APV Dryer Handbook - Umbc

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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
any of the other dryers previously mentioned.
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 is often an advantage
because the drying medium is used as a vehicle for the partially dried product.
Other operations such as product classification can also be carried out where
required. Another 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 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 multi-stage 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 10.
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 – owing to the fact that it is completely entrained – it is still
necessary to use backmixing techniques, on occasion, to produce a suitable feed.
A variety of feeding devices is 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