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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