Page 26 - Principles of Catalyst Development
P. 26
12 CHAPTER 1
A~
I. E~ternal diffusion
~
~ k = k
ext. dill g Sext
¥ 2 Internal diffusion Int "I
A
"
k
= k S
•
tnt. dlf f.
A
'" A 3 Adsorption
, ) ) I //)} / I k
odo
A 8 4 Surfoce reaction
1- 7
://////' 0
) k
+ 5 Desorption of B
6 Interna I diffusion of B
7. External diffusion of B
Figure 1.5. Steps in the catalytic reactions,
where kg is the mass transfer coefficient (cm s -I), P is the fluid density
(g cm- ), and DB is the bulk diffusivity of A in the fluid (cm 2 S-I).
3
This correlation gives kg with a fair degree of accuracy. Diffusion to
the surface is a first-order rate function with a rate constant
( 1.9)
where Sex, is the geometrical external surface of the particle per unit particle
volume. Equation (1.9) is strictly only valid if the reaction at the surface is
very rapid, but it is close enough for most estimates.
Equation (1.7) is good only for particle Reynolds numbers above 50.
This suffices for most industrial reactors. Problems arise with laboratory