Setting the Flash Zone Temperature
Calculate the EFV of the whole crude using the method described in the “▶ Introduction to Crude Oil and Petroleum Processing” chapter of this Handbook. This
EFV curve is at atmospheric pressure. The temperature of the flash at atmospheric
pressure is read off as the % volume vaporized to meet the amount of distillate
products and the overflash required by the process. This overflash is usually fixed at
between 3 % and 5 % volume on crude. Its purpose in the process is to provide that
extra heat in order to generate sufficient reflux downflow over the trays to satisfy the
prescribed degree of separation between the products.
This atmospheric flash temperature is now adjusted to the temperature at
the previously calculated partial pressure existing in the flash zone. This may be
done by reading the temperature at the partial pressure using the PVT curves
shown as Appendix 1 in the appendices to this chapter. This is the flash zone
temperature.
Other Features of the Flash Zone Section
Most crude oils contain sulfur in some form or other and organic chlorides which
are not removed in the pretreating desalter. Corrosion from these impurities is
particularly virulent in mixed phases (liquid/vapor) and at the elevated temperature
experienced in the unit’s flash zone and made more so by the presence of steam.
A cladding of 11/13 chrome is usually applied as a protective cover over the tower’s
carbon steel shell in this section. This cladding should be extended also to include
the residue stripper and the section of the tower containing the first four to six wash
trays above the flash zone. These trays and the residue stripping trays should also be
of 11/13 chrome alloy.
Effective separation of the distillate vapor phase and the unvaporized residue
phase is enhanced by the inclusion of a “swirl” at the inlet of the flash zone. The
crude from the fired heater is routed through this “swirl” which is an inverted trough
and extends around two-thirds of the tower circumference. The swirling action
caused by this forced flow of the mixed phases allows the lighter vapor phase to
separate to a large extent from the heavier liquid phase. The final separation occurs
on the top stripping tray.
The Fractionator Overhead System
The fractionator overhead equipment has four functions. These are:
• To condense the overhead vapors including the stripping steam
• To return and control the reflux condensate to the tower
• To collect and dispatch the overhead product
• To separate and dispose of the condensed steam
These functions are accomplished in several different ways. Some of these
overhead configurations are described in the following paragraphs.
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
135
Calculate the EFV of the whole crude using the method described in the “▶ Introduction to Crude Oil and Petroleum Processing” chapter of this Handbook. This
EFV curve is at atmospheric pressure. The temperature of the flash at atmospheric
pressure is read off as the % volume vaporized to meet the amount of distillate
products and the overflash required by the process. This overflash is usually fixed at
between 3 % and 5 % volume on crude. Its purpose in the process is to provide that
extra heat in order to generate sufficient reflux downflow over the trays to satisfy the
prescribed degree of separation between the products.
This atmospheric flash temperature is now adjusted to the temperature at
the previously calculated partial pressure existing in the flash zone. This may be
done by reading the temperature at the partial pressure using the PVT curves
shown as Appendix 1 in the appendices to this chapter. This is the flash zone
temperature.
Other Features of the Flash Zone Section
Most crude oils contain sulfur in some form or other and organic chlorides which
are not removed in the pretreating desalter. Corrosion from these impurities is
particularly virulent in mixed phases (liquid/vapor) and at the elevated temperature
experienced in the unit’s flash zone and made more so by the presence of steam.
A cladding of 11/13 chrome is usually applied as a protective cover over the tower’s
carbon steel shell in this section. This cladding should be extended also to include
the residue stripper and the section of the tower containing the first four to six wash
trays above the flash zone. These trays and the residue stripping trays should also be
of 11/13 chrome alloy.
Effective separation of the distillate vapor phase and the unvaporized residue
phase is enhanced by the inclusion of a “swirl” at the inlet of the flash zone. The
crude from the fired heater is routed through this “swirl” which is an inverted trough
and extends around two-thirds of the tower circumference. The swirling action
caused by this forced flow of the mixed phases allows the lighter vapor phase to
separate to a large extent from the heavier liquid phase. The final separation occurs
on the top stripping tray.
The Fractionator Overhead System
The fractionator overhead equipment has four functions. These are:
• To condense the overhead vapors including the stripping steam
• To return and control the reflux condensate to the tower
• To collect and dispatch the overhead product
• To separate and dispose of the condensed steam
These functions are accomplished in several different ways. Some of these
overhead configurations are described in the following paragraphs.
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
135