648
The Chemistry and Technology of Petroleum
The catalyst is usually supplied in the form of ferric oxide (Fe 2 O 3 ) and six-valent chromium oxide
(CrO 3 ) and is reduced by the hydrogen and carbon monoxide in the shift feed gas as part of the startop procedure to produce the catalyst in the desired form. However, caution is necessary since if the
steam/carbon ratio of the feedstock is too low and the reducing environment too strong, the catalyst
can be reduced further, to metallic iron. Metallic iron is a catalyst for Fischer–Tropsch reactions and
hydrocarbons will be produced.
Low-temperature shift catalysts operate at temperatures on the order of 205°C–230°C
(400°F–450°F). Because of the lower temperature, the reaction equilibrium is more controllable
and lower amounts of carbon monoxide are produced. The low-temperature shift catalyst is primarily used in wet scrubbing plants that use a methanation for final purification. PSA plants do not
generally use a low-temperature because since any unconverted carbon monoxide is recovered as
reformer fuel. Low-temperature shift catalysts are sensitive to poisoning by sulfur and are sensitive
to water (liquid) that can cause softening of the catalyst followed by crusting or plugging.
The catalyst is supplied as copper oxide (CuO) on a zinc oxide (ZnO) carrier and the copper must
be reduced by heating it in a stream of inert gas with measured quantities of hydrogen. The reduction of the copper oxide is strongly exothermic and must be closely monitored.
23.6.3 metHAnAtIon CAtAlysts
In wet scrubbing plants, the final hydrogen purification procedure involved is by methanation in
which the carbon monoxide and carbon dioxide are converted to methane:
CO H O CH H O
+
Æ
+
3 2
4
2
CO
H
CH
H O
2
2
4
2
4
2
+
Æ
+
The active agent is nickel, on an alumina carrier.
The catalyst has a long life, as it operates under ideal conditions and is not exposed to poisons.
The main source of deactivation is plugging from carryover of carbon dioxide from removal
solutions.
The most severe hazard arises from high levels of carbon monoxide or carbon dioxide, which
can result from breakdown of the carbon dioxide removal equipment or from exchanger tube leaks
that quench the shift reaction. The results of breakthrough can be severe, since the methanation
reaction produces a temperature rise of 70°C (125°F) per 1% of carbon monoxide or a temperature
rise of 33°C (60°F) per 1% of carbon dioxide. While the normal operating temperature during
methanation is approximately 315°C (600°F), it is possible to reach 700°C (1300°F) in cases of
major breakthrough.
23.7 HYDROGEN PURIFICATION
When the hydrogen content of the refinery gas is greater than 50% by volume, the gas should
first be considered for hydrogen recovery, using a membrane (Brüschke, 1995, 2003) or PSA unit
(Figure 23.5). The tail gas or reject gas that will still contain a substantial amount of hydrogen can
then be used as steam reformer feedstock. Generally, the feedstock purification process uses three different refinery gas streams to produce hydrogen. First, high-pressure hydrocracker purge gas is purified in a membrane unit that produces hydrogen at medium pressure and is combined with medium
pressure off-gas that is first purified in a PSA unit. Finally, low-pressure off-gas is compressed, mixed
with reject gases from the membrane and PSA units, and used as steam reformer feed.
Various processes are available to purify the hydrogen stream but since the product streams are
available as a wide variety of composition, flows, and pressures, the best method of purification
will vary. And there are several factors that must also be taken into consideration in the selection
The Chemistry and Technology of Petroleum
The catalyst is usually supplied in the form of ferric oxide (Fe 2 O 3 ) and six-valent chromium oxide
(CrO 3 ) and is reduced by the hydrogen and carbon monoxide in the shift feed gas as part of the startop procedure to produce the catalyst in the desired form. However, caution is necessary since if the
steam/carbon ratio of the feedstock is too low and the reducing environment too strong, the catalyst
can be reduced further, to metallic iron. Metallic iron is a catalyst for Fischer–Tropsch reactions and
hydrocarbons will be produced.
Low-temperature shift catalysts operate at temperatures on the order of 205°C–230°C
(400°F–450°F). Because of the lower temperature, the reaction equilibrium is more controllable
and lower amounts of carbon monoxide are produced. The low-temperature shift catalyst is primarily used in wet scrubbing plants that use a methanation for final purification. PSA plants do not
generally use a low-temperature because since any unconverted carbon monoxide is recovered as
reformer fuel. Low-temperature shift catalysts are sensitive to poisoning by sulfur and are sensitive
to water (liquid) that can cause softening of the catalyst followed by crusting or plugging.
The catalyst is supplied as copper oxide (CuO) on a zinc oxide (ZnO) carrier and the copper must
be reduced by heating it in a stream of inert gas with measured quantities of hydrogen. The reduction of the copper oxide is strongly exothermic and must be closely monitored.
23.6.3 metHAnAtIon CAtAlysts
In wet scrubbing plants, the final hydrogen purification procedure involved is by methanation in
which the carbon monoxide and carbon dioxide are converted to methane:
CO H O CH H O
+
Æ
+
3 2
4
2
CO
H
CH
H O
2
2
4
2
4
2
+
Æ
+
The active agent is nickel, on an alumina carrier.
The catalyst has a long life, as it operates under ideal conditions and is not exposed to poisons.
The main source of deactivation is plugging from carryover of carbon dioxide from removal
solutions.
The most severe hazard arises from high levels of carbon monoxide or carbon dioxide, which
can result from breakdown of the carbon dioxide removal equipment or from exchanger tube leaks
that quench the shift reaction. The results of breakthrough can be severe, since the methanation
reaction produces a temperature rise of 70°C (125°F) per 1% of carbon monoxide or a temperature
rise of 33°C (60°F) per 1% of carbon dioxide. While the normal operating temperature during
methanation is approximately 315°C (600°F), it is possible to reach 700°C (1300°F) in cases of
major breakthrough.
23.7 HYDROGEN PURIFICATION
When the hydrogen content of the refinery gas is greater than 50% by volume, the gas should
first be considered for hydrogen recovery, using a membrane (Brüschke, 1995, 2003) or PSA unit
(Figure 23.5). The tail gas or reject gas that will still contain a substantial amount of hydrogen can
then be used as steam reformer feedstock. Generally, the feedstock purification process uses three different refinery gas streams to produce hydrogen. First, high-pressure hydrocracker purge gas is purified in a membrane unit that produces hydrogen at medium pressure and is combined with medium
pressure off-gas that is first purified in a PSA unit. Finally, low-pressure off-gas is compressed, mixed
with reject gases from the membrane and PSA units, and used as steam reformer feed.
Various processes are available to purify the hydrogen stream but since the product streams are
available as a wide variety of composition, flows, and pressures, the best method of purification
will vary. And there are several factors that must also be taken into consideration in the selection