Environ. Sci. Technol. 2000, 34, 3191-3198
Kinetics of Catalytic Supercritical Water Oxidation of Phenol over TiO2 JIANLI YU AND PHILLIP E. SAVAGE* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
We oxidized phenol, a model pollutant, in supercritical water at 380-440 °C and 219-300 atm using bulk TiO2 as a catalyst in a tubular flow reactor. The phenol conversion and CO2 molar yield from this catalytic supercritical water oxidation (SCWO) are much higher than those from conventional noncatalytic SCWO of phenol under otherwise identical processing conditions. The selectivities to undesired phenol dimers decrease in the presence of TiO2, and the catalyst is stable and maintains its activity during phenol oxidation. All of these features are desirable for a catalytic SCWO waste treatment process. The rate of phenol disappearance over TiO2 was consistent with a power-law rate equation that is 0.69 order in phenol and 0.22 order in O2. The rate of disappearance of total organic carbon (TOC) exhibited reaction orders of 0.51 for the TOC concentration and 0 for the oxygen concentration. Both rates are independent of the water concentration. The catalytic kinetics for phenol disappearance were also consistent with the Mars-van Krevelen mechanism and with a Langmuir-Hinshelwood dual-site mechanism comprising reversible adsorption of phenol on one type of catalytic site, reversible dissociative adsorption of oxygen on a different type of site, and irreversible, ratedetermining surface reaction between adsorbed phenol and adsorbed oxygen. Our results show that the reactor volume for catalytic SCWO using TiO2 would be about onefourth that of the volume required for conventional, noncatalytic SCWO.
Introduction and Background At temperatures and pressures above its critical point (TC ) 374 °C, PC ) 218 atm), water is an excellent solvent for organic compounds. In addition, gases are completely miscible with supercritical water. Consequently, gas-liquid reaction systems can be made single-phase systems by operating at supercritical conditions. The chemical reactions can then take place without any interphase mass transfer limitation. Supercritical water could be used as a reaction medium for many purposes, such as chemical synthesis, fuel production, and waste treatment by oxidation (1, 2). The commercial supercritical water oxidation (SCWO) technology for wastewater treatment has been operated successfully at a reasonable cost (3-5). Conventional SCWO 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 to reduce the process temperature and pressure and thereby reduce the processing cost (6, 7). Unfortunately, the catalytic SCWO * Corresponding author e-mail:
[email protected]; phone: (734)764-3386; fax: (734)763-0459. 10.1021/es9914588 CCC: $19.00 Published on Web 06/16/2000
2000 American Chemical Society
literature contains few previous studies of the reaction kinetics, pathways, and mechanisms. This paper provides such information for the oxidation of phenol over bulk TiO2 in supercritical water. We selected TiO2 because transition metal oxides are known to be active catalysts for aqueousphase oxidation. In previous work, we examined a commercial, supported catalyst (8) and bulk MnO2 (9). TiO2 has been used as a SCWO catalyst before (10) but not for phenol. Phenol oxidation in aqueous solutions is important because phenol is a good “worst case” model pollutant for SCWO studies (11), it is a common pollutant in industrial waste streams, and it is an intermediate in the oxidation pathway of aromatic compounds. Matatov-Meytal and Sheintuch (12) 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 temperatures and pressures well below the critical point of water. Previous studies of catalytic oxidation of phenol in supercritical water showed that the phenol conversions and reaction rates were enhanced in the presence of CuO/ZnO/ CoO supported on a porous cement (13, 14); MnO2/CeO2, V2O5/Al2O3, and Cr2O3/Al2O3 (15, 16); Carulite 150, a CuOpromoted MnO2/Al2O3 commercial catalyst (8); and bulk MnO2 (9, 17).
Experimental Section We oxidized phenol in supercritical water in an isothermal, isobaric packed bed reactor. Oxygen was always present in stoichiometric excess. Table 1 provides a complete listing of the experimental conditions. The reactor system, experimental procedures, and most analytical methods have been described previously (9), so we give only an overview here. Two separate aqueous feed streams, one containing phenol and the other containing oxygen, were delivered to a Hastelloy C-276 mixing tee at the desired temperature and pressure. Deionized and degassed water was used to prepare the reactor feed solutions. We used an aqueous H2O2 solution as the oxygen source and verified experimentally that all of the H2O2 decomposed (2H2O2 ) O2 + 2H2O) before reaching the mixing tee. Phenol decomposition in the preheat tubing was negligible. The mixed stream entered a 12 cm long, 1/4 in. (6.4 mm) o.d. stainless steel tube reactor. Two porous Hastelloy disks (5 µm pore size) were seated at both ends of the catalytic reactor to keep the TiO2 (99.995%, 40-60 mesh, mixture of rutile and anatase phases, Puratronic, Alfa.AESAR Co.) particles inside. The specific surface area of the fresh TiO2 was 12.1 m2/g, as determined by a Micromeritics ASAP 2010 micropore size analyzer using BET analysis with N2 as the adsorbate. The reactor assembly and preheating lines reside in a temperature-controlled fluidized sand bath. After the mixture exited the reactor, it was quickly cooled and then depressurized. The cooled reactor effluent was then separated into gas and liquid phases, and the volumetric flow rates of the streams were measured. The gas phase was analyzed on-line by a gas chromatograph (GC) with a thermal conductivity detector. The liquid phase from the reactor effluent was periodically sampled and analyzed. Organic compounds in the reactor effluent were analyzed by high-performance liquid chromatography. To identify and quantify some of the aqueousphase products present in even lower concentrations, a liquid-liquid extraction and concentration protocol (8) was applied to the liquid samples. The concentrated extracts were analyzed by GC and GC-MS (GC with mass selective detector). We also analyzed the aqueous-phase reactor VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Phenol Oxidation over TiO2 in SCW reaction pressure (atm)
reaction temp (°C)
W/FA0 (kg of catatalyst -1 s mmol-1)
PhOH (mmol/L)
water concn (mol/L)
oxygen concn (mmol/L)
PhOH conv (%)
CO yield (%)
CO2 yield (%)
carbon tally (%)
250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 219 219 219 219 219 219 219 273 273 277 280 300 300 300 300 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 219 219 219 219 219 219 219 239
380 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 381 380 380 381 381 380 380 380 380 380 380 380 380 380 380 380 381 381 381 381 381 400 400 400 400 400 400 400 421 421 421 422 421 421 421 440 440 441 441 441 441 441 440 381 381 381 381 381 381 381 382
41.27 19.74 14.82 11.25 9.24 9.74 5.25 7.07 5.43 2.34 7.19 5.05 3.93 3.19 7.33 4.87 3.89 3.19 6.99 5.06 3.89 3.11 1.37 1.70 2.84 4.57 2.19 2.03 1.50 10.44 6.26 4.91 3.83 11.62 6.93 5.19 4.22 6.51 4.58 6.24 4.84 3.60 11.13 7.18 4.67 10.46 7.20 4.72 3.55 11.12 6.98 4.77 11.57 6.36 4.74 3.50 12.31 6.91 4.58 14.00 7.17 4.91 3.68 13.57 24.51 6.76 4.53 13.55 6.71 4.64 3.49 10.88
0.5 0.7 0.7 0.7 0.7 2.0 2.8 1.9 1.9 2.7 2.0 2.0 1.9 2.0 1.9 2.0 1.9 1.9 2.0 2.0 2.0 2.0 1.9 1.8 1.9 1.9 1.9 1.9 1.8 2.0 2.1 2.0 2.0 2.0 2.1 2.0 2.0 1.8 2.1 1.5 1.8 2.1 0.5 0.7 0.8 0.4 0.6 0.7 0.8 0.4 0.5 0.6 0.3 0.5 0.6 0.7 0.3 0.5 0.6 0.2 0.4 0.5 0.6 0.2 0.3 0.7 0.8 0.3 0.5 0.7 0.8 1.0
25.3 25.2 25.0 25.2 25.2 24.8 25.0 24.8 25.0 25.1 24.7 24.7 24.7 25.0 25.1 24.9 25.2 25.2 25.1 25.1 25.2 25.1 9.4 9.4 9.3 9.3 9.4 9.3 9.3 28.1 28.1 28.4 28.6 29.8 29.8 29.8 29.8 25.0 24.9 25.0 24.9 25.2 9.6 9.6 9.5 9.6 9.6 9.6 9.6 7.4 7.4 7.4 7.4 7.4 7.4 7.4 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 9.1 9.2 9.2 9.2 9.2 9.2 9.2 19.1
34.0 24.1 25.9 25.9 25.9 33.2 25.3 33.9 34.3 26.1 16.5 16.8 17.0 17.2 26.8 25.7 26.7 26.8 49.8 51.1 51.4 50.6 28.7 28.8 28.3 27.2 28.5 27.3 29.8 26.1 25.7 26.9 26.8 26.6 25.3 25.8 26.3 36.7 31.1 40.1 36.1 31.8 17.2 14.7 11.9 18.2 16.2 13.7 11.9 14.5 12.2 10.3 15.6 13.0 11.5 9.9 12.0 9.8 8.1 13.3 11.1 9.6 8.4 13.7 15.2 12.7 15.5 17.3 14.8 13.0 25.2 37.9
90 70 71 53 42 69 47 50 63 30 44 39 30 24 49 38 34 30 49 45 36 34 15 17 25 47 17 20 3 72 55 44 36 74 62 51 46 60 47 53 52 35 53 39 29 57 35 31 23 82 61 47 78 62 49 38 97 85 73 94 83 70 61 91 43 23 14 12 11 9 7 41
5.8 2.7 2.1 1.3 1.6 2.3 1.3 1.2 1.1 0.6 1.0 0.5 0.5 0.3 1.7 1.2 0.7 0.6 1.8 1.3 0.8 0.5 1.6 1.6 1.7 2.1 1.4 1.7 1.6 1.9 1.1 1.0 0.8 1.5 1.2 1.0 0.7 1.2 0.8 0.8 0.8 0.6 2.1 1.7 1.1 1.9 1.6 1.1 0.9 3.9 3.4 2.7 3.8 3.0 2.5 2.0 6.3 5.8 4.7 7.6 5.9 4.9 4.1 6.4 2.6 1.4 1.0 2.0 1.1 0.8 0.6 1.8
42 25 21 16 13 21 17 15 12 7 14 8 8 7 14 11 8 8 21 18 12 11 3 4 6 10 5 4 3 26 16 13 11 30 20 16 14 16 13 15 12 10 23 20 14 24 18 12 9 39 32 24 38 28 24 17 54 44 35 59 46 36 29 55 14 6 4 11 7 5 4 14
58 58 53 64 72 55 71 67 51 78 71 70 79 83 67 74 74 79 73 74 77 78 90 89 83 66 89 87 102 56 62 70 77 58 58 65 69 58 67 63 62 76 72 83 86 69 85 82 87 61 74 79 63 69 77 81 63 65 67 72 69 71 73 69 74 84 92 101 97 97 97 75
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TABLE 1. (Continued) reaction pressure (atm)
reaction temp (°C)
W/FA0 (kg of catalyst -1 s mmol-1)
PhOH (mmol/L)
water concn (mol/L)
oxygen concn (mmol/L)
PhOH conv (%)
CO yield (%)
CO2 yield (%)
carbon tally (%)
239 239 239 239 239 239 260 260 260 260 260 260 260
381 381 381 381 381 381 382 381 381 380 380 381 381
6.33 4.45 10.91 6.45 4.38 3.47 10.80 6.40 4.57 12.37 7.12 4.53 3.45
1.5 1.7 0.9 1.2 1.6 1.7 1.4 1.9 2.3 1.1 1.6 2.1 2.4
20.0 19.4 19.7 19.4 19.7 19.7 26.1 26.2 26.3 26.7 26.7 26.5 26.6
32.2 26.3 42.3 35.3 30.4 27.1 70.4 58.1 49.6 80.3 68.4 56.6 49.7
27 22 37 27 22 22 64 46 37 54 40 34 32
1.0 0.9 3.0 1.2 1.3 0.5 2.7 1.6 1.3 2.6 2.0 1.5 0.9
8 7 13 9 6 5 18 12 9 20 12 9 7
83 85 79 84 85 84 56 67 74 68 74 76 76
effluent for Ti by using a Perkin-Elmer 3100 atomic absorption spectrometer. We used a lamp with 35 mA current and chose 335.5 nm as the wavelength. The instrument’s sensitivity for Ti was 2.6 ppm. Product molar yields were 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 6, 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 provide the phenol conversion, the molar yields of CO and CO2, and the sum of the yields of phenol, CO, and CO2 (which we refer to as the carbon tally) for each experiment. The carbon tally is not a carbon balance. Its value being less than 100% simply means that organic products of incomplete oxidation were formed. Note that we intentionally selected experimental conditions that would produce both high and low phenol conversions. Our main interest is in the reaction kinetics and the development of reliable reaction rate laws requires experimental data obtained over a wide range of conversions. Figure 1 compares the phenol conversion and the molar yield of CO2 obtained from catalytic and noncatalytic SCWO of phenol as a function of the reactor residence time and W/FA0, the ratio of the catalyst mass to the phenol molar flow rate at the reactor entrance. The subscript 0 denotes a quantity at the reactor entrance. The values for catalytic SCWO are from our experiments. The values for noncatalytic SCWO were calculated from rate equations for phenol disappearance (18) and CO2 formation (19) during noncatalytic SCWO of phenol. These calculated results for noncatalytic SCWO represent the behavior expected if the TiO2 in the reactor were completely inert. The residence time used to calculate the noncatalytic results was determined as the reactor void volume divided by the volumetric flow rate through the reactor. Figure 1 shows that less than 25% phenol conversion would be expected from noncatalytic SCWO, but the experimental conversion was always higher and nearly reached 50% due to the presence of the TiO2 catalyst. Likewise, noncatalytic SCWO of phenol would have produced molar yields of CO2 of only a few percent, but the yields were increased to over 20% during catalytic SCWO of phenol. The catalyst thus enhanced not only the rate of phenol disappearance but also the rate of organic carbon conversion to CO2, which is the goal of SCWO treatment. The TiO2 catalyst is certainly active for phenol SCWO because it increases the phenol conversion and the CO2 yield.
FIGURE 1. Phenol conversion and CO2 molar yield from SCWO with and without TiO2 catalyst (T ) 380 °C, P ) 250 atm, [PhOH]0 ) 2.0 mmol/L, [O2]0 ) 51 mmol/L). We next consider the issue of the CO2 selectivity, which we define as the CO2 molar yield divided by the phenol conversion. The CO2 selectivity is important because SCWO of complex organic compounds such as phenol proceeds through a series of steps that produces products of incomplete oxidation. It is advantageous for a catalyst to reduce the yields of these products and increase the yield of CO2. Figure 2 shows as discrete points the experimental CO2 selectivity for catalytic SCWO over TiO2 at 380 °C and 250 atm with different phenol and oxygen feed concentrations. Two curves for noncatalytic SCWO of phenol, which were calculated from rate laws in the literature (18, 19) using concentrations representative of those used experimentally, are also shown. Figure 2 shows that the CO2 selectivities for catalytic SCWO are similar to those expected from noncatalytic SCWO at the same phenol conversion, which indicates that the use of the TiO2 catalyst does not enhance the CO2 selectivity. Taken together, the results in Figures 1 and 2 show that TiO2 enhances the rates of phenol conversion and CO2 formation but does not alter the CO2 selectivity at a given phenol conversion. Products of Incomplete Oxidation. The CO2 selectivities being less than 100% implies that some carbon resides in intermediate products that were formed during catalytic SCWO of phenol. Some products of incomplete oxidation in the liquid phase were identified to be 2-phenoxyphenol, 4-phenoxyphenol, xanthenone, 2,2-biphenol, 2-hydroxyVOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. CO2 selectivity from SCWO of phenol at 380 °C and 250 atm with and without TiO2 catalyst. (Discrete points are experimental data for catalytic SCWO, curves are for noncatalytic SCWO with [PhOH]0 ) 2.0 mmol/L, [O2]0 ) 50.7 mmol/L (solid curve) and [PhOH]0 ) 0.7 mmol/L, [O2]0 ) 25.4 mmol/L (dashed curve)).
FIGURE 4. Effect of phenol feed concentration on phenol conversion during catalytic SCWO (T ) 380 °C, P ) 250 atm, [O2]0 ) 26 mmol/L). CO2 during catalytic SCWO over TiO2. The most likely fate of this carbon is single-ring and ring-opening products, which are too numerous to permit complete identification and quantification.
Reaction Kinetics Analysis
FIGURE 3. Dimer concentration from phenol SCWO with and without TiO2 catalyst (T ) 380 °C, P ) 219 atm, [PhOH]0 ) 2.0 mmol/L, [O2]0 ) 27 mmol/L). benzaldehyde, dibenzofuran, dibenzo-p-dioxin, p-benzoquinone, m-xylene, o-xylene, p-xylene, and acetic acid. These byproducts were also detected in previous studies of both catalytic and noncatalytic SCWO of phenol (14, 20, 21). Formation of any products of incomplete oxidation is undesired, and formation of higher molecular weight, multiring products is especially problematic. One of the potential advantages of catalytic SCWO is that the catalyst might reduce the yield of multiring, dimeric products. Figure 3 compares the total concentration of the dimeric products identified in our experiments with that calculated for noncatalytic phenol SCWO (18) as a function of phenol conversion at 380 °C. As can be seen from this plot, the TiO2 catalyst offers the advantage of reducing the dimer concentration during SCWO of phenol. As noted in the previous section, the TiO2 catalyst had no measurable effect on the CO2 selectivity, but Figure 3 shows that it did reduce the dimer selectivity. Thus, a portion of the carbon that appeared as dimers during noncatalytic SCWO of phenol must appear in some form other than dimers or 3194
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In this section, we scrutinize the catalytic oxidation kinetics. Before investigating the reaction kinetics, however, we first applied the Mears and Weisz-Prater criteria (22) to determine whether there existed mass transfer limitations. Details concerning the application of these criteria to catalytic SCWO appeared in a previous paper (8). We found that all of the experiments had been done under conditions where the chemical reaction controlled the rate of phenol disappearance. To determine the effects of the concentrations of phenol, oxygen, and water individually on the reaction rate, we performed experiments wherein one concentration was varied and the other two were fixed at roughly constant values. Figures 4-6 display the results as plots of the phenol conversion (X) vs W/FA0. The derivative at any point on these curves is equal to the phenol disappearance rate because the mole balance equation for an isothermal, isobaric, plugflow packed-bed reactor is
dX rate ) d(W/FA0)
(1)
Figure 4 shows that the reaction rate and the phenol conversion at a given W/FA0 are enhanced by increasing the phenol concentration in the reactor feed. This behavior indicates that the global reaction order for phenol is greater than 0. Figure 5 shows that the phenol disappearance rate also increases with increasing oxygen concentrations. We therefore conclude that the global reaction order for oxygen is also greater than 0. Figure 6 shows that when the water concentration is increased from 9 to 30 mol/L (by changing the system pressure) with roughly constant phenol and oxygen concentrations, the phenol conversion at fixed W/FA0 does not appreciably change. Thus, the water concentration does not appear to affect the catalytic oxidation rate. This insensitivity to the water concentration was also observed in a previous phenol catalytic SCWO study (9). It is interesting then to note that the water density does influence the rate of noncatalytic SCWO of phenol (18). This striking difference regarding the role of water in catalytic and noncatalytic
FIGURE 5. Effect of oxygen feed concentration on phenol conversion during catalytic SCWO (T ) 380 °C, P ) 250 atm, [PhOH]0 ) 2.0 mmol/L).
FIGURE 6. Effect of water feed concentration on phenol conversion during catalytic SCWO (T ) 380 °C, [PhOH]0 ) 2.0 mmol/L, [O2]0 ) 27 mmol/L). oxidation rates may have mechanistic implications, but these are presently unclear. Phenomenological Rate Laws. Figures 4-6 show that the phenol disappearance rate is a function of the phenol and oxygen concentrations but not the water concentration. Accordingly, the global power-law rate equation is
rate ) -k[PhOH]a[O2]b
(2)
where k is the rate constant, which can be expressed in Arrhenius form k ) A exp(-Ea/RT), and a and b are the reaction orders for phenol and oxygen, respectively. [PhOH] is the concentration of phenol, and [O2] is the concentration of oxygen. Combining eqs 1 and 2, writing the phenol concentration as a function of conversion, taking the oxygen concentration to be conversion invariant, and solving the resulting differential equation leads to eq 3 as an explicit expression for the phenol conversion:
X ) 1 - (1 - (1 - a)k[PhOH] a0[O2] b0W/FA0)1/(1-a) if a *1 (3)
We used a nonlinear regression analysis to fit the experimental phenol conversions in Table 1 to eq 3 and thereby determine optimal values for the parameters a, b, A, and Ea in the power-law rate equation. Equation 3 applies only for cases where the O2 concentration is nearly constant in the reactor. Therefore, we used only the data from experiments with at least 100% excess oxygen in this kinetics analysis, which required the exclusion of 17 runs in Table 1. The reaction orders we obtained are a ) 0.69 (( 0.18) and b ) 0.22 (( 0.18). The values of the Arrhenius parameters A and Ea are 106.0(( 2.7) (M0.09 s-1 L g-1) and 135 ( 27 kJ/mol, respectively. The uncertainties reported here and elsewhere in this paper for power-law rate equations are the 95% confidence intervals. The global activation energy obtained here for catalytic SCWO of phenol over TiO2 is higher than the value of 52 kJ/mol determined previously (18) for the global activation energy for noncatalytic SCWO of phenol at comparable reaction conditions. A catalytic reaction having a higher activation energy than the corresponding noncatalytic reaction might appear to be a chemically troubling result, but it need not be. Such a result could occur if a rate-determining, low-activation-energy step in the noncatalytic reaction is replaced by a rate-determining, high-activation-energy step in the catalytic reaction. Differences in the preexponential factors could permit the catalytic rate to exceed the noncatalytic rate. Moreover, one must keep in mind that the kinetics parameters we report here are for a global, phenomenological rate law. The chemical significance of these parameters cannot be determined because the rate law does not correspond to any particular chemical mechanism. The global activation energy contains contributions from activation energies and reaction enthalpies for a large number of individual elementary steps. Without knowing the identities of these steps and how the effect of temperature on each step influences the global activation energy, there is no way to ascribe chemical meaning to the global activation energy. Therefore, comparing global activation energies may not be chemically meaningful. The main objective of a wastewater treatment technology is to convert organic compounds to CO2 completely. Thus, a rate law for CO2 formation is important for evaluating catalytic SCWO and possibly designing a catalytic SCWO reactor. Therefore, we have determined the global kinetics of CO2 formation during phenol oxidation in supercritical water over TiO2. The first step in the analysis was to determine the effects of the concentration of total organic carbon (TOC), O2, and water on the CO2 formation rate. We found that increasing the concentrations of TOC and O2 increased the rate, whereas changing the water density had a negligible effect on the rate. The quantitative analysis for the CO2 formation power-law rate equation is similar to that described above for the kinetics of phenol disappearance. The only difference is that the conversion, the feed concentration of phenol, and the phenol molar flow rate in equation 3 are replaced by the molar yield of CO2, the feed concentration of TOC, and the TOC molar flow rate, respectively. The experimental CO2 molar yields were then fit to this equation to find the optimal values for the parameters in the powerlaw rate equation for TOC disappearance. The nonlinear regression analysis led to a reaction order for oxygen of 0.034 ( 0.13, which is not statistically different from 0. Therefore, we took the oxygen reaction order to be 0 and performed a new nonlinear regression to determine the remaining parameters. The optimal values are a ) 0.51 (( 0.16), A ) 104.3(( 1.6) (M0.49 s-1 L g-1), and Ea ) 127 ((16) kJ/mol. Figure 7 is a parity plot that compares the experimental CO2 yields with those calculated from a rate law with the parameters above. All of the points in Figure 7 would fall on the diagonal line if the calculated values and the experimental data were VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Parity plot for power-law rate equation for CO2 formation.
TABLE 2. Comparison of CO2 Formation Rates for Catalytic SCWO of Phenol (250 atm, [PhOH]0 ) 1.4 mmol/L, [O2]0 ) 25 mmol/L) rate (mmol/kgcat-s)
T (°C)
bulk TiO2 (this work)
bulk MnO2 (9)
Carulite 150 (8)
380 400 420 440
0.11 0.23 0.45 0.83
0.29 0.56 1.04 1.86
62 132 245 419
in perfect agreement. It is clear that the power-law rate expression for CO2 formation provides a good correlation of the experimental data. The global activation energy of 127 kJ/mol obtained here for TOC disappearance during SCWO of phenol over TiO2 is statistically indistinguishable from the value of 135 kJ/mol obtained for the global activation energy for phenol disappearance. This near equality of the activation energies for these two processes will be surprising to some because it appears to be in opposition to the “refractory intermediate” concept popular in the wet oxidation literature. According to this concept, one expects the activation energy for TOC disappearance to be higher than that for phenol disappearance because the rate of TOC disappearance during wet oxidation is generally considered to be limited by the slow, highly activated oxidation of refractory intermediates. Previous work (11, 19) has shown, however, that this refractory intermediate hypothesis does not appear to be valid for the SCWO of phenol. Indeed, the global activation energy for phenol disappearance exceeds that for TOC disappearance even during noncatalytic SCWO (18, 19). The refractory intermediate concept, which appears to be generally applicable to wet oxidation, need not apply at the higher temperatures encountered during supercritical oxidation. Additionally, as we pointed our earlier in this paper, one must keep in mind that the activation energies we report are those for global, phenomenological rate laws. The chemical significance of the values of these parameters is unknown because the rate laws do not correspond to any particular chemical mechanisms. Table 2 compares the CO2 formation rates expected from phenol SCWO over TiO2, bulk MnO2, and Carulite 150, a commercial oxidation catalyst comprising MnO2 and CuO supported on amorphous Al2O3. The CO2 formation rates 3196
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given here for MnO2 and Carulite 150 differ from those in Table 2 of our previous report (9). These previous values were calculated incorrectly. We had used an erroneous rate constant for Carulite 150 and had misapplied the stoichiometric coefficient for CO2 when calculating the rate over MnO2. The preexponential factor in the CO2 formation rate law for MnO2 is 105.65 (M0.48 s-1 g-1), not 106.4 as reported previously (9). We regret not having identified these calculation errors sooner, but they have no effect on the conclusions drawn in our previous work. TiO2, MnO2, and Carulite 150 are the only catalysts for which CO2 formation rate laws have been published for phenol SCWO. The comparison in Table 2 shows that the CO2 formation rate associated with the commercial catalyst is much faster than that from bulk TiO2 and bulk MnO2. It is clear that Carulite 150 is much more active than TiO2 and MnO2 for catalytic SCWO of phenol and that TiO2 is the least active. The rates in Table 2 are expressed on a per mass of catalyst basis, whereas heterogeneous catalytic reactions occur on the surface of the catalyst. Therefore, it is instructive to consider the catalytic SCWO rates on a per unit surface basis. We measured the BET surface areas of the bulk TiO2 to be 12.1 m2/g, the bulk MnO2 used previously to be 6.6 m2/g, and the CARULITE 150 catalyst to be 241 m2/g. Using these specific surface areas and the rates in Table 2, we find that the CO2 formation rates at 380 °C are 2.6 × 10-4 for CARULITE 150, 4.4 × 10-5 for MnO2, and 9.3 × 10-6 mmol/m2-s for TiO2, respectively. Thus, even when considering rates on a surface area basis we still obtain the same relative activity ranking for these three materials. Mechanism-Based Rate Laws. The kinetics analysis performed to this point has used power-law rate expressions. Heterogeneous catalytic kinetics, however, are more often correlated by using Langmuir-Hinshelwood-HougenWatson (LHHW) rate laws. The LHHW rate law in eq 4 has previously provided a satisfactory correlation of experimental results for phenol disappearance during catalytic SCWO over other transition metal oxides (9, 14):
kK1K20.5[PhOH][O2]0.5 rate ) (1 + K1[PhOH])(1 + K20.5[O2]0.5)
(4)
Here k is the rate constant for the rate-determining, irreversible, surface reaction step involving adsorbed phenol and adsorbed oxygen atoms. K1 and K2 are the adsorption equilibrium constants for phenol and oxygen, respectively, on the catalyst surface. Phenol adsorbs on one type of site, and O2 adsorbs dissociatively on a distinctly different type of site. Substituting this rate law into the reactor mole balance equation (eq 1), writing the phenol concentration as a function of conversion, taking the oxygen concentration to be conversion invariant, and then solving the resulting differential equation leads to
W/FA0 )
1 + K20.5[O2]00.5 ln(1 - X) X0.5 0.5 K kK2 [O2]0 1[PhOH]0
(5)
This equation contains the phenol conversion as an implicit variable, so we used W/FA0, which is explicit, as the fitted variable in a nonlinear regression analysis to find numerical values for k, K1, and K2. The Arrhenius equation and the van’t Hoff equation (Ki ) Ki0e-∆Hi/RT) were used to correlate the effect of temperature on the rate constant and the adsorption equilibrium constants, respectively. Table 3 displays all of the Arrhenius and van’t Hoff parameters for the LHHW rate law (eq 4). In addition to LHHW mechanisms, the Mars-van Krevelen mechanism has also found frequent applicability for
TABLE 3. Parameters in LHHW Rate Law in Eq 4 for Phenol Disappearance during SCWO over TiO2 parameter
value
catalyst)-1 s-1)
108.8 191 100.3 33 10-3.4 57
A (mol (g of Ea (kJ/mol) K10 (L/mol) -∆H1 (kJ/mol) K20 (L/mol) -∆H2 (kJ/mol)
TABLE 4. Parameters in Mars-van Krevelen Rate Law in Eq 6 for Phenol Disappearance during SCWO over TiO2a parameter
value
A1 (L (g of catalyst)-1 s-1) E1 (kJ/mol) A2/β (L (g of catalyst)-1 s-1) E2 (kJ/mol) m
107.4(2.1 142 ( 27 104.9(4.5 128 ( 50 1.0 ( 0.4
a
Uncertainties reported in this table are the asymptotic standard errors.
hydrocarbon oxidation over transition metal oxides (23). This mechanism comprises two chemical reaction steps. The hydrocarbon molecule reacts with oxygen atoms at the surface of the catalyst with rate constant k1, and then the partially reduced surface is reoxidized by gas-phase oxygen with rate constant k2. According to this mechanism, lattice oxygen may (but is not required to) participate in the oxidation reaction. The lattice oxygen is then replenished by gas-phase oxygen. Equation 6 shows the rate law that results from the Mars-van Krevelen mechanism. β is a stoichiometric coefficient (moles of O2 required per mole of hydrocarbon) and m is the oxygen reaction order for the surface reoxidation reaction:
rate ) -
1 1 β + k1[PhOH] k2[O2]m
(6)
We are not aware of previous attempts to use Mars-van Krevelen kinetics to describe catalytic SCWO and therefore considered it worthwhile to do so. Combining this rate equation with the reactor mole balance, solving, and then rearranging leads to
W/FA0 )
ln(1 - X) β Xk1[PhOH]0 k2[O2]0m
(7)
We used nonlinear regression to fit the experimental data (W/FA0) to eq 7. Table 4 displays values for m and for the Arrhenius parameters associated with each rate constant. We note that our data led to m ) 1 as the optimized value of this exponent. A value of unity is common when fitting experimental data to this Mars-van Krevelen rate law (23). Note that the preexponential factor we report for k2 also includes the stoichiometric coefficient, β. We embedded this parameter within k2 because its value is not known a priori for phenol oxidation. The complete oxidation of phenol to form CO2 might not be due exclusively to heterogeneous catalytic reactions. It is likely that both heterogeneous and homogeneous reactions are important (9). Thus, the extent of oxidation that is heterogeneous and, consequently, the number of moles of oxygen needed to do this oxidation remain unknown. Thus far we have presented three different rate equations (power-law, LHHW, Mars-van Krevelen) for phenol disap-
FIGURE 8. Effect of time on stream on phenol conversion and CO2 molar yield (T ) 380 °C, P ) 250 atm, [PhOH]0 ) 1.9 mmol/L, [O2]0 ) 35 mmol/L, W/FA0 ) 5.2 kg of catalyst s-1 mmol-1). pearance during SCWO over bulk TiO2. The LHHW and Marsvan Krevelen rate laws are based on distinct chemical mechanisms whereas the power-law rate equation is entirely phenomenological. The power-law rate equation provided the best fit of the experimental phenol conversions, however. The sum of the squared residuals (Xcalc - Xexp)2 was 0.40 for this rate equation whereas it was 0.53 for the LHHW rate law and 0.46 for the Mars-van Krevelen rate law. Given that both the LHHW rate law and the Mars-van Krevelen rate law have roughly the same ability to describe the experimental results, we must conclude that the present kinetics results are incapable of discriminating between these two different mechanisms. One way to compare the activities of different SCWO catalysts with one another and with noncatalytic SCWO is to examine the effect of the different reaction rates on the reactor volumes required for treating a given waste stream. To make this comparison, we calculated the reactor volumes required to achieve 99.99% removal of phenol from a wastewater flowing at 20 g/min (at ambient conditions). The SCWO reaction conditions are 380 °C and 250 atm, and the concentrations of phenol and oxygen at reaction conditions at the reactor entrance were taken to be 2.0 and 30 mmol/L, respectively. The bed density for TiO2 is 1.4 g/cm3. The reactor volumes we obtained are 513 L with no catalyst (18), 132 L with bulk TiO2, 40 L with the Kranjc and Levec (14) catalyst, 24 L with bulk MnO2 (9), and 1.8 L with Carulite 150 (8) for SCWO. It is clear that catalytic SCWO requires much smaller reactor volumes and hence a lower capital investment for the reactor than noncatalytic SCWO. This finding is consistent with a published economic analysis of catalytic SCWO (7). It is also clear that, of the catalysts examined to date, the commercial, supported catalyst is the most active and bulk TiO2 is the least active.
Catalyst Deactivation To be applied in a commercial SCWO process, a heterogeneous oxidation catalyst must be stable and maintain its activity with use. Therefore, we conducted a time-on-stream study to investigate catalyst stability and activity maintenance during catalytic SCWO of phenol over TiO2. We evaluated the activity of TiO2 on the basis of phenol conversion and CO2 molar yield. We ran experiments using the same TiO2 catalyst in the reactor for over 120 h. At the end of this deactivation experiment, the used catalyst was recovered. Its specific surface area was measured as 3.2 m2/g, which is lower than that of the fresh material. VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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We attempted to maintain constant reaction conditions throughout the entire 5-day experiment, but there were some occasional deviations. Figure 8, which displays the results, shows that there are no apparent declines for the phenol conversion or CO2 molar yield with time on stream, which indicates no loss in activity for the TiO2 catalyst for 120 h run. Note that the phenol conversion and CO2 molar yield increased slightly during the last 2 days of the run. We believe these modest increases are due to the value of W/FA0 being slightly higher at the end of the run. In addition to examining the catalyst activity maintenance, we also examined catalyst stability by analyzing for Ti in the reactor effluent. The atomic absorption analysis revealed that if Ti was present in the effluent it was below the 2.6 ppm detection limit. Thus, Ti leaching out of TiO2 does not appear to be a problem when using TiO2 as a SCWO catalyst. All of these experimental results show that both the stability and activity of the TiO2 catalyst could be maintained for phenol SCWO for over 120 h operation.
Acknowledgments Financial support from the U.S. Department of Energy (DEFG22-95PC95213) is gratefully acknowledged.
Literature Cited (1) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41, 1723-1778. (2) Savage, P. E. Chem. Rev. 1999, 99, 603-621. (3) Caruana, C. M. Chem. Eng. Prog. 1995, April, 10-18. (4) Lyon, D. The Hazardous Waste Consultant 1999, 17 (1), A11A13. (5) McBrayer, R. N. The Hazardous Waste Consultant 1995, 13 (6), A4-A7. (6) Ding, Z.; Frisch, M. A.; Li, L.; Gloyna, E. F. Ind. Eng. Chem. Res. 1996, 35, 3257-3279.
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(7) Aki, S. N. V. K.; Abraham, M. A. Environ. Prog. 1998, 17, 246255. (8) Zhang, X.; Savage, P. E. Catal. Today 1998, 40, 333-342. (9) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1999, 38, 3793-3801. (10) Frisch, M. A. Catalyzed Supercritical Water Oxidation of Acetic Acid: Kinetics and Anatase Transformation. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1995. (11) Martino, C. J.; Savage, P. E. Environ. Sci. Technol. 1999, 33, 1911-1915. (12) Matatov-Meytal, Y. I.; Sheintuch, M. Ind. Eng. Chem. Res. 1998, 37, 309-326. (13) Krajnc, M.; Levec, J. Appl. Catal. B: Environ. 1994, 3, L101L107. (14) Krajnc, M.; Levec, J. Ind. Eng. Chem. Res. 1997, 36, 3439-3445. (15) Ding, Z.; Aki, S. N. V. K.; Abraham, M. A. Catalytic supercritical water oxidation; Hutchenson, K. W., Foster, N. R., Eds.; ACS Sympmposium Series 608; American Chemical Society: Washington, DC, 1995; pp 232-245. (16) Ding, Z.; Aki, S. N. V. K.; Abraham, M. A. Environ. Sci. Technol. 1995, 29, 2748-2753. (17) Oshima, Y.; Tomita, K.; Koda, S. Ind. Eng. Chem. Res. 1999, 38, 4183-4188. (18) Gopalan, S.; Savage, P. E. AIChE J. 1995, 41, 1864-1873. (19) Li, R.; Thornton, T. D.; Savage, P. E. Environ. Sci. Technol. 1992, 26, 2388-2395. (20) Thornton, T. D.; Savage, P. E. Ind. Eng. Chem. Res. 1992, 31, 2451-2456. (21) Thornton, T. D.; Douglas E. L., III; Savage, P. E. Environ. Sci. Technol. 1991, 25, 1507-1510. (22) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1998. (23) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991.
Received for review December 31, 1999. Revised manuscript received May 3, 2000. Accepted May 5, 2000. ES9914588
