Evaporation

In physics, evaporation is the change of state from liquid to aeriform (gas or vapor) which involves only the surface of the liquid. At boiling temperature, on the other hand, the boiling process occurs involving the entire volume of the liquid. Both processes represent the change of state from liquid to aeriform are collectively identified under the name of vaporization. The phenomenon of evaporation follows Dalton’s law of partial pressures.

In the process of evaporation, which is always endothermic, the enthalpy of the liquid subject to evaporation decreases, by subtraction of kinetic energy, of a specific amount called latent heat. In pure liquids (i.e. not in solution) this energy transfer occurs isothermally, i.e. without temperature variation.

Unlike the phenomenon of boiling, which occurs at characteristic temperatures for each substance (at fixed pressure) and involves the entire mass of liquid, evaporation occurs at any temperature, and involves only the free surface of a liquid. In these conditions a molecule evaporates from liquid if it acquires sufficient kinetic energy to move away from the free surface of the liquid, which happens in a completely random way or according to the laws of statistics. Evaporation differs from boiling because it takes place only at the surface of the liquid, while boiling takes place simultaneously in the whole mass of the liquid in which it occurs; moreover, for a given value of external pressure, boiling takes place only at a well determined temperature, while evaporation takes place at any temperature.

Evaporation is due to the tendency of the surface molecules of the liquid to escape the force of mutual attraction due to the kinetic energy they possess. The molecules that have an excess of kinetic energy gradually leave the liquid, while gradually the average kinetic energy of the remaining molecules decreases, or, equivalently, the temperature of the liquid decreases.

Therefore, to keep constant the temperature of the evaporating liquid, it would be necessary to give heat to the liquid: this is due, for example, to the feeling of coldness that is felt when a liquid evaporates from a wet hand: the liquid, evaporating, subtracts heat to the hand. The amount of energy required to evaporate the unit mass of liquid is called latent heat of evaporation. This amount of heat varies as the evaporation temperature varies, decreasing as it increases, and, for the same temperature, it is different for different liquids. Liquids having low latent heat of evaporation are called volatile, in reference to the ease with which they evaporate.

Generally evaporation does not stop until all of the liquid has evaporated, but this only happens if every molecule of vapor is removed from the vicinity of the liquid’s surface, for example by ventilation. The speed of evaporation of a liquid is in fact related not only to the area of the surface on which evaporation takes place and to the physical characteristics of the liquid, but also to the ventilation of the surface, that is to the possibility that the surface can be continuously freed from evaporated molecules.

During evaporation in a closed environment, for example in a vacuum, the evaporated molecules thicken near the surface; the pressure of this vapor increases and, keeping constant the temperature, we reach a point where the number of molecules that pass to the vapor in the unit of time is equal to the number of those that return to the liquid. A state of equilibrium is reached between evaporation rate and condensation rate with a corresponding apparent stop of evaporation.

The pressure of the vapor at which this occurs is called saturated vapor pressure, or tension, and the vapor, under these conditions, is called saturated. Saturated vapor pressure also depends naturally on temperature and increases as temperature increases.

Natural evaporation

In nature, evaporation is fundamental to the water cycle: rivers, lakes, seas and oceans release part of their water which reaches the earth’s atmosphere in the form of steam, where it is a function of air temperature, relative humidity and wind speed. The vapor by condensation is subsequently transformed into rain, snow or hail. The external source of energy is the radiant energy of the Sun.

Evaporation is a direct function of temperature and inverse function of the vapor pressure (relative humidity) of the environment: as the temperature increases, the evaporating flux increases and at the saturation of the environment, the equilibrium is reached, i.e. for every molecule that evaporates in a given time interval there is on average another one that returns to the liquid phase in the same time interval.

Also the air circulation, that is the wind, can favor evaporation. In saltworks the phenomenon of natural evaporation is used for the production of salt.

Industrial evaporation

In the technical field and industrial processes, evaporation means the operation of removing, in the form of vapor, a part of the solvent of a solution, to increase the concentration of this or to obtain the solute in the crystalline state. Rarely the procedure is also applied to liquid solutes, but in any case it is required that the solute has a vapor pressure practically negligible compared to that of the solvent.

The solution is brought to boiling, by administration of thermal energy, mostly through an exchange surface. In the case of temperature-sensitive solutes, evaporation is conducted at reduced pressure (vacuum evaporation) in order to lower the boiling temperature of the solution.

Evaporation in plant engineering is the name for two unitary operations in which:

  1. it is removed, through the administration of thermal energy, a non-volatile component of a solution or suspension (which is brought beyond its boiling point), thus obtaining a solution or suspension more concentrated in its other components;
  2. heat is exchanged in order to obtain the vaporization of a liquid. An example of this is the evaporator of a refrigeration cycle.

In the first case the objective of the operation is separation, while in the second case it is heat exchange. In both cases the equipment used to carry out the unitary process is called an evaporator.

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