Column packing, discrepancies

Just as in the case of the gas-phase Peclet number, significant discrepancies in the reported literature on the liquid-phase Peclet number exist. Kramers and Alberda2w showed that in a 15-cm-diameter column packed with 10-mm Raschig... 

For chromatographers who do not use a sufficient number of columns per year to economically justify the purchase of a HPLC column packing facility, or who cannot afford the time sometimes required to obtain consistent results, the purchase of prepacked columns is advised. Indeed, because some materials are difficult to pack, many manufacturers do not release the bulk material (see for example Tables 2 and 4). The quality and reliability of pre-packed columns has improved considerably over the last few years, and most manufacturers now provide a test chromatogram in which a standard procedure is used to calculate column performance often a written guarantee is supplied. The test report will specify the efficiency of the column expressed in terms of the number of theoretical plates per column or metre, and will also show the test conditions used to obtain the result (mobile phase composition, flow-rate, injection volume, detection wavelength and attenuation, temperature and test sample components). It will also indicate the back-pressure the column will typically exhibit, under the test conditions. This facility is extremely useful to the chromatographer who can, on receipt of the column, repeat the test procedure for the column. Any discrepancy in the result can be due to one or more of the reasons listed below. 

In fact, the experimental retention times are tR(mono) = 15.3 min and tR(di) = 21.0 min, and are thus 5 to 10% greater than the expected times. This discrepancy can be due to differences in packing procedures between the laboratory column and the columns used in the SMB or differences between the lots of resins. 

Significant literature on the axiaj dispersion in gas and liquid phases for countercurrent-flow packed-bed columns have been reported. Trickle- and bubble-flow regimes have been considered. Unlike the holdup, there is quite a discrepancy in the results of various investigators. Almost all the RTD data are correlated by a single-parameter axial dispersion model. A summary of the reported axial dispersion studies in countercurrent flow through a packed bed is given in Table 8-1. 

Arwickar (1981) reported some results for distillation under total reflux conditions of the system acetone-methyl acetate-methanol. The experiments were carried out in a laboratory scale column of 7.62 cm diameter packed with 0.635 cm Raschig rings. The simulation of total reflux operations using the nonequilibrium model is discussed by Krishnamurthy and Taylor (1985a). In simulations of Arwickar s experiments Taylor et al. used the correlations of Onda et al. (1968) to estimate the mass transfer coefficients in each phase and the effective interfacial area. The average absolute discrepancy between predicted and measured mole fractions was less than 2 mol% for acetone and methyl acetate and less than 4 mol% for methanol. 

Fig.4 Comparison of the results of three versions of solute exclusion method applied to a water-swollen dextran gel (Sephadex G-lOO). Determined with standard dextran fractions and oligosaccharides. Quantities on abscissa and ordinate are the same as those in Fig. 2. Observed discrepancy in total pore volume (height of plateau in the large diameter region) was attributed to the compression of gel when packed in a column. (Reproduced from ref. 32 with permission.)...

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