Chemical Engineering July 2020 - 26

lating the inlet valve. energy is saved
by the condensate-outlet scheme
only when eliminating the inlet-valve
pressure drop allows stepping down
the steam supply pressure. Since this
is infrequently the case, the vapor
inlet scheme remains competitive in
steam-heated reboilers. One uncommon
situation where the condensate
valve scheme wins is with tower-bottom
temperatures so high that ∆T is
limited, even at the highest available
steam pressure.
The capital costs of the inlet valve
scheme tend to be higher [6,13] because
the smaller ∆T generally increases
exchanger area to a greater
extent than flooding some of the tube
surface. The area difference appears
to be particularly high when turndown
ratios are high or when heating-medium
pressures are near critical [6].
In addition, the condensate-outlet
scheme uses a smaller control valve.
Fouling, corrosion, thermal stress
In steam-heated reboilers, the inlet
valve scheme (Figure 1a) condenses
steam at lower pressures and temperatures
than the outlet valve
scheme (Figure 1b). This reduces the
reboiler tube-wall temperatures and
suppresses reboiler fouling (process
side) and bottom product degradation.
This is a major advantage in
systems handling materials that can
thermally degrade or polymerize.
For thermally degradable and fouling
systems, the reboiler-wall temperature
can be further minimized
by adding a condensate pump (Figure
3a). The pump permits the reboiler
pressure to approach, or even
fall below, the condensate header
pressure. This minimizes the steam
condensation temperature, and in
turn, the tube-wall temperature.
Both the reboiler and pump may
need to be designed for vacuum.
In one such system [7], vacuum
was actually observed. To avoid
vacuum, the condensate drum can
be vented to an atmospheric or a
positive pressure header instead of
to the reboiler. As the condensate
is near its boiling point, enough net
positive suction head (NPSH) needs
to be provided, which may require
elevating the reboiler, or even the
tower skirt. A common NPSH for a
26
Condensate return
Steam supply
Pressure-balancing line
Heat exchanger
Process fluid
Process fluid
Air vent
Float-operated
steam trap
Float
mechanism
Condensate receiver
Figure 5. This diagram shows the steam-inlet control scheme with a pumping trap and float valve
(Reprinted with permission from Ref. 16)
centrifugal condensate pump [14]
is 8 ft, but this can be reduced to
2 ft with more expensive special designs.
One expert [14] recommends
batch pumps powered by gas or
steam pressure instead.
Reboiler fouling can be further reduced
by recycling some of the condensate
to desuperheat the steam
(Figure 3b). The condensate recycle
line must be adequately engineered,
or else it can be more trouble than it
is worth. good engineering requires
a temperature control and well-designed
injection nozzles when connecting
into the steam line to the
reboiler, as shown in Figure 3b, as
well as a well-designed impingement
plate at the reboiler entrance,
because condensate drops can be
highly erosive to the reboiler tubes
near the inlet. The author is familiar
with reboilers in polymerizing services
where adding the pump of
Figure 3b with well-designed condensate
recycle lines doubled the
run lengths between cleanings [7].
However, the author also encountered
cases in which a " shortcut "
version of this system, either with
no temperature control or with no
impingement plate, generated problems
that forced discontinuation of
the recycle.
Corrosion due to the condensate
level maintained in the reboiler
often occurs with the condensate
valve outlet scheme (Figure 1b). Although
condensate is pure water, it
becomes very acidic when CO2 and
other gases in the steam dissolve in it.
Steam supplied to a reboiler contains
various amounts of CO2, originating
from decomposition of residual carbonates
in the boiler feedwater. In a
vertical reboiler, the CO2 accumulates
near the top of the shell from where it
can be vented. However, in horizontal
reboilers, the CO2 will accumulate underneath
the partition baffle [11, 12],
and is absorbed by the condensate
near the steam-condensate interface
[14]. This will form corrosive carbonic
acid. In several reported cases [11,
14], a rust layer on the steam side
of the reboiler showed the level at
which steam condensate usually ran.
A small, continuous bleed from under
the partition baffle or corrosion inhibitors
can minimize this corrosion.
In horizontal thermosiphons reboilers
using the condensate outlet valve
control (Figure 1b), subcooled condensate
is a short vertical distance
away from condensing steam. When
the difference between the condensation
and tower-bottom temperatures
is large, the subcool can be
large (even more than 100°F). The
wall submerged in the subcooled
liquid can be much colder than the
wall where the steam condenses.
This generates tremendous thermal
stresses at the channel head. These
stresses are further aggravated when
the liquid level ascends or descends
- cold sections quickly heat up as
the condensate level is reduced or
hot sections quickly cool down as
the level increases. These thermal
stresses often cause leakage at the
channel head to the tubesheet gasket
[14,17,18]. The author and others
[12,17] are familiar with a number
of cases where this mechanism
caused severe channel-head leaks.
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