Page 232 - Introduction to chemical reaction engineering and kinetics

214 Chapter 8: Catalysis and Catalytic Reactions

and the rate law, from equations 8.5-46 and -49, may be written as

(-rA) = (l/k,, &kA) (8550)

Special forms of equation 8.5-50 arise depending on the relative importance of mass
transfer, pore diffusion, and surface reaction; in such cases, one or two of the three may
be the rate-controlling step or steps. These cases are explored in problem 8-18. The
result given there for problem 8-18(a) is derived in the following example.

If the surface reaction is rate controlling, what is the form of the rate law from equation
8.5-50, and what does this mean for kAg, cA$, r], and qO?

SOLUTION

If the surface reaction is the rate-controlling step, any effects of external mass transfer
and pore-diffusion are negligible in comparison. The interpretation of this, in terms of the
various parameters, is that kAg >> kA, cAs + c/Q,, and 7) and no both approach the value
of 1. Thus, the rate law, from equation 8.5-50, is just that for a homogeneous gas-phase
reaction:

(-rA) = kAC& (8.5-51)

The concentration profile for reactant A in this case iS a horizontal line at CA = cAg; this
can be visualized from Figure 8.9.

8.6 CATALYST DEACTIVATION AND REGENERATION
Despite advances in catalyst design, all catalysts are subject to a reduction in activ-
ity with time (deactivation). The rate at which the catalyst is deactivated may be very
fast, such as for hydrocarbon-cracking catalysts, or may be very slow, such as for pro-
moted iron catalysts used for ammonia synthesis, which may remain on-stream for sev-
eral years without appreciable loss of activity. Nonetheless, the design engineer must
account for the inevitable loss of catalyst activity, allowing for either regeneration of
the catalyst or its periodic replacement. Since these remedial steps are costly, both in
terms of capital cost and loss of production during shutdown, it is preferable to min-
imize catalyst deactivation if possible. In this section, we explore the processes which
cause deactivation, and how deactivation can affect the performance of a catalyst. We
also discuss methods for preventing deactivation, and for regeneration of deactivated
catalysts.

8.6.1 Fouling

Fouling occurs when materials present in the reactor (reactants, or products, or in-
termediates) are deposited upon the surface of the catalyst, blocking active sites. The
most common form of fouling is by carbonaceous species, a process known as “coking.”
Coke may be deposited in several forms, including laminar graphite, high-molecular-
weight polycyclic aromatics (tars), polymer aggregates, and metal carbides. The form
of the coke depends upon the catalyst, the temperature, and the partial pressure of the