Hydrocarbon Processing - May 2022 - 85

Heat Transfer
A. SHARMA, Process Engineer, Jaipur, India
Preferential baffle reboiler-Part 2
Distillation towers consume ~30% of
the energy in a refinery or petrochemical
facility. The reboiler is the stomach of the
distillation tower that consumes the energy
to separate the components. Any improvement
in design or operational flexibility
will save the energy and profitability
of the plant. Two main types of reboilers
are in use: a circulating (conventional) reboiler
and a once-through reboiler.
Part 1 of this article,1
which delivered a
brief introduction of the preferential baffle
reboiler and its working principle, was
published in the April issue. If designed
accurately, a preferential baffle reboiler
(baffle with hole) or baffle with underflow
can provide benefits; otherwise, it
can show detrimental effects.
Part 2 will describe the design of the
preferential baffle reboiler with the help
of hole size, shape of the hole, and elevation
of the hole in the baffle. It also provides
recommendations and guidelines
for optimum design. In this type of preferential
baffle reboiler, the elevation, shape
and diameter of the hole are all important.
Design and performance. A thermoshipon
reboiler's performance is affected
by many factors, including the diameter of
inlet and outlet piping, elevation of a column,
length of pipelines, Delta T (ΔT),
driving force (liquid head), etc.2
All factors
are fixed and considered during the
design phase, except the head of the liquid,
which may vary based on the level
in the column bottom sump. If the level
drops below a certain point, the head of
the liquid is reduced and will be unable to
push the effluent of the reboiler into the
column. In this scenario, the reboiler reduces
the circulation (or thermo-siphoning)
effect, but heat input to the reboiler
continues. This creates over-vaporization
of lighters and may cause polymerization
issues or an accumulation of heavies in the
reboiler. Even if the head is maintained
and the reboiler is heated sufficiently, the
accumulated heavies resist further circulation
through the reboiler and degrade
the thermosiphoning effect.3-9
Location of the hole. Part 1 mentioned
that the flow of the liquid through this baffle
hole should be unidirectional to make
it a preferential reboiler. The head of the
liquid and the height of the skirt support
of the column from the reboiler elevation
are responsible for thermosiphoning
(sending a mixture of liquid and vapor
into the column without any pump).
Therefore, the location of the hole in the
baffle should be higher than the level of
the liquid required to run the reboiler or
to circulate liquid through the reboiler
with the thermosiphoning effect (FIG. 5).
The driving force is equal to the resistance
force against the flow (Eq. 1):
DPdriving
= DPinlet
DPreboiler
where:
DPinlet
+ DPoutlet
+ DPtwo-phase
includes friction losses of inlet
line and entry loss, and DPdriving
is equal to H*density
DPoutlet includes friction losses of outlet
line and exit losses
DPtwo-phase is dual-phase losses, and is
equal to Hr*mixed-phase density
DPreboiler is the exchanger losses.
Many plants make a hole at the bottom
of the baffle. This may cause head loss if
an excess amount of product is withdrawn
from the product compartment and the
level in the reboiler compartment is reduced.
It can also starve the reboiler of
liquid (FIG. 6). Another problem with the
hole at the bottom of the baffle is that unheated
liquid (bottom tray liquid) can enter
directly into the product compartment
without going through a reboiler, which
would contain lighters and may result in
off-spec product. This diminishes the efficiency
of the distillation column. Many
Product
FIG. 5. Ideal elevation of hole.
+
(1)
Hr
H
operators run the tower at a high bottom
temperature to increase efficiency; this
negatively affects the reboiler as well as
the tower's productivity.
Shape of the opening (hole). In some
plants, the shape of the hole in the baffle
TABLE 1. Discharge coeffi cient of
diff erent shapes
Shape
Discharge
coeffi cient
(Cd)
Circular Square Rectangular
High
Low
Very low
Product
FIG. 6. Hole at bottom of baffle.
Hydrocarbon Processing | MAY 2022 85
Hydrocarbon Processing - May 2022

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