Page 399 - Chemical Process Equipment - Selection and Design

362 DISINTEGRATION, AGGLOMERATION. AND SIZE SEPARATION OF PARTICULATE SOLIDS
Materials suitable for prilling are those that melt without
decomposition, preferably have a low heat of solidification and a
high enough melting point to permit the use of ambient air for
cooling. Because of high viscosities, spray wheels are preferred to
spray nozzles. The wheels often are equipped with scrapers to
prevent clogging. Since several wheels are needed for capacity, they
are often arranged in line and the cross section of the tower is made
rectangular.
The density of the prills is reduced substantially when much
evaporation occurs: with 0.2-0.5% water in the feed, ammonium
nitrate prills have a specific gravity of 0.95, but with 3-5% water it
falls to 0.75. Prilled granules usually are less dense than those made
by layering growth in drum or fluidized bed granulators. The latter
processes also can make larger prills economically. To make large
prills, a tall tower is needed to ensure solidification before the
bottom is reached. The size distribution depends very much on the
character of the atomization but can be made moderately uniform.
Some commercial data of cumulative % less than size are:

% Lessthansize 0 5 50 95 100
Dia (rnrn) 1.2 1.6 2.4 3.5 4.8

Cooling of the prills can be accomplished more economically in
either rotary drums of fluidized beds than in additional tower
height. Fluidized bed coolers are cheaper and better because more
easily dust controllable, and also because they can be incorporated
in the lower section of the tower. After cooling, the product is
screened, and the fines are returned to the melter and recycled.

FLUIDIZED AND SPOUTED BEDS

In fluidized bed granulation, liquid or solution is sprayed onto or
into the bed, and growth occurs by agglomeration as a result of
binding of small particles by the liquid or by layering as a result of
evaporation of solution on the surfaces of the particles. The
granules grown by layering are smoother and harder. Some attrition
also occurs and tends to widen the size distribution range of the
product. Larger agglomerates are obtained when the ratio of
(C) droplet/granule diameters decreases. Increase in the rate of the
- (e) are lifted off the bottom and recirculated more thoroughly.
fluidizing gas and in the temperature of the bed decreases
penetration and wetting of the bed and hence leads to smaller
SIDE VIEW
CIRCULAR CROSS- SECTIONS -
smaller proportion of the particles and thus leads to larger size
flat face bevel SIDE VIEW SDE VIEW granule sizes. A narrower and more concentrated spray wets a
deep convex
product. The bed is often made conical so that the larger particles
standard canvex
(d)
0 Initial particle size distribution often is in the range of
0 50-250 pm. The product of Table 12.19(a) is 0.7-2.4 mm dia.
A wide range of operating conditions is used commercially.
TOP VIEW
TOP VIEW
Performance data are in Table 12.19. Gas velocities cover a range
of 3-20 times the minimum fluidizing velocity or 0.1-2.5 m/sec.
Bed expansion ratios are up to 3 or so. As in fluidized bed drying,
SIDE VIEW SIDE VIEW bed depths are low, usually between 12 and 24 in. Evaporation rates
oblong "foot ball" are in the range 0.005-1.0 kg/(sec)(m2).
Batch fluidized bed granulation is practiced for small
(C)
production rates or when the residence time must be long. Figure
12.17(a) and Table 12.19(a) are of an arrangement to make
Figure 12.14. Common shapes and sizes of pellets made by some granules as feed to pharmaceutical tabletting. A feature of this
agglomeration techniques. (a) Sizes and shapes of briquets made on equipment is the elaborate filter for preventing escape of fine
roll-type machines. (b) Catalyst pellets made primarily by extrusion particles and assuring their eventual growth. A continuous process
and cutting (Imperial Chemical Zndustries). (c) Some of the shapes for recovery of pellets of sodium sulfate from incineration of paper
made with tabletting machines. mill wastes is the subject of Figure 12.17(b) and Table 12.19(b).