Internal and External Mass Transport in Isothermal Pellets

4 Internal and External Mass Transport in Isothermal Pellets 

For a simple irreversible first order reaction we obtain  

If we introduce the ratio between the characteristic diffusion time in the pellet tp, and the external mass transfer time we will get a clear physical interpretation 

The overall effectiveness factor is defined as the ratio between the effective transformation rate and the rate at constant bulk concentration. 

For a catalyst in the form of a flat plate the effectiveness factor is given by i]p = tanh cpijcpi (Equation 2.174) and the overall effectiveness factor can be expressed as a function of the Thiele modulus and the Biot number. 

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