Pulsed Barrier Discharge in Gas-Liquid Two-Phase Flow for Water Treatment

Summary form only given. Pulsed barrier discharge is generated in a gas-liquid two-phase flow for decomposing persistent substances in water. This new method is proposed to extend the interfacial area between the nonthermal plasma and the treat water for efficient decomposition of persistent materials. Acetic acid solution as a persistent substance is filled in a plasma reactor that consists of two coaxial glass tubes of about 3 cm in diameter. A mesh and a sheet electrode are attached on the outer and the inner surface of the glass tubes, respectively. A bubbler placed in the bottom of the reactor generates many bubbles by feeding inert gases or oxygen. A gas-liquid two-phase flow is formed during the upward drift of bubbles in the gap of 1 ~ 2 mm between the glass tubes. The bubble diameter expands from a few mm to a few cm where the coaxial electrodes are mounted. Fast rising pulsed voltages of 6 kV peak are applied between the electrodes at the frequency of 10 kHz. The barrier discharge is generated inside of the expanded bubble and reacts with the water. The peak value of the current including both discharge current and displacement current is 18 ~ 22 A. A decomposition rate of acetic acid is evaluated by measuring the reduction amount of total organic carbon (TOC) in the solution and energy efficiency is obtained by dividing the TOC reduction-rate by the energy dissipated in the discharge. The decomposition rate strongly depends on both the void fraction in the two-phase flow and the gaseous species such as He, Ne, Ar or oxygen. Highest efficiency is obtained using Ne in that a stable barrier discharge is generated by lower driving voltage than that in Ar or oxygen discharge. The TOC concentration decreased from 10 ppm to 4 ppm within 20 minutes at the void fraction of 70 % with gas flow rate of 800 seem. High-speed photographs taken by an ICCD camera in the exposure time of 5-10 nsec show that the discharge area and the interfacial area change by the gaseous - species. In Ne gas, the interfacial area seems to be maximized. The emission spectra from OH radicals are not observed in He, Ne, and oxygen discharge but detected in Ar. This result suggests that OH radicals which react with the acetic acid are in the ground state. The decomposition efficiency decreases by increasing the discharge power density but higher decomposition rate is obtained at higher discharge power.