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Drying

[drahy-ing]

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 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.

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

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, so internal pores 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. 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 (superheated steam drying) involves steam drying of products containing 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.
Natural air drying takes place when materials are dried with unheated forced air, taking advantage of its natural drying potential. The process is slow and weather-dependent, so a wise strategy "fan off-fan on" must be devised considering the following conditions: Air temperature, relative humidity and moisture content and temperature of the material being dried. Grains are increasingly dried with this technique, and the total time (including fan off and on periods) may last from one week to various months, if a winter rest can be tolerated in cold areas.

Applications of drying

Grain Drying

Hundreds of millions of tonnes of wheat,corn, soybean, rice other grains as sorghum, sunflower seeds, rapeseed/canola, barley, oats, etc., are dried in grain dryers. 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 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% 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 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. 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 represent the physics and physical chemistry of drying. Drum Drying

The drum dryer technology has kept its position of importance. Today, in foods, potato puree is dehydrated as well as banana and tomato purees to produce dehydrated flakes

Spray drying

Spray drying is an important technique to produce dried powders. The principle is that a pumpable feed is first atomized, i.e, converted in a fog of droplets of about 100 micrometers in diameter, which 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, followed by instant coffee. Tomato powder is becoming very important. On the other hand, washing powder is an example product of the chemical process industry. The production of dehydrated natural flavors and essences is very important and is growing together with encapsulation, a technique devised to trap a volatile, but large molecule (as the flavor compound) inside a dry particle, the walls of which develop on drying and are more permeable to the water flux than to the flux of the larger volatiles. This principle of selective diffusion was first developed by the Dutch researcher Thijssen, in Eindhoven, during the 1970's, and is improved by adding maltodextrins (a product from controlled starch hydrolysis) to the dryer feed. Spray dryers differ in the type of atomizer, the relative directions of air and product flows, the chamber design, type of drying agent (air, nitrogen) in the system charactersitics (closed or open circuit), among other features. Equipment can be very large, of up to 20 m tall.

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