FUNDAMENTAL STUDIES OF OXIDATION KINETICS
AND SALT PRECIPITATION IN SUPERCRITICAL WATER
Research Supported by
NASA-Johnson Space Center
Contract No. NAG9-252
Final Report, Volume I: Research Summary
January 1987 - September 1992
J. W. Tester
P. A. Webley
F. J. Armellini
H. R. Holgate
J. C. Meyer
Energy Laboratory
and
Chemical Engineering Department
Massachusetts Institute of Technology
Cambridge, MA 02139
November
..
I
15, 1992
FUNDAMENTAL STUDIES OF OXIDATION KINETICS
AND SALT PRECIPITATION IN SUPERCRITICAL WATER
Final Report, Volume I: Research Summary
Table of Contents
1. Introduction and Scope ...................................
1
2. Project Objectives .......................................
2
3. Approach ............
3.1 Oxidation Studies
3.2 Salt Studies ....
........................................
........................................ 22
........................................ 4
4. Research Accomplishments .........
..............................
4.1 Experimental Oxidation Studies ............................
4.2 Elementary Reaction Modeling .............................
4.3 Salt Studies ..............
.............................
5. Suggested Future Work ...
5.1 Oxidation Studies .
5.2 Salt Studies .....
7
7
15
18
......................................
...................................... 2828
............................I......... 28
6. Research Impact ..............................................
29
7. Acknowledgements
............................................
30
8. Literature Cited ..............................................
30
9. Documentation of Research Contributions ...........................
9.1 Papers Published and Accepted for Publication .................
9.2 Completed Doctoral Theses ................................
9.3 Other Published and Forthcoming Papers .....................
32
32
33
34
i
1. Introduction and Scope
Future long-term space flights will require on-board water/waste recycling in a
partially or fully enclosed life support system. Development of suitable water
recycling systems is therefore of prime importance to the future of the space
program. Oxidation of the products of human metabolism in supercritical water has
been shown to be an efficient way to accomplish this recycling, since even refractory
compounds such as ammonia can be oxidized rapidly and completely without
formation of harmful by-products.
In the supercritical water oxidation (SCWO) process, the slurried metabolic
waste stream is mixed with air (or oxygen) and then heated and pressurized above
the critical point of pure water (374°C and 221 bars (3205 psia)). Due to its unique
solvation properties, supercritical water provides intimate single-phase contact
between oxygen and the waste, which results in high destruction efficiencies. Partial
oxidation of human wastes leads to the formation of smaller compounds, whose
oxidation is frequently the rate limiting step in the overall destruction of the waste.
A fundamental understanding of the oxidation kinetics and mechanisms of a wide
range of organic compounds in supercritical water is central to the successful design
and development of a SCWO unit.
In addition to the near-complete destruction of the organic matter, the
oxidation of metabolic waste in supercritical water also provides a means for the
separation of salts from the aqueous waste stream. Salts are a natural constituent of
human aqueous waste, and are also formed during the oxidation and subsequent
neutralization of certain metabolic wastes. Due to the low solubility of most salts in
supercritical water, the salts rapidly precipitate from the waste stream. Knowledge of
nucleation and crystal growth rates of salts in supercritical water is essential for
efficient salt separator design in the waste treatment process.
The work reported here covers the period from January 1987 through
September 1992 when research on oxidation of organic compounds in supercritical
water was supported by the National Aeronautics and Space Administration. During
that period several related projects in our laboratory received partial support from
other organizations including the Environmental Protection Agency, the Department
of Energy, and the Army Research Office. Research results from all studies
conducted in our laboratory relevant to the proposed use of SCWO for space
missions are summarized in this final report.
Volume I summarizes the major findings of our work, provides
recommendations for future studies, and discusses the impact of the research to
applications of SCWO for metabolic waste treatment and water recycle on long-term
space missions. Volume II contains the detailed documentation of our research in
the form of a complete set of indexed papers and reports published during the
project.
1
2. Project Objectives
The main objectives of our research project were to:
1.
Obtain a better understanding of the kinetics and mechanisms involved in the
oxidation of organics in supercritical water. Specifically, the effect of process
variables such as temperature, reactor residence time, feed concentration and
pressure on the rates of oxidation will be determined to aid in defining
optimum process conditions.
2.
Determine the effect (if any) of the reactor walls on the rate of oxidation.
This is essential if conclusions regarding reaction mechanisms and the effect of
process variables on oxidation rates are to be made. In addition, catalytic
effects could be exploited to achieve higher oxidation rates.
3.
Obtain a fundamental understanding of salt formation during the supercritical
water oxidation process, including determining salt crystallization mechanisms
and rates to provide basic data for developing efficient separation methods
and procedures.
3. Approach
3.1 Oxidation Studies
Our group at MIT has constructed an isothermal, isobaric, continuous-flow
reactor system for the measurement of oxidation kinetics of organic compounds in
supercritical water. A schematic of the experimental apparatus is shown in Figure 1.
The reactor consists of 4.71 m of 0.635-cm OD x 0.171-cm ID tube of corrosionresistant, high-strength Inconel 625 and is immersed in a high-temperature fluidized
sand bath for temperature control. The operating capability of our reactor is
25-700C, 1-250 atm and residence times of 5-20 seconds. Feed concentrations of
oxygen and organic are restricted by the solubility of oxygen and organic in water at
room temperature and at the pressure of the feed gas.
The reactor system allows us to measure the rate of oxidation of model
compounds, such as methanol and ammonia, as a function of temperature, feed
concentration and residence time. This information is useful for the design and
development of commercial-scale, supercritical water reactors. In addition,
measurement of the composition and concentration of the products of oxidation
allows chemical pathways to be determined. This information is useful for testing
theoretical and semi-empirical models describing oxidation in supercritical water.
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A packed bed reactor has also been constructed of Inconel 625 for the
determination of possible effects of the reactor wall material on the rates of
oxidation. The reactor is 61.67 cm long with an inner diameter of 0.912 cm and an
outer diameter of 1.429 cm. It is packed with -60+100 mesh (150-250 Mm)Inconel
625 beads. With a free volume of 15.63 cm3, the packed bed reactor has a surface-tovolume ratio 20.5 times that of the tubular reactor. Rates of oxidation, if affected by
the reactor surface, should be significantlydifferent in the packed bed reactor.
3.2 Salt Studies
An experimental apparatus capable of operating in either a static or dynamic
mode has been constructed for studying phase equilibria and precipitation
phenomena in aqueous salt solutions at sub- and supercritical conditions. The static
experiments are used to identify phase boundaries in salt-water systems, while the
dynamic or flow experiments are used to characterize salt formation at conditions
prevalent in the actual waste treatment reactor.
The primary component of the salt studies apparatus is an optically accessible
cell, which is shown in Figure 2. The cell is constructed from a high nickel, corrosion
resistant alloy, Inconel 625, and is capable of operating up to 600°C and 340 bars.
The cell is cubic in shape with an outer length of 12.7 cm and an inner working
volume of approximately 25.0 cm3. All six faces of the cell contain threaded ports, in
which either Inconel 625 window holders, plugs or flow tubes are inserted. All the
ports are interchangeable, which allows for many different cell configurations. The
cell is designed for both side and forward light scattering measurements up to angles
of 10° from the centerline.
A schematic of the experimental apparatus for studying phase behavior in saltwater systems is shown in Figure 3. The apparatus is assembled to perform
extinction measurements on a static solution. Light extinction or visual observation is
used to identify phase transformations and thus locate phase boundaries. The optical
equipment includes; a 10 mW He-Ne laser, a 5x beam expander, a silicon
photodetector, and a picowatt power meter. Twelve strip heaters (two attached to
each face ) heat the cell, which is encased with high temperature ceramic insulation.
A high performance liquid chromatography (HPLC) pump with a self-flushing head
keeps the pressure constant in the cell during the isobaric experiments. The entire
apparatus sits on an optical table, which dampens any undesirable room vibrations.
The dynamic experiments simulate the rapid mixing of a waste stream
containing dissolved salts with supercritical water in the SCWO process. Figure 4
shows the apparatus for the flow / shock crystallization experiments. The two feeds in
the experiments are a pure supercritical water (SCW) stream (typically at 550°C,
250 bar, and flowing at 10.2 g/min) and a cooler salt solution (0.5 to 10.0 wt%,
typically at 150°C, 250 bar, and flowing at 0.5 g/min). The Inconel 625 optical cell is
configured with two large flow tubes (0.912 cm ID) in the top and bottom ports, two
4
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5
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Figure 4. Schematic of apparatus for flow/shock crystallization experiments.
6
window holders in the front and back ports for photographic documentation and light
scattering measurements, and two Inconel 625 plugs in the other side ports. The salt
solution is injected co-axiallyinto the SCW stream as a jet using a water-cooled
nozzle, which passes down the center of the top flow tube. Particle collection runs
and laser transmission measurements are performed with the nozzle exit located in
the upper flow tube to assure complete mixing of the jet and the SCW stream. The
flow experiments are used to identify important mechanisms of salt growth and
estimate particle sizes of the solids formed in the process.
4. Research Accomplishments
4.1 Experimental Oxidation Studies
To date, we have measured the oxidation rates of a variety of model
compounds in supercritical water: hydrogen, carbon monoxide, ethanol, methane,
methanol, glucose, ammonia, and ammonia/methanol mixtures. A first-order
Arrhenius plot (in which the reaction is assumed to be first order in organic and zero
order in oxygen) for the individual model compounds is shown in Figure 5, and the
regressed global rate forms are shown in Table I. These compounds were chosen as
representative products of oxidation of larger organics such as cellulose and urea, and
would be the rate limiting step in the overall oxidation to carbon dioxide and
nitrogen. In particular:
(1)
The oxidation kinetics of carbon monoxide over the temperature range 400 to
540°C were determined at a pressure of 246 bar (24.6 MPa). In addition to
direct oxidation with oxygen, it was found that reaction of carbon monoxide
with water (the water-gas shift reaction) was significant. At 400°C, as much as
70% of the carbon monoxide was oxidized by water and not oxygen. This
fraction decreased with increasing temperature, to about 25% at 540°C. The
effects of temperature and concentration on direct and indirect oxidation
kinetics of carbon monoxide were correlated with global models. The
oxidation of carbon monoxide was found to be first order in carbon monoxide
and independent of oxygen concentration over the range investigated.
Elementary reaction models were also used in an attempt to model the
complex chemistry of the reaction pathways, with only some success.
(2)
Arrhenius parameters for ethanol oxidation were determined over the
temperature range 480 to 540°C, assuming the reaction was first order in
ethanol and zero order in oxygen. The major products of the reaction were
carbon monoxide, carbon dioxide, and acetaldehyde. The reaction exhibited
an apparent activation energy of 340 kJ/mol. The ethanol results have been
omitted from Figure 5 for clarity.
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The oxidation kinetics of methane over the temperature range 560 to 650°C
were determined at a pressure of 246 bar. The major products of the
oxidation were carbon monoxide and carbon dioxide. No methanol was
detected in the effluent. The oxidation was first order in methane and
0.66-order in oxygen over the range of concentrations investigated. The
dependence of the reaction on oxygen concentration accounts for the scatter in
the methane data in Figure 5. The activation energy was 179.1 kJ/mol over
the experimental temperature range. Attempted pyrolysis of methane gave no
conversion at 650C and 15 seconds residence time. Oxidation of methane in
the packed bed showed no significant rate increase over the tubular reactor.
Elementary reaction models were used to model methane oxidation, but the
models showed limited ability to reproduce experimental results.
(4)
The oxidation kinetics of methanol over the temperature range 450 to 550°C
were determined at a pressure of 246 bar. The oxidation was found to be
highly activated, going from zero conversion at 4500 C to complete conversion
at 540C, both at a residence time of 8 seconds, with an apparent activation
energy of 408.8 kJ/mol. The products of oxidation were carbon monoxide,
carbon dioxide, and hydrogen. The oxidation rate was found to be
approximately first order in methanol and zero order in oxygen over the
concentration range investigated. Attempted pyrolysis of methanol gave a
conversion of 2.2% at 5440 C and 6.6 seconds residence time. A single
oxidation experiment in the packed bed reactor showed no rate increase over
the tubular reactor. Elementary reaction models of methanol oxidation
correctly predicted the experimental conversions, although the predicted
activation energy was too high, no hydrogen was predicted in the effluent, and
the product CO/CO2 ratio was too high.
(5)
The oxidation kinetics of ammonia over the temperature range 650 to 700°C
were determined at a pressure of 245 bar and residence times of 10-15
seconds. Over this range conversions run from 0 to 15%. The product of
ammonia oxidation is exclusivelynitrogen; no NO or NO2 was detected. The
oxidation was roughly first order with respect to ammonia, and was weakly
dependent on oxygen concentration, although the typical power-law rate form
did not fit the data well. Instead, a catalytic model yielded a much better fit.
Duplicate runs in the packed bed reactor, in the temperature range 530 to
680°C, showed that the oxidation was strongly affected by the increased
presence of Inconel 625. Conversions up to four times higher were observed
in the packed bed reactor at operating conditions similar to those in the
tubular reactor, demonstrating that ammonia oxidation is at least partly
catalytic. Small amounts of N2 0 were detected in the effluent of the packed
reactor. A pyrolysis experiment at 700C yielded no decomposition of
ammonia to nitrogen and hydrogen.
10
(6)
The oxidation kinetics of mixtures of ammonia and methanol were examined
in the tubular reactor over the temperature range 480 to 520°C. The
conversion of methanol was slightly enhanced by the presence of ammonia
The conversion of ammonia was unaffected by the presence of methanol in the
tubular reactor, although in the packed reactor the rate of ammonia oxidation
was slightly retarded by the addition of methanol.
(7)
The oxidation kinetics of hydrogen were examined in the tubular reactor in the
temperature range 495 to 600°C and at a pressure of 246 bar. Over the range
of experimental conditions, the reaction was independent of oxygen
concentration and first-order in hydrogen concentration, with an activation
energy of 372 kJ/mol.
Figure 5 also shows that hydrogen is more refractory
than carbon monoxide up to a temperature of about 550C, indicating that
hydrogen oxidation was probably unimportant in the earlier carbon monoxide
oxidation experiments in which hydrogen was formed as a product. The
hydrogen oxidation reaction was also found to possess a distinct induction time
of approximately 2 s at 550°C; this observation represents the first reported
identification of an induction time in supercritical water oxidation kinetics (see
Figure 6).
(8)
Oxidation of carbon monoxide has been reexamined at 246 bar and 420 to
5930C in an updated tubular-reactor experimental apparatus over an extended
range of concentrations and fuel equivalence ratios (oxygen/carbon monoxide
feed ratios). Figure 7 shows rate data for the direct oxidation
(CO + 2 2 -, CO2) and indirectwater-gas-shift(CO + H 20 - CO + H2)
pathways. Reaction of the carbon monoxide with water during preheating of
the reactor feeds was quantified on the basis of heat-transfer experiments and
was found to be unimportant for the experimental conditions studied.
Regression of the newer carbon monoxide oxidation data to a global rate form
(for the direct-oxidation pathway) revealed a fractional-order dependence on
oxygen concentration, which had not been observed earlier. The global
reaction was first-order in carbon monoxide, with and activation energy of 134
ld/mol. Additional experiments showed that carbon monoxide oxidation in
supercritical water also possesses an induction time. Furthermore, hydrogen
formation (by the global water-gas shift pathway) during carbon monoxide
oxidation was facilitated by the presence of oxygen, and was very slow in the
complete absence of oxygen. Hydrogen formation was also strongly dependent
on the fuel equivalence ratio, with fuel-rich conditions favoring its formation.
Higher water concentrations favor hydrogen formation, and the water-gas shift
pathway is thus expected to be less important under practical supercritical
water oxidation conditions, where water concentrations are lower.
(9)
As shown in Figure 8, the oxidation of hydrogen and carbon monoxide, at 550
and 570°C, respectively, was strongly pressure (water-density) dependent over
the range of 118 to 263 bar (approximately a threefold variation in density,
from 0.03 to 0.08 g/cm3 ), with higher pressures favoring higher oxidation rates.
11
"Induction Times"
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6
Residence Time, s
7
8
9
10
Normalized Hydrogen Decay Profiles for Stoichiometric
H 2 -0 2 -H2 0 Mixtures, Demonstrating Effective Induction
Times.
Experimental conditions: 550±2 °C; [H20]
=
(4.25±0.08) x 10-3 moVcm3;
A-[H2]o= (3.06_0.03) x 10-4 moVcm , [02]o = (1.55±0.02) x 10 - moVcm 3 ;
O3-[H2]o= (2.06±0.02) x 10- moVcm 3 , [0210= (1.04_0.01) x 10- 6 moVcm 3 ;
3
x 10 - 6 moVcm 3 .
1-[H
2], = (1.065±0.015) x 10- moVcm , [0210 = (0.54+0.01)
3
Curves are exponential fits to data.
12
Reactor Temperature, °C
3
600 580 560 540 520 500 480 460
440
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Arrhenius Plot of Direct Carbon Monoxide Oxidation and
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Direct oxidation data include both the current data and the data of Helling and
Tester (1987).
13
Fluid Density, g/cm 3
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Effects of Operating Pressure on Apparent First-Order Rate
Figure 8a.
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conditions: [H2 0 = (1.06±0.02) x 10- mol/cm3, [0210= (0.53+0.01) x 10- 6
mol/cm 3 .
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Effects of Operating Pressure on Apparent First-Order Rate
Constant, k*, for Carbon Monoxide Oxidation at 570 °C. Experimental
conditions: [CO]o= (1.03+0.01) x 106 mol/cm3 , [0210= (0.51 0.01) x 10-6
movcm3 . Results for two residence times (3.4 s, 4.1 s) are shown.
14
The effective order with respect to water for stoichiometric carbon monoxide
oxidation at 570°C was 1.7. This dependence suggests that a slight kinetic
penalty is incurred for operation of the supercritical water oxidation process at
lower pressures (or lower water concentrations).
(10)
Limited studies of hydrogen and carbon monoxide oxidation at 246 bar and
550 and 560°C, respectively, in the packed bed reactor showed that the
additional Inconel 625 surface area tended to inhibit oxidation, most likely
through termination of free radicals. The inhibitory effect of the surface was
not so severe, however, suggesting that tubular-reactor kinetic results at higher
temperatures are representative of a homogeneous reaction pathway.
(11)
Glucose hydrolysis and oxidation in supercritical water were studied at 246 bar
and 425 to 600°C. Glucose hydrolysis was found to proceed rapidly in
supercritical water, with a conversion of 97% at 425°C for a reactor residence
time of 6 s; kinetic measurements of glucose disappearance were therefore not
feasible in our normal range of operating conditions. Typical HPLC
chromatograms for the liquid effluent from comparable hydrolysis and
oxidation experiments are shown in Figure 9. Clearly, the number of liquidphase products is greatly reduced under oxidizing conditions, with a
correspondingly higher extent of gasification. The products formed from
glucose hydrolysis are diverse but also undergo hydrolysis; at 600°C and a
6-second reactor residence time, glucose is converted completely to gases, even
in the absence of oxygen. The presence of oxygen accelerates the destruction
of the intermediate products, with no liquid-phase products found above
550C at a 6-second reactor residence time. The major, persistent
intermediate products of glucose hydrolysis and oxidation are furfural, acetic
acid, acetonylacetone, propenoic acid, and acetaldehyde in the liquid effluent,
and carbon monoxide, methane, ethane, ethylene, and hydrogen in the gaseous
effluent. Small quantities of methane, hydrogen, and possibly ethylene are
present in the glucose oxidation effluent at temperatures up to 600°C at a
reactor residence time of 6 seconds. The high-temperature stability of
methane is consistent with our earlier study of methane oxidation.
4.2 Elementary Reaction Modeling
An elementary reaction model for hydrogen oxidation (Figure 10A), with
certain modifications for high pressure, was largely successful in reproducing the
experimentally observed kinetic behavior, including the global reaction orders and
Arrhenius parameters and the effect of pressure. A similar model for carbon
monoxide oxidation (Figure 10B) was somewhat less successful, exhibiting a higher
overall activation energy than the data and lacking an oxygen dependence. Hightemperature data under stoichiometric and fuel-rich feed conditions were reproduced
well. However, results for fuel-lean conditions were not correctly predicted by the
carbon monoxide model. The principal effects of the high pressure (water
15
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Reactor Residence Time = 6 s
c
IL:
-
- -.
,,
.0o
60.0
Retention Time (min)
Figure 9.
Comparison of HPLC Chromatograms of Liquid Effluent
from Glucose Hydrolysis and Oxidation in Supercritical
Water.
Nominal initial conditions: 500 °C, 1 x 10 mol/cm3 glucose, 6 x 10- mol/cm3
oxygen (for oxidation), 6 s reactor residence time. UV detection at 210 nm.
16
He
OH +
H 20
'
H20 2
H
02+ M
H2 0
HO
M
HO2
Figure 10a.
Major Free-Radical Reaction Pathways in the Elementary Reaction
Network for Hydrogen Oxidation in Supercritical Water.
Arrow thicknesses indicate relative rates of reactions.
AA
OH +
CO 2
OH
OH
H2
HO0
H2 02
A
H2 0
H2 0
02
02
A-'
OH
HO,2
HO2
OH-
H 20
02
Figure 10b.
Major Free-Radical Reaction Pathways in the Elementary Reaction
Network for Carbon Monoxide Oxidation in Supercritical Water.
Arrow thicknesses indicate relative rates of reactions.
17
concentration) on the oxidation mechanism are threefold: a) the dissociation of
hydrogen peroxide (H2 0 2 - OH + OH) is at or near its high-pressure limit; b) the
dissociation (recombination) of the hydroperoxyl radical (H + 02 - HO 2) approaches
its high-pressure limit; and c) the rate of the HO 2 + H20 - H2 0 2 + OH branching
reaction is greatly accelerated by the high water concentrations (densities) present in
supercritical water relative to typical gas-phase oxidation conditions. The majority of
the pressure (density) dependence of the oxidation reactions is accounted for by the
effect of the changing water concentration on the rate of the branching reaction. The
models for hydrogen and carbon monoxide oxidation are both highly sensitive to the
rate constant of the HO2 + H 2 0 - H 20 2 + OH reaction. Experimental data could
be reproduced with the assumed reaction network only if the value of this rate
constant under supercritical water conditions is significantly lower than its most
probable gas-phase value.
4.3 Salt Studies
Experimental techniques have been developed to determine phase boundaries
and precipitation mechanisms in aqueous salt solutions at conditions prevalent in the
SCWO process. So far, two salts have been studied, sodium chloride and sodium
sulfate, which are both contained in metabolic wastes. Specifically:
(1)
An optical cell capable of operating up to 6000 C and 340 bars was designed
and constructed in cooperation with Harwood Engineering (Walpole, MA) for
direct in-situ measurements of salt solubility, nucleation, and deposition. The
ancillary equipment for operating the optical cell in both a static and a
dynamic mode and performing laser extinction measurements was assembled.
(2)
A series of isobaric experiments examining phase behavior in the sodium
chloride and water system at the typical SCWO reactor pressure of 250 bars
were performed. These experiments verified the ability of the static
experimental apparatus for estimating phase boundaries. Figure 11 shows the
results of the isobaric runs on the NaCl-H20 phase diagram at 250 bar.
Homogeneous sodium chloride solutions of known concentration were heated
at constant pressure until a phase boundary was crossed and a new phase was
nucleated. In these experiments, which were documented on video tape, the
nucleation was identified visually, and appeared as small vapor bubbles rising
in the cell.
(3)
Static isobaric phase equilibrium experiments were performed on the
Na 2SO 4 -H 2 0 system at 250 bar. The nucleated phase was solid salt, which
appeared as fine particles settling inside the optical cell and salt crystals with
needle/dendrite morphology growing on the inner window surface. These
results are plotted on the sodium sulfate-water phase diagram in Figure 12.
The results verify the phase behavior of this system, which is distinctly
different than the NaCl-H2 0 system under conditions encountered in the
Modar process.
18
500
,
450
0
a
liquid;olid region
E
F
400
350
0.01
0.1
1
10
100
wt% NaCI
compilation of Bischoff and Pitzer (1989)
0
A Parisod and Plattner (1981)
- - - prediction of Pitzer and Pabalan (1986)
Sourirajan and Kennedy (1962)
V
+
Bischoff, et al, (1986)
estimated from Unke, (1958)
v
Olander and Uander (1950)
Khaibullin and Borisov (1966)
Present study
Figure 11.
Results of sodium chloride static isobaric experiments shown on the
temperature-composition NaCI-H2 0 phase diagram at 250 bar.
19
450
.. ...
. .. .. ... .
. . . . . . . . . . . . . . . . . . .. . . . . . . . .
1250 bar
400
|
fluid-solid salt region
o0
4,
I..
a
350
Ione phase fluid region
300
v This Study
Ravich and Borovaya (1964)
r wCnCv. . . .
)
....
·. . .· . 1. .
. . . . . . . I. . . I. . . I,,
. . . . . . . . . . .. . . . I..
. . . . . I. L,,,,,,,
5
10
15
20
wt% Na2 SO4
Figure 12.
Results of sodium sulfate isobaric experiments shown on the temperature composition Na2 SO4 -H2O phase diagram at 250 bar.
20
(4)
The solubility of sodium chloride in water vapor was determined at
supercritical temperatures ranging from 450 to 550°C and sub- and
supercritical pressures varying from 100 to 250 bar. In the experiments, pure
water was fed through an Inconel 625 tube packed with solid salt. Figure 13
shows a representative sample of the results at 500°C, compared with data
from other investigators. Measured sodium chloride concentrations in this
study ranged from 0.9 to 101 ppm (by weight), and were regressed with the
following semi-empirical correlation,
logCNaa = 3.866 logpw
1233.4
T
T
+ 7.772
where log is the logarithm in base 10, CNaa is the saturated NaC1l
concentration in ppm, pwis the pure water density in g/cm3 , and T is the
temperature in Kelvins. The equation does a good job of representing our
data over the full pressure range. Hydrolysis of solid NaC1lto form NaOH and
HC1 was found as a possible explanation for the discrepancies between our
data and those of Martynova (1964) and Styrickovich et al. (1955) at pressures
below 150 bar. Also, Sourirajan and Kennedy (1962) may have had difficulties
with their vapor-phase NaCl detection (Bischoff et al., 1986), while Alekhin
and Vakulenko (1988) determined NaCl concentrations using a novel but
unproven in-situ radioactive tracer technique.
(5)
Solubility experiments with sodium sulfate at 500°C and 250 bar were also
performed. Measured sodium sulfate concentrations were 0.9 ppm + 0.2 ppm,
and exhibited unsteady behavior. Figure 14 compares our value at 500°C with
data from other investigators. As with NaCl, there is large scatter in the data.
Though, only an estimate of Na2SO4 solubility could be obtained, this value
was over two orders of magnitude lower than that for sodium chloride at
identical conditions. Future experiments with Na2SO4 will be performed in a
batch system, to hopefully explain the discrepancies in the solubility data.
(6)
The shock crystallization of sodium chloride was examined at a pressure of
250 bar and a final mixed stream temperature of about 550°C. Jet
concentrations ranged from 0.5 to 10.0wt% with an exit temperature of
approximately 150°C. Results from SEM analysis of precipitated solids, in-situ
laser extinction measurements, and visual observation of the jets, indicate that
sodium chloride solutions first pass through a vapor-liquid state before solid
salt is formed. As shown in Figure 11, the vapor-liquid region in the phase
diagram extends over a wide concentration range. The size and morphology of
precipitated solids were concentration dependent. As shown in Figure 15, the
solids formed from a dilute 0.5 wt% jet appeared shell-like with lengths from 5
to 20 im, while kernel-shaped particles with lengths from 10 to 50 Am were
formed from a 3.0 wt% jet. The effect of operating at a subcritical pressure of
200 bar was also examined. For NaCl, the subcritical pressure caused a
dramatic increase in particle size, most likely due to a phase boundary shift in
21
.9
-
.
I
I
I
I
I
I
.
.
.
I
.
.
.
I
.
11
I
.
.
I
.
I
, I
I
I
I
I
I
I
.
I
.
_
##
r(o
L
.
100
E
aa
^^
I11111
c
101
z
1
- 0
-
Al
V.
I I I , I , , I I .I
0
v
I I,
II ,
100
200
Pressure (bar)
I
I
I
300
This study
semi-empirical correlation (see text)
- - - Pitzer and Pabalan model prediction (1986)
Styrickovich, et al. (1955)
O Martynova (1964)
O Sourirajan & Kennedy (1962)
o Galobardes, et al. (1981)
* Bischoff, etal. (1986)
# Alekhin & Vakulenko (1988)
*
Figure 13.
Comparison of sodium chloride solubility measurements and model
prediction at 5000C.
22
1000
"
111
I I...I
11
I
.
.
I
EZ1
100
II.I
.
II
II
I Il
_m
_-
_
_
I
O
_-
__1
E
/
-
0c
I I
0
f
!
10
C
I-
c
1
z
0.1
*
*
0
A
....... I......
.... ...
,,,I
......
.... .... .. .
0.01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . . . . . . ,,....I........,I
0
100
200
300
400
500
600
700
Pressure (bar)
A
o
0
*
v
Figure 14.
Martynova and Samoilov, 1962
Martynova, 1976
Morey and Hesselgesser, 1951
Styrikovich et al, 1955
Present study
Comparison of sodium sulfate solubility measurement at 5000C
and 250 bar with data from other investigators.
23
(a)
(b)
Figure 15.
SEM photos of NaCI particles precipitated from a jet mixed with
supercritical water at 250 bar, (a) 3.0 wt% jet, (b) 0.5 wt% jet (Jet feed:
1.0 g/min, 110-140 0 C; SCW feed: 10.2 g/min, 540 0 C).
24
the NaCl-H2 0 system. Particles with lengths between 0.5 and 1.0 mm were
formed from a 3.0 wt% jet at 200 bar.
(7)
The shock crystallization of sodium sulfate was examined at a pressure of 250
bar and a final mixed stream temperature of about 550C. Jet concentrations
ranged from 0.5 to 5.0 wt% with an exit temperature of approximately 1500C.
Because of the phase behavior of the Na2SO4-H20 system at 250 bar (see
Figure 12), sodium sulfate solutions nucleate solids directly from a
homogeneous fluid phase. The precipitated solids appeared much finer and
also more agglomerated than the sodium chloride solids. As shown in Figure
16, the diameter of the primary sodium sulfate particles ranged from 1 to 3
microns, while the aggregates reached diameters up to about 20 microns. The
precipitation mechanism changed with pressure. At 200 bar, a small vaporliquid region exists in the system, which possibly caused some solid formation
from a concentrated liquid brine phase. The size range of the collected
particles increased to between 2 and 50 Am.
(8)
In-situ laser transmission measurements were performed to quantify the extent
of solid formation during the shock crystallization experiments. Transmission
is defined as the attenuation of light intensity by scattering, which was
calculated as the measured laser power with a pure water jet (Io) divided by
the laser power measured during salt injection (I). Thus, a transmission of
100% represents no measurable scattering or zero turbidity. Figure 17
compares the transmission values for various NaCl and Na2SO4 jets. At 250
bar, the transmission measured during shock crystallization experiments with
sodium chloride jets was always much greater than that for sodium sulfate jets
under identical run conditions. Although only rough estimates, the average
Na2SO4 particle sizes calculated from the data were all between 0.3 and 1.6
Aum,while the calculated NaCl particle sizes ranged from 5 Am for the 0.5 wt%
jet to 90 Aumfor the 10 wt% jet. Transmissions for the 200 bar jets were all
approximately 100%, which suggests a shift to higher average particle sizes due
to the subcritical pressure. These results are consistent with SEM analysis of
the collected particles, described above.
(9)
Solid precipitation from mixed NaCl/Na 2 SO 4 solutions was studied at 250 bar.
From laser transmission measurements, it was concluded that the increased
solubility of Na2SO4 in NaCl brines caused less fine particle nucleation, and
favored liquid brine formation. The collected solids were caked on the inner
walls of the cell, which suggests that concentrated liquid brines exist in the
NaCl-Na 2 SO 4 -H2 0 system up to temperatures of 6000C.
(10)
Certain individual growth mechanisms believed to be important during the
shock crystallization process were modeled. Diffusion-limited growth was
shown to be a possible explanation for the micron size of the sodium sulfate
precipitates. A time of only 10 ms was estimated to be required for the
diffusion-limited growth of a 1.0 gsmNa2SO4 particle in supercritical water
25
(a)
(b)
Figure 16.
SEM photos of Na2SO4 particles precipitated from a 3.0 wt% jet mixed
with supercritical water at 250 bar, (a) aggregates collected on a Hastelloy
disk, (b) primary particles collected on a Hastelloy frit (Jet feed: 0.5 g/min,
110-140 0 C; SCW feed: 10.5 g/min, 550°C).
26
100
· iTEo
B1
0
80
o
o
60
-
0
E
cr
C
40
.a,
2,.
il-
20
J
XI,
0
0
1
2
3
4
10
wt% Salt in jet feed
Figure 17.
In-situ laser transmission measurement during the shock
crystallization experiments as a function of jet concentration.
27
from a 3.0 wt% jet at 385°C. This time is much less than the 10 s residence
time available for growth during the shock crystallization experiments.
Agglomeration calculations showed that primary particle collisions in the bulk
SCW medium could not account for the large Na2 SO 4 aggregates collected
during some runs. The aggregates, which had spherical heads of diameter of
about 20 /zm and long tails (see Figure 16(a)), most likely formed near the jet
exit where enhanced particle concentrations and velocity gradients existed.
(11)
Thermodynamic modeling of solubility and non-ideal behavior of NaCl-H2 0
systems from sub- to supercritical conditions was performed. The saturated
vapor-phase was modeled with the Peng-Robinson equation of state, while
conventional activity coefficient models were applied to the saturated liquidphase. In their present form, these approaches do not satisfactorily describe
the very non-ideal nature of the system in the temperature, pressure, and
composition ranges tested.
5. Suggested Future Work
5.1 Oxidation Studies
Future efforts will be directed toward improving experimental capabilities.
Modifications to the experimental apparatus will be made to increase the range of
operating conditions (concentrations, residence times, and pressures) and to maximize
the flexibility of the system. With the improved experimental resources, temporal
evolution of reactant and product concentrations during model compound oxidation
will be examined in greater detail, to provide additional data for validation of
elementary reaction models. With additional data, modeling efforts may be extended
to more complex compounds, and models may be examined in greater detail to reveal
sources of shortcomings. Experimental efforts will also be extended to additional
model compounds, including acetic acid, dimethyl sulfoxide, and possibly
acetaldehyde. Furthermore, additional glucose hydrolysis and oxidation experiments,
possibly at subcritical temperatures and pressures, will be conducted to quantify
global oxidation kinetics and to identify the implications of operating under off-design
conditions. Finally, the role of hydrogen peroxide as a more efficient oxidant will be
explored both experimentally and theoretically.
5.2 Salt Studies
In the future, phase equilibria and precipitation phenomena in other salt-water
systems will be studied, such as, KC1-H2 0, NaSO 3 -H2 0, and Na 2 HPO 4 -H2 0. The
effect of other components, such as organics or gases, will also be explored, since
these components make up a significant percentage of the actual SCWO reactor
contents. Additional shock crystallization experiments to determine the effect of
28
nozzle diameter on NaCI and Na2 SO4 particle sizes and morphology are required for
proper extension of the results to systems of larger scale. Experiments using a lowangle light scattering instrument to measure in-situparticle size distribution were
hindered by laser power fluctuations. Before additional tests are conducted,
modifications to the experimental apparatus are necessary to decrease the
temperature gradients in the optical cell, especially from the window ports. Finally,
the diffusion-limited growth and agglomeration calculations should be extended to
estimate the effects of concentration and temperature gradients at the jet exit. These
gradients can be approximated from transport modeling. Numerical simulations of a
laminar jet exhibited unrealistically slow mixing rates, thus some degree of buoyancydriven turbulence must be considered.
6. Research Impact
The primary benefit of our research has been to provide fundamental
information for the design and operation of supercritical water oxidation systems.
Kinetic studies of model compounds have furnished global rate expressions which can
now be used in the design of future systems; in fact, ABB Lummus Crest and
MODAR, Inc. are currently using the global rate expression for ethanol oxidation in
their coupled kinetic/hydrodynamic design and modeling study of a commercial-scale
reactor. In addition, the studies have identified rate-limiting steps in the oxidation of
organics and metabolic wastes; for example, in the case of glucose, carbon monoxide
is the dominant rate-limiting intermediate at lower temperatures (450 to 550°C),
while methane and possibly ethylene are rate-limiting at higher temperatures (around
600°C). Experiments conducted during this project, have revealed identities of many
of the problematic liquid-phase products of the incomplete oxidation of glucose,
including furfural, acetaldehyde, acetic acid, acetonylacetone, and propenoic acid;
however, our studies have at the same time demonstrated that complete destruction
of all of these species may be accomplished very easily at temperatures near 600 °C.
Project research has improved our understanding of the role of water in the
oxidation process, and indicated that the primary effect of density/pressure results
from the change in the water concentration. Since practical SCWO systems in
general operate at much lower water concentrations than our experimental system,
the change in water concentration with operating pressure will be less drastic in a
practical system, and the kinetic penalty of operating at lower pressures will be much
more minor than observed in our studies. In practice, the SCWO process may
therefore be operated at lower pressures without seriously affecting oxidation kinetics.
Elementary reaction modeling has allowed interpretation of mechanistic
pathways in waste oxidation, and has provided an understanding of the process at a
level much more basic than that of global kinetics. By using theoretically consistent
modifications to combustion based free radical elementary reactions for H2 and CO
oxidation to correct for higher pressure and density, we were able to successfully
29
represent observed kinetic results, including the production of H 2 via the indirect
water-gas-shift pathway.
The salt studies have provided aqueous phase equilibrium data for two salts
contained in metabolic wastes (sodium chloride and sodium sulfate) at conditions
encountered in the MODAR process. Also, precipitation mechanisms have been
developed for the two salt-water systems. This information will aid in the design of
efficient salt separation devices for the SCWO process. Knowledge of phase
boundaries are required to predict where solid formation will occur in the system.
Estimates of particle sizes have also been obtained, which are needed to design both
impingement and filter salt separator units. For both salts, larger solids were formed
in the shock crystallization experiments at 200 bar. Although further studies are
necessary to determine the effects of gas dilution, our results for NaCl and Na2SO4
suggest a significant advantage by operating at lower, even subcritical, pressures, since
salt separation should become easier as particle sizes increase.
7. Acknowledgements
The authors would like to thank NASA for partial support of this work. We
are particularly grateful to Donald Price, Albert Behrend, and Ted Wydeven for their
interest and guidance in carrying out the project. Many present and former MIT
faculty members have provided thoughtful comments and discussions, while serving as
thesis committee members. They include Adel Sarofim, Janos Beer, Bill Deen, Jack
Howard, Preetinder Virk, and Mike Modell. We also thank Rich Helling (DOW
Chemical), for his efforts on the project prior to the NASA funded phase and his
helpful suggestions throughout the project, and MIT Students, David Stevenson,
Victor Antaramian, Jay Corbett, Ashley Shih, Matt DiPippo, Phil Marrone, and Bryce
Mitton for their capable and valuable assistance in the accomplishment of our
experimental objectives. Glenn Hong, William Killilea, K.C. Swallow,Alan Bourhis,
and Dave Ordway, of MODAR, Inc., provided generous assistance with both the
experimental and theoretical aspects of this project.
8. Literature Cited
Alekhin, Yu.V. and A.G. Vakulenko (1988), "'Thermodynamicparameters and
solubility of NaCl in water vapor at 300-500C up to 300 bar", Geochemistry
International, 25(5), pp. 97-110.
Bischoff, J.L. and KS. Pitzer (1989), "Liquid-vapor relations for the system NaClH 2 0: Summary of the P-T-x surface from 300 to 500 OC", Amer. J. ScL, 289,
pp. 217-248.
30
Bischoff, J.L, RJ. Rosenbauer, and K.S. Pitzer (1986), "The system NaCI-H2 0:
relations of vapor-liquid near the critical temperature of water and of vaporliquid-halite from 300 to 5000 C', Geochim. Cosmo. Acta, 50, pp. 1437-1444.
Galobardes, J.F., D.R. Van Hare and LB. Rogers (1981), "Solubility of sodium
chloride in dry steam", J. Chem. Eng. Data, 26, pp. 363-366.
Khaibullin, I. Kh. and N.M. Borisov (1966), High Temperature, 4, pp. 489-494.
Linke, W.F. (1958), Solubilities - inorganicand metal organiccompounds, VII ACS,
Washington DC.
Martynova, O.I. (1964), "Some problems of the solubilitiy of involatile inorganic
compounds in water vapor at high temperatures and pressures", Russ. J. of
Phys. Chem., 38, pp. 587-592.
Martynova, O.I. (1976), "Solubility of inorganic compounds in subcritical and
supercritical water", in High Temperatureand High PressureElectrochemistryin
Aqueous Solutions, D.deG. Jones and R.W. Staehle editors, National
Association of Corrosion Engineers, Houston, TX, pp. 131-138.
Martynova, O.I. and Yu.F. Samoilov (1962), 'The formation of solutions of inorganic
substances in water vapor", Russ. J. Inorg. Chem., 7(4), pp. 372-375.
Morey, G.W. and J.M. Hesselgesser (1951), 'The solubility of some minerals in
superheated steam at high pressures", Econ. GeoL, 46, pp. 821-835.
Olander, A. and H. Liander (1950), 'The phase diagram of sodium chloride and
steam above the critical point", Acta. Chim. Scand, 4, pp. 1437-1445.
Parisod, C.J. and E. Plattner (1981), "Vapor-liquid equilibria of the NaCI-H20 system
in the temperature range 300-400°C",J. Chem. Eng. Data, 26, pp. 16-20.
Pitzer, KS. and R.T. Pabalan (1986), "Thermodynamics of NaCl in steam", Geochim.
Cosmochim. Acta, 50, pp. 1445-1454.
Ravich, M.I. and F.E. Borovaya (1964), "Phase equilibria in the sodium sulphatewater system at high temperatures and pressures", Russ. J. Inorg. Chem., 9(4),
pp. 520-532.
Sourirajan, S. and G.C. Kennedy (1962), The system H 20-NaCI at elevated
temperatures and pressures",Amer. J. Sci, 260, pp. 115-141.
Styrikovich, M.A., I.Kh. Khaibullin and D.G. Tschvirachvili (1955), "Solubility of salts
in high pressure steam",Akad. Naukl SSSR Dokady, 100, pp. 1123-1126
(in Russian).
31
9. Documentation of Research Contributions
A complete listing of papers and reports presented and published during the
course of this project is given below. Copies of the actual papers are provided in
Volume II: Appendix of the final report. Copies of the thesis summaries (digests)
for the PhD dissertations of Paul Webley, Rick Holgate, and Fred Armellini are
included as well. Copies of complete theses are available upon request. Also for
completeness, earlier and forthcoming publications of our group related to SCWO
technology are listed in the final section.
9.1 Papers Published and Accepted for Publication
(1)
Webley, P.A. and Tester, J.W. (1988) "Fundamental kinetics and
mechanistic pathways for oxidation reactions in supercritical water."
SAE Technical Paper Series #881039, 18th Intersociety Conference on
Environmental Systems, San Francisco, CA, July 11-13.
(2)
Webley, P.A. and Tester, J.W. (1989) "Fundamental kinetics of
methanol oxidation in supercritical water." ACS Symp. Ser. 406:
SupercriticalFluid Science and Technology,K.P. Johnston and J.M.L
Penninger, eds. Washington, D.C.: American Chemical Society,
pp. 259-275.
(3)
Armellini, FJ. and Tester, J.W. (1990) "Salt separation during
supercritical water oxidation of human metabolic waste: Fundamental
studies of salt nucleation and growth." SAE Technical Paper Series
#901313, 20th International Conference on Environmental Systems,
Williamsburg, VA, July 9-12.
(4)
Webley, P.A., Holgate, H.R., Stevenson, D.M., and Tester, J.W. (1990)
"Oxidation kinetics of model compounds of human metabolic waste in
supercritical water." SAE Technical Paper Series #901333, 20th
International Conference on Environmental Systems, Williamsburg, VA,
July 9-12.
(5)
Webley, P.A. and Tester, J.W. (1991) "Fundamental kinetics of methane
oxidation in supercritical water." Energy & Fuels 5, pp. 411-419.
(6)
Webley, P.A., Tester, J.W., and Holgate, H.R. (1991) "Oxidation
kinetics of ammonia and ammonia-methanol mixtures in supercritical
water in the temperature range 530-700C at 246 bar." Ind. Eng. Chem.
Res. 30(8), pp. 1745-1754.
32
(7)
Armellini, FJ. and Tester, J.W. (1991) "Experimental methods for
studying salt nucleation and growth from supercritical water."
J. SupercriticalFluids 4, pp. 254-264.
(8)
Armellini, FJ. and Tester, J.W. (1992) "Solubility of sodium chloride in
sub- and supercritical water vapor." Presented at the AIChE National
Meeting, Los Angeles, CA, November 17-22, and accepted for
publication in Fluid Phase Equilibria.
(9)
Holgate, H.R. and Tester, J.W. (1991) "Fundamental kinetics and
mechanisms of hydrogen oxidation in supercritical water." Presented at
the Second International Symposium on Supercritical Fluids, Boston,
MA, May 20-22, and accepted for publication in Combustion Science and
Technology.
(10)
Holgate, H.R., Webley, PA, Tester, J.W., and Helling, R.K. (1992)
"Carbon monoxide oxidation in supercritical water: The effects of heat
transfer and the water-gas shift reaction on observed kinetics." Energy
and Fuels 6, pp. 586-597.
(11)
Tester, J.W., Webley, P.A., and Holgate, H.R. (1992) "Revised global
kinetic measurements of methanol oxidation in supercritical water."
Accepted for publication in Industrial and EngineeringChemistry
Research.
9.2 Completed Doctoral Theses
(12)
Webley P.A. (1989) "Fundamental oxidation kinetics of simple
compounds in supercritical water." Doctoral thesis, Department of
Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, MA.
(13)
Holgate, H.R. (1993) "Oxidation chemistry and kinetics in supercritical
water:. hydrogen, carbon monoxide, and glucose." Doctoral thesis,
Department of Chemical Engineering, Massachusetts Institute of
Technology, Cambridge, MA.
(14)
Armellini, FJ. (1993) "Phase equilibria and precipitation phenomena of
sodium chloride and sodium sulfate in sub- and supercritical water."
Doctoral thesis, Department of Chemical Engineering, Massachusetts
Institute of Technology, Cambridge, MA.
33
93
Other Published and Forthcoming Papers
(15)
Helling, R.K. and Tester, J.W. (1987) "Oxidation kinetics of carbon
monoxide in supercritical water." Energy & Fuels 1, pp. 417-423.
(16)
Helling, R.K. and Tester, J.W. (1988) "Oxidation of simple compounds
and mixtures in supercritical water: Carbon monoxide, ammonia and
ethanol." Environ. Sci TechnoL 22(11), pp. 1319-1324.
(17)
Tester, J.W., Holgate, H.R., Armellini, FJ., Webley, PA, Killilea, W.R.,
Hong, G.T., and Barner, H.E. (1991) "Supercritical water oxidation
technology: A review of process development and fundamental
research." Presented at the Third Annual Symposium on Emerging
Technologies for Hazardous Waste Management, Atlanta, GA, October
1-3, and to appear in an ACS Symposium Series volume, Emerging
Technologiesfor Hazardous Waste Management III, D.W. Tedder and
F.G. Pohland, eds.
(18)
Holgate, H.R. and Tester, J.W. (1992) "Oxidation of hydrogen and
carbon monoxide in sub- and supercritical water. I. Experimental
Results." To be submitted to the Journal of Physical Chemistry.
(19)
Holgate, H.R. and Tester, J.W. (1992) "Oxidation of hydrogen and
carbon monoxide in sub- and supercritical water. II. Elementary
reaction modeling." To be submitted to the Joumal of Physical
Chemistry.
(20)
Holgate, H.R., Tester, J.W., and Meyer, J.C. (1992) "Glucose hydrolysis
and oxidation in supercritical water." To be submitted to the AIChE
Journal.
(21)
Armellini, FJ. and Tester, J.W. (1992) "Precipitation of sodium chloride
and sodium sulfate from supercritical water: Morphological
characterization." To be submitted to the J. Supercrtical Fluids.
34