LVRPA photon absorption

LVRPA local volumetric rate of photon absorption... 

It is rather atypical that a photochemical reaction will proceed in a single molecular pathway. Thus, several elementary steps are involved. Normally, the majority of them are dark (thermal) reactions while, ordinarily, one activation step is produced by radiation absorption by a reactant molecule or a catalyst. From the kinetics point of view, dark reactions do not require a different methodological approach than conventional thermal or thermal-catalytic reactions. Conversely, the activation step constitutes the main distinctive aspect between thermal and radiation activated reactions. The rate of the radiation activated step is proportional to the absorbed, useful energy through a property that has been defined as the local volumetric rate of photon absorption, LVRPA (Cassano et ak, 1995 Irazoqui et al., 1976) or the local superficial rate of photon absorption, LSRPA (Imoberdorf et al., 2005). The LVRPA represents the amount of photons that are absorbed per unit time and unit reaction volume and the LSRPA the amount of photons that are absorbed per unit time and unit reaction surface. The LVRPA is a property that must be used when radiation absorption strictly occurs in a well-defined three-dimensional (volumetrical) space. On the other hand, to... 

Cm = 0.05 X 10 g cm, the reaction rate is proportional to the square root of the LVRPA throughout the reaction space. Consequently, in a photoca-talytic slurry reactor, where low, intermediate, and high photon absorption rates can coexist, the complete reaction rate equation must be used. 

Photon absorption rate by a material particle of the suspension. At this point we would like to know the LVRPA by the solid and to be able to isolate this value even if the liquid would also absorb radiation. To do this we need to model absorption by a material particle of the suspension. In the continuum mechanics sense, a material point in space is a volume for which every property can be well defined by a single value. For a catalytic suspension, it will be made of the liquid and the solid phases. Let us consider a small volume V of the suspension space representing this material particle. This volume is located at a point in space x (Figure 6.11). Any point inside V can be defined in terms of a local reference frame f. 

The LVRPA or the photon absorption rate as a function of position results in ... 

Three different but connected problems must be studied (i) the reaction kinetics model (ii) the development of the rate of electron-hole generation in a material particle of the solid suspension and (iii) the model for characterizing the radiation field to evaluate the local volumetric rate of photon absorption (LVRPA). Point (iii) has been already described in section 6.6.1 for quantum yield determinations. In the first part of this section, we will concentrate on problems (i) and (ii). 

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