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 ------- • 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. ------- 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 ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 PS 0000329 U S ENVIR PROTECTION REGJTGW 5 LIBKAKY 230 S DEARBORN STRfc'ET CHICAGO U, 60604 ------- |