Its Effects on Packing Efficiency

Effect of changes In local L/V ratio on packing efficiency. Example 9.1 illustrates a 40 percent reduction in efficiency resulting from a liquid maldistribution of the order of as little as 5 percent between two halves of a column. A similar analysis of an actual troublesome maldistribution case history (140a) also showed that it does not take much maldistribution to cause a major efficiency loss. Example 9.1 also Illustrates the following ... 

To determine the required size of an absorption or stripping unit, it is necessaiy to know not only the equilibrium solubility of the solute in the solvent and the material balance around the column but also the rate at which solute is transferred from one phase to the other within the tower. This rate directly affects the volume of packing needed in a packed tower, the degree of dispersion required in a spray contactor, and (somewhat less directly) the number of trays required in a tray tower. The last effect occurs as a result of the influence of mass transfer rate on tray efficiency which is discussed in a later section. Because of its direct effect on packed tower design and the importance of this type of contactor in absorption, this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theory is given in Chapter 2. 

As discussed earlier for packed columns, the rate and shape of the temperature-programming profile can have a marked effect on column efficiency. A non-linear (concave) rate of temperatureprogramming is preferable whenever this is feasible. The lower the rate of temperature-programming, the lower is the elution temperature of a given compound, but the longer is its elution time. In practice, the optimum temperature limits and the rate of programming must be determined empirically for a given sample and column. 

The HETP curve clearly shows, that for a packed column, the particle size has a profound effect on the minimum value of the HETP of a column and thus the maximum efficiency attainable. It would also appear that the highest efficiency column would be obtained from columns packed with the smallest particles. This will in due course be shown to be a fallacy, but what is true, is that the smaller the particle diameter the smaller will be the minimum HETP and thus, the larger the number of plates per unit length obtainable from the column. At this time it will suffice to point out that the total number of theoretical plates that can be obtained will depend on the length of the column which, in turn, must take into account the available inlet pressure... 

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