pressure and recycle gas-to-feed ratio and thereby influence the catalyst stability
(deactivation rate). Typically pressure swing adsorption (PSA) purified hydrogen is
used for high-pressure hydrocracking unit because their purity is near 100 %
hydrogen. Other sources of hydrogen such as the net gas from catalytic reforming
units are lower purity hydrogen and will reduce the recycle gas purity in the
hydrocracking unit.
Hydrogen Purity
The purity of hydrogen in the makeup gas to a hydrocracker will have a major
influence on the hydrogen partial pressure and recycle gas-to-feed ratio. Therefore,
the minimum purity on the makeup gas should be set to provide the minimum
recycle gas purity allowed. If the hydrogen plant is unable for some reason to
produce minimum hydrogen purity product, a purge of recycle gas off the highpressure separator may be possible to maintain the recycle gas purity requirements.
Nitrogen and Methane Content
The total of the nitrogen and methane contained in the makeup gas is only harmful
as a diluent, i.e., they will reduce the hydrogen partial pressure. As long as the
minimum hydrogen purity is maintained, nitrogen and methane will not affect the
unit. However, it should be noted that excessive quantities of molecular nitrogen
entering a hydrocracker in the makeup gas stream can cause a buildup of nitrogen in
the recycle gas since the nitrogen is not condensable. If this is the case, the nitrogen
will have to be removed from the reactor circuit by a small, continuous purge of
recycle gas off the high-pressure separator. Nitrogen and methane, however, will
increase the molecular weight of the recycle gas and increase the power requirement of the recycle gas compressor.
CO + CO 2 Content
The normal specification for CO plus CO 2 in the makeup gas stream to a hydrocracker is in the range of 20 ppm maximum. Larger quantities can have a harmful
effect on catalyst activity. CO is considered the worst impurity due to the fact that it
has a limited solubility in both hydrocarbon and water and will, therefore, build up
in the recycle gas. CO 2 , on the other hand, is much more soluble and is readily
removed from the system in the high-pressure separator liquids.
Both CO and CO 2 have similar effects on the hydrocracking catalyst. They are
converted on the active sites of the catalyst in the presence of hydrogen to methane
and water. This methanation of CO and CO 2 competes with the normal hydrocarbon
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(deactivation rate). Typically pressure swing adsorption (PSA) purified hydrogen is
used for high-pressure hydrocracking unit because their purity is near 100 %
hydrogen. Other sources of hydrogen such as the net gas from catalytic reforming
units are lower purity hydrogen and will reduce the recycle gas purity in the
hydrocracking unit.
Hydrogen Purity
The purity of hydrogen in the makeup gas to a hydrocracker will have a major
influence on the hydrogen partial pressure and recycle gas-to-feed ratio. Therefore,
the minimum purity on the makeup gas should be set to provide the minimum
recycle gas purity allowed. If the hydrogen plant is unable for some reason to
produce minimum hydrogen purity product, a purge of recycle gas off the highpressure separator may be possible to maintain the recycle gas purity requirements.
Nitrogen and Methane Content
The total of the nitrogen and methane contained in the makeup gas is only harmful
as a diluent, i.e., they will reduce the hydrogen partial pressure. As long as the
minimum hydrogen purity is maintained, nitrogen and methane will not affect the
unit. However, it should be noted that excessive quantities of molecular nitrogen
entering a hydrocracker in the makeup gas stream can cause a buildup of nitrogen in
the recycle gas since the nitrogen is not condensable. If this is the case, the nitrogen
will have to be removed from the reactor circuit by a small, continuous purge of
recycle gas off the high-pressure separator. Nitrogen and methane, however, will
increase the molecular weight of the recycle gas and increase the power requirement of the recycle gas compressor.
CO + CO 2 Content
The normal specification for CO plus CO 2 in the makeup gas stream to a hydrocracker is in the range of 20 ppm maximum. Larger quantities can have a harmful
effect on catalyst activity. CO is considered the worst impurity due to the fact that it
has a limited solubility in both hydrocarbon and water and will, therefore, build up
in the recycle gas. CO 2 , on the other hand, is much more soluble and is readily
removed from the system in the high-pressure separator liquids.
Both CO and CO 2 have similar effects on the hydrocracking catalyst. They are
converted on the active sites of the catalyst in the presence of hydrogen to methane
and water. This methanation of CO and CO 2 competes with the normal hydrocarbon
356
M. Bricker et al.