Chemical Engineering July 2020 - 25

generates, a maximum vapor flowrate
may be reached with condensate
still covering a portion of the
tubes. The remedy depends on the
cause of the problem. If there is a
sufficient pressure difference between
the reboiler and the condensate
header, it may be sufficient to
re-size the valve and the condensate
line, or both. If the pressure difference
is small, a condensate pot with
a pump (Figure 2a) may be needed.
The converse phenomenon can
be even more troublesome. When
the reboiler cannot condense vapor
as fast as the condensate valve removes
liquid, the liquid seal in the reboiler
may be lost, resulting in vapor
passing into the condensate system.
Without the seal, vapor tends to
channel through the reboiler, causing
a dramatic loss of heat transfer,
and in the case of steam, also hammering
in the condensate system.
One seal-loss incident in a reboiler
condensing refrigerant vapor, using
the control system in Figure 1b, has
been described in detail [7, 8]. many
other seal-loss incidents have been
experienced by the author and others
[11, 12]. losing the seal is more
likely to occur when the reboiler is
fouled or pushed to the limit and the
liquid level is low. To regain the seal,
the condensate valve needs to be
heavily throttled [7, 8, 11].
There are several ways of preventing
this seal loss. The scheme in Figure
2a, but without the pump, is one
option. Alternatively, the setup in Figure
2b can be used. Here, the flow
controller normally regulates the condensate
valve, with the level override
cutting in whenever the level falls too
low. The level override should not be
set at a vertical height much greater
than the bottom of the reboiler. Otherwise,
some of the tubes will always
be submerged and the effective reboiler
capacity will be reduced. Often,
especially in vertical reboilers, the liquid
level in the condensing side of the
reboiler can be monitored and used
for level override control, so the drum
can be omitted.
Finally, there are two " poor per168.6ºC
Intlet
164.4ºC
Damage
to the steam trap is apparent from
the small difference between the inlet and
outlet temperatures. The small difference in the
damaged trap is contrasted with the functioning
trap (4b, below), whose inlet and outlet
temperatures are considerably different.
Different colors of the inlet and outlet
correspond to different temperatures,
proving steam is held up in the system
Outlet
153.8ºC
166.2ºC
son's " solutions. One is to monitor the
condensate temperature leaving the
reboiler. With the Figure 1b scheme,
the condensate is usually subcooled.
A decline in subcool indicates an
imminent loss of seal, and can be
alarmed or used as an override
control. One thing to watch out
for is that the loss of subcool can
proceed rapidly, with the seal
lost before the corrective action
is taken. Another solution is adding
a steam trap between the
reboiler and the control valve.
For as long as it passes condensate,
the trap will have no effect.
Once the seal is lost, the trap will
prevent the steam from blowing
into the condensate header.
The downside of this approach is
that when the trap blocks steam,
further opening of the valve has
no effect on anything [3], and the
controller will tend to open the
valve widely.
Energy efficiency and
capital
Inlet
164.2ºC
Outlet
131.7ºC
18.0
Figure 4. The images show thermal scans of steam traps. In
4a, the trap is passing steam, so a small difference between
inlet and outlet temperatures is observed. In 4b, the properly
functioning steam trap shows its condensate is significantly
cooler than the steam (from Ref. 31, reprinted with permission,
courtesy of AIChE)
The condensate-outlet valve
scheme permits the reboiler
heating at a higher pressure because
it eliminates the pressure
drop in the inlet control valve.
This is a major advantage
and an overriding consideration
ChemiCal engineering www.Chemengonline.Com July 2020
when the reboil is supplied by refrigerant
vapor. Refrigeration-compressor
interstage pressures often " ride "
on the reboiler condensing pressure,
set to provide the desired reboiler
heat duty. For instance, if satisfactory
condensation can be provided by
condensing propylene refrigerant at
20°F, with a scheme like that of Figure
1b, it needs to be compressed to
70 psia (saturation pressure at 20°F)
and then run into the reboiler. With
the vapor inlet valve, it needs to be
compressed to a higher pressure -
at least 75 psia and most likely 80
psia - to overcome the pressure
drop of the valve. This means higher
compressor
power
consumption
and a larger compressor, a major
increase in both energy and capital
costs, which does not pay out. So
almost all refrigerated reboilers use
a condensate-outlet valve scheme.
The exception is when the compressor
interstage pressures are set
by alternative criteria, and there is
enough pressure to overcome the
reboiler-inlet-control-valve pressure
drop without increasing compressor
energy consumption or size.
Similar considerations apply when
36.3
reboil is supplied by condensing
overhead from the same or a different
tower. If from the same tower, the
overhead needs to be compressed (a
heat-pumped tower), in which case,
the reboiler inlet control valve will increase
the compressor power consumption
and size. If from a different
tower, the pressure drop across the
inlet control valve will either reduce
the reboiler ∆T or force higher pressure
operation in the tower whose
overhead is condensed, in turn increasing
its reflux and reboil requirements.
So almost all schemes that
heat one tower with the overheads
of the same or another tower use the
condensate outlet valve scheme.
With steam-heated reboilers, the
steam supply pressures are normally
set to suit the boilers or turbines
rather than the reboilers, and the
pressure differences between adjacent
supply pressures are generally
high. Sufficient pressure difference is
usually available between the steam
supply pressure and the steam pressure
required at a given reboiler, and
the difference can be utilized for regu25
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Chemical Engineering July 2020

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