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Ind. Eng. Chem. Res. 2010, 49, 5517–5522

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An Electron-Beam Irradiation/Catalytic Oxidation System for Purification of Aromatic Hydrocarbons/Air Mixture under Practical Gas-Flow Condition Teruyuki Hakoda,*,† Akihiko Shimada,† Atsushi Kimura,† Mitsumasa Taguchi,† Yumi Sugo,‡ Koshi Araki,§ Edgar B. Dally,§ and Koichi Hirota† Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233, Takasaki, Watanuki, Gunma, 370-1292, Japan, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai, Ibaraki, 319-1195, Japan, and Valence Corp., 2200 N. Rodney Parham, Suite 210, Little Rock, Arkansas 72212

An electron-beam (EB) irradiation/catalytic oxidation system was developed for the purification of a volatile organic compound (VOC) gas stream under a practical gas flow condition. This system consists of a compactsized electron accelerator and an ozone decomposition catalyst, which is MnO2. The removal of toluene and/or xylene and their mineralization were examined with and without catalytic oxidation for a gas stream at a flow rate of 500 Nm3 h-1. A combined catalyst bed enhanced the removal of VOCs and the mineralization of VOC and its irradiation organic byproducts. For example, for an initial concentration of 5 ppmv, the removal ratio of toluene and xylene increased from 60% to 91% and from 81% to 91%, respectively, at a dose of 4.4 kGy. The mineralization ratio increased from 42% to 100% by the catalytic treatment at doses higher than 9.3 kGy. Furthermore, the yield of CO2 to COx increased from 52-60% to 83-89% by the catalytic treatment. 1. Introduction Volatile organic compounds (VOCs) have been used in various industries as solvents, washing reagents, and basic materials for chemical synthesis. The emission of VOCs to the atmosphere results in the formation of toxic photochemical oxidants and suspended particulate matters through photochemical reactions.1 Several efforts have been made to reduce VOC emission by installing treatment technologies, reducing the quantities used, and recycling. VOCs can be treated by two methods: they can be recovered by adsorption, absorption, and condensation or can be eliminated by thermal incineration, catalytic incineration, and nonthermal plasma (NTP).2-4 Electron beam (EB) and electric discharge techniques are used to create NTPs that can treat VOCs with concentrations less than 100 ppmv (parts per million in volume) at a flow rate of 10-1-105 Nm3 h-1.5 Full-scale EB installations for the treatment of flue gases from coal combustion have been used in Poland for simultaneous removal of SO2 and NOx6 and in China for the removal of SO2.7 Pilot plant tests have been conducted in Japan in order to decompose/detoxify dioxins in off-gases from waste-incineration flue gases.8 Many studies on the electronbeam treatment of VOCs have been performed for aromatics5,9-13 and chlorinated alkenes14,15 at laboratory scale and for aromatics at a pilot scale.16-18 When air is irradiated with EBs, various oxidizing species such as OH radicals and O atoms are produced from air components through their ionization and excitation. In general, OH radicals play an important role of initiating the decomposition and removal of such pollutants due to its higher reaction rate constants for organics.19 On the other hand, most of O atoms rapidly react with O2 in air and produce inert O3. In the case of VOCs as a pollutant, VOCs are attacked by OH * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +81-27-346-9371. Fax: +81-27-3469687. † Quantum Beam Science Directorate, Japan Atomic Energy Agency. ‡ Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency. § Valence Corp.

radicals to produce organic acids and aldehydes as intermediate irradiation byproducts, which are in both gaseous and particulate forms, under EB irradiation.5,11,13,20 If these products are emitted in the atmosphere without further oxidation, they will be changed into photochemical oxidants and suspended particulate matters similar to the case of VOCs. Thereby, they should be oxidized into CO2 and CO, which do not contribute to the production of them, in not only the EB irradiation process but also other NTP processes. The irradiation organic byproducts of VOCs, which commonly have sticky characteristics, are likely to deposit to the surface of the inner wall of a vessel for irradiation.13 A hybrid system of 1 MeV EB irradiation and thermal catalysts (Pt-loaded ceramics) has been studied on a pilot scale for enhancing the decomposition ratio of toluene and the oxidation of its irradiation byproducts.21,22 In these studies, some portion of its catalytic oxidation was observed to be induced by the heat generated in EB irradiation. On the other hand, we have applied an O3 decomposition catalyst as catalyst to an EB-irradiated 100 ppmv xylene/air mixture in a laboratory-scale experiment.23 In general gas treatments using EB irradiation, most of the O3 produced by irradiation has not been used for the oxidation of gaseous pollutants because it is less reactive. An O3 decomposition catalyst can decompose such inert O3 and produce active oxygen such as O atoms, O-, and O2- over the catalyst at even lower temperatures.24 The resulting active oxygen has a potential to oxidize organics deposited on the catalyst surface. In our above experiment, the oxidation of organics, mainly gaseous irradiation byproducts into CO2, was accelerated by the application of the MnO2 bed under the condition that the carbon concentration of these organics was much higher than the concentration of O3. Thereby, the EB/MnO2 combination process is one of the economical methods that can use effectively irradiation energy for the removal and mineralization of VOCs. For the application to practical purification of a VOC gas stream, the availability of this process should be examined under pilot-scale conditions, for example, turbulent gas flow condition. Moreover, the

10.1021/ie100278k  2010 American Chemical Society Published on Web 05/07/2010

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Figure 1. A test facility for the treatment of VOC gas stream under practical gas flow conditions; the gray-colored tubes are stainless steel and whitecolored tubes are vinyl chloride.

possibility of the oxidation of undecomposed VOC and particulate byproducts should be studied. On the other hand, a mobile and self-shielded electron accelerator has been manufactured for the purpose of mainly irradiating a gas stream. This accelerator can generate 160 keV EBs with the maximum beam current of around 30 mA and irradiate 102-103 Nm3 h-1 gas streams in uniform dose rate distribution using a unique electron emitter. It has been mainly tested for purification of VOCs gases stripped from the contaminated groundwater in the United States. Thereby, this accelerator has the potential to be used as a compact and economical pilot-scale test facility of the EB/MnO2 catalyst process. In the present study, an EB/MnO2 system using such an electron accelerator was constructed for the pilot-scale test of decomposition of VOCs in a gas stream. Toluene and o-xylene were used as target VOCs, which have been frequently emitted to the atmosphere.1 Their input concentration was set as 5 and 10 ppmv to study the possibility of the oxidation of undecomposed VOCs and particulate byproducts over the MnO2 catalyst. The VOCs/air at a flow rate of 500 Nm3 h-1 was irradiated with EBs and subsequently introduced into a MnO2 catalyst bed. We discussed the effect of the irradiation, the catalytic oxidation effect, and their combination effect on the basis of the results from gas analysis with and without the irradiation and/or the catalytic oxidation. Furthermore, the availability of the catalyst was also discussed in terms of economical efficiency. 2. Experimental Section A test facility for the decomposition of VOCs in gas streams was composed of a compact-sized electron accelerator, a catalyst vessel, a generation system of a VOC sample gas, an activated charcoal (AC) bed for the prevention of VOC emission, a blower, and tubes for gas flow. The outline of this facility is shown in Figure 1. 2.1. A MnO2 Bed and a Catalyst Vessel. Our previous study demonstrated that the application of a pellet-type MnO2 bed enhanced the oxidation of the gaseous irradiation byproducts of xylene to CO2 over MnO2 under a laboratory-experimental condition for a 10 kGy EB irradiated 100 ppmv xylene/air mixture.23 The activity of the catalytic oxidation was maintained for a long time by heating the catalyst bed at temperatures higher than 373 K. In the present study, a honeycomb-type MnO2 block with a size of 150 mm (length) × 150 mm (width) × 50 mm (thickness) and a weight of 445 g (NHC-453E50RC, Nikki-Universal Co., Ltd.) was used as a catalyst suitable for the treatment of a turbulent high-flow rate gas stream. This catalyst was prepared by coating MnO2 on the surface of a honeycomb-ceramic block. The surface area of this catalyst was 145 m2 g-1 by measuring a broken piece of this block using an automatic surface-area analyzer (Macsorb HM Model-1201, Mountech Co., Ltd.). The

Figure 2. A catalyst bed, gas heater, and cooler in a catalyst vessel.

45 pieces of the block catalysts with total volume of 50 dm3 was installed in a catalyst vessel as shown in Figure 2. The gas hourly space velocity was estimated to be 1 × 104 h-1 under 500 Nm3 h-1 gas flow condition. The catalyst bed and gas stream was heated at temperatures higher than 373 K to keep the catalytic activity for a long time. Additionally, the gas stream downstream of the catalyst bed should be cooled to prevent the change of shapes of a vinyl chloride tube. In the present system, a fin-typed gas heater and fin-typed gas cooler using circulating water were installed upward and downstream of the catalyst bed, respectively, as shown in Figure 2. They were designed to heat a 500 Nm3 h-1 gas stream from 298 to 373 K and cool it from 373 K to room temperature. The electric capacity of this heating system was estimated as 13 kW by assuming 100% of heat efficiency. In the present system, the gas heating system was designed with an electric capacity of 19 kW in the catalyst vessel by considering a certain amount of energy loss. Three thermocouples were installed in the catalyst vessel downstream of the gas heater (the first position), the catalyst bed (the second position), and the gas cooler (the third position), separately. The temperature of the catalyst was controlled by monitoring the temperature at the first position. When the temperature at the second position came within 373 ( 2 K, we considered that the whole catalyst bed was heated uniformly to this temperature. All gas analysis was performed after 30 min since the catalyst was at above temperature. 2.2. A Test Facility for Gas Treatment. A compact-size electron accelerator with a maximum accelerated voltage of 160 kV and a maximum beam current of around 30 mA (Valence Infinity System, Valence Corp.) was used as an irradiation source for the sample gas. Actually, it was able to output EB current up to 33 mA. This accelerator is a self-shielded type and consists of a cylindrical electron emitter, a stainless steel chamber for gas irradiation, a control panel for operation, and a supply for high voltage. The electron emitter is installed in the center of the gas-irradiation chamber. This geometry is completely different from the electron emitter of conventional low-energy electron accelerators, as the electrons are emitted radially outward in a 360° pattern. The irradiation chamber is compartmentalized into eight V-shaped sections with the sample gas incoming from the input plenum converging inward from the outer diameter of the chamber toward the emitter from four sections, then around a gap at the surface of the emitter and then outward through four V sections to the outlet exit of the system through a separate output plenum. This design has the advantage that each unit volume of gas receives the same uniform dose. The irradiation window of the electron emitter is a titanium foil with a thickness of 20 µm welded on the metal frame of the electron emitter. The distance from the irradiation window to the wall of the reaction chamber is 25 cm. The penetration range of accelerated electrons at 160 keV is

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estimated to be about 18 cm by Monte Carlo simulation. The irradiation window was cooled by the gas stream and circulating water in the inside of the frame. Toluene and o-xylene (hereafter, xylene) were used as target VOCs. Atmospheric air in the test facility was used as the source of air. Air containing gaseous toluene and/or xylene was generated by bubbling air through a container holding the liquid VOCs which was controlled at constant temperatures. A sample gas containing toluene and/or xylene at input concentrations of 5.0 ( 0.2 and 10.0 ( 0.2 ppmv was prepared by dilution of these VOC gases with air at a flow rate of 500 Nm3h-1. These VOC concentrations are lower than those for actual exhaust gases from paint factories. This choice was made to examine the possibility of the catalytic oxidation of undecomposed VOCs and particulate byproducts besides that of gaseous byproducts under limited dose conditions. We introduced this sample gas into the irradiation chamber without EB irradiation and irradiated with EBs using a blower placed downstream of an AC bed, as shown in Figure 1. After EB irradiation, the sample gas flowed into the catalyst vessel and subsequently reached the pellet-type AC bed for prevention of the release of VOC/its irradiation products. The volume of the AC bed was 250 dm3, enough to remove undecomposed VOCs and its irradiation byproducts from 500 Nm3 h-1 gas stream. The gray tubes in Figure 1 indicates stainless steel tubes used for preventing VOC from adsorbing and the white tubes indicates vinyl chloride tubes. The concentrations of toluene and xylene in the sample gas was measured using a gas chromatograph (GC-8A, Shimadzu Co., Ltd.) with a packed column (BX-20 100/120, 2 mm i.d. × 3 m, GL Science Inc.) and a flame ionizing detector (FID) by introducing a fraction of the sample gas into the GC-FID before and after EB irradiation and after catalytic treatment. For the measurement of CO2 and CO (COx) concentrations, a fraction of the sample gas was coincidently collected in a Tedlar bag (inner volume 10 dm3) from three sampling ports. The concentrations of COx in the sample gas of the bag were measured using the GC-FID with a packed column (Shincarbon ST 50/ 80 3 mm i.d. × 1 m, Shinwa Chemical Industries Ltd.) and a methanizer (MTN-1, Shimadzu Co., Ltd.). The concentration of COx produced by EB irradiation and catalytic oxidation was calculated from the difference in their concentrations before and after each gas treatment. In the present study, an average dose of the irradiated gas was estimated in terms of O3 concentration produced from common irradiated air as follows. At first, common air at a flow rate of 10 dm3 min-1 was irradiated at certain doses using a conventional electron accelerator (HV, 170 kV max; current, 10 mA max) and the concentration of produced O3 was measured using an ozone analyzer (EG-2001, Ebara Jitsugyo Co., Ltd.). An average dose of gas irradiated using this accelerator was already calibrated using a gas dosimeter of a pure N2O gas.11,25 Then, the average dose of the irradiated sample gas at a flow rate of 500 Nm3 h-1 was estimated from this relationship between O3 concentration and dose. The concentration of O3 in the irradiated air increased linearly with beam currents up to 33 mA, as shown in Figure 3. An average dose was estimated from this O3 concentration; a beam current of 30 mA corresponded to an average dose of 13.9 kGy. 3. Results and Discussion The decomposition of VOCs in the sample gas by EB irradiation and that by EB/catalytic oxidation are described in the following two sections.

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Figure 3. The concentration of O3 (b) and estimated average dose (O) as a function of beam current of an electron accelerator.

Figure 4. Concentrations of toluene (solid marks) and xylene (open marks) in 500 Nm3 h-1 single and double VOC/air mixture as a function of dose: (a) in single VOC/air mixture and (b) in double VOCs/air mixture.

3.1. Decomposition of VOCs by EB Irradiation. The decomposition of 5 and 10 ppmv VOCs in the sample gases, which are toluene/air, xylene/air, and toluene/xylene/air, was studied by only EB irradiation. The changes of the VOC concentrations as a function of the average dose are shown in Figure 4. The concentrations of toluene and xylene decreased with doses for all sample gases. In general, the average dose required for the treatment of a VOC stream is a very important factor, in addition to the flow rate and concentration, to determine the outline of an EB system such as the specification of the electron accelerator and the design of the irradiation chamber.5 In the present study, the dose to decompose 90% input VOCs, D90, has been used as the EB energy required for the treatment of a VOC stream and is plotted in Figure 5. The data for 10 ppmv toluene in toluene/xylene/air was not plotted because 90% of decomposition was not obtained by EB irradiation at the maximum dose of 15.2 kGy. For single-VOC sample gas, best-fitting lines did not pass the origin. This result suggests that the EB irradiation to decompose VOCs at lower input concentrations requires higher doses because of OH sink reactions such as its bimolecular reaction and reaction with other radicals. On the other hand, for two-component sample gas, the D90 was higher than that for single-component sample gas because OH radicals are consumed by the reaction with both toluene and xylene. In practical gas purification, EB irradiation was carried out at the maximum output power of the electron accelerator to purify off-gases with the maximum flow rate. The relationship obtained in Figure 5 for an EB irradiation of practical-flow

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Figure 5. Average dose for 90% decomposition, D90, as function of the input concentration of VOC in single and double VOC/air mixture: (b) toluene and (2) xylene in double VOCs/air mixture and (O) toluene and (4) xylene in single VOC/air mixture.

simulated gases enables one to estimate the specification of an electron accelerator to be used under a certain targeted gas flow condition. The mineralization ratio, which is calculated from the ratio of [CO2] + [CO] to the carbon concentration of input VOCs, was obtained at the D90, where [CO2] and [CO] stand for the concentration of CO2 and CO, respectively. The mineralization ratios at the D90 were 27 ( 2% and 46 ( 4% for 5 and 10 ppmv toluene/air mixture and 31 ( 3% and 36 ( 2% for 5 and 10 ppmv xylene/air mixture. The mineralization ratio of xylene was almost constant independent of its input concentration, while that of toluene increased with its input concentration. In the present study, the reason for the increment of mineralization at higher input toluene concentration was not clear. The mineralization ratios at a dose of 15.2 kGy were 72 ( 3% for 5 ppmv toluene/xylene/air mixture and 33 ( 3% for 10 ppmv toluene/ xylene/air mixture. The yield of CO was 33-50% relative to COx. Further EB irradiation of the sample gas resulted in the increase in both concentrations of CO2 and toxic CO. Thereby, the combination of the catalytic oxidation should be required for the increase in the mineralization ratio with suppressing toxic CO formation. 3.2. EB/Catalytic Oxidation. Prior to the study of the oxidation of VOCs in air by the EB/catalyst process, we examined the possibility of thermal oxidation of VOCs over the surface of the MnO2 bed without EB irradiation. In the combination of the catalytic treatment, we cannot distinguish the decomposition of VOCs and its adsorption to the catalyst surface on the basis of the decrease in the VOC concentration. Thereby, the removal ratio will be used instead of the decomposition ratio in the following sections. 3.2.1. Catalytic Oxidation Using a MnO2 Bed without EB Irradiation. A sample gas containing toluene and xylene at both initial concentrations of 5 and 10 ppmv (a gas flow rate 500 Nm3 h-1) was heated to a temperature of 373 K by passing the gas through a heater in the catalyst vessel. The concentration of toluene and xylene was completely the same between the upstream and downstream gas heater. This result suggests that toluene and xylene are not decomposed over the surface of the heated fin-heater. The heated sample gas was introduced at a flow rate of 500 Nm3 h-1 in the catalyst bed. The gas temperature downstream from the catalyst bed was slightly elevated to 318 K for 10 min and became 373 K within a few minutes. This temperature-time profile suggests that the honeycomb-type catalyst bed has enough surface area to exchange the heat between the sample gas and the catalyst surface. It took 10 min to uniformly heat the whole catalyst bed up to 373 K by purging

Figure 6. Removal ratio of toluene (b, O) and xylene (2, 4) and the mineralization ratio (9, 0) with and without the catalytic treatment as a function of dose for 5 ppmv toluene/xylene/air mixture.

373 K gas at a flow rate of 500 Nm3 h-1. The sample gas downstream of the catalyst bed was analyzed 30 min after the catalyst bed reached a temperature of 373 K. As a result, the concentrations were 1.1 ppmv for toluene, 0.44 ppmv for xylene, 12.0 ppmv for CO2, and 1.1 ppmv for CO in a 5 ppmv toluene/ xylene/air mixture and 2.3 ppmv for toluene, 0.98 ppmv for xylene, 23.3 ppmv for CO2, and 1.0 ppmv for CO in a 10 ppmv toluene/xylene/air mixture. The removal ratios of toluene and xylene were 78% and 91% for initial concentrations of 5 and 10 ppmv, respectively. The production of COx suggests that a portion of toluene and xylene was thermally decomposed over the catalyst surface. The mineralization ratio by thermal oxidation was 17 ( 1% at both initial concentrations. However, in the case of pellet-type MnO2 bed heated to 373 K, the formation of COx was not observed and the decrease in VOC concentration was exclusively caused by adsorption of the catalyst. The difference in production of COx will be caused by the surface activity (or state) of two types of the catalyst bed. 3.2.2. Catalytic Oxidation Combined with EB Irradiation. The catalytic oxidation of coexistent VOCs and its irradiation organic byproducts in the sample gas was examined from the difference in the concentrations of VOCs and COx before and after the catalytic oxidation. The sample gas was analyzed at 30 and 60 min after the beginning of sample gas irradiation with EBs at each dose. The concentrations of them were constant for 30-60 min. In the present study, their concentrations measured at 30 min later were used to examine the catalytic effect. The removal ratios of toluene and xylene and the mineralization ratio were estimated from the changes of the concentrations of toluene, xylene, and COx with and without the catalytic treatment. The results for input VOC concentrations of 5 and 10 ppmv are shown in Figures 6 and 7, respectively. The yield of CO2 to COx was also plotted in these figures. For an input concentration of 5 ppmv, the removal ratios of toluene and xylene increased by the catalytic treatment. For example, the removal ratio of toluene increased from 60% to 91% and that of xylene varied from 81% to 91% at a dose of 4.4 kGy. The mineralization ratio also increased by 60% by the catalytic treatment and became 100% at doses higher than 9.3 kGy. This result suggests that not only the gaseous byproducts of VOCs but also undecomposed VOCs and oxidation-resistant particulate byproducts should be completely oxidized into COx by 9.3 kGy irradiation/catalytic oxidation. Furthermore, the yield of CO2 to COx increased from 52-60% to 83-89% at doses of 4.4-13.9 kGy by this combination treatment. This catalytic oxidation produced only CO2 as a final product from oxidation of VOCs, its byproducts, and CO. For an initial concentration

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the catalytic oxidation is profitable to reduce total initial installation cost, namely capital cost, for purification of VOC gas stream. As mentioned in the above sections, the catalytic oxidation enhanced the oxidation of target VOCs and their irradiation byproducts into CO2 and simultaneously reduced the concentrations of toxic CO and O3. We also conclude that the combination process is a safety method for purification of VOC gas stream under even the pilot-scale conditions. 4. Conclusion

Figure 7. Removal ratio of toluene (b, O) and xylene (2, 4) and the mineralization ratio (9, 0) with and without the catalytic treatment as a function of dose for 10 ppmv toluene/xylene/air mixture.

of 10 ppmv, the removal ratio of toluene at a dose of 4.4 kGy increased from 47% to 87% and that of xylene increased from 68% to 97% by the combination of the catalytic treatment. The mineralization ratio also increased from 40-50% to 81% at doses higher than 11 kGy. The yield of CO2 to COx was 42-50% at doses of 4.4-11 kGy by only EB irradiation and increased to 84-91% by the catalytic treatment. Thus, the catalytic treatment is effective to enhance the oxidation of target VOCs and their irradiation byproducts into CO2 under even the pilot-scale conditions. 3.3. Cost Merit in the Combination of the Catalytic Treatment. In the present experiment, the operation of the electron accelerator required 18.6 kW of electricity, which was measured using a current monitor (3287, Hioki), for EB irradiation at a beam current of 30 mA. This current included 2.8 kW of power to operate a chiller for cooling the water circulated in the EB emitter for cooling the irradiation window. If running water is used as the circulated water (a few dm3/ min), the electric power of 15.8 kW is necessary for generating 30 mA EBs. On the other hand, the electric power for heating the sample gas up to 373 K was also measured to be 13.2 kW. In painting factories, the temperature of off-gases is higher than room temperature because of the generation of heat in vaporization of solvents. The EB irradiation at higher doses results in the increase of gas temperature. Under these conditions, the electric power for gas heating will be lower. In our previous laboratory-scale experiment, we have observed that an average dose could be reduced by half to obtain certain mineralization ratios by combining the catalytic oxidation.26 If this dose-mineralization relationship was valid, the mineralization ratio obtained by 30 mA irradiation/ catalytic oxidation (electric power 15.8 + 13.2 kW) is equal to that obtained by 60 mA irradiation without the catalytic oxidation (an expected electric power of 31.6 kW). Thus, the electric power required for the operation of both oxidation methods was almost the same. The catalytic oxidation is not always profitable to reduce total electric power, namely running cost, for minimization of VOCs. This running cost is roughly estimated to be 5.5 US$ h-1 for 30 mA irradiation of a 500 Nm3 h-1 gas stream by considering only the electric power cost of 0.19 US$ kW h-1 (1 US$ ) 90 JPN Yen). On the other hand, the initial installation cost of the catalytic treatment was a few tens of times cheaper than that of an electron accelerator. In the present study, the total cost related to the catalytic treatment is 3.89 × 104 US$ and that of the electron accelerator is 5.56 × 105 US$. Of course, these costs can be reduced by the improvement and optimization of the treatment process, when they are mass-produced. Thereby,

An EB irradiation/MnO2 catalytic oxidation system was designed and constructed for the pilot-scale test of decomposition of toluene and xylene in a gas stream. This system consists of a compact-sized electron accelerator and a MnO2 catalyst bed. The VOCs/air at a flow rate of 500 Nm3 h-1 was irradiated with EBs and subsequently introduced into a MnO2 catalyst bed. The removal of VOCs and the mineralization of them were examined with and without a catalytic treatment for gas streams. The EB/catalyst system has enhanced the decomposition of VOCs, the mineralization of VOC/its irradiation byproducts, and the yield of CO2 to COx. For example, for an initial concentration of 5 ppmv, the removal ratio of toluene and xylene increased from 60% to 91% and from 81% to 91%, respectively, at a dose of 4.4 kGy. For an initial concentration of 10 ppmv, the removal ratio of toluene at a dose of 4.4 kGy increased from 47% to 87% and that of xylene increased from 68% to 97% by the combination of the catalytic oxidation. The mineralization ratio for an input concentration of 5 ppmv increased from 42% to 100% by the catalytic treatment at doses higher than 9.3 kGy and that for 10 ppmv increased from 40-50% to 81% at doses higher than 11 kGy. Furthermore, the yield of CO2 to COx increased from 52-60% to 83-89% for the case of 5 ppmv and from 42-50% to 84-91% for 10 ppmv by the catalytic treatment. Thus, it was observed that the catalytic oxidation is effective to enhance the oxidation of target VOCs and their irradiation byproducts into CO2 under even the pilot-scale conditions. On the other hand, the EB irradiation/catalyst system requires additional cost related to the installation of a catalyst bed. However, the initial installation cost of the catalytic treatment was a few tens of times cheaper than that of an electron accelerator. Thereby, the catalytic treatment is profitable to mainly reduce total capital cost for purification of a VOC gas stream. On the basis of these results, we can conclude that the combination treatment has an advantage for the purification of a VOC gas stream in terms of economy and from the point of view of safety. Literature Cited (1) Wiederkehr, P. Emission Reduction Programmes for VOC in Some OECD Countries. In Characterization and Control of Odours and VOC in the Process Industries; Vigneron, S., Hermia, J., Chaouki, J., Eds.; Elsevier: Amsterdam, 1994; pp 11-28. (2) Emission Standards Division. Control Techniques for Volatile Organic Compound Emissions from Stationary Sources; U.S. Environmental Protection Agency: Washington, DC, 1992. (3) Dueso, N. Volatile Organic Compounds Treatment Techniques. In Characterization and Control of Odours and VOC in the Process Industries; Vigneron, S., Hermia, J., Chaouki, J., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 263-276. (4) Hunter, P.; Oyama, S. T. Control of Volatile Organic Compound Emissions: ConVentional and Emerging Technologies; Wiley-Interscience: New York, 2000. (5) Hirota, K.; Sakai, H.; Washio, M.; Kojima, T. Application of Electron Beams for the Treatment of VOC Streams. Ind. Eng. Chem. Res. 2004, 43, 1185.

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ReceiVed for reView February 4, 2010 ReVised manuscript receiVed April 18, 2010 Accepted April 20, 2010 IE100278K