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

-------
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,

-------
    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.

-------
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
   Penalty for Private Use
   $300
      BULK RATE
POSTAGE & FEES PAID
         EPA
   PERMIT No. G-35
   EPA/600/SR-95/080

-------