Page 278 - Chemical Process Equipment - Selection and Design

9.5. CONTINUOUS TRAY AND CONVEYOR BELT DRYERS
TABLE 9.4. Examples of Products Dried in S
Equipment

1. Spray dryers: rubber chemicals, sulfonates, inorganic phosphates,
Pneumatic conveying dryer ceramics, kaolin, coffee, detergents, pharmaceuticals, pigments,
inks, lignosulfonate wood waste, melamine and urea formaldehyde
resins, polyvinyl chloride, microspheres, skim milk, eggs, starch,
yeast, silica gel, urea, salts
2. Drum dryers: potatoes, cereals, buttermilk, skim milk, dextrins,
yeasts, instant oat meal, polyacylamides, sodium benzoate,
propionates, acetates, phosphates, chelates, aluminum oxide,
m-disulfuric acid, barium sulfate, calcium acetate-arsenate-
carbonate-hydrate-phosphate, caustic, ferrous sulfate, glue, lead
arsenate, sodium benzene sulfonate, and sodium chloride
3. Vacuum drum dryers: syrups, malted milk, skim milk, coffee, malt
extract, and glue
4. Vacuum rotary dryers: plastics, organic polymers, nylon chips,
chemicals of all kinds, plastic fillers, plasticizers, organic thickeners,
cellulose acetate, starch, and sulfur flakes
4. Belt conveyor dryers: yeast, charcoal briquettes, synthetic rubber,
catalysts, soap, glue, silica gel, titanium dioxide, urea formaldehyde,
clays, white lead, chrome yellow, and metallic stearates
6. Pneumatic conveyor dryers: yeast filter cake, starch, whey, sewage
sludge, gypsum, fruit pulp, copper sulfate, clay, chrome green,
synthetic casein, and potassium sulfate
7. Rotary mukitray dryer: pulverized coal, pectin, penicillin, zinc
sulfide, waste slude, pyrophoric zinc powder, zinc oxide pellets,
calcium carbonate, boric acid, fragile cereal products, calcium
chloride flakes, caffein, inorganic fluorides, crystais melting near
100°F. prilled pitch, electronic grade phosphors, and solvent-wet
organic solids
8. Fluidized bed dryer: lactose base granules, pharmaceutical crystals,
weed killer, coal, sand, limestone, iron ore, polyvinyl chloride,
asphalt, clay granules, granular desiccant, abrasive grit, and salt
9. Freeze dryers: meat, seafood, vegetables, fruits, coffee,
concentrated beverages, pharmaceuticals, veterinary medicines,
and blood plasma
10. Dielectric drying: baked goods, breakfast cereals, furniture timber
blanks, veneers, plyboard, plasterboard, water-based foam plastic
slabs, and some textile products
11. lnfrared drying: sheets of textiles, paper and films, surface finishes
of paints and enamels, and surface drying of bulky nonporous
articles.

circulation belts are applied to granules more than about 3 mm in
narrowest dimension. When the feed is not in suitable granular
form, it is converted in a preformer to a size range usually of
3-15mm. Belts are made of chain mail mesh or metal with 2mm
perforations or slots of this width.
Several arrangements of belt dryers are shown in Figures
9.8(c)-(e). In the wet zone, air flow usually is upward, whereas in
the drier and cooling zones it is downward in order to minimize
(6) dusting. The depth of material on the belt is 1-sin. Superficial air
velocities of 5ft/sec usually are allowable. The multizone
Figure 9.5. Residence time distribution in particle dryers. (a) Four
types of dryers (McCormick, 1979). (b) Residence time distribution arrangement of Figure 9.8(e) takes advantage of the fact that the
of air in a detergent spray tower; example shows that 27% material becomes lighter and stronger and hence can be loaded
(difference between the ordinates) has a residence time between 24 more deeply as it dries. Each zone also can be controlled separately
and 32 sec [Place et al., Trans. Inst. Chem. Eng. 37,268 (1959)J. (c) for air flow and temperature. The performance data of Table 9.9
Fluidized bed drying of two materials (Vanacek et al., Fluidized Bed cover a range of drying times from 11 to 200min, and thermal
Drying, 1966). efficiencies are about 50%.
Laboratory drying rate data of materials on trays are best
obtained with constant air conditions. Along a belt conveyor or in a
particularly for materials that tend to degrade when they are tray-truck tunnel, the moisture contents of air and stock change
moved. From the point of view of drying, belt conveyors are of two with position. Example 9.3 shows how constant condition drying
types: with solid belts and air flow across the top of the bed, called tests can be adapted to belt conveyor operation. The effects of
convection drying, or with perforated belts and through circulation recycling moist air and of increasing the air velocity beyond that
of the air. The screw conveyor of Figure 9.8(f) has indirect heating. studied in the laboratory tests are studied in Example 9.4. Recycling
Solid belts are used for pastes and fine powders. Through does reduce drying time because of the increased air velocity, but it