Calculating Flash Zone Conditions in a Vacuum Unit
Flash zone conditions are easier to calculate for a “Dry” vacuum unit than for the
atmospheric crude unit. Indeed the flash zone conditions can be measured in
vacuum units with a greater degree of accuracy than in the case of the atmospheric
column.
The procedure for predicting flash zone conditions in this case follows a similar
route to that in the atmospheric unit case. The following steps describe this
procedure.
Step 1. Develop the EFV from the TBP curve of the reduced crude. The same
method that was used when discussing the flash zone of the atmospheric crude
unit in this chapter will apply in this case also. Remember that the EFV
calculated is for atmospheric pressure.
Step 2. Develop the material balance for the vacuum unit. As the flash zone
conditions are to be used in most calculations involving the vacuum tower, it
is best to develop the material balance at this stage. To do this, determine the
distillate cuts required, and, by using the respective crude assays, determine the
specific gravity for each cut using mid boiling points. The mid boiling point for
each distillate product only should be determined in this case. Use the method
described in the chapter titled “▶ Introduction to Crude Oil and Petroleum
Processing” of this Handbook to determine mol weight. Some of these will
need to be extrapolated.
Step 3. Set the overflash. Now in this area of the TBP and EFV curves, the slope of
the curve is quite steep. That is, there is a high temperature difference for each
percent of volume increase. In vacuum units, therefore, a 1–2 % overflash would
be realistic to meet a realistic coil outlet temperature.
Step 4. Determine the new flash temperature to include the overflash from the EFV
curve. This is the temperature at atmospheric pressure.
Step 5. Calculate the total pressure of the flash zone. Vacuum towers operate at
or below 5 mmHg at the top. Pressure drop through the tower should not be
more than 25 mmHg. Well-designed off-take trays and packing should be as
follows:
Grid packing
6–7 mmHg per 10 f. of packed height
Drawoff (chimney trays)
2–3 mmHg/tray
Step 6. There will be no partial pressure calculation of hydrocarbon vapor (as in the
case of the atmospheric unit) as there is no steam in the flash zone of a “dry”
vacuum tower. The total pressure calculated in Step 5 is the actual hydrocarbon
flash pressure. Using the vapor pressure curves, determine the flash zone temperature at the total flash zone pressure. This is the flash zone temperature that
will now be used for all remaining heat balances.
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
187
Flash zone conditions are easier to calculate for a “Dry” vacuum unit than for the
atmospheric crude unit. Indeed the flash zone conditions can be measured in
vacuum units with a greater degree of accuracy than in the case of the atmospheric
column.
The procedure for predicting flash zone conditions in this case follows a similar
route to that in the atmospheric unit case. The following steps describe this
procedure.
Step 1. Develop the EFV from the TBP curve of the reduced crude. The same
method that was used when discussing the flash zone of the atmospheric crude
unit in this chapter will apply in this case also. Remember that the EFV
calculated is for atmospheric pressure.
Step 2. Develop the material balance for the vacuum unit. As the flash zone
conditions are to be used in most calculations involving the vacuum tower, it
is best to develop the material balance at this stage. To do this, determine the
distillate cuts required, and, by using the respective crude assays, determine the
specific gravity for each cut using mid boiling points. The mid boiling point for
each distillate product only should be determined in this case. Use the method
described in the chapter titled “▶ Introduction to Crude Oil and Petroleum
Processing” of this Handbook to determine mol weight. Some of these will
need to be extrapolated.
Step 3. Set the overflash. Now in this area of the TBP and EFV curves, the slope of
the curve is quite steep. That is, there is a high temperature difference for each
percent of volume increase. In vacuum units, therefore, a 1–2 % overflash would
be realistic to meet a realistic coil outlet temperature.
Step 4. Determine the new flash temperature to include the overflash from the EFV
curve. This is the temperature at atmospheric pressure.
Step 5. Calculate the total pressure of the flash zone. Vacuum towers operate at
or below 5 mmHg at the top. Pressure drop through the tower should not be
more than 25 mmHg. Well-designed off-take trays and packing should be as
follows:
Grid packing
6–7 mmHg per 10 f. of packed height
Drawoff (chimney trays)
2–3 mmHg/tray
Step 6. There will be no partial pressure calculation of hydrocarbon vapor (as in the
case of the atmospheric unit) as there is no steam in the flash zone of a “dry”
vacuum tower. The total pressure calculated in Step 5 is the actual hydrocarbon
flash pressure. Using the vapor pressure curves, determine the flash zone temperature at the total flash zone pressure. This is the flash zone temperature that
will now be used for all remaining heat balances.
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
187