1529
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. TESTER
1
and JOSEPH W. BOZZELLI
2
1
Department of Chemical Engineering and Energy Laboratory
Massachusetts Institute of Technology
Cambridge, MA 02139, USA
2
Chemistry 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
(⬍975 K) and higher pressure (⬎220 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 C
6
H
5
OO predicted to be formed by C
6
H
5
reacting with O
2
. Through the adjustment of
the rate coefficients of two thermal decomposition pathways of C
6
H
5
OO, 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
Supercritical water oxidation (SCWO) is a reme-
diation process for treating aqueous organic wastes.
When organic compounds and oxygen are brought
together in water well above its critical point of 221
bar and 647 K, the organic is oxidized to carbon di-
oxide and water, heteroatoms are converted to their
corresponding mineral acids and can be neutralized
using a suitable base, and any nitrogen forms N
2
or
N
2
O [1]. The present working hypothesis maintains
that SCWO proceeds by free-radical reactions and
that the individual elementary reactions are similar
to those which would take place in combustion at
the temperature and pressure of SCWO. Further-
more, water, which serves as the reaction medium
and participates in reactions both as a reactant and
as a third-body collider, does not interfere with re-
action events through solvation effects.
The free-radical reaction pathway hypothesis has
received support by multiple attempts to model re-
actions using low-pressure combustion mechanisms
adapted to SCWO conditions. Previous modelingef-
forts yielded kinetic mechanisms describing the
oxidation of simple compounds such as hydrogen
[2–7], carbon monoxide [3,5,6], methane [5,6,8,9],
methanol [6,7,10–14], and phenol [15]. The model
predictions have been compared, with varying de-
grees of success, to experimentally measured species
concentration profiles.
An elementary reaction mechanism for benzene
oxidation under supercritical water conditions was
developed [16] based on a detailed kinetic mecha-
nism for benzene oxidation under combustion con-
ditions [17,18]. The present paper reports resultsob-
tained using this developed mechanism to identify
reactions controlling benzene oxidation under su-
percritical water conditions. The model predictions
are compared with benzene and phenol concentra-
tion profiles measured in our experimental investi-
gation of benzene SCWO at 750–860 K and sub- to
supercritical pressures (139–278 bar) under fuel-rich
to fuel-lean conditions for reactor residence times of
3–7 s [19]. The details of these experiments are re-
ported elsewhere [19], but briefly benzene SCWO
was studied in an 11 mL Hastelloy C276 tubular
plug-flow reactor. Temperature was controlled by
immersing the reactor in a fluidized sand bath.
Aqueous feed solutions of benzene and oxidant were
1530 ELEMENTARY REACTION KINETICS
pressurized, preheated, and delivered to the reactor.
The reactor effluent was immediately quenched and
analyzed for unreacted benzene and oxidation prod-
ucts. Upwards of 90% of the reacted benzene was
accounted for in phenol, carbon monoxide, carbon
dioxide, and methane.
The various mechanisms developed to model ben-
zene combustion successfully predict the oxidation
of benzene as well as many other stable and radical
intermediates [17,18,20–24]. The main shortcom-
ings of these models are their pronounced overpred-
ictions of the concentrations of C
6
H
5
,C
6
H
5
O, and
C
6
H
5
OH. Given that these species are the primary
products of benzene oxidation, this inaccuracy is
troubling. As noted by Chai and Pfefferle [25], the
current benzene oxidation models, developed pri-
marily for temperatures above 1600 K and fuel-rich
conditions, are not usable outside of the temperature
and stoichiometric conditions for which they were
adjusted, and the understanding of the detailed ox-
idation mechanism is particularly poor at 900–1300
K and fuel-lean conditions. Under SCWO condi-
tions, new reaction pathways may be accessible due
to the higher pressure, and thermal decomposition
pathways with high activation barriers which playim-
portant roles at combustion conditions may be in-
accessible due to the lower temperatures.Therefore,
comparisons of the predictions of a current benzene
combustion mechanism [17,18] against our data for
the SCWO of benzene will test the robustness of the
mechanism while furthering the understanding of
benzene oxidation.
Adaptation of Benzene Oxidation Mechanisms
from Combustion to Supercritical Water
Conditions
The most obvious modification necessary to make
low-pressure combustion mechanisms suitable for
SCWO conditions is adjustment of reaction rate co-
efficients for the effect of pressure. Unimolecular
rate coefficients are well known to depend on pres-
sure to a power which is unity in the low-pressure
limit, zero in the high-pressure limit, and fractional
in the intermediate-pressure or fall-off region. The
transitions to the low- and high-pressure limits occur
at reaction-specific pressures.
Recombination reactions exhibit the same pres-
sure dependence as unimolecular reactions when
only one reaction pathway is possible. When addi-
tional products can be formed by chemically acti-
vated pathways, a complex pressure and tempera-
ture dependence can result from the competition
between the stabilization and the decomposition
and/or isomerization pathways [26,27], and experi-
mentally measured rate coefficients cannot be ex-
trapolated directly to other temperatures and pres-
sures. The computer programs CHEMACT [27] and
CHEMDIS [28,29] implement the bimolecular
Quantum Rice Ramsperger Kassel (QRRK) analysis
of Dean [26] and allow the estimation of the pres-
sure and temperature dependence of the rate coef-
ficients for such reactions. Westmoreland et al. [30]
and Dean [26] discussed the governing bimolecular
and unimolecular QRRK equations and compared
predicted and measured rate coefficients. In the
present study, two important chemically activated
reactions, between H and O
2
and between phenyl
(C
6
H
5
) and O
2
, were evaluated.
H Ⳮ O
2
↔↔OH Ⳮ OHO*
2
The addition/elimination reaction between H and
O
2
is one of the most important chain-branching
steps in low-pressure combustion. The addition/
elimination pathway proceeds through the formation
of the activated intermediate , which can be sta-HO*
2
bilized to HO
2
or dissociate to OH and O. The rela-
tive importance of the two pathways depends on
pressure and temperature, with the HO
2
-forming
pathway favored at higher pressures.
Cobos et al. [31] studied the recombination reac-
tion forming HO
2
at 298 K and 1–200 bar and esti-
mated the high-pressure rate coefficient for recom-
bination by extrapolating measured fall-off curves.
Westmoreland et al. [30] used this high-pressurerate
coefficient in a bimolecular QRRK analysis of the
reaction between H and O
2
. The rates of both the
stabilization (to HO
2
) and addition/elimination (to
OH and O) pathways were successfully predicted
over wide ranges of temperature and pressure.
A bimolecular QRRK analysis similar to that of
Westmoreland et al., conducted using CHEMACT
with rate coefficients from Cobos et al. [31] and
Baulch et al. [32], predicted HO
2
to be the primary
product of the reaction between H and O
2
at 246
bar and 813 K (for which conditions the most com-
plete set of benzene concentration data are available
[19]) with a rate coefficient close to the high-pres-
sure limit [31]. Further calculations reveal excellent
agreement between the predicted and measured
rate coefficients for the recombination reaction in
the high- [31] and low-pressure limit [32] and the
addition/elimination pathway [32].
C
6
H
5
Ⳮ O
2
↔ Products
The identity and formation rate of products of the
reaction between phenyl and oxygen are a focus of
continuing study. In previous benzene oxidation
mechanisms, the products were set to phenoxy
(C
6
H
5
O) and O [17,18,20–23]. A semiglobal path-
way was also included in some of these mechanisms
[17,18,23]:
CH Ⳮ O ↔ 2CO Ⳮ CH Ⳮ C H (R1)
65 2 22 23
Frank et al. [33] studied the reaction of phenyl
with O
2
between 900 and 1800 K and 1.3 to 2.5 bar
ANALYSIS OF A BENZENE SCWO MECHANISM 1531
TABLE 1
Comparison of predicted and experimental rate
constants at 2.3 bar for the overall reaction
C
6
H
5
Ⳮ O
2
⇔ C
6
H
5
O Ⳮ O
T(K)
Measured by
Frank et al. [33]
Predicted
by QRRK
1000 1.2 ⳯ 10
12
1.6 ⳯ 10
12
1100 1.6 ⳯ 10
12
2.0 ⳯ 10
12
1200 2.0 ⳯ 10
12
2.2 ⳯ 10
12
and proposed two sets of products: (1) C
6
H
5
O and
O and (2) p-benzoquinone (C
6
H
4
O
2
) and H. The
second of these overall addition/elimination path-
ways was necessary to explain their observation of
fast initial H production. Rate coefficients were mea-
sured for both pathways and were included in a
mechanism by Tan and Frank [24].
Yu and Lin [34] performed a direct study on the
reaction between C
6
H
5
and O
2
at 297–500 K and
20–80 Torr. The overall reactions leading to C
6
H
5
O
and O or C
6
H
4
O
2
and H proceed first through the
formation of an energized C
6
H
5
OO radical
(C
6
H
5
OO*), which can undergo stabilization, isom-
erization, and/or dissociation to new products. Yu
and Lin measured the recombination rate coefficient
for C
6
H
5
Ⳮ O
2
by monitoring C
6
H
5
OO formation
and found it to be pressure independent under their
conditions. The sole reaction product was C
6
H
5
OO,
consistent with their Rice–Ramsperger–Kassel–
Marcus prediction that stabilization to C
6
H
5
OO
dominates over addition/elimination to C
6
H
5
O and
OorC
6
H
4
O
2
and H below 1000 K and between 20
and 80 Torr.
Since the reaction between C
6
H
5
and O
2
proceeds
through the formation of C
6
H
5
OO*, the rate coef-
ficients for the addition/elimination pathways mea-
sured at the conditions of Frank et al. [33] or used
in low-pressure mechanisms [17,18,20–24] are not
applicable at SCWO conditions. CHEMDIS was
used to calculate the rate coefficients for stabilization
(to C
6
H
5
OO) and addition/elimination (to C
6
H
5
O
and O). The high-pressure rate coefficient for
C
6
H
5
OO formation was taken from Yu and Lin [34],
and the high-pressure rate coefficient for dissocia-
tion of C
6
H
5
OO* to C
6
H
5
O and O was estimated
from microscopic reversibility and assuming the re-
verse reaction has a pre-exponential factor for dif-
fusion-controlled reactions (A ⳱ 10
12
cm
3
mol
ⳮ
1
s
ⳮ
1
) and no energy barrier (E
a
⳱ 0). At 813 K and
246 bar, the calculated stabilization rate coefficient
is two orders of magnitude larger than that for ad-
dition/elimination to C
6
H
5
O and O. A comparison
between the predicted and measured [33] rate co-
efficients for the addition/elimination pathway at 2.3
bar and from 1000 to 1200 K showed agreement to
within 10% to 20% (Table 1), indicating that the es-
timated value of k
⬁
for C
6
H
5
OO* dissociation to
C
6
H
5
O and O may not be a source of significant
error.
Since C
6
H
5
OO was predicted to be the main re-
action product of C
6
H
5
and O
2
, bimolecular and un-
imolecular reactions of C
6
H
5
OO were also incor-
porated into the benzene SCWO mechanism [16].
The rates of the unimolecular reaction were found
to dominate at supercritical water conditions. The
unimolecular decomposition reactions of C
6
H
5
OO
tested in the SCWO mechanism are
CHOO↔ CHOⳭ O (R2)
65 65
CHOO↔ CHO Ⳮ H (R3)
65 642
CHOO↔ CH Ⳮ CO (R4)
65 55 2
The rate coefficient of reaction R2 was calculated
using QRRK analysis and the reverse rate constant
by microscopic reversibility. Inclusion of reactions
R3 and R4 was necessary, as will be shown below, to
gain agreement between the predicted and experi-
mental benzene concentration profile. However, re-
actions R3 and R4 are not elementary reactions, and
rate coefficients for these overall steps were chosen
so as to affect the radical pool to give the best fit
between the mechanism predictions and the ben-
zene concentration profile measured at 813 K, 246
bar, and residence times of 3–7 s. Their reverse rate
coefficients were calculated by microscopic reversi-
bility. Since the present rate coefficients for reac-
tions R3 and R4 are semiempirical, any attempt to
use them in other models should be pursued with
caution. While the mechanisms by which reactions
R3 and R4 take place have not been experimentally
determined and likely involve never observed inter-
mediates, theoretical calculations using density func-
tional analysis show that reactions R3 and R4 involve
one common isomerization path through a dioxetane
cyclic intermediate. After this first isomerization, a
path to C
6
H
4
O
2
Ⳮ H is present. A second series
(unzipping process) is also present, along with an-
other isomerization series. The rate-controlling step
involves a 3,2,0 bicyclic (four plus five member ring)
tight transition state. CO
2
results from the unimo-
lecular decomposition of this bicyclic [35–37].
Justification for the inclusion of reaction R3 comes
from the detection of C
6
H
4
O
2
during benzene com-
bustion at both fuel-rich and fuel-lean conditions at
900–1300 K [25], our observation of C
6
H
4
O
2
in the
oxidation of benzene under supercritical water con-
ditions [16,19], and the incorporation of the overall
reaction of C
6
H
5
and O
2
to C
6
H
4
O
2
and H in the
benzene combustion mechanism of Tan and Frank
[24]. Since the reaction between C
6
H
5
and O
2
pre-
dominantly forms C
6
H
5
OO at 246 bar and 813 K,
reaction R3 was used in place of the addition/elim-
ination reaction suggested by Frank et al. [33] to
1532 ELEMENTARY REACTION KINETICS
Fig. 1. Benzene concentration profiles predicted by the
supercritical water benzene oxidation mechanism: T ⳱ 813
K; P ⳱ 246 bar; U ⳱ 1.0; [C
6
H
6
]
0
⳱ 0.6 ⳯ 10
ⳮ
3
mol/L;
(䊊) experimental data; — — mechanism of Shandross et
al. [17] and Shandross [18]; ——— SCWO mechanism in-
cluding reaction R3 C
6
H
5
OO ↔ C
6
H
4
O
2
Ⳮ H with k
f,3
⳱
4.0 ⳯ 10
8
s
ⳮ
1
and reaction R4 C
6
H
5
OO ↔ C
5
H
5
Ⳮ CO
2
with k
f,4
⳱ 1.6 ⳯ 10
8
s
ⳮ
1
; ------ SCWO mechanism with-
out reaction R3 or R4; —-- – SCWO mechanism including
only reaction R3 with k
f,3
⳱ 4.0 ⳯ 10
8
s
ⳮ
1
.
account for C
6
H
4
O
2
production. Further reactions
of C
6
H
4
O
2
were accounted for by incorporating a
recent submechanism [38] by which C
6
H
4
O
2
is fully
oxidized.
Reaction R4 is included to explain the very early
appearance of CO
2
in benzene oxidation under su-
percritical water conditions [16]. The CO
2
yields ex-
ceeded those of CO at all conditions studied and for
all conversions of benzene. Observations of CO
2
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 CO
2
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 CO
2
at low benzene conver-
sions and postulated CO
2
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 C
6
H
5
and O
2
leads to formation of
C
5
H
5
and CO
2
. In the present study, since C
6
H
5
OO
is the primary reaction product of C
6
H
5
and O
2
at
246 bar and 813 K, reaction R4 was included in place
of an addition/elimination reaction.
Comparison of Model Predictions and
Measurements of Benzene Oxidation under
Supercritical Water Conditions
The predictions of the benzene SCWO mecha-
nism [16], developed by adapting the benzene com-
bustion mechanism of Shandross et al. [17] and
Shandross [18] for the present conditions (813 K,
246 bar, and fuel equivalence ratio, U, of 1.0), are
compared in Fig. 1 (solid line) against the experi-
mental data (points). Also shown is the profile pre-
dicted by the original mechanism of Shandross et al.
[17] and Shandross [18] (long-dashed line), which
generally represents the predictions of low-pressure
benzene combustion mechanisms at SCWO condi-
tions. Dissociation reactions of C
6
H
5
OO to C
6
H
4
O
2
and H and C
5
H
5
and CO
2
were necessary in the
SCWO mechanism to improve the agreement be-
tween the predictions and the data. With their in-
clusion, the shape and position of the predicted
benzene concentration profile agree with the exper-
imental data. The absolute values of the rate coef-
ficients of reactions R3 and R4 were found not to
have significant effects on the model predictions as
long as the ratio of k
f,4
to k
f,3
is equal to 0.4. If this
ratio is larger or smaller than 0.4, the predicted ben-
zene reaction rate is too slow or too fast, respectively.
Without inclusion of reaction R3 or R4, the pre-
dicted benzene oxidation rate is too fast and the re-
action delay is too small (short-dashed line). The in-
duction time can be varied from 0 to a maximum of
⬃1 s by changing the rate coefficient of reaction
R3. The dashed-and-dotted line represents the
slowest benzene oxidation rate that can be achieved
through adjustment of k
f,3
(without including reac-
tion R4).
The net rates of formation or destruction of key
species by the individual reactions in the mechanism
were calculated to determine the controlling reac-
tions at 813 K and 246 bar with stoichiometric oxy-
gen. By comparing the net rates of all reactions in-
volving a single species, the primary destruction and
formation pathways were determined.
Benzene was found to react almost exclusively by
reactions R5 and R6:
CH Ⳮ OH ↔ CHOHⳭ H (R5)
66 65
CH Ⳮ OH ↔ CH Ⳮ H O (R6)
66 65 2
with reaction R6 accounting for over 97% of the ox-
idation rate of benzene at 813 K and 246 bar. The
rate coefficient used for reaction R6 [32] is the same
as that used by Shandross et al. [17] and Shandross
[18]. Although reaction R5 is recognized to proceed
by a chemically activated pathway [44], given the
relative unimportance of reaction R5 the rate coef-
ficient used [45] is the same as that used by Shan-
dross et al. [17] and Shandross [18]. The destruction
of phenyl radical (C
6
H
5
) formed by reaction R6 is
completely accounted for by reaction R7:
ANALYSIS OF A BENZENE SCWO MECHANISM 1533
Fig. 2. Phenol concentration profiles predicted by the
supercritical water benzene oxidation mechanism: T ⳱ 813
K; P ⳱ 246 bar: U ⳱ 1.0; [C
6
H
6
]
0
⳱ 0.6 ⳯ 10
ⳮ
3
mol/L;
(ⵧ) experimental data; — — mechanism of Shandross et
al. [17] and Shandross [18]; ——— SCWO mechanism in-
cluding reaction R3 C
6
H
5
OO ↔ C
6
H
4
O
2
Ⳮ H with k
f,3
4.0 ⳯ 10
8
s
ⳮ
1
and reaction R4 C
6
H
5
OO ↔ C
5
H
5
Ⳮ CO
2
with k
f,4
⳱ 1.6 ⳯ 10
8
s
ⳮ
1
; ------ SCWO mechanism with-
out reaction R3 or R4; —--– (almost hidden by ———)
SCWO mechanism including only reaction R3 with
k
f,3
⳱ 4.0 ⳯ 10
8
s
ⳮ
1
.
CH Ⳮ O ↔ C H OO (R7)
65 2 65
The fate of C
6
H
5
OO is the most critical unknown.
Comparisons with the data suggest that the radical-
forming, chain-branching loss channel to C
6
H
5
O
and O (reaction R2) can only be a minor channel,
and analysis of thermochemistry and the kinetics
from the density functional analysis supports the ra-
tio of 0.4 for k
f,4
/k
f,3
under SCWO conditions [37].
The reactions and associated rate constants of cy-
clopentadienyl (C
5
H
5
) and cyclopentadiene (C
5
H
6
)
in the Shandross mechanism were taken from the
mechanism of Emdee et al. [21], in which the C
5
H
5
submechanism was based on the outline presented
by Brezinsky [46], and the abstraction of H from
C
5
H
6
by HO
2
, OH, H, and O was estimated from
the analogous reactions with formaldehyde. In the
SCWO-mechanism, we computed rate coefficients
using CHEMDIS for the addition and combination
reactions of C
5
H
5
and C
5
H
6
at 246 bar from 300
to 1000 K using the data presented in Zhong and
Bozzelli [47,48]. Reactions with the highest rate co-
efficients at 813 K and 246 bar were included in
the benzene SCWO mechanism, according to
which C
5
H
5
from reaction R4 reacts primarily by
reaction R8:
CH Ⳮ HO ↔ CHOⳭ OH (R8)
55 2 55
C
5
H
5
O then undergoes ring-opening reactions,
leading eventually to CO and CO
2
.
Since the oxidation of benzene proceeds mainly
by reaction R6, the reaction delay (or induction
time) and the subsequent rate of benzene reaction
are determined by the rate of OH radical generation,
which, in the present mechanism, is primarily by re-
action R8.
If the global reactions R3 and R4 are not incor-
porated into the mechanism, reaction R2 is the dom-
inant C
6
H
5
OO destruction channel. With O formed
by reaction R2, excess OH is generated directly by
reaction R9
O Ⳮ HO↔ OH Ⳮ OH (R9)
2
and indirectly by the following series of reactions:
CH Ⳮ O ↔ CHOⳭ H (R10)
66 65
H Ⳮ O ↔ HO (R11)
22
HO Ⳮ HO ↔ HO Ⳮ O (R12)
22222
HO ↔ OH Ⳮ OH (R13)
22
Reaction R6 proceeds much too quickly, and the
present mechanism overpredicts the benzene oxi-
dation rate.
As stated previously, the rate coefficients of reac-
tions R3 and R4 were chosen to optimize model-
data agreement for the benzene concentration pro-
file. Including reaction R3 with a higher rate
constant than reaction R2 eliminates the rapid for-
mation of O and the subsequent overproduction of
OH by reaction R9. Since reaction R3 generates H,
OH formation proceeds through reactions R11, R12,
and R13, and inclusion of reaction R3 alone cannot
decrease the rate of benzene oxidation sufficiently
to bring the model into agreement with the data. As
discussed earlier, the reaction of H and O
2
produc-
ing OH and O is not important at SCWO conditions.
To gain model-data agreement, reaction R4 was in-
cluded. The inclusion of reaction R4 slows the ben-
zene oxidation rate, as C
5
H
5
is relatively unreactive.
Reaction R4 also provides a pathway for CO
2
for-
mation to account for the experimental observation
that CO
2
yields exceed those of CO for all measured
residence times and reaction conditions [19]. With-
out reaction R4, the model incorrectly predicts that
the CO
2
concentration remains below that of CO for
residence times less than 7 s. Even with reaction R4,
the model underpredicts the concentrations of CO
and CO
2
, reflecting inadequate chemistry for further
oxidation of intermediate species. A large fraction of
carbon remains as C
6
H
4
O
2
, and C
6
H
3
O
2
,C
5
H
5
O,
C
4
H
4
, and H
2
CCCCH are also significant.
Figure 2 demonstrates that with inclusion of re-
actions R3 and R4, the predicted C
6
H
5
OH concen-
tration profile agrees with the experimental data.
![Fig. 4. Comparison of predicted (lines) and measured (symbols) benzene concentrations at three fuel equivalence ratios (U): ( , – –) U 0.5; ( , ———) U 1.0 ; ( , - - - -) U 2.5. T 813 K; P 246 bar; [C6H6]0 0.6 10 3 mol/L.](/figures/fig-4-comparison-of-predicted-lines-and-measured-symbols-2ql7ajsk.webp)
![Fig. 3. Comparison of predicted (— —) and measured ( ) benzene conversions at various temperatures: s 6.2 s; P 246 bar; U 1.0; [C6H6]0 0.6 10 3 mol/L.](/figures/fig-3-comparison-of-predicted-and-measured-benzene-j3d4tk4w.webp)
![Fig. 2. Phenol concentration profiles predicted by the supercritical water benzene oxidation mechanism: T 813 K; P 246 bar: U 1.0; [C6H6]0 0.6 10 3 mol/L; ( ) experimental data; — — mechanism of Shandross et al. [17] and Shandross [18]; ——— SCWO mechanism including reaction R3 C6H5OO ↔ C6H4O2 H with kf,3 4.0 108 s 1 and reaction R4 C6H5OO ↔ C5H5 CO2 with kf,4 1.6 108 s 1; - - - - - - SCWO mechanism without reaction R3 or R4; —- -– (almost hidden by ———) SCWO mechanism including only reaction R3 with kf,3 4.0 108 s 1.](/figures/fig-2-phenol-concentration-profiles-predicted-by-the-w08rqvn3.webp)
![Fig. 1. Benzene concentration profiles predicted by the supercritical water benzene oxidation mechanism: T 813 K; P 246 bar; U 1.0; [C6H6]0 0.6 10 3 mol/L; ( ) experimental data; — — mechanism of Shandross et al. [17] and Shandross [18]; ——— SCWO mechanism including reaction R3 C6H5OO ↔ C6H4O2 H with kf,3 4.0 108 s 1 and reaction R4 C6H5OO ↔ C5H5 CO2 with kf,4 1.6 108 s 1; - - - - - - SCWO mechanism without reaction R3 or R4; —- -– SCWO mechanism including only reaction R3 with kf,3 4.0 108 s 1.](/figures/fig-1-benzene-concentration-profiles-predicted-by-the-2tjkb0tk.webp)