1784
Ind. Eng. Chem. Res. 1999, 38, 1784-1791
KINETICS, CATALYSIS, AND REACTION ENGINEERING Oxidation and Thermolysis of Methoxy-, Nitro-, and Hydroxy-Substituted Phenols in Supercritical Water Christopher J. Martino and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
We have examined the oxidative decomposition of m- and p-methoxyphenol, m-, and pnitrophenol, and resorcinol and the nonoxidative decomposition of o-, m- and p-methoxyphenol in dilute aqueous solutions at 460 °C and 25.3 MPa for residence times on the order of 5 s. The major products from methoxyphenol decomposition in the absence of added oxygen are phenol and hydroxyphenols. The thermolysis kinetics can be described by a power-law rate equation with a reaction order between 0.5 and 1.0, which is consistent with previous studies done at different reaction conditions. Comparing the thermolysis rates for methoxyphenols with those of other substituted phenols showed that the rates are sensitive to both the identity and the location of the substituent. For a given substituent location, NO2-substituted phenols reacted more rapidly than either CHO- or OCH3-substituted phenols. Additionally, the ortho isomer was always the most reactive. Phenol is a product of incomplete oxidation from the methoxyphenols and nitrophenols, but no phenol was observed when resorcinol was oxidized. The oxidation kinetics were correlated with power-law rate expressions. The experiments and subsequent kinetics analysis allowed us to separate and quantify the rates of thermolysis and oxidation individually. A comparison of these reactant disappearance rates shows that thermolysis accounts for about 5% of the total rate for m- and p-methoxyphenol during oxidation in supercritical water. Thermolysis accounts for up to 25% of the total rate for m- and p-nitrophenols. Introduction Supercritical water oxidation (SCWO) is a process technology for the safe and complete destruction of organic compounds in aqueous waste streams. It combines an oxidant with the waste stream, and oxidation occurs at temperatures and pressures above the critical point of water (Tc ) 374 °C, Pc ) 22.1 MPa). The types of chemical reactions that can occur at SCWO conditions include pyrolysis, hydrolysis, and oxidation. The first two reactions proceed even in the absence of an added oxidant. If the rates of these nonoxidative reactions are sufficiently rapid, then the compounds that are ultimately oxidized are the hydrolysis and pyrolysis products of the original organic waste rather than the waste itself. An improved understanding of the rates and reaction networks of both the oxidative and nonoxidative (hereafter referred to as thermolysis) decomposition reactions of organic compounds in supercritical water (SCW) would facilitate the safe treatment of wastes by SCWO. The emphasis of early SCWO studies was on demonstrating the high destruction efficiencies that can be obtained with this process. Thomason et al.1 summarize many of the destruction efficiency studies, which simply provide the high conversions obtained at the (typically severe) conditions studied. More progress toward elucidating the rates and pathways of the reactions in* Corresponding author. E-mail:
[email protected]. Phone: (734) 764-3386. Fax: (734) 763-0459.
volved with SCWO and subsequently engineering systems to manipulate them, however, comes from studies of the reaction kinetics and the intermediate oxidation products. SCWO reaction kinetics can be used for reactor analysis2 and design. The identification of intermediate products and the development of reaction networks are required to assess the environmental impact of the SCWO process with respect to products of incomplete oxidation. This article provides primary reaction networks and kinetics for the disappearance of methoxy- (-OCH3), nitro- (-NO2), and hydroxy- (-OH) substituted phenols in supercritical water both with and without added oxygen. We also quantify and separate the contributions of the oxidative and nonoxidative (thermolysis) disappearance pathways. The compounds used in the investigation are substituted phenols, which are common pollutants in wastewaters in the chemical processing industry. The work reported herein is part of a broader comparative study3 of the reactivities of a large number of different substituted phenols in SCW. Since our primary interest is in comparing the reactivities of different compounds, we performed experiments with many different phenols but at only a single temperature of 460 °C. Background Phenol SCWO has been studied extensively,4-14 but the oxidation of substituted phenols has received much less attention. The literature provides kinetics for
10.1021/ie9805741 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/19/1999
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1785
only chlorophenols,14-17 cresols and hydroxybenzaldehydes,18-20 hydroquinone,21 and ethylphenols and hydroxyacetophenones.22 There have been no previous reports regarding the SCWO reactions of the substituted phenols that are the subject of this article. Although oxidation of these compounds in SCW has not been explored, the reactivity of these and related compounds in SCW in the absence of oxygen has been investigated, as noted in recent reviews.23-25 Both nitrophenols and methoxyphenols react in SCW even in the absence of oxygen. We previously reported19 that nitrophenols disappear within a few seconds in SCW at 460 °C and 25.3 MPa. Klein and co-workers26-28 reported reaction products, kinetics, and mechanisms for the disappearance of o-methoxyphenol in SCW at 383 °C and in the absence of added oxygen. They found that catechol (o-dihydroxybenzene) was the major product whereas phenol and o-cresol formed in smaller quantities. Klein et al.29 later reported on the kinetics and products of the thermolysis reactions of p-methoxyphenol in SCW. The major products were hydroquinone (p-dihydroxybenzene) and methanol, and the minor products were anisole (methoxybenzene) and phenol. These previous reports revealed that methoxyphenols react in SCW via parallel hydrolysis and freeradical pyrolysis pathways. The relative rates of the two paths depend on the water density, among other factors. The previous thermolysis studies of methoxyphenols in SCW were done near the critical temperature of water and with reaction times on the order of tens of minutes. The time scale must be on the order of a few seconds, however, for hydrolysis to be important for SCWO processes. Therefore, we deemed it necessary to revisit the issue of the reactivity of methoxyphenols in SCW at higher temperatures and shorter times in the absence of added oxygen. Experimental Section The organic reactants were obtained either from Aldrich Chemical Co. or Eastman Chemical Co. The nominal purities were at least 99% for o- and pnitrophenol, p-methoxyphenol, and m-hydroxyphenol, 98% for o- and m-methoxyphenol, and 97% for mnitrophenol. These nominal purities were taken into account when calculating concentrations in reactor feedstocks and calibrating analytical equipment. All experiments were conducted in a Hastelloy C-276 tubular flow reactor system designed such that two streams can be preheated and fed separately into the reactor. The reactor system contained separate 1/16-in. (1.59-mm) o.d. and 1.08-mm. i.d. preheat sections for aqueous feed streams containing the organic compound and oxygen. The lengths of these preheat lines were 1 m for the organic-containing feed and either 2 or 4 m for the oxygen-containing feed. Oxidation experiments required use of both of the reactor feed streams whereas thermolysis experiments required use of only the organiccontaining feed stream. The preheated feed streams were mixed together in a mixing tee, which also housed a thermocouple. The reactor temperatures reported herein are those measured at this point. Previous experiments30 showed that reaction outcomes were insensitive to the precise geometry of this mixing tee, which suggests that mixing was sufficiently rapid. The mixed streams immediately entered the reactor section, which was a 1-m length of 1/8-in. (3.18-mm.) o.d. and 1.40-mm i.d. tubing. The residence times for all experi-
Table 1. Results from SCWO Reactions of m-Nitrophenol at 460 °C and 25.3 MPa τ (s)
[m-NO2]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
Yphenol (%)
C tally (%)
0.90 0.93 0.95 1.21 1.22 1.26 1.33 1.39 1.81 1.99 2.54 2.99
102 187 56.3 92.5 123 77.3 41.8 26.5 57.7 88.7 81.9 41.5
7.90 7.17 7.26 8.34 9.36 4.94 3.31 10.62 7.08 8.89 9.32 3.30
30.5 22.1 25.8 32.8 55.2 34.3 38.2 67.4 53.5 47.3 62.5 57.1
15.7 9.5 n.d. 24.1 24.4 22.1 23.1 50.2 36.1 36.0 46.2 41.7
13.6 2.4 n.d. 22.5 4.4 15.8 14.3 11.4 26.6 28.9 22.1 14.3
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
99 90 n.d. 114 74 104 99 94 109 118 106 99
ments were calculated as the volume of this reactor section divided by the volumetric flow rate at reaction conditions. This approach provides a consistent basis for the residence time, but it is a lower bound for the thermolysis experiments because it neglects the amount of time the fluid spends in the preheat line as it reaches the reaction temperature. The preheat lines, mixing tee, and reactor coil were housed in an isothermal fluidized aluminum-oxide bath equipped with a temperature controller. The pressure and temperature were nominally constant at 25.3 MPa and 460 °C, respectively, during each reaction experiment. These reaction conditions fall within the range of normal SCWO operating conditions.1 Upon emerging from the reactor, the effluent was rapidly cooled in two consecutive water-cooled tube-intube heat exchangers and decompressed in a backpressure regulator. The exiting stream was then separated into gas and liquid phases (at ambient conditions) in a liquid trap. The gas flow rate was measured with a bubble meter at the outlet of the system, and the liquid flow rate was measured by collecting samples of known volume over a known period of time. Both the gas and liquid phases were analyzed chromatographically to determine their compositions. The reactor, the operating procedures, and the analytical protocol used in this study have been described in greater detail elsewhere.18,19 We typically examined 12 unique steady-state reaction conditions (combinations of residence time and initial concentrations) for each compound for oxidation experiments and six for thermolysis trials. The oxidation experiments involved varying the initial reactant concentration at reaction conditions, [organic]0, by about an order of magnitude, the initial oxygen concentration at reaction conditions, [O2]0, by at least a factor of 2, and the reactor residence time, τ, by at least a factor of 3. Similarly, the thermolysis experiments (no oxygen added to the reactor) explored ranges of reactant concentrations and residence times. Results Tables 1 and 2 display the results of the oxidation experiments for nitrophenols in supercritical water at 460 °C and 25.3 MPa. Results from thermolysis experiments with nitrophenols were previously reported.19 Tables 3-5 contain the results of the thermolysis and oxidation experiments with methoxyphenols. No oxidation experiments were performed for the ortho isomers of the methoxy- and nitrophenols because these isomers were extremely reactive even in the absence of added
1786 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 2. Results from SCWO Reactions of p-Nitrophenol at 460 °C and 25.3 MPa τ (s)
[p-NO2]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
Yphenol (%)
C tally (%)
0.94 0.68 0.68 0.93 1.00 1.20 1.21 2.01 2.02
179 92.0 184 71.8 96.4 86.1 48.5 42.7 183
7.30 3.92 7.12 5.28 5.03 5.97 6.83 9.90 9.90
38.9 16.9 28.6 23.6 21.1 23.6 32.2 55.0 67.1
22.1 37.3 10.5 45.0 6.4 8.8 14.4 24.1 25.7
21.0 1.7 7.7 11.0 0.6 0.6 10.1 3.9 3.9
4.2 0.0 3.9 0.0 6.2 7.2 0.0 16.5 19.1
108 122 93 132 92 93 92 90 82
oxygen. The fast thermolysis reactions made it difficult to acquire meaningful oxidation kinetics data. Table 6 provides experimental results from the SCWO of mhydroxyphenol (resorcinol) at 460 °C and 25.3 MPa. The uncertainties reported for the conversions in Tables 3-5 are the 95% confidence intervals (CI), which were determined through the collection and analysis of multiple samples for the same reactor steady state. No uncertainty range is reported for cases where multiple samples were not analyzed. The product yields (Yi) reported for the phenolic compounds in these tables are calculated as the molar flow rate of the product in the reactor effluent divided by the molar flow rate of the
reactant at the reactor entrance. The CO and CO2 molar yields are calculated similarly except that these were also divided by the number of carbon atoms in the reactant. This normalized CO2 molar yield will be 100% if all of the organic carbon fed to the reactor is converted to CO2. The carbon tally reported in Tables 1-6 is the percentage of carbon atoms fed to the reactor that appear in the quantified products in the reactor effluent. The carbon tally will be 100% if all products are identified and quantified. Carbon tallies falling short of 100% indicate that carbon-containing products in addition to those identified were formed. We could not analyze the gaseous products from the thermolysis experiments because the gas flow rate was too small to quantify. Additionally, we did not attempt to quantify the yields of nitrogen-containing gases from the experiments with nitrophenols. Finally, our analytical protocol did not permit quantification of the yields of methane, methanol, and formaldehyde, potential products from the decomposition of methoxyphenols. Having now presented the experimental data, we next perform a quantitative kinetics analysis and discuss the reaction pathways. The following sections use the data in Table 1-6 to establish reaction rate laws and to assemble reaction networks for the decomposition of the substituted phenols in supercritical water.
Table 3. Results from Reactions of o-Methoxyphenol in Supercritical Water at 460 °C and 25.3 MPa τ (s)
[o-OCH3]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
Yphenol (%)
Yo-CH3 (%)
Yo-OH (%)
Yp-OH (%)
C tally (%)
1.39 1.39 3.11 3.16 6.53 6.65
57.5 914 917 57.5 57.4 916
0.0 0.0 0.0 0.0 0.0 0.0
48.5 ( 5.9 26.2 ( 3.2 49.2 ( 1.5 57.3 ( 8.9 77.7 ( 8.7 72.1 ( 1.4
11.1 1.6 4.5 22.9 41.7 10.6
2.7 1.8 4.1 5.3 5.8 7.4
0.8 9.6 14.6 10.0 12.7 19.8
2.3 1.2 2.1 4.7 6.1 2.6
66 86 73 80 80 64
Table 4. Results from Reactions of m-Methoxyphenol in Supercritical Water at 460 °C and 25.3 MPa τ (s)
[m-OCH3]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
Ym-CH3 (%)
Ym-CHO (%)
Yo-OH (%)
Ym-OH (%)
Yp-OH (%)
C tally (%)
1.36 1.36 3.08 3.10 5.84 5.85 0.55 0.55 0.67 0.68 1.52 1.52 1.78 1.79
828 79.8 831 79.7 829 79.8 51.7 442 417 53.7 417 48.9 56.0 440
0.0 0.0 0.0 0.0 0.0 0.0 3.5 3.5 7.2 7.2 3.7 3.7 6.9 7.0
0.3 ( 2.4 5.1 ( 1.8 2.9 ( 3.4 6.6 ( 1.2 3.6 ( 1.8 7.0 ( 4.7 20.6 ( 3.5 1.9 ( 2.7 6.0 ( 2.4 30.8 ( 2.0 19.0 ( 2.5 50.4 ( 2.5 69.5 ( 1.5 32.7 ( 2.0
0.0 0.0 0.0 0.0 0.0 0.0 11.3 5.4 11.3 18.6 18.0 27.3 40.9 25.8
0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.3 0.7 0.6 1.2 1.5 2.7 1.9
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.0
0.1 0.2 0.2 0.4 0.3 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.3 0.0 0.0 0.2
0.2 0.3 0.5 0.5 1.1 1.3 0.6 0.3 0.5 0.2 0.8 0.8 0.8 0.7
0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.3 0.5 0.5 1.4 0.9 0.0 1.4
100 95 98 94 98 95 91 104 107 89 102 80 75 97
Table 5. Results from Reactions of p-Methoxyphenol in Supercritical Water at 460 °C and 25.3 MPa τ (s)
[p-OCH3]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
Yphenol (%)
Yp-CH3 (%)
Ym-CHO (%)
Yp-CHO (%)
Yo-OH (%)
Ym-OH (%)
Yp-OH (%)
C tally (%)
1.37 1.46 3.11 5.87 6.69 0.55 0.55 0.67 0.68 1.55 1.56 1.81 1.85
898 92.8 898 92.7 898 424 44.1 435 44.3 44.5 423 462 46.8
0.00 0.00 0.00 0.00 0.00 3.68 3.69 7.09 7.17 3.65 3.70 6.69 6.83
7.5 ( 2.2 20.5 ( 2.5 11.5 ( 1.3 34.6 ( 5.4 23.8 ( 2.6 38.6 ( 0.6 72.1 ( 3.0 53.4 ( 0.5 86.4 ( 3.9 100.0 ( 0.0 78.1 ( 0.8 89.5 ( 0.3 98.2 ( 3.3
0.0 0.0 0.0 0.0 0.0 10.8 33.1 18.5 39.7 49.2 30.9 43.2 59.0
0.0 0.0 0.0 0.0 0.0 1.4 2.6 2.3 4.4 6.7 5.1 6.2 8.2
0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.3 0.3 1.3 0.8 0.5 1.1
0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.6 0.0 0.0 0.3 0.2 0.0
0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.3 0.0 0.0 0.5 0.4 0.0
0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.2 0.2 0.1 0.4 0.3 0.1
0.0 0.0 0.0 0.0 0.0 0.8 2.3 0.6 2.4 1.5 0.6 0.4 1.7
0.0 0.0 0.0 0.0 0.0 0.3 0.2 0.3 0.4 0.5 0.4 0.3 0.4
0.0 0.0 0.0 0.0 0.0 7.9 13.0 8.6 14.9 11.7 10.2 8.5 7.8
92 80 88 65 76 83 77 77 73 69 69 69 79
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1787 Table 6. Results from SCWO Reactions of Resorcinol at 460 °C and 25.3 MPa τ (s)
[m-OH]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
C tally (%)
0.68 0.69 0.69 0.91 1.12 1.13 1.21 1.22 1.22 1.23 1.24 1.52 3.82 3.91 4.05 4.54
28.0 275 560 175 335 16.7 40.0 62.0 344 617 113 286 42.7 385 34.0 71.3
3.50 5.89 3.51 7.74 5.69 5.71 4.97 9.67 4.61 9.64 5.05 5.70 10.82 11.04 5.25 5.83
96.7 36.0 -0.3 49.5 25.5 94.5 97.9 82.9 85.4 19.0 54.5 51.8 98.0 96.5 98.6 99.9
82.9 26.8 8.7 36.0 28.4 90.6 75.1 57.4 18.8 22.1 36.3 28.0 67.6 68.7 86.4 80.2
4.2 1.6 0.4 2.8 1.6 0.0 6.9 4.6 1.4 1.6 2.7 2.5 9.7 7.4 10.7 10.9
90 92 109 89 105 96 84 79 35 105 85 79 79 80 99 91
Thermolysis Kinetics
o-OCH3 m-OCH3 p-OCH3
(1)
where kt is the thermolysis rate constant and at is the thermolysis reaction order for the substituted phenol. Combining this rate expression with the design equation for an isobaric, isothermal, plug-flow reactor, the mathematics of which have been described in detail elsewhere,18 leads to the expression below for the reactant conversion, X.
X ) 1 - (1 + (at - 1)10k′t[organic]0at-1τ)1/(1-at) for at * 1 (2) The subscript 0 denotes the reactor entrance, and τ is the reactor residence time. Note that here, and throughout this report, k′ ) log10 k. We used eq 2 along with the nonlinear regression capabilities of the SimuSolv software package31 to find the values of the parameters at and k′t that best represent the experimental data in Tables 3-5. The sum of the squared differences between the experimental and calculated conversions served as the objective function. Table 7 displays the resulting thermolysis rate parameters along with their uncertainties, reported here and elsewhere in this paper as the 95% confidence intervals. The rate has units of mole per liter per second and the concentration is in mol per liter at the reaction conditions. Additional details about the regression
log kt
at
-1.23 ( 0.33 -3.55 ( 1.05 -2.24 ( 0.63
0.82 ( 0.09 0.57 ( 0.28 0.72 ( 0.17
Table 8. Pseudo-First-Order Rate Constants for the Thermolysis of Substituted Phenols in SCW at 460 °C, 25.3 MPa, and [organic] ) 250 µmol/L substituent
k (103 s-1)
o-CHOa
202 ( 73 15.2 ( 3.6 8.0 ( 4.0 258 ( 32 10.4 ( 3.4 57.2 ( 11.5 460 ( 109 32.3 ( 8.8 117 ( 12
m-CHOa p-CHOa o-OCH3 m-OCH3 p-OCH3 o-NO2a m-NO2a p-NO2a a
In this section we focus on the thermolysis data for methoxyphenols, which are contained in the first several rows of Tables 3-5. Each of the methoxyphenols underwent appreciable decomposition even in the absence of added oxygen. Although m-methoxyphenol only had thermolysis conversions ranging up to 7.0%, o- and p-methoxyphenol displayed conversions from 26% to 78% and 7.5% to 35%, respectively, for thermolysis alone. The significant level of reaction for these two methoxyphenols is consistent with the work of Klein and colleagues,26,29 who found that methoxyphenols decomposed in SCW at lower temperatures and much longer times than those used here. For all three methoxyphenols the reactant conversions from thermolysis are sufficiently high that thermolysis rate laws can be determined. We cast the thermolysis kinetics of methoxyphenol disappearance in the form of the power-law expression in eq 1
rate ) kt[organic]at
Table 7. Parameters in Global Rate Law (eq 1) for the Thermolysis of Methoxyphenols at 460 °C
From ref 19.
statistics, including the variances and covariances for each of the regressed parameters, are available elsewhere.3 The thermolysis rate expressions in the form of eq 1 adequately describe the thermolysis data, but of course they severely underpredict the kinetics of the oxidation reactions. Note that the reaction orders, at, for the methoxyphenols are all within the range 0.51.0. This result is consistent with the mechanism-based rate law Lawson and Klein26 developed for o-methoxyphenol disappearance in SCW. Their rate law is of the form rate ) R[organic] + β[organic]1/2, where R and β are parameters that depend on temperature and the water density. The parameters in Table 7 can be used along with those obtained in our previous study19 of thermolysis in SCW to examine the relative decomposition kinetics of different substituted phenols in SCW in the absence of added oxygen. There are now rate laws available for the thermolysis of methoxyphenols, nitrophenols, and hydroxybenzaldehydes in SCW at 460 °C and 25.3 MPa. To compare the relative reactivity of these compounds, we calculated a pseudo-first-order rate constant from each of the rate laws. Table 8 shows these pseudo-firstorder rate constants evaluated at a substituted phenol concentration of 250 µmol/L. Several trends appear in the data in Table 8, and these trends are independent of the precise substituted phenol concentration used to calculate the pseudo-firstorder rate constant. First, we observe that for all three substituted phenols the reactivity is a strong function of the location of the substituent. Indeed the pseudofirst-order rate constants for a given substituted phenol vary by more than an order of magnitude depending on the substituent location. Second, we observe that, for all three substituted phenols, the ortho isomer is always the most reactive. Finally, we note that, for a given substituent location, NO2-substituted phenols react more rapidly than either CHO- or OCH3-substituted phenols. We can explain the relative reactivities of the three methoxyphenols (ortho > para > meta) by recognizing that decomposition involves cleavage of the O-CH3 bond as an important step and that the relative locations of the -OH and -OCH3 substituents have a large effect on the strength of that bond. The O-CH3 bond is strongest in the meta isomer, about 2.8 kcal/mol weaker
1788 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 9. Parameters in Global Rate Law (eq 3) for the Disappearance of Methoxy-, Nitro-, and Hydroxyphenols during SCWO at 460 °C and 25.3 MPa m-OCH3 p-OCH3 m-NO2 p-NO2 m-OH
log k
a
b
-1.38 ( 0.45 -1.37 ( 0.11 -0.37 ( 1.27 3.17 ( 1.44 -2.98 ( 0.72
0.49 ( 0.06 0.65 ( 0.03 0.73 ( 0.26 1.32 ( 0.20 0.42 ( 0.10
0.51 ( 0.17 -0.05 ( 0.02 0.56 ( 0.37 1.05 ( 0.46 -0.26 ( 0.32
in the para isomer, and about 7.5 kcal/mol weaker in the ortho isomer.32 Thus, the relative reactivities observed in the present study for these three methoxyphenols (ortho > para > meta) is fully consistent with these relative O-CH3 bond dissociation energies. Disappearance Kinetics with Added Oxygen In this section we focus on the data in Tables 1, 2, and 4-6 obtained from experiments with added oxygen. Our goal here is to correlate these data for reactant disappearance by using the global power-law expression in eq 3
rate ) k[organic]a[O2]b
(3)
where k is the rate constant and a and b are the reaction orders. Note that this disappearance rate includes contributions from both oxidation and thermolysis. Note too that the reaction orders in eq 3 are simply parameters that will be determined by fitting experimental data. They should not be expected to embody mechanistic significance. We developed an expression for the reactant conversion, eq 4, by combining eq 3 with the design equation for a plug-flow reactor and then solving the resulting differential equation. We also wrote the organic concentration in terms of the conversion and took the oxygen concentration to be conversion-invariant because oxygen is always present in large stoichiometric excess. b 1/(1-a) X ) 1 - (1 + (a - 1)10k′[organic]a-1 0 [O2]0τ) for a * 1 (4)
We used eq 4 along with the nonlinear regression routines in SimuSolv31 to find the values of the parameters a, b, and k′ that best correlate the experimental reactant conversions. Table 9 displays the resulting parameters for the disappearance rates along with their 95% confidence intervals. Martino3 provides complete details about the regression protocol and statistics, including the variances and covariances for each of the regressed parameters. Having developed rate equations for reactant disappearance in both the absence and presence of added oxygen, we can now determine the extent to which the thermolysis reactions contribute to the overall kinetics of reactant disappearance for these substituted phenols during SCWO. We calculated the thermolysis contribution as the thermolysis rate from eq 1 divided by the total disappearance rate (from oxidation and thermolysis) from eq 3. Table 10 contains these calculated values for the thermolysis contribution at 460 °C, 25.3 MPa, and concentrations of the substituted phenol and oxygen representative of those used experimentally. We calculated the uncertainties in Table 10 from the propagation of errors formula.20 One needs the variances and covariances of the parameters in the oxidation and ther-
Table 10. Portion of the Total Reactant Disappearance Rate at SCWO Conditions Attributable to Thermolysis (Reactions Rates Are Compared at 460 °C, 25.3 MPa, [organic] ) 100 µmol/L, and [O2] ) 7 mmol/L)
a
compound
thermolysis contribution (%)
o-CHOa m-CHOa p-CHOa m-OCH3 p-OCH3 m-NO2 p-NO2
19.6 ( 8.5 22.0 ( 7.8 7.5 ( 4.2 4.1 ( 1.4 5.1 ( 1.1 14.0 ( 5.2 25.8 ( 4.2
Calculated from rate laws in refs 19 and 20.
molysis rate expressions to complete this calculation. Martino3 provides these data along with more details regarding this propagation of errors calculation. It is evident from the data in Table 10 that thermolysis reactions only account for about 5% of the total reactant disappearance rate for m- and p-methoxyphenols when an oxidant is present. We also see that thermolysis reactions likewise account for only a small portion of the reaction rate for SCWO of p-hydroxybenzaldehyde. It is also clear, however, that thermolysis reactions do contribute significantly to the total rate of disappearance of the other hydroxybenzaldehydes and the nitrophenols. For these compounds, the thermolysis reactions account for about 14-26% of the reactant disappearance rate that occurs in SCWO at the conditions investigated here. Recall too that thermolysis reactions are very rapid for the ortho isomers of methoxy- and nitrophenols. These compounds reacted so quickly in SCW in the absence of an added oxidant that we did not attempt to determine oxidation rate laws. Oxidation Kinetics The rate equations in the previous section describe the disappearance of reactant through both thermolysis and oxidation. Because the thermolysis rates are fast for several of these compounds, we desired to separate the contributions from thermolysis and oxidation so that we could determine the kinetics of the oxidation reaction alone. Therefore, we developed a rate expression that has the ability to account simultaneously, but separately, for both thermolysis and oxidation reactions occurring in a SCWO reactor. To accomplish this separation, we write the total rate of disappearance of the phenolic compound as the sum of the rate of thermolysis plus the rate of oxidation.
rate ) ratethermolysis + rateoxidation
(5)
We again used power-law kinetics for the rate equations. The governing differential equation for the plug-flow reactor system then becomes
-
d[organic] ) kt[organic]at + ko[organic]ao[O2]bo (6) dτ
The subscripts t and o on the kinetics parameters signify the thermolysis and oxidation contributions to the rate expression, respectively. The parameters kt and at have already been determined as shown in Table 7. Thus, the task at hand now is to determine values for the parameters ko, ao, and bo. To this end, we used a fourth-order Runge-Kutta method with 100 steps to integrate the governing differential equation numerically,33 and we performed
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1789 Table 11. Parameters in Separated Global Rate Laws (eq 6) for Oxidation and Thermolysis of Substituted Phenols (Values for kt and at for Methoxyphenols Are from Table 7 and the Values for Nitrophenols Are from ref 19) m-OCH3 p-OCH3 m-NO2 p-NO2
log kt
at
log k0
ao
bo
-3.55 -2.24 -2.82 -0.40
0.57 0.72 0.63 1.15
-1.34 -1.33 -0.09 4.14
0.49 0.63 0.75 1.38
0.54 0.00 0.68 1.45
the parameter estimation simultaneously using the Solver routine in a Microsoft Excel spreadsheet. The objective function that was minimized was the sum of the squares of the differences between the calculated and experimental reactant conversions. Thus, the parameters kt and at were fixed at the values contained in Table 7, and the experimental data in Tables 1, 2, 4, and 5 were then used to estimate values of ko, ao, and bo in eq 6 for nitro- and methoxyphenols. The final estimates for ko, ao, and bo did not differ significantly whether we held the thermolysis rate parameters constant or performed regressions, letting them vary freely. Table 11 displays the results of the parameter estimation protocol. Reaction Products and Pathways In this section we discuss the quantitative data for the product yields and use it to draw inferences regarding the reaction networks for the decomposition of methoxy-, nitro-, and hydroxyphenols in supercritical water. Methoxyphenols. Only o- and m-methoxyphenols exhibited yields of thermolysis products sufficiently high to quantify. o-Methoxyphenol produced the richer product spectrum. Consistent with the results of Lawson and Klein,26 we find that the major thermolysis products are o-hydroxyphenol (catechol) and phenol. o-Cresol and p-hydroxyphenol also formed, but in lower yields. Interestingly, the phenol yield exceeds the o-hydroxyphenol yield in the experiments with low o-methoxyphenol concentrations in the reactor feed, but the relative
amounts of these two products are reversed when the o-methoxyphenol concentration in the reactor feed was high. That o-hydroxyphenol is formed in higher yields at the higher o-methoxyphenol concentrations is consistent with previous results. Lawson and Klein26 used initial concentrations more than 2 orders of magnitude higher than those used in this study and found ohydroxyphenol in much higher yields than phenol. Both of these major products are likely to be primary products that form from methoxyphenol in parallel pathways. This observation, coupled with the observed effect of the reactant concentration on the product yields, suggests that the rate laws for these two paths must have different reaction orders with respect to the reactant. The molar yields of the products from SCWO of methoxyphenols appear in Figures 1 and 2 as a function of conversion. It is clear that oxidation of the methoxyphenols led to CO2 yields that always greatly exceeded the CO yields and the yields of the aqueous-phase products. That CO2 is the most abundant gas-phase product, even at the lowest conversions studied, is fully consistent with previous studies for the SCWO of phenol8 and other substituted phenols.16,20 The high CO2 yields relative to CO indicate that CO2 formation is not predominantly via CO oxidation. Rather, it is more likely that decarboxylation of carboxylic acid intermediate products is responsible for the high CO2 yields. The major aqueous-phase products from the oxidation of methoxyphenols are dihydroxybenzenes, and all three isomers were typically present among the products regardless of the specific methoxyphenol isomer used in the experiment. On the basis of our experience with dihydroxybenzenes, the isomers of which appear to undergo facile interconversion, we believe that only one of the dihydroxybenzene isomers is a primary product and the others are formed from isomerization. Combining the information above, we offer Figure 3 as a description of the major primary pathways for methoxyphenol oxidation in SCW. The fastest path is ring opening and CO2 evolution, but the other paths to dihydroxybenzene and phenol are also operative. In-
Figure 1. Molar yields of products from SCWO of m-methoxyphenol at 460 °C and 25.3 MPa.
Figure 2. Molar yields of products from SCWO of p-methoxyphenol at 460 °C and 25.3 MPa.
1790 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 3. Primary reaction paths for SCWO of methoxyphenols.
Figure 4. Primary reaction paths for SCWO of nitrophenols.
deed, these latter two paths are important even in the absence of oxygen. Ring opening of the methoxyphenol itself must occur and it must be the fastest path because the oxidation rates of the primary products (phenol and hydroxyphenols) are not sufficiently rapid to produce the high CO2 yields observed. Nitrophenols. Martino and Savage19 found that thermolysis of all three nitrophenols produced phenol as the major aqueous-phase product. The present study shows that phenol was also the main aqueous-phase product from p-nitrophenol oxidation. Interestingly, the phenol yields from oxidation were not much higher than they were from thermolysis, even though the nitrophenol conversion was much higher. These results imply that most of the phenol formed in the oxidation experiments came from the thermolysis pathways, and that the higher conversions were obtained because the presence of added oxygen made the oxidation paths, which eventually produce CO and CO2, available. As shown in Tables 1 and 2, SCWO of nitrophenols produced CO2 as the major product. As noted above, this behavior has been observed for all other substituted phenols studied to date. Unlike other substituted phenols, however, SCWO of nitrophenols often produced yields of CO that were comparable to the yields to CO2 (compare the CO and CO2 yields in Tables 1 and 2 with those for methoxyphenols in Figures 1 and 2). This higher relative yield of CO from the oxidation of nitrophenols highlights an intriguing difference in the SCWO behavior of nitrophenols. Figure 4 is a summary of the primary reaction paths for nitrophenol SCWO. Hydroxyphenols. There are no significant liquidphase products evident from the oxidation of resorcinol. Neither liquid or gas chromatographic analyses revealed product peaks other than those that can be attributed to small quantities of the other dihydroxybenzene isomers. In contrast to the behavior of the other substituted phenols discussed above, SCWO of resorcinol did not produce phenol. Rather, SCWO of resorcinol appears to form ring-opening products that then rapidly react further to form CO and CO2. CO2 is formed in high yields, with the reactions of greater than 50% conversion having CO2 yields from 19% to 91%. Summary and Conclusions This work extends current knowledge about the SCWO reaction kinetics and byproducts for phenolic
pollutants in wastewaters. Herein, we reported quantitative rate laws for the disappearance of methoxy-, nitro-, and hydroxyphenols via both thermolysis and oxidation reaction paths in supercritical water at 460 °C. Both methoxyphenols and nitrophenols are reactive at SCWO temperatures and time scales even in the absence of added oxygen. Results from the thermolysis experiments revealed that, for a given substituent position, the nitrophenol is more reactive than the corresponding methoxyphenol. Additionally, for both nitro- and methoxyphenols, the ortho isomer is the most reactive. Thermolysis accounts for about 5% of the total rate for SCWO of m- and p-methoxyphenols, but it accounts for up to 25% of the rate for m- and pnitrophenols at 460 °C. All of the oxidation experiments for methoxy-, nitro-, and hydroxyphenols produced CO2 as the product with the highest yield. The CO2 and CO yields from the oxidation of nitrophenols were often comparable, however, whereas the CO2 yield was always at least a factor of 6 greater than the CO yield for the methoxyphenols and resorcinol. Aqueous-phase products of incomplete oxidation included dihydroxybenzenes, and in the case of methoxyphenols and nitrophenols, phenol. There are three parallel primary paths for SCWO of methoxyphenols. These lead to ring-opening products, dihydroxybenzenes, and phenol. The first path is the fastest. There are two major parallel primary paths for SCWO of nitrophenols. One leads to phenol and the other to ring-opening products and ultimately CO and CO2. Acknowledgment This work was supported by grants from the U. S. Department of Energy University Coal Research Program (DE-FG22-92PC92536) and the National Science Foundation (CTS-9521698, CTS-9311300). Literature Cited (1) Thomason, T. B.; Hong, G. T.; Swallow, K. C.; Killilea, W. R. The MODAR Supercritical Water Oxidation Process. In Thermal Processes: Innovative Hazardous Waste Treatment Techchnology Series, Vol. 1; Freeman, H. M., Ed.; Technomic Publishing Co.: Lancaster, PA, 1990; Sec. 1.3. (2) Oh, C. H.; Kochan, R. J.; Charlton, T. R.; Bourhis, A. L. Thermal-Hydraulic Modeling of Supercritical Water Oxidation of Ethanol. Energy Fuels 1996, 10, 326-332. (3) Martino, C. J. Supercritical Water Oxidation of Monosubstituted Phenols: A Comparative Study of Reaction Kinetics and Products. Ph. D. Thesis, University of Michigan, Ann Arbor, MI, 1997. (4) Thornton, T. D.; Savage, P. E. Phenol Oxidation in Supercritical Water. J. Supercrit. Fluids 1990, 3, 240. (5) Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1992, 38, 321. (6) Thornton, T. D.; Savage, P. E. Phenol Oxidation Pathways in Supercritical Water. Ind. Eng. Chem. Res. 1992, 31, 2451-2456. (7) Thornton, T. D.; LaDue, D. E.; Savage, P. E. Phenol Oxidation in Supercritical Water: Formation of Dibenzofuran, Dibenzo-p-dioxin, and Related Compounds. Environ. Sci. Technol. 1991, 25, 1507. (8) Gopalan, S.; Savage, P. E. A Reaction Network Model for Phenol Oxidation in Supercritical Water. AIChE J. 1995, 41, 1864-1873. (9) Koo, M.; Lee, W. K.; Lee, C. H. New Reactor System for Supercritical Water Oxidation and its Application on Phenol Destruction. Chem. Eng. Sci. 1997, 52, 1201-1214. (10) Krajnc, M.; Levec, J. On the Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1996, 42, 1977-1984. (11) Rice, S. F.; Steeper, R. R. Oxidation Rates of Common Organic Compounds in Supercritical Water. J. Hazard Mater. 1998, 59, 261-278.
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1791 (12) Oshima, Y.; Hori, K.; Toda, M.; Chommanad, T.; Koda, S. Phenol Oxidation Kinetics in Supercritical Water. J. Supercrit. Fluids 1998, 13, 241-246. (13) Wightman, T. J. Studies in Supercritical Wet Air Oxidation. M. S. Thesis, University of California, Berkeley, Berkeley, CA, 1981. (14) Li., R.; Thornton, T. D.; Savage, P. E. Kinetics of CO2 Formation from the Oxidation of Phenols in Supercritical Water. Environ. Sci. Technol. 1992, 26, 2388. (15) Yang, H. H.; Eckert, C. A. Homogeneous Catalysis in the Oxidation of p-Chlorophenol in Supercritical Water. Ind. Eng. Chem. Res. 1988, 27, 2009. (16) Li, R.; Savage, P. E.; Szmukler, D. I. 2-Chlorophenol Oxidation in Supercritical Water: Global Kinetics and Reaction Products. AIChE J. 1993, 39, 178. (17) Lin, K. S.; Wang, H. P.; Li, M. C. Oxidation of 2,4Dichlorophenol in Supercritical Water. Chemosphere 1998, 36, 2075-2083. (18) Martino, C. J.; Kasiborski, J.; Savage, P. E. Kinetics and Products from o-Cresol Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1995, 34, 1941-1951. (19) Martino, C. J.; Savage, P. E. Thermal Decomposition of Substituted Phenols in Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 1385-1390. (20) Martino, C. J.; Savage, P. E. Supercritical Water Oxidation Kinetics, Products, and Pathways for CH3- and CHO-Substituted Phenols. Ind. Eng. Chem. Res. 1997, 36, 1391-1400. (21) Thammanayakatip, C.; Oshima, Y.; Koda, S. Inhibition Effect in Supercritical Water Oxidation of Hydroquinone. Ind. Eng. Chem. Res. 1998, 37, 2061-2063. (22) Martino, C. J.; Savage, P. E. Supercritical Water Oxidation Kinetics and Pathways for Ethylphenols, Hydroxyacetophenones, and other Monosubstituted Phenols. Ind. Eng. Chem. Res. 1999, 38, 1775-1783. (23) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Fundamentals and Applications. AIChE J. 1995, 41, 1723-1778.
(24) Savage, P. E. Organic Chemical Reactions in Supercritical Water. Chem. Rev. (in press). (25) Katritzky, A. R.; Allin, S. M.; Siskin, M. Aquathermolysis: Reactions of Organic Compounds with Superheated Water. Acc. Chem. Res. 1996, 29, 399-406. (26) Lawson, J. R.; Klein, M. T. Influence of Water on Guaiacol Pyrolysis. Ind. Eng. Chem. Fundam. 1985, 24, 203-208. (27) Townsend, S. H.; Abraham, M. A.; Huppert, G. L.; Klein, M. T.; Paspek, S. C. Solvent Effects during Reactions in Supercritical Water. Ind. Eng. Chem. Res. 1988, 27, 143-149. (28) Huppert, G. L.; Wu, B. C.; Townsend, S. H.; Klein, M. T.; Paspek, S. C. Hydrolysis in Supercritical Water: Identification and Implications of a Polar Transition State. Ind. Eng. Chem. Res. 1989, 28, 161-165. (29) Klein, M. T.; Mentha, Y. G.; Torry, L. A. Decoupling Substituent and Solvent Effects during Hydrolysis of Substituted Anisoles in Supercritical Water. Ind. Eng. Chem. Res. 1992, 31, 182-187. (30) Brock, E. E. Ph. D. Thesis, University of Michigan, Ann Arbor, MI, 1997. (31) Steiner, E. C.; Rey, T. D.; McCroskey, P. S. SimuSolv Modeling and Simulation Software Reference Guide; The Dow Chemical Co.: Midland, MI, 1990. (32) Suryan, M. M.; Kafafi, S. A.; Stein, S. E. The Thermal Decomposition of Hydroxy- and Methoxy-Substituted Anisoles. J. Am. Chem. Soc. 1989, 111, 1423-1429. (33) Carnahan, B.; Luther, H. A.; Wilkes, J. O. Applied Numerical Methods; John Wiley & Sons: New York, 1969.
Received for review September 8, 1998 Revised manuscript received November 30, 1998 Accepted December 17, 1998 IE9805741
