Photochemical reactor design

Photochemical reactors designed for this purpose may either be linked to (flash) distillation columns or be part of the distillation column itself, the latter being an advantageous solution for continuous production units. 

Photochemical reactor design involves simultaneous solution of the mass, energy, and momentum balance equations (as in normal reactors) along with equations for the radiation field and energy source (which are specific to photochemical reactors). Two approaches are possible (1) the intensity of the incident light, irrespective of the source, is used as the inlet boundary condition incidence models)-, (2) the emission from the source itself is part of the mathematical description emission models). The first approach has been extensively used but suffers from the weakness that the incident light is a function of scale, and hence a priori design from laboratory scale data tends to be uncertain. The second approach is formally correct, and involves no such uncertainty. 

An important consideration in photochemical reactor design is the rapid attenuation of light intensity with the distance from the wall. It defines the zone in which much of the reaction occurs and is called the optical thickness. A useful expression was developed (Fischer, 1978) for light intensity as a function of thickness d for 99% absorption from a lamp of radius r. ... 

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 ... 

This geometry of irradiation makes the most efficient use of the light emitted by an extended light source. In fact, this geometry is used in all immersion-type photochemical reactors, and most industrial photochemical production units are based on this design. 

Figure 20. Batch process design using several photochemical reactors (hv) in a parallel arrangement linked to a central reservoir (R) [18].

In designing modules of mono- or multilamp immersion-type photochemical reactors, again the concept of convergence of light distribution and reactor geometries is followed, and knowledge of light penetration in a suspension of optimal photocatalyst concentration is therefore essential. Optimal thickness of annular irradiated reaction volume is best determined by a spherical probe under conditions where only absorption by the photocatalyst has to be taken into account [12, 78, 98, 99]. The radiant power P = f(r) within the limits of r and rR, respectively, has been simulated by the Monte Carlo method on the basis of... 

Within the general concept of the ESVE models, Alfano et al. conceived a model for the radiant power profile of a tubular light source located in the focal axis of a parabolic reflector in order to analyze the design of a cylindrical photochemical reactor irradiated from the bottom [118]. Differences between experimental and calculated (ESVE) results were always less than 15%. 

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. 

Once the basic mechanism of photolysis [reactions (18) to (20)] is established, the kinetics of the photochemical reaction can be studied. The kinetics of photochemical reactions is dependent on factors such as the intensity and wavelength of the incident radiation, the optical path of the radiation, and the nature of the compound irradiated and the solution in which it is present. The performance of UV radiation will also depend on the photoreactor design. For example, in a batch photochemical reactor, the rate of compound removal due to direct photolysis, assuming the mechanism of reactions (18) to (20), is as follows [95] ... 

Jacob SM, Dranoff JS. Design and analysis of perfectly mixed photochemical reactors. Chem Eng Prog Symp Ser 1968 64 54-63. 

It is clear from this discussion that the design of mechanically agitated photochemical reactors requires a knowledge of eg, db, and aL. These parameters can be estimated from the correlations reported in Section II. For the multilamp reactor, the effects of internals on eg, db, and a L should be appropriately evaluated. No similar, reliable design procedures have yet been developed for solid-fluid photocatalytic reactors. 

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