Photoreactor

A photoreactor is a chemical reactor device which brings photons, a photocatalyst and reactants into contact, as well as collects the reaction products deriving from physicochemical transformations. It can assume different shapes and modes of operation and can be operated in various environments of temperature and pressure. Design a photoreactor means to establish type, mode of operation, size, and optimal operative conditions. Two general types of photoreactors exist such as batch and plug flow: in the first one the mixing of reacting mixture is favoured as high as possible in order to assume a perfectly mixed reactor while in the second type mixing is hindered in order to assume a perfectly segregated reactor. With respect to the operation mode, photoreactors are classified in three general types: (i) discontinuous reactor which during the operation does not exchange matter with the ambient; (ii) semicontinuous reactor which exchanges reagents or products with the ambient; and (iii) continuous reactor, for which there is a continuous and equal inlet and outlet of mass. The discontinuous reactor is simple and needs only few accessories; it is suitable for experimental kinetics studies and it is industrially used when the amount of matter to be treated is quite small. The semicontinuous reactor is flexible but more difficult to model; it allows a good control on the reaction rate, as the reaction may be affected by the addition of reagents or by the subtraction of products. The continuous reactor is suitable for industrial purposes when big amount of matter must be treated and/or the reaction rate is extremely high. It needs many accessories devices, but it is possible to achieve a very good control on the product quality (Loddo et al. 2009). The utilisation of radiation and solid catalysts in a synergic effect for performing chemical transformations of species present in liquid or gaseous mixtures makes the heterogeneous photocatalytic reactors more complex than the heterogeneous catalytic ones. The main components of a photocatalytic system are the photoreactor and the radiation. For thermal catalytic processes the reactor configuration is chosen on the basis of i) mode of operation; ii) phases present in the reactor; iii) flow characteristics; iv) needs of heat exchange; v) composition and operative conditions of the reacting mixture source (Braunet al. 1991). In addition to previous parameters the choice of the heterogeneous photoreactors is conditioned by the fact that their geometry and materials must guarantee the penetration of radiation all over the reacting mixture having in mind that the absorbed photon energy should be equal or higher than the band gap of the used photocatalyst. In the case of stirred photoreactors the presence of the photocatalyst, usually a powdered micro- or nano-crystalline semiconductor, affects in a complex way the depth of radiation penetration. Photoreactors and, thus, photocatalytic processes, are applicable in wastewater treatment, energy production, chemical synthesis, and greenhouse gas mitigation. They have the potential to address both the consumption of non renewable fossil fuels and global warming, two of the greatest problems facing humankind. The state-of-the-art in solar photoreactor design and assess those systems which are most applicable for industrial-scale implementation has been reviewed by Braham et al (2009). Designs for parabolic trough, compound parabolic, inclined plate, double skin sheet, rotating disk, water bell, fiber optic, and fixed/fluidized bed photoreactors are qualitatively discussed and compared. Compound parabolic photoreactors are most suited to near term applications at pilot-scale (>1000 L/day) due to their advantageous light collecting properties and well-known design methodology. Engineered-photocatalysts, photoreactor systems, process optimizations and modellings of the photooxidation processes for water treatment, a number of potential and commercial photocatalytic reactor configurations, in particular the photocatalytic membrane reactors, are discussed by Chong et al. (2010).The main technical barriers that impede commercialisation of semiconductor photocatalytic process was the post-recovery of the catalyst particles after water treatment. But, nowadays, membrane photoreactors have shown to solve this drawback (Molinari et al. 2002). The existence of mass transfer limitations in slurry photocatalytic reactors and experimental validation was made in a flat reactor that was part of a recycling system by Ballari et al (2010). When the photocatalytic reaction is not fast, employing catalyst loadings below 1 g L-1, irradiation rates at the reactor wall below 1 x 10-6 Einstein cm-2 s-1 and good mixing operation (Re > 1700), mass transport limitations in the bulk of the fluid can be considered virtually nonexistent. The photocatalyst with designed physicochemical properties is the key in the process. Recent research progress towards the design and preparation of porous photocatalysts with structural, compositional, and morphological properties suitable for use in a photocatalytic reactor for water treatment via advanced oxidation processes have been reviewed by Pan et al. (2010). Various synthetic strategies for fabricating porous photocatalysts with well-defined microscopic morphologies and nano/meso-scopic active nanobuilding blocks and the synthesis-component-structure-property relationship working in photocatalyst design is discussed. References Ballari MD,Alfano OM, Cassano AE(2010) Mass transfer limitations in slurry photocatalytic reactors: Experimental validation. CHEMICAL ENGINEERING SCIENCE 65:4931-4942 Braham RJ, Harris AT (2009)Review of Major Design and Scale-up Considerations for Solar Photocatalytic Reactors. IND. & ENG. CHEMISTRY RESEARCH 48:8890-8905 Braun AM, Maurette MT, Oliveros E (1991) Photochemical technology. J. Wiley & Sons: New York, US. Chong MN, Jin B,Chow CWK,Saint C(2010) Recent developments in photocatalytic water treatment technology: A review. WATER RESEARCH 44: 2997-3027 Loddo V, Augugliaro V, Molinari R, Palmisano L(2009) Photocatalytic Membrane Reactors, in: Simulation of Membrane Reactors.Basile A, Gallucci F (Eds). Chap. 10, pp. 401-443. Nova Science Publishers, New York, US Molinari R, Palmisano L, Drioli E, Schiavello M(2002) Studies on various reactor configurations for coupling photocatalysis and membrane processes in water purification. J. Membr. Science 206: 399-415 Pan JH, Dou HQ,Xiong ZG, Xu C, Ma JZ,Zhao XS(2010) Porous photocatalysts for advanced water purifications. JOURNAL OF MATERIALS CHEMISTRY 20: 4512-4528