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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.41

rather than volume even though it is common practice to label and purchase resins by vol-
ume. It is therefore necessary to fit the resin volume to the tank. If an inexact amount of
resin is blindly put into a tank that is going to employ countercurrent regeneration, the
chance of failure is pretty high. A packed bed demineralizer consists of a tank contain-
ing an upper distributor and an underdrain. The resin bed fills the entire tank except for
the amount of freeboard required to allow for the resin swelling and contraction of the
resin bed between the regenerated and exhausted forms. There is no separate regenerant
collector. The regenerant flow is performed countercurrently to the service flow such that
upflow service units have downflow regeneration. The direction of the service flow rate
may be upward or downward, but most of the packed bed designs built to date have up-
flow service and downflow regeneration.

External Regeneration
External regeneration is performed in a vessel other than the service vessel. This type of
regeneration is widely practiced by the service exchange companies and is the preferred
method in condensate polishing demineralizers in the electric power industry. The resin
becomes fully mixed as it is transferred from the service vessel, so there is no advantage
to CCR. Externally regenerated units are always regenerated in a downflow fashion. Resins
used in mixed beds must be separated prior to regeneration so that the acid regenerant
can contact the cation portion of the resin and the sodium hydroxide regenerant can con-
tact the anion portion of the resin. If the wrong chemical contacts the wrong resin, it can
cause fractured resin beads due to osmotic shock and precipitation of various salts, and
contamination of the resin bed.

Multiple-Step Regeneration

These types of regenerations are used in specialty applications, for example, where the resin
is neutralized after the acid or base regeneration. The salt forms of weakly ionized resins
have better kinetic properties and are often used for special purposes. The sodium form weak
acid cation resin is very selective for divalent and trivalent metals such as lead, copper, and
chromium. Weakly basic resins are used in the salt form for specialized exchanges such as
chromate removal. It is extremely difficult to convert weakly ionized resins directly to the
salt form, while an acid or base step followed by neutralization is relatively easy.

Regeneration
When the resin's capacity is exhausted and the leakage of undesirable ions rises in the ef-
fluent to an unacceptable point, the resin is regenerated. The regeneration process reverses
the exchange reaction under controlled conditions, thus leaving the resin in the desired
ionic form.

Salt Regeneration. High ionic concentrations are more effective regenerants and are
more efficient. On the other hand, sudden changes in ionic concentration cause ion ex-
change resins to rapidly change size. This sudden volume change osmotically shocks the
resin and can cause bead breakage. So it is necessary to control the rate of change and
total concentrations during regeneration. For salt regenerated exchangers, the best range
of salt concentration is between 8% and 15%, with 10% being the most commonly em-
ployed concentration.