R CH2N(CH3)3OH
R CH2OH
N(CH3)3
heat
R CH2N(CH3)3OH
R CH2N(CH3)2
CH3OH
heat
DOE-HDBK-1015/2-93
DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL
Principles of Water Treatment
CH-04
Rev. 0
Page 24
As solution passes through the resin column, the relative amounts of ammonium and
hydrogen ions on the resin change. The actual exchange process occurs primarily in a
relatively narrow band of the column rather than over the entire length. This band is
called the exchange zone. Assuming the column is vertical and that solution flows from
top to bottom, the resin above the exchange zone is depleted; that is, practically all the
exchange capacity has been used. Below the exchange zone, essentially none of the
resin's exchange capacity has been used. As more and more solution flows through the
column, the exchange zone gradually moves downward as more of the resin is depleted.
Eventually, as the exchange zone approaches the end of the column, small amounts of
NH begin to appear in the effluent. The point at which this occurs is called
4
+
breakthrough. If more solution passes through the resin, the concentration of NH in
4
+
the effluent increases until it is the same as the concentration in the influent. This
condition is called exhaustion and indicates that essentially all the exchange capacity of
the resin has been used. (Because of the equilibrium nature of the exchange process,
a small amount of the resin may remain in the hydrogen form, but not enough to remove
any more ionic impurities.) Note that because the exchange zone in this case was
narrow, a relatively small volume of solution takes the resin from breakthrough to
exhaustion.
Resin Overheating
The potential for elevated temperatures exists during most conditions of facility
operation, we will examine in detail the processes that occur if the resin in an ion
exchanger is overheated. Although the inert polystyrene basic structure of resin is
stable up to fairly high temperatures (approximately 300 F), the active exchange sites
are not. The anion resin begins to decompose slowly at about 140 F, and the
decomposition becomes rapid above 180 F. The cation resin is stable up to about
250 F. Because these temperatures are well below normal reactor coolant
temperatures, the temperature of the coolant must be lowered before it passes through
the ion exchange resin.
The anion resin (hydroxyl form) decomposes by either of two mechanisms with
approximately equal probability.
(4-24)
or
(4-25)
