Column distribution design/packing

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapour, counter-currently, up the column. In some gas-absorption columns co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. 

As we have discussed, uniformity of liquid distribution is essential to the attainment of the maximum packed bed efficiency. In those cases where a mixed vapor/liquid feed is used, or where the liquid feed would flash on entering the column, special designs are necessary. To avoid excessive turbulence in the liquid distributor, it is customary to install a device that separates the vapor and liquid phases ahead of the final distributor. Such a device used for small columns is located immediately above the distributor (Figure 10-15). For large diameter towers where feed... 

Where only about 5% or less of the liquid dow iiflow is to be withdrawn from the column, a special collector box can be installed within the packed bed. This box can remove small quantities of intermediate boiling components that otherwise w ould accumulate in a sufficient quantity to interfere with the fractionation operation. Such a collector box must be designed very carefully to avoid interference with the vapor distribution above it or... 

Having established that a finite volume of sample causes peak dispersion and that it is highly desirable to limit that dispersion to a level that does not impair the performance of the column, the maximum sample volume that can be tolerated can be evaluated by employing the principle of the summation of variances. Let a volume (Vi) be injected onto a column. This sample volume (Vi) will be dispersed on the front of the column in the form of a rectangular distribution. The eluted peak will have an overall variance that consists of that produced by the column and other parts of the mobile phase conduit system plus that due to the dispersion from the finite sample volume. For convenience, the dispersion contributed by parts of the mobile phase system, other than the column (except for that from the finite sample volume), will be considered negligible. In most well-designed chromatographic systems, this will be true, particularly for well-packed GC and LC columns. However, for open tubular columns in GC, and possibly microbore columns in LC, where peak volumes can be extremely small, this may not necessarily be true, and other extra-column dispersion sources may need to be taken into account. It is now possible to apply the principle of the summation of variances to the effect of sample volume. 

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