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UV processes
Separation and Purification Technology 54 (2007) 204–211
Degradation of indoor gaseous formaldehyde by hybrid VUV and TiO2/UV processes Liping Yang, Zhenyan Liu ∗ , Jianwei Shi, Yunqian Zhang, Hai Hu, Wenfeng Shangguan Research Center of Combustion and Environmental Technology, School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Huashan Road 1954, Shanghai 200030, PR China Received 15 January 2006; received in revised form 23 August 2006; accepted 7 September 2006
Abstract This paper presented a new hybrid process of vacuum ultraviolet (VUV) and TiO2 /UV for the decomposition of gaseous formaldehyde (HCHO) at a typical indoor concentration level. The performance was assessed by a preliminary technical and economic analysis. Concentration, flow velocity, relative humidity and light intensity were investigated as process variables. The synergistic or combination effect between VUV and TiO2 /UV was attained by comparing the experimental conversions with the calculated ones based on the purifier-in-series efficiency model. The results showed that the proposed hybrid process can be technically feasible and economically attractive for the decomposition of gaseous HCHO. The outstanding merit of the hybrid process is the complementation of advantages of VUV and TiO2 /UV. VUV improved the conversion of HCHO markedly on the basis of TiO2 /UV. Ozone produced by VUV could be decomposed by TiO2 /UV and its concentration was less than the maximum recommended by the WHO at the flow velocity of 0.75 m/s or more. There was weak combination effect between VUV and TiO2 /UV and the behavior was analyzed in terms of the actions of ozone. The hybrid process was more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. © 2006 Elsevier B.V. All rights reserved. Keywords: Vacuum ultraviolet; TiO2 /UV; Gaseous formaldehyde; Air purification
1. Introduction Environmental Protection Agency has validated that indoor air pollution is one of the top human health risks [1]. The studies on indoor air quality (IAQ) have been transited gradually to indoor volatile organic compounds (VOCs). Formaldehyde (HCHO), as a major indoor air contaminant, exists extensively in modern building materials and household products. Its removal is vital for improving IAQ and human being’s health due to a carcinogenic risk. Photocatalysis is a promising air purification technology for trace contaminant degradation because it can degrade a broad
Abbreviations: IAQ, indoor air quality; VOCs, volatile organic compounds; HCHO, formaldehyde; • OH, hydroxyl radicals; VUV, vacuum ultraviolet; HVAC, heating, ventilating and air-conditioning; PCO, photocatalytic oxidation ∗ Corresponding author. Tel.: +86 21 64076226 103; fax: +86 21 64076226 108. E-mail addresses: [email protected] (L.P. Yang), [email protected] (Z.Y. Liu), [email protected] (W.F. Shangguan). 1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.09.003
range of VOCs to H2 O and CO2 at room temperature and atmospheric pressure [2,3]. TiO2 has been extensively used as a photocatalyst and its band-gap energy is 3.2 eV. Under the irradiation of UV light with the band-gap energy or more, electrons are photoexcited to the conduction band from the valence band and holes are left in the valence band. Electron–hole pairs can quickly transfer to the surface of catalysts and then react with the adsorbed water and oxygen to generate highly reactive oxygen species such as hydroxyl radicals (• OH). However, the photocatalytic degradation rate of VOCs is rather low at ppb levels [4], which is more typically associated with IAQ issues [5]. Microwave or magnetic field can enhance photocatalysis [6,7], but they resulted in extra cost [8]. Vacuum ultraviolet (VUV) with high-energy photon (185 nm wavelength corresponds to a photon energy of 6.7 eV) can dissociate oxygen and water to form O(1 D) and • OH [8,9]: O2 + hν (< 243 nm) → O(1 D) + O(3 P)
(R1)
O(1 D) + H2 O → 2• OH
(R2)
O(1 D) + M → O(3 P) + M
(M = O2 or N2 )
(R3)
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O(3 P) + O2 + M → O3 + M
(R4)
H2 O + hν (185nm) → • OH + H
(R5)
Therefore, VUV is an economical and efficient technology enhancing photocatalysis. The VUV degradation of aqueous VOCs has been investigated [10–12]. In recent years, the application of this technology has been extended to the treatment of gaseous VOCs [8,9,13]. It was found that the photocatalytic mineralization efficiencies of gaseous toluene and benzene under the 254 and 185-nm UV irradiation were much higher than that under 254-nm or 365-nm UV irradiation. However, the results were obtained at rather low flow velocities (0.05–0.092 m/s [8], 0.0076 m/s [9] and 0.0076–0.03 m/s [13]) and accordingly were not applicable to heating, ventilating and air-conditioning (HVAC) systems with high flow velocities. We designed a hybrid air purification system, which was composed of a VUV lamp and a photocatalytic oxidation (PCO) reactor with 15 UV irradiation sources, to degrade indoor VOCs at higher flow velocities. The VUV lamp was not used as the light source of photocatalysis in order to avoid the formation of amount of ozone. In the present study, we investigated the degradation of gaseous HCHO by the hybrid process below 500 ppb (1 ppb corresponds to 0.0409 mol/m3 ), which are more typical HCHO concentration associated with IAQ issues [14]. Experiments were performed under different conditions such as concentration, flow velocity, relative humidity and light intensity. The outlet concentration of ozone was examined. The photolysis of ozone under the irradiation of 254-nm UV was investigated. The effect between VUV and TiO2 /UV was analyzed. The cost of the hybrid process was calculated. This study provided a valuable basis for the application of the hybrid process and provided an insight into the decomposition of ozone under the irradiation
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of 254-nm UV at relatively high flow velocities. In addition, most studies on the photocatalytic degradation of VOCs were done at relatively high concentrations and low flow velocities, so this study also provided an insight into the photocatalytic degradation of HCHO at ppb levels and relatively high flow velocities. 2. Experimental 2.1. Materials Precursor solution for TiO2 films was prepared by a sol–gel method. Tetrabutyl titante (Ti(OBu)4 , 20.42 ml) and diethanolamine (DEA, 5.76 ml) were dissolved in anhydrous ethanol (70.74 ml). After the solution was stirred for 1 h at room temperature, the mixture of deionized water (1.08 ml) and anhydrous ethanol (2.0 ml) was added dropwise to the solution. Subsequently, the resultant precursor solution was stirred at room temperature for 2 h, and then was sealed and placed in the dark for 24 h. Finally, a uniform, stable, and transparent sol of TiO2 was obtained through the above procedure. After treated by ultrasonic wave in ethanol to remove grease, foam nickels were washed with distilled water and dried at room temperature, then calcinated at 550 ◦ C for 10 min in air. TiO2 films were prepared by a dip-coating method. Foam nickels were dipped in the precursor solution for minutes and then rotated at high speed to form a wet gel film. After dried at room temperature for 12 h, it was calcined at 550 ◦ C for 45 min in air. 2.2. Experimental apparatus The schematic diagram of experimental apparatus is shown in Fig. 1. The reaction chamber was a simulated central air-
Fig. 1. Schematic diagram of experimental apparatus for degrading gaseous HCHO.
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conditioning case with the transverse section of 0.4 m × 0.6 m. The mixing duct was long enough to obtain a uniform concentration field. In addition, a fully developed velocity field was obtained to measure the flow velocity and the air state parameters reliably. The flow equalizer could make the extended airflow pass uniformly through the reaction chamber. A PCO reactor was placed in the reaction chamber, and a 15-W low-pressure mercury lamp with 80% output at 254 nm and 6% at 185 nm (1.4 × 10−6 einstein/s), was placed in front of the PCO reactor. The space irradiated by VUV is 0.168 m3 . The PCO reactor had 15 parallel-connected cells. Each cell was composed of a tubular foam nickel and an UV lamp (Fig. 1). The length and diameter of each cell were 0.44 and 0.098 m, respectively. A 15-W germicidal lamp emitting 254-nm UV light was located at the center of each cell. This position provided good irradiation for the foam nickels and the light intensity was 2.8 mW/cm2 . One end of each cell was sealed by a sealing board so that the air could enter each cell along the axial direction and then flow out through the holes of foam nickels along the radial direction. This configuration was flexible enough for large-scale applications and improved the poor mass transfer performance of annular reactors. Moreover, this configuration could decrease the face velocity by increasing windward areas and accordingly relatively long resident time of contaminations and low pressure drop could be obtained at high flow velocities. The generation dose of HCHO was controlled by a thermostatic saturator and a valve. The humidity was adjusted by a supersonic wave humidifier. The concentration of HCHO was determined by a Formaldemeter (PPM-400, England). The flow velocity was adjusted by a variable-frequency draft fan and was detected by a hot-wire anemometer (RHAT-301, China) in the mixing duct (in Section 3, the flow velocity in the reaction chamber was used). The concentration of ozone was measured by an ozone analyzer (8810, China). The light intensity was adjusted by using different types of mesh screens made of stainless steel [15] and was measured by a radiometer with a 254 nm sensor (UV-B, China). 2.3. Experimental method and procedure
Fig. 2. Schematic diagram of measurement procedure: (1) adsorption equilibrium phase, (2) degradation phase and (3) verification phase.
centration reached a steady state, and then the UV lamps were turned off. (3) Verification phase. When the adsorption equilibrium was reached again, if the concentration of HCHO was equal to the initial concentration, the data was considered to be credible due to the steady HCHO generation dose. This phase was also regarded as the adsorption equilibrium phase of the next degradation process. Both adsorption and degradation processes reached equilibrium within 15 min. The HCHO concentration was recorded every 5 min. For the creditability of data, the measurement procedure was carried out repeatedly. 3. Results and discussion 3.1. Effect of concentration The effect of inlet concentration on the conversion of HCHO at 0.6 m/s is shown in Fig. 3. The conversion in the TiO2 /UV process was about 12%. In our previous TiO2 /UV experiments, two wavelike foam nickels were placed at the cross-section of the reaction chamber, and two 15-W UV lamps emitting 254 nm were placed between two foam nickels. The conversion almost approached zero due to the very short resident
The indoor air filtered by active carbons was used as carrier gas. The purified air was discharged into another room with windows open. The concentration of HCHO was measured at the outlet of the experimental apparatus. After the water in the thermostatic saturator was heated to a certain temperature, the flow velocity was set, and then HCHO was introduced to the duct. The concentration of HCHO was further controlled by adjusting the valve. The schematic diagram of measuring procedure including three phases is shown in Fig. 2. The three phases are as follows: (1) Adsorption equilibrium phase. After the adsorption process reached equilibrium as indicated by the steady HCHO concentration in Fig. 2, the UV lamps were turned on. (2) Degradation phase. With the UV lamp on, a steady-state reaction phase was established when the outlet HCHO con-
Fig. 3. Effect of inlet concentration on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Flow velocity: 0.6 m/s; relative humidity: 50–60%.
L.P. Yang et al. / Separation and Purification Technology 54 (2007) 204–211
Fig. 4. Effect of flow velocity on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Inlet concentration: 260–290 ppb; relative humidity: 50–60%.
time. It was demonstrated that the method of increasing the resident time by increasing the windward area was effective. However, the conversion was still low. It was improved greatly by VUV, especially at relatively low concentrations, because lots of oxygen species such as O(1 D) and • OH were produced by VUV. The conversion of HCHO in the TiO2 /UV process increased slightly with the increase of concentration. The total active sites were much greater than those occupied by HCHO, so the HCHO adsorbed on TiO2 increased obviously with the increase of concentration, which caused the remarkable increase of the contact probability of HCHO with • OH on TiO2 . The conversion in the VUV and hybrid processes decreased with the increase of concentration. The degradation of HCHO by VUV is a bulk reaction and the contact time of HCHO with oxygen species is longer than that of the surface reaction, which resulted in a relatively full reaction of oxygen species. Consequently, the removal quantity of HCHO had a little change and the conversion decreased with the increase of concentration. 3.2. Effect of flow velocity The effect of flow velocity in the range of 0.3–0.94 m/s on the conversion of HCHO was investigated. For higher flow velocities, the degradations were not performed due to the unsteady generation dose of HCHO. The results are shown in Fig. 4. The conversion declined with the increase of flow velocity in all processes. In the VUV and hybrid processes, the number of photons absorbed by per unit volume air decreased with the increase of flow velocity, so the concentration of oxygen species produced by VUV dropped, which resulted in the decrease of the conversion. Fig. 5 shows the effect of flow velocity on the degradation rate of HCHO in the TiO2 /UV process. The reaction rate was calculated according to the following formula: r=
ηQCin A
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Fig. 5. Effect of flow velocity on the photocatalytic reaction rate of HCHO. Inlet concentration: 260–290 ppb; relative humidity: 50–60%.
flow velocity increased from 0.3 to 0.6 m/s and decreased when the flow velocity ranged from 0.6 to 0.94 m/s. In heterogeneous catalytic reactions, mass transfer and surface reaction are two important factors controlling overall reaction rates [16]. When the overall reaction rate is controlled by mass transfer, the reaction rate will increase with the increase of flow velocity. When the overall reaction rate is controlled by surface reaction, the reaction rate will decrease with the increase of flow velocity due to the decrease of resident time. The photocatalytic degradation of HCHO on TiO2 is a surface reaction and needs several steps [17]. Therefore, once the flow velocity exceeds a certain value, the photocatalytic reaction rate will decline evidently due to the too short resident time. 3.3. Effect of relative humidity The effect of relative humidity on the conversion of HCHO is shown in Fig. 6. In the TiO2 /UV process, the conversion depended weakly on relative humidity because HCHO does not need a lot of • OH and there is no perceptible competitive adsorption between water vapor and HCHO at low HCHO concentrations. In the VUV and hybrid processes, the conversion increased with the increase of relative humidity because more water molecules reacted with oxygen atoms and absorbed 185nm photons to generate more • OH (Eqs. (R2) and (R5)).
(1)
where η is the conversion of HCHO, Cin the inlet concentration of HCHO, Q the volume flow rate, and A is the reaction area of the PCO reactor. In Fig. 5, the reaction rate increased when the
Fig. 6. Effect of relative humidity on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Inlet concentration: 410–420 ppb; flow velocity: 0.6 m/s.
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Table 1 Conversions of HCHO and quantum yields under different photon fluxes and light intensities VUV
TiO2 /UV
F (einstein/s) at 185 nm
η1 (%)
Φ1 (%)
I
1.4 × 10−6
15.6 8.4 5.5 3.2 – – –
39 39 42 37 – – –
2.8 2.8 2.8 2.8 1.6 0.5 0.25
7.7 × 10−7 4.6 × 10−7 3.1 × 10−7 1.4 × 10−6 1.4 × 10−6 1.4 × 10−6
Hybrid process
(mW/cm2 )
at 254 nm
η2 (%)
Φ2 (%)
η (%)
12.5 – – – 12.5 12.3 11.9
0.31 – – – 0.56 1.8 3.5
23.8 18.5 16.2 14.3 23.4 22.7 21.9
Flow velocity: 0.6 m/s; concentration: 480–490 ppb.
3.4. Effect of light intensity The effect of light intensity on the conversion of HCHO and the quantum yield in the VUV and TiO2 /UV processes are shown in Table 1. The quantum yield, which indicates the light efficiency of processes, is defined as the ratio of the reaction rate to the absorption rate of photons. The quantum yield for the photooxidation of HCHO in the VUV process can be calculated according to the following formula: Φ1 =
(Cin − Cout )Q Fabs
(2)
where Cout is the outlet concentration of HCHO and Fabs is the 185-nm light flux absorbed by the photoreaction substance, which can be determined using the following equation [18]: Fabs = F (1 − 10−εlC )
(3)
where F is the 185-nm light flux emitted by the VUV lamp, ε the extinction coefficient of the photoreaction substance, l the light pathlength, and C is the concentration of the photoreaction substance. The quantum yield for the photocatalytic degradation of HCHO in the TiO2 /UV process can be calculated using the following formula: Φ2 =
rNA hc kIλ
(4)
where I is the incident light intensity on the surface of catalysts, λ the wavelength, k the optical absorption coefficient of foam nickels coated with TiO2 , NA the Avogadro’s number (6.023 × 1023 molecules/mol), h the Plank’s constant (6.626 × 10−34 J s), and c is the speed of light in a vacuum (3 × 108 m/s).
For the VUV process, the conversion of HCHO decreased linearly with the decrease of photon flux (the slope is 1 × 107 and the correlation coefficient is 0.99) and the quantum yield was about 40%. For the TiO2 /UV process, the conversion kept invariant at first and then decreased with the decrease of light intensity, while the quantum yield increased all along. It was demonstrated that the 2.8 mW/cm2 light intensity was high and a number of electron–hole pairs recombined. Electron–hole recombination and interfacial electron transfer are second-order and first-order process, respectively. As a result, the recombination rate of electron–hole pairs increases relative to the transfer rate of interfacial electrons when the light intensity increases [19]. Therefore, the quantum yield under high light intensities is much lower than that under low light intensities. In the hybrid process, the conversion of HCHO dropped significantly with the decrease of the 185-nm photon flux, while had little decrease with the decrease of the 254-nm light intensity. 3.5. Intermediate products and by-products Ozone is a by-product in the VUV process (Eq. (R4)) and the excessive ozone is a health hazard. In the VUV process, the concentration of ozone exceeded 50 ppb, which is the maximum recommended by the WHO (Table 2). In comparison, the concentration of ozone decreased greatly in the hybrid process and was less than 50 ppb at the flow velocity of 0.75 m/s or more. Ozone captured the photogenerated electrons either directly or indirectly [20]: O3 + e− → O3 −•
(R6)
O2 + e− → O2 −•
(R7)
Concentration of ozone (ppb)
Decomposition of ozone (%)
Table 2 Outlet concentrations of ozone in the VUV and hybrid processes Concentration of HCHO (ppb)
260 270 290 290 280
Flow velocity (m/s)
0.3 0.375 0.6 0.75 0.94
VUV
Hybrid process
291 235 148 119 95
104 84 59 50 45
64.3 64.3 60.1 58 52.6
L.P. Yang et al. / Separation and Purification Technology 54 (2007) 204–211
O2 −• + O3 → O2 + O3 −•
(R8)
Ozone could react with • OH [21]: O3 + • OH → HO2 • + O2
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Table 3 Decomposition of ozone under the irradiation of 254-nm UV No.
Flow velocity (m/s)
RH (%)
(R9)
Concentration of ozone (ppb)
Photolysis of ozone (%)
VUV
VUV–UV
Ia
In our experiments, it was found that ozone did not decrease after HCHO was introduced into the experimental apparatus in the VUV process. In the studies of Zhang and Liu [22,23], the conversions of hexane and toluene only by ozone were less than 5%. It was indicated that the oxidation reaction of gaseous VOCs by ozone was difficult. CO is a by-product in the photolysis of HCHO [24,25]. Obee and Brown [24] found the concentration of CO increased linearly with the increase of light intensity in the photolysis of HCHO. For the initial 3.3 ppm HCHO, they observed no CO at 7 mW/cm2 and 0.1 ppm CO at 25 mW/cm2 . In the present work, the photolysis of HCHO in the TiO2 -uncoated photoreactor under the light intensity of 2.8 mW/cm2 was investigated. A series of HCHO concentration measurement after UV lamps were turned on indicated that the photolysis was negligible. It was expected that the decomposition of HCHO in the VUV process was not a direct photolysis reaction, but a VUV-induced photooxidation reaction due to a minor emission at 185 nm and the competitive absorption of 185-nm photons between HCHO and oxygen. Formic acid is an intermediate product in the oxidation of HCHO by • OH to CO2 [17,26], but formic acid tends to be adsorbed strongly on the surface of catalysts and then quickly converted into CO2 [26]. Therefore, there was no undesirable product except a small quantity of ozone in the hybrid process. 3.6. Decomposition of ozone by UV In the study of Jeong et al. [13], when produced by 185-nm VUV, ozone was decomposed by 254-nm UV to generate • OH at flow velocities of 0.0076–0.03 m/s. The reaction equations are (R10) and (R2). O3 + hν (< 310 nm) → O(1 D) + O2
(R10)
Zhang and Liu [22] investigated the decomposition of ozone under the irradiation of 254-nm UV. The decomposition efficiency was over 90% when the concentration of ozone was in the range of 14.2–71 ppm and the flow velocity was in the range of 0.006–0.03 m/s. In the present work, we investigated the decomposition of ozone under the irradiation of 254-nm UV at relatively high flow velocities. The results are shown in Table 3. It was found that the decomposition efficiencies at high flow velocities and low ozone concentrations were rather low due to the poor contact of ozone with photons. It was illustrated that the decomposition of ozone in the hybrid process mainly resulted from TiO2 /UV but not from the photolysis and accordingly the PCO reactor was indispensable because it eliminated HCHO and ozone simultaneously.
1 2
0.375 0.75
55 57
230 108
228 107
0.9 0.9
IIb 1 2
0.375 0.75
55 57
230 108
222 105
3.5 2.8
a Two 15-W UV lamps emitting 254 nm were located at two sides of the VUV lamp in line. b Six 15-W UV lamps emitting 254 nm were placed behind the VUV lamp in two rows.
3.7. Effect of ozone produced by VUV on the conversion of HCHO on TiO2 For investigating the synergistic or combination effect between VUV and TiO2 /UV, namely, the effect of ozone produced by VUV on the degradation of HCHO on TiO2 , we compared the calculated conversions with the experimental ones in the hybrid process. The results are shown in Table 4, where ηcal is the calculated conversion and ηexp is the experimental conversion. The calculated conversion was attained in terms of the purifier-in-series efficiency model: η = 1 − (1 − η1 )(1 − η2 )
(5)
If there is no any synergistic or combination effect between VUV and TiO2 /UV, the calculated conversions should be equal to the experimental ones. In Table 4, the calculated conversions were a little larger than the experimental ones. There was weak combination effect between VUV and TiO2 /UV. Ozone did not promote the degradation of HCHO on TiO2 , but there was no obvious inhibition effect. Ozone has several actions: (1) ozone can produce additional • OH under the irradiation of 254-nm UV (Eqs. (R10) and (R2)) [22]; (2) ozone can more easily capture the photogenerated electrons on TiO2 than oxygen because the electron affinity of ozone is 2.1 eV and that of ozone is 0.44 eV (Eq. (R6)) [20]; (3) ozone is a scavenger to • OH (Eq. (R9)) [21,27]; (4) ozone may Table 4 Comparisons between the calculated conversions and the experimental ones in the hybrid process Concentration of HCHO (ppb)
190 290 410 480 260 270 290 280
Flow velocity (m/s)
0.6 0.6 0.6 0.6 0.3 0.375 0.75 0.94
VUV
TiO2 /UV
Hybrid process
η1 (%)
η2 (%)
ηcal (%)
ηexp (%)
31.6 22.4 17.1 15.6 53.8 33.8 15.9 12.8
11.1 11.1 11.8 12.3 19.1 18.1 7.1 3.4
39.2 31.0 26.9 25.9 62.6 45.8 21.8 15.8
37.4 28.4 25.3 23.8 61.5 43.2 20.7 15.2
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Table 5 Parameters used in the economic calculation
Lamp TiO2 -coated foam nickela Electricity a
4. Conclusions
Price
Lifetime
30 RMB/lamp 150 RMB/m2 0.61 RMB/kWh
6000–8000 h 3 months –
Including labor cost.
compete with VOCs for adsorption [23]. Because the decomposition efficiency of ozone was rather low at high flow velocities and low concentrations, the extra-formed • OH was little. The degradation of HCHO was not also detected in the photolysis of ozone by 254-nm UV. Furthermore, the action that ozone can capture the photogenerated electrons on TiO2 more easily than oxygen was inappreciable because the concentration of ozone is far less than that of oxygen. Therefore, the ozone produced by VUV did not promote the degradation of HCHO on TiO2 . On the other hand, the competitive adsorption of ozone with HCHO and the elimination of • OH by ozone were not obvious due to the small quantity of ozone. Therefore, the action that ozone inhibits the HCHO conversion on TiO2 was weak. 3.8. Cost analysis It is clear that a technically efficient process must also be feasible economically. In the economic calculation, energy and material costs are taken into account as major cost items. It was assumed that the air purification systems are operated 300 days a year and 24 h a day. Economic parameters used for the evaluation of total operating costs are given in Table 5. The operating costs of three processes calculated as RMB per kg removed HCHO are shown in Table 6. From the point of view of energy, the VUV process is an energy-saving process, but the excessive ozone was produced. The use of this hybrid process avoids the generation of the excessive ozone. At the same time, the hybrid process is more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. The operating cost of the hybrid process was 2752 RMB/year, which is rather small in comparison with that of HVAC systems. Therefore, the hybrid process was feasible economically for the degradation of indoor gaseous HCHO. Table 6 Operating cost VUV
TiO2 /UV
Hybrid process
Materials cost Lamps (RMB/year) TiO2 -coated foam nickel (RMB/year)
30 –
450 1218
480 1218
Energy cost (RMB/year)
66
988
1054
Operating cost (RMB/year)
96
2656
2752
364
19751
7983
Operating cost a
(RMB/kg HCHO)a
Operating conditions: 0.75 m/s flow velocity and 290 ppb concentration.
A hybrid air purification process, applicable to HVAC systems, was proposed and tested for the degradation of HCHO at a typical indoor concentration level. The experimental results showed the hybrid process could be technically feasible though ozone produced by VUV did not promote the degradation of HCHO on TiO2 because ozone was not decomposed to generate • OH at high flow velocities and low ozone concentrations. The conversion of HCHO was improved remarkably by the VUV process on the basis of the TiO2 /UV process, especially at relatively low concentrations. The excessive ozone produced by VUV was eliminated by TiO2 /UV. In the economic calculation, the VUV process was the most energy-saving process, but the excessive ozone was produced. Though the concentration of ozone can be reduced by decreasing the 185-nm photon flux, the conversion of HCHO will decrease linearly with the decrease of photon flux, which goes against our original intention to improve the conversion of HCHO. The hybrid process was more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. The operating cost was 2752 RMB/year, which is rather small in comparison with the operating cost of HVAC systems. Therefore, from the economic and technological point of view, the hybrid process was the most feasible method among them for removing indoor gaseous HCHO. Acknowledgement The study was supported by the Special Foundation of Nanometer Technology (no. 05nm05002) from Shanghai Municipal Science and Technology Commission (STCSM), China. References [1] US EPA, Characterizing air emissions from indoor sources, EPA report: EPA/600/F-95/005, US Environmental Protection Agency, Washington, DC, 1995. [2] W.F. Jardim, R.M. Alberici, Photocatalytic destruction of VOCs in the gasphase using titanium dioxide, Appl. Catal. B: Environ. 14 (1997) 55–68. [3] J. Zhao, X.D. Yang, Photocatalytic oxidation for indoor air purification: a literature review, Build. Environ. 38 (2003) 645–654. [4] C.H. Ao, S.C. Lee, Indoor air purification by photocatalyst TiO2 immobilizedon an activated carbon filter installed in an air cleaner, Chem. Eng. Sci. 60 (2005) 103–109. [5] L. Stevens, J.A. Lanning, L.G. Anderson, W.A. Jacoby, N. Chornet, Investigation of the photocatalytic oxidation of low-level carbonyl compounds, J. Air Waste Manage. 48 (1998) 979–984. [6] S. Kataoka, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, Photocatalytic oxidation in the presence of microwave irradiation: observations with ethylene and water, J. Photochem. Photobiol. A 148 (2002) 323–330. [7] M. Wakasa, S. Suda, H. Hayashi, N. Ishii, M. Okano, Magnetic field effect on the photocatalytic reaction with ultrafine TiO2 particles, J. Phys. Chem. B 108 (2004) 11882–11885. [8] P.Y. Zhang, J. Liu, Z.L. Zhang, VUV photocatalytic degradation of toluene in the gas phase, Chem. Lett. 33 (2004) 1242–1243. [9] J. Jeong, K. Sekiguchi, K. Sakamoto, Photochemical and photocatalytic degradation of gaseous toluene using short-wavelength UV irradiation with TiO2 catalyst: comparison of three UV sources, Chemosphere 57 (2004) 663–671.
L.P. Yang et al. / Separation and Purification Technology 54 (2007) 204–211 [10] T.M. Hashem, M. Zirlewagen, A.M. Braun, Simultaneous photochemical generation of ozone in the gas phase and photolysis of aqueous reaction systems using one VUV light source, Wat. Sci. Tech. 35 (1997) 41–48. [11] G. Heit, A.M. Braun, VUV-photocatalysis of aqueous systems: spatial differentiation between volumes of primary and secondary reactions, Wat. Sci. Tech. 35 (1997) 25–30. [12] F. Al-Momani, E. Touraud, J.R. Degorce-Dumas, J. Roussy, O. Thomas, Biodegradability enhancement of textile dyes and textile waste water by VUV photolysis, J. Photochem. Photobiol. A 153 (2002) 191–197. [13] J. Jeong, K. Sekiguchi, W. Lee, K. Sakamoto, Photodegradation of gaseous volatile organic compounds (VOCs) using TiO2 photoirradiated by an ozone-producing UV lamp: decomposition characteristics, identification of by-products and water-soluble organic intermediates, J. Photochem. Photobiol. A 169 (2004) 277–285. [14] I.M. Ritchie, R.G. Lehnen, HCHO-related health complaints of residents living in mobile and conventional homes, Am. J. Pub. Health 77 (1987) 323–328. [15] J.H. Wang, M.B. Ray, Application of ultraviolet photooxidation to remove organic pollutants in the gas phase, Sep. Purif. Technol. 19 (2000) 11–20. [16] O. Levenspiel, Chemical Reaction Engineering, John Wiley & Sons, Chichester, UK, 1972. [17] J.J. Yang, D.X. Li, Z.J. Zhang, Q.L. Li, H.Q. Wang, A study of the photocatalytic oxidation of HCHO on Pt/Fe2 O3 /TiO2 , J. Photochem. Photobiol. A 137 (2000) 197–202. [18] R.G. Zepp, Quantum yields for reaction of pollutants in dilute aqueous solution, Environ. Sci. Technol. 12 (1978) 327–329. [19] N.J. Peill, M.R. Hoffmann, Mathematical model of a photocatalytic fiberoptic cable reactor for heterogeneous photocatalysis, Environ. Sci. Technol. 32 (1998) 398–404.
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[20] P. Pichat, J. Disdier, C. Hoang-van, D. Mas, G. Goutailler, C. Gaysee, Purification/deodorization of indoor air and gaseous effluents by TiO2 photocatalysis, Catal. Today 63 (2000) 363–369. [21] Y.S. Shen, Y. Ku, Decomposition of gas-phase trichloroethene by the UV/TiO2 process in the presence of ozone, Chemosphere 46 (2002) 101–107. [22] P.Y. Zhang, F.Y. Liang, G. Yu, Q. Chen, W.P. Zhu, A comparative study on decomposition of gaseous toluene by O3 /UV, TiO2 /UV and O3 /TiO2 /UV, J. Photochem. Photobiol. A 156 (2003) 189–194. [23] P.Y. Zhang, J. Liu, Photocatalytic degradation of trace hexane in the gas phase with and without ozone addition: kinetic study, J. Photochem. Photobiol. A 167 (2004) 87–94. [24] T.N. Obee, R.T. Brown, TiO2 photocatalysis for indoor air applications: effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene, Environ. Sci. Technol. 29 (1995) 1223–1231. [25] H.M. Liu, Z.W. Lian, X.J. Ye, W.F. Shangguan, Kinetic analysis of photocatalytic oxidation of gas-phase formaldehyde over titanium dioxide, Chemosphere 60 (2005) 630–635. [26] F. Shiraishi, D. Ohkubo, K. Toyoda, S. Yamaguchi, Decomposition of gaseous formaldehyde in a photocatalytic reactor with a parallel array of light sources. 1. Fundamental experiment for reactor design, Chem. Eng. J. 114 (2005) 153–159. [27] J.A. Buckley, S.R. Cater, A.S. Amiri, Homogeneous photochemical destruction technology for contaminated air stream, in: Proceedings of the First International Conference on Advanced Oxidation Technologies for Water and Air Remediation (AOTs-1), London, Ont, Canada, 1994, p. 6.
Degradation of indoor gaseous formaldehyde by hybrid VUV and TiO2/UV processes Liping Yang, Zhenyan Liu ∗ , Jianwei Shi, Yunqian Zhang, Hai Hu, Wenfeng Shangguan Research Center of Combustion and Environmental Technology, School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Huashan Road 1954, Shanghai 200030, PR China Received 15 January 2006; received in revised form 23 August 2006; accepted 7 September 2006
Abstract This paper presented a new hybrid process of vacuum ultraviolet (VUV) and TiO2 /UV for the decomposition of gaseous formaldehyde (HCHO) at a typical indoor concentration level. The performance was assessed by a preliminary technical and economic analysis. Concentration, flow velocity, relative humidity and light intensity were investigated as process variables. The synergistic or combination effect between VUV and TiO2 /UV was attained by comparing the experimental conversions with the calculated ones based on the purifier-in-series efficiency model. The results showed that the proposed hybrid process can be technically feasible and economically attractive for the decomposition of gaseous HCHO. The outstanding merit of the hybrid process is the complementation of advantages of VUV and TiO2 /UV. VUV improved the conversion of HCHO markedly on the basis of TiO2 /UV. Ozone produced by VUV could be decomposed by TiO2 /UV and its concentration was less than the maximum recommended by the WHO at the flow velocity of 0.75 m/s or more. There was weak combination effect between VUV and TiO2 /UV and the behavior was analyzed in terms of the actions of ozone. The hybrid process was more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. © 2006 Elsevier B.V. All rights reserved. Keywords: Vacuum ultraviolet; TiO2 /UV; Gaseous formaldehyde; Air purification
1. Introduction Environmental Protection Agency has validated that indoor air pollution is one of the top human health risks [1]. The studies on indoor air quality (IAQ) have been transited gradually to indoor volatile organic compounds (VOCs). Formaldehyde (HCHO), as a major indoor air contaminant, exists extensively in modern building materials and household products. Its removal is vital for improving IAQ and human being’s health due to a carcinogenic risk. Photocatalysis is a promising air purification technology for trace contaminant degradation because it can degrade a broad
Abbreviations: IAQ, indoor air quality; VOCs, volatile organic compounds; HCHO, formaldehyde; • OH, hydroxyl radicals; VUV, vacuum ultraviolet; HVAC, heating, ventilating and air-conditioning; PCO, photocatalytic oxidation ∗ Corresponding author. Tel.: +86 21 64076226 103; fax: +86 21 64076226 108. E-mail addresses: [email protected] (L.P. Yang), [email protected] (Z.Y. Liu), [email protected] (W.F. Shangguan). 1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.09.003
range of VOCs to H2 O and CO2 at room temperature and atmospheric pressure [2,3]. TiO2 has been extensively used as a photocatalyst and its band-gap energy is 3.2 eV. Under the irradiation of UV light with the band-gap energy or more, electrons are photoexcited to the conduction band from the valence band and holes are left in the valence band. Electron–hole pairs can quickly transfer to the surface of catalysts and then react with the adsorbed water and oxygen to generate highly reactive oxygen species such as hydroxyl radicals (• OH). However, the photocatalytic degradation rate of VOCs is rather low at ppb levels [4], which is more typically associated with IAQ issues [5]. Microwave or magnetic field can enhance photocatalysis [6,7], but they resulted in extra cost [8]. Vacuum ultraviolet (VUV) with high-energy photon (185 nm wavelength corresponds to a photon energy of 6.7 eV) can dissociate oxygen and water to form O(1 D) and • OH [8,9]: O2 + hν (< 243 nm) → O(1 D) + O(3 P)
(R1)
O(1 D) + H2 O → 2• OH
(R2)
O(1 D) + M → O(3 P) + M
(M = O2 or N2 )
(R3)
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O(3 P) + O2 + M → O3 + M
(R4)
H2 O + hν (185nm) → • OH + H
(R5)
Therefore, VUV is an economical and efficient technology enhancing photocatalysis. The VUV degradation of aqueous VOCs has been investigated [10–12]. In recent years, the application of this technology has been extended to the treatment of gaseous VOCs [8,9,13]. It was found that the photocatalytic mineralization efficiencies of gaseous toluene and benzene under the 254 and 185-nm UV irradiation were much higher than that under 254-nm or 365-nm UV irradiation. However, the results were obtained at rather low flow velocities (0.05–0.092 m/s [8], 0.0076 m/s [9] and 0.0076–0.03 m/s [13]) and accordingly were not applicable to heating, ventilating and air-conditioning (HVAC) systems with high flow velocities. We designed a hybrid air purification system, which was composed of a VUV lamp and a photocatalytic oxidation (PCO) reactor with 15 UV irradiation sources, to degrade indoor VOCs at higher flow velocities. The VUV lamp was not used as the light source of photocatalysis in order to avoid the formation of amount of ozone. In the present study, we investigated the degradation of gaseous HCHO by the hybrid process below 500 ppb (1 ppb corresponds to 0.0409 mol/m3 ), which are more typical HCHO concentration associated with IAQ issues [14]. Experiments were performed under different conditions such as concentration, flow velocity, relative humidity and light intensity. The outlet concentration of ozone was examined. The photolysis of ozone under the irradiation of 254-nm UV was investigated. The effect between VUV and TiO2 /UV was analyzed. The cost of the hybrid process was calculated. This study provided a valuable basis for the application of the hybrid process and provided an insight into the decomposition of ozone under the irradiation
205
of 254-nm UV at relatively high flow velocities. In addition, most studies on the photocatalytic degradation of VOCs were done at relatively high concentrations and low flow velocities, so this study also provided an insight into the photocatalytic degradation of HCHO at ppb levels and relatively high flow velocities. 2. Experimental 2.1. Materials Precursor solution for TiO2 films was prepared by a sol–gel method. Tetrabutyl titante (Ti(OBu)4 , 20.42 ml) and diethanolamine (DEA, 5.76 ml) were dissolved in anhydrous ethanol (70.74 ml). After the solution was stirred for 1 h at room temperature, the mixture of deionized water (1.08 ml) and anhydrous ethanol (2.0 ml) was added dropwise to the solution. Subsequently, the resultant precursor solution was stirred at room temperature for 2 h, and then was sealed and placed in the dark for 24 h. Finally, a uniform, stable, and transparent sol of TiO2 was obtained through the above procedure. After treated by ultrasonic wave in ethanol to remove grease, foam nickels were washed with distilled water and dried at room temperature, then calcinated at 550 ◦ C for 10 min in air. TiO2 films were prepared by a dip-coating method. Foam nickels were dipped in the precursor solution for minutes and then rotated at high speed to form a wet gel film. After dried at room temperature for 12 h, it was calcined at 550 ◦ C for 45 min in air. 2.2. Experimental apparatus The schematic diagram of experimental apparatus is shown in Fig. 1. The reaction chamber was a simulated central air-
Fig. 1. Schematic diagram of experimental apparatus for degrading gaseous HCHO.
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conditioning case with the transverse section of 0.4 m × 0.6 m. The mixing duct was long enough to obtain a uniform concentration field. In addition, a fully developed velocity field was obtained to measure the flow velocity and the air state parameters reliably. The flow equalizer could make the extended airflow pass uniformly through the reaction chamber. A PCO reactor was placed in the reaction chamber, and a 15-W low-pressure mercury lamp with 80% output at 254 nm and 6% at 185 nm (1.4 × 10−6 einstein/s), was placed in front of the PCO reactor. The space irradiated by VUV is 0.168 m3 . The PCO reactor had 15 parallel-connected cells. Each cell was composed of a tubular foam nickel and an UV lamp (Fig. 1). The length and diameter of each cell were 0.44 and 0.098 m, respectively. A 15-W germicidal lamp emitting 254-nm UV light was located at the center of each cell. This position provided good irradiation for the foam nickels and the light intensity was 2.8 mW/cm2 . One end of each cell was sealed by a sealing board so that the air could enter each cell along the axial direction and then flow out through the holes of foam nickels along the radial direction. This configuration was flexible enough for large-scale applications and improved the poor mass transfer performance of annular reactors. Moreover, this configuration could decrease the face velocity by increasing windward areas and accordingly relatively long resident time of contaminations and low pressure drop could be obtained at high flow velocities. The generation dose of HCHO was controlled by a thermostatic saturator and a valve. The humidity was adjusted by a supersonic wave humidifier. The concentration of HCHO was determined by a Formaldemeter (PPM-400, England). The flow velocity was adjusted by a variable-frequency draft fan and was detected by a hot-wire anemometer (RHAT-301, China) in the mixing duct (in Section 3, the flow velocity in the reaction chamber was used). The concentration of ozone was measured by an ozone analyzer (8810, China). The light intensity was adjusted by using different types of mesh screens made of stainless steel [15] and was measured by a radiometer with a 254 nm sensor (UV-B, China). 2.3. Experimental method and procedure
Fig. 2. Schematic diagram of measurement procedure: (1) adsorption equilibrium phase, (2) degradation phase and (3) verification phase.
centration reached a steady state, and then the UV lamps were turned off. (3) Verification phase. When the adsorption equilibrium was reached again, if the concentration of HCHO was equal to the initial concentration, the data was considered to be credible due to the steady HCHO generation dose. This phase was also regarded as the adsorption equilibrium phase of the next degradation process. Both adsorption and degradation processes reached equilibrium within 15 min. The HCHO concentration was recorded every 5 min. For the creditability of data, the measurement procedure was carried out repeatedly. 3. Results and discussion 3.1. Effect of concentration The effect of inlet concentration on the conversion of HCHO at 0.6 m/s is shown in Fig. 3. The conversion in the TiO2 /UV process was about 12%. In our previous TiO2 /UV experiments, two wavelike foam nickels were placed at the cross-section of the reaction chamber, and two 15-W UV lamps emitting 254 nm were placed between two foam nickels. The conversion almost approached zero due to the very short resident
The indoor air filtered by active carbons was used as carrier gas. The purified air was discharged into another room with windows open. The concentration of HCHO was measured at the outlet of the experimental apparatus. After the water in the thermostatic saturator was heated to a certain temperature, the flow velocity was set, and then HCHO was introduced to the duct. The concentration of HCHO was further controlled by adjusting the valve. The schematic diagram of measuring procedure including three phases is shown in Fig. 2. The three phases are as follows: (1) Adsorption equilibrium phase. After the adsorption process reached equilibrium as indicated by the steady HCHO concentration in Fig. 2, the UV lamps were turned on. (2) Degradation phase. With the UV lamp on, a steady-state reaction phase was established when the outlet HCHO con-
Fig. 3. Effect of inlet concentration on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Flow velocity: 0.6 m/s; relative humidity: 50–60%.
L.P. Yang et al. / Separation and Purification Technology 54 (2007) 204–211
Fig. 4. Effect of flow velocity on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Inlet concentration: 260–290 ppb; relative humidity: 50–60%.
time. It was demonstrated that the method of increasing the resident time by increasing the windward area was effective. However, the conversion was still low. It was improved greatly by VUV, especially at relatively low concentrations, because lots of oxygen species such as O(1 D) and • OH were produced by VUV. The conversion of HCHO in the TiO2 /UV process increased slightly with the increase of concentration. The total active sites were much greater than those occupied by HCHO, so the HCHO adsorbed on TiO2 increased obviously with the increase of concentration, which caused the remarkable increase of the contact probability of HCHO with • OH on TiO2 . The conversion in the VUV and hybrid processes decreased with the increase of concentration. The degradation of HCHO by VUV is a bulk reaction and the contact time of HCHO with oxygen species is longer than that of the surface reaction, which resulted in a relatively full reaction of oxygen species. Consequently, the removal quantity of HCHO had a little change and the conversion decreased with the increase of concentration. 3.2. Effect of flow velocity The effect of flow velocity in the range of 0.3–0.94 m/s on the conversion of HCHO was investigated. For higher flow velocities, the degradations were not performed due to the unsteady generation dose of HCHO. The results are shown in Fig. 4. The conversion declined with the increase of flow velocity in all processes. In the VUV and hybrid processes, the number of photons absorbed by per unit volume air decreased with the increase of flow velocity, so the concentration of oxygen species produced by VUV dropped, which resulted in the decrease of the conversion. Fig. 5 shows the effect of flow velocity on the degradation rate of HCHO in the TiO2 /UV process. The reaction rate was calculated according to the following formula: r=
ηQCin A
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Fig. 5. Effect of flow velocity on the photocatalytic reaction rate of HCHO. Inlet concentration: 260–290 ppb; relative humidity: 50–60%.
flow velocity increased from 0.3 to 0.6 m/s and decreased when the flow velocity ranged from 0.6 to 0.94 m/s. In heterogeneous catalytic reactions, mass transfer and surface reaction are two important factors controlling overall reaction rates [16]. When the overall reaction rate is controlled by mass transfer, the reaction rate will increase with the increase of flow velocity. When the overall reaction rate is controlled by surface reaction, the reaction rate will decrease with the increase of flow velocity due to the decrease of resident time. The photocatalytic degradation of HCHO on TiO2 is a surface reaction and needs several steps [17]. Therefore, once the flow velocity exceeds a certain value, the photocatalytic reaction rate will decline evidently due to the too short resident time. 3.3. Effect of relative humidity The effect of relative humidity on the conversion of HCHO is shown in Fig. 6. In the TiO2 /UV process, the conversion depended weakly on relative humidity because HCHO does not need a lot of • OH and there is no perceptible competitive adsorption between water vapor and HCHO at low HCHO concentrations. In the VUV and hybrid processes, the conversion increased with the increase of relative humidity because more water molecules reacted with oxygen atoms and absorbed 185nm photons to generate more • OH (Eqs. (R2) and (R5)).
(1)
where η is the conversion of HCHO, Cin the inlet concentration of HCHO, Q the volume flow rate, and A is the reaction area of the PCO reactor. In Fig. 5, the reaction rate increased when the
Fig. 6. Effect of relative humidity on the conversion of HCHO in the VUV, TiO2 /UV and hybrid processes. Inlet concentration: 410–420 ppb; flow velocity: 0.6 m/s.
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Table 1 Conversions of HCHO and quantum yields under different photon fluxes and light intensities VUV
TiO2 /UV
F (einstein/s) at 185 nm
η1 (%)
Φ1 (%)
I
1.4 × 10−6
15.6 8.4 5.5 3.2 – – –
39 39 42 37 – – –
2.8 2.8 2.8 2.8 1.6 0.5 0.25
7.7 × 10−7 4.6 × 10−7 3.1 × 10−7 1.4 × 10−6 1.4 × 10−6 1.4 × 10−6
Hybrid process
(mW/cm2 )
at 254 nm
η2 (%)
Φ2 (%)
η (%)
12.5 – – – 12.5 12.3 11.9
0.31 – – – 0.56 1.8 3.5
23.8 18.5 16.2 14.3 23.4 22.7 21.9
Flow velocity: 0.6 m/s; concentration: 480–490 ppb.
3.4. Effect of light intensity The effect of light intensity on the conversion of HCHO and the quantum yield in the VUV and TiO2 /UV processes are shown in Table 1. The quantum yield, which indicates the light efficiency of processes, is defined as the ratio of the reaction rate to the absorption rate of photons. The quantum yield for the photooxidation of HCHO in the VUV process can be calculated according to the following formula: Φ1 =
(Cin − Cout )Q Fabs
(2)
where Cout is the outlet concentration of HCHO and Fabs is the 185-nm light flux absorbed by the photoreaction substance, which can be determined using the following equation [18]: Fabs = F (1 − 10−εlC )
(3)
where F is the 185-nm light flux emitted by the VUV lamp, ε the extinction coefficient of the photoreaction substance, l the light pathlength, and C is the concentration of the photoreaction substance. The quantum yield for the photocatalytic degradation of HCHO in the TiO2 /UV process can be calculated using the following formula: Φ2 =
rNA hc kIλ
(4)
where I is the incident light intensity on the surface of catalysts, λ the wavelength, k the optical absorption coefficient of foam nickels coated with TiO2 , NA the Avogadro’s number (6.023 × 1023 molecules/mol), h the Plank’s constant (6.626 × 10−34 J s), and c is the speed of light in a vacuum (3 × 108 m/s).
For the VUV process, the conversion of HCHO decreased linearly with the decrease of photon flux (the slope is 1 × 107 and the correlation coefficient is 0.99) and the quantum yield was about 40%. For the TiO2 /UV process, the conversion kept invariant at first and then decreased with the decrease of light intensity, while the quantum yield increased all along. It was demonstrated that the 2.8 mW/cm2 light intensity was high and a number of electron–hole pairs recombined. Electron–hole recombination and interfacial electron transfer are second-order and first-order process, respectively. As a result, the recombination rate of electron–hole pairs increases relative to the transfer rate of interfacial electrons when the light intensity increases [19]. Therefore, the quantum yield under high light intensities is much lower than that under low light intensities. In the hybrid process, the conversion of HCHO dropped significantly with the decrease of the 185-nm photon flux, while had little decrease with the decrease of the 254-nm light intensity. 3.5. Intermediate products and by-products Ozone is a by-product in the VUV process (Eq. (R4)) and the excessive ozone is a health hazard. In the VUV process, the concentration of ozone exceeded 50 ppb, which is the maximum recommended by the WHO (Table 2). In comparison, the concentration of ozone decreased greatly in the hybrid process and was less than 50 ppb at the flow velocity of 0.75 m/s or more. Ozone captured the photogenerated electrons either directly or indirectly [20]: O3 + e− → O3 −•
(R6)
O2 + e− → O2 −•
(R7)
Concentration of ozone (ppb)
Decomposition of ozone (%)
Table 2 Outlet concentrations of ozone in the VUV and hybrid processes Concentration of HCHO (ppb)
260 270 290 290 280
Flow velocity (m/s)
0.3 0.375 0.6 0.75 0.94
VUV
Hybrid process
291 235 148 119 95
104 84 59 50 45
64.3 64.3 60.1 58 52.6
L.P. Yang et al. / Separation and Purification Technology 54 (2007) 204–211
O2 −• + O3 → O2 + O3 −•
(R8)
Ozone could react with • OH [21]: O3 + • OH → HO2 • + O2
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Table 3 Decomposition of ozone under the irradiation of 254-nm UV No.
Flow velocity (m/s)
RH (%)
(R9)
Concentration of ozone (ppb)
Photolysis of ozone (%)
VUV
VUV–UV
Ia
In our experiments, it was found that ozone did not decrease after HCHO was introduced into the experimental apparatus in the VUV process. In the studies of Zhang and Liu [22,23], the conversions of hexane and toluene only by ozone were less than 5%. It was indicated that the oxidation reaction of gaseous VOCs by ozone was difficult. CO is a by-product in the photolysis of HCHO [24,25]. Obee and Brown [24] found the concentration of CO increased linearly with the increase of light intensity in the photolysis of HCHO. For the initial 3.3 ppm HCHO, they observed no CO at 7 mW/cm2 and 0.1 ppm CO at 25 mW/cm2 . In the present work, the photolysis of HCHO in the TiO2 -uncoated photoreactor under the light intensity of 2.8 mW/cm2 was investigated. A series of HCHO concentration measurement after UV lamps were turned on indicated that the photolysis was negligible. It was expected that the decomposition of HCHO in the VUV process was not a direct photolysis reaction, but a VUV-induced photooxidation reaction due to a minor emission at 185 nm and the competitive absorption of 185-nm photons between HCHO and oxygen. Formic acid is an intermediate product in the oxidation of HCHO by • OH to CO2 [17,26], but formic acid tends to be adsorbed strongly on the surface of catalysts and then quickly converted into CO2 [26]. Therefore, there was no undesirable product except a small quantity of ozone in the hybrid process. 3.6. Decomposition of ozone by UV In the study of Jeong et al. [13], when produced by 185-nm VUV, ozone was decomposed by 254-nm UV to generate • OH at flow velocities of 0.0076–0.03 m/s. The reaction equations are (R10) and (R2). O3 + hν (< 310 nm) → O(1 D) + O2
(R10)
Zhang and Liu [22] investigated the decomposition of ozone under the irradiation of 254-nm UV. The decomposition efficiency was over 90% when the concentration of ozone was in the range of 14.2–71 ppm and the flow velocity was in the range of 0.006–0.03 m/s. In the present work, we investigated the decomposition of ozone under the irradiation of 254-nm UV at relatively high flow velocities. The results are shown in Table 3. It was found that the decomposition efficiencies at high flow velocities and low ozone concentrations were rather low due to the poor contact of ozone with photons. It was illustrated that the decomposition of ozone in the hybrid process mainly resulted from TiO2 /UV but not from the photolysis and accordingly the PCO reactor was indispensable because it eliminated HCHO and ozone simultaneously.
1 2
0.375 0.75
55 57
230 108
228 107
0.9 0.9
IIb 1 2
0.375 0.75
55 57
230 108
222 105
3.5 2.8
a Two 15-W UV lamps emitting 254 nm were located at two sides of the VUV lamp in line. b Six 15-W UV lamps emitting 254 nm were placed behind the VUV lamp in two rows.
3.7. Effect of ozone produced by VUV on the conversion of HCHO on TiO2 For investigating the synergistic or combination effect between VUV and TiO2 /UV, namely, the effect of ozone produced by VUV on the degradation of HCHO on TiO2 , we compared the calculated conversions with the experimental ones in the hybrid process. The results are shown in Table 4, where ηcal is the calculated conversion and ηexp is the experimental conversion. The calculated conversion was attained in terms of the purifier-in-series efficiency model: η = 1 − (1 − η1 )(1 − η2 )
(5)
If there is no any synergistic or combination effect between VUV and TiO2 /UV, the calculated conversions should be equal to the experimental ones. In Table 4, the calculated conversions were a little larger than the experimental ones. There was weak combination effect between VUV and TiO2 /UV. Ozone did not promote the degradation of HCHO on TiO2 , but there was no obvious inhibition effect. Ozone has several actions: (1) ozone can produce additional • OH under the irradiation of 254-nm UV (Eqs. (R10) and (R2)) [22]; (2) ozone can more easily capture the photogenerated electrons on TiO2 than oxygen because the electron affinity of ozone is 2.1 eV and that of ozone is 0.44 eV (Eq. (R6)) [20]; (3) ozone is a scavenger to • OH (Eq. (R9)) [21,27]; (4) ozone may Table 4 Comparisons between the calculated conversions and the experimental ones in the hybrid process Concentration of HCHO (ppb)
190 290 410 480 260 270 290 280
Flow velocity (m/s)
0.6 0.6 0.6 0.6 0.3 0.375 0.75 0.94
VUV
TiO2 /UV
Hybrid process
η1 (%)
η2 (%)
ηcal (%)
ηexp (%)
31.6 22.4 17.1 15.6 53.8 33.8 15.9 12.8
11.1 11.1 11.8 12.3 19.1 18.1 7.1 3.4
39.2 31.0 26.9 25.9 62.6 45.8 21.8 15.8
37.4 28.4 25.3 23.8 61.5 43.2 20.7 15.2
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Table 5 Parameters used in the economic calculation
Lamp TiO2 -coated foam nickela Electricity a
4. Conclusions
Price
Lifetime
30 RMB/lamp 150 RMB/m2 0.61 RMB/kWh
6000–8000 h 3 months –
Including labor cost.
compete with VOCs for adsorption [23]. Because the decomposition efficiency of ozone was rather low at high flow velocities and low concentrations, the extra-formed • OH was little. The degradation of HCHO was not also detected in the photolysis of ozone by 254-nm UV. Furthermore, the action that ozone can capture the photogenerated electrons on TiO2 more easily than oxygen was inappreciable because the concentration of ozone is far less than that of oxygen. Therefore, the ozone produced by VUV did not promote the degradation of HCHO on TiO2 . On the other hand, the competitive adsorption of ozone with HCHO and the elimination of • OH by ozone were not obvious due to the small quantity of ozone. Therefore, the action that ozone inhibits the HCHO conversion on TiO2 was weak. 3.8. Cost analysis It is clear that a technically efficient process must also be feasible economically. In the economic calculation, energy and material costs are taken into account as major cost items. It was assumed that the air purification systems are operated 300 days a year and 24 h a day. Economic parameters used for the evaluation of total operating costs are given in Table 5. The operating costs of three processes calculated as RMB per kg removed HCHO are shown in Table 6. From the point of view of energy, the VUV process is an energy-saving process, but the excessive ozone was produced. The use of this hybrid process avoids the generation of the excessive ozone. At the same time, the hybrid process is more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. The operating cost of the hybrid process was 2752 RMB/year, which is rather small in comparison with that of HVAC systems. Therefore, the hybrid process was feasible economically for the degradation of indoor gaseous HCHO. Table 6 Operating cost VUV
TiO2 /UV
Hybrid process
Materials cost Lamps (RMB/year) TiO2 -coated foam nickel (RMB/year)
30 –
450 1218
480 1218
Energy cost (RMB/year)
66
988
1054
Operating cost (RMB/year)
96
2656
2752
364
19751
7983
Operating cost a
(RMB/kg HCHO)a
Operating conditions: 0.75 m/s flow velocity and 290 ppb concentration.
A hybrid air purification process, applicable to HVAC systems, was proposed and tested for the degradation of HCHO at a typical indoor concentration level. The experimental results showed the hybrid process could be technically feasible though ozone produced by VUV did not promote the degradation of HCHO on TiO2 because ozone was not decomposed to generate • OH at high flow velocities and low ozone concentrations. The conversion of HCHO was improved remarkably by the VUV process on the basis of the TiO2 /UV process, especially at relatively low concentrations. The excessive ozone produced by VUV was eliminated by TiO2 /UV. In the economic calculation, the VUV process was the most energy-saving process, but the excessive ozone was produced. Though the concentration of ozone can be reduced by decreasing the 185-nm photon flux, the conversion of HCHO will decrease linearly with the decrease of photon flux, which goes against our original intention to improve the conversion of HCHO. The hybrid process was more economical than the TiO2 /UV process with a cost reduction of 60% for removing per kg HCHO. The operating cost was 2752 RMB/year, which is rather small in comparison with the operating cost of HVAC systems. Therefore, from the economic and technological point of view, the hybrid process was the most feasible method among them for removing indoor gaseous HCHO. Acknowledgement The study was supported by the Special Foundation of Nanometer Technology (no. 05nm05002) from Shanghai Municipal Science and Technology Commission (STCSM), China. References [1] US EPA, Characterizing air emissions from indoor sources, EPA report: EPA/600/F-95/005, US Environmental Protection Agency, Washington, DC, 1995. [2] W.F. Jardim, R.M. Alberici, Photocatalytic destruction of VOCs in the gasphase using titanium dioxide, Appl. Catal. B: Environ. 14 (1997) 55–68. [3] J. Zhao, X.D. Yang, Photocatalytic oxidation for indoor air purification: a literature review, Build. Environ. 38 (2003) 645–654. [4] C.H. Ao, S.C. Lee, Indoor air purification by photocatalyst TiO2 immobilizedon an activated carbon filter installed in an air cleaner, Chem. Eng. Sci. 60 (2005) 103–109. [5] L. Stevens, J.A. Lanning, L.G. Anderson, W.A. Jacoby, N. Chornet, Investigation of the photocatalytic oxidation of low-level carbonyl compounds, J. Air Waste Manage. 48 (1998) 979–984. [6] S. Kataoka, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, Photocatalytic oxidation in the presence of microwave irradiation: observations with ethylene and water, J. Photochem. Photobiol. A 148 (2002) 323–330. [7] M. Wakasa, S. Suda, H. Hayashi, N. Ishii, M. Okano, Magnetic field effect on the photocatalytic reaction with ultrafine TiO2 particles, J. Phys. Chem. B 108 (2004) 11882–11885. [8] P.Y. Zhang, J. Liu, Z.L. Zhang, VUV photocatalytic degradation of toluene in the gas phase, Chem. Lett. 33 (2004) 1242–1243. [9] J. Jeong, K. Sekiguchi, K. Sakamoto, Photochemical and photocatalytic degradation of gaseous toluene using short-wavelength UV irradiation with TiO2 catalyst: comparison of three UV sources, Chemosphere 57 (2004) 663–671.
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