Regeneration - Spent Hydroprocessing Catalysts - page 169

coke removal should be noted, whereas the rate of oxidation with ozone (curve 2) was between
that observed for the conventional burn-off method and radiation thermal treatment method.
Under electron beam conditions, carbon-containing compounds are completely removed
within 10-15min. However, a careful control of the experimental conditions is necessary to
avoid losses of Mo due to sublimation. At the same time, in the case of the spent NiMo/Al 2 O 3
catalyst, the content of Ni promoter was not affected. There is little information suggesting
that the radiation thermal treatment has been used for catalyst regeneration on and/or near a
commercial scale.

6.3.5 Reductive Regeneration

In an ideal case, catalyst could be regenerated at the end of the operation by discontinuing the
feed supply while continuing the H 2 supply. In spite of this, there is little information on the
reductive regeneration of catalysts used for hydroprocessing of any petroleum feed. This may
result from the fact that a desirable level of coke removal could not be achieved even at much
higher temperatures than those applied during the oxidative regeneration [13] . For example,
during the reductive regeneration of the NiMo/Al 2 O 3 catalyst used for hydroprocessing of a
VGO, the evolution of the most of CH 4 formed via hydrogasification of coke required
temperature of more than 1000 K. As the result of this, sintering of the MoS 2 particles was
observed in addition to other structural changes as indicated by Scheffer et al. [436] . Almost
certainly, hydrogasification of coke catalysed by active metals was the source of the small CH 4
maximum at about 800 K.

It was noted that the reductive regeneration of Pt, Re, and Ir catalysts supported on -Al 2 O 3

proceeded at lower temperatures and at a much greater rate [437,438] than that of the
NiMo/Al 2 O 3 catalyst [13] . For the former catalysts, the maximum of CH 4 evolution indicating
coke removal occurred at about 850 K. This is not unexpected considering a high
hydrogenation activity of the noble metals containing catalysts, generally observed. In refinery
practice, noble metals (e.g., Pt and Pd) are part of the catalysts used for dewaxing of gas oil
fractions for the preparation of lube base oil. In this case, reductive regeneration has much
more potential compared with conventional hydroprocessing catalysts. In spite of its potential,
the reductive regeneration of dewaxing catalysts has not attracted much attention.

A brief comparison of the reductive regeneration with the oxidative regeneration was made by
Noguchi et al. [379] . The spent catalyst was from the hydroprocessing of the Arabian heavy
vacuum residue conducted at 713 K and 8.5MPa. The comparison revealed that the recovery
of surface area ( Fig. 6.31 ) and activity during the reductive regeneration was higher than that
during the oxidative regeneration in spite of about 3 wt.% of coke still left behind after
reductive regeneration compared with almost complete coke removal during the oxidative
regeneration. However, the latter was conducted in air at 923 K, whereas the reductive
regeneration at 713 K. Apparently, at 713 K, part of the coke in the vicinity of active metals