Photocatalysis reactor system

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

FIGURE 2.4. Schematic representation of the optical-fiber bundled array photocatalytic reactor system (Reprinted with permission from Environ. Sci. Tech., 32(3), N.J. Peill and M.R. Hoffmann, Mathematical model of a phtoocatlaytic piber-optic cable reactor for heterogeneous photocatalysis, 398-404. Copyright 1998 American Chemical Society ). 

Photocatalysis, i.e., using semiconductor particles under band gap irradiation as little micro reactors for the simultaneous reduction and oxidation of different redox systems, has been intensively studied during the last 25 years since the pioneering work of Carey et al [1]. The main focus of these studies seems to be the investigation of the principal applicability of photocatalytic systems for the efficient treatment of water and air streams polluted with toxic substances. Several review articles on this topic have recently been published [2]. In some cases, pilot-scale or even commercially available reactors have already been constructed, especially when titanium dioxide is used as the photocatalyst [3]. 

However, due to the inherent complexity of this minute photoelectro-chemical system, details of the underlying reaction mechanisms of photocatalysis are even today still far from being understood. In contrast to an ordinary photoelectrochemical cell which employs an external bias voltage to deliberately separate oxidation and reduction processes in different compartments of the reactor, in photocatalysis both processes occur on the surface of the same semiconductor particle, usually only separated by a distance of a few angstroms. Moreover, as is evident from basic principles, the reaction rate of the overall process will be limited by the... 

Sopajaree, K., Qasim, S. A., Basak, S., and Rajeshwar, K., 1999a, Integrated flow-reactor membrane filtration system for heterogeneous photocatalysis. Part I. Experiments and modeling of a batch - recirculated photoreactor, J. App. Electrochem., 29(5) 533-539. 

A newly-designed photoelectrocatalytic (PEC) reactor for CO2 reduction, which combines photocatalysis by Ti02 and electrocatalysis by carbon nanotubes (CNT), has recently been proposed (Fig. 7) [152]. A proton-conductive Nafion membrane connects the Ti02 and CNT. Irradiation of the combined system of nano-structured Ti02 deposited on a metal Ti electrode with Pt modified CNT deposited on carbon sheet caused water splitting to H2 and O2. A half-cell for the cathodic electrode, i.e., Pt or Fe modified CNT electrode, produces various organic molecules such as 2-propanol due to electrocatalytic reduction of CO2 on the electrode. The proposed PEC reactor is incomplete in its present state. However, these systems are expected to couple water splitting and CO2 reduction, and thus it may establish a new artificial photosynthetic system. 

A promising approach to overcoming these problems is the combined application of photocatalysis and membrane processes. Photocatalytic membrane reactors (PMRs) are useful for the catalyst separation and for the control of photo-oxidation products and/or by-products. TTie membrane may also ensure continuous operation in systems where the reaction of interest and the separation of product(s) can occur in a single step. The membrane process can be carried out without chemical additives and involves low energy costs. 

One of the key issues in the application of photocatalysis is separation and recovery of photocatalyst from the reaction media. A promising solution for this is coupling of photocatalysis with membrane technology. PMRs are systems in which the advantages of both photocatalytic reaction and membrane separation are combined. Due to the application of membrane processes, not only recovery and reuse of the photocatalyst but also elongation of the residence time of the substrates in the reactor, as well as selective separation of the reaction products, is possible. 

Sopajaree K, Qasim S A, Basak S and Rajeshwar K (1999a), An integrated flow reactor-membrane filtration system for heterogeneous photocatalysis. Part I Experiments and modelling of a batch-recirculated photoreactor , / Appl Electrochem, 29,533-539. 

Pressurized PMRs are systems in which photocatalysis is coupled with MF, UF or NF. These reactors can be operated either in batch or continuous... 

In Fig. 21.7 a laboratory scale PMR coupling photocatalysis with MF is shown. The PMR was applied for the removal of trichloroethylene (TCE) from water (Choo et al., 2008). The system was composed of a photocatalytic reactor (volume of 700 cm ) and a hollow fiber MF module (effective membrane surface area of 20.7 cm ). A UV-A light source was placed in the inner chamber of the photoreactor, whereas in the outer chamber the solution undergoing the photocatalytic reaction was flowing. Feed from the feed tank was pumped through the photoreactor to the membrane module. The PMR was operated either in batch or in continuous mode. In batch operations, the permeate and retentate were recycled to the photoreactor. In continuous mode, the permeate was discharged and the same volume of the solution was fed into the reactor. Thus the working volume of the photoreactor was maintained at a constant level. 

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