Batch Dryers

BATCH DRYERS

Hot Air Ovens

Ovens operating by passing hot air over the surface of a wet solid that is spread over trays arranged in racks provide the simplest and cheapest dryer. On small installations, the air is passed over electrically heated elements and once through the oven. Larger units may employ steam-heated, finned tubes, and thermal efficiency is improved by recirculating the air. This is controlled by manually set dampers, and a common operating position gives 90% recircula-tion and 10% bleed-off. The heater bank is placed so that the solids do not receive radiant heat and incoming air may be filtered. A typical hot air oven is illustrated schematically in cross section in Figure 7.7A.

The temperature-humidity sequence of the circulating drying air is pre-sented in Figure 7.7B. The incoming air, at a temperature and humidity given by point A, is heated at constant humidity to point B and passed over the wet solid. The humidity rises and the temperature falls as the adiabatic cooling line is followed until the air leaves the tray in condition C. It is then recirculated to the heater, and in Figure 7.7B, two further cycles are shown.

FIGURE 7.7 (A) A tray dryer. (B) Temperature-humidity sequence of drying air.

We have assumed that all heat is drawn from the air and transmitted across the stationary air layer in contact with the drying surface, as described earlier. Surface temperatures are, in fact, modified by heat absorbed and con-ducted from unwetted surfaces, such as the underside of the tray, and by radiation.

The chief advantage of the hot air oven, apart from its low initial cost, is its versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Thermostatically controlled air temperatures of between 408C and 1208C permit heat-sensitive materials to be dried. For small batches, a hot air oven is, therefore, often the plant of choice. However, the following inherent limitations have led to the development of other small dryers:

1. A large floor space is required for the oven and tray-loading facilities.

2. Labor costs for loading and unloading the oven are high.

3. Long drying times, usually of the order of 24 hours, are necessary.

4. Solvents can be recovered from the air only with difficulty.

5. Unless carefully designed, nonuniform distribution of air over the trays gives variation in temperature and drying times within the oven. Variations of ±78C in temperature have been found from location to location during the drying of tablet granules. Poor air circulation may permit local saturation and the cessation of drying.

If the material is of suitable granular form, drying times may be reduced to an hour or less by passing the air downward through the material laid on mesh trays. The oven in this form is called a batch through-circulation dryer.

Vacuum Tray Dryers

Vacuum tray dryers, as shown in Figure 7.8A, differing only in size from the familiar laboratory vacuum ovens, offer an alternative method for drying small quantities of material. When scaled up, construction becomes massive to

FIGURE 7.8 (A) Rotary vacuum dryer and (B) fluidized bed dryer.

withstand the applied vacuum, and cost is further increased by the associated vacuum equipment. Vacuum tray dryers are, therefore, only used when a def-inite advantage over the hot air oven is secured, such as low-temperature drying of thermolabile materials or the recovery of solvents from the bed. The exclusion of oxygen may also be advantageous or necessary in some operations.

Heat is usually supplied by passing steam or hot water through hollow shelves. Drying temperatures can be carefully controlled, and for the major part of the drying cycle, the material remains at the boiling point of the wetting liquid under the operating vacuum. Radiation from the shelf above may cause a significant increase in temperature at the surface of the material if high drying temperatures are used. Drying times are long and usually of the order of 12 to 48 hours.

Tumbling Dryers

The limitations of ovens, particularly with respect to the long drying times, have, where possible, promoted the design and application of other batch dryers. The simplest of these is the tumble drier for which the most common shape is the double cone shown in Figure 7.8A. Operating under vacuum, this provides controlled low-temperature drying, the possibility of solvent recovery, and increased rates of drying. Heat is supplied to the tumbling charge by contact with the heated shell and by heat transfer through the vapor. Optimum con-ditions are established experimentally by varying the vacuum, the temperature, and, if the material passes through a sticky stage, the speed of rotation. With correct operation, a uniform powder should be obtained as distinct from the cakes produced when static beds are dried. Some materials, such as waxy solids, cannot be dried by this method because the tumbling action causes the material to aggregate into balls.

A normal charge would be about 60% of the total volume, and for dryers 0.7 to 2 m in diameter, drying times of 2 to 12 hours may be expected. In studying the application of tumbler dryers to drying tablet granules, it was found that periods of 2 to 4 hours replaced times of 18 to 24 hours obtained with hot air ovens. The mixing and granulating capacity of the tumbling action has suggested that these operations could precede drying in the same apparatus.

Fluidized Bed Dryers

The term “fluidization” is applied to processes in which a loose, porous bed of solids is converted to a fluid system having the properties of surface leveling, flow, and pressure-depth relationships by passing the fluid up through the bed.

Fluidized bed techniques, employing air as the fluidizing medium, have been successfully applied to drying when the solid is of suitable physical form. The high interfacial contact between drying air and solids gives drying rates 10 to 20 times greater than that obtained during tray drying. A drying curve for this method is shown in Figure 7.9.

The dryer, illustrated in Figure 7.8B, consists of a basket of either plastic or stainless steel with a perforated bottom, which is mounted in the body of the drier and into which the material to be dried is placed. Heated air may be either blown or sucked through the bed. The air leaving the basket passes through an air filter and may be recirculated. Particle properties, such as shape and size distribution, affect fluidization, and a unit must have a variable air flow adjusted

FIGURE 7.9 Drying curves.

so that the material is fluidized but is not carried into the filters. For this reason, the material must have a fairly close size range or else elutriation of fine particles into the filters will take place.

Fluidized bed dryers are particularly suitable for granulated materials and are being increasingly used for tablet granulations provided product changeover is not too frequent. It may be advantageous to preform other materials, such as a dewatered filter cake, into granules solely to employ fluidized bed drying. If fluidizing conditions are ideal, the granulation will not require further grinding. Tray dryers, on the other hand, produce a caked product that may require mild comminution. Variation in temperature, which may be quite marked in tray dryers, is virtually eliminated in fluidized bed dryers by the intense mixing action. The floor space for a given capacity is smaller compared with a tray dryer. Machines vary in size, handling up to 250 kg. Drying times, maximum, minimum, and optimum air velocities, air temperature, and the tendency to cake and channel are established experimentally as these cannot be predicted accu-rately at present.

Considerable erosion and the production of large amounts of fines might be expected from the intense turbulent movement. Experience shows that the opposite is true. The particles are to some extent “padded” by the surrounding fluid so that either the amount of contact between particles is low or the impact energy is small.

Agitated Batch Dryers

Agitated batch dryers consist of a jacketed cylindrical vessel with agitator blades designed to scrape the bottom and the walls. The body may be run at atmo-spheric pressure or under vacuum. Pasty materials, which could not be handled in tumbling or fluidized bed dryers, may be successfully dried at rates higher than that can be achieved in an oven.

Freeze-Drying

Freeze-drying is an extreme form of vacuum drying in which the solid is frozen and drying takes place by subliming the solid phase (Dushman and Lafferty, 1962; Jennings, 1988; Nail, 1980; Pikal et al., 1984). Low temperatures and pressures are used. Establishing and maintaining these conditions, together with the low drying rates obtained, create a most expensive method of drying, which is only used on a large scale when other methods are inadequate.

There are two principal fields in which freeze-drying is extensively used. It is used when high rates of decomposition occur during normal drying. The second field concerns substances that can be dried at higher temperatures but are thereby changed in some way. Fruit juices, for example, are reputed to lose subtle elements of flavor and odor, and proteinaceous materials are partly denatured by the concentration and higher temperatures associated with con-ventional drying. Drying of blood plasma and some antibiotics are important large-scale applications of freeze-drying. On a smaller scale, it is extensively used for the dehydration of bacteria, vaccines, blood fractions, and tissues.

Freeze-drying is theoretically a simple technique. Pure ice exhibits an equilibrium vapor pressure of 4.6 mmHg at 08C and 0.1 mmHg at – 408C. The vapor pressure of ice containing dissolved substances will, of course, be lower. If, however, the pressure above the frozen solution is less than its equilibrium vapor pressure, the ice will sublime, eventually leaving the solute as a sponge-like residue equal in apparent volume to the original solid and, therefore, of low bulk density. The latter is readily dissolved when water is added, and freeze-drying has been called “lyophilic drying” or “lyophilization” for this reason. No concentration, in the normal sense of the word, occurs, and structural changes in, for example, protein solutions, are minimized.

In practice, many difficulties are encountered. Under conditions of high vacuum, water vapor must be trapped or eliminated. To maintain drying, heat must be supplied to the frozen solid to balance the latent heat of sublimation without melting the frozen solid. Difficulties become acute if, like blood plasma, the product is dried in the final container under aseptic conditions.

In the first stage of the process, the material is cooled and frozen. If the temperature of a dilute solution of a salt is slowly reduced, leveling occurs in the time-temperature curve just below 08C because of the liberation of the latent heat of fusion of ice, and pure ice separates. With further cooling, the solution becomes concentrated until the eutectic mixture is formed. This freezes to give a plateau in the cooling curve. It is a clear indication of complete freezing. If the concentration of the liquid eutectic mixture is small, the material may appear to be completely frozen at higher temperatures. Under these conditions, some drying from a liquid phase will occur, possibly with damaging results. This can be detected by measuring the electrical resistance of the ice that becomes infi-nitely great when the eutectic mixture freezes. Conversely, thawing gives a marked decrease in resistance, an effect that can be used to automatically control the state of the drying solid. Protein solutions do not give clearly defined eutectic points and are usually frozen to below –258C before drying. Freezing is carried out quickly to prevent concentration of the solution and to produce fine ice crystals. Some degree of supercooling may be induced, followed by a very quick freeze. Freezing may or may not be carried out in the drying chamber. If drying in final containers is necessary, small-scale operations may employ immersion in a coolant such as liquid air or isopentane. Larger-scale installa-tions may cool with a blast of very cold air. Alternatively, evaporative freezing, in which the liquid is cooled to near its freezing point and the system is rapidly evacuated, is employed. The evaporating liquid cools and freezes rapidly. Frothing caused by the evolution of dissolved gases may complicate this technique. For bulk drying, the liquid is placed in shallow trays on refrigerated shelves in the drying cabinet.

A suitable surface area to depth of solid ratio must be provided to facilitate drying. Thin layers of frozen liquid are used in bulk drying. The surface area of bottle-dried plasma may be increased by spinning in a vertical axis during freezing to give a frozen shell about 2 cm thick around the inside periphery of the bottle. Spinning also prevents frothing during evaporative freezing by inhibiting the formation of bubbles.

In plasma processing, freezing, and drying, handling must be carried out aseptically. This is maintained by a filter at the neck of the bottle that allows the passage of water vapor but prevents the ingress of bacteria. Similar precautions are taken during the drying of antibiotics.

Effective drying vacuum of 0.05 to 0.2 mmHg may be provided by directly pumping water vapor and permanent gases, originally present or derived from the drying material and from leaks, out of the system. Normal practice, how-ever, favors interposing a refrigerated condenser between the drying surface and the pump. This arrangement allows a smaller pump, handling mainly permanent gases, to be used but demands a low condenser temperature, such as –50^○C, to remove water vapor at the low operating pressure. A system for bulk drying in trays is represented diagrammatically in Figure 7.10A.

During drying, heat must be supplied to the drying surface. When drying a material, such as plasma, in a final container, a temperature gradient is established across the container wall and through the ice to the drying surface by means of a heater suitably mounted in relation to the container. The power dissipated by the heater must be carefully controlled so that melting does not occur at the ice-container junction, the point nearest to the heat source and at the highest temperature. At any time, the conditions prevailing are such that the rate of evaporation is approximately constant and temperatures and pressure adjust so that there is a temperature and pressure gradient from the drying surface to the condenser. As evaporation proceeds, a drying line recedes into the solid. With the thinning of the ice layer, the temperature gradient through the ice will be modified by the decreasing resistance to heat flow. An increase in the rate of drying due to increase in temperature and vapor pressure of the drying surface might, therefore, be expected. In practice, this is modified by the layer of dried plasma that offers considerable resistance to the flow of vapor.

FIGURE 7.10 (A) Equipment for freeze-drying bulk liquids in trays and (B) variations in tem-perature and pressure during the freeze-drying cycle for blood plasma.

The bacterial filter also causes a large, constant pressure drop. Evaporation of pure ice without the filter and plasma layer would be 300 times faster. When the plasma is nearly dry, its temperature is allowed to rise to about 308C to facilitate final drying. The total drying time is about 48 hours. The temperatures and pressure in the system during this period are shown, as a function of time, in Figure 7.10B.

If the product is not being dried in its final container, radiant heat may be used to provide the latent heat of sublimation. If the dried solid could be removed continuously, high drying rates are possible. Not only is heat provided directly to the drying surface, but there is also little danger of melting the ice at the container wall.