2. THEORICAL FOUNDATIONS

2.1 Drying concepts

To understand the theorical foundations of drying there are some definitions of basic concepts:

- Humidity: The moisture content of a solid can be expressed on dry or wet basis, but the drying of solids is more appropriate to refer the moisture on dry basis (mass of water that accompanies the dry solid).

- Moisture equilibrium (X*): When a wet solid is in contact with air temperature and humidity and certain constants, the conditions of equilibrium achieved between the air and the wet solid. The equilibrium conditions are reached when the partial pressure of water that accompanies the wet solid is equal to the vapor pressure of water in the air.

- Wet solid and hygroscopic solid: For a certain temperature, the vapor pressure of water contained in the wet solid increases with the humidity for all substances, reaching the value of the vapour pressure of pure water the temperature considered. Hygroscopic solid is called when the vapor pressure of water that accompanies the solid is less than the vapor pressure of water at the same temperature. Is called wet solid if the water vapor pressure of solid is equal to the vapor pressure.

- Free Moisture: Is the difference between the moisture of a solid and moisture equilibrium with the air in the given conditions: F = X - X *. It is the moisture that can be lost after some time of contact with air given and constant conditions.

- Humidity tied or bound water: The value of the equilibrium moisture of the solid in contact with saturated air; or moisture minimum required for this solid to finish behaving as hygroscopic.

- Moisture disconnected or disconnected water: Is the difference between the moisture and humidity associated solid, or moisture free air saturated with solid contact. If the solid has moisture disconnected behaves as wet.

- Critical Moisture: The humidity is critical to a solid point that separates the two periods of drying critical period ahead and critical period post.

- Critical period ahead: Is the time period in which the drying rate is constant, from the initial moisture to the critical moisture.

- Critical period post: Is the time period in which the drying rate decreases until it reaches a value of zero. This period begins with the critical humidity until moisture equilibrium.