Role of Chemical Potential Profile

In comparing separation techniques, we generally find a striking difference in methods based on continuous (c) chemical potential profiles and those involving discontinuous (d or cd) profiles. There is, for example, a glaring contrast in instrumentation, applications, experimental techniques, and the capability for multicomponent separations between the two basic static systems, Sc (e.g., electrophoresis) and Sd (e.g., extraction). Similarly, there 

We expand on the separation power exhibited along the flow axis. We noted earlier that flow is a powerful transport mechanism, capable of carrying components rapidly over considerable distances. Flow is also capable of keeping the components in fairly compact zones by virtue of its power to evacuate component material from one region as it carries it into another. These capabilities, as noted in Section 7.7, stem from the flow transport term, -vdc/dx, in the basic transport equation, Eq. 3.30. When acting alone, this term gives 

The above idea takes more concrete form when expressed in terms of the number N of theoretical plates. When migration distance X is replaced by total path length L and the general velocity term W replaced by u, Eq. 5.43 takes the form 

Equation 9.2 is useful conceptually in showing the power of flow expressed through high us and Ls. The downside of increased flow is that it usually generates high Dt. Thus the benefit of a high v in Eq. 9.2 is often cancelled by a large DT term. 

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