Packing efficiency practice

In design practice, a less rigorous parameter, HETP, is used as an index of packing efficiency. Tne HETP is the height of packed bed required to achieve a theoretical stage. The terms Hoe and HETP may be related under certain conditions ... 

It is seen from equation (15) that the maximum overload volume is linearly related to the (k ) value of the first eluted solute of the critical pair, the function (a-1) and the column dead volume, Consequently, the larger the column, either in length and/ or radius, the larger the sample volume can be, This assumes that the column is of such a size, that it can be efficiently packed with practical techniques and that the particle size of the packing is chosen such that the pump pressure available can provide the necessary mobile phase flow-rate. 

For these reasons, it is a common prectice (15,52,63) to design random packed beds for a pressure drop not smaller than 0.1 in of water per foot of packing. In practice, there are many columns that operate efficiently at a lower pressure drop. 

The basic design of Uquid-Hquid-extraction test plant with packed columns is shown in Fig. 7-1. It largely corresponds to the structure commonly used for gas/Hquid systems. A distributor with bore holes is used for the pre-distribution of the dispersed phase, with the hole diameter roughly matching the expected droplet size. A distributor is also used for the pre-distribution of the continuous phase, in order to achieve an even flow of both phases in the packed extractor. Practical experience has shown that a poor pre-distribution of the continuous and dispersed phases results in strong back-mixing, which, in turn, leads to a significant reduction of the extractor s separation efficiency [3,4, 10,21],... 

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... 

Use of HETP Data for Absorber Design Distillation design methods (see Sec. 13) normally involve determination of the number of theoretical equihbrium stages or plates N. Thus, when packed towers are employed in distillation appRcations, it is common practice to rate the efficiency of tower packings in terms of the height of packing equivalent to one theoretical plate (HETP). 

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