Diffusional falsification

Rosner DE. The apparent chemical kinetics of surface reactions in external flow systems Diffusional falsification of activation energy and reaction order. AIChE Journal 1963 9 321-331. 

For a PFR, c k,t) = Cf k)QKt()[-kri k)f where /7(k) is the catalyst effectiveness factor. Denoting as the overall asymptotic order for the mixture controlled by diffusion. Ho et al. showed that = ( + l)/2, a relationship similar to that in the single-reactant case. Hence diffusional falsification occurs for mixtures with n. Since > 1 in general, so n, < Gosselink and Stork " found for the HDS of a gas oil that the overall order changed from two to 1.4 in going from ground-up to 3 mm catalyst particles. Stephan et al." observed that the overall order for powder catalyst is three, vs. two for 5 mm pellets. 

However, cracking of a pellet or ore lump may increase the overall rate of reaction (for that particular pellet) because it provides an easy access by the gaseous reactant to the solid by reducing the diffusional resistance. It is important that investigators be aware of cracking phenomena in kinetic studies because these might lead to the falsification of the kinetic data and could also signal possible difficulties for the operation of industrial or even pilot scale units. 

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