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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.
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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.
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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.
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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
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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.
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