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Alternative Technologies for the Destruction of Chemical Agents and Munitions 7 Processes at Medium and High Temperatures Processes beyond those previously discussed might be applied to the destruction of the chemical weapons stockpile.1 In contrast to the liquid-phase processes reviewed in Chapter 6 that operate at low temperatures and atmospheric pressure, most of the technologies reviewed in this chapter operate under much more severe conditions. Many of these technologies, even the oxidative ones, produce gaseous waste streams that would require further oxidation in some form of afterburner. The technologies examined here can be grouped in the following categories: Moderate-temperature, high-pressure processes: Wet air and supercritical water oxidation are processes that occur in water at moderate temperatures and high pressures. High-temperature, low-pressure pyrolysis: Some technologies involve agent vaporization and decomposition upon heating. The gaseous products resulting from this pyrolysis will generally need to be oxidized further to destroy organic by-products of agent breakdown. Molten metal and plasma arc (electric arc) are such technologies and will be discussed below. Gasification and steam reforming are also in this category. High-temperature, low-pressure oxidation: One class of technologies, including catalytic fluidized-bed systems, molten salt, and catalytic oxidation, entails agent reaction with oxygen to produce carbon dioxide, water, gases, and other inorganic substances. Other processes: A hydrogenation process might be used to destroy agent or a sulfur process might be used to create a polymer while destroying agent. Reduction and pyrolytic processes will result in products that are themselves toxic, such as PH3 or H2S. These will require safe handling as well as final destruction to create stable products. 1 Much of the information presented in this chapter is based on presentations made at the committee's workshop (Appendix F).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions FIGURE 7-1 WAO flow diagram. Source: Adapted from Copa and Gitchel (1989).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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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Alternative Technologies for the Destruction of Chemical Agents and Munitions 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
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Alternative Technologies for the Destruction of Chemical Agents and Munitions FIGURE 7-2 SCWO flow sheet (MODAR type). Source: Adapted from Barner et al. (1991).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions Advantages and disadvantages. The advantages for molten salt oxidation are as follows: The process allows gaseous oxidation to occur at relatively low temperatures, thus minimizing the formation of such emissions as NOx. Most acid gases formed would be retained in the bed. Wet scrubbing would be minimized, possibly eliminated. This technology can be used for secondary oxidation after a primary agent detoxification process, such as hydrolysis or pyrolysis. It is a versatile technology, able to destroy agents, explosives, and propellants and to decontaminate metal parts. Energetic materials would have to be fed carefully (probably in slurry form) to avoid disastrous explosions. Disadvantages of the process include the following: The means of disposing of the salt removed from the system (blow-down) is currently uncertain; the salt is soluble. A better fundamental understanding is needed to avoid the possibility of superheated-vapor explosions. Development needs. This technology has been used on a small scale since 1950. Molten carbonate is a fairly well-known material, and there is some experience in using it for agent destruction on a laboratory scale. Specific development problems have not been identified, but a pilot plant and demonstration effort would be required. Catalytic Oxidation Technology description. Catalytic oxidation uses an oxidation catalyst with natural gas to heat the catalyst to an operating temperature of about 500°C (930°F). (It is also possible to preheat the feed stream by using other direct or indirect methods.) This technology is normally applied only to very dilute gas streams for final cleanup. With some modification, possibly with electrical heating, it could be used to replace the baseline combustion-based afterburner. The required operating temperature would depend on catalyst activity. For a temperature of 500°C and a space velocity of about 7,500 V/h/V, the catalyst can destroy volatile organic compounds containing halogens, sulfur, and phosphorus when they consist of about 0.1 percent of the gaseous feed. The catalysts are generally available on a cordierite monolith to avoid problems of drops in pressure and often contain noble metals on a gamma-alumina washcoat (Shaw et al., 1993; Wang et al., 1992; Yu et al.,
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Alternative Technologies for the Destruction of Chemical Agents and Munitions 1992). The technology is very similar to that used in the automobile industry to reduce emissions of unburned hydrocarbons and CO. Development status. Laboratory data available in the literature are adequate for commercial design (Shaw et al., 1993). Catalyst manufacturers such as Engelhard (Farrauto, 1992) and Allied (Lester, 1992) have announced the availability of commercial oxidation catalysts that are not degraded by elements such as chlorine, phosphorus and sulfur. However, few data are available in the technical literature to verify these claims. Proprietary catalysts (of unknown composition) were tested on chemical agents in air streams at high dilutions with successful results (Snow, 1992). A fun-scale test for the destruction of trichloroethylene will be conducted in early 1993 under the U.S. EPA Site Program (Shaw et al., 1992). Application to chemical weapons destruction. Laboratory results have demonstrated the destruction of trichloroethylene, methylene chloride, and hydrogen sulfide (to better than 99.9 percent) to the products hydrogen chloride, sulfur dioxide, and sulfur trioxide; little deactivation occurred after the initial drop in fresh catalyst activity of about 25 percent. This technology can be used to fully oxidize trace gases produced in well-mixed oxidizing systems that need an afterburner. A lab demonstration showed that oxidation can be promoted by using the support material (cordierite) without catalyst; the temperature required was much higher, 1000°C (1830°F) versus the 500°C (930°F) when using catalyst. The oxidation was not as complete as with catalysis; some chlorinated hydrocarbon species survived in the product. For use of highly active catalysts, the best application appears to be for treatment of gases from low-or medium-temperature processes, such as chemical and biological oxidation and WAO or SCWO when the concentration of catalyst poisons can be within acceptable limits and where heat must be added to bring the gases to reactive temperature. For high-temperature agent oxidation the heteroatom content can limit catalyst activity; however, the available high temperature allows use of a ragged low-activity catalyst. Waste streams. Liquid products of oxidation may need to be removed, possibly by using scrubbers or activated-carbon filters. Scrubber effluent liquids contain unreacted alkaline materials and salts of chlorine, fluorine, sulfur, and phosphorus. The spent sorbem will need conventional disposal. The solids will contain calcium chloride, fluoride, sulfate, and phosphate. Similarly, the spent catalyst will need to be discarded. Advantages and disadvantages. Catalytic oxidation is used commercially for the oxidation of trace hydrocarbons, because of its several advantages over
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Alternative Technologies for the Destruction of Chemical Agents and Munitions conventional incineration: reducing the need for fuel or other heating media, avoiding the need for high-temperature materials of construction, and offering the possibility of overload with catalytic solids to achieve high destruction levels. The disadvantages of catalytic oxidation are associated with the poisoning and deactivation of the active metals by the heteroatoms found in agent. Thus, system lifetime is highly uncertain. The technology is generally used for dilute gas streams. It would not be a reasonable technology for initial agent destruction. Development needs. The scaleup of catalytic oxidation is generally accomplished from tests on very small reactors. Thus, the testing of a catalytic oxidizer with agent heteroatoms should take less than 1 year. To the extent that the catalyst is adequate, the next step would be a full-scale test. Some catalyst development and pilot plant work would be required. OTHER PROCESSES Hydrogenation Processes Technology description. Catalytic hydrogenation is very widely practiced, particularly in the oil industry. It usually involves temperatures high enough to promote cracking and rearrangement reactions along with hydrogenation. The technology is now being pursued to convert hazardous wastes into useful materials. Organic materials containing a high concentration of sulfur are commonly treated with hydrogen to eliminate the sulfur (as H2S). The hydrogenation process is now being developed to attack chlorine similarly (for removal as HCl). Presumably other heteroatoms, such as P and F, would also be reactive with hydrogen under appropriate conditions; this remains to be demonstrated. Usual operating conditions are a temperature of up to 450°C (840°F) and pressure in the range of 116 to 1,450 psi (8 to 100 bars) with a large excess of hydrogen gas, typically about 80 percent by volume. The hydrogen reacts with the heteroatom and also saturates the remaining hydrocarbon. Figure 7-10 is a block design for the UOP process, a by-product conversion process.9 This process consists primarily of a reactor for removal of the chlorine in the material, followed by separation and recovery of the HCl and saturated hydrocarbon (Hedden, 1992). 9 For example, trichloroethylene reacts with hydrogen to form ethane and hydrochloric acid: .
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Alternative Technologies for the Destruction of Chemical Agents and Munitions FIGURE 7-10 Block flow diagram of the UOP HyChlor conversion process. Source: Hedden (1992).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions Status and database Laboratory and pilot plant equipment axe in place and could be used to examine surrogate materials for chemical agents. Little if any work has been done on fluorine and phosphorus compounds. The central requirement for this process is a suitable catalyst, one that will bring about the reaction desired and is rugged enough to have a long useful life, typically 1 year or more. Desulfurization catalysts are well-known. Catalysts for chlorine removal have been developed but are probably optimized for particular feedstocks. Different catalysts may be needed depending on the heteroatom to be removed. The UOP process has been used on several waste streams: PCBs, to form biphenyl and HCl; cresol, to form toluene and water; trichloroethylene, to form ethane and HCl; tetraethyl lead (treated with H2S), to form ethane and PbS; and waste streams from the manufacture of epichlorhydrin and vinyl chloride. Application to chemical weapons destruction. Hydrogenation is certainly capable of destroying chemical agents. The high conversion level needed is not typical, however, and would have to be demonstrated. The process is applicable to liquid feedstocks. Hydrotreating of propellants and explosives would not be useful. Special considerations. Care must usually be taken to prevent loss of catalyst activity. Contaminants can be destructive; for example, small concentrations of metals or heavy ends that crack and form coke on the catalyst can destroy catalyst activity. Almost any new feedstock will require extended process runs to ensure catalyst life. Hydrocracking is exothermic, but the large excess of hydrogen limits the temperature rise, allowing the hydrogenation processes to operate in a stable manner. Conversion amounts, for example, for sulfur removal, can be high. Test work would be needed to demonstrate the extremely high levels of destruction required for chemical agents. By-products and waste streams. The exact nature of the waste streams would have to be determined by work on the agents. Gaseous waste streams of HCl, HF, and H2S and possibly PH3 would be produced in a dilute mixture with hydrogen. They all may pose problems to catalyst performance. They could be separated from the hydrocarbon product by distillation and then would need to be treated appropriately. The hydrogen would be recycled. The gaseous products in this case represent safety and toxicity hazards. Liquid product would probably be burned. Combustion products might require a water or caustic wash, resulting in a waste aqueous stream.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions Agents might require distillation before being run over the catalyst. The residue from the distillation would be a waste stream that might, for example, be coked to a stable carbon residue. The composition of gas produced by coking is unknown; it might contain some of the agent heteroatoms and require further treatment. Ultimately, the catalyst would have to be removed and possibly reworked. Advantages and disadvantages. There are several advantages of the process: It is a closed system; material can be recycled if the processing is incomplete. Useful products are recovered, and waste streams are minimized; the products would probably be used as fuel within the plant. The operating pressure and temperature for the kinds of materials in chemical agents would most likely be at the low end of the scale, perhaps 360 psi (25 bars) and 400°C (750°F); these are conditions within the range of common industrial experience. The product streams are small in volume compared with those of oxidation processes; they could be retained easily for certification before release. The major disadvantages are as follows: The three chemical warfare agents would probably require the development of three catalysts; some of the heteroatoms would be expected to be poisons for normal hydrogenation catalysts, nitrogen and phosphorus in particular. Small system leaks would be hazardous and would require more than usual care. Substantial leakage in any enclosed space containing air could lead to an explosion; special mitigation measures will be needed. Development needs. Hydrogenation process development would require catalyst developments and process definition, including operating conditions and recovery and treatment of products. Development based on surrogate chemical agents could be achieved in a few months. Work would be necessary with actual agents, however, because of the presence of impurities that could affect the catalysts and process.
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Alternative Technologies for the Destruction of Chemical Agents and Munitions The Adams Process-Reaction with Sulfur Technology description. The Adams process is a patented method that relies on the reactivity of elemental sulfur vapor to destroy organic materials at temperatures of 500 to 600°C (930 to 1100°F) (Berkey and Hendricks, 1992). Gaseous and solid products are formed. Gases such as CS2, COS, HCl, and H2S require recovery or further destruction. The solid formed is a high-molecular-weight carbon-sulfur material of uncertain composition, probably containing some of the heteroatoms (P, Cl, F, or O) of the original organic materials. The process involves several steps (Figure 7-11). Liquid agent, possibly preheated, and sulfur vapor are fed to a reactor; an oxygen-free atmosphere is maintained by using nitrogen at a pressure slightly above atmospheric. The reactor suggested in a process patent is a ''rotating screw-type oven heated by electric induction heating coils to maintain temperature...'' (the ratio of sulfur vapor to organic has not been disclosed; there is presumably a range of compositions over which reaction is possible, analogous to upper and lower flammability limits in combustion with air) (Adams, 1990). The solid product formed is further heated to drive off unreacted sulfur vapor. The result is a black glassy product, primarily carbon and sulfur in a highly cross-linked molecular structure. No residual reactant (e.g., PCB) has been detected. Experiments suggest that about 90 percent of the carbon in the feed is converted to this solid product. The gas leaving the reactor contains nitrogen and unreacted sulfur vapor along with products of the reaction, such as CS2, H2S, COS, S2Cl2, CSCl2, and HCl. A series of gas-treating steps are envisioned: condensers to remove condensibles such as S, CS2, and S2Cl2; scrubbers to remove acid gases, such as HCl and H2S, which will require further disposal (the precise steps for separating all of the products do not appear to have been considered by the developers because they consider them to be existing technology); and recycle of fixed gases, primarily nitrogen. Status and history. The process has been tested at the University of Pittsburgh and at Picatinny Arsenal. Bench-scale tests carried out by the Center for Hazardous Material Research (University of Pittsburgh) demonstrated the application of the process to a number of organic and chlorinated organic materials. A pilot-scale continuous unit was operated by the National Environmental Technology Applications Corporation (University
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Alternative Technologies for the Destruction of Chemical Agents and Munitions FIGURE 7-11 A process flow sheet for the Adams process as presented by CHEMLOOP, L.P. Source: Berkey and Hendricks (1992).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions of Pittsburgh), where a number of chlorinated organic compounds were destroyed.10 According to the developers of the process, experimental work with chlorinated compounds has produced the following compounds: Reaction rates have not been measured but are believed to be high. Experimental work on chlorinated compounds has shown high destruction levels. Staging, with reactors in series, would ensure against the agent bypassing the system and ending up in the product. The detailed chemistry of the process does not appear to have been addressed. The solid residue is reported to be stable and insoluble, but the effects of phosphorus, fluorine, nitrogen, or oxygen in the feed have not been studied. The solid product is apparently produced in a range of particle sizes. Some buildup of deposits has been observed, and one plugging problem has been reported during continuous operation of the pilot unit. Application to chemical weapons destruction. In principle, the process of reaction with sulfur is applicable to liquid agents, propellants, explosives, metal parts, and dunnage. Such a broad array of treated materials would require various furnace designs to accommodate the corresponding range of reactivity, heat release, and feed type (solids and liquids). Propellants and explosives would generate fixed gases, and their reaction might be very rapid or explosive. The chemistry of these materials with sulfur has not been investigated. The process is probably not applicable to agent detoxified in a previous operation. Any large amount of water would have to be handled separately; the process would therefore not appear suitable as a process subsequent to chemical hydrolysis or bioprocessing. 10 Information presented at the committee's workshop (see Appendix F).
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Alternative Technologies for the Destruction of Chemical Agents and Munitions The reaction appears to be stable. The possibility of hazardous reactions or explosions needs to be considered, however, including the possible explosion of hydrocarbon-sulfur vapor mixtures and rapid reaction with air leaked into the system. By-products and waste streams. This process has waste streams that differ greatly from those of the other technologies. Many different carbon-sulfur-chlorine-hydrogen compounds have been observed in the produced gases. Other heteroatoms found in chemical weapons (F, P, and O) have not been studied. Conventional acid gas scrubbers such as Catacarb or other alkaline media could probably be used to remove most, if not all, gaseous components. Thus, the only gaseous emission would be N2. No liquid products are withdrawn from the reactor. Some of the products appear large enough, however, to condense at moderate temperature. A black polymer, which has not been characterized, is the only solid product. It appears to be very stable. Some of this product will appear as very fine dust dispersed in the gas. Intermediate or low-molecular-weight C-S polymers may be adsorbed on the solid, particularly on the fine dust dispersed in the sulfur vapor. Complete characterizations of these solids have not been reported. The adsorbed material may represent a disposal problem. Advantages and disadvantages. The Adams process is considered innovative and appears to incorporate most of the hazardous material in a very stable solid believed to be a carbon-sulfur polymer. The other by-products are well known and treatable with conventional technology. Furthermore, the operation at atmospheric pressure and temperatures of 500 to 600°C (930 to 1100°F) are not very demanding in terms of construction materials or energy needs. The major disadvantages of this process concerns the lack of knowledge of its chemistry. This omission makes it difficult to project scaleup problems and long-term stability of the solid products. Other problems include the need to process a gas containing fine solids in such conventional equipment as pipes and blowers. Solids produced by condensation from vapor are typically very fine dusts (e.g., 1 μm). Problems involving plugging, adsorption of agent or other hazardous gases and liquids on the solid, and the effect of electrostatics on the solid have not been considered. Development needs. An extensive development and scaleup program would be required to apply this process to the destruction of the chemical weapons stockpile and satisfy environmental standards. The development program would include pilot plant operations using agent surrogates followed by confirmation with some runs using agents. Ultimately, a full-scale
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Alternative Technologies for the Destruction of Chemical Agents and Munitions demonstration would be necessary. The process appears to present many uncertainties related to chemistry and solids handling; resolving these uncertainties would require a substantial research and development effort.
Representative terms from entire chapter: