OCR Text |
Show a small number (as few as one in the Solarchem system) of high energy lamps, or large numbers of low energy lamps (up to 72 in the Ultrox unit), to photolyze either hydrogen peroxide or ozone to generate hydroxyl radicals (OH) in the bulk fluid. These radicals, which are potent oxidizers, then react with the organic species present in the waste stream. These systems have proven somewhat successful in addressing contaminated groundwater sites, though they have encountered some difficulty treating water containing perhalogenated organics (e.g. carbon tetrachloride, tetrachloroethylene). This is due to the resistance of carbon-halogen bonds to O H attack at low temperature, in contrast to carbon-hydrogen bonds which are easily broken by hydrogen abstraction by O H. All of the photochemical detoxification technologies that have been developed to date suffer from the same weakness. At low temperature the photo-induced reactions just aren't energetic enough to build into the complex chain reactions that can result in the efficient destruction of the waste molecules and the products that result from the initial reactions. This problem is even greater in condensed phases (such as liquid water) where relatively slow diffusion rates (large Schmidt numbers) limits the interaction of the waste molecules and short-lived reactive radicals. One solution to this problem is to conduct the reactions at high temperatures and in the gas phase where the thermal energy is available to accelerate the reactions to acceptable rates and the diffusivities are high enough (small Schmidt numbers) to allow the reacting species to undergo a large number of collisions. The first reactor designed to destroy hazardous organic wastes using a high temperature photochemical process was developed by Babcock and Wilcox along with Veda Corporation in 1984.[11] Sponsored by the U.S. Department of Energy, a small bench-scale reactor was designed to incinerate P C B s using concentrated solar energy. Unfortunately, while the description of the reactor called for the sunlight to provide both heat and U V radiation, the actual design considerations relied almost entirely on the heat of combustion of the P C B waste, and even had provisions for removing the heat derived from the sunlight. Consequently, the role of the photochemical aspects of the reactor were never quantitatively described or demonstrated. The following year, we proposed a small research program to investigate the use of concentrated sunlight to destroy hazardous organic wastes.[12] With a strong background in conventional incineration research, the approach employed was to treat the system as a synergistic process combining aspects from both photochemical and thennal processes. Space does not permit a detailed discussion of the numerous results which were obtained from the research program that followed. Rather, the issues which are of immediate importance in successfully designing the P D U will be presented as part of the discussion of the overall laboratory results presented below. Theory of Photothermal Detoxification A useful way to visualize the fundamental principals of the photothennal process is through 3 III-19 |