Embed Size (px)
Citation preview
Demineralizers and Ion Exchangers
Ivelisse Ortiz-Hernandez, PhD.
1
Objectives1.1 LIST the three reasons for removing impurities from water prior to use in reactor systems.
1.2 DEFINE the following terms:a. Ion exchange e. Polymerb. Demineralize f. Mixed-bed demineralizerc. Cation g. Affinityd. Anion h. Decontamination factor
1.3 DESCRIBE the following:a. Resin beadb. Cation resinc. Anion resin
1.4 DISCUSS the following factors of ion exchange:a. Relative affinityb. Decontamination factor
2
Objectives
1.5 WRITE the reaction for removal of NaCl and CaSO4 by a mixed-bed ion exchanger suchas one containing HOH resin.
1.6 EXPLAIN the three basic methods used to remove dissolved gases from water.
1.7 LIST five filtration mediums used to remove suspended solids from water.
1.8 EXPLAIN how mixed-bed ion exchangers may be used to control pH.
1.9 DISCUSS resin malfunctions, including the following:
a. Channelingb. Breakthroughc. Exhaustion
1.10 LIST the maximum conductivity and approximate concentration of electrolyte for each level of purity for makeup water.
3
LIST the three reasons for removing impurities from water
• To minimize corrosion, which is enhanced by impurities.
• To minimize radiation levels in a reactor facility.– Some of the natural impurities and most of the corrosion products
become highly radioactive after exposure to the neutron flux in the core region.
• To minimize fouling of heat transfer surfaces.– Corrosion products and other impurities may deposit on core surfaces
and other heat transfer regions, which result in decreased heat transfer capabilities by fouling surfaces or blockage of critical flow channels.
– Fouling is defined as the accumulation of unwanted material on a surface.
4
STATE the purpose of ion exchange.
• It is the main process used to control the purity and pH of the reactor coolant.
• Many plants also use this process for feedwater chemistry control and water pretreatment.
5
DEFINE ion exchange
• Any process which results in the reversible exchange of ions contained in a fluid with those contained on a solid without a permanent change in the solid structure.
• Water is treated with an ion exchange resin.• These resins will replace undesirable ions with those
which are more acceptable within an aqueous process stream under a specific set of operating conditions.
6
DESCRIBE the two general types of demineralizer resins
• Ion exchange demineralizers use microscopic resin beads composed of an insoluble inert structural matrix and a chemically active functional group.
• The functional groups are molecules with exchangeable ions such as H+ or OH-, that can be safely released into the system.
• Cation resins exchange positively charged functional groups, for undesirable positive ions. – With their functional groups in the hydrogen form, R – H, “R” represents the
exchange resin and “H” represents the attached hydrogen ion.
• Anion resins exchange negatively charged functional groups for any undesirable negative ion. – The hydroxyl ion, OH- (R—OH) is commonly used as the functional group in an
anion resin.
7
DESCRIBE a typical ion exchange reaction
water.fresh form result wea As solution. in ions by the
exchanged are resin thein ions that theisresult net The
RR
:follows as is reactionnet The
arrow. by the indicated as reaction reversiblea is This
H
: waterform to
combine resin zingdeminerali by the released ions The
ions. chloride and ions sodium form
tosdissociateit water in dissolved is NaCl When
--
2
2
OHHClRNaClNaOHRH
OHOH
ClNaNaCl OH
8
DESCRIBE a typical ion exchange reaction
• The cation resin has a higher affinity for Na+ than for H+ and releases the H+ in the exchange reaction.
• The anion resin has a higher affinity for the Cl- than for the OH-, and releases the OH- in the exchange reaction.
• The reactions in the previous slide shows a typical demineralizer reaction.
• In reality, some of the water will leak through the resin allowing some untreated water to reach the reactor system.
• The greater the ionic charge the greater the affinity of the ion for the resin.
• Larger ions have a greater affinity than smaller ions.
9
DESCRIBE pH control utilizing the ion exchange process.
• pH is the measure of the relative acidity (or alkalinity) of a solution.• If a lithium form cation resin is used with a hydroxyl form ion, the effluent
will have a high pH and will be strongly basic, due to the exchange of lithium ions (Li+) for cation impurities and hydroxyl ions (OH-) for anion impurities.
• Cation exchange resins are classified as:– Strong acid– Intermediate acid– Weak Acid
• Anion exchange resins are classified as:– Strong Base– Intermediate base
• If you have sodium chloride, after the cation is exchanged we have HCl acid remaining in solution. (strong acid)
• If the cation in solution is magnesium and chloride ions are removed the result is magnesium hydroxide which is a weak base.
10
STATE the purpose of a demineralizer
• Demineralization is the removal of essentially all inorganic salts. In ion exchange demineralization hydrogen cation exchange converts dissolved salts to their corresponding acids, and basic anion exchange removes these acids.
• Purpose– Removal of ionic substances– Reduction of conductivity– Control of pH
11
Basic Definitions
• Regeneration is the treatment of the resin bed (chemical) to replace impure cations and anions.
• The spent regenerant containing the undesirable ions is then discarded to the plant wastewater system.
• Leakage is the very small, almost undetectable amounts of undesirable ions that continuously pass through the demineralizer without being exchanged.
12
Describe the principles of demineralizer operation
• The demineralizer system consists of one or more ion exchange resin columns, including a strong cation unit and a strong anion unit. The cation resins exchange hydrogen for raw water cations as shown below:
13
Describe the principles of demineralizer operation
• The anion resins exchange hydroxyl for raw water anions.
• In the example, the acids resulting from the cation exchange process react with the anion exchange resin. As a result we form water and the anions are embedded into the resin.
• Other weak acids are also removed because the resin is strongly basic.
14
Describe the principles of demineralizer operation
• This reactions are equilibrium. Not all ions will be removed by the demineralizer.
• The leakage will vary according to the demineralizer system used, the raw water mineral composition and the demineralizer regenerant level (the amount of acid and caustic used for regeneration).
• To minimize leakage the resins must be regenerated with reverse flow.
15
16
17
18
Describe the effect of demineralizer operation on water conduction
• Conductivity of the water decreases by removing the salts from solution and replacing them with protons and hydroxide ions.
• Variables monitored to check the performance of the demineralizer include:– Silica concentration and water conductivity
• Demineralizers that must remove silica use strong base anion resins, and both the silica content and conductivity are important water quality criteria in determining the effectiveness of resin.
• Both silica content and conductivity increase at the end of the service run.
19
DESCRIBE why silica is monitored.
• Demineralizers that must remove silica use strong base anion resins, and both the silica content and conductivity are important water quality criteria in determining the effectiveness of resin.
• The silica level, nearly constant during the entire service run, increases sharply at the end; conductivity, also nearly constant during the service run, drops briefly at the end and then rises as shown below:
20
• During the normal service run, most of the effluent conductivity is attributed to the small level of sodium hydroxide produced in the anion exchanger (a small amount of sodium always leaks through the resin).
• When the capacity of the anion resin is exhausted, silica leakage converts the sodium leakage to sodium silicate, a material less conductive than NaOH.
• A typical mixed bed demineralizer will rinse down to low conductivity and silica values after regeneration.
21
Three types of demineralizers are commonly used:
• Anion demineralizers, containing anion resin• Cation demineralizers, containing cation resin• Mixed bed demineralizers containing both
cation and anion resins. Used at the end of the water treatment chain as a water polisher.
22
STATE the two basic types of solutions used during resin regeneration.
• The cation exchange resin is regenerated with acid, typically hydrochloric or sulfuric acid.
• The anion exchange resin is regenerated with an alkaline solution. Sodium hydroxide (caustic soda) is the most common anion regenerant.
23
Regenerating a Mixed Bed Resin
• When regenerating a mixed bed resin, regenerant must not flow through the wrong resin. This can destroy a resin's ion exchange capability. One method of preventing a regenerant passing through the wrong resin is to transfer resins from the demineralizer to separate regeneration tanks.
• Once the resin has been regenerated for a specified time, it is ready for rinsing. In the rinsing step, resin is flushed with pure water to remove any residual regenerant and any insoluble materials which may have broken loose during the regeneration step.
• In a mixed bed resin, the resins must be remixed.
24
25
The resins are separated in two different regeneration tanks.
26
STATE the two basic types of filterdemineralizer construction.
• The deep-bed and the powdered resin filter demineralizer.• Deep Bed
– water enters at the top and then passes through the depth of mixed resin, through strainers which shouldn’t pass the resin beads, and exits at the outlet.
– As the water passes through the resin mixture, ion exchange takes place and mechanical filtration of suspended solids occurs.
– The effective length of time that a batch of resin can be used is called the operating cycle.
• Powder Resin Filter Demineralizer– These demineralizers use a filter element pre-coated with ground up
mixed-bed resins.
27
STATE the advantages and disadvantages of both deep-bed and powdered resin demineralizers.
• The main advantage of a deep-bed demineralizer is the large ionic capacity which will allow time for an orderly plant shutdown if a significant condenser tube leak occurs.
• Two disadvantages of a deep-bed demineralizer concern chemical regeneration and channeling.– The chemical regeneration of the resin requires large volumes of regenerate
solutions. Large amounts of water are needed to rinse and transfer the waste.
– Since water will take the path of least resistance through the resin bed, the overall flow distribution is uneven. This uneven distribution can deteriorate to the point where a “channel” or channels form in the resin bed. Filtration capability decreases over time.
• Excessive pressure drop due to the build up of solids will also be a problem (decreasing the operating cycle).
28
STATE the advantages and disadvantages of both deep-bed and powdered resin
demineralizers.• Powdered Resin Filter Demineralizers• Advantages:
– Radwaste requirements are lower. The powdered resin is not cleaned or regenerated. Filter is just disposed of.
– More efficient mechanical filtering device.• There are two major disadvantages to the Powdered Resin
Demins:– Lower ionic capacity—exhausts resins rapidly and allow leakage of
ions.– “Sloughing” – Potential for internal mechanical failure allowing water
to pass through the demineralizer without ion exchange.
29
30
Limitations of Ion Exchangers
1) Exhaustion,2) Differential pressure3) Temperature4) Radiation Exposure
31
Resin Exhaustion
• When a resin has reached the limit of its ability to undergo exchange, some of the impurity ions will pass through the resin without exchanging.
• This will result in an increase in conductivity of the effluent (in the case of a demineralizer).
• If an ion exchanger is used instead, conductivity will not be greatly affected.
• The demineralization factor is determined.• Specific conductivity in mhos, radioactivity in μc/ml, and
concentration of impurities in ppm are common parameters checked.
32
STATE the reason for sampling both the inlet and outlet conductivity of a demineralizer.
• To check for the conductivity and the demineralization factor (to determine if the demineralizer is operating correctly).
• Specific conductivity in mhos, radioactivity in μc/ml, and concentration of impurities in ppm are common parameters checked.
33
DEFINE the term “demineralization factor (DF)” as it applies to a demineralizer
• DF is a direct measure of demineralizer efficiency. Exhaustion of demineralizer resins can be predicted using a concentration–history curve which is a plot of the DF versus time.
34
tyConductivi Final
tyConductivi InitialDF
removed. whereimpurities theof %96
96
10010025
water. thefrom
removedty conductivi ofamount therepresents x where
25x-100
100
tyConductivi Final
tyConductivi InitialDF
:DFfor equation the Using100. isty conductivi initial that theassume Lets
x
x
35
tyConductivi Final
tyConductivi InitialDF
Question from NRC testing
The ion exchange efficiency of a condensate demineralizer is determined by performing a calculation using the...
A. change in conductivity at the outlet of the demineralizer over a period of time.
B. change in pH at the outlet of the demineralizer over a period of time.
C. demineralizer inlet and outlet conductivity.
D. demineralizer inlet and outlet pH.
36
Procedure: Use the DF to calculate the percentage of impurities removed. 100 – percentage removed = percentage of the initial conductivity remaining Fraction of conductivity remaining times the initial conductivity will give the final conductivity.
37
38
DESCRIBE the effect of excess differential pressure on demineralizer performance.
• The pressure drop across the demineralizer is a function of flow rate. Excess differential pressure results in an increased decay in the performance of the resin.
• As the demineralizer acts similar to a filter, a definite pressure drop (and flow rate) is desired.
• As corrosion products and suspended solids are accumulated over the service run, the flow resistance and therefore the ΔP, increases until eventually the filter performance is compromised.
– Filter is not able to remove unwanted ions as well.
39
As velocity increases pressure decreases.The smaller the area the greater the velocity.
• Monitoring the ΔP provides useful information on demineralizer performance.– Higher than normal ΔP may indicate the bed is clogged
due to buildup of corrosion products and suspended solids, or may indicate high flow through the demineralizer.
– Lower than normal ΔP indicates the demineralizer is operating at reduced efficiency.
40
DESCRIBE the effect of excess differential pressure on demineralizer performance.
• High differential pressure can be caused by resin overheating, crud buildup, and high flowrate through the resin.
• If excess solids are accumulated the rate of unwanted ion removal decreases.
41
STATE the purpose for a demineralizer differential pressure gauge.
• To determine the pressure drop in the demineralizer. As the amount of solids in the filter increases area for the water to flow will decrease and the velocity will increase.
• As velocity increases the differential pressure will increase.
42
As velocity increases pressure decreases.The smaller the area the greater the velocity.
DESCRIBE the reason for demineralizer flow limitations.
• Excessive flow can result in several adverse effects:
– May reduce the rate of ion exchange due to insufficient time for exchange to take place.
– Resin beads may be forced through the lower retention element into the demineralizer effluent.
– High flow can physically bread down resin beads into “fines” which will also pass through the retention element.
– High flow can result in channeling of the bed, which decreases mechanical filtration and results in very little exchange occurring.
43
DESCRIBE the effects of channeling in a demineralizer.
• decreases mechanical filtration and results in very little exchange occurring.
44http://www.lakos.com/downloads/literature/LS-brochures-English/LS-827_TechPaper_SandMediaFilterPrefiltration.pdf
DESCRIBE the reason for demineralizer temperature limitations.
• Inert resin bead structure is stable up to ~ 300°F
• Anion resin begins to decompose slowly at ~ 140°F, and rapid decomposition begins at ~180°F.
• Cation resin is stable up to 250°F.
45
Checking your knowledge
46
DESCRIBE the reason for demineralizer temperature limitations.
47
The alcohol formed has no exchange capability.
The amine has a lower exchange capability.
DESCRIBE the reason for demineralizer temperature limitations.
48
DESCRIBE the demineralizer characteristics that can cause a change in boron concentration.
• The hydroxide ions of the anion resin have a very high affinity for borate ions. When a new resin is placed in service, the hydroxide ions of the anion resin are readily replaced by the borate ions.
• If the new bed is not boron saturated, when placed in service, the bed will remove borate ions from the RCS water until the resins become saturated.
• As the bed becomes boron saturated, it loses the high affinity for borate ions and the affinity for chloride and iodide ions is greater than that for the borate ions.
49
OHBORROHBOH 23333 33
• Increasing temperature can affect the boron affinity for the resin– At lower temperature, the borate ion bonding to
the exchange site contains three boron atoms. – At higher temperatures, the borate ion contains
only one boron atom.
• At low temperature Boron is more effectively removed than at high temperatures.
50
STATE the reason for bypassingdemineralizers.
• In systems where it is possible to subject the demineralizer resins to high temperature (CVCS), the demineralizers have automatic features that bypass the demineralizers on high temperatures to protect the resins.
51
STATE the reason for using mixed bed demineralizers to process primary water.
• Mixed bed and cation ion exchangers are installed in the primary system to provide purification of primary water.
1) filter out suspended crud particles, 2) remove soluble ions by exchange
3) assist in the maintenance of pH of the primary water, reducing the rate of corrosion.
52
DESCRIBE plant evolutions which can cause crud bursts.
• Reactor trips• Rapid heat up or cool down• Reactor coolant pump starts• Chemical shock of the primary system
53
STATE the definition of boron saturated as it relates to a demineralizer
• The hydroxide ions of the anion resin have a very high affinity for borate ions. When a new resin is placed in service, the hydroxide ions of the anion resin are readily replaced by the borate ions.
• As the bed becomes boron saturated, it loses the high affinity for borate ions and the affinity for chloride and iodide ions is greater than that for the borate ions.
• If the new bed is not boron saturated, when placed in service, the bed will remove borate ions from the RCS water until the resins become saturated.
54
STATE the definition of lithium saturated as it relates to a demineralizer
• A lithium saturated resin is used in the same way as the boron saturated resin.
• If the resin is not saturated it will remove lithium from the water instead of adding it.
• pH can be raised by placing a lithium saturated mixed bed in service rather than a chemical addition.
55
DESCRIBE the effect of temperature on saturated ion exchangers.
• The boron affinity of a resin bed is affected by the temperature of the coolant passed through the bed.– At lower temperature, the borate ion bonding to the exchange site
contains three boron atoms.– At higher temperatures, the borate ion contains only one boron atom.
• At lower temperatures, the resins are more efficient at removing boron from the coolant than at high temperatures.
• A boron saturated resin bed will actually release boron as temperature is increased.
56
Characteristics of the Mixed Bed Demineralizer used in the plant
• The mixed bed demineralizers contain 30 ft3 of mixed resin. • The cation resin used is 99.7% Lithium-7 based and the anion resin is OH-
based resin. • This resin is purchased in the LiOH form or lithiated in the system. • Enriched lithium is used to convert the resin to the lithium form so the
production of tritium is minimized. • The anion fraction of the mixed bed is converted to the borate form when
the bed is initially placed in service. • The bed is designed to contain equal number of anion and cation
exchange sites so it is loaded with 1/3 cation resin and 2/3 anion resin. • Each mixed bed demineralizer is designed for up to 120-gpm letdown
flow. • Used only for one fuel cycle.
57
• Each bed is designed to provide a decontamination factor (DF) of 10 even with 1% failed fuel. This designed DF specification is for isotopes subject to ionic exchange excluding Rb-86, Mo-99, Cs-134, Cs-137.
• The cation bed is placed in service as needed to control these isotopes.
• The cation demineralizer is loaded with 20 ft3 of hydrogen form cation resin. It is placed in service as necessary to remove lithium and also to control Rb, Cs, and Mo as previously stated. It must also control Cs-137 to less than 1.0 ci/gm even with 1% failed fuel. – Maximum flow through the cation demineralizer is 60 gpm. – Used only for one cycle
58
Decontamination Factor
• % Eff. = ((DF-1)/DF) * 100
59
When the contaminants are mostly soluble, the resin should perform normally. When the contaminants are mostly insoluble, the resin ion exchange capability will have little effect on the particles and the bed will only remove them by mechanical filtration. This can cause DF values to decrease.
60
D/P Cell
• Diaphragm or bellows connected between 2 points in a system that can be used to measure:– Flow
– Pressure
– level
61
LOWPRESSURE
CONNECTION
MOVEABLEWALL
SPRING BELLOWS
HIGH PRESSURECONNECTION
SEALINGBELLOWS
LINKTO
POINTER
EQUALIZINGVALVE
H
L
D/P
OVERTRAVELSTOP
OVERTRAVELSTOP
62
• Low press. Inside bellows exerts a force on the movable wall
• Hi press. Outside the bellows exerts a force on the opposite side of the wall
• Greater the press. On hi side, the more the wall will compress the bellows against the spring
• Bellows type d/p cells have large movements over a full range of pressures
• Stronger/weaker springs can change the range
63
• Measure the difference between applied pressures across the bellows
• D/P cells have an equalizing valve for removing the instrument from service
• Prevents overloading measuring element by exposure to high pressure on one side only
• Follow procedure when placing D/P cells on service
OE22585, Unanticipated Boration When Placing A Mixed Bed Demineralizer In Service (Farley,
April 7, 2006 )• At the beginning of an outage, it is necessary to ensure low
lithium concentration in the RCS as part of the crud burst and chemical cleanup process.
• The 1A demineralizer was scheduled to be replaced a week prior to the shutdown to remove lithium.
• The new resin is a lithiated resin that must be flushed to decrease the release of lithium.
• The new resin was not prepared until the day of the shutdown and the lithium output after performing two 1000 gallon flushes the lithium output was to high and they did not have time for more flushes (concentration of 1.12 ppm).
64
• They decided to place the 1B demineralizer in service which has been used during the previous shutdown. This will release a low concentration of lithium and should be able to decrease its concentration.– Problem: The demineralizer contained large amounts of boron and
needed to be rinsed. They rinsed it once with 1000 gallons and the boron concentration was found to be 5.54ppm. The existing RCS boron content was 9ppm so they concluded that it was ok.
– When the demineralizer was put into operation it was found that 15 minutes later the temperature started to decrease. The operator informed the shift supervisor.
– The operator was instructed to bypass the demineralizer and it was later found that a sudden increase in the boron concentration caused the problem.
65
• Cause:– The 1A demineralizer was scheduled to be replaced but
proper procedures and scheduling was not set into place.– By not replacing the demineralizer with enough time, they
did not have enough time to flush the unit and prepare it for the outage.
– Due to lack of time, 1A could not be used and they decided to use 1B which was highly borated.
– They did not know the impact of using a borated column and did not know the concentration of solids.
– Self checking was not applied.
66
About Boron• During the reactor operation the concentration of lithium and
boron is controlled to keep the pH between 7 and 7.3.• Boron is used to control reactivity
– Boron concentration is decreased as the fuel is depleted.– The boron on the surface of the fuel pellet allows the core to have
more reactivity designed into it. – Due to the proximity of the boron to the uranium, the boron-10 is
converted to boron-11 which has a very low cross section for neutron absorption.
– The RCS boron concentration must be increased to make-up for this burn up until all of the boron-10 on the fuel pellets is converted to boron-11.
– Boron concentration is increased until one third of the activity is used and then is decreased over time.
67
68
• Lithium is removed by intermittent operation of the CVCS Cation Demineralizer.
69
70
71
