Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 80

operational difficulties caused by deposits of inorganic solids. Such function may be fulfilled
by a guard chamber filled with a low cost solid (e.g., alumina, bauxite, clays, etc.).
4.6.2.2 Deposits of Organometallic Origin
Arsenic occurring naturally in many crudes in an organic form has been receiving little
attention, although under hydroprocessing conditions, the As organic compounds are very
reactive. Also, deactivating effect of arsenic after repeated utilization-regeneration cycles is
quite evident as it is shown in Fig. 4.13 [204] . Thus, they are either converted to AsH 3 , which
ends up in gaseous products or readily decompose and remain adsorbed on the catalyst
surface. In fixed-bed reactors, very steep As gradient is observed between the inlet and outlet.
Once on the catalyst, As may be converted to a sulfide. Such conversion is favorable under
typical hydroprocessing conditions. Arsenic may be a sever poison, particularly for HDS. This
was more evident for regenerated catalysts than for the fresh catalysts [252] . In fact, in excess
of 0.3 wt.%, As prevented regeneration of the spent catalyst used during hydroprocessing
under moderate conditions.
The porphyrin forms of V and Ni are the main organometallic compounds in heavy feeds. They
are the main cause of the metal deposits formation on catalyst surface. In crude oils, most of
these metals are associated with the asphaltenes entities. The depolymerization of asphaltenes
entities is considered as a very first stage of the metal deposit formation. As the result of this,
porphyrins are released into the oil phase and become available for HDM reactions. Initial
stage of the metal deposition coincides with the final stage of the overall HDM of
metal-containing compounds, i.e., the separation of metal from the pyrrole ligand skeleton.
In most of heavy feeds, the predominance of the V-porphyrins compared with Ni-porphyrins
has been noted. Therefore, the metal deposition patterns are influenced by the former to a
much greater extent. Moreover, reactivity of the V-porphyrins is greater than that of the
Ni-porphyrins, particularly at low conversions [253] . This is confirmed by kinetic data in
Fig. 4.22 [207] from hydroprocessing of the Khafji AR. Thus, the reactivity difference was
maintained for the different particle size of the catalyst. The higher reactivity of V-porphyrins
is attributed to the presence of VO-group, which facilitates much stronger interaction with
catalyst surface than that of the Ni-porphyrins. At high conversions, e.g., under severe
hydroprocessing conditions, the rate of deposition of the Ni-porphyrins competes more
successfully with that of the V-porphyrins. Another way of looking at this issue is the change
in conversion between the inlet and outlet of the fix-bed of a catalyst. Thus, it is believed that
the conversion will increase towards the outlet of the fixed-bed. Then, contribution of Ni
compounds to the deposit formation increases in the same direction, as it was confirmed by
Tamm et al. [197] .
4.6.2.2.1 Vanadium containing deposits
Once on catalyst surface, transformation of the vanadyl group to a V-sulfide may be affected
because of the steric hindrance. This may prevent complete sulfidation of the V O group after