Drawoff Temperatures
Unlike the atmospheric crude distillation unit, the temperature of the vacuum tower
bottom (bitumen) will be essentially the flash zone temperature. There will be a
small difference, say, 2–3
F, below the actual flash zone temperature due to
overflash returning from the wash trays. Very often the overflash amount is
drawn off from below the wash section and either sent to fuel or blended into the
bitumen stream external to the tower. In this case the unquenched bitumen leaving
the tower will be at flash zone temperature. Again there is no steam present to
influence this temperature.
Side stream drawoff temperatures are easier to calculate for a vacuum tower than
was the case for the atmospheric tower. This is so because in a “dry vacuum”
column, there is no steam to influence partial pressures, and of course there is no
side stream stripping.
A method similar to the “Packie” method used for the atmospheric column is
used for the vacuum column drawoff. In this case however, it is only necessary to
determine the initial boiling point of the side stream EFV curve at the tower
condition to arrive at the drawoff temperature.
Note: It is the IBP of the actual EFV curve in this case, not the IBP of the flash
reference line as in the case of the atmospheric unit. Also there will be no “Packie”
correction factors required in this case.
The calculation steps for this procedure are as follows:
Step 1. Draw the EFV curve from the side stream TBP curve using the method
described earlier in this chapter. Only the 0, 10, 30, and 50 % vol section of the
curve need to be developed.
Step 2. Set the total pressure at the drawoff tray. If this is not available as plant data,
then use the criteria for pressure drop given in the item dealing with the
flash zone.
Step 3. Calculate the partial pressure of the side stream product at the drawoff tray.
To do this, consider all material lighter than the drawoff side stream to be inert.
Include in the inert estimates of air leakage and cracked hydrocarbon vapors as
described earlier in this chapter in the item dealing with tower overhead ejector
system. The total hydrocarbon vapor will include the overflow from the drawoff
tray. As a rule of thumb, estimate overflow as:
Top side stream
0.8 times product
Mid side stream
1.0 times product
Bottom side stream
1.5–2.0 times product
Step 4. Using the vapor pressure curves, relate the IBP temperature of the EFV to
the partial pressure determined in Step 3. This is the drawoff temperature, and
this will be the temperature for the respective side streams and pumparound
drawoff that will be used in the tower heat balances.
188
D.S.J. Jones
Unlike the atmospheric crude distillation unit, the temperature of the vacuum tower
bottom (bitumen) will be essentially the flash zone temperature. There will be a
small difference, say, 2–3
F, below the actual flash zone temperature due to
overflash returning from the wash trays. Very often the overflash amount is
drawn off from below the wash section and either sent to fuel or blended into the
bitumen stream external to the tower. In this case the unquenched bitumen leaving
the tower will be at flash zone temperature. Again there is no steam present to
influence this temperature.
Side stream drawoff temperatures are easier to calculate for a vacuum tower than
was the case for the atmospheric tower. This is so because in a “dry vacuum”
column, there is no steam to influence partial pressures, and of course there is no
side stream stripping.
A method similar to the “Packie” method used for the atmospheric column is
used for the vacuum column drawoff. In this case however, it is only necessary to
determine the initial boiling point of the side stream EFV curve at the tower
condition to arrive at the drawoff temperature.
Note: It is the IBP of the actual EFV curve in this case, not the IBP of the flash
reference line as in the case of the atmospheric unit. Also there will be no “Packie”
correction factors required in this case.
The calculation steps for this procedure are as follows:
Step 1. Draw the EFV curve from the side stream TBP curve using the method
described earlier in this chapter. Only the 0, 10, 30, and 50 % vol section of the
curve need to be developed.
Step 2. Set the total pressure at the drawoff tray. If this is not available as plant data,
then use the criteria for pressure drop given in the item dealing with the
flash zone.
Step 3. Calculate the partial pressure of the side stream product at the drawoff tray.
To do this, consider all material lighter than the drawoff side stream to be inert.
Include in the inert estimates of air leakage and cracked hydrocarbon vapors as
described earlier in this chapter in the item dealing with tower overhead ejector
system. The total hydrocarbon vapor will include the overflow from the drawoff
tray. As a rule of thumb, estimate overflow as:
Top side stream
0.8 times product
Mid side stream
1.0 times product
Bottom side stream
1.5–2.0 times product
Step 4. Using the vapor pressure curves, relate the IBP temperature of the EFV to
the partial pressure determined in Step 3. This is the drawoff temperature, and
this will be the temperature for the respective side streams and pumparound
drawoff that will be used in the tower heat balances.
188
D.S.J. Jones