GENERAL RESEARCH

The Effects of Mixing and Oxidant Choice on Laboratory-ScaleMeasurements of Supercritical Water Oxidation Kinetics

Brian D. Phenix,† Joanna L. DiNaro, Jefferson W. Tester,* Jack B. Howard, andKenneth A. SmithDepartment 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 supercriticalwater environments was evaluated in this study. The objectives were two-fold: (1) to comparethe use of hydrogen peroxide with dissolved oxygen as an oxidant and (2) to characterize thedynamics and intensity of mixing organic reactant and oxidant streams. Methanol was used asthe model organic as the oxidation rate exhibits a first-order dependence according to extensiveearlier studies. No statistically significant difference was observed in the reaction rates or productdistributions for the use of either dissolved oxygen gas or hydrogen peroxide that was preheatedand 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 effectivemixing times for the reactant and oxidant. Although hydrodynamic effects are certainlydependent 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 reducemixing times to 1 s or less.

Introduction

Supercritical water oxidation (SCWO) typically refersto a waste treatment or remediation process that derivesits effectiveness from the unique solvent properties ofwater at conditions well above its critical point of 221bar and 374 °C. When organic compounds and oxygenare brought together in supercritical water (SCW), theoxidation of the organic is rapid and complete to carbondioxide and water. Heteroatoms such as Cl, S, and Pare converted to their corresponding mineral acids (HCl,H2SO4, and H3PO4), which can be neutralized by usinga suitable base to produce salts of relativity low solubil-ity at supercritical conditions. If any organic nitrogenis present the resulting product is primarily molecularN2 with some N2O.1 NOx gases, typical undesiredbyproducts of combustion processes, are not formedbecause the temperature is too low for these oxidationpathways to be favored. Practical SCWO processesusually operate in the ranges 450-600 °C and 250-280 bar. Detailed reviews of the technology are availablefrom Modell,2 Tester et al.,3 Gloyna and Li,4 and Testerand Cline.5

Our research effort emphasizes the investigation ofthe hydrolysis (reaction in the absence of oxygen) andoxidation of simple organic compounds in supercriticalwater. Certain specific organic compounds, referred toas “model compounds”, were selected for study becausethey either are relatively refractory intermediates thatare produced in the oxidation of more complex com-pounds, are simulants for hazardous waste compounds,

represent wide classes of organic wastes, or are them-selves characteristic waste compounds. To that end,comprehensive kinetic studies involving measurementsof the oxidation and hydrolysis rates have been per-formed 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 methylenechloride,21-24 benzene,25-27 and methyl tert-butyl ether(MTBE).28,29 Many of these model compounds exhibitedoverall first-order kinetic behavior under oxidativeconditions.

Detailed reviews by Savage et al.30 cover reactions insupercritical fluids, including water. Siskin and Katritz-ky31 have also published a comprehensive review ofreactions in superheated water that provides a valuableanalysis and evaluation of important reactivity factorsof relevance to this work.

The present study was undertaken to provide a betterunderstanding of the differences between SCWO kineticdata measured in our reactor system and those mea-sured elsewhere. Although other investigators havereported the equivalence of using pure oxygen ordecomposed peroxide as the oxidant, the evidence hasbeen mostly anecdotal, with little or no published datashowing side-by-side comparisons under controlled con-ditions. Furthermore, other comparisons of kinetic dataon oxidation of the same model compound in differentlaboratories with different experimental setups haverevealed wide differences in the observed kinetics. Atthe outset, it became clear to us that there would bevalue in quantitatively characterizing the effects ofoxidant choice and mixing of reactant and oxidant onobserved rates in a single apparatus under well-definedconditions.

* To whom correspondence should be addressed. Tel.: 617-253-7090. Fax: 617-253-8013. E-mail: testerel@mit.edu.

† Current address: Merck and Company, Rahway, NJ.

624 Ind. Eng. Chem. Res. 2002, 41, 624-631

10.1021/ie010473u CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 01/11/2002

Two factors were investigated separately in thisstudy: the use of hydrogen peroxide as an alternativeoxidant to dissolved oxygen and the influence of reactantand oxidant mixing times on the observed rates ofreaction.

Experimental ApparatusAll experiments were conducted in a modified version

of the bench-scale, tubular plug-flow reactor that hasbeen described in detail by Holgate and Tester17 andMarrone et al.21 Briefly, separate aqueous organic andoxidant feed solutions were prepared. The organic andoxidant feeds were pressurized to reactor pressure anddelivered to the reactor system via two independentdigital HPLC pumps (Rainin, SD-200), which replacedthe older model pumps (LDC Analytical, minipumpmodel 2396) used by previous investigators at MIT.

The pressurized organic and oxidant feeds wereseparately preheated to the operating temperature. Thepreheater system consists of a direct ohmic preheatingsection followed by a preheating coil located in afluidized sand bath (Techne, FB-08) that houses thereactor. The direct ohmic heating (DOH) system re-placed the preheater sand bath used in earlier studiesin our laboratory.17,21 Heating in the DOH section isaccomplished by the application of a voltage acrossindependent 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 pre-heating coil is traced with heating tape. A 30-cm lengthof tubing connects the DOH preheater section to the5.2-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. This30-cm length of tubing is wrapped with resistive cableheaters [Watlow, p/n 62H24A6X, 1/16-in. (1.6-mm) i.d.× 2-ft (61-cm) length, 10 V, 240-W maximum] to preventheat loss.

The aqueous organic and oxidant feeds are mixed ina modified 1/8-in. (3.2-mm) HC-276 cross from High-Pressure Equipment (p/n 60-24HF2) at the reactorentrance. This cross was designed during the course ofthis study to minimize the mixing time. The organic andoxidant feeds enter through separate arms of the cross.A thermocouple is seated in the third arm, and thereactor is attached to the fourth. The reactor is a 4.71-mcoiled 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 of10.71 cm3. A thermocouple is seated in a 1/8-in. (3.2-cm)HC-276 tee at the reactor exit. A 26-cm length ofinsulated HC-276 tubing [1/4-in. (6.35-mm) o.d. × 1/16-in. (1.6-mm) i.d.] rises out of the sand and connects thereactor to a shell-and-tube heat exchanger. A spring-loaded, manual back-pressure regulator (Tescom, p/n26-3200) controls the system pressure. Upon passingthrough the back-pressure regulator, the effluent isflashed to atmospheric pressure, and the two-phaseeffluent is separated in a gas-liquid separator. Gassamples are taken from a sampling port with a syringe,and the flow rate of the gas stream is measured usinga soap-bubble flowmeter and a stopwatch. Liquid samplesare collected from the liquid effluent line, and the flowrate is measured using a class A volumetric flask and astopwatch.

Part I. Hydrogen Peroxide as an AlternativeOxidant

Background. A dissolved oxygen solution was usedas the oxygen source in all previous MIT SCWO studies

(e.g., see Holgate and Tester17). Because of the limitedsolubility of oxygen in water, the maximum attainabledissolved oxygen concentration is about 3930 ppm atambient temperature, given the 125-bar pressure ratingof the oxygen saturator used to prepare the dissolvedoxygen solution. To realize higher concentrations ofoxygen in the reactor, hydrogen peroxide was exploredas an alternative oxidant.

The use of hydrogen peroxide as an oxidant is basedon the assumption of complete decomposition of thehydrogen peroxide to oxygen and water (H2O2 f 1/2O2+ H2O). Researchers at Sandia National Laboratories(SNL) were the first to use hydrogen peroxide as thesource of oxygen in an SCWO system, but its use hassince been adopted by other research groups (e.g., seeBrock et al.32 and Krajnc and Levec33), including ourown. Whereas the assumption of complete breakdownof hydrogen peroxide to oxygen and water was not testedexperimentally at SNL, both Brock et al.32 and Krajncand Levec33 did verify complete decomposition in theirown reactor systems. An investigation was undertakenhere to validate oxygen delivery by hydrogen peroxidein our bench-scale, tubular plug-flow reactor by compar-ing methanol oxidation rates measured using hydrogenperoxide and dissolved oxygen. These tests also servedto demonstrate the absence of any kinetic effects dueto the presence of long-lived hydroxyl or peroxy radicals.

A careful distinction must be made between the useof hydrogen peroxide as an oxygen source and as aprimary oxidant. The use of hydrogen peroxide as a rateenhancer was explored in the oxidation of 2,4-dichloro-phenol and acetic acid.34 In separate experiments,hydrogen peroxide and oxygen were premixed with theorganics in batch reactors. The premixed solutions werethen heated to 400-500 °C. Under comparable condi-tions, the conversions of both compounds were higherwith hydrogen peroxide than with oxygen. This findingis not surprising given the premixing of the organicswith this strong oxidizer. A more recent study of theeffect of hydrogen peroxide on SCWO oxidation rateswas carried out by Bourhis et al.35 In these experiments,a cold water feedstream was spiked with hydrogenperoxide at a concentration of 0.75-3 wt % and mixedwith a pure organic waste stream. Oxidation wasinitiated 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 extentof reaction was inferred by measuring the axial tem-perature rise along the outer surface of the reactor andmonitoring the CO levels in the effluent. The additionof small amounts of hydrogen peroxide was found tosignificantly raise the temperature profile down thelength of the reactor, leading to the conclusion that therate of oxidation must have been increased.

The purpose here is not to exploit the rate-enhancingproperties of hydrogen peroxide but instead to ensurecomplete breakdown of hydrogen peroxide to oxygen andwater in the preheater and to verify the absence of anyresidual kinetic effects due to radical persistence.Experiments measuring the decomposition rate in SCWat SNL led to the development of the following rateexpression36

where koverall is the overall first-order rate constant for

koverall (s-1) ) kh (s-1) + kw (cm s-1) × (S/V) (cm-1)(1)

Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002 625

hydrogen peroxide decomposition and S/V is the surface-to-volume ratio of the reactor. Hydrogen peroxidedecomposition is catalyzed by metal surfaces, and henceboth homogeneous (kh) and heterogeneous (kw) reactionscontribute to the overall rate. The first-order rateconstants for the homogeneous and heterogeneous reac-tions were developed for temperatures of 300-420 °Cand pressures of 245-340 bar36

The aqueous hydrogen peroxide solution has ap-proximately a 6-s residence time in the final section ofthe preheater, which is isothermal and has a surface-to-volume ratio of approximately 37 cm-1. For a 6-sresidence time, eq 1 predicts 100% conversion of thehydrogen peroxide at temperatures above 400 °C, whichis a much lower temperature than is normally used inour experiments.

Oxygen Evolution Control Experiments. Twocontrol experiments were performed to measure themass of oxygen evolved from the breakdown of hydrogenperoxide in the reactor system. An aqueous hydrogenperoxide solution was fed to the reactor system at 500°C and 246 bar. Effluent flow rates for the evolved gasphase (which was analytically confirmed to be 100%oxygen) and water streams were measured. The con-centration of dissolved oxygen in the water was calcu-lated by Henry’s law. Using the measured effluentconcentration of oxygen in the gaseous and aqueousphases, the concentration of hydrogen peroxide in thefeed solution that would be necessary to produce thisconcentration of oxygen was back-calculated and com-pared to the measured concentration of hydrogen per-oxide in the feed solution. The results of experimentsat two different flow rates, shown in Table 1, revealedagreement to within about 10%.

Comparison of Oxidation Kinetics Using Oxy-gen and Hydrogen Peroxide. Even though the aboveexperiment indicates that the bulk of the H2O2 is con-verted to O2, such an experiment cannot be used todetermine whether minor amounts of H2O2 remain inthe reactor. Small quantities of H2O2 could affectoxidation kinetics, given that H2O2 will dissociate to OHradicals in SCW. Thirty-six experiments were con-ducted, 21 with H2O2 and 15 with dissolved oxygen, andthe results were compared to validate the hypothesisthat the rate is not a function of the oxidant. Experi-ments were conducted at 500 °C and 246 bar withresidence times of 1.4 to 4.0 s. Mean reaction or resi-dence time is defined by the ratio of reactor volume tothe volumetric flow rate under reaction conditions. Theinitial methanol concentration was maintained at 0.069wt % [21.5 mmol (millimolar)], and experiments wereconducted under fuel-rich conditions in an effort to

maximize the ability to discriminate between the twooxidants. The degree of fuel richness is specified explic-ity using the fuel equivalency ratio Φ, which is definedby Thus, Φ > 1 is fuel-rich, and Φ < 1 is oxygen-rich.

For all oxidation experiments reported in this study, Φwas set to 1.5 at the feed point to maintain fuel-richconditions. In the experiments using dissolved oxygen,the oxygen saturator pressure was maintained at 42bar. The concentration of hydrogen peroxide was pre-pared to deliver an equivalent concentration of oxygenupon complete decomposition.

Figure 1 shows the conversion of methanol as afunction of residence time using dissolved oxygen andhydrogen peroxide. The data convincingly demonstratethat the rates of oxidation are equal for the twooxidants. The concentrations under supercritical condi-tions (SCC) of CO and CO2, the primary oxidationproducts, are displayed in Figure 2. As is evident fromthese graphs, the calculated concentrations of CO andCO2 obtained using either oxidant are indistinguishablewithin 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 carbonmonoxide,9 and acetic acid19 reported the presence ofan induction period before the onset of oxidation. Theseinduction periods were estimated to be about 1-3 s induration by assuming a first-order dependence of thereaction rate on the fuel concentration and linearlyextrapolating data plotted as ln(C/C0) vs τ back to thepoint of zero conversion. The residence time correspond-ing to the extrapolated zero-conversion point was in-terpreted as a purely kinetic induction time attributableto the time necessary to establish the free-radical pool.These extrapolations were necessary because a directmeasurement of induction times was not possible in thereactor system used by these investigators as a resultof the overall uncertainty in the residence time arisingfrom uncertainties in the quench time, reactor volume,and flow rates. The predictions of elementary reactionmechanisms developed previously for combustion condi-tions and adapted to SCWO conditions were indeed ableto confirm the presence of these induction times, al-

Table 1. Results of Oxygen Evolution Experiments UsingH2O2 Feed Solutionsa

liquid effluentflow rate(mL/min)

back-calculated[H2O2] in feed

(wppm)

measured[H2O2] in feed

(wppm)

3.15 ( 0.01 3175 ( 48 3450 ( 2258.22 ( 0.02 3241 ( 57 3450 ( 225

a At 500 ( 1 °C and 246 ( 0.4 bar.

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)

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

Φ t(fuel in feed)/(actual oxygen in feed)

(fuel in feed)/(stoichiometric oxygen requirement)

626 Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002

though they were predicted to be shorter than thoseobserved.17,9

Much shorter induction times were measured by Riceet al.37 at Sandia National Laboratories (SNL) in a studyof methanol SCWO kinetics. Through the use of in situRaman spectroscopy, methanol concentrations weremeasured in their plug-flow reactor system at residencetimes of 0.2-2.7 s at 246 bar over a temperature rangeof 440-500 °C with an initial methanol concentrationof 1.5 wt % and a fuel equivalence ratio (Φ) of 0.85. Mostof these experiments were performed with residencetimes of less than 1 s. By extrapolating these very shortresidence time data, induction times of 0.13-0.69 s wereestimated. The induction times were found to decreasewith temperature, consistent with the observations atMIT.

Brock et al.32 also attempted to determine the induc-tion time for methanol SCWO in their isothermal,isobaric, tubular plug-flow reactor by the extrapolationof very short residence time data. Experiments wereconducted with residence times from 0.1 to 3.65 s, witha majority of the experiments performed at residencetimes of less than 1 s. Induction times from 0.09 to 0.5s were reported, decreasing with increasing tempera-ture, for methanol oxidation at 249 bar with tempera-tures ranging from 500 to 589 °C, initial methanolconcentrations from 0.02 to 0.05 wt %, and fuel equiva-lence ratios from 0.12 to 0.54.

The ability of Brock et al.32,38 to provide accurateestimates of induction times from short residence timedata is questionable, as their experimental apparatusis similar to ours. With no in situ techniques formeasuring real-time species concentrations, the reactionmixture must first be quenched for measurement ofmethanol concentrations in the liquid effluent samplesby GC. When applied to the measurement of very short

residence time data, as was done here, the residencetime is probably not known with high accuracy becauseof the relatively large contribution of systematic uncer-tainties. For example, the time necessary to quench thereaction and any uncertainties in determining the exactpoint in the reactor at which reaction stops could easilybe on the order of these very short residence timemeasurements. Additionally, there is significant scatterin the lower-temperature data obtained by Brock etal.,32,38 which further hinders a precise estimation of theinduction time. In contrast, the short residence timeexperiments by Rice and co-workers37,39,40,41 providemore accurate estimates of induction times by using anin situ technique, but they do not explicitly separate outmixing effects.

The present study was undertaken so that we mightbetter understand and reduce the effects of mixing onkinetic data measured in our reactor system. The above-mentioned studies all indicate that a reported inductiontime for oxidation is likely to be the combined result ofmixing effects, quenching effects, and a purely kineticinduction period. The fact that different apparent induc-tion times are measured in different reactor systemssuggests that the time required to mix the organic andoxidant feeds might sometimes dominate the kineticinduction time. Oxidation experiments were precededby a set of hydrolysis runs to characterize the extent ofmethanol decomposition in the absence of oxygen. Wedid not detect any conversion for residence times up to12 s. Under the supercritical conditions studied, nomeasurable corrosion was expected in the 316SS reactorsystem. In investigations of model hydrocarbon (H-C-O) compounds, only minimal corrosion rates have beenobserved at subcritical conditions in the preheatersection of the apparatus.

Methanol oxidation kinetics measured at 500 °C and246 bar with an initial methanol concentration of 0.069wt % and a Φ value of 1.5 are shown in Figure 3. Thesedata were measured using an 8.2-m-long [1/8-in. (3.18-mm) o.d. × 0.041-in. (1.04-mm) i.d.] 316SS reactor fittedwith the opposed-flow 1/8-in. HC-276 high-pressuremixing cross at the reactor entrance denoted as “con-figuration 1” in Figure 4.

Extrapolation of the conversion data to zero conver-sion yields an induction time (τind) of around 3 s. Thisis significantly longer than the 0.13-s τind reported byRice et al.37,39,40,41 at 500 °C; however, their experimentsused a significantly higher initial methanol concentra-tion, which might have had an effect on τind. We notethat, for the experimental conditions studied, the Rey-

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

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

Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002 627

nolds number of fluids passing into the mixing regionwas in the range Re ) 1500-3400.

The mixing cross used in these first experiments(configuration 1) was replaced with a cross that wasidentical in all respects except that the thermocoupleextended into the opposite arm of the cross leading tothe reactor (configuration 2). Measurements using cross2 in conjunction with the same 8.2-m-long reactor as inFigure 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 inFigure 5. The data from the 2.5- and 8.2-m reactorsusing cross configuration 2 are in agreement andindicate that τind is about 0.7 s, which is much shorter

than the 3.2-s τind measured in the 8.2-m reactor withcross configuration 1 (Figure 6).

On the basis of the experimental observation that theapparent induction time varies with the configurationof the mixing cross (which is actually a tee because athermocouple fully occupies one port), it was tentativelyconcluded that the thermocouple could enhance mixingif it were inserted deeply into the cross because it wouldthen cause higher velocities in the two streams. Thus,some fraction of the apparent induction time reportedin earlier MIT studies might in fact be due to a lack ofrapid mixing.

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

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

628 Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002

Prior Work on Mixing Tees. Mixing in tees hasbeen the subject of considerable prior work, with muchof it directed at the case in which the two enteringstreams are at right angles to each other rather thanat the case in which their directions are in oppositionto each other. Nonetheless, the work does provide someinsights. Much of the early work focused on the radialhomogeneity of a tracer or related quantity, such asthermal or electrical conductivity, in the flow down-stream of the tee.42-44 These techniques are best suitedto conditions under which the mixing is not rapid, andthey shed considerable light on the state of mixing atdownstream positions in the range of 15 < x/D < 120.More recent work has focused on conditions for whichthe mixing is more rapid and the downstream positionsof interest typically correspond to x/D < 3. Thesestudies45-49 have tended to use very rapid, well-characterized chemical reactions to track the state ofmixedness. Not surprisingly, the most important con-clusion from both kinds of studies is that good mixingrequires that the flow be fully turbulent. Perhaps moresurprisingly, it seems that mixing is essentially inde-pendent of Reynolds number if that parameter is inexcess of 10 000. For transition-range Reynolds num-bers, the sensitivity is significant.

Construction of Optimized Mixing Crosses. Twonew mixing crosses were fabricated in opposed-flow andside-entry configurations (configurations 3 and 4, re-spectively, in Figure 4) to enhance the rate of mixingbased on the above studies. Small inserts were config-ured 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 0.711cm (0.28 in.) and placed in the sidestream arms of thesecrosses through which the dilute organic and oxidantstreams enter. This reduction increased the ratio of theinlet feed stream-to-reactor velocities from approxi-mately 0.5 to around 24 at typical operating conditions.With this modification, the Reynolds number of the fluidin the inlet arms was increased from 1500-3400 to10 000-20 000.

Results. The SCW methanol oxidation rate was againmeasured using the same 2.5-m reactor as before withcrosses 3 and 4. Again, the methanol SCWO experi-ments were carried out at 500 °C and 246 bar for aninitial methanol concentration of 0.069 wt % and oxidantprovided by the decomposition of hydrogen peroxidewith Φ ) 1.5 at the mixing point. Identical profiles ofconversion versus time were measured using crosses 3and 4 (Figure 7) that were also in agreement with theconversions measured using cross 2.

With these new crosses, the observed τind decreasedfrom 3.2 s with cross 1 to between 0.5 and 1 s, as shownin Figures 6 and 7. Although a significant reduction,the observed τind is still significantly longer than the0.13-s value measured by Rice and co-workers37,39,41 atSNL at 500 °C with an initial methanol concentrationof 1.5 wt %. Our values are also slightly longer thanthose inferred by Brock et al.32,38 Whether a furtherreduction in mixing time could be achieved throughadditional improvements to the design of the crossremains unclear. However, it is very clear that aphenomenon originally thought to be purely kinetic innature has been shown, actually, to be a function of theefficiency of mixing.

ConclusionsThe use of an aqueous hydrogen peroxide solution has

been shown to be a viable means of generating molec-

ular oxygen in situ under supercritical water conditionsin a laboratory-scale reactor system. At the sametemperature, pressure, and methanol feed concentra-tions, the oxidation of methanol was found to proceedat the same rate using either aqueous hydrogen perox-ide or dissolved oxygen as the oxidant. Moreover, theconcentrations of the oxidation products in the reactoreffluent were identical for the two oxidants. Our experi-ments strongly suggest that, as long as the hydrogenperoxide has sufficient time to decompose fully, nosignificant rate enhancement is induced by the earlypresence of OH or other free radicals. Of course, theresults obtained here are specific to this reactor feedsystem, and similar experiments should be performedto verify complete hydrogen peroxide decomposition inother systems.

Mixing effects between organic feeds and oxidantsinfluence observed oxidation kinetics and their inter-pretation. The measured apparent induction time wasshown to be influenced by the geometry and flowconditions occurring within the mixing cross. Two newmixing crosses were designed to increase the intensityand rate of mixing to mitigate its effect on the inductiontime as much as possible. By reducing the innerdiameter of the oxidant and organic arms of the crossto increase their Reynolds numbers into the fullyturbulent region, the observed induction time wasreduced from 3.2 to 0.7 s.

AcknowledgmentThe authors gratefully acknowledge the partial sup-

port of the Army Research Office through its UniversityResearch Initiative (Grant DAAL03-92-G-0177) andAASERT (Grant DAAH04-94-G-0145) Programs, bothunder the supervision of Dr. Robert Shaw; SandiaNational Laboratories through its Strategic Environ-mental Research and Development Program (SERDP)under the direction of Dr. Steven Rice; and the NIEHSSuperfund program. We especially thank Dr. WilliamPeters of the MIT Energy Laboratory for his technicalassistance and insights. Other current and formermembers of the MIT supercritical fluids research groupenriched our work as well.

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Figure 7. Comparison of methanol conversion measured usingthe opposed-flow (configuration 3) and side-entry (configuration4) mixing crosses depicted in Figure 4 (T ) 500 °C, P ) 246 bar,[CH3OH]0 ) 0.069 wt %, Φ ) 1.5).

Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002 629

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

Accepted November 9, 2001

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