Ind. Eng. Chem. Res. 1997, 36, 4809-4814
4809
Kinetics of Wet Oxidation of Formic Acid and Acetic Acid Rajesh V. Shende and Vijaykumar V. Mahajani* Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India
Oxidation of lower molecular weight carboxylic acids such as formic, acetic, glyoxalic, and oxalic acids is often the rate-controlling step during wet oxidation (WO) of an aqueous waste stream exhibiting very high chemical oxygen demand (COD). The kinetics of WO of formic acid was studied in the absence and presence of a cupric sulfate as catalyst in the temperature range 150-240 °C and oxygen partial pressure range 0.345-1.380 MPa. Wet oxidation of acetic acid was carried out in the presence of cupric sulfate in the temperature range 215-235 °C. Homogeneous copper sulfate was found to be a very good catalyst for oxidation of formic acid and acetic acid. Introduction Wet oxidation (WO) is a very powerful technique for the treatment of effluent streams exhibiting high chemical oxygen demand (COD) and biological oxygen demand (BOD). This technique is gaining popularity among environmental engineers for treating waste streams to meet local discharge standards. Organic compounds present in effluents gradually degrade to low molecular weight compounds and finally to highly refractory lower molecular weight carboxylic acids (Mishra et al., 1995) during wet oxidation. For example, Devlin and Harris (1984) have studied wet oxidation of phenol and listed various acids such as formic, acetic, glyoxalic, and oxalic acids as the products of wet oxidation. The lower molecular weight carboxylic acids invariably exhibit resistance to further oxidation. This results in effluent stream exhibiting finite COD and BOD at the end of wet oxidation, and at times may not meet local discharge standards. Therefore, more severe oxidation conditions are required for complete oxidation of lower molecular weight refractory acids to CO2 and H2O. Since oxidation of these acids is difficult, their oxidation to CO2 and H2O often becomes the rate-controlling step in the overall wet oxidation process. However, severe oxidation conditions can be reduced by using appropriate catalyst. In order to have a thorough insight into the process and for better understanding of a reactor design, knowledge of kinetics of WO of lower molecular weight carboxylic acids is desired. Homogeneous and heterogeneous catalysts have been reported for wet oxidation. The heterogeneous catalytic system offers (Levec, 1976; Imamura et al., 1982) the advantage of ease of separation of the catalyst used. However, if we examine very critically the system, we foresee a danger of metallic catalyst getting leached into an aqueous stream by lower molecular weight acids, for instance, the most stable acetic acid. The aqueous stream, therefore, needs to be treated to meet discharge standards with respect to metallic catalyst. As a result, demetalation of an aqueous stream originating from a wet oxidation reactor using homogeneous or heterogeneous catalyst becomes necessary to meet discharge standards with respect to metal. However, the load on the demetalation unit will be lower in the case of heterogeneous catalytic system than for a homogeneous catalytic system. On the other hand, a homogeneous catalyst system is expected to result in a smaller reactor size due to the higher rate of wet oxidation as compared * To whom correspondence should be addressed. S0888-5885(97)00048-1 CCC: $14.00
to heterogeneous catalyst. Considering both systems, namely, the main reactor and auxiliary demetalation system, there would be an economic trade off between homogeneous and heterogeneous wet oxidation before arriving at the final decision. Recently, de Bekker and Heerema (1994) have reported a novel reaction system of a deep shaft reactor below the Earth’s surface. A homogeneous catalytic system would definitely then be advantageous for such a reactor from the process engineering point of view. Both acids are biodegradable, and it is unlikely that such a highly capital intensive wet oxidation would be used in destruction of these acids. As indicated earlier, these acids are formed during wet oxidation, and since acetic acid is much more stable, its destruction may become the rate-controlling step. Therefore, a process design engineer may need to know the kinetics of destruction of low molecular weight acids to design a reactor system. Chang and Lin (1993) have presented some information on wet oxidation of acetic acid using homogeneous CuSO4 as a catalyst in their paper on wet oxidation of spent caustic. Lin et al. (1996) have also reported studies on wet air oxidation of a mixture of maleic acid, fumaric acid, acetic acid, and acrylic acid. They have studied the catalytic effect of copper sulfate, zinc oxide, and cobalt oxide on the wet oxidation of a mixture of acids. Copper sulfate was observed to be the best catalyst among the above studied catalysts. However, there is practically no information available on homogeneously catalyzed wet oxidation of formic acid and acetic acid separately. It was, therefore, thought desirable to study separately homogeneously catalyzed wet oxidation of formic and acetic acids. The inorganic homogeneous catalyst used was in the form of copper sulfate. Our choice of copper sulfate as a wet oxidation catalyst was based on the following rationale of process engineering. The anion, Cl-, is wellknown to cause pitting corrosion and more so in very hostile conditions, namely, at temperatures above 200 °C and in the presence of oxygen during wet oxidation. We have experienced in our laboratory severe pitting corrosion during wet oxidation of organic waste containing chlorides. Further, NO3- causes stress corrosion and may lead to explosion. On the other hand, SO42is the least hostile of all. Therefore, it was thought desirable to use a sulfate salt. Noble metals and metallic elements like Co, Cr, Cu, and Fe are known for their catalytic activity during oxidation. Pd and Pt are oxidation catalysts during oxidation of alkanol to alkanoic acid under alkaline © 1997 American Chemical Society
4810 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
conditions (Mahajani, 1979). Cobalt-based catalytic formulations (naphthenate, toluate) are well-known catalysts for oxidation of paraffins to acetic acid, oxidation of p-xylene to terephthalic acid, etc. However, oxidation of acetic acid to carbon dioxide and water is more difficult because -CH3 is an electron donor to the C atom having dO and -OH groups, thereby stabilizing the whole molecule. It is a well-known fact that copper interacts well with CO, CO2, and H2 to form CH3OH; as a result, it is the main ingredient of commercial methanol catalyst. Further, copper also interacts well with H2O and CO in a shift reaction. It is the main element in low-temperature shift (LTS) and recently introduced medium temperature shift (MTS) conversion catalyst in ammonia manufacture. This led us to believe that copper could be the better catalyst in the oxidation of acetic acid to CO2 and H2O via interaction with the -COOH group. From the foregoing discussions, the choice of copper sulfate as a catalyst for oxidation of acetic acid is now obvious. Shende and Mahajani (1994a) have reported detailed studies on wet oxidation of glyoxalic acid and oxalic acid using copper as the homogeneous catalyst. Wet oxidation studies of formic acid and acetic acid have been recently reviewed by Mishra et al. (1995) in their review on wet oxidation. In the present investigation, we have reported kinetic studies of oxidation of formic acid with and without catalyst and kinetic studies of oxidation of acetic acid with the catalyst. A homogeneous catalyst in the form of CuSO4 was used for obtaining basic kinetic data from the process engineering point of view. The aqueous stream can be detoxified with respect to copper by the well-known ion exchange technique, or precipitation as a sulfide salt to meet local discharge standards. Since the entire investigation is from the environmental engineering point of view, all kinetic results are expressed in terms of chemical oxygen demand (COD). Materials and Analytical and Experimental Procedures Materials. Formic and acetic acids used were of analytical reagent grade obtained from s.d. Fine Chemicals, Bombay, India. Cupric sulfate was of analytical reagent grade, and it was used as a catalyst. Oxygen from a cylinder with a minimum stated purity of 99.5% was used for oxidation. Analytical Procedure. Chemical Oxygen Demand. The samples during wet oxidation were analyzed for COD content using the standard dichromate reflux method (Snell, 1967). Infrared (IR) Spectroscopy and Gas Chromatography (GC). In order to detect formic acid formed as an intermediate, if at all, during wet oxidation of acetic acid, some typical samples were subjected to GC and IR analysis (GC instrument used, Chemito-3865, Toshniwal Instruments India Ltd.; and IR instrument used, Perkin-Elmer-270-30 spectrophotometer). The glass column containing Porapak QS packing (length 2 m) was used for GC analysis in the FID mode of operation. Experimental Procedure. The wet oxidation was carried out in a 2-L SS 316 autoclave equipped with a six-bladed turbine agitator having a variable speed arrangement. The gas inlet, pressure gauge, release valve, cooling water feed line, and safety port were situated on the top of the reaction vessel. The experimental setup is depicted in Figure 1. The sample line
Figure 1. Schematic diagram of the experimental setup for wet oxidation: CW, cooling water; CY, gas cylinder; δ, sample condenser; GS, gas sparger/liquid sampling device; H, electric heater; PI, pressure indicator; R, reaction vessel/autoclave; RD, rupture disk; S, sample outlet; SI, speed indicator; T, thermowell; TI, temperature indicator; TIC, temperature indicator and controller.
and a thermocouple were well immersed in the reaction mixture. The autoclave was charged with a known concentration of formic/acetic acid. In the case of noncatalytic oxidation, the reaction mixture was heated to the desired temperature. Once the temperature was attained, the sample was withdrawn. This time was considered as “zero time” for a reaction. The reaction temperature was controlled using a temperature indicator controller (TIC). Oxygen was then sparged in the vessel to a predetermined pressure level. During sampling, the oxygen pressure was maintained in order to keep the system at constant pressure. Samples were collected periodically. In the case of catalytic WO a similar procedure was followed, except that copper catalyst was first added in the substrate solution and the reaction mixture was then charged into the autoclave. Results and Discussion Wet oxidation (WO) is a heterogeneous gas-liquid reaction and consists of the following steps taking place in a series at the macroscopic level: transfer of oxygen from the bulk gas phase to the gas-liquid interface (gas phase mass transfer); instantaneous saturation of the interface with respect to the solute gas, O2 in this case; transfer of the dissolved oxygen from the gas-liquid interface to the bulk of liquid (liquid phase mass transfer); chemical reaction in the liquid phase and desorption of a product like CO2 from the bulk to the gas phase. Owing to the high diffusivity of oxygen in the gas phase and its low solubility in water, the gas phase mass transfer resistance was estimated to be negligible in the range of operating temperatures used for oxidation studies. The liquid phase mass transfer resistance depends upon the level of turbulence in the liquid phase, and hence the effect of speed of agitation on the rate of the reaction may be deemed as the barometer to indicate the presence of liquid phase mass transfer resistance. The effect of speed of agitation on the rate of reaction was studied in the range 6.3-14.5 rps. The liquid side mass transfer resistance was completely eliminated at an impeller speed of 14.5 rps. Preliminary calculations based on oxygen consumption indicated that the reaction was not sufficiently fast enough to take place in gas-liquid diffusion film (Doraiswamy and Sharma, 1984). Thus it was ensured that the reaction was kinetically controlled under the experimental conditions employed.
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Due to very low concentrations of the substrate used in the aqueous solutions, the oxygen solubility was considered to be same as that in water and was estimated from the data published by Crammer (1980). By and large, the WO reaction scheme for any organic compound can be represented as organic substrate + O2
CO2 + H2O
(1)
low molecular weight acids (formic, acetic, etc.)
The destruction of organics via the wet oxidation technique is known to be combination of various free radical reactions (Li et al., 1991; Tufano, 1993):
O2 f O• + O•
(2)
O• + H2O f HO• + HO•
(3)
RH + HO• f R• + H2O
(4)
R• + O2 f ROO•
(5)
ROO• + RH f R• + ROOH
(6)
RH + O2 f R• + HO2•
(7)
RH + HO2• f RO• + H2O
(8)
where RH ) organic substrate resulting in COD. The destruction of acid (COD) to CO2 and H2O could be a combination of the various above reactions. However, for the ease of design, we consider lumped or global kinetics of COD destruction to CO2 and H2O. The order with respect to oxygen could vary between 0 and 1 depending upon the rate-determining reactions in the case of the free radical mechanism as mentioned before. The reaction continues until CO2 and H2O are formed. Since we are interested from an environmental point of view, the results are presented in terms of COD of the final stream. The results can be interpreted as follows:
-
d(COD) ) r ) k0 exp[-E/RT][COD]m[O2]n (9) dt
For a given temperature and oxygen partial pressure we have a pseudo mth order expression as
-
d(COD) ) k[COD]m dt
(10)
where
k ) k0 exp[-E/RT][O2]n
(11)
The data can be well fitted to m ) 1, for given temperature and time. Thus we have
-
d(COD) ) k[COD] dt
(12)
and therefore,
ln
(COD)0 (COD)f
) kt
(13)
Figure 2. Effect of temperature and oxygen partial pressure on COD reduction during noncatalytic WO of formic acid: (g) 150 °C, 0.69 MPa; (0) 200 °C, 0.69 MPa; (4) 210 °C, 0.69 MPa; (×) 225 °C, 0.35 MPa; (9) 225 °C, 0.69 MPa; (2) 225 °C, 1.38 MPa; (]) 240 °C, 0.69 MPa.
The variation in k with respect to temperature and oxygen concentration can then yield the energy of activation and nthe order with respect to oxygen. The data in the case of catalytic oxidation can also be analyzed in a similar manner using the following expression:
-
d(COD) ) r ) k0 exp[-E/RT][COD][O2]n[CuSO4]p dt (14)
Kinetics of Wet Oxidation of Formic Acid The reaction for oxidation of formic acid can be represented as
HCOOH + 1/2O2 f H2O + CO2
(15)
In the present investigation, oxidation was carried out from an environmental engineering point of view, and therefore, results are given in terms of COD. The kinetics of noncatalytic oxidation was carried out in the temperature range 150-240 °C and oxygen partial pressure range 0.345-1.380 MPa. In order to ascertain the absence of a mass transfer resistances (kGa and kLa), it was decided to study the destruction of COD as a function of speed of agitation by varying the impeller speed in the range 6.3-14.5 rps. It was observed that the rate of oxidation was independent of impeller speed, indicating the absence of liquid phase mass transfer resistance. The high oxygen partial pressures employed and the low solubility of oxygen meant the absence of gas phase mass transfer resistance. All subsequent measurements were carried out at an impeller speed of 14.5 rps. The effect of temperature and oxygen partial pressure on COD reduction during noncatalytic wet oxidation is shown in Figure 2. The COD reduction was 12.92% at 150 °C and 0.69 MPa oxygen partial pressure. However, about 95% COD reduction was observed at 240 °C. Over the entire temperature range studied, the reaction was found to obey first-order kinetics with respect to COD (m ) 1) though other orders were also tried. The reaction kinetics could be well explained by having m
4812 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 3. First-order kinetics plot for wet oxidation of formic acid: (0) 210 °C, 0.69 MPa; (4) 218 °C, 0.69 MPa; ()) 225 °C, 0.69 MPa; (g) 225 °C, 0.345 MPa; (+) 225 °C, 1.38 MPa.
Figure 5. Effect of temperature, oxygen partial pressure and catalyst loading on COD reduction of formic acid solution: (0) 150 °C, 0.690 MPa, 6.888 × 10-4 kmol/m3; (4) 165 °C, 0.345 MPa, 6.888 × 10-4 kmol/m3; ()) 165 °C, 0.690 MPa, 6.888 × 10-4 kmol/m3; (g) 165 °C, 1.380 MPa, 6.888 × 10-4 kmol/m3; (+) 180 °C, 0.690 MPa, 6.888 × 10-4 kmol/m3; (×) 165 °C, 0.690 MPa, 1.252 × 10-4 kmol/m3; (9) 165 °C, 0.690 MPa, 3.131 × 10-4 kmol/m3.
Figure 4. Energy of activation for noncatalytic wet oxidation of formic acid.
) 1, i.e., order with respect to substrate (formic acid (COD)) as 1. The first-order kinetics plot is shown in Figure 3. The effect of oxygen partial pressure on COD reduction was studied over the range of oxygen partial pressures 0.345-1.380 MPa at 225 °C. The order with respect to oxygen was found to be 0.86. This may be due to combination of reactions 2, 3, 5, and 7. This, therefore, indirectly presents evidence for a free radical mechanism. The Arrhenius plot showing ln k vs 1/T is exhibited in Figure 4. The energy of activation was found to be 28.94 kcal/(g mol), confirming the absence of diffusional resistance. Based on experimental findings, the global rate equation for noncatalytic wet oxidation of formic acid is given by
r1 ) 4.712 × 1010 exp[-14565/T][COD]1[O2]0.86 (16) Catalytic oxidation of formic acid using cupric sulfate (CuSO4) was studied in the temperature range 150-
Figure 6. Energy of activation for catalytic wet oxidation of formic acid.
180 °C and oxygen partial pressure range 0.345-1.035 MPa. The effect of temperature, oxygen partial pressure, and catalyst loading on COD reduction is shown in Figure 5. For the catalyst loading of 6.888 × 10-4 kmol/ m3, a maximum COD reduction of 97% was achieved at 180 °C in 1.5 h. In this interesting case, we have observed that the order with respect to COD is 1.5 and first order with respect to O2. The higher order with respect to COD (formic acid) may be due to a formic acid-Cu complex forming during oxidation. The Arrhenius plot for catalytic WO of formic acid is shown in Figure 6. The energy of activation was found to be 22.5 kcal/(g mol). This value confirms the absence of mass transfer resistance. Kinetic data for the catalytic wet oxidation of formic acid within the range of above variables could be
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4813
Figure 8. Energy of activation for catalytic wet oxidation of acetic acid. Figure 7. Effect of temperature, oxygen partial pressure and catalyst loading on COD reduction during catalytic WO of acetic acid: (g) 215 °C, 0.69 MPa, 3.13 × 10-3 kmol/m3; (0) 225 °C, 0.69 MPa, 3.13 × 10-3 kmol/m3; (4) 235 °C, 0.35 MPa, 3.13 × 10-3 kmol/ m3; ()) 235 °C, 0.69 MPa, 3.13 × 10-3 kmol/m3; (g) 235 °C, 1.04 MPa, 3.13 × 10-3 kmol/m3; (×) 235 °C, 0.69 MPa, 1.88 × 10-3 kmol/ m3; (9) 235 °C, 0.69 MPa, 6.262 × 10-3 kmol/m3.
catalyst loading was studied in the range 6.262 × 10-4 to 6.262 × 10-3 kmol/m3 using 0.69 MPa oxygen partial pressure at 235 °C. The final expression for the kinetics of catalytic WO of acetic acid based on COD reduction is as follows
r3 ) 8.479 × 109 exp[-11459.2/T][COD]1[O2]0.92 ×
correlated by the expression
r2 ) 1.752 × 1012 exp[-11302/T][COD]1.5[O2]1 × [CuSO4]0.44 (17) It was possible to achieve complete oxidation of formic acid (in terms of COD) within 3 h using 6.888 × 10-4 kmol/m3 catalyst loading at 180 °C. Thus the homogeneous copper catalyst was found to be effective at lower temperatures. Kinetics of Wet Oxidation of Acetic Acid Acetic acid is the most refractory acid among low molecular weight carboxylic acids (C1-C6 carboxylic acids) toward wet oxidation. Recently, Merchant (1992) has studied the kinetics of noncatalytic wet oxidation of acetic acid. He has reported only a 7% reduction in COD during WO of acetic acid at 275 °C in 5 h. In fact, it is the key refractory component that provides resistance for further oxidation, and therefore, the effluent stream exhibits finite COD after wet oxidation. Kinetics of catalytic wet oxidation using copper sulfate were studied in the temperature range 215-235 °C and oxygen partial pressure range 0.345-1.04 MPa. The effect of temperature, oxygen partial pressure, and catalyst loading on COD reduction is shown in Figure 7. It can be seen that as temperature, oxygen partial pressure, and catalyst loading increases, reduction in COD also increases. About 80% COD reduction was achieved after 5 h at 235 °C. The wet oxidation of acetic acid in the presence of catalyst was found to obey first-order kinetics with respect to substrate COD. A plot of ln k vs 1/T is shown in Figure 8. The energy of activation was found to be 22.77 kcal/(g mol). This value indicates that the reaction is free of mass transfer resistance. The effect of oxygen partial pressure was studied in the range 0.345-1.04 MPa using the catalyst loading 3.131 × 10-3 kmol/m3 at 235 °C. Also, the effect of
[CuSO4]1 (18) The order with respect to O2 is 0.92, which means attack by O• via OH• and by O2 both may be contributing simultaneously. During WO of acetic acid catalyzed by “CuSO4”, formic and oxalic acids were not observed as decomposition products in infrared and gas chromatographic analyses. Formic acid (present study) and oxalic acid (Shende and Mahajani, 1994a) in the presence of “CuSO4” under WO conditions were found to be highly reactive, and therefore, even if produced during catalytic WO of acetic acid, they might have disappeared instantaneously under the reaction conditions. Since acetic acid is more stable as compared to formic acid, severe reaction conditions were required. However in the presence of “CuSO4” wet oxidation of acetic acid was found to proceed even at a temperature of 215 °C. The waste stream can be easily detoxified with respect to copper by the ion exchange technique. Alternately, it can be precipitated as a CuS salt by adding Na2S/ FeS or a hydroxide under alkaline conditions. We, therefore, postulate that since homogeneous “CuSO4” can destroy acetic acid, it could be a very effective catalyst in wet oxidation of any organic substrate. Almost complete destruction of phenol (COD) was possible during catalytic wet oxidation in the presence of homogeneous cupric sulfate (Shende and Mahajani, 1994b). Conclusions The wet oxidation of formic and acetic acids catalyzed by cupric sulfate was proved to be an effective technique to meet the discharge standards with respect to COD and concentration of individual acids as well. The noncatalytic wet oxidation of formic acid was found to obey first-order kinetics with respect to substrate COD, whereas, in the case of catalytic oxidation the data could be well fitted to m ) 1.5. Wet oxidation of acetic acid was found to obey first-order kinetics with respect to
4814 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
substrate COD and it was found to be oxidized directly into carbon dioxide and water. The use of homogeneous CuSO4 catalyst may be very effective in destroying organic molecules via wet oxidation because it is effective in destroying formic and acetic acids that are formed in wet oxidation. Acknowledgment R.V.S. is grateful to University Grants Commission, India, for the financial support. Nomenclature CA0 ) initial substrate concentration, mg/L COD ) chemical oxygen demand, mg/L BOD ) biochemical oxygen demand, mg/L [O2] ) oxygen concentration, kmol/m3 E ) energy of activation, kcal/(g mol) m ) order with respect to COD n ) order with respect to oxygen p ) order with respect to catalyst r ) rate of oxidation reaction with respect to COD reduction, mg/L s R ) gas constant, kcal/(g mol K) k0 ) preexponential factor (unit dependent on n and p) k ) rate constant T ) temperature, K [COD]0 ) initial COD, mg/L [COD]t ) COD (mg/L) at time “t” (s) t ) time, s
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Devlin, H. R.; Harris, I. J. Mechanism of the Oxidation of Aqueous Phenol with Dissolved Oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387. Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions. Analysis, Examples and Reactor Design; John Wiley and Sons: New York, 1984; Vol. 2. Imamura, S.; Hirano, A.; Kawabat, N. Wet Oxidation of Acetic Acid Catalyzed by Co-Bi complex oxides. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 570. Levec, J.; Herskowitz, M.; 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 Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37, 1687. Lin, S. H.; Ho, S. J.; Wu, C. L. Kinetic and Performance Characteristics of Wet Air Oxidation of High-Concentration Wastewater. Ind. Eng. Chem. Res. 1996, 35, 307 Mahajani, V. V. Noble Metal Catalyzed Liquid Phase Oxidation of Alkanol to Alkanoic acid. Ind. Chem. Eng. 1979, 21, 741. Merchant, K. P. Studies in Heterogeneous Reactions. Ph.D. Thesis, University of Bombay, India, 1992. Mishra, V. S.; Mahajani V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. Shende, R. V.; Mahajani, V. V. Kinetics of Wet Air Oxidation of Glyoxalic Acid and Oxalic Acid. Ind. Eng. Chem. Res. 1994a, 33, 3125. Shende, R. V.; Mahajani, V. V. Detoxification of Phenolic Stream by Catalytic Wet Air Oxidation. A paper presented at the National Seminar on Clean Environment-Strategies, Planning and Management. Lucknow, Feb 1994b, India. Snell, F. D.; Ettre, L. S. Encyclopedia of Industrial Chemical Analysis; John Wiley and Sons: New York, 1967; Vol. 17. Tufano, V. A. Multi-Step Kinetic Model for Phenol Oxidation in High Pressure Water. Chem. Eng. Technol. 1993, 16, 186.
Received for review January 21, 1997 Revised manuscript received May 6, 1997 Accepted May 12, 1997X IE970048U
X Abstract published in Advance ACS Abstracts, September 1, 1997.
