Drying is the process of using heat to remove a liquid (such as water) from a material that contains the liquid. The material can be a wet solid or a liquid solution of a solid dissolved in a liquid. The drying process requires a source of heat and a sink that receives the vapor produced. The term drying is also applied to the removal of water vapor from a gas or mixture of gases. In scientific terms, drying is described as a "mass transfer" process. The process that leads to extreme drying is called desiccation.
The process of drying can be undertaken by various methods for a variety of applications. Examples range from drying hair after a shower to drying candy at a candy factory to drying semiconductor wafers. Wood drying is an integral part of timber processing, and food drying is often used to preserve food. The process known as freeze drying is used for the dehydration and preservation of pharmaceuticals, vaccines, blood, and some food products. Extensive technical literature is available on the subject of drying.
The extraction of liquid from a material can also be undertaken by methods such as centrifugation, decantation, and filtration. These methods, however, do not involve the use of heat and are not considered "drying."
The term "dehydration" may mean drying of water-containing products such as foods, but it is also used in other contexts. It is, for instance, applied to water removal by osmotic drive from a salt or sugar solution. In medicine, dehydration refers to a situation in which a person loses water by respiration, sweating, and evaporation, without recovering the "make-up" water required to allow the body to keep functioning normally.
Drying can be accomplished by various techniques, some of which are noted below.
Hundreds of millions of metric tons of grains and seeds—including wheat, corn, soybean, rice, barley, oats, sorghum, sunflower seeds, rapeseed/canola—are dried in grain dryers worldwide each year. In the main agricultural countries, drying involves the reduction of moisture from about 17-30 percent (by weight) to values between eight and 15 percent, depending on 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 percent (by weight); soybeans to 12.5 percent; sunflower seeds to eight to nine percent; and peanuts to nine percent.
Grain drying is carried out as a prerequisite for safe storage, to inhibit microbial growth. In addition, low storage temperatures are highly recommended to avoid degradative reactions and 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, and cross-flow dryers in the USA. In Argentina, both types are used. Continuous flow dryers may produce up to 100 metric metric tons of dried grain per hour. The depth 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 dryers.
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 cylinder in the center of the bin. Air passes through a path of grain some 0.50 m deep in the radial direction. The usual drying times range from one to four hours, depending on how much water must be removed, the air temperature, and 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. It is now possible to "simulate" the performance of a dryer with computer programs based on equations that represent the physics and physical chemistry of drying.
Spray drying is an important technique to produce dried powders. In this method, a pumpable feed is first atomized—that is, converted in a fog of droplets (each about 100 micrometers in diameter). The droplets dry very fast while falling by gravity, accompanied by heated air. The dried particles eventually exit through the bottom of the dryer and are separated from the drying air by a cyclone, or a system based on cyclones, plus bag filters or electrostatic precipitators.
Milk powder is possibly the most popular product, and tomato powder is becoming very important. On the other hand, washing powder is an example of the chemical process industry. The production of dehydrated natural flavors and essences is very important and is growing. Another technique is encapsulation, devised to trap a large, volatile molecule (such as the flavor compound) inside a dry particle, the walls of which develop on drying and are permeable to water flux but not to the flux of the larger volatiles. This principle of selective diffusion was first developed by the Dutch researcher Thijssen in Eindhoven, during the 1970s. Spray dryers differ in the type of atomizer, the relative directions of air and product flows, chamber design, type of drying agent (air, nitrogen) in the system characteristics (closed or open circuit), among other features. Equipment can be very large, up to 20 m tall.
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