Environmental and Safety Aspects of Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 110

Figure 5.5: Isothermal oxidation of spent catalyst particles [From ref. 332 . Reprinted with
permission].
The results in Fig. 5.5 [332] were obtained in air rather than in 2 vol.% O 2 as it is the case of
Fig. 5.4 [331] . In contrast to Fig. 5.4 , these experiments were conducted isothermally at
indicated temperatures. Prior to the experiments, the spent NiMo/Al 2 O 3 catalyst was extracted
by THF followed by pumping at 200 ◦ C overnight. After approaching a steady state, the flow
of air was replaced by the flow of N 2 ( Fig. 5.5a ). In this case, a slight decline in the weight at
200 and 250 ◦ C indicated the beginning of the decomposition of oxygen-containing surface
complexes. No weight loss was observed during the continuous oxidation ( Fig. 5.5b ). Under
these conditions, a decomposition of the complexes was more than offset by the formation of
additional complexes. A rapid weight loss at 300 ◦ C indicated the ignition of catalyst particles
aided either by the most volatile component of coke or by the oxidation of metal sulfides.
This is supported by a slight weight gain during the later stages of oxidation shown in
Fig. 5.5c .
With regard to the exposure of spent catalysts to air, some important conclusions may be
drawn from the results in Figs 5.4 and 5.5 [331,332] . First of all, it is a continuous
chemisorption of O 2 , which can be accelerated with increasing temperature, that deserves
attention. At sufficient concentration of the oxygen-containing complexes, the catalyst
particles may ignite, particularly in proximity of hot objects. Also, some activities, e.g.,
welding, cutting, etc., in a proximity of the storage of spent catalysts may increase the