Miscellaneous Mechanical Components
To return the ion exchanger to service, the drain valve is closed, the outlet valve is opened, and
the ion exchanger is ready for service.
Single-bed demineralizers are usually regenerated "in place." The resins are not pumped out to
another location for regeneration. The regeneration process is the same for cation beds and for
anion beds; only the regenerating solution is different. It is important to realize that if the ion
exchanger has been exposed to radioactive materials, the backwash, regeneration, and rinse
solutions may be highly radioactive and must be treated as a radioactive waste.
A mixed-bed demineralizer is a demineralizer in which the cation and anion resin beads are
mixed together. In effect, it is equivalent to a number of two-step demineralizers in series. In
a mixed-bed demineralizer, more impurities are replaced by hydrogen and hydroxyl ions, and
the water that is produced is extremely pure. The conductivity of this water can often be less
than 0.06 micromhos per centimeter.
The mixed-bed demineralizer shown in Figure 14 is designed to be regenerated in place, but the
process is more complicated than the regeneration of a single-bed ion exchanger. The steps in
the regeneration are shown in Figure 14.
Figure 14a shows the mixed-bed ion exchanger in the operating, or on-line mode. Water enters
through a distribution header at the top and exits through the line at the bottom of the vessel.
Regeneration causes the effluent water to increase in electrical conductivity.
The first regeneration step is backwash, as shown in Figure 14b. As in a single-bed unit,
backwash water enters the vessel at the bottom and exits through the top to a drain. In addition
to washing out entrained particles, the backwash water in a mixed-bed unit must also separate
the resins into cation and anion beds. The anion resin has a lower specific gravity than the
cation resin; therefore, as the water flows through the bed, the lighter anion resin beads float
upward to the top. Thus, the mixed-bed becomes a split bed. The separation line between the
anion bed at the top and the cation bed at the bottom is called the resin interface. Some resins
can be separated only when they are in the depleted state; other resins separate in either the
depleted form or the regenerated form.
The actual regeneration step is shown in Figure 14c. Dilution water is mixed with caustic
solution and introduced at the top of the vessel, just above the anion bed. At the same time,
dilution water is mixed with acid and introduced at the bottom of the vessel, below the cation
bed. The flow rate of the caustic solution down to the resin interface is the same as the flow rate
of the acid solution up to the resin interface. Both solutions are removed at the interface and
dumped to a drain.