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Prater Criterion

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Prater Criterion

The Weisz-Prater Criterion is a method used to estimate the influence of pore diffusion on reaction rates in heterogeneous catalytic reactions.[1] If the criterion is satisfied, pore diffusion limitations are negligible. The criterion is
N_{W-P} = \dfrac{\mathfrak{R} R^2_p}{C_s D_{eff}} \le 3\beta
Where \mathfrak{R} is the reaction rate per volume of catalyst, R_p is the catalyst particle radius, C_s is the reactant concentration at the particle surface, and D_{eff} is the effective diffusivity. Diffusion is usually in the Knudsen regime when average pore radius is less than 100 nm.
For a given effectiveness factor,\eta, and reaction order, n, the quantity \beta is defined by the equation:
\eta = \dfrac{3}{R^3_p} \int_{0}^{R_p} [1-\beta (1-r/R_p)^n] r^2\ dr
for small values of beta this can be approximated using the binomial theorem:
\eta = 1-\dfrac{n \beta}{4}
Assuming \eta \ge 0.95 with a 1st or zero order reaction gives values of \beta, 0.6 and 6 respectively. Therefore for many conditions, if N_{W-P} \le 0.3 then pore diffusion limitations can be excluded.[2]

References

  1. ^ Weisz, P. B.; Prater, C. D. (1954). "Interpretation of Measurements in Experimental Catalysis". Advances in Catalysis. Advances in Catalysis 6: 143.  
  2. ^ Vannice, M. Albert (2005). Kinetics of Catalytic Reactions. New York: Springer Science+Business Media. pp. 63–65. 
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