Environ. Sci. Technol. 2007, 41, 2028-2035
Evaluation of the Intrinsic Photocatalytic Oxidation Kinetics of Indoor Air Pollutants IGNASI SALVADO Ä -ESTIVILL,† DAVID M. HARGREAVES,‡ AND G I A N L U C A L I P U M A * ,† Photocatalysis & Photoreaction Engineering, School of Chemical, Environmental and Mining Engineering and School of Civil Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
This paper presents a methodology for the evaluation of the intrinsic photocatalytic oxidation (PCO) kinetics of indoor air pollutants. It combines computational fluid dynamics (CFD) modeling of the fluid flow in the reactor with radiation field modeling and photocatalytic reaction kinetics to yield a rigorous model of a flat-plate, single-pass, flowthrough photocatalytic reactor for indoor air purification. This method was applied to model the PCO of trichloroethylene (TCE) in humidified air and to derive kinetic parameters directly from kinetic data in an integral flow reactor. Steadystate PCO experiments of TCE over irradiated TiO2 (Degussa P25) thin films immobilized on glass supports were carried out at different radiation intensities, flow rates, and inlet substrate concentrations. The oxidation rate of TCE was found to be first-order on the incident photon flux and to follow a Langmuir-Hinshelwood type reaction kinetics rate law. Mass transfer resistances were observed at Reynolds numbers less than 46. Apparent quantum yields were found to be up to 0.97 mol Einstein-1. A comparison of the model prediction with the experimental results in an integral reactor yielded pollutant-specific kinetic rate parameters which were independent of reactor geometry, radiation field, and fluid-dynamics. The kinetic parameters would, therefore, be more universally applicable to the design and scale-up of photocatalytic reactors for indoor air purification.
Introduction In developed countries, many people spend well over 90% of their time indoors (1). Rates of respiratory disease and incidence of allergic responses such as asthma have increased in recent years, and there is concern that some of this increase can be associated with changes in the quality of the air in the indoor environment (2). The elimination of low/trace concentration gaseous indoor air pollutants is of particular concern because of their long-term effects on humans. Indoor pollutants arise from a number of sources including outdoor air, the material of construction, fittings and fixtures, furniture, and the occupants (2-3). When indoor chemical * Corresponding author phone: +44 (0) 115 9514170; fax: +44 (0) 115 9514115; e-mail:
[email protected]. † School of Chemical, Environmental and Mining Engineering, The University of Nottingham. ‡ School of Civil Engineering, The University of Nottingham. 2028
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 6, 2007
contamination is coupled with microbiological contamination of the building ventilation system (e.g., fungal spores), serious effect on the occupants may arise. Inadequate ventilation, chemical, and microbiological contamination are contributing factors to ‘‘sick building syndrome”. The efficient removal of indoor airborne particles and volatile organic compounds (VOCs) in public and private buildings would allow for a reduction in outdoor air supply rates. Consequently, when air conditioning (heating or cooling) is necessary, substantial energy savings may be realized. UV photocatalytic air purifiers have the potential to achieve the necessary reductions in indoor VOC levels (4-5). However, one of the unresolved issues in the commercial application of this photocatalytic oxidation (PCO) technology to the treatment of contaminated indoor air is the availability of reliable tools to aid the scale-up, design, and optimization of photocatalytic reactors (6). This is currently the main concern faced by the industry, and it prevents widespread use of photocatalytic air purifiers (7). In the absence of a valid photocatalytic reactor model, the PCO unit must be designed through an empirical, trial-anderror process that neither guarantees an energy-efficient design, nor provides an understanding of the practical capabilities and limits of the technology (6). In addition to all the standard design parameters that must be taken into account in the design of chemical reactors, photocatalytic processes must have regard for additional factors. These are to guarantee an efficient contact between the activating photons, the solid catalyst, and the pollutants which are usually present at low or trace concentration levels; to achieve high effectiveness of photon utilization to ensure maximum efficiency for the entire photocatalytic process. To evaluate the impact of each of the parameters that influence the above factors, it is essential to develop a mathematical model of the photocatalytic reactor. This involves coupling the radiation field model with the momentum balance, the mass balance, and the appropriate reaction kinetics. The rigorous modeling of the radiation field in photocatalytic reactors is particularly complex (8-9). Many of these studies have been successful in providing a detailed description of the effect of radiation intensity and photocatalyst optical properties on the photocatalytic process. However, thorough models which can predict pollutant conversion as a function of the operating parameters in real photocatalytic reactors have received little attention, especially with regard to gas-phase photocatalytic reactions (6). Furthermore, these models are essential for extracting intrinsic kinetic data from results collected in integral flow reactors (observed reactant conversion higher than 5%). Computational fluid dynamics (CFD) can be a powerful tool to model advection, diffusion, and reaction in integral gas-phase photocatalytic reactors as recently reported (9, 11-12). In this paper, CFD was integrated with radiation field modeling and photocatalytic reaction kinetics to yield a rigorous model of a well-controlled and reproducible flatplate, single-pass, flow-through photocatalytic reactor for air purification. Steady-state studies were carried out both mathematically and experimentally with the objective of predicting trichloroethylene conversion under different experimental conditions. This was selected as model pollutant since it is one of the most common VOC in indoor air (13). The effect of the intensity of the incident radiation (different lamp configurations), flow rate and substrate concentration have been modeled and compared with experimental results. The comparison of the model prediction with the experi10.1021/es061569o CCC: $37.00
2007 American Chemical Society Published on Web 02/10/2007
FIGURE 1. Planar and side views of the flat-plate photocatalytic reactor (right). Experimental setup for the photocatalytic oxidation experiments (left).
mental results yielded pollutant-specific kinetic parameters directly from experimental results in an integral reactor, which were also independent of reactor geometry, radiation field, and fluid-dynamics.
Experimental Section Figure 1 presents planar and side views of the photocatalytic reactor used for the gas-phase PCO of organic substrates. It consisted of a flat 75 mm-wide, 600 mm-long, stainless steel reactor that allowed the controlled distribution of the contaminated air flow over the catalyst. A 75 mm × 100 mm glass plate coated with the photocatalyst was located 270 mm from the inlet of the reactor and 170 mm from the outlet. The reactor was covered with a borosilicate glass (7.7 mm thick) sealed with a Viton gasket. This formed a 75 mm × 2.5 mm flow passage across the whole length of the reactor. The reactor inlet and outlet were designed to minimize backflow dispersion and to achieve uniform, fully developed flow before reaching the photocatalytic plate. The reactor was irradiated with five blacklight blue fluorescent lamps (Philips TL 8W/08 F8T5/BLB, 0.0155 m bulb diameter, 0.26 m bulb length and 1.2 W UV-A output). The lamp emitted a minute fraction of the total radiation at 324 and 325 nm and the rest between 343 and 400 nm with a maximum irradiance peak at 365 nm (14). The centerlines of the lamps were separated by 0.039 m. The radiation intensity at the photocatalytic surface was regulated by changing the number of lamps switched on (1, 3, or 5) and by adjusting the distance between the lamps and the reactor. UV radiation was measured with a radiometer (Cole-Parmer), equipped with a 365 nm sensor. The experimental setup consisted of a carrier gas and VOC delivery systems, the reactor and the analytical unit. The carrier gas line was split into two streams, one of which was bubbled through water to set the humidity for the reaction. The relative humidity of the re-joined streams was measured by a thermohygrometer (Testo 635). The gas flow rates were adjusted using calibrated flowmeters (ColeParmer). The VOC was continuously injected into the system as liquid, using an infusion syringe pump (Cole-Parmer). The temperature at the point of injection upstream was monitored by a temperature controller (Cole-Parmer) and maintained slightly above the boiling point of the VOC. At the reactor inlet, the temperature of the gas was 35 ( 2 °C.
The temperature gradient of the gas over the section of the reactor occupied by the catalytic plate was 1.3 °C, and the average temperature was 31 ( 2 °C. Visual observations and pressure measurements confirmed that no condensation of the vapors took place within the reactor. Trichloroethylene (TCE) (Fisher, for analysis, >99.8%) was used as a model pollutant. Ultra High Purity water produced by a NANOpure Diamond UV water purification system (18.2 MΩ cm-1, e1 ppb TOC) was used in the bubbling bottle to saturate the gas. Ultrahigh purity oxygen carrier gas (99.999%) was provided by BOC gases. The outlet gas mixture was separated using a GS-GASPRO capillary column (30 m, 0.32 mm i.d.) installed in a gas chromatograph (Agilent Technologies, GC-6890N) equipped with a thermal conductivity detector (200 °C) and a flame ionization detector (250 °C), with helium as the carrier gas. Reactor inlet and outlet gas samples were injected at 200 °C through an automatic sampling system. The valves allowed the injection of 0.25 mL gas samples from either the inlet or the outlet of the photoreactor every 2 min. The reactor outlet stream was vented to the atmosphere. A 7 wt % suspension of TiO2 Degussa P25 (primary particle size, 20-30 nm by TEM; specific surface area 52 m2g-1 by BET; composition 78% anatase and 22% rutile by X-ray diffraction) in ethanol was deposited over the glass plate, left overnight at room temperature to evaporate the solvent and dried at 120 °C for 2 h in an oven. Another layer was added to obtain a catalyst loading of 7 g m-2. In a typical experiment, the TCE concentration in the reactor was initially allowed to reach steady-state, which was monitored by the continuous analysis of the reactor inlet and outlet gas samples, the temperature of the reactor and the inlet pressure. Then, the UV-lamps were switched on and the reactor effluent was monitored until the concentration of TCE at the outlet reached a constant value. The conversion was calculated from the inlet and outlet TCE concentrations. The oxidation of TCE was carried out at 8% relatively humidity since this was proved to yield the highest conversion of TCE in the reactor (15-16).
Photocatalytic Reactor Model Radiation Field Model. Among the light emission models proposed in the literature for tubular lamps (10), the linear source spherical emission model (LSSE) (17) represents a VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2029
FIGURE 2. LSSE model predictions of the radiation intensity on the surface of the catalyst for different lamp arrangements and positions, using Philips TL 8W/08 F8 T5/BLB lamps. 〈Iw〉 ) 73.5 W m-2.
reasonable compromise between complexity and accuracy of predictions (18). It was, therefore, considered to be appropriate to simulate the light intensity distribution over the photocatalytic plate. This model considers the lamp to be a line source, with each point on this line emitting radiation in every direction and isotropically. The following assumptions were also made: (1) isothermal conditions; (2) constant density and diffusivity of the gas within the reactor; (3) negligible absorption, scattering or emission of radiation by the gaseous media occupying the space between the lamps and the catalyst; (4) constant attenuation coefficient of the borosilicate glass which covers the reactor, independent of wavelength and intensity; (5) negligible lamp radius compared to the distance of the lamp to the photocatalytic plate; (6) lamps axes parallel to reactor. The incident radiation intensity at any position (x, y) on the surface of the photocatalytic plate according to the LSSE model is as follows (17):
I(x,y)|z)0 ) x - xL,0 x - xL,0 - L rLIw arctan - arctan 4R R R
[
(
)
(
)]
(1)
where rL (m) is the radius of the lamp, Iw (W m-2) is the radiation intensity measured at the lamp wall, x (m) is the axial coordinate, y (m) is the reactor transversal coordinate, xL,0 (m) is the distance of the lamp ending from the axis origin, L (m) is the length of the lamp, and R (m) is the distance between the lamp axis and the point of interest on the surface of the plate (Figure S1, see the Supporting Information). In the presence of multiple lamps, with N lamps axially mounted above the reactor, the distance between the axis of lamp i and a point (x, y) on the surface of the photocatalytic plate can be defined as follows:
Ri ) xZ2 - (ylamp,i - y)2
(2)
where ylamp,i is the distance of the lamp axis from the origin (Figure S1, see the Supporting Information). The radiation 2030
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 6, 2007
intensity on the surface of the photocatalyst equals the sum of the contributions from each lamp. It follows
I(x,y)|z)0 )
∑
N lamps
(I(x,y)|z)0)i )
∑
( [ ( ) ( )]) rLIw,i
N lamps
4Z
arctan
arctan
x - xL,0
-
Ri x - xL,0 - L Ri
(3)
i
Equation 3 is strictly valid for monochromatic irradiation only. However, with polychromatic irradiation, as in this work, eq 3 can still be used by replacing Iw,i with its value averaged across the useful spectrum of the incident radiation, 〈Iw,i〉. The wavelength range which applies to the present case is λmin ) 324 nm, the minimum wavelength emitted by the lamp, and λmax ) 380 nm, the highest wavelength that can photoactivate the TiO2 photocatalyst. In actual fact the experimental photon flux I was measured with a UV radiometer fitted with a 365 nm sensor. Therefore, 〈Iw,i〉 was estimated by eq 4, where Isensor (W m-2) is the incident radiation measured with the radiometer, Pλ is the relative spectral response of the 365 nm sensor, and Wλ (W) is the radiant power of the lamp at wavelength λ.
〈I〉 ) Isensor
∫ ∫
380nm 324nmWλdλ
380nm 324nmWλPλdλ
(4)
Measurements were taken along the length of the lamp and around the circumference and the values were averaged across the axial and radial direction to calculate Iw,i for each lamp. Figure 2 shows model calculations of the radiation intensity on the plane where the surface of the catalyst lays (z ) 0) for different lamp arrangements and positions. The closer the lamp to the surface, the strongest is the impact of the geometry of the lamp which produces a less uniform
illumination of the catalyst. Increasing the distance between the lamps and the catalyst, gives a more homogeneous illumination of the surface, however, the radiation intensity decreases. The model was used to predict the optimal distance which allowed the most uniform illumination of the surface of the catalyst, while achieving radiation intensities comparable to UV-A radiation from sunlight. This was achieved at a distance of 0.122 m above the catalyst. The corresponding distribution of radiation intensity on the surface of the catalytic plate for 5, 3, and 1 lamp is shown in Figure S2 (see the Supporting Information). Figure S3 (see the Supporting Information) shows the experimental validation of the LSSE model, which was then integrated into the CFD simulations. The radiation emission model described above was essential to explore the effect of radiation intensity on reactor performance. Fluid-Flow Model, Transport Equation, and Kinetics. The numerical modeling of the fluid-flow through the reactor involves the solution of the Navier-Stokes equations, which are based on the assumptions of conservation of mass and momentum in a moving fluid. The conservation of mass is described by the partial differential equation (19),
∂F + ∇‚(Fv) ) 0 ∂t
(5)
where F (kg m-3) is the density and v is the velocity vector of the fluid. The conservation of momentum in a horizontal reactor is similarly described by the equation (19):
∂ (Fv) + ∇‚(Fvv) ) -∇p + ∇‚τj ∂t
(6)
where p (Pa) is the pressure and jτ is the stress tensor. For laminar, nonreacting flows, the Navier-Stokes equations were sufficient to define the fluid-dynamics of the system. For turbulent flow the two-equation k- turbulence model (19) was used. The first series of CFD simulations were concerned with determining the effect of inlet and outlet pipework configuration on the flow regime above the catalyst, to establish if this was fully developed laminar or turbulent, and to assess possible nonideal behavior of the reactor. The CFD simulations were run using FLUENT 6.2, Fluent Inc. A model of the exact geometry of the inlet, the outlet pipework and of the reactor (Figure S4, Supporting Information) was built. The domain was discretised into a sufficiently large number of control volumes or cells, using standard grid generation techniques. Contours of fluid velocity on a vertical plane through the center of the inlet pipework, for both the laminar and turbulent simulations, are presented in Figure S5 (see the Supporting Information). Both models demonstrated that after a redistribution of the flow at the entrance of the reactor, the velocity profile in the channel was laminar and fully developed. The turbulent simulation exhibited some recirculation of the fluid at the reactor entrance and exit, however, this did not propagate in the section occupied by the catalyst. The complete oxidation of trichloroethylene in the presence of humid air can be represented by the reaction scheme: TiO2, hv 3 Cl2C ) CHCl + O2 + H2O 98 2CO2 + 3HCl (7) 2
The concentration of the reagents and final products (TCE, O2, H2O, HCl, CO2) in each control volume of the grid was expressed in terms of molar fraction, yi, with n
∑y )1 i
i)1
(8)
where n is the number of species. The reaction was considered to be one step from reagents to final products. The effect of the main intermediate product (dichloroacetyl chloride) was neglected on the basis of the work of Jacoby et al. (20) which showed that it does not compete with TCE adsorption and reaction. Furthermore, in our experiments the concentration of reaction intermediates was undetectable. The change in the molar fraction of TCE as a result of the formation of intermediates could be neglected since oxygen was in vast excess. A transport equation for each species takes the following form:
∂ (Fyi) + ∇‚(Fvyi) ) -∇‚Ji + ri(MW)i ∂t with i ) 1, 2...n - 1 (9) where ri (mol s-1 m-3) is the net rate of formation of a species i, (MW)i is its molecular weight, and Ji is the diffusion flux. The rate term in eq 9 was added to circumvent the limitation of FLUENT to model surface reactions. Under normal circumstances, it should not appear in eq 9 and the reaction rate term should be introduced as a boundary condition on the diffusion flux at z ) 0. In this work, we adopt the Langmuir-Hinshelwood type TCE rate equation proposed by Jacoby et al. (20, 21) to model the rate of disappearance of TCE over the catalyst at constant humidity. However, we extend the rate equation to include the effect of radiation intensity (i.e., eq 3) as follows:
-rTCE ) [I(x,y)]n
kKCTCE 1 + KCTCE
(10)
where k is the apparent rate constant, which encompass the true surface reaction rate constant, the optical properties of the photocatalytic film (absorption and reflection), and the effect of relative humidity and intermediates. K is the adsorption constant of TCE with the catalyst and CTCE is the concentration of TCE in the control volumes bordering the surface of the catalyst. The reaction was considered to occur in the layer of cells adjacent the surface of the catalyst only. In the remaining cells the last term in eq 9 was zero. In this case, molecular diffusion, dispersion and advection contributed to develop transversal, axial and vertical concentration profiles of TCE in the reactor. The exponent n in eq 10 depends on the efficiency of electron-hole formation and recombination at the catalyst’s surface and takes a value between 0.5 and 1 providing the reaction is kinetically controlled (20). At weak intensities, the observed rate of oxidation is first-order with respect to radiation intensity, and shifts to half-order once the rate of electron-hole formation becomes greater than the photocatalytic rate, favoring electron-hole recombination (22). Equations 3, 6, and 8-10 represent the full mathematical description of the reactor. Since the velocity contour plot upstream of the catalytic plate was found to be fully developed in the laminar regime, a reduced geometry was used in the CFD simulations to reduce computational time to approximately 1 h in a standard PC. This included the catalytic plate and short sections of reactor upstream and downstream. At the upstream boundary, the laminar velocity profile was taken from the simulation of the entire reactor. At the downstream boundary, a pressure outlet of 1.1 bar was specified since this was experimentally measured in the actual reactor. The diffusion coefficient of TCE in air was taken to be 7.9 × 10-6 m2 s-1 (23). Figure 3 shows the transversal and axial profiles of TCE at the surface of the catalyst predicted by the CFD model for a typical operating condition of the reactor, with either one, three or five lamps illuminating the catalyst. With five lamps VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2031
FIGURE 4. Side view of TCE concentration in the reactor at the centerline using five lamp geometry. The vertical direction has been expanded 20 times. CTCE,inlet ) 25.1 µM, Flow rate ) 2.3 L min-1, T ) 304 K, 〈Iw〉 ) 73.5 W m-2, Z ) 0.122 m. the fluid-flow regime is laminar. The vertical gradient at the inner surface of the glass plate is zero because TCE cannot be transported through it. Conversely, there is a positive vertical gradient toward the catalytic plate since TCE reacts on this surface. Downstream from the catalytic plate there is a redistribution of the TCE profiles to form zero vertical gradients at the top and bottom walls. Furthermore, the concentration of TCE tends to become more uniform with distance from the catalytic plate as a result of transversal and axial diffusion and absence of reaction.
Experimental Results and Validation of Model
FIGURE 3. Contours plot of TCE concentration at z ) 0 in the reactor using one, three, and five lamp geometry. CTCE,inlet ) 25.1 µM, Flow rate ) 2.3 L min-1, T ) 304 K, 〈Iw〉 ) 73.5 W m-2, Z ) 0.122 m. illumination, a more uniform transversal profile of TCE can be observed compared to the one lamp configuration. Furthermore, it shows lower concentrations of TCE at the reactor outlet. Lower TCE concentrations at the centerline of the catalytic plate are formed as a result of the higher radiation intensity in this region (Figure S2, see the Supporting Information) and larger vertical gradients of TCE. The TCE vertical gradient is higher in the middle axis of the reactor compared to the other regions since this is the region with faster fluid flow. At the edges of the plate, near the side walls, there is a decrease in TCE concentration as a result of the longer residence time of the fluid traveling near the walls. Figure 4 shows the vertical and axial profiles of TCE at the centerline of the reactor. The developing profiles are the result of reaction, advection, and diffusion of TCE in the reactor. Strong gradients can be observed in both the vertical and the axial direction. The gradient in the axial direction occurs primarily as a result of advection with a negligible contribution of diffusion. Conversely, the gradient in the vertical direction occurs as a result of diffusion alone since 2032
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 6, 2007
Effect of Mass Transfer. As the flow rate increases, the reaction rate should become independent of the fluid velocity, and the observed reaction kinetics are released of mass transport limitations. Therefore, at high flow rates, the observed kinetics are controlled by the surface reactions only. Several authors have noted this effect in gas-phase photocatalytic reactors using immobilized TiO2 (20, 24-25). The determination of the surface reaction operational regime depends on reactor configuration and on the experimental conditions. Figure S6 (see the Supporting Information) shows the experimental TCE reaction rate as a function of the total flow rate at constant radiation intensity. It is clearly shown that mass transfer limitations were present at flow rates below 1.72 L min-1 (Re < 46), whereas surface reaction kinetics control was achieved at higher flow rates. In consequence, the experiments designed to evaluate the activity of the catalyst were carried out at a flow rate of 2.3 L min-1 (Re ) 61), to operate the reactor in the reaction controlled regime only. Apparent Quantum Yield. Kinetic data of photocatalytic reactions from different reactor configurations, reaction conditions, and catalyst type can be compared in terms of quantum yields (i.e., mole of pollutant converted per mole of photons absorbed). Since the absorption of photons depends on the chemical composition and physical characteristics of the photocatalytic film, the “apparent” quantum yield is often employed. The apparent quantum yield of TCE photocatalytic oxidation can be defined as the ratio of the number of moles of TCE consumed in the reactor per unit time (mol s-1) divided by the photons flux at the surface of the catalyst (Einstein s-1). Since a polychromatic radiation source was used, the overall photon flux was calculated by considering the energy of the photons emitted at the different wavelengths. Apparent quantum yields of TCE photocatalytic oxidation were found to be in the range from 0.28 to 0.97 mol Einstein-1. Table S1 (see the Supporting Information) compares the apparent
FIGURE 5. TCE peak areas and conversion as a function of time showing the response of the reactor to dark and illuminated periods. Flow rate ) 2.3 L min-1. CTCE,inlet ) 33.2 µM. quantum yields observed in this study with those calculated from the literature. The quantum yields observed here were found to be in the upper range. Comparing the results of different work presented in Table S1, TiO2 P25 appears to have higher quantum yield than Aldrich and Ishiara catalysts with regard to TCE oxidation. Furthermore, UVC radiation does not promote higher quantum yields compared to the results with UV-A radiation. Effect of Radiation Intensity, TCE Concentration, and Molar Flow Rate. The effect of radiation intensity was investigated in the range 6.2-28.1 W m-2 (average intensity across the whole photocatalytic plate) which was achieved by illuminating the catalyst with either one, three, or five lamps. The distance of the lamps from the catalyst was 0.122 m to achieve the most uniform illumination of the surface, as shown in Figure 2. Figure S2 (see the Supporting Information) shows the actual distribution of radiation intensity on the surface of the catalytic plate. The TCE inlet
concentration was varied in the range 25.1-40.5 µM. Figure 5 shows a typical steady-state/transient experiment with dark and illuminated periods using different lamp arrangements. The inlet TCE concentration was steady across a large time period. As the number of lamps was increased, the conversion of TCE increased as expected. The response time of the reactor to illumination and dark periods was rapid, less than 7 min, indicating that the reactor was operating under ideal conditions (absence of dead spaces and recirculations). Separate experiments showed that under illumination and in the absence of catalyst no TCE was degraded. Figure 6 shows conversion of TCE at different radiation intensities and concentrations. The photocatalytic degradation rate of TCE over thin film of Degussa P25 catalyst irradiated with UV-A blacklights lamps has been reported to be first-order on radiation intensity in the range 0-17 W m-2 (20). The results in Figure 6 show that TCE conversion increases linearly up to 28.1 W m-2 suggesting that in our experiments the reaction kinetics were not saturated with respect to the irradiation level. Note that conversion is proportional to the average reaction rate in the reactor. Consequently, the coefficient n in eq 10 was taken to be equal to one in the simulations. As the inlet concentration of TCE increased, the conversion was found to increase. The CFD model described above was fitted to these experimental results by adjusting the adsorption constant K, and the apparent rate constant k only, until the sum of the square of the residuals between the model and the results was minimized (Figure 6). The best fit values are reported in Table 1. Confidence limits for (10% change in the kinetic parameters are shown in Figure 6 for the experiments with 40.5 µM TCE concentration at the reactor inlet. Table 1 compares the kinetic parameters obtained in the present study with those in the literature using flat plate and tubular reactors. k′ and k′′ are the apparent rate constants given per unit surface area and per unit of catalyst mass,
FIGURE 6. TCE conversion as a function of inlet concentration and radiation intensity (average value on the catalyst surface). Comparison between experimental and CFD model. Flow rate ) 2.3 L min-1.
TABLE 1. Kinetic Parameter of TCE Photocatalytic Oxidation over TiO2 Thin Films K k′ k′′ [k′ × K] [k′′ × K] reference (mol m-2 s-1 W-1 m2) (mol g-1 s-1 W-1 m2) (m3 mol-1) (m s-1 W-1 m2) (m3 s-1 g-1 W-1 m2) this work
3.80 × 10-8
5.43 × 10-9
10-8
10-9
26
5.82 ×
1.82 ×
27
3.79 × 10-7
1.52 × 10-9
20
1.91 × 10-6
3.81 × 10-7
7.00 27.6 439 0.83
2.66 × 10-7
3.80 × 10-8
10-6
10-8
1.61 ×
5.02 ×
1.66 × 10-4
6.68 × 10-7
1.59 × 10-6
3.18 × 10-7
TiO2 and support P25 on flat plate P25 on tubular reactor walls P25 spread as a powder on a flat plate baffled reactor P25 on tubular reactor walls
% relative humidity 8.0 23.0
