Ind. Eng. Chem. Res. 2004, 43, 4841-4847
4841
Potential Explanations for the Inhibition and Acceleration of Phenol SCWO by Water Jeffrey T. Henrikson and Phillip E. Savage* Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136
This paper offers potential explanations for the inhibition and acceleration by water of phenol supercritical water oxidation rates. We report new experimental data at 380-465 °C that reveals the effect of water density on the oxidation reaction. At 380 and 400 °C, increasing the water density increases phenol conversion. At 442 and 465 °C, the phenol conversion decreases as water concentration increases from 2 to 9 mol/L. At 420 °C, the phenol conversion decreases as water density increases from 2 to 8 mol/L, but conversion increases as water density increases from 8 to 13 mol/L. We present new data and analyses showing that ion-ion reactions are not responsible for the observed effects. These effects are, however, quantitatively consistent with a model based on the existence of two competing rate-determining steps. In one possible scenario, one step has an increase in polarity along the reaction coordinate and the other has a decrease. It is also plausible that diffusion limitations at high temperatures and the increase in the dissociation of phenol into phenolate ions may play a part in water’s effect on phenol SCWO kinetics. SCWO and to offer plausible explanations for both its inhibition and acceleration of phenol SCWO.
Introduction Supercritical water oxidation (SCWO) is the controlled oxidation of organic material into water and carbon dioxide at conditions that exceed the critical point of water (T > 374 °C and P > 221 bar). It can be used for waste treatment and energy production. SCW has a low dielectric constant that enables small organic compounds and oxygen to be fully miscible in the solvent medium. Several reviews1,2 discuss the merits of SCWO relative to competing technologies. Phenol is a good model organic compound to study because it is soluble in liquid water, is prevalent in industry wastewater streams, and has been used extensively in previous work. While phenol makes a convenient model organic pollutant for SCWO research, it has also proven to provide apparently contradictory responses to changes in water density. Some3-5 have reported phenol SCWO rates that increased with water density. Others6,7 reported precisely the opposite effect. The inhibitive and accelerative effects of water density on phenol SCWO reported by different research groups would at first glance seem to be contradictory. However, it is plausible that the different groups simply studied different areas of the phenol SCWO parameter space and that density effects differ in these different regions. The different effects that water has on phenol SCWO rates might be due to the existence of two (or more) competing rate-controlling processes. For example, much of the seemingly contradictory data can be harmonized if the controlling process is inhibited by increasing density at low water densities and accelerated by increasing density at high water densities. This paper and its two predecessors7,8 are the first to concentrate exclusively on deciphering the role of water during phenol SCWO. The goal of this paper is to present new data detailing the effect of water density on phenol * To whom correspondence should be addressed. Tel.: (734) 764-3386. Fax: (734) 763-0459. E-mail:
[email protected].
Experimental Section We performed phenol SCWO experiments in two nominally isothermal, isobaric tubular flow reactors at 380, 400, 420, 442, and 465 °C and pressures ranging from 124 to 310 bar. The water density varied from 1.6 to 30 M, which is the largest reported density range to be investigated for SCWO of any compound. The two reactors had volumes of 17 and 67 cm3 and were used to gather phenol conversion data at similar reactor residence times over the wide water density range. We combined these new data with those collected previously8 at the above temperatures. In some of the previous work at 420, 442, and 465 °C, the reaction medium was a mixture (1/3 helium-2/3 water by moles). The experimental apparatus, analytical techniques, and explanation of the introduction of a helium-water solvent are discussed fully in a previous paper.7 Experimental Results Figures 1-3 display phenol conversion as a function of water density with all other reaction variables fixed for SCWO at 380, 400, 420, 442, and 465 °C. The complete data set is available in tabular form in the Supporting Information of this and a previous paper.8 The mean oxygen and phenol concentrations at reaction conditions are [φOH] ) 0.18 ( 0.03 mmol/L and [O2] ) 6.4 ( 0.8 mmol/L. These uncertainties represent the standard deviations. The reactor residence time for Figures 1-3 is either 40 or 50 s. Each point in Figures 1-3 is from a separate, experimentally determined profile of phenol conversion versus reactor residence time. All but four of the points in Figures 1-3 were found from linear interpolation between two experimental residence times. For these four, we extrapolated the experimentally investigated range of reactor residence times by no more than 4 s.
10.1021/ie030841p CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004
4842 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 1. Effect of water concentration on phenol conversion at 380 and 465 °C (τ ) 40 s). Figure 4. Effect of water concentration on global rate constant at 390 °C (data from Oshima et al.6).
Figure 2. Effect of water concentration on phenol conversion at 400 and 442 °C (τ ) 40 s).
Figure 3. Effect of water concentration on phenol conversion at 420 °C (τ ) 50 s).
Figures 1 and 2 show that at temperatures near the critical point of water, phenol conversion is relatively insensitive to the water density at water concentrations between 3 and 10 mol/L. The conversion increases steadily, however, as water concentration increases from 10 to 30 mol/L. In a previous paper,8 we indicated that the phenol conversion went through a minimum at 380 °C. Acquiring additional data at 380 °C, reported here in Figure 1, revealed that this minimum, if present, cannot be distinguished from the run-to-run experimental uncertainty. Figure 3 shows that as the water concentration increases from 2 to 8 mol/L the phenol conversion at 420 °C decreases. Then, as the water concentration increases further from 8 to 15 mol/L the phenol conversion begins to increase. The data show a clear minimum in the phenol conversion. Figures 1 and 2 show that the results at 442 and 465 °C are similar to those at 420 °C in that as water concentration increases from 2 to 8 mol/ L, phenol conversion decreases. Water concentrations higher than 8 mol/L were not attainable at the higher
temperatures due to limitations in the reactor system. Thus, it was not possible to determine whether minima in phenol conversion exist at 442 and 465 °C. As noted previously,8 the trends in Figures 1-3 are consistent with all previous results save those of Oshima et al.6 This group reported a negative global reaction order for water (-0.45) at water concentrations above 11 mol/L. In contrast, all other published accounts3-5,8 report exclusively an acceleration of phenol SCWO by water density in this region. Given this apparent discrepancy, we examined the results of Oshima et al. in more detail. These investigators explored only three water densities (11, 20, and 28 mol/L), and most of their data were obtained at 390 °C. We used the data at 390 °C to evaluate the influence of water density on phenol SCWO. We calculated the average reaction rate for each experiment and then fit these rate data to a powerlaw rate equation using an unweighted nonlinear regression. The resulting rate equation was 1.0 order in phenol, 0.23 order in oxygen, and -0.14 order in water. We next used the experimental data, along with the phenol and oxygen orders just determined, to calculate the global rate constant (kg) for each experiment at 390 °C.
kg ) rate/[phenol][O2]0.23
(1)
This rate constant has embedded within it the influence of the water density. Figure 4 presents the results for the three water densities examined. It does appear that the rate constant decreases (slightly) between water densities of 11 and 20 mol/L. From 20 to 28 mol/ L, however, the rate constant appears to increase (slightly) with increasing density. Thus, these data show only a modest inhibition effect of water on phenol SCWO kinetics between 11 and 20 mol/L and a modest acceleration effect from 20 to 28 mol/L. Moreover, the runto-run variation in the rate constant at 11 mol/L exceeds by nearly a factor of 3 the difference between the mean rate constants at the different densities. This level of run-to-run variation, which is not unusual in these difficult experiments, makes it difficult to assign with confidence any effect of water density on the kinetics from these data because of the comparatively small variation in the rate constant with density. The key experimental findings from the present phenol SCWO work are as follows. At 380 and 400 °C the phenol disappearance rate is largely insensitive to water density up to about 10 mol/L, but at higher
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4843
densities the rate increases steadily as water concentration increases. At 442 and 465 °C, the phenol disappearance rate decreases steadily as water concentration increases up to 8 mol/L. The data at 420 °C reveal that phenol conversion decreases as water concentration increases from 2 to 8 mol/L. The phenol conversion then reaches a minimum and increases as water concentration is increased from 8 to 15 mol/L. Discussion of Results The previous section showed that water inhibits phenol SCWO at low densities and high temperatures and it accelerates phenol SCWO at higher densities and lower temperatures. In this section, we examine different physical and chemical phenomena that might explain the ability of water to both inhibit and accelerate phenol SCWO. Before embarking on this exploration, we emphasize that explaining the influence of density on the rate of a complex, multistep, supercritical-fluid-phase chemical reaction is rarely a simple task. An approach that has proved profitable is to use concepts previously developed for elementary reactions in the gas phase and in solution.1 The assumption is that a supercritical fluid is not too much unlike either a dense gas or an expanded liquid, depending on its density. One must keep in mind, however, that much remains unknown about the molecular physics of supercritical fluids and elementary reactions therein. Essential processes such as energy transfer and diffusion in supercritical fluids remain a topic of research,9-11 and our understanding of them is evolving. Likewise, radical combination rate constants have been found to display unusual density dependence12 in supercritical fluids. Thus, there is no guarantee that extrapolating concepts developed for gas- or liquid-phase reactions into the supercritical fluid region will be adequate. Previous work has demonstrated that the ability of water to both inhibit and accelerate phenol SCWO cannot be attributed to potential parasitic effects related to phase behavior or flow regimes.7,8 Moreover, it has been established that at temperatures between 420 and 465 °C and water concentrations below 4.5 mol/L it is the water concentration and not the system pressure or the total fluid density that causes the observed effects.5,7 Additionally, exercising a detailed chemical kinetics model for phenol SCWO revealed that, if the model adequately captures the chemistry, the observed effects cannot be attributed to water participating in elementary reaction steps as a reactant, product, or collision partner.8 The balance of this section is devoted to a consideration of four different effects that could conceivably be involved in the observed behavior. These are the influence of water density on (1) a possible competition between ionic and free-radical chemistries, (2) the dielectric constant of water, which can influence rates of reaction with a change in polarity along the reaction coordinate, (3) the dissociation of phenol, and (4) the rates of diffusion-controlled reactions. First, however, we consider one experimental detail that could conceivably alter the interpretation of the observed results. This detail is the presence of induction times in some of the phenol SCWO conversion profiles. Induction times can arise from incomplete mixing of the reactor feed streams at the reactor entrance or from the intrinsic chemistry. In either case, if there exists an
Figure 5. Effect of water density on pseudo-first-order rate constant at 400 °C.
Figure 6. Effect of water density on pseudo-first-order rate constant at 420 °C.
induction time and if it varies with the reaction conditions, comparing phenol conversions, as done in Figures 1-3, might not be a wholly adequate means of quantifying the effect of water density on phenol SCWO. To assess whether induction times had an effect on the water density results evident in Figures 1-3 we examined the effect of density on pseudo-first-order rate constants for phenol disappearance. These rate constants were calculated two different ways. One way assumed there was no induction time (linear regression of ln(1 - X) vs τ was forced through the origin). The other way allowed for the existence of an induction time by allowing the y-intercept to deviate from the origin. By obtaining these two different pseudo-first-order rate constants at each temperature and pressure it is possible to determine whether induction times alter the observed trends. We plotted these pseudo-first-order rate constants as a function of water density for each temperature. Figures 5 and 6 display results from experiments at 400 and 420 °C (along with curves that will be discussed later). The two sets of pseudo-firstorder rate constants in each plot are very similar and show the same trend with increasing water density. This result shows that induction times, which were 5.2 ( 2.3 s at the 95% confidence level, did not affect the trends evident in Figures 2 and 3. We conclude that the occasional appearance of induction times is not responsible for the effect of water density observed on the phenol conversion in this work. Ionic Chemistry. One would expect SCWO reactions at 380-465 °C to follow a free-radical mechanism. Nevertheless, at 380 °C and high water densities it is conceivable that an ionic mechanism could account for a portion of the disappearance of phenol. There is ample evidence13-15 of ionic reactions taking place in SCW. To distinguish between a free radical and ionic reaction we
4844 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 1. Dielectric Constant of Pure Water at Reaction Conditions28 121 bar 310 bar
380 °C
400 °C
420 °C
442 °C
465 °C
1.35 11.29
1.31 6.80
1.28 3.25
1.26 2.43
1.24 2.10
added salt (NaCl) to change the ionic strength of the reaction medium. If the disappearance of phenol were caused in part by a reaction between two ions, then increasing the ionic strength would either raise or lower the rate of phenol destruction depending on the charges on the ions.16 We compared phenol SCWO conversion rates at 380 °C and [H2O] ) 19 mol/L with and without added salt (6.3 mmol/L). NaCl at this concentration is fully soluble in supercritical water at these conditions.17,18 The absolute difference in the conversion of phenol at a given residence time for the two experimental profiles was less than (7%, which is within the (10% standard error for our equipment.7 This result indicates that the destruction of phenol in SCWO is not controlled by reactions between ions. The result is also consistent with earlier work5 that showed a negligible effect of added salt on phenol SCWO (at 400 °C). Dielectric Constant. The dielectric constant of a reaction medium can affect the stability of a transition state relative to the reactants. For supercritical water, the dielectric constant rises as the pressure (and water density) increases and the water takes on more liquidlike properties. Table 1 displays the range of values the dielectric constant of pure water possesses at the experimental conditions. At 380 °C the dielectric constant increases by a factor of 8 over the range of pressures examined. On the other hand, at 442 and 465 °C the same range of pressures fails to even double the dielectric constant. Kirkwood theory16 leads to eq 2 as the relationship between the medium dielectric constant () and the rate constant (k) for an elementary reaction between neutral species A and B. 2
ln k ) ln ko -
2
2
µ ( - µA - µB
16πodAB3kBT
(2)
ko is the hypothetical rate constant at ) ∞, µi is the dipole moment, o is the permittivity of a vacuum, dAB is the sum of the radii of the reactants, and kB is the Boltzmann constant. Equation 2 indicates that a plot of ln k vs 1/ at a given temperature should be linear. A positive slope indicates that the transition state is less polar than the reactants and a negative slope implies that the transition state is more polar than the reactants. One hypothesis that could account for the observed effects of water density on phenol SCWO kinetics is that there exists a density-driven shift in ratedetermining steps. The rate-determining step in the low water density region would involve a decrease in polarity along the reaction coordinate from reactants to transition state. The rate-determining step in the high water density region would involve an increase in polarity as reactants form the transition state. To test the hypothesis that there is a density-driven shift in the rate-determining step for phenol SCWO we took the global rate constant, k, to be the sum of the rate constants k1 and k2 for the two rate-determining steps. This model for the global kinetics, along with eq 2, leads to eq 3 as the relationship between the observed rate constant and the dielectric constant.
k ) a exp(m1/) + b exp(m2/)
(3)
We used Scientist, a statistical software package, to fit the experimental data at a given temperature to eq 3, where a, b, m1, and m2 are empirical parameters. The rate constants modeled at 400 °C were those obtained by neglecting any induction times, whereas those modeled at 420 °C were obtained by allowing for the presence of an induction time. Figures 5 and 6 compare the model calculations and the experimental rate constants at 400 and 420 °C. Model comparisons were performed only at these two temperatures because it was these experimental data that showed a possible change in the rate-determining step with density. The calculated rate constants compare well with the experimental rate constants indicating that the model is able to describe the experimental data. Thus, one possible explanation for the inhibiting and accelerating effects of water density on phenol SCWO is that different steps control the phenol disappearance rate in different water density regions and that these steps have different changes in polarity along the reaction coordinate. Unfortunately, the mechanism for phenol SCWO is not known with enough certainty to allow meaningful speculation regarding the identities of the different rate determining steps. Phenol Dissociation. Every acid or alcohol dissociates to some degree in any solvent. The extent of dissociation depends on the receptivity of the solvent to a proton and the ability of the ion formed to disperse the electric charge. Phenol dissociates in water to form hydrogen and phenolate ions (φOH S φO- + H+). The aromatic ring on the phenolate helps to disperse the negative charge. Sue et al.19 measured the equilibrium constant (KφOH) for the dissociation of phenol into phenolate in supercritical water. Using a semiempirical model, they obtained eq 4 as a correlation for this equilibrium constant.
( ) (
∆Hro 1 1 ln KφOH ) ln Kr R T Tr
)( )
2 ZφO-2 1 1 R ZH+ + (4) T rH+ r - r φO
Kr, ∆Hr°, Tr, and r are the reference state values of the equilibrium constant, enthalpy of phenol dissociation, absolute temperature, and dielectric constant, respectively. Zi and ri are the charge and radius of the ions, and R is a constant. The equilibrium constant is related to species concentrations (molal units) according to eq 5.
KφOH )
[H+]γH+[φO-]γφO[φOH]
(5)
γH+ and γφO- are the activity coefficients of the ions in water. The activity coefficient of phenol was taken by Sue et al. to be unity. The activity coefficients for the individual ions are difficult to estimate. The mean ionic activity coefficient (γ( ) [γH+γφO-]1/2), however, can be approximated by using the Debye-Hu¨ckel limiting law20
log γ( )
(1.8248 × 106)Fs1/2 (Z+Z-)I1/2 (T)3/2
(6)
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4845
Figure 8. Test for partial diffusion control. Figure 7. Effect of water density on phenolate concentration.
where Fs is the solution density (kg/L) and I is the ionic strength of the solution. Given the low ionic strengths in our phenol SCWO experiments, the Debye-Hu¨ckel limiting law should be adequate. Using eqs 4-6 along with a charge balance and the ion product of water,21 we calculated the phenolate concentration in supercritical water at the different temperatures and densities used in this investigation. Figure 7 shows that the phenolate concentration increases by as much as 11 orders of magnitude over the range of water densities investigated. If the phenolate ion oxidizes more rapidly than phenol, then this densitydependent speciation could account for the accelerating effect of SCW on phenol SCWO at the higher densities. There is some limited evidence to support phenolate having a higher rate of reaction than phenol with oxidants. The rate constant for phenol reacting with chlorine dioxide is 0.4 M-1 s-1, whereas it is 4.9 × 107 M-1 s-1 for phenolate.22 The rate constant for phenol reacting with hydroxyl radical at 298 K is 6.4 × 109 M-1 s-1, whereas it is 9.6 × 109 M-1 s-1 for phenolate.23 These are the only data in the literature that directly compare phenol and phenolate reactions with oxidants. Unfortunately, rate constants are not available for these and other elementary steps at supercritical water oxidation conditions. To summarize, the phenolate concentration increases with density by many orders of magnitude, and there is reason to believe that phenolate is more reactive than phenol in SCWO. These observations could explain why the phenol conversion increases in Figures 1-3 as the water density increases. These observations do not, however, explain why the phenol conversion decreases as water density increases at low water concentrations. One possibility for this behavior is that the overall reaction is diffusion-limited at low water densities. The next section discusses this topic. Diffusion-Limited Reaction. The rate of a chemical reaction can be controlled by the intrinsic kinetics or by the ability of the reactants to encounter each other (diffusion). At 420-465 °C and low water densities, the rate of phenol disappearance decreases as water density increases. The diffusion coefficients for species in SCW also decrease with increasing density in this region. In fact, the variation of some diffusion coefficients with water density24,25 closely resembles the variation of the phenol conversion with water density at temperatures greater than 420 °C. These diffusion coefficients decrease sharply with increasing water densities up to about 11 mol/L and then the solute diffusivity becomes insensitive to water density changes. The similarity in how water density influences both diffusion coefficients
and phenol conversion suggests that at these higher temperatures the intrinsic kinetics of phenol SCWO might approach the maximum rate allowed by diffusion. The rate of diffusion might be a limiting factor in the reaction. One way to test for diffusion control is to calculate the global activation energy. When diffusion is rate controlling, the apparent activation energy should be around 5 kcal/mol. Our data at 420, 442, and 465 °C and a water density between 2.4 and 3.0 mol/L yielded an activation energy of 23.9 ( 9.6 kcal/mol, where the uncertainty represents the standard deviation. The high activation energy makes it unlikely that phenol SCWO is wholly diffusion controlled. There remains the possibility, however, that the reaction could still be partially diffusion controlled or diffusion limited. If an elementary reaction is diffusion limited, the apparent rate constant (k) can be related to the rate constants for the intrinsic reaction (kCHEM) and for a completely diffusion controlled reaction (kD).16
k)
kCHEM kCHEM 1+ kD
(7)
From the Smoluchowski and Stokes-Einstein equations one can express the diffusion-controlled rate constant as
kD )
8kBT 3η
(8)
so that
3 η 1 1 + ) k kCHEM 8kB T
()
(9)
where η is the viscosity of water and T is absolute temperature. This development indicates that a plot of 1/k vs (η/T) should be linear for an elementary bimolecular reaction that is diffusion limited. To test the hypothesis that phenol SCWO at temperatures above 420 °C and at water concentrations less than 8 mol/L is partially diffusion controlled, we plotted the inverse of the global pseudo-first-order rate constant for phenol disappearance against η/Τ. The plot (Figure 8) shows that the data at 420 and 465 °C do exhibit a linear trend, which is consistent with the overall reaction being diffusion-limited in this region. The data at 442 °C are less conclusive. The best-fit lines through the data possess y-intercepts less than 0, which is inconsistent with eq 9. This failure could mean that the
4846 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
reaction is not partially diffusion controlled or it could mean that the theory underpinning the plot has broken down. The simple theory assumes hydrodynamic control of diffusion, whereas diffusion in SCW at low densities might be better described by kinetic theory instead.25 Furthermore, it neglects all intermolecular interactions. Finally, it does not include the potential influence of solvent clustering and local density enhancements around solutes in supercritical fluids. Such effects are known to occur in water conditions well above the critical temperature.15,26,27 Given these limitations in the theory, the failure of the experimental data at 420 °C to provide a positive y-intercept might reflect a breakdown in the underlying assumptions rather than the absence of partially diffusion-controlled reactions. That phenol SCWO might be diffusion limited at high temperatures may seem, at first glance, inconsistent with phenol SCWO rates being affected only by the water density and not by the total system density. Indeed, experiments at identical water densities but different total densities (manipulated by addition of helium) resulted in indistinguishable phenol conversions.7 One possible way to rationalize this apparent inconsistency is to speculate that water preferentially solvates the diffusing reactants such that they are largely unaware of the presence of helium when it is added to the reaction medium. In this scenario, the local water concentration around the diffusing reactants is about the same in both experiments, and this local density, rather than the bulk density, could more strongly influence the diffusivity. If this local density enhancement occurs, then it is conceivable that a diffusion-limited reaction could proceed at roughly the same rate in pure water at one pressure and a denser helium-water mixture at a higher pressure. Having some evidence consistent with partial diffusion control of phenol SCWO, we retain it as a possible explanation for the phenol conversion decrease at 420, 442, and 465 °C as the water concentration increases from 2 to 8 mol/L. Summary and Conclusions We collected phenol SCWO data over a wide range of water densities. The experiments showed that water inhibits the reaction rate at high temperatures and low densities and it accelerates the rate at lower temperatures and high densities. This effect of water on phenol SCWO cannot be attributed to experimental artifacts, or to water acting as a reactant, product, or collision partner. Furthermore, phenol SCWO kinetics is not controlled by ion-ion reactions, and the oxidation chemistry most likely involves a free-radical mechanism. One plausible cause for water’s inhibition of phenol SCWO at low water densities is that the ratecontrolling process is diffusion limited in this region. An alternate explanation is that the rate-controlling step experiences a decrease in polarity as the reactants form the transition state. A possible cause of water accelerating phenol SCWO at high densities is that the rate-controlling step in this region has a transition state that is more polar than its reactants. A second possibility is that phenolate ions, the concentration of which increases by 11 orders of magnitude in this region, are oxidized much more rapidly than phenol. Acknowledgment We acknowledge financial support for this work from the National Science Foundation (CTS-9903373).
Supporting Information Available: Table with conditions used in and results from phenol SCWO experiments. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723. (2) Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hong, G. T.; Barner, H. E. Supercritical Water Oxidation TechnologysProcess Development and Fundamental Research. ACS Symp Ser. 1993, 518, 35. (3) Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1992, 38, 321. (4) Gopalan, S.; Savage, P. E. A Reaction Network Model for Phenol Oxidation in Supercritical Water. AIChE J. 1995, 41, 1864. (5) 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. (6) Oshima, Y.; Hori, K.; Toda, M.; Chommanad, T.; Koda, S. Phenol Oxidation Kinetics in Supercritical Water. J. Supercrit. Fluids 1998, 13, 241. (7) Henrikson, J. T.; Savage, P. E. Water Density Effects on Phenol Oxidation in Supercritical Water. AIChE J. 2003, 49, 718. (8) Henrikson, J. T.; Chen Z.; Savage, P. E. Inhibition and Acceleration of Phenol Oxidation by Supercritical Water. Ind. Eng. Chem. Res. 2003, 42, 6303. (9) Sekiguchi, K.; Shimojima, A.; Kajimoto, O. Intramolecular and intermolecular vibrational energy relaxation of CH2I2 dissolved in supercritical fluid. Chem. Phys. Lett. 2002, 356, 84. (10) Sekiguchi, K.; Shimojima, A.; Kajimoto, O. Intramolecular vibrational redistribution of CH2I2 dissolved in supercritical Xe. Chem. Phys. Lett. 2003, 370, 303. (11) Ohmori, T.; Kimura, Y.; Hirota, N.; Terazima, M. Diffusion of Transient Radicals in Alcohols and Cyclohexane from Ambient to Supercritical Conditions Studied by the Transient Grating Method. J. Phys. Chem. B 2003, 107, 5958. (12) Oum K.; Sekiguchi K.; Luther K.; Troe J. Observation of unique pressure effects in the combination reaction of benzyl radicals in the gas to liquid transition region. Phys. Chem. Chem. Phys. 2003, 5, 2931. (13) Westacott R. E.; Johnston, K. P.; Rossky, P. J. Stability of Ionic and Radical Molecular Dissociation Pathways for Reaction in Supercritical Water. J. Phys. Chem. B 2001, 105, 6611. (14) 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. (15) Penninger, J. M. L.; Kersten, R. J. A.; Baur, H. C. L. Hydrolysis of diphenyl ether in supercritical watersEffects of dissolved NaCl. J. Supercrit. Fluids 2000, 17, 215. (16) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Collins: New York, 1987. (17) Shin, H. Y.; Matsumoto, K.; Higashi, H.; Iwai, Y.; Arai, Y. Development of a solution model to correlate solubilities of inorganic compounds in water vapor under high temperatures and pressures. J. Supercrit. Fluids 2001, 21, 105. (18) DiPippo, M. M.; Sako, K.; Tester, J. W. Ternary phase equilibria for the sodium chloride-sodium sulfate-water system at 200 and 250 bar up to 400 °C. Fluid Phase Equilib. 1999, 157, 229. (19) Sue, K.; Murata, K.; Matsuura, Y.; Tsukagoshi, M.; Adschiri, T.; Arai, K. Flow-through electrochemical cell for pH measurement of organic acid aqueous solutions at subcritical and supercritical conditions. Fluid Phase Equilib. 2002, 194, 1097. (20) Tester, J. W.; Modell, M. Thermodynamics and Its Applications, 3rd ed.; Prentice Hall PTR: Upper Saddle River, NJ, 1997. (21) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance, 0-1000 °C, 1-10000 Bars. New International Formulataion and Its Background. J. Phys. Chem. Ref. Data 1981, 10, 295. (22) Tratnyek, P. G.; Hoigne, J. Kinetics of Reactions of Chlorine Dioxide (OClO) in Water. II. Quantitative StructureActivity Relationships for Phenolic Compounds. Water Res. 1994, 28, 57.
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4847 (23) Matthews, R. W.; Sangster, D. F. Measurement by Benzoate Radiolytic Decarboxylation of Relative Rate Constants for Hydroxyl Radical Reactions. J. Phys. Chem. 1965, 69, 1938. (24) Hyun, J.; Johnston, K. P.; Rossky, P. J. Structural and Dynamical Origins of Ionic Mobilities in Supercritical Water. J. Phys. Chem. B 2001, 105, 9302. (25) Kubo, M.; Levy, R. M.; Rossky, P. J.; Matubayasi, N.; Nakahara, M. Chloride Ion Hydration and Diffusion in Supercritical Water Using a Polarizable Water Model. J. Phys. Chem. B 2002, 106, 3979. (26) Tucker, S. C. Solvent Density Inhomogeneities in Supercritical Fluids. Chem. Rev. 1999, 99, 391.
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Received for review November 14, 2003 Revised manuscript received March 5, 2004 Accepted May 20, 2004 IE030841P
