Photoreactor Concepts

Besides the mechanical wiping techniques several other efficient countermeasures against inorganic and organic fouling or film formation on quartz sleeves are possible  

R reflector. Specifications for AOX elimination were 30 LP Hg lamps with P S ofl20W each, UV irradiated area A ca. 4 m, volume flow rate of wastewater V = 8 m per day reproduced by permission of E. Thomanetz (University of Stuttgart, Germany). 

The contact-free photoreactors include the vertical falling film design (Fig. 8-5), the inchned flat bed system (Fig. 8-6) and the batch photoreactor (Figs. 8-7 and 8-8). These reactor configurations do not have a quartz/water interface (as in situations A, B, C, Fig. 8-3), so that the problems related to fouling and film formation are effectively circumvented. Such photoreactor types are even suited for the UV treatment of suspensions and cloudy wastewaters (cf Kim and Thomanetz, 1995). 

However, in some contact-free batch reactor (CFBR) installations the MP Hg lamp must be protected by a quartz pane from squirting water or from volatile organic compounds that evaporate from the water phase. 

Many research activities in the field of AOPs and AOTs are related to photoreactor development, reactor modeling, optimization and scaling-up of photoreactors. Several recent examples are collected in Tab. 8-1. 

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The complex field of photochemical engineering is governed by the reaction system, the lamp technology and by chemical engineering (see Fig. 3-3). Suitable photoreactor concepts can only be obtained by an appropriate combination of these three aspects. Nevertheless, many commercially available photoreactor systems are still based on empirical developments coupled with successive improvements, and on the expertise of the manufacturers. 

Some work has been done on photoreactor concepts that would collect sunlight using dish solar concentrators that direct concentrated sunlight onto the aperture of a light pipe or optical fiber system. An optical fiber system would then carry light into the reactor and disperse it evenly throughout the bioreactor volume. Such a... 

Fig. 8.10 The Enviolet photoreactor module consists of a borosilicate glass pipe with a MP Hg lamp (P e i = 12 kW). Special flow regulators create a rotational flow of water with Reynolds numbers Re of up to 10. Thus, a highly turbulent and abrasive flow condition is maintained throughout the UV treatment that prevents deposits on the quartz sleeve reproduced by permission of a.c.k. aqua concept GmbH (Karlsruhe, Germany).

Although different solar photoreactors have been developed in the last 20 years, each one with its own advantages and limitations, there is still room for new designs and innovative ideas. Nowadays, CPC appear as one of the most promising alternatives among solar photocatalytic reactors. The concepts of nonimaging optics, with its emphasis in efficient energy collection can also be a very useful in future developments. 

Although these results are preliminary, they demonstrate the feasibility of the concept of photocatalytic microstructured reactors, and their potential to increase by various orders of magnitude the catalyst coated surface per reaction liquid volume [124[. A critical problem to solve is the maximization of the illumination efficiency. The availability of a multitude of low-intensity light emitting sources on the micro- or even nanoscale near the catalyst particles appears to be a novel and promising approach to achieve uniform and maximized illumination. Combination of the latter approach with equipment to overcome the mass transfer limitations may prove to be the significant improvement that photoreactors need for industrial implementation. 

The general methodology for modeling slurry photoreactors has been reviewed by Cassano and Alfano (2000). We will apply these concepts to the evaluation of absolute, true values of quantum yields. 

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