External mass transfer and intraparticle diffusion limitations

4 EXTERNAL MASS TRANSFER AND INTRAPARTICLE DIFFUSION LIMITATIONS 

In a catalytic reactor, the fluid flows through the catalyst particles and may face a resistance caused by the concentration gradient between the bulk fluid and the external particle surface. This resistance (interparticle or external mass transfer limitation) must be added to intraparticle diffusion limitation. 

The external concentration Cao is known (it is the bulk concentration), but a concentration gradient ranging from Ca to Cas may exist within the thin film around the particle, Cas being concentration at the external surface of the particle or at the 

Curve n shows a lower concentration gradient outside the particle, compared to curve 1, but followed by a concentration gradient inside the pore due to the intraparticle diffusion limitation. On the other hand, curve III does not have influence of external limitations, representing the reaction rate in the presence of pore-diffusion resistance as shown in the following equation  

The effectiveness factor depends on the Thiele modulus defined by Equation 18.13. 

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Defining a new factor that comprises both external mass transfer and intraparticle diffusion limitations (Fogler, 2000) ... 

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