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 176 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: .
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-10 Block flow diagram of the UOP HyChlor conversion process. Source: Hedden (1992). 177
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 178 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.
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 179 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.
