Environmental Engineering Reference
In-Depth Information
contaminant metals was similar as that of the fresh catalyst. This amount of metals represented
the coverage of one-half of monolayer. Apparently, most of these metals were deposited on the
bare support. During hydroprocessing of a heavy feed, the activity of the decoked catalyst
rapidly declined with the further addition of metals and approached that of the fresh catalyst
containing the same amount of metals. This suggests that the metals in excess of 8 wt.%
deposited near or on the active sites and, as such, caused permanent deactivation. Then, such
catalysts may be too heavily contaminated with Ni and V to be regenerated, particularly when
the intention was the reuse of the regenerated catalyst for hydroprocessing of the same heavy
feed. It is uncertain how this catalyst would perform during the upgrading of metals and
asphaltenes free feed. It should be again kept in mind that these observations were made for a
particular catalyst and they may differ for other type of spent hydroprocessing catalysts. It is
again emphasized that choice of the conditions used for testing regenerated catalysts requires
attention for obtaining reliable information. Thus, as it was pointed out by Dufresne et al.
[307,366-368] , the difference between the activity of the fresh and regenerated (contaminated
with metals) catalyst was usually greater at start of the run than after several weeks/months of
operation. An example of this specific behavior had been shown on the residue catalyst
contaminated with the overall contaminants content as high as 21 wt.% [416] . After
regeneration, the catalyst was less active for HDM and HDS than a fresh catalyst on start of
run, but, after one month of the operation, the activity difference was negligible for HDM and
still existing but reduced for HDS. So, in practical situation, the catalyst reuse has to be
carefully examined on a case by case basis either for the same application or for cascading in
less severe applications.
6.2.4.3 Chemical Structure
Spectroscopic techniques have been used to identify phases present in spent-regenerated
catalysts. Also, for regenerated catalysts, the dispersion of active phase and/or potential loss of
active metals due to the interaction with the support have been the focus of attention. Useful
information could be obtained by using extended X-ray absorption fine spectroscopy
(EXAFS), X-ray photoelectron spectroscopy (XPS), IR, X-ray diffraction spectroscopy
(XRD), and other techniques.
The XRD technique was used to identify phases after regeneration of the spent NiMo/Al 2 O 3
catalyst either in 2% O 2 or in air [331] .In2%O 2 , only
-Al 2 O 3 and MoO 3 peaks were
detected. The Ni-containing species were not observed because of a high dispersion. After
regeneration in air, the additional species such as Al 2 (MoO 4 ) 3 was formed. The
-Al 2 O 3 ,
-Al 2 O 3 were found in spent regenerated catalyst containing contaminant metals
(e.g., V and Ni) even in 2% O 2 . This signified that the contaminant metals accelerated
sintering of
-Al 2 O 3 , and
-Al 2 O 3 . For this catalyst, crystalline phases of V 2 O 5 , FeO, NiAl 2 O 4 , and
NiMoO 4 were observed after regeneration in air. Teixeira da Silva et al. [416] suggested that
-NiMoO 4 is the precursor to the NiMoS active phase. At higher temperature, the former
 
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