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Hydrocracking
Additional capital cost savings from once-through hydrocracking result from the reduced
overall required hydraulic capacity of the plant for a given fresh feed rate as a result of the
elimination of a recycle oil stream. Hydraulic capacity at the same fresh feed rate is 30%–40%
lower for a once-through plant compared with one designed for recycle. Utilities savings for a
once-through versus recycle operation arise from lower pumping and compression costs as a
result of the lower design pressure possible and also lower hydrogen consumption. Additional
savings are realized as a result of the lower oil and gas circulation rates required, since recycle
of oil from the fractionator bottoms is not necessary. Lower capital investment and operating
costs are obvious advantages of once-through hydrocracking compared with a recycle design.
This type of operation may be adaptable for use in an existing gas oil hydrotreater or atmospheric resid desulfurization plant. The change from hydrotreating to hydrocracking service
will require some modifications and capital expenditure, but in most cases, these changes will
be minimal.
The fact that unconverted oil is produced by the plant is not necessarily a disadvantage. The
unconverted oil produced by once-through hydrocracking is a high quality, low sulfur, nitrogen
material that is an excellent feed stock for a fluid catalytic cracking or ethylene pyrolysis furnace or
as a source of high-viscosity index lube oil base stock. The properties of the oil are a function of the
degree of conversion and other plant operating conditions.
One disadvantage of once-through hydrocracking compared to a recycle operation is a somewhat
reduced flexibility for varying the ratio of gasoline to middle distillate that is produced. A greater
quantity of naphtha can be produced by increasing conversion and production of jet fuel plus diesel
can also be increased. But selectivity for higher boiling products is also a function of conversion.
Selectivity decreases as once-through conversion increases. If conversion is increased too much, the
yield of desired product will decrease, accompanied by an increase in light ends and gas production.
Higher yields of gasoline or jet fuel plus diesel are possible from a recycle than from a once-through
operation.
Middle distillate products made by once-through hydrocracking are generally higher in aromatics content of poorer burning quality than those produced by recycle hydrocracking. However, the
quality is generally better than produced by catalytic cracking or from straight run. Middle distillate
product quality improves as the degree of conversion increases and as hydrogen partial pressure is
increased.
The hydrocracking process employs high-activity catalysts that produce a significant yield of
light products. Catalyst selectivity to middle distillate is a function of both the conversion level
and operating temperature, with values in excess of 90% being reported in commercial operation.
In addition to the increased hydrocracking activity of the catalyst, percentage desulfurization and
denitrogenation at start-of-run conditions are also substantially increased. End of cycle is reached
when product sulfur has risen to the level achieved in conventional vacuum gas oil (VGO) hydrodesulfurization (HDS) process.
An important consideration, however, is that commercial hydrocracking units are often limited by design constraints of existing vacuum gas oil hydrotreating units. Thus, the proper choice
of catalyst(s) is critical when searching for optimum performance. Typical commercial distillate
hydrocracking (DHC) catalysts contain both the hydrogenation (metal) and cracking (acid sites)
functions required for service in existing desulfurization units.
22.3 OPTIONS FOR HEAVY FEEDSTOCKS
The major goal of residuum hydroconversion is cracking of residua with desulfurization, metal
removal, denitrogenation, and asphaltene conversion. Residuum hydroconversion process offers
production of kerosene and gas oil, and production of feedstocks for hydrocracking, fluid catalytic
cracking, and petrochemical applications. The processes that follow are listed in alphabetical order
with no other preference in mind.
Hydrocracking
Additional capital cost savings from once-through hydrocracking result from the reduced
overall required hydraulic capacity of the plant for a given fresh feed rate as a result of the
elimination of a recycle oil stream. Hydraulic capacity at the same fresh feed rate is 30%–40%
lower for a once-through plant compared with one designed for recycle. Utilities savings for a
once-through versus recycle operation arise from lower pumping and compression costs as a
result of the lower design pressure possible and also lower hydrogen consumption. Additional
savings are realized as a result of the lower oil and gas circulation rates required, since recycle
of oil from the fractionator bottoms is not necessary. Lower capital investment and operating
costs are obvious advantages of once-through hydrocracking compared with a recycle design.
This type of operation may be adaptable for use in an existing gas oil hydrotreater or atmospheric resid desulfurization plant. The change from hydrotreating to hydrocracking service
will require some modifications and capital expenditure, but in most cases, these changes will
be minimal.
The fact that unconverted oil is produced by the plant is not necessarily a disadvantage. The
unconverted oil produced by once-through hydrocracking is a high quality, low sulfur, nitrogen
material that is an excellent feed stock for a fluid catalytic cracking or ethylene pyrolysis furnace or
as a source of high-viscosity index lube oil base stock. The properties of the oil are a function of the
degree of conversion and other plant operating conditions.
One disadvantage of once-through hydrocracking compared to a recycle operation is a somewhat
reduced flexibility for varying the ratio of gasoline to middle distillate that is produced. A greater
quantity of naphtha can be produced by increasing conversion and production of jet fuel plus diesel
can also be increased. But selectivity for higher boiling products is also a function of conversion.
Selectivity decreases as once-through conversion increases. If conversion is increased too much, the
yield of desired product will decrease, accompanied by an increase in light ends and gas production.
Higher yields of gasoline or jet fuel plus diesel are possible from a recycle than from a once-through
operation.
Middle distillate products made by once-through hydrocracking are generally higher in aromatics content of poorer burning quality than those produced by recycle hydrocracking. However, the
quality is generally better than produced by catalytic cracking or from straight run. Middle distillate
product quality improves as the degree of conversion increases and as hydrogen partial pressure is
increased.
The hydrocracking process employs high-activity catalysts that produce a significant yield of
light products. Catalyst selectivity to middle distillate is a function of both the conversion level
and operating temperature, with values in excess of 90% being reported in commercial operation.
In addition to the increased hydrocracking activity of the catalyst, percentage desulfurization and
denitrogenation at start-of-run conditions are also substantially increased. End of cycle is reached
when product sulfur has risen to the level achieved in conventional vacuum gas oil (VGO) hydrodesulfurization (HDS) process.
An important consideration, however, is that commercial hydrocracking units are often limited by design constraints of existing vacuum gas oil hydrotreating units. Thus, the proper choice
of catalyst(s) is critical when searching for optimum performance. Typical commercial distillate
hydrocracking (DHC) catalysts contain both the hydrogenation (metal) and cracking (acid sites)
functions required for service in existing desulfurization units.
22.3 OPTIONS FOR HEAVY FEEDSTOCKS
The major goal of residuum hydroconversion is cracking of residua with desulfurization, metal
removal, denitrogenation, and asphaltene conversion. Residuum hydroconversion process offers
production of kerosene and gas oil, and production of feedstocks for hydrocracking, fluid catalytic
cracking, and petrochemical applications. The processes that follow are listed in alphabetical order
with no other preference in mind.