Total column porosity

This is the fraction of the column volume that remains available for the mobile phase after packing. There are two contributions to the total column porosity. One part of the volume available to the mobile phase is in between the particles (interparticle space). For uniform, spherical particles this is about 40% of the column volume. The second contribution is due to the very porous structure of materials with large specific surface areas. This makes a significant part of the intraparticle volume available to the mobile phase (usually 20 to 30% of the column volume). 

Assumptions column dimensions, t = 100 mm, total column porosity e, = 0.7, skeleton porosity 5 = 0.3, silica skeleton density 2.2 g/cm. ... 

Other important information that has to be derived from the standard chromatogram are the column porosities and the void fraction. These parameters have to be carefully determined as they will be the basis for detailed modeling and simulation of the purification process. As illustrated in Fig. 2.4, the total volume of a packed column (Vc) is divided into two sub-volumes the interstitial volume of the fluid phase (Vint) as well as the volume of the stationary phase (Vads) (Eq. 2.4)... 

Total porosity, ej The ratio of the total mobile phase volume, Vm, to that of the total column volmne, Vc ... 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

The lowest theoretical interparticular volume of perfectly packed uniformly sized spherical beads is calculated to be about 26% of the total available volume. In practice, even the best packed columns still contain about 30-40% void volume in addition to the internal porosity of the beads. The problem of interparticular volume does not exist in systems in which a membrane is used as the separation medium. Both theoretical calculations and experimental results clearly document that membrane systems can be operated in a dead-end filtration... 

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