A Criterion for Neglecting Diffusion

The importance of diffusion in a tubular reactor is determined by a dimensionless parameter, 3 Ai/R2 = aL/(uR2), which is the molecular diffusivity of component A scaled by the tube size and flow rate. If SCff/- 2 is small, then the effects of diffusion will be small, although the definition of small will depend on the specific reaction mechanism. Merrill and Hamrin1 studied the effects of diffusion on first-order reactions and concluded that molecular diffusion can be ignored in reactor design calculations if 

The Merrill and Hamrin criterion was derived for a first-order reaction. It should apply reasonably well to other simple reactions, but reactions exist that are quite sensitive to diffusion. Examples include the decomposition of free-radical initiators where a few initial events can cause a large number of propagation reactions, and coupling or cross-linking reactions where a few events can have a large effect on product properties. 

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