Figure 1. Comparative performance curves for Thruflo and conventional units.
would be between seven and eight for the same duty. The advantage is more apparent when it is seen that respective floor areas occupied are 55 ft2 for the Thruflo dryer pictured in Figure 2 and 245 ft2 in the case of conventional units using transverse airflow.
Reference to the drying curves for the processing of materials in solid or filter cake form or, in fact, in the case of wet powders, clearly indicates that the ultimate rate-governing factor is the rate of diffusion of moisture from the wet mass. This becomes increasingly so during the falling rate period of drying. This situation, however, can be
Figure 3. Double-cone vacuum dryer.
improved by preforming the product in order to increase the effective surface area presented to heat and mass transfer. The logical extension of this technique is to total dispersion drying (flash or pneumatic dryers, fluid beds, etc) where discrete particles can be brought into contact with the hot gas. This produces rapid heat transfer with correspondingly short drying times.
Batch-type fluidized-bed dryers have, therefore, superseded forced-convection units in many cases, notably in the drying of pharmaceuticals. These machines generally are available in a range of standard sizes with batch capacities of 50-200 lb, although much larger units are made for special applications.
When considering this type of dryer, it is important to ensure that the feed material can be fluidized in both initial and final conditions. It also should be remembered that standard fan arrangements are not equally suitable for a variety of materials of different densities. Therefore, it is necessary to determine accurately the minimum fluidizing velocity for each product.
If the feedback is at an acceptable level of moisture content for fluidization, this type of dryer provides many advantages over a batch-type unit. Simplified loading and unloading results in lower labor costs, high thermal efficiencies are common, and the drying time is reduced to minutes as opposed to hours in conventional units. Current developments of this type of equipment now include techniques for the simultaneous evaporation of water and the granulation of solids. This makes the units ideal for use in the pharmaceutical field.
The various batch dryers referred to operate by means of forced convection, the transfer of thermal energy being designed to increase the vapor pressure of the absorbed moisture while the circulated air scavenges the overlying vapor. Good conditions thus are maintained for continued effective drying. Alternatively and where the material is thermosensitive—implying low temperatures with consequently low evaporative rates—some improvement can be effected by the use of subatmospheric dryers, ie, by reducing the vapor pressure. Several different configurations
are in use and all fall into the category of conduction-type dryers. The most usual type of heating is by steam, although hot water or one of the proprietary heat-transfer fluids can be used.
Two particular types are the double-cone dryer (Fig. 3) with capacities of <400 ft3 and the agitated-pan dryer not normally larger than 8 ft diameter, where average evaporative rates per unit wetted area usually are in the region of 4 lb/ft2 h. These units are comparatively simple to operate and, when adequately insulated, are thermally quite efficient, although drying times can be extended. They are especially suitable for applications involving solvent recovery and will handle powders and granules moderately well. They do, however, suffer from the disadvantage with some materials that the tumbling action in double-cone dryers and the action of the agitator in agitated-pan machines can produce a degree of attrition in the dried product that may prove unacceptable.
Similarly, quite large rotary vacuum dryers are used for pigment pastes and other such materials, especially where organic solvents present in the feedstock have to be recovered. These units normally are jacketed and equipped with an internal agitator that constantly lifts and turns the material. Heat transfer here is entirely by conduction from the wall of the dryer and from the agitator. Owing to the nature of their construction, initial cost is high relative to capacity. Installation costs also are considerable. In general, these dryers find only limited application.
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