Page 154 - Principles of Catalyst Development

142 CHAPTER 7
which does not penetrate pores smaller than 1.2 x 10 nm at atmospheric
3
pressure. For this reason particle density is also called mercury density.
There is a relationship between particle and skeletal densities, d p and d"
and porosity, (), given by

(7.1)

7.3.1.4. Packing Density

Also called bulk or bed density, the volume in this case includes the
void space between particles. This general definition poses some problems
since measured volumes differ, for example, for various vessels, packing
methods, and agitation. This is overcome by specifying dimensions of the
container used and the method of packing. From a practical viewpoint,
packing methods should be the same as those used in the reactor filling.
Packing density, dh , and particle density are related through the void
fraction, E; thus we have

(7.2)

Values in the example in Table 7.1 show that considerable errors result
if these densities are confused. A good practice is to always specify the
volume included, as in Table 7.1.

7.3.2. Particle Size

Measurement of the particle size of macroscopic pellets, extrudates,
and spheres presents no problems. Dimensions can be determined directly
or sample material sieved for irregular particles. Smaller distributions in
powders, such as cracking catalysts, require specialized procedures: optical
and electrical imaging, light scattering, light shadowing, elutriation,
sedimentation, electrical resistance, impaction, and nozzle pressure
drop. L~)().~) ).~)'.1)41 For cracking catalysts and other powders in the range
0.6-160 ftm, the preferred approach is to use an electronic particle size
analyzer. An electric current path of small dimensions is modulated by
individual particle passage through apertures, producing individual pulses
proportional to the particle volume. The sample is suspended in an elec-
trolyte, dispersed ultrasonically, and forced through restricting apertures.
Distributions are found from the number of particle passing through pro-
gressively smaller openings.(2))) Table 7.2 gives typical results.