temperature and appear in the splitter bottoms and are subsequently desulfurized in
the selective desulfurization reactor(s). The effluent from the SHU
™ reactor is
routed to a splitter which separates the light cracked naphtha (LCN) as an overhead
product which is essentially sulfur-free. The splitter bottoms is routed to the
selective desulfurization reactors HDS-1 and HDS-2 (Fig. 14). The second desulfurization reactor operates at a higher temperature. Decomposition of the remaining
sulfur compounds, due to the higher temperature, limits the remaining recombinant
mercaptans to meet the desired sulfur specification.
At high feed sulfur levels, the amount of H 2 S produced is so high, and the
formation of mercaptans is so extensive that low product sulfur levels cannot be
achieved without separation of the H 2 S between two stages of desulfurization. This
type of flow scheme is represented below by two-stage SelectFining (see Fig. 15).
The FCC gasoline enters the DIOS reactor to saturate diolefins. The effluent of the
DIOS reactor is mixed with recycle gas, heated to reaction temperature, and routed
to the first selective desulfurization reactor. The effluent of the first desulfurization
reactor is cooled, stripped of H 2 S, mixed with recycle gas, heated to reaction
temperature, and routed to the second selective desulfurization reactor. Finally,
the effluent of the second selective desulfurization reactor is stripped of H 2 S and
routed to product blending/storage.
Process Conditions
Typical process conditions are shown in Table 5.
FCC Feed Pretreating
Hydrotreating the feed to an FCC is usually practiced to remove sulfur and limit
SOx emissions but also impacts the FCC in other ways. Hydrotreating the feed to
the FCC results in hydrogen addition to the feed due to saturation of aromatics, thus
rendering the resulting molecules easier to crack in the FCC and increasing
conversion and gasoline yield. An additional benefit is the reduction in the sulfur
content of the FCC gasoline, thus reducing the severity required of the FCC
gasoline posttreater and consequently limiting olefin saturation and octane loss.
FCC feedstocks are vacuum gas oils boiling in the range of ~650–1,100
F.
Sulfur levels may vary widely from ~0.5 to 3 wt%. Nitrogen levels may vary from
500 to 3,000 wppm. Additionally, depending on the end point, metals (Ni and V)
may also be present. While decreasing the sulfur content of the FCC feed has
beneficial effects, reducing the nitrogen content and adding hydrogen will increase
conversion and gasoline yield.
Feedstock Characteristics
Generally the chemical nature of the FCC feed is similar to that of distillates but of
higher molecular weight.
Hydrotreating in Petroleum Processing
403
the selective desulfurization reactor(s). The effluent from the SHU
™ reactor is
routed to a splitter which separates the light cracked naphtha (LCN) as an overhead
product which is essentially sulfur-free. The splitter bottoms is routed to the
selective desulfurization reactors HDS-1 and HDS-2 (Fig. 14). The second desulfurization reactor operates at a higher temperature. Decomposition of the remaining
sulfur compounds, due to the higher temperature, limits the remaining recombinant
mercaptans to meet the desired sulfur specification.
At high feed sulfur levels, the amount of H 2 S produced is so high, and the
formation of mercaptans is so extensive that low product sulfur levels cannot be
achieved without separation of the H 2 S between two stages of desulfurization. This
type of flow scheme is represented below by two-stage SelectFining (see Fig. 15).
The FCC gasoline enters the DIOS reactor to saturate diolefins. The effluent of the
DIOS reactor is mixed with recycle gas, heated to reaction temperature, and routed
to the first selective desulfurization reactor. The effluent of the first desulfurization
reactor is cooled, stripped of H 2 S, mixed with recycle gas, heated to reaction
temperature, and routed to the second selective desulfurization reactor. Finally,
the effluent of the second selective desulfurization reactor is stripped of H 2 S and
routed to product blending/storage.
Process Conditions
Typical process conditions are shown in Table 5.
FCC Feed Pretreating
Hydrotreating the feed to an FCC is usually practiced to remove sulfur and limit
SOx emissions but also impacts the FCC in other ways. Hydrotreating the feed to
the FCC results in hydrogen addition to the feed due to saturation of aromatics, thus
rendering the resulting molecules easier to crack in the FCC and increasing
conversion and gasoline yield. An additional benefit is the reduction in the sulfur
content of the FCC gasoline, thus reducing the severity required of the FCC
gasoline posttreater and consequently limiting olefin saturation and octane loss.
FCC feedstocks are vacuum gas oils boiling in the range of ~650–1,100
F.
Sulfur levels may vary widely from ~0.5 to 3 wt%. Nitrogen levels may vary from
500 to 3,000 wppm. Additionally, depending on the end point, metals (Ni and V)
may also be present. While decreasing the sulfur content of the FCC feed has
beneficial effects, reducing the nitrogen content and adding hydrogen will increase
conversion and gasoline yield.
Feedstock Characteristics
Generally the chemical nature of the FCC feed is similar to that of distillates but of
higher molecular weight.
Hydrotreating in Petroleum Processing
403