Translucent packed bed structures for high throughput photocatalytic reactors

Chemical Engineering Journal 361 (2019) 725–735

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Translucent packed bed structures for high throughput photocatalytic reactors

T

Thomas Claesa, Anouk Dilissena, M. Enis Leblebicib, Tom Van Gervena,

⁎

a b

KU Leuven, Department of Chemical Engineering, Celestijnenlaan 200F, B-3001 Heverlee, Belgium KU Leuven Lab4U, Faculty of Industrial Engineering, Agoralaan Building B, B-3590 Diepenbeek, Belgium

HIGHLIGHTS

GRAPHICAL ABSTRACT

translucent packed bed micro• Astructure was assessed for use within photocatalytic reactors.

light source with homogeneous light • Afield was built using a verified raytracing model.

benchmarked, the reactor was • When proven to perform as a scalable array of microreactors.

reactor had the highest energy • The efficiency among immobilized catalyst photoreactors.

ARTICLE INFO

ABSTRACT

Keywords: Photocatalysis Reactor engineering LED Light distribution Structured reactors Photocatalytic degradation

Translucent photocatalytic reactor structures are investigated as a possible alternative to numbering up as a method for the scale-up of microreactors. The structure and the light source design is elaborated to introduce this concept. The light field was characterized using a ray tracing algorithm. A rectangular reactor made from glass was designed using borosilicate spheres small enough to create an array of interconnected microchannels in the reactor. It was found that ray tracing can be used as a proper tool to easily design multiple-LED light sources and predict respective irradiance patterns. The performance of the reactor was assessed using the apparent rate constant, the space-time yield and the photocatalytic space-time yield, a recently introduced performance parameter which takes into account the lamp power and the reactor productivity. The apparent rate constant of the reactor for an incoming irradiance of 191 W m−2 was found to be 0.82 min−1, which is, to our knowledge, in the range of microreactors and 1–2 orders of magnitude higher than any high throughput immobilized reactor in literature. With a photocatalytic space-time yield of 0.657 m3 day−1 m−3 reactor kW−1 our reactor was amongst the best reported performers in terms of productivity and energy efficiency. This performance is related to the high specific illuminated surface area of 4267 m2 m−3 and high catalyst load of 1.9 g L−1.

1. Introduction Ever since the discovery of the photocatalytic properties of TiO2, photocatalysis and photochemistry in flow has become a promising technology for a wide area of applications such as water purification,

⁎

pharmaceutical ingredients synthesis and energy storage [1,2]. The biggest scientific challenge in viable photocatalytic processes is to cope with slow apparent reaction rates and energy inefficiency of industrial scale reactors. The mismatch between the timescale of the light-catalyst interactions (∼1 µs) [3] and the substrate diffusion inside

Corresponding author. E-mail addresses: [email protected] (T. Claes), [email protected] (M.E. Leblebici), [email protected] (T. Van Gerven).

https://doi.org/10.1016/j.cej.2018.12.107 Received 29 August 2018; Received in revised form 11 December 2018; Accepted 19 December 2018 Available online 19 December 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 361 (2019) 725–735

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the porous catalyst (∼1 s) is causing illuminated areas inside the catalyst layer without any substrate molecule to react. The solution to this problem of internal mass transfer (1) is the use of a very thin catalyst layer. This implies a catalyst load problem (2) and an efficient illumination problem (3) since a large part of the photon energy will leave at the back of the catalyst layer. The problem of internal mass transfer problems in the catalyst layer was already addressed by multiple authors [4–7]. Experiments and models show that the optimal film thickness differs from 1 µm to 5 µm depending on the operational conditions of the experiments (e.g. light intensity) and the layer characteristics such as porosity and pore size which are mostly dependent on the coating technique. However, the criterion for a favorable photocatalytic system is often defined based on the reaction rate. This does not indicate that the inner parts of the layer are efficiently used. Studies, which select the catalyst coating thickness based on the rate criterion alone, usually have thick (> 5μm) catalyst coating. Thin layers mean that the catalyst loading, and the resulting reaction rate, will be low. This is correct for designs where only one surface is coated along the light-path (such as parallel plate reactor). The photons that are not absorbed by this single coated wall, escape and are therefore lost. To solve the catalyst load problem, illumination problem and internal mass transfer issues designs are proposed which are attempting to create illuminated surface area. Monoliths, optical fiber reactors, or a combination of a monolith structure with optical fiber illumination are examples in literature which try to increase the illuminated surface area [8–11]. Both reactors were proven to work but with low energy efficiency. The monoliths are not transparent for light which means light energy gets lost on the reactor walls while the optical fiber reactors have a low surface area and less-efficient reverse illumination. The microreactor from Visan et al. is an excellent example of a reactor that solves the mentioned problems simultaneously [5]. The micro-photoreactor is an immobilized catalyst reactor. It is designed to run in a continuous mode with a characteristic length of less than a millimeter. Microreactors prove that when the three limitations are solved, the apparent reaction rates can be improved drastically due to the fast intrinsic kinetics (up to 1 s−1 for the microreactor from Visan et al.). External mass transfer can be tackled using small channels while the internal mass transfer limitations are solved using thin catalyst layers. Both properties result in a high surface to volume ratio which means high catalyst loads (up to ∼50 g L−1 for layers thicknesses of 1000 nm) are possible [5]. In addition, the microreactor channels can be illuminated intensively. The main concern in microreactor engineering is the scalability and subsequently the productivity. Photocatalytic microreactors are very fast, but not very productive (low volume) with respect to their energy consumption. To achieve a throughput of industrial size, thousands of microreactors are needed in parallel. However, the light and material distribution in such a scaledup multi-reactor environment is not straightforward to carry out efficiently [12]. The solution to the applicability problem can therefore be achieved by a replicating array of microreactor structures with a translucent material for efficient light distribution. The concept of using an array of structures is gaining attention in non-photocatalytic processes. These reactors tempt to scale up microreactors by incorporating multiple channels in a structure [13]. This idea can be extended to photocatalytic processes where the goal is to create efficiently illuminated surface area by designing optically translucent (micro) structured reactors. This way, a high catalyst load can be achieved while keeping the coating thickness low. The photons that are not utilized by one surface can still be used to excite another. For gas-solid reactions, this principle has proven to achieve high apparent rate constants. Verbruggen et al. [14] used glass spheres as a catalyst support in an annular reactor configuration and reached first order rate constants up to 4 s−1. This example shows an improvement of 4 to 8 times compared to conventional annular gas-solid reactors (kapp ∼ 0.5 – 1 s−1) while the lamp power in these designs were much

higher (20 W vs. annular reactors: 48–150 W) [15,16]. The good performance of gas reactors in general is due to high effective diffusivity of the gas but the gas reactor of Verbuggen et al. makes the difference by the incorporated small channels inside the bed (∼800 µm) and the higher illuminated specific surface area (3000 m2 m−3 vs 250–720 m2 m−3). For liquid-solid reactions, these designs require improvement. The reactor of Vaiano et al. is an example of a liquid-solid reactor using a packing of Pyrex spheres with a diameter of 4.3 mm. They used CFD to calculate the ideal thickness [17]. This reactor had an apparent first order rate constant of ∼10−5 s−1, which is not an improvement comparing to other immobilized photocatalytic reactors. Analyzing their design a few possible reasons for this performance can be identified. Firstly, they use beads of 4.3 mm with a bed porosity of 0.55. This means that the void space between the beads is large (> 1 mm) and no microreactor channel scale is reached. Secondly, they use air as an oxygen source by saturating the reactor fluid before entering the reactor. Since the saturation concentration of oxygen in water using air is approximately 10 ppm, only a low concentration of oxidizing agent is present. Lastly, the illumination source existed out of cylindrical UV lamps which are not directive which means light energy gets lost. Another promising design is the silicium carbide reactor from Kouamé et al. for the photocatalytic degradation of Diuron, a herbicide. The reactor is composed out of a TiO2 coated foam. The reactor of Kouamé et al. reaches first order apparent rate constants in the order of ∼10−5 s−1. The main reason for the rather low performance of the foam is the inefficient illumination using a tubular light source and the low translucency of the foam, since it is not made from a UV transparent material [18]. The gas reactors prove that structured reactors are promising and showing possibilities for photocatalytic reactor scale-up. However, for liquid-solid reactions and more specifically wastewater treatment reactors, no such performance was seen. To investigate if we can reach microreactor kinetics in structured reactors for liquid-solid reactions, a reactor must be built which has no limiting reagent (oxidizing agent), which reaches microreactor length-scale in its internals and which has an efficient illumination source designed for the particular reactor. To evaluate the resulting design we have to observe the efficiency of both the light source and the reactor built. These have to be compared with microreactors, other structured reactors and conventional slurry reactors. In this work, the design of a photocatalytic reactor and custom LED light source will be presented which is a step forward to overcoming the productivity problem while keeping the microreactor reaction rates. An extensive comparison with literature will be performed showing where the presented microstructured photoreactor stands along existing reactors ranging from lab to pilot scale. 2. Reactor design and experimental method 2.1. Reactor design and setup The photocatalytic setup is shown in Fig. 1. An inlet reservoir was coupled to a peristaltic pump (Ismatec VC MS/CA8) to supply the reactor with a reaction solution containing methylene blue trihydrate (Fisher Scientific) and H2O2 (CHEM-LAB, Belgium). A UV–VIS spectrophotometer (Avantes Avaspec 2048L) performed online absorbance measurements of the reactor output at 666 nm (absorption peak methylene blue) before being collected in the outlet reservoir. The light source is an LED array and is connected to movable pillars on a rail system to reproducibly mount the light source at a distance of 4 cm from the reactor. Important design choices for an LED array light source for the given application are the type of LED, emission wavelength, total power output, inter-LED distance and LED emission angle. These design parameters were evaluated using a ray tracing algorithm implemented in COMSOL multiphysics (version 5.3a) to achieve a 726

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Fig. 1. Schematic overview of the experimental setup.

homogeneous light field with minimum edge effects and with a sufficient light source to surface efficiency. The validation of the ray tracing model was performed by irradiance measurements using an Ocean Optics QE65-pro spectrophotometer. The irradiance patterns will be discussed in the results section. For this design, an LED was chosen with an emission wavelength of 375 nm and a maximum optical output of 17 mW. The chosen wavelength is dependent on the chosen catalyst material which is in this case P25 TiO2 (λabs < 400 nm). The optical power output determines the irradiance and heat generation while the emission angle determines the directivity. Highly directive LEDs with a small emission angle require a high LED density to achieve a homogeneous light field at small source to reactor distances. This high LED density leads to heating problems. The relative intensity profile in function of the emission angle supplied by the manufacturer (Roithner Lasertechnik) and is given in the supplementary data (Fig. S1) The emission angle of the selected LED is 30°, which is a compromise between directivity and LED density on the board. The next step is the determination of the amount of LEDs and the distance between the LEDs. This determines the size of the homogeneous irradiance field. Furthermore, the amount of LEDs fixes the total power output and the source to reactor distance for which the light field is becoming homogeneous. A picture and a schematic of the final design is presented in Fig. 2. To sufficiently illuminate the reaction zone of 5 × 15 cm (see next section), 192 LEDs were ordered in a 8 × 24 grid with an inter LED distance (t) of 8 mm. The maximum input power was 100 mW and the maximum optical output power 17 mW corresponding to a total theoretical optical energy output of 3.264 W for 192 LEDs. The maximum power input and optical output power were calculated using the data of a single LED supplied by the manufacturer. The photocatalytic reactor was fixed to the rail system via two flanges. A schematic and picture of the reactor is given in Fig. 3. As seen on Fig. 3A the reactor is assembled from an inlet, outlet and middle part. On Fig. 3B a picture of the reactor shows that the inlet part and the middle part are filled with packing. The inlet part is filled with uncoated borosilicate beads (Sigma Aldrich) with a diameter of 3 mm which serves as flow distributor while the outlet part is kept empty. The actual reacting microstructure consisting of TiO2 P25 coated beads (same as the ones in the inlet distributor) is situated in the middle part

Fig. 2. (Top) Photograph of the designed LED source (board length: 275 mm, board width: 125 mm). (Bottom) Schematic of the designed LED source.

fixed between two glass foams. The reacting structure has a length of 15 cm. This is an important parameter which was used to couple the light source to the bed. The structure is the most important part of the reactor and needs to be transparent for the used wavelength. In this work spherical beads are used as a base structure. Although spherical beads have the lowest surface to volume ratio among different geometries, they are automatically self-ordering in crystal structures and they are relatively more straightforward to coat. In Fig. 4A and B a graphical representation of a stacked bead bed with a face centered cubic packing (left) and a 2D close packing (right) is shown. A perfectly stacked bed of equally sized spheres has a porosity of 0.26 when not taking into account wall effects. To investigate the pore size, the capillary orifice model can be used [19]. Table 1 shows the mean hydraulic diameter at 0.26, 0.30 and 0.35 porosity, calculated using the following equations:

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Fig. 3. (A) Schematic image of the used photocatalytic reactor. (B) picture of the photocatalytic reactor.

dh =

2 dp 3(1

M=1+

Table 1 The mean hydraulic diameter of a stacked bed of spheres for different sphere diameters using the capillary orifice model.

(1)

)M 2d p 3D (1

)

(2)

where, dh is the hydraulic diameter in mm, dp is the particle diameter in mm, ε the bed porosity and D is the characteristic bed diameter which is in this case the width of the reactor and equal to 49 mm. For an ideal closed packing with 0.26 porosity, the channels are already sub millimeter scale. For a non-ideal closed packing with 0.35 porosity, the sphere diameter has to be below 3 mm to reach microreactor scale. Table 2 shows the surface to liquid volume ratio calculated for a reactor volume of 150 mL. The surface to volume ratio is dependent on the structure porosity and the particle size. For bead sizes from 3 mm to 1 mm the range is in between 5000 and 17000 m2 m−3 which is in microreactor and slurry reactor range [20]. For beads with a diameter of 3 mm, non-porous TiO2 layers and a thickness of 250 nm, a catalyst load of ∼4 g L−1 can be reached. In Table 3 the bed characteristics are presented as measured before the photocatalytic experiments.

dp (mm)

dh26 (mm)

dh30 (mm)

dh35 (mm)

5 4 3 2 1 0.5

0.956 0.794 0.619 0.430 0.224 0.115

1.154 0.960 0.750 0.522 0.273 0.140

1.429 1.191 0.933 0.651 0.341 0.175

Table 2 Surface to volume ratio in a perfectly packed bead bed reactor with a volume of 150 mL with porosity of 0.26. dp (mm)

2 −3 A V−1 ) liq (m m

3 2 1

5692 8538 17077

Fig. 4. (A) 3D representation of a face centered cubic packing of equally sized spheres. (B) 2D representation of the pore size in a packing of perfectly stacked spheres. 728

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Table 3 Photocatalytic reactor structure characteristics. V*empty (mL)

Vstructure (mL)

porosity

Theor. Cat. load (mg g−1 bead)

Exp. Cat. load (mg g−1 bead)

Exp. Cat. load (g L−1)

Surface area (m2 m−3)

149

50

0.33

1

0.49**

1.9

4267

* Vempty indicates the volume of the photocatalytic reaction zone without catalyst bed. ** Part of the catalyst is not deposited on the beads during the coating procedure (see Section 2.2).

The bed porosity, measured by determining the volume of liquid inside the reactor, using a measurement cylinder was found to be 0.33, which is in between the theoretical densest possible bed with a porosity of 0.26 and the theoretical densest possible random stacking with porosity 0.38 [21]. This means the bed structure is composed of both regularly and randomly structured parts. Taking into account that these values are not accounting for wall effects, our porosity is sufficiently low to obtain an array of microchannels. Using Table 1, it is verified that at a porosity of 0.33 (experimentally measured) the mean hydraulic diameter is still less than 1 mm. The experimental catalyst load is lower than the theoretical catalyst load. This is due to losses during the coating and washing procedure of the bed.

2.4. Kinetics and performance analysis The apparent first order kinetic constant was determined assuming the ideal plug flow reactor (PFR) model and the theoretical residence time (calculated with the measured flow rate and measured reacting volume). The Langmuir-Hinshelwood kinetics are used for determining the kinetic constants. The Langmuir-Hinshelwood expression is a widely used model for heterogeneous (photo)catalytic reactions. A derivation can be found in Perry’s Chemical Engineers’ handbook [25]. For the rate expression, a simplified version of the Langmuir-Hinshelwood kinetics can be used since the initial concentration of methylene blue is very low (< 10−3 M) [26–30].

2.2. TiO2 coating The coating procedure is based on the work of Verbruggen et al. [14]. Firstly, a suspension of 250 mg TiO2 (Evonik P25) in 150 mL of ethanol (Fisher Scientific) was prepared and stirred overnight. Simultaneously, 250 g of borosilicate beads (Sigma-Aldrich) was cleaned by stirring the beads overnight in an ethanol solution. Then, the beads were dried and equally divided in mass over five different petri dishes to obtain a mono layer of glass beads. Afterwards, the TiO2 suspension was placed in a ultrasonic bath (VWR ultrasonic cleaner) for 30 min. Each petri dish was then filled with 30 mL suspension, which is sufficient to cover the beads completely. Finally, the beads were shaken gently to ensure a homogeneous deposition of the TiO2 on the beads. The beads were dried at 60 °C and subsequently calcined at 350 °C for 2 h. XRD measurements on the samples showed no changes in crystal structures due to the calcination step. After the calcination step, the beads were thoroughly washed with deionized water to remove any loose material. To quantify the catalyst loading, a sample of 5 g was taken from the beads and the layer was physically removed by stirring it vigorously in water overnight. The obtained TiO2 suspension was digested using a microwave digester (Berghoff) and a mixture of 10 mL sample fluid, 9 mL HNO3 (Acros Organics), and 3 mL of HF (Acros Organics). The digested solution was measured using ICP-OES (Perkin-Elmer). It was found that a total of 49% of the 250 mg TiO2 feed was deposited on 250 g borosilicate beads.

dC kKC = dt 1 + KC

(3)

dC = kKC = k 'C for C < 10 3M dt

(4)

where k is the reaction rate constant at full surface coverage and K is the equilibrium adsorption constant. The space-time yield and apparent first order rate constant k’ can now be calculated using:

C =k C0

ln

(5)

C and C0 can be extracted from the experimental data, while τ is calculated using the reactor volume and the flow rate. The apparent first order rate constant can be used in reactor models calculating the STY. The STY is a measure for productivity in volume per day per volume reactor. For the slurry reactors or PFR reactors in loop the continuously stirred tank reactor (CSTR) model is used to predict the STY [12].

=

Ca0 Ca kapp Ca

STYcstr =

Vr

(6)

=

Vr Ca0 Ca k app Ca

(7)

For a PFR reactor operated in continuous mode the PFR model can be used.

=

2.3. Photocatalytic experimental procedure

ln

Ca Ca0

(8)

kapp

STYpfr =

Every experiment was performed in three steps. First, ultrapure water (18.2 MΩ, Sartorius Arium Pro) was pumped through the reactor. Secondly, a 10 ppm methylene blue trihydrate (MB) solution containing 100 ppm of H2O2 is introduced as a concentration step. The hydrogen peroxide acts as electron acceptor to improve the degradation process. The amount of hydrogen peroxide (molar ratio H2O2 to the pollutant equal to 100) is based on an earlier study of Jamali et al. [22]. A higher H2O2 concentration will lead to radical scavenging [23,24]. The light source was activated when the outlet concentration matched the inlet concentration. When the output concentration was stable minimum 5 min, the conversion was determined taking the mean absorbance over 5 min of stable operation. The maximum standard deviation on the mean absorbance was 3–5% for flow rates under 30 mL min−1 and < 2% for flow rates above 30 mL min−1.

( ) kapp

( )

ln

Ca Ca0

(9)

To assess the productivity of the reactor combined with the energy consumption, photocatalytic space-time yield was used [12]. It is calculated using the space-time yield and the standardized lamp power [31]. The standardized lamp power is a measure for the energy density of a particular volume. It is calculated as follows:

LP =

P × 1m3 Vreactor

(10)

where LP is the standardized lamp power (W or kW), P is the lamp power (W or kW) and Vreactor (L or m3) the reactor volume. The equation is multiplied with 1 m3 to normalize the unit of reaction volume. Secondly, the space-time yield is divided by the standardized 729

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Fig. 5. Irradiance distribution at different source to reactor distances (d) calculated by the ray tracing algorithm.

lamp power.

PSTY =

STY LP

distance from the LED board. There are two observations, the maximum intensity and the broadness of the distribution. Both can be predicted by the ray tracing model. The model calculates that there is a transition from a flat distribution to an inhomogeneous distribution at 4 cm. However, experimental measurements indicate that the transition from flat to inhomogeneous distribution happens at 5 cm. Whether this transition relevant for the application is dependent on the desired accuracy of the irradiance field. The uneven current distribution of the LEDs, the difference between the electrical current/optical output characteristics of the LEDs and the differences in the emission profile between the data sheet and reality are the reasons for this behavior. The quality of the distribution can be quantified using the standard deviation on the modeling data in the area we are interested in which is for our reactor the area between 2 cm and 7 cm (our reactor is 5 cm wide). Choosing the modeling data as reference for this calculation cancels out experimental and practical variability and allows us to only observe the deviation due to the light profile. For 2 cm, 3 cm, 4 cm, 5 cm and 8 cm the relative standard deviation is equal to 3.8%, 4.8%, 6.3%, 7.6%, 9.6% respectively. For all distances, the relative standard deviation stays within a limit of 10% but the mean power output has to be taken into account, which is lower at 8 cm distance than at 4 cm distance. Considering the model results, experimental results and the setup constraints, the LED source to reactor distance was fixed at 4 cm for all the experiments. Energy evaluation of the light source is also performed to assess the total optical energy transmission from the source to the reactor. The

(11)

Where PSTY is the photocatalytic space-time yield in treated mass (g) or volume (m3) per time (day), reactor volume (m3) and standardized lamp power. 3. Results and discussion 3.1. Light source The modelling results of the ray tracing algorithm were validated by irradiance measurements. Fig. 5 shows the simulation irradiance patterns at different distances from the LED board for surfaces with a length of 250 mm (y-axis) and a width of 100 mm (x-axis). At LED to reactor distances smaller than 2 cm, the light field is characterized by small high intensity zones (point sources). At distances higher than 2 cm the intensity field becomes homogeneous. This is the case until an LED source to reactor distance of 5 cm when the intensity field starts to diverge. Since an ideal operating distance is characterized by a homogeneous intensity field, the reactor can be operated at distances from 2 cm to 4 cm. The designed light source was experimentally verified by measuring the irradiance over the x-axis at y = 125 mm, which is in the middle of the LED board. The experimental intensity profiles are plotted together with the simulation results in Fig. 6 at 2 cm, 3 cm, 4 cm, 5 cm and 8 cm 730

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Fig. 6. Experimental (full lines) and modeled (dashed lines) irradiance of the light field in function of the width (x coordinate) measured at y = 125 mm.

Fig. 7. (A) Efficiency scheme of the electrical energy transformation to light energy and to the surface of the reactor. The power on the surface was measured at a source to reactor distance of 4 cm. (B) Efficiency of the LED board in function of the applied current.

731

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energy which flows from the power source as electrical energy to the reactor surface as light energy, will be wasted due to LED source electronics, LED efficiency and the imperfections in the optical path. In Fig. 7 a scheme is shown to identify the different bottlenecks in the illumination part of the reactor. Firstly, the incoming power of 23.39 W is sent to the LEDs and resistances on the board. The resistances are meant to stabilize the current through the LEDs and dissipate 9.79 W of the incoming power as heat. Fig. 7B presents the efficiency of this step in function of the applied current. If both the energy loss in the resistances and the efficiency of the LEDs are taken into account only 9.9% of the energy is transformed into light energy. In further calculations we always make use of the electrical input of the LED source itself. The light output of the LEDs (2.31 W) is optical energy which is transferred to the reactor surface area (see Fig. 7A, d = 4 cm). Since the interesting emission area is the homogeneous part of the light field, energy is lost at the borders of this area. The amount of energy reaching the surface at a source to reactor distance of 4 cm is equal to 1.43 W (measured). This value can also be calculated by multiplying the irradiance with the total surface. The model predicted that 1.4 W would reach the surface of the reactor which is close to the measured value. This energy loss is only present in small LED arrays where the border of the LED source is an important part of the emission field. In scaled up versions with bigger emission areas this efficiency would approximate 100%.

the liquid volume and the flow rate. The first order apparent rate constant is calculated from the plug flow reactor model according to Eq. (5). The STY is calculated using Eq. (9) while PSTY is calculated using Eq. (11). To find the optimal operating point of the reactor, an investigation of external mass transfer must be carried out. Usually this can be done by performing kinetic experiments in function of the flow rate. Fig. 8B indicates that two different regimes are present. When the flow rate (Q) < 110 mL min−1 the reaction is mass transfer limited and the apparent first order rate constant scales with the mass transfer coefficient and thus with Re1/3 or Q1/3. When Q > 110 mL min−1 the apparent rate constant becomes independent of flow rate and the reactor is running in the reaction rate controlled regime [32]. The maximum apparent first order rate constant is 0.818 min−1 found at a flow rate of 216 mL min−1. Fig. 8C depicts the space-time yield in function of the flow rate. The STY indicates the volume that can be treated at a specific conversion. The disadvantages of the STY benchmark is the dependency on a defined conversion of the pollutant and the dependency on the irradiance and illumination efficiency, which is a major parameter determining the reactor performance and energy efficiency. Higher irradiances will increase the space-time yield, but this relationship is not linear and thus the energy efficiency will decrease. The photocatalytic space-time yield which is defined in Eq. (11), is a benchmark taking into account both energy consumption and productivity of the reactor. The PSTY is plotted against the experimental flow rate in Fig. 8D. Since the kapp, STY and PSTY are similarly dependent on the external mass transfer coefficient and flow rate the three graphs are following the same trend. The highest STY and PSTY found for this reactor are equal to 170.87 m3 day−1 m−3 reactor and 0.657 m3 day−1 kW−1 m−3 reactor,

3.2. Reactor operation and performance In Fig. 8, the residence time (tres), apparent first order rate constant (k), space-time yield (STY) and photocatalytic space-time yield (PSTY) are plotted against the flow rate. The residence time is calculated using

Fig. 8. Results of the experimental tests performed on the reactor. The error bars represent the standard deviation on the experimental results for minimum 3 replicates except for the data point at 12.0 mL min−1 which has only 1 repetition. (A) Residence time in function of the flow rate. (B) Apparent first order rate constant in function of the flow rate. (C) Space-time yield in function of the flow rate. (D) Photocatalytic space-time yield in function of the flow rate. 732

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Table 4 PSTY and STY calculations for various photocatalytic reactors. Design principle

kapp (min−1)

Reactor volume (L)

LP (W)

LPstand(W)

STY (m3 day−1 m−3 reactor)

PSTY (m3 day−1 m−3 reactor kW−1)

Refs.

Slurry reactors MEM(3) AR(1) MEM(1) EISR(1) RAR(1) MEM(2) ARAR ASAR AR(2) SAR EISR(2) EISR(3)

N/A 0.1070 0.1981 0.0033 0.0770 0.1130 0.0200 0.0196 0.0160 0.0052 0.0350 0.0530

12.00 2.00 1.00 1.00 1.10 1.20 3.90 3.90 1.53 3.90 0.25 0.75

6144.0 8.0 30.0 150.0 40.0 100.0 120.0 120.0 46.1 120.0 66.9 125.0

512.00 4.00 30.00 150.00 36.36 83.33 30.77 30.77 30.22 30.77 267.76 166.67

2.88 × 103 1.54 × 10−1 2.86 × 10−1 6.80 × 10−1 1.11 × 10−1 1.63 × 10−1 2.88 × 10−2 2.83 × 10−2 2.31 × 10−2 7.50 × 10−3 5.05 × 10−2 7.68 × 10−3

5.63 × 100 3.86 × 10−2 9.52 × 10−3 4.54 × 10−3 3.05 × 10−3 1.95 × 10−3 9.37 × 10−4 9.18 × 10−4 7.63 × 10−4 2.44 × 10−4 1.88 × 10−4 4.61 × 10−5

[33] [34] [35] [36] [37] [38] [39] [39] [40] [39] [41] [42]

0.05 10.00 0.30 0.34 0.30 1.10 3.00 0.90 0.84 1.25 3.20 × 10−5 1.14 × 10−4 1.50 × 10−6 3.25 × 10−5 0.30

13.6 30.0 80.0 3.1 80.0 40.0 105.0 500.0 81.0 120.0 1.2 200 120.0 120 500.0

272.00 3.00 266.67 9.06 266.67 36.36 35.00 555.56 96.89 96.00 38125.00 1754385.96 80000000.00 3692307.69 1666.67

1.71 × 102 4.47 × 10−2 1.13 × 100 3.03 × 10−2 5.42 × 10−1 6.34 × 10−2 3.60 × 10−2 2.31 × 10−1 3.49 × 10−2 2.77 × 10−2 1.04 × 101 1.00 × 102 3.76 × 103 1.53 × 102 1.59 × 10−3

6.28 × 10−1 1.49 × 10−2 4.23 × 10−3 3.34 × 10−3 2.03 × 10−3 1.74 × 10−3 1.03 × 10−3 4.15 × 10−4 3.60 × 10−4 2.88 × 10−4 2.74 × 10−4 5.70 × 10−5 4.70 × 10−5 4.16 × 10−5 9.51 × 10−7

[43] [17] [4] [17] [37] [6] [11] [44] [45] [46] [47] [5] [48] [10]

Immobilized catalyst reactors OUR WORK 0.8180 SDR(1) 0.0310 PBR(1) 0.0054 FPR(1) 0.0210 PBR(2) 0.0026 RAR(2) 0.0440 FPR(2) 0.0250 OFMR(1) 0.1600 SDR(2) 0.0242 CDR 0.0192 MR(2) 0.05 MR(4) 0.4791 MR(1) 18.0000 MR(3) 0.73544 OFR(1) 0.0011

*Abbreviations: Membrane reactor (MEM); Annular reactor (AR); Aerated rotating annular reactor (ARAR); (Aerated) slurry annular reactor ((A)SAR); Externally illuminated slurry reactor (EISR); Packed bed reactor (PBR); Flat plate reactor (FPR); Optical fiber monolith reactor (OF(M)R); Microreactor (MR); Spinning disc reactor (SDR); Corrugated drum reactor (CDR).

respectively. This means that if we scale up this reactor from a volume of 50.10−6 to 1 m3 a volume of 170.87 m3 water can be treated with 99.9% conversion of the pollutant every day. The performance of this reactor is promising, but the question remains how this performance compares to other reactors in literature and more specifically microreactors. To assess our reactor a literature review was performed based on the work of Leblebici et al. [12]. Table 4 presents an overview of the literature data for 29 wastewater treatment reactors which was used to calculate power, standardized lamp power (Eq. (10)), volume and kinetic constants. The method

to calculate the STY is given in Section 2.4. For comparison, the STY and PSTY are plotted on a logarithmic scale in Fig. 9. The graph compares the performance of 29 different wastewater treatment photocatalytic reactors by STY and PSTY. The reactors are ordered according to reactor type (slurry or immobilized) and descending PSTY. The best performing reactor is the slurry reactor with a membrane separation unit from Benotti (MEM(3)). The reactor is illuminated with a wavelength of 254 nm and 185 nm which causes photolysis of the organic pollutants and which is less prone to mass transfer problems. Secondly, they test multiple components not reaching 99.9% Fig. 9. Space-time yield and photocatalytic spacetime yield for different reactors taken from literature. The reactors are ordered according to declining PSTY and type (slurry or immobilized bed). The blue reactor is our work, the green reactors are two examples of packed bed reactors and the yellow reactors are the microreactors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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A high STY can only lead to a high PSTY when the illumination part of the reactor works efficiently. This means that both standardized lamp power and STY are playing an important role and gives different information about the reactor setup. Microreactors are situated in this area because of their very low volumes and very high lamp powers per volume of reactor. The light source is often irradiating non-reactive zones due to the small channel size of the microreactor. Microreactors are fast and productive with respect to their volume (high STY) but have low flow rates. Ideal applications for these reactors would be the production of high added value chemicals due to the energy costs being less important. The aim of this research was to design an immobilized reactor which has the STY of a microreactor combined with a high PSTY. These reactors are found at the top right corner of the graph. They combine a good illumination efficiency with fine mass transfer properties. The key is the thickness of the applied catalyst layer. It is known that for photocatalytic systems the reaction rate is related to the irradiance by a power law due to internal mass transfer problems in the catalyst layer [4,5,18]. This means that thinner catalyst layers are more energy efficient than thicker catalyst layers if the photon energy leaving the back of the layer is also consumed. The structure reported here allows to consume photon energy that is not consumed by another catalyst layer. In this design, the 3 mm beads assemble a structure with a reacting surface to reacting volume ratio close to 5000 m2 m−3 and a channel size of less than 1 mm. Both values are in the range of microreactor scale [20]. Secondly, the catalyst load is equal to 1.9 g L−1 which is higher than most immobilized reactors and which can compete with microreactors. Lastly, the light source is specifically designed to fit the reactor, this minimizes the surface energy losses and boosts the photocatalytic space-time yield. The latter can be shown by assuming the edge losses of the light source proportional to the surface to edge ratio of the LED grid. If the surface of the LED grid is multiplied by 10, the surface to edge ratio will increase and the illumination efficiency will rise from 62% to 92%. The PSTY will increase proportionally to this number.

conversion, which is the used conversion to evaluate the STY in this work. Thirdly, the slurry itself possesses good mass transfer properties. However, it remains unclear whether the nanoparticle catalyst is fully separated from the wastewater. The reactor from Danion et al., OFR(1), has the lowest PSTY and STY among the dataset [10]. The main issue is the inefficient illumination of the optical fibers itself. A 500 W mercury lamp is used to illuminate the fibers externally, which leads to light energy losses on the illuminated cross section of the reactor. In comparison, the internally illuminated monolith reactor (OFMR(1)) from Lin et al. has a two order of magnitude higher STY and three orders of magnitude higher PSTY [11]. In the design of Lin, the optical fibers are illuminated directly from a well-directed lamp which minimizes light energy loss. Optical fibers can help solving photon transfer, but only if they are an addition to the illumination system because direct illumination will be always more efficient to transfer light energy. Among the immobilized bed reactors there are two important groups indicated in yellow and green. The yellow reactors are the microreactors and are all characterized by a very high productivity if scaled up to big volumes, but a low energy efficiency. This is due to their small volumes in comparison with the used lamp power. The structured reactors from the green group have the highest productivity among the higher throughput reactors. Although their high productivity, their PSTY is low. This is due to the light setup of the reactor system which is consisting out of cylindrical UV lamp and the structure design. Our reactor, indicated in blue, is the best performing immobilized catalyst reactor in the dataset with a productivity close to the best slurry system and microreactor system. To get a better understanding of the difference between STY and PSTY a standardized comparison of the STY and PSTY is made in Fig. 10. The standardization is performed dividing the STY and PSTY by its maximum among the dataset. This means that the best performer in terms of PSTY is present on the top of the graph while the best performer in terms of STY is plotted on the right side of the graph. Three types of reactors can be distinguished. Reactors with a high PSTY and low STY are reactors that are energy efficient because of low lamp powers or high volumes. They can be found on the top left of the graph. A low lamp power leads to a lower local reaction rate in the catalyst layer and thus a higher mean concentration of reactants in the layer. This reduces the mass transfer issues in the reactor which means photon energy is more efficiently used. High volumes decrease the standardized lamp power and thus increase the PSTY. The AR(1) from Vela et al. [34] and SDR(1) from Yatmaz et al. [43] are examples of reactors with a low lamp power. To scale up reactors with high PSTY but low STY, large volumes are required. It is not advised to design reactors in this area since a large amount of reactor volume will remain unused. Reactors with a low PSTY but high STY are reactors with very small volumes or high lamp powers with a rather low illumination efficiency.

4. Conclusion A study was performed on the design, application and benchmarking of a micro structured photocatalytic reactor. It was shown that micro structured reactors together with custom light source design show great potential towards scale-up and optimization of photocatalytic reactors. The novelty in this work is threefold. Firstly, an easy method is provided to design a customized LED-source using a ray tracing algorithm. Secondly, it was proven that structured reactors can increase the productivity and the energy efficiency of a photocatalytic reactor. It was shown that a translucent packed bed reactor can reach the productivity of a microreactor while increasing the photocatalytic space time yield by 3 to 4 orders of magnitude. Finally, an extensive comparison with the literature was provided. To the best of our knowledge, our reactor is performing among the best in literature when comparing STY, PSTY and the apparent first order rate constant. The large illuminated surface area, custom light source and high catalyst load are the main arguments for this conclusion. Although we used a degradation reaction to test the viability of structured reactor design in photocatalysis, this work can also be significant for other types of applications like carbon dioxide reduction, water splitting, pharmaceutical ingredient synthesis and especially oxidations using molecular oxygen. For this to happen more work has to be performed to relate the properties of the structure to the photocatalytic performance. Acknowledgment

Fig. 10. Photocatalytic space-time yield plotted against the space-time yield for various photocatalytic reactors reported in literature.

M.E. Leblebici acknowledges fundamental research postdoctoral fellowship of Research Foundation – Flanders 39715. 734

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Appendix A. Supplementary data

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