Methanol Oxidation in Supercritical Water
J. Phys. Chem., Vol. 100, No. 39, 1996 15835
coefficients for free radicals cannot be calculated with any real
accuracy. It is noteworthy that at temperatures above 550 °C
supercritical water behaves as a nearly ideal gas. Consequently,
using high-temperature data to validate a DCKM allows one to
focus exclusively on the reaction mechanism and kinetics
without having the confounding influence of thermodynamic
nonidealities.
We are not aware of any published data that simultaneously
fulfill all three of the requirements outlined above (simple
hydrocarbon, concentration profiles at different temperatures,
high temperatures). Previous experimental studies of meth-
ane4,11,12 and methanol10,13 SCWO were designed and performed
for purposes other than testing a DCKM. Most previous
experiments were performed at temperatures where fluid-phase
nonidealities could influence the reaction rates. Moreover, the
data obtained from high-temperature experiments consist of
concentrations at isolated times, rather than complete concentra-
tion profiles. This lack of suitable data led us to initiate the
present work. We oxidized methanol in supercritical water at
temperatures up to 589 °C, and we measured the temporal
variations of the reactant and products’ concentrations. This
paper presents these new experimental results and a comple-
mentary DCKM.
provide additional details about the analytical protocol. This
analytical protocol for the liquid phase provided good separation
of methanol and formaldehyde (a likely oxidation product), but
the formaldehyde peaks in the reactor effluent samples were
too small to allow reliable quantitative analysis. Therefore, we
estimated the yield (Y is the moles of product formed per mole
of methanol fed to the reactor) of formaldehyde by using the
measured yields of methanol, CO, and CO2 with the assumption
that the carbon mass balance depends only on CO, CO2,
methanol, and formaldehyde as shown by eq 1.
YF ) 1 - YM - YCO - Y
(1)
CO2
Experimental Results
Table 1 displays the experimental conditions investigated and
the results obtained. Methanol conversions from less than 10%
to essentially 100% were obtained by changing the residence
time at each temperature. CO and CO2 were the major gas-
phase products, and the yield of CO almost always exceeded
that of CO2. Exceptions occurred for experiments where the
gas yield was low (around 1%) and thus subject to an uncertainty
that was comparable to the yield itself. Exceptions also occurred
at very high methanol conversions where presumably much of
the CO had been further oxidized to CO2. Formaldehyde was
present as an aqueous-phase product in some of the experiments,
but its yield was almost always less than 20%.
Experimental Section
Experimental Procedure. We conducted methanol oxida-
tion experiments in supercritical water at temperatures between
500 and 589 °C and at 246 atm in an isothermal, isobaric, tubular
plug-flow reactor. The reactor was a Hastelloy C-276 tube,
and we used three different inner diameter-length combinations
(0.108 cm × 33 cm, 0.108 cm × 100 cm, and 0.140 cm × 100
cm) to explore a wide range of residence times.
Global Kinetics of Methanol Disappearance
The global kinetics of methanol disappearance can be
conveniently examined and compared with previous work by
assuming that the global rate of this reaction is proportional to
the methanol concentration and independent of the O2 concen-
tration and the water density. This type of pseudo-first-order
analysis has been employed in previous SCWO studies.
Moreover, there is a growing interest in computational fluid
dynamics studies of SCWO reactors,15 and reliable but simple
kinetics models are required.
For first-order kinetics, a plot of ln(1 - X), where X is the
methanol conversion, against the residence time should give a
straight line at each temperature. Figure 1 shows that the data
can be fit by a straight line. The line does not pass through the
origin, but rather intersects the x-axis at a positive residence
time. The x-intercepts for each line provide estimates of the
length of the induction period, and the slopes provide estimates
of the rate constants. The kinetics parameters that result from
this analysis appear in Table 2. The uncertainties in Table 2
and elsewhere in this paper are the 95% confidence intervals.
The data in Table 2 show that the length of the induction period
decreases as the temperature increases. This trend is consistent
with recent results from Rice et al.,10 but if we extrapolate their
results to our higher temperatures, the “predicted” induction
times are much shorter than those we determined experimentally.
We note that Rice et al. used much higher methanol concentra-
tions in their experiments, and they found that the kinetics were
not truly first order at lower temperatures (<470 °C).
All water was distilled, deionized, and degassed prior to use.
Aqueous solutions of methanol (Fisher, 99.9% purity) and of
oxidant were preheated to the reaction temperature separately
in two 0.108 cm i.d. × 2 m long Hastelloy C-276 preheat lines
immersed in an isothermal fluidized sand bath. The two
solutions were mixed at the reactor entrance and the reactor
temperature was measured using a thermocouple. In all
experiments, molecular oxygen (O2) was the oxidant present at
the reactor entrance. In most of the experiments, the O2 was
produced from the decomposition of H2O2 (Fisher, 99.9%) in
the preheater line. Complete conversion of H2O2 to O2 and
water in the preheater line was verified experimentally. In other
experiments, high-pressure, gas-phase O2 was dissolved in water
to produce the oxidant stream. In all experiments, methanol
oxidation occurred as the reactants traveled through the tubular
reactor. In a given experiment, the residence time in the reactor
was fixed by controlling the total flow rate of the reactants while
maintaining a nearly constant molar ratio of methanol to O2 at
the reactor conditions (temperature and density). Immediately
upon leaving the reactor, the effluent stream was cooled in two
consecutive water-cooled heat exchangers. The pressure was
then reduced to ambient conditions, and the gaseous and liquid
phases were separated. Martino et al.14 provide additional
details about the reactor system and the operating procedure.
Analytical Techniques. The amounts of CO and CO2 in
the gas phase were determined by a gas chromatograph (GC)
with a thermal conductivity detector (TCD). The GC housed a
10 ft stainless steel column packed with 100/120 mesh Supelco
Carboseive S-II. The liquid phase was collected and then
analyzed by a GC with a flame ionization detector (FID) to
measure the amount of methanol remaining. This GC housed
a 6 ft glass column packed with 80/100 mesh HayeSep P. The
analysis was done isothermally at 150 °C. Savage et al.12
Figure 2 shows an Arrhenius plot of the pseudo-first-order
rate constant, k1, for methanol disappearance after the induction
period. The Arrhenius parameters corresponding to k1 are A )
1021.3(5.3 s-1 and Ea ) 78.4 ( 20.1 kcal/mol. Tester et al.13
reported kinetic measurements for methanol oxidation in su-
percritical water over the temperature range 450-530 °C and
obtained a global activation energy of 97.7 ( 20.4 kcal/mol. In
their kinetics analysis, however, they did not consider the
induction period, possibly because their data did not allow it to
be determined. Rice et al.10 also investigated methanol oxidation