The atmospheric residue. This is the bottom product from the atmospheric
distillation of the crude oil. Most crude oils are distilled in the atmospheric crude
oil tower to cut the atmospheric residue at a +650
F up to a +680
F cut point.
Cutting the residue heavier than +680
F risks the possibility of thermal cracking with
heavier coke lay down and discoloring of the distillate products. Those atmospheric
crude towers that do operate at higher cut points minimize the cracking by recycling
the cold quench into the bottom of the tower (below the bottom stripping tray) and
minimizing the residue holdup time in the tower. The atmospheric residue may be
routed to the fuel oil pool as the precursor to several grades of finished fuel oil
products. The other options for this stream in a modern refinery are as follows:
• Feed to a vacuum distillation unit (this is the most common option).
• Feed to a thermal cracker (visbreaker or coking unit). See the chapter on
“▶ Upgrading the Bottom of the Barrel.”
• Feed to a deep oil fluid catalytic cracker. Refer to the chapter “▶ Fluid Catalytic
Cracking (FCC) in Petroleum Refining.”
• Feed to a hydrocracker or hydrotreater. Refer to chapters titled “▶ Hydrocracking in Petroleum Processing” and “▶ Hydrotreating in Petroleum Processing.”
The vacuum distillation of atmospheric residue. In modern refinery practice, the
distillation of atmospheric residue is accomplished under high vacuum conditions in
a specially designed tower whose internal equipment ensures a very low pressure
drop. Normally the vacuum conditions in the flash zone of the tower allow about the
same percentage of distillate based on the tower feed to be cut in this tower as the
distillate on whole crude in the atmospheric unit. Again the flash zone temperature in
the vacuum unit is kept below 700
F. Usually there are two or three vacuum
distillates from this tower. In a pure energy-related refinery, there will be two. The
heavier of the two (say to a cut range of 750–930
F) will be the feed to a distillate
hydrocracker (see the chapter “▶ Hydrocracking in Petroleum Processing”) or to a
fluid catalytic cracker (see the chapter “▶ Fluid Catalytic Cracking (FCC) in Petroleum Refining”). In both of these cases, however, a small heavier cut is taken off and
returned to the tower bottom in order to correct the bottom distillate Conradson
carbon content to meet the specification required for either of the two downstream
processes. This distillate product is usually titled HVGO (heavy vacuum gas oil).
For those refineries which produce lube oils as nonenergy products, this bottom
distillate may be split into two side streams in order to provide the flexibility
required in the production of the lube oil blending stock specifications (see the
chapter on “▶ Non-energy Refineries in Petroleum Processing”).
The light vacuum distillate is taken off as a top side stream and is usually routed
to a hydrodesulfurizer to be sent either to the gas oil pool as heating oil stock or
routed to the fuel oil pool as blending stock. This side stream is a cut range of
680–750
F and is usually labeled LVGO (light vacuum gas oil).
The vacuum residue. This is the bottom product from the vacuum distillation
unit. Just as in the case of the atmospheric residue, it has several options for its use
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D.S.J. Jones and S.A. Treese
distillation of the crude oil. Most crude oils are distilled in the atmospheric crude
oil tower to cut the atmospheric residue at a +650
F up to a +680
F cut point.
Cutting the residue heavier than +680
F risks the possibility of thermal cracking with
heavier coke lay down and discoloring of the distillate products. Those atmospheric
crude towers that do operate at higher cut points minimize the cracking by recycling
the cold quench into the bottom of the tower (below the bottom stripping tray) and
minimizing the residue holdup time in the tower. The atmospheric residue may be
routed to the fuel oil pool as the precursor to several grades of finished fuel oil
products. The other options for this stream in a modern refinery are as follows:
• Feed to a vacuum distillation unit (this is the most common option).
• Feed to a thermal cracker (visbreaker or coking unit). See the chapter on
“▶ Upgrading the Bottom of the Barrel.”
• Feed to a deep oil fluid catalytic cracker. Refer to the chapter “▶ Fluid Catalytic
Cracking (FCC) in Petroleum Refining.”
• Feed to a hydrocracker or hydrotreater. Refer to chapters titled “▶ Hydrocracking in Petroleum Processing” and “▶ Hydrotreating in Petroleum Processing.”
The vacuum distillation of atmospheric residue. In modern refinery practice, the
distillation of atmospheric residue is accomplished under high vacuum conditions in
a specially designed tower whose internal equipment ensures a very low pressure
drop. Normally the vacuum conditions in the flash zone of the tower allow about the
same percentage of distillate based on the tower feed to be cut in this tower as the
distillate on whole crude in the atmospheric unit. Again the flash zone temperature in
the vacuum unit is kept below 700
F. Usually there are two or three vacuum
distillates from this tower. In a pure energy-related refinery, there will be two. The
heavier of the two (say to a cut range of 750–930
F) will be the feed to a distillate
hydrocracker (see the chapter “▶ Hydrocracking in Petroleum Processing”) or to a
fluid catalytic cracker (see the chapter “▶ Fluid Catalytic Cracking (FCC) in Petroleum Refining”). In both of these cases, however, a small heavier cut is taken off and
returned to the tower bottom in order to correct the bottom distillate Conradson
carbon content to meet the specification required for either of the two downstream
processes. This distillate product is usually titled HVGO (heavy vacuum gas oil).
For those refineries which produce lube oils as nonenergy products, this bottom
distillate may be split into two side streams in order to provide the flexibility
required in the production of the lube oil blending stock specifications (see the
chapter on “▶ Non-energy Refineries in Petroleum Processing”).
The light vacuum distillate is taken off as a top side stream and is usually routed
to a hydrodesulfurizer to be sent either to the gas oil pool as heating oil stock or
routed to the fuel oil pool as blending stock. This side stream is a cut range of
680–750
F and is usually labeled LVGO (light vacuum gas oil).
The vacuum residue. This is the bottom product from the vacuum distillation
unit. Just as in the case of the atmospheric residue, it has several options for its use
58
D.S.J. Jones and S.A. Treese