Page 227 - Principles of Catalyst Development

216 CHAPTER 8
model components have given a better understanding of this effect, as shown
with data in Table 8.9. (246) Initially most of the product comes from hydro-
genolysis. A small amount of poison decreases hydrogenolysis but not acid
cracking, as measured by the concentration of C 1 hydrocarbons. Finally,
all hydrogenolysis is suppressed but with some loss of activity. Yields of
isomers, however, increase, as seen in Figure 8.17.
In spite of these improvements, deactivation in catalytic reforming was
severe and regeneration was required every few days. This was accomplished
with extra "swing" reactors or moving bed designs, both expensive to build
and operate. Another solution was to increase pressure from about 15 to
35 atm. Higher hydrogen pressure reduces hydrogenolysis and accelerates
hydrogenation of the carbon. Processes then ran three to six months before
regeneration. However, aromatic yield from the dehydrogenation of naph-
thenes and cyclization of n-paraffins decreased and octane number suffered.
A breakthrough came with the discovery of bimetallic reforming
catalystsY 1) When platinum is combined with rhenium, the stability
improvement is dramatic. Rhenium appears to inhibit both coking and
sintering, so that deactivation during use and the frequency of regeneration
is decreased. Use of iridium was a further advance, giving not only prolonged
life of over ten years but higher activity. Comparisons are shown in Fig.
8.23. Not only was operation easier but plant configuration was simplified
with large savings in capital cost. Problems exist with bimetallics, however:
which tend to segregate during regeneration, requiring more complicated
treatment. But that seems a small price to pay.

TABLE 8.9. H 2S Poisoning of Ni-LiY Effect on Product
Distribution n-C 6 Hydrogenolysis and Isomerization"

Moles of product
Fraction of Ni poisoned with H 2S
Product 0 0.05 0.10 0.15 0.20
-------
C, 56.5 32.8
C 2
C, 0.89 2.10 0.64 0.25 0.20
iC. (LOS 0.23
/lC. 1.63 2.69
ie, 0.24 0.53
/lC, 2.33 3.43
2MP 0.62 1.66 3.43 2.42 1.01
3MP 0.32 1.42 0.84 0.66 0.49
/le" 85.5 91.2 95.4 96.7

u Reference 246.