SYNERGETI EFFET OF UV LIGHT ON TOLUENE DEOMPOSITION Y DIELETRI ARRIER DISHARGE R. Pyagay, 1 J-S. Kim, 2. Ahn, 2 Y-S. Yim 2 1 hemistry department, Lomonosov Moscow State University, Moscow 119-992, Russia 2 Sudo Premium Engineering o., LTD, Seoul 150-093, Korea ASTRAT Decomposition of toluene in air was studied by dielectric barrier discharge (DD) on the one hand as well as UV light as assistance to DD on the other hand. Input energy to reaction chamber, flow rate of contaminated air, and concentration of toluene were studied as parameters influencing on the decomposition rate of toluene by two methods. Increasing the input energy from 2.8 kv to 4.2 kv at flow rate 1 SLM was increased the decomposition rate of toluene from 3.5% to %. However the decomposition rate of toluene was decreased with increasing both the flow rate and the concentration of toluene in a treated air. It was discovered synergetic effect of UV light assistance to DD on toluene decomposition. The decomposition rate of toluene by UV assisted DD was at least 3 times higher than by the DD only, and 2 times higher than the sum of decomposition rates by the DD and UV light made separately. Ozone concentration was decreased by UV assisted DD in comparison with other one and this rate depends on flow rate of a treated air. This effect was discussed due to reaction activities of different radicals obtained by interaction of ozone and UV light. KEYWORDS Dielectric arrier Discharge, UV, Toluene, Decomposition, Ozone. 1. INTRODUTION Plasma air treatment is a subject of many investigations to use it for indoor air pollution control [1-4]. The main advantage of it is a decomposition of gaseous air pollutants like volatile organic compounds (VO) at room temperature. The decomposition rate of air pollutants is mainly depended on a supplied discharge voltage, i.e. plasma density. The higher discharge voltage the higher decomposition rate. At the same time a decomposition rate of gaseous air contaminants by plasma treatment accompanied to the formation of harmful by-products such as an ozone, carbon monoxide, and aerosol particles [5, 6] that will be as lower as possible. The formation rate of these compounds also depends on supplied discharge voltage. The lower supplied energy the lower the amount of byproducts. In order to overcome this duality the researchers [7, 8] paid their attentions to develop combination systems based on plasma treatment and other additional methods like catalyst [7], water scrubber [8]. In the study [9] was shown that the utilization of ozone catalyst coated onto the fan wheel in plasma air cleaner allowed to increase discharge voltage up to 20% without changing an ozone concentration in outlet of air cleaner. Assistance of UV light to dielectric barrier discharge (DD) plasma, hereafter (DDUV) on the decomposition of toluene in closed chamber was experimentally studied in []. In this study the DDUV was detailed study for toluene decomposition in air flows. The experiments were carried out: under DD plasma conditions, under UV irradiation conditions, and for combination of DD plasma and UV irradiation. 2. EXPERIMENTAL 2.1. Reaction chamber A closed plastic chamber (4.5 L) was used in this study, fig. 1, as a reaction chamber. Inside of a Robert Pyagay: Tel: + 82-2-26374632 ext1, Fax: + 82-2-26345797 E-mail address: Piagai@sudo.co.kr
chamber was placed air cleaner system consisting of dielectric barrier discharge device and ultraviolet lamp. The DD device was the glass tube coated inside with a silver film as a high voltage electrode and a stainless steel net covering the glass as a ground one. The high voltage 60 Hz frequency A power supply was used for formation of barrier discharge plasma in surrounding of glass tubes. In table 2 are shown the discharge currents, which are generated in dependence of high voltages supplied to DD. The ultraviolet lamp was Hg-lamp PL-L 18W (Philips), irradiation of power of which was 5.5 W. Gaseous toluene was prepared by bubbling toluene liquid with air at 285 K and injected into chamber to form a known initial concentration of toluene. An air flow was controlled by mass flow controller. Figure1. Set-up for study of toluene decomposition by UV assisted DD plasma. PP- peristaltic pump; MF mass flow controller; M mixing chamber; G gas bubbler; DD - dielectric barrier discharge plasma device; UV ultraviolet lamp; PS power supply; TVO total volatile organic compound. 2.1. Analyses The concentration of toluene in the chamber was controlled by PGM-7420, O and O 2 by PGM- 52 IAQRAE Rae analyzers (USA). Ozone concentration was measured by 49 ozone calibrator (USA) and Gastec glass tube. Plasma power generated by DD at different high voltages supplied to DD was measured by withstanding tester TOS5050A (KIKUSUI, Japan). The decomposition rate (DR) of toluene by DD plasma was calculated by: O - P DR P = ---------- (1) O where DR P is decomposition rate of toluene by plasma treatment; O and P are the concentrations of toluene without and with plasma treatment, respectively. The decomposition rate of toluene by other methods was calculated similarly. Table 1. Plasma currents generated by DD at different high voltages supplied to DD. HV, kv I, ma VAR, W 2.8 3.2 3.6 4.2 0.19 0.23 0.27 0.31 0.53 0.74 0.97 1.30
Selective oxidation rate of toluene was described as a parameter (S) which is defined by: ([O 2 ] P [O 2 ] O ) S = ------------------------------ (2) ([ 7 H 8 ] O - [ 7 H 8 ] I )*7 where [O 2 ] P and [O 2 ] O are carbon dioxide concentrations in air after and before DD and DDUV treatments of contaminated air, respectively. [ 7 H 8 ] O and [ 7 H 8 ] I are toluene concentrations in air before and after DD and DDUV treatments of contaminated air, respectively. 3. RESULTS 3.1. Efficiency improvement by UV irradiation In order to improve DD method for removal of toluene, a decomposition of it was studied in the presence of UV. In fig. 2 are shown the decomposition of toluene by three methods: DD, UV, and DDUV at different flow rates of air. High voltage supplied to DD tube was 2.8 kv. The decomposition rates of toluene by DD as well as UV irradiation are nearly the same. Maximum decomposition rate is 3.5 % for DD and 2 % for UV at flow rate of air 1 standard liter per minute (SLM). However, this value is dramatically increased by using DDUV. The decomposition rate of toluene is increased up to % for the same flow rate. This value is two times higher than the sum of decomposition rate of toluene obtained by DD and UV when they used separately. This trend is kept at higher flow rate of air, at 1.5 and 2 SLM. It means some synergetic effect takes place at DDUV utilization. Toluene decomposition rate, % 8 6 4 2 0 D 1 1.5 2 Flow rate, SLM Figure 2. Decomposition rate of toluene as a function of air flow rates by DD (), DDUV (), and UV light only (D).Initial concentration was 34 ppm. High voltage supplied to deodorizer was 2.8 kv. This phenomenon is strengthened with increasing high voltages supplied to DD. In figure 3 is shown the decomposition rate of toluene by two methods at different voltages supplied to DD. The flow rate of air was 1 SLM. Synergetic effect is as much more as higher supplied voltage to DD. The difference in the decomposition rate between two methods was 33% at supplied voltage of 3.6 kv whereas its value was about 7 % at 2.8 kv one. It should note, it was used similar UV light for all experiments only and hence irradiation power of UV light was the same for all experiments.
50 Toluene decomposition rate, % 40 30 20 D 0 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 High voltage, kv Figure 3. Decomposition rate of toluene as a function of high voltages supplied to DD (), UV light only (), and DDUV (D). Initial concentration of toluene was 34 ppm. Flow rate of air was 1 SLM. 45 Decomposition rate of toluene, % 40 35 30 25 20 15 5 D 0 18 20 22 24 26 28 30 32 34 oncentration of toluene in air, ppm Figure 4. Decomposition rate of toluene as a function of initial concentrations of toluene in air by DD (), UV light only (), and DDUV (D). Flow rate was 1 SLM. High voltage supplied to deodorizer was 2.8 kv. Synergetic effect for toluene decomposition by DDUV is kept for different initial concentration of toluene. As seen from figure 4 the decomposition rate of toluene by DDUV is much more than of
DD. Moreover, the difference between two decomposition rates is as much more as lower the initial concentration of toluene. The difference in the decomposition rate between two methods was about 34 % at initial concentration of around 19 ppm whereas its value was % only for initial concentration of around 32 ppm. 3.2 Ozone concentration. It is well known ozone formation is a main problem by utilizing DD for air pollution control because it is harmful gas for human body and its concentration strongly regulated by the occupational safety health administration. Therefore, formation of ozone concentration during air purifying by DD and DVDUV was one of a separate study. In Figure 5 is shown ozone concentration in air by DD and DDUV at different high voltages supplied to DD. Ozone is dramatically reduced by DDUV in comparison of DD. Differences in ozone concentration between two methods is higher the higher voltages supplied to DD. It was about 9 ppm at 3.2 kv whereas that value was about 5 ppm at 2.8 kv. It means UV irradiation reduces ozone formed by DD. As for ozone formation in air by UV lamp it was zero for all time. 18 Ozone, ppm 16 14 12 8 6 2.8 2.9 3.0 3.1 3.2 High voltage, kv Figure 5. Formation of ozone in air by DD () and DDUV () at different high voltage supplied to DD. Flow rate of air was 2 SLM. UV lamp was "PL-P 18W". In figure 6 is shown ozone concentration in air by two methods at different flow rate of air at fixed high voltage supplied to DD. Reducing behavior of UV irradiation for ozone formed by DD is kept at higher flow rate of air. However, this effect decreases with increasing of flow rate of air. The difference between two methods at 1 SLM was about 18 ppm whereas at 4 SLM was 3 ppm, only. 3.3 Toluene oxidation rate. The other parameter which must be controlled by application of DD for indoor air cleaning is selective oxidation rate (S) of toluene because the uncompleted oxidation products like aldehydes, organic acids, etc may be formed. The higher oxidation rate the higher the quality of air cleaner. In table 2 are shown the oxidation rate of toluene by two methods at different flow rates of air and supplied voltages. Oxidation of toluene depends on both flow rates of air and high voltages supplied to DD. At 2.8 kv an oxidation rate by DD can not determined due to low decomposition rate of toluene and hence formation rate of carbon dioxide is too low to detect it by our device. As concerns an
oxidation rate of toluene by DDUV at 2.8 kv there is not complete oxidation even at flow rate of air 1 SLM in here. However, complete oxidation takes place at higher plasma power, beginning with 3.2 kv in both flow rates: 1 and 2 SLM. Unfortunately, we can not find oxidation rate at higher flow rate of air because decomposition rate of toluene is not enough to produce carbon dioxide to detect it by our device. 30 25 20 Ozone, ppm 15 5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Flow rate of air, SLM Figure 6. Formation of ozone by DD () and DDUV () at different flow rate of air. High voltage supplied to DD was 2.8 kv. UV lamp was "PL-P 18W". Table 2. Oxidation rate of toluene by DD and DDUV. High voltage, kv 2.8 DD 2.8DDUV 3.2 DD 3.2 DDUV 3.6 DD 3.6 DDUV FR, SLM Toluene decomposed, ppm O 2 in air measured O 2 in air theoretically S, % 1 1.3 Not detected - 2 0.7 Not detected - 1 3.4 23 44 2 2.1 Not detected 15-1 2.7 20 19 0 2 1.0 7 0 1 9.6 80 68 0 2 4.4 35 30 0 1 4.5 30 31 0 2 1.5 0 1 14.5 0 1 91 2 5.7 40 39 0 4. DISUSSION Synergetic effect for decomposition of toluene by DDUV is a very interesting and potentially useful phenomenon. In order to explain this phenomenon it is necessary to understand toluene oxidation mechanism by DD as well as UV. Sekiguchi et al [11] investigated the toluene hydroxylation by DD plasma. They considered two types
of oxygen atoms are responsible for toluene oxidation: O( 3 P) an atomic oxygen in the ground state; O( 1 D) an exited oxygen atom created by impact with O 2. The O( 1 D) is the strong oxidant for toluene to produce oxidation product such as O and OH by breaking the aromatic structure whereas the O( 3 P) is a effective reagent for partial oxidation []. On other hand, Atkinson et al [12] showed OH radical is key reactive specie for most organics. The most organics react with the OH radical with rate constants of 5x -15 cm 3 molecule s -1 at 298 K whereas an ozone reacts with toluene at the reaction rate constant of 1.2±0.6 * -20 cm 3 molecule s -1 at 298 K [12]. Influence of UV irradiation on the toluene oxidation was investigated by Wang et al [13]. They postulated the main species for toluene oxidation are also OH radical and O( 1 D) which can be occurred by: H2O + hν = H + OH. (3) H2O + hν = H 2 + O( 3 P) or O( 1 D) (4) According to this review, synergetic effect obtained at toluene decomposition by DDUV may be associates with the formation of additional amount of hydroxyl radical which can be formed from ozone due to [12]: O 3 + hν (λ 3 nm) = O( 1 D) + O 2 ( 1 g ) (5) O( 1 D) + H 2 O = 2OH (6) In this case it is clearly reducing of ozone concentration by DDUV in comparison to DD. As for an selective oxidation rate as well as decomposition one of toluene by DD plasma and DDUV, they may be depend on several parameters like high voltage supplied to DD, volume of reaction chamber, flow rate of air, relative humidity of air, and etc. More detail study of influence of above first four parameters on a selective toluene conversion will be a subject of our further investigation. 5. ONLUSIONS Decomposition of toluene by DD plasma with and without UV light assistance was carried out in air flow. The discharge voltage was varied from 2.8 to 4.2 kv and flow rate from 1 to 4 SLM. The toluene concentration in air was changed from 19 to 32 ppm. The experimental data indicated that synergetic effect was discovered in decomposition of toluene in air by UV assisted DD plasma. The decomposition rate of toluene was dramatically increased in comparison with DD plasma only. The difference in the decomposition rate between two methods depends on discharge voltage, flow rate of air, and a concentration of toluene in air. These differences were as higher as higher discharge voltage and lower both flow rate of air and toluene concentration. Ozone concentration in air by two methods was also studied. Utilization of UV assisted DD plasma was significantly reduced an ozone concentration in comparison with DD plasma only. The reduction rates were as higher as higher discharge voltage and lower flow rate of air. 6. REFERENES 1. T. Oda et al (2002) Nonthermal plasma processing for dilute VOs decomposition. IEEE Trans. Ind. Appl. 38, 873-978. 2. T. Oda (2003) Non-thermal plasma processing for environmental protection: dilute VOs decomposition in air. J. Electrostatics. 57, 293-311. 3. K. Nishikawa and H. Nojima (2001) Air purification effect of positively and negatively charged ions generated by discharge plasma at atmospheric pressure. Jpn.J.Appl.Phys., 40. L835-L837. 4. S.L. Daniels (2002) On the ionization of air for removal of noxious effluvia (Air ionization of indoor environments for control of volatile and particulate contaminants with non-thermal plasmas generated by dielectric-barrier discharge). IEEE Trans. Plasma Science, 30 Issue: 4, 1471-1481. 5. K. Urashima and J. S. hang (2000) Removal of volatile organic compounds from air streams and industrial flue gas by non-thermal plasma rechnology. IEEE Trans.Dielect. Elect. Insulat, 7, 602-14.
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