Evaporation

In physics, evaporation is the change of state from liquid to aeriform (gas or vapor) involving only the surface of the liquid. At boiling temperature, on the other hand, the boiling process takes place in the entire volume of the liquid. Both processes representing the change of state from liquid to aeriform are collectively referred to as evaporation.

Evaporation differs from boiling in that it takes place only at the surface of the liquid, whereas boiling takes place simultaneously throughout the entire mass of the liquid in which it occurs; moreover, for a given value of external pressure, boiling takes place only at a well-defined temperature, whereas 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 with excess kinetic energy gradually leave the liquid, while the average kinetic energy of the remaining molecules gradually decreases, i.e. the temperature of the liquid decreases. Therefore, in order to keep the temperature of the evaporating liquid constant, it would be necessary to give heat to the liquid: this is due, for example, to the feeling of coldness one gets when a liquid evaporates from a wet hand: the liquid, by evaporating, takes heat from the hand.

The amount of energy required to evaporate a unit mass of liquid is called the latent heat of evaporation. This amount of heat varies as the evaporation temperature changes, decreases as it increases, and is different for different liquids at the same temperature. Liquids that have a low latent heat of evaporation are called volatile, referring to the ease with which they evaporate.

In general, evaporation will not stop until all of the liquid has evaporated, but this will only happen if every vapor molecule is removed from the vicinity of the liquid’s surface, for example, by ventilation. In fact, the rate of evaporation of a liquid depends not only on the area of the surface over which evaporation occurs and the physical properties of the liquid, but also on the aeration of the surface, i.e., whether the surface can be kept continuously free of evaporated molecules.

Evaporation in a closed environment, such as a vacuum, causes the evaporated molecules to condense near the surface; the pressure of this vapor increases and, if the temperature remains constant, a point is reached where the number of molecules entering the vapor per unit of time is equal to the number of molecules returning to the liquid. That is, a state of equilibrium is reached between the rate of evaporation and the rate of condensation, with a corresponding apparent cessation of evaporation. The vapor pressure at which this occurs is called the saturated vapor pressure or tension, and the vapor under these conditions is called saturated. Saturated vapor pressure is also naturally temperature-dependent and increases with temperature.

Thermodynamic description

In the process of evaporation, which is always endothermic, the enthalpy of the evaporating liquid is reduced by a certain amount, called the latent heat, by subtracting the kinetic energy. In pure liquids (i.e. not in solution), this energy transfer is isothermal, i.e. without temperature change.

Unlike the phenomenon of boiling, which occurs at temperatures characteristic of each substance (at a fixed pressure) and involves the entire mass of the liquid, evaporation occurs at any temperature and involves only the free surface of a liquid. Under these conditions, a molecule evaporates from the liquid when it gains sufficient kinetic energy to move away from the free liquid film, which happens completely randomly, that is, according to the laws of statistics.

The phenomenon of evaporation follows Dalton’s law of partial pressures.

Natural evaporation

In nature, evaporation is fundamental to the water cycle: rivers, lakes, seas, and oceans release some of their water, which enters the Earth’s atmosphere as water vapor, depending on air temperature, relative humidity, and wind speed. The vapor is later transformed by condensation into rain, snow, or hail. The external source of energy is radiant energy from the sun. Evaporation is a direct function of temperature and an inverse function of the vapor pressure (relative humidity) of the environment: as the temperature increases, the evaporative flux increases, and when the environment is saturated, equilibrium is reached, i.e. for every molecule that evaporates in a given time interval, there is on average one molecule that returns to the liquid phase in the same time interval.

Air circulation, i.e. wind, can also promote evaporation. In salt works, the phenomenon of natural evaporation is used to produce salt.

Industrial evaporation

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

The solution is brought to boiling by the application of thermal energy, usually through an exchange surface. In the case of temperature-sensitive solutes, evaporation is carried out under reduced pressure (vacuum evaporation) in order to lower the boiling temperature of the solution.

In plant engineering, evaporation is the name given to two unified operations in which:

  1. a non-volatile component of a solution or suspension is removed (brought above its boiling point) by the application of thermal energy, thus obtaining a solution or suspension more concentrated in its other components;
  2. exchanging heat to obtain evaporation of a liquid. An example 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 unit process is called an evaporator.

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