Regeneration - Spent Hydroprocessing Catalysts - page 163

generated during oxidative regeneration are rather small. Moreover, a large volume of
oxidizing gas ensures further dilution of these toxic compounds. It is therefore believed that
their concentrations in atmosphere should never approach lethal limits, particularly in a
properly ventilated enclosures. Nevertheless, as the matter of awareness, a potential of HCN
and AsH 3 evolution is being indicated and it should not be overlooked completely. The
performance of some hydroprocessing catalysts may be improved by doping with fluoride, i.e.,
up to 3 wt.% on fluorine. So far, no attention has been paid to the potential release of the
fluorine containing compounds (e.g., HF) during oxidative regeneration of spent catalysts. It
was indicated earlier that some precautions may be necessary during all stages of handling
spent catalysts due to the presence of fluoride. Although much less toxic than fluorine,
phosphorus may also be present in spent catalysts. Other untypical species, which may be
released during regeneration, involve Ni carbonyls [384] . It is desirable that the potential
release of rather unusual species such as metal carbonyls as well as fluorine and phosphorus
containing species is investigated. Continuous efforts to improve performance of
hydroprocessing catalysts may result in the formation and release of other toxic compounds
during oxidative regeneration.

6.2.6 Other Oxidizing Agents

Copperthwaite et al. [420,421] demonstrated that ozone is rather efficient oxidizing agent of
coke at relatively low temperatures (150-190 ◦ C). Most likely, the oxidation may involve the
following set of reactions:

+ = +

C O 3 CO O 2

O 3 = O + O 2

O + C = CO

CO + O 3 = CO 2 + O 2

CO + 0.5O 2 = CO 2

During the reaction with coke, ozone reacted preferentially with aromatic carbon. At these
temperatures, the involvement of O 2 during the oxidation of coke may be limited to the
formation of surface complexes. A direct comparison of regeneration in ozone with the
conventional regeneration was made by Solovetskii et al. [422] . As the results in Fig. 6.28
show, the advantage of regeneration with ozone is quite evident. At the same time, the
radiation thermal treatment was the most efficient method. Presumably, ozone may be suitable
for regeneration of catalysts that are unstable at higher temperatures. However, a commercial