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
