Packing is used in counter-current liquid/liquid contactors to facilitate mass transfer. The heavier phase is introduced from the top, flows downward and exitst he column at the bottom. The lighter phase on the other hand, enters at the bottom and exits the column at the top. Depending on the process, one of the liquids is the continuous phase and the other is dispersed phase. Special internals are used to introduce the two liquid phases, especially the dispersed phase. Selection and arrangement of the internals depends on which phase (light or heavy) is continuous and which is dispersed. In all cases, the use of feed pipes for directing the feed, light and heavy, to the disperser are recommended to controlvelocity.
In contactors where the light phase, feed which enters the bottom of the tower, is dispersed, packed beds are supported by the light phase disperser (model 561). In addition to supporting the packing, the plates allow proper dispersion or formation of small droplets that rise through the continuous phase. In breaking the dispersed liquid into small droplets, the light phase disperser provides maximum initial contact area between the two phases. Because the droplets tend to coalesce in the packing, beds are typically limited to a depth of 6 to 8 ft (1.5 to 2.5m). Multiple beds, each supported by a light phase disperser are recommended where a total of more than 8 ft (2.5m) of packing is required.
When the heavy phase, feed which enters the top of the tower, is dispersed, the heavy phase disperser is used above the top bed. When multiple beds are required, the heavy phase disperser is also used to support the upper beds, collect and disperse the heavy phase to the beds below. The bottom bed is supported by conventional support plate (see models SPL521 or SPM522). The heavy phase disperser, although structurally different, is hydraulically inverted when compared to the light phase disperser. In heavy phase dispersed contactors, the same bed depth recommendations apply as with light phase dispersement.
It is generally recommended to disperse the phase with the higher flow rate to generate maximum interfacial contact. The exception to this rule is when the higher volumetric flow rate phase has higher viscosity or preferentially wets the packing surface.
Surfactants may alter surface properties to the extent that the performance of a liquid-liquid contactor cannot be predicted.
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