Homogeneous photochemical reactors

SCALING-UP OF A HOMOGENEOUS PHOTOCHEMICAL REACTOR WITH RADIATION ABSORPTION... 

The ideas described in this section can be easily extended to more complex reacting systems either from the chemistry point of view - for example to include the parallel oxidation reaction with hydrogen peroxide or ozone - or to deal with other lamp-reactor configurations. A comprehensive, tutorial review for homogeneous photochemical reactors has been published (Cassano etai, 1995) that provides most of the required methods. 

Equation (45) can be solved by applying different photoreactor models. The literature reports several photochemical reactor models for both homogeneous and heterogeneous reactors [11,108,109]. In practice, annular photoreactors are often used (see Fig. 5) therefore, models for this type of reactor are considered here. For other types of reactors, attention should be given to other publications [109]. 

The PTEF is a dimensionless quantity, as required by thermodynamic consistency. The PTEF definition can be broadly applied, covering various kinetic models and being appropriate for various photochemical reactors, either homogenous or heterogeneous. 

Several solid-state photochemical reactions have been investigated with polycrystalline samples suspended in solvents. Solvents such as water, where the reactant and the product are likely to be insoluble, are usually chosen and a surfactant is added to maintain the suspension. There are at least two apparent advantages to this method. First of all, photochemical equipment commonly used for fluid samples can be readily adopted to solid-state reactions. Secondly, it is expected that all microcrystals in a powdered sample will be homogeneously exposed to the incident light in a well-stirred reactor. Interestingly, while several examples of solid-to-solid reactions in suspended crystals have been documented, there are some cases where the solvent is incorporated into the phase of the final product. In a report by Nakanishi et al. [134] it was shown that p-formyl cinnamic acid (51, Scheme 33) forms mirror-symmetric dimers. While irradiation of crystals suspended in hexane gave amorphous cyclobutanes in 85% yield, suspension of the crystals in water gave a 100% yield of a crystalline photodimer with one water molecule of crystallization. 

The PTEF factor (PTEF= p = Qused/Qa) equates the used energy in the photochemical transfomiation and the photon energy absorbed by the photocatalyst. The PTEF is of general applicability with its application not being restricted to a specific chemical species, reaction order, reactor geometry or reactor type (i.e. homogenous or heterogeneous). 

As was discussed earlier, ozone plays an important part in the chemistry of the troposphere, where its excess is harmful, and in stratospheric chemistry, where its shortage is also detrimental. Ozone can decompose by several mechanisms thermal, photochemical, homogeneous catalysis reactions and under the action of solid surfaces. In the laboratory, the latter effect can be controlled by a suitable treatment of the reactor walls, as well as by a study of the rate of reaction as a function of the surface/volume ratio. In order to eliminate photochemical and homogeneous catalysis reactions, the chemical reaction must be carried out in the absence of radiations and catalytic additives, such as halogenated substances. The mechanism put forward to interpret the thermal reaction, can be written as follows ... 

Photochemical syntheses, as well as photodegradation studies, are usually performed in irradiation reactors a variety of different designs are commercially available, depending on the specific application requirements. For synthetic photochemistry, immersion well reactors are commonly used (Fig. 14.4). The source, typically a low or medium pressure mercury lamp, is housed in a double-walled quartz jacket, which allows water-cooUng and/or filtering of excitation radiation. The solution to be irradiated surrounds the lamp source, enabling homogenous irradiation. 

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