Removal of the Chlorofluorocarbon 1, 1, 2-Trichloro-1, 2, 2

Diesel particulate matter and NOx removals using a pulsed corona surface discharge. S. Yao , M. Okumoto , T. Yashima , J. Shimogami , K. Madokoro , E...
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Ind. Eng. Chem. Res. 1998, 37, 1428-1434

Removal of the Chlorofluorocarbon 1,1,2-Trichloro-1,2,2-Trifluoroethane in Gas by a Corona-Discharge Reactor Noriaki Sano,* Hajime Tamon, and Morio Okazaki Department of Chemical Engineering, Kyoto University, Kyoto 606-01, Japan

Two types of corona-discharge reactors, a deposition-type reactor in which negative ions deposit at the anode and a wetted-wall reactor in which negative ions are absorbed into a liquid film on the anode, are applied to removals of C2F3Cl3. By the deposition-type reactor, the removal efficiency from N2 increases with the decrease of the inlet concentration, suggesting that this reactor can be applied to remove extremely low concentrations of C2F3Cl3. When the C2F3Cl3 concentration is high, there is an optimum concentration of coexisting H2O to give the maximum removal efficiency. When O2 is mixed, the removal efficiency decreases. The removal mechanism is discussed on the basis of dissociative electron attachment, radical reaction, and particle formation. When the C2F3Cl3 concentration is high, the wetted-wall reactor shows a higher removal efficiency than the deposition-type reactor. On the other hand, when the C2F3Cl3 concentration is low, the result is opposite. Introduction Ultrahigh gas purification is necessary in various fields: (1) removal of indoor air pollutants, (2) purification of gas used for semiconductor industries, and (3) removal and decomposition of toxic gases exhausted from nuclear fuel recycling, incineration, and other chemical processes. Among them, many kinds of halogen compounds such as dioxins from incineration, radioactive iodine compounds, and chlorofluorocarbons (CFCs) are required to be removed to keep the environment. There are many kinds of gas removal methods widely researched and developed (Kohl et al., 1979). Since their removal efficiencies in conventional gas purification such as adsorption and absorption are limited by equilibrium, it is difficult to achieve an extremely high removal efficiency of gas impurities. Under such a situation, it is important to propose and develop new gas purification methods. Recently, gas discharge has been applied to decompose gas components as a new gas purification method. A plasma induced by a pulsed corona or a surface corona (Masuda et al., 1987 and 1991; Clements et al., 1989; Kato et al., 1995) and an electron beam (Kawamura, 1989) have been used. Those methods use high-energy electrons to produce oxidizing radicals and reducing radicals from the wet air or the air containing ammonia. Air pollutants react with the active radicals to be converted to solid particles, liquids, and some gases which are easily removed from gas stream. On the other hand, low-energy electrons also must be useful for gas purification. We have proposed a gas purification principle using low-energy electrons based on the extremely high selectivity of electron attachment (Tamon et al., 1989 * Correspondence concerning this article should be addressed to N. Sano, Department of Chemical Engineering, Himeji Institute of Technology, 2167 ShoSha, Himeji 671-22, Japan. Telephone: 81-792-67-4845. Fax: 81-792-67-4830. E-mail: [email protected].

and 1995). By the electron attachment reaction, a lowenergy electron is captured by a gas molecule, producing a negative ion. The reactivity of electron attachment depends on electron energy, the structure of the gas molecule, and its electron affinity (Caledonia, 1975; Massey, 1976; Christophorou, 1996). To use the selectivity of electron attachment for gas purification, corona discharge reactors were used (Tamon et al., 1995 and 1996; Sano et al., 1996 and 1997a-c). These reactors consist of a wire cathode and a cylindrical anode. Electrons drifting from the cathode to the anode collide with gas molecules, and the gas molecules of the high reactivity of electron attachment are converted to negative ions. The negative ions also drift to the anode, and the removal is completed when the negative ion species are removed at the anode surface. If the negative ion species deposit at the anode surface, the removal is completed by their depositing there. We call this reactor a “deposition-type reactor” (Tamon et al., 1995). However, some negative ion species may not deposit at the anode surface. In this case, making a liquid film on the anode surface is effective to absorb the negative ion species there. We call this reactor a “wetted-wall reactor” (Sano et al., 1996). Our previous articles have shown that this method can be applied to the removal of iodine compounds (Sano et al., 1996 and 1997a,b), sulfur compounds (Tamon et al., 1995; Tamon et al., 1996), and some organic compounds: skatole and acetaldehyde (Sano et al., 1997c). From that research, it was found that the removal efficiency depends not only on the reactivity of the electron attachment but also on other effects such as the influences of reaction byproducts, reactivity with O3, and formations of negative ion clusters. Since those additional effects depend on the property of each removed species, it is necessary to obtain engineering data in the removal of different kinds of gas species by the presented reactors. It can be considered that the halogen compounds can react with electrons to produce negative ions by dissociative electron attachment because the electron af-

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finities of the halogen atoms constituting the compounds are quite high. 1,1,2-Trichloro-1,2,2-trifluoroethane (C2F3Cl3) had been commonly used as a solvent in semiconductor industries. Currently, it is an important problem to dispose of the used C2F3Cl3. However, if C2F3Cl3 is released to ambient air without being decomposed, C2F3Cl3 can reach the stratosphere to cause destruction of the ozone layer (Molina and Rowland, 1974). Then it is necessary to decompose C2F3Cl3 or convert it to other compounds. In this study, two kinds of corona-discharge reactors, deposition-type and wetted wall reactors, are used to remove C2F3Cl3 from N2. Also the influence of coexisting O2 and H2O is studied in the removal by the deposition-type reactor.

Figure 1. Removal of C2F3Cl3 from N2 (SV ) 18.9 h-1).

Results and Discussion Experimental Section The literature schematically described the experimental apparatus with the deposition-type reactor (Tamon et al., 1995 and 1996; Sano et al., 1996) and that with the wetted wall reactor (Sano et al., 1996). In both reactors, a brass pipe was used as the anode. The inner diameter and the length of the anode were 38 mm and 280 mm, respectively. A wire cathode was stretched in the center of the cylindrical anode. The diameter of the cathode was 0.3 mm. The cathode was connected to a direct power supply (Nichicon Co., DCG-50k2T), and the anode was grounded. In the wetted wall reactor, if distilled water is circulated to make the liquid film, as in the previous study (Sano et al., 1996), the concentration of Cl2 in the gas flow becomes high by stripping, since Cl compounds of reaction byproducts from dissociation of C2F3Cl3 are dissolved in the liquid film. From this consideration, tap water was continuously supplied instead of circulating distilled water. The reactor was at atmospheric pressure. The dc voltage applied on the cathode to generate the discharge current was -10 to approximately -15 kV. The electron energy was higher near the cathode than around the anode because of the electric field profile inside the reactor. The concentration of C2F3Cl3 was adjusted by mixing a commercial standard gas with N2. The concentration of H2O was adjusted by bubbling N2 through distilled water in a temperature-controlled bath. The concentration of O2 is adjusted by mixing pure O2 with the inlet gas of the reactor. To measure the concentration of C2F3Cl3 in the gas flow, a gas chromatograph (Shimadzu Corporation, GC14B) with a flame ionization detector (FID) was used. To analyze halogen compounds of reaction byproducts, F2, Cl2, HF, and HCl, the outlet gas was introduced to an absorber which contained a solution of 0.1 N NaOH mixed with 1 M H2O2. F2, Cl2, HF, and HCl were expected to become F- and Cl- immediately after they were absorbed in the solution. Ion electrodes (electrode 8002-06T and 8010, HORIBA Ltd.; ion meter N-8F, HORIBA Ltd.) were used to analyze F- and Cl- in the solution to calculate the concentration of F and Cl in the gas. The concentration of H2O was measured with a dew point hygrometer (Yokogawa Electric Corporation, model 2586). When there were reaction byproducts detected with the gas chromatograph, a gas chromatograph mass spectrometer (GCMS) (Shimadzu Corporation, MS-QP1000S) was used to identify the reaction byproducts.

Removal of C2F3Cl3 from N2. Figure 1 shows the removal results of C2F3Cl3 from N2. Since the prevention of releasing C2F3Cl3 is important, the removal efficiency, Ψ, is defined by eq 1 even if reaction byproducts of halogen compounds are produced.

Ψ ) 1 - Cout/Cin

(1)

Cout and Cin are respectively the outlet and inlet concentrations of C2F3Cl3. To observe the influence of the inlet concentration of C2F3Cl3, Cinwas adjusted to 50, 100, and 400 ppm. One can see in this figure that Ψ increases when Cin decreases. This result indicates that C2F3Cl3 of extremely low concentration can be removed from N2 by this reactor. The electric power consumed, for instance, for the removal of 50 ppm C2F3Cl3 at 1.5 mA to achieve Ψ ≈ 1 is 13.4 W by using the present experiment. However, it must be noted that the power consumption depends on the structure of the reactor so that the power efficiency for removal can be improved by changing the reactor structure (Tanthapanichakoon et al., 1998). Analysis of Reaction Byproducts from Deposition-Type Reactor. Several reaction byproducts were observed in the outlet gas in the removal of C2F3Cl3 with a FID gas chromatograph. The amounts of reaction byproducts were negligibly small except for that of one byproduct. If the concentration of the byproduct is assumed to be proportional to the FID peak area with the same proportional coefficient as that of C2F3Cl3, the amount of byproduct was about 5-10% of the amount of C2F3Cl3 removed. A GCMS was used to identify this reaction byproduct. As a result, the reaction byproduct was identified as C2HF3Cl2 produced by the reaction with the H2O impurity in the reactor. Except the organic reaction byproduct, it is possible that halogens such as F2 and Cl2 are formed from Fand Cl- produced by the dissociative electron attachment to C2F3Cl3 because those ions release electrons at the anode surface. Also Cl-, F-, and H2O impurities in the reactor may produce HF and HCl. To measure the concentrations of those halogen compounds, the outlet gas was introduced to an absorber which contained a solution of 0.1 N NaOH mixed with 1 M H2O2. F2, Cl2, HF, and HCl were supposed to become F- and Climmediately after they were absorbed in the solution. The ion electrodes were used to measure the concentrations of F- and Cl- in the solution. As a result, 0.81.5 mol each of F and Cl was produced from 1 mol of C2F3Cl3 removed from N2 when Ψ was clearly below 1.0.

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Figure 2. Weight change of deposition on the anode surface in the removal of C2F3Cl3 from N2 (Cin ) 100 ppm, Q ) 1.83 × 10-6 m3‚s-1, I ) 0.2 mA, Ψ ) 0.61, parallel plate reactor).

It was found that if the discharge current increased even after Ψ was almost 1.0, Cl was removed by the excess discharge current. Analysis of Deposit on Anode of DepositionType Reactor. The deposition layer which was formed at the anode surface during the removal experiment of C2F3Cl3 from N2 was analyzed using the plate-anode reactor which had two 50 mm × 200 mm plate anodes parallel to each other at a 50-mm interval and a wire cathode of 0.3-mm diameter in the center of those anodes. A gas containing 100 ppm C2F3Cl3 balanced with N2 was supplied to the reactor with 1.83 × 10-6 m3‚s-1, and the deposit on the anode was obtained at the discharge current 0.2 mA. Ψ was about 0.61 during this removal. After 22 h of the removal, the anodes were detached, and the weights of these anodes were measured. After this, the reactor was set with the same anodes and the removal was restarted until the total removal time reached 48 h. Figure 2 shows the change of the deposit weight, G, at the anode with time. In fact, the weight of the deposit increases by 10-20% when the anode was taken out to room air because the deposit may absorb water vapor in air. Then one can think that the weight of the deposit measured can be overestimated by 10-20%. In the same figure, the weight of the deposit estimated by eq 2 is also shown. The estimated value is based on the analytical results that 1 mol of Cl and 1 mol of F are contained in the outlet gas if 1 mol of C2F3Cl3 is removed. The estimated value was obtained with an assumption that 5 or 10% of the removed C2F3Cl3 was converted to an organic reaction byproduct, C2HF3Cl2.

G ) QCinΨ(p/RT){MC2F3Cl3 - MF - MCl MC2HF3Cl2}t (2) where Q is the gas flow, M is the molecular weight,  is the conversion of the removed C2F3Cl3 to C2HF3Cl2 ( ) 0.05 or 0.10), and t is the removal time. Though the measured G is overestimated by 10-20% because of the water vapor absorbed, as described above, the value measured is larger than the ones estimated. This may be from the assumption that the FID signal is proportional to the concentration of C2HF3Cl2 with the same proportional coefficient as that of C2F3Cl3 in the C2HF3Cl2 measurement. Therefore, it can be concluded that G can be roughly estimated by eq 2. This consideration indicates that the components of C2F3Cl3 removed by the dissociative electron attachment deposit at the anode surface.

Figure 3. Apparatus for removal of C2F3Cl3 from a F, Cl compounds-N2 mixture.

The deposit formed at the anode in the removal of C2F3Cl3 from N2 was a layer of black particles. When the anode was detached to room air, the deposition layer changed into a transparent liquid, increasing its weight. The deposit was dissolved into distilled water, and the solution was analyzed by the FID and a TOC meter (Shimadzu Corporation, TOC-5000). It was suggested that the deposit contained organic compounds even though it was only a qualitative analysis. Influence of Reaction Byproducts on Removal Efficiency of Deposition-Type Reactor. Since halogen compounds with Cl and F are produced as reaction byproducts, it is necessary to study the influence of those halogen compounds on the removal efficiency because those halogens may have a high probability for electron attachment. The influence of the halogen compounds on the removal efficiency should depend on the selectivity of C2F3Cl3 for halogen compounds for electron attachment. For this study, an experimental apparatus using two reactors was prepared as shown in Figure 3. Reactor #1 was used to produce reaction byproducts. A gas containing 400 ppm C2F3Cl3 balanced with N2 was supplied to reactor #1 with 1.67 × 10-6 m3‚s-1, and the concentration of C2F3Cl3 at the gas outlet of reactor #1 became 50 ppm by the discharge current 2.9 mA. The outlet gas of reactor #1 contained 210 ppm F and 525 ppm Cl. This gas was supplied to reactor #2. When the discharge current was generated in reactor #2, one could observe the removal efficiency of C2F3Cl3 from the F, Cl compounds-N2 mixture. For comparison, the removal efficiency of 50 ppm C2F3Cl3 from N2 was also measured at the same gas flow rate. Those results are shown in Figure 4. In this figure, it is shown that Ψ in the presence of F, Cl compounds is lower than that from N2. It seems that the reaction byproducts of F, Cl compounds react with electrons and that the probability of electron attachment to C2F3Cl3 molecules decreases, resulting in the decrease of Ψ. The rate constants for electron attachment to F2 and Cl2 are reported by Ayala et al. (1981). They reported that the rate constants of F2 and Cl2 for dissociative electron attachment to

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Figure 4. Removal of C2F3Cl3 from a F, Cl compounds-N2 mixture (Cin ) 50 ppm, Q )1.67 × 10-6 m3‚s-1).

produce Cl- and F- at 300 K are respectively 1.87 × 109 and 6.62 × 108 m3‚mol-1‚s-1. Compared with the rate constants of sulfur compounds determined by the deposition-type reactor (Tamon et al., 1995), those values can be considered as large enough to capture electrons inside the reactor. Removal Mechanism of C2F3Cl3 from N2 by Deposition-Type Reactor. To evaluate the removal efficiency, it is convenient to define electron efficiency, the mean number of gas molecules removed per one electron, ne, by eq 3.

ne ) Nr/Ne0

(3)

where Nr is the number of gas molecules removed per unit time and Ne0 is the number of electrons produced by the corona discharge per unit time. By eq 3, it is found that about five molecules of C2F3Cl3 are removed by one electron at 0.1 mA in the removal of 400 ppm C2F3Cl3 from N2. To explain such a high removal efficiency, one may consider formations of negative ion clusters, F-(C2F3Cl3)m or Cl- (C2F3Cl3)n because one electron can contribute to remove several molecules if such ion clusters are formed and removed (Tamon et al., 1996; Sano et al., 1997a,c). However, admixing F and Cl to the inlet gas of the reactor decreases the removal efficiency of C2F3Cl3, as shown in Figure 4. Hence, the formation of such negative ion clusters does not contribute to the removal because the mixed F and Cl increase the concentrations of F- and Cl- by their dissociative electron attachment. On the other hand, one can consider that radical reactions are included in the removal mechanism. It is thought that electron attachment to C2F3Cl3 produces F- and Cl-, as expressed by eqs 4 and 5.

C2F3Cl3 + e- f C2F3Cl2 + Cl-

(4)

C2F3Cl3 + e- f C2F2Cl3 + F-

(5)

One can consider a hypothesis that the radicals (C2F3Cl2 and C2F2Cl3) are reactive with C2F3Cl3 to decompose it according to eqs 6 and 7.

C2F3Cl3 + C2F3Cl2 f decomposed products (6) C2F3Cl3 + C2F2Cl3 f decomposed products (7) The particles formed from the decomposed products

Figure 5. Long time removal of C2F3Cl3 (SV ) 18.9 h-1, I ) 0.1 mA).

Figure 6. Removal of C2F3Cl3 from a N2-H2O mixture (Cin ) 400 ppm, SV ) 18.9 h-1).

become charged by the corona discharge and deposit at the anode surface. Long Time Removal of C2F3Cl3 from N2 by Deposition-Type Reactor. To observe whether the high removal efficiency of C2F3Cl3 can be kept for a long time, a long-time removal experiment of C2F3Cl3 from N2 at a low I was conducted. A fresh brass anode was used in the removal. The experimental results are shown in Figure 5. Here, three values of Cin were adopted. In this figure, Ψ for Cin ) 50 ppm becomes constant when the removal begins. However, for the removal at relatively high Cin, Ψ decreases until around t ) 50 h. After that, Ψ becomes constant. When Cin is low, the reaction byproducts generated do not influence the removal efficiency of C2F3Cl3 because their concentrations are low. On the other hand, the removal efficiency at high Cin is thought to be influenced significantly by the reaction byproducts. After starting the discharge, the reaction byproducts of halogen compounds that inhibit the C2F3Cl3 removal are adsorbed on the anode. Therefore, the removal efficiency is relatively high. When the adsorption of the byproducts approaches equilibrium on the anode surface, the concentration of the byproducts increases in the gas phase, and the removal efficiency of C2F3Cl3 decreases because the byproducts inhibit the C2F3Cl3 removal. After the adsorption equilibrium is achieved, the concentrations of the byproducts become constant in the gas phase and the removal efficiency becomes constant. Influence of Coexisting H2O on Removal of C2F3Cl3 by Deposition-Type Reactor. Figures 6 and 7 show the removal of C2F3Cl3 from an N2-H2O mixture. When Cin is 400 ppm, there is an optimum concentration of H2O at CH2O ) 0.82% to give the highest Ψ, as shown in Figure 6. When H2O was mixed in the reaction gas, the reaction byproducts of Cl compounds

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Figure 7. Removal of C2F3Cl3 from a N2-H2O mixture (Cin ) 50 ppm, SV ) 18.9 h-1).

Figure 9. Removal of C2F3Cl3 from N2 by deposition-type and wetted-wall reactors (Q ) 1.67 × 10-6 m3‚s-1).

Figure 8. Removal of C2F3Cl3 from a N2-O2 mixture (SV ) 18.9 h-1).

decreased significantly at the gas outlet, and they became negligibly small at CH2O ) 0.82%. Since the reaction byproducts, F2 (or HF) and Cl2 (or HCl), inhibit the C2F3Cl3 removal, the decrease of the halogen byproducts should contribute to the improvement of the removal efficiency. At CH2O ) 1.23%, however, the removal efficiency is lower than that at CH2O ) 0.82%. Since the coexisting H2O does not influence the removal of C2F3Cl3 at low Cin, as shown in Figure 7, the decrease of Ψ by mixing H2O at high CH2O is not caused by the electron attachment to H2O. Thus, this decrease may be caused by reaction byproducts produced from reactions of the excessive H2O with the radicals or the ions shown by eqs 4 and 5. Such byproducts are removed in the reactor, so that we have not detected those at the gas outlet, and the removal of such byproducts can consume electrons to decrease Ψ. The influence of coexisting H2O is not significant for Cin ) 50 ppm. This may be because the concentrations of the halogen byproducts are very low and the effect decreasing the halogen byproducts is not significant. Also the formation of the byproducts which decreases Ψ at high CH2O may be negligible. Hence, the coexisting H2O does not inhibit the removal of C2F3Cl3. Concerning the formation of other reaction byproducts, the organic reaction byproduct, C2HF3Cl2, observed in the removal from N2 is produced also in the presence of H2O. Influence of Coexisting O2 on Removal of C2F3Cl3 by Deposition-Type Reactor. Figure 8 shows the removal of C2F3Cl3 from an N2-O2 mixture. Compared with the removal from N2, the removal efficiency of C2F3Cl3 from an N2-O2 mixture is significantly low. This tendency is opposite to those for the removals of most sulfur compounds (Tamon et al., 1996), iodine compounds (Sano et al., 1997a,b), and organic compounds (Sano et al., 1997c). It may be attributed to the facts

that electrons are consumed by the reaction with the coexisting O2 and that C2F3Cl3 has less chance to react with an electron in an N2-O2 mixture than in N2. Another possibility of the decrease of Ψ by mixing O2 is that the radical reactions of C2F3Cl3 or the formation of the particles of the removed components could be inhibited by the coexisting O2. Zook et al. (1991) have reported that several buffer gases enhance thermal electron attachment in some halogen compounds. However, it is thought that this effect is not significant for C2F3Cl3 because the mixing O2 does not improve the removal efficiency. Concerning the byproduct formation in the presence of O2, the halogen compounds (Cl2 or HCl, and F2 or HF) and the organic reaction byproduct, C2HF3Cl2, observed in the removal from N2 were produced. Removal of C2F3Cl3 from N2 by Wetted Wall Reactor. Figure 9 shows the removal of C2F3Cl3 from N2. In this figure, one can compare the removal efficiency of the wetted wall reactor with that of the deposition-type reactor. When Cin ) 400 ppm, the removal efficiency is higher for the wetted wall reactor than for the deposition-type reactor. This result can be ascribed to the fact that halogen compounds produced as reaction byproducts are absorbed into the liquid film. Since coexisting halogen compounds are inhibitors for the removal of C2F3Cl3 as suggested above, this absorption should improve the removal efficiency. When Cin ) 50 ppm, a different tendency is obtained. Here the deposition-type reactor shows a higher removal efficiency than that of the wetted wall reactor. One of the possible reasons for this result is as follows. A low concentration of O2 is stripped from the liquid film because a certain amount of O2 is dissolved in the liquid film. It is estimated by a measurement of O2 using a zirconia-type oxygen analyzer (Toray Industries, Inc., LC-700) that more than 200 ppm O2 existed in the outlet gas of the wetted wall reactor when pure N2 was supplied to the reactor. When the concentration of C2F3Cl3 is quite low, the concentration of coexisting O2 becomes relatively high compared to the concentration of C2F3Cl3 and the influence of coexisting O2 becomes large to decrease Ψ. One may consider another hypothesis that the coexisting H2O vaporized from the liquid film consumes electrons and the probability of collision of C2F3Cl3 molecules with electrons decreases. However, it was observed in the deposition-type reactor that the coexist-

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ing H2O does not inhibit the removal of C2F3Cl3 at Cin ) 50 ppm. Thus this hypothesis does not seem reasonable. Conclusively, the wetted wall reactor is not suitable to remove C2F3Cl3 of extremely low concentration but suitable when the concentration of C2F3Cl3 is relatively high. Conclusions In the removal of C2F3Cl3 from N2 by the depositiontype reactor, the removal efficiency increases with a decrease of the inlet concentration of C2F3Cl3. This result suggests that C2F3Cl3 of extremely low concentration can be removed from N2. It is found that about five C2F3Cl3 molecules are removed by one electron and that the halogen compounds of byproducts capture electrons, resulting in the decrease of the removal efficiency. The removal mechanism is considered as follows. (1) The dissociative electron attachment to C2F3Cl3 produces negative ions, F- and Cl-, and radicals, C2F3Cl2 and C2F2Cl3. (2) The radicals react with C2F3Cl3 to decompose it. (3) The decomposed products form particles. (4) The particles become charged in the corona discharge region. (5) The charged particles are collected at the anode surface. A long time removal experiment indicates that the high removal efficiency from N2 is kept even after the anode surface is covered with the deposit. When the inlet concentration of C2F3Cl3 is relatively high in the presence of H2O, there is an optimum concentration of H2O to give the maximum removal efficiency. In case that the inlet concentration of C2F3Cl3 is relatively low, the influence of the coexisting H2O does not influence the removal of C2F3Cl3. When O2 is mixed, the removal efficiency of C2F3Cl3 decreases because electrons are captured by O2 or because the radical reactions and the formation of the particles are inhibited by the coexisting O2. The wetted wall reactor was applied to the removal of C2F3Cl3 from N2. When the inlet concentration of C2F3Cl3 is relatively high, the removal efficiency is higher for the wetted wall reactor than for the deposition-type reactor. This result can be ascribed to the fact that coexisting halogen compounds produced as reaction byproducts are absorbed into the liquid film. However, when the inlet concentration of C2F3Cl3 is relatively low, the deposition-type reactor shows a higher removal efficiency than that of the wetted wall reactor because O2 may be stripped from the liquid film. Acknowledgment We are grateful to Masaaki Hirade and Toshiki Nagamoto for experimental contributions. Financial support was supplied by the Ministry of Education, Science, Sports and Culture of Japan through Grant in Aid on Development of Scientific Research No. 0555211 (1993). We gratefully acknowledge the financial support of the Secom Science and Technology Foundation (1995) and of the Japan Securities Scholarship Foundation (1996). Further N.S. is grateful to the Japan Society for the Promotion of Science (JSPS) for the financial support of the JSPS Research Fellowships for Young Scientists (1996). Nomenclature C ) concentration, ppm G ) deposit weight, g

I ) discharge current, mA M ) molecular weight, g‚mol-1 ne ) electron efficiency p ) pressure, Pa R ) gas constant, Pa‚m3‚mol-1‚K-1 T ) temperature, J t ) time, s  ) conversion of the removed C2F3Cl3 to C2HF3Cl2 Ψ ) removal efficiency Subscripts in ) gas inlet out ) gas outlet

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Received for review September 2, 1997 Revised manuscript received January 2, 1998 Accepted January 10, 1998 IE970615D