Packed Column Capacity

As the vapor flows upward, a pressure drop develops in the vapor phase in the direction of vapor flow as a result of friction against the packing and the falling liquid. The liquid flows down by gravity against this pressure drop, resulting in some steady-state liquid-to-vapor-flow ratio, L/.  

A packed column (or a trayed column) designed for a given service is usually expected to operate over a range of F/V ratios in anticipation of variations in separation requirements, feed flows and compositions, and other factors. 

If the vapor rate is increased, the pressure drop increases, which raises the resistance to the liquid flow. At a certain vapor rate, the pressure drop rises to a point where liquid flow is practically blocked and liquid fills the column, resulting in a condition known as flooding. The column ceases to function as a separation device. A larger diameter column would be required to handle these flows. 

Below certain vapor rates and pressure drops, channeling takes place, where the vapor and liquid pass each other with no appreciable contacting. To prevent this from happening, a smaller column diameter should be selected. 

In general, columns should be designed such that pressure drop is maintained between 0.01 in. of water per foot of packing for vacuum columns and 1 in. of water per foot of packing for atmospheric fractionators. Columns should be operated above 50% and below 90% of the flooding vapor rate. 

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The column can operate actually up to the flooding point F. But it is easy to see, that die maximal efficient capacity (MEC or C ) is reached in point G. That is why the detenninadon of this point is very important. The rate in it provides perfectly stable operation because it has been determined from the sepmation efficiency. This appro h to packed column capacity has been verified the investigation of Kunesh et al. [49]. The maximum hydraulic capacity of the packing is about 20% higher than dus in point G [3, p. 145]. 

Used in virtually all organic chemistry analytical laboratories, gas chromatography has a powerful separation capacity. Using distillation as an analogy, the number of theoretical plates would vary from 100 for packed columns to 10 for 100-meter capillary columns as shown in Figure 2.1. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

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. 

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. 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

Calculation of column diameter (for packed columns, this is usually based on flooding conditions, and, for plate columns, on the optimum gas velocity or the liquid-handling capacity of the plate)... 

Packed columns are gaining ground on trayed columns. Lieberman states that based on his design and operating experience, a properly designed packed tower can have 20-40% more capacity than a trayed tower with an equal number of fractionation stages. 

The silica dispersion showed the smallest retention volume. It should be noted, however, that the authors reported that the silica dispersion required sonicating for 5 hours before the silica was sufficiently dispersed to be used as "pseudo-solute". The retention volume of the silica dispersion gave the value of the kinetic dead volume, /.e., the volume of the moving portion of the mobile phase. It is clear that the difference between the retention volume of sodium nitroprusside and that of the silica dispersion is very small, and so the sodium nitroprusside can be used to measure the kinetic dead volume of a packed column. From such data, the mean kinetic linear velocity and the kinetic capacity ratio can be calculated for use with the Van Deemter equation [12] or the Golay equation [13]. 

As packing factor, F, becomes larger by selection of smaller sized packing gas capacity for the column is reduced and pressure drop will increase for a fixed gas flow. 

Eichel, F. G., Capacity of Packed Columns in Vacuum Distillation, Chem. Eng. Sept 12 (1966) p. 197. 

Excellent open tubular columns may now be purchased, providing a number of stationary phases of differing polarity on WCOT and SCOT columns, and whose efficiency, greatly improved sample detectability, and thermal stability surpass those exhibited by packed columns their chief disadvantage is that they have a lower sample capacity than packed columns.65,66... 

Packing gel permeation column. Suspend 50 g of Bio-Beads in the GPC eluting mixture and allow the beads to swell overnight. Pour the suspension all at once into the chromatographic column (capacity ca 180 mL). Once the gel has settled to a height of... 

The reduced plate height is reasonably constant, independent of the capacity factor for a packed column, while equation (1.82) indicates Ithat the reduced plate height, at least for small values... 

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