DEMINERALIZERS
DOE-HDBK-1018/2-93
Miscellaneous Mechanical Components
During the regeneration step, it is important to maintain the cation and anion resins at their
proper volume. If this is not done, the resin interface will not occur at the proper place in the
vessel, and some resin will be exposed to the wrong regenerating solution. It is also important
to realize that if the ion exchanger has been involved with radioactive materials, both the
backwash and the regenerating solutions may be highly radioactive and must be treated as liquid
radioactive waste.
The next step is the slow rinse step, shown in Figure 14d, in which the flow of dilution water
is continued, but the caustic and acid supplies are cut off. During this two-direction rinse, the
last of the regenerating solutions are flushed out of the two beds and into the interface drain.
Rinsing from two directions at equal flow rates keeps the caustic solution from flowing down
into the cation resin and depleting it.
In the vent and partial drain step, illustrated in Figure 14e, the drain valve is opened, and some
of the water is drained out of the vessel so that there will be space for the air that is needed to
re-mix the resins. In the air mix step, (Figure 14f) air is usually supplied by a blower, which
forces air in through the line entering the bottom of the ion exchanger. The air mixes the resin
beads and then leaves through the vent in the top of the vessel. When the resin is mixed, it is
dropped into position by slowly draining the water out of the interface drain while the air mix
continues.
In the final rinse step, shown in Figure 14g, the air is turned off and the vessel is refilled with
water that is pumped in through the top. The resin is rinsed by running water through the vessel
from top to bottom and out the drain, until a low conductivity reading indicates that the ion
exchanger is ready to return to service.
External Regeneration
Some mixed-bed demineralizers are designed to be regenerated externally, with the resins being
removed from the vessel, regenerated, and then replaced. With this type of demineralizer, the
first step is to sluice the mixed bed with water (sometimes assisted by air pressure) to a cation
tank in a regeneration facility. The resins are backwashed in this tank to remove suspended
solids and to separate the resins. The anion resins are then sluiced to an anion tank. The two
batches of separated resins are regenerated by the same techniques used for single-bed ion
exchangers. They are then sluiced into a holding tank where air is used to remix them. The
mixed, regenerated, resins are then sluiced back to the demineralizer.
External regeneration is typically used for groups of condensate demineralizers. Having one
central regeneration facility reduces the complexity and cost of installing several demineralizers.
External regeneration also allows keeping a spare bed of resins in a holding tank. Then, when
a demineralizer needs to be regenerated, it is out of service only for the time required to sluice
out the depleted bed and sluice a fresh bed in from the holding tank. A central regeneration
facility may also include an ultrasonic cleaner that can remove the tightly adherent coating of
dirt or iron oxide that often forms on resin beads. This ultrasonic cleaning reduces the need for
chemical regeneration.
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