Technical
Paper
Oxygen Removal by Catalyzed
Carbon Beds
Author: William S. Miller
EPRI Condensate Polishing Workshop, October 29,
30 and 31, 1985, Richmond, Virginia
Introduction
This paper will discuss a chemical process for
removing dissolved oxygen from either make-up
water supplies or condensate. The process involves
the addition of hydrazine to the oxygenated influent, then passing the mixture through activated
carbon media to catalyze the reaction, followed by
a suitable ion exchange bed downstream to
remove any carbon impurities or hydrazine overfeed. The reaction is stoichiometric and rapid even
at cold (35°F [1°C]) influent temperatures and consistently produces an effluent analyzing less than
10 ppb dissolved oxygen from influents containing
12 to 14 ppm dissolved oxygen. The process is
already proven commercially. Since May 1983,
numerous utility and industrial users have used this
process to produce over 500 million gallons (2 million m3) of deoxygenated water in a variety of
application modes.
Hydrazine Reaction
The reaction of hydrazine and oxygen can
be expressed:1
N2H4
(Hydrazine)
+ 02
→
(Oxygen)
N2
+ 2H20
(Nitrogen) (Water)
Since the reaction products are inert nitrogen gas
and water, hydrazine has a unique advantage over
other chemical oxygen scavengers such as sodium
sulfite or isoascorbic acid, which actually add dissolved solids to the boiler feed water. However, the
reaction kinetics for the hydrazine and oxygen
reaction are poor and prohibitively slow at 35°F to
80°F (1°C to 12°C) range common to make-up
water supplies. For instance, an 11 fold excess of
hydrazine will only reduce the dissolved oxygen
from 9 to 7 ppm over a two hour period at 70°F (21°C).
Even large excesses of catalyzed hydrazine, which
contains metal or organometal additives, do not
sufficiently increase reaction rates for low temperature applications. For the above example, using an
11 fold excess of catalyzed hydrazine still requires a
reaction time of 10 to 30 minutes at 21°C (70°F)
while allowing extremely large excesses of hydrazine and catalyst to remain in the boiler feed water.3
Activated Carbon Catalysis
Considering the relative sluggish reaction rates just
discussed, the rapid rate of the hydrazine - oxygen
reaction when catalyzed by an activated carbon
bed is astonishing. At 35°F (1°C) reactions are still
completed in seconds rather than minutes or hours.
Production of 10 ppb dissolved oxygen levels is
attained immediately on start-up and can often be
maintained for a period of several hours after shutdown of the hydrazine feed. This “flywheel effect” is
most useful for applications designed to maintain
low dissolved oxygen levels in storage tanks or during boiler layups.
Equipment requirements are minimal. A chemical
feed pump, a pressure vessel to contain either
granular or powdered carbon, followed by a downstream ion exchange unit is all that is required.
We have not found the need for a special grade
of carbon. To date, most of our commercial
experience is with a coal-based carbon. We have
used products from three different manufacturers
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TP1052EN.doc Jun-09
with equal success. Laboratory tests on wood
based carbons also showed satisfactory deoxygenation capability.
Process Kinetics
To study the time required for complete reaction,
pilot tests where conducted with fiberglass pressure
tanks containing 1 cubic foot of 12 x 40 mesh coal
based carbon. A feed of 12 ppm hydrazine was
added to a 51°F (10.5°C) influent water supply containing about 9 to 10 ppm dissolved oxygen and
passed downflow through the carbon column at
various flow rates. Both Norfolk and Virginia City
water (240 Mmhos) and a two-step D.I. influent (10
mmho) were tested. The effluent was analyzed by a
direct reading in-line Rexnord Model #340-0 dissolved oxygen analyzer, backed up by colorimetric
tests using Chemets manufactured by Chemetrics.
Results are summarized in Table 1.
Table 1: Deoxygenation Tests
Dissolved O2- Carbon Effluent
obtained. This contradicts actual commercial experience, which has presented no problems in attaining less than 10 ppb dissolved oxygen effluent
quality in the operating range of pH 5.5 to 10.
Carbon Leachables
One of the few limitations for the hydrazine - carbon deoxygenation process is the need to provide
downstream ion exchange treatment to remove
impurities leached from the carbon during the
process and any overfeed of hydrazine.
Theoretically, there is no need to overfeed hydrazine as the carbon catalyst will allow stoichiometric
operation. From a practical view, it is advisable to
feed 10-20% excess to allow for subtle changes in
influent flow rate, dissolved oxygen content, and
hydrazine pumping inaccuracies. Hydrazine is
effectively removed from solution to non-detectable
concentrations (<5 ppb) by a hydrogen form strong
acid cation resin. The selectivity of the sodium form
of the resin is insufficient to obtain a commercially
acceptable capacity for hydrazine.
When considering the carbon catalyzed process
one must not overlook the potential for effluent
contamination due to the carbon media. In addition
to carbon fines, several dissolved ionic salts will be
leached into solution. The ionic leachables from
coal based carbon is shown in Table 2:
Table 2: Coal Based Carbon Leachables
As shown, the reaction rates and attainable effluent
quality are markedly improved over the previously
stated rates for non-carbon catalyzed process. All
full-scale installations have obtained the effluent
qualities predicted even when starting with raw
lake or river water at 35°F (1°C).
While the results in Table 1 were confirmed with a
minimum of three repeats, I still find the difference
in kinetic rates between the city supply versus the
D.I. supply difficult to explain. Since the effluent
qualities are attainable within five minutes of startup, the difference cannot be explained by overloading organics on the carbon with city water. The use
of two-step demineralized water does introduce a
pH variable, i.e. pH 8.5 for two-step versus pH 7.3
for the city supply. A paper presented at the 1985
Liberty Bell Corrosion Course suggests better kinetics at higher pH, but the data also shows that below
pH 9, effluent qualities below 100 ppb were not
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These leachables could come either from the ash
content of the carbon or perhaps from the water
used to initially wet, transfer, and backwash the carbon.
The concentration of the leachables in the carbon
effluent decreases slowly with time but even after
64 bed volumes there are still significant levels of
impurities being added to the effluent. The data
presented in Table 3 was obtained by passing 0.1
Mmho water containing 8 ppm dissolved oxygen
through a bed of coal based carbon at a flow rate
of 1 GPM/FT3.
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Table 3: Carbon Leachables vs. Time
(0.004 m3/h) deionized water analyzing 860 ppb TOC
through a 1 ft3 (0.03 m3) bed of virgin activated carbon. The deionized water was prepared by taking
an alum coagulated municipal surface supply analyzing 3,000 ppb TOC and passing it through a
“tired” mixed bed whose anion component was about
30% low in salt splitting capacity.
Table 4: TOC Removal by Activated Carbon ppb TOC
Since the deionized influent analyzed only 5 ppb
silica, the amount of silica leachable is very significant and requires a downstream strong base
anion resin in the hydroxide form for removal.
Additional leaching tests with a deionized feed containing 8-10 ppm hydrazine did not alter the type or
concentration of leachables. Surprisingly, start/stop
operation did not produce an exaggerated concentration of leachables upon restart, but simply maintained the smooth downward trend shown in Table
3. Most importantly, a properly regenerated mixed
bed downstream of the carbon column has no
trouble in removing any of the carbon leachables
and has often provided an upgrade of D.I. influent
water quality.
Actual operating data obtained over a one-year
period from a 500 gpm (2 m3/h) installation is
shown in Table 5. The influent was a coagulated
and filtered municipal surface supply which was
deionized by strong acid cation, strong base, type I,
gel anion, activated carbon with hydrazine feed for
deoxygenation, followed by mixed bed polishing.
Table 5: TOC Removal — System Profile ppb TOC
TOC Removal
The removal of organics by activated carbon is well
documented in the literature. Factors such as the
chemical structure of the organic compound, concentration, temperature, pH, carbon pore and particle size can affect adsorptive capability.
For the general category of organics, sometimes
referred to as “naturally occurring organics” commonly found in make-up water supplies, activated
carbon is only effective for removing 30% to 70% of
the total influent TOC. In fact, if one performs TOC
profiles across surface water supplied demineralizing systems which contain an activated carbon column, one would find that most of the organic
removal is attained by a type I strong base anion
resin. The introduction of hydrazine has not shown
any benefit in reducing the organic removal capability of activated carbon compared to a carbon
column without a hydrazine feed.
Tables 4 and 5 present some TOC removal data for
a coal base activated carbon having a surface area
960 to 1200 ft2/0.04oz. (800 to 1000 m2/g). The
results in Table 4 were obtained by passing 1 gpm
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During this time period over 100 million gallons
(400,000 m3) were processed through 360 ft3 (10m3)
of activated carbon.
To engineer and install a system to produce parts
per billion levels of dissolved oxygen makes little
sense unless these levels can be maintained while
the water is being stored. Storage tanks open to
atmosphere will not maintain low oxygen levels in
the stored water. The common approach to the
problem is to engineer either a floating bladder or
nitrogen blanket system on the storage vessel. Both
systems have been found acceptable for maintaining less than 10 ppb dissolved oxygen levels. To
prevent air ingress or nitrogen loss through a storage tank overflow pipe, an overflow “wier box” or
“u-bend” full and overflowed periodically has been
used successfully.
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Should the nitrogen blanket be “lost”, it could take
an extended time for storage tank dissolved oxygen
levels to return to normal. The data in Table 6 are
from a 500,000 gallon (2000 m3) storage tank
designed with a top fill and bottom drain.
Table 6: Storage Tank Dissolved Oxygen
The major process features include the following:
1. Low temperature operation (1°C)
2. Effective on both raw and demineralized supplies
3. Immediate and consistent effluent quality
below 10 ppb O2
4. No downstream impurities are added
5. Flywheel effect prevents immediate quality degradation upon loss of chemical feed
Potential applications include deoxygenation of
make-up supplies, maintenance of low dissolved
oxygen levels during system lay-ups and in storage
tanks, and full flow deoxygenation of condensate.
A patent has been allowed for the process and
issuance is scheduled for December 3, 1985.
During the period this data was obtained, the daily
draw from the tank was only slightly less than the
gallons added which averaged 400,000 gpd (1500
m3/day). The average quality being fed to the tank
was 1 to 3 ppb dissolved oxygen.
Addendum
Table 7: Deoxygenation Experience
As stated previously, most of the low level dissolved
oxygen measurements were made with an in-line
Rexnord dissolved oxygen analyzer. Some interesting differences in analyses have been obtained
depending how the analyzer is installed.
If the analyzer, which requires a constant 50 ml/min
flow, is fed by plastic tubing the recorded oxygen
levels will be higher than if hard piping is used. In
one case, braided nylon tubing was used as the
feed line to the analyzer and a range of 5 to 10 ppb
dissolved oxygen was indicated. With no other
changes being made except to run stainless steel
tubing to the instrument, the indicated oxygen values immediately and consistently dropped to 0.5 to
2 ppb dissolved oxygen.
This observation, which is discussed in the Rexnord
operating manual, is apparently due to the ability of
plastic tubing to breathe, allowing air to contaminate the sample.
References
1. Hercules Bulletin WMC-l23, Boiler Oxygen
Corrosion.
2. Hydrazine and Its Derivatives, Schmidt, E.W., pg.
829, John Wiley & Sons, New York, NY, 1984.
3. IBID, pg. 828
4. Carmen, C., Oxygen Reduction Via Activated
Carbon Precoat, 23rd Annual Liberty Bell Corrosion Course 4, Sept. 1985.
Summary
The process for removing dissolved oxygen by
addition of hydrazine to the influent and passing
the mixture through activated carbon followed by
downstream ion exchange treatment is a simple,
effective, and commercially available process.
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