Hydrocarbon Processing - April 2022 - 45

Heat Transfer
A. SHARMA, Process Engineer, Jaipur, India
Preferential baffle reboiler-Part 1
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.
A preferential baffle reboiler is simply a combination of a
once-through and circulating reboiler, and is more efficient than
the circulating reboiler and more flexible than a once-through
reboiler. This thermosyphon, or forced reboiler, can take liquid
directly from the tray as well as the product. It is also called a
baffle with an underflow or a baffle with a large hole.
This article will describe the principle of the preferential
baffle reboiler and its working mechanism, as well as the hole
sizing in the baffle and some design guidelines. The reasons the
preferential baffle is required and what the fluid flow direction
should be through the hole of the baffle are discussed.
Two types of reboiler are available based on the driving
force: forced and thermosyphon.
Circulating reboiler. For a circulating reboiler, the name itself
suggests that liquid circulates through the reboiler continuously.
The liquid outlet of the reboiler always recirculates through
the reboiler. The tower bottom product and reboiler inlet are
the same and all properties are identical. The bottom sump of
the column is always filled with liquid, so the circulating reboiler
is not limited by liquid and there is little chance of dry out.
However, this depends on the level of the sump of the column
due to the head of the level for thermosyphons.
A
B
The reboiler outlet temperature is always higher than the
tower-bottom temperature.1
The drawback of the circulating reboiler
is its separation efficiency. Due to remixing of the reboiler
outlet liquid with the bottom tray liquid, the liquid inlet temperature
is always higher, which reduces the logarithmic mean temperature
difference (LMTD) across the reboiler and reduces
the separation in the reboiler. This is why a circulating reboiler
is not considered as a theoretical separation stage. Lighter components
can pass downstream at the bottom with liquids.
FIG. 1A shows the circulating reboiler configuration without
a baffle, with a variable head based on level. This is applicable
when product withdraws and is less often compared to reflux ratio
or reboiler circulation.2 FIG. 1B shows the circulating reboiler
with a baffle at the bottom of the column. This is a constant
head reboiler and does not depend on the level of the product
in the compartment. Liquid always flows over the baffle. It is
applicable when the withdrawal of product is 70%-80% of the
circulation rate or higher than the reflux rate.
Once-through reboiler. Matching its name, the reboiler circulates
liquid through the reboiler only once. The liquid inlet
to the reboiler is drawn directly from the bottom tray of the
column, and the reboiler return liquid is the product of the column.
The product of the column and the reboiler inlet liquid
are different from each other and at different temperatures. The
temperature of the reboiler inlet liquid is colder than the product
temperature, so the log-mean temperature of the reboiler is
higher. This is a theoretical stage of the column.
Liquid from the bottom tray should not bypass the reboiler,
a main part of the column. As per Lieberman,1
" A once-through
A
B
Product
Product
FIG. 1. A circulating reboiler without baffle (A), and a circulating
reboiler with baffle (B).
Product
Product
FIG. 2. A once-through thermosyphon reboiler without baffle (A),
and a once-through thermosyphon reboiler with baffle (B).
Hydrocarbon Processing | APRIL 2022 45
Hydrocarbon Processing - April 2022

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