Countercurrent ion exchange in a column or series of columns performs very well in comparison to batch contacting, but truly countercurrent operation with both the resin and the fluid moving would be even more efficient. There are several ways of doing this. The concept of the Higgins unit is shown in the next sketch:

The loop contains ion exchange resin. The feed flows through the resin, and the spent stream exits through a screen that holds back the resin. Similarly, the eluant flows through the resin to exit as the eluate. At frequent intervals, a sudden brief shock is applied to the fluid in the reverse direction. This pulse conveys the resin a short distance in the opposite direction to achieve true countercurrent flow.

Bulges or compartments to make it easier to prevent short circuiting of the liquid streams. This makes use of head losses in smaller connecting pipes so that the easiest path is from the inlet to the desired outlet. The thin arrows in the lines connecting the compartments indicates that there is a small flow of liquid. This dilutes the feed and eluant streams slightly. The motion due to pulses moves the resin in the direction opposite from the thin arrows.

If elution must be separate from regeneration of the resin, there must be three sections and their associated piping.

Other inventions have taken advantage of a small difference in specific gravity between the resin in its regenerated form and in its state with the ions being collected from the solution. The resin will classify according to specific gravity in a gently fluidized system. Such designs must avoid mechanical metering of the resin as this can crush some of the beads.

Another means for conveying the resin is to retain it in a mesh basket. This is akin to having tea bags of resin and dragging them countercurrent to the flow of solution.

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