Removal of Mixed Volatile Organic Compounds of Benzene, Toluene, and Xylene in a Multistage Corona Discharge System under Excess Air

Abstract

In this study, the oxidative decomposition of a mixture of three volatile organic compounds (VOCs)benzene, toluene, and xylenein a nonthermal plasma corona discharge (CD) under an excess air condition was examined to determine the influence of voltage, frequency, feed flow rate, and stage number. An initial total VOC concentration of 100 ppm (v/v) with a unity weight ratio of these three VOCs in air was fixed for the first experiment using a single-stage pin-and-plate (PP)­CD system, revealing that the optimum conditions were a voltage of 8 kV, a frequency of 500 Hz, and a feed flow rate of 50 cm3/min, corresponding to a very short residence time of 0.46 s. Under these conditions, the use of two stages could maximize the removal of both the total and individual VOCs with the highest reduction in power consumption of 45%, indicating the advantage of a multistage PPCD plasma system. The oxidative decomposition pathway mechanisms of the mixed VOCs were initiated by the direct electron collision of VOC molecules to split the methyl group of xylene to yield toluene and then the methyl group of toluene to yield benzene. Next, the benzene ring was opened by electron collision to yield various low-molecular-weight (MW) hydrocarbons (HCs) The formation of radical oxygen species (ROS) also occurred in the plasma zone, resulting from the collision between generated electrons and oxygen molecules. The low-MW HCs were subsequently oxidized by ROS to yield mostly carbon dioxide and carbon monoxide with small portions of lower-MW HCs.