Page 155 - Principles of Catalyst Development

CATALYST CHARACTERIZATION 143
TABLE 7.2. Typical Results for a
Particle Size Distribution Silica-
Alumina Cracking Catalyst

Size range Distribution
(JLm) (wt% )

0-20 3
20-45 35
45-60 25
60-90 25
90+ 12

7.3.3. Mechanical Properties

Mechanical stresses experienced by catalyst particles during handling
and use are considerable. The nature of these stresses is discussed in Chapter
Six. Properties that relate to stress resistance are crushing strength, attrition
loss, and loss on ignition.

7.3.3.1. Crushing Strength

The basis for evaluating crushing strength is the Huit equationl2141

F/ D~ = C (7.3 )

where F is the force necessary to crush a particle of diameter D" and C is
a constant which depends upon (I) the ultimate tensile strength of the
material, (2) the plasticity of the solid powder, (3) the grain density, (4)
porosity, (5) pelleting pressure, (6) binders, and (7) devices used in testing.
With such an array of variables, it is not surprising that existing correlations
and techniques are empirical, with no scientific foundation. However,
although only relative, these tests do index performance of particular
formulations. Most commonly found are variations of axial and radial
crushing tests, shown in Fig. 7.5. A statistical number of particles are crushed,
either along the axial direction for cylinders or the radial direction for
extrudates, and an average taken. Values range from 10 to 100 kg em 2 for
axial and l-lOkgcm- ' for radial tests. The process designer knows from
experience that particles with certain specifications give satisfactory results.
Although initially based on performance in similar reactors, these
specifications are continually updated.
A much more satisfactory procedure is to measure bulk crushing
strength, which more closely approximates packed beds and is applicable