Ind. Eng. Chem. Res. 1999, 38, 3793-3801
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Catalytic Oxidation of Phenol over MnO2 in Supercritical Water Jianli Yu and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
Bulk MnO2 was used as a catalyst for phenol oxidation in supercritical water at 380-420 °C and 219-300 atm in a flow reactor. The bulk MnO2 catalyst enhances both the phenol disappearance and CO2 formation rates during supercritical water oxidation (SCWO), but it does not affect the selectivity to CO2 or to the phenol dimers at a given phenol conversion. The role of the catalyst appears to be accelerating the rate of formation of phenoxy radicals, which then react in the fluid phase by the same mechanism operative for noncatalytic SCWO of phenol. The rates of phenol disappearance and CO2 formation are sensitive to the phenol and O2 concentrations but independent of the water density. Both power-law and dual site LangmuirHinshelwood-Hougen-Watson (LHHW) rate laws were developed to correlate the catalytic kinetics. Our results show that SCWO reactor volumes can be reduced by an order of magnitude if bulk MnO2 is used as the catalyst and by yet another order of magnitude if a supported oxidation catalyst is used. Introduction Supercritical water oxidation (SCWO) is a promising waste treatment technology that can destroy organic compounds in industrial wastewater. The oxidation reactions occur in an aqueous phase under conditions that exceed the critical point of water (TC ) 374 °C, PC ) 218 atm). In 1994, the first commercial SCWO plants for treating industrial wastewater became operational in Texas and Germany, and a pilot plant began operating in Japan.1-3 These processes rely on homogeneous, free radical reactions to convert organic carbon to CO2. Recently, there has been increasing interest in the use of heterogeneous catalysts in SCWO. Catalysts can increase the oxidation rates, reduce the residence times and temperatures required for treatment, and possibly provide control over competing reaction pathways that is difficult to achieve in noncatalytic processes. A review article4 fully describes the potential advantages of catalytic SCWO for destroying organic wastes. Additionally, Aki and Abraham5 recently reported that catalytic SCWO could be more economical than incineration and other hydrothermal oxidation technologies. The potential applications for catalytic SCWO are numerous, but the body of literature is relatively small. There have been few previous studies of the governing reaction kinetics, networks, and mechanisms. In this article we provide such information for the catalytic SCWO of phenol over bulk MnO2. Background The catalytic oxidation of phenol in aqueous solutions has received considerable attention because phenol is a common pollutant in industrial waste streams and it is an intermediate in the oxidation pathway of aromatic compounds. It is also a good “worst case” model pollutant for SCWO studies.6 Matatov-Meytal and Sheintuch7 provide a good review of the previous studies of the heterogeneous catalytic oxidation of phenol in an aqueous phase. Most previous work focused on oxidation at * Corresponding author. E-mail:
[email protected]. Phone: (734)-764-3386. Fax: (734)763-0459.
low temperatures (80-210 °C) and pressures (1-32 atm).7-25 Some of these studies8,12,13 involved the use of MnO2 as the catalyst. The first reported catalytic oxidation of phenol in supercritical water (SCW) was done at 380-390 °C and 230-235 bar using CuO and ZnO supported by a porous cement.26 High conversions were obtained in the presence of the catalyst. Ding et al.27,28 made similar observations for MnO2/CeO2 or V2O5 catalysts. They also noted that MnO2/CeO2 is more active and stable than V2O5 and Cr2O3/Al2O3. Krajnc and Levec29 did more experiments and developed a Langmuir-Hinshelwood rate law for catalytic oxidation of phenol in SCW at 400-440 °C and 230-250 bar. They also identified several products of incomplete oxidation and classified them into four groups: dimers (4-phenoxyphenol, 2-phenoxyphenol, dibenzo-p-dioxin, biphenol, and dibenzofuran), single-ring compounds (1,4-benzenediol and p-benzoquinone), organic acids (formic acid and acetic acid), and gases. Recently, Zhang and Savage30 reported that the catalytic SCWO of phenol over a CuO + MnO2/Al2O3 commercial catalyst (CARULITE 150) at 380-430 °C and 250 atm resulted in a conversion of phenol to CO2 much higher than that produced by noncatalytic oxidation. They observed no deactivation after several days of continuous operation. The commercial catalyst was so active that the rate of pore diffusion limited the oxidation rate and made the kinetics analysis difficult. Nevertheless, these investigators were able to develop a simple first-order rate law that adequately described the experimental data. One of the pressing needs7 in this field, however, is the development of reliable rate laws for catalytic SCWO that could be used for engineering purposes. Therefore, we initiated the experiments described herein with bulk MnO2, which is a component in but is much less active than the optimized commercial catalyst used by Zhang and Savage.30 Experimental Section All experiments were carried out in an isothermal, isobaric packed bed reactor. The experimental condi-
10.1021/ie990277b CCC: $18.00 © 1999 American Chemical Society Published on Web 09/04/1999
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Table 1. Summary of Phenol Oxidation over MnO2 in SCW reaction pressure, reaction W/FA0, kg PhOH water atm temp, °C cat. s/mmol concn, mmol/L concn, mol/L 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 219 219 219 219 219 219 219 219 219 219 273 273 273 273 273 273 273 273 273 273 273 300 300 300
381 380 381 381 381 381 381 381 380 381 381 381 381 381 381 381 381 381 381 380 381 381 381 381 381 382 382 381 381 382 382 382 382 382 385 385 385 385 391 390 391 391 391 390 390 421 421 421 421 421 381 381 379 391 391 391 391 391 391 391 380 380 380 380 391 391 391 390 390 390 390 381 383 381
8.26 5.47 4.32 3.51 4.20 3.98 2.81 2.14 1.68 2.02 2.11 1.49 1.10 0.93 5.74 2.77 3.78 2.96 1.15 6.29 2.73 1.91 1.44 1.12 5.38 2.91 1.94 1.46 1.14 2.83 1.91 1.46 1.36 1.15 9.64 16.97 11.50 8.66 4.30 2.93 2.00 6.20 3.22 2.32 1.79 14.90 10.46 15.40 10.69 8.06 1.65 0.63 0.53 4.73 2.57 1.88 4.59 2.69 1.82 1.42 2.96 2.14 1.63 1.33 5.41 2.64 2.08 8.22 3.63 2.34 1.72 3.37 3.27 2.31
0.68 0.70 0.68 0.68 1.08 1.38 1.35 1.37 1.39 2.72 2.18 2.58 2.69 2.62 1.32 1.62 1.32 1.33 2.03 1.29 1.60 2.00 1.99 2.03 1.32 1.54 1.90 1.95 1.97 1.85 1.90 1.91 1.90 1.95 0.30 0.18 0.23 0.27 0.60 0.78 0.96 0.38 0.62 0.79 0.91 0.09 0.10 0.07 0.09 0.10 1.52 2.05 2.09 0.34 0.50 0.59 0.28 0.41 0.52 0.59 2.07 2.04 2.02 1.99 0.88 1.41 1.60 0.49 0.95 1.27 1.54 2.40 1.88 1.84
24.9 25.3 25.0 25.1 24.9 25.2 25.1 25.2 25.5 25.1 25.2 24.9 25.1 25.2 25.1 25.0 24.5 24.9 25.2 25.4 24.9 25.2 25.1 25.1 24.4 24.2 24.1 24.4 24.7 24.0 24.1 24.2 24.2 24.3 20.6 20.6 20.4 20.6 12.6 12.7 12.7 12.5 12.7 13.1 12.9 7.4 7.4 7.4 7.4 7.4 9.2 9.2 9.6 7.6 7.6 7.6 7.5 7.5 7.6 7.6 28.0 28.0 28.2 28.1 21.3 21.4 21.3 21.9 22.1 22.0 22.3 29.4 28.9 29.6
oxygen concn, mmol/L
PhOH Conv, %
25.5 25.3 25.5 25.7 30.3 24.8 25.1 24.9 25.2 24.8 30.6 25.9 25.2 26.0 21.9 19.1 21.1 21.5 15.8 34.9 29.7 24.8 24.8 24.2 51.4 46.2 38.2 37.9 38.3 30.0 29.5 29.8 29.9 29.4 16.5 24.3 20.9 18.6 24.0 21.2 17.8 27.9 23.9 22.1 19.4 7.4 6.3 8.4 7.3 6.4 38.0 29.5 31.7 14.8 11.9 10.2 15.8 13.5 11.5 10.2 29.1 29.7 30.3 30.6 39.2 30.6 27.4 47.2 40.2 34.7 30.9 23.0 30.4 32.6
87 69 60 50 71 70 58 46 36 62 58 43 45 45 84 60 72 53 39 82 57 49 35 32 87 72 69 62 49 76 65 52 43 44 70 69 57 49 60 39 26 41 27 22 20 44 35 40 34 30 47 28 23 49 27 19 41 21 19 14 83 67 56 46 87 66 55 61 51 39 33 93 92 74
% yield CO CO2 3.1 2.3 1.6 1.4 2.1 2.4 1.2 0.9 0.8 1.2 1.6 0.9 0.5 0.7 0.1 0.8 1.2 0.9 0.1 1.6 0.9 0.5 0.4 0.3 2.4 1.4 1.2 0.8 0.4 1.6 1.0 0.6 0.4 0.3 1.1 2.0 1.2 1.2 1.7 0.5 0.3 0.9 0.4 0.3 0.2 2.2 1.7 2.2 1.7 1.1 1.2 0.5 0.4 1.0 0.3 0.0 0.5 0.0 0.0 0.0 3.1 1.8 1.3 0.8 2.7 1.1 1.2 2.8 1.6 0.9 0.6 4.1 4.8 2.9
38 25 19 14 26 25 12 9 6 13 13 7 4 3 6 11 13 8 2 22 10 7 4 3 44 25 20 13 11 30 19 12 10 8 24 32 20 14 9 4 3 11 7 6 7 19 14 23 16 13 7 2 3 15 5 2 7 3 2 1 27 15 12 7 22 8 7 24 11 7 4 49 45 23
carbon tally, % 55 58 60 65 57 57 55 64 71 52 57 65 60 60 22 51 43 56 63 41 54 58 70 71 60 55 52 52 62 56 55 60 67 64 55 66 64 66 51 65 77 71 80 84 87 77 80 85 84 84 61 75 80 67 79 83 66 82 83 87 48 50 57 62 37 44 53 65 62 69 73 59 58 52
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3795 Table 1. (Continued) reaction pressure, reaction atm temp, °C 300 300 300 300 300 300 300 300 300 300
381 381 391 391 390 391 391 391 391 391
W/FA0, kg cat. s/mmol
PhOH concn, mmol/L
water concn, mol/L
oxygen concn, mmol/L
PhOH Conv, %
1.79 1.40 5.18 2.65 2.78 1.94 6.33 3.17 2.14 1.58
1.83 1.90 1.17 1.77 1.76 2.11 0.77 1.29 1.65 1.95
29.6 29.6 25.8 25.7 26.2 26.1 26.1 26.0 26.0 26.0
32.8 31.6 50.2 39.0 40.6 33.9 58.0 48.5 41.9 36.6
62 51 99 93 90 82 95 91 81 72
% yield CO CO2 1.7 1.1 0.0 6.6 2.8 2.0 4.0 3.2 2.1 1.5
6 10 95 49 22 15 62 31 17 12
carbon tally, % 45 60 96 63 35 35 72 43 38 42
Figure 1. Experimental Apparatus for Catalytic SCWO.
tions are 380-420 °C and 219-300 atm. The reactant concentration ranges for phenol and oxygen at the reaction conditions are 0.05-2.82 and 6.3-123 mmol/ L, respectively. Oxygen was always present in excess. The water density varied between 0.13 and 0.54 g/cm3. Table 1 provides a complete listing of the experimental conditions. The configuration of the reactor system, which is shown in Figure 1, allows for two aqueous feed streams, one with an H2O2 solution and the other with an organic compound dissolved in water. The water used to prepare these feed solutions is deionized and then degassed overnight prior to use. Aqueous solutions of H2O2 and phenol are loaded into separate feed tanks and blanketed with about 500 psi of helium. Two Eldex model AA-100-S metering pumps are then used to draw material from the feed tanks and get the feed streams to the desired operating pressure. The feed streams are next preheated to the reaction temperature by flowing through preheating lines that are 2-m long, 1/16-in. (1.6 mm) o.d. Hastelloy C-276 tubing. The H2O2 decomposes in the preheating line according to 2H2O2 f 2H2O + O2 so that O2 is the oxidant fed to the reactor. The preheated feed streams then mix in a Hastelloy C-276 mixing tee, where the temperature is measured by a thermocouple, and the mixed stream enters the reactor. The reactor is a 12-cm long, 1/4-in. (6.4 mm) o.d.
stainless steel tube with two porous Hastelloy disks (5 µm pore size) at each end. The catalyst, MnO2 (99%, 60-230 mesh, Aldrich Co.), was packed in the reactor. The reactor assembly and preheating lines reside in a temperature-controlled Techne model SBL-2 fluidized sand bath. A Techne model TC-8D temperature controller that is capable of holding the temperature constant to within (1 °C controls the temperature. After the mixture has exited the reactor, it is cooled in two consecutive tube-in-tube heat exchangers and depressurized in a Tescom model 44 back-pressure regulator. The cooled reactor effluent is then separated into gas and liquid phases, and the volumetric flow rates of the streams are measured. The gas phase is analyzed by an online gas chromatograph (GC) with a thermal conductivity detector. The liquid phase from the reactor effluent is sampled and then analyzed by using a reverse-phase, isocratic, high-performance liquid chromatograph (HPLC). A Supelco C-18 column and a Supelco C-610 column are employed to determine the concentration of unreacted phenol and the concentration of organic acids, respectively. To identify and quantify some of the aqueous-phase products present in low concentrations, a liquid-liquid extraction and concentration protocol is applied to the liquid samples from phenol oxidation. This concentrated extract sample is then analyzed by gas chromatography. An HP model
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
Figure 2. Phenol conversion from SCWO with and without MnO2 catalyst (T ) 380 °C, P ) 250 atm, [PhOH]0 ) 1.9 mmol/L, [O2]0 ) 30 mmol/L).
5890 gas chromatograph with an HP model 5970 mass selective detector (GC-MS) is used to identify the reaction products. Quantification of the products is completed using an HP model 5890 GC with a flame ionization detector. A 50-m × 0.2-mm × 0.11-µm film thickness HP-5 capillary column is used in both GCs, and we employ an oven temperature program. More detailed analytical procedures for the gas and liquid phases have been reported previously.30-37 Product molar yields are calculated as the molar flow rate of the product in the reactor effluent divided by the molar flow rate of phenol into the reactor. For CO2 and CO, the molar yields were normalized by dividing by six, the number of carbon atoms in phenol. Thus, the maximum possible CO2 molar yield is 100%, which corresponds to complete conversion of the carbon in phenol to CO2. Results Table 1 presents a complete list of the experimental conditions and results. We report the phenol conversion, the yields of CO and CO2, and the carbon tally for each experiment. The carbon tally is the sum of the yields of phenol, CO, and CO2. It is not a carbon balance. The carbon tally being less than 100% simply means that organic products of incomplete oxidation have been formed. The main gaseous product is CO2, but small amounts of CO (
