United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/080
June 1995
&EPA Project Summary
Supercritical Water Oxidation
Model Development for Selected
EPA Priority Pollutants
Earnest F. Gloyna, Lixiong Li
The research summarized here in-
volves the use of supercritical water
oxidation (SCWO) technology as a
method to destroy hazardous organic
wastes. Kinetic models and reaction
pathways for selected EPA priority pol-
lutants were developed. Critical engi-
neering issues were evaluated. These
results were used to develop SCWO
process strategy, improve reactor de-
signs, and optimize operating condi-
tions.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The goals of this project were to assess
the performance of SCWO in treating se-
lected EPA priority pollutants and to en-
hance the development of SCWO
processes. The project was executed in
three phases. The first phase (year one)
involved batch SCWO studies of five model
compounds: acetic acid, 2,4-dichlorophenol
(2,4-DCP), pentachlorophenol, pyridine,
and 2,4-dichlorophenoxyacetic acid me-
thyl ester (2,4-D methyl ester). The sec-
ond phase (year two) consisted of detailed,
continuous-flow tests involving both kinetic
and mechanistic studies of 2,4-DCP and
pyridine. The third phase (years one and
two) dealt with the evaluation of critical
engineering issues such as corrosion and
chromium speciation.
The task of pollution control has moved
well beyond conventional technology. The
present and future challenges of reducing
toxic organic waste and sludge volume
have overwhelmed existing waste man-
agement concepts. Based on 1984 esti-
mates, extrapolated to 1990, hazardous
wastes produced by industry range from
280 to 395 million metric tons/yr. Indus-
tries and municipalities continue to pro-
duce large amounts of biological sludges
that must be dewatered, destroyed by
burning, or disposed of by land farming.
In addition, the federal government has
large quantities of stored munitions and
other organic wastes that must be treated.
Listed among the military items requiring
demilitarization are 340,000 tons of stored
munitions. In addition, 30,000 tons of mu-
nitions are created each year. Presently,
detonation and incineration costs are about
$800/ton and $3,000/ton, respectively. In-
novative and economical approaches must
be found to manage existing contaminants
and future stockpiles of unacceptable
wastes.
Today, two of the national goals for
hazardous waste management are: (a)
greater than 99.99% destruction efficiency;
and (b) treatment systems that are "To-
tally Enclosed Treatment Facilities." The
supercritical water oxidation process can
accomplish these objectives. The concept
offers unique, economical, and innovative
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solutions. By recovering heat from the ef-
fluent using heat exchangers, the SCWO
process can become thermally self-sus-
taining, with wastes having a chemical
oxidation demand of about 30 g/L or
higher.
Since the early 1980s, a number of
universities and companies have investi-
gated the treatment of hazardous wastes
in supercritical water. Early emphasis was
on demonstrating the SCWO treatment
concept and establishing treatability char-
acteristics. During the last few years, how-
ever, it has become apparent that both
fundamental and technical aspects of pro-
cess design and commercial-scale devel-
opment need to be addressed. In
particular, design models for special
wastes need to be developed.
Experiments and Observations
Experimental apparatus, test conditions,
key observations, and data evaluation for
batch and continuous-flow studies, as well
as engineering evaluation, are summa-
rized below.
Batch Study
Batch experiments were carried out at
three temperatures (400°C, 450°C, 500°C),
a constant water density (0.3 g/mL), and
reaction times varying from 2 min to 20
min. The reactors were made of Stainless
Steel 316 tubing (0.85 cm I.D. and 1.27
cm O.D.). The effective volume of a U-
shaped reactor was 20 ml_. Heat was
provided by a fluidized sand bath. While
in the sand bath, each reactor was me-
chanically vibrated to enhance mixing and
heat transfer. Feed concentrations ranged
from 40 mg/L to 3000 mg/L. Oxygen was
used as the oxidant. The model com-
pounds were analyzed by chromatographic
techniques. Significant results were noted
as follows:
• Destruction efficiencies of >99.99%
were observed for pentachlorophenol
at a temperature of 500°C and a re-
action time of 2 min.
• Destruction efficiencies of >99% were
observed for 2,4-DCP and 2,4-D me-
thyl ester at a temperature of 500°C
and a reaction time of 10 min.
• Acetic acid and pyridine, when com-
pared with the chlorinated aromatics,
were relatively refractory, but destruc-
tion efficiencies >99% were observed
at a temperature of 500°C and a re-
action time of 20 min.
• Qualitatively, as determined by GC
analyses, SCWO of 2,4-DCP and 2,4-
D methyl ester produced noticeable
amounts of intermediate compounds
(about 20) at either lower tempera-
tures (< 450°C) or shorter reaction
times (< 5 min), and the number of
these intermediates reduced to about
three when the temperature and re-
action times were > 450°C and > 5
min.
• SCWO of acetic acid, 2,4-DCP, and
pyridine followed pseudo-first-order
reaction kinetics. Activation energies
for acetic acid, 2,4-DCP, and pyri-
dine, respectively, were 106, 28.5, and
91.5 kJ/mole.
• Pyridine and 2,4-DCP were recom-
mended for more detailed kinetic and
mechanistic studies involving continu-
ous-flow SCWO reactor systems.
Continuous-Flow Study
The continuous-flow experiments were
conducted using a plug-flow reactor setup.
One reactor was made of Stainless Steel
316 tubing (0.635 cm O.D. and 0.165 cm
wall thickness). A second reactor was
made of coiled Hastelloy C-276 tubing.
The feed flow rate was 35 g/min. The
tests were performed at temperatures vary-
ing from 400° to 520°C, residence times
ranging from 2 sec to 11 sec, >200%
excess oxygen, and a pressure of 27.6
MPa. The Reynolds number ranged from
7400 to 8200. Feed concentrations varied
from 300 mg/L to 800 mg/L for 2,4-DCP
and from 1000 mg/L to 3000 mg/L for
pyridine. The major findings are summa-
rized as follows:
• More than 10% of the 2,4-DCP was
hydrolyzed by supercritical water at
temperatures above 450°C. The rate
of hydrolysis was first-order with re-
spect to the concentration of 2,4-DCP.
The activation energy and
preexponential factor were 209 kJ/
mole and 10122 sec'1, respectively.
• The overall oxidation and hydrolysis
reaction rate (r) for 2,4-DCP was
found to be r = A exp(-Ea/RT) [2,4-
DCP][O2]035, where the activation en-
ergy (Ea) and preexponential factor
(A) were 88.9 kJ/mole and 1055
sec-1(mole/L)-°35, respectively.
• Nine intermediate compounds were
identified during the SCWO of 2,4-
DCP: 2-chlorophenol, 4-chlorophenol,
2,6-dichlorophenol, phenol, chloride,
acetic acid, formic acid, carbon diox-
ide, and carbon monoxide. Based on
these compounds, a simplified reac-
tion pathway for the SCWO of 2,4-
DCP was developed.
• Less than 5% of the pyridine was
hydrolyzed by supercritical water at
the highest tested temperature, 522°C.
Therefore, the SCWO rate was ap-
proximated by the overall oxidation
and hydrolysis reaction rates. The
SCWO rate for pyridine was found to
be r = A exp(-Ea/RT) [Pyridine][O2]02,
where Ea and A were 210 kJ/mole
and 10131sec1(mole/L)-°2, respectively.
• Seventeen intermediate compounds
were found in the effluent derived from
the SCWO of pyridine. These com-
pounds included carboxylic acids, di-
carboxylic acids, amines, ammonia,
carbon dioxide, and carbon monox-
ide. Based on these compounds, a
simplified reaction pathway for the
SCWO of pyridine was developed.
Engineering Evaluation
Material Performance
Three nickel alloys (Stainless Steel 316,
Hastelloy C-276, and Monel 400) were
evaluated. The experiments were con-
ducted using a batch reactor setup at three
temperatures (300°C, 400°C, and 500°C),
three pH conditions (2.1, 5.8, and 8.6),
varying water densities (0.09 g/cc to 0.3
g/cc), fixed oxygen loading (2.1 MPa), con-
stant chloride concentration (420 mg/L),
and uniform coupon exposure time (100
hr). The following observations were made:
• Both localized (pitting and crevice)
corrosion and uniform corrosion were
apparent in all three alloys under the
test conditions, and additionally, se-
lective leaching of the Monel 400 al-
loy was observed at supercritical water
temperatures ranging from 400°C to
500°C.
• For both Stainless Steel 316 and
Hastelloy C-276 and a given pH,
higher corrosion rates were observed
at test temperatures of 300°C and
500°C as compared to 400°C.
• Generally, the lowest pH condition
(2.1) created the most severe corro-
sion.
• For Stainless Steel 316, the least cor-
rosion, 0.03 mils per year (mpy), oc-
curred at a temperature of 400°C and
pH of 5.8, and the worst corrosion,
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1.89 mpy, corresponded to a tem-
perature of 300°C and pH of 2.1.
For Hastelloy C-276, the least corro-
sion, 0.06 mpy, occurred at a tem-
perature of 400°C and pH of 5.8, and
the worst corrosion, 1.33 mpy, corre-
sponded to a temperature of 500°C
and pH of 5.8.
Among the three alloys, Monel 400
displayed the most corrosion under
the test conditions.
Chromium Speciation
Generally, in an SCWO process, chro-
mium it can be introduced from the influ-
ent or it can result from corrosion of the
reactor system. Since some alloys cur-
rently used for SCWO studies contain high
chromium contents, the concentration of
chromium species in reactor effluents could
reach a unacceptable level.
The chromium speciation study was con-
ducted with the use of a vertical, concen-
tric-tube reactor. The reactor was made of
Stainless Steel 316 which contained 16
wt% chromium. Tests were conducted at
varying temperatures (300°C to 450°C),
feed flow rates (45 g/min to 120 g/min),
and a fixed pressure (25 MPa). Chromium
concentrations of the influent and effluent
were monitored. Both municipal and in-
dustrial sludges were used. The following
observations were made:
• The speciation of chromium, trivalent
or hexavalent, showed direct correla-
tion with the effluent pH. When the
effluent pH was less than 7, trivalent
chromium was the only detectable
chromium species, and when the ef-
fluent pH was greater than 7, both
trivalent and hexavalent chromium
corrosion products were produced.
• The level of hexavalent chromium in
the treated effluent decreased more
than 10 times, 0.046 mg/L to 0.004
mg/L, when the process temperature
changed from 300°C to 400°C (pH
7.9).
• At 400°C, the concentration of pre-
cipitated hexavalent chromium at the
reactor bottom (0.288 mg/L) was
much higher than the effluent
hexavalent chromium concentration
(0.004 mg/L), whereas at 300°C, the
hexavalent chromium concentration at
the reactor bottom (0.035 mg/L) was
comparable to the effluent hexavalent
chromium concentration (0.046 mg/
L).
• The concentrations of trivalent chro-
mium in the treated effluents de-
creased only 50%, 0.39 mg/L to 0.16
mg/L, when the process temperatures
changed from 300°C to 400°C.
• The precipitation of hexavalent chro-
mium was due to a substantial de-
crease in solubility of chromic and
chromate salts in supercritical water.
Chromium separation by precipitation
was affected by temperature, specific
co-ions, and co-ion concentration.
• Soluble trivalent chromium was re-
tained in the mass that settled in the
reactor bottom. Co-precipitation with
insoluble and associated soluble salts
appeared to be the mechanism by
which the trivalent chromium was re-
moved.
Conclusions and
Recommendations
• Refractory and chlorinated organic
compounds, such as acetic acid, 2,4-
DCP, pentachlorophenol, pyridine, and
2,4-D methyl ester, were effectively
destroyed by the SCWO process.
• These five model compounds exhib-
ited a wide range of reactivity in
SCWO environments, indicating the
effect of chemical and structural fea-
tures of each compound on the over-
all reaction rate.
• Kinetic models were developed for
2,4-DCP, pyridine, and acetic acid.
• Mechanistic studies involving 2,4-DCP
and pyridine provided an insight into
the possible reaction pathways and
by-product transformation.
• The breakdown of complex organic
molecules under SCWO conditions
produced a large number of unstable
compounds and a small number of
relatively stable, lower-molecular
weight, intermediate compounds.
• By adjusting the reaction conditions,
the type and amount of intermediates
produced can be controlled, resulting
in more efficient reactor design and
higher destruction efficiency.
• For a given temperature, the highest
corrosion rate occurred at the lowest
pH in the test conditions, which ranged
from pH = 2.1 to pH = 8.6. For a
given pH, higher corrosion rates were
observed at 300°C and 500°C than at
400°C.
• The relative formation of chromium
species (trivalent and hexavalent) can
be controlled by adjusting the pH of
the reaction media.
• Chromium species, hexavalent in par-
ticular, can be precipitated effectively
because of the limited solubility of
chromate salts in supercritical water.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
816760-02-0 by the Separations Research
Program, Center for Energy Studies, and
Environmental and Water Resources En-
gineering, Department of Civil Engineer-
ing, The University of Texas at Austin,
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
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Earnest F. Gloyna and Lixiong Li are with the University of Texas at Austin,
Austin, TX 78712
Ronald J. Turner is the EPA Project Officer (see below).
The complete report, entitled "Supercritical Water Oxidation Model Develop-
ment for Selected EPA Priority Pollutants," (Order No. PB95-230975; Cost:
$17.50 subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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EPA/600/SR-95/080
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