Page 204 - Principles of Catalyst Development

CATALYST DEACTIVATION 193
An example is found in hydrotreating with molybdenum-containing
catalysts. During regeneration, coke is burned off with air. Although precau-
tions are taken to avoid reactor hot spots, these sometimes occur. Molybdena
volatizes above 800°C, so that yellow crystals deposit downstream, and the
catalyst particles turn white. Activity is irreversibly lost. Another case is
found with nickel methanation catalysts. If the catalyst bed cools below
150°C in the presence of carbon monoxide-containing gas, nickel carbonyl
vaporizes from the particle, creating a very toxic hazard as well as loss of
nickel.
Long-term volatization is also found in steam reforming of naphtha.
Coke formation from heavier hydrocarbons is controlled with potassium,
which promotes carbon-steam reactions. In the presence of steam, however,
potassium slowly forms KOH which volatilizes, resulting in accelerated
coke formation. The solution is an ingenious example of the catalyst desig-
ners expertise. Less than 10% kalsilite (K 20, A1 20 3 , Si0 2 ) is included in
the catalyst formulation. In the presence of steam and CO 2 , kalsilite slowly
decomposes to K 2CO) and KOH, always providing enough alkali to remove
carbon, even though it eventually vaporizes. (49) Lifetimes of 4-5 years have
been achieved with these catalysts.

8.3.4. Phase Change
All components in the catalyst must be maintained in their most active
state. Phase changes are thermal phenomena and are distinguished from
compound formation. High surface area aluminas, for example, change to
low area phases when heated. Alumina-based catalysts are often regenerated
by burning the carbon. Over a period of time, the surface area declines and
activity drops, due to thermal sintering during regeneration. This is one
reason why the activity does not return to initial conditions, as shown in
Fig. 8.2. This phase change is controlled by adding silica as a promoter.
Segregation of components is also considered a phase change. Highly
dispersed alloys are notorious in their tendency to form nonhomogeneous
crystallites, with more volatile components diffusing to the surfaces. This
is found many times with copper-nickel alloys, confusing much of the early
work in metal catalysts. (262) There is also evidence that rhenium and iridium
promoters of platinum in catalytic reforming catalysts undergo some degree
of segregation upon regeneration. (II)

8.3.5. Compound Formation

Compounds form between components and the reactive atmospheres,
rendering the catalyst less active. Elevated temperatun:s accelerate the