UV-PCO DEVICE FOR INDOOR VOCS REMOVAL: INVESTIGATION ON MULTIPLE COMPOUNDS EFFECT WH Chen *, JS Zhang and ZB Zhang Dept. of Mechanical and Aerospace Engineering, Syracuse University, Syracuse, NY 3244, USA ABSTRACT Current design models for UV-PCO devices often assume that the air contains only one VOC species or all the VOCs in the air can be treated on a non-interacting basis. However, trace-level multiple VOCs co-exist in most indoor environments. In this study, a UV-PCO reactor was tested in a full-scale chamber under low VOC concentration levels. Multiple versus single VOC tests were performed on selected groups of compounds. Removal efficiency for each compound was calculated. It was found that the interference effect among test VOCs were generally small in the 2-VOC and 3-VOC mixture tests performed on toluene, ethylbenzene, octane, decane and dodecane. However, in the 6 VOC mixture test, the interference effect among different VOCs became quite obvious, and compounds with lower removal efficiency in the single compound test appeared to also have relatively lower efficiency and more obvious delay period in the initial reaction. Results, although limited, indicate that interference between multiple VOCs may not be neglected for the PCO reactor for the indoor applications where the number of VOCs species is large and the TVOC concentration is high. INDEX TERMS VOCs, Photocatalytic oxidation, Indoor air quality, Multiple component system INTRODUCTION Volatile organic compounds (VOC) represent a major group of indoor contaminants. They are possible causes of sick building syndrome and may have adverse health effect. Application of ultraviolet photocatalytic oxidation (UV-PCO) for indoor VOCs removal has received growing interests in recent years. Different types of models, from simple effectiveness-ntu method (Zhang et al., 2003) to -D lumped parameter model (Hall et al., 998) and more complex 3-D CFD model (Hossain et al., 999), have been developed for analyzing the removal performance of PCO reactors. All these models assume that the air contains only one VOC species or all the VOCs in the air can be treated on a non-interacting basis. However, on the other hand, experiments results have confirmed the interference effect among different VOCs for the VOC mixture, especially under high concentration levels. For example, Turchi et al. (995) tested a mixture of isopropanol, acetone and methanol with a concentration of approximately 00 ppmv per compound using an annular tube flow reactor. They found that the behavior of the mixture can be described by a Langmuir-Hinshelwood (L-H) model with the effect of competition between different VOCs for the same type of adsorption sites considered in the denominator. Lichtin et al. (996) tested 4 binary mixtures of VOCs over Degussa P-25 TiO 2 with concentrations ranged from 50 ppm to % using either a batch reactor or an annular tube flow reactor. Both promotion and inhibition of removal of one component by the other as well as absence of significant effects have been observed. They suggested that the inhibition of removal could be a consequence of competition between reactants for adsorption sites on the catalyst, modification of the catalyst surface by adsorption of or reaction with a reactant, or strong adsorption of an intermediate product of degradation of the other reactant. As for the promotion of removal of some VOCs by trichloroethylene (TCE) and perchloroethylene (PCE), they attributed the promotion effect to the production of a transient product of decomposition of these reactants which can initiate or propagate chain oxidation and suggested that atomic chlorine plays an important role. No quantitative relationships were developed in this paper. Obee et al. (999) tested the mixture of butanol and propanal with various combinations of concentrations (0 2.7 ppm) using a glass-plate flow reactor. They found that small amount of butanol (i.e..0 ppm) could significantly reduce the oxidation of propanal, although the oxidation rates for each individual compound lied in the linear range at these low concentrations. However, the decrease of butanol oxidation rate by the addition of small amount of propanal was not significant. No quantitative relationships were developed in this paper. Since most of IAQ applications involve multiple VOC species at low concentrations (ppb to very low ppm), it is important to determine how significant the interference effects can be under typical indoor concentration levels among different VOCs and to describe these effects quantitatively in model whenever possible. * Corresponding author email: wchen3@syr.edu 398
This paper presents full-scale chamber test results of a UV-PCO reactor under low concentration levels (i.e. < mg/m 3 for most VOCs) using single compounds, binary VOC mixtures, a triple VOC mixture and a 6-VOC mixture, respectively. No such data have been reported previously in the literature. The purpose is to assess the significance of interference effects among different VOCs for indoor applications. RESEARCH METHODS PCO Reactor for Testing The PCO reactor tested was a prototype structured honeycomb in-duct unit. It consists of two layers of honeycomb monoliths coated with TiO 2 -based catalyst and a bank of three 25 W germicidal UV lamps centered in between. Test Procedures All the tests were conducted in a 54.4 m 3 full-scale stainless steel chamber (4.88m x 3.66m x 3.05m high or 6ft x 2ft x 0ft high) using a pull-down test procedure under a controlled temperature of 23 ± 0.5 o C and relative humidity of 50 ± 5%. The PCO reactor was installed in the test box on the return duct of chamber HVAC system and the recirculation flow rate through the reactor was controlled at 360 m 3 /h (800 CFM) for all the tests. Between each test, the PCO reactor was flushed by humid clean air for at least overnight. An empty chamber test was performed first to investigate the possible effects of chamber characteristics (i.e. air tightness and sink effect) on test results for the air cleaners. Detailed description of the pull-down test procedure and empty chamber test results can be found in Chen et al. (2005). The dynamic period was hour for all the tests, except for the 6 VOC mixture test which had a 2-hour dynamic period. Test VOCs Table lists the tests performed during this study. Two aromatic hydrocarbons (toluene and ethylbenzene) and three alkanes (octane, decane and dodecane) were selected for single VOC, binary and triple VOC mixture tests. A mixture of 6 VOCs, which cover major chemical categories and a wide range of molecular weight and boiling point for VOCs commonly found indoors, was also tested. Detailed information of the challenge VOC mixture and their properties can be found in Chen et al. (2005). Table Summary of Tests Performed Test Compound Test Date Note Toluene 2/3/04 Toluene (II) 2/9/04 Repeat test Ethylbenzene 2/20/04 Toluene + Ethylbenzene 2/2/04 Octane 2/25/04 Decane 2/27/04 Decane (II) 5/3/04 Repeat test Dodecane 5/0/04 Dodecane (II) 5/24/04 Repeat test Dodecane (III) 5/25/04 Repeat test Dodecane (IV) 5/26/04 Repeat test Dodecane (V) 5/27/04 Repeat test Octane + Decane 2/26/04 Octane + Decane + Dodecane 5/4/04 6 VOC mixture: Toluene, Ethylbenzene, Octane, Decane, Dodecane, Undecane, Hexane, Dichloromethane, Tetrachloroethylene,,2-Dichlorobenzene, Formaldehyde, Acetaldehyde, n-hexanal, 2-Butanone, Cyclohexanone, and 2-Butanol 3//04 VOC Sampling and Analysis For quantitation of individual VOC, air samples were collected on the return duct of the chamber using sorbent tubes (Tenax TA, 0.2mg). These sample tubes were then analyzed by either an ATD GC/MS (Automated Thermal Desorber Gas Chromatograph/Mass Spectrometer) or GC/FID (Flame Ionization Detector) system to determine the concentration of each individual compound excluding formaldehyde and acetaldehyde. For formaldehyde and acetaldehyde, DNPH-Silica cartridges were used to collect samples and then analyzed by a HPLC system. The measurement uncertainty for individual VOC was estimated to be ± 5%. 3982
RESULTS AND DISCUSSIONS Figure shows the measured VOC concentration decay vs. time for each compound tested. Concentration (mg/m^3) 0 0. dodecane dodecane (II) dodecane (III) dodecane (IV) dodecane (V) dodecane in 3-VOC mixture dodecane in 6-VOC mixture C o nc en tra tio n (m g /m ^3 ) 0 0. decane decane (II) decane in 2-VOC mixture decane in 3-VOC mixture decane in 6-VOC mixture 0.0 0 2 3 4 (a) Dodecane 0.0 0 2 3 4 (b) Decane C o n c e n tra tio n (m g /m ^ 3 ) 0 0. octane occane in 2-VOC mixture octane in 3-VOC mixture occane in 6-VOC mixture Concentration (mg/m^3) 0 0. toluene toluene (II) toluene in 2-VOC mixture toluene in 6-VOC mixture ethylbenzene ethylbenzene in 2-VOC mixture ethylbenzene in 6-VOC mixture 0.0 0.0 0 2 3 4 0 2 3 4 (c) Octane (d) Toluene and Ethylbenzene Figure Experimental data: single compound vs. multiple compounds Assuming perfect mixing in the chamber and neglecting sink effect and air leakage rate, the mass-balance for a test VOC under full-recirculation mode during the dynamic period can be written as: V dc dt = Q η C(t) ( t 0) (C = C 0 at t = 0) () where, V test chamber volume, C contaminant concentration inside the chamber, Q volumetric air flow rate through the PCO reactor η - single-pass removal efficiency or fractional conversion as stated in Zhang (2003) Cin Cout η = C in If η does not change during the test period, an analytical solution can be obtained: Q η t V C( t) = C e (2a) or 0 Q C( t)) = ln( C ) η t V ln( 0 (2b) 3983
As shown by Figure, the VOC concentration decay followed the exponential decay well in all tests except for the results from 6 VOC mixture test. Therefore, the single-pass efficiency was calculated by least-square fit of experimental data to Eq. (2b) for these tests and results are summarized in Table 2. For the 6 VOC mixture test, the removal characteristics was different for different VOCs. For some compounds (i.e. toluene, octane), the removal rates were slow at the beginning and became larger later during the test period, which made the direct fitting of all experimental data to Eq. (2b) inappropriate. Therefore, a 2-h average removal efficiency was defined based on the time-averaged VOC concentration during the test period (Chen et. al, 2005) and reported in Table 2. Results show that the maximum difference was 0% for all the repeat tests, suggesting a good repeatability of the test method. For the 2-VOC and 3-VOC mixture tests, the addition of other compound either somewhat reduced the removal efficiency of the target VOC or had no significant effect. For example, the removal efficiency of dodecane was almost the same in the 3-VOC mixture test as that in single compound test, while the removal efficiency reduced by a factor of 2 (largest in these tests) for octane in the 3-VOC mixture test. On the other hand, the high R 2 values of least-square fit indicate that the removal efficiency was nearly constant within the tested concentration range, which further suggests that the reaction was in the linear range ( st order reaction with a constant rate coefficient) with respect to each compound in the mixture. This seems contradictory to the decrease of removal efficiency observed for some compound (i.e. octane in 3-VOC mixture test), because fitting a simple st order reaction well means that there was no obvious competitive adsorption between different VOCs in the mixture for the available sites on the catalyst surface. Therefore, the removal efficiency should not have obvious decrease unless mechanism other than competitive adsorption took into effect. However, since the decrease of removal efficiency was within a factor of.4 for most compounds in the 2-VOC and 3-VOC mixture test, the results are still meaningful and suggest that the interference effects among test VOCs under the experimental concentration levels were generally small considering the uncertainty of VOC concentration measurement. As for the 6 VOC mixture test, the interference effect among different VOCs became quite obvious. The reactions of some compounds (i.e. octane, toluene) on the catalyst surface seemed to be prohibited by the coexistence of other VOCs at relatively high concentration levels at the beginning, which was possibly due to the competition of the available adsorption sites. Once the other more reactive VOCs have been decomposed, the reactions for these compounds became faster. The 2-h average efficiency for each compound in the 6 VOC mixture test was significantly smaller than that obtained from single compound test, suggesting that the interference effect became stronger as more VOCs were added into the mixture and the TVOC concentration went up. In addition, compounds with lower removal efficiency in the single compound test appeared to also have relatively lower efficiency and more obvious delay period in the initial reaction during the 6 VOC mixture test. Table 2 Summary of Calculated Single-pass Efficiency for Each Test Compound Test VOC Compound Single-pass Fit by Regression Relative Efficiency η Slope (Qη/V) R 2 Effectiveness* (%) Toluene.698 0.997 6.8.00 Toluene (II).673 0.998 6.7 0.99 Toluene in 2-VOC.042 0.995 4.2 0.6 Toluene in 6-VOC.8** 0.27 Ethylbenzene.936 0.998 7.7.00 Ethylbenzene in 2-VOC.43 0.997 5.7 0.73 Ethylbenzene in 6-VOC 2.6** 0.34 Octane.609 0.999 6.4.00 Octane in 2-VOC.304 0.994 5.2 0.8 Octane in 3-VOC 0.80 0.998 3.2 0.50 Octane in 6-VOC.** 0.7 Decane 2.42 0.999 8.6.00 Decane (II).938 0.999 7.8 0.90 Decane in 2-VOC 2.095 0.997 8.4 0.98 Decane in 3-VOC.504 0.999 6.0 0.70 Decane in 6-VOC 2.5** 0.29 Dodecane 2.204 0.998 8.8.00 Dodecane (II) 2.320 0.999 9.3.05 Dodecane (III) 2.376 0.998 9.5.08 Dodecane (IV) 2.373 0.998 9.5.08 Dodecane (V) 2.365 0.997 9.5.07 3984
Dodecane in 3-VOC 2.05.000 8.4 0.96 Dodecane in 6-VOC 4.0** 0.45 * Relative Effectiveness was calculated as the ratio of single-pass efficiency of a compound in mixture to that of a compound as a single test VOC; ** The efficiency was 2-h average removal efficiency. CONCLUSION AND IMPLICATIONS A full-scale PCO reactor has been tested on selected VOC and mixtures to evaluate the effect of interference between multiple VOCs on removal efficiency for indoor applications. Results show that:. UV-PCO is a promising technology. The tested PCO reactor did totally remove all the test compounds (except dichloromethane) in the 6 VOC mixture test, although the relative time required for degradation was different for different compounds. 2. Whether the interference effect for multiple VOC system has to be considered depends on the number of coexisting VOC species, their properties as well as TVOC concentration. In the 2-VOC and 3-VOC mixture tests performed on toluene, ethylbenzene, octane, decane and dodecane, the removal efficiency was nearly constant and the interference effect among test VOCs were generally small within the tested concentration range. However, in the 6 VOC mixture test, the interference effect became quite obvious and the affinity of a compound for the adsorption sites on the catalyst surface might have played an important role on the relative quickness of its removal in the mixture. Brown (2002) reported that the maximum mean TVOC concentration in established dwellings was only 0.32 mg/m 3, in which neglecting the interference effect in modeling may be acceptable. However, in new dwellings and offices after renovation, the TVOC concentration could reach as high as mg/m 3 (Brown, 2002). Treating the VOCs on a non--interacting basis may significantly overestimate the performance of PCO reactor for this situation. 3. Literature review of single compound test results show that the reaction rate is in the linear range under low concentrations (i.e., less than several ppm) for most VOCs. In this concentration range, the interference effect should not occur significantly based on the competitive adsorption theory. More systematic research is needed to determine whether the L-H model with the effect of competition between different VOCs for adsorption sites considered can predict the mixture performance based on the single VOC test data under the concentration levels in typical indoor applications. ACKNOWLEDGEMENT The authors are grateful for the financial support from Niagara Mohawk - a National Grid Company, NYSERDA, STAR Center and CASE Center at Syracuse University. REFERENCES Brown S.K., 2002. Volatile organic pollutants in new and established buildings in Melbourne, Australia, Indoor Air 2, pp 55-63. Chen W., Zhang J.S., and Zhang Z., 2005. Performance of air cleaners for removing multiple volatile organic compounds in indoor air, ASHRAE Transactions, (), pp 0-4. Hall, R. J., Bendfeldt P., Obee T. N., and Sangiovanni J. J., 998. Computational and Experimental Studies of UV/Titania Photocatalytic Oxidation of VOCs in Honeycomb Monoliths, J. Adv. Oxid. Technol. Vol. 3, No. 3, pp 243 252. Hossain, Md. M., Raupp G. B., Hay S.O., and Obee T.N., 999. Three-Dimensional Developing Flow Model for Photocatalytic Monolith Reactors, AIChE Journal, Vol. 45, No. 6, pp. 309-32. Lichtin N.N., Avudaithai M., Berman E., and Grayfer A., 996. TiO 2 -photocatalyzed oxidative degradation of binary mixtures of vaporized organic compounds, Solar Energy Vol. 56, No. 5, pp 377 385. Timothy N. Obee, and Stephen O. Hay, 999. The Estimation of Photocatalytic Rate Constants Based on Molecular Structure: Extending to Multi-component Systems, J. Adv. Oxid. Technol. Vol. 4, No. 2, pp 47 52. Turchi C.S., Rabago R., and Jassal A.S., 995. Destruction of volatile organic compound (VOC) emissions by photocatalytic oxidation (PCO): Benchscale test results and cost analysis, Sematech Inc., Technology Transfer # 95082935A ENG. Zhang Yinping, Yang Rui, and Zhao Rongyi, 2003. A model for analyzing the performance of photocatalytic air cleaner in removing volatile organic compounds, Atmospheric Environment 37, pp 3395 3399. 3985