Environ. Sci. Technol. 1991, 25,1507-1510
COMMUNICATIONS Phenol Oxidation in Supercritical Water: Formation of Dibenzofuran, Dlbenzo-p-dioxin, and Related Compounds Thomas D. Thornton, Douglas E. LaDue, 111, and Phllllp E. Savage"
Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48 109-2136
Introduction Supercritical water is a good medium for the complete oxidation of organic compounds, and this chemistry forms the basis for a novel technology currently being advanced for the ultimate destruction of hazardous wastes. Above its critical point (T,= 374 OC, P, = 218 atm), water has a high solubility for both organics ( I ) and oxygen ( 2 ) ,so a single phase containing a homogeneous mixture can exist a t reaction conditions. This elimination of potential interphase transport limitations coupled with the moderately high operating temperatures and pressures leads to rapid oxidation rates. Much of the previous research dealing with oxidation in supercritical water has been devoted to either demonstrating the technology (3-7) or measuring the disappearance kinetics for relatively simple compounds (8-12). Only very limited work (3,12),however, has been devoted to identifying and quantifying the reaction products from the supercritical water oxidation of organic compounds. Such research is clearly important from an environmental viewpoint, especially when one considers that incineration, another thermal oxidation process, can produce undesired, high molecular weight condensation products such as dibenzofurans and dibenzo-p-dioxins from phenols (13-18). We have undertaken and completed this study to determine whether the oxidation of phenol, a representative organic pollutant, in near- and supercritical water can lead to the formation of similar high molecular weight products.
Experimental Section Oxidation reactions were accomplished in both batch and flow reactors. For the flow reactor studies, phenol oxidation was carried out in an isothermal reactor that was constructed from a 1-or 4-m length of l/gin.-o.d. Hastelloy C-276 tubing. The reactor feed streams were prepared by dissolving high-pressure oxygen into deionized water in a stirred vessel and loading a mixture of phenol dissolved in deionized water into a second vessel. The two feed streams were pumped and preheated separately in 2-m lengths of 1/16-in.-o.d.Hastelloy C-276 tubing coiled inside a temperature-controlled fluidized sand bath. Employing this split feed arrangement ensured that the phenol and oxygen did not contact each other until the streams had been pressurized and preheated to the desired reaction conditions. We verified that the phenol did not react in the preheater line by pyrolyzing phenol in supercritical water in a batch reactor, under conditions identical with those used in the flow reactor studies, and observing no measurable conversion and detecting no reaction products. The feed streams were mixed at the reactor inlet, and after passing through the heated reactor, the reactor effluent was cooled rapidly in a heat exchanger, depressurized, and separated into liquid and vapor phases. Additional details 0013-936X/91/0925-1507$02.50/0
regarding the reactor design and operation have been reported elsewhere (19). The batch reactor studies were completed in constantvolume reactors constructed from 3/8-in.-o.d.316 stainless steel tubing fitted with 316 stainless steel Swagelok end caps. The reactors were loaded with a measured volume of an aqueous stock solution of phenol in order to achieve the desired reactor pressure for each reaction temperature. The loading and sealing of the reactors were completed in an oxygen-filled glovebag. Thus, phenol, water, and oxygen were the only compounds initially present within each reactor. After the reactors were loaded and sealed, they were next immersed in a preheated, isothermal fluidized sand bath. When the desired residence time was reached, the reactors were removed from the sand bath and quenched in an ambient temperature water bath. Each reactor was opened, and the liquid-phase product was retained for further analysis. The concentration of unreacted phenol was determined by reverse-phase, high-performance liquid chromatography (HPLC), and more details are available elsewhere (19). Prior to further analysis, the liquid product samples were subjected to a benzene solvent extraction protocol in which the liquid product was contacted with benzene in a 5:l volume ratio. The extraction step transferred the organic compounds in the aqueous phase into the benzene, thereby concentrating the samples. The benzene layer was transferred to a vial and retained for further analysis. Identification of the reaction products was accomplished using a Hewlett-Packard (HP) Model 5890 gas chromatograph (GC) with a Model 5970 mass selective detector and a computer workstation. Quantification of the product concentrations was accomplished using an H P 5890 GC equipped with a flame ionization detector and an H P 7673 autosampler/injector. In both gas chromatograph systems, a 12 m X 0.2 mm X 0.33 pm film thickness HP-1 capillary column separated the sample constituents. The concentrations of each of the products were determined by first preparing a set of aqueous standard solutions containing different concentrations of each product. We next subjected these standards to the solvent extraction protocol used for the reaction products, separated the benzene and aqueous phases, injected l pL of the benzene phase into the GC, and recorded the integrated area of the resulting peak on the chromatogram. Plotting these integrated areas as a function of the concentration of the compound in the standard solution resulted in a linear calibration. We used these calibrations to determine the concentrations of the different reaction products in the samples taken from the flow reactor. Multiple injections of samples withdrawn from the same vial revealed that the GC analyses were reproducible as the integrated peak area varied by *6% at the 95% con-
@ 1991 American Chemical Society
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Abundance 2.0e+07
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fidence level. Additionally, background analyses verified that the deionized water, phenol, and benzene used in the experiments were each free of high molecular weight impurities.
Results and Discussion The nominal reaction conditions used in our experiments included temperatures of 300,380, and 420 "C and pressures of 218, 233, and 278 atm. We intentionally operated at temperatures somewhat lower than the temperatures of 400-650 "C typically proposed for commercial applications of supercritical water oxidation technology (5, 7) so that the products from the incomplete oxidation of phenol could be detected. The operating pressure was comparable to that proposed for commercial operation. The initial phenol concentrations were from 100 to 1000 parts per million on a mass basis (ppm), and the initial oxygen concentrations ranged from the precise stoichiometric amount required for the complete conversion of phenol to carbon dioxide and water (0% excess oxygen) to 11 times the stoichiometric amount (1000% excess oxygen). Through the combination of both flow and batch reactor work, a wide range of residence times could be tested. The residence times investigated in the flow reactor ranged from 3 to 109 s, and much longer residence times from 300 to 29 000 s (5 min to about 8 h) were tested in the batch reactor. These residence times covered a broader range than those proposed for commercial operation. Gas chromatography-mass spectrometry analysis of the benzene extracts revealed the presence of a very large number of individual reaction products. Figure 1 displays a representative total ion chromatogram, which corresponds to a sample produced by oxidizing 875 ppm phenol with 40% excess oxygen at 380 "C and 233 atm for 33 s in the flow reactor. We tentatively identified each of the compounds by searching the computer library of mass spectra and selecting the one that best matched its mass 1508
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spectrum. The structures that appear near the individual peaks in Figure 1 summarize the results of the computer matches of the mass spectra. Note that these identifications remain tentative for most of the compounds. Four of the compounds (4-phenoxyphenol, 2,2'-biphenol, dibenzofuran, and dibenzo-p-dioxin), however, were positively identified by comparing both their retention times and mass spectra with those of authentic compounds. Additionally, we are confident that the identification of 2-phenoxyphenol is also correct because of the very good match of its mass spectrum with the library spectrum. Table I provides a summary of representative experimental results obtained under the different reaction conditions. Inspection of the data in Table I reveals that a t shorter residence times 2-phenoxyphenol, 4-phenoxyphenol, and 2,2'-biphenol were typically present in higher concentrations than were dibenzofuran and dibenzo-pdioxin. A t longer residence times, however, dibenzofuran was typically the product present in the highest concentrations. In no experiment did the dibenzo-p-dioxin concentration ever exceed 0.3 ppm, and for several sets of operating conditions it was not detected as a product. Figure 2 displays the temporal variations of the concentrations of the five products whose identities are certain. These data were obtained by operating the flow reactor at 380 "C and 278 atm and using a feed stream with an initial phenol concentration of 100 ppm and 800% excess oxygen. At short reaction times the concentrations of the two phenoxyphenol isomers were the highest. These concentrations increased rapidly with time until they reached maximum values near 13 s. As time increased, however, the concentrations of the phenoxyphenols decreased until they were present in very low concentrations a t the long reaction times. The concentration of 2,2'-biphenol showed a similar trend with time, but its concentration was always much lower than that of the phenoxyphenols. The dibenzofuran concentration was always greater than 0.9 ppm, and it increased slowly with reaction
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The data in Figure 2 indicate that the 2,2'-biphenol and phenoxyphenols were oxidized on roughly the same time scale as was phenol, the original reactant. The batch reactor results in Table I, however, show that the dibenzofuran was more resistant to degradation. For example, oxidation of 750 ppm phenol a t 420 "C and 278 atm for 3600 s (1 h) resulted in a dibenzofuran concentration of
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these products formed from the supercritical water oxidation of phenol are also formed from phenol incineration (13-18). This observation suggests that there may be a relationship between the two thermal oxidation processes, even though supercritical water oxidation operates at much lower temperatures and much higher pressures than does incineration.
Literature Cited (1) Connolly, J. F. J . Chem. Eng. Data 1966, 11, 13. (2) Pray, H. A,; Schweickert, C. E.; Minnich, B. H. Znd. Eng. Chem. 1952, 44, 1146.
(3) Modell, M. U.S. Patent 4 543 190, 1985. (4) Thomason, T. B.; Modell, M. Hazard. Waste 1984,1,453. (5) Modell, M. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGrawHill: New York, 1989; p p 8.153-8.168. (6) Staszak, C. N.; Malinowski, K. C.; Killilea, W. R. Environ. Prog. 1987, 6, 39. (7) Bramlette, T. T.; et al. Destruction of DOE/DP Surrogate Wastes with Supercritical Water Oxidation Technology. Sandia National Laboratory Report, SAND90-8229; 1990. (8) Helling, R. K.; Tester, J. W. Energy Fuels 1987, I , 417. (9) Helling, R. K.; Tester, J. W. Environ. Sci. Technol. 1988, 22, 1319. (10) Rofer, C. K.; Streit, G. E. Kinetics and Mechanism of Methane Oxidation in Supercritical Water. Los Alamos
National Laboratory Report, LA-11439-MS (DOE/HWP64); 1988. (11) Wightman, T. J. M.S. Thesis, University of California, Berkeley, 1981. (12) Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988,27, 2009. (13) Choudhary, G. G.; Olie, K.; Hutzinger, 0. In Chlorinated Dioxins and Related Compounds: Impact on the Environment; Hutzinger, O., Frei, R. W., Merian, E., Pocchiari, F., Eds.; Pergamon: Oxford, U.K., 1982; pp 275-301. (14) Eklund, G.; Pedersen, J. R.; Stromberg, B. Nature 1986, 320, 155. (15) Stehl, R. H.; Lamparski, L. L. Science 1977, 197, 1008. (16) Ballschmiter, K.; Swerev, M. Fresenius 2. Anal. Chem. 1987,328, 125. (17) Born, J. G. P.; Louw, R.; Mulder, P. Chemosphere 1989, 19, 401. (18) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983,17, 121. (19) Thornton, T. D.; Savage, P. E. J. Supercrit. Fluids 1990, 3. 240.
Received for review March 27,1991. Revised manuscript received May 24,1991. Accepted May 27,1991. This work was supported in part by the National Science Foundation (CTS-8906859 and CTS-8906860), the Shell Faculty Career Initiation Fund, and the Amoco Foundation.
Sonochemical Destruction of Chlorinated Hydrocarbons in Dilute Aqueous Solution H. Mlchael Cheung," Ashlsh Bhatnagar, and Greg Jansen The University of Akron, Department of Chemical Engineering, Akron, Ohio 44325-3906
Introduction
tained for the other compounds.
Power ultrasound in the range of 20-100 kHz has found application in cleaning (the common laboratory ultrasonic cleaner operates in this frequency range), plastic welding, emulsification, and chemical reactivity. Most sonochemical research to date has focused on synthetic aspects of ultrasound. Both homogeneous and heterogeneous ultrasound can produce sonochemistry, a recent review by Suslick (1)provides a concise overview of the current state of the art. A more extensive tutorial in sonochemistry can be found in the book by Mason (2). Homogeneous sonochemistry results from the formation of cavitation bubbles in the solvent. The bubble collapse leads to surprisingly high local temperatures and pressures. Locally the temperature and pressure may reach 5000 "C and 500 atm, respectively (3). These rather extreme conditions are very short lived, but have been shown to produce several reactive species in aqueous systems including HzOz,HOz,H', and OH' (4). The reactions that take place in aqueous solution are similar to combustion (5)though strong reduction as well as oxidation reactions have been observed (6). We are currently exploring the use of homogeneous ultrasound in destroying chlorinated hydrocarbons in dilute aqueous solution. We have thus far examined the sonochemical destruction of methylene chloride, carbon tetrachloride, l,l,l-trichloroethane, and trichloroethylene in concentrations in the 100-1OOO ppmv range. Ultrasound appears to be quite effective in the destruction of the compounds examined thus far. Quantitative results with GC/MS have been obtained for methylene chloride, qualitative results from pH measurements have been ob-
Experimental Section The experimental setup consisted of a Heat Systems W-385 ultrasonicator and a 2-L glass reaction vessel equipped with a stainless steel cooling coil. The W-385 ultrasonicator is capable of delivering 475 W of ultrasonic energy. The temperature and pH of the reactor contents were continuously monitored. The sonicator was operated at its maximum output setting and typically delivered 40% (approximately 250 W) of its rated power to the reactor. The chlorinated hydrocarbons were all of a t least 99% purity and were used as received. A Hewlett-Packard GC/MS (5890 GC, 5970 mass sensitive detector, and 9133 Chem Station) with a 12 m X 0.2 mm X 0.33 pm film thickness H P Ultra 1 capillary column coated with cross-linked methylsilicone gum was used for determination of methylene chloride concentrations. Regular tap water was used in the cooling coils to prevent temperature rise due the sonication. The temperature was not controlled but ranged from 15 to 20 "C. We are planning to install a temperature-controlled circulation bath on the reactor. Temperature and pH data were recorded at regular intervals with the sonicator on standby. The sonicator interferred with the stability of the pH reading. For methylene chloride, 1 O - m L samples were withdrawn for GC/MS analysis. The analytical protocol involved combining the 10-mL sample with 1 mL of hexane and shaking for 2-5 min by hand. After at least 10 min of equilibration, 1pL of the hexane phase was injected into the GC/MS. A calibration curve utilizing aqueous samples of known
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