The Effects of Mixing and Oxidant Choice on Laboratory-Scale

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GENERAL RESEARCH The Effects of Mixing and Oxidant Choice on Laboratory-Scale Measurements of Supercritical Water Oxidation Kinetics Brian D. Phenix,† Joanna L. DiNaro, Jefferson W. Tester,* Jack B. Howard, and Kenneth A. Smith Department of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room E40-479, Cambridge, Massachusetts 02139

The use of laboratory-scale equipment to measure intrinsic oxidation kinetics in supercritical water environments was evaluated in this study. The objectives were two-fold: (1) to compare the use of hydrogen peroxide with dissolved oxygen as an oxidant and (2) to characterize the dynamics and intensity of mixing organic reactant and oxidant streams. Methanol was used as the model organic as the oxidation rate exhibits a first-order dependence according to extensive earlier studies. No statistically significant difference was observed in the reaction rates or product distributions for the use of either dissolved oxygen gas or hydrogen peroxide that was preheated and fully decomposed before mixing with methanol at supercritical water conditions (500 °C, 246 bar). The intensity of mixing was shown to be an important factor in determining effective mixing times for the reactant and oxidant. Although hydrodynamic effects are certainly dependent on the design and geometry of the mixing tee in the reactor system, fully turbulent (Re > 10 000) cross-flow between entering oxidant and organic streams was found to reduce mixing times to 1 s or less. Introduction Supercritical water oxidation (SCWO) typically refers to a waste treatment or remediation process that derives its effectiveness from the unique solvent properties of water at conditions well above its critical point of 221 bar and 374 °C. When organic compounds and oxygen are brought together in supercritical water (SCW), the oxidation of the organic is rapid and complete to carbon dioxide and water. Heteroatoms such as Cl, S, and P are converted to their corresponding mineral acids (HCl, H2SO4, and H3PO4), which can be neutralized by using a suitable base to produce salts of relativity low solubility at supercritical conditions. If any organic nitrogen is present the resulting product is primarily molecular N2 with some N2O.1 NOx gases, typical undesired byproducts of combustion processes, are not formed because the temperature is too low for these oxidation pathways to be favored. Practical SCWO processes usually operate in the ranges 450-600 °C and 250280 bar. Detailed reviews of the technology are available from Modell,2 Tester et al.,3 Gloyna and Li,4 and Tester and Cline.5 Our research effort emphasizes the investigation of the hydrolysis (reaction in the absence of oxygen) and oxidation of simple organic compounds in supercritical water. Certain specific organic compounds, referred to as “model compounds”, were selected for study because they either are relatively refractory intermediates that are produced in the oxidation of more complex compounds, are simulants for hazardous waste compounds, * To whom correspondence should be addressed. Tel.: 617253-7090. Fax: 617-253-8013. E-mail: [email protected]. † Current address: Merck and Company, Rahway, NJ.

represent wide classes of organic wastes, or are themselves characteristic waste compounds. To that end, comprehensive kinetic studies involving measurements of the oxidation and hydrolysis rates have been performed at MIT on carbon monoxide,6-10 ethanol,11 ammonia,7,12,13 methane,14 methanol,12,15,16 hydrogen,9,10,17 glucose,18 acetic acid,19 thiodiglycol,20 methylene chloride,21-24 benzene,25-27 and methyl tert-butyl ether (MTBE).28,29 Many of these model compounds exhibited overall first-order kinetic behavior under oxidative conditions. Detailed reviews by Savage et al.30 cover reactions in supercritical fluids, including water. Siskin and Katritzky31 have also published a comprehensive review of reactions in superheated water that provides a valuable analysis and evaluation of important reactivity factors of relevance to this work. The present study was undertaken to provide a better understanding of the differences between SCWO kinetic data measured in our reactor system and those measured elsewhere. Although other investigators have reported the equivalence of using pure oxygen or decomposed peroxide as the oxidant, the evidence has been mostly anecdotal, with little or no published data showing side-by-side comparisons under controlled conditions. Furthermore, other comparisons of kinetic data on oxidation of the same model compound in different laboratories with different experimental setups have revealed wide differences in the observed kinetics. At the outset, it became clear to us that there would be value in quantitatively characterizing the effects of oxidant choice and mixing of reactant and oxidant on observed rates in a single apparatus under well-defined conditions.

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Two factors were investigated separately in this study: the use of hydrogen peroxide as an alternative oxidant to dissolved oxygen and the influence of reactant and oxidant mixing times on the observed rates of reaction. Experimental Apparatus All experiments were conducted in a modified version of the bench-scale, tubular plug-flow reactor that has been described in detail by Holgate and Tester17 and Marrone et al.21 Briefly, separate aqueous organic and oxidant feed solutions were prepared. The organic and oxidant feeds were pressurized to reactor pressure and delivered to the reactor system via two independent digital HPLC pumps (Rainin, SD-200), which replaced the older model pumps (LDC Analytical, minipump model 2396) used by previous investigators at MIT. The pressurized organic and oxidant feeds were separately preheated to the operating temperature. The preheater system consists of a direct ohmic preheating section followed by a preheating coil located in a fluidized sand bath (Techne, FB-08) that houses the reactor. The direct ohmic heating (DOH) system replaced the preheater sand bath used in earlier studies in our laboratory.17,21 Heating in the DOH section is accomplished by the application of a voltage across independent 9.5-m lengths of 1/16-in. (1.6-mm) o.d. × 0.01-in. (0.25-mm) wall Hastelloy (HC-276) tubing. Approximately the last 0.5 m of the 9.5-m DOH preheating coil is traced with heating tape. A 30-cm length of tubing connects the DOH preheater section to the 5.2-m coiled length of 1/16-in. (1.6-mm) o.d. × 0.01-in. (0.25-mm) wall HC-276 tubing in the sand bath. This 30-cm length of tubing is wrapped with resistive cable heaters [Watlow, p/n 62H24A6X, 1/16-in. (1.6-mm) i.d. × 2-ft (61-cm) length, 10 V, 240-W maximum] to prevent heat loss. The aqueous organic and oxidant feeds are mixed in a modified 1/8-in. (3.2-mm) HC-276 cross from HighPressure Equipment (p/n 60-24HF2) at the reactor entrance. This cross was designed during the course of this study to minimize the mixing time. The organic and oxidant feeds enter through separate arms of the cross. A thermocouple is seated in the third arm, and the reactor is attached to the fourth. The reactor is a 4.71-m coiled length of Inconel 625 tubing [1/4-in. (6.35-mm) o.d. × 0.067-in. (1.7-mm) i.d.] with an internal volume of 10.71 cm3. A thermocouple is seated in a 1/8-in. (3.2-cm) HC-276 tee at the reactor exit. A 26-cm length of insulated HC-276 tubing [1/4-in. (6.35-mm) o.d. × 1/16in. (1.6-mm) i.d.] rises out of the sand and connects the reactor to a shell-and-tube heat exchanger. A springloaded, manual back-pressure regulator (Tescom, p/n 26-3200) controls the system pressure. Upon passing through the back-pressure regulator, the effluent is flashed to atmospheric pressure, and the two-phase effluent is separated in a gas-liquid separator. Gas samples are taken from a sampling port with a syringe, and the flow rate of the gas stream is measured using a soap-bubble flowmeter and a stopwatch. Liquid samples are collected from the liquid effluent line, and the flow rate is measured using a class A volumetric flask and a stopwatch. Part I. Hydrogen Peroxide as an Alternative Oxidant Background. A dissolved oxygen solution was used as the oxygen source in all previous MIT SCWO studies

(e.g., see Holgate and Tester17). Because of the limited solubility of oxygen in water, the maximum attainable dissolved oxygen concentration is about 3930 ppm at ambient temperature, given the 125-bar pressure rating of the oxygen saturator used to prepare the dissolved oxygen solution. To realize higher concentrations of oxygen in the reactor, hydrogen peroxide was explored as an alternative oxidant. The use of hydrogen peroxide as an oxidant is based on the assumption of complete decomposition of the hydrogen peroxide to oxygen and water (H2O2 f 1/2O2 + H2O). Researchers at Sandia National Laboratories (SNL) were the first to use hydrogen peroxide as the source of oxygen in an SCWO system, but its use has since been adopted by other research groups (e.g., see Brock et al.32 and Krajnc and Levec33), including our own. Whereas the assumption of complete breakdown of hydrogen peroxide to oxygen and water was not tested experimentally at SNL, both Brock et al.32 and Krajnc and Levec33 did verify complete decomposition in their own reactor systems. An investigation was undertaken here to validate oxygen delivery by hydrogen peroxide in our bench-scale, tubular plug-flow reactor by comparing methanol oxidation rates measured using hydrogen peroxide and dissolved oxygen. These tests also served to demonstrate the absence of any kinetic effects due to the presence of long-lived hydroxyl or peroxy radicals. A careful distinction must be made between the use of hydrogen peroxide as an oxygen source and as a primary oxidant. The use of hydrogen peroxide as a rate enhancer was explored in the oxidation of 2,4-dichlorophenol and acetic acid.34 In separate experiments, hydrogen peroxide and oxygen were premixed with the organics in batch reactors. The premixed solutions were then heated to 400-500 °C. Under comparable conditions, the conversions of both compounds were higher with hydrogen peroxide than with oxygen. This finding is not surprising given the premixing of the organics with this strong oxidizer. A more recent study of the effect of hydrogen peroxide on SCWO oxidation rates was carried out by Bourhis et al.35 In these experiments, a cold water feedstream was spiked with hydrogen peroxide at a concentration of 0.75-3 wt % and mixed with a pure organic waste stream. Oxidation was initiated when this mixture was combined with a SCW/ air stream in a 6.2-m × 0.925-cm i.d. tubular reactor. The hydrogen peroxide concentration never exceeded 5% of the stoichiometric oxygen requirement. The extent of reaction was inferred by measuring the axial temperature rise along the outer surface of the reactor and monitoring the CO levels in the effluent. The addition of small amounts of hydrogen peroxide was found to significantly raise the temperature profile down the length of the reactor, leading to the conclusion that the rate of oxidation must have been increased. The purpose here is not to exploit the rate-enhancing properties of hydrogen peroxide but instead to ensure complete breakdown of hydrogen peroxide to oxygen and water in the preheater and to verify the absence of any residual kinetic effects due to radical persistence. Experiments measuring the decomposition rate in SCW at SNL led to the development of the following rate expression36

koverall (s-1) ) kh (s-1) + kw (cm s-1) × (S/V) (cm-1) (1) where koverall is the overall first-order rate constant for

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Table 1. Results of Oxygen Evolution Experiments Using H2O2 Feed Solutionsa liquid effluent flow rate (mL/min)

back-calculated [H2O2] in feed (wppm)

measured [H2O2] in feed (wppm)

3.15 ( 0.01 8.22 ( 0.02

3175 ( 48 3241 ( 57

3450 ( 225 3450 ( 225

a

At 500

(

1 °C and 246

(

0.4 bar.

hydrogen peroxide decomposition and S/V is the surfaceto-volume ratio of the reactor. Hydrogen peroxide decomposition is catalyzed by metal surfaces, and hence both homogeneous (kh) and heterogeneous (kw) reactions contribute to the overall rate. The first-order rate constants for the homogeneous and heterogeneous reactions were developed for temperatures of 300-420 °C and pressures of 245-340 bar36

kh (s-1) ) 1013.7(1.2 exp[-180 ( 16 (kJ/mol)/RT]

(2)

kw (cm s-1) ) 103.3(0.3 exp[-62.5 ( 4.4 (kJ/mol)/RT] (3) The aqueous hydrogen peroxide solution has approximately a 6-s residence time in the final section of the preheater, which is isothermal and has a surfaceto-volume ratio of approximately 37 cm-1. For a 6-s residence time, eq 1 predicts 100% conversion of the hydrogen peroxide at temperatures above 400 °C, which is a much lower temperature than is normally used in our experiments. Oxygen Evolution Control Experiments. Two control experiments were performed to measure the mass of oxygen evolved from the breakdown of hydrogen peroxide in the reactor system. An aqueous hydrogen peroxide solution was fed to the reactor system at 500 °C and 246 bar. Effluent flow rates for the evolved gas phase (which was analytically confirmed to be 100% oxygen) and water streams were measured. The concentration of dissolved oxygen in the water was calculated by Henry’s law. Using the measured effluent concentration of oxygen in the gaseous and aqueous phases, the concentration of hydrogen peroxide in the feed solution that would be necessary to produce this concentration of oxygen was back-calculated and compared to the measured concentration of hydrogen peroxide in the feed solution. The results of experiments at two different flow rates, shown in Table 1, revealed agreement to within about 10%. Comparison of Oxidation Kinetics Using Oxygen and Hydrogen Peroxide. Even though the above experiment indicates that the bulk of the H2O2 is converted to O2, such an experiment cannot be used to determine whether minor amounts of H2O2 remain in the reactor. Small quantities of H2O2 could affect oxidation kinetics, given that H2O2 will dissociate to OH radicals in SCW. Thirty-six experiments were conducted, 21 with H2O2 and 15 with dissolved oxygen, and the results were compared to validate the hypothesis that the rate is not a function of the oxidant. Experiments were conducted at 500 °C and 246 bar with residence times of 1.4 to 4.0 s. Mean reaction or residence time is defined by the ratio of reactor volume to the volumetric flow rate under reaction conditions. The initial methanol concentration was maintained at 0.069 wt % [21.5 mmol (millimolar)], and experiments were conducted under fuel-rich conditions in an effort to

Figure 1. Comparison of methanol conversion as a function of time using dissolved oxygen and hydrogen peroxide as oxidants (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

maximize the ability to discriminate between the two oxidants. The degree of fuel richness is specified explicity using the fuel equivalency ratio Φ, which is defined by Thus, Φ > 1 is fuel-rich, and Φ < 1 is oxygen-rich. Φt

(fuel in feed)/(actual oxygen in feed) (fuel in feed)/(stoichiometric oxygen requirement)

For all oxidation experiments reported in this study, Φ was set to 1.5 at the feed point to maintain fuel-rich conditions. In the experiments using dissolved oxygen, the oxygen saturator pressure was maintained at 42 bar. The concentration of hydrogen peroxide was prepared to deliver an equivalent concentration of oxygen upon complete decomposition. Figure 1 shows the conversion of methanol as a function of residence time using dissolved oxygen and hydrogen peroxide. The data convincingly demonstrate that the rates of oxidation are equal for the two oxidants. The concentrations under supercritical conditions (SCC) of CO and CO2, the primary oxidation products, are displayed in Figure 2. As is evident from these graphs, the calculated concentrations of CO and CO2 obtained using either oxidant are indistinguishable within the experimental scatter of the data. Part II. Exploration of Mixing Effects Experimental Evidence of Mixing Time Effects. Previous SCWO kinetic studies of hydrogen,17 carbon monoxide,9 and acetic acid19 reported the presence of an induction period before the onset of oxidation. These induction periods were estimated to be about 1-3 s in duration by assuming a first-order dependence of the reaction rate on the fuel concentration and linearly extrapolating data plotted as ln(C/C0) vs τ back to the point of zero conversion. The residence time corresponding to the extrapolated zero-conversion point was interpreted as a purely kinetic induction time attributable to the time necessary to establish the free-radical pool. These extrapolations were necessary because a direct measurement of induction times was not possible in the reactor system used by these investigators as a result of the overall uncertainty in the residence time arising from uncertainties in the quench time, reactor volume, and flow rates. The predictions of elementary reaction mechanisms developed previously for combustion conditions and adapted to SCWO conditions were indeed able to confirm the presence of these induction times, al-

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Figure 3. Methanol conversion as a function of time using mixing cross 1 (see Figure 4) (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

Figure 2. Comparison of CO and CO2 concentrations as functions of time using dissolved oxygen and hydrogen peroxide as oxidants (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

though they were predicted to be shorter than those observed.17,9 Much shorter induction times were measured by Rice et al.37 at Sandia National Laboratories (SNL) in a study of methanol SCWO kinetics. Through the use of in situ Raman spectroscopy, methanol concentrations were measured in their plug-flow reactor system at residence times of 0.2-2.7 s at 246 bar over a temperature range of 440-500 °C with an initial methanol concentration of 1.5 wt % and a fuel equivalence ratio (Φ) of 0.85. Most of these experiments were performed with residence times of less than 1 s. By extrapolating these very short residence time data, induction times of 0.13-0.69 s were estimated. The induction times were found to decrease with temperature, consistent with the observations at MIT. Brock et al.32 also attempted to determine the induction time for methanol SCWO in their isothermal, isobaric, tubular plug-flow reactor by the extrapolation of very short residence time data. Experiments were conducted with residence times from 0.1 to 3.65 s, with a majority of the experiments performed at residence times of less than 1 s. Induction times from 0.09 to 0.5 s were reported, decreasing with increasing temperature, for methanol oxidation at 249 bar with temperatures ranging from 500 to 589 °C, initial methanol concentrations from 0.02 to 0.05 wt %, and fuel equivalence ratios from 0.12 to 0.54. The ability of Brock et al.32,38 to provide accurate estimates of induction times from short residence time data is questionable, as their experimental apparatus is similar to ours. With no in situ techniques for measuring real-time species concentrations, the reaction mixture must first be quenched for measurement of methanol concentrations in the liquid effluent samples by GC. When applied to the measurement of very short

residence time data, as was done here, the residence time is probably not known with high accuracy because of the relatively large contribution of systematic uncertainties. For example, the time necessary to quench the reaction and any uncertainties in determining the exact point in the reactor at which reaction stops could easily be on the order of these very short residence time measurements. Additionally, there is significant scatter in the lower-temperature data obtained by Brock et al.,32,38 which further hinders a precise estimation of the induction time. In contrast, the short residence time experiments by Rice and co-workers37,39,40,41 provide more accurate estimates of induction times by using an in situ technique, but they do not explicitly separate out mixing effects. The present study was undertaken so that we might better understand and reduce the effects of mixing on kinetic data measured in our reactor system. The abovementioned studies all indicate that a reported induction time for oxidation is likely to be the combined result of mixing effects, quenching effects, and a purely kinetic induction period. The fact that different apparent induction times are measured in different reactor systems suggests that the time required to mix the organic and oxidant feeds might sometimes dominate the kinetic induction time. Oxidation experiments were preceded by a set of hydrolysis runs to characterize the extent of methanol decomposition in the absence of oxygen. We did not detect any conversion for residence times up to 12 s. Under the supercritical conditions studied, no measurable corrosion was expected in the 316SS reactor system. In investigations of model hydrocarbon (H-CO) compounds, only minimal corrosion rates have been observed at subcritical conditions in the preheater section of the apparatus. Methanol oxidation kinetics measured at 500 °C and 246 bar with an initial methanol concentration of 0.069 wt % and a Φ value of 1.5 are shown in Figure 3. These data were measured using an 8.2-m-long [1/8-in. (3.18mm) o.d. × 0.041-in. (1.04-mm) i.d.] 316SS reactor fitted with the opposed-flow 1/8-in. HC-276 high-pressure mixing cross at the reactor entrance denoted as “configuration 1” in Figure 4. Extrapolation of the conversion data to zero conversion yields an induction time (τind) of around 3 s. This is significantly longer than the 0.13-s τind reported by Rice et al.37,39,40,41 at 500 °C; however, their experiments used a significantly higher initial methanol concentration, which might have had an effect on τind. We note that, for the experimental conditions studied, the Rey-

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Figure 4. Schematics of mixing crosses used in the methanol oxidation experiments. The thermocouple is schematically represented by a cigar-shaped, elliptical element; for example, in configuration 1, it is located vertically upward.

Figure 5. Methanol conversion as a function of time using mixing cross 2 (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

nolds number of fluids passing into the mixing region was in the range Re ) 1500-3400. The mixing cross used in these first experiments (configuration 1) was replaced with a cross that was identical in all respects except that the thermocouple extended into the opposite arm of the cross leading to the reactor (configuration 2). Measurements using cross 2 in conjunction with the same 8.2-m-long reactor as in Figure 3 and with a 2.5-m-long 316SS [1/8-in. (3.18-mm) o.d. × 0.041-in. (1.04-mm) i.d.] reactor are shown in Figure 5. The data from the 2.5- and 8.2-m reactors using cross configuration 2 are in agreement and indicate that τind is about 0.7 s, which is much shorter

Figure 6. Assumed first-order plots of ln(1- x) vs τ for the methanol data of Figure 5 (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

than the 3.2-s τind measured in the 8.2-m reactor with cross configuration 1 (Figure 6). On the basis of the experimental observation that the apparent induction time varies with the configuration of the mixing cross (which is actually a tee because a thermocouple fully occupies one port), it was tentatively concluded that the thermocouple could enhance mixing if it were inserted deeply into the cross because it would then cause higher velocities in the two streams. Thus, some fraction of the apparent induction time reported in earlier MIT studies might in fact be due to a lack of rapid mixing.

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Prior Work on Mixing Tees. Mixing in tees has been the subject of considerable prior work, with much of it directed at the case in which the two entering streams are at right angles to each other rather than at the case in which their directions are in opposition to each other. Nonetheless, the work does provide some insights. Much of the early work focused on the radial homogeneity of a tracer or related quantity, such as thermal or electrical conductivity, in the flow downstream of the tee.42-44 These techniques are best suited to conditions under which the mixing is not rapid, and they shed considerable light on the state of mixing at downstream positions in the range of 15 < x/D < 120. More recent work has focused on conditions for which the mixing is more rapid and the downstream positions of interest typically correspond to x/D < 3. These studies45-49 have tended to use very rapid, wellcharacterized chemical reactions to track the state of mixedness. Not surprisingly, the most important conclusion from both kinds of studies is that good mixing requires that the flow be fully turbulent. Perhaps more surprisingly, it seems that mixing is essentially independent of Reynolds number if that parameter is in excess of 10 000. For transition-range Reynolds numbers, the sensitivity is significant. Construction of Optimized Mixing Crosses. Two new mixing crosses were fabricated in opposed-flow and side-entry configurations (configurations 3 and 4, respectively, in Figure 4) to enhance the rate of mixing based on the above studies. Small inserts were configured from 0.0254-cm (0.01-in.) i.d. × 0.158-cm (1/16-in.) o.d. 316SS tubing cut to a length of approximately 0.711 cm (0.28 in.) and placed in the sidestream arms of these crosses through which the dilute organic and oxidant streams enter. This reduction increased the ratio of the inlet feed stream-to-reactor velocities from approximately 0.5 to around 24 at typical operating conditions. With this modification, the Reynolds number of the fluid in the inlet arms was increased from 1500-3400 to 10 000-20 000. Results. The SCW methanol oxidation rate was again measured using the same 2.5-m reactor as before with crosses 3 and 4. Again, the methanol SCWO experiments were carried out at 500 °C and 246 bar for an initial methanol concentration of 0.069 wt % and oxidant provided by the decomposition of hydrogen peroxide with Φ ) 1.5 at the mixing point. Identical profiles of conversion versus time were measured using crosses 3 and 4 (Figure 7) that were also in agreement with the conversions measured using cross 2. With these new crosses, the observed τind decreased from 3.2 s with cross 1 to between 0.5 and 1 s, as shown in Figures 6 and 7. Although a significant reduction, the observed τind is still significantly longer than the 0.13-s value measured by Rice and co-workers37,39,41 at SNL at 500 °C with an initial methanol concentration of 1.5 wt %. Our values are also slightly longer than those inferred by Brock et al.32,38 Whether a further reduction in mixing time could be achieved through additional improvements to the design of the cross remains unclear. However, it is very clear that a phenomenon originally thought to be purely kinetic in nature has been shown, actually, to be a function of the efficiency of mixing. Conclusions The use of an aqueous hydrogen peroxide solution has been shown to be a viable means of generating molec-

Figure 7. Comparison of methanol conversion measured using the opposed-flow (configuration 3) and side-entry (configuration 4) mixing crosses depicted in Figure 4 (T ) 500 °C, P ) 246 bar, [CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

ular oxygen in situ under supercritical water conditions in a laboratory-scale reactor system. At the same temperature, pressure, and methanol feed concentrations, the oxidation of methanol was found to proceed at the same rate using either aqueous hydrogen peroxide or dissolved oxygen as the oxidant. Moreover, the concentrations of the oxidation products in the reactor effluent were identical for the two oxidants. Our experiments strongly suggest that, as long as the hydrogen peroxide has sufficient time to decompose fully, no significant rate enhancement is induced by the early presence of OH or other free radicals. Of course, the results obtained here are specific to this reactor feed system, and similar experiments should be performed to verify complete hydrogen peroxide decomposition in other systems. Mixing effects between organic feeds and oxidants influence observed oxidation kinetics and their interpretation. The measured apparent induction time was shown to be influenced by the geometry and flow conditions occurring within the mixing cross. Two new mixing crosses were designed to increase the intensity and rate of mixing to mitigate its effect on the induction time as much as possible. By reducing the inner diameter of the oxidant and organic arms of the cross to increase their Reynolds numbers into the fully turbulent region, the observed induction time was reduced from 3.2 to 0.7 s. Acknowledgment The authors gratefully acknowledge the partial support of the Army Research Office through its University Research Initiative (Grant DAAL03-92-G-0177) and AASERT (Grant DAAH04-94-G-0145) Programs, both under the supervision of Dr. Robert Shaw; Sandia National Laboratories through its Strategic Environmental Research and Development Program (SERDP) under the direction of Dr. Steven Rice; and the NIEHS Superfund program. We especially thank Dr. William Peters of the MIT Energy Laboratory for his technical assistance and insights. Other current and former members of the MIT supercritical fluids research group enriched our work as well. Literature Cited (1) Killilea, W. R.; Swallow, K. C. The Fate of Nitrogen in Supercritical Water Oxidation. J. Supercrit. Fluids 1992, 5 (1), 72.

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Received for review May 29, 2001 Revised manuscript received November 9, 2001 Accepted November 9, 2001 IE010473U