647
Hydrogen Production
deactivation occurs, there is a simultaneous temperature increases with a concomitant increase in
coke formation and deposition. In other zones, where the hydrocarbon-to-hydrogen ratio is lower,
there is less risk of coking.
Coking depends to a large extent on the balance between catalyst activity and heat input with the
more active catalysts producing higher yields of hydrogen at lower temperature thereby reducing the
risk of coking. A uniform input of heat is important in this region of the reformer since any catalyst
voids or variations in catalyst activity can produce localized hot spots leading to coke formation
and/or reformer failure.
Coke formation results in hot spots in the reformer that increases pressure drop, reduces feedstock
(methane) conversion, leading eventually to reformer failure. Coking may be partially mitigated by
increasing the steam/feedstock ratio to change the reaction conditions, but the most effective solution may be to replace the reformer catalyst with one designed for higher boiling feedstocks.
A standard steam–methane reforming catalyst uses nickel on an α-alumina ceramic carrier that
is acidic in nature. Promotion of hydrocarbon cracking with such a catalyst leads to coke formation
from higher boiling feedstocks. Some catalyst formulations use a magnesia/alumina (MgO/Al 2 O 3 )
support that is less acidic than α-alumina that reduces cracking on the support and allows higher
boiling feedstocks (such as LPG) to be used.
Further resistance to coking can be achieved by adding an alkali promoter, typically some form
of potash (KOH) to the catalyst. Besides reducing the acidity of the carrier, the promoter catalyzes
the reaction of steam and carbon. While carbon continues to be formed, it is removed faster than it
can build up. This approach can be used with naphtha feedstocks boiling point up to approximately
180°C (350°F). Under the conditions in a reformer, potash is volatile, and it is incorporated into
the catalyst as a more complex compound that slowly hydrolyzes to release potassium hydroxide
(KOH). Alkali-promoted catalyst allows the use of a wide range of feedstocks but, in addition to
possible potash migration, which can be minimized by proper design and operation, the catalyst is
also somewhat less active than conventional catalyst.
Another option to reduce coking in steam reformers is to use a pre-reformer in which a fixed-bed
of catalyst, operating at a lower temperature, upstream of the fired reformer is used. In a pre-reformer,
adiabatic steam–hydrocarbon reforming is performed outside the fired reformer in a vessel containing high nickel catalyst. The heat required for the endothermic reaction is provided by hot flue gas
from the reformer convection section. Since the feed to the fired reformer is now partially reformed,
the steam–methane reformer can operate at an increased feed rate and produce 8%–10% additional
hydrogen at the same reformer load. An additional advantage of the pre-reformer is that it facilitates higher mixed feed preheat temperatures and maintains relatively constant operating conditions
within the fired reformer regardless of variable refinery off-gas feed conditions. Inlet temperatures
are selected so that there is minimal risk of coking and the gas leaving the pre-reformer contains only
steam, hydrogen, carbon monoxide, carbon dioxide, and methane. This allows a standard methane
catalyst to be used in the fired reformer and this approach has been used with feedstocks up to light
kerosene. Since the gas leaving the pre-reformer poses reduced risk of coking, it can compensate to
some extent for variations in catalyst activity and heat flux in the primary reformer.
23.6.2 sHIFt ConversIon CAtAlysts
The second important reaction in a steam reforming plant is the shift conversion reaction:
CO H O CO H
+
Æ
+
2
2
2
Two basic types of shift catalyst are used in steam-reforming plants: iron/chrome hightemperature shift catalysts, and copper/zinc low-temperature shift catalysts.
High-temperature shift catalysts operate in the range of 315°C–430°C (600°F–800°F) and consist primarily of magnetite (Fe 3 O 4 ) with three-valent chromium oxide (Cr 2 O 3 ) added as a stabilizer.
Hydrogen Production
deactivation occurs, there is a simultaneous temperature increases with a concomitant increase in
coke formation and deposition. In other zones, where the hydrocarbon-to-hydrogen ratio is lower,
there is less risk of coking.
Coking depends to a large extent on the balance between catalyst activity and heat input with the
more active catalysts producing higher yields of hydrogen at lower temperature thereby reducing the
risk of coking. A uniform input of heat is important in this region of the reformer since any catalyst
voids or variations in catalyst activity can produce localized hot spots leading to coke formation
and/or reformer failure.
Coke formation results in hot spots in the reformer that increases pressure drop, reduces feedstock
(methane) conversion, leading eventually to reformer failure. Coking may be partially mitigated by
increasing the steam/feedstock ratio to change the reaction conditions, but the most effective solution may be to replace the reformer catalyst with one designed for higher boiling feedstocks.
A standard steam–methane reforming catalyst uses nickel on an α-alumina ceramic carrier that
is acidic in nature. Promotion of hydrocarbon cracking with such a catalyst leads to coke formation
from higher boiling feedstocks. Some catalyst formulations use a magnesia/alumina (MgO/Al 2 O 3 )
support that is less acidic than α-alumina that reduces cracking on the support and allows higher
boiling feedstocks (such as LPG) to be used.
Further resistance to coking can be achieved by adding an alkali promoter, typically some form
of potash (KOH) to the catalyst. Besides reducing the acidity of the carrier, the promoter catalyzes
the reaction of steam and carbon. While carbon continues to be formed, it is removed faster than it
can build up. This approach can be used with naphtha feedstocks boiling point up to approximately
180°C (350°F). Under the conditions in a reformer, potash is volatile, and it is incorporated into
the catalyst as a more complex compound that slowly hydrolyzes to release potassium hydroxide
(KOH). Alkali-promoted catalyst allows the use of a wide range of feedstocks but, in addition to
possible potash migration, which can be minimized by proper design and operation, the catalyst is
also somewhat less active than conventional catalyst.
Another option to reduce coking in steam reformers is to use a pre-reformer in which a fixed-bed
of catalyst, operating at a lower temperature, upstream of the fired reformer is used. In a pre-reformer,
adiabatic steam–hydrocarbon reforming is performed outside the fired reformer in a vessel containing high nickel catalyst. The heat required for the endothermic reaction is provided by hot flue gas
from the reformer convection section. Since the feed to the fired reformer is now partially reformed,
the steam–methane reformer can operate at an increased feed rate and produce 8%–10% additional
hydrogen at the same reformer load. An additional advantage of the pre-reformer is that it facilitates higher mixed feed preheat temperatures and maintains relatively constant operating conditions
within the fired reformer regardless of variable refinery off-gas feed conditions. Inlet temperatures
are selected so that there is minimal risk of coking and the gas leaving the pre-reformer contains only
steam, hydrogen, carbon monoxide, carbon dioxide, and methane. This allows a standard methane
catalyst to be used in the fired reformer and this approach has been used with feedstocks up to light
kerosene. Since the gas leaving the pre-reformer poses reduced risk of coking, it can compensate to
some extent for variations in catalyst activity and heat flux in the primary reformer.
23.6.2 sHIFt ConversIon CAtAlysts
The second important reaction in a steam reforming plant is the shift conversion reaction:
CO H O CO H
+
Æ
+
2
2
2
Two basic types of shift catalyst are used in steam-reforming plants: iron/chrome hightemperature shift catalysts, and copper/zinc low-temperature shift catalysts.
High-temperature shift catalysts operate in the range of 315°C–430°C (600°F–800°F) and consist primarily of magnetite (Fe 3 O 4 ) with three-valent chromium oxide (Cr 2 O 3 ) added as a stabilizer.