Decomposition Ed

Decomposition of CCl4 and CHCl3 on in 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 Shortened title (an additional shortened running title not more than 35 characters including spaces, to be placed on the top of pages), for example: "Decomposition of CCl4 and CHCl3" Abstract The decomposition of chlorinated hydrocarbons, CCl4 and CHCl3, in gliding arc plasma was examined. The effects of initial concentrations, total gas flow rates, and power consumption have been were investigated. The conversion of the hydrocarbons mentioned above was relatively high. It could reach 80% for CCl4 and 97% for CHCl3. Under atmospheric air as the a carrier gas, the reaction was occurred at exothermic, reaction and the main products were CO2, CO, and Cl2. The transformation into CCl4 was also detected for the decomposition reaction of CHCl3. The conversion of these compounds was increased rose with the increasing applied frequency frequency of power supplied and decreasing total gas flow rate. Key words: Plasma, Gliding Arc, CCl4, CHCl3, decomposition reaction

†

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

1. Introduction Chlorinated volatile organic compounds (CVOCs) are released to the atmosphere in flue gases from a variety of industrial processes. At various industrial processes, chlorinated volatile organic compounds (CVOCs) are released into the atmosphere with flue escaping gases. Chlorinated hydrocarbons have been gained widespread accepted acceptance 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. Unfortunately, these compounds are toxic and harmful to the environment [24], and serious effort was made to reduce their emissions. The current technology for the elimination of them these emissions is high temperature (> 800oC) incineration [5]. This method allows influent of CVOCs directly combusted with air. This method allows to combust the influent of CVOCs in air. Although this method is very simple, some problems have been reported. While the combustion was not perfect (incomplete combustion), the reaction tended to yield a large amount of complex chlorinated products. [6] This method is very simple, but due to incomplete combustion, a large amount of complex chlorinated products can be emitted [6] Catalytic oxidation, which is employing employs metal and metal oxide catalysts, has been also 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. It has been reported that this method of conversion is very effective and the product selectivity is very good, but the catalyst applied was easily deactivated by impurities and solid products. Also, this method requires temperature heater to achieve the activated catalyst temperature and reaction rate. Besides, this method requires the use of a heater to achieve the necessary temperature of catalyst activated and satisfactory rate of reaction. Other limitation to use this method in industrial plant is the input flow rate which was very small. One more limitation on the use of this method at industrial plants is. very low input flow rate. Due to overcome such those problems, many studies are carried out on the application of new technologies. To overcome the problem of the elimination of CVOC emissions, many studies have been carried out on the search for new advanced technologies. Plasma-assisted technology is one of technologies are the emerging (what do you mean writing emerging? may be, originating? or promising?) and effective technologies for destroying CVOCs. Plasmaassisted technologies, such as Electron Beams [8], RF plasma [9], surface discharge reactors [10], dielectric barrier discharge reactors [11], pulsed discharge reactors [12], and discharge reactors of capillary-tube type [13] have been studied and develop developed. In this study,

gliding arc plasma was used to decompose chloroform (CHCl 3), and carbon tetrachloride (CCl4). Compared to the previous above plasma-assisted methods, gliding arc plasma has a great chance to be utilized for industrial chemical applications. to be used in chemical industry. It achieved higher flame This method allowed the attaining of higher temperature and power energy necessary to destruct toxic input materials [14]. One favor of this method was this method can handle very high input flow rate. The possibility to treat very high input flow rate is one of the advantages of this method.

2. Experiment Experimental setup Figure 1 The schematic diagram of experimental setup is shown in Figure 1. CCl4, CHCl3 and atmospheric air have been were used as a source gas. The details of each part of the system are described in the next following section. 2.1. Plasma reactor and applied power system applied 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 are supplied with a teflon seal comprised comprising two electrodes made of stainless steel. The length of the electrodes was 150 mm. The electrodes were of length 150 mm. The separation of the electrodes in the narrowest section was 1.5 mm. The gas mixture was introduced injected between these electrodes through a capillary of inner diameter 0.8 mm. A thermocouple, located 10 cm above the electrode, has been was provided to measure the outlet gas temperature. A high frequency AC power supply (Auto electric, A1831) with a maximum voltage of 10 kV and a maximum ampere current of 100 mA was connected to gliding arc electrode the electrode of gliding arc to generate plasma. Figure 2 shows typical waveforms of voltage and discharge current used obtained in this experiment. Figure 2 2.2. Input gas Chlorinated hydrocarbons that were used as the starting material are: The following chlorinated hydrocarbons were used as the initial material: a. Chloroform: CHCl3, molecular weight 119.38, purity 99.0%, purchased from Junsei

Chemical Co., Ltd., concentration 1, 3, 5, 8% v/v (is it volume percent?) b. Carbon tetrachloride: CCl4, molecular weight 153.82, purity 99.5%, purchased from Kanto Chemical Co., Inc., concentration 1, 3, 5, 8% v/v (is it volume percent?) Atmospheric air was used as the a carrier gas and controlled by a Mass Flow Controller Tylan, FC-280S. The total flow rate of mixed gas (air + chlorinated compounds) flow rate was 180, 240, and 300 Nl/hr. The composition of the outlet mixture was analysed before and after the plasma operation. 2.3. Measurement system Measuring system The composition of outlet gas composition was examined by a quadrupole mass spectroscopy spectroscope (Balzers QMS 200), with the Quadstar 421 software. This software was used for the qualitative and quantitative analysis of the to qualitatively and quantitatively analyse reactants and products. Two GCs (this abbreviation occurs for the first time, please give its full form ) have been were used to analyse the quantitative measure the amount of products. The contents of CCl4 and CHCl3 in the gas mixture before and after the reaction ware were 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. GC-TCD (YoungLin M600D, Column: SK Carbon) was used to determine if CO and CO 2 are available in outlet gas mixture. Chlorine gas (Cl2) was determined by bubbling the gas reacted through 0.05 M aqueous KI (What does 'KI' mean? Please. explain) during measuredexperiment time and followed by with subsequent iodometric titration of 0.05 M of Na2SO3. To evaluate the performance of system, selectivity and conversion were used and formulated as: The performance of reactor is described in terms of selectivity and the conversion of reaction and are formulated as follows: for the 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

for the decomposition of CHCl3:

(3) (4)

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

(8)

Conversion of CHCl 3 =

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

(9)

These parameters were used to study the effect of the initial concentration of chloromethane, 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: registered by a digital oscilloscope (Agilent 54641A) : 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. In this study all experimental data were taken 30 minutes after the start of gliding arc referred to the onset of outlet temperature of bulk gas measured by thermocouple. 3. Results and Discussion 3.1. Effect of initial concentration and total gas flow rate. Figure 3 Figure 3a shows the trend of chlorinated hydrocarbons in at various values of initial concentration and total gas flow rate. Figure 3a shows the influence of initial concentration and total gas flow rate on the degree of conversion of chlorinated hydrocarbons injected. The conversion gradually decreased when the initial concentration increased rose. The This conversion also decreased with increasing increase in total gas flow rate. The maximum conversion of CCl4 could reach 80% at the a concentration of 1% and a total air flow rate of 180 Nl/hr. At Under the same conditions, CHCl3 conversion the conversion of CHCl3 could can reach higher value (97%). Although conversion decreased, the removal efficiency has increased with increasing rise of initial concentration. It was proposed that Cl◦ initiated and gave positive effect on decomposition process. It was supposed that Cl, released during the decomposition process,

gave positive effect on decomposition process.

Table 1 Besides the conversion, different type of compounds, initial concentration, and total gas flow rate also produced different value of consuming discharge power. Besides the influence on conversion, different types of compounds, initial concentration, and total gas flow rate exert influence on discharge power consumed. The stability of compounds under plasma treatment has the plays an important role in destruction reaction. Table 1 shows that to destruct CCl4, it spent more energy compared to CHCl3. Table 1 shows that more energy is required to destruct CCl4 as compared to CHCl3. The chemical bond and structure stability of CCl4 structure was were stronger than those of CHCl3 [1]. Stronger form of compound spent more energy to initiate the arc plasma. It also produced lower value of conversion (comparison between CCl 4 and CHCl3 conversion in figure 1 Stronger stability of compound requires higher energy to initiate the arc plasma and is responsible for lower value of conversion (as can be seen from the comparison between the data for the conversion of CCl4 and CHCl3, see Figure 1 (wrong Figure number. Probably, it shall be Figures 3a and 3b. Please, correct and label the Figures a) and b)). This phenomenon has the same tendency when chlorinated hydrocarbons were treated by oxidation combustion reaction [15]. This phenomenon showed the same tendency when chlorinated hydrocarbons are treated with combustion [15]. The total gas flow rate was also has significant effect on power discharged (may be, consumed?). With increasing the total gas flow rate rise, the total power was decreased. Physical study about of this phenomenon has been established in term of Paschen’s law [16]. This phenomenon can be interpreted in the context of the Paschen’s law [16]. Based on QMS spectra result and GC analysis, the gas product was dominated by CO, CO2, and Cl2. According to the results of QMS spectra measurements and GC analysis, CO, CO 2, and Cl2 dominated the gas products. The number amount of other C-Cl compounds was relatively small. In case of CCl4, the selectivity of final product went to Cl2 gas was high the selectivity of final product, Cl2 gas, was high. It could reach from 42% up to 78.9%. Although not as high as in the case of CCl4, the product probability (may be, selectivity?) of Cl2 on at the decomposition of CHCl3 was also high, from 33% up to 50.5%. Increasing CCl4 initial concentration has decreased the product selectivity and it means the product distribution would be varied. Increase of initial concentration of CCl4 leads to reduced product selectivity, thus allowing a variation of the product distribution. This phenomenon was not occurred in case of CHCl3. The selectivity of Cl2 gas was relatively constant. This phenomenon did not occur in the case of CHCl3, when the selectivity of Cl2 gas was relatively constant.

In the case of CO and CO2 production, the selectivity has the same trend as those of Cl2, especially for in case of CCl4 decomposition. It could be proposed as kind of C vacancy due to lose Cl species (Please, clarify: what could be proposed (may be supposed - We can suppose that?), and what does it mean ' kind of C vacancy due to lose Cl species'). When CCl4 was destructed into Cl2, it will remain C radical as an intermediate species C radical remained as 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. This heightens the chance for C radicals (C vacancy) to collide with other compounds, thus raising the possibility of C radicals collision to collide with oxygen and CO or CO2 as compared with production of other kind of composition. CO and CO2 have the stable structure and the concentration of oxygen gas in the air-carrier stream was is 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). The processing of the QMS spectra of oxygen (O2) has shown that the concentration of oxygen (O2) in the output product (after treatment with plasma) is 510% lower than its concentration in inlet stream (before treatment with plasma). One possible reason was the losing oxygen was reacted and consumed into CO and CO2. One of possible reasons of this fact is the consumption of oxygen in the reaction associated with CO and CO2 formation. Comparison Ratio of produced CO2 to CO at in the final product stream was around 1/10 up to 1/5 about 1/10 - 1/5. Decomposition of CHCl3 was also produced CCl4 At the decomposition of CHCl3, CCl4 was one of the products formed. The selectivity was did not exceed more than 12%. During the experiment, another trace compounds were also detected, such as COCl 2 and HCl. traces of other compounds, such as COCl2 and HCl were also detected. Liquid product was also produced and that might be counted considered as remaining species of C and Cl. 3.2. Effect of frequency As was mentioned above, supplied the discharge power supplied to plasma system has important role on was of considerable importance in for 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.. Among three variables that have influence on the product distribution in this experiment, only power frequency could be adjusted manually. Voltage and current were automatic fixed set automatically into specific value after initial breakdown of after the start-up of gliding arc.

Figure 4 Figure 5 Figure 6 The effect of power frequency is presented shown in figure 4. The maximum power frequency was 20 kHz. As shown in can be seen from Figure 4, the conversion increased with the increasing frequency increase. For CCl4, the rate of conversion reached 4% higher per 1 kHz increment of frequency and was slightly lower for CHCl3. Increasing frequency leads to increase the supplied discharge power The increase of frequency leads to higher discharge power supplied. Figure 5 shows the increasing discharge power increase due to increasing higher 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 in frequency affect AC waveform. and results in the increase in the number of peaks per cycle and the energy supplied to the system The change of patter of power wave form due different applied power frequency in power waveform due to varied power frequency applied 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. Figure 7 shows that by varying frequency or discharge power, we obtain different production selectivity of Cl2 at the same initial concentration and flow rate. It means that power frequency or power discharge has the importance to be a factor to decide the way of reaction. This testifies to the fact that power frequency and discharge power are the parameters influencing the reaction pathway. With the increasing power frequency rising, the selectivity of Cl2 was getting increased also increased. It could be proposed that in the higher energy field, the plasma reaction tent to produce Cl2 gas as the final product. It can be supposed that at higher energy plasma reactor is likely to produce Cl2 gas as the final product.

3.3. QMS Spectroscopy 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 QMS spectra recorded under the conversion of CCl4 at the 180 Nl/hr feed stream containing atmospheric air with 1% of CCl4 are shown in Figure 8. (There are no Figs 8a) and 8b); there are Figs.8 - 'before' and 'after'!)

Figure 8 +

The baseline spectra of CCl4 were 83/85 ( CCl 2+ ), and 117 ( CCl 3 ). The basic spectra of CCl4 are +

characterized by lines 83/85 ( CCl 2+ ), and 117 ( CCl 3 )(see the upper part of Figure 8 ) (what mean the numbers (83/85; 117)? AMU? Please, clarify. Besides, take into account that in the + upper part of Fig. 8 there is no line CCl 3 ). Please, check this Figure ). CO2 has spectra lines at

44 ( CO2+ ) and CO spectra was collide with (What does it mean? May be - coincide with?) the minor N2 spectra at 29 ( CO + / N 2+ ). The major N2 spectra lines, it self, was are located at 28 ( N 2+ ). But, because As the concentration of CO in the air was quite small, CO compound could be negligible in this spectroscopy. lines of CO compound can be indistinguishable under spectroscopic measurements.. 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 Cl2 ( Cl 2+ ). After treatment with plasma the peak of CCl 2+ (83/85) vanished simultaneously with the decrease in intensity of line 117, and new peak, 71, was formed appeared , which is identified as Cl2 ( Cl 2+ ).(see Figure 8, the lower part) The intensity of CO2 line was also increased also, but the difference (between cases before and after discharge?) was very small. High increment has been occurred on spectra characterizes line 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. As N2 is relatively stable gas and there is no source of N compound, that could be responsible for N2 formation, it can be said that the rising intensity of line 28 can be attributed to the production of CO during discharge burning. Figure 9 The mass spectroscopy of CHCl3 before and after treatment with plasma reaction is presented in figure 9. Figure 9a shows the components lines of components at in the 240 Nl/hr feed stream containing compressed air mixed with 1% of CHCl3. The baseline spectra lines of CHCl3 were 82/84/86 ( CCl 2+ ). CO2 has spectra at 44 ( CO2+ ), but it was collided this spectra

+

coincided with the major spectra of N2O ( N 2 O ) that has possibility to can exist in gas products. CO line was at 29 ( CO + ), but it was also collided coincided with the minor of N2 spectra 29 ( N 2+ ). It made was difficult to distinguish them using QMS. In this experiment, CO and CO 2 species were analysed by gas chromatography. When the plasma was applied burnt, figure 9b, the peak intensity of 83/84/86 got decreased, and produced some new peaks appeared, 63/65, +

which is are known as COCl2 ( COCl ), 69/71 as Cl2 ( Cl 2+ ), 117 as CCl4 ( CCl 3+ ) (see Figure 9b).

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 to form CO. Other Another one was: COCl + M = CO + Cl + M

(14)

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

(15) (16)

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

(17)

CCl 3 + O2 = COCl 2 + ClO

(18)

It means that oxygen and oxygen radical make significantly significant contribution to held this reaction. The production of O radical came from is the result of dissociation reaction [Penetrante, et. al, 1995 - please, give the reference]: 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 obtained 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 can proceed in compliance with formula 21, as the main path to produce CCl4 is in the decomposition of CHCl3, CCl 3 + Cl = CCl 4 4. Conclusion The performance of decomposition of CCl4 and CHCl3 decomposition on in gliding arc plasma at atmospheric pressure related with have been studied at different initial concentration, total gas flow rate, and input power frequency, was studied. Gliding arc plasma could can generate effective electrons and ions effective enough to fragment decompose molecules. The maximum conversion of CCl4 was 80% and that of CHCl3 was 97% for the stream of feed input gas stream containing 1% of v/v (volume percent?) and with a total air flow rate of 180 Nl/hr. The major final products were CO, CO2, and Cl2. Cl2 The selectivity of Cl2 was relatively high and reached amounted up to 78.9% for CCl4 and 50.5% for CHCl3. The conversion (of CHCl3 ?) into CCl4 was found in the case of CHCl3 decomposition, but it did not exceed than 12%. COCl2 has plays an important role as intermediate species to produce Cl2 and CO. Mostly CO2 was produced by CO reaction, from these result which it could can be concluded that gliding arc plasma system was very useful effective for the decomposition of CCl4 and CHCl3. Development was needed to remove some traces of unwanted compounds, such as COCl2 , in the final product. Acknowledgement This study was supported by Program of the National Research Laboratory Program of the Korea Minister Ministry of Science and Technology of Korea.

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Figure captions Figure 1. Schematic diagram of experimental setup Figure 2. Voltage and current profile Figure 3. Influence of the initial concentration of injected chlorinated hydrocarbons and total gas flow rate on its conversion. (■ 180 Nl / min; ● 240 Nl/min; ▲ 300 Nl/min) Figure 4. Effect of applied the frequency of input power frequency on the conversion of 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. 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. 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 Figure 7. Effect of applied different power frequency on Cl 2 gas production selectivity. Data was taken at 1% v/v of CCl4 and 1% v/v of CHCl3. Figure 8. QMS spectrogram of CCl4 decomposition before and after treatment with gliding plasma (taken at applied frequency 20kHz, 1% of CCl4, total gas flow rate 180 Nl/h); (a) before (b) after gliding plasma on (- there is no captions (a), (b) in these Figures) Figure 9. QMS spectrogram of CHCl3 decomposition (taken at applied frequency 20kHz, 1% of CHCl3, total gas flow rate 180 Nl/h); (a) before and (b) after treatment with gliding plasma on