Where total dispersion of the product in a heated gas stream can be achieved with a significant increase in evaporative rates, pneumatic or continuous fluid-bed dryers are preferred. The capital cost of these alternatives generally is lower and maintenance is limited to such components as circulating fans and rotary valves. When considering these two types of dryers, it is convenient to examine them together since both share similar characteristics. Both employ forced convection with dispersion of the feedstock, and as a result of the intimate contact between the drying medium and the wet solids, both exhibit much higher drying rates than do any of the other dryers mentioned previously.
In a fluidized-bed dryer, the degree of dispersion and agitation of the wet solids is limited, whereas in a pneumatic dryer, the degree of dispersion is total and the material is completely entrained in the gas stream. This often is turned to advantage as the drying medium is used as a vehicle for the partially dried product. Other operations such as product classification also can be carried out where required. A further feature of fluid-bed and flash dryers is that the method of operation allows many temperature-sensitive materials to be dried without thermal degradation due to the rapid absorption of the latent heat of vaporization. This generally permits high-rate drying, whereas in other types of dryers lower temperatures would be necessary and correspondingly larger and more costly equipment would be required.
A very good degree of temperature control can be achieved in fluid-bed dryers and the residence time of the material can be varied either by the adjustment of the discharge weir or by the use of multistage units. Similarly, the residence time in the flash dryer can be adjusted by the use of variable cross-sectional area and, therefore, variable velocity. In addition, multiple-effect columns can be incorporated to give an extended path length or continuous recirculatory systems employing both air and product recycle can be used as illustrated in Figure 9.
Generally speaking, the residence time in fluidized-bed dryers is measured in minutes and in the pneumatic dryer in seconds. Both dryers feature high thermal efficiencies, particularly where the moisture content of the wet feed is sufficiently high to produce a significant drop between inlet and outlet temperatures. While the condition of the feed in the pneumatic dryer is somewhat less critical than that in the fluid-bed dryer because it is completely entrained, it still is necessary to use backmixing techniques on occasion in order to produce a suitable feed. A variety of feeding devices are used with these machines.
In fluid-bed dryers, special attention must be paid to the nature of the proposed feed since one condition can militate against another. To some extent, this is reflected in the range of variation in the figures given in Table 1 for evaporative rates. If a large or heavy particle is to be dried, the fluidizing velocities required may be considerable and involve high power usage. In such circumstances, if the moisture content is low and the surface/mass ratio also is low, the thermal efficiency and evaporation would be low. This would make selection of a fluid-bed dryer completely unrealistic and probably would suggest a conventional rotary dryer for the application.
Another case is that in which the minimum fluidizing velocity is so low that a dryer of very large surface dimensions is necessary to obtain the required thermal input. This also occurs in problems of fluid-bed cooling and usually is overcome by the introduction or removal of thermal energy by additional heating or cooling media through an extended-surface heat exchanger immersed in the bed.
With both types of dispersion dryer many configurations are available. While the power requirements of each usually is well in excess of other dryers because of the use of high-efficiency product recovery systems, the smaller size of the fluid-bed dryer compared with conventional rotaries and the fact that the flash dryer can be arranged to fit in limited floor space makes them very attractive.
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