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The Chemistry and Technology of Petroleum
compressor. Sometimes with sour feeds, the first-stage recycle gas is scrubbed with an amine system to remove hydrogen. If the feed sulfur level is high, this option can improve the performance of
the catalyst and result in less costly materials of construction.
The distillation section consists of a hydrogen sulfide (H 2 S) stripper and a recycle splitter. This
latter column separates the product into the desired cuts. The column bottoms stream is recycled
back to the second-stage feed. The recycle cut point is changed depending on the light products
needed. It can be as low as 160°C (320°F) if naphtha production is maximized (for aromatics) or as
high as 380°C (720°F) if a low pour point diesel is needed. Between these two extremes, a recycle
cut point of 260°C–285°C (500°F–550°F) results in high yields of high smoke point low freeze
point jet fuel.
Overall, a single-stage once-through (SSOT) unit resembles the first stage of the two-stage plant.
This type of hydrocracker usually requires the least capital investment. The feedstock is not completely converted to lighter products. For this application, the refiner must have a demand for highly
refined heavy oil. In many refining situations, such an oil product can be used as lube oil plant feed
or as FCC plant feed or in low sulfur oil blends or as ethylene plant feed. It also lends itself to stepwise construction of a future two-stage hydrocracker for full feed conversion.
A single-stage recycle (SSREC) unit converts heavy oil completely into light products with a
flow scheme resembling the second stage of the two-stage plant. Such a unit maximizes the yield of
naphtha, jet fuel, or diesel depending on the recycle cut point used in the distillation section. This
type of unit is more economical than the more complex two-stage unit when plant design capacity
is less than about 10,000–15,000 bbl/day. Commercial SSRFC plants have operated to produce low
pour point diesel fuel from waxy Middle Fast vacuum gas oils. Recent emphasis has been placed on
the upgrading of lighter gas oils into jet fuels.
Building on the theme of one- or two-stage hydrocracking, the once-through partial conversion
(OTPC) concept evolved. This concept offers the means to convert heavy vacuum gas oil feed into
high-quality gasoline, jet fuel, and diesel products by a partial conversion operation. The advantage
is lower initial capital investment and also lower utilities consumption than a plant designed for total
conversion. Because total conversion of the higher molecular weight compounds in the feedstock
is not required, once-through hydrocracking can be carried out at lower temperatures and in most
cases at lower hydrogen partial pressures than in recycle hydrocracking, where total conversion of
the feedstock is normally an objective.
Proper selection of the types of catalysts employed can even permit partial conversion of heavy
gas oil feeds to diesel and lighter products at the low hydrogen partial pressures for which gas oil
hydrotreaters are normally designed. This so-called mild hydrocracking has been attracting a great
deal of interest from refiners who have existing hydrotreaters and wish to increase their refinery’s
conversion of fuel oil into lower boiling higher value products.
Recycle hydrocracking plants are designed to operate at hydrogen partial pressures from about
1200–2300 psi depending on the type of feed being processed. Hydrogen partial pressure is set
in the design in part depending on required catalyst cycle length, but also to enable the catalyst
to convert high molecular weight polynuclear aromatic and naphthene compounds that must
be hydrogenated before they can be cracked. Hydrogen partial pressure also affects properties of
the hydrocracked products that depend on hydrogen uptake, such as jet fuel aromatics content and
smoke point and diesel cetane number. In general, the higher the feed end point, the higher the
required hydrogen partial pressure necessary to achieve satisfactory performance of the plant.
Once-through partial conversion, hydrocracking of a given feedstock may be carried out at
hydrogen partial pressures significantly lower than required for recycle total conversion hydrocracking. The potential higher catalyst deactivation rates experienced at lower hydrogen partial pressures
can be offset by using higher activity catalysts and designing the plant for lower catalyst space
velocities. Catalyst deactivation is also reduced by the elimination of the recycle stream. The lower
capital cost resulting from the reduction in plant operating pressure is much more significant than
the increase resulting from the possible additional catalyst requirement and larger volume reactors.
The Chemistry and Technology of Petroleum
compressor. Sometimes with sour feeds, the first-stage recycle gas is scrubbed with an amine system to remove hydrogen. If the feed sulfur level is high, this option can improve the performance of
the catalyst and result in less costly materials of construction.
The distillation section consists of a hydrogen sulfide (H 2 S) stripper and a recycle splitter. This
latter column separates the product into the desired cuts. The column bottoms stream is recycled
back to the second-stage feed. The recycle cut point is changed depending on the light products
needed. It can be as low as 160°C (320°F) if naphtha production is maximized (for aromatics) or as
high as 380°C (720°F) if a low pour point diesel is needed. Between these two extremes, a recycle
cut point of 260°C–285°C (500°F–550°F) results in high yields of high smoke point low freeze
point jet fuel.
Overall, a single-stage once-through (SSOT) unit resembles the first stage of the two-stage plant.
This type of hydrocracker usually requires the least capital investment. The feedstock is not completely converted to lighter products. For this application, the refiner must have a demand for highly
refined heavy oil. In many refining situations, such an oil product can be used as lube oil plant feed
or as FCC plant feed or in low sulfur oil blends or as ethylene plant feed. It also lends itself to stepwise construction of a future two-stage hydrocracker for full feed conversion.
A single-stage recycle (SSREC) unit converts heavy oil completely into light products with a
flow scheme resembling the second stage of the two-stage plant. Such a unit maximizes the yield of
naphtha, jet fuel, or diesel depending on the recycle cut point used in the distillation section. This
type of unit is more economical than the more complex two-stage unit when plant design capacity
is less than about 10,000–15,000 bbl/day. Commercial SSRFC plants have operated to produce low
pour point diesel fuel from waxy Middle Fast vacuum gas oils. Recent emphasis has been placed on
the upgrading of lighter gas oils into jet fuels.
Building on the theme of one- or two-stage hydrocracking, the once-through partial conversion
(OTPC) concept evolved. This concept offers the means to convert heavy vacuum gas oil feed into
high-quality gasoline, jet fuel, and diesel products by a partial conversion operation. The advantage
is lower initial capital investment and also lower utilities consumption than a plant designed for total
conversion. Because total conversion of the higher molecular weight compounds in the feedstock
is not required, once-through hydrocracking can be carried out at lower temperatures and in most
cases at lower hydrogen partial pressures than in recycle hydrocracking, where total conversion of
the feedstock is normally an objective.
Proper selection of the types of catalysts employed can even permit partial conversion of heavy
gas oil feeds to diesel and lighter products at the low hydrogen partial pressures for which gas oil
hydrotreaters are normally designed. This so-called mild hydrocracking has been attracting a great
deal of interest from refiners who have existing hydrotreaters and wish to increase their refinery’s
conversion of fuel oil into lower boiling higher value products.
Recycle hydrocracking plants are designed to operate at hydrogen partial pressures from about
1200–2300 psi depending on the type of feed being processed. Hydrogen partial pressure is set
in the design in part depending on required catalyst cycle length, but also to enable the catalyst
to convert high molecular weight polynuclear aromatic and naphthene compounds that must
be hydrogenated before they can be cracked. Hydrogen partial pressure also affects properties of
the hydrocracked products that depend on hydrogen uptake, such as jet fuel aromatics content and
smoke point and diesel cetane number. In general, the higher the feed end point, the higher the
required hydrogen partial pressure necessary to achieve satisfactory performance of the plant.
Once-through partial conversion, hydrocracking of a given feedstock may be carried out at
hydrogen partial pressures significantly lower than required for recycle total conversion hydrocracking. The potential higher catalyst deactivation rates experienced at lower hydrogen partial pressures
can be offset by using higher activity catalysts and designing the plant for lower catalyst space
velocities. Catalyst deactivation is also reduced by the elimination of the recycle stream. The lower
capital cost resulting from the reduction in plant operating pressure is much more significant than
the increase resulting from the possible additional catalyst requirement and larger volume reactors.