Manuscript

Decomposition of CCl4 and CHCl3 on Gliding Arc Plasma 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- Decomposition of chlorinated hydrocarbons, CCl4 and CHCl3, in gliding plasma was examined. The effects of initial concentrations, total gas flow rates, and power consumption have been investigated. The conversion was relatively high. It could reach 80% for CCl4 and 97% for CHCl3. Under atmospheric air as the carrier gas, the reaction was occurred at exothermic reaction and the main products were CO2, CO, and Cl2. Transformation into CCl4 was also detected for CHCl3 decomposition reaction. The conversion of these compounds was increased with the increasing applied frequency and decreasing total gas flow rate.

Key words: Plasma, Gliding Arc, CCl4, CHCl3, decomposition reaction

1. Introduction

Chlorinated volatile organic compounds (CVOCs) are released to the atmosphere in flue gases from a variety of industrial processes. Chlorinated hydrocarbons have been †

Correspondence addressed. E-mail:[email protected], Tel:+82-19-352-1981

widespread accepted as solvents and chemical intermediates in the manufacture of herbicides and plastics [1]. Unfortunately, emissions of these compounds are harmful to the environment and toxic [2-4]. Much effort was made to reduce it. The current technology for elimination them is high temperature (> 800 oC) incineration [5]. This method allows influent of CVOCs directly combusted with air. Although this method is very simple, some problems have been reported. While the combustion was not perfect (incomplete combustion), the reaction tend to yield a large amount of complex chlorinated products. [6] Catalytic oxidation which is employing metal and metal oxide catalyst has been investigated also [7]. It has been reported that using this method, the conversion and the product selectivity were very good, but the catalyst become easily deactivated by impurities and solid product. Also, this method requires temperature heater to achieve the activated catalyst temperature and reaction rate. Other limitation to use this method in industrial plant is the input flow rate which was very small. Due to overcome such those problems, many studies are carried out on the application of new technologies. Plasma-assisted technology is one of the emerging and effective technologies for destroying CVOCs. Plasma-assisted technology, such as Electron Beam [8], RF plasma [9], surface discharge reactors [10], dielectric barrier discharge reactors [11], pulsed discharge reactors [12], and capillary-tube type discharge reactors [13], have been studied and develop. In this study, gliding arc plasma was used, to decompose chloroform (CHCl3), and carbon tetrachloride (CCl4). Compare to the previous plasma methods, gliding arc plasma has great chance to be utilized for industrial chemical applications. It achieved higher flame temperature and power to

destruct the toxic input material [14]. One favor of this method was this method can handle very high input flow rate.

2. Experiment setup

Figure 1

The schematic diagram of experimental setup is shown in figure 1. CCl4, CHCl3 and atmospheric air have been used as source gas. Details of each part of the system are described in the next section.

2.1. Plasma reactor and applied power system

The reactor was made from a quartz-glass tube of inner diameter 45 mm and length 300 mm. The upper part and bottom of the reactor supplied with a teflon seal comprised two electrodes made of stainless steel. The length of the electrodes was 150 mm. The separation of the electrodes in the narrowest section was 1.5 mm. The gas mixture was introduced between the electrodes through a capillary of inner diameter 0.8 mm. A thermocouple, located 10 cm above the electrode, has been provided to measure the outlet gas temperature. A high frequency AC power supply (Auto electric, A1831) with a maximum voltage 10 kV and a maximum ampere 100 mA was connected to the gliding arc electrode to generate plasma. Figure 2 shows typical waveforms of voltage

and discharge current used in this experiment.

Figure 2

2.2. Input gas

Chlorinated hydrocarbons that were used as the starting material are: a. Chloroform: CHCl3, molecular weight 119.38, purity 99.0%, purchased from Junsei Chemical Co., Ltd., concentration 1, 3, 5, 8% v/v b. Carbon tetrachloride: CCl4, molecular weight 153.82, purity 99.5%, purchased from Kanto Chemical Co., Inc., concentration 1, 3, 5, 8% v/v. Atmospheric air was used as the carrier gas and controlled by a Mass Flow Controller (Tylan, FC-280S). The total mixed gas (air + chlorinated compounds) flow rate was 180, 240, and 300 Nl/hr. The composition of the outlet mixture was analyzed before and after the plasma operation.

2.3. Measurement system

The outlet gas composition was examined by quadrapole mass spectroscopy (Balzers, QMS 200) with Quadstar 421 software. This software was used for the qualitative and quantitative analysis of the reactants and products. Two GCs have been used to analyze

the quantitative amount of products. The contents of CCl4 and CHCl3 in the gas mixture before and after the reaction ware determined by GC-FID (YoungLin M600D, Column: Bentone). And for determination of CO and CO2 in the outlet gas mixture, a GC-TCD (YoungLin M600D, Column: SK Carbon) was used. Chlorine gas (Cl 2) was determined by bubbling the reacted gas through 0.05 M aqueous KI during measured-experiment time and followed by iodometric titration 0.05 M Na2SO3. To evaluate the performance of system, selectivity and conversion were used and formulated as: for decomposition of CCl4: Selectivity of Cl 2 =

moles of Cl 2 formed × 100% 2 × moles of CCl 4 converted

(1)

Selectivity of CO =

moles of CO formed × 100% moles of CCl 4 converted

(2)

Selectivity of CO2 =

moles of CO2 formed × 100% moles of CCl 4 converted

Conversion of CCl 4 =

moles of CCl 4 consumed × 100% moles of CCl 4 introduced

(3)

(4)

for decomposition of CHCl3: Selectivity of Cl 2 =

2 × moles of Cl 2 formed × 100% 3 × moles of CHCl 3 converted

(5)

Selectivity of CO =

moles of CO formed × 100% moles of CHCl 3 converted

(6)

Selectivity of CO2 =

moles of CO2 formed × 100% moles of CHCl 3 converted

(7)

Selectivity of CCl 4 =

moles of CCl 4 formed × 100% moles of CHCl 3 converted

Conversion of CHCl 3 =

moles of CHCl 3 consumed × 100% moles of CHCl 3 introduced

(8)

(9)

Those parameters were used to study the effect of initial chloromethane concentration, total gas flow rates, and power frequency. The discharge power was calculated as a product of voltage and current by oscilloscope (Agilent 54641A), defined as: Discharge power = ∫ (V (t ) × I (t ) ) dt × frequency Watt

(10)

In this study, each data of experiment was taken after 30 minutes from the start of gliding plasma operation refers to the stability of bulk gas outlet temperature which has measured by thermocouple.

3. Result and Discussion

3.1. Effect of initial concentration and total gas flow rate.

Figure 3

Figure 3a shows the trend of chlorinated hydrocarbons in various values of initial concentration and total gas flow rate. Conversion gradually decreased when the initial

concentration increased. The conversion also decreased with increasing total gas flow rate. The maximum conversion of CCl4 could reach 80% at the concentration 1% and total air flow rate 180 Nl/hr. At same condition, CHCl3 conversion could reach higher value, 97%. Although the conversion decreased, the removal efficiency has increased with increasing initial concentration. It was proposed that Cl◦ initiated and gave positive effect on decomposition process. Table 1

Beside the conversion, different type of compounds, initial concentration, and total gas flow rate also produced different value of consuming discharge power. Stability of compounds under plasma treatment has the important role in destruction reaction. Table 1 shows that to destruct CCl4, it spent more energy compared to CHCl3. The chemical bond and structure stability of CCl4 was stronger than CHCl3 [1]. Stronger form of compound spent more energy to initiate the arc plasma. It also produced lower value of conversion (comparison between CCl4 and CHCl3 conversion in figure 1). This phenomenon has the same tendency when chlorinated hydrocarbons were treated by oxidation combustion reaction [15]. Total gas flow rate was also has significant effect on power discharged. With the increasing total gas flow rate, the total power was decreased. Physical study about this phenomenon has been established in term of Paschen’s law [16]. Based on QMS spectra result and GC analysis, the gas product was dominated by CO, CO2, and Cl2. The number of other C-Cl compounds was relatively small. In case of CCl4, the selectivity of final product went to Cl2 gas was high. It could reach 42% up to 78.9%. Although not as high as CCl4, product probability of Cl2 on CHCl3

decomposition was also high, 33% up to 50.5%. Increasing CCl4 initial concentration has decreased the product selectivity and it means the product distribution would be varied. This phenomenon was not occurred in case of CHCl3. The selectivity of Cl2 gas was relatively constant. In case of CO and CO2 production, the selectivity has the same trend as Cl2 especially for CCl4 decomposition. It could be proposed as kind of C vacancy due to lose Cl species. When CCl4 was destructed into Cl2, it will remain C radical as an intermediate species. It opened chance for C radical (C vacancy) to collide with another compounds. Higher possibility to collide with oxygen to produce CO or CO2 compared to other kind of composition. CO and CO2 have the stable structure and the concentration of oxygen gas in the air-carrier stream was high (~20%). Based on calculation of QMS spectra of oxygen (O2) produced that the concentration of oxygen (O2) in the product stream (after plasma process) was 5-10% lower than concentration in inlet stream (before plasma process). One possible reason was the losing oxygen was reacted and consumed into CO and CO2. Comparison of produced CO2 to CO at the final product stream was around 1/10 up to 1/5. Decomposition of CHCl3 was also produced CCl4. The selectivity was not exceeded more than 12%. During the experiment, another trace compounds were also detected, such as COCl2 and HCl. Liquid product was also produced and might be counted as the remaining C and Cl species.

3.2. Effect of frequency

As mentioned above, supplied discharge power to the plasma system has important role on the decomposition reaction. This factor was also important to govern the product distribution. In this experiment, there were three variables that gave influence on its value. Among these variables, only power frequency that could be adjusted manually. Voltage and current were automatic fixed into specific value after initial breakdown of gliding arc. Figure 4

Figure 5

Figure 6

The effect of power frequency is presented in figure 4. The maximum power frequency was 20 kHz. As shown in figure 4, the conversion was increased with the increasing frequency. For CCl4, the rate of conversion reached 4% higher per 1 kHz increment of frequency and slightly lower for CHCl3. Increasing frequency leads to increase the supplied discharge power. Figure 5 shows the increasing discharge power due to increasing power frequency. Increasing frequency would give effect on AC wave form. Higher frequency produced more number of peaks per cycle and supplied more energy to the system. The change of patter of power wave form due different applied power frequency is shown in figure 6.

Figure 7

Figure 7 shows the different value of frequency or power discharge also gives different

value of Cl2 gas production although the initial concentration and flow rate were same. It means that power frequency or power discharge has the importance to be a factor to decide the way of reaction. With the increasing power frequency, the selectivity of Cl2 was getting increased also. It could be proposed that in the higher energy field, the plasma reaction tent to produce Cl2 gas as the final product.

3.3. QMS Spectroscopy

Figure 8

A QMS spectroscopy of the conversion of CCl4 is shown in figure 8. Figure 8a describes the condition of components in the 180 Nl/hr feed stream containing +

atmospheric air with 1% of CCl4. The baseline spectra of CCl4 were 83/85 ( CCl 2 ), and + + 117 ( CCl 3 ). CO2 has spectra at 44 ( CO2 ) and CO spectra was collide with minor N2

+

+

+

spectra at 29 ( CO / N 2 ). Major N2 spectra, it self, was located at 28 ( N 2 ). But, because the concentration of CO in the air was quite small, CO compound could be negligible in this spectroscopy. When the plasma was applied, figure 8b, the peak intensity of 83/85 got disappeared together with decreasing intensity of spectra 117 and +

produced a new peak, 71, which is known as Cl 2 ( Cl 2 ). Intensity of CO2 was increased

also but the difference was very small. High increment has been occurred on spectra 28. Because N2 was relatively stable gas and no source of N compound to produce N2, it could be said that the increasing intensity of spectra 28 due to production of CO by plasma. Figure 9

The mass spectroscopy of CHCl3 before and after plasma reaction is presented in figure 9. Figure 9a shows the components in the 240 Nl/hr feed stream containing compressed + air mixed with 1% of CHCl3. The baseline spectra of CHCl3 were 82/84/86 ( CCl 2 ).

+ + CO2 has spectra at 44 ( CO2 ) but it was collided with major N2O spectra ( N 2 O ), +

which has possibility to exist in the gas product. CO was at 29 ( CO ) but it was also + collided with minor N2 spectra 29 ( N 2 ). It made difficult to distinguish them using

QMS. In this experiment, CO and CO2 were analyzed by gas chromatography. When the plasma was applied, figure 9b, the peak intensity of 83/84/86 got decreased and + produced some new peaks: 63/65, which is known as COCl2 ( COCl ), 69/71 as Cl2 (

Cl 2+ ), 117 as CCl4 ( CCl 3+ ).

3.4. Reaction Pathway

The kinetic reaction of CO, CO2, and Cl2 formation which were produced from chlorinated hydrocarbons has been studied and presented [8, 17-22]. The major pathways responsible for the formation of Cl2 were: COCl 2 + Cl = COCl + Cl 2

(11)

CHCl 3 + Cl = CHCl 2 + Cl 2

(12)

COCl + Cl = CO + Cl 2

(13)

However, the third reaction (formula 13) was also one of the primary reaction paths for the formation of CO. Other one was: COCl + M = CO + Cl + M

(14)

It was seen that CO formation was initially through COCl2. The major reaction path for the formation of CO2 was come from CO: CO + ClO = CO2 + Cl

(15)

CO + O + M = CO2 + M

(16)

in which ClO forms via: CCl 4 + O = CCl 3 + ClO

(17)

CCl 3 + O2 = COCl 2 + ClO

(18)

It means oxygen and oxygen radical have significantly contribution to held the reaction. The production of O radical came from dissociation reaction [Penetrante, et. al, 1995]: e + O 2 = e + O( 3 P ) + O ( 3 P )

(19)

e +O 2 = e + O ( 3 P ) + O ( 1D )

(20)

Cl radical in this formula can be produced from: CCl 4 = CCl 3 + Cl

(21)

CCl 3 + O = COCl 2 + Cl

(22)

CCl 3 + Cl 2 = CCl 4 + Cl

(23)

Reverse reaction could be happened on formula 21 as the main path way to produce CCl4 in decomposition of CHCl3, CCl 3 + Cl = CCl 4

4. Conclusion

The performance of CCl4 and CHCl3 decomposition on gliding arc plasma at atmospheric pressure, related with initial concentration, total gas flow rate, and input power frequency, was studied. Gliding plasma could generate effective electrons and

ions enough to fragment the molecules. The maximum conversion of CCl4 was 80% and CHCl3 was 97% for feed gas stream containing 1% of v/v and total air flow rate 180 Nl/hr. The major final products were CO, CO2, and Cl2. Cl2 selectivity was relatively high and reached up to 78.9% for CCl4 and 50.5% for CHCl3. Conversion into CCl4 was found in case of CHCl3 decomposition, but not exceed than 12%. COCl2 has an important role as intermediate species to produce Cl2 and CO. Mostly CO2 was produced from CO reaction. From these result, it could be concluded that the gliding arc plasma system was very useful for decomposition of CCl4 and CHCl3. Development was needed to remove some traces unwanted compounds, such as: COCl2 in the final product.

Acknowledgements

This study was supported by National Research Laboratory Program of Korea Minister of Science and Technology.

REFERENCES

[1]

Kirk-Othmer, Encyclopedia of Chemical Technology (John Wiley & Sons), Vol. 5, 3rd edition, New York, pp. 668-713

[2]

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[3]. International Agency for Research on Cancers (IARC), Monographs on the evaluation of carcinogenic risk to humans, Supplements 7, 1987. [4]

U.S. Environmental Protection Agency, Report: Greenhouse gases and global warming potential values, April 2002.

[5]. L. C. Lou and Y. S. Chang, Combust. Flame, 109, 188 (1997). [6]. P. H. Taylor and B. Dellinger, Environ. Sci. Technol., 22, 438 (1988). [7]. R. M. Alberici and W. F. Jardim, Appl. Catal. B: Environ., 14, 55 (1997). [8]. L. Prager, H. Langguth, S. Rummel, and R. Mehnert, Radiat. Phys. Chem., 46(46), 1137 (1995) [9]. W. J. Lee, C. Y. Chen. W.C. Lin, Y. T. Wang, and C. J. Chin, J. Hazardous Materials, 48, 51 (1996). [10]. T. Oda, T. Takahahshi, and K. Ya,aji, IEEE Transactions on Industry Applications, 38(3), 873 (2002). [11]. R. G. Tonkyn, S. E. Barlow, and T. M. Orlando, J. Appl. Phys., 80(9), 4877 (1996). [12]. T. Yamamoto, K. Ramanathan, P. A. Lawness, D. S. Ensor, J. R. Newsome, N. Plaks and G. H. Ramsey, IEEE Transactions on Industry Applications, 28(3), 528 (1992). [13]. H. Kohno, A. A. Berezin, J. S. Chang, M. Tamura, T. Yamamoto, A. Shibuya and S. Honda, IEEE Transactions on Industry Applications, 34(5), 953 (1998). [14]. A. Czernichowski, Pure & Appl. Chem., 66 (6), 1301 (1994). [15]. P. H. Taylor and B. Dellinger, Environ. Sci. Technol., 22 (4), 438 (1988). [16]. J. D. Cobine, Gaseous Conductor Theory and Engineering Application (Dover

Publications, Inc., 1958), pp. 160-177. [17]. J. C. Lou, and Y. S. Chang, Combust. Flame, 109, 188 (1997). [18]. M. Koch, D. R. Cohn, R. M. Patrick, M. P. Schuetze, L. Bromberg, D. Reilly, K. Hadidi, P. Thomas, and P. Falkos, Environ. Sci. Technol., 29, 2946 (1995). [19]. H. Nichipor, E. Dashouk, A. G. Chmielewski, Z. Zimek, and S. Bulka, Rad.Phys. Chem., 57, 519 (2000). [20]. W-D. Chang, and S. Senkan, Environ. Sci. Technol., 23, 442 (1989). [21]. C. Feiyan, S.O. Pehkonen, and M. B. Ray, Water Research, 36, 4203 (2002). [22]. Y-P. Wu, and Y-S. Won, Combust. Flame, 122, 312 (2000)

v e n t to a tm o s p h e r e F ID G C B e n to n e

TC D G C S K C a rb o n

C o m p re s s e d a ir

Q u a d ra p o le M ass S p e c t ro m e t e r

C h lo r in a te d h y d ro c a rb o n

K I s o lu t i o n

P la s m a R e a c to r

Figure 1. Schematic diagram of experimental set up

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

1e-5

Time (s)

Figure 2. Voltage and current profile

2e-5

CCl4

CCl4 conversion (%)

100

80

60

40

0

2

4

6

8

10

8

10

initial CCl4 concentration (% v/v)

CHCl3

CHCl3 conversion (%)

100

80

60

40

0

2

4

6

Initial CHCl3 concentration (% v/v)

Figure 3. Influence of initial concentration of injected chlorinated hydrocarbons and total gas flow rate on its conversion. (■ 180 Nl/min; ● 240 Nl/min; ▲ 300 Nl/min)

concentration (% v/ v)

flowrate (Nl/ hr)

power (Watt)

1 3 5 8 1 3 5 8 1 3 5

180 180 180 180 240 240 240 240 300 300 300

242.6 240.5 239.3 229.6 236.8 234.2 229.4 219.6 227.7 225.6 218.2

selectivity CO+CO2 Cl2 48.7 32.4 28.9 25.4 48.0 49.0 41.0 41.7 26.7 25.2 18.2

78.9 73.0 68.0 65.0 63.3 55.1 48.4 45.6 51.6 48.0 42.0

(a)

concentration (% v/ v)

flowrate (Nl/ hr)

power (Watt)

CO+CO2

selectivity CCl4

1 3 5 8 1 3 5 8 1 3 5

180 180 180 180 240 240 240 240 300 300 300

234.5 232.1 226.8 212.8 222.1 216.6 205.1 175.0 220.0 212.6 197.4

31.1 40.3 42.5 48.6 31.1 34.4 41.0 43.8 22.2 27.4 24.6

6.4 9.7 10.9 11.8 9.4 10.0 10.2 11.8 5.3 7.9 10.0

Cl2 33.0 34.9 33.8 42.7 38.8 48.3 47.4 50.5 43.3 47.5

(b)

Table 1. Effect of initial concentration and total gas flow rate on product distribution of (a) CCl4 and (b) CHCl3 decomposition

CCl4

CCl4 conversion (%)

100

80

60

40

20

14

16

18

20

22

20

22

Power frequency (kHz)

CHCl3

CHCl3 conversion (%)

100

80

60

40

20

14

16

18

Power frequency (kHz)

Figure 4. Effect of applied input power frequency on CCl4 and CHCl3 conversion (■  180 Nl/min;  ●  240 Nl/min;  ▲  300 Nl/min). Data was taken at 1% v/v of CCl4 and 1%

v/v of CHCl3.

350

Discharge power (Watt)

300

250

200

CCl4; 180 Nl/hr

150

CCl4; 240 Nl/hr CHCl3; 180 Nl/hr

100

CHCl3; 240 Nl/hr

14

16

18

20

22

Power frequency (kHz)

Figure 5. Effect

of applied different power frequency on consumed discharge power. Data

was taken at 1% v/v of CCl4 and 1% v/v of CHCl3.

1000 15 kHz 16.25 kHz 17.5 kHz 18.75 kHz 20 kHz

800

Power (Watt)

600

400

200

0

-200 -2e-5

-1e-5

0

1e-5

2e-5

Time (sec)

Figure 6. Effect of applied different power frequency on power-profile wave form. Data was taken at 5% of CHCl3 and total gas flow rate 180 Nl/min

80

CCl4; 180 Nl/hr CCl4; 240 Nl/hr

Selectrivity (%)

CHCl3; 180 Nl/hr 60

CHCl3; 240 Nl/hr

40

20

0 14

16

18

20

22

Power frequency (kHz)

Figure 7. Effect of applied different power frequency on Cl2 gas production selectivity. Data was taken at 1% v/v of CCl4 and 1% v/v of CHCl3.

before

1.4e-9 1.2e-9

Intensity

1.0e-9 CO and N2 8.0e-10 6.0e-10 4.0e-10 2.0e-10

CO2

Cl2

CCl4 CCl4

0.0

after

1.4e-9 1.2e-9 CO and N2

Intensity

1.0e-9 8.0e-10 6.0e-10 4.0e-10 CO2

2.0e-10

CCl4

Cl2

0.0 0

20

40

60

80

100

120

140

160

180

200

AMU (m/z)

Figure 8. QMS spectrogram of CCl4 decomposition (taken at applied frequency 20kHz, 1% of CCl4, total gas flow rate 180 Nl/h); (a) before (b) after gliding plasma on

1.2e-9

(a) before

1.0e-9

intensity

8.0e-10

6.0e-10

4.0e-10

2.0e-10

CHCl3

1.2e-9

(b) after

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 9. QMS spectrogram of CHCl3 decomposition (taken at applied frequency 20kHz, 1% of CHCl3, total gas flow rate 180 Nl/h); (a) before (b) after gliding plasma on