Criterion for Neglecting Radial Diffusion

The importance of diffusion in a tubular reactor is determined by a dimensionless parameter = 3j L/(uR ), which is the molecular diffusivity of component 

A scaled by the tube size and flow rate. If SSJ/R is small, then the effects of diffusion will be small, although the detinition of small will depend on the specific reaction mechanism. Merriii and Hamrin (1970) 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 crosslinking reactions where a few events can have a large effect on product properties. 

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