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the removal of a liquid, usually water, from solid, liquid, and gaseous substances. As a rule, drying removes moisture that is bound to the material physicochemically (by adsorption or osmosis) and mechanically (moisture in macrocapillaries and mi-crocapillaries). Moisture that is bound chemically cannot be removed by drying. The purpose of drying is to retain the physico-chemical properties of materials, to ensure, in many cases, the preservation of materials over prolonged periods, and to eliminate excess weight in shipping. In industry, the drying of moist solid materials is usually carried out during the preparation of the materials for processing, use, or storage.
Drying is a process accompanied by heat and mass exchange between the drying agent, for example, air or flue gases, and the moisture of the material being dried. The vapor pressure of the liquid on the surface of a solid material increases with temperature, and the vapor diffuses into the flow of the drying agent. The concentration gradient in the material’s moisture resulting from this process forces the moisture to move from the deeper layers to the surface at a rate dependent on the character of the bond between the moisture and the material. In natural drying, where there is no forced movement of the drying agent (free evaporation), the process proceeds slowly; it is speeded up when a heated stream of drying agent flows past the material, that is, when artificial drying is employed. This article will deal only with artificial drying and the various types of industrial dryers.
The selection of drying conditions, such as temperature, pressure, and speed of the drying agent, depends on the physico-chemical properties of the material being dried. Among the properties to be considered are tendencies to contract (wood), to form thick crusts on the surface (certain salts), and to undergo an increase in brittleness or thermostability (paper).
Depending on the method of heat supply, dryers are classified as convection (direct contact between the material being dried and the stream of preheated drying agent), contact (contact between the material being dried and a heated surface), freeze (removal of moisture in the frozen state under a vacuum), dielectric-heat (removal of moisture through the action of high-frequency electric fields), and radiant-energy (drying from infrared radiation).
Convection dryers of various designs (compartment, rotary, pneumatic, fluidized-bed, spray) are widely used in industry. In the basic type of convection dryer (Figure 1, a), the drying agent, which is first heated in a heater to the maximum allowable temperature, passes through the dryer and comes into direct contact with the material being dried (food products, medicines, chemical compounds). The drying agent is heated and passed through the dryer only once, which is the distinctive feature of this type of dryer.
In drying materials that are not thermostable, for example, polyethylene, the drying agent is only partially heated in the main heater and is then fed into the drying chamber at a temperature permissible for the material being dried. The agent obtains the balance of the heat necessary for drying through additional heaters mounted in the drying chamber.
Dryers in which part of the heated air is recirculated (Figure 1, b) are often used in drying such materials as wood and formed ceramic products. Recirculating the air lessens the difference in the temperature and moisture content between the air at the dryer’s inlet and outlet and ensures more uniform drying. Dryers in which inert gases or air circulates in a closed path are used to dry flammable and explosive materials or to extract valuable products (alcohols, ethers) from the material being dried. The dryer’s design will depend on the task at hand.
Rotary dryers—used for drying finely divided and bulk materials (nitrogen fertilizers, iron pyrites, potassium chloride, grain)—consist of a cylinder having internal flights for showering and mixing the material in order to improve contact with the drying agent (Figure 2). The cylinder is mounted either horizontally, with protruding rings resting on support rollers, or at a slight inclination
(0.5°–3°). The diameter of the cylinder may be 3,500 mm, and the length is 3.5–7 times the diameter. The cylinder rotates slowly (0.5–8 rpm).
Pneumatic dryers—for drying granular materials (coal, adipic acid) with a stream of hot drying agent—consist of a one-piece or sectional vertical conveying duct (Figure 3). The material being dried is moved through the duct by a stream of drying agent, whose speed exceeds the free-fall velocity of the largest granules (usually 10–40 m/sec). The brevity of the contact (1–5 sec) makes this dryer suitable for materials that are not thermostable, even when the drying agent is at a high temperature.
In fluidized-bed dryers, because of the possibility of an intensive intermingling of the material and accelerated heat and mass exchange, the drying agent can be used at elevated temperatures. Combining simplicity of design with high productivity and ease of automation, these dryers have found a variety of uses in the chemical industry and in nonferrous metallurgy.
Spray dryers are used to dry liquid substances of elevated viscosity (milk, blood, albumin), which are sprayed into a stream of hot drying agent (Figure 4). Because of the large surface area of
the sprayed material, the process of moisture evaporation is intense, and drying time is short (15–30 sec). With extremely rapid drying, the surface temperature of the particles approximates the adiabatic evaporation temperature of the pure liquid, even when the drying agent is at a high temperature. The material being dried, which is in the form of emulsions, suspensions, or solutions, is sprayed by mechanical or pneumatic atomizers. The dryers are fitted with units for trapping entrained particles of the material being dried.
Continuous tray dryers are used for bulk and fibrous materials (artificial fibers, certain polymers). Here, the material being dried moves over an endless belt (or over several consecutively arranged belts) stretched between a driving drum and a driven drum (Figure 5). Drying is accomplished by hot air or flue gases that move either parallel or perpendicular to the belt.
Contact dryers, such as drum dryers, are used to dry liquid materials and pastes (xanthates of alkali metals) at atmospheric pressure or under a vacuum. Drum dryers include single-drum and double-drum types, the principal component of which is a slowly rotating drum (2–10 rpm) into which steam is introduced through a hollow journal and from which the condensate is removed. The material being dried is applied as a thin film (1–2 mm) to the surface of the drum and, after drying, is removed with a knife. Single-drum and double-drum vacuum dryers are shown in Figure 6.
Freeze dryers are used for drying food products and medicinal preparations (antibiotics, blood plasma) while preserving the main biological properties of the material. Here, moisture is removed in the frozen state under a vacuum (residual pressure of 6.65–332.5 newtons/m2, or 0.05–2.5 mm Hg) at a temperature of approximately 0°C. Most of the moisture (60–85 percent) is evaporated in the chamber, with the remaining amount removed by
vacuum drying involving the application of heat (at a temperature of 30°–45°C). The heat needed for drying is supplied to the material from hot surfaces or by radiation from heated screens. Since with freeze drying there is no oxidation from atmospheric oxygen and no change in the dimensions of the product, it is possible to obtain high-quality products that approach fresh products in organoleptic indexes and content of vitamins and odoriferous and other substances.
Dielectric-heat dryers are used principally to dry materials that have high resistance to the internal movement of moisture (pencils, thin casting molds). High-frequency currents produced by special generators are used to heat the material being dried throughout the material’s entire thickness, thereby accelerating the drying process. It is possible to regulate temperature and moisture content throughout the entire volume of the material. Under the action of high-frequency electric fields, ions and electrons in the material change their direction of motion synchronously with changes in sign of the charge at the capacitor plates, dipole molecules acquire a rotatory motion, and nonpolar molecules are polarized by virtue of the displacement of their charge. These processes, which are accompanied by internal friction, lead to the evolution of heat and to a warming of the material being dried. This type of drying can be used for plastics, rubber products, and other materials possessing dielectric properties.
The drying of solid materials is a common process in the chemical, food-processing, paper, woodworking, building-materials, leather, and textile industries. In foundry work, drying is used for strengthening and imparting required physicomechanical properties to molds and cores, as well as for eliminating excess moisture from the paints and polishes applied to the surface of the molds and cores. The drying of liquids is carried out with desiccants, such as phosphoric anhydride, concentrated sulfuric acid, and anhydrous calcium chloride, that do not react with the water-binding liquids being dried.
The drying of gases (air, flue gases) is carried out chiefly by the methods of absorption and adsorption. The absorption method is based on the absorption (dissolution) of moisture from gases using liquid solvents (absorbents) that do not react chemically with the gas being dried. Common absorbents include solutions of diethylene glycol, triethylene glycol, glycerol, calcium chloride, and caustic alkalies, although the use of calcium chloride is limited because of the corrosive effect on equipment. Systems for drying gases through absorption include absorbers, desorbers, various heat-exchange units, and pumps for displacing the solutions.
Adsorption methods are based on the adsorption of moisture from gases by solid substances with high porosity known as adsorbents, among them bauxites, activated alumina, silica gel, and zeolites (molecular sieves). These adsorbents are readily regenerated, and they adsorb 3–12 percent of the moisture (by weight). Adsorption units for drying gases include sorbent-filled adsorbers and heat-exchange equipment (heaters, coolers). The desorption of moisture (regeneration) is carried out by blowing a stream of hot gas or superheated steam through a layer of saturated adsorbent.
Other methods of drying gases are based on the condensation or freeze-out of moisture with decreasing temperature. These methods are carried out in alternately operating heat exchangers, where the gas is cooled with water or a low-temperature coolant; in the latter case, the moisture contained in the gas precipitates in the form of snow or frost. Increasing the pressure has a favorable effect on the drying of gases by the cooling method.
Gases are sometimes dried by bringing them into contact with solid hygroscopic substances, in particular, caustic potash or caustic soda. Here, the gases to be dried are passed through units filled with the absorbent. The drying of gases often precedes the gases’ fractionation by methods of rectification or partial condensation, as well as the transport of fuel gases through pipelines.
REFERENCESLykov, M. V. Sushka v khimicheskoi promyshlennosti. Moscow, 1970.
Krischer, O. Nauchnye osnovy tekhniki sushki. Moscow, 1961. (Translated from German.)
Lykov, A. V. Teoriia sushki, 2nd ed. Moscow, 1968.
Romankov, P. G., and N. B. Rashkovskaia. Sushka vo vzveshennom sostoianii, 2nd ed. Leningrad, 1968.
Kasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 9th ed. Moscow, 1973.
Gersh, S. la. Glubokoe okhlazhdenie, 3rd ed., parts 1–2. Moscow-Leningrad, 1957–60.
Guigo, E. I., N. K. Zhuravskaia, and E. I. Kaukhcheshvili. Sublimatsionnaia sushka v pishchevoi promyshlennosli, 2nd ed. Moscow, 1972.
V. L. PEBALK
An operation in which a liquid, usually water, is removed from a wet solid in equipment termed dryers. The use of heat to remove liquids distinguishes drying from mechanical dewatering methods such as centrifugation, decantation or sedimentation, and filtration, in which no change in phase from liquid to vapor is experienced. Drying is preferred to the term dehydration, which usually implies removal of water accompanied by a chemical change. Drying is a widespread operation in the chemical process industries. It is used for chemicals of all types, pharmaceuticals, biological materials, foods, detergents, wood, minerals, and industrial wastes. Drying processes may evaporate liquids at rates varying from only a few ounces per hour to 10 tons per hour in a single dryer. Drying temperatures may be as high as 1400°F (760°C), or as low as -40°F (-40°C) in freeze drying. Dryers range in size from small cabinets to spray dryers with steel towers 100 ft (30 m) high and 30 ft (9 m) in diameter. The materials dried may be in the form of thin solutions, suspensions, slurries, pastes, granular materials, bulk objects, fibers, or sheets. Drying may be accomplished by convective heat transfer, by conduction from heated surfaces, by radiation, and by dielectric heating. In general, the removal of moisture from liquids (that is, the drying of liquids) and the drying of gases are classified as distillation processes and adsorption processes, respectively, and they are performed in special equipment usually termed distillation columns (for liquids) and adsorbers (for gases and liquids). Gases also may be dried by compression.
Drying of solids
In the drying of solids, the desirable end product is in solid form. Thus, even though the solid is initially in solution, the problem of producing this solid in dry form is classed under this heading. Final moisture contents of dry solids are usually less than 10%, and in many instances, less than 1%.
The mechanism of the drying of solids is reasonably simple in concept. When drying is done with heated gases, in the most general case, a wet solid begins to dry as though the water were present alone without any solid, and hence evaporation proceeds as it would from a so-called free water surface, that is, as water standing in an open pan. The period or stage of drying during this initial phase, therefore, is commonly referred to as the constant-rate period because evaporation occurs at a constant rate and is independent of the solid present. The presence of any dissolved salts will cause the evaporation rate to be less than that of pure water. Nevertheless, this lower rate can still be constant during the first stages of drying.
A fundamental theory of drying depends on a knowledge of the forces governing the flow of liquids inside solids. Attempts have been made to develop a general theory of drying on the basis that liquids move inside solids by a diffusional process. However, this is not true in all cases. In fact, only in a limited number of types of solids does true diffusion of liquids occur. In most cases, the internal flow mechanism results from a combination of forces which may include capillarity, internal pressure gradients caused by shrinkage, a vapor-liquid flow sequence caused by temperature gradients, diffusion, and osmosis. Because of the complexities of the internal flow mechanism, it has not been possible to evolve a generalized theory of drying applicable to all materials. Only in the drying of certain bulk objects such as wood, ceramics, and soap has a significant understanding of the internal mechanism been gained which permits control of product quality.
Most investigations of drying have been made from the so-called external viewpoint, wherein the effects of the external drying medium such as air velocity, humidity, temperature, and wet material shape and subdivision are studied with respect to their influence on the drying rate. The results of such investigations are usually presented as drying rate curves, and the natures of these curves are used to interpret the drying mechanism.
When materials are dried in contact with hot surfaces, termed indirect drying, the air humidity and air velocity may no longer be significant factors controlling the rate. The “goodness” of the contact between the wet material and the heated surfaces, plus the surface temperature, will be controlling. This may involve agitation of the wet material in some cases.
Drying equipment for solids may be conveniently grouped into three classes on the basis of the method of transferring heat for evaporation. The first class is termed direct dryers; the second class, indirect dryers; and the third class, radiant heat dryers. Batch dryers are restricted to low capacities and long drying times. Most industrial drying operations are performed in continuous dryers. The large numbers of different types of dryers reflect the efforts to handle the larger numbers of wet materials in ways which result in the most efficient contacting with the drying medium. Thus, filter cakes, pastes, and similar materials, when preformed in small pieces, can be dried many times faster in continuous through-circulation dryers than in batch tray dryers. Similarly, materials which are sprayed to form small drops, as in spray drying, dry much faster than in through-circulation drying.
Drying of gases
The removal of 95–100% of the water vapor in air or other gases is frequently necessary. Gases having a dew point of -40°F (-40°C) are considered commercially dry. The more important reasons for the removal of water vapor from air are (1) comfort, as in air conditioning; (2) control of the humidity of manufacturing atmospheres; (3) protection of electrical equipment against corrosion, short circuits, and electrostatic discharges; (4) requirement of dry air for use in chemical processes where moisture present in air adversely affects the economy of the process; (5) prevention of water adsorption in pneumatic conveying; and (6) as a prerequisite to liquefaction.
Gases may be dried by the following processes: (1) absorption by use of spray chambers with such organic liquids as glycerin, or aqueous solutions of salts such as lithium chloride, and by use of packed columns with countercurrent flow of sulfuric acid, phosphoric acid, or organic liquids; (2) adsorption by use of solid adsorbents such as activated alumina, silica gel, or molecular sieves; (3) compression to a partial pressure of water vapor greater than the saturation pressure to effect condensation of liquid water; (4) cooling below dew point of the gas with surface condensers or coldwater sprays; and (5) compression and cooling, in which liquid desiccants are used in continuous processes in spray chambers and packed towers—solid desiccants are generally used in an intermittent operation that requires periodic interruption for regeneration of the spent desiccant.
Desiccants are classified as solid adsorbents, which remove water vapor by the phenomena of surface adsorption and capillary condensation (silica gel and activated alumina); solid absorbents, which remove water vapor by chemical reaction (fused anhydrous calcium sulfate, lime, and magnesium perchlorate); deliquescent absorbents, which remove water vapor by chemical reaction and dissolution (calcium chloride and potassium hydroxide); or liquid absorbents, which remove water vapor by absorption (sulfuric acid, lithium chloride solutions, and ethylene glycol).
The mechanical methods of drying gases, compression and cooling and refrigeration, are used in large-scale operations, and generally are more expensive methods than those using desiccants. Such mechanical methods are used when compression or cooling of the gas is required.
Liquid desiccants (concentrated acids and organic liquids) are generally liquid at all stages of a drying process. Soluble desiccants (calcium chloride and sodium hydroxide) include those solids which are deliquescent in the presence of high concentrations of water vapor.
Deliquescent salts and hydrates are generally used as concentrated solutions because of the practical difficulties in handling, replacing, and regenerating the wet corrosive solids. The degree of drying possible with solutions is much less than with corresponding solids; but, where only moderately low humidities are required and large volumes of air are dried, solutions are satisfactory. See Filtration, Heat transfer, Humidification, Unit operations