Proceedings of the Combustion Institute, Volume 28, 2000/pp. 1529–1536
ANALYSIS OF AN ELEMENTARY REACTION MECHANISM FOR BENZENE OXIDATION IN SUPERCRITICAL WATER
JOANNA L. DINARO,1 JACK B. HOWARD,1 WILLIAM H. GREEN,1 JEFFERSON W. TESTER1 and JOSEPH W. BOZZELLI2 1Department of Chemical Engineering and Energy Laboratory Massachusetts Institute of Technology Cambridge, MA 02139, USA 2Chemistry and Chemical Engineering Department New Jersey Institute of Technology Newark, NJ 07102, USA
A benzene supercritical water oxidation (SCWO) mechanism, based on published low-pressure benzene combustion mechanisms and submechanisms describing the oxidation of key intermediates, was developed and analyzed to determine the controlling reactions under SCWO conditions of 750–860 K, 139–278 bar, and equivalence ratios from 0.5 to 2.5. To adapt the combustion mechanisms to the lower temperature (Ͻ975 K) and higher pressure (Ͼ220 bar) conditions, new reaction pathways were added, and quantum Rice–Ramsperger–Kassel theory was used to calculate the rate coefficients and, hence, product selectivities for pressure-dependent reactions. The most important difference between the benzene oxidation mech- anism for supercritical water conditions and those for combustion conditions is reactions in supercritical
water involving C6H5OO predicted to be formed by C6H5 reacting with O2. Through the adjustment of the rate coefficients of two thermal decomposition pathways of C6H5OO, whose values are unknown, the model accurately predicts the measured benzene and phenol concentration profiles at 813 K, 246 bar, stoichiometric oxygen, and 3–7 s residence time and reproduces the finding that the carbon dioxide con- centration exceeds that of carbon monoxide at all reaction conditions and levels of benzene conversion. Comparison of the model predictions to benzene SCWO data measured at several different conditions reveals that the model qualitatively explains the trends of the data and gives good quantitative agreement without further adjustment of rate coefficients.
Introduction [2–7], carbon monoxide [3,5,6], methane [5,6,8,9], methanol [6,7,10–14], and phenol [15]. The model Supercritical water oxidation (SCWO) is a reme- predictions have been compared, with varying de- diation process for treating aqueous organic wastes. grees of success, to experimentally measured species When organic compounds and oxygen are brought concentration profiles. together in water well above its critical point of 221 An elementary reaction mechanism for benzene bar and 647 K, the organic is oxidized to carbon di- oxidation under supercritical water conditions was oxide and water, heteroatoms are converted to their developed [16] based on a detailed kinetic mecha- corresponding mineral acids and can be neutralized using a suitable base, and any nitrogen forms N or nism for benzene oxidation under combustion con- 2 ditions [17,18]. The present paper reports results ob- N2O [1]. The present working hypothesis maintains that SCWO proceeds by free-radical reactions and tained using this developed mechanism to identify that the individual elementary reactions are similar reactions controlling benzene oxidation under su- to those which would take place in combustion at percritical water conditions. The model predictions the temperature and pressure of SCWO. Further- are compared with benzene and phenol concentra- more, water, which serves as the reaction medium tion profiles measured in our experimental investi- and participates in reactions both as a reactant and gation of benzene SCWO at 750–860 K and sub- to as a third-body collider, does not interfere with re- supercritical pressures (139–278 bar) under fuel-rich action events through solvation effects. to fuel-lean conditions for reactor residence times of The free-radical reaction pathway hypothesis has 3–7 s [19]. The details of these experiments are re- received support by multiple attempts to model re- ported elsewhere [19], but briefly benzene SCWO actions using low-pressure combustion mechanisms was studied in an 11 mL Hastelloy C276 tubular adapted to SCWO conditions. Previous modeling ef- plug-flow reactor. Temperature was controlled by forts yielded kinetic mechanisms describing the immersing the reactor in a fluidized sand bath. oxidation of simple compounds such as hydrogen Aqueous feed solutions of benzene and oxidant were
1529 1530 ELEMENTARY REACTION KINETICS pressurized, preheated, and delivered to the reactor. CHEMDIS [28,29] implement the bimolecular The reactor effluent was immediately quenched and Quantum Rice Ramsperger Kassel (QRRK) analysis analyzed for unreacted benzene and oxidation prod- of Dean [26] and allow the estimation of the pres- ucts. Upwards of 90% of the reacted benzene was sure and temperature dependence of the rate coef- accounted for in phenol, carbon monoxide, carbon ficients for such reactions. Westmoreland et al. [30] dioxide, and methane. and Dean [26] discussed the governing bimolecular The various mechanisms developed to model ben- and unimolecular QRRK equations and compared zene combustion successfully predict the oxidation predicted and measured rate coefficients. In the of benzene as well as many other stable and radical present study, two important chemically activated intermediates [17,18,20–24]. The main shortcom- reactions, between H and O2 and between phenyl ings of these models are their pronounced overpred- (C6H5) and O2, were evaluated. ictions of the concentrations of C6H5,C6H5O, and ↔↔ C6H5OH. Given that these species are the primary H O2 HO2* OH O products of benzene oxidation, this inaccuracy is The addition/elimination reaction between H and troubling. As noted by Chai and Pfefferle [25], the O is one of the most important chain-branching current benzene oxidation models, developed pri- 2 steps in low-pressure combustion. The addition/ marily for temperatures above 1600 K and fuel-rich elimination pathway proceeds through the formation conditions, are not usable outside of the temperature of the activated intermediateHO* , which can be sta- and stoichiometric conditions for which they were 2 bilized to HO or dissociate to OH and O. The rela- adjusted, and the understanding of the detailed ox- 2 tive importance of the two pathways depends on idation mechanism is particularly poor at 900–1300 pressure and temperature, with the HO -forming K and fuel-lean conditions. Under SCWO condi- 2 pathway favored at higher pressures. tions, new reaction pathways may be accessible due Cobos et al. [31] studied the recombination reac- to the higher pressure, and thermal decomposition tion forming HO at 298 K and 1–200 bar and esti- pathways with high activation barriers which play im- 2 mated the high-pressure rate coefficient for recom- portant roles at combustion conditions may be in- bination by extrapolating measured fall-off curves. accessible due to the lower temperatures. Therefore, Westmoreland et al. [30] used this high-pressure rate comparisons of the predictions of a current benzene coefficient in a bimolecular QRRK analysis of the combustion mechanism [17,18] against our data for reaction between H and O . The rates of both the the SCWO of benzene will test the robustness of the 2 stabilization (to HO ) and addition/elimination (to mechanism while furthering the understanding of 2 OH and O) pathways were successfully predicted benzene oxidation. over wide ranges of temperature and pressure. A bimolecular QRRK analysis similar to that of Westmoreland et al., conducted using CHEMACT Adaptation of Benzene Oxidation Mechanisms with rate coefficients from Cobos et al. [31] and from Combustion to Supercritical Water Baulch et al. [32], predicted HO2 to be the primary Conditions product of the reaction between H and O2 at 246 bar and 813 K (for which conditions the most com- The most obvious modification necessary to make plete set of benzene concentration data are available low-pressure combustion mechanisms suitable for [19]) with a rate coefficient close to the high-pres- SCWO conditions is adjustment of reaction rate co- sure limit [31]. Further calculations reveal excellent efficients for the effect of pressure. Unimolecular agreement between the predicted and measured rate coefficients are well known to depend on pres- rate coefficients for the recombination reaction in sure to a power which is unity in the low-pressure the high- [31] and low-pressure limit [32] and the limit, zero in the high-pressure limit, and fractional addition/elimination pathway [32]. in the intermediate-pressure or fall-off region. The transitions to the low- and high-pressure limits occur ↔ at reaction-specific pressures. C6H5 O2 Products Recombination reactions exhibit the same pres- The identity and formation rate of products of the sure dependence as unimolecular reactions when reaction between phenyl and oxygen are a focus of only one reaction pathway is possible. When addi- continuing study. In previous benzene oxidation tional products can be formed by chemically acti- mechanisms, the products were set to phenoxy vated pathways, a complex pressure and tempera- (C6H5O) and O [17,18,20–23]. A semiglobal path- ture dependence can result from the competition way was also included in some of these mechanisms between the stabilization and the decomposition [17,18,23]: and/or isomerization pathways [26,27], and experi- ↔ mentally measured rate coefficients cannot be ex- CH65O 22CO CH 22C 23 H (R1) trapolated directly to other temperatures and pres- Frank et al. [33] studied the reaction of phenyl sures. The computer programs CHEMACT [27] and with O2 between 900 and 1800 K and 1.3 to 2.5 bar ANALYSIS OF A BENZENE SCWO MECHANISM 1531
TABLE 1 within 10% to 20% (Table 1), indicating that the es- Comparison of predicted and experimental rate timated value of kϱ for C6H5OO* dissociation to constants at 2.3 bar for the overall reaction C6H5O and O may not be a source of significant ⇔ C6H5 O2 C6H5O O error. Since C6H5OO was predicted to be the main re- Measured by Predicted action product of C6H5 and O2, bimolecular and un- T(K) Frank et al. [33] by QRRK imolecular reactions of C6H5OO were also incor- porated into the benzene SCWO mechanism [16]. 1000 1.2 1012 1.6 1012 The rates of the unimolecular reaction were found 1100 1.6 1012 2.0 1012 to dominate at supercritical water conditions. The 1200 2.0 1012 2.2 1012 unimolecular decomposition reactions of C6H5OO tested in the SCWO mechanism are ↔ CHOO65CHO 65 O (R2) and proposed two sets of products: (1) C6H5O and ↔ CHOO65CHO 642 H (R3) O and (2) p-benzoquinone (C6H4O2) and H. The second of these overall addition/elimination path- CHOO↔ CH CO (R4) ways was necessary to explain their observation of 65 55 2 fast initial H production. Rate coefficients were mea- The rate coefficient of reaction R2 was calculated sured for both pathways and were included in a using QRRK analysis and the reverse rate constant mechanism by Tan and Frank [24]. by microscopic reversibility. Inclusion of reactions Yu and Lin [34] performed a direct study on the R3 and R4 was necessary, as will be shown below, to reaction between C6H5 and O2 at 297–500 K and gain agreement between the predicted and experi- 20–80 Torr. The overall reactions leading to C6H5O mental benzene concentration profile. However, re- and O or C6H4O2 and H proceed first through the actions R3 and R4 are not elementary reactions, and formation of an energized C6H5OO radical rate coefficients for these overall steps were chosen (C6H5OO*), which can undergo stabilization, isom- so as to affect the radical pool to give the best fit erization, and/or dissociation to new products. Yu between the mechanism predictions and the ben- and Lin measured the recombination rate coefficient zene concentration profile measured at 813 K, 246 for C6H5 O2 by monitoring C6H5OO formation bar, and residence times of 3–7 s. Their reverse rate and found it to be pressure independent under their coefficients were calculated by microscopic reversi- conditions. The sole reaction product was C6H5OO, bility. Since the present rate coefficients for reac- consistent with their Rice–Ramsperger–Kassel– tions R3 and R4 are semiempirical, any attempt to Marcus prediction that stabilization to C6H5OO use them in other models should be pursued with dominates over addition/elimination to C6H5O and caution. While the mechanisms by which reactions OorC6H4O2 and H below 1000 K and between 20 R3 and R4 take place have not been experimentally and 80 Torr. determined and likely involve never observed inter- Since the reaction between C6H5 and O2 proceeds mediates, theoretical calculations using density func- through the formation of C6H5OO*, the rate coef- tional analysis show that reactions R3 and R4 involve ficients for the addition/elimination pathways mea- one common isomerization path through a dioxetane sured at the conditions of Frank et al. [33] or used cyclic intermediate. After this first isomerization, a in low-pressure mechanisms [17,18,20–24] are not path to C6H4O2 H is present. A second series applicable at SCWO conditions. CHEMDIS was (unzipping process) is also present, along with an- used to calculate the rate coefficients for stabilization other isomerization series. The rate-controlling step (to C6H5OO) and addition/elimination (to C6H5O involves a 3,2,0 bicyclic (four plus five member ring) and O). The high-pressure rate coefficient for tight transition state. CO2 results from the unimo- C6H5OO formation was taken from Yu and Lin [34], lecular decomposition of this bicyclic [35–37]. and the high-pressure rate coefficient for dissocia- Justification for the inclusion of reaction R3 comes tion of C6H5OO* to C6H5O and O was estimated from the detection of C6H4O2 during benzene com- from microscopic reversibility and assuming the re- bustion at both fuel-rich and fuel-lean conditions at verse reaction has a pre-exponential factor for dif- 900–1300 K [25], our observation of C6H4O2 in the fusion-controlled reactions (A 1012 cm3 mol 1 oxidation of benzene under supercritical water con- 1 s ) and no energy barrier (Ea 0). At 813 K and ditions [16,19], and the incorporation of the overall 246 bar, the calculated stabilization rate coefficient reaction of C6H5 and O2 to C6H4O2 and H in the is two orders of magnitude larger than that for ad- benzene combustion mechanism of Tan and Frank dition/elimination to C6H5O and O. A comparison [24]. Since the reaction between C6H5 and O2 pre- between the predicted and measured [33] rate co- dominantly forms C6H5OO at 246 bar and 813 K, efficients for the addition/elimination pathway at 2.3 reaction R3 was used in place of the addition/elim- bar and from 1000 to 1200 K showed agreement to ination reaction suggested by Frank et al. [33] to 1532 ELEMENTARY REACTION KINETICS account for C6H4O2 production. Further reactions of C6H4O2 were accounted for by incorporating a recent submechanism [38] by which C6H4O2 is fully oxidized. Reaction R4 is included to explain the very early appearance of CO2 in benzene oxidation under su- percritical water conditions [16]. The CO2 yields ex- ceeded those of CO at all conditions studied and for all conversions of benzene. Observations of CO2 yields always exceeding those of CO were also re- ported in the SCWO of phenol [15,39] and substi- tuted phenols [40–43], leading to speculation of pathways for CO2 formation which do not involve CO. Additionally, in benzene combustion at 900– 1300 K and at 350 Torr, Chai and Pfefferle [25] mea- sured high amounts of CO2 at low benzene conver- sions and postulated CO2 production by routes other than the reaction of OH and CO. Carpenter [35], Barckholtz et al. [36] and our work [37] show that the reaction of C6H5 and O2 leads to formation of Fig. 1. Benzene concentration profiles predicted by the C5H5 and CO2. In the present study, since C6H5OO supercritical water benzene oxidation mechanism: T 813 3 is the primary reaction product of C6H5 and O2 at K; P 246 bar; U 1.0; [C6H6]0 0.6 10 mol/L; 246 bar and 813 K, reaction R4 was included in place (᭺) experimental data; — — mechanism of Shandross et of an addition/elimination reaction. al. [17] and Shandross [18]; ——— SCWO mechanism in- ↔ cluding reaction R3 C6H5OO C6H4O2 H with kf,3 8 1 ↔ 4.0 10 s and reaction R4 C6H5OO C5H5 CO2 8 1 Comparison of Model Predictions and with kf,4 1.6 10 s ; ------SCWO mechanism with- Measurements of Benzene Oxidation under out reaction R3 or R4; —--– SCWO mechanism including 8 1 Supercritical Water Conditions only reaction R3 with kf,3 4.0 10 s . The predictions of the benzene SCWO mecha- nism [16], developed by adapting the benzene com- slowest benzene oxidation rate that can be achieved bustion mechanism of Shandross et al. [17] and through adjustment of kf,3 (without including reac- Shandross [18] for the present conditions (813 K, tion R4). 246 bar, and fuel equivalence ratio, U, of 1.0), are The net rates of formation or destruction of key compared in Fig. 1 (solid line) against the experi- species by the individual reactions in the mechanism mental data (points). Also shown is the profile pre- were calculated to determine the controlling reac- dicted by the original mechanism of Shandross et al. tions at 813 K and 246 bar with stoichiometric oxy- [17] and Shandross [18] (long-dashed line), which gen. By comparing the net rates of all reactions in- generally represents the predictions of low-pressure volving a single species, the primary destruction and benzene combustion mechanisms at SCWO condi- formation pathways were determined. tions. Dissociation reactions of C6H5OO to C6H4O2 Benzene was found to react almost exclusively by and H and C5H5 and CO2 were necessary in the reactions R5 and R6: SCWO mechanism to improve the agreement be- tween the predictions and the data. With their in- ↔ CH66OH CHOH 65 H (R5) clusion, the shape and position of the predicted ↔ benzene concentration profile agree with the exper- CH66OH CH 65H 2 O (R6) imental data. The absolute values of the rate coef- ficients of reactions R3 and R4 were found not to with reaction R6 accounting for over 97% of the ox- have significant effects on the model predictions as idation rate of benzene at 813 K and 246 bar. The long as the ratio of kf,4 to kf,3 is equal to 0.4. If this rate coefficient used for reaction R6 [32] is the same ratio is larger or smaller than 0.4, the predicted ben- as that used by Shandross et al. [17] and Shandross zene reaction rate is too slow or too fast, respectively. [18]. Although reaction R5 is recognized to proceed Without inclusion of reaction R3 or R4, the pre- by a chemically activated pathway [44], given the dicted benzene oxidation rate is too fast and the re- relative unimportance of reaction R5 the rate coef- action delay is too small (short-dashed line). The in- ficient used [45] is the same as that used by Shan- duction time can be varied from 0 to a maximum of dross et al. [17] and Shandross [18]. The destruction ϳ1 s by changing the rate coefficient of reaction of phenyl radical (C6H5) formed by reaction R6 is R3. The dashed-and-dotted line represents the completely accounted for by reaction R7: ANALYSIS OF A BENZENE SCWO MECHANISM 1533