Solar photocatalytic reactors

Solar-grade silicon, production of, 22 507-508 Solar heat control, use of gold in, 12 703 Solarization effect, 19 203 Solar photocatalysis, 23 23-24 Solar photocatalytic detoxification, 19 76 Solar photocatalytic processes, 19 100-101 Solar photocatalytic reactor, using deposited titania, 19 99 Solar photoreactors, 19 95-99 Solar salt harvesting, 22 802, 806-808 Solar spectrum, 23 2 Solar still, 26 89-92 Solar thermal converters, 23 10-13 Solar transmittance, for thin films, 23 19 Solatene, 24 558 Solder, 3 53... 

Camilo A. Arancibia-Bulnes, Antonio E. Jimenez, and Claudio A. Estrada, Development and Modeling of Solar Photocatalytic Reactors... 

The modeling of radiation absorption within solar photocatalytic reactors has been addressed by some researchers, but is one of the areas where less work has been carried out. Both heuristic and first principles methods. 

Different types of solar photocatalytic reactors have been developed over time. Two main distinctions have been made (i) concentrating versus nonconcentrating reactors and (ii) slurry versus fixed catalyst reactors. These two classifications are not mutually exclusive, and other categories may also be established, for instance, a very important is the one of tubular solar photoreactors. 

A second characteristic of UV solar radiation is fhat, even for very clear afmospheres, if is composed in similar amounfs of both beam and diffuse radiation (Hulstrom et al., 1985). The first is defined as the radiation arriving directly from the sun, while the second is the solar radiation that has been scattered by gases and aerosols after entering the earth s atmosphere. This second t)/pe of radiafion reaches fhe ground in a more or less diffuse manner that is, with similar intensity from all directions in the sky. In this respect, the situation encountered in solar photocatalytic reactors is quite different from fhe one encounfered in solar fhermal collectors. The latter are able to use the whole solar spectrum, and in that case diffuse radiation accounts for a much smaller fraction of fhe global irradiance. 

Figure 8 Reaction rate optical factor as a function of catalyst concentration for a parabolic trough solar photocatalytic reactor. Adapted from Arancibia-Bulnes and Cuevas 2004, with permission from Elsevier.

The MCM has been used to simulate tubular solar photocatalytic reactors, like parabolic troughs (Arancibia-Bulnes et al., 2002a), CPC (Arancibia-Bulnes et al., 2002b), and also of flat plate geometry (Cuevas et al., 2004). Also it has been used to simulate flat lamp reactors (Brucato et al., 2006) or to obtain optical coefficients by comparison with transmission results from an experimental cell (Yokota et al., 1999). 

Even though this chapter is devoted mostly to solar photocatalytic reactors, we would like to discuss the modeling of an annular lamp reactor, as a different example of the application of the PI approximation. This problem was studied (Cuevas et al., 2007) with reference to a particular reactor known as photo CREC-water II (Salaices et al., 2001, 2002). Equation (38) is again written in cylindrical coordinates. Nevertheless in this case the... 

This chapter reviews some of the main topics involved in the design and modeling of solar photocatalytic reactors, with particular emphasis on the authors research experience. Solar photons are source of energy that initiates photocatalytic degradation. Thus, proper consideration of radiative processes is key to address this subject. The determination of the directional and spectral characteristics of solar UV radiation, the interaction of the catalyst with radiation inside reaction spaces, the optical design of solar collectors, and the optical properties of the materials involved are all subjects where these concepts are necessary. Therefore, developments in this area should be solidly grounded on the fields of solar collector optics and radiative transfer, besides the more traditional chemical engineering aspects involved. This requires a multidisciplinary approach. 

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. 

Braham, R. J. Harris, A. T. Review of Major Design and Scale-up Considerations for Solar Photocatalytic Reactors. Ind. Eng. Chem. Res., 2009, 48, 8890-8905. 

Most of the reactor designs tested for the photooxidation of organic pollutants by solar radiation are Ti02 slurry reactors. The implementation of solar photocatalytic reactors has occurred concurrently with advances in thedesign of solar thermal collectors, given the important characteristics shared by these units. There are, however, specific constraints for the design of solar photocatalytic reactors. 

Solar photocatalytic reactors can be operated in either continuous single pass mode or discontinuous batch mode. In the continuous single pass mode, complete oxidation of the contaminant is achieved in a single pass with the water flow rate being adjusted for fixed solar flux densities. On the other hand, batch mode operation requires a set volume of water to be treated with varying solar flux densities (Alfano et al., 2000). Table 2.4 reports the main types of solar reactors reported in the technical literature. 

Regarding solar photocatalytic reactors the following characteristics and performance indicators can be mentioned... 

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