A dryer (or drier) is a machine or apparatus used to remove moisture. It may be a small laboratory oven taking a few grams of moist material or a large industrial unit handling tonnes of wet feed per hour. An industrial dryer is never a stand-alone unit; it is part of a drying system which includes the feeder, the heaters and product collectors besides the actual drying section itself. The Drying of natural products has been accomplished from the beginning of time by the use of wind and sun as agents to drive off moisture. Because of the vagaries of weather, alternative methods have been gradually adopted, such as drying indoors in specially-heated and-ventilated rooms. Today, we still talk of drying chambers, which are rooms shrunk to the size of a cabinet or vessel. The variety of materials to be dried and the corresponding range of dryer designs available defies simple classification. However, there are three principal factors that define the nature of a dryer:
The method of material conveyance through the drying section;
The method of heating the material;
The pressure and temperature of operation.
These factors will be considered in turn.
More than any other factor, the method of conveying the feed through the dryer governs the outward appearance of a dryer and limits its operating parameters. While free-flowing granules can be handled in a variety of ways, more awkward materials like loose fibres, which can tangle together, and very wet sticky feeds require special techniques. The following table illustrates the range of conveying methods adopted industrially.
Figure 1. Some batch-drying methods, (a) Exposed heap; (b) through-circulated heap; (c) cross-circulated layers on trays; (d) fluidized heap; (e) agitated pan. After Keey (1992).
Figure 2. Continuous-drying methods with undisturbed or cascading solids, (a) through-circulated band dryer; (b) vibrated-band dryer; (c) suction-drum dryer; (d) rotary cascading dryer; (e) multitubular rotary cascading dryer; f through-circulation rotary dryer. After Keey (1992).
Most modern dryers are operated continuously, or semicontinuously over the working day, as a continuous dryer will require less labor, fuel and floor space than a batch dryer of the same capacity.
However, batch drying would be chosen whenever the production rate is small (under 200 kg h−1), or a large number of products have to be handled in the same unit, or whenever large bulky items have to be dried under extensive and complex schedules, for which the drying of porcelain sanitary ware and sawn timber boards provide examples.
Dryers may be indirectly- or directly-heated. The heating medium in an indirect dryer is separate from the carrier gas. Steam may be used in conjunction with convective heating coils or supplied to a jacket surrounding the drying chamber. In freeze-drying units, hot oil is circulated around a heated platen which supports the drying material. For many direct dryers, a heated airflow passes straight through the drying chamber and the dried product is discharged or separated from the outlet air stream. The air may be indirectly heated through a heat exchanger. Alternatively, the dryer can be directly-fired or supplied with flue gas from a burner. If such a gas is used as a drying medium and the exhaust from the dryer is partially recycled to the intake, then the atmosphere within the drying chamber can become inertized. This scheme is often adopted should there be an explosion hazard with a dried product that is finely divided. On the other hand, care has to be taken in obtaining a clean flue gas with direct firing to avoid contamination of the product. With particularly sensitive products, it is wiser not to use direct firing. Even without direct firing, many installations for foods and light-colored products will need an air filter to keep out dirt and other specks of foreign matter.
There are various modes of heating the moist material. Convection is perhaps the commonest. In convective heating, the carrier gas for the evaporated moisture is preheated before passing over or through the material, and the drying conditions are readily controlled by the temperature and humidity of the warm gas. The temperature of the solid is always less than the gas temperature and once the solids have warmed up, they may remain close to Wet-bulb Temperature for a significant period of time in the early stages of drying. When bulky porous materials are dried, such as wound textile bobbins and board timber, another quasi-steady temperature appears intermediate between the Wet- and Dry-bulb Temperatures within the material. This temperature is thought to be associated with the movement of an evaporative front into the material [Keey (1978)]. An example of a continuously-worked convective dryer is shown in Figure 3. The material to be dried is placed in shelves of trays which are stacked on trolleys and slowly moved through a drying tunnel. Heaters are placed in the air space above the train of trolleys, and internal fans circulate the air around or over the drying material. Some heat is received by radiation from the heating coils and by gas radiation, as well as by conduction through the contact of the goods with the supporting base. These additional heat transfer mechanisms can significantly boost convection in cases where the solids are supported on trays or bands. Likewise in rotary cascading dryers, heat transfer to the raining curtain of particles is not solely by convection with the crossflowing air stream; gas radiation is important, and some heat can be transferred from the hot cylindrical wall as the particles rest in the lower quadrants of the revolving drum.
If the material to be dried is very thin or wet, then heat may be suppled by way of Conduction. All the heat now passes through the material itself, from hot surfaces supporting or confining it, so the material's temperatures are higher than in convective heating. For this reason, thermally-sensitive materials, if in the form of slurry, would be dried under vacuum to reduce operating temperatures. Figure 4 shows a twin-drum dryer which can accept a lump-free slurry. The rotating drums drag the slurry around their slowly-revolving peripheries while knives peel off the dried product, which falls into a conveying duct. With single-drum units, the wet paste may be splashed onto the surface of the drum, or the drum may dip into a pool of feed material, as is the commoner practice. Thin materials, in the form of sheets, can be drawn over and under a series of internally-heated cylinders to provide discontinuous thermal contact. The drying section of a paper-making machine is designed on this basis. In this instance, besides the conductive heat transfer, there is some adsorption of moisture by pressing felts and additionally, local thermal boosting by radiators is sometimes adopted for moisture-profile adjustment.
Energy may be supplied by various forms of electromagnetic radiation, whose wavelengths varies between those of solar radiation to those of microwaves (0.2 m to 0.2 μm). Longer-length radiation within this waveband barely penetrates beyond the exposed surface of the material, which normally only absorbs significant incident radiation at certain wavelengths. Radio-frequency (RF) and microwave energy, however, can penetrate a material significantly and may be regarded in some cases as providing volumetric heating. The cost of electrical energy compared with other energy sources confines the application of electrical heating methods to modest throughputs of high-value material or to finishing operations achieving a more uniformly-dried product. In general, the energy output of the various electromagnetic emitters is restricted. For example, infrared emitters are limited to about 10 kW per unit, and where a particular power source greater than this is required, a number of individual emitters would be installed. However, the use of a number of emitters gives flexibility in heating arrangements; further flexibility can be achieved by varying individual voltages on the heaters, depending on the local moisture content. Two further attractive features of the use of shorter-wave RF and microwave energy are the volumetric and selective nature of heating. The latter property arises because most materials to be dried are nonmetallic, and are thus good electrical insulators with a low-loss factor, whereas moisture such as water has a high-loss factor. Moisture may evaporate in situ. In general, it is easier and cheaper to build a RF heater than a microwave unit, as complex arrangements are needed in a continuous process with microwave sources to allow the material being dried to be conveyed through the unit without excessive leakage of radiation at the inlet and outlet ports. There is some advantage in combining RF heating with convective drying, as dielectric heating (providing 10 to 20% of the total) can be used to raise moisture temperature, and thus enhance the convective drying rate.
The thermal sensitivity of the material fixes the temperature-time limits for safe drying. For many materials, the rate of thermal degradation follows an Arrhenius relationship, and the maximum permissible working temperature falls exponentially with an increase in holding time. Polymers may be safely dried in a fluidized-bed dryer, for example, in which the dwell-time of the particles may be of the order of 10 to 20 min while in a static bed, with its inherently slower drying rates, drying conditions would have to be very mild and the associated dryer large and costly. Spray dryers for the manufacture of milk products can be operated with an air-inlet temperature of the order of 200°C since the residence time of particles in the drying chamber is very short, of the order of 20s or less. Nevertheless, it is sometimes necessary to dry thermolabile materials under vacuum to reduce process temperatures. Freeze-drying can only be undertaken below the triple-point pressure of 630 Pa. Sensitive pharmaceutical products are dried in agitated vacuum pans. Degrade of timber for high-quality end use can be reduced by seasoning under vacuum, rather than by a lengthier kiln-drying schedule at atmospheric pressure.
The selection of a complete drying installation includes many considerations other than the drying characteristics of the wet feedstock. These factors include the storage and delivery of the feed material, any equipment for performing it or blending back dried fine particles, the means of conveying the material as it dries, the equipment for collecting the dried product, and ancillary plants for the supply of heat, vacuum or refrigeration [Keey (1992)]. Past experience in operating equipment will be a guide in the case of an existing product or drying process, and careful consideration of past choices normally reveals some deficiencies which can be rectified. Simple bench tests can reveal considerable semiquantitative information about a material's drying behavior under proposed drying conditions, leading to the elimination of some types of dryer. There have been many attempts to provide first guides for the selection of a dryer for a particular job. Simple decision trees are set out by van't Land (1984) for batch and continuous dryers, and these are reproduced in Figure 5 and 6. More advanced methods based on expert systems are currently being developed [Kemp (1994)].
Such methods provide general indications and frequently, special considerations apply. When high-valued materials are to be dried, the costs of drying may relate more to product losses or dust emissions than to costs of investment and energy use. Design of the dryer may then be secondary to the careful specification of appropriate gas-cleaning technology. The dominance of material losses on processing is also related to the safe handling of toxic materials and the prevention of hazards in drying solvent-wet solids. To avoid losses, sophisticated drying assemblies can often be justified, such as a vacuum batch chamber for drying pharmaceutical granules by microwave heating. Other possibilities include the design of hybrid equipment in which several unit operations can be carried out in sequence in the same vessel. Thus, an agitated vacuum dryer might be used for crystallization and filtration before drying or blending and granulation afterwards.
Cook and DuMont (1991) have provided a description of modern drying practice, including methods of improving the performance of existing dryers, besides the selection and commissioning of drying units.
Cook, E. M. and DuMont, H. D. (1991) Process Drying Practice, McGraw Hill, New York.
Keey, R. B. (1978) Introduction to Industrial Drying Operations, Pergamon, Oxford.
Keey.R. B. (1992) Drying of Loose and Particulate Materials, Hemisphere, New York.
Kemp, I. C. (1994) A new algorirthmn for dryer selection, Proc. 9th Internat. Drying Symp., Gold Coast, 1-4 Aug.
Sloan, C. P. (1967) Drying systems and equipment, Chem. Eng., 74(14), 169-200.
van't Land, C. M. (1984) Selection of industrial dryers, Chem. Eng., 91(5), 53-61.