Continuous Dryers

CONTINUOUS DRYERS

Although many types of continuous dryers are available, the scale of the operation for which they are designed is rarely appropriate for pharmaceutical manufacture. As with most continuous plant items, the cost is dis-proportionately high for small units. Spray and drum dryers provide an exception to this comment because residence times in the dryers are short and thermal degradation is minimized. Under some conditions, freeze-drying may be the only practicable alternative.

Spray Dryers

As the name implies, the solution or suspension to be dried is sprayed into a hot airstream and circulated through a chamber. The dried product may be carried out to cyclone or bag separators or may fall to the bottom of the drying chamber and be expelled through a valve. The chambers are normally cylindrical with a conical bottom, although proportions vary widely. A typical spray dryer is illustrated in Figure 7.11 (Masters, 1991; Sacchetti and Van Oort, 2006).

FIGURE 7.11 Spray dryer.

FIGURE 7.12 Schematic diagrams of atomizers for spray drying.

The process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, finally, removal and collection of the dry particles.

Atomization may be achieved by means of single fluid or two fluid nozzles or by spinning disk atomizers. The single fluid nozzle, illustrated in Figure 7.12, operates by forcing the solution under pressure through a fine hole into the airstream. An intense swirl is conferred on the liquid before it emerges from the orifice. This causes the jet to break up. In the two fluid nozzles, shown in Figure 7.12, a jet of air simultaneously emerges from an annular aperture con-centric with the liquid orifice. Both types are subject to clogging and severe erosion, so neither is well suited to spraying suspensions. The spinning disks are most versatile and consist, in their simplest form, of a mushroom-shaped disk spinning at 5000 to 30,000 rpm. Other designs include the slotted disk (Fig. 7.12), which will spray thick suspensions and, if special feeding arrangements are used, pastes. The main factors that determine the size of the droplets are the viscosity and surface tension of the liquid, the fluid pressure in the use of nozzles, or, for spinning disks, their size and speed of rotation. A reasonably uniform and controllable size within the range 10 to 500 μm is desirable.

In vertical spray dryers, the flow of the drying gas may be concurrent or countercurrent with respect to the movement of droplets. The movement of the gas is, however, complex and highly turbulent. Good mixing of droplets and gas occurs, and the heat and mass transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give very high evaporation rates. The residence time of a droplet in the dryer is only a few seconds (5–30 seconds). Since the material is at wet bulb temperature for much of this time, high gas temperatures of 150^○C to 200^○C may be used even with thermolabile materials. Although the temperature of the material rises above the wet bulb temperature at the end of the process, the drying gases will be cooler and the material will be almost dry, a condition in which many materials are thermally less sensitive. For these reasons, it is possible to dry complex vege-table extracts, such as coffee or digitalis, milk products, spore suspensions, and other labile materials without significant loss of potency or flavor.

Drying is considered to take place by simple evaporation rather than by boiling, and it has been observed that a droplet reaches a terminal velocity within about 30 cm of the atomizer. Beyond this, there is no relative velocity between the droplet and the drying gas unless the former is very large. The droplets may dry to form a solid, spherical particle. If, however, the emerging solids form a skin, internal pressure may inflate the particle, and the final dry form will be hollow spheres that may or may not have a blow hole. These xenospheres may also fragment so that the final product occurs as agglomerates of finely divided solids. It has been found experimentally that the product’s bulk density, which is lowest for xenospheres and highest for fragmented solids, increases as the inlet air temperature is lowered and as the drop size increases. A higher feed concentration also increases the bulk density because drops of the same size give spheres with thicker walls.

These attractive physical characteristics lend further advantage to spray drying. The product often has excellent flow and packing properties that greatly facilitate handling and transport. As an example, spray-dried lactose is a widely used tablet excipient, which will flow, pack, and compact without prior gran-ulation. Similarly, a slurry of fillers and other excipients could be granulated by spraying and drying. After adding an active principle, the mix could be com-pressed without further processing.

The capital and running costs of spray dryers are high, but if the scale is sufficiently large, they may provide the cheapest method. When thermolabile materials are dried on a small scale, costs will be 10 to 20 times greater than that for oven drying. Air used to dry fine chemicals or food products is heated indirectly, thus reducing thermal efficiency and increasing costs. In some other installations, hot gases from combustion may be used directly.

Drum Dryers

The drum dryer consists of one or two slowly rotating, steam-heated cylinders. These are coated with solution or slurry by means of a dip feed, illustrated in Figure 7.13, in which the lower portion of the drum is immersed in an agitated trough of feed material or, in the case of some double drum dryers, by feeding the liquor into the gap between the cylinders, as shown in Figure 7.13. Spray and splash feeds are also used. When dip feeding is employed, the hot drum must not boil the liquid in the trough. Drying takes place by simple evaporation rather than by boiling. The dried material is scraped from the drum at a suitable point by a knife.

FIGURE 7.13 Drum dryers.

Drying capacity is influenced by the speed of the drum and the temper-ature of the feed. The latter may be preheated. With the double drum dryer, the gap between the cylinders determines the thickness of the film.

Drum dryers, like spray dryers, are relatively expensive in small sizes, and their use in the pharmaceutical industry is largely confined to drying thermo-labile materials where the short contact time is advantageous. Drums are nor-mally fabricated from stainless or chrome-plated steel to reduce contamination. The heat treatment to which the solid is subjected is greater than that in spray drying, and the physical form of the product is often less attractive. During drying, the liquid approaches its boiling point and the dry solids approach the temperature of the drum surface.