Environmental Engineering Reference
In-Depth Information
influence of operating conditions may be much more pronounced than that of the origin of
vacuum distillates and type of catalyst. In one case of hydroprocessing of a VGO, catalyst was
exposed to a temperature runaway under low H 2 pressure. For this catalyst, more severe
regeneration conditions were necessary for achieving a desirable level of the coke
removal.
The content of contaminant metals in residues derived from conventional crude was generally
low. It is, therefore, believed that spent catalysts, particularly those used in the downstream
layers and/or reactors of a multistage system, can also be suitable for regeneration. However,
for such spent catalysts, the number of regeneration-utilization cycles will be less than that for
the spent catalysts from hydroprocessing of lighter feeds derived from a conventional crude.
Similarly, for vacuum residues (VR), the regeneration of catalysts from the fourth or even third
stage reactors of the multistage processes (e.g., atmospheric residue desulfurization [ARDS]
and HYVAHL processes) can be performed without difficulties. This was confirmed by the
results published by Al-Dalama and Stanislaus [12] showing a significant recovery of the
surface area and porosity of catalysts taken from the third and fourth reactor of the ARDS
process, compared with a little recovery for the catalysts taken from the first and second
reactors. If not reused directly, i.e., in the same reactor, the spent-regenerated catalysts can be
cascaded to different catalytic operation. This option becomes attractive in larger refineries
with several catalytic reactor units in operation. Various options for cascading regenerated
catalysts in petroleum refinery as well as other potential uses of regenerated catalysts are
evaluated in several following chapters.
A high content of contaminant metals complicates upgrading of residues derived from heavy
crudes. In this case, spent catalysts may be non-regenerable, particularly if upgrading was
conducted in the ebullated bed reactors. The situation may be elevated by pretreating the heavy
feeds, e.g., using solvent deasphalting method. The quality of deasphalted oil (DAO) produced
is influenced by the type of solvent employed. There is always a trade-off between the yield
and quality of DAO. For example, the contamination with metals increases with increasing
yield of DAO. This will have an adverse effect on the regenerability of spent catalysts used for
hydroprocessing of the DAO, i.e., the number of regeneration-utilization cycles will decrease.
In some cases, the quality of DAO derived from a heavy feed may approach that of a residue
derived from a conventional crude.
Information on the regeneration of catalysts used for hydroprocessing of heavy feeds and at the
same time deactivated only by coke is limited. The results shown in Fig. 6.1 [367,368] were
obtained during the regeneration of a VGO. They indicate that almost complete carbon removal
could be readily achieved. In the same studies, several catalysts used for hydroprocessing of
lighter feeds were also included. The main differences included the higher temperatures
required for the removal of sulfur from the spent catalyst used for hydroprocessing of the VGO
than that for the lighter fraction. Moreover, the regeneration lasted longer because of the
 
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