United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-81-017 May 1981
Project Summary
Effect of Scrubbing Operating
Conditions on Adipic Acid
Degradation
John C. S. Chang
A series of adipic acid degradation
tests have been conducted at the
EPA/IERL-RTP limestone SOzscrub-
ber to investigate the effects of opera-
ting variables on unaccountable adipic
acid loss.
This Project Summary was develop-
ed by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The use of buffering additives such as
adipic acid in limestone flue gas desul-
furization(FGD) wet scrubbers has been
shown to benefit both SO2 removal and
limestone utilization. However, substan-
tial loss of adipic acid was experienced
during the long-term scrubber tests.
These losses were in excess of that ex-
pected from the scrubber solution
discharged with the filter cake solids.
Tests have also shown that the adipic
acid loss rate is considerably higher
during forced oxidation runs than
during natural oxidation conditions.
Part of the loss was attributed to the
oxidative degradation of adipic acid in
the limestone FGD system.
In December 1979, a series of lime-
stone-adipic acid factorial tests were
conducted at the Shawnee test facility.
Data from these tests indicate that the
unaccountable adipic acid loss became
negligible when the pH values were
dropped below 5. Furthermore, it was
shown that no degradation loss was ob-
served at pH below 5 even under forced
oxidation conditions. Since the quench-
ing of the degradation reactions can
substantially reduce the buffer additive
consumption rate, the favorable
operating conditions shown by the
Shawnee results are very important to
the application of adipic acid in full scale
FGD systems. Under EPA Contract 68-
02-3176, Task 4, twelve adipic acid
degradation tests were conducted in the
EPA/IERL-RTP Iimestone/S02 scrub-
ber to investigate the effects of opera-
ting variables on unaccountable adipic
acid loss.
The specific objectives of the project
were to:
• verify the Shawnee low-pH re-
sults of zero adipic acid degrada-
tion; and
• conduct parametric studies to
identify the effect of key operating
variables on adipic acid degrada-
tion and other losses.
The effects of operating variables in-
vestigated include:
• pH change from 5.6 to 4.6
• sources of adipic acid
• total sulfite level
• forced oxidation at pH 4.6
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• fly ash effect
• addition of metal catalysts such as
ferrous, ferric, and manganous
ions.
Results
The adipic acid tests were conducted
with a three-stage turbulent contact
absorber (TCA). Both natural and forced
oxidation scrubber configurations were
employed. Each run consisted of
approximately 100 hours of continuous,
steady-state operation. Dry adipic acid
powder was added to the hold tank at a
constant rate every 2 hours. The adipic
acid concentration in the scrubbing
liquor was analyzed by ion chromato-
graph and checked by total carbon
analysis and silicic acid titration. The
unaccountable adipic acid loss was ob-
tained by the material balance:
Unaccountable loss = In (Adipic
Acid Cone, x total Liquor Dis-
charged)
Adipic acid degradation and copreci-
pitation with the scrubber solids were
the two most likely causes of the unac-
countable loss. It was the purpose of
this project to investigate the changes in
the unaccountable losses under various
operating conditions, especially at pH
values below 5.0.
The major results of each test are
listed in Table 1. The first four runs
show that adipic acid loss cannot be
quenched by only lowering the pH to
below 5.0. However, the adipic acid
degradation rate can be decreased by
reducing the destructive free radical
concentrations by increasing total
sulfite. The possible effect of fly ash was
ruled out by runs E, F, and G. Run H
shows that forced oxidation can
aggravate the adipic acid degradation
loss even at low pH. The last four runs
indicate that manganous ion can inhibit
the oxidative degradation of adipic acid
at low pH. Analyses of the filter cake
samples show that an appreciable
amount of adipic acid loss (up to 40
percent) was caused by coprecipitation
with calcium sulfite. Very high natural
oxidation (55 percent) was observed for
Run I. Therefore, high natural oxidation
coupled with low pH and manganous
ion may be the cause of the low
unaccountable adipic acid loss for Run I.
Table 1. Unaccountable Adipic Acid Losses of Limestone-TCA Scrubber Tests
Unaccountable adipic
Run acid loss, g/hr Effects investigated
A 5.7
B 5.26
C 7.65
D 8.0
E 7.36
F 7.66
G 7.57
H 14.35
1 2.6
J 7.43
K 5.4
L 5.4
low pH
low pH,
low pH,
low pH.
low pH,
sources of adipic acid
Fe3*. low sulfite level
low sulfite level
Duke fly ash
replicate of D
low pH,
low pH,
low pH,
high pH,
low pH,
low pH,
Shawnee fly ash
forced oxidation
Fe3\ and Mn2+
Mnz+
Mn^
Fe3+ and Mnz+ (replicate 1)
Conclusions
Based upon material balance data
from the pilot-scale adipic acid degrada-
tion tests conducted in the EPA/IERL-
RTP limestone/SO2 scrubber, several
observations were made:
• adipic acid degradation could not
be totally quenched by only
lowering the pH values below 5.0.
• pH change did not have a signifi-
cant effect on unaccountable
adipic acid loss. There may be
some other factors which increase
the adipic acid degradation rate at
both high and low pH values.
• an appreciable amount of adipic
acid loss was caused by coprecipi-
tation with calcium sulfite
• forced oxidation aggravates the
adipic acid degradation loss, even
at pH below 5.0.
The adipic acid loss could be reduced
under the following conditions:
• high sulfite concentration—the
adipic acid degradation could be
decreased by lowering the de-
structive free radical concentra-
tions by high total sulfite.
• in the presence of manganous ion
at low pH—the metal ion may act
as an inhibitor to the oxidative
degradation reaction at low pH.
• high natural oxidation—the adipic
acid coprecipitation loss may be
reduced with the high natural
oxidation.
A comparison of the adipic acid degra-
dation (loss) data from four different test
facilities was made. Most of those data
support the conclusions reported here.
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John C. S. Chang is with Acurex Corporation. Morrisville, NC 27560.
R. H. Borgwardt is the EPA Project Officer (see below).
The complete report, entitled "Effect of Scrubbing Operating Conditions on
Adipic Acid Degradation," (Order No. PB 81-163 305; Cost: $6.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
* US. OOVemMNTPHNtlNa OFFICE 1M1 -757-012/7119
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