Chemical Engineering January 2018 - 55

present in the feedgas stream. The
resulting materials tend to polymerize
during the regeneration process,
and form a thick, messy substance
that is very corrosive. This substance
also inhibits effective heat transfer in
the reboilers.
The corrosion rate depends upon
various factors, including temperature
and velocity. If the glycol solution
is not properly cleaned, it may
result in glycol oxidation or thermal
decomposition in the reboiler due to
higher temperature, which will lead
to corrosion.
Fouling can result in leakage
and poor heat transfer in plateand-frame
heat exchangers,
which will increase heat flux in the
reboiler system.
Low pH
The most troublesome corrosive
contaminants, including products
of glycol oxidation or thermal decomposition,
as well as acid gases
absorbed from the gas stream, can
also lead to conditions of low pH.
The ideal glycol pH should be in the
range of 7 to 7.5. Practically, it may
not be feasible to maintain this range
for a long period, but it is both recommended
and possible to maintain
system pH above 6 continuously. If
glycol circulation rates result in more
acid-gas absorption, low operating
temperatures may occur, which may
in turn lead to accelerated corrosion
and glycol decomposition.
Recommendations
There are some suitable design and
operating strategies to minimize the
operating and maintenance problems
that result from the presence
of contaminating species in the glycol.
The main design and operating
recommendations are detailed in the
1,200
1,000
800
600
400
200
10
Acid gas (H2S and CO2), mole %
FIGURE 4. The concentration of acid gas in the glycol negatively impacts the effectiveness of the process
to remove water from the gas
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16.6
following paragraphs.
Optimize glycol circulation. The
glycol circulation rate and the absorption
of BTEX and acid gas in
the circulating glycol are directly
proportional, as seen in Figure 2.
Hence, reducing the glycol circulation
rate is the most effective way
of decreasing the absorption of
BTEX and acid gas. While reducing
the circulation rate will increase the
number of theoretical stages (mass
transfer stage) in the contactor column's
design to achieve the desired
outlet water content specifications,
it does help to reduce issues like
foaming, fouling and high glycol
losses. A lower circulation rate can
also decrease BTEX emissions and
reduce the reboiler duty. However,
it should be ensured that when adjusting
the glycol circulation rate, the
system can still meet the minimum
wetting rate and achieve adequate
liquid distribution in the contactor,
which is essential for effective
mass transfer between the gas
and glycol to facilitate the required
water removal.
Avoid hydrocarbon carryover and
condensation. The glycol is chemically
reactive and needs to be protected
against contamination. Major
operating problems can arise due
to inadequately designed glycol
contactor-inlet scrubbers or separators.
An inlet-gas scrubber or separator
can be provided upstream of
the contactor to avoid liquid hydrocarbon
carryover to the contactor.
The integral scrubber, as part of the
contactor, should not be used as
the primary separator. The primary
separator must be be properly sized
with suitable internals to remove liquid
hydrocarbons, free water, solids
and other chemical agents. Even
small quantities of contaminated
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Water content in gas, mg/std. m3

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Chemical Engineering January 2018

Table of Contents for the Digital Edition of Chemical Engineering January 2018