Drying is aㅜㅜ 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. Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Evaporation is the process by which Molecules in a Liquid state (e A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature
In the most common case, a gas stream, e. g. , air, applies the heat by convection and carries away the vapor as humidity. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative 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. Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. 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". Centrifugation is a process that involves the use of the Centrifugal force for the separation of mixtures used in industry and in laboratory settings 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. Dehydration ( hypohydration) is the removal of Water ( hydro in ancient Greek) from an object Osmosis is the Diffusion of a solvent (frequently water through a semi-permeable membrane, from a solution of low solute concentration (high water potential 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. In living organisms a respiratory system functions to allow Gas exchange. Evaporation is the process by which Molecules in a Liquid state (e Physiology (from Greek grc φύσις physis, "nature origin" and grc -λογία -logia) is the study of the mechanical physical
Drying may be either a natural or an intentional process.
The process of extreme drying is called desiccation. Desiccation is the state of extreme dryness or the process of extreme drying
There is very extensive technical literature on this subject, including several major textbooks and a dedicated scientific journal (Drying Technology ).
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.
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 is an important technique to produce dried powders. Spray drying is a commonly used method of drying a liquid feed through a hot gas 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 is 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 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 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 1970's. 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.