Rate definition

Figure 7.17 shows a summary of the available conditions of water electrolysis [72]. For each configuration there exists a range of performance. Conventional electrolyzers, which nevertheless are still the most common in the current production of H 2 on the intermediate and small scale, show high overpotential and a relatively small production rate. Membrane (SPE) and advanced alkaline electrolyzers show very similar performance, with somewhat lower overpotential but a much higher production rate. Definite improvements in energy consumption would come from high temperature (steam) electrolysis, which is, however, still far from optimization because of a low production rate and problems of material stability. 

Following the reaction rate definition of the form given in eq. (3.1), if component i is a reaction product the rate is positive if it is a reactant that is being consumed, the rate is negative thus, the rate of disappearance of the reactant is -r,. In environmental applications, as we are interested in the disappearance of a pollutant, the rate is expressed as -r, which is positive. The rate of disappearance is used in Chapters 3 and 5, where for simplicity it is referred to as the reaction rate. 

It is noteworthy that the form of the rate (r = / (state of the system) does not actually depend on our choice of reaction rate definition. Only the rate coefficients and their dimensions change with each rate definition (Levenspiel, 1972). 

Stage IV Declining Rate. Definition. The declining rate stage is defined as a period of decreasing rate of reaction which tends to approach a limit in the amount of reactant consumed, or product produced (zero rate), under the prevailing conditions of temperautre, irradiance, partial pressure of oxygen, concentration of reactants, etc. 

In catalysed reactions it is often more convenient to define the production rate per unit volume of catalyst or even per unit mass of catalyst, instead of the production per unit volume reaction volume. Eqn. (8.2) expresses the relations between the various rate definitions ... 

Wirr) or the in-adiated reactor volume (V,>r) should be carefully considered as two key parameters. This makes photocatalytic rate definition and kinetic parameter calculations phenomenologically sound and meaningful. 

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