Chromatography nonequilibrium effects

The apparent dispersion coefficient in Equation 10.8 describes the zone spreading observed in linear chromatography. This phenomenon is mainly governed by axial dispersion in the mobile phase and by nonequilibrium effects (i.e., the consequence of a finite rate of mass transfer kinetics). The band spreading observed in preparative chromatography is far more extensive than it is in linear chromatography. It is predominantly caused by the consequences of the nonlinear thermodynamics, i.e., the concentration dependence of the velocity associated to each concentration. When the mass transfer kinetics is fast, the influence of the apparent axial dispersion is small or moderate and results in a mere correction to the band profile predicted by thermodynamics alone. 

In this system the bands (zones) broaden because of diffusion effects and nonequilibrium. This broadening mechanism is fairly symmetrical and the resulting elution bands approach the shape of a Gaussian curve. This system best explains liquid or gas partition chromatography. The system may be viewed in two ways ... 

Assuming that zone broadening is caused only by nonequilibrium, calculate the effective diffusion coefficient for a zone in gas chromatography using the typical parameters teq = 1(T2 s, v = 10 cm/s and R = 0.20. After 1 min calculate how far the zone has migrated and its width 4[Pg.248]

There is a very important difference, however. Notice, particularly, that the width of the three bands in any of the three figures is independent of the D values. You see, in using the binomial expansion to represent the results of a multistage separation process, only the role played by equilibrium considerations was considered. To obtain results that match experimental reality, however, this must be amended to include the effects of diffiision and other related nonequilibrium processes. This will be dealt with in the next section, which concerns chromatography. 

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