ABSTRACT
Fluidization is one of the most commonly used techniques for drying particulate solids. The technique relies on a high-velocity hot gas stream that creates a fl uid bed with special hydrodynamic and heat/mass-transfer characteristics. Although well entrenched in industry, fl uid bed dryers have the following disadvantages that limit their application:
A relatively high pressure drop that calls for high-pressure blowers• Particle shape restricted to those approaching sphere or cuboid• Bed height limited to between the minimum height that will • ensure uniform fl uidization and the maximum height allowed by the pressure drop Restricted particle size and size distribution; polydisperse mate-• rials cause excessive entrainment, whereas both fi ne and coarse materials do not fl uidize well Susceptible to aggregative fl uidization or channeling resulting in • bed instability and reduced transfer characteristics
The negative effects from the last two constraints can be alleviated or even eliminated by applying periodic fl ow of the fl uidizing gas, which creates the so-called pulsed fl uidized bed (PFB). Although simple shut-off and start-up of a fl uidizing gas stream with frequency from 0.5 to 4 Hz is used in some dryers (e.g., Anonymous, 1990), in most cases gas pulsation is generated by a butterfl y valve installed in the gas inlet duct and rotated at a specifi ed angular velocity. Thus, the instantaneous superfi cial gas velocity varies across the bed area from zero to the maximum. Better hydrodynamics of such a pulsated fl uid bed are, however, offset by additional pressure drops due to the on-off operation of the valve as well as by the back-pressure developed that can create problems with direct gas fi ring. Vibration of the dryer body is also of important concern, although in a design patented by Sztabert et al. (1978), pulsation of a gas stream is used to vibrate the supporting grid in a special type of the vibrated fl uid bed. Figure 6.1 presents the schematic of a PFB dryer manufactured by Bühler-Miag GmbH (Germany) where pulsation of a fl uidizing gas is obtained by the rotation of a dual butterfl y valve with wings displaced by 90°. Such a design greatly reduces the negative effects of a single pulsator as the gas shut-off phase is alternated between two longitudinal segments of the bed.
