Internal column porosity

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),... 

High performance monolithic columns were prepared from styrene and divinyl-benzene (PSDVB, 200 pm i.d.) (Oberacher et al., 2004). The monoliths possess 5-300 nm pores with porosity of ca. 50% and 20% for external and internal pores, respectively, with specific surface areas of 30-40 m2/g. The column showed permeability K= 3.5 x 10 15m2 in water and slightly less in acetonitrile. The pore size... 

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... 

Reversed-phase high-performance liquid chromatography (RP-HPLC) columns (C8, C18) from semipreparative (7 mm internal 0) down to capillary (<1 mm). Phase porosity of 300 A and granulometry of 7 p,m are preferred. 

The extraction column to be used in the process will have an internal gauze packing - Sulzer BX, that was chosen due to its large interfacial area, high porosity and low pressure drops. 

Thus, for a given sort of molecules and temperature, Z- explicitly depends only on the total surface area of the column and on the true flow-rate. It is evidently a result of general validity — the size and internal geometry of the volume (including its eventual packing) are irrelevant. Notice that a- must be the true surface area, which would take into account roughness and open porosity of real surfaces. 

Internal porosity. The total porosity, e or ej, is the volume fraction of the column that is available to the mobile phase. The external porosity, e, is the volume fraction of the column that is available to the mobile phase percolating through the bed while the internal or particle porosity, characterizes the volume available to the stagnant mobile phase. Chromatographers define the internal porosity of the column as the difference between the total and the external porosities. So, for chromatographers. 

The confusion that a combination of these different definitions can generate, together with the difficulties encoimtered in the determination of some of the column characteristics involved (particularly the internal and the external porosities) makes useful a careful consideration of these issues. Given the stage of sophistication that the modeling of chromatography has now reached, it is not possible to tolerate errors, confusions, or approximations in the definitions nor in the estimations of the critical parameters related to the porosities, the velocities, and the equilibrium constants, nor to accept that more errors be made in the estimation of these parameters than those that are always involved in any measurement process. 

Almost all stationary phases used in chromatography have a bimodal pore size distribution. The first mode corresponds to the macropores or throughpores that allow the percolation of the column by the stream of mobile phase. The second distribution corresponds to the mesopores that combine to give the conventional internal porosity distribution described in the previous section. The mesopores are responsible for most of the specific surface area necessary to provide the retention and the saturation capacity that are needed to permit the retention of the mixture components in a good solvent, a condition for chromatographic separation. Nonporous particles have been used with only moderate success because very weak solvents must be used to achieve sufficient retention, which often causes solubility problems, and the saturation capacity of these particles is small. The terms of macro- and meso-pores apply as well to columns made of packed particles and to monolithic columns. 

Morbidelli et al. [41] discussed a numerical procedure for the calculation of numerical solutions of the GRM model in the case of an isothermal, fixed-bed chromatographic column with a multicomponent isotherm. These authors considered two different models for the inter- and intra-particle mass transfers. These models can either take into account the internal porosity of the particles or neglect it. They include the effects of axial dispersion, the inter- and intra-particle mass transfer resistances, and a variable linear mobile phase velocity. A generalized multicomponent isotherm, initially proposed by Fritz and Schliider [34] was also used ... 

Internal or intraparticle porosity, Fraction of the column volume contained inside the particles. Also called the fractional pore volume. Fraction occupied by the stagnant mobile phase. 

Fc is calculated from the internal cross-section of a column, the average linear velocity of the mobile phase u, and where appropriate, a term, ej, to account for the porosity of the particles in packed columns, cj is approximately 0.4 for solid particles, 0.8 for porous packings and 1.0 for columns... 

Because of the smaller particle size and internal porosity of LC packings, the area covered by the molecular monolayer stationary phase is vastly greater than that of the thick polymer film stationary phase in GC columns of similar volume. This restores the analyte capacity of these columns to a level compatible with typical chromatographic detector designs. 

Packed columns are as important as tray columns in the process industiy. Due to novel developments of packing elements the industrial use of packed columns is steadily increasing. In packed columns there exists a genuine countercurrent flow of gas and liquid as is shown in Fig. 5.4-14. An intimate contact between gas and liquid phases is established by packings that represent a solid structure with high porosity and large internal surface. The liquid proceeds downward in form of thin films or rivulets. Decisive for a good performance are a low pressure drop of the gas and a Uquid flow that is uniform over the cross section of the column. 

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