Criteria for Importance of Diffusional Limitations

We have now seen how possible pore diffusion problems can be evaluated compute the generalized modulus and use Figs. 3.6.a-3 or 3.6.b-l to see if the value of t) is less than unity. In the design situation this procedure can be used since is presumably known, but when determining kinetic constants from laboratory or pilot plant data this can t be done since k is udiat is being sought. Thus, criteria for the importance of pore diffusion, independent of k are also useful. There are two main types that are generally used. 

The first is a classical test and involves performing experiments with two sizes of catalyst Then from Eq. 3.6.b-8, if one assumes that and D, are the same (this 

the two sizes will give two different values to the moduli (only the ratio can be determined from Eq. 3.6.C-1) and so if the two observed rates are the same, til = tj2 and the operation must be on the horizontal part of the curve (i.e., no pore diffusion limitations). At the other extreme, tj = l/ , and so 

Therefore, in this case, the observed rates are inversely proportional to the two pellet sizes. For intermediate degrees of pore diffusion limitation, the ratio of rates will be less than proportional to L2/L1. A graphical procedure was given by Hougen and Watson [99] in which a line of coordinates to 

The other method, which can be used for a single particle size, is called the Weisz-Prater criterion [93] and is found for a first-order reaction by solving Eq. 3.6.a-16 for k  

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