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–3–

Heterogeneous Processes and Reactor

Analysis

3.1 INTRODUCTION TO HETEROGENEOUS PROCESSES

v
In the releant literature, many definitions of reaction rates can be found, especially in the
case of catalytic systems. Depending on the approach followed, a catalytic reaction rate can
be based on catalyst volume, surf or mass. Moreo in practical applications, rates are ace, , v er
often expressed per volume of reactor. Each definition leads to different manipulations and
special attention is required when switching from one expression to another. In the follow-
ing, the v arious forms of catalytic reaction rates and their connection is going to be presented.
Starting from the fundamental rate defined per active site, the reader is tak en step –by step
to the rate based on the volume of the reactor and the concept of the overall rate in two- and
three-phase systems.
The analysis in this chapter mainly concerns catalytic reactions. However, the basic prin-
ciples are applicable to an though with dif y heterogeneous process, ferent terminology and
levels of importance. Concerning adsorption and ion exchange, only the reaction rate per unit
mass of solid phase ( r ) and per unit volume of reactor ( m R ) are used in practice, whereas the
concepts analyzed in the overall rate and rate-controlling sections are equally applicable to
ion exchange and adsorption.

3.1.1 Reaction rate in heterogeneous catalysis: from active sites to
reactor level

vel Fundamental—Active site le
As mentioned in Chapter 2, a catalytic reaction is not catalyzed oer the entire surface of v
the catalyst but only at certain actie sites (F 1999). Then, the reaction rate of any
v
ogler
,
reaction component i at a fundamental leel for catalytic reactions can be defined with
v
ws: respect to actie sites as follo v
1d N
r (3.1)
t
n t d
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