Environmental Engineering Reference
In-Depth Information
Table 7.2: Surface area, pore volume, and HDS activity of fresh, spent, and rejuvenated catalysts
[From ref.
475
. Reprinted with permission].
Catalyst
Agent
Surface area
(m
2
/g)
Pore volume
(mL/g)
HDS activity
%
% Recovery
Fresh
240
0
.
40
61
100
Spent
52
0
.
12
17
28
Method A
Oxalic
63
0
.
15
22
34
Method A
Oxalic
+
H
2
O
2
141
0
.
27
52
85
Method B
Oxalic
+
H
2
O
2
181
0
.
43
59
97
Method C
Oxalic
+
H
2
O
2
197
0
.
46
36
59
Method C
Oxalic
180
0
.
42
37
61
Method A: leached but not decoked; method B: leached and decoked; method C: decoked and leached.
HDS: hydrodesulfurization.
the HDS activity approached 85% of the activity of the fresh catalyst in spite of the coke still
being present on the catalyst. On the basis of the extensive testing program, which has been
conducted at KISR, one may conclude that the catalyst rejuvenation using organic agents is
approaching a commercial stage
[475,476,480-485]
. As the consequence, the rejuvenated
catalyst may be cascaded for less severe applications and/or at least used as the solid for guard
beds. In both cases, economics are a decisive factor.
Apparently, leaching with oxalic acid can be further optimized by varying concentration of the
acid and temperature.
Figure 7.4 [480]
shows that at 25
◦
C, increasing the concentration of
oxalic acid from 0.33 to 1.0M had little incremental effect on leaching of V. The 0.33M
concentration represents about 3 wt.% solution, whereas in the study of Beuther and Flinn
[479]
1 wt.% solution was successfully tested. The wide range of concentration suitable for
leaching offers flexibility when optimizing other leaching parameters. For example, at
concentration of 0.66M, increasing temperature from 25 to 50
◦
C about doubled the amount of
V leached out (
Fig. 7.5
)
[480]
.
McVicker et al.
[486]
reported that buffering of oxalic acid solution at varying concentrations
enhanced significantly selectivity for leaching contaminants metals. At the same time, with
buffered solution, little removal of catalytically active metals from catalyst was observed in
spite of long contact times of oxalic acid solution with contaminated catalyst even at elevated
extraction temperatures.
Several combinations of the organic acids with different agents were also evaluated. In
agreement with results in
Table 7.2
, the results in
Table 7.3 [475]
show that the leaching
efficiency can be enhanced in the presence of H
2
O
2
. This was attributed to the oxidation of the
