Experimental and Numerical Characterization of a Falling Film Plasma Reactor for the Degradation of Organic Pollutants in Water

Abstract

An experimental and numerical characterization of a falling film plasma reactor for the degradation of aqueous organic pollutants in batch operation mode is reported. A pulsed corona discharge in humid air is excited by short voltage pulses (<100 ns) generated by a capacitive-storage power source and a high-pressure gas spark-gap. Indigo carmine is chosen as the reference pollutant. A volume of 20 L of indigo carmine solution with an initial concentration of 20 mg/L are completely decoloured after 11 min treatment for a reactor mean power of 33 W. The electrical-energy efficiency per order and the energy yield of the process are calculated to be 0.25 kWh/m3 and 101 g/kWh, respectively. The generation of reactive species is also assessed in both the liquid and gas phases. Very low concentrations of NO2– and NO3– ions are found, practically not causing water acidification. The main gaseous species produced by the corona discharge are O3 and HO2· radicals. In addition, a kinetic model of the reactor is presented and compared with measured data. The numerical results indicate that reactions in the stagnant liquid film next to the gas-liquid interface are essential to explain the measured removal rates. The rapid kinetic regime of the liquid film strongly accelerates the uptake rates of HO2· (rapidly converted to O2–·) and O3, which far exceed the uptake rates predicted by the mass transfer coefficient for a reactionless film.