With the advent of
higher pressure boilers and more restrictive feed water requirements of
turbine manufacturers, the ion exchange equipment manufacturers were
called on to meet the challenge of purifying very large quantities of
water which contained very small quantities of both dissolved and
suspended contaminants. The major dissolved contaminants were sodium,
chloride, and silica, while the suspended contaminants were oxides of
iron and copper.
Conventional mixed bed
demineralizers consisted of in-place regenerated resins. The anion resin
was regenerated with caustic and the cation resin with acid. While the
new condensate polishers were similar in nature to the conventional
mixed bed demineralizers, the high operating pressure and flow rate
required changes. The polishers were externally regenerated to limit the
cost of duplicating the regenerant headers, valves, and piping. With
external regeneration facilities, the freshly regenerated resins could
be rinsed down to quality without risking contamination of the
high-pressure boilers with traces of regenerant acid or caustic.
Initially, most of the
power plant piping contained substantial quantities of copper in the
condenser tubing. In order to reduce corrosion of the systems, ammonia
was added to raise the pH. However, in the process of providing
corrosion protection the ammonia dissolved some of the copper piping,
and therefore industry decided to use all iron piping. Ammonia was added
to raise the pH to between 8.8 and 9.6. The presence of ammonia in the
condensate feeding the ion exchange polishers created a higher load on
the hydrogen-form cation resin. Run lengths grew shorter as the ammonia
concentration in the feed water was increased higher and higher to
reduce iron transport in the system. The relatively short length of run
required more and more regenerant, and problems were encountered in
performing the number of regenerations required to keep the units on
line. The industry then studied its early attempts at operating the
polishers beyond the ammonia break. However, as the ammonia started
leaking into the effluent, the sodium started to increase.
Let us examine the sodium equilibrium reaction with ammonia. We see that
sodium in the influent to a polisher in the ammonia form produces
ammonia in the effluent, with sodium being removed on the resin. The
rate constant for this reaction is 0.75.
Assuming that the only cations present are sodium and ammonium, the sum
of the sodium on the resin and the ammonium on the resin would be 100%
Rearranging the equation:
We see that sodium leakage in the effluent at equilibrium is a function
of both the sodium on the resin and ammonia in the feed.
1.) As sodium on the
resin increases, so does the sodium in the effluent (at constant ammonia
2.) As the ammonium
concentration is increased (pH of the condensate), the sodium in the
effluent will increase.
3.) As the ammonium
concentration is increased, less sodium can be tolerated on the resin
Further treatment of the subject ammonia, sodium, and kinetics is well
presented in papers by Salem(1), Emmett(2), and Darji(3).
How then can we reduct the level of sodium on the resin? Emmett(2) has
provided data on the regenerant level required to attain specific resin
purity. This data holds for cation resin by itself, but what about that
cation resin which has been backwashed into the anion resin and is
suspended in the anion resin layer or the inert resin layer? How do we
deal with the contamination of this resin which occurs during the sodium
hydroxide regeneration of the anion? The key to solving the problem of
sodium is to obtain the best possible separation between the cation and
the anion resins.
Careful selection of resin screen sizes and densities is necessary to
avoid cation resin beads in the anion resin layer. Since the degree of
fluidizatin of a resin bead is a function of both its size, or diameter,
and its density, large low-density beads and small higher density beads
can find themselves side by side when fluidized during a separation
Clumping of resins compounds the problem, especially when the resins are
new. Clumping is caused by a strong surface charge on the resin which,
by means of electrical attraction, draws resin beads together. This is
especially true between the hydroxide-form anion resin and hydrogen-form cation resin. Once clumped, these resins resist separation. They also
swell due to entrained water between the agglomerated clumps and you may
find that the resin which fit in the tank very nicely with the cation
below the separation point has now expanded above that point. Some resin
manufacturers have addressed this problem by treating the anion resin
with a proprietary chemical to partially foul the surface of the resin
to prevent clumping. The cation and anion resins or cation and inert
resins, when clumped, cling together, resisting ordinary efforts to
separate them. A number of remedies have proven helpful in this regard,
amongst which are the use of Triton X-100 in a 1/1000 dilution, or a
complete exhaustion of the mixed bed resins, for example, using sodium
Once the resins have been separated as well as possible, it is still
necessary (in a transfer demineralizer with a three-tank regeneration
facility) to sluice the anion resin over to the anion regeneration tank.
Usually a small amount of anion resin (1/4" to 1/2") has been left
behind to minimize the transfer of cation resin fines along with the
anion resin into the anion regeneration tank. Once in the anion
regeneration tank, this cation resin either forms shall beads or
fragments from resin attrition, would be regenerated to the 100% sodium
Ion exchange resin manufacturers have provided the industry with a long
desired goal, an inert resin which will form an interface between the
cation and the anion resin. The inert resin, with size and density
carefully selected, provides a separating layer between the cation and
the anion resins after backwash. The intermixed zone has been reduced as
far as possible by the selection of resins which separate well without
using inert resins. The inert resins further aids with the separation
and also dilutes the intermixed resin. The use of inert permits transfer
of the anion resin without either leaving behind a little bit of the
anion resin, or taking along a little bit of the cation resin. These
resins in effect optimize the separation. What we are doing, in effect,
is providing a cation- and anion-free space between the cation and the
anion-free space between the cation and the anion bulk resins. Reality,
as always, is somewhat less attractive than the ideal since no
separation is really 100%. But we do have a much-improved system.
Three major processes have been developed in the last two decades to
minimize sodium in the resin and hence reduce leakage.
1.) Ammonium hydroxide treatment of anion resin.
hydroxide treatment of anion resin.
3.) Floating anion
resin with sodium hydroxide.
The first two process use ammonia or lime to treat the anion resin after
its regeneration with sodium hydroxide. The purpose of these treatments
is to address the problem of the cation resin which has been transferred
over with the anion resin. This resin has been regenerated with caustic
and is now in the 100% sodium form. The last process makes use of a
sodium hydroxide solution of density intermediate between that of the
cation and anion resin to float the anion resin in the anion
regeneration tank while permitting the cation to sink to the bottom.
This separates out the cation resin, which has been converted to the
sodium form. Let us take a look in detail at each of these three
The ammonia wash system uses a weak solution of ammonium hydroxide,
which is either passed once through the anion resin to waste, or
recycled downflow through the regenerated anion resin, then down through
the ammonia form cation resin before it has been regenerated. The
ammonia will strip off sodium from those cation beads or fines in the
anion vessel, which were converted to the sodium form during the anion
regeneration. Recycle ammoniation deposits the sodium removed from the
cation resin in the anion tank onto the cation resin in the cation
regeneration tank. The caution resin is later regenerated with sulfuric
acid. The cation resin regeneration level will have to be of such
magnitude as to remove the additional sodium load as well as that sodium
which already exists on the resin.
The time required to strip the sodium from the contaminated cation resin
in the anion tank is directly proportional to the actual quantity of
cation resin contamination in with the anion resin. In practice, the
weak ammonia solution is either passed through or recirculated for
anywhere between 2 hours and 24 hours until the sodium is removed. If a
lot of cation resin has been carried over with the anion resin, this
time can become considerable.
The lime wash system uses a very dilute filtered calcium hydroxide lime
solution to remove the sodium from the cation fines, exchanging calcium
for the sodium on the resin. Some people have been reluctant to use the
lime for fear of any calcium entering the boiler. The calcium itself is
very strongly held on the resin, and the sodium and ammonia are unlikely
to remove any significant quantity of calcium. Any precipitated or
suspended calcium in the form of hydroxide or carbonate is removed by a
backwash followed by a downflow rinse. An alternate method is in situ
manufacture of calcium hydroxide by passing a calcium salt through
hydroxide form anion resins as mentioned by Connelley (5). The lime
solution is then filtered and applied as before. The lime required to
exchange the sodium will depend on the amount of cation resin present.
The more cation resin present, the longer the time required.
In the third process, a sodium hydroxide solution of selected density
used to float the anion resin. These permit the cation resin fines to
sink to the bottom of the anion regeneration vessel while the anion
resin is being regenerated. The floating anion resin, free of cation
fines, is transferred to the mix and hold vessel for rinsing. The cation
resin is left in the bottom of the anion regeneration vessel, and is
subsequently transferred into the cation regeneration vessel along with
the next charge of resin for regeneration.
We note that, as the ammonia in the feed increased, both pH and hydroxyl
ion concentration followed. For the anion resin, the increase in
hydroxyl ion concentration tends to reverse the equilibrium, creating
more bisilicate ions in the equilibrium reaction. Looking at the
The equilibrium constant K,
has been assigned an approximate value of 5 by Venderbosch
Rearranging the equation as we have done with the cation resin, we see
that the bisilicate ion concentration is a function of the hydroxyl ion
concentration and the amount of bisilicate or silica on the resin.
We can now see that:
1.) At any given pH,
the silica concentration in the effluent will increase proportional to
the silica concentration on the resin.
2.) As the pH
increases, so does the silica concentration in the effluent.
3.) To maintain a
low silica concentration as pH (hydroxide concentration) is increased,
less silica can be permitted on the resin.
The same three points are true for the chloride-hydroxide equilibrium.
You may be willing to concede by now that the amount of cation resin in
the sodium form, or anion resin in the chloride, bisilicate, or
bisulfate forms, is important to the leakage of the same ionic species
in an environment of ammonia. So what? How can this possibly have any
bearing on methods of polisher operation? This becomes abundantly clear
to anyone who has ever attempted to operate beyond the first few signs
of ammonia in the effluent.
At the very beginning, we started to discuss ammonia. Almost all
polishers start off life (after regeneration) as H - OH
cycle mixed beds. Under these conditions the cation equilibrium is one
between hydrogen and sodium until the ammonia (if any) works its way
down the bed. With a driving force of OH-
or NH4+ ions absent, the
polishers do not require the same degree of conversion to the
regenerated forms as when the ammonia does come through. The name of the
game definitely becomes trying to avoid deterioration of effluent
quality as the run passes from the hydrogen cycle to the ammonia cycle.
How can we observe the progress of the run and make intelligent
decisions concerning the status of the resin? At the very start of the
run, a recycle rinse, or rinse to drain, will push out that water which
has been sitting idle in the standby polisher in contact with the
resins. In a short time, we will be able to determine the health of the
polisher by measuring both inlet and outlet specific and cation, or Larson
Lane.¹ conductance as well as ionic sodium and silica.
The inlet specific conductance, when compared to the inlet cation
conductance, lets us know how much ammonia is being fed. The pH would
also be a means of verifying ammonia feed. The inlet cation conductance,
with the influence of any masking ammonia removed, permits us to examine
the sum of any influent chlorides and sulfates as their corresponding
acids. The effluent specific conductance (usually 0.1 microsiemens)
informs us that the ammonia is being removed by the polisher The
effluent cation conductance (if greater than the specific conductance)
indicated anion leakage of chlorides and/or sulfates. The Larson Lane
column acts are an analytical tool to change the ammonia or sodium
chloride into the corresponding acid, namely hydrochloric. The quantity
of anion leaking can be estimated by dividing the cation conductance
less the pure water blank, which is 0.055 at
, by approximately
0.007 and obtaining the leakage in ppb as CaCO3. For example, a cation
conductance of 0.13 microsiemens would indicate an anion leakage of
Sodium and silica leakage are generally determined by means of
continuous inline analyzers. Grab samples are very susceptible to
contamination and are not reliable in the range of sodium and silica
concentration generally found in condensate.
Suspended matter can be determined using membrane filter stain tests and
comparing the stain to a standard chart, or by means of total iron
and/or copper analytical testing.
The diagnostic tool used to monitor suspended solids during the run is
pressure drop across the bed. When a resin bed becomes fouled with crud,
the pressure drop will increase or, if no flow-balancing devices are in
use, the flow through that unit with the most service (more crud) will
On the H-OH cycle, the service run continues until the ammonia is no
longer being completely removed by the cation resin (often 3-5 days).
The first indication of ammonia break is a gradual rise in the specific
conductance of the polisher effluent. As the ammonia and the hydroxide
concentration increase in the effluent, the concentration of both sodium
and silica start to increase. The level of sodium or silica leakage, as
mentioned previously, is determined by the regeneration level that the
resin receives or, correspondingly, the concentration of sodium or
silica on the resin and on the degree of intermixing and on any
condenser leakage that may have occurred.
When operating beyond the ammonia break, the run proceeds until the
inlet and outlet conductivities are the same. At that point, the cation
resin is fully ammoniated. Ammonia is now passing unchanged through the
cation resin. Sodium influent to the polisher is removed by the cation
resin and the equivalent amount of ammonium ion exits the polisher. The
level of sodium leakage is determined by the influent pH, which is also
now the effluent pH, and the amount of sodium on the resin. Similarly,
the chloride and silica are exchanged for hydroxide anions and their
leakage is controlled by the equilibrium reactions discussed earlier.
Once the cation resin is fully ammoniated, the run can continue until
the cation resin is either exhausted with respect to sodium and leakage
exceeds what is tolerable, or when the pressure drop reaches intolerable
levels. High silica caused by anion exhaustion is also a reason for
terminating the run.
In spite of the best metallurgy and the tightest condensers, sooner or
later all plants experience condenser leakage. The seriousness of a
condenser leak is proportional to the concentration of solids in the
cooling water, and the magnitude of the leak. What matters to the
polisher is the quantity of solids, both dissolved and suspended, which
are flooding into it and occupying its capacity. During a condenser
leak, the influent dissolved solids are increased by the cooling
water, which is leaking through the ruptured condenser tube into the
recirculating condensate stream. The cation resin will exchange ammonium
ions for sodium and hardness, while the anion will exchange hydroxide
ions for alkalinity, chlorides, sulfates and silica. The increase in the
effluent ammonium and hydroxide levels causes a shift in the equilibrium
and the sodium, chloride and silica levels in the effluent will rise.
The magnitude of the leak and the degree of regeneation of the resins,
as well as how much sodium, silica, and chloride are on the resin, will
determine if the plant can continue to operate with the leak. The run
length and the time required to regenerate will decide if the leak can
be tolerated for a longer period. When operating with a detectable
condenser leak, increased regenerant is recommended to remove the added
ionic contaminants. In order to keep the sodium and silica leakage in
range, operation is usually restricted to the H-OH cycle during a
condenser leak and the run is terminated when ammonia starts leaking
into the effluent.
What ratio of cation resin to anion resin should we use? Resin ratio is
a term which has been loosely tossed around a great deal. Consider the
2:3 cation:anion ratio (by volume), which is the so-called
stoichiometdcally equivalent mixture. Referring to Table 1, we find that
the capacity of an 8% crosslinked cation (measured in the sodium form)
is 2.0 meq/mL. That of a porous gel anion (measured in the chloride
form) is 1.1 meq/mL. With a 2 parts cation: 3 parts anion ratio, each
cubic foot of mixed resin will have an ultimate capacity of 0.8 meq/mL.
of both cation and anion capacities as shown.
H+ Na+ OH-
* 8% 1.8 2.0 *Type 1 1.25 1.45
*10% 2.0 2.2 **Type 1 1.1 1.3 *Gel
Macro 1.7 1.75 Macro 0.9 1.0 **Porous gel
If the same resins, the 8% gel cation and the porous get anion, are
supplied in the hydrogen and hydroxide forms, the picture changes. The
cation capacity decreases from 2.0 to 1.8 meq/mL. and the anion capacity
decreases from 1.3 to 1.1 meq/mL. The cation contributes 0.72 meq/mL. to
the mixture and the anion only 0.66 meq/mL.
With a 10% gel cation, the cation in the hydrogen form has a capacity
of 2.0 meq/mL. and contributes 0.8 to the anion¹s 0.66 meq/mL. Suffice
it to say that the 2:3 ratio should be used as a guide and not as a
panacea, especially since we are talking about a balance of ultimate
capacities, not operating capacities.
How do you intend to design your polisher? Will it operate exclusively
on the H-OH cycle with a pH 9.2 influent? With a normally leak-free
environment, the cation resin will be hard at work removing ammonia
while the anion will be on hand just in case of a condenser leak. This
does not need a balanced system. A 2:1 ratio of cation:anion would seem
appropriate with the anion there as an "insurance policy."
If the designer is using a brackish cooling water with total dissolved
solids in excess of 500 ppm, and is anticipating certain pinholes leaks,
more anion may be justified. If the "design" leak condition would limit
operation to the H-OH cycle, then the cation load would consist of the
leak itself, namely the calcium, magnesium and sodium in the leak, plus
the ammonia in the feed. Perhaps a 1:1 ratio by volume might be
With operation beyond the ammonia break, consideration of operating
conditions becomes critical. The quantity of sodium, chloride and silica
which can be removed is determined by the pH of the influent condensate.
Some of the anion capacity is required to remove the car-bon dioxide,
which has entered the cycle along with air in leakage. Any leak which
would contribute chlorides, sulfates, CO2, silica and alkalinity would
also have entered the calculation.
We have considered condensate polishers which operate on the ammonia
cycle. Some condensate polishers operate exclusively on the H+ -
OH-cycle. Typical of these are those used in purifying condensate for
use in a boiling water reactor. In this case, these stoichiometrically
equivalent ratio would have to be considered since the NRC Regulatory
Guide #1.56 requires that the capacity utilization of both anion and
cation be limited to half the total or ultimate capacity.
During operation for extended run lengths, the resin in the polishers
can become fouled with crud or oxides of iron and copper. In order to
successfully regenerate these resins, they must be cleaned and this can
be accomplished by a combination of air scrub and backwash. To
successfully clean resin, it helps to have the capacity of scrubbing and
backwashing the resins individually after separation in order to obtain
sufficient fluidization of the cation.
1. Salem, E., "A Study of the Chemical and
Physical Characteristics of Ion Exchange Media Used in Trace Contaminant
Removal," American Power Conference, Chicago, IL, April 1969.
2. Emmett, J. R. and Grainger, P.M., "Ion
Exchange Mechanism in Condensate Polishing," International Water
Conference, Pittsburgh, PA, October 1979.
3. Darji, J. D. and McGilbra, H. F., "Ion
Exchange Equilibrium - A Key to Condensate Polisher Performance," American
Power Conference, Chicago, IL, April 1980.
4. Venderbosch, H. W., Snel, A., Overman, L. J.
and Kema, N. V., "Concerning the Capacity of Ion Exchange Resins When
Removing Trace Impurities from Water," VGB Feedwater Conference 1971.
5. Connelley, E. J., "New Ion Exchanger
Method for Extended Run Operation of Utility Condensate Demineralzers,"
Industrial Water Engineering, March/April 1979.