Ind. Eng. Chem. Res. 1996, 35, 307-314
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Kinetic and Performance Characteristics of Wet Air Oxidation of High-Concentration Wastewater S. H. Lin,* S. J. Ho, and C. L. Wu Department of Chemical Engineering, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan, ROC
Treatment of high-concentration wastewater from a chemical company by wet air oxidation (WAO) is studied. Experiments were conducted to investigate the effects of the operating pressure and temperature on the chemical oxygen demand (COD) removal. Both air and oxygen were employed as the oxygen supply for the WAO treatment process. Catalytic effects of copper sulfate, cobalt oxide, and zinc oxide on the WAO efficiency were also examined. It was found that over 50% of COD removal can be easily realized in an hour of WAO oxidation. Two kinetic models (first-order and generalized) were employed to represent the experimental data, and their fit to those experimental data was compared. The rate coefficients and the kinetic parameters of the two models were determined also. Introduction Organic compounds are involved in the manufacturing of a wide variety of commercial chemical products. Processings of these organic compounds invariably incur various types of wastewater that contain significant amounts of toxic organic compounds. This is especially true in heavy chemical and petrochemical industries. Discharge of these wastewaters without treatment into a natural water body is environmentally unacceptable because the wastewater discharge can upset the water quality of the receiving water body. In addition to the toxicity of the organic compounds, very often the dissolved oxygen (DO) concentration of the receiving water body polluted by the untreated chemical wastewater can fall below the level deemed necessary for normal aquatic life. Hence, increasingly stringent restrictions have been imposed by government on the concentration of these organic compounds in the wastewater for safe discharge. Treatment of these wastewaters has thus become an integral part of the chemical and petrochemical industries. Traditionally, activated sludge treatment is the most widely used method to deal with various kinds of chemical wastewater because of its simplicity and relatively low cost (Metcalf & Eddy, 1991). However, the microorganisms in the activated sludge system, even well acclimatized, can only deal with chemical wastewaters containing a relatively low concentration of organic compounds due primarily to the low biodegradibility and inhibitory effects of those organic compounds (Peoples et al., 1972; Beltrame and Carniti, 1982). The chemical wastewaters containing a high concentration of organic compounds, in excess of 10 000 mg/L for instance, as those occurring in many heavy chemical and petrochemical industries need to be treated by other methods. The wet air oxidation (WAO) treatment process represents a typical one of these methods. The WAO process has been subjected to numerous investigations by researchers in the past decades as a potential alternative to incineration because of its capability to oxidize high-concentration organic compounds in the chemical wastewaters (Li et al., 1991; Mishra et al., 1995). The WAO treatment of organic compounds in the aqueous phase at elevated temperatures and pressures followed a rather complex reaction * Author to whom correspondence is addressed. Fax: 8863-455-9373.
0888-5885/96/2635-0307$12.00/0
pathway (Devlin and Harris, 1984). However, the oxidation process has been shown to be rapid and its efficiency high indeed (Li et al., 1991; Mishra et al., 1995). Typical operating pressures and temperatures of a WAO treatment process can be over 10 MPa and 300 °C, respectively (Li et al., 1991; Mishra et al., 1995). Due to its severe operating conditions, the WAO treatment process does have several inherent disadvantages. Maintaining the WAO treatment process at these operating conditions is energy consuming, implying that it is a relatively costly process in comparison with other conventional wastewater treatment methods. Furthermore, the presence of corrosive chemical compounds in the wastewaters along with high temperatures and pressures renders the reaction environment in a WAO reactor highly corrosive (DeAngelo and Wilhelmi, 1983; Thomas, 1990). Such a severe reaction environment makes material selection for the WAO reactor a difficult task. Therefore, from the practical standpoint, it is highly desirable to operate the WAO treatment process at a sufficiently lower temperature and pressure which still enables retention of good oxidation efficiency of the WAO process. Scrutiny of the past literature (Li et al., 1991; Mishra et al., 1995) reveals that a large number of previous investigations of the WAO treatment process employed simulated wastewaters which consisted of a single organic compound. Such chemical wastewaters permitted easy control of the wastewater composition and concentration which considerably facilitates kinetic studies of the WAO treatment process and evaluation of the process performance characteristics. The WAO treatment process has also been employed by several investigators in dealing with the aerobically or anaerobically activated sludge (Li et al., 1991; Mishra et al., 1995). In comparison, relatively few works have been devoted to the WAO treatment of real high-concentration wastewaters generated by the heavy chemical or petrochemical industries. Consideration of those chemical wastewaters can reveal many perspectives which eluded that of simulated chemical wastewaters. The purpose of this study is to investigate the performance characteristics and reaction kinetics of the WAO process treating high-concentration wastewater from a petrochemical plant. Two kinetic models were employed to represent the experimental chemical oxygen demand © 1996 American Chemical Society
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volume, COD, mg/L pH composition
a
Figure 1. Experimental setup.
(COD) removal data, and their fit to those experimental data was compared. Experimental Studies The experimental setup of the WAO treatment process is shown in Figure 1. The reaction vessel is a highpressure Parr reactor (Model 4563, Parr Instrument, Inc., USA). It is made of SS-316 stainless steel and has an effective volume of 600 mL. The reactor is equipped with a six-bladed turbine-type impeller mixer. A thermal sensor, cooling coil, and external heating element are also provided in the reactor for temperature control to an accuracy of (1 °C. The operating pressure of the oxidation reaction was controlled by a regulator in the exit air line. The gas exiting the reactor passed through water-cooled cold trap to condense out the possible volatile organic components carried by the gas. Both bottled air or oxygen were employed as the oxygen supply for the oxidation reaction. The petrochemical company from where the sample wastewater was obtained is a large producer of methyl tetrabutyl ether (MTBE), methyl ethyl ketone (MEK), and sec-butyl alcohol (SBA) in Taiwan. The raw material employed by the manufacturer is liquefied petroleum gas containing primarily C4 organic compounds. The manufacturing processes of the company produce over a dozen streams of wastewater. Among them, three wastewater streams have unusually high polluting strength. The total volume of the three wastewater streams is 45 m3/day, which constitutes about 3% of the total volume of wastewaters produced daily by the plant. However, in terms of chemical oxygen demand (COD) concentration, the three wastewater streams have 80% of the total amount of pollutants in the wastewater. The general characteristics of these three wastewater streams are listed in Table 1. It is apparent that the wastewater stream no. 1 contains a high concentration of strong base (NaOH), while the two other wastewater streams contain primarily various organic acids. Mixing the three wastewater streams according to their respective daily production generates a wastewater with an initial COD concentration of 24 500 mg/L and a pH of 4. The COD concentration of the mixture is approximately 28 times higher than that of the final wastewater produced by the plant. To deal with the wastewater, the plant is currently employing a sophisticated wastewater treatment system which consists of various physical, chemical, and biological treatment units. Although its treatment efficiency is still satisfactory, the current wastewater treatment system has been slightly overloaded
1
2
3
4.6 42 800 12.9 SBAa C8-C12 Oil 6-7% NaOH SO42+
26.4 23 700 2.1 maleic acid fumaric acid acetic acid acrylic acid C16-C24 oil
13.6 12 000 2.5 maleic acid fumaric acid acetic acid acrylic acid C16-C24 oil
SBA ) sec-butyl alcohol.
and the treatment cost is high too. To reduce the load of the current treatment system and the treatment cost, a logical and plausible approach is to separate the three high-concentration wastewater streams from the rest and treat them separately by the WAO process. By doing this, the rest of the wastewater streams can then be treated by a simple treatment procedure, resulting in an alleviation of the capacity problem and a significant cost reduction of the overall wastewater treatment system. In the experiments, 300 mL of the mixture of the three high-concentration wastewater streams was first put in the reactor. After the reactor was sealed, it was rapidly pressurized and heated to the desired pressure and temperature. The operating temperature was chosen to be within the range between 150 and 250 °C. Both air and pure oxygen were tested in the present study, and the air or oxygen flow rate was controlled to within the range between 0.5 and 3.5 L/min. The operating pressure was kept at or below 5 MPa (50 atm) which was sufficient to keep the WAO reaction in the liquid phase at the chosen operating temperature. The mixer was set at 300 rpm to minimize the gas/liquid interficial resistance of oxygen dissolution in liquid and to keep the reactor content well mixed. Samples were taken periodically during and at the end of the treatment period for analysis. The COD concentrations of the samples were determined according to the standard method (APHA, 1992). Results and Discussion To keep the wet air oxidation in the liquid phase, a sufficient pressure must be maintained in the reactor, preventing the liquid from vaporization. This implies that the applied pressure must exceed the vapor pressure of the liquid mixture at a given temperature. The temperature-dependent vapor pressure of the liquid mixture is difficult to predict theoretically due to the complexity of the liquid wastewater composition. Such a relationship, however, can be established experimentally using the Parr reactor system. After the liquid wastewater was placed in the reactor, it was sealed and all inlet and outlet valves were closed. The reactor temperature was raised to a desired value and maintained steadily at that level for over an hour, an equilibrium between the vapor phase and the liquid wastewater was established. The temperature and the equilibrium vapor pressure were registered. Figure 2 shows the temperature/vapor pressure relationship of the present chemical wastewater sample. In comparison with that of pure water, there appears to be a vapor pressure elevation of the wastewater sample and the vapor pressure elevation tends to increase with a decrease in temperature. For instance, at 150 °C, the
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Figure 2. Vapor pressure vs temperature of the high-concentration wastewater.
Figure 3. Effect of air flow rate on the COD removal at 200 °C and 3 MPa.
vapor pressure of the wastewater is about 8% higher than that of water, while, at 250 °C, the vapor pressure elevation is about 1.5% only. Based on the relation shown in this figure, the reactor pressure to be applied could be easily chosen for a given temperature. The oxygen required for the aqueous phase oxidation of the organic compounds was primarily supplied by the air flow. In fact, the air flow also provided a good mixing effect on the reaction system, much like that of the mixer. Figure 3 demonstrates the influence of air flow rate on the COD removal. The effect is seen to be quite strong, particularly at low air flow rate at or below 1.0 L/min. An increase in the air flow rate from 1.0 to 3.5 L/min does not seem to provide a proportional increase in the COD removal as it does at or below 1.0 L/min. An air flow rate of 1.0 L/min was thus considered as a good choice and was adopted for other experimental runs in the present study. It is further noted that the COD removal achieved at 1.0 L/min air flow rate and in one and a half hours of WAO treatment is more than 60% already. This translates to that about
Figure 4. Effect of pure oxygen flow rate on the COD removal at 200 °C and 3 MPa.
50% of the total wastewater pollution load of the plant has been removed by the WAO treatment process. Such a reduction of total wastewater pollution load considerably alleviates the downstream wastewater treatment burden. Test runs were also conducted using pure oxygen in lieu of air. Effects of various oxygen flow rates on the COD removal are displayed in Figure 4. This figure reveals that the difference in the COD removal due to different oxygen flow rates is not as pronounced as that of the air flow rates shown in Figure 3. One possible explanation is that the dissolved oxygen (DO) in the liquid as provided by the oxygen flow, even at 0.5 L/min, is more than sufficient for the WAO treatment process, and hence a further increase in the oxygen flow rate tends to provide the mixing effect only. Comparison of Figures 3 and 4 also reveals that the COD removal using oxygen was significantly elevated when compared to that using air at the same flow rate. For instance, after 60 min of WAO treatment, the 32% COD removal at 0.5 L/min air flow rate was markedly increased to 56% using oxygen at the same flow rate. The above observations clearly indicate that, to obtain a good COD removal for the WAO treatment using air, a flow rate at or above 1.0 L/min would be needed, while using pure oxygen, the flow rate should be kept as low as 0.5 L/min or even below. The major role played by the pressure is to maintain the wastewater oxidation in the aqueous phase. Certainly, a high pressure maintained in the reactor will also enhance the dissolved oxygen (DO) concentration in the liquid phase which is beneficial to the wastewater oxidation. Figure 5 shows the effect of pressure on the COD removal. It is surprising to see that the pressure effect is only marginal, significantly smaller than those of other operating parameters shown in the previous figures. This indicates that the DO enhancemment effect of high pressure is negligible. Hence, a low operating pressure of 2.5 MPa would be advantageous for the present WAO treatment system. In fact, according to practical design experiences, an operating pressure above 3 MPa is usually to be avoided because, above that pressure level, there is a very rapid increase in the capital cost of the air compressor required for the wastewater oxidation system.
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Figure 5. Effect of pressure on the COD removal at 200 °C and 1.0 mg/L air flow rate.
Figure 6. Effect of temperature on the COD removal at 3 MPa and 1.0 mg/L air flow rate.
The single most important operating variable of the WAO treatment process is the temperature. The effect of this variable on the COD removal is demonstrated in Figure 6 in which a constant pressure was maintained in the reactor while the tempertaure was varied from 150 to 240 °C. The COD removal is seen to increase markedly with an increase in temperature from 150 to 225 °C. Above 225 °C, the temperature effect is drastically reduced. In fact, the difference in the COD removal between 225 and 240 °C is essentially nil after 50 min of WAO treatment. Hence for practical purpose, operating the WAO treatment process at 225 °C would definitely be more economic than other temperatures in terms of effective COD removal and energy consumption. A common feature revealed in the above figures is that the COD removal tends to level off as the time of WAO oxidation becomes sufficiently long. This is due presumably to the fact that a large number of shortchain organic compounds was generated by the WAO treatment process and those short-chain organic com-
Figure 7. Effects of various catalysts on the COD removal at 200 °C, 3 MPa, and 1.0 mg/L air flow rate.
pounds are much more resistant to WAO oxidation than the organic compounds present in the original wastewtaer (Li et al., 1991; Mishra et al., 1995). One way to deal with those short-chain organic compounds is to elevate the operating pressure and temperature to the supercritical level (Li et al., 1991; Mishra et al., 1995) which can be very costly and is usually deemed uneconomical. A potential alternative is to utilize catalysts, as suggested by many previous investigators (Sadana and Katzer, 1974; Levec and Smith, 1976; Imamura et al., 1982; Ito et al., 1989; Lin and Wu, 1995). The present study considers the latter approach. The catalysts employed included copper sulfate (CuSO4), cobalt oxide (Co2O3), and zinc oxide (ZnO). While copper sulfate is a soluble catalyst, the two other catalysts are insoluble in the wastewater. Figure 7 compares the COD removal using these three catalysts and that without at 200 °C. In the three catalytic WAO oxidations, 500 mg/L of catalyst was utilized in the experimental runs. A pronounced increase in the COD removal using CuSO4 over that without catalyst is apparent in this figure, elevating the COD removal at 60 min of WAO treatment from 53% to 72%. The improvement, however, becomes less appreciable with Co2O3 and ZnO, especially at the latter stage of the WAO treatment process. One possible reason for the high catalytic effectiveness of CuSO4 might be that it was a homogeneous reaction, while those of Co2O3 and ZnO were heterogeneous. Two kinetic models have been proposed for representing the experimental kinetic data of the WAO treatment process. The first one is to use two first-order reactions (Joglekar et al., 1991; Lin and Chuang, 1994). This kinetic model represents well the WAO treatment process of many phenolic wastewaters (Joglekar et al., 1991; Lin and Chuang, 1994). The second one, known as the generalized model, was proposed by Li et al. (1991). This model was based on the assumption that some of the organic compounds present in the wastewater are directly oxidized to the end products (CO2 and H2O) in the WAO treatment process, while the rest is first converted to an intermediate product (acetic acid) which is then further oxidized to the end products. The generalized kinetic model is represented by the following reaction pathway:
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 311 A + O2
k1
k2
D k3
B + O2
where A is the initial and other relatively unstable intermediate organic compounds, B the refractory intermediate product (acetic acid), D the end products (CO2 and H2O), and ki the associated rate coefficient. The main reason for adopting this reaction mechanism is the presence of acetic acid in the wastewater during the WAO treatment, as observed by many investigators (Li et al., 1991; Mishra et al., 1995). The first-order kinetics is given by
ln(C/C0) ) -kt
(1)
in which Co and C are respectively the initial COD concentration and that at time t and k is the rate coefficient. The rate coefficient is related to the reaction temperature according to the Arrhenius equation
k ) A exp(-∆E/RT)
(2)
Figure 8. First-order kinetic model fit of the COD removal data at 1.0 L/min air flow rate and 5 MPa.
where A is the frequency factor, ∆E the activation energy, R the gas constant, and T the temperature. Assuming a first-order kinetics for all reaction paths, the generalized kinetic model is given by (Li et al., 1991)
{
k2 [A + B] ) exp(-k3t) + k1 + k2 - k3 [A + B]0 k1 - k3 exp[-(k1 + k2)t] k1 + k2 - k3
}
(3)
where [A] and [B] represent the concentrations of A and B, respectively, in the reaction pathway. In terms of COD concentration, [A + B]0 and [A + B] are respectively the same as C0 and C in eq 1. The ratio of the formation rate coefficient of acetic acid (k2) to that of the end products (k1) was defined by Li et al. (1991) as the point selectivity (R)
R ) k2/k1
(4)
The rate coefficients for the first-order kinetic model, as given by eq 1, and for the generalized kinetic model, as given by eq 3, were determined by least-squares curve fitting of the respective model to the experimental data. It should be noted that adoption of the two firstorder reaction kinetics is based on the fact that the WAO oxidation of the original organic compounds proceeds in two steps. The organic compounds are converted into small-molecule intermediates in the first step of the WAO reaction. Those intermediates are subsequently reduced to final products (carbon dioxide and water) in the second step. Figures 8 and 9 show the first-order kinetic model fit of the COD removal data using air and pure oxygen, respectively. For the WAO treatment using air, the two first-order kinetics fit the measured data well for the test runs at 150 and 175 °C, as shown in Figure 8. However, for those test runs above 175 °C, the model fit does not appear to be as good as that at lower temperatures. Similar general characteristics of the kinetic model fit of the measured data using oxygen are observed in Figure 9. Based on the model fit of the measured data shown in Figures 8 and 9, the kinetic rate coefficient for each
Figure 9. First-order kinetic model fit of the COD removal data at 0.5 L/min oxygen flow rate and 5 MPa.
oxidation step was obtained by using eq 1. The reaction rate coefficients obtained for both reaction steps were highly temperature dependent. The relationship between the reaction rate coefficients and the temperature could then be plotted according to eq 2 for each reaction step to see if it followed the Arrhenius correlation. Figure 10 demonstrates the rate coefficient plots for the first and second reaction steps using 1 L/min air flow rate. Similar plots for the rate coefficient plots using 0.5 L/min oxygen flow rate are displayed in Figure 11. It is apparent that the rate coefficients for the second reaction step in Figure 10 do not follow the Arrhenius correlation at all, while those for other first-order reaction steps do observe the correlation fairly well. The frequency factors and the activation energies for the three rate coefficients that follow eq 2 are listed in Table 2. The rate coefficients of the first reactions are seen to be significantly larger than those of the second reactions, reflecting the much slower WAO oxidation of the intermediates (acetic acid).
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Figure 10. Arrhenius plot for the first-order rate coefficients pertaining to the WAO treatment shown in Figure 8.
Figure 11. Arrhenius plot for the first-order rate coefficients pertaining to the WAO treatment shown in Figure 9. Table 2. Frequency and Activation Energy of a First-Order Reactiona first reaction A, air oxygen a
min-1
8 635 74 563
second reaction
∆E, kJ/mol
A, min-1
∆E, kJ/mol
50.8 56.6
0.0364
7.35
A ) frequency factor. ∆E ) activation energy.
The model fits for the WAO treatment processes using air and oxygen based on eq 3 are displayed in Figures 12 and 13, respectively. The kinetic parameters in eq 3 were obtained by a nonlinear least-squares curvefitting method (Johnson, 1980). Both figures show significantly better model fits of the experimental data for all temperatures than those demonstrated in Figures 8 and 9. Similar model fits for the WAO treatment using air and catalysts (CuSO4 and ZnO) are displayed in Figures 14 and 15. The two figures also show very
Figure 12. Generalized model fit of the COD removal data at 1.0 L/min air flow rate and 5 MPa.
Figure 13. Generalized model fit of the COD removal data at 0.5 L/min oxygen flow rate and 5 MPa.
good fits of the generalized model to the experimental data. The kinetic parameters for the model fit demonstrated in Figures 12-15 are listed in Table 3 as a function of temperature. Also given in this table is the point selectivity as defined by eq 4. This table clearly shows a strong temperature dependence of the kinetic parameters. Both k1 and k2 increase significantly with an increase in temperature. But k3 does not show any clear trend with temperature. The reason for such k3 dependence characteristics on temperature is not exactly known. It may be due to the fact that k3 is about 1 order of magnitude smaller than k1 or k2. Hence, the accuracy of k3 as a function of temperature was somehow obscured in the numerical curve-fitting process. Another important point to note in Table 3 is the value of R which is larger than 0.5 for most of the cases studied here. This implies that there is a strong presence of acetic acid in the present WAO treatment process. A strong presence of acetic acid in the WAO treatment
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Figure 14. Generalized model fit of the COD removal data at 1.0 L/min air flow rate, 5 MPa, and 500 mg/L of CuSO4.
Figure 15. Generalized model fit of the COD removal data at 1.0 L/min air flow rate, 5 MPa, and 500 mg/L of ZnO. Table 3. Kinetic Parameters of the Generalized Model air
oxygen
air + CuSO4
air + ZnO
T, °C
k1, min
k2, min
k3, min
R
150 175 200 225 240 175 200 225 240 175 200 225 240 200 225 240
0.117 0.157 0.211 0.253 0.267 0.0209 0.0250 0.0554 0.0854 0.0838 0.0971 0.1120 0.1270 0.0688 0.0944 0.1020
0.0878 0.1068 0.1174 0.1190 0.1220 0.0194 0.0215 0.0424 0.0606 0.0666 0.0742 0.0827 0.0881 0.0549 0.0701 0.0738
0.004 05 0.006 41 0.007 70 0.006 70 0.006 21 0.005 34 0.005 28 0.003 88 0.004 10 0.008 15 0.008 50 0.005 94 0.005 75 0.008 00 0.004 17 0.003 86
0.7504 0.6805 0.5562 0.4704 0.4569 0.9282 0.8600 0.7658 0.7093 0.7947 0.7642 0.7385 0.6937 0.7980 0.7423 0.7235
process was also experimentally observed by Wu et al. (1987) and Foussard et al. (1989). The Arrhenius plots for k1 are displayed in Figure 16. The frequency factors (A) and activation energies (∆E)
Figure 16. Arrhenius plots of the rate coefficients based on the generalized model pertaining to the WAO treatment shown in Figures 12-15.
Figure 17. Point selectivity vs the reciprocal of temperature for various WAO reactions fitted by the generalized model.
obtained from these plots are given in Table 4. This table reveals that the activation energies for the WAO treatment processes using oxygen without catalyst or air with catalyst (CuSO4 or ZnO) are significantly smaller than that using air alone, implying a much improved treatment efficiency for the former WAO processes. The temperature dependence of R can be described by the following exponential equation (Li et al., 1991)
R ) m exp(n/T)
(5)
which is displayed in Figure 17. The constant parameters, m and n, are given in Table 4. Conclusions Experimental results of wet air oxidation (WAO) of high-concentration chemical wastewater containing various organic acids and inorganic compounds indicated
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Table 4. Kinetic Parameters of k1 and r k1 air oxygen air with CuSO4 air with ZnO
R
A, min-1
∆E, kJ/mol
m
n
15.18 18.50 2.04 12.72
17.03 42.45 11.93 20.47
0.0361 0.1129 0.2966 0.2214
1294.5 950.3 444.8 605.6
that over 50% reduction of the chemical oxygen demand (COD) concentration of the high-concentration wastewater could be easily achieved by the WAO treatment process in about an hour employing relatively mild reaction temperatures and pressures. The experimental data also revealed that the pressure effect on the COD removal was relatively small, and hence a low pressure at 2.5 MPa was sufficient for the present WAO treatment of the high-concentration chemical wastewater under 250 °C temperature. An air flow rate above 1.0 L/min was observed to provide marginal improvement in the COD removal and was considered unnecessary. Using oxygen in lieu of air, the same COD removal efficiency could be achieved at a much smaller gas flow rate. Temperature was found to be the most important operating variable of the WAO treatment process. But above 225 °C, the temperature effect on the COD removal became marginal. The first-order kinetic model and the generalized model were employed to represent the experimental data. The results clearly showed that the generalized model fit the experimental data significantly better than the first-order alternative. The model parameters obtained by nonlinear least-squares curve fitting displayed a strong temperature dependence. The kinetic model fitting confirmed the presence of a significant amount of refractory compound (acetic acid) during the WAO treatment process. Acknowledgment The authors are grateful to the National Science Council, Taiwan, ROC, for financial support (under the Grant NSC83-0402-E-155-002) of this project. Literature Cited APHA Standard Methods for Water and Wastewater Examination, 17th ed.; American Public Health Association: Washington, DC, 1992.
Beltrame, P. L.; Carniti, P. Inhibiting Action of Chlorophenol on Biodegration of Phenol and its Correlation with Structural Properties of Inhibitors. Biotechnol. Bioeng. 1988, 31, 821. DeAngelo, D. L.; Wilhelmi, A. R. Wet Air Oxidation of Spent Caustic Liquors. Chem. Eng. Prog. 1983, 80, 218. Devlin, H. R.; Harris, I. J. Mechanism of the Oxidation of Aqueous Phenol with Dissolved Oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387. Foussard, J. N., Debellefontaine, H.; Besombes, V. J. Efficient elimination of organic liquid wastes. J. Environ. Eng., ASCE 1989, 115, 367. Imamura, S., Kinunaka, H.; Kawabata, N. Wet Oxidation of Organic Compounds Catalyzed by Co-Bi Complex Oxides. Bull. Chem. Soc. Jpn. 1982, 55, 3679. Ito, M., Akita, K.; Inoue, H. Wet Oxidation of Oxygen and Nitrogen Containing Organic Compounds Catalyzed by Co(III) Oxides. Ind. Eng. Chem. Res. 1989, 28, 894. Joglekar, H. S.; Samant, S. D.; Joshi, J. B. Kinetics of Wet Air Oxidation of Phenol and Substituted Phenols. Water Res. 1991, 25, 135. Johnson, K. J. Numerical Methods in Chemistry; Marcel Dekker: New York, 1980. Levec, J.; Smith, J. M. An Active Catalyst for the Oxidation of Acetic Acid Solutions. AIChE J. 1976, 22, 919. Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetics Model for Wet Air Oxidation. AIChE J. 1991, 37, 1687. Lin, S. H.; Chuang, T. S. Combined Treatment of Phenolic Wastewater by Wet Air Oxidation and Activated Sludge. Toxicol. Environ. Chem. 1994, 44, 243. Lin, S. H.; Wu, Y. F. Catalytic Wet Air Oxidation of Phenolic Wastewaters. Environ. Technol. 1995, in press. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal and Reuse, 3rd ed.; McGraw-Hill: New York, 1991. Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. Peoples, R. F.; Krishnan, P.; Simonsen, R. N. Nonbiological Treatment of Refinery Wastewater. J.-Water Pollut. Control Fed. 1972, 44, 2120. Sadana, A. J.; Katzer, J. R. Catalytic Oxidation of Phenol in Aqueous Solution over Copper Oxide. Ind. Eng. Chem. Fundam. 1974, 13, 127. Thomas, A. J. Corrosion Behavior of High Grade Alloys in the Supercritical Water Oxidation of Sludge. M.S. Thesis, University of Texas, Austin, TX, 1990. Wu, Y. C.; Hao, O. J.; Olmstead, D. G.; Hsieh, K. P.; Scholz, R. J. Wet air oxidation of anaerobically digested sludge. J. Water Pollut. Control Fed. 1987, 59, 39.
Received for review April 18, 1995 Revised manuscript received August 7, 1995 Accepted September 20, 1995X IE950251U X Abstract published in Advance ACS Abstracts, November 15, 1995.
