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242 3. Heterogeneous Processes and Reactor Analysis

Then, the combined (transitional) dif f icient is fusion coef

1 1
D t (3.604)
D F D k

v s equation: The tortuosity could be ealuated using the Bruggemann’
1
(3.605)
p

p
or using the Mackie and Meares equation (Perry and Green, 1999),
(2 ) p
(3.606)
p

p
For gel-type exchange resins and for macropore resins with a low degree of cross-
linking, the following equation is proposed for the solid diffusion coef icient (Helf f ferich,
1962):
  2
D s D f  p  (3.607)
 2 p 

which is Fick’s or molecular diffusion in large pores, as defined before.
Although there are a lot of data in the literature regarding diffusion coeficients in liq- f
uids or their calculation from molecular properties (Appendix I, Section I.2), it is not the
case for diffusion coeficients in solids, where the phenomena appearing are more com-
f
plex. In solids, the molecule may be forced to follo w a longer and tortuous path due to the
blocking of the cross-sectional area, and thus the diffusion is somehow impaired. Se eral v
models have been developed to take into consideration this effect in the estimation of dif-
fusion coefficients, leading, ho to a variety of results. er v , we