Fluidized and Spouted Bed Dryers

After a review of the customary calculation methods [97] for both essential classes of dryers (convective and contact dryers), the dimensioning methods for spray dryers [98, 99], fluidized and spouted bed dryers [100, 101, 102], cascading rotary dryers [103], pneumatic conveying dryers [104], conductive-heating agitated dryers [105] and layer dryers [106] were presented. They all confirmed the initially made conclusion that the scaling up of dryers is still made today without dimensional analysis and the model theory based thereupon. 

Because of the high gas velocities, solids loading ratios, and momentum loss in the collision zone, the pressure loss in ISDs is much greater than in pneumatic dryers, but it is comparable with that of fluidized and spouted bed dryers [44,45], The impinging stream configurations can, however, compete in various aspects with the classical systems for drying of particulates and pastes (Table 21.11). 

Intermittently fluidized or spouted bed dryers have been shown to afford several advantages. Jumah et ai. (1996) have shown experimentally and by mathematical modeling that a rotating jet spouted bed batch dryer for grains can save up to 30 percent of energy while... 

When drying large particles, which are difficult to fluidize, a spouted bed or one of its many variants may be used instead of the conventional fluidized bed in which the air is distributed uniformly through a low open area-supporting grid. In a spouted bed, the air enters as a jet and causes a regular recirculatory motion of the particles within the vessel. Due to several limitations, the conventional spouted bed has not found significant commercial application as a dryer, however. 

Commonly used direct dryers in polymer plants are rotary warm air, fluidized bed, flash, spray, tunnel, and various vibrating and spouted bed (SB) types. All these have a common disadvantage. The amount of air or hot gas required is fairly large, which causes the auxiliary equipment needed (e.g., air heaters, blowers, and dust collectors) to be sized accordingly the thermal efficiency is also lower than that of indirect dryers. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Stable fluidization requires a distribution of particle sizes, preferably in the range of a few hundred microns. Normally a size of 4 mm or so is considered an upper limit, but the coal dryers of Tables 9.15(a) and (b) accommodate sizes up to 0.5 in. Large and uniformly sized particles, such as grains, are dried successfully in spouted beds [Fig. 9.13(f)] Here a high velocity gas stream entrains the solid upward at the axis and releases it at the top for flow back through the annulus. Some operations do without the mechanical draft tube shown but employ a naturally formed central channel. 

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