Page 141 - Principles of Catalyst Development
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CATALYST PREPARATION 129
lengths by a knife rotating outside the end plate or allowed to break up as
it falls onto a moving belt on its way to the drier.
Extrudates are irregular in shape and length, have higher porosities
and lower strengths than pellets, but are less expensive to make. Many
commercial products are now available as extrudates.
6.S.3. Spheres
Hydrogels that age rapidly, such as alumina, silica, and silica-alumina,
are formed into spheres by a technique similar to the historical method for
making musket balls. The hydrogel is forced through holes in a plate at the
top of a column containing oil in which the gel is immiscible, as shown in
Fig. 6.27. Assuming a spherical shape, the drop falls slowly through the oil,
hardening as it ages. This process is accelerated by increa:~ing the pH down
the column with flowing ammonia up the tower. At the bottom, the spheres
are separated, dried, calcined, and sieved. Very uniform spheres are used
in moving and ebullating bedsy89J
ALUMINA
SLURRY
PERFORATE!)
PLATE
o
o OIL
o
o
o HARDENING
SPHERES
o
o
o
o
o
AMMONIA
Figure 6.27. Production or spht'res using the column method.