tray temperature as possible. If the feed is well below its bubble point on entering
the tower, several trays below the feed tray are taken up for heat transfer before
effective mass transfer can begin. This could prevent the specified product separation occurring and tray efficiency in this section of the tower falls off dramatically.
Feed condition can be checked by bubble point calculation and a flash calculation.
Hot Feed
This situation is probably the more serious regarding feed condition. If the feed
enters at a temperature far above its bubble point, its resulting enthalpy will be such
as to reduce the reboiler duty. This will occur automatically as the tower must
always be in heat balance. The tower controls will maintain the product quantity
and split. However, if the reboiler duty is drastically reduced, insufficient stripper
vapors will be available for the stripping function. Poor separation will result.
As a rule of thumb, the stripping vapor to the bottom tray must be at least 70 %
mole of the bottom product made. In super fractionation, such as a deisopentanizer,
this figure would be at least 80–100 % of bottoms made.
Heat balances as shown will quickly determine the stripping vapor status.
Ideal Feed Condition
Ideally the feed should enter the tower close to the feed tray temperature. Usually
then at the inlet pressure, the feed will be in a mixed phase with the vapor portion
very close in quantity to the distillate product. As the feed to these units is generally
heated by the bottom product heat exchange, the approach temperatures are always
a consideration. To maintain good feed conditions, however it is often beneficial to
include a separate steam (or hot oil) feed preheater.
Entrainment
A common cause of poor plant performance at high throughout or high reflux rates
is liquid entrainment or carryover from tray to tray. Very often in a high load and
entrainment situation, the problem is further aggravated by increasing reflux to
attempt separation improvement.
A well-designed light ends tower can operate up to about 120 % of allowable
flood before substantial carryover occurs. Loading above this figure would result in
some degree of entrainment.
Downcomer Backup and Flooding
If tower loadings are increased well above allowable flood point, there is a real
danger that downcomers become unable to cope with the liquid load. They would
fill, and the tower would be in a state of flood. This will be very apparent with very
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D.S.J. Jones
the tower, several trays below the feed tray are taken up for heat transfer before
effective mass transfer can begin. This could prevent the specified product separation occurring and tray efficiency in this section of the tower falls off dramatically.
Feed condition can be checked by bubble point calculation and a flash calculation.
Hot Feed
This situation is probably the more serious regarding feed condition. If the feed
enters at a temperature far above its bubble point, its resulting enthalpy will be such
as to reduce the reboiler duty. This will occur automatically as the tower must
always be in heat balance. The tower controls will maintain the product quantity
and split. However, if the reboiler duty is drastically reduced, insufficient stripper
vapors will be available for the stripping function. Poor separation will result.
As a rule of thumb, the stripping vapor to the bottom tray must be at least 70 %
mole of the bottom product made. In super fractionation, such as a deisopentanizer,
this figure would be at least 80–100 % of bottoms made.
Heat balances as shown will quickly determine the stripping vapor status.
Ideal Feed Condition
Ideally the feed should enter the tower close to the feed tray temperature. Usually
then at the inlet pressure, the feed will be in a mixed phase with the vapor portion
very close in quantity to the distillate product. As the feed to these units is generally
heated by the bottom product heat exchange, the approach temperatures are always
a consideration. To maintain good feed conditions, however it is often beneficial to
include a separate steam (or hot oil) feed preheater.
Entrainment
A common cause of poor plant performance at high throughout or high reflux rates
is liquid entrainment or carryover from tray to tray. Very often in a high load and
entrainment situation, the problem is further aggravated by increasing reflux to
attempt separation improvement.
A well-designed light ends tower can operate up to about 120 % of allowable
flood before substantial carryover occurs. Loading above this figure would result in
some degree of entrainment.
Downcomer Backup and Flooding
If tower loadings are increased well above allowable flood point, there is a real
danger that downcomers become unable to cope with the liquid load. They would
fill, and the tower would be in a state of flood. This will be very apparent with very
226
D.S.J. Jones