Learn how the life expectancy of resin is affected by usage and regeneration.
Ion exchange resins do not last forever. They are hydrophilic plastic beads that shrink and swell in use as they exchange for various ions and are then regenerated with a variety of salts, acids, and bases. Cycling between exhaustion and regeneration leads to rapid changes in size which weaken the beads physically and cause them to crack and break. Their use often requires or results in fluidization of the beads, either by backwash, turbulence during the exhaustion cycle, or resin transfer. Fluidization causes beads to collide with one another, leading to erosion of the beads surface as small pieces of the bead break off.
The physical degradation of the beads aside, their very function and the contaminants they attract (despite their ability to be regenerated) can impact their life expectancy overtime. The filtration properties of resin often result in suspended solids becoming trapped among the beads. Their coalescing and agglomeration properties can result in dissolved contaminants collecting on the surface and submicron particles growing into much larger ones. Thermal and oxidative forces damage the plastic chemically and either weaken the plastic or cleave the ion exchange functional group or both. Foulants in the feedwater or in the regenerant chemicals can cause loss of capacity, coat the surface of the beads, fill up the void spaces in between the beads, and in some cases exchange into the resin at such high preference they effectively poison the exchange sites and prevent other ions from exchanging in and out.
And all this is before the resin is physically lost through a breach in the underdrain retention screens or lost by excessive backwash flow rates. Considering the risks, it’s a wonder that IX resins last as long as they do.
Managing Expectations
Estimation of expected resin life and resin replacement intervals is perhaps the most difficult cost parameter of the overall IX system OPEX to predict in advance. This is because the factors that determine need for replacement depend not only on how hard the resin is used and what the feedwater is like, but also on feedwater variability, performance expectations, level of risk the end user is willing to assume, how closely the system performance is monitored and the ability to be proactive about addressing problems.
The shortest resin life the author has experienced was an anion failure after two weeks use in the hydroxide form. The cation resin upstream was damaged by chlorine but not so severely that replacement was necessary on the grounds of capacity loss. Nonetheless the degraded cation resin released polystyrene sulfonate leachables at a concentration sufficient to poison the new anion resin within a few days of use. The impaired anion was not only unable to remove CO2 and silica, it also didn’t remove all the chlorides and sulfates, resulting in an effluent pH from the demineralizer below 5.
The longest resin life the author has experienced was a salt form cation resin (water softening) more than 25 years old and still performing at about 3/4 of its original design capacity. This resin was used in a municipal gravity type softener with very low exhaustion flow rate, modest brine dose, clean well water as feed source, and excellent attention by the plant operators.
These two examples are the exception, not the norm, and it is possible to define resin life with reasonable accuracy, given a few provisos regarding event-oriented problems that trigger immediate need for replacement.
Before getting into the heart of this discussion, the best estimate of resin replacement intervals going forward is historical. If a site has been replacing resin about every three to four years, it’s a pretty safe bet the next replacement will be at a similar interval. The importance of keeping track of replacements and the reason for them, along with other records of system performance cannot be overstated.
Factors that Affect Resin Life
Number of Exhaustion and Regeneration Cycles
Salt form resins are good for around 3,000 to 5,000 exhaustion and regeneration cycles. Hydrogen and hydroxide form resins around 1,000 to 3,000 cycles. Externally regenerated polisher and portable exchange resins are good for 300 to 600 transfer cycles. Past this point resins seem to get old and tired and even if they still have good bead integrity and capacity, they just don’t work the way they did when they were new.
Water Temperature
Cold, clean feedwater is ideal. Resins used in warm climates don’t last as long. Water temperature less than 60ºF extends life while water temperature above 80ºF limits life. Above approx 160ºF the presense of oxygen becomes an increasingly important factor in resin life and above 190ºF oxygen attack becomes the dominant factor. Acrylic anion resins in the hydroxide form have exceptionally short life if water temperature is greater that 75ºF (less than 2 years)
Chlorine and Other Oxidants
Chlorine and other oxidants damage resin chemically and physically. Aside from physical resin loss through a breach in a distributor screen or excess backwash flow rate, oxidation is the number one reason for resin replacement.
Highly crosslinked resins hold up better than lower crosslinked one. Anion resins generally hold up to chlorine better than cation resins. Acid form cation resins deteriorate faster than salt or base forms. Anion resins downstream of oxidized acid form cation resins are rapidly impaired by the cation leachables.
Operation and Maintenance
Systems with excellent operation and maintenance protocols last longer than systems that are maintained only when something breaks. Resins used in municipal systems generally last longer than resins used in industrial systems. Resins that are frequently cleaned do not last as long as resins with adequate pretreatment.
Aqueous Solutions
Resins used in solutions other than water and in aqueous solutions with high ionic strength (TDS greater than approximately 5000 ppm) do not last as long as resins used in water, especially if they are frequently regenerated. Damage is exclusively physical.
Weak Versus Strong Resins
The weak resins (weak acid cation and weak base anion) have greater swelling and shrinking between exhausted and regenerated forms and tend to have higher physical breakage than the strong resins (strong acid cation and strong base anion)
Resin Tranfer Versus Stagnancy
Systems that frequently transfer resin have higher physical breakage than systems where the resin remains in a single vessel its entire life.
Resin transfer damage is exclusively physical, never chemical. Pumped resins are damaged faster than resins transferred by syphon or water/air pressure.
Exhaustion Direction
Upflow exhaustion results in greater physical breakage then downflow exhaustion.
Flow Rate Stabilty
Fluctuating flow rates or water hammer results in greater physical breakage than constant flow and gentle changes in pressure.
Pressure Loss
A large pressure loss (greater than approximately 20 psid) across a resin bed leads to greater physical breakage.
Factors Affecting Resin Life & the Most Common Grounds for Replacement
(In the absence of oxidants, overly warm water, suspended solids, fouling, or physical mishandling)
1 Strong Acid Cation (SAC) - Chlorine attack, fouling, and physical loss can affect all forms.
2 Weak Acid Cation (WAC) - Physical breakage can occur due to large swell factors between regenerated and exhausted forms. Also suspended solids fouling and precipitants that cement the beads together can affect all types.
3 Strong Base Anion (SBA) - Loss of salt splitting capacity can affect all types while salt form and Type 1 hydroxide form can also be affected by organic fouling.
4 Weak Base Anion (WBA) - Loss of capacity and fouling are the most common concerns.
5 Mixed Bed (MBD) - Loss of capacity and fouling are the most common concerns.
6 Chelating - Loss of capacity and fouling are the most common concerns.
Closing Thoughts
The opinions offered in this discussion are those of the author through more than 50 years of working with ion exchange resins and are offered in good faith. They do not, however, represent a guarantee of any kind regarding resin life. Some applications punish IX resins yet remain the best available technology for certain types of ion separations. Sometimes the conditions of use will result in short life no matter what but in most cases of short life, improvement is possible.
ResinTech’s founder, Michael Gottlieb built the business on the premise that “we help our customers build better working ion exchange systems”. If you are reading this document and believe your resin life is shorter than it should or could be, you are welcome to discuss possible solutions with a knowledgeable Ion Exchange Technologist here at ResinTech.
About the Author
Peter Meyers, the former Technical Director for ResinTech, Inc., had a fascination with ion exchange that began in high school chemistry class and never ended. Over the course of his 28-year career at ResinTech, Meyers became one of the water treatment industry’s most prolific technical writers and a highly sought-after presenter and session leader. He wrote scores of articles for virtually every industry trade publication and spoke over a hundred times at conferences and trade shows that serviced the water purification market. His expertise covered a wide range of ion exchange topics from demineralization, condensate polishing, and water softening to industrial process design and operation. Mr. Meyers’ name also appears on five U.S. Patents related to ion exchange technology, most notably “Method of making and using modified anion exchange materials with metal inside the materials” which is the basis for ResinTech’s arsenic removal resin, ASM-10-HP.