Page 25 - Principles of Catalyst Development
P. 25
CATALYTIC FUNCTIONS 11
TABLE 1.3. Texture Parameters
Parameter Nomenclature Units
Surface area Sg m 2 g- 1
Pore volume Vg cm] g-I
Pellet density d p g(cm 3 of pellet)-'
Porosity 8 = V.dl'
Average pore radius r, = (2 V.I Sg) x 10 3 nm
Pore size distribution PSD
During formulation, microparticles are compressed, and grain boun-
daries flow together to produce rigid, strong particles. Interstices between
the microparticles create a macropore structure with radii greater than
1.5 nm. Measurement of pore size distribution shows typical bimodal shapes.
Most of the surface area is found in the mesopores, with the combined
meso- and macropores amounting to approximately 95% of the total area.
Important terms and units used to characterize texture are given in Table
1.3. Measurement of these parameters is discussed in detail in Chapter 7.
1.4. STEPS IN THE CATALYTIC REACTION
The catalytic mechanism extends beyond the surface to involve phy-
sical diffusion to and inside the particle. Combining these leads to the steps
shown in Fig. 1.5.
1.4.4. External Diffusion
First the reacting molecule, A, diffuses to the external surface of the
particle. Motion of A through the fluid outside the particle is governed by
external or bulk diffusion. The reader should consult standard references
for additional discussion.(2l) Useful correlations have been found between
the mass transfer factor, jD, and the dimensionless particle Reynolds
number:
50 < N Re < 1000 ( 1.7)
where N Re = (2R)G/ /-t, R is the particle radius (em), G is the linear mass
velocity (g S-I), and /-t is the viscosity (poise).
The mass transfer factor is given by
. k gP ( /-t )2/3
JD =- -- (1.8)
G pDB