What's Inside your Water Softener: A Closer Look at Resin

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Author: Claire Beaureagard - Technical Representative

Water Softener Resin Beads
Water softening is a process in which water flows through a bed of resin to exchange the hardness ions, calcium and magnesium, for sodium ions.  When the resin has reached its capacity for holding hardness ions, the water softener initiates a regeneration cycle.  During this cycle, a sodium chloride brine solution flows through the resin and effectively reverses the process by exchanging sodium ions for hardness ions, and flushing the hardness ions down the drain.
Water Softener Cutout

What Is Resin?

Resin is the ion exchange media used commonly in water softening applications.  The most widely used resin in the industry is polystyrene-type gel resin.  This resin has a very porous, skeletal structure and each bead ranges in size from 0.3-1.2mm, containing approximately 45% moisture.  The building blocks of this type of resin are Polystyrene and Divinylbenzene (DVB).  To better understand the function of the resin bead and the failure mechanisms associated with it, consider the following analogy where the spherical sponge represents polystyrene and the elastic bands represent DVB.
Resin Rubber Band Ball
Several elastic bands are wrapped around the sponge that is compressed more and more with each elastic band that’s added.  The “bead” becomes stronger and more compact with this process known as “crosslinking”.  Crosslinking varies from 2-20% DVB content, but the most commonly used in softening applications are 8% and 10%.  The 10% crosslinked resin offers up to 50% longer life and 10% additional capacity than the 8% crosslinked resin. A higher degree of crosslinking leads to a decreased bead size and therefore a greater number of beads allowed per cubic foot of resin.  More beads per cubic foot effectively allows for more functional groups to attract hardness ions, resulting in a greater capacity.

How Long Does it Last?

Contrary to popular belief, resin does not last forever.  Throughout the life of a water softener, resin is under constant attack from hydraulic shock, oxidation, osmotic shock, general attrition, fouling and more. Resin manufacturers often use ten years as a general rule for expected lifetime, but this can change significantly depending on the conditions to which the resin is subjected.
Water Softener Resin Beads

What are the Failure Mechanisms?

There are many failure mechanisms associated with the resin bead and the following are brief descriptions of some of the most common:

Water Hammer (Hydraulic Shock)

The sudden interruption of high pressure water flow causes the resin beads to “slam” against the side of the tank and will lead to cracked and/or broken beads.  Avoiding fast acting solenoid valves in the system design is advisable to minimize this risk.

Oxidation (Chlorine Attack)

Consider chlorine attack to be analogous to “snipping away” the elastic bands around the spherical sponge causing the bead to lose strength, swell, and retain higher moisture content.  The number of ion exchange sites will remain unchanged with swelling but the resin beads now occupy a larger volume within the tank.  This can lead to cracked and/or broken beads.

Osmotic Shock

By nature, resin beads swell and contract as they exhaust and regenerate.  As time goes on, these beads will eventually crack and/or break.  The expansion rate during the backwash stage of regeneration is a function of flow rate and incoming temperature.  As such, the osmotic stress is greater in low temperature, high backwash-rate systems.

Resin Attrition

An increased pressure drop across the resin bed can often be attributed to a high percentage of cracked and/or broken beads.  Broken bead particles tighten the bed surface by filling the void spaces with bead particulate.  “Fines” will eventually leave the system in the backwash stage of regeneration and a reduced capacity in the softener will be observed due to the decrease in exchange sites.

Metal Fouling

Oxygen is introduced with brine during every regeneration cycle so iron fouling is to be expected in any system with elevated iron levels in the incoming water.   This iron oxide precipitant cannot be removed by regular salt regeneration and effectively plugs up resin exchange sites that would otherwise be available for softening, resulting in reduced capacity.
Water Softener Pre-Treatment

What Can You Do?

First and foremost, understanding the incoming water source is critical for predicting the life of the resin and properly maintaining the system.  The incoming water should be tested prior to installing a new water softener system and the type of resin should be carefully selected.  After the system is installed, the following are recommendations and available options to consider:
  1. Log Books: Daily hardness levels, water meter readings, flow rates and inlet/outlet pressure readings should be recorded on a regular basis for each softener. Monitoring these trends is a great way to identify when the system performance is declining and additional testing, cleaning, and/or replacing resin should be considered.

  2. Resin Testing: Core samples can be obtained from the resin bed and analyzed for total capacity, moisture content, percent broken and more.

  3. “Topping Up”: Some degree of resin attrition and resin loss is to be expected in any system. Oftentimes, a simple “top-up” of the tank is advisable to increase overall capacity.

  4. Chemical Additive: In extreme cases, a chemical additive such as Klenzoid's Resinklenz Fe should be considered.  This regeneration aid prevents iron from precipitating into iron oxide, improving performance and extending the life of the resin significantly.

  5. Chemical Cleaning: In heavily fouled systems, resin can be cleaned using a low pH solution such as Citric Acid.  In chemical cleanings, uniform exposure and contact time is critical.

  6. Filtration: Pre-filtration can be considered in cases to remove iron and other foulants from entering the softener system and fouling the resin.

  7. Resin Replacement: Though the initial cost for higher strength, higher crosslinked resin is greater, it may be the better option in the long-run and the economics should be considered.
Hands on Water Softener Pre-Treatment

Conclusions

Water softeners are a critical piece of equipment in many industries where failures can quickly lead to costly breakdowns.  Considering resin to be a maintenance item rather than the commonly forgotten about “beads inside that tank” is a great start to improved management of your water system.

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Claire Beaureagard has a degree in Mechanical Engineering from The University of Western Ontario. As a Technical Representative, she works with our clients to achieve the lowest operational costs for their heat transfer systems through well-maintained water treatment programs. Claire has a collection of unicycles she enjoys riding in her spare time.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity - any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.
Did we pique your interest on chloride-cycle dealkalizers? Click here to learn more...


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.
Are you a chemistry nerd? Click here to see the chemistry behind the release of carbon dioxide...


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.
Click here if you want to impress your water treatment professional with your knowledge of amines...


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.
Interested in this option? Click here to learn more about clean steam generators specifically.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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