Nonuniform separation systems

An extended discussion of zone spreading and plate height in nonuniform separation systems is beyond the scope of the present chapter. However, later we account for flow nonuniformities due to gas compressibility in gas chromatography. More generally, the treatment developed by the author [8,16] for chromatographic columns can be expanded to describe most other zonal systems. 

The strength of flow is that it provides the most powerful and versatile mechanism of transport available for separative displacement. The weakness of flow—other than its nonselectivity—is its nonuniformity. For most flow systems, v varies widely from point to point in the flow space. The different us carry component molecules downstream at different rates, thus leading to the broadening of component zones. We must understand the fundamentals of flow in order to control this broadening while still enjoying the significant advantages of flow transport. 

The origin of DT is interesting. We have already noted that F( + ) systems are selective for different solutes because the enrichment processes acting in a direction perpendicular to flow work in concert with (and require) nonuniform flow. However, it is the nonuniformity in flow that tends to increase DT in Eq. 9.2, thus reducing separative efficiency. The positive and negative sides of nonuniform flow require that optimization be undertaken carefully. More details are given below. 

The separation efficiency of an asymmetrical flow FFF system has been known to be higher than that of a conventional symmetrical channel. Because an asymmetrical channel utilizes only one frit, nonuniformity of flow that could arise from the imperfection of frits can be reduced. In addition, the initial sample band can be kept narrower in an asymmetrical channel, due to the focusing-relaxation procedure, which is an essential process in an asymmetrical channel. The relaxation processes, which provide an equilibrium status for sample components, are necessary in both symmetrical... 

Obsolete chemical weapons that have been in storage since the decades following World War II constitute the U.S. chemical stockpile and are differentiated from nonstockpile materiel. Facilities in the United States that have been constructed to destroy this stockpile employ assembly line systems for separating the agent from the munition. This is feasible because the munitions are overwhelmingly in a good and consistent condition. Leakers and other occasional nonuniform munitions that are periodically encountered can cause problems out of proportion to their numbers, however. 

Dispersion, the tendency for ordered molecules to decrease gradients and local concentration, is caused by both molecular diffusion and nonuniform bulk liquid motion. High dispersion rates may be advantageous for mixing and chemical reactions, but are undesirable in separation and purification applications. For separations, minimizing dispersion improves resolution and sensitivity [3] and yields improved dynamics for concentration and purification [4]. As a consequence, the physical processes that lead to dispersion have been a subject of intense interest for more than a century. In recent years, the development of the concept of the micro-total analysis system (p,TAS) or labs on a chip has motivated further exploration of dispersion in microchannel flows. 

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