leave the top of the tower to be condensed and collected in the overhead drum.
A portion of this stream is returned as reflux, while the remainder is delivered to the
light end processes for stabilizing and further distillation.
The side stream distillates shown in the diagram are:
• Heavy gas oil (has the highest boiling point)
• Light gas oil (will become diesel)
• Kerosene (will become jet fuel)
A “Pumparound” section is included at the light gas oil drawoff. This is simply
an internal condenser which takes heat out of that section of the tower. This in turn
ensures a continued reflux stream flow below that section. The product side streams
are stripped free of entrained light ends in separate stripping towers. These towers
also contain fractionation trays (usually four but sometimes as many as ten), and the
side stream drawn off the main tower enters the top tray of its respective stripper.
Steam is injected below the bottom tray and moves up the tower to leave at the top,
together with the light end stripout, and is returned to the main fractionator at a
point directly above the side stream drawoff tray. These side stream stripper towers
are often stacked one above the other in a single column in such a way as to allow
free flow from the side stream drawoff tray to its stripper tower. On a few occasions,
where the particular side stream specification requires it, the stripping may be
effected by reboiling instead of using steam. One such requirement may be in the
Kero side stream if this stream is to be routed directly into jet fuel blending and
therefore must be dry.
The residue (unvaporized portion of the crude) leaves the flash zone to flow over
four stripping trays countercurrent to the flow of stripping steam. This stripping
steam enters the tower below the bottom stripping tray. Its purpose primarily is to
strip the residue free of entrained light ends. The fact that this steam enters the flash
zone also enhances the flashing of the crude in this zone by creating a reduced
partial pressure for the liquid/vapor separation. This becomes an important factor in
the design and operation of the atmospheric crude distillation unit. The stripped
residue leaves the bottom of the unit to be routed either through the unit’s heat
exchanger system and either to product storage or to some downstream processing
unit such as a vacuum distillation unit or a thermal cracker.
Development of the Material Balance for the Atmospheric Crude
Distillation Unit
The knowledge of the material balance in any refining process is important both for
ensuring its proper design and later for its proper operation. Because of the relative
number of streams involved, this is particularly so in the case of the atmospheric
crude distillation unit. The operation of this unit also is critical to the performance
of downstream units such as catalytic crackers and catalytic reformers. The material
balance for any specific operation required of the unit (e.g., maximizing naphtha
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
129
A portion of this stream is returned as reflux, while the remainder is delivered to the
light end processes for stabilizing and further distillation.
The side stream distillates shown in the diagram are:
• Heavy gas oil (has the highest boiling point)
• Light gas oil (will become diesel)
• Kerosene (will become jet fuel)
A “Pumparound” section is included at the light gas oil drawoff. This is simply
an internal condenser which takes heat out of that section of the tower. This in turn
ensures a continued reflux stream flow below that section. The product side streams
are stripped free of entrained light ends in separate stripping towers. These towers
also contain fractionation trays (usually four but sometimes as many as ten), and the
side stream drawn off the main tower enters the top tray of its respective stripper.
Steam is injected below the bottom tray and moves up the tower to leave at the top,
together with the light end stripout, and is returned to the main fractionator at a
point directly above the side stream drawoff tray. These side stream stripper towers
are often stacked one above the other in a single column in such a way as to allow
free flow from the side stream drawoff tray to its stripper tower. On a few occasions,
where the particular side stream specification requires it, the stripping may be
effected by reboiling instead of using steam. One such requirement may be in the
Kero side stream if this stream is to be routed directly into jet fuel blending and
therefore must be dry.
The residue (unvaporized portion of the crude) leaves the flash zone to flow over
four stripping trays countercurrent to the flow of stripping steam. This stripping
steam enters the tower below the bottom stripping tray. Its purpose primarily is to
strip the residue free of entrained light ends. The fact that this steam enters the flash
zone also enhances the flashing of the crude in this zone by creating a reduced
partial pressure for the liquid/vapor separation. This becomes an important factor in
the design and operation of the atmospheric crude distillation unit. The stripped
residue leaves the bottom of the unit to be routed either through the unit’s heat
exchanger system and either to product storage or to some downstream processing
unit such as a vacuum distillation unit or a thermal cracker.
Development of the Material Balance for the Atmospheric Crude
Distillation Unit
The knowledge of the material balance in any refining process is important both for
ensuring its proper design and later for its proper operation. Because of the relative
number of streams involved, this is particularly so in the case of the atmospheric
crude distillation unit. The operation of this unit also is critical to the performance
of downstream units such as catalytic crackers and catalytic reformers. The material
balance for any specific operation required of the unit (e.g., maximizing naphtha
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
129