Photoreactor Design Aspects and Modeling of Light

Abstract

Geometry of the photoreactors depends mainly on the application as well as on the available irradiation source. Additionally, the following factors also need to be considered during the design of photoreactors: (1) type and particle size of the photocatalyst; (2) distribution of the photocatalyst (fixed or suspended); (3) type, content, and distribution of pollutants; (4) mass transfer; (5) fluid dynamics (laminar or turbulent flow); (6) temperature control; (7) reaction mechanism; and (8) reaction kinetics. This chapter deals with the general classification and description of photoreactors used for reaction carried out in the gas and liquid phase. Different types of photoreactors are described in relation to their applications.

7.1 Introduction

Since heterogeneous photocatalysis has become an important area of research, it has been applied to various environmental problems including air, water, and wastewater treatment [1], destruction of microorganisms such as bacteria and viruses (disinfection processes) [2, 3], inactivation of cancer cells [4, 5], energy production (hydrogen generation by water splitting, biomass conversion, as well as CO₂ conversion into useful hydrocarbons) [6–12], remediation of oil spills [13], and chemical synthesis [14]. Nevertheless, the accomplishment of the photocatalytic processes at required scale stipulates the use of a photoreactor, a device which allows to contact photons, a photocatalyst, and reactants, as well as to collect the reaction products. In this regard, there are two critical factors and major challenges in the design of photoreactors: (1) how to provide the efficient illumination of the photocatalyst (for a high activity, a large area has to be illuminated) and (2) how to adapt photoreactors for utilization of irradiation provided by different sources. Due to the fact that scaling up of photocatalytic reactors is a difficult and complex process, there are some additional factors that also need to be considered, such as the (1) type and particle size of the photocatalyst; (2) distribution of the photocatalyst (fixed or suspended); (3) type, content, and distribution of pollutants; (4) mass transfer; (5) fluid dynamics (laminar or turbulent flow); (6) temperature control; (7) reaction mechanism; and (8) reaction kinetics.

The design of reactor geometry and selection of a photocatalytic reactor depend on the experimental conditions and the specific application. Moreover, the design of large-scale photoreactors must take into account the capacity, ruggedness, reliability, and ease of use. Figure 7.1 illustrates the main types of reactors used for air and wastewater treatment, water splitting, and CO₂, glycerol, and biomass photoconversion. In the liquid phase, the photocatalyst particles are usually suspended in a fluid phase, although other reactor configurations have also been proposed such as packed and fluidized beds with an immobilized photocatalyst. In gas-phase photocatalysis, the most common photoreactors are tubular, annular, and flat-plate types.

Fig. 7.1

Main types of photoreactors depending on the application

Regarding the photocatalyst structural configuration, thin-film powder layer and/or fluidized bed, coated wall-parallel, and honeycomb/foam monolithic reactors are probably the most representative. For photochemical water splitting, batch-type photoreactor is most frequently used configuration in lab-scale investigations. In the case of solar photoreactor systems, there are two of the major design issues: (i) whether to use a suspended or a supported photocatalyst and (ii) whether to use concentrated or non-concentrated sunlight.

The most popular reactors are (1) parabolic trough reactors (PTRs), (2) compound parabolic collecting reactors (CPCRs), and (3) non-concentrating flat-plate reactors which are the double-skin sheet reactor (DSSR).

In addition, it is necessary to pay attention to:

• The type of irradiation: photoreactors can be irradiated using artificial UV light, UV polychromatic lamps, or solar radiation

• The position of the irradiation source: immersed light source, external light source, and distributed light sources such as reflectors or optical fibers

This review deals with the general classification and description of photoreactors used for reaction carried out in the gas and liquid phase. Different types of photoreactors are described in relation to their applications.

7.2 Gas-Phase Photoreactors

Photocatalytic gas-phase reactor should contain two parts: (i) the reactor structure and (ii) source of light. Ideally, the structure of a photocatalytic reactor for air purification should have (i) light source irradiating directly on the photocatalyst surface, (ii) high specific surface area of photocatalyst, and (iii) high mass transfer, low pressure drop, and long residential time. Many types of photocatalytic reactors are designed. Annular, plate, slurry, honeycomb monolith, packed bed, and fluidized bed reactors are the most popular, but in the literature, other types are also described: powder layer reactor, with aerosol generator, with optical fibers, and others. However, most of the studies are only based on laboratory scale. Therefore, one of the challenges in the development of photocatalysis for environmental application is the design of efficient reactor that can be used to large-scale commercial application.

7.2.1 Reactors for Photocatalytic Degradation of Volatile Organic Compounds (VOCs)

Generally, the annular reactors are composed of two or more concentric cylindrical tubes mostly made of Pyrex glass. The photocatalyst is coated on the inner wall of the outer cylindrical tubes. The light source is located at the central part of the cylindrical tube. The photocatalyst film coated on the wall of the surface should be thin enough to let all the photocatalyst be irradiated by the light source. Furthermore, source of light can be located outside the reactor, and then thin film of the photocatalyst is coated on the surface of two or more concentric cylindrical tubes. The airflow is provided along the axial direction through the annulus between the lamp and the tube. Figure 7.2a shows one of the types of the annular reactor. Different types of the annular reactors have been used by several research groups for photocatalytic degradation of volatile organic compounds [15–19].

Fig. 7.2

Main types of reactors used for air treatment (a) annular, (b) packed bed, (c) honeycomb monolith, and (d) plate

Tomasic et al. used the annular fixed bed photocatalytic reactor (total volume of the reactor was 0.485 dm³) to study the degradation of toluene in the gas phase [15]. The P25-TiO₂ thin film was coated on the internal glass surface of the outer tube of the annular reactor. Fluorescent blacklight blue lamp was placed in the central part of the reactor. Tomasic et al. used mathematical models of the photocatalytic reactor (1D model and 2D models based on ideal flow and laminar flow conditions) to understand complex reaction pathways and the reactor’s limitation. The obtained models were verified by comparing the computer simulation data with the experimental results. It was found out that photocatalytic reaction carried out in the annular photocatalytic reactor was mainly limited by the interphase mass transfer. Imoberdorf et al. studied the performance of single and multi-annular photocatalytic wall reactor configurations by using a two-dimensional, reaction–diffusion–convection model and reliable intrinsic reaction kinetics for the photocatalytic degradation of perchloroethylene [16]. The effect of (i) the reactor volume, (ii) the photocatalytic surface area, and (iii) the annulus width on photocatalytic degradation of perchloroethylene in single annular reactor was studied. In the case of multi-annular reactor configurations, the effect on the reactor conversion of (i) the type of flow pattern and (ii) the thickness distribution of TiO₂ films was investigated. It was found that the performance of reactors was strongly influenced by external diffusive resistances; single- and multi-annular photocatalytic reactors present high values of reactor irradiation incidence and photocatalyst irradiation absorption efficiencies. Vincent et al. investigated the photocatalytic oxidation of gaseous 1-propanol by using annular reactor (total volume was 0.0664 dm³) [17]. The fiberglass support impregnated of P25-TiO₂ was placed between two Pyrex glass tubes. The fiberglass support area exposed to UV irradiation was 0.36 dm². 18 W fluorescent tube used as a source of light was located in the center of the reactor. The influence of kinetic parameters such as pollutant concentration, incident light irradiance, contact time, and humidity content has been studied. The authors concluded that the reactor efficiency could be improved in order to reduce the by-product concentrations with other experimental conditions (such as a higher contact time) [17].

The plate reactor, shown in Fig. 7.2d, is the simplest type of photoreactor used for photocatalytic degradation of volatile organic compounds. There are two basic types of plate reactors – with inner and outer source of irradiation. The typical form of this reactor is square or rectangular box, made of different materials (such as stainless steel, plexiglass, or polycarbonate), that is resistant to UV light. Photocatalyst samples used in plate reactors are in the form of powders or flat shape located at the bottom of the reactor. In the case of the plate reactors with inner source of irradiation, a lamp is placed at the upper part of the reactor. In the second one, reactors are equipped with a quartz or borosilicate window, which allows the light passage from lamp into the photocatalyst sample. The advantages of the plate reactor are small pressure drop, the possibility of obtaining large reaction rates, and simplicity. However, the major disadvantage of this type of reactor is the smaller reaction area. Salvado-Estivill et al. used a two-dimensional (2D) analysis of a flat-plate reactor for photocatalytic oxidation of trichloroethylene (TCE) in gas phase under different experimental conditions [20]. The reactor was made of stainless steel (75 mm wide and 600 mm long). A glass plate coated with the photocatalyst (P25-TiO₂) was placed 270 mm from the inlet of the reactor and 170 mm from the outlet. The plate reactor was irradiated by blacklight blue fluorescent lamps. It was found that a two-dimensional model of a flat-plate photocatalytic reactor was shown to approximate closely the experimental results of the photocatalytic oxidation of trichloroethylene. Demeestere et al. used flat-plate reactor to study the photocatalytic degradation kinetics of gaseous trichloroethylene [21]. The reactor was made of stainless steel, with two photocatalyst (P25–TiO₂)-coated glass plates located in the reactor. 18 W blacklight blue lamp used as a source of light was placed over the reactor. The effect of trichloroethylene inlet concentrations (100–500 ppmv), gas residence times (2.5–60.3 s), and relative humidity (0–62 %) has been investigated. The authors concluded that a trimolecular Langmuir–Hinshelwood model could not fit the experimental results adequately. Therefore, a new kinetic model has been developed, which was based on linear trichloroethylene adsorption–desorption equilibrium and first-order reaction kinetics. Mo et al. studied the by-products during photocatalytic degradation of toluene in a plate reactor [22]. The reactor was made of stainless steel, which two photocatalyst (P25–TiO₂)-coated glass plates located in the reactor. The UVC lamps (Philips Hg lamps) were used to irradiate the photocatalyst plate from the top of the reactor through a quartz glass. It was found that acetaldehyde, methanol, acetone, benzaldehyde, formic acid, ethanol, and acetic acid were the main by-products in the gas-phase toluene degradation.

The packed bed reactors are simple, easy-constructing, and efficient reactor. This type of reactor consists of cylindrical tube made of Pyrex glass, metal, or others. The photocatalyst is located in the central part of the reactor. The source of light can be placed inside or outside reactors. Arabatzis et al. proposed a new packed bed reactor for photocatalytic degradation of volatile organic compounds (VOCs) (see Fig. 7.2b) [23]. The form of this reactor was a cylindrical container made of metal. This container was used to concentrate the emitted light energy from the irradiation source (Sylvania F15w T8/BLBlue lamps). The porous photocatalyst was located on the outer wall of the central glass tube. This reactor has been optimized using theoretical prediction of the conversion factor as a function of the volume, reaction, and molecular feed. Ibhadon et al. presented theoretical study and kinetic modeling of a new packed bed photocatalytic reactor [24]. These results have been confirmed by experimental study on the degradation of benzene, toluene, and xylene. A cylindrical metal container was used to concentrate emitted light energy from four irradiation sources (Sylvania F15WT8/BLBlue lamps). In the central part of cylindrical metal container transparent to UV light, glass tube was located. This tube was filled with the porous P25–TiO₂ photocatalyst. It was found that theoretical and experimental conversion factor was similar and amounted to 96.7 % and 95 %, respectively. This study showed efficient way to design and optimize a packed bed photocatalytic reactor for degradation of VOCs. Fu et al. studied the effect of reaction temperature and water vapor content on the photocatalytic degradation of ethylene using packed bed reactor [25]. The reactor was made of Pyrex tube and was illuminated by four fluorescent UV bulbs. The tube with the bulbs was placed in an insulated cylindrical glass container. It was found that the reaction temperature has a strong influence on the rate of photocatalytic degradation of ethylene under UV light and TiO₂ or Pt/TiO₂ used as photocatalysts. The cause of enhanced photoactivity which was observed at increased reaction temperatures may be due to an enhanced desorption of water from both types of photocatalysts at higher operating temperatures.

Honeycomb monolith reactors are commonly used in automobile exhaust emission control and for NO_x reduction in power-plant flue gases by catalytic reduction, but they also can be used for photocatalytic reactions in the gas phase (see Fig. 7.2c). This type of reactors contains certain number of channels of circular or square cross section. The photocatalyst is coated onto the inner walls of channels as a thin film. The irradiation source is located in front of the channels. Wang et al. used honeycomb monolith reactor for modeling of formaldehyde photocatalytic degradation using computational fluid dynamics [26]. It was found that distance between the monolith and lamp should be decreased when the number of lamps increases to achieve an optimal configuration. The choice of an optimal number of lamps depends on the flow rate over the monolith. Taranto et al. used an aluminum honeycomb monolith reactor, coated with a thin film of P25–TiO₂ for methane and toluene degradation in the gas phase [27]. As the irradiation source, low-pressure mercury lamps were used. Different types of honeycomb monolith reactors have been used by several research groups for photocatalytic degradation of volatile organic compounds [28, 29].

Fluidized bed reactors are made of transparent container; the treated airstreams pass through container filled with the photocatalyst bed. The light source is located outside of the reactor. The photocatalyst has good contact with the treated airstreams. Fluidized bed reactors can be used to treating fairly high airstreams. Palma et al. used fluidized bed reactor for the intensification of gas-phase photocatalytic oxidative dehydrogenation of cyclohexane [30]. UV irradiation was provided by a two ultraviolet-light-emitting diode (UV-LED) modules located in front of the Pyrex windows. A mathematical modeling was based on Langmuir–Hinshelwood (LH)-type kinetic model. It was found that proposed mathematical model describes the performance of the photocatalytic fluidized bed reactor well for all operating conditions. Hajaghazadeh et al. studied the photocatalytic oxidation of methyl ethyl ketone under UVA light in a fluidized bed reactor [31]. The reactor was made of two parallel quartz windows incorporated in a stainless steel frame. 40 UVA-LEDs were used as an irradiation source and were located in the contact with reactor’s quartz windows. Commercial TiO₂ such as P25, PC50, and PC500 was used as a photocatalyst. It was found that the photocatalytic activity depends on the surface area of the photocatalyst.

The batch reactor is the simple type of photoreactors used for VOC degradation. Typically, the batch reactor consists of a chamber made of Pyrex glass. The photocatalyst is located in the lower part of the chamber. The irradiation source is located outside the reactor. Amama et al. used cylindrical batch reactor for photocatalytic degradation of trichloroethylene [32]. The reactor (total volume 0.11 dm³) was made of Pyrex glass. TiO₂ coated on glass fiber cloth by sol–gel process was used as a photocatalyst and illuminated by eight symmetrically arranged fluorescent blacklight lamps which were located at a fixed distance from the reactor. The authors suggested that photocatalytic degradation of trichloroethylene to carbon dioxide did not occur in the gas phase but mainly at the surface of TiO₂. Additionally, it was found that mineralization yield of trichloroethylene and by-product formation could be affected by pretreatment step of TiO₂, such as preillumination, prehydroxylation, and prechlorination of photocatalyst surface.

Debono et al. used batch reactor for photocatalytic oxidation of decane at ppb levels [33]. This reactor consisted of a Pyrex glass chamber (total volume 120 dm³) was illuminated by nine PL-L-40 Philips UV lamps. The photocatalyst used for experiments (TiO₂–P25) was placed in the lower part of the reactor chamber. It was found that formaldehyde, acetaldehyde, and propanal were the main by-products formed in the gas phase during photocatalytic degradation of decane. Moreover, the amounts of these compounds were linearly dependent on the initial decane concentration.

To conclude, many types of photocatalytic reactors for photocatalytic degradation of VOCs have been designed. More examples of reactors are compiled in Table 7.1. The kinetic reaction and mass transfer rate are two of the main parameters having influence for performance of a photocatalytic reactor. Other influencing factors of the reactor efficiency include light of source and intensity, contaminant concentration, humidity, temperature, surface area, and activity of photocatalyst.

Table 7.1 Reactors used for photocatalytic degradation of volatile organic compounds

7.2.2 Reactors for Photocatalytic Degradation of Inorganic Pollutants

There are various photocatalytic reactors used for oxidation of inorganic pollutants in the gas phase [66–69]. Soylu et al. used flow reactor for photocatalytic oxidation of NO_x [70]. TiO₂–Al₂O₃ photocatalyst was placed on polymethyl methacrylate (PMMA) sample holder inside the reactor. The irradiation was provided by 8 W UVA lamps located outside the reactor. It was found out that TiO₂–Al₂O₃ photocatalyst showed remarkable photocatalytic NO_x oxidation and storage performance in relation to the TiO₂–P25. Dong et al. used flow reactor for photocatalytic NO removal on BiOI surface under the influence of visible light [71]. Photoreactor (4.5 dm³) was in the form of rectangular box, made of stainless steel, and covered with quartz glass. Testing BiOI film on sample dish was located in the middle of the reactor. A LED lamp was vertically located outside the reactor above the sample dish. Wang et al. used continuous flow reactor for the oxidation of NO from a gaseous phase [72]. Photoreactor was made of Pyrex glass with “Z” type and was irradiated by one 125 W Hg arc lamp located outside the reactor. The volume of the reactor was 340 dm³. The reactor and source of light were set in a hollow chamber which was coated with tinfoil. Various surfaces platinized TiO₂ were placed in the bottom part of the reactor. Portela et al. used continuous flow flat reactor for photocatalytic oxidation of H₂S [73]. Various photocatalysts were coated on the glass plates. The reactor with a top borosilicate glass window was irradiated by two 8 W UVA lamps. Sheng et al. used continuous flow reactor for photocatalytic oxidation of NO [74]. The woven glass fabric immobilized with photocatalyst was placed into reactor with a “Z” type, made of cylindrical Pyrex glass. 125 W Hg arc lamp used as an irradiation source was located outside the reactor.

Several research groups used fixed bed reactor for photocatalytic oxidation of NO_x, SO₂, and H₂S in the gas phase [75–78]. Liu et al. used fixed bed reactor made of double concentric quartz tubes for oxidation of NO_x and SO₂ [75]. 125 W high-pressure mercury lamp was located in the center of inner tube and was used as an irradiation source. The reactor was placed inside a black box. Cu doped titanium dioxide supported by multi-walled carbon nanotubes was placed in the outer tube. Ou et al. studied photocatalytic oxidation of NO under the influence of visible light using a fixed bed continuous flow reactor [77]. This reactor was made of tubular quartz. The 350 W Xe lamp was vertically placed, parallel with the reactor. The photocatalyst powder (g-C₃N₄/BiVO₄) was mixed with silica sand and packed in the reactor. The photocatalytic activity test showed that the maximum conversion of NO was 40 % when the concentration of NO was about 400 ppm under the visible light irradiation Wang et al. used bed reactor for photocatalytic decomposition of H₂S under the influence of visible light [78]. These experiments were carried out in a glass tubular reactor. The 100 W lamp was located outside the reactor. A shutter window was located between the lamp and the reactor to remove UV radiation.

Lafjah et al. studied photocatalytic oxidation of H₂S in the gas phase using single pass annular Pyrex reactor [79]. This reactor was made of two coaxial tubes, between which the contaminated air was passed through. The irradiation source (8 W blacklight tube) was located inside the internal tube. The photocatalyst powder was placed on the inner surface of the external tube. Tellez et al. used annular reactor for photocatalytic oxidation of H₂S [37, 80]. This type of annular reactor has been described previously.

The plate reactors are often used for photocatalytic oxidation of NO_x in the gas phase [81–86]. These types of reactors were described in the previous subsection. Ao et al. studied photocatalytic oxidation of NO₂ using plate reactor with inner source of irradiation (6 W UV lamp) [81]. The reactor’s surface was coated by a Teflon film. TiO₂ powder was coated on the glass fiber filter. Moreover, the plate reactor made of stainless steel with inner source of irradiation was used by Chen et al. for photocatalytic oxidation of NO_x [60, 82]. Yu et al. used plate reactor with outer source of irradiation for removal of NO [83]. This reactor was made of non-adsorbing plastic material. Top of the reactor was covered with a borosilicate plate. The photocatalyst was illuminated by 25 W cool daylight lamps. Other types of the reactors used for photocatalytic oxidation of inorganic pollutants were described elsewhere [87, 88].

7.2.3 Reactors for Photocatalytic Inactivation of Bacteria

Lin et al. used reactor with the commercial TiO₂ filter for photocatalytic inactivation of Bacillus subtilis and Penicillium citrinum [89]. 8 W fluorescent blacklight lamp was placed above the surface of the filter and glass slide. Photocatalyst-coated filter and irradiation source were located inside the chamber. The spore suspensions of bacteria were dropped directly onto the center part of the TiO₂ filter. TiO₂ filter had a large pore size about 500 μm. Chotigawin et al. used photocatalytic HEPA filter for microorganism disinfection [90]. Two photocatalytic HEPA filters were located into the closed loop chamber side by side. The photocatalyst was irradiated by five 36 W UVA lamps. The photocatalytic filters were made by dip coating a HEPA filter in a P25–TiO₂ slurry. S. epidermidis, B. subtilis, A. niger, and P. citrinum were used as the model of microorganism. Vohra et al. investigated the disinfection effectiveness of commercial titanium dioxide coated on the fabric filters for Bacillus cereus, Staphylococcus aureus, Escherichia coli, Aspergillus niger, and MS2 bacteriophage inactivation [91]. These experiments were carried out in the recirculation duct. The form of this reactor is rectangular in cross section while the lower duct portion is circular.

Keller et al. used the photocatalytic reactor which is a Vigreux-like Pyrex tubular reactor for photocatalytic inactivation of Escherichia coli as the model bacteria in airstream [92]. This reactor was made of Pyrex glass. Four 8 W blacklight tubes were located outside the reactor. The photocatalyst was coated on the inside of the tube. This technical solution of the reactor allowed better contact between the solid photocatalyst and flowing bacterial contamination. The reactor consisted of an aerosol generator and a bacterial cultivation medium.

Guo et al. studied photocatalytic inactivation of Escherichia coli K12 placed in Petri dish irradiated by two 8 W fluorescent lamps [93]. It was found, that photocatalytic inactivation of microorganism by TiO₂, based on generation of reactive oxygen species (ROS), are followed by action of the generated ROS on the target organism. It was stated that photocatalytic inactivation of bacteria involved oxidative damage of cell walls, membranes, enzymes, and nucleic acids by ROS. Modesto et al. used plate reactor for inactivation of bacteria in the gas phase [94]. Escherichia coli, Bacillus subtilis, and Staphylococcus aureus were used as the model of bacteria. The reactor was made of wooden medium density fiber (MDF) plates of 15 mm thickness. Six glass plates coated with the photocatalyst were located at the lateral walls of the chamber. Four 4 W blacklight lamps were located along the chamber. The suspension of microorganisms in the airstream passed through the reactor. TiO₂, Ag–TiO₂, Pd–TiO₂, and Fe–TiO₂ were used as photocatalysts.

7.2.4 Reactors for Photocatalytic CO₂ Conversion

Photocatalytic CO₂ conversion is carried out in two major system types: (i) two phases and (ii) three phases. Two-phase systems include (i) gas photocatalyst and (ii) liquid photocatalyst. Table 7.2 shows various types of reactors that can be applied for photocatalytic CO₂ conversion in two-phase and three-phase system. It could be concluded that the convective mass transfer rate of CO₂, reaction rate, and surface area of the photocatalyst are the main factors for efficient photocatalytic CO₂ conversion.

Table 7.2 Various types of reactors used for photocatalytic CO₂ conversion in two-phase and three-phase systems

Zhao et al. studied photocatalytic reduction of CO₂ in fixed bed reactor [100]. Gas mixture of CO₂, H₂O, and methanol was introduced into a cylindrical reactor made of stainless steel and quartz window. Ag/TiO₂ photocatalyst was coated on the glass fiber filter and placed at the bottom of the reactor. A 150 W solar simulator was located outside the reactor. The same reactor has been used by Liu et al. for photocatalytic reduction of CO₂ in the presence of Cu/TiO₂ photocatalyst [98]. Shi et al. used fixed bed reactor for photocatalytic conversion of CH₄ and CO₂ to acetone production [109]. The experiments were carried out in a continuous flow quartz fixed bed reactor. A 125 W ultrahigh pressure mercury lamp was located in the center part of the reactor. The photocatalyst bed was placed along the reactor’s wall. Cu/CdS–TiO₂/SiO₂ was used as the photocatalyst. Wang et al. used fixed bed reactor for CO₂ reduction with H₂O under simulated solar irradiation [99, 110]. This reactor was made of a stainless steel with the volume of 1.5 dm³. Photocatalyst powder was placed on the stainless steel omentum located in the center of the reactor. A 300 W Xe arc lamp was put at the top of the quartz window. A moist glass wool was placed between the bottom of the reactor and photocatalyst. The bottom of glass wool support was moisturized of deionized water.

Ola et al. used honeycomb monolith reactor for CO₂ conversion using Pd and Rh–TiO₂ photocatalyst under the influence of ultraviolet irradiation [107]. The optical fibers were uniformly distributed in the monolith and located into a cylindrical reactor made of Pyrex glass. The irradiation was carried out by the high-pressure mercury lamp through the quartz window. The reactor was covered in aluminum foil and located in the gloved box. The experiments were also carried out in the slurry batch annular reactor to comparison quantum efficiency. It was found that the quantum efficiency of the monolith reactor was 23.5 times higher than that of the slurry batch annular reactor due to the high surface area of the monolith and the elimination of uneven light distribution via the optical fibers. Tahir and Amin used microchannel monolith reactor for photocatalytic CO₂ reduction [111, 112]. The reactor was made of a stainless steel cylindrical vessel with a total volume of 0.15 dm³. The monolith has been coated with photocatalyst and located in the center of the cylindrical reactor, equipped with a quartz window for passing light irradiations from 200 W mercury lamp. The reactor was fitted with heating and cooling jacket to adjust the reactor temperature. The photocatalytic experiment was carried out in a microchannel monolith photoreactor, and its performance was compared with a cell-type photoreactor. It was found that the quantum efficiency achieved in the cell-type reactor was much lower compared to the microchannel monolith reactor due to higher illuminated surface area, higher photon energy consumption, and better utilization of monolith reactor volume.

Nguyen et al. used continuous circular reactor made of Pyrex glass with a quartz window for reduction of CO₂ over ruthenium dye-sensitized TiO₂ metal-doped photocatalysts under concentrated natural sunlight [113]. Photocatalyst was coated on the optical fibers. High-pressure Hg lamp or concentrated natural sunlight was used as the irradiation source. The concentrated natural sunlight was collected by using a solar concentrator and transmitted via an optical cable and focused on the window of the reactor. Wu and Lin used optical fiber reactor for photocatalytic reduction of CO₂ to methanol [104]. Photocatalysts coated on 120 fibers with 16 cm long were located into the reactor. Both sides of the reactor were sealed using O-rings and illuminated from the quartz window of one side by an Hg lamp. The reactor was covered using aluminum foil to avoid the light from the outside during the reaction. Wu et al. studied this same optical fiber reactor for CO₂ reduction using TiO₂, Cu/TiO₂, and Ag/TiO₂ films coated on 216 fibers as photocatalysts [103].

7.3 Liquid-Phase Photoreactors

There are many types of reactors that can be used in the liquid-phase photocatalytic reactions. The selection usually depends on the experimental conditions and the application. Different water contaminants, ranging from hazardous contaminants of pesticides, herbicides, and detergents to pathogens, viruses, coliforms, etc., can be effectively removed in liquid-phase photoreactors. Table 7.3 summarizes various model compounds and microorganisms commonly used in the photocatalytic reactions. Examples of these various photocatalytic degradation processes and inactivation of the microorganisms will be considered in the following sections.

Table 7.3 Overview of model compounds and microorganisms used for photocatalytic applications

Liquid-phase heterogeneous photoreactors can be generally divided into three main groups based on their design characteristic such as [114–116]:

• 1. State of the photocatalyst: reactors with suspended photocatalyst particles (slurry) and reactors with photocatalyst immobilized on the inert surfaces

• 2. Type of illuminations: artificial light or solar light

• 3. Position of the irradiation source: external light source, immersed light sources, and distributed light sources (such as reflectors or optical fibers)

While fundamental principles of the photocatalytic processes are relatively well understood, the design and modeling of photocatalytic reactors still require consideration. It is particularly essential in the case of scaled reactors processing large volumes of water and using high levels of irradiation [115, 117].

7.3.1 State of the Photocatalyst

7.3.1.1 Slurry Reactors

Slurry reactors are the most common and conventional reactors in photocatalytic technology [136]. In a slurry system, the catalysts are suspended in the liquid phase with the help of mechanical or gas-promoted agitation.

These show the largest photocatalytic activity compared with the immobilized photocatalyst and provide a high total surface area of photocatalyst per unit volume which is one of the most important factors configuring a photoreactor. However, these reactors require an additional downstream separation unit for the recovery of photocatalyst particles [116, 137]. Table 7.4 summarizes the advantages and disadvantages of both slurry and immobilized systems.

Table 7.4 The advantages and disadvantages of slurry and immobilized-type reactors [115, 116, 138, 139]

The effects of operational parameters on the photocatalytic slurry reactors are systematically investigated to achieve optimum reactor design for more effective photocatalytic water treatment process [115]. Nishio et al. examined the influence of light intensity, initial dye concentration, photocatalyst loading, and initial solution pH on the decolorization rate of Orange II in an external UV light irradiation slurry photoreactor using zinc oxide (ZnO) as a semiconductor photocatalyst. The experiments were performed in a Pyrex glass cylindrical reactor of 0.08 m inside diameter and 0.55 m height. The working volume was 2 dm³. Around the cylindrical photoreactor were located three 15 W near UV fluorescent lamps (352 nm) and externally irradiated the solution. The distance between the lamp and the photoreactor surface was 0.025 m. The photocatalytic reactor as well as lamps was totally covered with an aluminum foil. It was observed that the dye removal efficiency increased as initial pollutant concentration decreased and UV light intensity increased. The highest efficiency was achieved for ZnO concentration being around 1000 mg/dm³ and pH was around 7.7 [140]. McCullagh presented a novel photoreactor based on a slurry continuous flow reactor configuration for methylene blue (MB) photodegradation in the presence of TiO₂ photocatalyst. This configuration combines the high surface area contact of photocatalyst with pollutant of a slurry reactor and also provides a high illumination of photocatalyst. Moreover, on the inside wall of the reactor vessel, reactor has a unique array of weir-like baffles which continuously remove catalyst from aqueous, enabling the catalyst to be exposed to UV irradiation as the reactor vessel rotates perpendicular to the light source. Experimental results indicated that developed novel reactor configuration exhibited a high UV light penetration characteristic as well as very effective mass transfer rate [141]. In another study, Subramanian et al. reported phenol degradation studies in an annular slurry reactor under various operating and design conditions. The photoreactor had concentric transparent acrylic stationary outer cylinder and inner cylinder rotating at specified revolutions per minute. Authors studied the influence of pollutant concentration (10–50 mg/dm³), inner cylinder rotation speed (0–50 rpm), catalyst loading (0–8 g/dm³), annular gap width (7.5, 17.5, and 32.5 mm), as well as mode of illumination: continuous or periodic on the photocatalytic performance. It was clearly demonstrated that the performance of the reactor was improving with the increased content of catalyst, but controlled periodic illumination had no significant influence on reactor efficiency over the regular continuous irradiation. Moreover, rotation of the inner cylinder was necessary only in the case of high gap width configuration at high catalyst loadings [142].

Wang et al. investigated photocatalytic disinfection of gram-negative Pseudomonas fluorescens and gram-positive Macrococcus caseolyticus spoilage bacteria under various conditions. The reactor system consisted of a magnetic stirrer, a black UV light lamp, and a baker which was exposed to the irradiation from the top. The light intensity was measured using a digital light intensity meter. It was demonstrated that increased photocatalyst contents and UVA light intensity resulted in increased microorganisms killing. Moreover, effectiveness of suspended photocatalyst depended on the initial bacterial population – nano-TiO₂ was more effective against M. caseolyticus than Pseudomonas fluorescens bacteria [133].

For the photocatalytic reduction of CO₂, in 1979, Inoue et al. introduced a slurry reactor in which catalysts were suspended in water [143]. Until 2000, slurry-type reactors were widely considered for reduction of CO₂ under UV or visible irradiation. On the other hand, Tahir and Amin suggested that this type of reactor is not efficient for enhancing the photocatalytic activity due to the low surface area and complicated separation process required to isolate the miniature catalyst grains [144]. Furthermore, one of the limitations for CO₂ photoreduction in the liquid phase is due to its low solubility in water. Therefore, Rossetti et al. developed an innovative concept of photoreactor, allowing to operate under high pressure (up to 20 bar). The proposed stainless steel reactor was effectively employed to improve CO₂ solubility in a liquid solvent even at high temperature. The suspension was saturated with CO₂ at various temperature and pressure and then irradiated with a 125 W medium-pressure Hg vapor lamp (range of emission: 254–364 nm). The results showed a strong dependence of product distribution on temperature and pressure. An increase of pressure caused increase in CO₂ concentration in the liquid phase and preferred the formation of liquid fuels such as CH₃OH and HCOOH [145].

Priya et al. developed two slurry photocatalytic reactors: batch reactor (BR) (see Fig. 7.3d) and batch-recycle reactor with continuous supply of inert gas (BRRwCG) and compared their performance in the process of hydrogen production. The photoreactors of capacity 300 m leach were made of plexiglass material which was transparent to the solar light. The photocatalytic powders were kept suspended using magnetic stirrer in the BR and gas bubbling and recycling of the suspension in the BRRwCG. The higher generation of hydrogen was observed in the case of BRRwCG due to the recycling of solution and continuous purging of inert gas, enabling the fast desorption of products [146].

Fig. 7.3

Main types of photoreactors used for water and wastewater treatment (irradiated by sun or UV lamps): (a) compound parabolic collector (CPC); (b) parabolic trough reactors (PTR); (c) double-skin sheet reactor (DSSR); (d) slurry, wall, fixed bed reactor; (e) batch reactor with outer source of irradiation; and (f) batch reactor with inner source of irradiation

7.3.1.2 Immobilized Reactor

Photocatalytic reactors with immobilized photocatalyst are those in which the photocatalyst is fixed to support by physical surface forces or chemical bonds. These reactors extend the benefit of not requiring catalyst recovery and permit the continuous operation [114, 137].

Typical photocatalyst supports are:

• Sand [147]

• Polymer films [148]

• Alumina [149]

• Glass beads [150]

• Zeolite [151]

• Activated carbon [152]

• Silica gel [153, 154]

• Stainless steel [155, 156]

• Carbon fiber [157]

Recently, Li et al. designed novel double-cylindrical-shell (DCS) photoreactor for degradation of rhodamine B (RhB) and methyl orange (MO). The photoreactor was developed by immobilizing TiO₂-coated silica gel beads on the outside surface of interior quartz glass tube of the DCS reactor. In order to optimize designed photocatalytic reactor, the operational parameters such as flow rate, initial concentration, and repetitive operation for the degradation of dye were studied. The developed novel reactor exhibited higher efficiency, lower energy consumption, and better repetitive operation performance for the degradation of RhB and MO as compared with reported slurry and thin-film photoreactors [158]. Behnajady et al. described the construction and performance of a continuous flow photoreactor with immobilized TiO₂ on glass plates for photodegradation of C.I. Acid Red 27(AR27). The photocatalytic reactor consisted of four quartz tubes connected through means of polyethylene tubes from the top to the bottom. Three glass plates loaded with TiO₂–P25 were put into the quartz tubes. Four low-pressure mercury UV lamps were placed in front of the quartz tubes. The results showed that removal efficiency of AR27 increased linearly with increasing the light intensity, but it decreased when the flow rate increased [159].

The reports about photocatalytic disinfection of water commonly use slurry photoreactor, reaching a high efficiency to inactivate microorganisms. However, some efforts have been also concentrated on using immobilized systems, usually exhibiting to be less active and requiring more irradiation time as compared with suspended systems [160, 161]. Grieken et al. developed wall and fixed bed reactors for inactivation of Escherichia coli. TiO₂ photocatalyst was immobilized in an annular reactor in two different ways: on the inner reactor wall and on the surface of glass rings used in packed fixed bed reactor. The effect of the increase in the TiO₂ layer thickness has been evaluated, and the results have been compared with those obtained for increasing concentrations of TiO₂ slurries (see Fig. 7.3d). Although immobilized systems were less photoactive than slurry system, they exhibited a higher resistance to the inhibition by organic matter, leading to comparable irradiation time to obtain microorganism concentration below detection limit in wastewater [162].

Hsu et al. immobilized S-doped ZnO nanorods on stainless steel mesh as novel hierarchical photocatalysts for water splitting to hydrogen production. Polymer additive enabled the growth of nanorods on the total surface of wire mesh. The surface texture and photocatalytic hydrogen production performance from salt water under UV light irradiation in a reactor loaded with these photocatalysts were tested. The highest evolution rate was achieved due to increased surface area of the hierarchical immobilized photocatalyst, enhanced light trapping, as well as liquid flow among wire meshes [163].

7.3.2 Type of Irradiation

7.3.2.1 Artificial Light

One of the most challenging parameters in the design of photoreactors is the appropriate illumination of catalyst. Therefore, the important aspects in design consideration for photocatalytic reactors are light wavelength, light intensity, as well as type of irradiation source [115]. There are main types of artificial irradiation sources including: (i) arc lamps, (ii) fluorescent lamps, (iii) incandescent lamps, (iv) lasers, and (v) light-emitting diodes (LEDs). Arc lamps are often named according to the gas contained in the bulb, including neon, argon, xenon, krypton, sodium, metal halide, and mercury. Additionally, mercury lamps can be grouped in low, medium, and high-pressure mercury lamp categories [164] (Table 7.5).

Table 7.5 Overview of artificially illuminated liquid-phase photoreactor

Swarnalatha et al. studied photocatalytic oxidation of 2,6-dinitrophenol using different catalysts: TiO₂, TiO₂–P25, CdS, WO₃, and ZnO. The annular-flow photocatalytic reactor used in this research was a cylindrical plastic vessel, in which the mercury lamp is surrounded by a quartz glass tube to belay it from direct contact with an aqueous solution flowing by an annulus between the inner surface of the vessel and the outer surface of the quartz glass tube. It was demonstrated that aqueous TiO₂–P25 suspension exhibited the highest efficiency in photocatalytic degradation at the wavelength of 254 nm using an annular-flow-type reactor equipped with an 8 W low-pressure mercury lamp. Moreover, the effect of irradiation time and pH on the efficiency of degradation was investigated. The pollutant degradation in the presence of P25 was found to incrementally increase with increasing irradiation time at an optimum pH of 8. Complete degradation of the 2,6-dinitrophenol occurred after 3 h of irradiation [172]. In another study, Han et al. investigated photocatalytic degradation of p-chlorobenzoic acid (p-CBA) in aqueous solution using two kinds of low-pressure mercury lamps: UV lamp emitted at 254 nm and the vacuum UV lamp emitted at both 254 nm and 185 nm. The lamp was put in the center of the photocatalytic reactor with quartz tube protection (outer diameter 25 mm). Oxygen or air was used as a bubbling gas which was implemented to the reactor through a porous glass plate with a flow rate of 200 cm³/min. It could be seen that degradation of p-chlorobenzoic acid was more effective in the presence of vacuum UV lamp than in the case of UV lamp when the same power lamps were used in research [173].

Chen et al. investigated photocatalytic disinfection of Escherichia coli K12 using natural sphalerite (NS) as a photocatalyst under various spectra and intensities of visible light emitted by LEDs. The photocatalytic test was performed in the reactor equipped with 16 LED lights and compared with results obtained for two other visible light sources such as fluorescent tube and xenon lamp. Moreover, photocatalytic disinfection of microorganisms was compared under various single spectra: blue, green, yellow, and red color LEDs. It was shown that the most effective wavelength ranges for photocatalytic inactivation of bacteria are 440–490 and 570–620 nm. Moreover, a positive dependence was observed between the disinfection efficiency and the visible light intensity. The results showed also that NS caused complete inactivation of E. coli within 8 h irradiation using white LEDs [174]. In another study, Benabbou et al. examined photocatalytic inactivation of Escherichia coli K12. The disinfection experiments were carried out in a Pyrex reactor in which an HPK 125 W lamp emitting in the 200–400 nm range was used as irradiation source. Moreover, various optical filters were used to modify lamp emission spectrum. The light intensity was controlled by grids with various sizes of mesh, which were put on the lamp. The effect of different types of UV light, including UVA, UVB, and UVC was also examined, and modification of the light radiation intensity was discussed. It was found that the addition of photocatalyst at low concentration improved the inactivation of bacteria in the presence of UVA and UVB, but negative effect was noted under UVC. Furthermore, the photocatalytic efficiency increased as a function of light intensity, no matter the experimental conditions [175].

Kočí et al. studied the effect of reactor geometry on the photocatalytic reduction of CO₂ using ZnS nanoparticles deposited on montmorillonite as a catalyst. The photocatalytic experiments were performed in two homemade batch annular reactors with three quartz tubes of various diameters: 3.5, 4.0, and 4.5 cm. The photocatalyst was suspended in NaOH solutions, and after saturation by CO₂, the suspension was illuminated using UV 8 W Hg lamp (254 nm). It was demonstrated that for both reactors, the highest activity of the photocatalytic reduction was obtained in a configuration where the lamp touched the surface of the liquid in the reactor and the configuration of the reactor was not annular. Moreover, it was suggested that one of the most important factors in the slurry reactors is appropriate mixing but its implementation is difficult in apparatus of annular configuration [102].

Hernández-Gordillo et al. investigated photocatalytic activity of CdS photocatalyst for the hydrogen production from either methanol–water or sulfide/sulfite solution in the presence of blue light energy. The photocatalytic tests were performed in a glass homemade photoreactor without any cooling system. The solution was irradiated using blue light emitted by LED lamps of very low power (3 W) which were placed in appropriate positions to allow complete illumination of the suspended catalysts. It was shown that the amount of hydrogen generated linearly increased as a function of the number of LED lamps, achieving to a hydrogen production of 9.54 μmol/h. This study suggested that the hydrogen production depended very strongly on the lamp intensity [176]. In another study, Gomathisankar et al. investigated photocatalytic hydrogen production from aqueous methanol solution using Cu-deposited ZnO photocatalyst. The photocatalytic test was carried out in the Pyrex column vessel reactor. The spout of vessel was hermetic closed with septum and aluminum insulating. The optical filter (λ > 400 nm) was used for the visible light irradiation. A xenon lamp (500 W) was located on the side of the photoreactor and used as a light source. The light intensity was controlled by a UV radiometer equipped with a sensor of 320–410 nm wavelengths. It was demonstrated that Cu-deposited ZnO had the response to the visible light for the hydrogen production. Furthermore, under the optimal conditions, the photoactivity was about 130 times higher than those showed for bare ZnO photocatalyst [177].

7.3.2.2 Solar Light

The implementation of solar photocatalytic reactors has occurred concurrently with advances in the design of solar thermal collectors. There are specific constraints for the design of solar photocatalytic reactors such as [178, 179]:

• The wastewater must be exposed to ultraviolet solar radiation; therefore, the collector must be made of UV transparent materials.

• Temperature negligible affected the photocatalytic process, so no insulation is required.

• Construction should be economical and efficient with a low pressure drop.

Solar photocatalytic reactors can be divided into concentrating and non-concentrating (one sun) systems depending on received irradiation [180]. Non-concentrating solar reactors use intensities equal or lesser than natural solar irradiation, while concentrating solar reactors require intensities that surpass irradiations equivalent to one sun [116].

In the concentrating design, solar radiation is collected in a photocatalytic reactor by a reflecting surface, and because of this, for the same light-harvesting area, the reactor volume is smaller than in the case of non-concentrating system [180]. The most promising type of concentrating solar reactor is parabolic trough collector (PTC) which is demonstrated to be efficient for wastewater treatment. PTCs consist of platform that has one or two motors controlled by single- or dual-axis solar tracking system that maintain the collector aperture plane perpendicular to incoming solar radiation (see Fig. 7.3b) [181].

Non-concentrating photoreactors have no moving parts or solar tracking devices (see Fig. 7.3c). This kind of reactor does not concentrate radiation, and because of this, efficiency is not limited by factors connected with reflection, concentration, or solar tracking. In this system, optical efficiency is higher as compared with concentrating reactors. Moreover, non-concentrating system can utilize the diffuse and direct portion of the solar UVA [182]. One-sun collectors are usually cheaper than PTCs because their elements are simpler, and the surface required for their installation is smaller [178].

Compound parabolic collectors (CPCs) belong to the most promising photocatalytic solar reactors which combine the advantages of parabolic trough concentrator and non-concentrating system [178]. CPCs are low-concentration static collectors with reflective surface and can be designed for any given reactor shape (see Fig. 7.3a) [182]. The CPC reflectors are usually made from polished aluminum because of its high reflectivity in the UV range and high resistance to the environmental conditions. Pipes and valves are manufactured from polyethylene; photoreactor tube is made of borosilicate 25 glass due to high transmission in the UV range of its material. Water flows through the borosilicate tubes to a tank by a centrifugal pump, allowing a turbulent regime inside the photocatalytic reactor [183].

Zayani et al. investigated performances of solar pilot plant for photocatalytic removal of azo dye used as a model pollutant. Experiments were carried out in thin-film fixed bed reactor with an illuminated area of 25 m². Effect of important operating parameters including flow rate, catalyst loading, and initial dye concentration on photocatalytic treatment kinetic was examined for optimization which will be necessary in designing large-scale photoreactors. Furthermore, the photodegradation kinetic of total organic carbon (TOC) was discussed in terms of Langmuir–Hinshelwood model [184]. Xu et al. developed novel optical fiber reactor (OFR) in which side-glowing optical fibers (SOFs) were used as light transmission medium as well as photocatalyst supporter. The SOF was made up of quartz core with a silicon cover which can emanate light from side surface more uniformly and transmit light for longer distance. Furthermore, SOF was flexible and can be entwined into any shapes. It was demonstrated that novel reactor can collect solar light efficiency while occupying smaller surface as compared with traditional solar collectors. It was observed that 79 % of 4-chlorophenol decomposed under sunlight irradiation during 8 h [185].

Vidal et al. presented the first pilot-plant study about solar photocatalysis for bacterial inactivation. Researchers constructed a new low-cost compound parabolic concentrator (CPC) prototype containing: solar collector (Pyrex photoreactor tubes, aluminum reflective surface), flowmeter, pump, sensors (pH, O, T, UV radiation), pipes, fittings, and tanks (PVC). This solar photoreactor has an area of 4.5 m² and it was tilted at local latitude to maximize the available solar irradiation. It was observed 5-log reduction for E. coli and Enterococcus faecalis (initial concentration: 10²–10⁴ CFU/cm³) after 30 min of solar irradiation (solar UV value: 25 W/m²) [186]. In another study, McLoughlin et al. compared three different solar collectors for the disinfection of water heavily contaminated with Escherichia coli. It was demonstrated that three lab-scale solar photoreactors which were constructed using Pyrex tubing and aluminum reflectors of compound parabolic, parabolic, and V-groove profiles all enhance the effect of natural solar irradiation. Among these three collector shapes, compound parabolic reflector promoted the most efficient inactivation of bacteria. Moreover, researchers carried out the tests to assess the improvement to disinfection which could be achieved using TiO₂-coated Pyrex rods fixed within the reactors. However, this solution caused only a slight improvement in performance of the compound parabolic reactor and no enhancement to overall disinfection performance in either the parabolic and V-groove reactors [187]. Alrousan et al. carried out solar photocatalytic disinfection of water using compound parabolic collector and P25 immobilized on borosilicate glass tube. Researchers tested several photoreactor configurations such as (1) borosilicate glass tubes (1.5 m in length) of diameter 50 mm dip coated with TiO₂–P25, (2) uncoated 50 mm borosilicate glass tubes, (3) 32 mm borosilicate glass tube externally dip coated with TiO₂, and (4) uncoated 32 mm borosilicate glass tube. Each configuration was examined using one tube and one CPC mirror, with an irradiated surface of 0.2 m² and a total volume of treated water of 7 dm³ The most effective configuration was the concentric tube arrangement with CPC [188].

In spite of the still insufficient efficiency of direct photocatalytic water splitting, there are few trials in large-scale application of hydrogen photoreactors in the literature [189]. Jing et al. developed CPC-based solar reactor for photocatalytic hydrogen production that consisted of solar collector, Pyrex photoreactor tubes, reflective surface, and flow meter; fitting, pipes, and tanks; and pump and sensors. The photocatalytic performance was investigated for various design parameters such as tube radius, flow velocity, photocatalyst, as well as sacrificial agent concentrations. In optimal conditions, this photoreactor had higher hydrogen rate per unit volume than in the case of lab-scale reactor which could be caused by the design of tubular reactor properly illuminated by CPC on one side [190]. In another study, Villa et al. tested the simultaneous photocatalytic hydrogen production under direct solar irradiation at pilot-plant scale. The experiments were performed in a compound parabolic collector (CPC) composed of Pyrex glass tube placed on the fixed platform. A centrifugal pump with a flow rate of 20 dm³/min enabled the recirculation of the aqueous slurry from the tank to the tubes of the photoreactor. The hydrogen was generated from aqueous solutions of formic acid, glycerol, as well as a real wastewater. The highest hydrogen production was obtained with aqueous solution of formic acid after 5 h of irradiation. However, the tests with real wastewater gave moderate amount of hydrogen, suggesting the possible use of such waters for hydrogen production in the future [191].

7.3.3 Position of the Irradiation Source

The arrangement of light source is another important aspect of photocatalytic reactor design. In the case of immersed-type reactor configuration, the lamp is placed inside the unit (see Fig. 7.3e). In external-type reactor, the lamp is located outside the reactor (see Fig. 7.3f). Light has to pass through reactor wall to get the water body. In this kind of reactor, the light intensity and evenness of UV fluence rate (UV-FR) are usually lower than that in the other two types for the same power consumption [192, 193]. Another type is the distributed reactor where light is transported from the source to the photocatalytic reactor using reflectors or light guides [194]. The distributive-type reactor usually is characterized by higher and more uniform irradiation inside the reactor than the external-type reactor.

Effects of different lamp arrangements on photocatalytic reactor performance have not been well studied. Recently, Xu et al. used computational fluid dynamics (CFD) simulation software FLUENT to simulate microorganism particle motion in various UV water disinfection reactors. The influence of lamp arrangements on the UV-FR field and log reduction of different UV water disinfection photoreactors were studied under various flow rates and constant UV dosage. In the experiment, direction, number, and orientation of lamps were diverged. The results showed that overall effects on the reactor log reduction were complex. Higher water flow rate reduced “barrier” effect in reactors with multiple lamps, lowering log reduction. This study provided new approach for understanding the effect of lamp arrangement on the performance of photodisinfection reactor [193]. Palmisano et al. performed the validation of a two-dimensional model describing the behavior of a batch cylindrical photoreactor, externally irradiated by 1–6 UV fluorescent lamps coupled with a modified Langmuir–Hinshelwood kinetics. Experimental runs were performed at different 4-nitrophenol concentration, Degussa TiO₂–P25 amounts, and under various irradiation configurations. The proposed model allows to determine the behavior of the photoreactor in a wide range of operating conditions: various catalyst and substrate loadings as well as radiations have been applied [195]. Moreover, in the literature, there are a few other reports about models for externally irradiated cylindrical reactors [196, 197].

7.4 Light Modeling

Most of the work in the field of design and modeling of photoreactors was done by Cassano and Alfano [16, 180, 198, 199]. In the case of photoreactor modeling, three main components should be considered: (1) thermal energy balance, (2) multicomponent mass conservation, and (3) photon balance (radiation energy). Balance of photons should be considered independently from the thermal energy balance since the energy useful in photochemical processes is generally negligible. The radiation energy used in the most majority of photochemical processes can be attributed to a range of wavelengths between 200 and 600 nm. Local volumetric rate of energy absorption (LVREA), defined as the rate of the radiation-activated step and proportional to the absorbed energy, was preliminarily introduced by Irazoqui et al. [200]. The LVREA depends on the photon distribution in the reaction space. To begin any photochemical reactions, absorption of a photon by a molecule resulted in formation of an excited state is a necessary step. Following absorption of radiation, a few pathways, different from the desired reaction, could be predicted, such as (1) a different, parallel reaction, (2) phosphorescence, (3) fluorescence, (4) deactivation by chemical quenching, etc. In a single-photon absorption process, the rate of radiation-activated step is proportional to the rate of energy absorbed (LVREA). The proportionality constant is the primary reaction quantum yield, defined as:

$$ {\Phi}_{prim,\ v} = \frac{numbermole{c}_{prim}}{numberphot{o}_v absorb.} $$

where:

• numbermolec _prim is the number of molecules following the expected path in the primary process.

• numberphot _v absorb. is the number of absorbed quanta of radiation.

In most cases, radiation may be arriving at one point inside a photochemical reactor from all directions in space. For a photochemical reaction to take place, this radiation has to be absorbed by an elementary reacting volume described as spectral incident radiation (G _v expressed in W/m²):

$$ {G}_v = {\displaystyle \underset{\Omega}{\int }}{I}_vd\Omega $$

where:

• I _v is the spectral specific intensity (W/m² · sr).

• Ω is the unit direction vector (coincides with the axis of an elementary cone of solid angle dΩ).

Thus, to evaluate the LVREA in the case of polychromatic radiation, we have to know the spectral intensity at each point inside the reactor, according to the following equation [198]:

$$ {e}^a = {\displaystyle \underset{v1}{\overset{v2}{\int }}}{\displaystyle \underset{\theta 1}{\overset{\theta 2}{\int }}}{\displaystyle \underset{\phi 1}{\overset{\phi 2}{\int }}}{\kappa}_v{I}_v \sin \theta d\phi d\theta dv $$

where:

(θ ₁ , θ ₂ ) and (Φ ₁ , Φ ₂ ) are the integration limit that define the space from which radiation arrives at the point of incidence.

7.5 Conclusions

Gas- and liquid-phase photoreactors discussed in this chapter specify the diversity in photocatalytic reactor design along with their potential applications. The following conclusions could be pointed based on the current state of the art in this field:

• 1. Photoreactors could be generally classified into three main groups based on their design characteristics such as (i) state of the photocatalyst, reactors with suspended photocatalyst particles (slurry) and reactors with photocatalyst immobilized on the inert surfaces; (ii) type of illuminations, artificial light or solar light; and (iii) position of the irradiation source, external light source, immersed light sources, and distributed light sources (such as reflectors or optical fibers).

• 2. Solar-driven large-scale photoreactors are mainly used for water/wastewater treatment and disinfection.

• 3. Local volumetric rate of energy absorption (LVREA) is defined as the rate of the radiation-activated step in the photochemical reaction and depended on the photon distribution in the reaction space.

The advantages and disadvantages of liquid- and gas-phase photoreactors are briefly summarized in Table 7.6.

Table 7.6 Summary of the principal advantages and disadvantages of gas- and liquid-phase photoreactors

The industrial application of photocatalytic processes is still limited due to the high cost of UV irradiation light as well as the problem with separation and reusing of photocatalysts after reaction. It could be also assumed that quantum yield in gas-phase reaction is much higher than that one in liquid-phase reaction due to lower light scattering. Therefore, solar-driven or low-powered UV lamp-irradiated (e.g., light-emitting diodes) photoreactors are crucial for broader-scale application of photocatalytic processes. Moreover, the future prospect of photocatalysis cannot rely only on the design of the photoreactors but also on the development of more effective photocatalysts. Photocatalysts used during the processes must achieve greater conversion efficiencies at lower irradiation energies. Finally, visible light-absorbing materials will be the most important component in wide-scale technology.