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OCR for page 230
APPENDIX
Supercritical Water Oxidation
Two of the technology providers, Lockheed Martin
and General Atomics, propose using supercritical wa-
ter oxidation (SCWO) for the final destruction of the
hydrolysate from both agents and energetics. This ap-
pendix provides a general description of SCWO and
lists the findings of the 1998 National Research Coun-
cil report on SCWO of VX hydrolysate (NRC, 1998),
which are considered to be applicable to this study.
BASICS
The foundation of SCWO technology is that the fluid
properties of water change dramatically above its criti-
cal temperature (374°C; 705°F) and pressure (218 atm;
3,204 psi). Supercritical water functions like a dense
gas with special properties, such as high organic solu-
bility, complete miscibility with permanent gases and
organics, high diffusivity, low viscosity, and a low di-
electric constant similar to that of nonpolar liquids. In-
organic salts become almost completely insoluble un-
der supercritical conditions. These properties make
supercritical water an excellent oxidation medium for
the destruction and mineralization of most organic
compounds to simple compounds, such as carbon di-
oxide, nitrogen, and water.
In the SCWO process, dissolved organics, oxygen
(or other oxidants), and water are reacted above the
critical temperature and pressure of water. Oxidation
of the organics is spontaneous, and complete mineral-
ization to carbon dioxide and water is achieved in a
few seconds of residence time. Although the SCWO of
organics is exothermic, a supplementary source of fuel
is normally required for the oxidation of organics in
230
dilute aqueous solutions. A simplified flow sheet for a
typical SCWO waste-treatment system is presented in
Figure F- 1.
HYDROLYSATES OF AG ENTS AN D EN ERG ETICS
Testing by the Army and Army contractors has
shown that SCWO can achieve high destruction effi-
ciencies for the organic constituents in VX hydrolysate
(NRC, 1998~. Testing has also demonstrated that
SCWO is capable of destroying chemical agent (GA,
1997~. For example, the SCWO oxidation of the nerve
agent GB proceeds according to the following
chemical equation:
F
H3C-P=0+6.5 O2 - HF+4CO2+0.5P2O5+4.5H2O
OCH(CH3~2
In the presence of caustic, which is a component of
agent hydrolysate, the HF formed during oxidation of
GB will react with sodium hydroxide to form NaF salt
and water as follows:
HF+NaOH - NaF+H2O
Because SCWO of organics is nonspecific (i.e., or-
ganic material is oxidized indiscriminately, regardless
of the source), the hydrolysate from agent neutraliza-
tion and energetics deactivation can either be combined
or processed separately in the SCWO reactor.
OCR for page 231
APPENDIX F
Atmospheric Alternate
air oxygen
\A/O eta I
auxiliary
fuel
l
SCWO
reactor
1
hi'.
,. ~ .
.. ~ . .
......
.......
.......
. . . .
~600°C
_ Quench
· ~ separator
| recovery |
A}
Interchanger
FIGURE F-1 Typical flow sheet for supercntical water oxidation. Source: NRC, 1998.
TECHNICAL CHALLENGES
Although SCWO has been under development for
more than 20 years, two major problem areas have
prevented its commercial application until recently
(Modell, 1985):
· rapid and excessive corrosion of reactor materials
as a result of the severe reaction conditions and
the formation of acid compounds when heteroat-
oms are present in the reactor feed
plugging of reactors by the deposition of inorganic
salts formed during SCWO in the presence of dis-
solved inorganic compounds and heteroatoms
.
These two problems are generally exacerbated when
the feed stream contains acid- and salt-forming heteroa-
toms, such as C1, F. P. and S. which are present in the
hydrolysates of agents and energetics.
Other problem areas in the application of SCWO are
that process kinetics and the effects of process param-
eters on process design and efficiency are not com-
pletely understood. For example, Li and Egiebor (1994)
reported that the feed stream preheating rate has a signifi-
cant effect on the destruction efficiency for organics.
231
Off-gas
treatment
(if required)
Vent gas
to
atmosphere
| Pressure | _
I let-down I B i
A| Pressure l
I let-down | Liquid
effluent
The more rapid the preheating, the higher the oxidation
efficiency. This was attributed to increased pyrolysis
and polymerization of the organic contaminants during
slow preheating to form highly refractory compounds
resistant to SCWO. Furthermore, Webley and Tester
(1991 ~ reported that global kinetic models failed to pre-
dict the reaction rates during the SCWO of methane
and carbon dioxide. Thus, the fundamental physics of
SCWO are not completely understood.
KEEPING UP WITH ONGOING SCWO
TESTI NG
SCWO has been selected by the Army for the treat-
ment VX hydrolysate at the Newport, Indiana, site. The
design of that SCWO system is well under way, but
significant testing still remains to be done. The Army
also plans to use SCWO for the destruction of smokes
and dyes at the Pine Bluff Arsenal, Arkansas; the Navy
is currently testing two different SCWO designs for
onboard treatment of shipboard wastes. The Army
should continue to monitor these tests closely and fac-
tor the results into its decision to implement SCWO for
follow-on ACWA programs.
OCR for page 232
232
ALTERNATIVE TECHNOLOGIES FOR DEMILITARIZATION OF ASSEMBLED CHEMICAL WEAPONS
FINDINGS AND RECOMMENDATIONS FROM
THE 1998 NRC REPORT
The following paragraph and the subsequent find-
ings and recommendations are taken directly from the
NRC report Using Supercritical Water Oxidation to
Treat Hydrolysate from VX Neutralization (NRC,
1998~. They are reproduced here because the commit-
tee considers them applicable to the SCWO technolo-
gies evaluated in this study.
Excerpt
Chemical neutralization of VX nerve agent results
in the production of a liquid hydrolysate stream that
has greatly reduced toxicity compared to the original
nerve agent but requires further treatment to meet the
requirements of the Chemical Weapons Convention
and to be suitable for disposal. After considering sev-
eral approaches, the U.S. Army has selected SCWO
(supercritical water oxidation) as the primary process
for treating the hydrolysate from VX neutralization
prior to ultimate disposition. The integration of SCWO
into the complete process for the destruction of VX
stored at Newport, Indiana, also requires an evaporator
system after SCWO treatment to allow water to be re-
cycled back into the neutralization process. The evapo-
ration system also produces a dry solid waste stream
consisting of salts produced during the neutralization
and SCWO treatment steps. Excess condensed water
from the evaporator is expected to be of relatively high
purity and suitable for discharge. The technology se-
lected for the evaporation process step is mature with
considerable full-scale design and operations experi-
ence. In contrast, treatment of the hydrolysate will be a
new application for SCWO. Thus, the findings and rec-
ommendations presented here focus on the use of
SCWO for the treatment of VX hydrolysate.
Findings
Finding 1. Limited pilot-scale testing has demonstrated
the ability of SCWO to achieve high destruction effi-
ciencies for the organic constituents of VX hydroly-
sate. Effluent from SCWO treatment of VX hydroly-
sate has been shown to have negligible acute toxicity in
intravenous testing in mice, gavage testing in rats, and
dermal testing in rabbits. The separation of salts in the
effluents from SCWO through an evaporator system
should produce relatively pure water suitable for dis-
charge and solid salts suitable for disposal. Treatment
requirements for VX hydrolysate are less stringent than
they are for VX because the hydrolysate has low toxic-
ity relative to the agent. However, criteria for process
destruction efficiency and final disposal standards have
not been established.
Finding 2. Using SCWO to treat VX hydrolysate is
significantly different and more complex than previous
applications. SCWO systems on a pilot scale have been
used to treat several other types of wastes, but SCWO
is in commercial operation at only one site. There has
been only limited pilot-scale or operational-scale expe-
rience with wastes that are similar to VX hydrolysate
in being highly corrosive and salt-laden. Operation with
VX hydrolysate or appropriate surrogates at design
conditions, equipment configuration, or approximate
scale for full-scale operations has not been demon-
strated. A vertical cylindrical reactor is the only reactor
configuration that has been successfully demonstrated
to date at pilot scale for the treatment of VX hydroly-
sate and similar waste streams. Additional development
and pilot-scale testing of SCWO technology will be
necessary to ensure sustained, reliable operation of a
full-scale integrated treatment system. Sufficient time
appears to be available in the Army's implementation
schedule for the Army to carry out development and
testing for using SCWO at the Newport site, provided
they are carried out expeditiously.
Finding 3. Pilot-scale operation of SCWO in a vertical
cylindrical reactor at the temperature and pressure nec-
essary for the effective destruction of hydrolysate con-
stituents has been limited to one eight-hour and two
less than two-hour tests. During pilot-scale testing
with hydrolysate, the following factors were identi-
fied that could create difficulties in sustaining system
performance:
· Large quantities of insoluble salts were produced,
which must be effectively managed within, and
downstream of, the SCWO reactor.
· Unexpected fluctuations were observed in tem-
perature, pressure, and salt expulsion from the
SCWO reactor.
OCR for page 233
APPENDIX F
· High levels of corrosion and erosion of materials
of construction were observed in the reactor liner
and pressure let-down valves.
· The sustained performance and reliability of the
pressure let-down system was not demonstrated.
Although at this point in development the Stockpile
Committee cannot be certain, it believes that a SCWO
system for the treatment of VX hydrolysate with suffi-
cient sustained performance can be achieved with
additional development and testing.
Finding 4. Limited bench-scale and pilot-scale tests
have demonstrated operating regimes under which
SCWO can effectively destroy carbon-phosphorus
bonds and oxidize the organic constituents present in
VX hydrolysate. The demonstrated conditions for high
levels of destruction (> 99 percent) include tempera-
tures between 640°C (11 84°F) and 730°C (1346°F) and
pressures between 231 and 258 aim (3395 to 3792 psi).
At temperatures and pressures below this regime, ef-
fluent from SCWO processing may contain significant
concentrations of residual organic species that are dif-
ficult to destroy, including constituents with carbon-
phosphorus bonds.
A basis for the reliable scale-up and operation of
SCWO technology for the treatment of VX hydroly-
sate has not yet been demonstrated. Fundamental
knowledge about the following processes within the
SCWO reactor is still not available:
· the number and characteristics of the physical
phases, including large quantities of entrained and
adhered solids and potentially liquid, gas, and
supercritical fluid phases
· fluid dynamics and mixing processes complicated
by relatively high loadings of insoluble salts
· heterogeneous and homogeneous reaction mecha-
nisms and kinetics
· salt nucleation, particle growth, agglomeration and
adhesion mechanisms, and kinetics
Because the understanding of fundamental processes
is limited and the process operational data and experi-
ence are sparse, empirical design and engineering judg-
ment will be required for the selection of a prudent
scale for development prior to full-scale demonstration.
This is common engineering practice.
233
Finding 5. Alkaline VX hydrolysate and its destruc-
tion products under SCWO reaction conditions create
an extremely corrosive and erosive environment that
requires the careful selection of materials of construc-
tion. Although preliminary data indicate that certain
noble metals, such as platinum and gold, may have ac-
ceptable properties, the data currently available are in-
sufficient for the selection of materials of construction.
The Army has initiated further testing of materials of
construction.
Finding 6. Process monitoring and control strategies
for the management of salts within the SCWO reactor
and the destruction of the organic constituents of the
hydrolysate have not been demonstrated.
Recommendations
Recommendation 1. A pilot-scale SCWO process fa-
cility with the critical characteristics of the full-scale
design should be constructed and operated to further
define operating characteristics and demonstrate sus-
tained continuous operation of the process. Objectives
for process development and demonstration should
include:
· operation with either hydrolysate or a suitable sur-
rogate to demonstrate reliable operation for periods
similar to full-scale design operating cycles
· the development and validation of process moni-
toring and control strategies for salt management
and the destruction of organic constituents
· the definition of stable operating regimes, includ-
ing the temperature, pressure, and the use of the
oxidant (liquid oxygen or compressed air) selected
for full-scale operation
· the definition of a basis for process scale-up,
operation, and maintenance of a full-scale system
· the development and demonstration of a reliable
pressure let-down system
Because the understanding of the fundamental pro-
cess mechanisms and operating characteristics is lim-
ited, the committee recommends that the pilot-scale
system be within an order of magnitude of the total
mass and heating throughput of a full-scale design unit.
Based on testing and reactor scale-ups to date, a verti-
cal cylindrical reactor configuration is recommended
OCR for page 234
234
ALTERNATIVE TECHNOLOGIES FOR DEMILITARIZATION OF ASSEMBLED CHEMICAL WEAPONS
as the system that will probably require the least
amount of additional development. Other reactor con-
figurations may perform at required levels but would
require significant additional development.
Recommendation 2. Testing of materials of construc-
tion should be carried out as necessary to finalize the
selection of materials for critical components, includ-
ing the SCWO reactor and the pressure let-down sys-
tem. Additional pilot-scale testing indicated in Recom-
mendation 1 should include fabrication with the
materials of construction selected from testing smaller
samples and evaluation of corrosion and erosion rates
for critical components.
Recommendation 3. Flexibility and redundancy of
critical components should be incorporated into the
design of the full-scale system to allow for uncer-
tainties about the basis for scale-up and operation.
Trade-offs should be evaluated to establish an ap-
propriate balance between two 100 percent capacity
SCWO reactors or a greater number of smaller reac-
tors. The analysis should consider performance un-
certainties associated with process scale-up and com-
plexity, as well as the reliability of operating several
reactors in parallel.
Recommendation 4. The Army should make provi-
sions for targeted research and development to resolve
problems identified during pilot-scale testing and the
full-scale implementation of SCWO technology.
Recommendation 5. Requirements for process de-
struction efficiencies and final disposal standards for
all effluent streams from SCWO treatment should be
clearly defined to ensure that the final design meets
regulatory standards.
REFERENCES
GA (General Atomics). 1997. Assessment of Technologies for As-
sembled Chemical Weapon Demilitarization. Proposal submit-
ted in response to U.S. Army Solicitation No. DAAM01-97-R-
0031, September 15, 1997. San Diego, Calif.: General Atomics.
Li, L., and N.O. Egiebor. 1994. Feedstream preheating effect on
supercritical water oxidation of dissolved organics. Energy and
Fuels 8: 1126-1130.
Modell, M. 1985. Processing Methods for the Oxidation of
Organics in Supercritical Water. U.S. Patent 4,543,190
(September 24, 1985~.
NRC (National Research Council). 1998. Using Supercritical Wa-
ter Oxidation to Treat Hydrolysate from VX Neutralization.
Committee on Review and Evaluation of the Army Chemical
Stockpile Disposal Program, Board on Army Science and Tech-
nology. Washington, D.C.: National Academy Press.
Webley, P.A., and J. Tester. 1991. Oxidation kinetics of carbon
monoxide and methane in supercritical water. Energy and Fuels
5: 411-416.
Representative terms from entire chapter:
water oxidation
