Drying

For the food conservation method, see drying (food).

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.

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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 heat is supplied by contact conduction or radiation (or microwaves) while the produced vapor is removed by the vacuum system. Another indirect technique is drum drying, where a heated surface is used to provide the energy and aspirators draw the vapor outside the room.

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 a 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 bring dehydrated blood to assist war casualties. Now freeze drying is increasingly used to preserve some foods, especially for backpackers going to remote areas. The method may keep protein quality intact, the same as the activity of vitamins and bioactive compounds.

In turn, 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 from a salt or sugar solution. In medicine, dehydration is the situation by which 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]).

Methods of drying

• Application of heated air (convective or direct drying). Air heating reduces air relative humidity, which is the driving force for drying. Besides, higher temperatures speed up diffusion of water inside the solids, so drying is faster. However, product quality considerations limit the applicable rise to air temperature. Too hot air almost completely dehydrates the solid surface cause pores to shrink and almost close, leading to crust formation or "case hardening".
• 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. It keeps biological properties of proteins, and retains vitamins and bioactive compounds. Pressure may be reduced by a vacuum pump or steam nozzle. If using a vacuum pump, the vapor produced by sublimation is removed from the system by converting it into ice in a condenser, operating at very low temperatures, outside the freeze drying chamber.
• Supercritical drying (aka superheated steam drying) involves steam drying products with water. Strange as it seems, this is possible because the water in the product is boiled off, and joined with the drying medium, increasing its flow. It is usually employed in closed circuit and allows a proportion of latent heat to be recovered by recompression, a feature which is not possible with conventional air drying, for instance. May have potential for foods if carried out at reduced pressure, to lower the boiling point.

Applications of drying

Cereal Drier

Hundreds of millions of tonnes of wheat,corn, soybean, rice other grains as sorghum, sunflower seeds, rapeseed/canola, barley, oats, etc., are dried in cereal driers. In the main agricultural countries, drying comprises the reduction of moisture from about 17-30%w/w to values between 8 and 15%w/w, depending of the grain. The final moisture content for drying must be adequate for storage. The more oil the grain has, the lower its storage moisture content will be (though its initial moisture for drying will also be lower). Cereals are often dried to 14% w/w, while oilseeds, to 12.5% (soybeans), 8-9% (sunflower) and 9% (peanuts). Drying is carried out as a requisite for safe storage, in order to inhibit microbial growth. However, low temperatures in storage are also highly recommended to avoid degradative reactions and, especially, the growth of insects and mites. A good maximum storage temperature is about 18°C. The largest dryers are normally used "Off-farm", in elevators, and are of the continuous type: Mixed-flow dryers are preferred in Europe, while Cross-flow dryers in the USA. In Argentina, both types are usually found. Continuous flow dryers may produce up to 100 metric tonnes of dried grain per hour. The path of grain the air must traverse in continuous dryers range from some 0.15 m in Mixed flow dryers to some 0.30 m in Cross-Flow. Batch dryers are mainly used "on-farm", particularly in the USA and Europe. They normally consist of a bin, with heated air flowing horizontally from a narrow-diameter cylinder through a perforated metal sheet, placed in the center of the bin. Air passes through a path of grain some 0.50 m deep in radial direction and leaves the system through another perforated sheet. The usual drying times range from 1 h to 4 h depending on how much water must be removed, the air temperature, and the grain depth. In the USA, continuous counterflow dryers may be found on-farm, adapting a bin to slowly drying the grain, and removing the dried product using an auger. Grain drying is an active area of manufacturing and research. Now it is possible to "simulate" the performance of a dryer with computer programs based on equations that represents the physics and physical chemistry 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

See also

• Desiccation