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 163 Advantages and disadvantages. The destruction process for agent in this case is a two-step process: an initial steam reforming at high temperature followed by combustion. The possibility of agent breakthrough is therefore remote. Gasifiers are very high throughput devices, and the treatment of agent would have little effect on the operation. They axe easily controlled and quite stable. The inventory of agent would be small because of the very high processing rate possible. The disadvantages of this process axe inherent in the primary purpose of the equipment, which is to gasify coal. It would be difficult to combine agent destruction with any ongoing industrial process, considering the lethal nature of chemical agents. Furthermore, industrial designs would need to be revised and scaled down for application to chemical weapons destruction. Development needs. Gasification is commercial technology. Safe feed mechanisms to bring the agent or the energetics into the gasifier need to be developed. Operation of gasifiers with the materials containing the heteroatoms of chemical agents would need to be demonstrated. Such problems as attack on refractory lining and metal corrosion could be expected. The process would also need to be integrated with the subsequent combustion and gas cleanup system. Consequently, substantial design and development would be required. Synthetica Detoxifier Technology description. Distinct from many of the other proposed unit operations, the Synthetica Steam Detoxifier consists of a series of processes integrated into an overall equipment package, complete with controls, monitoring equipment, and other features (Figure 7-8). The same figure is repeated in Appendix K and shows an estimated heat and material balance. Agent destruction takes place by high-temperature pyrolysis followed by low-pressure gasification. This series of processes treats organic materials as follows: ⢠Evaporation and treatment of the feed material: The feed material is evaporated into high-temperature steam, and pyrolysis and some reaction with steam occurs. Alternative evaporators might be used. A promising evaporator for applications to chemical warfare agents is the moving bed evaporator (MBE), which consists of a descending bed of uniform-size solid balls about 3/8 in. in diameter. Evaporator capacity is specified as up to 3 drums per day by the developer, Synthetica Technologies, Inc., Richmond, Calif. Hot gas enters the bottom at about 700°C (1290°F). Feed, such as
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 164 FIGURE 7-8 Schematic flow sheet for the Synthetica Steam Detoxifier. Source: Synthetica Technologies, Inc. evaporator (MBE), which consists of a descending bed of uniform-size solid balls about 3/8 in. in diameter. Evaporator capacity is specified as up to 3 drums per day by the developer, Synthetica Technologies, Inc., Richmond, Calif. Hot gas enters the bottom at about 700°C (1290°F). Feed, such as chemical warfare agent, is injected at about the midpoint. The solids contain a reactive alkaline material.
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 165 temperature of about 425°C (795°F). The gas consists primarily of C-H-O species. Yet another form of evaporator for this type of system is simply a drum of the feed material bathed in an atmosphere of hot gas, that is, steam and reaction products, at a temperature as high as 650°C (1200°F). This approach might be preferred for handling solid propellant or high explosives. Development work would be needed to establish desirable operating conditions in this case, because these materials are reactive and potentially explosive. ⢠Further destruction in the detoxification reactor: The vapor leaving the evaporator, consisting of remaining organics, steam, and products of the steam-reforming process, passes to a detoxification reactor. In this reactor the material is heated electrically to a higher temperature, typically about 1300°C (2370°F). Reaction rates at such high temperatures are rapid and the product is believed to closely approximate thermodynamic equilibrium. A small excess of steam drives the products predominantly to CO and H2, with negligible residues of larger molecules, such as compounds with benzene rings. Chemical warfare agents should be completely destroyed at this point. ⢠Gas stream cleanup: The gas is given a final ''polishing'' by beds of activated carbon and of Selexsorb, a patented zeolite adsorption bed that adsorbs and neutralizes acid gases. These treatments require the gases to be cooled from the temperature in the detoxification reactor of 1250°C (2280°F) to about 175°C (350°F). The cooling is accomplished by a series of heat exchangers: a ceramic exchanger and a series of stainless steel exchangers, followed by addition of 100°C steam. Although not shown on the flow diagram, a heat balance indicates that external cooling will also be needed. The exchangers also permit the cooled gases at 175°C (350°F) to be reheated to 700°C (1290°F) for use in the first unit, the MBE. The actual product gas is a small slip stream removed from the treated gas before the gas is recycled at about 175°C (350°F). It consists primarily of CO, H2, and steam, with trace quantities of light hydrocarbons such as CH4. It could possibly be diluted and oxidized at a lower temperature in a catalytic oxidizer. ⢠Gas recycle: The recycle gas stream carrying heat to the evaporator is a relatively large stream. As a result of the recycling, most of the gas goes through the detoxification reactor and subsequent cleanup beds several times before being released to the catalytic oxidizer and the atmosphere. The unit operates at a slight negative pressure; any gas leak would be from ambient air into the equipment. Status and database. A series of full-size detoxifier tests was conducted under a California state grant program. The feed used was generally a mixture of materials, such as 66 percent acetone, 32 percent xylene, 1 percent
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 166 trichloroethane, and 1 percent dichlorobenzene (Galloway, 1989). High levels of destruction were demonstrated at 1150°C (2100°F), well below the maximum temperature claimed for the detoxification reactor (about 1500°C, or 2730°F; see Appendix K; flow rates can also be found in Galloway, 1989). Two commercial units have been installed and tested at customer sites. Reaction rates are high at the very high temperature used. It is assumed that the gas composition from the detoxification reactor approaches equilibrium. The effects of changing feed composition, the ratio of steam to organics materials, and temperature have been demonstrated by calculation of equilibrium compositions. These calculations also show that high temperatures and excess steam limit the formation of aromatics such as benzene to very low amounts. At moderate temperatures-those less than about 1250°C (2280°F)-CH4 is a significant equilibrium product. It represents a few percent depending on the steam content. The MBE, with its bed of uniformly sized balls containing alkaline agent, is a fairly new development. It is reported to be in operation but no details of its operability, extent of reaction, solids handling problems, and other features are known. Proprietary information has been held until suitable patent protection has been obtained. Application to chemical weapons destruction. Chemical warfare agents are expected to be as reactive as materials that have already been tested. High levels of destruction are anticipated at the higher temperatures (greater than 1500°C) that the equipment is designed to meet. There is no direct experience with organics containing F, P, or S, although there has been some test work on pesticides containing P (Galloway, 1992, personal communication). Equilibrium calculations for the high temperatures used suggest these atoms should all go primarily to acid products (HF, H2S, and P4O6), which can be adsorbed. Materials tested to date have had a much lower content of heteroatoms than do neat chemical agents. Propellants and explosives would probably be reacted first in the drum feed evaporator (a simple dram). Their behavior would need to be evaluated. Presumably they will react (oxidize) spontaneously at some high temperature. The amount of these materials fed into the unit at any one time might be critical. If the materials reached a spontaneous ignition temperature they would burn up very quickly, liberating a large quantity of heat and a very large burst of gas. Detonation would be possible if a critical amount of these materials were exceeded. Because of the minor amounts of metallic compounds they contain, a small quantity of solid residue should be expected from both propellants and explosives. Metal parts can be treated in the drum feed evaporator and should reach the 3X level of decontamination. Dunnage could possibly be handled similarly.
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 167 Special considerations. Corrosion in the system has been controlled by suitable construction materials. Most components of the feed system, such as piping, are 316-L stainless steel; ceramics are used in the highest temperature zones. The system has been run hot, above the condensation point of water and acid gas (about 180° C, or 360°F). Thus, electrolytic corrosion has been avoided. Data have been reported with feeds containing chlorine but not the other heteroatoms (in particular, fluorine) found in chemical agents. The corrosiveness of their products needs to be explored. Propellant and explosive would presumably be fed to the unit intermittently. They would need to be removed from their containers and fed in small increments. Detonation is possible if a critical size of batch feed is exceeded (very small in the case of explosive). The potential for sudden gas generation and detonation will need to be addressed in the evaporator design and may preclude the use of the equipment for energetic materials. By-products and waste streams. Product gas has passed through an extensive cleanup system, adsorbent beds, and catalytic oxidation. Organic matter should thus be largely eliminated, to a destruction level of 99.9999 percent or more. The spent solid adsorbents (carbon and Selexsorb) will have to be managed appropriately. Other waste streams will include any solid residue remaining from the high-temperature evaporation and spent caustic slurry with salts of the acid gases that have been absorbed. These will show up separately or as a mixture, depending on the front-end evaporation process. The solid waste from treating chemical agents would be relatively large primarily because of the salts formed, amounting to more than double the weight of the original agent. This waste will require further stabilization (oxidation) before disposal bemuse of the presence of phosphite or sulfide salts. The solid waste from disposing of propellant or explosive should be very small. Upsets in the operation (e.g., a power failure) are possible. Provision could be made for all waste streams to be retained until analytical work has shown that they can be released appropriately. Estimates of the stream sizes and heat requirements for GB destruction are given in Appendix K. Advantages and disadvantages. The advantage of steam reforming lies in the product distribution obtained. The reaction with steam leads to different products than reaction with oxygen. Equilibrium calculations indicate that troublesome products such as NOx, SOx, and solid particulates are not formed at the very high reforming temperatures used. Trace amounts of some stable aromatics, such as benzene, toluene, styrene, naphthalene, or dichlorobenzene, could be formed. However, the amounts formed can be controlled by the
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 168 mount of excess steam supplied and by temperature. In any case, final gas cleanup is provided. The process operates essentially at atmospheric pressure. The reactions in steam reforming are endothermic, with the required heat supplied electrically. Reactions would likely proceed with no large temperature transients as a consequence. The system is totally integrated and could be operated as an enclosed system, with no discharge of product before analysis and verification. A major structural problem resulting in a significant air leak could lead to fires. However, the control system has instrumentation and shutdown features to handle usual problems such as air leaks. Some solid residue should be anticipated in the evaporator. Chemical agents contain poorly characterized impurities, including some metallic compounds, which can have high boiling points; they will probably decompose on heating to form solid carbonaceous residue. These solids will presumably be left in the evaporator; they will be mixed with acid salts in the MBE. The residues may be considered toxic bemuse of their metal and fluoride content. The process is very energy intensive. Electric power consumption for destruction of 1 ton of agent per day is estimated to be 335 kW (with 24-hour operation). Because all of the materials will ultimately be oxidized to final stable products, the overall process is exothermic. All of the exothermic heat of reaction, as well as the energy of electric power consumption, must be removed by suitable cooling. Phosphorus and sulfur in agents are recovered as solid phosphite and sulfide salts. These are not stable materials and will require further oxidation. Development needs. The Synthetica process consists of several steps in series, with no intermediate storage. Process integration will have to be demonstrated. Some of the technology is novel or unusual: ⢠The moving bed evaporator is being developed; a prototype is operating. ⢠The detoxification reactor operates at extremely high temperatures (1200°C). The gas being heated and reacted will contain small amounts of very corrosive chemicals, such as HF or HCl, which might cause operational difficulties. ⢠The heat exchangers operate at very high temperatures and with some very large temperature differences. Industrial experience has shown thermal stress to result in cracking and leakage under these conditions. Thus, further work in several areas would be needed to use the Synthetica process for chemical demilitarization. Surrogate materials with appropriate concentrations of heteroatoms (F, P, and S) would need to be tested to validate equilibrium product calculations and to evaluate corrosion,