Kinetic Study of Acetaldehyde Photocatalytic Oxidation with a Thin

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Kinetic Study of Acetaldehyde Photocatalytic Oxidation with a Thin Film of TiO2 Coated on Stainless Steel and CFD Modeling Approach Ame´lie Queffeulou,† Laurent Geron,‡ Catherine Archambeau,‡ Herve´ Le Gall,† Paul-Marie Marquaire,† and Orfan Zahraa*,† Laboratoire Re´actions et Ge´nie des Proce´de´s, UPR 3349 CNRS, ENSIC, 1 rue GrandVille, BP 20451, 54001 Nancy Cedex, France and ArcelorMittal Lie`ge Research, BouleVard de Colonster, 4000 Lie`ge, Belgique

Removal of low-ppm concentrations of acetaldehyde, a common indoor air pollutant, by photocatalysis is investigated in an annular photoreactor with a thin film of TiO2 coated on a stainless steel plate. Numerical residence time distribution is performed by CFD and characterized with the dispersion model. No byproducts are detected, and complete carbon balance is achieved, allowing the assumption that all of the eventually formed byproducts are converted into carbon dioxide and water. The dependence of the reaction rate on light intensity is studied, showing a first-order tendency in the experimental conditions. Modeling of fluid dynamics and photocatalytic reaction is realized with a CFD approach, considering that intrinsic kinetics is independent of reactor geometry, radiation field, and fluid dynamics. Kinetic parameters determined in a batch reactor are used to calculate the concentration distribution in the annular reactor. In terms of conversion yield, model prediction and experimental results are found in good agreement. 1. Introduction Volatile organic compounds (VOC) represent a major group of indoor contaminants. They can have adverse health effects on occupants such as headache, irritation, and nausea. These phenomena are known as sick building syndrome. Sources of air pollution are numerous: cigarette smoke, building materials, human activities, etc. In indoor air, contaminants exist in concentrations below 100 parts per billion (ppb) and the total number of compounds can reach several hundred.1 Photocatalysis is a promising method for air purification as the process occurs at room temperature and pressure and is able to oxidize low-concentration pollutants to water and carbon dioxide.2,3 TiO2 is the most frequently used photocatalyst; this semiconductor is chemically inert, environmentally friendly, and able to efficiently catalyze reactions.3 Photocatalytic TiO2 can be either used as a powder or coated on a support.4 This work focuses on the photocatalytic degradation of acetaldehyde using a stainless steel foil coated with a thin film of TiO2. Acetaldehyde is a common indoor air pollutant and has been detected at concentrations around 10-100 ppb in the indoor environment.1 This aldehyde has a threshold limit value (TLV) of 25 ppm and is also found as a byproduct in the photocatalytic degradation of ethanol,5,6 propanol,7 and MEK.8,9 Most investigations of VOC destruction by photocatalysis are done (for analysis facility reasons) at high concentrations (several hundred ppm1). However, in this study, experiments are performed at low concentrations of acetaldehyde, in the range of few ppm. Photocatalytic degradation of this compound is investigated in an annular photoreactor where a stainless steel foil is coated with a thin film of TiO2. The main objective of the present work is to predict the performance of this annular reactor considering both kinetics and hydrodynamics interaction using a CFD approach. Intrinsic kinetic parameters are determined in a well-mixed batch reactor. * To whom correspondence should be addressed. E-mail: orfan. [email protected]. † Laboratoire Re´actions et Ge´nie des Proce´de´s. ‡ ArcelorMittal Lie`ge Research.

Imoberdof’s group10 already proposed a method to predict the performance of a multiannular photoreactor from kinetic data obtained in a plug flow reactor in the case of the photocatalytic destruction of perchlorethylene. In this work, the velocity field was previously calculated with a mathematical expression for laminar flow. The CFD approach has been successfully used in studying photocatalytic reactions, particulary in order to determine intrinsic kinetic parameters,11,12 to predict the performance of photoreactors10,11,13,14 and design photocatalytic reactors.11,13-17 For our study, first, experimental photocatalytic degradation of acetaldehyde in the annular photoreactor was carried out. The expression of reaction rate, based on experimental results, can be then obtained ensuring that the degradation process is chemically controlled. Second, a surface reaction model, based on the intrinsic parameters determined in the batch reactor, is coupled with a CFD code in order to determine the distribution of acetaldehyde. The mass transport equation is solved by setting the chemical reaction as a boundary condition. Finally, experimental results and simulation are compared in terms of conversion yield to validate the methodology. 2. Experimental Section 2.1. Experimental Setup. The apparatus is designed to perform experiments with gaseous pollutants concentration in the range of a few ppm. The experimental setup is divided into three parts: the polluted humid air preparation, the reactor, and the analytical system. The photocatalyst is a thin TiO2 film deposit on stainless steel foil which is obtained by physical vapor deposition. The thickness of the TiO2 layer is about 100 nm, and the main phase is anatase. TiO2 deposit is homogeneous on the total steel surface.18 From a gas bottle of acetaldehyde diluted in nitrogen (Air Liquide), the flow rate of acetaldehyde is controlled using a mass flow regulator. Simulatneously, compressed air is cleaned and dried with filters and divided into two streams, each one being controlled by a mass flow regulator. One air stream bubbles through pure water, which is placed in a thermostatic bath. Relative humidity rate is controlled with the saturated vapor by means of temperature adjustment. The

10.1021/ie9017308  2010 American Chemical Society Published on Web 03/03/2010

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Figure 3. Geometry of the annular photoreactor. Figure 1. Experimental setup: (1) gaseous acetaldehyde; (2) dry air; (3) VOC path; (4) dry air path; (5) humid air path; (6) H2O saturator; (7) mixing; (8) annular photoreactor; (9) UV lamp; (10) gas chromatograph; (11) outlet.

second stream is used to dilute the concentration of pollutant and fix its relative humidity (Figure 1). An annular configuration has been chosen for three main reasons. This kind of geometry allows a homogeneous light flux on the photocatalyst and an optimal utilization of photons. Then, an efficient contact between the polluted atmosphere and the catalyst can be achieved. Finally, an annular reactor can be designed to obtain a fluid flow close to plug flow. The reactor can be assimilated to an ideal reactor and allows an easy access to kinetics. This choice of geometry was possible according to the catalyst properties; the steel foil has a thickness of 0.3 mm and is easily malleable; thus, catalyst can be rolled and placed into the reactor directly inside the outer wall. The nonalteration of the TiO2 morphology by this stress has been checked by SEM (scanning electron microscope) measurements as shown in Figure 2. The annular reactor is composed of three concentric glass tubes, the outer one containing the steel foil with the catalyst at the inside of the outer wall. A black light fluorescent lamp (Philips TL 18 W centered on the wavelength 365 nm) has a central position. Then, a liquid circulation is used to control temperature and light flux (the latter by using an optical filter realized with a colorant, nigrosine in water19). Finally, the circulation of polluted air occurs along the photocatalyst-coated foil which is placed on the outer surface of the annular space. Three identical photocatalyst foils with a height of 10 cm and surface area of 198 cm2 can be placed into the reactor along the lamp. The maximum volume of the photoreactor is 196 cm3. The annular space is 3.5 mm (Figure 3). Acetaldehyde concentration and CO2 concentration measurements are performed by gas chromatography (PR 2100 Perichrom at 38 °C, equipped with flame ionization detectors at 260 °C). For acetaldehyde analysis, a MXT-5 Restek (5% diphenyl 95% dimethyl polysiloxane, 30 m, 0.53 mm i.d., 5

µm df) column is used. For CO2 analysis, Porapak T (0.45 m) followed by Porapak Q (2.5 m) columns are used to separate CO2 from air and then a methanization oven is used to convert CO2 into CH4. 2.2. Experimental Parameters and Protocol. During experiments, temperature and pressure in the reactor are set, respectively, at 30 °C and 1030 mbar. The temperature and pressure difference between the bottom and the top of the reactor is, respectively, less than 1 °C and 4 mbar when the steady state is reached. Consequently, the gas volume flow rate can be considered as constant between the inlet and the outlet of the reactor. Thus, the conversion rate can be expressed as X)

Cin - Cout Cin

(1)

where Cin and Cout are, respectively, the acetaldehyde concentrations at the inlet and the outlet of the reactor. Space time is defined as τ)

V Q

(2)

where V is the photoreactor volume and Q the volume flow rate. The height of photocatalyst foils is 30 cm, and the volumetric flow rate varies from 305 to 995 N mL · min-1, which leads to theoretical space time from 11 to 35 s. The inlet concentration of acetaldehyde varies from 2 to 60 ppm. Relative humidity is set at 5%, which is equivalent to 2000 ppm of water. Incident light intensity is controlled by the optical filter according to the concentration of nigrosine solution. Six values of incident UV between 2.5 to 39.5 W · m-2 on the photocatalyst are investigated. First, polluted humid atmosphere goes through the system. When the steady state is reached between the inlet and the outlet of the reactor in terms of pollutant concentration (this takes about 30 min), the ultraviolet lamp is switch on. Samples at the inlet and outlet are then taken.

Figure 2. SEM images of deposed TiO2 on stainless steel (a) before and (b) after rolling.

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Figure 4. Numerical RTD at 995 (corresponding to a theoretical residence time of 11 s) and 305 N mL · min-1 (corresponding to a theoretical residence time of 35 s).

2.3. Reactor Characterization. In order to characterize the annular reactor, a gas residence time distribution (RTD) is determined through CFD simulation. At steady state, the air velocity field is calculated in the annular reactor with a CFD software, FLUENT 6.3, considering that air is incompressible and using the value of flow rate at the reactor entrance. Turbulence phenomena are taken into account in the Navier-Stokes equation through the k-ε model. The choice of the turbulence model is not important, since, as shown below, the flow regime is approximately laminar. At unsteady state, the concentration distribution of acetaldehyde is calculated at each time step in the whole photoreactor after a pulse of injection at the entrance of the reactor. The pollutant is assimilated to a passive tracer. Its concentration distribution is evaluated considering the transport of the pollutant by flow and taking into account molecular diffusion. The flow rate of pollutant going out of the reactor with time is obtained, and results are given in such a way that the area under the curve of RTD E(t) is unity

∫

∞

0

E(t) dt ) 1

(3)

The RTD curve (Figure 4) can be described by an axial dispersion model,20 thus yielding a global axial dispersion coefficient D (m2 · s-1). The Pe´clet number defined with the reactor length characterizes the spread in the whole tube Pe )

uL D

(4)

with u being the mean velocity in the tube (m · s-1) and L the length of the reactor (m). For τ of 11 and 35 s with an effective average gas residence time of 11.4 and 36 s, Pe´clet number values of 272 and 113 are obtained. The annular photoreactor can therefore be assimilated to a plug flow reactor as the Pe´clet number20 is larger than 100. 2.4. Evaluation of the Incident Light Intensity on the Catalyst. An ultraviolet lamp (Philips TL 18 W) is used with a circulation of a colorant, nigrosine, at different concentrations to vary the value of the incident light intensity on the catalyst surface. Evaluation of this intensity is achieved by coupling experimental measurements, performed with a radiometer (VLX) centered on 365 nm, and modeling with optical software, SPEOS. First, a cell composed of two glass plates with 2 values of the distance between the plates where nigrosine solution is placed is used to determine the absorption coefficient of nigrosine solution by considering a Beer-Lambert law and measuring the light flux with the radiometer. For each nigrosine solution, absorption is considered to be constant with the wavelength in the domain of the lamp spectrum according to measurements performed with a spectrophotometer. Then, by knowing the absorption coefficient of each nigrosine solution,

Table 1. Experiments at Constant Space Time with Cin ) 40 ppm hTiO2 (cm)

Q (N mL · min-1)

τ (s)

Iincident (W · m-2)

X (%)

30 20 10 30 20 10 30 20 10

995 663 332 535 357 178 765 510 255

11 11 11 20 20 20 14 14 14

39.5 39.5 39.5 39.5 39.5 39.5 15.5 15.5 15.5

45.5 44 50 80.5 71 84 24 26.5 28

For a given independent of mass transfer acetaldehyde is

value of space time, conversion yield is approximately the flow rate. In these experimental conditions, external is therefore not the limiting step. Degradation of thus in a chemical-controlled regime.

lamp spectrum, reactor geometry, and optical properties of materials, the modeling of the incident UV intensity on the catalyst in the annular reactor is performed with the optical software. 3. Experimental Results and Discussion 3.1. External Mass Transfer. The photocatalytic degradation of volatile organic compounds is governed by three mechanisms: mass transfer, adsorption/desorption phenomena, and photochemical reactions. To ensure that mass transfer is not the limiting step, experiments at constant space time with varying flow rate have been performed, which implies adjustment of the height of the TiO2 plate hTiO2, in other words, the reactor volume. Results, in terms of conversion yield, in accordance with experimental conditions (height of the TiO2 plate, flow rate, incidence intensity) are summarized in Table 1. 3.2. Effect of Initial Concentration and Space Time. The effect of initial pollutant concentration and space time on the conversion yield was investigated for each value of the incident light irradiance. Experimental results obtained for Iincident ) 8 W · m-2 are represented in Figures 5 and 6. The conversion yield decreases with initial concentration of acetaldehyde, which is a common behavior in photocatalytic reactions. As expected, the conversion yield increases with space time. 3.3. Determination of the Photocatalytic Reaction Rates. In a plug flow reactor, for a constant inlet concentration, reaction rate is determined experimentally as the derivate of the outlet concentration with respect to space time. For the six values of incident light intensity, curves of outlet concentration versus space time are established for different values of initial concentrations. Experimental results obtained for Iincident ) 8 W.m-2 with Cin ) 7 ppm are represented in Figure 6. The Langmuir-Hinshelwood kinetic model is widely used to describe the photocatalytic degradation of a VOC21 and was

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Figure 5. Effect of initial concentration on the conversion yield with Iincident ) 8 W · m-2; Q ) 995 N mL · min-1.

Figure 6. Effect of space time on the outlet concentration with Iincident ) 8 W · .m-2; Cin ) 7 ppm.

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Figure 7. Linear regression to determine Langmuir-Hinshelwood kinetic parameters for Iincident ) 8 W · m-2.

Figure 8. Comparison of reaction rates parameters for Iincident ) 8 W · m-2.

successfully used to describe acetaldehyde removal.22,23 The disappearance rate of a single pollutant is expressed as r)

kKC 1 + KC

(5)

with k being the reaction rate constant, K the adsorption equilibrium constant, and C the pollutant concentration. Considering the geometric catalyst surface as the extensive parameter, acetaldehyde mass balance in a plug flow reactor combined with expression 5 leads to a straight line equation XFe 1 S 1 )kK k Q ln(1 - X) Q ln(1 - X)

(6)

with Fe being the molar flow rate at the inlet of reactor and S the total geometric catalyst surface. For each incident light intensity, linear regressions are realized and straight lines are indeed obtained. As an example, results for Iincident ) 8 W · m-2 are represented in Figure 7. From the intercept point and slope, parameters k and K are determined and the initial surface reaction rate is then evaluated. Values of rates determined by these two different methods are compared for all cases; the relative difference is less than 10% (Figure 8). 3.4. Determination of the Influence of the Incident Light Intensity on the Photocatalytic Degradation Rate of Acetaldehyde. The following power law, r R In, is usually used to link the dependence of the degradation rate with the photon flux.24 According to the literature, the reaction rate increases linearly with ultraviolet intensity (n ) 1) until a given value and then increases as the square root of the light intensity (n )

Figure 9. Reaction rate constant versus UV incident light.

0.5); this phenomenon can be explained by a predominant recombination of electron-hole pairs versus charge transfer. For even higher values of light intensity, the order of the reaction is 0 with regard to the light flux (n ) 0) and the mass transfer of VOC to the catalyst surface is the limiting step.24 In the annular reactor, under the present experimental conditions, experimental results show that the dependence of the reaction rate constant k on total UV incident light (from 345 to 400 nm) is almost linear (Figure 9). Comparatively, Obee and Brown25 found experimentally an order of 0.55 for values of light intensity between 10 and 40 W · m-2 and initial concentrations of toluene, formaldehyde, and 1,3-butadiene in the range of a few ppm. Vincent et al.26 found experimentally an order of 0.38 concerning the photocatalytic degradation of acetone, with an initial concentration of a hundred ppm and values of light intensity between 2.1 and 40 W · m-2. These authors26 showed that an order between 0.25 and 0.5 can be explained by

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rate. According to the experimental results, the dependence of degradation rate on concentration follows a LangmuirHinshelwood law, the byproducts are not taken into account, and a linear dependence of the degradation rate on UV light is assumed. The photocatalytic surface reaction rate assimilated to a mass flux is expressed as follows r ) k0

Figure 10. Carbon mass balance with Iincident ) 8 W · m-2; Cin ) 7 ppm.

electron-hole recombination and also by hydroxyl radicals’ recombination. 3.5. Generation of Byproduct. No byproduct is detected in the gas phase during photocatalytic degradation of acetaldehyde, and the conversion yield is constant with time, which suggests that no byproduct is accumulated on the surface. In fact, acetic acid and formaldehyde are mostly obtained from the photocatalytic degradation of acetaldehyde27,28 in our experimental conditions; these intermediates could be converted rapidly into water and carbon dioxide. Furthermore, considering only two species, acetaldehyde and carbon dioxide, a complete carbon balance is achieved. Indeed, total carbon mass at the reactor outlet (generated carbon dioxide and outlet acetaldehyde) is equal to the total carbon mass at the reactor inlet (acetaldehyde) at (5%. Mass balance of carbon is presented in Figure 10 for given experimental conditions, considering the inlet and outlet concentrations of acetaldehyde, respectively, Cin and Cout, and the outlet concentration of carbon dioxide CCO2. 4. Prediction of the Annular Photoreactor Performance with a CFD Approach In this section, performances of the annular photoreactor are evaluated with a CFD approach. Expression of reaction rate is based on the experimental results exposed above and on intrinsic kinetic parameters of the couple VOC-catalyst which are determined in another reactor. Intrinsic kinetics should be independent of reactor geometry, radiation field, and fluid dynamics. Therefore, intrinsic kinetic parameters determined in a given reactor geometry can be used in other reactor configuration knowing the radiation field and flow.10,12 4.1. Principle of Flow and Surface Reaction of Photocatalysis Coupling. As for RTD simulation, the transport equation of acetaldehyde is solved using the CFD software, FLUENT 6.3, at the air mass flow rates studied experimentally and catalyst geometries used. Photocatalytic reaction is considered to occur only on the surface of TiO2 and is imposed as a Neumann boundary condition expressing the mass flux in the concentration distribution calculation. Finally, from the concentration distribution, the conversion yield is calculated according to this expression X)

A

inlet

Cu b·b n dS -

A

A

outlet

Cu b·b n dS inlet

Cu b·b n dS

(7)

with b u ·b n being the normal velocity vector. 4.2. Surface Reaction Model of Photocatalysis and Parameters Determination. The chemical reaction rate is described through a theoretical surface model of the degradation

(A [ STiO2

1 NAvogadro

∫

λgap

0

])

I(λ) Kn dλ dS (mol · s-1) Ehν(λ) 1 + Kn (8)

with n being the number of acetaldehyde moles (mol), k0 the intrinsic kinetic constant, STiO2the surface of illuminated TiO2 (m2), NAvogadro the Avogadro number (mol-1), λ the wavelength of photons (nm), λgap the gap wavelength of TiO2 (nm), I(λ) the amount of energy received at the considered wavelength (W · m-2 · nm-1), and Ehν(λ) the energy of a photon at the considered wavelength (J). The double integral represents the number of efficient photon moles Nph received by the TiO2 surface Nph )

A

STiO2

[

1 NAvogadro

]

∫

I(λ) dλ dS(mol · s-1) (9) Ehν(λ)

λgap

0

Results provided by the optical software are the incident light intensity at a specific spatial position Iincident(r b); this term can be decomposed into three contributions Iincident(b) r ) |I|I¯S(b) r

∫

I (λ) spectrum λ

dλ (W · m-2)

(10)

(1) the norm of the intensity |I|, (2) the dependence on the spatial position jIS(r b), and (3) the dependence on the wavelength jIλ(λ); this last contribution is independent of the system geometry and depends only on the lamp spectrum. Consequently, Nph can be written as

Nph )

A

STiO2

[

∫

λgap

1 I NAvogadro incident

0

∫

Iλ(λ) dλ Ehν(λ)

I (λ)dλ spectrum λ

]

dS(mol · s-1)

(11)

The two integrals inside the brackets are calculated from the lamp spectrum. Whatever the reactor configuration, if the same pollutant and exactly the same catalyst are used, the adsorption coefficient and the intrinsic yield remain identical. 4.3. Determination of the Conversion Yield from the Concentration Distribution Simulation in the Annular Reactor and Comparison with Experimental Data. 4.3.1. Determination of Surface Reaction Model Parameters in a Batch Reactor. Photocatalytic removal of acetaldehyde is studied in a well-mixed batch reactor. The volume of the reactor is 0.5 m3, the photocatalyst geometric surface is 20 × 20 cm2 (made with the same stainless steel foil as described above), and four ultraviolet lamps are used (Philips Cleo, 15 W electrical power). The incident UV intensity on the catalyst of 67 W · m-2 is evaluated by modeling with the optical software SPEOS. Acetaldehyde concentration is followed with a photoionization detector (PID) (IAQRAE PGM-5210). This detector contains a UV lamp composed of an inert gas with an energy amount of 10.6 eV. All molecules with a ionization potential below 10.6 eV are ionized and generate a signal.

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Figure 11. Experimental concentration of acetaldehyde in the batch reactor and modeling with a Langmuir-Hinshelwood model.

In this case also, the experimental curve of acetaldehyde concentration versus time can be modeled using a LangmuirHinshelwood law (Figure 11) r)-

KC dC )k dt 1 + KC

(12)

By integrating eq 12, the following implicit analytical expression for the pollutant concentration is obtained t 1 1 ln(C0 /C) + ) kK C0 - C k C -C 0

Figure 12. Velocity field (cm · s-1) for a flow rate at the entrance of the reactor of 995 N mL · min-1. The velocity field is characteristic of a laminar flow in a tube; a parabolic profile between the inner face and the outer one can be observed.

(13)

Performing a linear regression from experimental data, Langmuir-Hinshelwood kinetic parameters are determined: k ) 0.08 ppm · min-1 and K ) 0.4 ppm-1 . To obtain the intrinsic kinetic constant k0, the surface model of the photocatalysis reaction developed previously (eq 8) has to be extended to a volume one. This is possible if the homogenization time of the pollutant in the whole volume is shorter than the reaction time. As acetaldehyde concentration is uniform in the whole volume, the reaction rate, expressed as follows, characterizes the evolution of the pollutant concentration in the whole space r ) k0

1 Vspace

(A [ STiO2

1 NAvogadro

∫

λgap

0

])

I(λ) dλ dS × Ehν(λ) KC (mol · m-3 · s-1)(14) 1 + KC

with Vspace (m3) being the volume of the considered space and C (mol · m-3) the concentration of the pollutant. 4.3.2. Results of CFD Modeling and Comparison with Experimental Data. Simulation of the acetaldehyde concentration distribution in the annular photoreactor is performed with the CFD software FLUENT 6.3 at steady state with the values of k0 and K determined experimentally in the batch photoreactor. Figure 12 shows the velocity field at half-height of the reactor for a flow rate at the entrance of 995 N mL · min-1. The flow in the reactor is laminar; indeed, the number of Reynolds is ∼10. Since in laminar flow Taylor and Aris showed that axial dispersion is given by the expression Dax/Dm ) 1 + (Re · Sc)2/192, where Re ) u dhF/µ and Sc ) µ/FDm, the axial distribution is then the sum of two contributions: Dax ) Dm + u2 dh2/192Dm, where Dm is the molecular diffusion equal to 1.4 × 10-5 m2/s for acetaldehyde, dh )(D - d), is the diameter of the annular space of the reactor, and u is the axial mean velocity. Thus, under our experimental conditions, the calculated Pe´clet number Pe ) uL/Dax yields a value of 300, superior to 100,

Figure 13. Normalized concentration distribution of acetaldehyde for a quarter of reactor for Iincident ) 8 W · m-2, Q ) 995 N mL · min-1, τ ) 11 s, and Cin ) 9 ppm.

which is the commonly accepted limit for plug flow. This reactor can therefore be considered as a plug flow reactor. This analysis is confirmed by the results of CFD simulation presented in Figure 13, showing the normalized concentration distribution of acetaldehyde for a quarter of reactor, where it appears clearly that the concentration profile is nearly flat. Results obtained in terms of conversion yields are on the same order of magnitude as the experimental results obtained in the annular photoreactor. For all cases, the relative difference is smaller than 35%; results obtained for Iincident ) 8 W · m-2 are shown in Figure 14.

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leading to completion of a mathematical model of the photocatalytic reaction. The methodology presented in this paper can be used for any design and any VOC if the same photocatalyst and the same pollutant are used in the considered configurations. For example, the performance of a pilot-scale air purifier can be evaluated from intrinsic parameters determined in a laboratory reactor. Moreover, knowing intrinsic kinetics, this CFD approach can be used in the optimal design of an air purifier. Acknowledgment The authors are grateful to Pr. E. Plasari and Dr. G. Wild for the numerous fruitful discussions during the preparation of this manuscript. Literature Cited

Figure 14. Comparison of experimental and predicted conversion yield for Iincident ) 8 W · m-2.

Values of conversion yields obtained by simulation have the same order of magnitude as experimental ones, and variations with initial concentration and space time are relatively small. 5. Conclusion The following conclusions are drawn from the study of the photocatalytic oxidation of gas-phase acetaldehyde in an annular photoreactor made up TiO2 thin films on stainless steel. (a) Experimental results show that generated products are carbon dioxide and water. The reactor is shown to be a plug flow reactor according to numerical RTD realized with CFD: the reaction rate can be modeled by a monomolecular Langmuir-Hinshelwood law, and contact time is a key parameter in the degradation of acetaldehyde. In our experimental conditions, the dependence of reaction rate on incident UV intensity is first order. (b) Coupling CFD and the theoretical model of the photocatalytic reaction, based on experimental data obtained in another reactor, gives results in good agreement with those obtained experimentally. Predicted conversion yields of acetaldehyde are of the same order of magnitude as those obtained experimentally. It is worth noting that intrinsic kinetic parameters of the couple VOC-catalyst are determined in a batch reactor and that no adjustable parameters are used. The influence of humidity, temperature, and presence of other pollutants on acetaldehyde conversion should be investigated,

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ReceiVed for reView November 2, 2009 ReVised manuscript receiVed February 11, 2010 Accepted February 12, 2010 IE9017308