Scrubber adipic acid degradation

Laboratory Investigation of Adipic Acid Degradation in Flue Gas Desulfurization Scrubbers... 

Tests conducted at the Shawnee Test Facility indicated that adipic acid added to their limestone FGD scrubber did not degrade at pH s below 5. Since these unexpected but favorable results were important to the future application of adipic acid as an FGD additive, independent verification was desired. Radian was contracted by the EPA to carry out a systematic study of the effects of scrubber operating conditions on adipic acid degradation. 

Meserole, F.B. Lewis, D.L. and Kurzawa, F.T. "Further Study of Adipic Acid Degradation in FGD Scrubbers," Final Report, EPA-600/7-80-152, Environmental Protection Agency, Research Triangle Park, NC, August, 1980. 

The addition of adipic acid to limestone-based FGD wet scrubbers results in improved limestone utilization and enhanced S02 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 

The topics presented in this paper include a description of the bench-scale system, the experimental approach, and the results of degradation testing. Also included are the results of batch precipitation experiments designed to study coprecipitation of adipic acid in scrubber waste solids. 

The batch precipitation tests show dramatic effects of adipic acid slurry concentration and solid phase oxidation fraction on coprecipitation of adipic acid in scrubber solids. Real world scrubbers would probably never operate at adipic acid concentrations as high as those tested and would also not likely ever produce pure phase calcium sulfite hemihydrate. Therefore, the magnitude of the results observed is somewhat a product of the laboratory test conditions. The results do, however, establish the potential importance of adipic acid coprecipitation and, hence, the need for analysis of scrubber solids for adipic acid when determining adipic acid chemical degradation rates by a mass balance calculation approach. 

Organic acids are normally stable to oxidation, but laboratory and pilot plant results (18) have shown that adipic acid oxidizes in conjugation with sulfite oxidation in the scrubber. This paper reports oxidative degradation rate of adipic acid as a function of pH and Mn concentration (19). Results are also presented on sulfopropionic, sulfosuccinic, succinic, hydroxypropionic, and hydroxyacetic acids (20). 

Fumaric acid degraded 2 to 4 times faster than adipic acid at pH 5.5 with 1 mM Mn (FI). The carbon-carbon double bond is apparently more susceptible to oxidation. Therefore, unsaturated acids should be sulfonated to avoid degradation. Since fumaric acid does not sulfonate rapidly in the scrubber, it is not an attractive buffer alternative. 

The odor has been identified as that of valeric acid, CH3(CH2)3 COOH, an intermediate product formed by side reactions that degrade adipic acid at scrubber operating conditions. At Shawnee, this odor was rarely noticed and was not a problem. 

Formic and acetic acids are most attractive, but would probably be volatile under scrubber conditions (8). Succinic and lactic acids would not be cost-effective if purchased at market price. Fumaric acid is more subject to oxidative degradation. Phthalic and Benzoic acids may give undesirable aromatic degradation products. Therefore, the most useful buffers appear to be hydroxypropionic, sulfosuccinic, fumaric, sulfopropionic, adipic, and hydroxyacetic. 

---