tower and product is eliminated. There is also an advantage in better control of
the tower top temperature by the flow control of the pumparound itself. This
too can be reset by temperature, but as a former plant operator, the author is not
particularly in favor of temperature reset as a finite means of control. There is
no other apparent advantage to this system. Indeed, the inclusion of additional
trays in the tower for the pumparound is quite expensive. The trays would need
to be of Monel, and some Monel lining of the shell will be required in this
section as protection against chloride attack.
Some Design Considerations
Corrosion
The top section of the atmospheric distillation tower is very prone to corrosion from
HCl. To combat this corrosion and to protect the carbon steel shell from damage,
Monel is used in tray manufacture and as lining for the tower shell. This material is
also used to line the first condenser shell interior, and the tubes may also be
fabricated in Monel. Ammonia (or amine) either in its anhydrous form or as a
solution is injected into the tower top and at specific intervals into the top section
tray spaces. The purpose of the ammonia is to neutralize as far as possible the HCl.
The injection of the ammonia is controlled by the pH reading of the condensate
water leaving the boot of the product condensate drum.
Corrosion is most virulent at the liquid interface on trays. Consequently, in most
modern fractionating towers, Monel lining of the shell is usually applied as strips on
the top four or six trays. The strips installed at the interface of the liquid/vapor on
these trays will be about 4–6 in. wide. The injection of anhydrous ammonia in the
case of configuration number 2 does cause problems in the formation of ammonium
chloride crystals occurring in the “dry” section of the condenser system. That is the
section where the hydrocarbon is condensing, but the steam is still in its vapor
phase. The injection of aqueous ammonia solution instead of the anhydrous form
solves this problem.
Sulfur compounds are also a problem with respect to corrosion, particularly in
the “cold” liquid phase of the distillate product drum. The drum shell in this case is
gunite lined. This lining is applied to the bottom section of the main drum and
extended to include the “boot” in which the condensed water is concentrated.
Gunite is best described as a cement lining and is applied in the same way as
refractory lining, by plastering it onto the shell. Other lining materials are also used.
The Distillate Drum
The design of the distillate drum includes space for the complete separation of the
aqueous phase (condensed steam) from the hydrocarbon phase. It is important for
the hydrocarbons to be as free of water as possible to protect the downstream
units. The performance of these units such as hydrotreaters and, more particularly,
the catalytic reformer are badly affected by the presence of water in their feed
streams.
140
D.S.J. Jones
the tower top temperature by the flow control of the pumparound itself. This
too can be reset by temperature, but as a former plant operator, the author is not
particularly in favor of temperature reset as a finite means of control. There is
no other apparent advantage to this system. Indeed, the inclusion of additional
trays in the tower for the pumparound is quite expensive. The trays would need
to be of Monel, and some Monel lining of the shell will be required in this
section as protection against chloride attack.
Some Design Considerations
Corrosion
The top section of the atmospheric distillation tower is very prone to corrosion from
HCl. To combat this corrosion and to protect the carbon steel shell from damage,
Monel is used in tray manufacture and as lining for the tower shell. This material is
also used to line the first condenser shell interior, and the tubes may also be
fabricated in Monel. Ammonia (or amine) either in its anhydrous form or as a
solution is injected into the tower top and at specific intervals into the top section
tray spaces. The purpose of the ammonia is to neutralize as far as possible the HCl.
The injection of the ammonia is controlled by the pH reading of the condensate
water leaving the boot of the product condensate drum.
Corrosion is most virulent at the liquid interface on trays. Consequently, in most
modern fractionating towers, Monel lining of the shell is usually applied as strips on
the top four or six trays. The strips installed at the interface of the liquid/vapor on
these trays will be about 4–6 in. wide. The injection of anhydrous ammonia in the
case of configuration number 2 does cause problems in the formation of ammonium
chloride crystals occurring in the “dry” section of the condenser system. That is the
section where the hydrocarbon is condensing, but the steam is still in its vapor
phase. The injection of aqueous ammonia solution instead of the anhydrous form
solves this problem.
Sulfur compounds are also a problem with respect to corrosion, particularly in
the “cold” liquid phase of the distillate product drum. The drum shell in this case is
gunite lined. This lining is applied to the bottom section of the main drum and
extended to include the “boot” in which the condensed water is concentrated.
Gunite is best described as a cement lining and is applied in the same way as
refractory lining, by plastering it onto the shell. Other lining materials are also used.
The Distillate Drum
The design of the distillate drum includes space for the complete separation of the
aqueous phase (condensed steam) from the hydrocarbon phase. It is important for
the hydrocarbons to be as free of water as possible to protect the downstream
units. The performance of these units such as hydrotreaters and, more particularly,
the catalytic reformer are badly affected by the presence of water in their feed
streams.
140
D.S.J. Jones