Environmental Engineering Reference
In-Depth Information
Figure 7.12: Vanadium leached versus molybdenum leached for coked and decoked catalysts with
oxalic acid
+
hydrogen peroxide reagent [From ref.
484
. Reprinted with permission].
for V removal was achieved when malonic acid was combined with Al(NO
3
)
3
than when the
same oxidizing agent was added to oxalic acid
[483]
.
7.1.2 Kinetics of Rejuvenation
It is postulated that in the case of organic acids, leaching process comprises both complexation
of metals in solution released by catalyst dissolution and liquid-solid reaction. As part of the
latter reaction, metals are released from the solid to the solution with the aid of organic agents
such as depicted in Reactions
{7.6}
,
{7.7}
, and
{7.8}
. It is believed that the complexation in
the solution proceeds at a much higher rate than the liquid-solid reaction. Also, the rate of
complexation in solution is much greater than the rate of metal dissolution in water. It is
however believed that the liquid-solid reactions in which metals are released from the solid to
the solution with the aid of organic agents dominate rejuvenation process. In this case,
diffusion phenomena are expected to play an important role. This is supported by the shape of
leaching curves (
Fig. 7.10
)
[490]
indicating a faster leaching during the initial stages, followed
by a gradual slow down indicating a growing involvement of diffusion in overall reaction.
Thus, the early stages of leaching involve the chemically controlled removal of metals from
the external surfaces of particles with only a limited involvement of diffusion.
Kinetic treatment of different leaching systems developed by Levenspiel
[491]
was adapted by
Marafi et al.
[480]
to study kinetics of the rejuvenation process. Among several models,
progressive conversion or shrinking particle model and shrinking core (or constant particle
size) model are relevant to leaching processes employed during rejuvenation. Rate equations
for two cases, i.e., particle constant size and particle changing size are summarized below.
7.1.2.1 Particle Constant Size
Liquid phase diffusion controls
t
τ
X
B
(7.1)
