Project Summary

United States

Environmental Protection

Agency

Supercritical Water Oxidation

Model Development for Selected

EPA Priority Pollutants

Earnest F. Gloyna, Lixiong Li

National Risk Management

Research Laboratory

Cincinnati, OH 45268

Research and Development

The research summarized here involves 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 pollutants were developed. Critical engineering issues were evaluated. These

results were used to develop SCWO

process strategy, improve reactor designs, and optimize operating conditions.

This Project Summary was developed

by EPA’s National Risk Management

Research Laboratory, Cincinnati, OH,

to announce key findings of the research project that is fully documented

in a separate report of the same title

(see Project Report ordering information at back).

Introduction

The goals of this project were to assess

the performance of SCWO in treating selected EPA priority pollutants and to enhance 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 methyl ester (2,4-D methyl ester). The second 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 management concepts. Based on 1984 estimates, extrapolated to 1990, hazardous

wastes produced by industry range from

280 to 395 million metric tons/yr. Industries and municipalities continue to produce 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 munitions are created each year. Presently,

detonation and incineration costs are about

$800/ton and $3,000/ton, respectively. Innovative 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 “Totally Enclosed Treatment Facilities.” The

supercritical water oxidation process can

accomplish these objectives. The concept

offers unique, economical, and innovative

EPA/600/SR-95/080 June 1995
2

solutions. By recovering heat from the effluent using heat exchangers, the SCWO

process can become thermally self-sustaining, 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 investigated the treatment of hazardous wastes

in supercritical water. Early emphasis was

on demonstrating the SCWO treatment

concept and establishing treatability characteristics. During the last few years, however, it has become apparent that both

fundamental and technical aspects of process design and commercial-scale development 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 summarized 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 Ushaped reactor was 20 mL. Heat was

provided by a fluidized sand bath. While

in the sand bath, each reactor was mechanically 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 compounds 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 reaction time of 2 min.

• Destruction efficiencies of >99% were

observed for 2,4-DCP and 2,4-D methyl ester at a temperature of 500 ̊C

and a reaction time of 10 min.

• Acetic acid and pyridine, when compared with the chlorinated aromatics,

were relatively refractory, but destruction efficiencies >99% were observed

at a temperature of 500 ̊C and a reaction 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 temperatures (< 450 ̊C) or shorter reaction

times (< 5 min), and the number of

these intermediates reduced to about

three when the temperature and reaction 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 pyridine, respectively, were 106, 28.5, and

91.5 kJ/mole.

• Pyridine and 2,4-DCP were recommended for more detailed kinetic and

mechanistic studies involving continuous-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 varying 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 summarized 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 respect to the concentration of 2,4-DCP.

The activation energy and

preexponential factor were 209 kJ/

mole and 1012.2 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]

0.35, where the activation energy (Ea) and preexponential factor

(A) were 88.9 kJ/mole and 105.5

sec-1(mole/L)-0.35, 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 dioxide, and carbon monoxide. Based on

these compounds, a simplified reaction 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 approximated by the overall oxidation

and hydrolysis reaction rates. The

SCWO rate for pyridine was found to

be r = A exp(-Ea/RT) [Pyridine][O2]

0.2

,

where Ea and A were 210 kJ/mole

and 1013.1 sec-1(mole/L)-0.2, respectively.

• Seventeen intermediate compounds

were found in the effluent derived from

the SCWO of pyridine. These compounds included carboxylic acids, dicarboxylic acids, amines, ammonia,

carbon dioxide, and carbon monoxide. 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 conducted 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), constant 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, selective leaching of the Monel 400 alloy 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 corrosion.

• For Stainless Steel 316, the least corrosion, 0.03 mils per year (mpy), occurred at a temperature of 400 ̊C and

pH of 5.8, and the worst corrosion,
3

1.89 mpy, corresponded to a temperature of 300 ̊C and pH of 2.1.

• For Hastelloy C-276, the least corrosion, 0.06 mpy, occurred at a temperature of 400 ̊C and pH of 5.8, and

the worst corrosion, 1.33 mpy, corresponded 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, chromium it can be introduced from the influent or it can result from corrosion of the

reactor system. Since some alloys currently 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 conducted with the use of a vertical, concentric-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 industrial sludges were used. The following

observations were made:

• The speciation of chromium, trivalent

or hexavalent, showed direct correlation with the effluent pH. When the

effluent pH was less than 7, trivalent

chromium was the only detectable

chromium species, and when the effluent 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 precipitated 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 chromium in the treated effluents decreased 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 chromium was due to a substantial decrease 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 retained 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 removed.

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 exhibited a wide range of reactivity in

SCWO environments, indicating the

effect of chemical and structural features of each compound on the overall 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 particular, can be precipitated effectively

because of the limited solubility of

chromate salts in supercritical water.

The full report was submitted in fulfillment of Cooperative Agreement No. CR816760-02-0 by the Separations Research

Program, Center for Energy Studies, and

Environmental and Water Resources Engineering, Department of Civil Engineering, The University of Texas at Austin,

under the sponsorship of the U.S. Environmental 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 Development 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

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-95/080