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Suggested Citation:"Wet Air 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 138
Suggested Citation:"Wet Air 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 139
Suggested Citation:"Wet Air 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 140
Suggested Citation:"Wet Air 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 141
Suggested Citation:"Wet Air 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 142
Suggested Citation:"Wet Air 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 143
Suggested Citation:"Wet Air 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 144
Suggested Citation:"Wet Air 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 145

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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 138 MODERATE-TEMPERATURE, HIGH-PRESSURE PROCESSES Organic materials may be oxidized in the presence of water at moderate temperature and high pressure. Temperatures used are in the range 200 to 650°C, which is low compared with the usual combustion temperatures of about 1500°C (2732°F). The pressures used in this method are high, from 360 to 4,000 psi (25 to 275 bars). Processes have been developed at the low and high ends of both the pressure and temperature ranges. Wet air oxidation (WAO) is carried out in the liquid water phase, with pressures exceeding saturation pressure. Supercritical water oxidation is carried out at much higher temperatures and pressures, exceeding the critical temperature and pressure of water, that is, 374°C (705°F) and 3,205 psi (221 bars), respectively. The fluid properties under these conditions are very different from those of the liquid water used in WAO. For example, organic substances are completely soluble in water whereas salts are almost insoluble. In practice, the suggested operating pressures sometimes overlap; temperatures, however, differ by 200°C or more. In many respects these processes are an alternative to incineration: they are broadly applicable to any oxidizable organic compound and could be used to treat chemical warfare agents, propellants, and explosives (solid materials would require comminution and would be fed as a slurry). These processes could also be used to oxidize the products of agent pretreatment, such as the products of hydrolysis. Both of these oxidation processes offer some major advantages. Objectionable pollutants such as nitrogen oxides, dioxins, and particulates do not form at the relatively low temperatures used. Some nitrogen may show up as N2O or NH3, depending on its form in the feed material and the severity of the oxidation. Product volumes can be controlled to be small enough that dosed systems are practical, allowing products to be analyzed and their safety confirmed before release to the atmosphere. WAO is reviewed below and is followed by a review of supercritical water oxidation (SCWO). Wet Air Oxidation Technology description. In WAO, oxidizable materials, usually organic materials, are oxidized in a dilute, aqueous, liquid matrix at temperatures of 200 to 300°C (392 to 572°F); the corresponding pressures required to maintain a liquid phase are in the range 230 to 1,250 psi (16 to 186 bars). The process is applicable to material in solution or to suspended solids in water (Copa and Lehmann 1992; Copa and Gitchel, 1989; Zimmerman, 1958).

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 139 In this process, air (or air enriched with oxygen) and an aqueous feed mixture are compressed to the required pressure (Figure 7-1). Heat is added as needed, and the mixture flows to a reactor. Oxidation of material in the feed releases heat and raises the reactor temperature further; some of this reaction heat may be recovered in a heat exchanger as shown. The amount of heat added and the amount of reaction heat used will depend on the concentration of organics in the water. Higher concentrations of organic matter will release more heat and lead to a greater temperature rise in the reactor. Most applications have been to wastewater with low concentrations of organic matter, about I percent (by weight) or less. Experience is limited to reactor temperatures of less than 350° C (660°F) with the organic content of the feed usually less than 5 percent, generally I to 2 percent. Feeds with higher concentrations could be processed, but there is little experience with such conditions and their greater temperature increases. Status and database Approximately 200 WAO plants are operating worldwide. A variety of waste streams have been treated with this technology, including spent caustics, sludges in municipal and industrial wastewater treatment, wastewater from chemical production processes, pulp and paper wastes, and military wastes (Copa and Lehmann, 1992). In all these applications, organic and inorganic compounds are converted to simple products.2 The process has been applied to pesticides with chemical structures similar to those of nerve agents, achieving greater than 99 percent destruction of malathion, parathion, and glyphosate.3 Application to chemical weapons destruction. Experience with pesticides and such materials as chlorinated compounds indicates that WAO could 2 Organic compounds with carbon, hydrogen, and oxygen are converted to carbon dioxide (CO2), water, and low- molecular-weight compounds such as acetic acid; sulfur sulfate ion ; phosphorus phosphate ion ; chlorine chloride ion (Cl-); and nitrogen ammonium , N2 , nitrate ion , and nitrous oxide (N2O). For inorganic substances, sulfides sulfate and cyanides CO2 and . The particular nitrogen end products depend on the organic nitrogen compound converted. 3 Reported destruction efficiencies for pesticides are: malathion, 99+ percent at 200°C; dyfonate, 99 + percent at 260°C; parathion, 99+ percent at 260°C; glyphosate, 99+ percent at 260 to 280°C; complete destruction of pesticides at 280°C. Glyphosate contains a phosphorus atom double-bonded to one oxygen and single-bonded to two oxygens; parathion has a phosphorus atom double-bonded to sulfur and single-bonded to three oxygens; and parathion and dyfonate have a phosphorus atom double-bonded to a sulfur and single-bonded to a sulfur and two oxygen atoms (Copa and Lehman, 1992).

PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-1 WAO flow diagram. Source: Adapted from Copa and Gitchel (1989). 140

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 141 destroy chemical agents. It could also be used to oxidize the products from a pretreatment of the agent, such as hydrolysis. Reaction rate. Kinetic data on the rate of oxidation of large organic molecules in WAO processes are not available, although there have been tests on destroying compounds containing phosphorus. In WAO such molecules break down rapidly but yield a substantial mount of low-molecular-weight material that then oxidizes much more slowly. Some small organic molecules remain for further treatment. The rate of oxidation and the weight percent of organic compounds remaining as small, partially oxidized materials (e.g., acetic acid) depends on temperature and pressure. In typical applications, the weight percent remaining would be 25 percent of the weight of the original material. The P-C bond (GB or VX) is believed to react slowly. Table 7-1 summarizes recommended operating temperatures and saturation pressures (pressures at the boiling point) for chemical warfare agent destruction by WAO. In actual operation in previous applications, however, the pressure is maintained substantially higher than the pressures shown, as much as twice as high. From experience (Copa and Lehman, 1992), agent destruction of 98 to 99 percent should be expected in a single reactor with a residence time of 1 to 2 hours. WAO has been applied to some propellants and to wastewaters containing nitro-compounds from the manufacture of propellants and explosives. Solid propellants and high explosives would have to be fed to the reactor as a slurry in water. Special considerations. High destruction efficiency will require the reactor to operate much like a plug flow,4 which is difficult to achieve in a reactor with a long residence time and a very slow flow rate. Reactors have been built with internal baffles to suppress longitudinal mixing and they have also been built as separate vessels in series (three reactors in series). For the very high destruction efficiency required for chemical warfare agents, it appears that several reactors in series would be preferred. In such an arrangement-for example, hydrolysis followed by two WAO units, with each achieving 98 to 99 percent conversion-a very high overall destruction efficiency can be achieved. (Three reactors in series, each achieving 99 percent destruction, would attain an overall destruction of 99.9999 percent.) The products from a chemical treatment of agent (hydrolysis) could be effectively mineralized by a WAO process; they are in a dilute aqueous 4 Plug flow in a tube assumes that properties of the reacting mixture are uniform at any cross section of the tube and change in the longitudinal direction.

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 142 TABLE 7-1 Recommended WAO Operating Temperatures and Saturation Pressures for Destruction of Chemical Warfare Agents and Propellants Agent/Component Temperature (°C) Saturation Pressure (bars) Sarin 260 47 VX 260 47 Mustard 200-240 15-34 Propellants 260-280 47-64 Source: Copa and Lehman (1992). solution with excess caustic, which is needed for pH control in a WAO unit. The stoichiometry for hydrolysis of GB followed by WAO might then appear approximately as shown in Equation 1.5 Corrosion is a concern with WAO conditions, particularly for materials containing chlorine, fluorine, sulfur, and phosphorus, which all form adds in solution on agent oxidation. In the example shown above, excess caustic is limited; more may be needed to neutralize such adds and reduce corrosion. If hydrolysis and WAO are used sequentially, a large excess of caustic would be used to ensure complete hydrolysis (see Chapter 6), and this caustic would provide the pH control needed in the WAO process. In such an arrangement, caustic would certainly be added to react with the strong adds to form, for example, sodium fluoride and sodium phosphate as in the above equation. Addition of caustic beyond that required by the stoichiometry would 5 The partially oxidized organic compound shown here is the sodium salt of acetic acid. Other materials would show up in actual practice. More NaOH than that shown might be used for corrosion control. Material balance for GB destruction is shown in Appendix K.

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 143 need to be evaluated. With limited addition of caustic, an acidic solution is formed by the oxidation process. If more caustic is needed for corrosion control, the additional caustic would react with CO2 to form sodium bicarbonate or sodium carbonate, adding greatly (up to threefold) to the solid salts that must be handled in the process but reducing the quantity of waste gas volume. The fluoride ion is particularly corrosive. A current limitation for WAO processing is 200 ppm of fluoride. Because fluorine is 14 percent of GB by weight, this corrosion limitation implies a GB feed concentration of only 1,500 ppm (i.e., 0.15 percent). Insoluble deposits also appear to aggravate corrosion; the environment under a solid deposit may differ significantly from that of the bulk liquid. Scaling of heat exchangers may also pose a problem, caused, for example, by the hardness of water containing calcium, silicon, or iron salts. Such scaling is usually handled by acid washing (for example, to remove CaSO4 scale). Even though a basic compound would be added to control WAO pH, the reaction conditions would still be very aggressive. Components sensitive to corrosion include the reactor, heat exchangers, piping, valves, and tanks., which would require materials resistant to corrosion.6 For example, high-chromium materials, such as Hastelloy C-276, could be expected to stand up very well in the WAO environment. Titanium has been used in experimental work, but it cannot be used in a pure oxygen environment because of its flammability. Like combustion, WAO is highly exothermic, and its reaction rates are sensitive to temperature. WAO temperature excursions are limited, however, by the presence of a large mount of water. Most of the industrial wastes treated with WAO have had organic contents of less than I or 2 percent. Larger throughputs with higher concentrations are possible but would result in larger heat releases and the possibility of larger temperature excursions. In practice, the organic concentration in the feed water should preferably be limited to 5 percent or less. If temperature and pressure excursions occur outside of the normal operating regime, an alarm system is triggered and the system shuts down. The use of pure oxygen rather than air as the oxidizing agent would reduce gas production by 85 percent or more and make it easier to operate the process as a closed system in which the gaseous emissions would be stored and analyzed before release to the environment. Some fixed gas is considered desirable, however, for stable operation (Copa, 1992). Any variations in 6 Applicable materials include 304L and 316L stainless; Carpenter 20CB-3; Incoloy 800 and 825; Inconel 600 and 625; Hastelloy C-276, G-3, and C-22; and titanium grades 1, 2, 3, 7, 11, and 12.

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 144 oxidation will go to changing reactor temperature and to evaporation (or condensation) of water. The latter is important for system stability and will depend on the presence of a fixed gas. Thus, although pure oxygen has been used in WAO, enriched air (e.g., 50 percent oxygen) is preferred. By-products and waste streams Gas leaving a WAO unit is said to be free of most of the objectionable pollutants associated with combustion gases, such as the usual oxides of nitrogen, dioxins, furans, and particulate matter in the gas phase. For a WAO process using air, typical effluent gas composition is shown in Table 7-2. This has been estimated for oxidation of GB; estimates for excess O2 and for carbon monoxide (CO) and hydrocarbons are based on experience with other materials. Gas composition and volume will differ if the gas used is air enriched with oxygen. Some final gas cleanup may be required. Small concentrations of CO and some trace hydrocarbons can be eliminated by a thermal or catalytic oxidizer (described later in this chapter). An activated-carbon-bed adsorber would also ensure against discharging chemical agent or polar organics to the atmosphere. A substantial fraction (by weight) of organics in the feedstock will remain in the water as small oxygenated species; typically, 70 percent by weight is oxidized to CO2 and water and 30 percent remains as acetic acid and other organic compounds. The feed to a subsequent treatment unit (for biological or chemical oxidation) would have requirements of 5 to 10 g/L of biological oxygen demand (BOD) and 10 to 20 g/L of chemical oxygen demand (COD). For a biological treatment process, water is usually removed from solids and the solids are then sent to a landfill. For WAO some additional treatment may be needed because of the solubility of the large quantity of inorganic salt that remains. Advantages and disadvantages. There are several major advantages of WAO compared with the baseline technology of incineration: • The gas effluent is free of SO2, dioxins, and particulate matter, and the only N oxide reported is N2O. • The large water dilution could be a significant advantage in treating energetic materials (propellants and explosives). Explosion or detonation should not occur if there is adequate mixing, although this would require demonstration. • WAO is particularly well-suited for treatment of dilute wastewater. It has not been used commercially for feedstocks containing concentrated organic compounds that require dilution.

PROCESSES AT MEDIUM AND HIGH TEMPERATURES 145 TABLE 7-2 Estimated Effluent Gas Composition for Two-Step Destruction of GB, Hydrolysis Followed by WAO (Using Air) Component Typical Gas Composition (by volume) Oxygen (O2 ) 3-6 percent Nitrogen (N2) 78-82 percent Carbon Dioxide (CO2) 8-12 percent Carbon Monoxide (CO) 10-1,000 ppm Hydrocarbons 100-1,000 ppm Solids 0 WAO also has several major disadvantages: • It operates at high pressures (e.g., at 1,450 psi, or 100 bars). This pressure is well within common industrial practice. Nevertheless, there might be some concern about operating with agent under high pressure; thus, it might be preferable to use WAO to oxidize products from pretreatment of agent, such as the products of hydrolysis. • WAO does not oxidize all of the material completely. A posttreatment, usually biodegradation, is required to meet Resource Conservation and Recovery Act (RCRA) standards and Clean Air Act National Emission Standards for Hazardous Waste Pollutants (NESHAP). Development needs. Several steps would be needed for WAO to be used in chemical weapons destruction: • Corrosion testing on possible construction materials would be required bemuse of the fluoride, chloride, sulfate, and phosphate ions present in solution. • Pilot plant work could be done on related compounds to establish the reaction conditions and the treatment process for the liquid product. Process conditions must be set on the basis of experimental work, ultimately with the materials of actual interest. The amount of caustic agent required,

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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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