Criteria for the Estimation of Transport Effects

For the catalyst development and optimization as well as for the correct reactor design, it is important to ascertain the influences of transport phenomena on the reaction kinetics. It is essential that criteria for estimating transport effects are based on what is measurable or observable [31,32]. 

One way to estimate the influence of transport processes is to use directly experimental results observed under given experimental conditions. In general, the experimentalist has information concerning observed reaction rates, bulk reactant concentrations, and temperature, as well as the catalyst pellet form and dimensions. With these details at hand, the Weisz module can be estimated. For example, for spherical catalytic particles, see Equation 2.180. 

The most general of the criteria (Equation 6 in Table 2.2) ensures the absence of any internal and external concentration and temperature gradient. But a problem may arise because of compensation between mass and heat transport. This situation will occur if y Therefore, it may be better to respect separately the criteria for isothermicity. 

It is disturbing that criteria for the absence of heat effects are based on the true activation energy, which is not observable, if mass transfer affects the rate of reaction. A critical discussion of the experimental results and a prudent application of the criteria are, therefore, indispensable. 

In this chapter, the fundamentals of chemical reaction engineering are presented. The basic definitions along with the material balance of different types of ideal reactors and their design equations are discussed. 

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In contrast to the experimental criteria of Tables 2 and 3, which apply to power law kinetics only, the criteria of Tables 4 and 5 can be used for arbitrary forms of rate expressions, although normally not for systems with more than one reaction. It is interesting to note that, in a recent publication, Gonzo [40] reports on the use of general theoretical criteria for the estimation of transport effects in networks of multiple catalyzed reactions. 

A substantial number of a priori criteria for the estimation of transport effects on catalytic reaction rates has been reported by a number of workers. These criteria are generally derived on the premise that one does not desire the net transport effect to alter the true rate by more than some arbitrarily specified amount, normally 5%. Because of the uncertainty involved in knowing some of the parameters, the philosophy in applying these criteria should be conservative one does not look to marginal satisfaction of their requirements, but perhaps to be an order of magnitude better. 

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