Ind. Eng. Chem. Res. 2001, 40, 5481-5486
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Decomposition of Carbon Tetrachloride by a Pulsed Corona Reactor Incorporated with In Situ Absorption Liwei Huang, Katsuhiko Nakajyo, Takaaki Hari, Shoji Ozawa, and Hitoki Matsuda* Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan
The decomposition of carbon tetrachloride by a wire-in-tube pulsed corona reactor was tested to investigate the influence of the reaction agents H2 and O2 on the plasma decomposition of carbon tetrachloride, as well as on the control of unwanted products. It was found that the decomposition of carbon tetrachloride was higher with 2% H2 in N2 gas and lower with 2% O2 in N2 gas, compared to the decomposition in N2 atmosphere. Cl2 and ClCN were regarded as the major products from CCl4 decomposition in N2 atmosphere. HCl was the major FTIR-detected product from CCl4 decomposition in a 2% H2/N2 gas mixture, whereas the products CO2, CO, and COCl2 were detected when carbon tetrachloride was decomposed in a 2% O2/N2 gas mixture. To prevent the production of unwanted byproducts such as ClCN and COCl2 from CCl4 decomposition, a combination of nonthermal plasma and in situ absorption by coating a layer of Ca(OH)2 on the surface of the grounding electrode was proposed. It was demonstrated that the Ca(OH)2 sorbent in the plasma reactor played an effective role as a scavenger participating in the CCl4 decomposition reaction by in situ capturing of the unwanted intermediates. Introduction Nonthermal plasmas are regarded as an effective means of removing multifarious air pollutants contained in gas streams.1,2 A variety of nonthermal plasma techniques have been employed for the treatment of volatile organic compound (VOC) species such as carbon tetrachloride (electron beam,3-5 pulsed corona,4 dielectric packed bed,6 and hybrid plasma-catalyst7 methods), dichloromethane (dielectric packed bed and pulsed corona8-10), formaldehyde (dielectric barrier11), trichloroethylene (silence discharge12,13), and trichlorotrifluoroethylene (dielectric packed bed14). The decompositions of VOCs by nonthermal plasmas undergo the steps of initiation, propagation, and termination of the concerned reactions. The dissociation and/or excitation of the targeted VOC molecules by direct electron impacts and the production of active reaction radicals and ions by electron dissociation and ionization of background gas molecules represent the initial step. The subsequent chain reactions between the produced radicals and VOC molecules propagate until stable products are formed. Because, in a nonthermal plasma, the electrical energy injected into gas stream goes mainly into the production of a reaction seed of energetic electrons, rather than into gas heating, the system can be kept at room temperature. This means that the nonthermal plasma technique for VOC decomposition is remarkably more energy-efficient than traditional thermal decomposition or catalytic oxidation processes. However, a problem associated with this novel technology is that unexpected byproducts and some toxic compounds are inevitably formed from the decomposition of some VOCs, especially for halogenated VOCs, as reported in the above-cited studies.4,10,13,14 In other words, the plasmainduced chemical reaction seems inherently difficult to control in terms of the generation of unfavorable prod* Corresponding author. Phone: +81-52-7893382. Fax: +8152-7895619. E-mail:
[email protected].
ucts. Therefore, it is essential to prevent the production of secondary pollutants for this technology to be used in practical applications. In this study, we investigated the decomposition of carbon tetrachloride (CCl4) by a pulsed corona reactor, as a successive study to work on pulsed corona plasma for dichloromethane decomposition.15 In comparison to previous studies on carbon tetrachloride decomposition, we investigated the effect of H2 and O2 on the decomposition behaviors of carbon tetrachloride. With the aim of preventing the production of harmful byproducts produced from the plasma decomposition of CCl4, a technique of plasma decomposition combined with in situ gas absorption by an alkaline sorbent was devised to capture the undesirable species from the plasma decomposition reaction field. Experimental Arrangement Plasma reactors can be designed in different configurations depending on electrical power supplies.16 In this study, a typical wire-tube combination corona reactor was adopted for the experiment, as shown in Figure 1. Such reactors have larger discharge volumes and lower pressure drops than packed-bed-type plasma reactors and allow for easy loading of gas sorbents. The reactor consists of a Pyrex glass tube with an aluminum film attached to the inner wall as the grounding electrode and a coaxial stainless steel wire of 0.5 mm diameter as the corona wire. The inner diameter of the tube is 28 mm. The aluminum film is 0.3 mm thick and 500 mm long. For the corona reactor combined with in situ absorption, a Ca(OH)2 layer of about 1-mm thickness was coated on the grounding electrode, the surface of the aluminum film. To make the Ca(OH)2-coated electrode, a Ca(OH)2 slurry formed by mixing 60% water and 40% Ca(OH)2 power (reagent) was evenly painted onto the aluminum film of the grounding electrode. After the Ca(OH)2 layer was naturally dried in air for about 1 h, the whole reactor was placed into a stove for heating at 110 °C in air for 3 h to form the electrode.
10.1021/ie010172k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001
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Figure 1. Schematic drawing of the pulsed corona reactor.
Figure 3. Decomposition of CCl4 under different gas compositions.
standard gases. Spectra for species such as COCl2 were identified by referring to standard compilations.17 In this study, only the CCl4 concentration was qualitatively analyzed. The calibration of the FTIR instrument was performed by a series of certified concentrations of CCl4 between 50 and 300 ppm for measurements of the CCl4 concentration in the sample gas. The accuracy of the measured concentration can be estimated to be within 5% of the true value. To ensure reproducibility, the concentration of CCl4 under certain operation conditions was measured several times until the difference between the nearest three times was within 1%. Figure 2. Schematic diagram of the pulse-forming circuit.
A high pulsed voltage (HPV) was applied to the wire electrode. A typical pulse-forming circuit using a rotating spark gap system was adopted to produce the pulsed voltage; the circuit schematic is shown in Figure 2. It consists of two parts, a high dc-forming circuit and a pulse-forming circuit. A capacitor C was charged by a high dc supply through resistor R. When the rotating spark gap (RSG) reached the point of conduction, the energy stored in the capacitor was injected into the reactor. The width of the voltage pulse was determined to be less than 300 ns, and the rising width was within 50 ns. The pulse repetition rate was set at 50 pulses/s. The effects of the Ca(OH)2 layer in the reactor on the discharge characteristics were also measured. We did not find remarkable changes in the current or voltage waveform with the coating, except that the onset discharge voltage increased from 20 to 22 kV. The prepared gas samples were fed into the reactor at a fixed flow rate of 500 mL/min in all experiments, and the CCl4 concentration in all employed sample gas mixtures was adjusted to 200 ppm. The gas samples were analyzed before and after the reactor using an online FTIR spectrometer (Shimadzu, FTIR-8700) with a gas cell of 10-cm path length. The decomposition of CCl4 is defined as
η)
Cin - Cout × 100% Cin
where Cin and Cout are the inlet and outlet concentrations of CCl4, respectively. Identifications of the spectra of CCl4, CO2, and CO in FTIR were performed by comparison with those of
Results and Discussion Decomposition of Carbon Tetrachloride in the Reactor without Absorbent. As the first step of the experiment, we examined the decomposition of 200 ppm of carbon tetrachloride in the three carrier gases of pure N2, 2% H2 in N2, and 2% O2 in N2 using the pulsed corona reactor containing the aluminum film of the grounding electrode that was not coated with Ca(OH)2 sorbent. The gas flow rate was 500 mL/min, corresponding to a gas residence time of about 37 s in the reactor. The result of the decomposition is shown in Figure 3 as a function of the pulse peak voltage imposed on the reactor. It was found that the decomposition of CCl4 increased with increasing pulse voltage from 18 to 22 kV (corresponding to power consumptions of about 300900 J/L) in all carrier gases. However, differences in decomposition efficiency were observed. The decomposition of CCl4 increased when 2% H2 was added to N2 gas and decreased when 2% O2 was added to N2 gas compared with the decomposition in pure N2 gas, showing that the additive gases H2 and O2 have different influences on CCl4 decomposition behavior. The analysis of the decomposition products was implemented by an on-line FTIR scan with a pulse peak voltage of 20 kV (corresponding to a power consumption of about 620 J/L). The results are shown in Figure 4, where Figure 4a is the FTIR pattern of CCl4 before electrical discharge. Parts b-d of Figure 4 are the FTIR patterns of the decomposition products in the carrier gases of pure N2, 2% H2 in N2, and 2% O2 in N2, respectively. In N2 atmosphere, CCl4 decomposition seemed to proceed with some products such as Cl2 that could not be detected by FTIR, as suggested by Penetrante et al.4 Only a product peak near that of CCl4 was
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Figure 4. FTIR pattern of the CCl4 decomposition products: (a) CCl4, 200 ppm, no discharge; (b) CCl4 decomposition in pure N2 gas; (c) CCl4 decomposition in a 2% H2/N2 gas mixture; and (d) CCl4 decomposition in a 2% O2/N2 gas mixture.
detected, as indicated by the arrow in Figure 4b, which was thought to be an intermediate product from CCl4 decomposition. In the gas mixture of 2% H2 and 98% N2, the major detectable product of CCl4 decomposition was HCl, as shown in Figure 4c, whereas CO2, CO, and COCl2 were the identified products from CCl4 decomposition in the gas mixture of 2% O2 and 98% N2, as shown in Figure 4d. An unconfirmed product was also detected from CCl4 decomposition in the 2% O2/N2 gas mixture, as indicated by the arrow in Figure 4d. Decomposition Mechanism. The decomposition of CCl4 by nonthermal plasma treatment is known to be mainly initialized by a dissociative electron attachment reaction18
e- + CCl4 f CCl3 + Cl-
(1)
In N2 gas, the CCl3 thus produced is scavenged by reaction with N radical19
CCl3 + N f ClCN + 2Cl
(2)
Therefore, the detectable product, as indicated by the arrow in Figure 4b, is identified as the product ClCN. The production of ClCN was also detected by Tonkyn et al.5 using a dielectric barrier/packed-bed plasma reactor for CCl4 decomposition in pure nitrogen. The Cl radicals produced recombine to form Cl2 by reaction 3, which might be the principal product from the decomposition of CCl4 in N2 gas.
Cl + Cl + M f Cl2 + M
(3)
With 2% O2 in N2 gas, the CCl3 radical is most readily removed by reaction with O2 or O radical3,4,7
CCl3 + O2 f CCl3O2
(4)
CCl3 + O f COCl2 +Cl
(5)
In addition to the above two reactions, the direct reaction between CCl4 and O radical might make an additional contribution to the decomposition of CCl4
CCl4 + O f ClO + CCl3
(6)
ClO + O f Cl + O2
(7)
The intermediate product CCl3O2 might appear as the unconfirmed peak in Figure 4d, which then proceeds through a series of radical reactions to form the final products CO and CO2. Although reaction 4 is faster than reaction 2, the experimental results indicate that the decomposition of CCl4 in pure N2 gas is higher than that in the 2% O2/ N2 gas mixture. This observation is attributed to the electronegative property of O2, which could result in an increase of the minimum reduced electric field (E/N) required to generate the plasma in the reactor. Therefore, direct dissociation of CCl4 by electron attachment, the initiation step of the entire reaction, might be inhibited to some degree by the lack of enough energetic electrons. Penetrante et al.4 explained this phenomenon by the reason that the effective electron mean energy of the discharge plasma in N2 is higher than that in air. Falkenstein19 also investigated the influence of the O2 concentration on VOC removal and suggested that the production of O3 by the scavenging of atomic oxygen might decrease the removal efficiency of VOCs. With 2% H2 in N2 gas, the decomposition of CCl4 might proceed according to reactions 1 and 8-14.12,21,22
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Cl + H2 f HCl + H
(8)
H + CCl4 f HCl + CCl3
(9)
CCl3 + H2 f CHCl2 + HCl
(10)
CHCl2 + H2 f CH2Cl + HCl
(11)
CH2Cl + H2 f CH3 + HCl
(12)
CH3 + H2 f CH4 + H
(13)
CH3 + CH3 f C2H6
(14)
As described in a previous paragraph, a large amount of HCl was detected by FTIR scan from CCl4 decomposition in the 2% H2/N2 gas mixture, which means that most of the CCl4 was dechlorinated to form HCl. The missing carbon might exist in the form of hydrocarbons such as CH4 after reaction. The higher CCl4 decomposition in the H2/N2 gas mixture than in the O2/N2 gas mixture can be explained as follows: (1) Hydrogen is not an electronegative gas, which means that it did not inhibit the production of energetic electrons in the gas stream. (2) Both excited H2 molecules and H atoms actively participated in CCl4 decomposition reactions, where the final stable product of HCl is produced in each reaction, thus avoiding reverse reactions that might occur in other gases. Effect of Reaction Temperature. In a nonthermal plasma, only the light particles of electrons can be accelerated to the temperature of several tens of thousands of Kelvin, but the heavy molecules of the main stream are maintained at ambient temperature. The electron-impact dissociation and ionization of the background gas molecules depend on the energy of the accelerated electrons rather than on the thermal temperature of the gas stream. However, the reactions between the radicals produced and the VOC molecules are gas-temperature-dependent.12 Therefore, raising the temperature might influence the radical reaction rates. A few previous papers have investigated the influence of temperature on the nonthermal decomposition of CCl4. Penetrante et al.4 investigated the decomposition of CCl4 in dry air at temperatures of 25, 120, and 300 °C and found no significant changes in the CCl4 decomposition efficiency. In our experiments, we found that the decomposition of CCl4 increased significantly when the reaction temperature was raised to 100 °C for the three kinds of gas compositions investigated, as shown in Figure 5. In particular, an increase in CCl4 decomposition from 73 to 99% in 2% H2/N2 was found when the reaction temperature increased from room temperature to 100 °C. As a result, an increase in HCl production was also detected with increasing reaction temperature from room temperature to 100 °C, as shown in Figure 6. It was indicated that the influence of temperature on the decomposition of CCl4 is largely dependent on the gas composition. Compared with the experiment by Penetrante et al. using an input energy density with a maximum of 200 J/L, a much higher power load of 620 J/L was used in our experiment, which might also lead to a higher CCl4 decomposition. Corona Reactor Combined with In Situ Absorption. To implement the idea of preventing the production of unwanted species by removing the reaction intermediates such as dissociated chlorine from the CCl4
Figure 5. Decomposition of CCl4 at a temperature of 100 °C.
Figure 6. HCl yield from CCl4 decomposition in a 2% H2/N2 gas mixture.
Figure 7. CCl4 decomposition in the corona reactor combined with in situ absorption.
decomposition process, thereby promoting the decomposition reaction in the right direction, a technique of nonthermal plasma combined with in situ absorption was devised. We employed Ca(OH)2 as the sorbent, which was introduced into corona reactor as described in the Experimental Arrangement section. Figure 7 shows the experimental results obtained from the reactor combined with in situ absorption. In comparison with the results shown in Figure 3, the decomposition of CCl4 is lower for the new reactor combined with Ca(OH)2 sorbent, when the pulse voltage applied to the
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Figure 9. Comparison of CCl4 decomposition by the reactor with in situ absorption and with downstream absorption.
Figure 8. FTIR pattern of the products from the corona reactor combined with in situ Ca(OH)2 absorption: (a) in N2 gas, (b) in a 2% H2/N2 gas mixture, and (c) in a 2% O2/N2 gas mixture.
reactor is 20 kV. This observation indicates that the layer of Ca(OH)2 coated on the electrode functioned as a dielectric barrier between the two electrodes, causing unstable corona discharge in the reactor. As a matter of fact, the rotating spark gap was difficult to conduct under these conditions. When the pulse voltage was increased to 22 kV, a stable corona discharge was formed. Under these stable corona discharge conditions, CCl4 decomposition increased dramatically. The decomposition products from this new reactor were also analyzed by an on-line FTIR scan. The results are presented in Figure 8, where Figure 8a-c shows the FTIR patterns of the decompositions in N2, 2% H2 in N2, and 2% O2 in N2, respectively. From a comparison of Figures 8 and 4, it was found that the decomposition products such as ClCN in N2, HCl in the 2% H2/N2 gas mixture, and COCl2 and CCl3O2 in the 2% O2/N2 gas mixture disappeared in the FTIR patterns with the new reactor combined with Ca(OH)2 absorption. It was therefore considered that the Ca(OH)2 coated on the electrode played an important role not only in the removal of unfavorable byproducts but also in promoting the CCl4 decomposition reaction. To confirm the evi-
dence of Ca(OH)2 participation in CCl4 decomposition, we used an ion chromatograph to analyze the water solution of the surface layer after the decomposition experiment. The presence of Cl- ion in the water solution was confirmed. Therefore, it was ascertained that the chlorine derived from CCl4 decomposition was captured by the surface layer, which might exist in the form of CaCl2. It should be pointed out that the actual reactions between the gas and the surface layer might be more complex than we expected. For example, the removal of the product of HCl by the surface layer of Ca(OH)2 is thought to form CaCl2, but the removal of ClCN seems rather complex. There are two possibilities. One is that the capture of Cl- ion from the gas phase by the surface layer of Ca(OH)2 prevents the production of ClCN. The other one is that ClCN is also captured by the surface layer and is then decomposed further to form the products such as CaCl2, CO, and NO. To further support our argument on the role of the surface layer in CCl4 decomposition, a comparison test in which a Ca(OH)2-coated tube of the same size as the corona reactor but without the Ca(OH)2 coating was placed downstream of the corona reactor was also performed. We found that, with such a combination, although the unwanted byproducts such as COCl2 were also removed, the CCl4 decomposition was much lower, as shown in Figure 9. The gas composition was 200 ppm in the O2/N2 gas mixture with a variable concentration of oxygen. The pulse voltage applied to the corona reactor was 22 kV. In the figure, the upper line is the decomposition by the reactor with in situ absorption, and the line below is the decomposition by the reactor combined with downstream absorption. This result also demonstrated that the decomposition of CCl4 was promoted by the in situ capture of the chlorine-derived reaction intermediates such as Cl-, ClCN, and COCl2 produced by plasma dissociation of the CCl4 molecule. Conclusions We have experimentally investigated the decomposition of carbon tetrachloride in nitrogen gas with additives of hydrogen and oxygen using a tubular pulsed corona reactor. It was found that the decomposition of carbon tetrachloride is higher in N2 gas with 2% H2 and lower with 2% O2 compared with the decomposition in a N2 gas atmosphere. The difference is ascribed to the different electrical properties of H2 and O2 in terms of electrical discharge and their activities in the CCl4
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degradation reaction process to form stable products. The main products of CCl4 decomposition are Cl2 and ClCN in N2 gas; HCl and hydrocarbons such as CH4 in the 2% H2/N2 gas mixture; and CO2, CO, COCl2, and probably CCl3O2 in the 2% O2/N2 gas mixture. When CCl4 decomposition was implemented with the corona reactor combined with in situ absorption, the production of unwanted species such as ClCN and COCl2 was suppressed. It was demonstrated that the Ca(OH)2 sorbent in the reactor played an important role as a scavenger participating in the CCl4 decomposition reaction through the in situ capture of unwanted intermediates. The influence of reaction temperature on the decomposition was also investigated, showing that CCl4 decomposition increases with increasing reaction temperature and that almost 100% of 200 ppm of CCl4 in a 2% H2/N2 gas mixture flowing at a rate of 500 mL/min was decomposed at 100 °C with a power consumption of 620 J/L. Literature Cited (1) Penetrante, B. M., Schultheis, S. E., Eds. Nonthermal Plasma Techniques for Pollution Control. Part B. Electron Beam and Electrical Discharge Processing; Springer: Berlin, 1993. (2) Vercammen, K. L. L.; Berezin, A. A.; Lox, F.; Chang, J. S. Non-Thermal Plasma Techniques for the Removal of Volatile Organic Compounds in Air Streams: A Critical Review, J. Adv. Oxid. Technol. 1997, 2, 312-329. (3) Koch, M.; Cohn, D. R.; Patrick, R. M.; Schuetze, M. P.; Bromberg, L.; Reilly, D.; Hadidi, K.; Thomas, P.; Falkos, P. Electron Beam Atmospheric Pressure Cold Plasma Decomposition of Carbon Tetrachloride and Trichloroethylene. Environ. Sci. Technol. 1995, 29, 2946-2952. (4) Penetrante, B. M.; Hsiao, M. C.; Bardsley, J. N.; Merritt, B. T.; Vogtlin, G. E.; Wallman, P. H.; Kuthi, A.; Burkhart, C. P.; Sayless, J. R. Electron Beam and Pulsed Corona Processing of Carbon Tetrachloride in Atmospheric Pressure Gas Streams. Phys. Lett. A 1995, 209, 69-77. (5) Nichipor H.; Dashouk E.; Chmielewski, A.G.; Zimek, Z.; Bulka, S. A Theoretical Study on Decomposition of Carbon Tetrachloride, Trchloroethylene and ethyl chloride in dry air under the influence of an electron beam. Radiat. Phys. Chem. 2000, 57, 519-525. (6) Tonkyn, R. G.; Barlow, S. E.; Orlando, T. M. Destruction of Carbon Tetrachloride in a Dielectric Barrier/ Packed-Bed Corona Reactor. J. Appl. Phys. 1996, 80, 4877-4886. (7) Yamamoto, T.; Mizuno, K.; Tamori, I.; Ogata, A.; Nifuku, M.; Michalska, M.; Prieto, G.; Catalysis-Assisted Plasma Technology for Carbon Tetrachloride Destruction. IEEE Trans. Ind. Appl. 1996, 32, 100-104.
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Received for review February 21, 2001 Revised manuscript received June 26, 2001 Accepted August 30, 2001 IE010172K
