processing the LCO in a hydrocracking unit becomes an attractive alternative. The
difficult sulfur species in LCO can be more effectively removed in the highpressure hydrotreating section of the hydrocracking unit. Furthermore, the LCO
quality is further improved by the higher operating pressure of the hydrocracking
unit, since additional aromatic saturation can take place to improve the cetane
qualities of the stream.
Deasphalted oil is another feedstock that some refiners find economical to send
to a hydrocracking unit. Processing DAO in a hydrocracking unit requires additional catalyst system considerations to account for the higher metal content (Ni and
V) as well as the potential for the presence of very heavy molecules as condensedring aromatics. DAO is created in a solvent deasphalting unit that separates on the
basis of molecular type rather than boiling point. Therefore, for moderate to heavy
lifts, increasing extent of extraction, in the SDA unit the heaviest molecules in the
DAO are not adequately characterized by boiling point due to the limitations of the
current distillation test methods. The higher metal content in DAO is addressed by
specifying an adequate amount of metal trapping material in the hydrotreating
section of the hydrocracking unit. Due to the presence of very heavy molecules,
the potential for a higher deactivation rate should also be considered when selecting
the design operating pressure and sizing both the hydrotreating and hydrocracking
catalyst beds.
Optimization Between the Hydrotreating and Hydrocracking
Function
In a typical hydrocracking flow scheme, the hydrotreating section effluent is
directly routed to the inlet of the hydrocracking section without any removal of
the H 2 S and NH 3 that are formed from the hydrotreating reactions. Ultimately, the
catalyst loading for the hydrotreating section depends on the concentration of
contaminants, organic sulfur and nitrogen, the fresh feed, and the permissible
level of these contaminants entering the hydrocracking section. There is some
degree of flexibility in the level of contaminants permitted to enter the hydrocracking section, particularly organic nitrogen, from the hydrotreating section. The
permissible concentration will depend on the relative reactor volumes available
for loading hydrotreating versus hydrocracking catalyst, the target conversion rate,
and the desired product quality from the hydrocracking unit.
Residual Oil Hydrodesulfurization
Residual oil, as the name implies, is the material remaining after distillation.
Atmospheric residue (AR) is the bottoms of the atmospheric column and generally
has the boiling range of ~650
F+. Vacuum gas oil is contained within AR. Vacuum
residue (VR) is the bottoms of the vacuum column. With the vacuum gas oil (VGO)
removed, the boiling range of the vacuum residue is ~1,050
F+.
Hydrotreating in Petroleum Processing
411
difficult sulfur species in LCO can be more effectively removed in the highpressure hydrotreating section of the hydrocracking unit. Furthermore, the LCO
quality is further improved by the higher operating pressure of the hydrocracking
unit, since additional aromatic saturation can take place to improve the cetane
qualities of the stream.
Deasphalted oil is another feedstock that some refiners find economical to send
to a hydrocracking unit. Processing DAO in a hydrocracking unit requires additional catalyst system considerations to account for the higher metal content (Ni and
V) as well as the potential for the presence of very heavy molecules as condensedring aromatics. DAO is created in a solvent deasphalting unit that separates on the
basis of molecular type rather than boiling point. Therefore, for moderate to heavy
lifts, increasing extent of extraction, in the SDA unit the heaviest molecules in the
DAO are not adequately characterized by boiling point due to the limitations of the
current distillation test methods. The higher metal content in DAO is addressed by
specifying an adequate amount of metal trapping material in the hydrotreating
section of the hydrocracking unit. Due to the presence of very heavy molecules,
the potential for a higher deactivation rate should also be considered when selecting
the design operating pressure and sizing both the hydrotreating and hydrocracking
catalyst beds.
Optimization Between the Hydrotreating and Hydrocracking
Function
In a typical hydrocracking flow scheme, the hydrotreating section effluent is
directly routed to the inlet of the hydrocracking section without any removal of
the H 2 S and NH 3 that are formed from the hydrotreating reactions. Ultimately, the
catalyst loading for the hydrotreating section depends on the concentration of
contaminants, organic sulfur and nitrogen, the fresh feed, and the permissible
level of these contaminants entering the hydrocracking section. There is some
degree of flexibility in the level of contaminants permitted to enter the hydrocracking section, particularly organic nitrogen, from the hydrotreating section. The
permissible concentration will depend on the relative reactor volumes available
for loading hydrotreating versus hydrocracking catalyst, the target conversion rate,
and the desired product quality from the hydrocracking unit.
Residual Oil Hydrodesulfurization
Residual oil, as the name implies, is the material remaining after distillation.
Atmospheric residue (AR) is the bottoms of the atmospheric column and generally
has the boiling range of ~650
F+. Vacuum gas oil is contained within AR. Vacuum
residue (VR) is the bottoms of the vacuum column. With the vacuum gas oil (VGO)
removed, the boiling range of the vacuum residue is ~1,050
F+.
Hydrotreating in Petroleum Processing
411