Page 225 - Principles of Catalyst Development

214 CHAPTER 8
of potassium. Other alkalis and alkaline earths function equally well, but
potassium is the most effective.
The other alternative is to remove the coke either at intervals or as it
forms. Reactions (S.lO) offer pathways, but in many processes, for example,
catalytic cracking, the reactions are not compatible with the process. In
these situations, the catalyst is regenerated at intervals with mixtures of air
and steam via reactions (5), (7), and (S) in (S.lO). Regeneration intervals
vary with rate of coke buildup and cover a wide range from minutes for
catalytic cracking, weeks for catalytic reforming, to months for hydrotreat-
ing. The usual procedures are to purge the bed with steam or nitrogen into
which air is admitted slowly enough to control exothermic temperature
increases. (2571
In catalytic cracking, regeneration is an integral part of the process
energy balance. The catalyst is fluidized and passes continuously through
reaction and regeneration stages. Cracking is endothermic and the necessary
heat is provided by combustion of coke in the regenerator. Coke level and
energy requirements are designed to match. Catalyst designers must beware
not to invent catalysts that are too coke resistant! Most heat is recovered
with complete combustion to carbon dioxide, but this raises regeneration
temperatures and causes catalyst sintering. For many years, coke was
removed by partial combustion and heat recovered in an afterburner (CO
oxidation). This was expensive and less efficient. More sinter-resistant
catalysts have been developed, but a better solution is to add minute (ppm)
amounts of platinum or other oxidation components to the cracking catalyst.
The level is low enough not to promote serious dehydrogenation coking,
but high enough to catalyze complete combustion at controllable
temperatures. (2hl i This practice is now becoming widespread.
Regeneration of catalysts in other processes is not so critical since
energy balance is not required. Combustion promoters are not necessary.
However, regeneration facilities must be provided and the plant shut down
for a certain period-both expensive. For less frequent regenerations, it is
advantageous to replace the catalyst with a fresh load and regenerate off-site.
A considerable industry is growing around these operations.

8.3.8.2. Dehydrogenation Coking

When dehydrogenation catalysts (metals, oxides, or sulfides) are pres-
ent, then the cascade of reactions leads to carbon in a form somewhat
different from that on acidic sites. Figure 8.22 shows the process whereby
dehydrogenation and associated hydrogenolysis lead to carbon fragments
C \. The carbon is reactive and exists as a surface carbide-type or as
pseudographite. With acid supports, these fragments migrate from the