Hydrocarbon Processing - April 2022 - 47

Heat Transfer
The second challenge faced during the startup of the column
is starting the tower first on total reflux, so direct feed-in can begin
whenever upstream is ready. Total reflux means there is no
feed to the tower and no product withdrawn, only circulation
with inventory. When the feedrate to the reboiler from the bottom
tray is lower and the liquid thrown by the reboiler return
(effluent) to the other compartment is higher (reboiler intake
from one compartment and return to the other), the level in the
reboiler compartment is depleted and this compartment will
dry. Reboiler return goes to the product compartment and its
level increases continuously. So, the level of reflux drum level
will reduce and the bottom would fill up. One solution is to have
an equalization line to equalize the liquid in both compartments
during the total reflux run. This solution is provided by Kister.4
To avoid these two problems, many designers put a hole
in the bottom baffle, which is called a preferential baffle. This
means the reboiler has the ability to take liquid from both compartments.
This solution is a combination of a once-through
and a circulating reboiler. The theoretical separation efficiency
of the preferential reboiler is higher than a circulating reboiler
and lower than once-through reboiler.6
This preferential baffle
provides operational flexibility during the startup of the column
running on total reflux. This hole in the baffle (which makes
it preferential) equalizes the level in both compartments. This
baffle does not need any seal welding or to be fully tight.
FIG. 4 shows the configuration of the preferential baffle. In the
baffle, there is a man-way to inspect both sides of the baffle, but
it should be tight at the time of box-up before startup. In some
plants-to make it a preferential baffle-the man-way of this
baffle is often open to equalize the level. However, this is an incorrect
methodology.
An accurate hole size matters in the preferential baffle. The
direction of flow must always be from the product compartment
to the reboiler compartment. This will reduce the LMTD
slightly, but the bottom product composition will be unaffected.
If the hole diameter (size) is larger than normal, then flow can be
from the reboiler compartment to the product compartment and
lighter liquid can go with the bottom product and pressurize the
downstream. If the hole diameter is smaller than normal, then the
liquid can accumulate and disturb the bottom level. The hole size
should be such that liquid always flows from the product sump to
the reboiler sump. The liquid level in the product compartment
should be 4 in.-5 in. higher than the reboiler compartment.
The elevation of the hole in the baffle can also make a difference
in design. The elevation of the hole should be such that the
head of the thermosyphon reboiler is always maintained. As the
withdrawal of product increases, the level is reduced and liquid
can flow from the reboiler to the product compartment. The elevation
and size of the hole should be calculated based on liquid
hydraulics and the required head of the thermosyphon reboiler.
For preferential baffle reboiler designs, one additional level
gauge or level transmitter should be provided to ensure the level
in the reboiler inlet compartment.
Takeaway. The proper design of a preferential baffle reboiler
can avoid startup and liquid accumulation issues, which can potentially
destroy the whole column. As noted by Kister,7
50% of
column problems were due to level accumulation and reboiler
return nozzle submerging in the liquid. The bottom sump deHydrocarbon
Processing | APRIL 2022 47
sign must be checked during the design stage. This may reduce
50% of the issue before it happens.
Part 2 will appear in the May issue.
ACKNOWLEDGMENT
This work is based on the author's experience and is not affiliated with any
company. The author wishes to express his gratitude to Krishna, who supported and
advised him on this article.
LITERATURE CITED
1
Lieberman N. P. and E. T. Lieberman, A working guide to process equipment,
McGraw-Hill Education, 4th Ed., 2014.
2 Kister, H. Z., Distillation operation, McGraw-Hill Education, New York, 1990.
3
4 Kister, H. Z., Distillation troubleshooting, John Wiley & Sons Inc., 2006.
5
6
7
Maat, C., S. Sudhar, K. Bellard, J. Barton and J. Martin, " Troubleshooting oncethrough
reboilers, " AIChE, March 2021.
Sloley, A. W., " Properly design thermosyphon reboilers, " Chemical Engineering
Progress, March 1997.
Design Practice Committee Fractionation Research Inc., " Reboiler circuits for
trayed columns, " Chemical Engineering, January 2011.
Kister, H. Z., " What caused tower malfunctions in the last 50 years, " Institute of
Chemical Engineers (IChemE), Vol. 81, Part A, January 2003.
ABHISHEK SHARMA is a Process Engineer at an ethylene plant,
and has more than 4 yr of experience in the steam cracker unit.
He completed his B.Tech degree in chemical engineering from
the National Institute of Technology Raipur, India with honors
and completed a process equipment design course from IIT
Roorkee. He is an active professional member of the American
Institute of Chemical Engineers and an associate member of
IChemE. He has published three articles and is involved in the smooth operation
and troubleshooting of the plant.
https://hindustanpetroleum.com/hpcareers/ https://hindustanpetroleum.com https://hindustanpetroleum.com
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