Photoreactor geometry

Another area which has gained interest in recent years is the utilization of supported catalyst in solar photoreactors, as opposed to suspended catalyst. Different geometries have been proposed for supporting the catalyst, which are strongly linked to photoreactor geometry. Results have been in some cases very good, despite of the fact that traditionally suspended catalyst has been considered more effective. 

Figure 9 Different photoreactor geometries compared by Bandala et al. 2004, reprinted with permission from Elsevier.

As discussed previously, several solar photoreactor geometries can be reduced to cylindrical glass tubes externally illuminated by different types of reflectors, like parabolic troughs, CPC, V-grooves, or without reflector, directly illuminated by the sun. In this section the general solution of the PI approximation for this t)q5e of photo reactors is reported. This general solution is applicable to any particular reactor if the flux distribution impinging on the wall of the tubular reaction space is known. 

For preparative purposes, increased conversions can be obtained by increasing the radiant flux with a corresponding adaptation of the size and geometry of the photoreactor and by diluting the reactant in an inert solvent such as acetonitrile. 

Fig. 4.16 Schematic representation of an excimer flow-through photoreactor. Two excimer lamps are coupled via flanges. They are used with inward directed radiation geometry (Oppenlander et al., 1995).

Particular problems of photochemical engineering are related to the scaling-up of photoreactors. This is mainly due to problems of lamp technology related to the variations of the radiant exitance M with the increase of the lamp s geometry and electrical input power. Thus, to carry out a reasonable scaling-up and optimization of photoreactors the radiant exitance M or the radiant density (expressed as the ratio of radiant power P to the arc length I of the lamp in W cm , see Tab. 4-1) of the lamps used must be fixed (Braun et al., 1993 a). This, however, is a challenge for the manufacture of lamps. 

The intramolecular aziridination of 2-(alkenyl)phenyl azides was best performed under pho-tolytic conditions116-117, generating the nitrene (ca. 0.001 M solution in cyclohexane, 350 nra, Rayonet Photoreactor)118. The nitrene addition reaction proceeded with complete diastereose-lectivity, the double bond geometry was retained in the aziridine thus produced. The alkaloid ( )-virantmycin was synthesized from the aziridine 2117. 

Chemical actinometry is especially suitable for photochemists, since the actinometric solution can be substituted by the sample of interest without changing the geometry and experimental conditions. This is of special value in the case of liquid solutions. By these means the problems with multiple reflection in solution or at thin layers as well as with inhomogeneous distribution of the irradiation or special geometries of the photoreactor can be overcome. 

Figure 6.1 Geometry of the continuous flow, annular photoreactor. Adapted from Cassano...

A pilot plant scale, tubular (annular configuration) photoreactor for the direct photolysis of 2,4-D was modeled (Martin etal, 1997). A tubular germicidal lamp was placed at the reactor centerline. This reactor can be used to test, with a very different reactor geometry, the kinetic expression previously developed in the cylindrical, batch laboratory reactor irradiated from its bottom and to validate the annular reactor modeling for the 2,4-D photolysis. Note that the radiation distribution and consequently the field of reaction rates in one and the other system are very different. 

In addition to previous parameters, the choice of the heterogeneous photoreactors is related to their geometry and materials in order to 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 the depth of radiation penetration in a complex way. 

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 appUcatimis. 

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