Column end effects

In spite of occasional pronounced drop end effects, they are commonly neglected in practice. This is especially justified in industrial units, where drop formation is rapid and the constant-velocity region is quite large, usually due to overdesign of the column. However, column end effects, associated with the longitudinal-dispersion and flow patterns in spray columns, strongly affect the transfer efficiency of the column. 

Column end effects, though on a larger scale, are sometimes considered analogous to those associated with drop formation. The latter undoubtedly contributes to increase the former. However, no quantitative relationships between drop and column end effects are as yet known for countercurrent operation. As suggested by Vermeulen and others, the concentration (or temperature) of each incoming stream increases or decreases abruptly on entering the column. In contrast, the concentration pattern for each outgoing stream becomes flat as it approaches its outlet, and no discontinuity is... 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

If a local concentration of solute is placed at the midpoint of a tube filled with either a liquid or a gas, the solute will slowly diffuse to either end of the tube. It will first produce a Gaussian distribution with a maximum concentration at the center and finally, when the solute reaches the end of the tube, end effects occur and the solute will continue to diffuse until there is a constant concentration throughout the length of the tube. This diffusion effect occurs in the mobile phase of a packed LC column but the end effects are never realized. The diffusion process is depicted in figure 2. 

The pressure change is measured for each capillary at the apparent shear rate. Regression analysis is then used to obtain the slope and intercept for the function of pressure change (column two) in Table 3.5 with respect to the length divided by the radius (L/R, column six). The slope of the function is 0.376 MPa, and the intercept is 1.5 MPa. The regressed pressure change is obtained from the slope and the intercept, and the pressure change corrected for the end effects are as follows ... 

The low frequency baseline noise of the viscometer can be substantially reduced by careful filtration of samples and regular checking and maintenance of column end fittings and fractional sections of tubing in the system. Figure 5 shows the effect of column screen replacement on the stability of the baseline signal at a flow rate of 1.0 ml/min. 

The results in Table 9.2 apply when no end effects are present. Sutterby (S7) determined simultaneous wall and end correction factors for the creeping flow range. His correlations are shown in Fig. 9.3 where the cylindrical column has closed ends a distance apart and the center of the spherical particle is distance Z from one end of the tube. The curve for D/L = 1.0 and Z/L = 1/2 is... 

Matrix schemes which assign end effects to each end of a helical segment are also proposed. A 3 x3 statistical weight matrix B using this weighting scheme is used here. Row index states of amino acid residues i-1 and /, while columns index states of amino acid residues i and /+ 1. 

Most of the properties change somewhat from one end to the other of industrial columns for effecting separations, so that the mass transfer coefficients likewise vary. Perhaps the property that has the most effect is the mass rate of flow which appears in the Reynolds number. Certainly it changes when there is a substantial transfer of material between the two phases in absorption or stripping and even under conditions of constant molal overflow in distillation processes, the mass rate of flow changes because of differences of the molecular weights of the substances being separated. As a practical expedient, however, mass transfer coefficients are evaluated at mean conditions in a column. 

Malyusov ei al. (M6), 1955 in very short wetted-wall columns (no rippling) (3) flowing over spheres. Entry and end effects studied, also effects of adding surfactant. Study of distillation in wetted-wall columns, taking into account the effects of laminar and turbulent flow of vapor phase. 

As the bed depth increases, end effects (i.e., mass transfer in the region of liquid introduction and in the region where liquid drips from the packing supports) become less important. Such end effects tend to lower the HETP observed in short columns, such as pilot-plant columns. 

The column diameter dc is also a factor for both packed and open tubular columns. In both cases, the zone spreading is a function of the square of the diameter. Other parameters that enter into this term include the diameter of the coil of the column, which is usually avoided by using uncoiled (straight) columns in LC end effects that cause broadening as the sample enters and leaves the column and the multipath effect, which arises from the fact that each analyte molecule can take a different path... 

End Effects Analysis of the mass-transfer efficiency of a packed column should take into account that transfer which takes place outside the bed, i.e., at the ends of the packed sections. Inlet gas may very well contact exit liquid below the bottom support plate, and exit gas can contact liquid from some types of distributors (e.g., spray nozzles). The bottom of the column is the more likely place for transfer, and Sil-vey and Keller [Chem. Eng. Prog., 62(1), 68 (1966)] found that the... 

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