Other Packing Considerations

Liquid Holdup Liquid holdup is the liquid present in the void spaces of the packing. Reasonable liquid holdup is necessary for good mass transfer and efficient tower operation, but beyond that, it should be kept low. High holdup increases tower pressure drop, the weight of the packing, the support load at the bottom of the packing and tower, and the tower drainage time. Most important, when distilling thermally unstable materials, excessive holdup raises product degradation and fouling, and with hazardous chemicals, increases undesirable inventories.

The effect of liquid and gas rates on the operating holdup is shown in Figs. 14-73 and 14-74. In the preloading regime, holdup is essentially independent of gas velocity, but is a strong function of liquid flow rate and packing size. Smaller packings and high liquid rates tend to have greater holdup.

Holdup can be estimated by using Buchanan's correlation [Ind. Eng. Chem. Fund. 6, 400 (1967)], as recommended in previous editions of this handbook. More recent correlations by Billet and Schultes [IChemE. Symp. Ser. 104, A159 (1987)], by Mackowiak

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