cited. Abatement of trichloroethylene using DBD

2014 Dielectric barrier discharge plasma was used to oxidize trichloroethylene (TCE) in 21% of O 2 in carriers of N 2 and He. The degradation products of TCE were analyzed using gas chromatography mass spectrometry. TCE was decomposed completely at optimum energy density of 260 and 300 J/l for He and N 2 , respectively and its conversion followed zero order reaction. The TCE removal efficiency is decreased in humid air due to interception of reactive intermediates by OH


Introduction
Volatile organic compounds (VOCs) are the major pollutants in air, and are harmful to human health and environment. TCE which is halogenated VOCs is widely used as solvent, cleaning agents and extractants in various industrial fields. 1 Several methods including absorption, adsorption and oxidation (thermally and catalytic) have been used for removal of VOCs. An alternative method for oxidation of VOCs is using electrons, free radicals and ions. This method includes electron beam irradiation, microwave and discharge plasma. Plasma treatment due to its reliability, energy efficiency and possibility of post-processed gas treatments, attracts more attention. Plasma in its different form such as RF, pulsed corona discharge and dielectric barrier discharge (DBD) can be used for degradation of VOCs at wide range of flow rates and concentrations. Due to its atmospheric pressure and room temperature, and also presence of dielectric barriers which prevent contamination of analyte gas with electrodes, DBD plasma has been used widely for abatement of VOCs. 2 DBD plasma can generate radicals, ions and activated molecules for decomposition of TCE. The optimum density of these reactive species depends on the plasma conditions such as power, carrier gas and configurative parameters such as dielectric material and gap distance. 3 Figure 1 shows the schematic of experimental system. N 2 and He containing 21% O 2 were selected as carrier gases of 1000 ppm TCE. The products of TCE conversion were analyzed using GC/MS. The conversion of TCE was calculated as following:

Experimental
where C 0 and C are inlet and outlet concentration of TCE, respectively. The DBD plasma reactor consists of a cylindrical quartz tube as a dielectric barrier, a stainless steel rod inner electrode and an external aluminum electrode. A CTP-2000K plasma generator was utilized to transform input AC voltage to applied voltage in the range of 0-10 kV. The power is given by the following equation: where V = voltage (kV), I = current (mA) and Ф = phase shift. Specific input energy (SIE) of the discharge was calculated by following relation: SIE (J/l) =power (W)/ gas flow rate (l/s)  Figure 2 shows the effect of input voltage (V) and frequency on applied voltage. The applied voltage depends linearly on the input voltage. For the same input voltage, He has lower slope. One possible reason is that He has lower electron collision cross-section compared to nitrogen and the electron density would be higher accordingly. 3    products, come off at t= 1.30 and t= 1.40 min. The peaks at t= 4.88, t= 5.87, t= 6.55 and t= 6.97 min are attributed to hexanal, heptanal, octanal and nonanal, respectively.  Conversion is tended to increase as the SIE is increased. The 100% conversion of TCE is achieved at 260 and 300 J/l for He and N 2 , respectively. These energies are lower as compared to the energy required during the thermal destruction. This shows that plasma reaction would be energetically feasible for TCE decomposition with low energy consumption.

Mechanism
During plasma discharge, accelerated electrons impact main constituents of carrier gas 5  He + e ି → He * + e ି Other reactive species which can be produced during discharge plasma are O, O ଶ ି , N ଶ ି . Since the electron temperature of the plasma in the discharge chamber is lower than the ionization energy of nitrogen atom (Table 1), then the dissociation species N + may not be observed. 5 Compared to other species, ozone is long-lived species and can make important role in the decomposition process of TCE. It can break double bond of TCE to form phosgene: TCE also reacts with atomic oxygen: The dissociative electron may initiate TCE decomposition as follow: C ଶ HCl ଷ + e → C ଶ HCl ଶ • + Cl ି Cl radical can react with TCE: The resulting alkyl radical reacts with O 2 to form a peroxy radical, which reacts with another peroxyradical to give an alkoxy radical: The alkoxy radical may lose a Cl atom to form the DCAC: DCAC, which was identified as a primary product, should be oxidized further to give CO and CO 2 : The TCE removal efficiency is decreased in humid air due to interception of reactive intermediates by OH radicals: Figure 6 presents outlet concentration of TCE at various residence times for two initial TCE concentrations. Concentration of TCE has linear relation with residence time and prolonging the residence time provides longer time for TCE molecules to be attacked by radicals, resulting in higher conversion of VOCs. For modeling the TCE conversion reaction, the DBD reactor may be regarded as an integral plug flow reactor and a simple power-law kinetic model can be used, when interference of reaction products and effect of mass and heat transfer are neglected:

Kinetic studies
where (-rTCE) is reaction rate of TCE conversion, k is reaction rate constant and C TCE is concentration of TCE.
Assuming n=0, conversion of TCE (x) is as follow: Since temperature of reactor is constant, k which depends only on temperature is also constant and x depends mainly on C 0 . In Fig. 6, decreasing C 0 from 1000 ppm (circle) to 700 ppm (square) leads to increase of TCE conversion. This dependency of conversion on initial TCE confirms the assumption of zero order kinetic.

Conclusion
DBD plasma system was used effectively to decompose 1000 ppm TCE in He and N 2 carriers. Close to 100% removal of TCE was achieved at SIE 260 and 300 J/l for He and N 2 carriers, respectively. CO 2 , phosgene, nonanal, octanal, heptanal and hexanal were the major products detected from TCE decomposition in DBD plasma at SIE=100 J/l.
The increase of residence times can increase removal of TCE. But generally high residence time is not favored because it could finally result in decrees of energy efficiency and removal rate.