Page 184 - Principles of Catalyst Development

172 CHAPTER 7
Activities fall naturally into two types: (1) kinetic and (2) practical.
Kinetic expressions of various kinds are essential in research and process
reactor design. Rate equations and associated parameters direct the chemist
toward a better understanding of mechanisms, from which improved
catalysts are designed, and the engineer toward models, from which proces-
ses are optimized. Considerable care and accuracy must go into measure-
ment of these quantities. Practical activities have limited purposes, for
example screening of catalyst candidates, optimization of preparational
parameters, determination of process parameters, deactivation studies, diag-
nosis of malfunctioning catalysts, and product quality checks on new catalyst
charges. Usually, relative results are sufficient, with speed and productivity
the dominant factors.

7.5.1. Kinetic Activities
Kinetic activities are based on the prevailing rate equation

(7.18 )

with

where

1 dC .. ,
----
V dl
is the rate per unit volume of either reactor, V" fluid, V" or catalyst \1;.; k,
is a rate constant on a vol ume basis; I( C A ) is a rate equation concentration
term, which may be simple order or complex with adsorption terms; kv.o is
a preexponential term; and E is the activation energy. Equation (7.18)
suggests three ways to express activity: as a rate, a rate constant, or an
Arrhenius constant.

7.5.1.1. Reaction Rates

This is the most direct and specific. In catalysis, it is usual to express
rate in terms of catalyst volume, \1;, but alternate forms are

1 de", . . .
-- - -- -moles per Ul1It tIme per Ul1lt mass of catalyst (7.19)
'fi
W dt ( specl crate )