Demineralizers and Ion Exchangers Ivelisse Ortiz-Hernandez, PhD.
Objectives 1.1 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. Polymer b. Demineralize f. Mixed-bed demineralizer c. Cation g. Affinity d. Anion h. Decontamination factor 1.3 DESCRIBE the following: a. Resin bead b. Cation resin c. Anion resin 1.4 DISCUSS the following factors of ion exchange: a. Relative affinity b. Decontamination factor
Objectives 1.5 WRITE the reaction for removal of NaCl and CaSO4 by a mixed-bed ion exchanger such as 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. Channeling b. Breakthrough c. Exhaustion 1.10 LIST the maximum conductivity and approximate concentration of electrolyte for each level of purity for makeup water.
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.
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.
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.
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.
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.
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.
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
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.
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:
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.
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.
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.
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:
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.
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.
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.
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.
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.
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).
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.
Limitations of Ion Exchangers 1) Exhaustion, 2) Differential pressure 3) Temperature 4) Radiation Exposure
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.
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.
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.
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.
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.
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. 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.
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.
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. 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.
DESCRIBE the effects of channeling in a demineralizer. • decreases mechanical filtration and results in very little exchange occurring. http://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.
DESCRIBE the reason for demineralizer temperature limitations. The alcohol formed has no exchange capability. The amine has a lower exchange capability.
DESCRIBE the reason for demineralizer temperature limitations.
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 willremove 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.
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.