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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 180 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, CSCl 2, 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
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-11 A process flow sheet for the Adams process as presented by CHEMLOOP, L.P. Source: Berkey and Hendricks (1992). 181
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 182 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).
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 183 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
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 184 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.
