Page 203 - Principles of Catalyst Development

192 CHAPTER 8
plugging, channeling, pressure drop increase, and irregular bed perform-
ance. This in turn leads to hot spots, with associated thermal and coking
effects. Thus particle failure initiates more serious deactivation by other
mechanisms.
Loss of mechanical strength is unlikely, unless assisted by thermal and
chemical effects. For example, an increase in the pressure of steam reforming
units produced a gradual loss of volatile silicates. Since pellets were
cemented together with calcium silicate type binders, silicate loss led to
weakening of the catalyst rings. The corrective action was a change in the
method of preparation and formulation. (49) Another example is found with
alumina-based pellets. Sulfate impurities that are not removed during pre-
paration react with aluminum under regeneration conditions to form
aluminum sulfate, weakening the pellet. This also occurs when heavy metals
deposit during residual oil desulfurization. (36) Regeneration in air at elevated
temperatures produces vanadium oxides which act as a flux to change
particle binding. Particle failure is usually obvious, but the contributing
factors may not be.

8.3.2. Fouling
In this sense, fouling means deposition of reactor debris on the particles.
Scale, rust, and other corrosion products are all possibilities, in addition
to chemical components from up-stream units. Particles removed from
reactors often have red-brown iron oxide crusts on the outside. Calcium
compounds are also found. The most severe cases occur in processing coal
and coal-derived liquids, which contain large amounts of inorganic mineral
matter.
At best, these materials clog the outside of particles, plugging pores
and blocking active surfaces. At the worst, particles are cemented together,
resulting in loss of void space within the reactor and complicating catalyst
removal. Once again, the analytical procedures discussed in Chapter 7 are
helpful to indicate the presence of this type of deactivation. There is no
cure, the catalyst can only be replaced, and care exercised to avoid future
contamination by the use of screens and guards.

8.3.3. Component Volatization

Elevated temperatures can result in loss of active components or pro-
moters through vaporization. Loss of activity and promoter function follows,
with added complications from corrosive deposition on downsteam plant
equipment. Events of this type are easily detected in laboratory operations,
so we must assume that, in process use, they are only found with unexpected
upsets or on a long-term basis.