Revised Manuscript

Treatment of Dichloromethane Using Gliding Arc Plasma

Antonius Indarto Clean Technology Research Center, Korea Institute of Science & Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea Phone number: 02-958-6558 email: [email protected]

Corresponding Author

Jae-Wook Choi Clean Technology Research Center, Korea Institute of Science & Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Hwaung Lee Clean Technology Research Center, Korea Institute of Science & Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Hyung Keun Song Clean Technology Research Center, Korea Institute of Science & Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Total counted words: 5253 (including: 1 table and 8 figures)

1

Abstract- Decomposition of dichloromethane (CH2Cl2) using a gliding plasma was examined and reported in this paper. The effects of initial concentrations of CH 2Cl2, total gas flow rates, and input frequency have been studied to evaluate the performance of gliding arc on CH2Cl2 decomposition. Using atmospheric pressure air as the carrier gas, experimental results indicate that the maximum conversion of CH2Cl2 was 95.1% at a total gas flow rate of 180 L/hr containing 1% by volume of CH2Cl2. The reaction occurred at an exothermic condition and gaseous products are dominated by CO, CHCl3, and Cl2. CO2 and CCl4 are also detected in the product stream in small amounts. The conversion of CH2Cl2 increases with the increasing applied voltage and decreasing total gas flow rate.

Key words: Plasma, Gliding Arc, CH2Cl2 decomposition

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Introduction

Nowadays, gaseous wastes release to the atmosphere is growing in both amount and complexity following the growth of industry and improvement of our living status. Various Chlorine-based compounds which are used in the widespread industrial sectors have a high potential to emit some chlorinated volatile organics compounds (CVOC), such as dichloromethane (CH2Cl2). The Environmental Protection Agency (EPA) Toxic Release Inventory reports that U.S. industrial discharges of dichloromethane amounted to 29,000 metric tons in 1994 (Keene et al. 1999). This compound has an atmospheric half-life of several months. Radical-chlorine atoms from CVOC will initiate and participate in ozonedepleting reactions in the stratosphere (Sanhueza 2001; US EPA 1999; Khalil and Rasmussen 1999). It will create a big environmental problem when the compound is released without any treatments (Shah and Singh 1988). This situation shows that the rising of released concentration of CVOC into the environment, together with its suspected toxicity and carcinogenic, has increased the demand for finding effective methods to reduce it. The most widely adopted technique for the treatment of chlorinated VOC effluents is thermal combustion or incineration (Lou and Chang 1997). This method will process influent of CVOC by direct oxidation reaction with air at high temperature range which is between 8001,100oC. The main problem is relating to the incomplete combustion reaction. The reaction will produce numerous amounts of another complex chlorinated compound (Taylor and Dellinger 1988). Catalytic oxidation is a well-known process for treating chloromethane. Unfortunately, the most active catalyst for the oxidation usually comes from noble and expensive metals, such as Platinum and Ruthenium, supported by alumina. High temperature process is also required to

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increase the reaction rate and to overcome chloride poisoning (Alberici and Jardim 1997). Other limitation of plasma-assisted process is small flow rate of emission input process. In recent years, many studies have been carried out on the application of new technologies to destruct the emission of CVOC. Plasma-assisted technology, such as RF plasma (Lee et al. 1996), surface discharge reactors (Oda et al. 2002), dielectric barrier discharge reactors (Tonkyn et al. 1996), pulsed discharge reactors (Yamamoto et al. 1992), and capillary-tube type discharge reactors (Kohno et al. 1998), have been developed. Among non-thermal plasmas, gliding arc plasma has a good energy efficiency conversion. This reason makes gliding arc plasma a bright prospect to be utilized for industrial chemical reactions (Fridmann et al. 1999). In this study, gliding arc plasma was used, as an attractive method due to better production of radical species, higher flow rates, and higher concentration of input material, to destruct dichloromethane (CH2Cl2). However, destruction of chloromethane, carbon tetrachloride (CCl4) and chloroform (CHCl3), in gliding arc plasma has been studied previously (Krawczyk and Ulejczyk 2003a; 2003b). The author reports that high destruction efficiency can be achieved using this method.

Experimental Setup Input Gas

Figure 1

A schematic diagram of experimental setup is shown in Figure 1. Liquid CH2Cl2 which was used as the input material has a 99.0% purity (purchased from the Junsei Chemical Co.). The carrier gas was atmospheric air and the flow rates were controlled with a Mass Flow

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Controller (Tylan, FC-280S). The initial gas CH2Cl2 concentration was set as 1, 2, 3, and 4% by volume. The flow rate of air to the plasma reactor was varied by 180, 240, and 300 L/hr. Before entering the reactor, atmospheric air first passed through a scrubber and mixed with CH2Cl2. Liquid CH2Cl2 was introduced by a syringe pump (KD Scientific, Model 100) and a heater tape rolled covering the input line to vaporize the liquid CH2Cl2 into gas phase before it was mixed with air. The composition of the outlet stream was analyzed before and after plasma operation.

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, the length of the electrodes being 150 mm. The distance of the electrodes in the narrowest section is only 1.5 mm. The gas mixture is fed between the electrodes with a capillary of inner diameter 0.8 mm. 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) is connected to the gliding arc electrode to generate the plasma. Figure 2 shows the typical waveform of voltage and discharge current used in these experiments.

Figure 2

Measurement system The outlet gas composition is examined by an electron-impact ionization quadrupole mass spectrometer (Balzers, QMS 200) with software Quadstar 421. This software is used for both 5

qualitative and quantitative analysis of the inputs and products. Two GCs are used to analyze the quantitative amount of products; CH2Cl2, also CHCl3 and CCl4, in the gas mixture before and after the reaction are determined by GC-FID (YoungLin M600D, Column: Bentone), for CO and CO2 by GC-TCD (YoungLin M600D, Column: Carbon Sieve). Chlorine gas (Cl2) is determined by bubbling the reaction gas through 0.05 M aqueous KI for a given time, followed by iodometric titration with 0.05 M Na2SO3 (Skoog et al. 2000). To evaluate the performance of system, selectivity and conversion, is used and defined as: Conversion of CH 2 Cl 2 =

Selectivity of CO =

moles of CO formed × 100% moles of CH 2 Cl 2 converted

Selectivity of CO2 =

moles of CO2 formed × 100% moles of CH 2 Cl 2 converted

Selectivity of CHCl 3 =

Selectivity of CCl 4 =

Selectivity of Cl 2 =

Carbon balance =

moles of CH 2 Cl 2 consumed × 100% moles of CH 2 Cl 2 introduced

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

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

moles of Cl 2 formed × 100% moles of CH 2 Cl 2 converted

(1)

(2)

(3)

(4)

(5)

(6)

moles of CO formed + moles of CO2 formed + moles of CHCl 3 formed moles of CH 2 Cl 2 converted

(7) The supplied power was calculated as a product of voltage and current by oscilloscope (Agilent 54641A).

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Results and Discussion Effect of concentration

Figure 3

CH2Cl2 was carried out at concentration range between 1% and 4% by volume and diluted in atmospheric pressure air. In this study the experimental data was obtained 30 minutes after the initiation of the plasma of gliding arc referred to the stable temperature of the bulk gas measured by thermocouple. Figure 3 shows the effect of CH2Cl2 initial concentrations variation on its conversion at a fixed power frequency of 20 kHz. Conversion efficiency gradually decreases when the initial concentration of CH2Cl2 increases. The maximum conversion reaches 95.1% at 1% of CH2Cl2 concentration and total air flow rate of 180 L/hr. Based on QMS spectra result (figure 8), the gaseous products are dominated by CO, CHCl3 and Cl2. Small amount of CO2 and CCl4 are also detected. CO2 is detectable when the initial concentration of CH2Cl2 is higher than 3%. Compared to the yields of CO, the yields of CO 2 was 4-7 times lower than CO yields in the products. In this study, mass balance analysis of C (carbon) component is calculated based on the total concentration of CO, CO2 and CHCl3. Table 1

Table 1 shows that more than half of reacted CH2Cl2 is converted into CO and CO2 as the products of the plasma reaction. High production of CO and CO2 reflects that oxygen, which exists in the air at concentration of 20%, has a significant role in the decomposition process. Less than 15% of reacted CH2Cl2 is transformed into other chloromethane products, such as CHCl3 and CCl4. As shown in table 1, when the initial concentration of CH2Cl2 is increased

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from 2% to 4%, the reaction selectivity of CO gradually decreases from 58.2% to 50.5% at total gas flow rate of 180 L/hr and 50.2% to 48% at 240 L/hr. This result indicates that higher concentration of CH2Cl2 in the plasma reaction reduces the possibility of product to form CO. However, at lower concentration of CH2Cl2, high-active state oxygen e.g. O (3P and 1D) or O2 (a1∆) enhances reaction with CH2Cl2 and other chlorine-contained compounds rather than at higher concentration of CH2Cl2. In the presence of atomic oxygen, CH2Cl2 can be removed by reaction (Fitzsimmons et al. 2000) O + CH2Cl2  CHCl2 + OH

(8)

The production of OH in the system can help the destruction process by OH + CH2Cl2  CHCl2 + H2O

(9)

CO is produced via oxidation of CH2Cl2 (Fitzsimmons et al. 2000; Hosson and Smith 1999; Ho et al. 1992; Thuner et al. 1999; Bilde et al. 1999). CH2Cl2  CH2Cl + Cl

(10)

CH2Cl + O2  CH2ClO2

(11)

CH2ClO2 + CH2ClO2  2 CH2ClO + O2

(12)

CH2ClO + O2  HCOCl + HO2

(13)

CH2ClO  HCOCl + H

(14)

O, Cl + HCOCl  CO + ClO, Cl

(15)

Other CO production pathways via secondary OH reaction and third body species are also proposed (Lou and Chang 1997; Fitzsimmons et al. 2000; Ho et al. 1992). HCOCl + Cl  COCl + HCl

(16)

OH + HCOCl  COCl + H2O

(17) 8

COCl  Cl + CO

(18)

A different phenomenon is found in chloroform (CHCl3) production. CH2Cl2 transformation into CHCl3 is higher when initial concentration of CH2Cl2 is increased. Following references (Hosson and Smith 1999; Ho et al. 1992; Thuner et al. 1999; Bilde et al. 1999), CHCl 3 production is dependent on the presence amount of Cl and CHCl2. CHCl2 + Cl  CHCl3

(19)

Mostly, the source of Cl is obtained from unimolecular dissociation of CH2Cl2 CH2Cl2  CH2Cl + Cl

(20)

Increasing the initial CH2Cl2 concentration in the input stream will produce higher amount of atomic Cl. Cl will easily collide with intermediate species CHCl2, produced by reaction 8 and 9, to form stable gases such as CHCl3. The ratio between total C (carbon) atom from gaseous products e.g. CO, CO 2, CHCl3, CCl4 and total C atom from reacted CH2Cl2 has a range between 0.5 and 0.75. It suggests that the major end products of plasma processing of CH2Cl2 diluted in atmospheric air will be CO, CO2, CHCl3, and CCl4. The remaining C possibly goes to the solid (shoot) or liquid production which is also produced during experiment. However, in term of energy consumption, higher initial concentration of CH2Cl2 increases the energy consumption of plasma process, around 5-10 Watt for every 1% increase in the concentration of CH2Cl2. Increasing amount of CH2Cl2 has a significant effect on alteration of voltage and current waveform which effects on power consumption. In the presence of CH2Cl2 molecule, gliding plasma spends more energy to destruct or break the chemical bond of CH2Cl2 to achieve a plasma state. Increasing concentration of CH2Cl2 also has a significant influence on the increment of gas temperature. A stable temperature is achieved after the

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gliding plasma system is on for 30 min. Compared with a pure-air gliding arc, the gas temperature increases by 5-25oC when the inlet stream contains CH2Cl2. It shows that the destruction of CH2Cl2 is an exothermic reaction that releases heat to the surrounding environment. The rising amount of destructed CH2Cl2 will produce more heat from the destruction reaction.

Effect of total gas flow rate

Figure 4

The effect of total gas flow rate, which is related to the residence time of CH2Cl2 in the reactor, was also examined. Figure 4 shows the CH2Cl2 conversion vs. total gas flow rates at frequency of 20 kHz. The conversion of CH2Cl2 decreases when the total gas flow rate is increased. Increasing total gas flow rate reduces the residence time of CH2Cl2 in the reactor and hence the chance and time of CH2Cl2 to collide with electrons or other high-energy state atoms or molecules. Figure 5

Figure 5 clearly shows that the selectivity CO and CO 2 decreases with the increasing value of total gas flow rate. Following above explanation, increasing total gas flow rate reduces the chance of collision reaction between of CH2Cl2 and high-energy state species as well as oxygen (atom or molecule). However, the carbon balance is also decreased with increasing total gas flow rate.

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Effect of applied frequency

Figure 6

Figure 7

Figure 6 shows the effect of various applied frequency of input power on the CH 2Cl2 destruction at 2% of CH2Cl2 concentration and the total gas flow rate of 180 L/min. The removal efficiency decreases from 90% to 55% when the applied frequency is decreased from 20 kHz to 15 kHz. Decreasing applied frequency decreases the supplied energy to the reactor, i.e., the energy source used to dissociate the molecule of CH2Cl2. Figure 7 shows that reaction selectivity of CO relatively increased when the applied frequency was also increased. On the other hand, the selectivity of CHCl3 is decreased. Increasing frequency linearly correlates with increasing numbers of radical and high-active oxygen that come out from atmospheric air, and tends to produce CO and CO2 as the gaseous end-product. This is the possible reason why the selectivity of CO and CO2 increases with the applied frequency.

QMS spectra Figure 8

QMS spectrum of the conversion of CH2Cl2 is shown in figure 8. Figure 8 (top) shows the peak components in the input stream. The baseline spectra of CH2Cl2 are m/z 83(CCl2+) and 85(CH2Cl2+) and this spectra was also owned by CHCl3. To distinguish between CH2Cl2 and CHCl3, we used a FID-GC as the analysis method. When the plasma is on, as shown in Figure 8 (bottom), peak intensity of m/z 83 and 85 are decreased and some new peaks appear, e.g. m/z 61(COCl+), 71(Cl2+), and 118(CHCl3+). It is evident that COCl exists in the product 11

stream because COCl has an important role in CO production as an intermediate species.

Conclusion The performance of CH2Cl2 conversion in gliding arc plasma at atmospheric pressure was studied. Gliding plasma shows a good performance on the destruction of CH2Cl2 molecules. The maximum conversion of CH2Cl2 was 95.1% for feed gas stream containing 1% of CH2Cl2 and air flow rate of 180 L/hr. The effect of the initial concentrations of CH2Cl2, gas flow rates, and input power frequencies were also experimentally investigated. Destruction of CH2Cl2 in gliding plasma environment occurred in an exothermic reaction. CO, CO2, Cl2 and CHCl3 are the major product with small amount of CCl4. CO and CO2 selectivity achieved ~50% in all range of experimental condition. Transformation into other chloromethane compounds included a maximum of 15% for CHCl3 and 7% for CCl4. It is concluded that gliding arc plasma is adequate for CH2Cl2 reduction.

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

References Alberici, R.M., and Jardim, W.F. 1997. Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide, Appl. Catal. B: Environ., 14: 55. Bilde, M., Orlando, J.J., Tyndall, G.S., Wallington, T.J., Hurley, M.D., and Kaiser, E.W. 1999. FT-IR Product Studies of the Cl-Initiated Oxidation of CH3Cl in the Presence of NO. J. Phys. Chem. A, 103: 3963. Fitzsimmons, C., Ismail, F., Whitehead, J.C., and Wilman, J.J. 2000. The Chemistry of 12

Dichloromethane Destruction in Atmospheric-Pressure Gas Streams by a Dielectric Packed-Bed Plasma Reactor. J. Phys. Chem. A, 104: 6032. Fridman, A., Nester, S., Kennedy, L.A., Saveliev, A., and Mustaf-Yardimci, O. 1999. Gliding arc gas discharge. Prog. Energy Combust. Sci., 25: 211. Hasson, A.S., and Smith, I.W.M. 1999. Chlorine Atom Initiated Oxidation of Chlorinated Ethenes: Results for 1,1-Dichloroethene (H2C=CCl2), 1,2-Dichloroethene (HClC=CClH), Trichloroethene (HClC=CCl2), and Tetrachloroethene (Cl2C=CCl2). J. Phys. Chem. A, 103: 2031. Ho, W.-H., Barat, R.B., and Bozzelli, J.W. 1992. Thermal Reactions of CH2Cl2 in H2/O2 Mixtures: Implications for Chlorine Inhibition of CO Conversion to CO2. Combust. Flame, 88: 265. Keene, W.C., Khalil, M.A.K., Erickson, et al. 1999. Composite global emissions of reactive chlorine from anthropogenic and natural sources: reactive chlorine emissions inventory. J. Geophys. Res., 104: 8429. Khalil, M.A.K., and Rasmussen, R.A. 1999. Atmospheric methyl chloride. Atmos. Environ., 33: 1305. Kohno, H., Berezin, A.A., Chang, J.S., Tamura, M., Yamamoto, T., Shibuya, A., and Honda, S. 1998. Destruction of Volatile Organic Compounds Used in a Semiconductor Industry by a Capillary Tube Discharge Reactor. IEEE Trans. Industry Applicat., 34(5): 953. Krawczyk, K., and Ulejczyk, B. 2003. Decomposition of Chloromethanes in Gliding Discharges. Plasma Chem. Plasma Process., 23(2): 265. Krawczyk, K., and Ulejczyk, B. 2003. Influence of Water Vapor on CCl4 and CHCl3 Conversion in Gliding Discharge. Plasma Chem. Plasma Process., 24(2): 155. Lee, W.J., Chen. C.Y., Lin, W.C., Wang, Y.T., and Chin, C. J. 1996. Phosgene formation from the decomposition of 1,1-C2H2Cl2 contained gas in an RF plasma reactor. J. Hazardous Mat., 48: 51. Lou, J.C., and Chang, Y.S. 1997. Thermal Oxidation of Chloroform. Combust. Flame, 109: 188 Oda, T., Takahahshi T. and Yaaji K. 2002. Nonthermal Plasma Processing for Dilute VOCs Decomposition. IEEE Trans. Industry Applicat., 38(3): 873. Sanhueza, E. 2001. Hydrochloric acid from chlorocarbons: a significant global source of background rain acidity. Tellus, 53B: 122. Shah, J.J. and Singh, H.B. 1988. Distribution of volatile organic chemicals in outdor and indoor air. Environ. Sci. Technol., 22: 1381. Skoog, D.A., West, D.M., Holler, F.J., and Crouch, S.R. 2000. Analytical Chemistry, An Introduction. 7th Ed., Saunders College Publishing. Taylor, P.H., and Dellinger, B. 1988. Thermal Degradation Characteristics of Chloromethane Mixtures. Environ. Sci. Technol., 22: 438. Thuner, L.P., Barnes, I., Becker, K.H., Wallington, T.J., Christensen, L.K., Orlando, J.J., and Ramacher, B. 1999. Atmospheric Chemistry of Tetrachloroethene (Cl2C=CCl2): Products of Chlorine Atom Initiated Oxidation. J. Phys. Chem. A, 103: 8657. Tonkyn, R.G.., Barlow, S.E., and Orlando, T.M. 1996. Destruction of carbon tetrachloride in a dielectric barrier/packed-bed corona reactor. J. Appl. Phys., 80(9): 4877. 13

U.S. Environmental Protection Agency (EPA). 1999. Greenhouse gases and global warming potential values. Yamamoto, T., Ramanathan, K., Lawness, P.A., Ensor, D.S., Newsome, J.R., Plaks, N., and Ramsey, G.H. 1992. Control of Volatile Organic Compounds by an AC Energized Ferroelectric Pellet Reactor and a Pulsed Corona Reactor. IEEE Trans. Industry Applicat., 28(3): 528. v e n t to a tm o s p h e re

F ID G C C o lu m n : B e n to n

M FC A t m o s p h e r ic a ir

S y r in g p u m p

TCD GC C o lu m n : S ie v e C arb o n

P la s m a R e a c to r

S cru b b e r

H e a te r ta p e

Figure 1. Schematic of experimental set up.

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M ass S p e c tro m e te r

6000

4000

Voltage (V)

2000

0

-2000

-4000

-6000 0.4

-2e-5

-1e-5

0

1e-5

2e-5

-2e-5

-1e-5

0

1e-5

2e-5

Current (A)

0.2

0.0

-0.2

-0.4

-0.6

Time (s)

Figure 2. Voltage and current profile of the applied electric power input.

15

100

90

Conversion (%)

80

70

60 180 L/hr 50

240 L/hr 300 L/hr

40 0.5

1

1.5

2

2.5

3

3.5

4

CH2Cl2 concentration (%)

Figure 3. Effect of initial CH2Cl2 concentration on CH2Cl2 conversion (obtained at frequency of 20 kHz).

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4.5

concentration (% v/ v) 2 3 4 2 3 4

flowrate (L/ hr) 180 180 180 240 240 240

CO+CO2 58.23 55.82 50.52 50.20 56.67 48.01

CHCl3 4.45 11.02 14.92 3.95 6.24 8.39

selectivity (%) CCl4 2.17 0.00 6.66 2.76 4.82 1.35

HCl 2.55 6.94 11.43 1.04 3.68 7.73

Cl2 12.84 12.15 15.68 12.51 8.24 10.76

Table 1. Effect of initial CH2Cl2 concentration on product selectivity

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100

90

Conversion (%)

80

70

60 1% 2%

50

3% 4% 40 150

170

190

210

230

250

270

Total Gas Flow Rate (L/hr)

Figure 4. Effect of total gas flow rate on CH2Cl2 conversion (obtained at frequency of 20 kHz).

18

290

310

CO and CO2 selectivity

Selectivity (%)

60

45

30 2% 3% 4% 15 0

Carbon balance

Carbon balance

0.75

0.60

0.45 2% 3% 4%

0.30 0.00 160

180

200

220

240

260

280

300

Total gas flow rate (L/hr)

Figure 5. Effect of total gas flow rate on CO2 and carbon balance.

19

320

100

Conversion (%)

80

60

40

20

0 14

15

16

17

18

19

20

21

Frequency (kHz)

Figure 6. Effect of applied input power frequency on CH2Cl2 conversion (obtained at the initial CH2Cl2 concentration of 2% and the total gas flow rate of 180 L/hr).

20

(CO+CO2) and CHCl3 selectivity

70

60 15

50

40 10 30

(CO+CO2) selectivity (%)

CHCl3 selectivity (%)

20

CHCl3 selectivity (CO+CO2) selectivity

5

20

0

0

Carbon balance

0.8

Carbon balance

0.6

0.4

0.2

0.0 0

14

15

16

17

18

19

20

21

Frequency (kHz)

Figure 7. Effect of applied input power frequency on (CO+CO2) and CHCl3 selectivity and carbon balance.

21

5e-10

before

Intensity

4e-10

3e-10

2e-10

1e-10

CH2Cl2, CHCl3 0

after

5e-10

Intensity

4e-10

3e-10

2e-10

1e-10

CO2 Cl2

CCl4

0 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

AMU (m/z)

Fig 8. QMS spectra of CH2Cl2 decomposition (obtained at the initial CH2Cl2 concentration of 2% and the total gas flow rate of 180 L/hr)

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