Page 186 - Principles of Catalyst Development

174 CHAPTER 7
TABLE 7.14. Rate Constants for First-Order Kinetics

Rate constant Unit parameter Form Units

surface k, em' 5- 1
mass of catalyst k,Sg em' g - 1 S 1
volume of catalyst k,Sgdl'
volume of reactor k,Sgdp(l - E)
volume of fluid k,Sgdl' (I - e) IE

(7.21)

The activities of series of catalysts are represented with kOb' values. Values
outside the ranges of concentrations and temperatures are often found by
extrapolation. This is a very dubious procedure. Equation (7.21) is in reality
an approximation of a Langmuir-Hinshelwood expression of the type

(7.22)

The exponents nand m depend on the values of KAPA and KBPB, which
in turn are temperature dependent. The rate constant so derived is

(7.23 )

where nand m are concentration and temperature dependent. Expressions
such as equation (7.21) should only be used over the limited range in which
measurements were made. Comparisons from one catalyst to another are
very uncertain.

7.5./.3. Arrhenius Parameters
On occasion, the parameters, K",o and E, are used to specify activity.
This method is only used when it is certain that one or the other is constant,
in which case, why not use the complete rate constant? A constant value
of K"o or E cannot be assumed since there are many cases in catalysis of
compensation effects, where K,,,o increases as E decreases or vice versa.(251)

7.5.2. Experimental Reactors
It is now appropriate to introduce the topic of experimental reactors
used to measure kinetic rate data. Much has been written on this subject
and many reviews are available.'Xh,252.2S1,254) Here we shall consider only