liquid will contain entrained light ends from the vapor. The actual temperature
will therefore be somewhat lower than the calculated, and this difference is
provided by Fig. 9. Establish the actual temperature of the side stream drawoff.
Note this will also be the temperature of the pumparound liquid.
Side stream stripping. The side stream enters a steam stripper at the drawoff
temperature. It is steam stripped, and the amount of stripping steam and the
corresponding stripout may be read off from Fig. 10. The steam introduced below
the bottom stripping tray is usually superheated at 50 psig. The temperature of the
stripout and steam leaving the top of the stripper to enter the main tower can be
taken as 5
F below the temperature of the stripper feed. The mol weight of the
stripout is taken as the average of the vapors lighter than the drawoff. A heat
balance can now be carried out over the stripper with an unknown being the stripped
product enthalpy and temperature. The product outlet temperature and enthalpy is
calculated by solving the equation:
Heat in ¼ Heat out
Side stream strippers for each product drawoff are usually stacked in one single
column. It becomes an exercise in layout to locate these stripper sections. It begins
with establishing the height of the bottom stripper section which will ensure
sufficient head to provide available NPSH for the product rundown pump. Thereafter, each successive section is located to ensure free flow of the liquid feed into
the respective stripper and to minimize the length of the vapor return line to the
main tower.
The Pumparounds
The mechanism of the pumparounds and their purpose has already been described
and discussed. It remains now to examine how to evaluate and detail quantitatively
their size in terms of the duty they will be required to perform. This will be followed
with some criteria that need to be observed in their design.
Total pumparound duties. This begins by establishing the tower bottom temperature (residue outlet temperature) and then conducting an overall tower heat
balance. To calculate the tower bottom temperature, consider the diagram in
Fig. 11 and the following calculation steps:
Step 1. Establish the material entering the top stripping tray. This will be the total
residue product which now includes the overflash as a liquid and the stripout
material (also as liquid). This is taken as being at the flash zone temperature.
Step 2. Calculate the total heat input to the bottom stripper. This will include the
heat into the top stripping tray plus the heat in the stripping steam (enters below
the bottom tray).
Step 3. Calculate the heat out of the stripper. This includes the bottom product at an
unknown temperature and enthalpy, the stripout from the top stripping tray, and
the steam from the top stripping tray. Assume the stripout and the steam will be
146
D.S.J. Jones
will therefore be somewhat lower than the calculated, and this difference is
provided by Fig. 9. Establish the actual temperature of the side stream drawoff.
Note this will also be the temperature of the pumparound liquid.
Side stream stripping. The side stream enters a steam stripper at the drawoff
temperature. It is steam stripped, and the amount of stripping steam and the
corresponding stripout may be read off from Fig. 10. The steam introduced below
the bottom stripping tray is usually superheated at 50 psig. The temperature of the
stripout and steam leaving the top of the stripper to enter the main tower can be
taken as 5
F below the temperature of the stripper feed. The mol weight of the
stripout is taken as the average of the vapors lighter than the drawoff. A heat
balance can now be carried out over the stripper with an unknown being the stripped
product enthalpy and temperature. The product outlet temperature and enthalpy is
calculated by solving the equation:
Heat in ¼ Heat out
Side stream strippers for each product drawoff are usually stacked in one single
column. It becomes an exercise in layout to locate these stripper sections. It begins
with establishing the height of the bottom stripper section which will ensure
sufficient head to provide available NPSH for the product rundown pump. Thereafter, each successive section is located to ensure free flow of the liquid feed into
the respective stripper and to minimize the length of the vapor return line to the
main tower.
The Pumparounds
The mechanism of the pumparounds and their purpose has already been described
and discussed. It remains now to examine how to evaluate and detail quantitatively
their size in terms of the duty they will be required to perform. This will be followed
with some criteria that need to be observed in their design.
Total pumparound duties. This begins by establishing the tower bottom temperature (residue outlet temperature) and then conducting an overall tower heat
balance. To calculate the tower bottom temperature, consider the diagram in
Fig. 11 and the following calculation steps:
Step 1. Establish the material entering the top stripping tray. This will be the total
residue product which now includes the overflash as a liquid and the stripout
material (also as liquid). This is taken as being at the flash zone temperature.
Step 2. Calculate the total heat input to the bottom stripper. This will include the
heat into the top stripping tray plus the heat in the stripping steam (enters below
the bottom tray).
Step 3. Calculate the heat out of the stripper. This includes the bottom product at an
unknown temperature and enthalpy, the stripout from the top stripping tray, and
the steam from the top stripping tray. Assume the stripout and the steam will be
146
D.S.J. Jones