Photochemical reactor design systems

The reaction system is one of the most important parts of choosing a basic module for the design of a photochemical reactor. This might be explained by enumerating some of the corresponding parameters and describing their impact on reactor geometry and operational conditions ... 

Figure 1. Schematic diagram of a reactor designed for long path-length interferometric absorption measurements upon a photochemically induced reaction system (Courtesy of K. H. Becker). 

The two broad classes of photochemical reactors are the batch processors and the continuous processors. The batch processor is simple in design, but costly in operation, because it requires the loading of the reactant, the unloading of the product and the cleaning of the reactor vessel all operations which involve human intervention. Batch processing is used as a rule in laboratory synthesis, but industrial applications prefer continuous systems for reasons of efficiency. Still, it must be accepted that batch processing will be used for many small-scale industrial syntheses. 

Chemat and his coworkers [92] have proposed an innovative MW-UV combined reactor (Fig. 14.7) based on the construction of a commercially available MW reactor, the Synthewave 402 (Prolabo) [9[. It is a monomode microwave oven cavity operating at 2.45 GHz designed for both solvent and dry media reactions. A sample in the quartz reaction vessel could be magnetically stirred and its temperature was monitored by means of an IR pyrometer. The reaction systems were irradiated from an external source of UV radiation (a 240-W medium-pressure mercury lamp). Similar photochemical applications in a Synthewave reactor using either an external or internal UV source have been reported by Louerat and Loupy [93],... 

When dealing with the design of the equipment for carrying out a photochemical reaction, several aspects must be considered. Some of them are common to the design of conventional thermal reactors, such as the kinetic characteristics of the reactions involved, the phases of the system, the necessity... 

The main difference between photochemical and thermal reaction is the presence of a radiation-activated step. The rate of reaction of this step is proportional to the local volumetric rate of energy absorption (LVREA). For any emission model, the LVREA is a function of the spatial variables, of the physical properties and geometrical characteristics of the lamp-reactor system, and some physicochemical properties of the reacting mixture. The most important design parameter that is pertinent in photochemical and photocatalytic reactions is the effective attenuation coefficient. 

The insertion of a photochemical derivatization system between the chromatographic column and the detector carries the risk of band broadening. To minimize the danger of this various different designs have been tried out [54,55], and the latest, a commercially available photochemical derivatization reaction module, consists of Teflon tubing formed into a KOT reactor wrapped round the UV lamp (Figure 7). 

Regarding the photocatalyst structural configuration, thin-film powder layer and/or fluidized bed, coated wall-parallel, and honeycomb/foam monolithic reactors are probably the most representative. For photochemical water splitting, batch-type photoreactor is most frequently used configuration in lab-scale investigations. In the case of solar photoreactor systems, there are two of the major design issues (i) whether to use a suspended or a supported photocatalyst and (ii) whether to use concentrated or non-concentrated sunlight. 

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