Chemical Engineering April 2022 - 47

Feature Report
Gas Trapping Can
Unsettle Distillation
Columns
Operational troubleshooting guidance is provided for several scenarios involving
trapped non-condensable gases in distillation towers
N
on-condensable gases are often
present in distillation towers, either
expectedly or unexpectedly.
When they do appear, or when
the quantity of non-condensable gases
varies from the design or expectation, they
can adversely affect condensation, cause
instability or lead to excessive venting
and product loss. Some of the causes of
gas trapping issues are well known, while
others remain obscure. This article briefly
describes the better-known issues, while
focusing on those that are more obscure,
presenting experiences in which non-condensable
gas trapping led to operating
problems. Gas trapping issues in distillation
towers can be classified into three groups:
* Gas trapping due to tower or auxiliary
equipment hardware
* Gas trapping due to liquid head
* Unexpected quantity of gas (high or low)
entering the tower
Each of these points are addressed and
discussed in this article, with some guidelines
for preventing and overcoming gastrapping
issues.
Equipment-related gas trapping
Gas-trapping issues associated with the
tower equipment itself, or its auxiliary equipment
hardware, are some of the more common
and better-known issues, and are therefore
only discussed briefly here.
Condensers and reboilers using a condensing
heating medium. Gas trapping in
condensers and reboilers is well known, and
is discussed at length by many references
[1-5]. A famous statement by Smith [3] is that
to troubleshoot a condenser, one needs to
ask three questions: " Is it clean? Is it vented?
Is it drained? " Prof. Bell, the legendary heatHenry
Z. Kister
Fluor Corp.
IN BRIEF
EQUIPMENT-RELATED
GAS TRAPPING
GAS TRAPPING BY
LIQUID HEAD
(a)
EXCESSIVE GAS IN THE
TOWER
DEFICIENT OR VARIABLE
GAS QUANTITY
CLOSING THOUGHTS
(b)
FIGURE 1. Illustrated here are the locations of vents on
condensing exchangers to remove non-condensable gas.
As shown in 1a, adding properly located vents in a horizontal
condenser can eliminate inert blanketing [9 ]. In 1b, a
tubesheet vent is installed in a steam-heated vertical reboiler
[8 ]. (Reprinted courtesy of Chem. Eng.)
transfer expert, estimated that inadequate
venting of non-condensable gases accounts
for half of condenser malfunctions [2]. Twenty
years later, a malfunction survey [6] reinforced
his statement with a slightly smaller fraction
of 41%.
Accumulation of even a small fraction of
non-condensable gas can impair condensation.
The non-condensable gas increases
the vapor-phase resistance to heat transfer.
In addition, the gas depresses the condensate
partial pressure, lowering the condensing
temperature and therefore the log mean
temperature difference. The diffusional reCHEMICAL
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Chemical Engineering April 2022

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