Methanol as a Cosolvent and Rate-Enhancer for the Oxidation

Ind. Eng. Chem. Res. 2002, 41, 9-21

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Methanol as a Cosolvent and Rate-Enhancer for the Oxidation Kinetics of 3,3′,4,4′-Tetrachlorobiphenyl Decomposition in Supercritical Water Gheorghe Anitescu† and Lawrence L. Tavlarides* Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244

Experimental results and kinetic analyses are presented for the dechlorination/oxidation of the 3,3′,4,4′-tetrachlorobiphenyl (T4CB) congener in supercritical water (SCW) at 25.3 MPa and 673773 K. A plug-flow reactor is employed with nominal T4CB feed concentrations of 2.97-5.48 µmol/L in methanol solutions (1.71 mmol/L) using H2O2 as the oxidant. Conversions vary from 20.2% (5.76 s at 673 K) to 99.5% (20.7 s at 773 K). The overall conversion is independent of the concentration of O2 and follows apparent second order with Arrhenius parameters of A ) 1020.7(0.1 s-1 (mol/L)-1 and Ea ) 219 ( 2 kJ/mol. Methanol functions as a rate enhancer by inducing a free-radical reaction pathway of consecutive dechlorinations leading to biphenyl, which then oxidizes to mineral products. Positively identified reaction intermediate products are lower chlorinated PCB congeners (3,3′,4- and 3,4,4′-TriCB; 3,3′-, 3,4-, 3,4′-, and 4,4′-DiCB; and 3- and 4-CB), biphenyl, CO, and CO2. Successive global reaction networks with second-order kinetics using both individual and lumped pseudo intermediates and final products are shown to represent well the main features of the reaction pathway. Conversions of the individual T4CB in methanol are compared with those of T4CB in Aroclor 1248/methanol solutions under similar conditions. The kinetic parameters for the two cases have very similar values, which are also close to those of Aroclor 1248, suggesting the possibility of treating this complex PCB mixture as a pseudo T4CB. Significant thermal decomposition of T4CB/methanol in SCW occurs at temperatures higher than 773 K (12.1% at 10.4 s). 1. Introduction The main goal of this study is to extend the supercritical water oxidation (SCWO) process from aqueous organic waste treatment to the destruction of solid and water-insoluble organic compounds such as polychlorinated biphenyls (PCBs). Methods for destroying PCBs are facing unusually challenging problems because of the very high chemical stability, low water solubility, and slow kinetics of these compounds.1-4 These methods include incineration, chemical and electrochemical dechlorination, gas-phase chemical reduction, bioremediation, and advanced oxidation technologies (e.g., SCWO, radiation, photolysis).5 Difficulties are encountered because of restricted areas of application, narrow ranges of initial concentrations, generation of harmful products, long process times, prohibitive costs, etc. The incineration of PCBs generates very harmful pollutants such as polychlorinated dibenzofurans/dioxins (PCDFs/Ds). Moreover, because PCBs themselves result from the incineration of chlorinated pollutants and because they were used as fire retardants, this method is inappropriate for PCB destruction. SCWO is drawing much attention because of its attractive features such as cleanness, rapidity, and wide range of application. Essentially complete conversion of organic to inorganic compounds occurs on the time scale of a few minutes and utilizes a totally enclosed treatment facility under temperatures much lower than * Corresponding author. E-mail: [email protected]. Fax: 315-443-1243. Tel.: 315-443-1883. † On leave from: Department of Physical Chemistry, Bucharest University, Bucharest 7034, Romania.

those used for incineration. SCWO technology can effectively destroy a large variety of industrial and highrisk wastes.6-14 The process is conducted at temperatures and pressures above the critical point of water (647 K and 22.1 MPa) and is considered applicable to aqueous streams containing up to 20% organics.10 The process is environmentally sound and economically competitive with alternative waste treatment technologies.15-19 Also, it can utilize fuel-like energy released by the pollutants and does not require dewatering steps. Detailed reviews of the SCWO process can be found in Modell,7 Tester et al.,10,20 Gloyna et al.,21,22 Savage et al.,23,24 and Schmieder and Abeln.14 Different classes of liquid and solid (water-soluble) organic compounds, including chlorinated aromatics,25-31 have been subjected to SCWO conditions. However, only a few investigations on the decomposition of liquid PCBs in SCW are known. Modell first reported a destruction efficiency of over 99.99% at 783 K, 25.3 MPa, and 3.7 min for a transformer fluid containing PCBs in a benchscale unit.7 Oe32 reportedly achieved near-complete decomposition of PCBs fed at up to 7% using a benchscale apparatus. Hatakeda et al.33,34 reported SCWO of the liquid 3-CB congener and a PCB mixture (Kaneclor KC-300). A 99.99% destruction removal efficiency of trace PCBs in sludge has recently been reported by Crain et al.35 These experiments have been focused on the ability of the SCWO process to achieve high conversions rather than on the acquisition of kinetic information, which requires voluntarily accepted incomplete conversions. These studies are quite promising, but the reaction pathways through which these results are obtained are not well-established. Some stable products

10.1021/ie0103714 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/02/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002

of PCB SCWO, such as PCDDs/Fs, under particular conditions, are more hazardous than the starting material, and their formation is a factor to consider in the design of SCWO processes. None of the studies cited provides a systematic pathway analysis to determine reactor optimum (least severe) conditions or the nature and concentrations of reaction products. The lack of suitable and comprehensive data on PCB oxidation led us to initiate research work in the field of SCWO. In addition, a two-stage supercritical fluid technology process of PCB extraction from soils and sediments with SCCO2/MeOH fluids followed by SCWO of the soil/sediment extracts is under investigation at Syracuse University. The second step proposes oxidation of the PCB/MeOH extract. Therefore, a systematic kinetic study of these systems was initiated with methanol oxidation36 over a wide range of conditions (673-773 K and 3-50 s) to assess the capabilities of this technique for generating accurate data in a reliable and quick manner and for serving as a bridge for the SCWO of PCB/MeOH solutions. As SCWO technology has been applied only for aqueous solutions or slurries of organic compounds, we propose to extend this process to solids that are insoluble in liquid water by delivering them separately from the water-oxidant line into the reactor as liquid solutions with a cosolvent. Previous results37 show that methanol is a PCB reaction-rate enhancer (accelerator) and that it would be a supplemental fuel for autothermal reactors. One approach to elucidate the chemistry of PCBs in the SCWO process involves the selection of and experimentation with single congeners that serve as models of important reactive moieties within complex PCB mixtures. In this article, the oxidation kinetics of 3,3′,4,4′-tetrachlorobiphenyl (T4CB) from methanol solutions in SCW is presented and discussed. The solid T4CB is a representative PCB coplanar congener with a dioxin-like structure and the most toxic component of Aroclor mixtures, providing useful information about meta- and para-chlorine reactivity.1-4 2. Experimental Section 2.1. Apparatus and Procedure. The T4CB decomposition/oxidation experiments in supercritical water were conducted in a high-pressure, isothermal plug-flow tubular reactor with limits of operation of up to 873 K and 69 MPa. The experimental setup used is described in detail elsewhere,36,37 so only the most relevant features are given here. The experimental system consists of three major subsystems: feeding, reactor, and separation. T4CB/ MeOH (1.71 mmol/L) and oxidant (H2O2/H2O solutions) are delivered to the reactor in separate lines. The initial T4CB feed concentration is limited by the solubility of the solid PCB congener in methanol and by the GC performance in identifying reaction products in the effluent stream. This low initial solute concentration is, nonetheless, advantageous for research purposes because it sustains the isothermality of the SCWO process. The variation of reactor residence time is achieved by changing either the reactor length or the feed rate. Reactor lengths are selected to be long enough to obtain sufficient PCB conversion for the support of kinetic studies and short enough to preserve unstable reaction intermediates needed for the development of reaction pathways.

The preheated organic feed and supercritical oxidant are mixed in a mixing block, and the combined stream then flows through the reactor housed in an isothermal fluidized sand bath with preheating coils. The temperature is monitored at both ends of the reactor ((1.5 K), and the pressure ((1.4%) is fixed by manually controlling the total flow rate of the effluent stream using a special designed micrometric valve. The stream pressure is first dropped to ambient conditions, and then the hot effluent mixture is cooled, resulting in the separation of gaseous and liquid phases in either one of two separators connected in parallel. This approach prevents significant corrosion and PCB precipitation at the end of the reactor. After steady state is reached (5-10 min), the reaction products of the effluent stream are depressurized, cooled, separated into gaseous and liquid phases, and analyzed chromatographically. 2.2. Analytical Technique. Chromatographic analysis is performed with three Hewlett-Packard 5890 series II gas chromatographs. The gas- and liquid-phase products are analyzed by on-line GC/TCD and by offline capillary GC/ECD, GC/FID, and/or GC/MSD, respectively. The GC was calibrated with standard solutions containing certified concentrations of PCB congeners in hexane and methanol. Chromatographic errors for the compounds in the liquid and gas samples were determined by replicate analysis to be less than 4 and 6%, respectively. A chemical analysis using the NIST library of spectra was performed with the GC/MSD chromatographic technique. The above analytical methods permit positive identification of most of the stable reaction intermediates and final products except for formaldehyde, which can coelute with methanol. 2.3. Reactants. The oxidant is O2, supplied for most of the experiments as a solution of hydrogen peroxide of 6 wt % concentration prepared from 30 wt % H2O2/ H2O solution (purum p.a., Fluka). Neat T4CB solid congener is from AccuStandard. The purity of methanol (Optima, Fisher Scientific) is >99.9%, and the water is distilled and deionized. 2.4. Residence Time Calculation. Residence time (τ) is calculated by dividing reactor volume (V) by the volumetric flow rate under supercritical conditions (vSC)

τ ) V/vSC

(1)

Using the mass balance of the materials in the liquid (L) and supercritical (SC) phases, the residence time can be calculated with the equation

τ ) V(FSC,0/FL)/vL

(2)

The residence time is in seconds, the volume of the reactor is in milliliters, the densities of both phases are in grams per milliliter, and the initial flow rate of the liquid reactants (vL) is in milliliters per second. The density of water under reaction conditions, calculated using NBS Steam Tables,38 is used instead of the density of the supercritical fluid mixture (97 wt % H2O, 1.6 wt % O2, and 1.4 wt % MeOH). However, the latter, estimated with the Peng-Robinson equation of state, is within the experimental error range of the former. 3. Results and Discussion 3.1. Thermal Degradation of T4CB in SCW. Thermal degradation experiments in SCW were conducted to determine whether significant conversions occur as

Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002 11 Table 1. Conversions (X%) for Thermal Decomposition of 3,3′,4,4′-TetraCBa at 25.3 MPa

a

T (K)

τ (s)

X

673 723 773 823 873

9.6 12.6 10.4 9.1 8.1

1.9 5.6 12.1 26.4 43.5

1.71 mmol/L in MeOH.

a result of this reaction and should be considered along with the oxidation reactions. Experiments were performed at five temperatures (673, 723, 773, 823, and 873 K) by using deionized water without H2O2. Under the experimental conditions, the residence time values were about 10 s. For each time value, two or three replicate runs were performed. The T4CB conversions are presented in Table 1. An empirical correlation of these conversions with the temperature was established to estimate the T4CB disappearance over the temperature range of the experiments: ln X ) -9298/T + 14.51, with regression coefficient r2 ) 0.997. Positively identified reaction products were lower chlorinated PCB congeners (3,3′,4- and 3,4,4′-TriCB; 3,3′-, 3,4-, 3,4′-, and 4,4′-DiCB; and 3- and 4-CB) and biphenyl. On the basis of these data and the fact that T4CB undergoes nearcomplete conversion by SCWO under similar reaction conditions at a temperature of 773 K, the extent of thermal degradation (12.1% at 773 K and 10.4 s) does not significantly interfere with the SCWO process. Moreover, essentially the same main step of successive dechlorination appears to govern both thermal degradation and SCWO, suggesting similar reaction mechanisms other than direct oxidation. 3.2. T4CB/MeOH Oxidation in SCW. The experiments on decomposition of T4CB/MeOH in SCW with oxygen were conducted to characterize the reactivity of this PCB congener in the presence of methanol and to provide the necessary information for global kinetic and reaction pathway analyses. The benchmark SCWO experiments of T4CB without methanol, for a direct comparison to those with methanol, could not be conducted because of the low solubility in water and difficulty in delivering neat PCB. The main variable parameters in SCWO experiments with T4CB are the residence time and the temperature. The residence times of the experiments ranged from 3.1 s at 773 K to 25.6 s at 673 K. The flow rates of H2O2/H2O and T4CB/ MeOH liquid solutions combined with the two reactors resulted in a significant overlap in residence times for each isotherm, which permited the reproducibility of the data to be verified. The experiments were conducted isothermally at the five different temperatures 673, 698, 723, 748, and 773 K. In addition to replicate runs (two or three for most of the experiments, with standard deviations less than 6%), duplicate samples were analyzed by combined chromatographic methods. Previous experiments using 5-12 wt % H2O2/H2O concentrations36,37 and the present work with 6-10 wt % show that the conversion of PCBs/MeOH is independent of the initial oxygen concentration for values ranging from 100 to 200% of the stoichiometric requirements. A reaction order of 0 for oxygen was also reported in most publications, and recent results for CH4/MeOH SCWO39 show that the reactant and product yields are largely insensitive to changes in the concentration of O2. In our case, these results support the assumption that O2 does not participate directly in the dechlorina-

Table 2. Global Conversions for SCWO of 3,3′,4,4′-Tetra-CBa at 25.3 MPa τ1 (s) 5.8 9.6 11.0 12.8 15.4 19.2 25.6 4.4 7.3 8.4 9.8 11.7 14.6 19.5 3.8 6.3 7.2 8.4 10.1 12.6 16.8 3.4 5.7 7.5 11.3 15.1 22.6 3.1 5.2 6.9 8.3 10.4 13.8 20.7

X1,exp

X1,cal

T ) 673 K 20.2 22.0 31.1 32.6 35.8 35.6 39.3 39.2 43.7 43.8 49.0 49.4 55.8 56.6 T ) 698 K 43.1 41.5 53.7 54.3 58.7 57.7 63.1 61.5 65.0 65.8 69.4 70.8 77.5 76.5 T ) 723 K 68.1 66.3 75.6 76.9 79.5 79.3 81.4 81.8 84.6 84.5 87.9 87.3 91.0 90.3 T ) 748 K 83.4 84.8 87.6 90.5 92.8 92.8 95.2 95.2 97.1 96.4 97.9 97.6 T ) 773 K 93.3 93.9 96.5 96.4 98.0 97.3 97.5 97.8 98.7 98.2 99.1 98.7 99.5 99.1

RD1 (%) 8.9 4.9 -0.5 -0.2 0.1 0.8 1.4 -3.7 1.1 -1.7 -2.6 1.2 2.0 -1.3 -2.7 1.8 -0.3 0.5 -0.2 -0.8 -0.8 1.7 3.3 0.0 -0.0 -0.8 -0.3 0.7 -0.2 -0.7 0.3 -0.5 -0.5 -0.4

τ2 (s) 6.3 8.4 12.6 16.8 25.2 32.2 40.2 53.7 5.7 7.5 11.3 15.1 22.6 28.9 36.1 48.2 5.2 6.9 10.4 13.8 17.8 20.7 24.8 31.1 41.4 62.2 4.5 6.0 9.1 12.1 18.1 21.8 27.2 36.3 54.4

X2,exp

X2,cal

T ) 723 K 78.1 78.5 84.0 83.1 88.7 88.2 91.1 91.0 93.7 93.9 94.4 95.3 95.9 96.2 96.4 97.2 T ) 748 K 88.8 90.5 93.2 92.8 96.0 95.2 96.6 96.4 97.3 97.6 98.0 98.2 99.1 98.5 99.3 98.9 T ) 773 K 95.9 96.0 97.2 97.1 98.4 98.1 99.03 98.56 99.36 98.89 99.41 99.06 99.51 99.22 99.60 99.38 99.70 99.55 99.86 99.70 T ) 823 K 99.48 99.30 99.64 99.48 99.76 99.66 99.83 99.75 99.89 99.84 99.91 99.87 99.95 99.89 99.97 99.92 99.98 99.95

RD2 (%) 0.5 -1.1 -0.6 -0.1 0.2 0.9 0.3 0.8 1.9 -0.5 -0.8 -0.2 0.3 0.1 -0.6 -0.4 0.13 -0.17 -0.30 -0.48 -0.47 -0.36 -0.29 -0.22 -0.16 -0.16 -0.19 -0.16 -0.10 -0.08 -0.05 -0.04 -0.06 -0.05 -0.03

a 1.71 mmol/L in MeOH (1) and 0.52 mmol/L in Aroclor 1248 /MeOH (2).

tion step of PCBs but rather in the oxidation of the bulk methanol to produce free radicals, which, in turn, participate in the Cl-abstraction step. Thus, the reaction rate can be assumed to be independent of the concentration of O2 over the above concentration range. Accordingly, most of the experiments are conducted at a nominal H2O2/H2O concentration of 6 wt %, providing an excess of O2 of ∼20 mol % (0.144 mol/L or 1.6 mol % under the reaction conditions). Judging from previous studies, it was determined that complete decomposition of H2O2 to H2O and O2 occurred in the preheater under the conditions of our experiments.36,40 If V∆t is the volume of an effluent sample and v0 is the volumetric flow rate of the organic stream, then the conversion of T4CB (X1) can be expressed as the ratio of the reacted amount to the initial amount charged for steady-state run time ∆t

X1 ) 1 - V∆tC1τ/(v0∆tC1m)

(3)

The T4CB concentrations in the methanol solution feed and in the condensed effluent aqueous solutions are C1m and C1τ, respectively. The experimental results for the disappearance of T4CB in supercritical water at 25.3 MPa and the above temperatures for initial T4CB feed concentrations of 2.97-5.48 µmol/L (reaction conditions) in methanol solution are presented in Table 2. Overall, conversions ranged from 20.2% (5.8 s, 673 K) to 99.5%

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(20.7 s, 773 K), depending on the residence time and temperature. The accuracy of these data was determined through propagation error calculations to be within (0.04(1 - X1), with the lower errors for the higher conversions (relative deviation RD < 1% for X1 > 0.8). The conversion increased rapidly with temperature and was essentially complete at 773 K and beyond 5 s. This destruction level of T4CB is very high when one considers the remarkable chemical stability of PCBs. In the experiments with T4CB/MeOH, as the bulk amount of methanol is oxidized (99.95% MeOH vs 0.05% T4CB), it is difficult to obtain a carbon mass balance for T4CB. Nevertheless, we have verified the reliability of our technique by checking the total carbon mass balance for the runs at 773 K, which closed to within experimental accuracy (97 ( 6%). 3.3. Oxidation of T4CB in Aroclor 1248/MeOH Solution in SCW. It is certainly of interest to compare the T4CB conversions in the Aroclor1248 (A1248) mixture with those of this work mainly to determine whether significant differences exist between the two cases as a result of the potential interactions and competitive reactions among PCB components of the Aroclor mixture. A1248/MeOH solution (18.1 mmol/L) was oxidized previously in SCW at 25.3 MPa and 723, 748, 773, and 823 K.37 T4CB is one of the major components of this mixture (2.73 mol % among 55 congeners in the feed concentration), and both PCBs contain an average of 48.6% chlorine and have the same molecular weight (292 g/mol).2 The results obtained from A1248/MeOH SCWO are included in Table 2, and for an easier comparison with the data reported in the section 3.2, the results for three common isotherms are presented in Figure 1 and show that the data overlap within experimental precision. Overall, conversions range from 78.1% (6.29 s, 723 K) to 99.98% (54.4 s, 823 K) depending on residence time and temperature. At 823 K and 54.4 s, the highest value of 99.98% conversion for T4CB was obtained. In accordance with the results presented in sections 3.2 and 3.3, it is clear that T4CB decomposition within A1248 system does not depend on other PCB congeners as no higher chlorinated components of A1248 can produce T4CB by dechlorination. Further, the disappearance rates of T4CB in the two sets of experiments are quite similar to those of overall A1248. This similarity would permit one to consider T4CB as a representative congener for simulating this complex mixture. 3.4. Methanol Influence on PCB Conversion. When organics are fed in an organic cosolvent that is oxidized itself, one might expect a significant co-oxidation effect on the compound under study. The oxidation of the cosolvent generates free radicals, which could enhance the decomposition of the PCBs as compared to PCB decomposition without any cosolvent present. We have observed that methanol is, in many aspects, an attractive cosolvent for SCWO and that it has an enhancing effect on the PCB disappearance kinetics. Methanol is easy to oxidize, so it is likely that methanolderived reactive intermediates are primarily responsible for dechlorination of the PCBs. Indeed, numerous elementary reactions in SCWO of methanol produce hydrogen species.41 Our preliminary data on the SCWO of liquid PCBs without methanol, compared to the data with methanol, show that the system with methanol is

Figure 1. 3,3′,4,4′-TetraCB (T4CB) conversion versus residence time at 25.3 MPa: calculated using eq 5 (s) and experimental for T4CB/MeOH (*, 673 K; 2, 698 K; 9, 723 K; b, 748 K, [, 773 K) and for T4CB/A1248/MeOH (0, 723 K; O, 748 K; ], 773 K).

dechlorinated significantly faster, in agreement with the findings recently reported for CH4/MeOH mixtures.39 A comparison of model predictions from this work and our previous study37 with reported data for kinetic experiments executed without methanol shows that similar performances can be achieved under less severe conditions or in a significantly shorter reaction time. For example, under Modell’s conditions,7 we estimate that we can achieve the same performance for A1248 (99.99% destruction efficiency) in a methanolic solution in 10 s rather than 3.6 min at 783 K, or at the same reaction time of 222 s, we can decrease the temperature by about 80 K.37 For the T4CB/MeOH system, essentially complete destruction can be achieved under the above conditions or the temperature can be dropped to near the critical value, respectively. Further, the literature reports42 show that, for most compounds, temperatures over 823 K and residence times near 20 s afford conversion efficiencies of greater than 99.95%. Compared with these results, our models indicate that similar conversions for far more chemically stable PCBs in methanol can be achieved either at 793 K and 20 s or at 823 K and 10 s for A1248/MeOH mixtures and complete destruction for T4CB/MeOH. Also, SCWO is much faster than photodechlorination and is effective at much higher concentrations of PCBs in SCW: 99% conversion of 3,3′,4,4′-TetraCB can be achieved through the former technique (this article, Table 2) in 10 s at 500 °C, whereas through the latter approach,43 it is reached in 30 min. The MeOH-enhanced rates of these reactions as functions of the reactor operating variables

Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002 13

will provide information for design and operation of SCWO units under the least severe conditions. 4. Global Kinetics of T4CB Disappearance The kinetics for SCWO of T4CB may be conveniently examined by assuming that the global rate of the disappearance reaction is proportional to the T4CB concentration in the reactor at a given time and independent of the concentrations of water, methanol, and oxygen

-r1 ) kC1R

(4)

This type of rate law expression typically captures the general trends in the SCWO data, being empirical and correlative with parameters determined by fitting the model to experimental data, but it does not capture the details of the complex oxidation chemistry. Combining the rate law of eq 4 with the definition of conversion (eq 3) and the design equation for a constant-volume, plug-flow reactor,44 and then analytically integrating, leads to

X1 ) 1 - [1 + (R - 1)10b exp(-Ea/RT) [T4CB]R-1τ]1/(1-R) (5) Here, 10b exp(-Ea/RT) represents the rate constant k with the Arrhenius frequency factor A ) 10b and the energy of activation Ea as fitting parameters, along with the global reaction order R. These parameters were determined over all temperatures by best fit of the experimental data. The regression technique was performed with the Microcal Origin 6.0 software package by minimizing the objective function defined as the sum of the squares of the differences between the experimental and predicted conversions (eq 5). For both the T4CB/MeOH and T4CB/A1248/MeOH sets of SCWO experiments, the values of R values of R (1.94 ( 0.02) indicate an apparent second order reaction. Also, the values of the Arrhenius parameters, A ) 1020.7(0.1 s-1 (mol/L)-0.94 and Ea ) 219 ( 2 kJ/mol for the first case and A ) 1020.1(0.3 s-1 (mol/L)-0.94 and Ea ) 204 ( 4 kJ/ mol for the second case, are nearly the same. The calculated conversion values for both cases considering the fitting parameters in eq 5 are presented in Table 2, along with relative deviations defined as

RD ) 100 × (Xcal - Xexp)/Xexp

(6)

The overall absolute average relative deviations (AARD ) Σ|RDi|/Σi) for the two sets of data in Table 2 are 1.4 and 0.4%, which shows good agreement between the calculated values and the experimental data. Two observations can be made regarding the generality of the above rate law expression: (1) The rate values are obtained over a limited range of initial concentrations of T4CB for both cases (0.79-5.48 µmol/L overall). However, our work on the SCWO of A1248 indicates that an order-of-magnitude-higher initial PCB concentration (e.g., 57.5 µmol/L) does not significantly change the rate law expression (R ) 2.09, A ) 1017.0, and Ea ) 186 kJ/mol).37 Any of these rate laws are still valid for a 60-times-higher initial concentration when 99.94% of PCB is converted at 68 s residence time, in good agreement (-0.05%) with the calculated value of 99.99% (unpublished data). (2) When considering a constant

Figure 2. GC/FID chromatogram of 3,3′,4,4′-TetraCB (peak 9) SCWO (698 K, 25.3 MPa, 7.31 s): 1, biphenyl; 2, 3-chlorobiphenyl; 3, 4-chlorobiphenyl; 4, 3,3′-dichlorobiphenyl; 5, 3,4-dichlorobiphenyl + 3,4′-dichlorobiphenyl; 6, 4,4′-dichlorobiphenyl; 7, 3,3′,4trichlorobiphenyl; 8, 3,4,4′-trichlorobiphenyl.

initial feed concentration of excess oxidant, the fitting procedure cannot meaningfully discriminate between A and [O2]β in the rate law equation, as both are constant. For β ) 0, the expression is obviously valid. If β * 0 (in our case, β ) -0.19 ( 0.12), the value of [O2]β is lumped with A. 5. Reaction Products and Pathways Knowledge of the reaction pathways and their relative rates is central to the development of a strategy for minimizing the formation of the undesired products, because different strategies could be adopted for different types of networks.45-48 Although elucidating the elementary reaction mechanisms could accelerate the understanding of the fundamentals of SCWO chemistry, such an approach on PCBs, even through lumped elementary steps,49 is sufficiently complex that a kinetic/ thermodynamic database for elementary reaction steps is not yet available. Nevertheless, a reaction path analysis for identifying important reaction steps and relatively stable chemical species in the system certainly provides a significant insight into the underlying chemistry of PCB oxidation in SCW. 5.1. Reaction Intermediate Compounds. One of the focal points of this study is the identification and quantification of the yields of the stable products from experiments in which incomplete oxidation is programmed to occur in order to examine whether any harmful stable products can be formed. Positively identified reaction products are all of the lower chlorinated PCB congeners (3,3′,4- and 3,4,4′-TriCB; 3,3′-, 3,4-, 3,4′-, and 4,4′-DiCB; and 3- and 4-CB), biphenyl, CO, and CO2. Some small amounts of CH4 were obtained at temperature of 773 K and short residence times. Figure 2 displays a representative GC/FID chromatogram that corresponds to an oxidation experiment at 698 K, 25.3 MPa, and 7.31 s and clearly shows lower chlorinated PCB congeners and biphenyl as typical incomplete reaction intermediates. Peak 5 in Figure 2 represents both overlapped 3,4- and 3,4′-DiCB congeners as these isomers coelute under the GC conditions of this chromatogram. Moreover, no PCDFs/Ds and other aromatic derivatives among reaction products were identified at the level of GC/ECD limit of detection (∼0.1 ppb). Although one rationally would expect that

14


Figure 3. Competitive PCB reaction pathways in SCW: (1) mainly with methanol; (2-4) without methanol; (4) with substoichiometric O2 .

single-ring aromatics and oxygen-containing compounds should be produced in this reaction, because of the low PCB concentrations of the feed, the potential unstable intermediates undergo fast reactions and become undetectable in the effluent streams. Reaction intermediates with rates of formation comparable to their rates of disappearance have no chance of significant accumulation. However, recent preliminary experiments that we conducted with ∼10 times more concentrated solutions of biphenyl/MeOH (5 wt %) compared to T4CB/MeOH (0.5 wt %) showed numerous reaction intermediates, including acetophenone, phenol, benzaldehyde, 1-phenyl-1,2-propandione, benzoic acid methyl ester, 1-phenyl 2-propen-1-one, hydroxybiphenyl isomers, benzene, acetic acid, oxiacetic acid, acetaldehyde. Also, some experiments with neat PCBs show chlorinated benzenes, hydroxylated PCBs, PDDFs, and PCB oligomers/polymers, suggesting that competitive reaction pathways occur during PCB SCWO without methanol. 5.2. Competitive SCWO Pathways. The above findings indicate a complex PCB reaction pathway network for complete oxidation to CO2 and H2O. A proposed network structure for a PCB congener under different conditions of the reaction medium is shown in Figure 3: (1) dechlorination to lower chlorinated congeners, which undergo further dechlorination to biphenyl, followed by oxidation to open-ring products; (2) reaction with OH radicals to form PCB hydroxylated intermediates; (3) cleavage of 1-1′ C-C bond, leading to chlorinated benzene compounds and thereafter to open-ring and final products; and (4) formation of PCB oligomers. Pathways 1 and 2-4 largely appear to occur in the presence and in the absence of methanol, respectively, with 4 favored by insufficient O2. If pathway 1 is largely dominant in the presence of methanol and “crosstalk” with other pathways is minimal, then pathway 1 can be considered independent under particular SCWO conditions and analyzed as follows. In the presence of methanol (route 1), the main reaction pathway for these systems appears to involve a successive dechlorination of congeners to biphenyl, which then serially reacts to form open-ring products, leading to

CO2 (bold arrows and capital letters for the reaction products). In the absence of methanol, PCBs undergo parallel, competing reaction pathways (routes 2-4). 5.3. Dechlorination Networks. As chlorinated compounds typically have chlorine atoms removed under reductive conditions (photodechlorination,43 anaerobic biodechlorination,50 SCWO in the presence of methanol37), the global reaction network for the SCWO of T4CB/MeOH can be written in the broadest sense as consecutive dechlorination and oxidation reactions involving at least 10 main species (Figure 4A). The dechlorination step of the reactions might occur because of hydrogen species produced by methanol oxidation, including the water-gas shift reaction. Also, a similar network for T4CB in 2-propanol was proposed for a photodechlorination process, which implies a similar free-radical reaction mechanism.43 To characterize, in a simpler way, the dechlorination step of T4CB/MeOH, a three-level lumping strategy is also proposed (Figure 4B-D). This method can be useful when applied to numerous products formed through similar pathways with similar kinetics, shrinking their number to representative pseudoproducts. Consequently, in the first lumped network derived from the detailed network (level A), TriCB, DiCB, and CB congeners are lumped separately (level B). Frequently, the ratedetermining step in the reaction of complex compounds is the oxidation of a lower-molecular-weight intermediate (e.g., methanol, CO, benzene, phenol, biphenyl) that is itself a partial oxidation product of the initial reactant.48,51-53 Accordingly, in the next lumping steps, all PCB intermediates are lumped together (level C) and with biphenyl (level D) as a global pseudo intermediate. All further ring-open products (ROPs) of biphenyl oxidation are lumped as a pseudo final product that is assumed to have the same molar mass as biphenyl and is calculated by difference using material balance. On the basis of our previous study37 and the present work, we assume that a simplified successive global reaction network with second-order kinetics represents the main features of this system. 5.4. Reaction Rate Constants. For ease in reaction pathway analysis, dimensionless yields of the reactants/


Figure 4. Proposed reaction pathways for dechlorination of 3,3′,4,4′-TetraCB (delivered in methanol solutions) in SCW: detailed reaction network (level A) and successively lumped reaction networks (levels B-D). ROPs ) ring-open products.

products defined as moles of compound i in the effluent stream per mole of initial reactant fed into the reactor are employed (Yi ) Ci/C1,0). Assuming that the reaction rates are independent of the oxygen and water concentrations for a given initial concentration of methanol, a set of ordinary differential equations (ODEs) can be conveniently written for the R-order reaction network proposed in Figure 4A in the form

dY1/dt ) -(k/1 + k/2)Y1R / / dY2/dt ) k/1Y1R - (k/3 + k4/1 + k4/2 )Y2R

(7)

dY10/dt ) k/11Y8R + k/12Y9R - k/13Y10R dY11/dt ) k/13Y10R with k/i ) ki(C1,0)R-1 in s-1 as pseudo rate constants. Although the analytical solutions of this type of ODE system (Riccati) are known to be difficult to obtain, for the lumped level D equations, the following analytical solutions are found54

Y1D ) 1/(k/1Dτ + 1)

(8)

Y2D ) (ak/1D/2k/2D){1 + 2(1 - a)/ [a - (2 - a)(k/1Dτ + 1)a-1]}/(k/1Dτ + 1) (9) Y3D ) 1 - Y1D - Y2D

(10)

with a ) 1 + (1 + 4k/2D/k/1D)1/2. Numerical integration techniques (Runge-Kutta) with parameter optimization by a nonlinear leastsquares method was employed for the above system to determine the k/i parameters for the case R ) 2. Figure 5, with the ordinate truncated at Yi ) 0.2 to amplify

the details, provides the molar yields of seven identified PCB intermediate products and biphenyl at different residence times for the five temperatures of the SCWO experiments (section 3.2). The molar yields obtained from the above lumped reaction networks (levels B-D) exhibit shapes and trends similar to those in level A. The results for the level D network are shown in Figure 6. At all temperatures, the yields of the intermediates decrease with increasing residence time after reaching a maximum. This observation suggests that these products undergo further dechlorination. Although the yields of each of the individual products are always low (typically less than 10%), the data are sufficiently accurate to execute a complete pathway analysis of all of the intermediates (with 3,4- and 3,4′CB lumped because of the GC coelution), along with kinetic parameter determination (Table 3). The Arrhenius plots of ln(ki) versus 1000/T, shown in Figure 7 for the 13 reactions of the level A network, correlate the data quite well and provide the basis for calculating the activation energies and frequency factors in Table 3. We note that at the higher temperatures, the problem of multiple optima for k/i occurred for this detailed network. The problem was avoided by imposing some restrictions on the k/i ranges on the basis of the values obtained by a lumped strategy where the multiple optima did not occur. The calculated rate constants and the Arrhenius parameters for the lumped reaction networks (Figure 4B-D) are also presented in Table 3. In particular, optimized values for k1D and k2D are obtained by fitting the experimental data with a nonlinear regression method. The energies of activation calculated from the results of Table 3 are Ea1 ) 228 ( 5 kJ/mol and Ea2 ) 242 ( 8 kJ/mol. The energy of activation value from k1D is close to the regression value (219 kJ/mol) for overall conversion when the partial reaction order R was unconstrained and determined to be 1.94, as expected.

16


Figure 5. Dechlorination of 3,3′,4,4′-tetrachlorobiphenyl (T4CB): comparison of predicted (s) and measured (symbols) yields of T4CB ([) and intermediate products as a function of time at 673, 698, 723, 748, and 773 K. Other symbols represent (9) 3,3′,4- and (0)3,4,4′TriCBs; (b) 3,3′-, (*) (3,4- + 3,4′)-, and (O) 4,4′-DiCBs; (2) 3- and (4) 4-CBs; and (×) biphenyl.


Figure 6. Dechlorination of 3,3′,4,4′-tetrachlorobiphenyl (T4CB): comparison of predicted (s) and measured (symbols) yields of T4CB ([), lumped PCB intermediates and biphenyl (9), and lumped ring-open products (2) (level D in Figure 4) as a function of time at 673, 698, 723, 748, and 773 K.

18


Table 3. Rate Constants (ki × 10-6 L mol-1 s-1) and Arrhenius Parameters for the Detailed (Level A) and Lumped (Levels B-D) Reaction Networks (Figure 4) of 3,3′,4,4′-TetraCB SCWO at 25.3 MPa rxn

T ) 673 K

T ) 698 K

T ) 723 K

T ) 748 K

T ) 773 K

Ea (kJ/mol)

log(A)

1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12A 13A 1B 2B 3B 4B 5B 1C 2C 3C 1D 2D

0.005 0.004 0.12 0.48 0.66 0.14 1.70 0.19 0.07 1.82 0.65 1.01 0.13 0.009 0.34 0.26 0.40 0.13 0.009 0.03 0.11 0.009 0.014

0.023 0.015 0.34 1.77 1.90 0.51 3.66 0.70 0.14 5.86 3.13 4.71 0.29 0.04 1.13 0.46 1.88 0.40 0.04 0.08 0.35 0.04 0.04

0.09 0.06 2.46 17.2 21.6 2.77 20.0 2.78 0.77 26.1 16.6 20.1 0.58 0.15 11.9 2.76 12.2 0.75 0.15 0.78 0.59 0.15 0.18

0.30 0.22 3.49 35.8 65.2 5.30 37.1 5.64 1.20 49.4 33.1 54.4 2.08 0.48 29.3 5.08 20.8 2.08 0.47 1.68 2.11 0.47 0.55

1.02 0.55 8.84 236 307 12.1 237 35.3 6.06 260 169 249 7.12 1.84 164 39.0 111 7.12 1.84 13.7 7.51 1.55 2.73

229 ( 2 218 ( 5 192 ( 22 266 ( 20 273 ( 20 197 ( 16 210 ( 22 217 ( 16 189 ( 21 208 ( 15 233 ( 14 233 ( 8 172 ( 17 229 ( 4 270 ( 19 214 ( 27 237 ( 18 167 ( 17 227 ( 4 267 ( 25 177 ( 16 228 ( 5 242 ( 8

21.5 ( 0.1 20.5 ( 0.4 20.0 ( 1.6 26.3 ( 1.4 26.9 ( 1.4 20.5 ( 1.2 22.4 ( 1.6 22.1 ( 1.2 19.5 ( 1.5 22.4 ( 1.1 24.0 ( 1.0 24.1 ( 0.6 18.4 ( 1.2 21.7 ( 0.3 26.4 ( 1.4 21.8 ( 2.0 24.0 ( 1.3 18.0 ( 1.2 21.6 ( 0.3 25.1 ( 1.8 18.7 ( 1.1 21.6 ( 0.3 22.7 ( 0.6

Also, this value is nearly identical to the value for other detailed/lumped reaction networks (229 ( 2/1A, 218 ( 5/2A, 229 ( 4/1B, 227 ( 4/1C kJ/mol). The value of Ea2 is within the range of energies of activation for PCB intermediates. The calculated molar yields agree well with the experimental yields (Figure 6) and are nearly identical to the results obtained by numerical integration. Although the SCWO of T4CB is a complicated process, this two-parameter kinetic model captures the major reaction pathways. The analytical solutions of the temporal variations of the molar yields provide a powerful approach to representing a system of lumped intermediates with sparse or insufficiently precise data, avoiding the problem of multiple optima solutions when numerical integration methods are employed. These solutions confirm the validity of the results obtained by numerical integration methods that are successful here because of the high accuracy and precision of the experimental procedures used to produce nonscattered data. The similarity between the values of the Arrhenius parameters for both unlumped and lumped PCB intermediates suggests that the dechlorination step of the process is essentially the same for all of the PCB congeners. 6. Summary and Conclusions A kinetic study of the dechlorination/oxidation of T4CB in SCW in the presence of methanol was conducted over a temperature range of 673-773 K at 25.3 MPa and residence times of 3.1-25.6 s. The disappearance kinetics of T4CB under the experimental conditions was shown to follow an apparent second-order decomposition that is independent of the excess oxygen concentration. The kinetic model was confirmed by comparison with the oxidation kinetics of T4CB in an A1248 mixture, and the values of the reaction rate coefficients (Arrhenius preexponential factor and energy of activation) were found to be essentially the same. The formulated rate expression is a useful tool for assessing the optimum operating conditions for destroying PCBs or for reactor design purposes.

The results of this study demonstrate that the use of methanol as a second solvent for delivering waterinsoluble chlorinated organics provides the benefit of a significantly enhanced reaction rate. Indeed, the experimental results show that the presence of methanol in the reactor feed stream significantly accelerates the rate of T4CB disappearance through a free-radical dechlorination step. This result is contrary to the conventional wisdom that the addition of other organic compounds along with a given reactant in SCW makes the SCWO process less effective. The reaction pathway of T4CB, in the presence of methanol, proceeds through consecutive dechlorinations involving all possible lower chlorinated congeners leading to biphenyl which undergoes oxidation, resulting in the observed products CO and CO2. Competing phenomena of production and decomposition of intermediate byproducts (all tri-, di-, and mono-chlorobiphenyl isomers and biphenyl) occur in the SCWO process. Chlorine abstraction from PCB molecules by hydrogen species, produced through methanol oxidation, avoids the reported formation of PCDDs/Fs. These compounds, which are more harmful than PCBs and which form through OH radical addition mechanisms in the absence of methanol, are not detected in our experiments within the analytical limits (∼0.1 ppb). In the absence of methanol, PCBs undergo parallel, competing reaction pathways through hydroxylated compounds, chlorinated benzenes, and oligomers/polymers as primary SCWO products. A global reaction network of 13 coupled reactions with second-order kinetics was shown to capture the detailed step mechanism of successive dechlorination of PCB congeners, followed by ring-opening oxidation of biphenyl, leading to mineral products. Successively simplified global reaction networks were shown to represent the main features of this reaction process by using lumped pseudo intermediates and a lumped pseudo final product. The reaction rate coefficients of this model are determined through data regression using both an analytical solution (for the simplest network) and a


Figure 7. Arrhenius plot of the reaction rate constants (network A, top in Figure 4).

numerical integration method to represent the molar yields of T4CB and the intermediate/lumped congeners and ring-open products. A comparison of the model predictions from this work and from our previous study with the reported conversions of PCBs in the literature shows that similar performances can be achieved under less severe conditions or in a significantly shorter reaction time. For example, at 773 K and 20.7 s, the high value of 99.5% conversion for T4CB/MeOH (1.71 mmol/L) can be obtained. This conversion value is very high when one considers the remarkable chemical stability of PCBs. Significant thermal decomposition of T4CB in supercritical water, tested in the temperature range of 673873 K at 25.3 MPa, occurs only at temperatures higher than 773 K (12.1% at 10.4 s) and sharply increases toward 873 K (43.5% at 8.1 s).

Acknowledgment Financial support from the National Institute of Environmental Health and Sciences through Superfund Basic Research Program Grant P42 ES-04913 is acknowledged. The analytical solution of the Riccati differential equations provided by Marin and Nicoleta Gosoniu is gratefully acknowledged, as is the help from Mr. Wentau Xu in using MatLab software for the numerical integration method. Literature Cited (1) Bolgar, M.; Cunningham, J.; Cooper, R.; Kozoski, R.; Hubball, J.; Miller, D. P.; Crone, T.; Kimball, H.; Janooby, A.; Miller, B.; Fairless, B. Physical, Spectral, and Chromatographic Properties of all 209 Individual PCB Congeners. Chemosphere 1995, 31 (2), 2687. (2) Erickson, M. D. Analytical Chemistry of PCBs; Lewis: Chelsea, MI, 1992.

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Received for review April 27, 2001 Revised manuscript received October 8, 2001 Accepted October 18, 2001 IE0103714

Methanol as a Cosolvent and Rate-Enhancer for the Oxidation

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