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Separation Process Engineering: Includes Mass Transfer Analysis, Third Edition by Phillip C. Wankat

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15.6 Difficulties with Fickian Diffusion Model

One difficulty with the Fickian diffusion model should be obvious from the discussion in Section 15.2.3. One has to select a somewhat arbitrary basis velocity or plane of reference to calculate the convective and diffusive fluxes. The values and indeed the meaning of the convective and diffusive fluxes may change when the basis velocity is changed. Although irritating, this difficulty is not considered to be major because the total fluxes of A and B, which is the purpose of the calculation, do not change.

There is an additional irritating problem. First, for liquid binary systems, DAB usually depends on concentration, and although Eq. (15-23) is often reasonably accurate, the activity coefficient ...

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