Revised Manuscript

Gliding Arc Plasma Processing for Decomposition of Chloroform

Antonius Indarto† , Jae-Wook Choi, Hwaung Lee, and Hyung Keun Song Korea Institute of Science & Technology, Clean Technology Research Center, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Abstract- The decomposition of chloroform diluted in air was studied. The experiment was carried out by using a gliding arc plasma. Different values of initial concentrations of chloroform, total gas flow rates, and input power frequencies have been used to investigate this effects on the conversion reaction products both qualitatively and quantitatively. Experimental results indicate that the maximum conversion of chloroform was 97% at a total gas flow rate of 180 L/h containing 1% chloroform. Using air as carrier gas, decomposition of chloroform produces CCl4, CO2, CO, and Cl2 as the main products. Small amounts of HCl and COCl2 are also detected. Liquid products were also produced. Keywords: Plasma, gliding arc plasma, chloroform, decomposition

†

Corresponding author: E-mail:[email protected] 1

1. Introduction

Chloroform (CHCl3) is classified as toxic and carcinogenic and it is an important task to remove it from indoor or outdoor ambient atmosphere [1,2]. Chloroform is used as solvent in production of various chemicals, and existed in some pesticide formulations. Chloroform is suspected to have Global Warming Potential (GWP) and to contribute to the destruction of the ozone layer in the stratosphere [3]. Two approaches are generally used to remove this pollutant from air, i.e. by thermal or catalytic oxidation. Several procedures of thermal oxidation have been developed and established [4,5]. This process requires high burning-temperature (between 760 - 980oC) and relatively long residence time to complete the oxidation reaction [4]. To avoid these problems, many researchers have proposed the use of catalysts. Although this could reduce the duration of residence time, the removal efficiency was found to be lower than by thermal oxidation. At low temperature, the catalyst’s lifetime is short [6] and different catalysts when the target material consists different VOC species. To overcome these problems, gliding arc discharge has a bright prospect, especially for industrial scale application. Gliding arc discharge is the subject of renewed interest for applications to various chemical reactions [7]. Therefore, the main recent improvement for utilizing gliding arc discharge are more powerful electrical discharges under thermal non-equilibrium conditions [8], short resident time, and concentration capacity. Kinetic destruction of chloroform using photosensitized oxidation [9], thermal oxidation [10], or photocatalysis [11,12] have been studied previously but, unfortunately, the reaction pathway under gliding arc discharge condition was not studied in detail. In the

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present work, the reaction mechanism of decomposition of chloroform diluted in compressed air was studied by gliding arc discharge. The main products CO, CO2, Cl2, and CCl4 were the object of analysis.

2. Experimental setup

Figure 1

A diagram of experimental setup is shown in Fig. 1. Chloroform and compressed air are used as input gas. The setup is described in detail in the following section.

2.1. Plasma reactor and applied power system

The reactor was made from a quartz-glass tube of an inner diameter of 45 mm and a length of 300 mm. Top and bottom of the reactor are equipped with teflon seals the lower comprising two electrodes made of stainless steel, length of the electrodes being 100 mm. The distance of the electrodes in the narrowest section is 1.5 mm. The gas mixture is fed between the electrodes with a capillary of 0.8 mm inner diameter. A thermocouple, located 10 cm above the electrode, is provided to measure the outlet gas temperature. A high frequency AC power supply (Auto electric, A1831, Korea) is connected to the gliding arc electrode to generate the plasma. In this experiment, the total power can not be adjusted; it is a function of the breakdown voltage and changes into stable-equilibrium automatically when the arc is formed. The supplied power is

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calculated for equilibrium condition when the plasma has been operated for 30 minutes. Figure 2 shows the typical waveforms of voltage and discharge current used in these experiments.

Figure 2

2.2. Input gas

Chloroform (CHCl3) (Junsei Chemical Co., Japan) purity of ~99.0%, is varied in its concentration to 1, 3, 5, 8% volume of total gas flow. Air used as carrier gas and is controlled by a Mass Flow Controller (Tylan, FC-280S) at flow rates of 180, 240, and 300 L/h. Air entering the reactor first passes through a scrubber and is mixed with chloroform. Chloroform is introduced by passing a portion of air through a bubble flash placed in a water bath. The concentration of chloroform is controlled both by the temperature of the water bath and the gas flow rate through the flask. The input lines are heated by means of heating tape to avoid the condensation. The composition of the mixture is analyzed at the outlet reactor with the plasma operation on and off.

2.3. System of Measurements

The outlet gas composition is examined by an electrone-impact ionization quadrupole mass spectrometer (Balzers, QMS 200) with software Quadstar 421, for the qualitative and quantitative analysis of reactants and products. Two GC’s are been: chlorinated 4

methane compounds in the product gas mixture is qualitatively determined by GC-FID (YoungLin, M600D, Korea, Column: 5 m x 2.5 cm i.d. Bentone), for CO and CO2 by GC-TCD (YoungLin, M600D, Korea, Column: 5 m x 2.5 cm i.d. SK Carbon). Chlorine gas (Cl2) is determined by bubbling the reacting gas through 0.05 M aqueous KI for a given experiment time, followed by iodometric titration with 0.05 M Na2SO3 [13]. The evaluation of system as to its performance, selectivity and conversion, is formulated as: Selectivity of (CO + CO2 ) =

Selectivity of Cl 2 =

moles of (CO + CO2 ) produced × 100% moles of chloroform converted

2 × moles of Cl 2 produced × 100% 3 × moles of chloroform converted

Conversion of chloroform(η ) =

moles of chloroform converted × 100% moles of initial chloroform

(1)

(2)

(3)

These parameters are used to study the effect of initial chloroform concentration, total gas flow rate, and input power frequency. The supplied power is shown as function of voltage and current determinated by oscilloscope (Agilent, 54641A, USA) and calculated to power term by following equation: Supplied power = ∫ (V (t ) × I (t ) ) dt × frequency (Watt )

(4)

In this study the experimental data were taken 30 minutes after the initiation of the gliding arc plasma referred to the stable temperature of the outlet bulk gas measured by thermocouple.

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3. Results and Discussion

With the gliding arc plasma, the destruction of chloroform (CHCl3) produces CO2, CO, Cl2, and CCl4, as the main products. The yields of CO and Cl2 are higher than that of CO2 and CCl4. The concentration of CO2 is 3-5 times lower than the concentration of CO. CCl4 is formed at yields between 8 to 15% by mol basis. HCl and COCl2, are also detected by mass spectrometer (QMS), but in low concentration. The destruction performance was studied as functions of initial chloroform concentration, total gas flow rate, and input frequency. The gaseous product analysis is focused on CO, CO2, and Cl2 compounds because these are the main products and can be measured precisely.

3.1. Effect of initial concentration.

Figure 3

Figure 3a shows the effect of various initial concentrations of chloroform on its conversion at an input power frequency of 20 kHz. The maximum conversion reached 97% at the lowest concentration of chloroform, 1%, and a total gas flow rate of 180 L/h. Conversion of chloroform gradually decreases when the initial concentration increases. The decrement rate reaches 2.5% per 1% of increase of initial chloroform concentration in the inlet stream. One factor that contributes to the decrease is the power supplied to the reactor which decreases when the initial concentration of chloroform increases (Fig. 3b). Thus, less energy is supplied for initiation of gas molecules. The supplied power is highly dependent on the breakdown process producing the arc by 6

breaking bonds or exciting molecules [8]. Chloroform has less strong chemical bonds compared to components such as O2 and N2 that are mostly existing in the inlet stream. It means that the required energy to initiate the first arc plasma is less also. Higher concentrations of chloroform decrease the required breakdown energy which decreases the supplied power automatically. Therefore, as mentioned above, the stabilized equilibrium power is automatically adjusted by the system. Different values of initial concentration of chloroform have significant effects on the energy efficiency of the system. The energy consumed per converted chloroform is rising from 18.6 to 114 Watt⋅h/L when the concentration of chloroform is decreasing from 8 % to 1 %. At a higher initial concentration of chloroform the collisions are more effective although the percentage of conversion is lower. Small increments of gas temperature due to increasing initial chloroform concentration are measured, reaching 250oC and rising 2-3oC per 1 % increment of initial chloroform. It reflects the energy released from the exothermic plasma reaction and calculated following formula 5 and 6.

[(

Q = m ⋅ C p

) un−reacted species + ∑ ( m ⋅ C p ) products ] × (Tg ,t − Tg ,0 )

∆H reaction = Q − W power supply

(5)

(6)

When the plasma reaction occurs, a portion of chemical energy is converted to thermal energy and rises the temperature. Compared to the supplied electrical energy, exothermic plasma reaction constitutes 70-80% of the energy increasing the product gas temperature (calculated using formula 5 and 6). Higher initial concentrations of chloroform produce more heat. By calculating the converted chloroform, the ratio leads

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to 97.03 (1%×180 L/h ×97.03%) and 274.44 (3%×180 L/h ×91.48%) for the chloroform concentration at 1% and 3% and total gas flow rate 180 L/h. This means that the molecule collision probability at higher concentration of chloroform, i.e. 3%, is approximately 3 times higher than that at lower concentration, i.e. 1%. The yields of gas products, (CO+CO2) and Cl2, is shown in Fig. 3c and d. No significant differences in yields of (CO+CO2) are observed at different initial concentrations of chloroform. The maximum yield of (CO+CO2) is around 50%. It shows that almost half fraction of reaction produced CO or CO2. Conversion into CCl4 was detected not exceed than 15% in all experimental conditions. The remaining carbon possibly transforms into solid (shoot) or liquid products which are also produced during experiment. In the case of Cl2, the yields increase slightly with increasing chloroform concentration. Oxygen and nitrogen in the input stream should be considered as factors for determining the product selectivity. Oxygen and nitrogen can be transformed into high-energy state species or radicals by collision. Calculated from mass spectra data, the difference of oxygen concentration before and after the plasma reaction approaches 10% and for nitrogen 5%, independent of the initial chloroform concentration. The role of radical species, such as oxygen and oxygen radical to maintain the decomposition process, has been studied before [14, 15].

3.2. Effect of total gas flow rate

Figure 4

The effect of total gas flow rate, determining the residence time of chloroform in the 8

reactor, was examined. Figure 4a shows the chloroform conversion according to the various total gas flow rates at initial chloroform concentrations of 1, 3, 5 % and power supply frequency of 20 kHz. The conversion of chloroform decreases with the increase of total flow rate. Raising the total gas flow rate reduces the residence time of chloroform in the reactor and the chance and time of a molecule to collide with electrons and other high-energy state species which have enough energy to destroy carbon-hydrogen or carbon-chloride bonds. Lower conversion is also caused by decreasing power as gas flow rate increased (Fig. 4b). This phenomenon has been observed previously [16]. In terms of energy efficiency the effect of total gas flow rate is rather smaller. The consumed energy per converted chloroform decreases from 149 to 122 Watt⋅h/L when the flow rate increases from 180 to 300 L/h. With increasing of total gas flow rate (Fig. 4b), the (CO+CO2) yield increases, Fig. 4c, while the yield of Cl2 tends to decrease.

3.3. Effect of applied frequency

Figure 5

Decomposition of chloroform increases slightly from 92 % to 97 % when the frequency of input power is increased from 15 to 20 kHz at 180 L/h (Fig. 5a). The same trend is observed at 300 L/h with yields between 42% to 60%. Radu et al. mention that a change

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in frequency will change the basic breakdown mechanism [17]. Higher frequency will increase the extent of sudden-fluctuating pulsed current and voltage peak per cycle. Furthermore, it will increase the total power supplied to the plasma (fig. 5b) which provides more energy for the gas ionization process. However, the ratio of power consumed versus converted chloroform is increased from 92 Watt⋅h/L at 15 kHz to 142 Watt⋅h/L at 20 kHz. It means that at lower frequency, although the conversion of chloroform is lower, the energy efficiency is better that at higher frequency. Increasing frequency decreases the yields of (CO+CO2) (Fig. 5c). However, the yield of Cl2 increases from 18% to 25% at 180 L/h and achieves maximum of 38% at a total gas flow rate 300 L/h (Fig. 5d).

3.4. Mass Spectra

Figure 6

A mass spectrum of the chloroform decomposition mixture is shown in Fig. 6. The main fragment of chloroform is m/z 82/84/86 ( CCl ) but overlaps with N2O ( CO2+

CO +

+ 2

). CO2 has its molecular ion at m/z 44 (

) which may form as gas product. CO is at 28 (

N 2O + ) but overlaps minor N2 spectra 29 (

). Some additional peaks are m/z 63/65, for N 2+

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+

+

+

COCl2 ( COCl ), m/z 70/72 for Cl2 ( Cl 2 ), m/z 117 for CCl4 ( CCl 3 ).

4. Conclusions

The performance of chloroform conversion in gliding arc plasma at atmospheric pressure, in respect to initial concentration of chloroform, total gas flow rate, and input power frequency, was studied. Gliding plasma generates enough active species to fragment chloroform molecules. The maximum conversion of 97% is achieved for a feed gas stream containing 1% of chloroform and a total air flow rate 180 L/h. Four gaseous compounds, CO, CO2, CCl4 and Cl2, are the major products. Cl2 yields are relatively high and reached maximum of 50%. Yields of CO and CO2 reaches up to 78% mol at 15 kHz of frequency and total gas flow rate at 180 L/h. Conversion into CCl4 was detected in case of chloroform decomposition, but not exceeds than 15%. COCl2 has an important role as intermediate species to produce Cl2 and CO. Development is still needed to remove some traces of unwanted compounds in the final product, such as: COCl2 and CCl4 which are also classified as toxic gases.

Acknowledgements

This work was supported by the National Research Laboratory of the Korea Minister

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Ministry of Science and Technology of Korea.

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[5]

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vent to atmosphere FID GC Bentone

MFC- 1

Compressed air

TCD GC SK Carbon

Quadrapole Mass Spectrometer

Scrubber MFC- 2 Plama zone Capilary tube

CHCl3

Electrode

waterbath KI solution

Plasma Reactor

Figure 1. Schematic diagram of experimental set up

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6000

4000

Voltage (V)

2000

0

-2000

-4000

-6000 0.4

Current (A)

0.2

0.0

-0.2

-0.4

-0.6 -2e-5

-1e-5

0

Time (s)

Figure 2. Voltage and current profile

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1e-5

2e-5

CHCl3 conversion

CHCl3 conversion (%)

100

80

60

40

180 L/h 240 L/h 300 L/h

20 300

Supplied Power

Power (Watt)

280 260 240 220 200 180 L/h 240 L/h 300 L/h

180 160

Molar yields (%)

60

(CO + CO2)

50 40 30 180 L/h 240 L/h 300 L/h

20 10

Molar yields (%)

60

Cl2

50 40 30 180 L/h 240 L/h 300 L/h

20 10 0

2

4

6

initial CHCl3 concentration (%)

8

10

Figure 3. Effect of initial chloroform concentration on (a) chloroform conversion, (b) supplied power, (c) yields of (CO + CO2) , and (d) of Cl2.

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CHCl3 conversion

CHCl3 conversion (%)

100

80

60

1% 3% 5%

40

Supplied Power

Power (Watt)

300

250

200

1% 3% 5%

150

Molar yields (%)

60

(CO + CO2)

50 40 30 20

1% 3% 5%

10

Molar yields (%)

60

Cl2

50 40 30 20

1% 3% 5%

10 160

180

200

220

240

260

280

300

Total gas flow rate (L/h)

Figure 4. Effect of total gas flow rate on (a) chloroform conversion, (b) supplied power, (c) yields of (CO + CO2), and (d) of Cl2.

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320

CHCl3 conversion

CHCl3 conversion (%)

100

80

60

40 180 L/h 300 L/h

20

Supplied Power

Power (Watt)

300

250

200

150 180 L/h 300 L/h

100

(CO + CO2)

Molar yields (%)

80

60

40

20

180 L/h 300 L/h

Molar yields (%)

60

Cl2

50 40 30 20 180 L/h 300 L/h

10 14

15

16

17

18

19

20

21

Frequency (kHz)

Figure 5. Effect of applied power frequency on (a) chloroform conversion, (b) supplied power, (c) yields of (CO + CO2), and (d) of Cl2.

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1.2e-9

1.0e-9

Intensity

8.0e-10

6.0e-10

4.0e-10

COCl2

2.0e-10

Cl2 0

20

40

60

CHCl3 CCl4 80

100

120

140

160

180

200

AMU (m/z)

Figure 6. Mass spectrum of chloroform decomposition (taken at applied frequency 20 kHz, 1% of chloroform, total gas flow rate 240 L/h).

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