Inverse packed column

Flooding and Loading Since flooding or phase inversion normally represents the maximum capacity condition for a packed column, it is desirable to predict its value for new designs. The first generalized correlation of packed-column flood points was developed by Sherwood, Shipley, and Holloway [Ind. Eng. Chem., 30, 768 (1938)] on the basis of laboratory measurements primarily on the air-water system. 

All packing materials produced at PSS are tested for all relevant properties. This includes physical tests (e.g., pressure stability, temperature stability, permeability, particle size distribution, porosity) as well as chromatographic tests using packed columns (plate count, resolution, peak symmetry, calibration curves). PSS uses inverse SEC methodology (26,27) to determine chromatographic-active sorbent properties such as surface area, pore volume, average pore size, and pore size distribution. Table 9.10 shows details on inverse SEC tests on PSS SDV sorbent as an example. Pig. 9.10 shows the dependence... 

It Is seen that, in a similar manner to the packed column, the optimum mobile phase velocity is directly proportional to the diffusiv ty of the solute in the mobile phase, However, in the capillary column the radius (r) replaces the particle diameter (dp) of the packed column and consequently, (u0pt) is inversely proportional to the column radius. 

Equation (13) gives the minimum analysis time that can be obtained from an open tubular column, when separating a mixture of defined difficulty, under given chromatographic conditions. It is seen that, in a similar manner to the packed column, the analysis time is inversely proportional to the fourth power of the function (a-1) and inversely proportional to the inlet pressure. The contribution of the function of (k ), to the analysis time is not clear and can be best seen by calculation, it is also seen (perhaps a little surprisingly) that the analysis time Is compiete y independent of the diffusivity of the solute in the mobile phase but Is directly proportional to the viscosity of the mobile phase. 

Flow rate To force the mobile fluid from inlet to outlet of a column, the inlet pressure must be higher than the outlet pressure. When the temperature is changed, the fluid viscosity changes, and then either the flow rate changes for the same inlet and outlet pressures or the pressure drop must be altered to maintain the same flow rate. The flow rate of a fluid through a packed or capillary column is inversely proportional to the viscosity. For example, from Darcy s law the flow of a gas through a packed column is given by ... 

Inverse gas chromatography has proven to be a particularly important technique for the investigation of polymers, with most studies making use of packed columns. IGC also has been recently extended to the investigation of fibers and of polymers coated on capillary columns. The preparation of each of these columns is very important to overall success. 

Diffusivity data are available only for a limited number of polymer-solvent systems. This paper describes research that has led to the development of the use of capillary column inverse gas chromatography (IGC) for the measurement of diffusion coefficients of solute molecules in polymers at infinite dilution. The work has resulted in a precise, rapid technique for the diffusion measurements that circumvents the many problems attendant to classical sorption methods and packed column IGC methods. Initial results of the program appeared in two recent publications (1,2)- Some of the material introduced in those papers is discussed here to present background for... 

Relative retentions..the a values..usually vary Inversely with column temperature, but are most strongly affected by the choice of liquid phase. In packed column chromatography, the choice of liquid phase Is usually the most effective route by which separation efficiency Is Influenced. In capillary GC, however, there Is normally such an abundance of theoretical plates that the choice of liquid phase Is a relatively unimportant parameter for many analyses. In some cases however. It does become desirable (or even necessary) to select a liquid phase in which the relative retentions of certain solutes Is larger. Until quite recently, this posed a real problem with the fused silica capillary column, because the more polar liquid phases, l.e. those In which relative retentions are usually greater, coated fused silica only reluctantly, and produced columns whose useful lives were quite limited. The development of stable bonded phase columns ( ) eventually overcame this difficulty (vide Infra). 

Particle size and size distribution define the quality of the support material and are the key determinants of efficiency and back-pressure of the column.1-3 The effect of dp on H is discussed in Chapter 2. For a well-packed column, Hmin is approximated to 2-2.5 dp. Also, since the van Deemter equation C term is proportional to dp2, columns packed with small particles have much less efficiency loss at high flow rates.14 However, since column back-pressure is inversely proportional to dp2, columns packed with sub-3-pm particles can easily exceed the pressure limit of most HPLC instruments at 6,000 psi. Note that decreasing particle size while keep the L constant can increase column efficiency and peak resolution (Figure 3.5A) and also increase peak height and sensitivity (Figure 3.5B). 

The mobile-phase mass-transfer coefficient C i is known to be inversely proportional to the diffusion coefficient of the analyte in the mobile phasic For packed columns, C is proportional to the sqiiare of the particle diameter of the packing material, dp. For open tubular columns, is proportional to the square of the... 

Packed columns are not likely to be affected by inverse response, but may in principle experience a related problem. In the loading regime, a rise in vapor flow increases liquid holdup in the column. With scheme 16.4e, raising boilup will therefore induce a drop in bottom level, which in turn will reduce boilup. If sufficiently vigorous, this phenomenon can unsettle column boilup. The expectation of this behavior is based on first principles only the author is not aware of any troublesome reports of this behavior. 

From the 1980s, the chromatographic broadening technique has been substituted in the literature by inverse gas chromatography (IGC). The two main IGC methods for the measurement of diffusion coefficients in liquids are packed column IGCP and capillary column IGCP ... 

The column is the most important part of the system, as its function is to encourage repetitive partitioning of each solute molecule between the gas and the liquid or solid phase. Packed columns are commonly used with the polymer or blend to be analysed coated on to a non-reactive substrate (Chromasorb W) which is then packed into the stainless-steel GC column. More recently capillary columns have been employed. Here the polymer is dynamically or statically coated as a uniform film on to the walls of the fused silica capillary. Inverse gas chromatography is a very flexible analytical technique however, full commercialisation of the technique has yet to be realised. 

In packed columns, the liquid flows in counter-current to the gas, in the form of films and runlets. This applies in particular to small, non-perforated packing elements with low void fraction, e.g. e = 0.4—0.6 m m . The void space of the packing elements contains dead space, which is filled with more and more Uquid as the Uquid load ul increases. Flooding occurs when the entire column is filled with Uquid. In the case of high specific liquid loads ul and very low gas velocities uy, i.e. at very high phase flow ratios at the flooding point Xq, the so-called phase inversion occurs through the formation of bubbles, whereby the gas phase is dispersed and the Uquid now forms the continuous phase. 

Scaha and Simeoni (2001) developed an assay of six xanthene dyes in lipsticks using an inverse snpercritical flnid-extraction (SEE) method for sample preparation. The SEE extraction prodnced recoveries that were comparable to those with a conventional liquid-hquid extraction method. The separation of the extracted dyes was performed by LC with a cyano-propyl packed column, and eluted isocratically with aqueous sodium acetate (0.02 M, pH 4.5) acetonitrile methanol (55 35 10). The spectra were recorded with a variable-wavelength UVA IS detector. 

Fortunately the back pressure (resistance to flow presented by the packed column) is inversely proportional to the square of the column diameter. Hence, by increasing the column diameter from 4.6 to 150 mm, a pressure reduction factor of over one thousand times would be expected. However, in order to maintain a proportional linear velocity of mobile phase through the larger column, the back pressure will remain... 

If the packing surface is discontinuous in nature, a phase inversion occurs, and gas oubbles through the liquid. The column is not unstable and can be brought back to gas-phase continuous operation by merely reducing the gas rate. Analogously to the flooding condition, the pressure drop rises rapidly as phase inversion occurs. 

To convert an available HETP value for a given system and column diameter to a different packing in the same series (X or S), assume HETP is inversely proportional to the specific surface (consult manufacturer for this detail). 

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