Henrietta Essie Whyte
email: whytehenrietta@yahoo.com

Albert Subrenat

Cécile Raillard

Valérie Hequet

Understanding the influence of media geometry on the degradation of acrylonitrile: Experimental and CFD analysis

Keywords: Photocatalytic oxidation, Indoor air treatment, Acrylonitrile, Single-pass removal efficiency, Computational fluid dynamics, Photocatalytic media geometry Graphical Air flow distribution Irradiance distribution Acrylonitrile

The media geometry design is a parameter that influences the efficiency of PCO device for indoor air treatment. However, there is little information in the literature to show how it impacts degradation efficiency. In this work, media geometry influence on the degradation of acrylonitrile is investigated experimentally and with numerical simulations. The experiments were conducted in a laboratory closed loop multi-pass reactor with the aim of comparing degradation efficiencies using plane and pleated media geometries. The experimental results showed a higher removal efficiency for the pleated media. Numerical simulations provided some information to better connect the increase in efficiency to the change in media geometry considering and modeling the media as a porous zone. The simulation results showed that pollutants had lower velocity in pleated media thus contact time was increased by 76%.Additionally,the more heterogeneous distribution of velocity and irradiance in the pleated media could counteract the influence of the contact time and possibly explain the lower than expected experimental results.

Introduction

Air treatment technologies are used to improve indoor air quality by reducing the level of pollutants and can be utilised alone or in combination with HVAC systems. Technologies that can be implemented in commercial devices for removing pollutants are mainly mechanical filtration, adsorption, ozonation, ultraviolet irradiation and photocatalytic oxidation (PCO).

These technologies control pollutants and allow air quality levels to be maintained with reduced outdoor air supply and concomitant energy savings [START_REF] Bergeron | Decreasing Airborne Contamination Levels in High-Risk Hospital Areas Using a Novel Mobile Air-Treatment Unit[END_REF][START_REF] Le | Air purification equipment combining a filter coated by silver nanoparticles with a nano-TiO 2 photocatalyst for use in hospitals[END_REF][START_REF] Perraud | Air filtration and prevention of aspergillary pneumopathies. Preliminary comparative study of two mobile units for bacteriological air purification with recycling[END_REF][START_REF] Rosenbaum | Use of a portable forced air system to convert existing hospital space into a mass casualty isolation area[END_REF][START_REF] Schoenleber | Évaluation d'un dispositif mobile de décontamination pour la maîtrise de l'air au bloc opératoire[END_REF]. These devices can be considered for use in hospitals operating rooms (OR). The OR air may contain different types of pollutants (chemical gases, biological contaminants and particulate matter) due to specific activities (disinfection, cleaning, surgical process and anesthesia) that are performed [START_REF] Barrett | Surgical smoke: a review of the literature[END_REF][START_REF] Dascalaki | Air quality in hospital operating rooms[END_REF]. Among the different air cleaning technologies used for the removal of gaseous chemical pollutants, photocatalytic oxidation seems to be more and more employed.

Photocatalytic oxidation is an advanced oxidation process that uses a catalyst (often TiO 2 ) and UV light to decompose a broad spectrum of Volatile Organic Compounds (VOCs) into innocuous final products like CO 2 and H 2 O. The performance of PCO is affected by several factors such as air velocity, light intensity, initial concentration, relative humidity, and presence of co-pollutants. The influence of these key parameters has been greatly studied in the literature [START_REF] Ao | Photodegradation of formaldehyde by photocatalyst TiO2: effects on the presences of NO, SO2 and VOCs[END_REF][START_REF] Chen | Photocatalytic Oxidation of Multi-Component Systems -An Investigation Using Toluene/Ethylbenzene, Octane/Decane/Dodecane and Formaldehyde/Acetaldehyde[END_REF][START_REF] Dillert | Influence of inlet concentration and light intensity on the photocatalytic oxidation of nitrogen(II) oxide at the surface of Aeroxide® TiO2 P25[END_REF][START_REF] Kozlov | Influence of humidity and acidity of the titanium dioxide surface on the kinetics of photocatalytic oxidation of volatile organic compounds[END_REF][START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Raillard | Kinetic study of ketones photocatalytic oxidation in gas phase using TiO2-containing paper: effect of water vapor[END_REF][START_REF] Sleiman | Photocatalytic oxidation of toluene at indoor air levels (ppbv): Towards a better assessment of conversion, reaction intermediates and mineralization[END_REF][START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF][START_REF] Yu | Indoor air purification using heterogeneous photocatalytic oxidation. Part I: Experimental study[END_REF]. However, the influence of the media geometry has not been greatly explored.

For the same reactor volume, the performance of the degradation can be increased by modifying the media geometry such that it (i) has a high surface area to allow contact with as large a quantity of reactants as possible, (ii) allows a better utilization of the light and (iii) allows a longer contact time between the reactants and the photocatalyst [START_REF] Birnie | Photocatalytic reactors: design for effective air purification[END_REF]. Destaillats et al. [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF] studied the degradation of seven VOCs using plane media geometry and a pleated media geometry whilst Batault [START_REF] Batault | Influence de l'adsorption et des paramètres opératoires sur le traitement photocatalytique de composés organiques volatils en mélange dans les conditions de l'air intérieur[END_REF] studied the influence of plane and pleated geometry on the degradation of toluene. Both authors noticed an increase in the degradation efficiency when the media geometry was changed from a plane to a pleated configuration. They attributed the difference in performance of the two configurations to difference in (i) TiO 2 content, (ii) contact time within media and (iii) irradiance distribution within media. Whilst Destaillats et al. [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF] suggested that the increase in contact time is as a result of the decreasing velocity of pollutants within the pleated media, Batault [START_REF] Batault | Influence de l'adsorption et des paramètres opératoires sur le traitement photocatalytique de composés organiques volatils en mélange dans les conditions de l'air intérieur[END_REF], suggested that the increase in the contact time is as a result of the longer path length travelled by the pollutants when the media was changed from plane to pleated.

Additionally, both authors agreed that for the same average light intensity received on the photocatalytic media (plane and pleated), a modification of the irradiance distribution can also have an influence on the global efficiency of the PCO. These parameters are difficult to measure or confirm experimentally. An effective approach to solve this issue is through the use of Computational Fluid Dynamics (CFD).

CFD is a useful tool to investigate photocatalytic reactors for air treatment [START_REF] Boyjoo | Some aspects of photocatalytic reactor modeling using computational fluid dynamics[END_REF].

It allows for comprehensive analysis of photocatalytic reactor through the simulation of fluid dynamics, species mass transport, chemical reaction kinetics, and irradiance distribution. It is also a cost effective approach as it helps to reduce the number of experiments necessary to predict the behavior of a process. [START_REF] Michalsky | Experimental and CFD analysis of photocatalytic gas phase vinyl chloride (VC) oxidation[END_REF] used experimental and CFD modeling to investigate the flow field and the photocatalytic degradation of vinyl chloride in an annular reactor and reported good agreement of conversion values between the model to experimental results. Castrillion and de Lasa [START_REF] Romero-Vargas Castrillón | Flow field investigation in a photocatalytic reactor for air treatment (Photo-CREC-air)[END_REF] Chong et al. [START_REF] Chong | Simulations of photodegradation of toluene and formaldehyde in a monolith reactor using computational fluid dynamics[END_REF] modeled flow field, radiation and kinetics of toluene and formaldehyde through a porous monolith photocatalytic reactor. The simulation gave them information about the local flow field and light intensity profile. They also observed a good agreement between model predictions and experimental values for conversion profiles. Wang et al. [START_REF] Wang | Modeling of Formaldehyde Photocatalytic Degradation in a Honeycomb Monolith Reactor Using Computational Fluid Dynamics[END_REF] conducted a CFD simulation to enable them describe the local UV flux within a porous monolith channel. They also investigated the monolith to lamp spacing and the number of lamps to achieve the optimal configuration that would provide higher formaldehyde conversion yields. The results of the model were compared experimentally in a corrugated wall, bench scale reactor constructed in the laboratory. Passalia et al. [START_REF] Passalía | A methodology for modeling photocatalytic reactors for indoor pollution control using previously estimated kinetic parameters[END_REF] used a methodology based on previously determined kinetic parameters of a simple geometry, continuous reactor and applied them to a corrugated wall reactor to predict its performance for the degradation of formaldehyde. Their results showed that the kinetic expression obtained was independent of the chosen reactor geometry and size. They found that the overall pollutant conversion showed good agreement between model predictions and experiments, with a root mean square error less than 4%. Using CFD helped them with the simulation and resolution of velocity and concentrations fields in a complex geometry reactor that otherwise would have been more difficult to achieve. [START_REF] Queffeulou | Prediction of photocatalytic air purifier apparatus performances with a CFD approach using experimentally determined kinetic parameters[END_REF] were able to successfully use CFD to predict and validate the performance of a batch reactor for the removal of acetaldehyde. With their proposed model, they found that the predicted times of acetaldehyde removal are of the same order magnitude than those obtained experimentally.

To the best of the authors' knowledge no study has been done on using CFD to explain the influence of the change of media geometry on the degradation efficiency of a pollutant.

In this work, the influence of media geometry on the degradation of acrylonitrile is studied for the first time. Acrylonitrile is a compound of concern found in surgical smoke, one of the main pollutants in an OR. Numerical simulations by CFD are also conducted to better understand the influence of the change of media geometry on the efficiency in terms of the air flow and light irradiance.

Materials and methods

Experimental Set-up

The experimental set-up used in this study is presented in Fig. 1. It consists of the VOC generation bench, the photocatalytic reactor and the sampling and analytical devices.

Fig. 1 : Schematic representation of the closed-loop photocatalytic reactor

Generation of pollutant concentrations

To generate the desired concentration of acrylonitrile, the process started first by generating clean humid air. To do this, compressed air from the laboratory network was passed through an air zero generator Claind AZ 2020 to eliminate VOCs present in the air, then through a Pressure Swing Adsorption (PSA) device remove humidity as well as CO 2 . The air is then humidified to 50 % by bubbling it through a water bottle at an average room temperature of 20°C.

Acrylonitrile (C 3 H 3 N) was supplied by Air products in certified commercial gas cylinder balanced in nitrogen at 150 bars. Acrylonitrile is mixed with the humid air zero to dilute it before injection into the reactor. The desired concentration of 2 ppm is achieved by adjusting the gas flow rates using mass flow controllers (MFC).

Photocatalytic reactor

The reactor that was used to perform experiments is a 420-L multi-pass reactor that has been described in previous articles [START_REF] Batault | How chemical and physical mechanisms enable the influence of the operating conditions in a photocatalytic indoor air treatment device to be modeled[END_REF][START_REF] Maudhuit | Adsorption phenomena in photocatalytic reactions: The case of toluene, acetone and heptane[END_REF][START_REF] Raillard | Use of experimental designs to establish a kinetic law for a gas phase photocatalytic process[END_REF][START_REF] Whyte | Photocatalytic oxidation of isoflurane, an anesthetic gas: the influence of operating parameters[END_REF]. It is a closed-loop reactor built made up entirely of stainless steel to minimize surface losses of pollutants. It is equipped with a variable speed fan which is used to control the air flow in the reactor and allows a flow rate from 28 to 300 Nm 3 .h -1 . For the present study, the flow was set at 148 Nm 3 .h -1 . The reactor also has a tranquilization chamber from where the pollutants are introduced into the reactor and sampling is done. A honeycomb is placed at the inlet of the chamber to provide a homogeneous flow distribution. The photocatalytic media and UV lamps are housed in the photocatalytic module. A Quartzel ® photocatalytic media was used in this work. It is a commercial media by Saint-Gobain Quartz.

It consists of SiO 2 fibers coated with TiO 2 deposited through a sol-gel method and pressed into a felt with average thickness of 17.5 ± 1.5 mm. The BET specific surface area was measured at 112 ± 1 m 2 .g -1 with a Micrometrics ASAP 2020 by N 2 adsorption. The media was irradiated by two 18-W UVC fluorescent tubes (Phillips TUV PL-L series). A Vilber Laurmat VLX-3W radiometer equipped with a calibrated CX-254 nm sensor was used to measure the irradiance over the entire photocatalytic media surface. The average light intensities were achieved by using a variable voltage supply to modify the power of the lamps. In this work, the light intensity was maintained at an average of 4.5 mW.cm -2 . Prior to each experimental run, the reactor and photocatalytic media were cleaned by flushing with humid zero air under UV irradiation for several hours.

Two photocatalytic media geometries were used in this work; plane and pleated. For the plane media configuration (Fig. 2), the photocatalytic media has a square surface with an area of 0.04 m 2 . The media is placed perpendicularly to the flow whilst the lamps are placed horizontally to media and are also 3 cm away from the media surface. The pleated configuration consists of the photocatalytic media folded into four pleats creating two triangles at 60° with a surface area of 0.072 m 2 . The lamps are placed vertically within the triangles. The lamps were also placed 3 cm from the middle of the media pleats.

Sampling and analytical methods

To monitor acrylonitrile concentrations over the period of the degradation, samples were taken onto stainless-steel cartridges packed with Carbopack B (Perkin Elmer). The Automatic Clean Room Sampling System (ACROSS) by Tera Environnement was used to sample at a rate of 200 mL.min -1 for 2.5 min. The analysis was carried out using a Thermal Desorber/Gas Chromatograph/Flame Ionization Detector/Mass Spectrometer (TD/GC/FID/MS). The column used in the GC was a polar column from Restek (Rxi-624Sil MS) which was connected to a FID for the quantification of compounds and a MS for the identification of compounds. The TD/GC/FID/MS analysis conditions are described in Table 1. The limits of detection was calculated by multiplying the background noise by 3. It was calculated as 0.1 ppm for isoflurane and 0.075 ppm for acrylonitrile.

Step

GC

Oven temperature program 35°C (5min), 15°C.min -1 to 150°C (2 min), 20°C.min -1 to 250°C (5 min)

Table 1 : TD/GC/FID/MS analysis conditions

Calculating the single-pass removal efficiency

In this work, the indicator that was chosen to compare the efficiency of the plane and pleated media configurations is the single-pass removal efficiency. The reactor used in this work is a multi pass reactor which provides information on the concentration degradation profiles over the entire duration of the experiment. It however does not provide direct information on the single-pass removal efficiency. This indicator has to be determined indirectly by employing mathematical models. In this work the model that is used has been developed by Dumont and Héquet is explained in their paper [START_REF] Dumont | Determination of the Clean Air Delivery Rate (CADR) of Photocatalytic Oxidation (PCO) Purifiers for Indoor Air Pollutants Using a Closed-Loop Reactor[END_REF]. The model, describes a first order decay relationship of concentration vs. time in a closed-loop reactor with a global apparent kinetic constant, including chemical kinetics and reactor dynamics. The model does not explicitly account for the light effect or the diffusion phenomena and is given as follows:

(1)

𝐂 = 𝑪 𝟎 𝐞𝐱𝐩 { -𝒕 𝝉 𝑹 │ [𝟏 -𝐞𝐱𝐩 ( -𝜶 ) ] }
In Eq. ( 1) the term α is the single-pass removal efficiency and represents the fraction of the total flow treated during the time τ R (residence time in the reactor). C is the pollutant concentration at time t and C 0 is the initial pollutant concentration. Numerical resolutions were carried out using Excel ® Solver which is based on the least-square method.

CFD Model Development

The simulation was done using the commercial software ANSYS 14.5. The geometry was defined to meet the dimensions and characteristics of the photocatalytic module of the closedloop reactor using ANSYS design modeler. The photocatalytic reactor volume was discretized into 503123 structured and unstructed cells. This number of cells was found to be high enough to give mesh-independent results and provided a satisfactory capture of the flow and irradiance qualities at a reasonable computational time.

Fluid dynamics

The simulation of fluid dynamics in the photocatalytic module was performed considering a three dimensional, steady state turbulent flow. The fluid was air and was considered as incompressible and isothermal with constant physical properties. The photocatalytic media was simulated as a porous zone. The conservation of mass is achieved by solving the classical continuity equation. For the conservation of momentum, since the flow is in the turbulent regime, the Reynolds-Averaged Navier Stokes (RANS) equation is solved. However since the reactor also contains a porous region which is the photocatalytic media, the equations are modified to account for the porosity of the media. The modified continuity and the momentum equations are given below as Eq. ( 2) and Eq. ( 3) respectively (ANSYS Inc., 2013;[START_REF] Wang | Modeling of Formaldehyde Photocatalytic Degradation in a Honeycomb Monolith Reactor Using Computational Fluid Dynamics[END_REF].

(2)

∂(𝜺)𝝆 ∂𝒕 + ∇ ( 𝜺𝝆𝒗 ) = 𝟎 ∂(𝜺𝝆𝒗) ∂𝒕 + ∇ ( 𝜺𝝆𝒗𝒗 ) = -𝜺∇𝒑 + ∇𝜺𝝆𝒈 + ∇𝜺𝝉 + 𝑺 𝑴 (3)
where ρ, v, τ, g, p and ε are the density of the fluid, the velocity of the fluid, stress tensor, gravity, pressure and porosity of the media respectively. S M is the momentum sink term which accounts for the loss of momentum or pressure drop due to an obstacle which in this case is the media. It is given by the modified Darcy law (Eq. ( 4)) as:

(4) 𝑺 𝑴 = µ 𝒌 𝒗 + 𝟏 𝟐 𝑪𝝆𝒗 𝟐
where k is the permeability, C is the inertial resistance factor, v is the velocity, µ is the dynamic viscosity. The media was assumed to be a porous body with porosity of 0.875. The permeability and inertial resistance factor are calculated 1.45 x10 -8 m 2 and 823 m -1 respectively. The parameters were calculated from experimental data. This was done by simply measuring and plotting a graph of the pressure drop as a function of velocity. By using the regression model from experimental results and the modified Darcy equation (Equation 4), the permeability and inertial resistance factor is calculated.

The inlet velocity was set as 1m.s -1 whilst at the outlet, atmospheric pressure was applied. A no-slip boundary condition was imposed at the reactor walls.

Radiation field modeling

To model the irradiance distribution within the reactor the radiative transport equation (RTE) needs to be solved. The RTE is an integro-differential equation which describes the photonic rays with their corresponding energy loss due to absorption and out-scattering and gain due to in scattering. It describes the movement of a bundle of light rays travelling in a direction s having a solid angle Ω, and intensity I. The generic form of the equation is given in Eq. ( 5) as (ANSYS Inc., 2013):

(5)

𝑑𝐼(𝑠𝛺) 𝑑𝑠 + 𝑎𝐼(𝑠𝛺) + 𝜎𝐼(𝑠𝛺) = 𝑎𝑛 2 𝜎 𝑆𝐵 𝑇 4 𝜋 + 𝜎 𝑠 4𝜋 ∫ 4𝜋 0 𝐼(𝑠 , 𝛺)𝜙(𝑠 , .𝑠)𝑑𝛺 ′
Where s is the direction, s' is the scattering direction, a is the absorption coefficient, n is the refractive index, σ is the scattering coefficient, σ SB is the Stefan-Boltzmann constant, T is the local temperature, Φ is the phase function, Ω' is the scattering solid angle. Numerical resolution of the RTE was done using the discrete ordinate (DO) method which solves the RTE for a finite number of discrete solid angles, each one associated with a vector direction. When using the DO method, the spatial discretization of the computational region is taken directly from the mesh grid topology. However, the directional discretization for the RTE was explicitly specified using an angular discretization of the sphere octant of 10 × 10 divisions (enough to avoid the well-known "ray effect") and 3 × 3 pixelation (enough to overcome angle overhanging).

It is assumed that there is negligible scattering, emission or absorption by the fluid medium thus in the fluid medium Eq. ( 5) reduces to:

= 0 (6) 𝒅𝑰(𝒔𝜴) 𝒅𝒔
However, there is a gradual loss of light due to absorption through the photocatalytic media. As stated in the literature [START_REF] Boyjoo | Some aspects of photocatalytic reactor modeling using computational fluid dynamics[END_REF][START_REF] Chong | Simulations of photodegradation of toluene and formaldehyde in a monolith reactor using computational fluid dynamics[END_REF] for immobilized systems, the scattering and reflection are assumed to be negligible. Therefore within the photocatalytic media, Eq. ( 7) is reduced to:

(7)

𝑑𝐼(𝑠𝛺) 𝑑𝑠 + 𝑎𝐼(𝑠𝛺) = 0
The absorption coefficient (a) is therefore a necessary user input. It is related to the UV-Transmittance (UVT) by the Beer-Lambert Law which is given in Eq. ( 8) as:

(8) 𝑈𝑉𝑇 = exp ( -𝑎𝑥)
where x is the path length travelled by the light inside the photocatalytic media.

The UVT of the media was found to be 0.6% therefore the absorption coefficient is calculated as 292 m -1 .

Finally, as a boundary condition for the resolution of the RTE, a surface emission model was considered for the UV lamps, as this model has been previously found to be appropriated to represent fluorescent UV lamps [START_REF] Boyjoo | Lamp emission and quartz sleeve modelling in slurry photocatalytic reactors[END_REF][START_REF] Boyjoo | Some aspects of photocatalytic reactor modeling using computational fluid dynamics[END_REF]. The lamp wall was set as semitransparent purely diffuse wall with an external irradiance. All the remaining walls except the reactor wall were set as zero-thickness and also semi-transparent. The reactor wall was defined as diffusely reflecting opaque stainless walls with a reflectivity of 0.15. The temperature was fixed to 1 K in all domains to inactivate calculations of radiation emission.

Solution strategy and convergence criteria

ANSYS CFX solver was used to solve the fluid dynamics equations whilst ANSYS Fluent was utilized to solve the RTE equations. The first order high resolution scheme was employed for fluid dynamics whilst the second order upwind method was used to solve the RTE. The convergence was ensured by imposing a residual value of 10 -6 for continuity and momentum and 10 -4 for incident radiation. It took 1130 iterations and 6 hours to achieve convergence for fluid dynamic modeling whilst it took 1100 iterations and about 14 hours to achieve convergence for radiation field modeling.

Results and discussion

Degradation profile of acrylonitrile

Fig. 3 shows acrylonitrile degradation experimental points fitted with the model (Eq. 1) for the plane and pleated configuration. Under the experimental conditions of C 0 = 2 ppm, v = 1 m.s -1 , I = 4.5 mW.m -2 and RH = 50 % the complete degradation of acrylonitrile was achieved within 60 minutes and 20 minutes for plane and pleated respectively. Both curves were characterized by a mono-exponential profile. The model was shown to fit satisfactorily to the experimental points based on the co-efficient of determination (R 2 ) of 0.99. Under the experimental conditions, the single-pass removal efficiency (α) was then calculated as 0.012 and 0.027 respectively for plane and pleated configurations. This indicated that in one pass, 1.2% of acrylonitrile would be eliminated from the total flow that passed through the plane media whilst for the pleated media 2.7% would be eliminated in one-pass.

Influence of the media geometry on the single-pass removal efficiency during photocatalytic degradation of acrylonitrile

In this work, to study the influence that the change in media geometry had on the degradation of acrylonitrile, experiments were carried out with a pleated media configuration and the results compared to reference experiments carried out with the plane media configuration. In order to further highlight the influence of the contact time, the average light intensity was kept constant for both plane and pleated media geometries. To ensure that the average light intensity received on the pleated media was the same as the plane media, numerical simulations were used and are detailed in section 3.3.2.

It was observed that the single-pass removal efficiencies increased from (12.08 ± 0.11) x 10 -3

for the plane media to (27.42 ± 0.74) x 10 -3 for the pleated media. It is evident that the pleated media geometry improves the PCO performance. This is in accordance with what Destaillats et al. [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF] and Batault [START_REF] Batault | Influence de l'adsorption et des paramètres opératoires sur le traitement photocatalytique de composés organiques volatils en mélange dans les conditions de l'air intérieur[END_REF] observed. Three main reasons may account for this efficiency increase: (i) an increase in the surface area translates to an increase in the amount of catalyst/active sites available for pollutant removal (ii) increase in the contact time between pollutants and media (iii) better distribution of light intensity.

To eliminate the effect of the increase in catalyst quantity due to increase in media area, the removal efficiencies obtained for both configurations were divided by their respective surface areas. The results are shown in Table 2. It can be observed that the surface corrected values of the single-pass removal efficiency (α) for the pleated media is 27 % higher than that of the plane media. This means that there is an increase in the efficiency not only due to the increase in catalyst quantity but probably also due to the increase in contact time and the possibly the better distribution of irradiance within the media.

Table 2 : Surface corrected single-pass removal efficiencies of acrylonitrile calculated for the plane and pleated configurations

Indeed, the contact time between the pollutants and the photocatalytic media and the distribution of irradiance is different in the two geometries. It is assumed that the contact time is globally increased in the pleated geometry. As previously discussed, Destaillats et al. [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF] state that the increase in contact time is as a result of the decreasing velocity of pollutants whilst for Batault [START_REF] Batault | Influence de l'adsorption et des paramètres opératoires sur le traitement photocatalytique de composés organiques volatils en mélange dans les conditions de l'air intérieur[END_REF], the increase in the contact time is as a result of the longer path length travelled by the pollutants. Additionally, they state that for the same average light intensity received on the photocatalytic media (plane and pleated), a modification of the irradiance distribution can also have an influence on the global efficiency of the PCO. These parameters are difficult to measure or confirm experimentally, therefore numerical simulations can be used to provide some information to be able to better connect the increase in efficiency to the change in geometry.

Geometry A (m 2 )

α/A (m -2 ) Plane 0.040 0.300 ± 0.003 Pleated 0.072 0.380 ± 0.010

Influence of the media geometry on the single-pass removal efficiency during photocatalytic degradation of acrylonitrile

To better understand the role of the media geometry in improving the removal efficiencies, numerical simulations were done to investigate the behavior of air flow and light irradiance in the photocatalytic reactor for both configurations.

Fluid dynamics modeling

The aim of this section is to be able to describe the behavior of the flow in the reactor and in the media and also determine the contact time within the media. The results for the plane configuration are first discussed followed by those of the pleated configuration.

Fig. 4 shows the results for the flow distribution from the inlet to the outlet of the photocatalytic module along a plane for the plane media geometry. From Fig. 4, it is observed that the flow is homogeneous from the inlet until it encounters the lamps where the flow is obliged to move around the lamps. This change in course presented by the lamps, causes zones of higher velocity above and beneath the lamps and zones of lower velocity behind the lamps. However, once inside the media, the flow homogenizes again and remains homogeneous when it exits the media and subsequently the reactor. Due to the fact the lamps are quite close to the media (3 cm), this behavior of having zones with higher and lower velocities compared to the inlet velocities is replicated at the inlet of the media as shown in Fig. 5a. The range of velocities at the media inlet is 0.9 -1.39 m.s -1 . Once inside the media, the flow becomes quite homogeneous as shown in Fig. 5b. The range at this distance is 1.12 -1.15 m.s -1 . The flow is still homogeneous when it exits the media as shown in Fig. 5c The streamlines were then studied and the results are shown in Fig. 6. It shows that the flow is parallel with some small recirculation zones behind the lamps but these are not significant enough to affect entire flow. The streamlines within the media showed that the flow is orthogonal as it moves through the media signifying that the path length within the media is equal to the media thickness.

Inlet Outlet

Fig. 6 : Streamlines through the photocatalytic module in plane media configuration

To verify the simulation results, the velocity distribution along the height of the reactor at a distance of 8 cm on the x-axis and 6 cm before the media surface was measured experimentally.

A Veloport 20 portable air velocity meter was placed in the reactor and the velocity at several distances along the height of the reactor was recorded. Experimental measures were triplicated.

As observed in Fig. 7, the simulated and experimental results were quite similar signaling that the simulation was quite representative of what was occurring in real conditions.

Fig. 7 : Comparison between simulated and experimental velocity values along the height of reactor at a distance of 6 cm before media

The average velocity that is received on the media surface and also within the media is calculated to be 1.14 m.s -1 . This value is about 14% higher than the inlet velocity of 1 m.s -1 . For fluid flow through a porous medium, the superficial velocity (v 0 ) is the flow rate Q (m 3 .s -1 ) divided by the cross-sectional area A (m 2 ). In our case, the presence of SiO 2 fibers and TiO 2 particles within the photocatalytic media reduces the area available for fluid flow i.e. to conserve fluid continuity with the entering superficial flow, the fluid has to move through a smaller area; hence the velocity within the photocatalytic media (v) is greater than that of the superficial velocity. The true velocity (v) is related to the superficial velocity by the following expression [START_REF] Holdich | Fluid flow through porous media[END_REF]:

(9) 𝑣 = 𝑣 0 𝜀
where ε is the porosity. Here v 0 = 1 m.s -1 , ε = 0,875 then v = 1/0.875 = 1.14 m.s -1 .

In knowing the average velocity within the media and the path length, the contact time (τ) was then calculated as 15.3 ms using the equation: )

Where e is the path length is equal to the thickness of the media (m) which is 17.5 mm and v is the average true velocity in media (m.s -1 ).

For the pleated media geometry, the behavior of the flow as it moves across the photocatalytic module is shown in Fig. 8. Similarly to the plane configuration, the flow in the pleated configuration is homogeneous until it encounters the lamps. At this point, the presence of the lamps and the pleats causes the flow to lose its homogeneity. Additionally, as a result of the influence of the presence of the pleats, the flow exiting the media is quite heterogeneous.

Fig. 8: Velocity contour showing flow behavior through the reactor in the pleated configuration

Fig. 9a shows the velocity profile received at the media inlet. It can be observed that the flow is heterogeneously distributed on the surface with the maximum velocities at the regions surrounding the pleat joints. The range on the surface is found to be 0.07 -2.03 m.s -1 . Unlike the plane configuration, the flow does not homogenize within the media as shown in Fig. 9b.

The range of velocity 8 mm into the media is found to be 0.17-0.72 m.s -1 . At the exit of the media the minimum velocities are found at the pleat joints as shown in Fig. 9c and the range of velocities for the entire surface is 0.0098 -0.7 m.s -1 . The streamlines show that the flow is parallel but once it enters the media becomes orthogonal as shown in Fig. 10. A closer look at the media shows that the flow enters perpendicularly into the media but at a distance of about 3 mm within the media the flow becomes orthogonal and remains orthogonal until the exit of the media. This means that the path length travelled within the media could be assumed to be equal to the thickness of the media. At the inlet surface of the media, the average velocity received is about 1.5 m.s -1 and is 32%

higher than what is received averagely by the plane configuration. This increase could probably be explained by the fact that presence of the pleats creates a canal effect (which means that area reduces) such that the velocity increases in order to maintain the same flow-rate. However once inside the media the increase in the surface area causes the velocity to be reduced to an average of about 0.65 m.s -1 which is about 43 % lower than the average velocity within plane configuration. Since the distance travelled (path length) was equivalent to the thickness of the media, the contact time within the pleated media is calculated as 26.9 ms.

Radiation field modeling

The objective of this section is to evaluate and compare the distribution of irradiance received within the reactor and media for the two geometry configurations. The simulation results are discussed first for the plane configuration then for the pleated configuration.

The irradiation behavior through the reactor for the plane media geometry is shown in Fig. 11.

It can be seen that the irradiance is the highest around the lamps but rapidly decays as the distance from the lamps increases. This is attributed to the effect of geometric attenuation which is explained as the reduction in the radiation quantity due to the distance between the point of interest and the source and excludes absorption by any matter present. UV radiometers are developed to replicate and preserve an ideal cosine response in order to accurately measure irradiance. However, some of these radiometers may have angular responses that deviate from the ideal cosine response [START_REF] Kozlov | Development of Multi-Stage Photocatalytic Reactors for Air Purification[END_REF][START_REF] Michalsky | Experimental and CFD analysis of photocatalytic gas phase vinyl chloride (VC) oxidation[END_REF][START_REF] Zibordi | Effects of cosine error in irradiance measurements from field ocean color radiometers[END_REF]. When the sensor is located directly in line with the light source a more accurate reading is obtained but once the angle of incidence is increased the error between the ideal cosine response and the detector reading increases and the sensor underestimates the irradiance that is received at that point [START_REF] Berni | System for measuring the angular response of radiometers[END_REF][START_REF] Feister | A method for correction of cosine errors in measurements of spectral UV irradiance[END_REF].

For the pleated configuration, the emissive power in the simulation was changed until the average irradiance received on the pleated media surface was similar to that received on the plane media surface.

Once this was done, the irradiance distribution was then studied. Fig. 14 shows the irradiance distribution within the reactor. The results presented show that similarly to the plane configuration the irradiance was highest around the lamps and decreased as the distance from the lamps increased due to the effect of geometric attenuation.

Fig. 14 : Irradiance contours showing irradiance distribution through reactor for pleated media configuration

The distribution received on the surface of the media was also studied. First of all, Fig. 15a shows that the irradiance is not homogeneously distributed on the surface and secondly that the highest irradiance is found in the areas directly behind the lamps and decreased as the distance from the lamps increased. It is also observed that the region around pleat joints is the lowest illuminated region. This trend is replicated through the media as shown in Fig. 15b andc. The distribution of irradiance is different for both media geometries and the light intensity is more heterogeneous (bigger range in values) in the pleated than in the plane. Simulation results also show that for both media geometry there are regions where the irradiation is quiet low. It can be assumed that the degradation in these regions would be less effective. For the plane media geometry, the lowest irradiated regions are the corners of the media irrespective of the location (inlet, 8mm within media and outlet) in the media. In the case of the pleated media, the lowest illuminated region is the region around pleat joints. Depending on the location in the media the irradiance changed. Therefore using a criterion range of 0.06 W.m -2 to 13 W.m -2 , it represented 1.5 % of the total surface of the plane media and 18 % of the total pleated media surface.

Light distribution within the photocatalytic media

Monolith photocatalytic media are known to offer low-pressure drop and high catalyst surface area however the light intensity is known to quickly decline through the media [START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Sauer | Acetone Oxidation in a Photocatalytic Monolith Reactor[END_REF]. Fig. 17 shows the irradiance profile through the photocatalytic media for both configurations. The irradiance is highest at the media inlet but decreases sharply to 23% at 50% distance within the media for both configurations. At 85% media distance it drops below 1% for the plane media and 3% for the pleated configuration. Beyond this distance the media is not sufficiently illuminated and is subjected to operation under dark conditions.

Wang et al. [START_REF] Wang | Modeling of Formaldehyde Photocatalytic Degradation in a Honeycomb Monolith Reactor Using Computational Fluid Dynamics[END_REF] found similar results when they simulated the irradiance profile through a cordierite ceramic monolithic media with a thickness of 25.4 mm. In plotting a graph of the dimensionless UV flux to distance they found that at a distance of 3.2 mm the UV light intensity decreased to 30 % of the incoming flux and to 1 % at a distance of 9.5 mm. These results suggest that a shorter media width would be preferable to reduce the dark zones or the provision of lamps on both sides of the photocatalytic media to allow a better utilization of the media Fig. 17 : Irradiance profile within photocatalytic media for plane and pleated configurations

Calculation of effective-surface single-pass removal efficiencies

In section 3.2, the surface corrected single-pass removal efficiencies (α/m 2 ) were found to be 27% higher for the pleated geometry than for the plane geometry as shown previously in Table 2. As previously discussed, the light intensity received on and within the media remains the same and the simulations show a 76 % increase in contact time by using the pleated media geometry. Thus, ideally if all parameters are constant for both configuration and only the contact increases, we expect that the surface corrected single-pass removal efficiencies (α/m 2 ) obtained from the experiments would be similar to this number but the experiments showed a much lower improvement of 27 %. (i) It was observed that the lowest region of illumination where the media would be operating under near dark conditions and would not be very effective represented 18 % of the total media surface compared to the 1.5 % in the plane media. This subsequently means that effective surface of the pleated media was further decreased.

If the surface corrected single-pass removal efficiency (α/m 2 ) is calculated considering the new "efficient surface" for both plane and pleated media, it is observed that it is 48 % higher for the pleated geometry than for the plane geometry. Quantitative results are presented in Table 4.

Table 4 : Surface corrected single-pass removal efficiencies (α/m -2 ) of acrylonitrile calculated for the plane and pleated configurations and the corrected surface areas (A') for each configuration (ii) It was also shown in the simulation that the flow and irradiance distribution within the pleated geometry was more heterogeneous (bigger range in values) than the plane geometry (Table 5). This means that even though the global average velocity was decreased and the contact time was higher within the pleated media, the higher level of heterogeneity could explain why the α values were lower than expected for the pleated configuration. Some authors have concluded that when light intensity distribution of the surface is homogenous it increases the overall photonic efficiency and better reaction rates are achieved whilst with heterogeneous distribution, areas of high light intensity will experience low photonic efficiency as a possible result of Geometry A' (m 2 ) α α/A' (m -2 ) Plane 0.039 0.01203 ± 0.00011 0.308 ± 0.003 Pleated 0.059 0.02742 ± 0.00074 0.46 ± 0.01 the electron hole recombination [START_REF] Kozlov | Development of Multi-Stage Photocatalytic Reactors for Air Purification[END_REF][START_REF] Martín-Sómer | Influence of light distribution on the performance of photocatalytic reactors: LED vs mercury lamps[END_REF].

Table 5 : Range of velocities and irradiance on different sections within the media

Conclusively, the use of pleated media geometry improves the degradation efficiency by decreasing the velocity of pollutants within the media and consequently increasing the contact time between the pollutants and the media. However, the more heterogeneous distribution of the velocity in the pleated media counteracts the influence of the contact time. Additionally, even though the average the light intensity on the media is the same for both media, the more heterogeneous distribution in the pleated geometry also hinders the effect of increase in contact time.

Conclusions

In this work, the influence of the change in media geometry on the degradation of acrylonitrile, a pollutant present in hospital ORs was studied. The experiments were performed using plane and pleated media geometries placed in a 420-L multi-pass laboratory reactor. The use of a model developed by Dumont and Héquet enabled the calculation of a performance indicator, the single pass removal efficiency. This quantitative descriptor enabled the study of the influence of the change in media geometry on the degradation of acrylonitrile.

The experimental results showed that under similar experimental conditions, the single-pass removal efficiency was increased when the pleated media geometry was used compared to the plane media geometry. The use of CFD allowed the simulation of velocity and irradiance distribution at the media level for both geometries that otherwise would have been more

difficult to achieve experimentally. The results of the air flow and irradiance distribution obtained from simulation enabled the conclusion that the more heterogeneous distribution of velocity and light intensity counteracted and hindered the effect of the increase in contact time.

CFD simulations helped to highlight important behavior with respect to flow and the irradiance in a photocatalytic unit and could be useful to help investigate configurations that could improve the contact time and irradiance but also better homogeneity of these variables in the media.

CFD could also be further used to study species transport and chemical reaction kinetics that would help to strengthen predictive capabilities and lessen the number of experiments thus consequently providing an economic advantage.

  photocatalytic reactor by combining CFD modeling of the fluid flow in the reactor with

Fig. 2 :

 2 Fig. 2: Representation of the photocatalytic module showing position and arrangement of

Fig. 3 :

 3 Fig. 3: Experimental degradation curve of acrylonitrile during use of plane and pleated media configurations fitted with model developed by Dumont and Héquet under reference experimental conditions of C 0 = 2 ppm; v = 1 m.s -1 ; I = 4.5 mW.cm -2 ; RH = 50 %

Fig. 4 :

 4 Fig. 4 : Velocity contour showing flow behavior through the reactor in the plane configuration with inlet velocity of 1m.s -1

Fig. 5 :

 5 Fig.5 : Velocity contour showing flow behavior on (a) the surface of the photocatalytic media (b) 8mm distance in media (c) outlet of the media in the plane configuration

Fig. 9 :

 9 Fig. 9 : Velocity contour showing flow behavior on (a) the surface of the photocatalytic media (b) 8mm distance in media (c) outlet of the media in the pleated configuration

Fig. 10 :

 10 Fig. 10 : Streamlines through the photocatalytic module and media in pleated media configuration

Fig. 11 :

 11 Fig. 11 : Irradiance contours showing irradiance distribution through reactor for plane media configuration

Fig. 12 :Fig. 13 :

 1213 Fig. 12 : Irradiance distribution on media surface; (a) simulation (b) experimental

Fig. 15 :Fig. 16 :

 1516 Fig. 15 : Irradiance contour showing flow behavior on (a) the surface of the photocatalytic media (b) 8mm distance in media (c) outlet of the media in the pleated configuration

Table 3

 3

	shows the

Variable Geometry Media inlet 8 mm in media Media outlet

	Plane	13.22 -94.41	4.38 -33.47	0.06 -1.03
	Irradiance (W.m -2 )
	Pleated 5.80 -112.90	0.33 -24.50	0.05 -6.74

Table 3 : Range of irradiance on different sections within the media

 3

Acknowledgements

The authors would like to express their gratitude to ATA Medical and IMT Atlantique for their financial support and also to Eric Chevrel, Patrick Brion, Francois-Xavier Blanchet, Yvan Gouriou and Katell Chaillou for their invaluable technical assistance and to Eric Dumont for his help with the modeling. Inc., 2013. Ansys CFX Solver Modeling Guide, 14.5. ed. Canonsburg.

ANSYS

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.