Page 33 - Principles of Catalyst Development
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CATALYTIC FUNCTIONS 19
1.5. DIAGNOSIS OF CATALYTIC FUNCTION
In order to understand and modify the functions of a catalyst in a
process, it is necessary to determine whether or not rates are determined
by physical or chemical steps. Responses to process parameters and catalyst
adjustments are different for the two regimes. Diffusional resistance, in
particular, causes unexpected complications. We have seen how low
effectiveness factors decrease conversion and disguise kinetics, but selec-
tivity also can be decreased.(32) In addition, poisoning of pore mouth sites
in conjunction with low diffusion results in a much more rapid activity
decline than otherwise.
Problems with diffusion-controlled processes are eased by changing
catalyst properties such as pore size distribution and particle size or process
conditions such as linear velocity. Chemical difficulties are treated through
modification of the active components of the catalyst.
The remainder of this chapter deals with determining the dominant
regime, physical or chemical, prevailing under a given set of conditions.
1.5.1. Interactions between Regimes
Figure 1.8 was designed to demonstrate interactions between physical
and chemical rates and to serve as a "first pass" for a process diagnosis. It
is calculated for vapor-phase conditions in the temperature range 300-500°C,
molecular masses from 2 to 500, and "average" values of D eff • As a teaching
tool it serves its purpose without further justification; as a diagnostic device
it is useful only within these approximations. However, errors due to
deviations from these assumptions are not large considering the range of
values. An order of magnitude error in the regime of strong internal diffusion
is not serious if the objective is to decide whether further checking is
necessary.
Procedures outlined in previous sections were used to compute the
curves in Fig. 1.8. Three variables are shown. The abscissa is the chemical
rate applicable if all the surface is available. It is the chemical rate built
into the catalyst. The ordinate represents values of an index that combines
all diffusional tendencies of the particle. Moving in the direction of more
diffusion control decreases this index. For example, higher porosity
increases Deff (providing it is not accomplished by decreasing the pore
radius), increasing total pressure decreases DB (and thus D eff ), and increas-
ing the pelIet diameter, D p , lowers both the effectiveness factor and mass
transfer coefficient. Parameters on the curves are pellet rates. The ratio
between the pellet rate and the intrinsic chemical rate is the effectiveness
factor.