Kinetic Study of Ozone Photocatalytic Decomposition Using a Thin

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Kinetic Study of Ozone Photocatalytic Decomposition Using a Thin Film of TiO2 Coated on a Glass Plate and the CFD Modeling Approach Xin Wang,† Xin Tan,†,‡ and Tao Yu*,†,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China § TU-NIMS Joint Research Center, Tianjin University, Tianjin 300072, PR China ‡

S Supporting Information *

ABSTRACT: The kinetics of ozone photocatalytic decomposition in a flow-through reactor using a thin film of TiO2 coated on a glass plate is investigated. The Langmuir−Hinshelwood kinetic model provides a good description of the ozone decomposition. The effect of light intensity on reaction rate is also studied, showing a transition in the kinetic order with respect to light intensity occurred from 0.75 to 1.0 mW·cm−2 under the experimental conditions. Fluid dynamics and surface photocatalytic reaction modeling by the computational fluid dynamic (CFD) approach is then proposed. The parameters determined in the kinetic experiment are used to calculate the ozone concentration distribution in the flow-through reactor under a given radiation field. In terms of conversion yield, the model predictions agree closely with the experimental results within the range in which the results are examined. This study presents a simple example of the photocatalytic reaction process modeling. Knowledge of the intrinsic kinetics allows the universal application of this CFD approach to the optimization and design of photocatalytic reactors. reactions to convert pollutants to harmless products.9,10 TiO2, as a photocatalyst used extensively in various redox reactions, is a chemically inert, easy to produce and use, and environmental friendly semiconductor that can efficiently catalyze reactions.11 Recent reports reveal the use of computational fluid dynamics (CFD) as a powerful tool for simulating photocatalytic reactions in a photoreactor. Mohseni and Taghipour12 employed both CFD modeling and experimental approaches to study vinyl chloride removal in an annular photocatalytic reactor. The authors reported that the model agreed well with the experimental data. Salvado-Estivill et al.13 combined CFD modeling with radiation field modeling and photocatalytic reaction kinetics to model trichloroethylene oxidation in a flatplate reactor. Hossain et al.14 presented a three-dimensional convection-diffusion-reaction model to simulate formaldehyde oxidation in a monolith photocatalytic reactor. Queffeulou et al.15 modeled fluid dynamics and photocatalytic reaction with the CFD approach using kinetics parameters previously determined in a batch reactor. The model prediction and the experimental results were found to be consistent. To the best of our knowledge, very few studies that are mainly focused on ozone photocatalytic decomposition have been published.16−18 Furthermore, no systematic study on the kinetics of ozone photocatalytic decomposition has been reported. This work focuses on the kinetics of ozone photocatalytic decomposition using a glass plate coated with a thin film of TiO2, and a simple method using the CFD approach to predict photoreactor performance based on kinetics data is proposed.

1. INTRODUCTION Ozone is a powerful oxidant that poses an extreme hazard to human health. Ozone can cause headaches, laryngitis, and damage to the human respiratory system at levels as low as 0.1 to 1 ppm; higher levels of ozone can be life threatening.1 Given its high toxicity, the common presence of ozone sources in the work environment is alarming. In an enclosed working space, photocopiers and laser printers can emit ozone, with maximum concentrations reaching 2000 ppm.2 Therefore, methods for the decomposition of ozone to harmless products have attracted great interest. Ozone treatment can be performed using several methods. One method of treatment involves the use of activated carbon filters;2 however, this method is limited by the incapacity for regeneration of activated carbon filters, and these filters require frequent replacement and subsequent disposal. Another method of treatment is irradiation of ozone with UVC light, i.e., 190 nm ≤ λ ≤ 280 nm, because ozone favors photolysis3 within this wavelength range. However, this treatment method is typically not very effective because ozone only absorbs weakly in this region. Another major method of ozone treatment is thermal catalysis. The most popular catalyst for this purpose is manganese oxide (MnO2) on a support material such as γalumina or titanium oxide.4,5 However, this treatment requires heating for effective decomposition. A room-temperature catalytic process is preferable to the thermal process, especially in indoor use, because extra heating and recooling before exhaust are unnecessary. Limited by these drawbacks, photocatalysis may be an emerging promising technology for ozone treatment. Photocatalysis has already been successfully used in the elimination of a wide range of organic and inorganic compounds.6−8 This technology can be applied at room temperature and atmospheric pressure, and it can utilize redox © 2014 American Chemical Society

Received: Revised: Accepted: Published: 7902

September 23, 2013 February 16, 2014 April 23, 2014 April 23, 2014 dx.doi.org/10.1021/ie403144w | Ind. Eng. Chem. Res. 2014, 53, 7902−7909

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and its dimensions were 75 and 600 mm in width and length, respectively. A 75 mm × 100 mm glass plate coated with the photocatalyst was loaded at a distance of 270 mm from the inlet of the reactor and at a distance of 170 mm from the outlet. The reactor inlet and outlet were designed to minimize back-flow diffusion and to achieve uniform, fully developed flow over the photocatalytic plate. A gas flow channel was generated between the catalyst film and the quartz plate, which served as the top cover of the reactor. The flow passage was 5 mm in height, forming a 75 mm × 5 mm flow passage across the entire length of the reactor. The photoreactor was made of polytetrafluoroethylene to minimize gas adsorption onto the reactor surface. The reactor was irradiated with up to 2 UV black light lamps (365 ± 25 nm, Philips, 0.011 m bulb diameter, and 0.20 m bulb length). The lamp emitted with a peak intensity of 365 nm and provided UV illumination to the photocatalyst. The centerlines of the lamps were separated by 0.029 m. These 2 UV black light lamps with a mirror reflector on the one side integrated the radiation source. The lamp was fixed directly above the catalyst film, and the radiation flux density at the photocatalytic surface was regulated by adjusting the distance between the lamps and the reactor. UV radiation was measured with a UV radiometer (Photoelectric Instrument Factory of Beijing Normal University, model UV-A) equipped with a 365 nm sensor. The UV intensity projected onto the catalyst film was determined by averaging 20 measurements at various positions on the plate. 2.3. Experimental Procedure. The experimental setup consisted of a gas source, the ozone generator, the photoreactor, and the monitor unit. The experiments were conducted at room temperature under atmospheric pressure. Prior to the experiment, the inlet ozone concentration was monitored by an ozone detector (Shenzhen Jishunan Technology Co., Ltd.). The gas flow was switched to the photoreactor after the initial ozone concentration stabilized, and the UVA lamp was switched on when the outlet ozone concentration reached approximately 90% of the initial ozone concentration. The ozone concentration rapidly decreased immediately after the gas flow was switched to the reactor, and the concentration gradually increased when the UVA lamp was switched off. However, the effluent concentration of ozone rapidly decreased to a sustained constant level when the UVA lamp was switched on. The photocatalytic reaction rate was determined only after the outlet ozone concentration reached a constant sustained level under UVA irradiation. Catalyst-free and irradiation-free control experiments were conducted to exclude any possible transformation of ozone other than the photocatalytic decomposition induced by TiO2. Ozone photolysis is a possible transformation pathway under UV light;3 however, no detectable decomposition was observed in the catalyst-free control experiment. This result can be attributed to the negligible absorption of ozone (adsorption peak at 254 nm)3 of the photon energy when λ > 340 nm.

In this study, a kinetics experiment on ozone photocatalytic decomposition is first conducted in a flow-through reactor. The expression of the reaction rate as well as the kinetic parameters can then be obtained based on the experimental results. Second, the effect of the incident radiation intensity is modeled, and the optimal lamp position is determined from the modeling results. Third, a surface reaction model based on the results of the experiment and the incident radiation intensity modeling is coupled with a CFD code to determine the ozone distribution in the reactor. The species transport equation is solved by setting the chemical reaction as a boundary condition. Finally, the simulation and experimental results are compared in terms of conversion yield to validate the methodology.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Photocatalyst Preparation. Ozone was produced using an ozone generator (model 3S-A) supplied with a constant oxygen feed from a cylinder. The instrument converts oxygen in the feed gas into ozone by corona discharge. A commercial TiO2 photocatalyst (P25) containing 20% rutile and 80% anatase was provided by Degussa AG Company. The TiO2 particles were immobilized using the doctor-blading method. Briefly, a TiO2 slurry was prepared by grinding 0.6 g of Degussa P25 TiO2 powder into a slurry of poly(ethylene glycol) 600 (0.5 g)−acetic acid (0.1 mL)−distilled water (0.2 mL), followed by addition of 10 mL anhydrous ethanol. The slurry was then deposited onto the glass plate with a glass slide using the doctor-blading technique. The coated plate was heated at 70 °C for approximately 5 min on the heating plate to evaporate the solvent, and subsequent heating was conducted in an oven at 70 °C for 30 min. Finally, the glass plate was sintered in a muffle furnace at 400 °C for a holding period of 30 min and subsequently cooled inside the furnace itself to room temperature. The coated area of the plate was 72.5 mm × 100 mm, and the coated plate was then fixed in a homemade photoreactor. 2.2. Photoreactor and Experimental Setup. Figure 1 shows the schematic of the photoreactor and the experimental setup. Ozone photocatalytic decomposition experiments were conducted using the flow-through reactor. The reactor was flat,

3. CFD MODELING The numerical simulation of the transport and chemical reaction was performed by solving the conservation equations describing the convection, diffusion, and reaction of each species in the photocatalytic reactor. Laminar flow in the flowthrough reactor was considered. The Reynolds number was confirmed to be below 737. Simulation of this photocatalytic decomposition reactor, including the photocatalytic surface reaction, was performed using a three-dimensional, steady-state

Figure 1. Schematic diagram of the experimental setup. (1) Gas cylinder, (2) regulator, (3) mass flow controller, (4) ozone generator, (5) flow passage, (6) catalyst, (7) quartz window, (8) UV black light lamps, and (9) ozone detector. 7903

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of electron−hole formation and recombination at the catalyst surface.21 The radiation flux density on the surface of the photocatalytic plate was modeled by the line source spherical emission (LSSE) model.22 The model was based on the following assumptions: (1) isothermal conditions; (2) negligible absorption, scattering, or emission of radiation by the gaseous media in the reactor; (3) negligible lamp radius compared with the distance of the lamp from the photocatalytic plate; (4) lamp axes parallel to the reactor; and (5) negligible attenuation coefficient of the quartz glass that covers the reactor. According to the LSSE model22

laminar model. The governing equations for modeling the system are described below. Mass conservation is described by the partial differential equation: ∂ρ + ∇·(ρv ⃗) = 0 ∂t

(1)

where ρ is the density, v⃗ is the velocity vector of the fluid, and t is the time. The conservation of momentum in the flowthrough reactor is as follows: ∂ (ρv ⃗) + ∇·(ρvv⃗ ⃗) = −∇p + ∇·τ + ρg ∂t

(2)

where p is the pressure, τ is the viscous stress tensor, and g is the gravitational force. The species transport equation takes the following form: ∂ (ρYi ) + ∇·(ρvY ⃗ i ) = −∇·Ji ⃗ + ri(MW)i ∂t

I(x , y)|z = 0 =

⎛ x − x L,0 − L ⎞⎤ − arctan⎜ ⎟⎥ ⎝ ⎠⎦ R

(3)

R i = [Z2 − (ylamp, i − y)2 ]1/2

I(x , y)|z = 0 =

where n⃗ is a unit vector normal to the surface, and rsi is the rate of production/depletion of species i as a result of surface reaction. Under UVA light, the overall photocatalytic decomposition of ozone can be expressed as TiO2 , hv

=

(I(x , y)|z = 0 )i

⎛r I ⎡ ⎛ x − x L,0 ⎞ ⎜ L W, i ⎢arctan⎜ ⎟ ⎜ 4R ⎢ ⎝ Ri ⎠ i ⎣ N lamps ⎝

∑

⎛ x − x L,0 − L ⎞⎤⎞ − arctan⎜ ⎟⎥⎟⎟ Ri ⎝ ⎠⎥⎦⎠ i

(5)

kKCs k KC = 0 s [I(x , y)]α 1 + KCs 1 + KCs

∑ N lamps

In this case, the Langmuir−Hinshelwood (L-H) kinetic model,19,20 which is widely applicable in the photocatalytic decomposition kinetics of pollutants, was considered for the overall disappearance of ozone attributed to adsorption onto the photocatalyst surface and decomposition reaction at the surface. Hence, the depletion reaction rate of ozone in the system in eq 4 is given by s −ris = −rozone =

(8)

where ylamp,i is the distance of the lamp axis from the origin and Z is the vertical distance of the lamp axis from the catalyst surface. The radiation flux density on the catalyst surface equals the sum of the contributions from each lamp and takes the following form:

(4)

2O3 ⎯⎯⎯⎯⎯⎯⎯→ 3O2

(7)

where rL is the radius of the lamp, IW is the radiation intensity measured at the lamp wall, x is the axial coordinate, y is the reactor lateral coordinate, xL,0 is the distance of the lamp ending from the axis origin, L is the length of the lamp, and R is the distance between the lamp axis and the point of interest in the surface of the plate. In the case of multiple lamps, the distance between the lamp i axis and the point of interest in the catalyst surface with N lamps axially mounted above the top surface of the reactor can be defined as

where ri is the net rate of production or depletion of the species i by chemical reaction, (MW)i is its molecular weight, and Ji ⃗ is the diffusion flux of species i. In the photocatalytic reaction, the chemical reaction was assumed to occur only at the TiO2 surface. The chemical reaction rate is thus described using a theoretical surface decomposition rate. For the surface reaction, the concentration of the species on the reaction surface is based on a balance between the convection/diffusion of each species to/from the surface and consumption/production rate at the surface. Because the flow pattern over the catalyst surface was fully developed laminar flow, the convection transport in the vertical direction could be neglected compared to transport by diffusion. Therefore, the boundary condition of ozone adsorption and reaction on the catalytic surface could be expressed as Ji ⃗ ·n ⃗ = ris

rLIW ⎡ ⎛ x − x L,0 ⎞ ⎟ ⎢arctan⎜ ⎝ R ⎠ 4R ⎣

(9)

The radiation field was computed externally based on eq 9 using MatLab v.8.0.0 software, and the calculated field was then introduced into the computation using user-defined functions (UDF). The “laminar finite rate” model, in which the transport equations are solved for species mass fractions with a predefined chemical reaction mechanism, was selected for reaction modeling. The reaction rate term rsi appeared as a source term in the species transport equation and was included to calculate the surface reaction. In this case, the reaction was considered to occur in the layer of cells adjacent the surface of the catalyst only, at a rate which was equal to that of the surface reaction. In the rest cells the last term in eq 3 was zero. Under normal circumstances, the reaction term should not be in eq 3 and should be introduced as a boundary condition on the diffusion flux at the reaction wall surface.16,23

(6)

where k is the apparent rate constant, which is a function of temperature, light flux, and catalyst properties; K is the Langmuir adsorption equilibrium constant; Cs is the ozone concentration near the catalyst surface; k0 is the intrinsic rate constant independent of light intensity; I(x,y) is the radiation flux density at the surface of the catalyst; and the exponent a takes a value between 0 and 1, which depends on the efficiency 7904

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The three-dimensional physical domain of the photoreactor was discretized with approximately 266,042 hexahedral cells. The physical and chemical properties of the species (that is, ozone and oxygen as the carrying medium) were specified to calculate the coefficients of the governing equations described above. Velocity inlet with ozone mass fractions and outflow were specified as boundary conditions for the inlet and outlet flows, respectively. The no-slip boundary condition was used at the wall surface of the reactor. A segregated implicit solution algorithm was used to perform the numerical experiments. The convergence criterion of 10−5 was specified for each scaled residual component of mass, velocity, and ozone concentration.

4. RESULTS AND DISCUSSION 4.1. Kinetics of Ozone Photocatalytic Decomposition. In general, mass transfer contributes to the overall behavior of the reaction rate in flow-through reactors. To verify any gasside mass transfer influence, the flow rate was increased from 0.15 to 0.25 L·min−1. The subsequent reaction rate remained unchanged, indicating that the reaction rates obtained at 0.15 L· min−1 were kinetically controlled (see the Supporting Information). Consequently, subsequent experiments to evaluate the intrinsic photocatalytic kinetics were conducted at flow rates above 0.15 L·min−1. The intraparticle masstransport resistance is negligible because of the thin-film configuration and light penetration limitation. The flow pattern in a flow-through reactor at steady state can be described approximately by the plug-flow model when the bulk transport limitation is small, which is given by the following equation:7,24,25 C 0 dC C0 CS dCs V /Q s = 0 =∫ =∫ s kKCs C C h F −rozone

(

=

K (C0 − C) + ln(C0/C) kK

1 + KCs

Figure 2. Langmuir−Hinshelwood model fitting results in a flowthrough reactor at a constant light intensity of 1.0 mW·cm−2 (C0 = 2− 14 ppm, Q = 0.15−0.55 L·min−1).

The data on ozone removal efficiency (Figure 3) showed that the ozone removal rate increased with increasing light intensity.

)

(10)

where V is the effective volume of the reactor, Q is the volumetric flow rate through the reactor, h is the thickness of reactor volume, F is the molar feed rate at the entrance of the reactor, and C0 is the initial concentration of O3. The quantity, V/Q is known as the contact time for the reaction or the average time at which a molecule passes through the reactor. By changing the form of eq 10, we obtain the following: V /Q h h ln(C0/C) = + (C 0 − C ) k kK (C0 − C)

Figure 3. Effects of light intensity on ozone conversion efficiency (C0 = 8.687 ppm, Q = 0.25 L·min−1). Error bars represent the standard deviations of duplicate measurements.

(11)

The data were also interpreted in terms of ozone reaction rate (Figure 4). A nonlinear power law was observed in the experiments,19,26 and a modified L-H kinetic model was used to account for the effect of light intensity:

The two parameters k and K can be obtained by the linear fitting of a plot of ln (C0/C)/(C0-C) versus (V/Q)/(C0-C) from experimental data. By manipulating the different values of initial ozone concentration and residence time, kinetic experimental data were collected at the light intensity of 1.0 mW·cm−2. Figure 2 indicates that the experimental data are in good agreement with the integral rate law analysis. Values of k = 4.23 × 10−7 mol·m−2·s−1 and K = 6274.5 m3·mol−1 were calculated from the intercept and the slope in Figure 2. To obtain the intrinsic kinetic constant k0 from the surface model of the photocatalytic reaction previously developed in eq 6, the exponent a should also be known. Therefore, the effects of light intensity on the reaction rate were also studied in this kinetic experiment. The experiments were conducted in the surface-kinetics-controlled regime (C0 = 8.687 ppm, Q = 0.25 L·min−1) at various light intensities from 0.25 to 2.5 mW·cm−2.

s −rozone =

k 0KCs α I = r′I α 1 + KCs

(12)

where k0 is the intrinsic rate constant independent of light intensity. The exponent α varies from 0 to 1 and can be estimated by a nonlinear fitting of the experimental data, as shown in Figure 4. Almost all of the few studies21,27,28 on photocatalytic reaction rate versus incident light intensity in systems show that at weak intensity, the reaction rate is first-order with respect to radiation intensity; the rate shifts to half-order at relatively high intensity once the rate of electron−hole formation becomes greater than the photocatalytic reaction rate, favoring electron−hole 7905

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4.2. Radiation Flux Density Profiles on the Surface of the Photocatalyst. Figure 5 shows the model calculations of

Figure 4. Effects of light intensity on the reaction rate of ozone (C0 = 8.687 ppm, Q = 0.25 L·min−1). Error bars represent the standard deviations of duplicate measurements. Figure 5. LSSE model predictions of the radiation flux density distribution on the surface of the catalyst at different lamp positions relative to the catalyst surface.

recombination.21 At sufficiently high intensity, the reaction rate becomes a constant independent of light intensity, and the mass transfer limit is encountered. In Figure 4, the effect of light intensity is divided into two regimes. At a sufficiently weaker intensity (below 1.0 mW· cm−2), the reaction rate increased with increasing light intensity, and the reaction rate with respect to radiation intensity was nearly first-order. However, the curves of intensity versus reaction rate dramatically leveled off at higher intensities (1.0 to 2.5 mW·cm−2). The exponent α in eq 12 depends on the efficiency of electron−hole formation and recombination at the catalyst surface.19 Piecewise-fitting results (Figure 4) showed significant differences in the α values obtained at low intensity (below 1.0 mW·cm−2) and high intensity (1.0 to 2.5 mW·cm−2). The piecewise-fitting results show that the transition was estimated to occur at 0.75 to 1.0 mW·cm−2. At low intensity (0.25 to 1.0 mW·cm−2), the α value was 0.6118, and the electron−hole pairs were consumed more rapidly by chemical reactions than by recombination reactions. At high intensity (1.0 to 2.5 mW· cm−2), the α value from the fitting results was 0.1009, which is less than 0.5. This result was also reported in several previous studies25 and can be explained by the fact that the rate of electron−hole formation becomes greater than the photocatalytic reaction rate, leading to faster electron−hole selfrecombination. Concurrently, the external mass transfer could also begin to play a role in this regime. At even higher intensity, the reaction rate is expected to reach a plateau where the system is mainly limited by mass transfer.29,30 Based on the kinetic experimental results discussed above, the intrinsic kinetic parameters of the ozone photocatalytic decomposition are summarized and listed in Table 1. These parameters are used in eq 6 and applied in the reactor for ozone decomposition modeling.

the radiation flux density on the catalyst plate at different distances between the catalyst surface and the lamp. The closer the lamp was to the surface, the less uniform illumination distribution with a strong light intensity peak was formed on the central line of the catalyst surface in the x direction (Figure 5a). Increasing the distance between the lamp and the catalyst resulted in a more homogeneous illumination of the surface of the catalyst. However, the radiation flux density inevitably decreased. This phenomenon can be explained by the rapid decrease in light intensity as a result of the loss of the photon flux scattered to other outer spaces. The light intensity distribution is more uniform on the catalyst surface as a result of the scattering effect. Figure 6 shows the model prediction of the average and the standard deviation of the catalyst surface irradiation at different distances between the catalyst surface and the lamp. The average and standard deviation of the radiation flux density of the experimental measurement agreed well with the model prediction. The standard deviation of the irradiance decreased considerably with increasing distance because of the expanded range of the effective irradiation and the scattering effect. The average irradiance decreased with increasing distance. Therefore, the optimal range of distances that allow good homogeneity in photocatalyst surface illumination can be achieved without great loss in incident radiation energy. As shown in Figure 5, the standard deviation for d = 3.5 cm was decreased by 65% compared with the case for d = 2 cm, which indicates a large improvement in the homogeneity of the illumination on the catalyst surface. Furthermore, the irradiance on the photocatalyst surface when d = 3.5 cm was approximately 1.0 mW·cm−2 (Figure 5c), which was the exact value of the upper limit of the first-order regime in the r ∼ Ia

Table 1. Kinetic Parameters of Ozone Photocatalytic Decomposition over TiO2 Thin Film Coated on Glass Plate k0 (mol·m−2s−1·mW−0.6118(−0.1009)·cm1.2236(0.2018)) −7

4.23 × 10

K (m3·mol−1)

a

I (mW·cm−2)

6274.5

0.6118 0.1009

0.025−1.0 1.0−2.5

7906

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observed as a result of the reaction and convection of the fluid in the reactor. In addition, the decrease in ozone concentration is more dramatic on the catalyst surface, which is closer to the reactor inlet, because the concentration level of ozone is relatively high and leads to a faster reaction rate. Lower ozone concentrations at the centerline of the catalytic plate are formed as a result of the higher radiation flux density in this region. The lower ozone concentrations at the edges of the plate near the side walls are formed as a result of the longer residence time of the fluid traveling near the walls. Figure 8 shows the steady-state concentration profiles of ozone in the vertical and axial directions at the centerline of the

Figure 6. Model prediction and experimental measurement of the average and standard deviation of the radiation flux density on the surface of the catalyst at different lamp positions. Error bars indicate the standard deviations.

power law relationship. The optimal light power utilization, namely, quantum efficiency, is achieved in the first-order domain.21 Therefore, the optimal distance (d = 3.5 cm) was obtained using the LSSE model, which caused the high effectiveness of photon utilization to ensure maximum efficiency in the entire photocatalytic reaction. 4.3. CFD Modeling Results and Comparison with Experimental Data. Simulation of the ozone photocatalytic decomposition in the flow-through reactor was performed with the experimental values of k0, K, and a. The kinetic model of the photocatalytic reaction was then integrated into the CFD simulation using UDF. Figure 7 shows the ozone concentration profiles on the surface of the photocatalyst surface in the transversal and axial directions as obtained by the CFD model for a typical operating condition. Strong gradients in the axial direction can be

Figure 8. Side view of ozone concentration in the reactor at centerline (C0 = 2.219 ppm, Q = 1.50 L·min−1, I = 1 mW·cm−2).

reactor. The developing profiles of ozone concentration are the result of ozone convection, diffusion, and reaction in the reactor. Strong gradients in both the vertical and axial directions are present. The concentration gradient of ozone in the vertical direction is zero at the top glass plate surface because ozone cannot be transported through the top surface. However, the concentration gradient of ozone in the vertical direction equals the reaction rate at the surface of the photocatalytic plate. The gradient in the axial direction occurs primarily as a result of convection with a negligible contribution of diffusion. As a result of transversal and axial diffusion and absence of reaction, the ozone concentration also becomes more uniform with the distance from the photocatalytic plate. The simulation results obtained in terms of ozone conversion yields were compared with the experimental results obtained in the flow-through reactor. The conversion yield values obtained by the simulation for I = 1 mW·cm−2 are shown in Figure 9. As the flow rate increased, the conversion yield decreased, as expected, both in the simulation and the experimental results. The relative differences between the simulation results and the experimental results are small, and the simulation and experimental values were consistent.

5. CONCLUSIONS The following conclusions are drawn from the study of the photocatalytic decomposition of gaseous ozone in a flowthrough reactor. (a) The kinetics of ozone photocatalytic decomposition can be modeled by the Langmuir−Hinshelwood rate law. The reaction rate was affected by light intensity. The influence of light intensity on the reaction rate followed a power law, and a

Figure 7. Contour plot of ozone concentration at the photocatalyst surface in the reactor (C0 = 2.219 ppm, Q = 1.50 L·min−1, I = 1 mW· cm−2). 7907

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Key Basic Research and Development Program of China (973 program, No. 2014CB239300).

■

Figure 9. Modeling and experimental results of the ozone conversion yield at different flow rates (C0 = 2.219 ppm, Q = 0.50−2.50 L·min−1, I = 1 mW·cm−2). Error bars represent the standard deviations of duplicate measurements.

transition in the kinetic order with respect to light intensity occurred from 0.75 to 1.0 mW·cm−2. (b) The LSSE model was able to simulate irradiation distribution on the catalyst surface as a function of lamp positions. The model also showed that an optimal range of distances, which allows the good homogeneity of photocatalyst surface illumination, exists with the high efficiency of light utilization in the photocatalytic reaction. (c) The CFD model can predict the steady state of the ozone concentration profile by calculating advection, diffusion, and chemical reaction in the flow-through reactor. This modeling approach for a photocatalytic reactor with surface reaction for ozone decomposition at various flow rates and ozone concentrations predicted results that are in good agreement with the experimental data. This study presented a simple example of photocatalytic reaction process modeling. The method presented in this paper is not limited to the Langmuir−Hinshelwood rate law but can also be used for any rate law expression. Knowledge of the intrinsic kinetics allows the universal application of this CFD approach to the optimization and design of photocatalytic reactors.

■

Greek Letters

ρ = density (kg·m−3) a = exponent of light intensity (dimensionless) λ = wavelength τ = viscous stress tensor

Subscripts

■

0 = reactor entrance s = catalyst surface W = lamp wall x = direction of the reactor axial coordinate y = direction of the reactor lateral coordinate z = direction of the reactor vertical coordinate

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Effect of mass transfer. This material is available free of charge via the Internet at http://pubs.acs.org.

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NOMENCLATURE C = substrate concentration (mol·m−3) d = vertical distance of the lamp axis from catalyst surface (m) F = molar feed rate (L·min−1) g = gravitational force (m·s−2) h = thickness of reactor volume (m) I = radiation flux density (W·m−2) Ji ⃗ = diffusion flux of species i (mol·m−2·s−1) k = apparent rate constant (mol·m−2·s−1·W−1·m2) K = Langmuir adsorption equilibrium constant (m3·mol−1) L = lamp length (m) n⃗ = unit vector normal to the surface (dimensionless) p = pressure (Pa) Q = volumetric flow rate (L·min−1) rL = lamp radius (m) ri = net rate of production or depletion of species i by chemical reaction (mol·m−3·s−1) R = distance of the lamp axis from a position of interest (m) t = time (s) v⃗ = velocity vector of the fluid (m·s−1) V = effective volume of the reactor (m3) xL,0 = distance of the lamp ending from the axis origin (m) ylamp = distance of the lamp axis from the origin (m) Yi = mass fraction of species i (dimensionless)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-22-23502142. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research and Development Program of China (973 program, No. 2012CB720100), the Natural Science Foundation of Tianjin (No.13JCQNJC05700), the New Teacher Fund of Ministry of Education of China (No. 20130032120019), and the National 7908

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