Page 231 - Principles of Catalyst Development

220 CHAPTER 8
As mentioned earlier, metal porphorins also adsorb on hydrotreating
catalysts, especially when using heavier residua fractions. The same prob-
lems develop, i.e., excessive coke and run-away regeneration. Pore-shape
control is practiced, as in Fig. S.24, for the same reasons. The larger
porphorin molecules are excluded to the outer surface of the particles.
Ultimately, metal buildup becomes so severe that the catalyst must be
replaced. Extraction methods for removing these deposits are available, but
as yet have not proven economical. It is interesting that discarded catalysts
may become a useful source of nickel and vanadium metal. The best
solutions appear to be external guards with cheaper adsorbents, or larger
hydrotreating beds, which allow for metal trapping at the top.

8.3.8.3. Dissociative Coking

Dissociative coking(CX') occurs when carbon monoxide dissociates on
a catalytic site, as follows:

2CO ~ C + CO 2 (S.12)

This reaction is found primarily in steam reforming of hydrocarbons, where
other coking possibilities also exist. With methane a dehydrogenation
reaction

(8.13)

occurs. Equilibrium constants for these are shown In Fig. S.26, together
with the third reaction

(8.14 )

Carbon formation is a major concern in methane and naphtha steam
reforming, but Fig. 8.26 shows that under reaction conditions only reaction
(8.13) is possible in the temperature range commonly used, 600-750°C.
Thermodynamics favors the reverse of reactions (S.12) and (8.14), and
carbon is removed, but the catalyst must have sufficient activity to counter-
balance coke deposition via reaction (8.13). As discussed earlier, this is
accomplished by promotion with potassium. Nevetheless, interesting inter-
actions occur, which may lead to reactor failure. Figure 8.27 illustrates the
case for a catalyst deactivated either by sulfur or carbon deposition. Below
the equilibrium line, we expect carbon formation. At the inlet of the reactor
tube, the temperature dips as the endothermic reaction absorbs heat. If it
drops below the equilibrium line, then coke forms and removal rates must