Hydrocarbon Processing - May 2022 - 86

Heat Transfer
liquid accumulation rate. This does not affect
the thermosiphoning of the reboiler.
FIGS. 9 and 10 show a front view of a
Cd
FIGS. 9 AND 10. A front view of a baffle
with different hole shapes and a side view
of the baffle, respectively.
H/D
even and fine to reduce friction losses.
FIG. 7. Cd of different shapes-the Cd reduces
with diameter and head.
90 TPH
210 TPH
4 in.
20 TPH
Product
190 TPH
FIG. 8. Hole diameter and height of liquid
above hole.
is rectangular, square or circular, or any
random cut (TABLE 1). It is important to
highlight that the issue with different
shapes would be the friction factor and
discharge coefficient of liquid. The discharge
coefficient (Cd) of liquid through
any hole is the ratio of actual flow to ideal
flow. The Cd is inversely proportional to
the friction factor; therefore, friction losses
should be the lowest and the discharge
coefficient should be the highest.
FIG. 7 shows a simple comparison on
practical values. The discharge coefficient
depends on the diameter (d) of the hole
(circular shape) and the head (H) of the
liquid in the product compartment. As the
diameter of the hole or cut in the baffle
increases, velocity reduces and fewer friction
losses lead to a higher discharge coefficient.
This should be optimized.
The head of the liquid in the product
compartment also changes the friction
losses. A higher head enhances the velocity
and increases friction losses. Therefore,
the minimum head should be there.
The cut in the baffle should be smooth,
86 MAY 2022 | HydrocarbonProcessing.com
300 TPH
Size of the hole in the baffle. The diameter
of the hole should be determined
based on the liquid load in the product
compartment. In Part 1, it was seen in
the calculation of the liquid that 20 tph
(tons/hr) were accumulated there. As a
result, the size or diameter of the hole is
determined by the amount of liquid that
accumulates
per hour. Recommendation:
The liquid velocity through the hole
should be approximately 1.4 m/sec, and
the height of the liquid in the product
compartment above the hole should be 4
in., or 100 mm (FIG. 8).
For 20 tph (28.57 M3
/hr with a density
of 700 kg/m3) of flow toward the reboiler
compartment, the hole size should
be 42.5 mm, or approximately 2 in. Based
on this recommendation (Eq. 2):
Q (flow in m3
/hr) =
A × V (m/sec) × 3,600
(2)
where A is the area of the hole and the velocity
(V) is 1.4 m/sec.
Similarly, the hole diameter in the baffle
can be calculated based on the liquid
accumulation rate. A large-diameter hole
in the baffle would equalize both levels
of the compartments and the chances of
bidirectional flow through this hole. This
baffle reboiler has the preference of taking
liquid from the reboiler compartment as
well as the product compartment.
The liquid should not go from the
reboiler compartment to the product
compartment. If the efficiencies of oncethrough,
circulating and preferential baffle
reboilers are compared, the preferential
baffle reboiler earns the second rank
while providing operational flexibility.
FIG. 8 shows an ideal design of the
baffle with a hole. The liquid head in the
product compartment is 4 in. above the
hole to make it unidirectional towards the
reboiler sump, and the hole in the baffle is
circular with a 2-in. diameter based on the
baffle with different hole shapes and a side
view of the baffle, respectively. The circular
hole in the baffle is beveled towards
the reboiler. This reduces the formation
of eddies and friction losses of flowing
liquid. All level transmitters should be in
the product compartment. It might be assumed
that with a hole in the baffle, level
transmitters will work on any side; however,
there is always a delta (difference) in the
height of liquid on both sides. A high-high
level alarm for the product compartment
is helpful to avoid the submerging of the
reboiler return line into the liquid pool.
Takeaway. The proper design of a preferential
baffle reboiler can avoid startup
and liquid accumulation problems and
provide optimum benefits. Recommendations
to enhance the design of the preferential
baffle reboiler include:
1. The hole in the baffle should be
circular rather than a rectangular
or square.
2. The diameter of the hole should
be calculated based on a velocity
of 1.4 m/sec.
3. To avoid thermosiphon flaws, the
elevation or altitude of the hole
must be determined using
a reboiler head calculation.
4. The hole or cut in the baffle should
be smooth and beveled towards
the reboiler compartment.
5. The baffle with an underflow
design should be selected rather
than the circulating reboiler;
otherwise, a once-through
reboiler is preferred.
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
Complete literature cited available online at
www.HydrocarbonProcessing.com.
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 participates in the smooth
operation and troubleshooting of the plant.
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