Columns 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). 

Capacity parameters (C) i.e. those parameters which affect the phase ratio film thickness, surface area, column diameter (open columns), porosity (packed columns). 

In packed columns, there are two parameters which may be varied independently in order to optimize the column characteristics, i.e. the diameter of the column and the diameter of the particles. In open columns, only the column diameter may be varied. Additionally, the phase ratio may be varied by changing one of the capacity parameters (see section 3.5). For packed columns these parameters include the surface area of the packing material, the column porosity and the stationary phase film thickness. For open columns only the latter parameter is relevant. 

Equation (2) is valid for surface potential of up to 50 mV. For packed capillary columns, the dependence of electroosmotic flow velocity on operating parameters based on Eq. (2) was examined and it was found that electroosmotic flow velocity increases with column porosity, particle diameter, and concentration of bulk electrolyte [21],... 

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

In this equation, c is the concentration in the fluid phase and q is the quantity in the solid phase. The column porosity e (expressed as phase ratio f = (1 -e)/e) defines the fraction of the fluid phase in the column. Furthermore, u stands for the linear velocity and t and x are the time and space coordinates, respectively. All contributions leading to band-broadening are lumped in a simplifying manner into an apparent dispersion coefficient, D p. In equation (21-2), it is assumed that the two phases are constantly in equilibrium expressed by the adsorption isotherms. Due to the nonlinear character of the isotherm equations, the solution of equation (21-2) requires the use of numeri-... 

The above numbers have been estimated by assuming as follows Column porosity = 0.7 retention factor = 0 and theoretical plate number = 10,000. 

A packed column in gas chromatography had an inside diameter of 5.0 mm. The measured volumetric flow rate at the column outlet was 50 mL/min. If the column porosity was 0.45, what was the linear flow velocity in cm/s ... 

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)... 

The apparent particle density, pp, is the ratio of the particle weight to the particle volume, or sum of the imaccessible volumes and the internal pore volume. This is the most practical definition. However, it carmot be as accurate as densities usually are because of the difficulty associated with defining accurately the external column porosity (see previous subsection),... 

Since e is usually between 0.65 and 0.80, the fluctuations of F will be several times larger than those of e. This has important consequences as it magnifies the effect of column to column porosities [26,45,50,51]. In the following we consider that each column has its own value of the phase ratio, F,. ... 

In the foUowing discussion, we examine only the effect of differences in the column porosity. Let be the porosity of column k. As discussed earlier Eqs. 17.51 and 17.52 define a sufficient number of criteria to allow the correct choice of the operating conditions in an SMB operating tmder linear conditions. This set of conditions is equivalent to the one derived by Storti et al. [16], the so called triangle method (see Subsection 17.10). However, both sets of conditions are based on the assiunption that all columns have identical characteristics and an infinite efficiency. 

Figure 17.17 Evaluation of the separation area of the Troger s base enantiomers. Solid line Anal5Ttical Solution with the Henry coefficients derived from the analytical chromatograms. Dashed line Analytical Solution with the Best Fit Langmuir Isotherm Data. Symbols +, calculated Umits of the separation area based on the correct isotherm and the true porosity of each column o calculated limits of the separation area based on the average column porosity experimental conditions of the experiments performed. K. Mihlbachler, A. Seidel-Morgenstem, G. Guiochon, AlChE J., 50 (2004) 611 (Fig. 6). Reproduced by permission of the American Institute of Chemical Engineers. 1997 AlChE. All rights reserved.

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