Page 23 - Principles of Catalyst Development
P. 23
CATALYTIC FUNCTIONS 9
TABLE 1.2. Common Catalyst Particles
Type Characteristics
Pellets Made in high-pressure press
Shape: cylindrical, very uniform, rings
Size: 2-10 mm diam
Use: packed, tubular reactors
Extrudates Squeezed through holes
Shape: irregular lengths
circular, star or lobe cross section
Use: packed, tubular reactors,
ebulating beds
Spheres Made by aging liquid drops
Size: 1-20 mm
Use: packed tubular reactors,
moving beds
Granules Fusing and crushing, particle granulation
Size: 8-14 to 2-4 mesh
Use: packed tubular reactors
Flakes Powder encapsulated in wax
Use: liquid phase reactors
Powders Spray-dried hydrogels
Size: <100""m
Use: fluidized reactors,
slurry reactors
dramatic result. It would appear that the larger the particles the better. This
is true if diffusion is not a problem. If it is a problem, larger particles lead
to lower conversions,(21) so some compromise is necessary.
Another parameter of concern to the process designer is the mechanical
crushing strength of the particle. If the particle fractures under the weight
of the bed or the force of the fluid passing through it, then small fines lodge
in interstices between larger particles, causing plugging, uneven flow, hot
spots, and pressure drop. Fortunately, crushing strength is fairly indepen-
dent of particle size.(22) Other factors during preparation and formulation
are, however, critical. These are treated in Chapter 6.
We now turn to the question of texture within the particle, i.e., surface
area, pore shape, and size distribution. Particles are formulated by
agglomerating microparticles produced during a precipitation phase, as
shown in Fig. 1.4. Approximately 100 Il-m in size, these microparticles consist
of a complex porous solid. Pores typically range from 1.5 to 15 nm in radius.
Formerly termed micropores, these channels are now called mesopores.
The name micropores is reserved for those less than 1.5 nm in radius, usually
found in zeolites.