Chemical Engineering July 2020 - 29

Vent
Liquid
in
HLL=850mm
I.D.=500
NLL=650mm
(a)
7º
LLL=450mm
LT
Condensate
in
Channel
baffle
(c)
Condensate out
Figure 8. These diagrams show a condensate pot with a channel baffle directing
the subcooled condensate away from the pot liquid surface. In 8a, it is at elevation;
8b shows the view from the top of the channel baffle; and 8c depicts the view from
the inlet nozzle
an environmental issue and even a safety issue. Some
plants completely disallow this practice. In other plants,
the sewer system may be free of volatiles, in which case,
there is neither an environmental nor a safety issue, and
this practice is accepted.
Another shortcut solution in a tower that has a spare
reboiler [7] is to start the tower up using a fouled reboiler.
Once the tower reaches full rates, the clean reboiler is
put into operation and the fouled one cleaned. Along
similar lines, injecting nitrogen into the reboiler to lower
the heat-transfer coefficient has been successful, but in
at least one case, some nitrogen and a small amount of
oxygen ended up in the condensate and caused major
problems in the deaerator.
The vapor inlet scheme may also be troublesome when
there is a small pressure difference between the reboiler
heating medium and the condensate header (for example,
with steam reboilers using 15- to 35-psig steam).
The problem is identical to that described above, but it
is caused by insufficient ∆P rather than by oversizing, low
rates or clean surface. In this case, the above solutions
(except the Figure 3c scheme) will also apply. In two different
kettle reboilers [17, 22], adding a condensate pump
drawing directly from the kettle draw compartment solved
such a problem. One expert [18] prefers using the Figure
2b scheme when the ∆P is small.
Critical flow issues
Tuning with the vapor-inlet valve scheme can be troublesome
when flow across the valve changes from noncritical
to critical upon reboiler turndown [4, 26]. As boilup
rate falls, so does the pressure downstream of the valve.
When the ratio of upstream to downstream pressure exceeds
a critical value, critical flow is established through
the valve, where the downstream pressure no longer affects
the vapor flowrate. The controller dynamics are different
under critical and noncritical flow. A loop tuned for
noncritical flow tends to be unstable when flow becomes
critical, while a loop tuned for critical flow tends to be
(b)
90º
Liquid
in
7º
sluggish when flow becomes noncritical [26].
It is best to design the system to operate over its normal
range in one flow regime or another. A level condensate
seal pot (Figure 3c) can keep up the downstream
pressure during turned-down conditions, thereby avoiding
this problem. Alternatively, installing a pressure regulator
upstream of the flow controller (Figure 6) will lower
the pressure upstream of the inlet valve. Both techniques
also minimize valve erosion at high pressure drops. The
author is familiar with several pressure-regulator systems
that have worked very well.
Tube leaks
In many cases, small tube leaks from the heating side
(typically steam) to the process side can be tolerated,
but even a small leak from the process side to the heating
side cannot. In others, the converse applies. When
choosing the preferred reboiler control scheme, the possibility
of a tube leak must be considered.
With the valve in the condensate outlet, the pressures
on the heating and process side are fixed and known, so
the leak direction is usually known. However, the quantity
leaking will depend on the degree to which the hole is
submerged. If the leak is from the heating side above the
liquid level, it will be mostly vapor, and the quantity leaking
will be small, due to its low density. On the other hand,
if the leak is below the liquid level, it will be liquid, and
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Chemical Engineering July 2020

Table of Contents for the Digital Edition of Chemical Engineering July 2020
