle of water evaporation is based, for example, the open-air drying laundry or disappearance of water puddles. The effect of evaporative cooling by water is the basis for the effect of thermoregulation through sweating by the skin removed from the heat of evaporation and it is thereby cooled.

In ecology, meteorology and climatology between transpiration ( sweating leaf evaporation) and evaporation is (evaporation of water on unbewachsenem / vacant land or water areas designated ) distinguished as forms of evaporation, whereby both summarizes also for evapotranspiration.

The uptake of water into the atmosphere through evaporation thereby takes place on the earth's surface, so for example, of water, soils and plants. Depending on the evaporation is mainly on the following factors:

• Air temperature
• Humidity
• Sunlight (season)
• Wind speed or wind direction also requires
• Surface characteristics (soil type, etc.) and vegetation
• Water content of the soil and rainfall

Due to the various parameters on which the evaporation is dependent whose determination is very difficult and expensive. Usually the evaporation is therefore not measured but only estimated with the aid of mathematical models to an approximation. The resultant evaporation per unit of time, so to speak, the evaporation rate is referred to as an evaporation rate.

One distinguishes the potential evaporation, which is the basis of the meteorological conditions in principle possible evaporation rate from the actual evaporation, which, involving the real existing water content, for example, of the soil. The potential evaporation is always greater than or equal to the actual evaporation. During times of drought, so especially in arid climates, the two values ​​can differ greatly.

Evaporation of other substances

Evaporate Volatile solvents such as acetone, diethyl ether also easy ( and their vapors will accumulate in the room air ). The principle of evaporative solvents is also applied to paints, the drying characterized by a coating ( and often additional chemical cross-link to a paint film).

Calculation and measurement

The evaporation can be measured only with great effort, mostly by Evaporimeter or lysimeter. Is measured is known as the Grass reference evaporation, which is used as the metrological synonym due to the rather theoretical definition of the potential evaporation. A large number of different approximation formulas, which adapted to various factors used to calculate the evaporation Much more common, however, are able to serve. Their errors are directed primarily by the respectively available data, which in terms of factors such as use, vegetation cover, root depth and hydrological soil properties is particularly problematic. However, approximation formulas based on standard meteorological measurements only reach a very limited accuracy in general.

Examples of the technical use of the evaporation

The open evaporation is due to the use of environmental and solar energy a rather more energy-saving process. That is why it is also used on a large scale, where the product stability allows. In the lithium extraction is the brine prior to transport in the Salar de Atacama, Chile, or in Silver Peak, USA, partially concentrated in huge solar ponds through evaporation by a factor of 40. This may be up to two years, the cycle time by more than 10 solar ponds. Another example is the extraction of sea salt from sea water. In Dampier, Australia, this salt marshes are operated on an area of ​​more than 9000 ha. Although the evaporation in solar ponds in Germany can not be used because of the high rainfall and relatively low sunlight. But here, the evaporation is used for salt production in Gradierwerken.

Another technical application of evaporation is the solar drying of sewage sludge. The resulting dewatered and in wastewater treatment sludge is applied to a large area in drying halls with transparent building envelope ( film, polycarbonate or glass roofing ). Irradiating the sun heats the stored sludge, whereby the steam pressure in the sludge in relation to the air above it is raised, and evaporates water from the sludge. The moist air is discharged via a computer-controlled ventilation of the drying room. Thus, from the waste sludge renewable alternative fuel with a calorific value of 8-11 MJ / kg DM (equivalent to approximately 2-3 kWh / kg TS; Conversion: 1 MJ = 0.2778 kWh) produced the fossil in coal-fired power stations and cement works fuels replaced. The largest solar sludge drying plant with 7,200 sqm drying area is currently being operated in Nicaragua after the WendeWolf process.

The necessary energy required for the evaporation process made ​​people already in antiquity advantage to cool drinks and other foods. These were contained in porous clay pots and by the evaporation of a small portion of the liquid through the vessel wall through the remaining larger rest could be kept relatively cool. Even with felt or leather covered canteens use this cooling effect, for which they must be moistened. Ask a widespread practical use of the principle of evaporation humidifier dar. Other applications include the measurement of humidity using a psychrometer.