Practical considerations, such as the cost of compression, catalyst life, etc., limit the
purity of the recycle gas to a minimum value usually in the range of 70–80 mol%.
Lower hydrogen purities are detrimental to the performance of the unit since higher
temperatures must be used to achieve the desired product quality. The purity of the
recycle gas is determined by the following factors:
• The purity of the makeup gas and introduction of light ends into the reactor
circuit
• Vapor/liquid equilibrium at the separator that forms the recycle gas
• The amounts of light hydrocarbons and H 2 S that are allowed to accumulate in
the recycle gas
In most instances, the makeup gas H 2 purity cannot be easily manipulated since
it is fixed by the operation of the reformer or the hydrogen manufacturing plant. The
light hydrocarbons present in the recycle gas enter the system with the makeup gas
in addition to those being formed in the reactor and must be vented from the highpressure separator to prevent their accumulation in the recycle gas. The amount of
hydrogen required is determined by:
(a) Chemical hydrogen consumption – the hydrogen consumed during the
hydrotreating reactions.
(b) Solution losses – the hydrogen that is removed from the reactor circuit
dissolved in the liquid hydrocarbon leaving the high-pressure separator.
(c) Mechanical losses – the hydrogen lost through the makeup and recycle gas
compressors’ packing vents and seals. This value may be roughly estimated at
3–5 % of the combined chemical consumption plus solution losses.
(d) Venting losses – the hydrogen lost in the purge stream from the high-pressure
separator to maintain recycle gas purity.
The H 2 S formed in the reactors can reach equilibrium values as high as 5 mol% in
the recycle gas. This concentration of H 2 S has an inhibiting effect on the activity of
the catalyst. Therefore, in many cases it is desirable to remove the H 2 S from the
recycle gas. The removal of H 2 S is performed in a scrubber where the recycle gas is
contacted with an amine (generally MEA, DEA, or MDEA) solution. In this manner,
the H 2 S content of the recycle gas can be reduced to the parts per million range.
Another method to increase hydrogen purity is membrane separation. This
system removes the hydrogen from the vent gas and recycles it back with the
makeup hydrogen.
Feed Contaminants and Catalyst Poisons
Temporary
Process variables influence catalyst life by affecting the rate of carbon deposition on
the catalyst. There is a moderate accumulation of carbon on the catalyst during the
Hydrotreating in Petroleum Processing
431
purity of the recycle gas to a minimum value usually in the range of 70–80 mol%.
Lower hydrogen purities are detrimental to the performance of the unit since higher
temperatures must be used to achieve the desired product quality. The purity of the
recycle gas is determined by the following factors:
• The purity of the makeup gas and introduction of light ends into the reactor
circuit
• Vapor/liquid equilibrium at the separator that forms the recycle gas
• The amounts of light hydrocarbons and H 2 S that are allowed to accumulate in
the recycle gas
In most instances, the makeup gas H 2 purity cannot be easily manipulated since
it is fixed by the operation of the reformer or the hydrogen manufacturing plant. The
light hydrocarbons present in the recycle gas enter the system with the makeup gas
in addition to those being formed in the reactor and must be vented from the highpressure separator to prevent their accumulation in the recycle gas. The amount of
hydrogen required is determined by:
(a) Chemical hydrogen consumption – the hydrogen consumed during the
hydrotreating reactions.
(b) Solution losses – the hydrogen that is removed from the reactor circuit
dissolved in the liquid hydrocarbon leaving the high-pressure separator.
(c) Mechanical losses – the hydrogen lost through the makeup and recycle gas
compressors’ packing vents and seals. This value may be roughly estimated at
3–5 % of the combined chemical consumption plus solution losses.
(d) Venting losses – the hydrogen lost in the purge stream from the high-pressure
separator to maintain recycle gas purity.
The H 2 S formed in the reactors can reach equilibrium values as high as 5 mol% in
the recycle gas. This concentration of H 2 S has an inhibiting effect on the activity of
the catalyst. Therefore, in many cases it is desirable to remove the H 2 S from the
recycle gas. The removal of H 2 S is performed in a scrubber where the recycle gas is
contacted with an amine (generally MEA, DEA, or MDEA) solution. In this manner,
the H 2 S content of the recycle gas can be reduced to the parts per million range.
Another method to increase hydrogen purity is membrane separation. This
system removes the hydrogen from the vent gas and recycles it back with the
makeup hydrogen.
Feed Contaminants and Catalyst Poisons
Temporary
Process variables influence catalyst life by affecting the rate of carbon deposition on
the catalyst. There is a moderate accumulation of carbon on the catalyst during the
Hydrotreating in Petroleum Processing
431