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( 3 ) Kirk-Othmer Encyclopedia of Chemical Engineering, 3rd ed.;Wiley-Interscience: New York, 1987; Vol. 3, pp 98, 109.
Davld E. Klmbrough," Janice R. Wakakuwa California Department of Health Services Southern Californla Laboratory 1449 West Temple Street Los Angeles, Cailfornia 90026-5698
Comment on "Phenol Oxidation in Supercritical Water: Formation of Dibenzofuran, Dibenro-p-dioxin, and Related Compounds" SIR: In their communication, Thornton, LaDue, and Savage (I) report the results of experiments conducted on the supercritical water oxidation (SCWO) of phenol. They mention that the low temperatures used in their experiments (300, 380, and 420 "C), are lower than the temperatures proposed for commercial applications of SCWO and explain that the low temperatures were necessary to obtain detectable quantities of reaction intermediates. Obviously it is necessary to operate at conditions which do not provide good destruction efficiencies in order to obtain detectable concentrations of organic reaction intermediates in SCWO experiments. It is one of the advantages of SCWO over other waste destruction technologies, however, that optimization of the destruction efficiency is easily accomplished by a combination of on-line effluent quality monitoring and on-the-fly modification of operating parameters. We wish to underscore this point and convey some results of our own experiments on the supercritical water oxidation of both chlorinated dibenop-dioxins and precursor molecules at the temperatures proposed for commercial applications. The results shown in Table I are from SCWO of a waste sample in a continuous-flowreactor with a residence time of 5 s in the temperature range 600-630 OC at 25.6 MPa pressure. The waste contained a mixture of tetra- and octachlorinated dibenzo-p-dioxins and tetra- and octachlorinated dibenzofurans in the range of 0.4-3 mg/L and several potential precursor molecules including chlorinated benzenes, phenols, and anisoles with concentrations in the range of 1-50 g/L. At least 99.99% destruction efficiency was achieved for the dibenzofurans and dibenzo-p-dioxins. The analyses were performed by Midwest Research Institute, one of the few laboratories which could achieve sufficiently low detection limits for dioxins and furans to demonstrate this level of destruction efficiency at the time the test was conducted in March 1985. Although there were some detectable concentrations of chlorinated dibenzofurans in the gas and liquid effluents and detectable octachlorodibenzo-p-dioxinin the liquid effluent, all of the measured concentrations were very close to the analytical detection limits. The desired destruction efficiencies were demonstrated with these data, and no attempt was made to repeat the test under different conditions which may have yielded no detectable concentrations of any compound in either the gas or liquid effluents. These data clearly demonstrate that at normal commercial operating temperatures SCWO is capable of efficiently destroying chlorinated dibenzo-p-dioxins and dibenzofurans and does not produce these compounds from precursor molecules. Our results, when compared to those of Thornton, LaDue, and Savage, suggest that, in SCWO, as in municipal 0013-936X/92/0926-1849$03.00/0
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waste incineration (2, 3), there may be competition between reactions which produce dioxins and furans at low temperatures and those which destroy them at higher temperatures. Dioxin and furan formation in municipal waste incineration is a complex process which is dependent on variables including temperature, residence time, excess oxygen concentration, flow rate, and total particulate matter. Under the conditions proposed for commercial applications of SCWO, the reactions which destroy dioxins and furans are obviously favored over the formation reactions. With optimization of the operating parameters it will be possible to produce no significant concentrations of these compounds in the gas or liquid effluents. Registry No. TCDBF, 30402-14-3; TCDBD, 141456-91-9; OCDBF, 39001-02-0;OCDBD, 3268-87-9;PhOH, 108-95-2;DBF, 132-64-9.
Literature Cited Thornton, T. D.; LaDue, D. E., 111; Savage, P. E. Environ. Sci. Technol. 1991,25,1507-1510. Vogg, H.; Stieglitz, L. Chemosphere 1986, 15 (9-12), 1373-1378. Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987,21, 1080-1084.
Kathleen C. Swallow,” Wllllam
R. Klllllea
MODAR, Inc. Natick, Massachusetts 0 1760
SIR We thank Swallow and Killilea for their interest in our work in SCWO, and we are gratified that MODAR has found it to be of sufficient interest to merit comment. We also appreciate their publishing data dealing with the destruction of chlorinated dibenzo-p-dioxins and dibenzofurans by SCWO. The only comment made by the authors with which we disagree is that their data “clearly demonstrate that at normal commercial operating temperatures SCWO ... does not produce these compounds [chlorinated dioxins and furans] from precursor molecules”. Their data confirm only that SCWO can destroy these compounds. The data do not address the issue of the formation of these products. For instance, chlorinated dioxins and furans may have been formed from precursor molecules in their experiments and then subsequently destroyed. Experiments at high destruction efficiencies provide no information about the formation
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of intermediate products in the reaction network. Experiments at low and moderate conversions are required to address this issue. This is precisely the type of experiment we completed (1). Our previous results for phenol oxidation in SCW do have implications to commercial SCWO processes. Simplified SCWO process flow sheets show that the temperature at the reactor entrance is -400 “C and that the exothermic oxidation reaction raises the temperature to -600 “C at the reactor exit (2). Thus, the conditions used in our experiments approximate the proposed commercial conditions near the reactor entrance. Therefore our previous work with phenol oxidation ( I ) , which showed that the formation of higher molecular weight products accounted for 42% of the phenol that reacted at 380 “C and 278 atm, suggests that similar products may form from phenolic reactants near the entrance of a commercial SCWO reactor. Of course, the results of Swallow and Killilea suggest that these products can be effectively destroyed at the higher temperature downstream in the reactor. To summarize, we agree with Swallow and Killilea that SCWO is a promising technology for waste destruction and that high destruction efficiencies can be achieved for most compounds. We also share their view that “with optimization of the operating parameters it will be possible to produce no significant concentrations of these compounds [dioxins and furans] in the gas or liquid effluents”. Research currently underway in our laboratory is focused on identifying the values of the process variables that minimize the formation of these products. Registry No. PhOH, 108-95-2.
Literature Cited (1) Thornton, T. D.; LaDue, D. E., 111; Savage, P. E. Enuiron. Sci. Technol. 1991,25, 1507-1510. (2) Modell, M. Supercritical Water Oxidation in Standard
Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. H., Ed.; McGraw-Hill: New York, 1989; Section 8.11.
Thomas D. Thornton, Douglas E. LaDue, 111, Phillip E. Savage‘ Department of Chemical Engineering The University of Michigan Ann Arbor, Michigan 48109-2136
