Hydrocarbon Processing - April 2022 - 46

Heat Transfer
thermosyphon reboiler not only functions as the bottom theoretical
stage of a tower, it also functions as if it is a superfractionation
stage. This means that it is more important than any
individual tray in the tower. This is especially true with a multicomponent
tower bottoms product. "
While this type of reboiler is often considered the most effective
out of all types of reboilers due to its superior thermal
efficiency, it also has many drawbacks. If the bottom tray is
leaking, then the liquid will bypass the reboiler and reduce efficiency.
Reboiler draw-off plugging or seal pan damage may also
restrict the liquid to the reboiler and overflow to a bottom sump.
Many failures can occur, such as a hole in the bottom tray or in
the collation box, a broken bottom tray, or weeping through the
bottom tray.3
FIG. 2A shows the once-through reboiler without baffle. The
reboiler takes liquid from the bottom tray and returns it to the
bottom sump. FIG. 2B shows the once-through reboiler with baffle.
All the liquid from the bottom tray is diverted towards the
reboiler compartment; the reboiler takes liquid from this compartment
and returns it to another compartment, also called the
product compartment.
The once-through with baffle reboiler has two major problems:
hydraulic balance and a startup issue.
When the liquid circulation rate through the reboiler is higher
than (double or more) the product withdrawal rate, the reboiler
takes the liquid from the reboiler compartment (which comes
directly from the bottom tray) and the return (effluent) of the reboiler
sends the liquid and vapor mixture into the product compartment,
where the vapor and liquid separate from each other
and disengage. Disengaged vapor flows upward, and liquid dumps
into the product compartment and is withdrawn as a product. If
this disengaged liquid rate in the product compartment is higher
than the product withdrawn, then liquid accumulation will start
and the level of the product compartment will increase continuously.
The liquid will fill up until baffle overflow or level control
of the bottom product increases, which disturbs the downstream
process. If the reboiler-return nozzle elevation is lower than the
height of the baffle, then the return nozzle will dip into the liquid,
which may reduce the density of the liquid and the level transmitter
would show a lower liquid level than the actual level.
Another problem with a higher liquid level is that the returnnozzle
submerges, dipping into the liquid and resulting in slug
flow or creating a pressure surge that can dislodge the tray of
packing in the tower.4
Reboiler return liquid may impinge on liquid,
which also causes level fluctuation. An example of material
balance issue (hydraulics) is shown in FIG. 3. The reboiler circulation
rate is 300 tph and the thermosyphon reboiler produces
30% vapor and 70% liquid as a return. That equals 210 tph as a
liquid product and 90 tph as vapor.
All the vapor produced by the reboiler will move up towards
the trays, and 210 tph of liquid will dump into the product sump
compartment. The product withdrawal rate is 190 tph, which
means 20 tph accumulates every hour. The level of the product
compartment increases, which increases the bottom flowrate
if the level controller is in auto mode; otherwise, the product
sump fills up until it overflows into the reboiler compartment.
90 tph
210 tph
+20 tph
Product
190 tph
300 tph
Product
FIG. 3. An example of material balance issue (hydraulics).
46 APRIL 2022 | HydrocarbonProcessing.com
FIG. 4. Preferential baffle reboiler.
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Hydrocarbon Processing - April 2022

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