CHF3 Decomposition by Dielectric Barrier Discharge Reactor

Decomposition of CHF₃ by a Dielectric Barrier Discharge Reactor

Duc Ba Nguyen and Won Gyu Lee*

Abstract

Oxidation of CHF₃ was investigated in a dielectric barrier discharge reactor was immersed in an electrical insulating oil. The feed gases was mixed of CHF₃, O₂ and N₂ with O₂/N₂ ratio of 21/79 volume/volume. The results obtained that 98.98% of CHF₃ in the feed gases were destruction under: applied voltage of 7kV, frequency of 30 kHz; total flow rate of 100 ml/min with initial CHF₃ concentration of 5%. Herein, selectivity of CO and CO₂ in the products was 3.42% and 91.18%, respectively. Decomposition of CHF₃ could be increased by improved plasma condition such as increasing applied voltage, increased frequency and decreased initial CHF₃ concentration in the feed gases.

Keywords: CHF₃, dielectric barrier discharge, electrical insulating oil, plasma reaction, oxidation of CHF₃

Introduction

Decomposition of trifluoromethane (CHF₃) is high potential reduce greenhouse gases. Because the 100 years global warming potential of CHF₃ is 12000 [1]. Several methods have been employed for the decomposition of CHF₃ such as thermal process [2-4], catalyst [5, 6]; plasma or combined plasma with catalyst (CPC) [7-11].

Thermal oxidation is one of effective CHF₃ decomposition [12]. However, HF acid and formation fluorinated compounds existed in the exhaust gas stream along with high operation temperature (1473 K) [2]. It mean that the process is high cost, require material reactor and concern environmental. Thus, other process are required for treatment of exhaust gas such as absorbed acids, cooling process and decomposition of fluorinated compounds before ambient atmospheric emission [2, 11]. Catalyst methods could be reduced operation temperatures in the abatement of CHF₃. However, HF formation and also operation temperature above 500 ⁰C lead to reducing effective of catalyst [13-15]. Several above challenges could be solved by plasma or CPC, including, non-thermal plasma (NTP) is attractive and effective decomposition of CHF₃ [16, 17]. Decomposition of CHF₃ in NTP is lead to interaction between of high energy electrons, radicals and gas molecules. Herein, NTP could be generated high energy electrons and radicals under high energy electrical. Therefore, decomposition of CHF₃ could be performed at room temperature, ambient atmospheric pressure, fast conversion and easy realization by plasma method. However, several researchers have been reported the decomposition of CHF₃ by catalyst or CPC with several thousand parts per million of CHF₃ in the feed gases [18, 19]. It demonstrated that process yields were low. Moreover, the emission source of CHF₃ is semiconductor industries, air condition, polystyrene industries and commercial refrigeration. So that the gas waste included CHF₃ and air. Therefore, abatement of CHF₃ in the gas waste is need before into atmosphere.

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In this study, decomposition of CHF₃ with Zero Air (21% O₂ and 79% N₂) performed in a coaxial dielectric barrier discharge reactor under initial CHF₃ concentration was not less than 5.0% (v/v). The reactor was immersed in an electrically insulating oil bath. Effect of several factors on the reaction investigated, namely, applied voltage, applied frequency, initial CHF₃ concentration in the feed gas. These factors were examined on the decomposition of CHF₃ and product components.

Experimental

The schematic of the experiment setup is shown in Fig. 1. A system is composed of four main parts: a feed gas system, an AC high voltage pulse power supply, a plasma reactor, and an analysis system. The reactor has an inner stainless steel stick as the power electrode that is 15 mm in diameter. The power electrode was placed inside a quartz tube as a dielectric barrier. Its outer diameter was 20 mm, and its thickness was 1.5 mm. Therefore, the discharge gap was fixed at 1.0 mm. Copper foil was wrapped around the quartz tube as the ground electrode, and its length was 200 mm. Thus, the discharge volume was about 10 ml. The plasma reactor was immersed in an electrically insulating oil bath (transformed oil provided by Michang Oil, KSC2301). The volume of electrical insulating oil bath was about 5000 ml.

AC pulse power supply with 2 kW capacity was used for plasma ignition, which had a supply voltage and a frequency up to 30 kV (peak-to-peak) and 30 kHz, respectively. The electrical power was controlled by manual adjustment of the applied voltage level. The power waveforms were recorded by an oscilloscope (Tektronix 2012B). Fig. 2 showed a typical voltage, current, and discharge power waveforms generated under the process condition: total flow rate of 100 ml/min with CHF₃ in the feed of 5% (v/v), frequency of 30 kHz; applied voltage of 7 kV. Discharge power was integral of current and voltage as shown in the equation below:

Discharge power (P), (1)

All of the experiments were performed at ambient atmospheric pressure and room temperature. The volume of gas products was measured by a soap-bubble flow meter. The composition of the gas products was analyzed by a gas chromatograph (GC, Younglin YL6100GC) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). A Carboxen^TM 1010 PLOT capillary column was used in the GC column and the flow rate of Ar as a carrier was 6.0 ml/min. The products of plasma reaction with mixing of CHF₃, O₂ and N₂ included N₂O, NO₂, COF₂, F₂, CF₄, CO, CO₂, CHF₃, O₂, N₂ and so on [19]. However, the GC analysis could detect the reaction products including CO, CO₂, and CHF₃. According to the analysis of the products, the overall conversion, carbon balance and selectivity were defined as follows:

(2)

(3)

(4)

(5)

Results and discussion
1. Effect of applied voltage

An applied voltage is important factor in the plasma process. Which is usually used to ignite and sustain glow discharge. Moreover, the degree of plasma reaction depend on the level of applied voltage, for example conversion of reactants and selectivity of products [19, 20]. The effect of applied voltage on the reaction was investigated under applied voltage from 4 to 7 kV, total flow rate of 100 ml/min with CHF₃ concentration of 5% in the feed gases; frequency of 30 kHz. The results was shown as in fig 3. At applied voltage of 4 kV, the conversion of CHF₃ obtained at 0%, however, the conversion of CHF₃ was increased sharply from 5 to 7 kV applied voltage in fig 3 (a). The results demonstrated that energy input at applied voltage of 4 kV into discharge zone was not enough for dissociation of gases molecules. It due to lack of electron and radical formation for plasma reaction. However, electron and radicals for plasma reaction could be formed when applied voltage above of 4 kV. In addition, bond-dissociation energy of F-CHF₂ and H-CF₃ were 539.9 and 445.2 kJ/mol^-1 at 298 K, respectively [21]. Discharge power increased sharply from 8 to 41 W, when applied voltage increased from 5 to 7 kV. It was caused of increasing CHF₃ conversion in these experiments. Consequently, concentration of CHF₃ in the gas outlet was 0.054% at applied voltage of 7 kV as shown in fig 3(b).

An applied voltage was also effect on the component of gas outlet. The concentration of CO₂ were increased significantly by increasing applied voltage from 5 to 7 kV, while, the concentration of CO were changed slightly during those experiments as shown in fig 3(b). In fact that, more radical and molecules in the discharge zone could be formed under high discharge power such as F, H, CF₃, CF₂, COF₂, COF, CO, CO₂, F₂ and so on [19]. Therefore, conversion of reactants and products formation increased. Moreover, the selectivity of CO₂ increased from 40% to 89% when applied voltage from 5 to 6 kV. Consequently, it increased slightly at applied voltage of 7 kV as shown in fig 3 (c). On the contract, the selectivity of CO decreased slightly from 5 to 6 kV and then it decreased gradually at applied voltage of 7 kV. The results due to increasing radical oxygen formation in the discharge zone when applied voltage increased from 5 to 7 kV. Carbon balance decreased slightly, when applied voltage increased from 6 to 7 kV. It mean that total selectivity of CO and CO₂ decreased. In fig 3 (c) shown that decreased selectivity of CO caused of reducing carbon balance. As the results, the maximum conversion of CHF₃ obtained at 98.98% under applied voltage of 7 kV, frequency of 30 kHz, total flow rate of 100 ml/min and CHF₃ concentration in the feed gases of 5%. Herein, the selectivity of CO₂ and CO 91.18% and 3.42% in the product, respectively.

Effect of initial CHF₃ concentration

Conversion of reactants could be improved by reducing initial amount of reactants in the feed. However, it caused of decreasing yield processing. Effect of initial CHF₃ concentration on the reactions were investigated under applied voltage of 7 kV, frequency of 30 kHz and total flow rate of 100 ml/min. The results was shown in fig 4. The conversion of CHF₃ decreased slightly from 98.98% to 95.94% when initial CHF₃ concentration in the feed increased from 5% to 15%. It was as shown in fig 4 (a). The results demonstrated that conversion rate of CHF₃ depended slightly on the range of initial CHF₃ concentration. It was due to increased amount of CHF₃ molecules in the discharge zone together with decreasing power discharge when initial CHF₃ concentration increased as shown in fig 4 (a). Because of total flow rate constant, if CHF₃ molecules increased then Nitrogen and oxygen molecules decreased. Moreover, bond-dissociation energy of O-O was 498.36 kJ/mol^-1 at 298K. It is low than bond-dissociation energy of F-CHF₂ (539.9 kJ/mol^-1) but higher than that of H-CF₃ (445.2 kJ/mol^-1) at 298 K [21]. At initial CHF₃ concentration of 15%, the ratio of CHF₃/O₂ in the feed was 1/1.19. Several reason above due to conversion of CHF₃ depended slightly in the range of initial CHF₃.

Initial concentration of CHF₃ was effective on the concentration of CO₂ in the products. However, it did not significantly on the concentration of CO and CHF₃ in the products as shown in fig 4 (b). In the detail, concentration of CO₂ increased from 4.79% to 14.20% when initial CHF₃ concentration increased from 5% to 15%. On the other hand, concentration of CO were increased from 0.18% to 0.38%, while, increasing CHF₃ concentration remain from 0.05% to 0.69%, respectively. In fig 4 (c) presented that the selectivity of CO and CO₂ were decreased slightly by increasing initial CHF₃ concentration in the feed gases. They caused of decreasing carbon balance during increasing initial CHF₃ concentration in the feed. As the results, reactant conversion and products selectivity were depending slightly on the initial CHF₃ concentration from 5% to 15% in these experiments.

Effect of frequency

Frequency of applied power is important factor along with voltage. Because they effected on the discharge power (equation 1) and applied power waveform. The effect of frequency on the reaction were investigated under condition of 7 kV applied voltage and 100 ml/min total flow rate with 5% CHF₃ concentration in the feed. The results was shown in fig 5. It showed that the conversion of CHF₃ increased significantly from 10 to 20 kHz; then it increased slightly at frequency of 30 kHz. While, discharge power increased gradually when frequency increased from 10 to 30 kHz as shown in fig 5(a). One of reason increased CHF₃ conversion was increased discharge power when applied frequency increased from 10 to 30 kHz.

Applied frequency also effect on the component of products. It was shown in fig 5 (b). Concentration of CO₂ increased significantly from 2.89% to 4.79%, while, concentration of CO decreased from 0.36% to 0.18% when applied frequency increased from 10 to 30 kHz. Although, decreased CO concentration rate is twice when applied frequency from 10 to 30 kHz but it was small compared with concentration of CO₂ in the products. As the same trend of concentration in products, the selectivity of CO₂ increased, while, the selectivity of CO decreased when increased applied frequency as shown in fig 5 (c). The results presented that trend of CO₂ and CO selectivity were opposed. It was caused of carbon balance did not change significantly from 10 to 20 kHz. However, carbon balance were increased when applied frequency increased from 20 to 30 kHz. It could be explained by that the selectivity of CO₂ increased was more than reducing of CO selectivity at frequency of 30 kHz. Therefore, total molecules of CO and CO₂ were increased when increased applied frequency from 20 to 30 kHz. In addition, carbon balance depending on total molecules of CO and CO₂ (Equation 3). Consequently, increasing of applied frequency was not only increasing CHF₃ conversion and CO₂ selectivity but also reduced the selectivity of CO.

Conclusion

Destruction of CHF₃ with zero air by a coaxial dielectric barrier discharge immersed in the electrically insulating oil bath was investigated. Several factors were effect on the reaction has been studied such as applied voltage, frequency and initial reactant concentration. The conversion of CHF₃ was improved by increasing applied voltage and frequency; decreasing initial concentration of CHF₃ in the feed gases. One of reason was the factors effect on the discharge power in the plasma reaction. The results demonstrated that the reactor was potential for CHF₃ decomposing with initial reactant concentration was from 5% to 15% in the feed gases. More 95% of CHF₃ in the feed could be destructed to CO, CO₂ and so on, herein, total selectivity of CO and CO₂ was more than 85% in the products.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (2010-0007450).

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List of figure

Fig.1.Schematic diagram of the experimental setup

Fig. 2. Typical signal of the voltage, current, and discharge power (total flow rate = 100 ml/min; CHF₃ in feed= 5% of volume; applied voltage= 7 kV; frequency=30 kHz).

Fig. 3. Effect of applied voltage on (a) conversion of CHF₃ and discharge power, (b) component of products; and (c) carbon balance and selectivity of products (total flow rate = 100 ml/min; CHF₃ in feed= 5% of volume; frequency=30 kHz).

Fig. 4. Effect of initial concentration of CHF₃ on (a) conversion of CHF₃ and discharge power; (b) component of products; and (c) carbon balance and selectivity of products (total flow rate = 100 ml/min; applied voltage = 7 kV; frequency=30 kHz).

Fig. 5. Effect of frequency on (a) conversion of CHF₃ and discharge power; (b) component of products; and (c) carbon balance and selectivity of products (total flow rate = 100 ml/min; CHF₃ in feed= 5% of volume; applied voltage = 7 kV).