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3.1 Introduction to Heterogeneous Processes 67

C

R δ
Figure 3.2 Two-phase system.

This is the rate of disappearance of the reactant A and
S the total surface of the catalyst, m 2
k the rate coef m icient, f 3 /m 2 s
s
C the concentration of reactant at the outer surface of the catalyst particle,
s
mol/m 3 .
The rate of mass transfer from the bulk of the gas to the catalyst surface is:
r k C ( C ) (3.31)
g g G s
where:
k the mass transfer coeficient in the gas f m/s ilm, f
g
C the concentration of reactant in the bulk gas phase, mol/m 3 .
G
At steady state,
1d N

( r ) r kC k C ( C ) (3.32)
s g s s s g G s
S d t
From the equality of the rates, the concentration of reactant at the outer surface of the cat-
alyst particle can be expressed in terms of bulk concentration:

k g
C s C G (3.33)
k s s k
g
Then, the oerall reaction rate ( v r ov ) can be also expressed in terms of the bulk concentra-
tion of the reactant:

k g
k C
r ov r ( s r ) g s s k C G ov G (3.34)
k s s k
g
where k ov v f erall coef is the oicient in m/s.