Method and Apparatus for Photothermal Destruction of Toxic Organic Compounds (122)
There has long been interest in using photolytic reactions (reactions induced by exposure to light) to destroy hazardous organic wastes at relatively low temperatures (i.e., room temperature). Unfortunately, attempts to exploit these reactions for large-scale hazardous waste remediation have experienced only limited success. Specifically, most photochemical processes offer relatively small capacity (small throughput rates) and an inability to completely mineralize the targeted wastes. For special cases such as aqueous waste streams, these problems have been partially addressed by using indirect photochemical reactions involving a highly reactive photolytic initiator such as hydrogen peroxide or heterogeneous catalysts. Recently, the University of Dayton Research Institute has developed a photolytic detoxification process that is extremely clean and efficient and offers the speed and general applicability of a combustion process.
The photothermal detoxification unit (PDU) uses photothermal reactions conducted at temperatures higher than those used in conventional photochemical processes (200 to 500°C versus 20°C) , but lower than combustion temperatures (typically greater than 1,000°C). At these elevated temperatures, photothermal reactions are energetic enough to destroy wastes quickly and efficiently without producing complex and potentially hazardous byproducts.
The PDU is a conceptually simple device consisting of an insulated reactor vessel illuminated with high intensity ultraviolet (UV) lamps. Site remediation technologies that generate high temperature gas streams (e.g., thermal desorption, in-situ steam stripping, etc.) can incorporate the PDU with only slight modifications to their equipment. Other technologies (e.g., soil vapor extraction, air stripping, etc.) could easily incorporate a PDU fitted with a preheater to precondition the process stream. Furthermore, the PDU could be equipped with conventional air pollution control devices for removal of acids and suspended particulate from the treated process stream.
The PDU has proven extremely effective in destroying polychlorinated biphenyls, polychlorinated dibenzodioxins, polychlorinated dibenzofurans, aromatic and aliphatic ketones, aromatic and chlorinated solvents, as well as brominated and nitrous wastes found in soil, sludges, and aqueous streams. The PDU can be incorporated with most existing and proposed remediation processes for clean, efficient, on-site waste destruction operation. Specifically, high temperature processes can incorporate the PDU directly; low temperature vapor extraction technologies can use the PDU fitted with a preheater; and ground water remediation processes can use the PDU in conjunction with an air stripping operation.
The PDU technology was accepted into the U.S. EPA's Emerging Technology Program in August 1992, and development work began in December 1992. Through prior programs dating back to 1985 with the U.S. Department of Energy the effectiveness of the process has been thoroughly investigated using relatively long wavelength UV light (i.e., concentrated sunlight with wavelengths greater than 300 nm). Limited data have also been generated at shorter wavelengths (higher energy) using pulsed laser and continuous xenon arc UV illumination systems.
Data from a laboratory scale-photothermal detoxification unit (LS-PDU) has shown the PDU process performs as expected for aromatic hydrocarbons and chlorinated aromatic wastes (e.g., m-xylene, toluene, dichlorobenzene, tetrachlorodibenzodioxin, etc.), and better than expected for relatively light chlorinated solvents (e.g., trichloroethylene, tetrachloroethylene, etc.). A general PDU reactor model and a detailed prototype design will soon be available for predicting full-scale system performance and cost analysis.
U.S. Patent 5,417,825 issued May 23, 1995.
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