Practical Supercritical Water Reactor for Destruction of High Concentration Polychlorinated Biphenyls (PCB) and Dioxin Waste Streams

Treatment technology is urgently needed for safe disposal of polychlorinated biphenyls (PCBs) that have been stored at a number of sites. In this work, we report on development of a new practical reactor for handling high concentration PCB streams that can include dioxins. In selecting materials of construction, we performed corrosion tests with 20 wt% HCl solutions at temperatures from 350 to 600°C and at pressures from 16.5 to 25 MPa. As a result of these tests, we found that titanium-palladium alloy to be suitable for temperatures above 380°C and tantalum metal to be suitable for temperatures below 450°C. A vessel-type reaction cartridge was tested and the trichlorobiphenyl (3-CB) could be decomposed to a concentration of less than 3 μg L −1 in the effluent (DRE. 99.9998%), which satisfied emission standards. However, dioxin was detected in the effluent to have a concentration of 17 000 pg-TEQ L −1 (DRE > 99.979%). Therefore, internals of the reaction cartridge were designed to have a vessel + plug flow reactor configuration. A reaction kinetics study on the oxidation of dioxin in supercritical water allowed for estimation of appropriate residence times for insuring dioxin destruction. The modified reaction cartridge prevented fluid bypassing and provided a minimum guaranteed residence time that was longer than 110 s. Treatment of PCB using the modified cartridge was found to satisfy the emission standards of both PCBs and dioxins and gave DRE of 99.99999% for 3-CB with a dioxin in the effluent of less than 10 pg-TEQ L –1 and less than 50 pg-TEQ m –3 for liquid and gas effluent, respectively, which satisfy current Japanese dioxin emission standards.