Microscopic view, mass transfer, diffusion

In choosing between these two models, one needs to consider the specific process. The use of mass transfer coefficients represents a lumped, more global view of the many process parameters that contribute to the rate of transfer of a species from one phase to another, while diffusion coefficients are part of a more detailed model. The first gives a macroscopic view, while the latter gives a more microscopic view of a specific part of a process. For this reason, the second flux equation is a more engineering representation of a system. In addition, most separation processes involve complicated flow patterns, limiting the use of Pick s Law. A description of correlations to estimate values of k for various systems is contained in Appendix B. 

From a microscopic point of view, devolatilization of aqueous-phase dispersions is a mass-transfer process, which involves the following steps (1) diffusion of the VOCs to the particle surface (2) transfer from the polymer surface to the aqueous phase (3) diffusion through the aqueous phase and (4) transfer from the aqueous phase to the gas phase (Figure 18.6) [72]. 

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