Environmental Engineering Reference
In-Depth Information
to the formation of H
2
SO
4
and finally to Al
2
(SO
4
)
3
involving the following set of tentative
reactions:
V
2
O
5
+
2SO
2
=
V
2
O
3
+
SO
3
SO
3
+
H
2
O
=
H
2
SO
4
Al
2
O
3
+
H
2
SO
4
=
Al
2
(SO
4
)
3
The formation of Al
2
(SO
4
)
3
was evident particularly on the exterior of catalyst particles. This
is in the agreement with the preferential accumulation of V on the exterior of the spent catalyst
particles generally observed. Similar observation was made by Yoshimura et al.
[383]
.
It has been reported that regeneration of the spent CoMo/Al
2
O
3
catalyst proceeded more
readily than that of the spent NiMo/Al
2
O
3
catalysts
[384]
. This was attributed to the formation
of Ni carbonyl. If so, Ni carbonyl could arise from the secondary reaction involving CO and
Ni, i.e.:
+
=
Ni
x
CO
Ni(CO)
x
An additional study is necessary to confirm the occurrence of this reaction during the oxidative
regeneration of spent hydroprocessing catalysts.
Using a set of thermodynamic data, Yoshimura et al.
[385]
concluded that the oxidation
tendency of Mo sulfide was much greater than that of Co sulfides. The latter were oxidized to
both CoSO
4
and Co oxides, in contrast to MoS
2
, which was only oxidized to Mo oxides. The
decomposition of CoSO
4
below 973 K had little probability to occur. Therefore, this portion of
Co became unavailable for the formation of the Co-Mo-S phase. After repeated
utilization-regeneration cycles, the loss of Co to CoSO
4
formation may be one of the reasons
for the decline in catalyst activity. In this regard, experimental evidence was provided by
Mahadjev et al.
[386]
. The information on behavior of Ni under similar conditions as used by
Yoshimura and Furimsky
[387]
indicated the formation of NiSO
4
, which could not be removed
completely during the subsequent catalyst resulfidation. It is therefore desirable that during
regeneration the conditions should be established under which the formation of the sulfates of
promoting metals is minimized.
An important contribution to the understanding of the mechanism of oxidative regeneration
was made by Yoshimura and Furimsky
[387]
, who studied the oxidation of the sulfided
Co/Al
2
O
3
, Mo/Al
2
O
3
, and CoMo/Al
2
O
3
catalysts, using varying concentrations of O
2
in the
oxidation mixture. For the Mo/Al
2
O
3
catalyst, a significant decrease in the amount of Mo
4+
was observed after oxidation below 533 K. However, most of the Mo still remained in a sulfidic
form. At 653 K, most of the Mo
4+
was converted to Mo
6+
. At the same time, the amount of
