Environmental Engineering Reference
In-Depth Information
Catalysts prepared in accordance with the above steps contained 1-3 wt.% of V, 1-3 wt.% of
Ni, 2-8 wt.% of Mo and 1-7 wt.% of W and had a specific surface area of 100-150 m
2
/g and
pore sizes of 120-180 A. The catalysts were effective for reduction of nitrogen oxides, i.e.,
90% or more. In this catalyst, vanadium which facilitates the oxidation of sulfur dioxide to
sulfur trioxide was present at a low amount. This was favorable for selective reduction of
nitrogen oxides contained in an exhausted gas together with large amounts of sulfur
components at high temperature in the presence of ammonia as a reducing agent.
It was reported that the spent-decoked CoMo/Al
2
O
3
catalyst was active during the reduction of
sulfur oxides to elemental sulfur
[547]
. However, no details on the type of chemical process
employed were given in this brief information. However, Zulfuganov et al.
[548]
reported that
in order to achieve an efficient conversion of sulfur oxides to elemental sulfur, the oxidative
regeneration of the spent CoMo/Al
2
O
3
at 500
◦
C was necessary.
The spent-decoked CoMo/Al
2
O
3
and NiMo/Al
2
O
3
catalysts were used for the direct
decomposition of H
2
S in the mixture with either H
2
or CH
4
[549]
. The decomposition to H
2
and elemental sulfur was significantly enhanced in the presence of the spent catalysts
compared with little decomposition observed over
-Al
2
O
3
. Presulfiding the catalysts did not
influence the conversion of H
2
S. In every test, the CoMo/Al
2
O
3
catalyst was more active than
NiMo/Al
2
O
3
catalyst. The H
2
S aided reduction of CO
2
was catalyzed by these catalysts as
well. However, the spent-decoked catalysts favored the formation of COS, whereas CO was
the main product over the corresponding fresh catalysts.
It is believed that the utilization of the spent hydroprocessing catalysts in non-petroleum
application has not been fully explored. For example, these spent catalysts can still possess
adequate activity for various gas-solid applications. Various types of the Mo- and
W-containing catalysts have been used commercially. The presence of V suggests that after an
adequate pretreatment the spent catalysts may be suitable for various oxidation reactions,
whereas the presence of Ni offers a possibility of utilizing such catalysts in hydrogenation
reactions.
9.3 Gas Treatment Sorbents
The presence of various transition metals (e.g., Ni, Co, Mo, W, Fe, V, etc.) indicates the
potential of spent hydroprocessing catalysts for preparation of sorbents for removing H
2
S from
gaseous streams. Such gases are produced during gasification of carbonaceous solids,
carbonization of coal, coking of heavy petroleum feeds. In addition, the H
2
S containing gases
are by-products of refining operations. The removal of H
2
S from the gaseous streams is an
essential requirement prior to their utilization, i.e., fuel, steam reforming, oligomerization,
pyrolysis for olefin production, etc.
