54
evaluation was performed using 4,6-DMDBT and a straight-run gas oil feedstock.
Maleic acid was found to extract Co from the CoMoO 4 species forming a cobalt
maleate complex. The addition of maleic acid resulted in more rapid sulfidation of
Mo without releasing Co significantly from the cobalt maleate complex before
reaching 300 °C, which resulted in the formation of higher proportion of
Co-promoted MoS 2 phase and enhanced HDS catalytic performance. SantolallaVargas et al. [53] studied in situ reactivation of spent HDS catalysts with molybdenum
acetylacetonate
previously
washed
with
xylene
and
2,6-bis-(1-hydroxy-1,1-diphenyl-methyl)pyridine and further investigated for the
HDS of straight-run gas oil. Aromatic carbons and coke with larger crystallite size
were removed from the spent catalyst by washing chemicals. In situ reactivation
with molybdenum acetylacetonate could help in the deposition of Mo on the catalyst and increase its surface content, which otherwise could decrease during the
leaching process. This novel method of regeneration/rejuvenation was claimed to
enhance the MoS 2 species, lower the carbon concentration, and shorten the coke
crystallite size resulting in a higher gas-oil HDS activity.
Regeneration and reuse of residue hydroprocessing catalysts gain significance
due to generation of a large volume of these catalysts in refineries due to their high
deactivation rates. However, regeneration of residue HDS catalysts is more difficult
compared to distillate/naphtha HDS catalysts due to the presence of high concentrations of metals such as vanadium and also the requirement of severe regeneration
conditions. Iwamoto [54] reviewed the regeneration of residue HDS catalysts and
showed the effects of the presence of vanadium on the regeneration activity.
Vanadium was found to increase the regeneration temperature due to its oxidative
properties apart from reducing the capacity for activity recovery and decreasing
catalyst strength.
7 Catalysts for Different Hydrotreating Feedstocks
Proper selection of the catalyst plays an important role in getting the most out of
their performance in terms of activity, selectivity, and stability. Some aspects that
need to be considered for selection of catalysts with regard to particular hydrotreating application are feedstock type and properties, process conditions, and required
product yields and properties. Due to the differences in the properties of different
feeds such as naphtha, distillates, vacuum gas oils, and heavy residues, the catalyst
design and selection criteria vary significantly among them. Apart from molecular
weight distribution between these feedstocks, there are significant variations in the
properties such as asphaltenes, metals, and heteroatoms. Due to these differences,
the catalysts used for hydrotreating of residues are not the same as those used for
distillates or naphtha.
Activity, selectivity, and stability are the three important characteristics that are
considered for catalyst selection for any particular HDT application. Activity is
defined as the ability of the catalyst to increase the rate of specific reaction such as
G. Valavarasu and B. Ramachandrarao
evaluation was performed using 4,6-DMDBT and a straight-run gas oil feedstock.
Maleic acid was found to extract Co from the CoMoO 4 species forming a cobalt
maleate complex. The addition of maleic acid resulted in more rapid sulfidation of
Mo without releasing Co significantly from the cobalt maleate complex before
reaching 300 °C, which resulted in the formation of higher proportion of
Co-promoted MoS 2 phase and enhanced HDS catalytic performance. SantolallaVargas et al. [53] studied in situ reactivation of spent HDS catalysts with molybdenum
acetylacetonate
previously
washed
with
xylene
and
2,6-bis-(1-hydroxy-1,1-diphenyl-methyl)pyridine and further investigated for the
HDS of straight-run gas oil. Aromatic carbons and coke with larger crystallite size
were removed from the spent catalyst by washing chemicals. In situ reactivation
with molybdenum acetylacetonate could help in the deposition of Mo on the catalyst and increase its surface content, which otherwise could decrease during the
leaching process. This novel method of regeneration/rejuvenation was claimed to
enhance the MoS 2 species, lower the carbon concentration, and shorten the coke
crystallite size resulting in a higher gas-oil HDS activity.
Regeneration and reuse of residue hydroprocessing catalysts gain significance
due to generation of a large volume of these catalysts in refineries due to their high
deactivation rates. However, regeneration of residue HDS catalysts is more difficult
compared to distillate/naphtha HDS catalysts due to the presence of high concentrations of metals such as vanadium and also the requirement of severe regeneration
conditions. Iwamoto [54] reviewed the regeneration of residue HDS catalysts and
showed the effects of the presence of vanadium on the regeneration activity.
Vanadium was found to increase the regeneration temperature due to its oxidative
properties apart from reducing the capacity for activity recovery and decreasing
catalyst strength.
7 Catalysts for Different Hydrotreating Feedstocks
Proper selection of the catalyst plays an important role in getting the most out of
their performance in terms of activity, selectivity, and stability. Some aspects that
need to be considered for selection of catalysts with regard to particular hydrotreating application are feedstock type and properties, process conditions, and required
product yields and properties. Due to the differences in the properties of different
feeds such as naphtha, distillates, vacuum gas oils, and heavy residues, the catalyst
design and selection criteria vary significantly among them. Apart from molecular
weight distribution between these feedstocks, there are significant variations in the
properties such as asphaltenes, metals, and heteroatoms. Due to these differences,
the catalysts used for hydrotreating of residues are not the same as those used for
distillates or naphtha.
Activity, selectivity, and stability are the three important characteristics that are
considered for catalyst selection for any particular HDT application. Activity is
defined as the ability of the catalyst to increase the rate of specific reaction such as
G. Valavarasu and B. Ramachandrarao