Once the diene content of a cracked naphtha blend is addressed, the next
consideration focuses on the monoolefin molecules. The olefin saturation reactions
occur fairly rapidly in the main hydrotreating reactor and result in a substantial
amount of heat release. The reaction occurs so readily that at start of run conditions
the majority of the olefins are saturated within the first bed of a hydrotreating
reactor, resulting in a significant heat rise across the first bed, and smaller temperature rises in the following bed. It is important to manage the heat released due to
olefin saturation in order to prevent coking, catalyst sintering, or uneven catalyst
utilization across the different catalyst beds. This can be achieved by recycling
some of the olefin-free reactor product with the fresh feed in order to dilute the
olefins available for saturation. Ultimately, the proper management of olefin heat
release is required to achieve the desired catalyst cycle length in the main
hydrotreating reactor.
Potential for Recombination with Cracked Stock
Due to presence of olefins in coker naphtha material, the potential for mercaptan
recombination exists. The below equation describes the equilibrium between mercaptans and olefin and H 2 S present in the reactor. It is desirable to favor the lefthand side of the equation so that the sulfur content from the feed is present in the
off-gas from the unit instead of the liquid product. This can be done by installing a
relatively small posttreat reactor that operates at a lower condition than the main
hydrotreating reactor, which favors the left-hand side of the equation and reduces
the amount of mercaptan sulfur than can be found in the hydrotreated naphtha
material:
R ¼ R
0
þ H 2 S $ R-R
0 -SH
Typical Operating Conditions
Table 4 compares the general range of operating conditions for the main
hydrotreating reactor for both a straight-run naphtha hydrotreating unit and a
coker naphtha hydrotreating unit.
Table 4 Naphtha hydrotreater operating conditions
SR naphtha only
SR and coker naphtha
Pressure, psig (barg)
300–500 (21–35)
500–800 (35–50)
Main HT reactor LHSV, h
À1
4–12
2–8
Gas-to-oil ratio, scf/bbl (Nm
3
/m
3
)
250–400 (40–75)
900–3,000 (150–500)
394
P. Kokayeff et al.
consideration focuses on the monoolefin molecules. The olefin saturation reactions
occur fairly rapidly in the main hydrotreating reactor and result in a substantial
amount of heat release. The reaction occurs so readily that at start of run conditions
the majority of the olefins are saturated within the first bed of a hydrotreating
reactor, resulting in a significant heat rise across the first bed, and smaller temperature rises in the following bed. It is important to manage the heat released due to
olefin saturation in order to prevent coking, catalyst sintering, or uneven catalyst
utilization across the different catalyst beds. This can be achieved by recycling
some of the olefin-free reactor product with the fresh feed in order to dilute the
olefins available for saturation. Ultimately, the proper management of olefin heat
release is required to achieve the desired catalyst cycle length in the main
hydrotreating reactor.
Potential for Recombination with Cracked Stock
Due to presence of olefins in coker naphtha material, the potential for mercaptan
recombination exists. The below equation describes the equilibrium between mercaptans and olefin and H 2 S present in the reactor. It is desirable to favor the lefthand side of the equation so that the sulfur content from the feed is present in the
off-gas from the unit instead of the liquid product. This can be done by installing a
relatively small posttreat reactor that operates at a lower condition than the main
hydrotreating reactor, which favors the left-hand side of the equation and reduces
the amount of mercaptan sulfur than can be found in the hydrotreated naphtha
material:
R ¼ R
0
þ H 2 S $ R-R
0 -SH
Typical Operating Conditions
Table 4 compares the general range of operating conditions for the main
hydrotreating reactor for both a straight-run naphtha hydrotreating unit and a
coker naphtha hydrotreating unit.
Table 4 Naphtha hydrotreater operating conditions
SR naphtha only
SR and coker naphtha
Pressure, psig (barg)
300–500 (21–35)
500–800 (35–50)
Main HT reactor LHSV, h
À1
4–12
2–8
Gas-to-oil ratio, scf/bbl (Nm
3
/m
3
)
250–400 (40–75)
900–3,000 (150–500)
394
P. Kokayeff et al.