Drying

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Technically, drying is a mass transfer process resulting in the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid (hereafter product) to end in a solid state. To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In the most common case, a gas stream, e.g., air, applies the heat by convection and carries away the vapor as humidity. Other possibilities are vacuum drying, where the source of heat may be by conduction or radiation (or microwaves) and the vapor is removed by the vacuum system. Another possibility is drum drying, where a heated surface is used in connection with aspirators to draw the vapor outside the site. Freeze drying or lyophilization is a drying method where the solvent is frozen prior to drying and is then sublimed, i.e., passed to the gas phase directly from the solid phase, below the melting point of the solvent. Freeze drying is often carried out under high vacuum to allow drying to proceed at some reasonable rate. This process avoids collapse of the solid structure, leading to a low density, highly porous product, able to regain the solvent quickly. In biological materials or foods, freeze drying is regarded as one of the best if not the best method to retain the initial properties. It was first used industrially to produce dehydrated vaccines, and to dehydrate blood to bring it to assist war casualties. Now freeze drying is increasingly used to dry some foods, because the method may keep protein quality intact, the same as the activity of vitamins and bioactive compounds.

In this regard, the mechanical extraction of the solvent, e.g., water, by centrifugation, is not considered "drying". The ubiquitous term dehydration may mean drying of water-containing products as foods, but its meaning is more vague, as it is also applied for water removal by osmotic drive (salt or sugar solution), or else in medicine as the condition whereby a person loses water by respiration, sweating and evaporation and does not incorporate, for whatever reason, the "make-up" water required to keep the normal physiological behavior of the body.

Drying may be either a natural or an intentional process.

The process of extreme drying is called desiccation.

There is very extensive technical literature on this subject, including several major textbooks and a dedicated scientific journal (Drying Technology [1]).

[edit] Methods of drying

Application of dry heated air (convective or direct drying). This causes evaporation of the surface water, which is replaced by water from inside the solid until the whole has dried out.
Indirect or contact drying (heating through a hot wall), as drum drying, vacuum drying.
Dielectric drying (radiofrequency or microwaves being absorbed inside the material) It is the focus of intense research nowadays. It may be used to assist air drying or vacuum drying.
Freeze drying Is increasingly applied to dry foods, beyond its already classical pharmaceutical or medical applications
Supercritical drying, e.g., superheated steam drying Steam dry products with water. Strange as it seems, this is possible because the water in the product being dried is boiled off, and joined with the drying medium, increasing the flow. It is usually employed in closed circuit and allows a proportion of latent heat to be recovered, which is impossible with conventional air drying, for instance.

[edit] Applications of drying

Devices commonly called dryers are used for efficient drying of various things: hair after a shower, candies at candy factories, semiconductor wafers
Most processes giving a solid product involve a drying step
Drying is often used to preserve food
The production of anhydrous alcohol requires azeotropic distillation, or a membrane process. The 96° mixture of ethanol-water cannot be separated by distillation, as it constitutes an azeotrope ("boiling without variation", from the Greek)
Wood drying is an integral part of timber processing