Column resistance factor

For mechanically rigid and physically-chemically robust particles, the relationship between column performance, in terms of the separation impedance or column resistance factor (CRF), such that pressure drop and the flow rate is linear over a wide range of flow rates and inversely proportional to square of the particle size, and can be approximated by the relationships... 

In practice, columns are usually compared in terms of the chromatographic permeability (K) and the column resistance factor (4). K is defined by ... 

The terms (1/1 SOS Xef /(1 — Sff)

). This parameter is known as the Knox quality of a column. Rearranging Eqs. (6) and (7) shows the relationship between pressure drop and particle... 

In pHPLC, there are numerous types of columns used. The comparison and characterization of these columns are often discussed in terms of thermodynamic properties and kinetic characteristics. The retention factor, k, selectivity, a, and the peak asymmetry are believed to be representative parameters for the thermodynamic properties, while the kinetic characteristics are often expressed in dimensionless magnitudes of reduced plate height, h, separation impedance, E, and flow resistance factor, ( ). 3... 

When the sample is introduced into the column, usually in the form of a zone of vapor, it takes the form of a narrow band. During transit through the column, various factors influence the width of this band, which is continuously increased due to various dispersion processes. These include diffusion of the solute, resistance to mass transfer between and within phases, and the influence of flow irregularities and pertur-bations.f A simple concept, the theoretical plate, carried over from distillation processes, has been used to compare columns and account for the degree of dispersion that influences bandwidth. A chromatographic column may be considered to consist of numerous theoretical plates where the distribution of sample components between the stationary and mobile phase occurs. Hence, a measure of the efficiency of a GC column may be obtained by calculating the number of theoretical plates, N, in the column from ... 

In 1990/1991, Madkowiak [26, 27] described an additional method for determining the pressure drop of packed columns with any type of column internal throughout the entire operating range up to flooding point, based on the knowledge of the resistance factor ]tlv for two-phase counter-current flow. The equations and numerical examples can be found in Chap. 5. 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

It is seen that, for GC packed columns operated under the conditions assumed, the two factors contributing to dispersion by resistance to mass transfer are of the same order of magnitude. Consequently, equations (20) and (21) cannot be simplified and must be used in their existing form for all optimization procedures using packed GC columns. If the conditions differ significantly from those assumed, then by using the same procedure the possibility of modifying expressions (20) and (21) can be reexamined. 

Figure 1.4 Variation of the resistance to mass transfer in the mobile phase, C , and stationary phase, Cj, as a function of the capacity factor for open tubular columns of different internal diameter (cm) and film thickness. A, df 1 micrometer and D, 5 x 10 cm /s B, df 5 micrometers and D, 5 x 10 cm /s and C, df - 5 Micrometers and 0, 5 x 10 cm /s.

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