Regeneration - Spent Hydroprocessing Catalysts - page 145

contribution from the conversion of metal sulfides to corresponding oxides. For sulfided
catalyst, the plot obtained from results determined from the yields of products in the fixed bed
reactor exhibited a curvature at low temperature. This resulted from a higher rate of oxidation
at lower temperature. However, in view of the presence of highly reactive components in coke,
the higher rate of coke oxidation should be expected. This would involve hydrogen containing
entities (e.g., alkyl substituents), which can be identified in Figs 4.20 and 4.21 . In other words,
the structure of coke oxidized at lower temperatures was not the same as that of coke oxidized
at high temperatures. Therefore, a continuous change in the structure of coke and its reactivity
associated with it, is another parameter deserving attention during determination of the
activation energies during the oxidative regeneration of spent catalysts. In the study of
Alwarez et al. [46] , the activation energy of about 157 kJ/mol was obtained. However, in this
case, the rate constants were estimated at later stages of burn-off. Most likely, under such
conditions, the rate constants determination was affected by diffusion.
6.2.2.2 Diffusion Controlled Kinetics
The particle size and porosity of spent catalysts are among parameters controlling kinetics in
the diffusion controlled region during regeneration. However, parameters that influence
chemically controlled burn-off are important as well. This is supported by the burn-off profiles
in Figs 6.11 and 6.13 [374] obtained for the commercial catalyst, which was crushed to obtain
particle size of 100-200mesh. At 350 ◦ C, the entire burn-off appeared to be chemically
controlled. Thus, a low burn-off rate ensures that O 2 can diffuse into particle interior before it
is consumed near the particle exterior. In this case, a uniform burn of coke across the particle is
expected. To a lesser extent, the same was true for burn-off at 450 ◦ C, although the
involvement of diffusion effects becomes more evident.
According to Fig. 6.11 [374] , at the latter stages of burn-off at 500 ◦ C, the rate of chemical
reactions is much higher than that of the diffusion of O 2 into the catalyst particle interior.
Then, two regions, i.e., the one chemically controlled and the other diffusion controlled, are
clearly present. In the former region occurring at 500 ◦ C, the coke, which is readily accessible
to O 2 , undergoes rapid combustion. Gradually, the rate is slowing down before the coke
combustion region, which is dominated by diffusion, is attained. Under such conditions, a
coke-catalyst interface ( Fig. 6.15 ) [13] is being developed in the interior of catalyst particles.
Such interface cannot develop under chemically controlled conditions ensuring an even radial
burn (e.g., at 350 ◦ Cin Fig. 6.11 ). A rapid reaction of coke with O 2 does not allow the latter to
diffuse further into particle interior before it is consumed. Equations (6.4) and (6.7) were
derived above to describe kinetics in the high rate burn-off region.
It should be noted that the events described above are applicable to spent catalysts containing a
small amount of coke. For example, for fluid catalytic cracking (FCC) catalysts, an even radial
distribution of coke was observed at less than 6 wt.% of the coke [397,398] . However, particle