Ind. Eng. Chem. Res. 1992,31, 2451-2456
2451
Phenol Oxidation Pathways in Supercritical Water Thomas D. Thornton' and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
We oxidized aqueous solutions of phenol in batch and flow reactors a t temperatures of 300, 380, and 420 "C,pressures of 218 and 278 atm, and reaction times between 1.2 and 29040 s. Gas chromatographic and mass spectrometric analysis of the oxidation products extracted from the aqueous reactor effluent permitted identification and quantification of the concentrations of multiring compounds such as 2- and 4-phenoxyphenol,2,2'-biphenol, dibenzofuran, and dibenzo-p-dioxin. The Delplot methodology was used to discriminate between primary and nonprimary products. This analysis revealed that 2- and Cphenoxyphenol, 2,2'-biphenol, and dibenzofuran formed directly from phenol whereas dibenzo-p-dioxin formed later in the reaction network. The literature suggested that 2-phenoxyphenolwas the most likely dibenzo-p-dioxin precursor. A reaction network consistent with these observations was developed and shown to be in quantitative agreement with the experimental results. The reaction model revealed that the formation of the primary, multiring producta from oxidation a t 380 "C and 278 atm accounted for 43% of the phenol that reacted. This result suggests that compounds such as dibenzofurans and dioxins could be formed during the supercritical water oxidation of phenolic wastes near the entrance of commercial-scale reactors where the design temperature is around 400 O C .
Introduction Oxidation in supercritical water (SCW) is a technology being developed for the ultimate destruction of organic wastes (Modell, 1982,1985,1989). The process involves the reaction of organics and oxygen in an aqueous phase at conditions that exceed the critical temperature and pressure of water (T,= 374 "C,P, = 218 atrn). Operating at supercritical conditions leads to a homogeneous reaction mixture in which organics, water, and oxygen can exist in a single phase. Under these reaction conditions, organic carbon can be converted to C02,hydrogen to H20,chlorine to HC1, and nitrogen to N2 or N20. It is well established that oxidation in SCW can lead to very high destruction and removal efficiencies (DRE) for hazardous organica. The reaction pathways through which these high DREs are obtained are not well established, however. Additionally, little mearch appears to have been devoted to identifying the products of incomplete oxidation of representative organic pollutants. In one study, Yang (1988) and Yang and Eckert (1988) oxidized p-chlorophenol in near-critical and supercritical water, and they identified CO and C02as the major gaseous products and ethylene, ethane, methane, and hydrogen as trace products. 1,4Benzoquinone was the dominant liquid-phase product, and hydrochloric acid was also detected. They reported quantitative information for only CO, C02, and HC1. More recently, our research group identified nearly 40 products from phenol oxidation in SCW (Thornton and Savage, 1990; Thornton et al. 1991). These products included permanent gases, ring-opening products such as carboxylic acids, single-ring products such as dihydroxybenzenes, and multiring products. Several of the compounds in the last group are environmentally significant because they are potentially more hazardous than phenol, the starting material. Of the products in this last group, the most abundant and environmentally significant were 2- and 4phenoxyphenol,2,2'-biphenol,dibenzofuran, and dibenzo-p-dioxin. Accordingly, we determined the concentrations of these products at different reaction conditions, and Thornton et d.(1991) have reported preliminary results from these analyses.
* To whom correspondence should be addressed.
Present address: Warrensville Research and Environmental Science Center, BP America, Cleveland, OH 44128.
In this paper, we report additional data that permit resolution of the reaction pathways for the formation of these multiring producta. We then provide a quantitative assessment of the relative rates of the different pathways in the reaction network for a specific set of reaction conditions. A knowledge of the reaction pathways and their relative rates is central to developing a strategy for minimizing the formation of these undesired multiring products because different strategies could be adopted for different types (e.g., series, parallel) of networks. Thus, these new data and the corresponding reaction engineering analysis constitute the first steps toward the goal of identifying SCW oxidation process conditions and configurations that would minimize or eliminate the formation of multiring products.
Experimental Section We oxidized phenol at temperatures of 300,380,and 420 "C and pressures of 218 and 278 atm in both plug flow and batch reactors. Reaction times ranged from 1.2 s to approximately 8 h. Details about the reaction conditions employed appear in Tables I and 11. The isothermal, isobaric flow reactor, which has been described previously (Thornton and Savage, 1990,1992), was constructed from l/&in.-o.d. Hastelloy C-276 tubing. The reactor feed streams were prepared by dissolving oxygen into deionized water in one feed tank and loading a previously prepared aqueous solution of phenol into a second feed tank. The reaction mixture was always greater than 99.5 mol ?% water. We worked with these dilute solutions so that the adiabatic temperature rise would be restricted to less than 1"C and so that a single fluid phase would exist in the reactor. High-pressure metering pumps were used to pressurize the two feed streams, which were then preheated separately by flowing through Hastelloy tubing immersed in a temperature-controlled,isothermal, fluidized sand bath. The preheated feed streams were mixed at the reactor inlet, and after passing through the reactor, the mixture was cooled and depressurized. This product stream was then separated into liquid and vapor phases prior to analysis. Each batch reactor comprised a length of 3/8in.-o.d. 316 stainless steel tubing fitted with 316 stainless steel Swagelok caps. The reactors were loaded with a known volume of an aqueous stock solution of phenol inside an atmospheric pressure, oxygen-filled glovebag. After the reactors
0888-588519212631-2451$03.00/00 1992 American Chemical Society
2412 Ind. Eng.Chem. Res., Vol. 31, No. 11,1992 Table I. Summary of Flow Reactor Experiments temp ("(2) 420 420 420 420 420 420 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 381 380 380 380 381 380 380 377 380 380 379 380 381 299 300 299 300 300 300 300 300 300
press. (atm)
278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 279 278 218 218 218 218 218 218 279 278 279 278 278 278 278 278 278
phenol init mncn (ppm) 98 104 92 98 94 92 973 951 965 98 96 94 94 98 96 104 92 101 98 99 106 100
90 102 84 99 96 243 243 242 243 252 242 239 225 242 246 248 258 247 249 226 258 234 242 242 244 218 230 232 271 240
excea oxygen
7
(9%)
(8)
781 651 871
4.3 8.2 12.0 16.1 25.7 40.8 16.0 31.7 65.3 3.5 4.1 6.1 8.3 9.9 12.9 13.4 17.1 24.1 32.3 39.7 47.6 63.4 68.8 73.3 91.8 96.1 97.7 12.3 16.2 16.3 31.7 31.9 41.2 48.7 69.1 69.1 78.1 1.2 2.8 5.7 7.0 12.6 25.9 t0.8 26.5 33.2 46.9 67.5 77.9 89.1 100.9 108.8
668 789 868
5 13 6 1013 985 984 950 923 859 796 976 858 816 819 736 841 912 747 1070 848 872 228 242 243 241 204 280 238 293 286 276 213 180 204 193 253 173 327 294 279 284 333 318 311 224 275
\
were loaded and sealed, they were immersed in an isothermal f l u i d i d sand bath that had been preheated to the desired reaction temperature. We used the steam tables (Haaret al., 1984) along with a knowledge of the reactor volume and the specif'ic volume and temperature of the reaction mixture to estimata the reaction pressure. When the desired holding time had elapsed, the reactors were removed from the sand bath and immersed in an ambient-temperaturewater bath to quench the reaction. This holding time includes the time required for the reactor to reach the sand bath temperature and the time required for the reactor to cool. On the basis of our experience with thistype of microbatch reactor, we estimate the heat-up time to be about 2-3 min and the cooling time to be a few seconds. After cooling,each reactor was opened, and the liquid phase was retained for analysis. The amount of phenol that remained unconverted was determined by reverse-phase, high-performance liquid chromatography. The multiring reaction products were
phenol conv (I) 16.0 26.0 29.2 40.2 47.2 74.8 32.3 61.1 72.9 15.5 17.1 20.8 16.2 27.0 37.0 48.6 46.8 59.3 70.8 77.2 85.7 94.6 98.5 95.4 97.8 97.3 97.7 27.4 29.7 41.4 54.1 55.8 68.6 81.9 99.4 94.5 93.1 4.8 4.3 6.9 10.3 13.2 28.7 26.2 20.6 27.5 40.4 78.2 82.0 78.8 95.6 95.2
2PP 7.6 10.1 12.9 16.0 15.3 9.0 57.7 36.3 23.0 3.2 3.7 4.7 4.3 4.5 9.6 4.0 7.6 7.1 5.7 4.6 2.8 1.0 0.2 0.7 0.3 0.5 0.3 21.9 18.9 19.3 16.3 16.7 14.8 5.9 0.0 0.7 2.6 3.1 4.5 7.5 8.1 11.5 14.8 3.7 3.8 4.0 4.0 0.9 1.5 2.2 0.0 0.0
product concentration (ppm) 4PP BP DBF 2.1 0.0 1.6 2.1 2.5 0.0 0.0 3.1 2.1 3.1 2.7 0.0 2.5 3.1 0.0 1.1 0.0 2.2 12.2 32.6 1.5 8.3 15.1 2.9 5.6 6.5 2.4 1.1 2.0 0.2 2.4 0.4 1.4 3.0 0.9 1.6 2.6 0.6 1.5 2.4 0.4 1.6 4.0 0.7 1.3 1.2 1.8 1.7 2.8 0.3 0.9 2.5 0.2 1.3 1.6 0.0 1.9 0.0 1.0 2.1 0.0 0.7 2.6 0.0 0.0 2.3 0.0 0.0 1.6 0.0 0.0 1.4 0.0 1.0 0.0 0.0 0.0 1.8 0.0 0.0 2.4 4.4 2.3 8.3 4.9 2.0 6.8 3.5 1.3 6.8 3.0 3.8 4.9 2.5 3.0 5.4 4.1 1.5 3.4 1.0 2.3 2.0 0.0 0.0 0.9 0.0 0.0 3.1 1.0 0.0 2.9 1.3 0.7 0.8 1.6 1.1 0.9 1.1 2.8 1.9 3.2 1.0 0.7 1.1 5.0 2.7 1.1 5.4 0.9 0.1 6.0 1.3 0.0 2.4 6.0 1.5 0.0 5.8 0.2 6.1 2.7 0.0 0.2 1.2 0.7 0.0 2.2 1.2 0.1 3.4 0.0 0.1 0.0 0.2 0.0 0.0
DBD tr < 0.1 tr < 0.1 tr < 0.1 tr < 0.1 tr < 0.1 0.0 0.1 0.2 0.0 0.0 0.0 0.1 0.1 0.3 0.0 0.2 0.0 0.1 0.2 0.1 0.2 0.0 0.0 0.0 0.0 0.0 0.1 0.2 0.1 0.0 0.2 0.2 0.3 0.1 0.0 0.0 0.1
tr < 0.1 tr < 0.1 tr < 0.1 0.0 tr < 0.1 tr < 0.1 tr < 0.1 tr < 0.1 0.0 0.0 0.0
tr < 0.1 0.0 0.0 0.0
extracted from the aqueous phase into benzene. The identities of these products were confirmed using a gas chromatograph with a mass spectrometric detedor, and their concentzations were determined by capillary column gas chromatography using flame ionization detection. Additional details about these analyses and the experimental procedure have been reported previously ("hornton and Savage, 1990; Thornton et al., 1991). Results Tables I and 11,which summarhe the results of the flow and batch reactor experiments, respectively, provide the concentrations of 2-phenoxyphenol (2PP), &phenoxyphenol (OP),29-biphenol (BP),dibenmfuran @BF), and dibenzo-p-dioxin (DBD)and the phenol conversions achieved at a large number of different reaction conditions. The 367% excess oxygen value listed in Table II for experiments at 380 OC and 218 atm differs from the value of 250% reported in our earlier communication (Thornton
Ind. Eng. Cham. Res., Vol. 31, No. 11,1992 2463 Table 11. Summary of Batch h c t o r Experiments excess temp preee. phenolinit oxygen (‘C) (atm) concn (ppm) (96) holding time (e) 420 278 750 263 300 420 263 600 278 750 900 420 263 278 750 1800 420 263 278 750 3600 420 263 278 750 28800 420 263 278 750 151 380 300 250 278 600 250 278 151 380 900 250 151 380 278 1800 250 278 151 380 3600 250 151 380 278 29040 250 151 380 278 367 218 300 590 380 218 600 367 590 380 900 367 590 380 218 367 380 1800 218 590 218 3600 367 590 380 218 367 590 28800 380 -58 278 300 300 450 300 600 278 -58 450 218 930 -58 450 300 -58 1800 300 278 450 300 -58 3720 278 450 300 -58 28800 278 450
et al., 1991). This earlier value is in error. The flow reactor data in Table I reveal that 2-phenoxyphenol was usually the most abundant product at low conversions. Furthermore, its concentration profile always exhibited a maximum, and this maximum occurred at conversions leas than 40%. The Cphenoxyphenol and 2,2”-biphenolconcentration profiles also showed maxima at similar conversions, but theae maximum concentrations were lower than those for 2-phenoxyphenol. The concentration of dibenzofuran was typically low, and it tended to increase slowly with conversion without ever achieving a distinct maximum. Finally, of the five high molecular weight products of interest, dibenzo-p-dioxinwas the one present in the lowest concentrations. Only trace amounts (leas than 0.1 ppm) were detected from reactions at 300 and 420 O C , but higher concentrations were observed at 380 OC. The batch reactor data in Table 11 permit evaluation of the persistence of each of the five high molecular weight compounds. The phenoxyphenols and biphenol were degraded on roughly the same time scale as phenol, but dibenzofuran was much more resistant to oxidative degradation in SCW. Indeed, dibenzofuran was still present even after oxidation at 420 “Cfor 14 whereas essentially complete conversion of phenol occurred at between 5 and 10 min at this temperature. The relatively low concentrations of dibenzo-p-dioxin prevent unequivocal assessment of its persistence.
Reaction Pathway Resolution The results presented in the previous section provide ineight to the phenol oxidation pathways. For example, Table I showed that the temporal concentration profiles for the phenoxyphenola and biphenol exhibited maxima, a feature characterietic of intermediate products in a series reaction network. Moreover, the concentration of dibenzofuran appeared to increase with residence time, behavior that suggests the absence of kinetically significant demmpdtion reactions for dibenzofuran on the time d e of the flow reactor experiments. The long persistence of dibenzofuran in the batch reactor experiments is also consistent with thie conclusion. The concentrations of dibenzo-p-dioxinwere too low and the data posseseedtoo
phenol conv (% ) 93.6 100.0 100.0 100.0 100.0 100.0 65.2 67.3 100.0 100.0 100.0 100.0 68.2 100.0 100.0 100.0 100.0 100.0 17.2 41.9 48.8 66.0 100.0 100.0
0.00
2PP 3.5 0.0 0.0
0.0 0.0 0.0 3.9 2.2 0.0 0.0 0.0 0.0
11.0 0.0 0.0 0.0 0.0 0.0 3.7 2.9 3.4 4.6 0.6 0.0
0.05
product concentration (ppm) 4PP BP DBF 0.0 0.0 12.5 0.0 0.0 8.4 0.0 0.0 8.1 0.0 0.0 9.7 0.0 0.0 9.7 0.0 0.0 0.0 1.9 15.0 7.8 1.4 15.0 5.9 0.4 0.0 2.4 0.3 0.0 2.5 0.4 0.0 3.7 0.0 0.0 0.0 2.1 0.0 11.2 0.0 0.0 8.0 0.0 0.0 3.6 0.0 0.0 5.6 0.0 0.0 3.8 0.0 0.0 8.6 7.7 16.3 0.4 5.3 15.7 0.4 5.2 15.7 0.6 7.7 15.5 1.4 0.5 0.0 1.3 0.6 0.0 0.8
0.10
0.15
0.20
0.25
DBD 0.3 0.0 0.0 0.0 0.0 0.0 0.3 0.3 0.0 0.0 0.0 0.0 0.6 0.2 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0
0.30
Conversion
Figure 1. Firstrank Delplot for phenol oxidation in SCW (nominal reaction conditions: 380 “C, 218 atm, 250 ppm phenol, 200% exma oxygen).
much scatter to ascertain any clear temporal trends. A more definite delineation of the reaction network can be obtained by employing the Delplot methodology, which has been recently described by Bhore et al. (1990). This technique uses a series of plots (termed Delplots) constructed from readily available experimental data (i.e., product yields and reactant conversions) to establish the order of appearance of products in a reaction network. The firsbrank Delplot, which provides discrimination between primary and nonprimary products, examinee the products’ selectivities (i.e., moles product formed per mole reactant r e a d ) as a function of the reactant conversion. primary products, by definition, possess positive intercepts on a f i t - r a n k Delplot, and nonprimary products possess intercepts equal to zero. Figure 1provides a f i b r a n k Delplot for the five high molecular weight products of intereat in this study. These particular data, obtained from a series of flow reactor experiments at nominal reaction conditions of 380 OC,218 atm, 250 ppm phenol, and 200% excess oxygen, were selected because low-conversion data ensure the most reliable
2454 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992
/-&a-
Other PmdYCtll
DBF
Figure 2. Pathways to multiring products from phenol oxidation in SCW.
product assignments. This is because the y-intercept of the Delplot contains the key information, and extrapolating low-conversion data allows the intercept to be determined with a measure of confidence. Product selectivities at low conversions are often difficult to measure accurately, however, because the uncertainty in the concentration measurements (about 5% for phenol in these experiments) can lead to larger uncertainties in the selectivities and conversions. These uncertainties contribute to the scatter present in the data in Figure 1. Nevertheless, Figure 1 clearly shows that 2-phenoxyphenol was the product with the highest selectivity, and that its intercept is nonzero. Thus, 2-phenoxyphenol is a primary product of phenol oxidation in SCW. Likewise, the initial selectivities to 4-phenoxyphenol, 2,2'-biphenol, and dibenzofuran are all nonzero, and these are all primary products. Finally, dibenzo-p-dioxin was not detected in any of these six experiments, so it must be a nonprimary product. 2-Phenoxyphenol is the most likely dioxin precursor because it is the only multiring primary product with two oxygen atoms in the same positions on the aromatic ring as in dibenzo-p-dioxin. In fact, the incineration and pyrolysis literature (e.g., Nilaeon et al., 1974; Born et al., 1989; Shaub and Tsang, 1983) confirms that dibenzo-p-dioxins can form from 2-phenoxyphenol precursors. Thus, the most likely secondary pathway for dibenzo-p-dioxin formation proceeds from 2-phenoxyphenol. A set of reaction pathways consistent with the Delplot analysis and these insights from the incineration and pyrolysis literature is shown in Figure 2. These pathways are also consistent with Thornton's (1991) analysis of a more complex reaction network. The reaction network in Figure 2 includes parallel primary reactions that form the phenoxyphenob, 2,2'-biphenol, dibenzofuran, and several of the other products (e.g., single-ring and lower molecular weight products) that have been detected from phenol oxidation in SCW.This last primary pathway eventually leads to COP All of the primary multiring products except dibenzofuran undergo secondary decomposition reactions that also presumably lead to the eventual formation of COP Additionally, a secondary pathway from 2-phenoxyphenol accounts for the formation of dibenzo-p-dioxin. Both dibenzofuran and dibenzo-p-dioxin were taken to be stable on the time scale of the flow reactor experiments. Reaction Pathway Model Having deduced a reaction network that is qualitatively consistent with the experimental observations and the
literature, we next sought to develop a quantitative reaction model based on these pathways. A comprehensive model would incorporate the effects of process variables such as temperature, pressure, and concentrations to identify process conditions and confiiations that would minimize the formation of the undesired multiring products. The present set of data in Tables I and XI do not yet admit the development of such a complete model. Nevertheless, we can use the available data to take the first step toward such a model and illustrate the type of analysis required. This analysis will also allow us to assess the relative rates of the different pathways in the reaction network for a specific set of reaction conditions. For reactions conducted with a negligible change in specific volume in an isothermal, isobaric, plug-flow reactor, the mole balance can be written as dCi _ -- ri (1) dr where Ciis the concentration of species i in the reaction mixture, T is the residence time, and ri is the rate of formation of species i. Thus, the differential equations that describe the concentrations of multiring products as functions of time, assuming pseudo-fust-order kinetics for all of the reactions in Figure 2, are dCphenol -dr - -kACphenol
where kA = 2kl + 2kz + 2k3 + 2k4 + k,; (3)
dC4PP -= (4) k2Cphenol - k8C4PP dr dCBP (5) d r = k3Cphenol - k9CBP ~ --C D B-Fk4Cphenol dr ~CDBD -(7) dr - k7c2PP We employed pseudo-fust-order kinetics because doing so facilitates analytical solution of the set of differential equations and because the rate of phenol disappearance is first order (Thornton and Savage, 1992). The factor of 2 that multiplies k1-k4 in the expression for k A accounts for the stoichiometry implicit in reactions 1 through 4. That is, for every one molecule of phenoxyphenol, biphenol, or dibenzofuran formed, two molecules of phenol must disappear. Equations 2-7 can be solved simultaneously with the initial condition Cphenol = Cphen,,l,O and all other Ci = 0 at T = 0 to obtain analytical expressions for the concentrations of phenol and each of the reaction products. The resulting equations are: Cphenol = Cphenol,O exp(-kAT) (8)
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2488
CDBD= klk7Cphenol,0
k6
+ k7 - k A
[
exp(-kA7) - 1 kA
+ k 7 ) d -1 + exp(-(k6 k, + k7
1
(13)
These equations can be used with different sets of experimental data in Table I to determine optimized numerical values of the reaction rate constants. Thornton (1991) provides the details of the optimization procedure employed to determine the reaction rate constants, but we will give a brief overview here. The sum k~ = 2kl + 2k2 2k3 2k4 + k5 is known from our previous study of the kinetics of phenol disappearance (Thornton and Savage, 1992). Thus, eq 9 contains only the two parameters kl and k6 + k, as unknowns, and these can be determined by fitting the data in Table I for the 2-phenoxyphenol concentration. Moreover, the parameters k2 and k8 can be determined by fitting the data for the 4-phenoxyphenol concentration profile to eq 10. Likewise, k, and k9 can be determined from eq 11,k4 from eq 12, and k7 from eq 13. Following the procedure outlined above, and using the set of data in Table I that was obtained from flow reactor experimenta at nominal conditions of 380 “C, 278 atm, 100 ppm phenol, and 800% excess oxygen, leads to the pseudo-fmt-order rate constants displayed in Table 111. That these rate constants give a faithful representation of the trends in the experimental data can be verified by substituting their values into eqs 8-13, calculating the temporal variations of the product concentrations, and comparing these calculated concentrations with the experimental data. Figure 3 provides a plot that allows such comparison. The solid curves represent the concentrations calculated using eqs 8-13 and the rate constants in Table ID,and the discrete points represent the experimentaldata from Table I. Clearly, the model calculations and experimental data display the same trends. Finally, the values of the rate constants in Table I11 provide insight into the relative significanceof the different steps in the reaction network. For instance, the fraction of phenol that reacts to form higher molecular weight products can be calculated as (2k1 + 2k2 + 2k3 + 2k4)/kk Substituting the values in Table I11 into this expression reveals that 43% of the phenol that reacted went into the primary formation of phenoxyphenols, 2,2’-biphenol, and dibenzofuran. This indicates that the formation of these higher molecular weight products is important in the overall reaction network describing phenol oxidation in SCW.
+ +
Implications to Commercial SCW Oxidation Processes Commercial application of SCW oxidation technology (Modell, 1989) envisions mixing a preheated aqueous, organic-containing stream with compressed air. This mixture, at about 400 OC, is then fed to the oxidation reactor. As exothermic oxidation reactions proceed, the temperature of the reaction mixture increases with typical reactor exit temperatures being near 650 OC. The hot reactor effluent can then be used to preheat the reactor feed stream. Given this process description, it is evident that the temperatures used in the present study would most closely correspond to those existing near the reactor inlet. Thus, the present reaults lead us to speculate that SCW oxidation
Table 111. Pseudo-First-Order Rate Constants for Reaction Network in Figure 2: Phenol Oxidation at 380 OC, 278 atm, 100 ppm Phenol, 800% Excess Oxygen reaction k (8-l) reaction k (8-l) 1 4.7 x 10-3 6 6.5 X 2 3.0 x 10-3 7 5.0 X lo-’ 1.2 x 10-1 3 7.8 X lo-’ 8 4 5.4 x 10-4 9 1.7 X lo-’ 5 2.4 X c
6.0e-44
20
0 3.0e-51
40
60
80
100
I
I A 2.0e-54
20
0 8.0e-61
c
A
A
40
60
80
100
0
0
Dibenzofuran
t ’1
6.0e-6
2.b-6 2,2 ‘-Biphenol
-1
,~~~ Dibenzo-pdiarin ,
**
O.Oe+
0
20
40
60
80
100
Residence Time (sed
Figure 3. Temporal variations of product concentrations from phenol oxidation in SCW (nominalreaction conditions: 380 O C , 278 atm, 100 ppm phenol, 800% excess oxygen). (a) Phenol. (b) 2Phenoxyphenol and 4-phenoxyphenol. (c) 2,2’-Biphenol,dibenzofuran, and dibenzo-p-dioxin.
of phenolic wastes could produce phenoxyphenols, biphenols, dibenzofurans, and dibenzo-p-dioxins near the entrance of a commercial-scaleSCW oxidation reactor via primary and secondary reaction pathways. The production of such compounds would be especially significant if chlorinated phenolics were present in the waste stream, because certain chlorinated versions of these phenol dimers are more hazardous. Of course, the production of these compounds does not necessarily guarantee their survival. For example, 2- and 4-phenoxyphenol and 2,2’-biphenol degraded on roughly the same time scale as phenol, so if a high DRE for phenol were achieved in the reactor, then these compoundswould also probably be degraded. Other compounds, such as dibenzofuran and perhaps dibenzop-dioxin, degraded on a longer time scale than that required for phenol. Therefore, these compounds would be among the products most resistant to SCW oxidation. Summary and Conclusions 1. Dibenzofuran, 2- and 4-phenoxyphenol, and 2,2’-biphenol are primary products of phenol oxidation in SCW
Ind. Eng. Chem. Res. 1992,31, 2456-2459
2456
at temperatures near the critical temperature. Dibenzop-dioxin appears later in the reaction network. 2. The phenoxyphenols and biphenol react on roughly the same time scale as phenol, but dibenzofuran has a higher persistence and is more resistant to oxidative degradation in SCW. 3. The primary reactions leading to the multiring products accounted for 43% of the phenol that initially reacted at 380 O C and 278 atm. Thus, the formation of undesired and potentially hazardous higher molecular weight compounds constitutes an appreciable portion of the initial reaction chemistry under these conditions. 4. The formation of potentially hazardous reaction products is a factor to consider in the design of SCW oxidation pmeasea for phenolic wastes. Further research is required to identify the process variables and process confiiations that minimize the formation of such products. Acknowledgment We thank Doug LaDue for experimental assistance. This project was supported by the National Science Foundation (CTS-8906860, CTS-8906859, and CTS9015738) and the Shell Faculty Career Initiation Fund. 2417-10-9; H, WStw NO. PhOH, 108-95-2; o - P ~ O C ~ H ~ O p-PhOC6H40H, 831-82-3; o-OHC6H4C6H4-o-OH,1806-29-7; dibenzofuran, 132-64-9; dibenzo-p-dioxin, 262-12-4.
Literature Cited Bhore, N.; Klein, M. T.; Bischoff, K. B. The Delplot Technique: A New Method for Reaction Pathway Analysis. Ind. Eng. Chem. Res. 1990, 29, 313.
Born, J. G. P.; Louw, R.; Mulder, P. Formation of Dibenzodioxins and Dibenzofurans in Homogeneous Gas-Phase Reactions of Phenols. Chemosphere 1989,19,401-406. Haar, L.; Gallagher, S.; Kell, G. S.NBSINRC Steam Tables; Hemisphere Publishing: Washington, DC, 1984. Modell, M. Processing Methode for the Oxidation of Organics in Supercritical Water. U.S. Patent 4,338,199, July 6, 1982. Modell, M. Processing Methods for the Oxidation of Organics in Supercritical Water. US. Patent 4,543,190, Sept 24, 1985. Modell, M. Supercritical-Water Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hik New York, 1989; Section 8.11. Nileson, C.; Andersaon, K.; Rappe, C.; Westermark, S. Chromatographic Evidence for the Formation of Chlorodioxina from Chloro-2-Phenoxyphenole. J. Chromutogr. 1974, W,137-147. Shaub, W. M.; Tsang, W. Dioxin Formation in Incinerators. Environ. Sci. Technol. 1983, 17, 721-730. Thornton, T. D. Phenol Oxidation in Supercritical Water: Reaction Kinetics, Products, and Pathways. Ph.D. Thesis,The University of Michigan, 1991. Thornton, T. D.; Savage, P. E. Phenol Oxidation in Supercritical Water. J. Supercrit. Fluids 1990, 3, 240. Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AZChE J. 1992, 38, 321-327. Thornton, T. D.; LaDue, D. E. IIJ; Savage, P. E. Phenol Oxidation in Supercritical Water: Formation of Dibenzofuran, Dibenzo-pdioxin,and Related Compounds. Environ. Sci. Technol. 1991,25, 1507. Yang, H. H. Homogeneous Catalysis in the Oxidation of p-Chlorophenol in Supercritical Water. Ph.D. Thesis, The University of Illinois, 1988. Yang, H. H.; Ekkert, C. A. Homogeneoua Catalysis in the Oxidation of p-Chlorophenol in Supercritical Water. Znd. Eng. Chem. Res. 1988, 27, 2009.
Received for review March 23, 1992 Revised manuscript received August 3, 1992 Accepted August 17, 1992
Kinetics of the Catalyzed Supercritical Water-Quinoline Reaction Zhuangjie Li and Thomas J. Houser* Chemistry Department, Western Michigan University, Kalamazoo, Michigan 49008-3842
The kinetics of the catalyzed reaction between supercritical water and quinoline was studied over the temperature range of 400-500 "C.The reaction rate is first-order with respect to both quinoline and ZnCla (catalyst) and inversely proportional to water concentration. These observations are consistent with a Langmuir-Hinshelwood heterogeneous mechanism which involves competitive adsorption on the catalytic surface, with water much more strongly adsorbed. The Arrhenius parameters for the rate constant were an activation energy of 112 kJ/mol and preexponential fador of 1.7 X los mol/(L*g.s). This activation energy is well below the C-C and C-N bond energies in aromatic heterocycles. Introduction The possible use of supercritical fluid extraction (SFE) of coal to obtain cleaner, more versatile fluid products has been of significant interest. Some fluids have the opportunity to participate as reactants at process conditions, which may yield extracts of very different compositions than those obtained from other treatments and which will be dependent on the fluid used. Thermodynamic consideration of SFE leads to the prediction that the enhanced solubility (volatility) of the solute may be several orders of magnitude (Gangoli and Thodos, 1977; Williams, 1981; Whitehead and Williams, 1975). Thus,this method combines many of the advantages of distillation with those of extraction. Because of this interest in SFE and in the destruction of hazardous materiala by supercritical water (SW)oxidation, several studies have been reporting some of the basic chemistry that may be taking place during coal
extraction and oxidation at these conditions (Houser et al., 1986,1989;Abraham and Klein, 1985; Tounsend and Klein, 1985; Lawson and Klein, 1985; Helling and Tester, 1987,1988; Thornton and Savage, 1990; Jin et al., 1990). One of the major concern of the current program ie the removal of nitrogen from organic model compounds thought to be representative of structures found in fossil fuels and that may be present during the destruction of hazardous materials. Because of the difficulty of removing heterocyclic nitrogen, experiments were initiated by extensively examining the reactivities of quinoline (Q)and isoquinoline, as well as briefly examining the reactivities of other compounds (Houser et al., 1986). The selection of water as the fluid was based on its physical and chemical properties (Frank, 1968). Zinc chloride was chosen as a catalyst because of its reported catalytic activity for hydrocracking aromatic structures (Salim and Bell, 1984).
0888-5886f 92f 2631-2456$03.O0f 0 0 1992 American Chemical Society
