Environmental Engineering Reference
In-Depth Information
Asaoka et al. [257] showed that in the presence of a catalyst, there is a significant difference
between the metal deposition patterns in pure H 2 and that of the H 2 +H 2 S mixture. In the latter
case, the precursor was converted to deposits at the first contact with catalyst surface. Then, the
deposits progressively penetrated into the catalyst particle interior. Also, the amount of deposit
was decreasing from the inlet towards the outlet of the reactor. Spectroscopic evaluations of
the deposits (formed in H 2 +H 2 S) identified V 3 S 4 as the predominant composition [257] .In
V 3 S 4 , V was present partly as V 4+ and partly as VO with the proportion of the latter increasing
towards the catalyst particle exterior. Contrary to this, Loos et al. [255] observed the formation
of V 2 S 3 rather than V 3 S 4 . However, the latter authors used the model VO-containing porphyrin
rather than the heavy feed. Kim and Massoth [258] pointed out that the structure of the V
deposits formed during hydroprocessing of real feeds may differ from that formed during the
treatment with model V-porphyrins. This was indicated by rather different effect of deposits on
catalyst functionalities. Thus, the catalyst was much more deactivated by the real deposits than
by those formed using model V-compounds. The difference between the V/S ratio of the
model deposits and the real feed deposits should be noted as well.
4.6.2.2.2 Nickel and mixed deposits
The product of the reaction of Ni-porphyrins with H 2 S may be at least a partially sulfided Ni.
The main HDM proceeds via hydrogenolysis of the Ni N bond releasing metallic Ni. After
deposition on the catalyst surface, Ni is sulfided via established mechanism. Under typical
hydroprocessing conditions, the complete sulfidation of Ni would lead to the formation of
Ni 3 S 2 -sulfide. A partially sulfided Ni and/or an oxosulfide form of Ni may be present as well.
The radial distribution of the Ni-sulfides formed non-catalytically via reaction with either H 2
or H 2 S should differ markedly from that formed catalytically via established HDM
mechanism. The former shall deposit physically predominantly on the exterior of catalyst
particles in a “skin-like” form, whereas the Ni-containing deposit formed as part of the HDM
reactions should be distributed more evenly. It may be rather difficult to distinguish between
these two types of the Ni-containing deposits on catalyst surface.
The overwhelming evidence suggests that initially, the metal deposition occurred
predominantly on the bare surface of the catalyst support [259-262] . The thickness and/or size
of the deposit were increasing progressively with time on stream. The multilayer deposit
would consist of the mixture of V-sulfides (e.g., VS 2 ,V 2 S 3 and V 3 S 4 ) and V-oxosulfides as
well as Ni-sulfides (e.g., Ni 3 S 2 ). The simultaneous deposition of V and Ni supports the
formation of mixed sulfides (Ni X V Y S Z ). The formation of a mixed (Fe,V)S 4 sulfide was
reported by Embaid et al. [263] for the Fe containing heavy feed. The ratio of the V to either
Ni or Fe in the mixed sulfide deposit will change from the exterior towards the center of the
catalyst particle, i.e., in the case of Ni, the V/Ni ratio will decrease as more Ni porphyrins than
V porphyrins can penetrate deeper into the catalyst particle interior. At the same time, the V/Fe
ratio may increase towards the particle interior because most (if not all) of the Fe deposited on
 
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