Diagnostic Experimental Criteria for the Absence of Internal

Petersen [1965a, b] has demonstrated that with realistic values of the mass transfer and diffusion parameters, external transport limitations will never exist unless internal diffusion limitations are also present. 

A comprehensive study by Mehta and Aris [1971] provides graphs for nth-order reactions. A brief summary of their results follows  

12 DIAGNOSTIC EXPERIMENTAL CRITERIA FOR THE ABSENCE OF INTERNAL AND EXTERNAL MASS TRANSFER LIMITATIONS 

In kinetic studies it is advised to avoid experimentation in which external transport limitations are experienced. These can be easily avoided by adapting the flow rates. Yet, Koros and Nowak [1967] and Madon and Boudart [1982] have drawn attention to the lack of generality of the usual diagnostic test for external mass and heat transfer limitations. In this experimental test, the conversion (or the rate) is measured at constant space-time, W/Fao, but the flow velocity is varied by adapting simultaneously ITand Fao- If the conversion (or the rate) is not affected, the conclusion is drawn that there are no external transport limitations. At the low Reynolds numbers encountered in laboratory reactors, the mass and heat transfer coefficients are quite insensitive to changes in flow rates, so that these have to be varied over a very wide range. 

The experimental test for the effect of internal mass transfer that compares rates at various particle sizes has to be interpreted with skill, because changes in the flow velocity may affect the heat transfer between the bed and the wall and, therefore, the temperature field. What Koros and Nowak [1967] 

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