A simple way to maintain interfacial area and dispersion in a vertical gravity-flow column, and to reduce axial mixing, is to fill the column with loose packing to provide tortuous flow paths. Typical packing consists of ceramic rings or saddle shapes, dumped in random arrangement. The

^ Figure 4.29 The spray column of Elgin [Е1]. (From Treybal [T2], by

liquid permission.)

packing reduces the available space for liquid flow and also introduces frictional drag, so the liquid throughout per unit of cross-sectional area is less than for spray columns. Neutron poisons can be incorporated into the packing to increase the criticality safe diameter.

Spray column contactors were used in the first large-scale solvent extraction plants at Hanford, Washington, for recovering plutonium from irradiated natural uranium and in the first chemical processing plant at Idaho for recovering enriched 235 U [L2].

Although packed columns are simple and have no moving parts, their large space requirements have resulted in the replacement of packed columns by pulsed columns, or by other more compact contactors, in more recent installations for reprocessing irradiated reactor fuel.