Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec H3G 1M8, CanadaDECTRON International Inc., Montreal, Quebec, Canada
i g h l i g h t s
An innovative experimental set-upwas designed and constructed fortesting UV-PCO.Test methodologies were developedto examine UV-PCO air cleaners forVOCs removal.VOCs type, inlet concentration, flowrate, irradiance, and RH have influ-ence on PCO.Gas-phase ozonation with a variety ofcompounds was examined in a ductsystem.Formation of by-products generatedfrom incomplete conversion wasinvestigated.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 23 February 2013eceived in revised form 6 July 2013ccepted 9 July 2013vailable online xxx
eywords:
a b s t r a c t
Photocatalytic oxidation (PCO) is a promising technology that has potential to be applied in mechanicallyventilated buildings to improve indoor air quality (IAQ). However, the major research studies were done inbench-top scale reactors under ideal reaction conditions. In addition, no study has been carried out on theinvestigation of the ozonation and photolysis effect using a pilot duct system. The objective of this study isthe development of methodologies to evaluate the performance of PCO systems. A systematic parametricevaluation of the effects of various kinetic parameters, such as compound’s type, inlet concentration,
airflow rate, light intensity, and relative humidity, was conducted, and new interpretations were providedfrom a fundamental analysis. In addition, the photolysis effect under vacuum ultraviolet (VUV) irradiationfor a variety of volatile organic compounds (VOCs) was examined for the first time in a pilot duct system.The performance comparison of ultraviolet C (UVC)-PCO and VUV-PCO was also discussed due to thepresence of ozone. Moreover, the formation of by-products generated with or without ozone generation
aluat
was fully compared to ev
. Introduction
Indoor air quality (IAQ) has received enormous attention forts impact on occupants’ health, comfort, and work performance.
Traditional dilution ventilation has limitation on protecting build-ing occupants against chemical and/or biological agents andreducing energy consumption. The technology of adsorption fil-tration, such as granular activated carbons and zeolites, has beenwidely studied due to their promising removal performance. How-
ever, high pressure drop and adsorbent regeneration are the mainobstacles in the applications of such adsorption air cleaners.
Heterogeneous photocatalytic oxidation (PCO), as a promis-ing advanced oxidation technology, has been suggested as an
with four UV lamps arranged in two banks (Fig. 1). The verticaldistance between the surfaces of the UV lamps and the PCO filterswas approximate 5 cm, and the distance between two lamps was
L. Zhong et al. / Journal of Hazar
lternative and energy efficient method to improve IAQ throughhe photocatalytic degradation of volatile organic compoundsVOCs) [1]. In literature, numerous researches have been carriedut to examine the PCO of gaseous contaminants [2–7]. However,ajority of available PCO data are based on laboratory bench-top
quipped with a small PCO reactor where experiments werearried out under ideal reaction conditions, i.e. low volumetricirflow rates, tested with one or a few VOCs, etc. Therefore, theest results based on the ideal experimental conditions could beroblematic and may not be scaled up to predict the performance
n full-scale systems. Although a few researches have explored theeasibility of the PCO technology applied in HVAC systems [8–10],hese applications are mainly aimed at designing a portable PCOir cleaner employed in a closed room or a chamber. Few studies11,12] aimed to explore the PCO performance as a single-pass waymployed in an HVAC system, and only several key parameters,uch as residence time, irradiance type, and filter type, have beenxamined.
The application of ozone (O3)-producing lamps in ultravioletUV)-PCO air cleaners inevitably introduces O3 into a duct system.3 is a very powerful and strong oxidant. In the past half century,inetics and mechanism of the gas-phase reactions of O3 with VOCsnder conditions relevant to the atmosphere were well examined
n the atmospheric science field [13]. It is found that the action of O3s extremely selective, and O3 usually plays a positive role to removenly alkenes and other VOCs containing unsaturated carbons. It isf interest to examine the ozonation effect in this project using
dynamic system with a relatively high O3 concentration (ppm),ather than using a traditional static chamber system with a low3 concentration (ppb). Also, the photolysis impact on the removalf VOCs under VUV irradiation is an important photochemical phe-omenon to be explored. Recently, a few studies investigated theemoval performance of UV-PCO for toluene and benzene using aench-top photocatalytic flow reactor with ozone-producing UV
amps [3,14,15]. To the best of our knowledge, no study has beenarried out on the investigation of the ozonation and photolysisffect for a wide range of VOCs using a pilot duct system, which isne of the contributions of this study.
The principal objective of this research is to develop method-logies to evaluate the performance of UV-PCO systems for IAQpplications. Therefore, this paper demonstrates a systematic eval-ation of in-duct UV-PCO air cleaners equitably and thoroughlynder the conditions relevant to the actual applications for a wideange of VOCs. In addition, the ozonation effect on the performancef in-duct PCO air cleaners has been fully examined for the firstime. Moreover, a parametric evaluation of the effects of variousinetic parameters, such as VOCs’ type, inlet pollutant concentra-ion, airflow rate, light intensity, and RH, on the PCO efficiency haseen conducted for extending the existing knowledge on the PCOechnology in indoor air applications. The formation of by-productsenerated from incomplete conversions has also been examined,hich may have a profound impact on PCO technological develop-ents.
. Experimental
.1. Materials
Two commercially available PCO air filters, titanium dioxideTiO2) coated on fiberglass fibers (TiO2/FGFs) and TiO2 coated onarbon cloth fibers (TiO2/CCFs), were examined in this study. The
hysical properties of the two systems were characterized by scan-ing electron microscopy (SEM) for morphology and N2 adsorption
sotherm for BET surface area and pore structure, which are givenn Table 1.
Low-pressure mercury lamps of each 18.4 W (Ster-L-Ray,Atlantic Ultraviolet Inc.) were employed: a G18T5L/U germicidal(UVC) lamp with a peak wavelength of 254 nm, and a G18T5VH/Uozone producing (VUV) lamp with a maximum emission at 254 nmand a minor emission at 185 nm. All lamps were powered by bal-lasts for ionization of the mercury vapor.
Eight reagent grade chemicals were selected as representativeof indoor air contaminants [16], which included toluene (99.9%), p-xylene (99.9%), 1-butanol (99.9%), n-hexane (96%), octane (95%),MEK (99.9%), and acetone (99.5%) from Fisher Scientific Inc.(Canada), and ethanol (99%) from SAQ (Société des alcools duQuébec – Québec Alcohol Board).
2.2. Experimental set-up
To develop a methodology for evaluating the performance ofUV-PCO, an innovative UV-PCO system was built up, and theschematic diagram of the test apparatus is shown in Fig. 1. The testrig was made of four parallel aluminum ducts with 0.3 m × 0.3 m(1 foot × 1 foot) inner cross section area. The system was able toprovide up to 255 m3/h (150 cfm) airflow rates and was equippedwith a radial fan with speed control mounted at the end of eachduct. This was an open-loop mode system, and the laboratoryair was introduced directly to the system after passing through apleated fabric pre-filter. The air containing evaporated VOCs wasintroduced into the PCO system through a stainless steel tube andmixed with laboratory air at the gas mixer chamber. The conditionsof inlet mixer gases were monitored for humidity and temperatureby a sensor (HMT 100, Vaisala) mounted at the center of the mixerchamber. The well-mixed gases were evenly fed into four ducts.
The upstream of each of the ducts was fitted with a perforatedstainless steel cross tube to collect air samples and an electroniclow-flow probe at the center to monitor the airflow rate. The UV-PCO reactor was designed to be versatile, so that different in-ductUV-PCO filters with various geometries could be installed. In thisstudy, three ducts were equipped with three PCO filters irradiated
Fig. 1. Schematic diagram of the UV-PCO system.
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32 L. Zhong et al. / Journal of Hazar
3.3 cm. Each lamp was 8.6 cm away from the reactor wall. In onef ducts, only two VUV lamps were installed without the PCO fil-ers in order to examine the ozonation effect. Two pressure tapsere mounted before and after each of the PCO reactors. After theV-PCO reactor, there was a probe installed at the center of eachuct to monitor RH and temperature of air stream at downstream,espectively. Downstream was also fitted with two cross samplingubes and one bulkhead union to provide ports for the collection ofOCs and ozone. Detailed prequalification tests of the test rig arevailable in previous publication [17].
The effluent stream was then introduced into the adsorptionodule containing carbon and chemical absorbents to trap the
esidual VOCs and the generated by-products. For the applica-ion of O3 producing lamps, metal honeycomb coated with MnO2ost-filters were installed at the end of the duct system for resid-al O3 decomposition. The number of layers of MnO2 post-filtersmployed in experiments were determined by the O3 concentra-ion generated downstream, and, for all experiments, the outlet O3oncentrations after the duct system were controlled in less than0 ppb.
.3. Contaminant generation system
The selected VOCs were injected using an automatic syringeump (KD Scientific). The laboratory compressed air was used ashe carrier gas and its flow rate was controlled by a mass flowontroller (Omega FMA 5400/5500). A chemically inert polyte-rafluoroethylene (PTFE) tube was used as a contaminant vaporine, through which vaporized chemical was passed into the injec-ion port. The port was placed on the top of the test rig in order tovoid condensation of the VOCs on the interior duct surface. A per-orated cross stainless steel tube with a diameter of 4.8 mm, whichas connected with the tube transporting gaseous pollutants, was
nstalled at the center of the duct system to uniformly distributeOCs in the four-duct system.
.4. Analytical methods
The inlet and outlet concentrations of VOCs and gaseous by-roducts were qualitatively and quantitatively monitored by annline calibrated photo-acoustic multi-gas monitor (B&K 1302)quipped with an auto sampler (CBISS MK3) and an offline cal-brated high performance liquid chromatography (HPLC, Perkinlmer). The concentration of ozone in each effluent stream waseasured by a calibrated six-channel ozone analyzer (Model 465L)hich was programmed to alternatively and continuously take a
ample from the downstream of each duct with an accuracy of ±1%f reading.
For the HPLC analysis, potential carbonyl by-products wererapped on a high purity silica adsorbent coated with 2,-dinitrophenylhydrazine (2,4-DNPH) (Supelco LpDNPH S10L).ample eluate was separated and analyzed by the HPLC withV detection (360 nm) equipped with a C18 Brownlee validatedicro-bore column (150 mm × 4.6 mm ID, 5 �m film thickness).cetonitrile and distilled water were used as mobile phase with aow rate of 1.0 mL/min. A gradient analysis method was developed:he ratio of 70% acetonitrile/30% water was held for 6 min, then theatio increased to 100% acetonitrile/0% water and maintained for
min, and finally the ratio returned back to 70% acetonitrile/30%ater for 4 min.
The irradiance of 254 nm and 185 nm on the surface of TiO2 fil-er was monitored by two calibrated UV radiometers (Steril-Aire
nd ILT900-R). The locations of nine irradiance measurement portsere equally arranged at the center of the duct. The average of nine
rradiance values represented the average irradiance on the surfacef the PCO filter. The average irradiances of 254 nm and 185 nm
aterials 261 (2013) 130– 138
under the airflow of 170 m3/h were 25–36 W/m2 and 1–3 W/m2,respectively.
2.5. Experimental conditions and procedure
The target concentration of the selected challenge gases wasa sub-ppm level (0.25–2 ppm) which closely represented indoorair pollution conditions. All PCO experiments were carried out atdifferent setup values for the parameters, except humidity wasunregulated in this system. Table 2 summarizes the detailed exper-imental conditions based on which to fully examine the impacts ofvarious experimental parameters on the UV-PCO performance.
The three-step injection procedure was developed and adoptedfor all UV-PCO tests, which included the following details. First, thepreparation work contained calibration of the air sampling pumpsand the sampling cartridges, and calibration of the online multi-gas monitor for the selected VOCs. Meanwhile, the sampling andmeasurement system was established by placing the sampling linesand sampling pumps in position and connecting potassium iodide(KI) ozone scrubber (selective removal of O3 and no VOCs lost byadsorption) in the sampling lines, setting up the real-time test sys-tem to monitor the airflow rate, temperature, and RH in each duct,setting up the online measurement system to monitor VOCs andO3 at upstream and downstream of each duct, and setting up anappropriate contaminant generation system.
Second, the proper PCO filters and UV lamps were installed in thedesignated position in each PCO reactor. The fans were turned onand were set at an appropriate airflow rate; the multi-gas analyzerand the O3 monitor were turned on to measure the backgroundfor 30 min; and then UV lamps were switched on to get a stableUV output. When the experimental conditions became stable, thePCO reaction could be initiated by first injection of a challenge VOCwith an appropriate injection rate, and the real-time concentra-tion was recorded by the online measurement system. Once thesteady-state condition was reached, DNPH samples were taken atsampling rate of 1.3 L/min for 1.5 h to explore the generation ofby-products. For all VOCs at inlet concentration of 500 ppb, DNPHsamples were taken twice in four ducts to check the repeatabilityof by-products. Upon DNPH sampling completion, the UV-PCO testwas ended by stopping the injection while the measurement wascontinued. Then the UV-PCO test was repeated with second injec-tion and third injection. In order to avoid of catalyst deactivationresulting from the high concentration, the order of injection ratewas in accordance with the expected concentration from low tohigh. The whole duration of a UV-PCO test for a VOC with threeconcentration levels lasted approximate 10 h.
Before starting of a UV-PCO test for another VOC, each set ofPCO filters was irradiated under UV lamps with fans running fora period of around 10 h to regenerate catalyst and to remove theresidue VOCs in the PCO system.
2.6. Quantification method
The experimental data collected from the upstream and thedownstream measurement ports is employed to calculate the effec-tiveness of a UV-PCO air cleaner. Single-pass removal efficiency, �t
(%), is determined by the amount of the removed pollutant fromthe air stream after it goes through the air cleaner, and it is definedas follows:
�t = Q (Cup,t − Cdown,t)QC
× 100 = Cup,t − Cdown,t
C× 100 (1)
up,t up,t
where �t (%) is the single-pass efficiency of a pollutant; Cup,t
(mg/m3) is the contaminant concentration at upstream as a func-tion of time; Cdown,t (mg/m3) is the contaminant concentration at
L. Zhong et al. / Journal of Hazardous Materials 261 (2013) 130– 138 133
Single VOC Toluene, p-xylene, ethanol, 1-butanol,acetone, MEK, hexane, octane
– –
Inlet concentration (ppm) 0.25–2 ±5% of the reading ±0.27Volumetric flow rate (m3/h) 41–255 (25–150 cfm) ±1.3% of the reading ±12.2 (7.2 cfm)
dt
3
3
3
c(tohtafsckaitgditootpstvsoti(ps
TR
RH (%) 10–60
Light intensity (W/m2) 16–43
Temperature (◦C) 20–25
ownstream as a function of time and Q (m3/s) is the airflow ratehrough an air cleaner.
. Results and discussion
.1. Influence of the targeted molecules
.1.1. Effect of VOCs typeFig. 2 presents the conversion of each VOC at an inlet
oncentration of 500 ppb under three experimental scenariosTiO2/FGFs + UVC, TiO2/FGFs + VUV, and TiO2/CCFs + VUV). For twoypes of air filters, the order of single-pass removal efficiencyf the selected chemical classes follows the sequence of alco-ols > ketones > aromatics > alkanes. These observations agree withhe photocatalytic oxidation rates reported by Hodgson et al. [11]nd Obee and Hay [18]. This sequence is also in the same trendound in the adsorption capacity of TiO2 coated air filters for theelected chemical classes [14]. This implies that adsorption pro-ess, to be more specific, the intermolecular force is one of theey factors influencing the photocatalytic activity. For non-polarlkanes adsorbed in the solid phase dispersion forces are the mainntermolecular force, which is weaker than van der Waals interac-ions for aromatics. Due to the high dipole moment of the carbonylroup, dipole–dipole interactions for ketones are stronger than vaner Waals attractions for aromatics. In addition to van der Waals
nteractions, hydrogen bonding plays a greater role for the attrac-ion between alcohols and hydrated catalyst surface. It can be alsobserved from Fig. 2 that single-pass UV-PCO removal efficiencyf TiO2/CCFs filter is distinctly higher than that of TiO2/FGFs fil-er for all VOCs. Photocatalytic activity depends not only on theroperties of challenge VOCs but also on the features of substratesupporting TiO2. Generally, the larger specific surface area helpso increase photocatalytic activity since more active sites are pro-ided through coating of TiO2 nano-particles on the larger BETurface of TiO2/CCFs [19]. According to Table 1, the BET surface areaf TiO2/CCFs was 887.7 m2/g, which was roughly 8 times higherhan that of TiO2/FGFs leading to a higher photocatalytic activ-ty for TiO2/CCFs. It is worth mentioning that RH was unregulated
15–45%) for all tests, and the cross influence of RH on the UV-PCOerformance for different VOCs was not considered here. Table 3hows the values of RH for the tests of the targeted molecules.
±0.17% of the reading ±2.4±5% of the reading ±1.0±0.17 ◦C ±0.8
TiO2/FGFs filter presents hydrophilic property, and VOCs withhigh polarity show higher affinity to the surface of TiO2/FGFs filter,whereas TiO2/CCFs filter belongs to a non-polar substrate whichprefers to adsorb non-polar VOCs. In addition, the lighter VOC inthe same chemical class always shows higher photocatalytic activ-ity than the heavier VOC for the TiO2/FGFs filter, which is opposingfor the TiO2/CCFs filter. This is attributed to the fact that for theTiO2/FGFs filter with less adsorptive ability, intermediates of smallmolecular weight generated from UV-PCO of light VOCs are lesscompetitive with light VOCs for adsorption and photocatalytic reac-tion at active sites, and they are also more easily further oxidizedor desorbed under humid conditions resulting in more active sitesavailable. However, for the TiO2/CCFs filter with strong adsorp-tion ability, van der Waals interaction is the dominant force, whichincreases with molecular weight. Hence, the heavier VOC of eachgroup demonstrates more active in the UV-PCO.
3.1.2. Effect of inlet concentrationThe UV-PCO experiments were conducted using three differ-
ent inlet concentration levels with all selected VOCs to examine itseffect on the removal performance of UV-PCO. Usually the inletconcentration was 250 ppb, 500 ppb and 1000 ppb, except thatmaximum 800 ppb was selected for 1-butanol due to difficult evap-oration and minimum 500 ppb was used for acetone because ofdetection limit. The effect of the inlet concentration on the single-pass removal efficiency for various VOCs is shown in Fig. 3. Thetrend of a lower inlet concentration resulting in higher removalefficiency was observed for all VOCs. The same behavior was alsoobserved by Jeong et al. [3] when they studied the photodegra-dation of toluene in the range of inlet concentrations from 0.6to 20 ppm under VUV irradiation. This can be interpreted by thelimited adsorption capacity of the fixed active sites at the catalystsurface. The amount of molecules effectively participating in theUV-PCO reaction is not enhanced in the same ratio as an increase ofthe inlet concentration resulting in a decrease of removal efficiency.Moreover, the competitive effect between multiple by-productsand the challenge VOC to some extent inhibits the adsorption ofa VOC, especially when its inlet concentration is high. Hence, com-pared with the challenge concentrations, the number of the activesites resulting from low BET surface area of TiO2/FGFs is a limitingfactor in this study. This result agrees with the conclusion madeby Sleiman et al. [5] that PCO is suitable for the photodegradationof gaseous effluents at low ppb concentration levels. It should benoted that although RH was not constant between tested VOCs, itwas almost constant for each individual one for three concentrationlevels (shown Table 3).
3.2. Influencing parameters
3.2.1. Effect of airflow rate
Fig. 4 shows the effects of airflow rate on the conversion of
ethanol. The airflow rate varied from 69 m3/h (45 cfm) to 255 m3/h(150 cfm) for the TiO2/FGFs filter and from 41 m3/h (25 cfm) to170 m3/h (100 cfm) for the TiO2/CCFs filter. It is evident from the
134 L. Zhong et al. / Journal of Hazardous Materials 261 (2013) 130– 138
Fig. 2. Single-pass removal efficiencies of various VOCs under three different experimenirradiance = 27–36 W/m2).
Table 4PCO conditions at different airflow rates.
PCO filter Airflowrate (m3/h)
Pressure dropat PCO (Pa)
Face velocity(m/s)
Residencetime (ms)
TiO2/FGFs 69 8 0.21 4589 13 0.26 37
127 26 0.38 25187 47 0.56 17255 78 0.76 12
TiO2/CCFs 41 17 0.12 7981 45 0.24 40
riht[sri
oe
Fa
127 99 0.38 25170 188 0.51 19
esults that the conversion decreased gradually with an increasen airflow rate. The same behavior was also observed for acetone,exane and toluene when the airflow rates were increased. Also,hese trends are in accordance with results reported previously3,6,11]. In addition, the curve of ethanol result is somewhat con-istent with the finding reported by Hodgson et al. [11] that theelationship between the reaction efficiency and the residence time
s approximated reasonably well by an exponential function.
The airflow rate of 41–255 m3/h corresponding to a face velocityf 0.12–0.76 m/s (shown in Table 4) was used in this study. How-ver, by changing the airflow rate, the residence time differed a lot.
ig. 3. The effect of inlet concentration on conversion of various VOCs under UVC irrnce = 27–30 W/m2).
Here, residence time is defined as the thickness (0.95 cm) of the airfilter divided by the face velocity. Table 4 presents the residencetime and the pressure drop for the tested airflow rates of two airfilters. Decreasing the airflow rate helps to increase the residencetime so that more VOCs can be adsorbed to the catalyst surfaceand adsorbed molecules have more chances to participate in thereactions with hydroxyl radicals, and then to be oxidized. As a con-sequence, a higher conversion rate can be achieved. It was observedthat the pressure drop increased as the airflow rate enhanced fortwo filters, which was in accordance with the conclusion reportedby Destaillats et al. [12]. The pressure drop of the TiO2/CCFs fil-ter increased more compared to that of TiO2/FGFs due to the highresistance resulting from the high BET surface area of the TiO2/CCFsfilter. It should be noted that in the case of VUV irradiation, O3concentrations were varied when airflow rate changed. Photolysisalso plays a role for the removal of VOCs, which will be discussedin Section 3.3.1. Therefore, a lower airflow rate results in highersingle-pass removal efficiency. The multi-pass method may be analternative to relatively extend the residence time of VOCs for thePCO technology applied in a HVAC system.
3.2.2. Effect of light intensityThe effect of light intensity on the performance of UV-PCO
air cleaners was examined at 170 m3/h airflow rate with the
adiation for the TiO2/FGFs air filter (RH = 15–45%, airflow rate = 170 m3/h, irradi-
L. Zhong et al. / Journal of Hazardous Materials 261 (2013) 130– 138 135
nvers
lalel1ccaaci[ld
3
id
F(
exposure, the conversion of ethanol for TiO2/CCFs and TiO2/FGFsdecreased from 40% to 23% and from 30% to 13%, respectively. For
Fig. 4. The effect of airflow rate on ethanol, acetone, hexane, and toluene co
ighter VOCs of each selected group (ethanol, hexane, acetone,nd toluene). The configurations of one, two, three, and four UVCamps standing in a row with two TiO2/FGFs filters were used inach duct, and the vertical distance between air cleaners and UVamps was kept constant. The range of irradiance employed was6–43 W/m2. The experimental results of single-pass removal effi-iency for tested VOCs are shown in Fig. 5. The increase trend ofonversion rate with an increase of the irradiance was observed forll VOCs. The trend was correctly described by a power function,nd the power exponent was in the range of 0.4–0.6, which is inonsistent with the reaction order of 0.5 for high absorbed lightntensity (greater than 10–20 W/m2) reported by Obee and Brown20]. It should be noted that the removal efficiency of ethanol wasower than that of acetone due to the possible reason of partialeactivation of the catalyst in this case.
.2.3. Effect of RHWater vapor plays a dual role in the UV-PCO through the follow-
ng ways: on the one hand it provides hydroxyl radicals by chemicalecomposition of adsorbed water; on the other hand excessive
y = 0.04 11x0.4223
R² = 0.931 2
y = 0.0175x0.585 0
R² = 0.9119y = 0.0228 x0.483 4
R² = 0.9688
y = 0.0072 x0.471 3
R² = 0.931 20%
5%
10%
15%
20%
10.0 20.0 30.0 40.0 50.0Sin
gle-
pass
rem
oval
effi
cien
cy
Irr adian ce (W/m2)
TiO2/FGFs+UVCAcetone Etha nol Toluene Hexane
ig. 5. The effect of irradiance on ethanol, acetone, hexane, and toluene conversioninlet concentration = 500 ppb, flow rate = 170 m3/h, RH = 55–62%).
water vapor competes with challenge VOCs for the same surfaceof TiO2. The optimal humidity level is determined by the balancebetween conversion promotion through chemical processes andinhibition through physical interactions [1].
Due to the limitation of the test facility, RH inside the duct wasuncontrolled. RH examining experiments were conducted whenthe laboratory RH conditions achieved to the expected levels. Theeffect of water vapor on the conversion of VOCs was examined byapplying humidity levels from 10% (2300 ppm) to 60% (16,000 ppm)to 500 ppb ethanol. Fig. 6 shows the single-pass removal efficiencyfor ethanol as a function of RH at different experimental scenar-ios. A decrease in the conversion rate of ethanol from 26% to11% was observed when RH of air increased from 10% to 57% forthe TiO2/FGFs filter under UVC illumination. In the case of VUV
the tested other VOCs, such as acetone, hexane, and toluene, watervapor also shows inhibition effect on their conversion rates. These
O3=1017 .1p pb
1269.2
1119.4
1428.4
1077.1
O3=237 2.51112.9
1321.7
1061.1 903.9
5%
10%
15%
20%
25%
30%
35%
40%
45%
0% 20% 40% 60%
Sin
gle-
pass
rem
oval
effi
cien
cy
RH
Ethano l TiO2/CC Fs+VU V
TiO2/FGFs+VUV
TiO2/FGF s+UVC
Fig. 6. The effect of RH on ethanol at different experimental scenarios (inlet con-centration = 500 ppb, flow rate = 170 m3/h, irradiance = 24–36 W/m2).
136 L. Zhong et al. / Journal of Hazardous Materials 261 (2013) 130– 138
Table 5Rate constants k for the gas-phase reactions of O3 with various compounds.
Compound k (cm3 molecule−1 s−1) T (K) Techniqueb Lifetimea
d-limonene, pinene)10−14–10−16 295 ± 2 S/F-CL 0.03–3 h
Monocyclic aromatic (except styrene, i.e. toluene, xylene) ≤10−20 297 ± 2 S-CL ≥30 yearsOxygen-containing compounds not containing double c–c
bonds (i.e. alcohols, ketones)≤10−20 297 ± 2 S/F-CL/IR/CA ≥3 years
a The rate constants were calculated when an O3 concentration of 1 × 1012 molecule cm−3 (i.e. around 40 ppb at ground level) was used.b S stands for static system; F stands for flow system; MS stands for mass spectrometry; IR stands for infrared absorption spectroscopy; FTIR stands for Fourier transform
infrared absorption spectroscopy; UV stands for ultraviolet absorption; CL stands for chemiluminescence; and CA stands for chemical analysis.
ethanol (ppb) 1-butano l (p pb) he xane (ppb) octane (ppb
Fig. 7. Single-pass removal efficiencies of various VO
bservations can be interpreted as the competition for adsorptionetween the VOC and water molecules. In addition, the presencef abundant water may enhance the possibility of electron–holeecombination, which is an unfavorable process for PCO of VOCs.ccording to ASHRAE Standard 55-2010 [21], a RH between 40% and0% is recommended for a healthy and comfortable indoor environ-ent. Compared with VOCs concentrations (typically ppb levels) in
he context of indoor air applications, water vapor exists in largexcess so that it is unlikely that hydroxyl radical concentrationsecome rate limiting. Competitive adsorption and electron–holeecombination can be deemed as the dominating interaction pro-esses under the condition that the RH is achievable in buildingsnd HVAC systems.
.3. Ozone influence
.3.1. Ozonation and photolysisWhen only VUV lamps are presented in a duct, a target com-
ound could be broken down by photolysis effect as well aszonation effect. The gas-phase reactions of O3 with various classesf organics under the conditions relevant to the atmosphere wereiscussed by Atkinson and Carter [13], and the bimolecular rateonstants are summarized in Table 5. From this table, it can beound that O3 usually plays a positive role in removing alkenes. Forlkanes, aromatics and oxygen-containing organics, the reactionsre very slow, at the room temperature with the bimolecular rateonstant of ≤10−20 cm3 molecules−1 s−1. Hence, these reactionsre of negligible atmospheric importance. Therefore, the dominantrocess tends to be photolysis since photons with an energy at the
ltraviolet wavelength can affect the chemical bonds.
The photons output relates with the number of VUV lamps. Here,xperiments to investigate the photolysis effect were conductedy placing different numbers of VUV lamps in each duct so as to
ene (ppb) p- xylene (ppb) acetone (ppb) MEK (ppb)
hree concentration levels by photons (RH = 35–58%).
establish four photon energy levels. Eight types of single compoundwith three concentration levels were employed using the samemethodology described in Section 2.5. Fig. 7 shows the single-passremoval efficiencies with similar concentrations for various VOCsat the 170 m3/h (100 cfm) airflow rate in the absence of any PCOfilter. It clearly shows acetone and MEK were scarcely removed.This low reactivity in the chemical reaction is due to the weakerelectron withdrawing power of the carbonyl group compared tothe hydrocarbons. Hence, ketones interact with photons much lessreadily than the other VOCs do. In a duct system, the photolysiseffect on the elimination of ketones is negligible. The results alsoindicate the strong interaction of photons with the heavier VOCsin the same class resulting in higher removal efficiency, especiallyfor aromatics. For example, when toluene and p-xylene were atthe similar inlet concentrations, the single-pass removal efficiencyof p-xylene by the photolysis effect was around twice of toluene.Moreover, these results further show that under the condition ofthe same amount of VUV lamps, an increased concentration of achallenge VOC significantly reduced the removal efficiency.
3.3.2. Ozone-involved UV-PCOThe electron affinity (EA) of O3 is 2.103 eV, and is considerably
larger than that of O2 (0.44 eV) or the oxygen atom (1.46 eV) [22].Thus, excited electron resulting from absorbance of UV photon withTiO2 is captured more efficiently in the presence of O3. The possi-ble set of reaction steps considered in this scenario is described asfollows:
e−CB + O3 → O− + O2 (2)
e−CB + O2 → O−
2 (3)
O−2 + O3 → O2 + O−
3 (4)
L. Zhong et al. / Journal of Hazardous Materials 261 (2013) 130– 138 137
Table 6Comparison of gas-phase UV-PCO by-products generated with or without ozone.
characteristics of the catalysts. Therefore, by knowing the intrin-
(1.4–6.0)
a UVC (with TiO2/FGFs). VUV (with TiO2/FGFs or TiO2/CCFs) (the HPLC detection l
−3 + H2O → OH · +OH− + O2 (5)
The enhancement of electron capture rate due to the partici-ation of O3 in PCO process not only reduces the possibility ofecombination of electron–hole pairs, but also more effectivelyroduces hydroxyl radicals through complicated chain reactions.onsequently, the single-pass removal efficiency of TiO2/VUV wassually higher than that of TiO2/UVC for the tested VOCs (seeig. 3) due to the higher generation rate of hydroxyl radicalsn the presence of O3. Photolysis effect during TiO2/VUV pro-ess also played a significant role. Jeong et al. (2005) and Yangt al. [3,23] also came to the same conclusion that employ-ent of VUV in PCO technology may be more effective and more
conomical than the TiO2/UVC process for the treatment of gastreams.
.3.3. Gas-phase by-products generated with or without ozoneThe formation of by-products in the photocatalytic oxidation
f the selected VOCs was investigated. Table 6 lists all the valueanges of major gas-phase UV-PCO by-products detected by thePLC when the challenge gas was around 500 ppb. Formaldehydend acetaldehyde were produced as the UV-PCO reaction productsor all experiments in the absence or in the presence of ozone. Usu-lly, yields of formaldehyde and acetaldehyde are proportional tohe inlet concentrations of a challenge VOC. It is evident that theormation of by-products generated from incomplete oxidation byhe UV-PCO is closely related to the nature of a challenge VOC. Forxample, the amount of acetaldehyde produced from the PCO ofthanol was 10 times higher than its amount produced from theCO of other VOCs. Hence, the by-products derived from each VOCere somewhat different, which were propanal and butanal for 1-
utanol, benzaldehyde for toluene, tolualdehyde for p-xylene, andtc.
In the case that VUV lamps were employed in the absence ofn air filter, the detected by-products generated by the photo-ysis were not significantly different from those produced from
V-PCO, but they were generated at lower concentrations. In addi-
ion, compared with UVC-PCO, the involvement of O3 induced byhe VUV lamps enhanced the mineralization of acetaldehyde andutanal as a by-product of partially oxidized ethanol and 1-butanol,
f each analyte is 3 ng).
respectively. This is attributed to more hydroxyl radicals generatedfrom photolysis of O3. Moreover, more products were producedwhen employing the VUV lamps than those using the UVC lampsdue to the photolysis. Detailed discussion about the impacts ofdifferent experimental conditions on the by-products generationcould be found in Farhanian et al. [24].
4. Conclusion
An innovative UV-PCO duct system experimental set-up wasdesigned and constructed, and in-duct test methodologies weredeveloped to investigate the performance of UV-PCO air cleanersfor various VOCs removal. The single-pass removal efficiency of twotypes of air filters (TiO2/FGFs and TiO2/CCFs) under UVC or UVVillumination in removing VOCs with various physical propertiesranks as follows: alcohols > ketones > aromatics > alkanes. It wasobserved that the PCO removal efficiency increased by decreasingof the inlet concentration, reducing the airflow rate, increasingof the irradiance, or decreasing of RH, respectively. Competitionof adsorption sites, bond energy, residence time, and the num-ber of effective hydroxyl radicals are new interpretations of theseobservations based on a fundamental analysis of reaction mecha-nisms.
For the first time, it was investigated that the photolysis underVUV irradiation played a critical role in the removal of alkanes, alco-hols, and aromatics at three concentration levels in a duct system.Additionally, the removal efficiency reduced with an increase in theinlet concentrations of a challenge VOC. Furthermore, the conver-sion rates of the VOCs by the VUV-PCO were higher than that bythe UVC-PCO due to the presence of ozone.
Formation of by-products had a close relationship with thePCO reaction mechanisms of different VOCs. The appearance andthe concentrations of the by-products depended on the experi-mental conditions, the nature of a challenge VOC, as well as the
sic kinetics, the design of a PCO air purifier working under optimalconditions can reduce the generation of by-products, which is thefuture research direction to enhance the practicability of the PCOtechnology.
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38 L. Zhong et al. / Journal of Hazar
cknowledgements
The authors would like to express their gratitude to the Naturalciences and Engineering Research Council of Canada for suppor-ing this research project through a CRD grant, and Circul-Aire, Inc.or the financial support and providing the support for the designnd construction of the experimental set-up.
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