Environmental Engineering Reference
In-Depth Information
Figure 9.10: Effect of hydrothermal treatment on hydrodesulfurization (HDS) and
hydrodevanadization (HDV) activity reprocessed catalyst [From ref.
524
. Reprinted with
permission].
It was observed that hydrothermal treatment resulted in the conversion of
-alumina to
boehmite according to the following reaction
[529]
:
Al
2
O
3
+
H
2
O
γ
-alumina
⇒
2AlO(OH)
boehmite
Boehmite formed in this way was well crystallized having large size of crystallites.
Calcination of boehmite (
Fig. 9.1
)at500
◦
C yielded
-alumina with large pores. In alumina
supports, porosity originates from the space between particles
[530]
. Therefore, when large
size alumina particles are packed to form extrudates, the space between the particles, i.e., pore
size and pore volume, will be large. Then, rehydration of
-alumina in the spent catalyst and
its transformation to well crystalized boehmite were responsible for the pore enlargement.
Consequently, the activity of the reprocessed catalyst was significantly enhanced.
9.1.2 Other Preparation Methods
Lee et al.
[531]
found that active reforming catalysts can be prepared using the metals
containing extract obtained by selective leaching of spent residue hydrotreating catalysts with
either oxalic acid or citric acid. The significant difference between selectivity of these acids is
indicated by very low content of Ni and Mo in the extract from oxalic acid leaching, as it is
shown in
Table 9.5 [531]
. Moreover, the selectivity of oxalic acid for V was much greater than
that for Ni and Mo. This extract, containing 1.8% V and small amounts of Mo (187 ppm) and
Ni (111 ppm), was used for catalyst preparation. High surface area (340 m
2
/g) amorphous
silica was used as support. The prepared catalyst showed significant dehydrocyclization
