Page 38 - Principles of Catalyst Development
P. 38
24 CHAPTER 2
FLUID FLOW HIGH ACTIVITY
Flow distribution Chemical activity
Low pressure drop High specific
Mechanical active surface
strength Porous pellet
PELLET
LONG LIFE
STABILITY
Resistent to
Sintering
POisoning
Fouling
Figure 2.1. Engineering features of the catalyst.
catalyst for hydrotreating, CoMo/ AI 20], is found in a wide variety of shapes
and sizes. Pellets, extrudates, and spheres, 1-10 mm, are all available,
depending on whether the feedstock is light naphtha, gas oil, or vacuum
residua. (161
High activity and selectivity is engineered by selecting correct chemical
components, using preparational methods to give the required surface area,
and formulating the pellet to ensure good access to active sites. Achieving
activity alone is not sufficient unless balanced with these other requirements.
Lifetime stability requires resistance to deactivation due to sintering
(loss of active surface through crystallite growth), poisoning (elimination
of active sites by strong chemisorption), and fouling (blockage of the pores
and surface by carbon and debris). Although also sensitive to process
conditions, stability is best achieved through addition of catalyst com-
ponents.
An interesting example that demonstrates similar chemistry for different
process conditions is found in the reversible reaction
(2.1 )
Both forward and reverse reactions are catalyzed by the same material,
i.e., nickel.'171 However, the state of the nickel is profoundly influenced by