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Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 146
Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 147
Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 148
Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 149
Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 150
Suggested Citation:"Supercritical Water Oxidation." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 151

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 146 reactor temperature and pressure, reactor flow rates, reactor configurations, and other variables would all have to be set. • Bench scale and pilot plant testing and demonstrations with chemical agents would be conducted at military sites. • Tests on intermediate products, such as material from the hydrolysis of agents, would also be conducted at a military site; simulants could be tested elsewhere. • Energetic materials, propellants, and explosives have been treated with WAO. A demonstration program would be needed for the slurry mixture, concentration levels, and other requirements of the process as applied to chemical weapons. The development work required would take at least 1 year, after which construction could begin on a full- scale demonstration unit. Supercritical Water Oxidation Technology description. In SCWO, organic materials, including materials containing heteroatoms such as chlorine, can be effectively oxidized by O2 in an aqueous medium above the critical point of pure water, that is, at 374°C (705°F) and 3,205 psi (221 bars). At temperatures above 500°C (930°F), high conversions are possible with short reactor-residence times. Hydrocarbons can be converted completely to CO2 and water. Supercritical water is an attractive medium for the oxidation reactions because it offers high solubility for both organic compounds and oxygen; the usual transport and mixing problems associated with reaction of two or more phases reacting are absent. There are many research groups worldwide actively pursuing aspects of SCWO. This report was current at the time it was written. The properties of supercritical water, which are quite different from those of liquid water at ambient conditions, resemble more closely those of steam. The dielectric constant for supercritical water is about 2 at 450°C (840°F) and 250 bars, and the ionic dissociation constant falls from its usual value of 10-14 to a value of about 10-23. As a result, supercritical water acts as a nonpolar fluid. Its solvation properties resemble those of a low-polarity organic fluid; hydrocarbons are highly soluble, whereas inorganic salts are almost totally insoluble. The SCWO flow sheet (Figure 7-2) resembles that of WAO, but operation is at higher temperatures and pressures, resulting in different products. Air or oxygen and the feed mixture are compressed to the required pressure. Heat is added as needed, and the mixture flows to a reactor. The reaction raises the temperature to the final level desired. Oxidation occurs

PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-2 SCWO flow sheet (MODAR type). Source: Adapted from Barner et al. (1991). 147

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 148 spontaneously. Heat transfer equipment is included to recover some of the process heat and to cool products before release. The feed to the reactor may include a caustic solution if the organics contain heteroatoms such as the Cl, F, P, and S contained in chemical warfare agents. The products of their oxidation are then solid salts, which are almost completely insoluble. The formation of solids with plugging of the reactor has been a major problem in developing SCWO, but reactor designs are now available that may handle this problem. Like WAO, SCWO can be considered an alternative to incineration, because it is broadly applicable to any oxidizable organic compound. It could be used to destroy chemical warfare agents or to oxidize the products from a pretreatment process for the agent, such as hydrolysis. It could also handle slurries of propellant or explosive. The process has been operated to produce very little waste gas by substituting pure oxygen for air. Status and database. The SCWO process has been under active development for more than 10 years (Modell, 1989). The largest pilot plant has a capacity of about 1500 gallons per day. Laboratory and pilot plant units have been operated for treating hazardous wastes. Exploratory work has been done with surrogate compounds similar to chemical warfare agents and propellants. Commercial application appears imminent. There is a substantial research program on supercritical phenomena, and particularly on oxidation reactions, supported significantly by such federal agencies as the Army Research Office, Department of Energy, National Science Foundation, National Aeronautics and Space Administration, and, more recently, Advanced Research Projects Agency. The phase behavior of mixtures of water with many other materials under near-critical conditions has been reported for: benzene; C2-C7 alkanes; naphthalene; 1,3,5-trimethyl benzene; and permanent gases such as nitrogen, oxygen, hydrogen, CO2, and CH4. All compounds studied to date are completely miscible with water above 400°C (750°F) and 250 bars. However, inorganic salts have very low solubilities, even including such salts as NaCl that are very soluble in liquid water. Some kinetic data on oxidations have been reported. In general, complex organic molecules break up rapidly to smaller oxygenated species. Further oxidation of these fragmentation products is slower, and eventually the oxidation of simple compounds such as CO and NH3 becomes rate limiting. Global kinetic data have been gathered for a number of small molecules, such as ammonia and methane (see Figure K-1). Destruction efficiencies for bench-scale and pilot-plant systems have been reported for a large number of substances including species containing chlorine, phosphorus, sulfur, and nitrogen (see Appendix K; Helling, 1986; Helling and Tester, 1987, 1988;

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 149 Holgate and Tester, 1991; MODAR, 1992; Modell, 1989; Tester et al., 1991; Tester, 1992; Thomason et al., 1990). The solvation characteristics of supercritical water affect chemical reactions. Reaction rates can be rapid and oxidation reactions can proceed to completion in contrast to the WAO process. Hydrocarbons are converted to CO2 and water. Heteroatoms are converted to inorganic compounds in high oxidation states that can be precipitated as salts with the addition of some base. SCWO has not been applied to compounds containing fluorine. Sulfur goes to sulfate, phosphorus to phosphate, chlorine to chloride, and nitrogen primarily to N 2, with some N2O. To reduce oxides of nitrogen in SCWO may require some after treatment. The process is operated with a concentration of organics in water of 10 to 20 percent (by weight), which is much higher than in WAO. The larger organic content is needed to maintain the higher reaction temperature. The amount of water present is large enough to stabilize operation, probably with little opportunity for temperature excursions, although this will need verification. Application to chemical weapons destruction. SCWO should be effective in destroying chemical warfare agents or their products from pretreatment. It could probably also handle propellant and explosives; they would have to be ground to a fine particle size and fed as a slurry. The process could also decontaminate metal parts, but this would require intermittent operation, an inconvenience for such a high-pressure process. Special considerations. Two significant and interrelated problems for SCWO have received much attention: corrosion and solid salt precipitation from the reacting medium. Chemical agents, propellants, and explosives could be oxidized in an SCWO reactor with no formation of solids. The products formed, however, would be acidic and likely very corrosive. Oxidation of GB, for example, would produce HF and P2O5: This process would be carried out as shown in the flow sheet of Figure 7-2. The GB-water mixture fed to the reactor would contain about 10 percent GB. GB content would be chosen to raise the temperature of the feed entering the reactor by approximately 400°C (750°F), to the final reactor temperature. The strong acid gases leaving the reactor would dissolve in the liquid water after

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 150 cooling and pressure decline (letdown); the gas would consist primarily of unused oxygen, nitrogen (if air was used as the oxidant), and CO2. The process described would appear applicable to propellants and explosives (fed as slurries). However, the strong acids formed from chemical agents may require the addition of caustic for corrosion control; the salts formed would be insoluble solids under SCWO conditions. Alternatively, the preferred feed may be a hydrolyzed agent that would also result in insoluble solids in the SCWO reactor. Reactor design to eliminate the plugging associated with solids is still under development. The overall reaction with GB (hydrolysis followed by oxidation) would be The caustic used to neutralize the strong acids formed in the oxidation is twice the amount required for hydrolysis alone. More caustic could be used to increase pH but would react with CO2 to form solid sodium carbonate (Na2CO3), greatly increasing the solids burden in the final discharge. The final solution is usually slightly acidic to avoid Na2CO3 formation. The precise mode of operation to best control corrosion and solids plugging will need to be developed. Solids formation in the reaction affects both the reactor design and the flow sheet. Reactor design, for example, may include two temperature levels; a lower temperature will result in a subcritical condition with liquid water present to dissolve the solid salts. Flow sheets have been modified to avoid solid precipitation in the inlet lines, that is, by maintaining subcritical conditions. Various reactor discharge and cooling arrangements have been suggested for separating or redissolving the solids. Very high destruction efficiencies will require reactor designs that provide no opportunity for bypassing or short-circuiting through the reactor. That is, reactors will need to operate as plug-flow reactors or will require staging. One preferred reactor when solids are present is a larger vessel, which would not operate as a plug-flow reactor. A second reactor in series, possibly a tubular design, would probably be necessary for very high conversion (99.9999 percent) of the organic feed content. Corrosion is expected to be a severe problem for SCWO, with fluoride and chloride ions being particularly aggressive. No data appear to have been reported on fluorine compounds. Reactor construction materials and much of the upstream and downstream equipment must be corrosion resistant as well as high in strength. High- nickel alloys (Inconel, Hastelloy C276) have been

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 151 used. Ceramic materials may be used in some locations but no details have been disclosed. It has been suggested that corrosion may be a less serious problem at supercritical conditions than at lower temperatures because of the special physical properties of the supercritical fluid, that is, its lower dielectric constant and a lower electrolytic corrosion. However, no quantitative information has been disclosed. Subcritical conditions will exist in the outlet lines, and electrolytic corrosion should be expected there. Experience with WAO revealed an additional problem with solids deposits: corrosion (pitting) was increased underneath the deposits. This may also be a factor in SCWO. Methods are needed for pressure letdown and to remove brines or slurries from the reactor and salt separator. Such methods have been achieved satisfactorily in existing pilot plant equipment. Chemical agents will have a much larger yield of solid products than most materials tested to date. By-products and waste streams. The usual gaseous effluent of SCWO treatment of chemical warfare agents consists of the oxidation products CO2, N2, and some N2O and residual O2 and N2 (from the air). If acid gases are not neutralized by added caustic, then the gas effluent might also contain HF, HCL, SO2, or P2O5, depending on the feed. The aqueous liquid product will contain the acidic components or corresponding salts. Excess caustic will react with CO2 to form solid Na2CO3. As noted previously, usual operation runs very slightly acidic with the CO2 as gas. Excess caustic may be needed if corrosion proves to be too severe, although it would have a pronounced effect on the product. The Na2CO3 formed would greatly increase the solids to be handled while reducing the gas discharge. Advantages and disadvantages. Major advantages of SCWO include the following: • The gas effluent is claimed to be free of the usual troublesome components found in typical incineration processes, such as NOx, dioxin, and particulate matter. Some final gas treatment may be provided as a precaution against undesirable gaseous effluents resulting from system upsets. • The system could be treated as a dosed system, with no discharge of products until analyses are complete. The use of oxygen instead of air reduces gas discharge volume; in conjunction with CO2 removal as carbonate, the gases discharged could be reduced (theoretically) almost to zero. • Dilute aqueous wastes with up to 20 percent organic content are particularly well suited to SCWO. The process is thus well adapted to handling products from a prior aqueous treatment.

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The U.S. Army Chemical Stockpile Disposal Program was established with the goal of destroying the nation's stockpile of lethal unitary chemical weapons. Since 1990 the U.S. Army has been testing a baseline incineration technology on Johnston Island in the southern Pacific Ocean. Under the planned disposal program, this baseline technology will be imported in the mid to late 1990s to continental United States disposal facilities; construction will include eight stockpile storage sites.

In early 1992 the Committee on Alternative Chemical Demilitarization Technologies was formed by the National Research Council to investigate potential alternatives to the baseline technology. This book, the result of its investigation, addresses the use of alternative destruction technologies to replace, partly or wholly, or to be used in addition to the baseline technology. The book considers principal technologies that might be applied to the disposal program, strategies that might be used to manage the stockpile, and combinations of technologies that might be employed.

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