BASIC ION EXCHANGE SYSTEM TROUBLESHOOTING

Some people thought that when their ion exchange system (softener, dual bed, mixed or polisher) does not perform according to their expectation, they simply think that the resin has a problem. More often than not, the resin is NOT the problem. For whatever reason, many service technicians blame the resin before they thoroughly troubleshoot the system. There are several accounts when unit seems to be working perfectly but the treated water is bad, they easily relate the problem with the “bad” resin. Here’s the truth. Yes, resin do fail. RESIN gets old like the rest of us. It can become fouled. But that is not a resin fault. Yes, resin beads break into fines and are backwashed out of the unit. This is not a fault of the resin because the beads are not made of steel. FOR WATER SOFTENERS – First, make certain that all mechanical components of the system are functioning properly. Easy to say, but for some reason, some people do not regularly do this in enough detail, either on purpose or due to lack of sufficient knowledge. Second, be on a lookout for a change in the feed water. Take note if your feed water is chlorinated, as this will chemical oxidize your resin. If your source is a private well, Dissolved Iron present in the feed water can foul your resin. Third, look for changes in water usage (daily water consumption or increased flow rates or leaks after the system), as your system may be treating more than it can handle. Fourth, having done all that, check how much resin is in the softener. Finally, while you are in there, take a look at the resin. How old is it? Does it appear to be mushy? Does it appear to be fouled? Is it lighter in color than it should be? Is it darker in color than it should be? The general stability of ion exchange RESIN depends on several factors. Deterioration can be physical, chemical or a combination of both. RESIN beads can crack and/or break due to OSMOTIC SHOCK – The shrinking and swelling effect resin is exposed to during regeneration and the service run. These "plastic" beads shrink during the service run and swell during regeneration. Think about that when you are bending a metal coat hanger and it finally breaks. Too high a salt dose or concentration will cause excessive swelling. Running the RESIN to complete exhaustion increases the shrinking. Fully exhausted RESIN swells faster even with a standard salt dose and concentration. Pressure on the beads caused by high feed water pressure or by excessive resin fines or sediment causing increased pressure on the beads will cause bead cracking and breaking. Broken beads (fines) and sediment if not adequately backwashed out will aggravate the condition. Chlorine in the feed water will degrade the RESIN by decrosslinking – the dissolving effect of Chlorine on the Divinylbenzene (DVB) Crosslinking in all RESIN. Iron or Manganese fouling (accumulation of Iron or Manganese within the beads will cause bead cracking and breaking. Frequent defouling procedures on the resin bed will cause damage. The list of causes of shortening of the RESIN LIFE is a long one. If all things are equal and the softener is properly operated and maintained and the feed water quality is not of a RESIN damaging nature, you should expect between Two (2) million to Ten (10) million gallons processed per cubic foot of RESIN or 10 years and longer.

Improving the Quality of Deionizers

When minerals dissolve in water, they dissociate into positively charged cations and negatively charged anions. The demineralization process exchanges these ions with hydrogen (H+) and OH– in the ion exhange resins producing "pure" water in the effluent.

Although both the cation and the anion resin are responsible for the quality of a deionization (DI) system effluent, it is the cation resin that is the big contributor to leakage. Improving the leakage characteristics of DI cation will reduce effluent conductivity, drop the pH and will lead to better silica removal efficiency.

The effluent of a dual bed DI system is generally alkaline or high pH (8.5 to 10). This is because the low pH of the cation reaction (pH 2.5 to 3.0) causes some of the residual sodium left on the resin from the previous regeneration to "leak" during the run. Sodium is the least tightly held of the cations. In the anion bed, this sodium is converted to sodium hydroxide (NaOH), which causes the high pH. One ppm of Na+ in the effluent will give a pH of about 9.5 and 4 ppm will result in a pH of around 10.0. This pH will also give rise to an increase of silica in the final effluent (leakage from the anion). It is, therefore, apparent that improving the leakage performance of the cation resin is key to overall good product quality from the system.

One way to reduce overall cation leakage is to increase the acid dosage during regeneration of the cation resin. In fact, the regenerant level is about the only controllable variable. However, there is a question in the economics for increasing the acid dose compared to the increase in capacity and reduction of leakage. Typical acid levels are approximately 8 pounds per cubic feet of resin.

Merlin TV Series Fansite - Merlin TV Show

Merlin TV Series Fansite - Merlin TV Show

Cation and Anion Leakage and the Factors Affecting Them

As feed water flows down the cation bed, cations are exchanged for H+, thus reducing the pH. The higher the total dissolved solids (TDS), the lower the pH and the higher the leakage of sodium. Higher percentages of influent sodium also will increase the sodium leakage because more sodium will be left on the cation exchanger after regeneration. This is why it is not recommended to soften feed water going to a DI system. Since the acid produced by the cation exchanger affects leakage, water that is high in chlorides and sulfates will cause a higher Na+ leakage. Conversely, waters that are high in bicarbonate will give more favorable cation leakage characteristics. One way to reduce overall cation leakage is to increase the acid dosage during regeneration. In fact, the regenerant level is the only controllable variable. However, there is a rapid drop off in the economics for increasing the acid dose compared to the increase in capacity and reduction of leakage. Typical acid levels are approximately 8 pounds per cubic feet of resin.

On the other hand, anion resins love strong acidic anions such as chlorides and sulfates but have a tougher time removing the weak ionized acidic anions such as bicarbonates and silica. This especially is true if there is a considerable Na+ leakage from the cation, which converts to NaOH in the anion exchanger. Efficient operation of the anion exchanger is not concerned with the overall TDS level because the removal of the acidic anions results in a neutral pH, as shown in the reaction:

HCl + R*OH → RCl– + HOH(H2O)

However, the anion is affected adversely by high levels of weak acids (which we already have deemed beneficial to the operation of the cation.) Again, increasing the regenerant level will improve the operation of an anion but the gains are limited by economics. Typical caustic levels are approximately 8 to 9 pounds per cubic feet of anion.

Typical two-bed effluent with 400 to 500 ppm feed water would be a conductivity of about < 5 microsiemens, a pH of 9.5 and silica leakage of 20 ppb.

GUIDELINES FOR SELECTING RESIN ION EXCHANGE OR REVERSE OSMOSIS FOR FEED WATER DEMINERALISATION

Prepared by: Purolite International

FACTORS TO BE CONSIDERED

a) Reliability
Both RO and IX are well established reliable technologies with a good track of performance world wide. Future technical developments are not expected to have a major influence on plant and process costs.

b) Feed Water Pre-treatment Required
Both processes require pre-treatment to remove suspended solids to a low level to avoid fouling. However IX is more tolerant of suspended solids and RO requires additional pre-treatment by micro-filtration. Membranes are also subject to scaling by hardness present in the feed water and require either a softening plant as part of the feed water pre-treatment or the addition of anti-scaling chemicals.

c) Quality of Treated Water
IX can produce demineralised water with a conductivity of less than 0.5uS/cm from a simple SAC-SBA combination and less than 0.1μS/cm with the addition of a mixed bed SAC/SBA unit.

Even the best performing RO plants cannot meet the treated water quality of a simple IX plant and a subsequent IX unit is required to achieve boiler feed water quality. (SAC is strong acid cation resin, SBA is strong base anion resin).

d) Flexibility
Ion exchange plants tend to be more flexible than RO, for example in terms of performance over a wider range of temperature variations and the ability to recover from high suspended solids in the feed.

e) Fouling by Organics
Both RO membranes and IX resins can be fouled by organics present in the feed water. IX resins are much more easily cleaned than RO membranes without long plant shut down and use cheap cleaning chemicals; salt and sodium hydroxide.

However RO has a place in producing demineralised water and when used in combination with IX can produce the highest quality boiler feed water. The role of RO is in treating high TDS waters and in order to establish guidelines for selection of RO versus IX it is necessary to carry out detailed cost comparisons.

Softener Design for Co-current and Counter-current Operation

To design a co-current or counter-current plant, determine the resin operating capacity based on a set of operating conditions and then apply correction factors for the specific conditions of the design.

- First, you have to have feed water quality analysis. Determine Hardness Concentration and TDS concentration of feed water in ppm as CaCO3. To determine concentration of TDS in ppm as CaCO3, determine concentration of all ions present in water (cations and anions) usually in ppm, and convert to ppm as CaCO3. To convert ppm to ppm CaCO3:

ppm of ion x (molecular weight of CaCO3/atomic weight of element)x(valence of element/valence of CaCO3)

Say we need to convert 9.2 ppm (mg/L) of Na ion to ppm CaCO3 (mg/L CaCO3),

MW of CaCO3 = 100

MW of Sodium = 23

Valence of Sodium = 1

Valence of CaCO3 = 2

Na (ppm CaCO₃) = 9.2 ppm x (100/23)x(1/2) = 20 ppm

- Set desired regenerant level, refer to resin data sheet (60-320 g/L). Determine Base Operating Capacity, C_B (Kgr/ft³) based on Figure 1:

- Set Service Flowrate (5-10 GPM/ft²) and determine Correction Factor C₁ at the set Service Flowrate and TDS concentration of feed water based on Figure 2:

- Calculate Operating Capacity (Kgr/ft³): 0.9(C_B x C₁), applying the 90% design factor.

- LEAKAGE is calculated as follows:

Determine Base Leakage, K_b @ set regenerant level based on Figure 3:

And Correction factor, K₁, for the TDS value based on Figure 4

Hence permanent (kinetic) leakage (as ppm CaCO3)= K_b x K₁

- Calculate the capacity required to handle the total exchangeable cation content of the feed for the desired feed rate and cycle time. First, determine the total cation content in the feed water as ppm CaCO3. Subtract from this value the sodium content of the feed as ppm CaCO3. The resulting number is the total exchangeable cation content as ppm CaCO3, divide this by 17.1 to obtain grains/U.S. gallon (grpg).

- Resin Volume = (feed rate, in GPM x Cycle time in minutes x total exchangeable cation, in grpg)/(operating capacity in Kgr/ft³ x 1000)

Calculate the flow rate per unit volume. If this number is outside the range of 1 - 5 gpm/ft³, modify the cycle length and resin volume to bring it within this range.

- Size the bed to this volume, keeping bed depths ≥36 inches (0.91 m). Calculate Softener Tank diameter from set Service Flowrate in #3. Resin Volume must be 60% of tank volume.

Packed Bed DI System: A new but proven counter-current configuration in Ion Exchange Technology

In a Packed Bed DI System, feed water enters the packed bed upward in the service cycle and downwards in the regeneration cycles. In the upward service cycle, the resin bed is lifted up in a compacted state, which minimizes the need for regular backwashing. As the water progressively comes into contact with the more regenerated resin, which is the resin in the upper portion of the bed since the regeneration is downward, high quality water production is ensured with lower ionic leakage.

Effluent from the regeneration of Packed Bed DI System is generally neutral due to the equivalence of cations to anions, also, acid and caustic are both introduced into the packed bed at the same time during regeneration.

Fully packed resin tanks give higher throughput compared to conventional systems. Counter current regeneration provides better water quality compared to co-current design. It also facilitates efficient and shorter regeneration time.

Dealkalization by Ion Exchange

Dealkalization is the reduction of Alkalinity in water. Alkalinity is caused by bicarbonates, carbonates or hydroxides in water. Two primary methods of Dealkalization by Ion Exchange are executed using the ff:

a. Weak Acid Cation (WAC)
b. Strong Base Anion (SBA) Chloride Form

Strong Base Dealkalization method utilizes SBA resins in the Chloride (Cl) form and is the most commonly used method for Dealkalization in the USA. The SBA method is most commonly used to dealkalize water for commercial applications such as boiler feed or RO pretreatment. It can also be used to dealkalize water for residential applications.

Weak Acid Dealkalization is preferred when the influent water is high in hardness and alkalinity and has hardness to alkalinity ratio of 1 or more. This process uses weak acid cation resin to exchange hydrogen for hardness that is associated with alkalinity. The treatment is most effective if followed by degasification to remove CO2.

Factors that Affect Resin Life

Ion exchange resins are manufactured to last for a long time. However, there are factors that can impact resin life. Some of the major factors are described below.

Temperature
Ion exchange resins have a recommended maximum operating temperature as indicated in their product data sheets. These temperature maxima are intended only as guides. Thus, a temperature limitation does not mean that the resin will be unstable above and stable below this temperature. It should also be recognized that thermal degradation is proportional to the product of time and temperature. When exposed to higher than the recommended temperature, however, the resin will often lose its functional groups, which will result in loss of capacity and reduced resin life.

Oxidation
Oxidants attack the polymer crosslinks, which weakens the bead structure, or by chemically attacking the functional groups. One of the most common oxidants encountered in water treatment is free chlorine (Cl2). Hydrogen peroxide (H2O2), nitric acid (HNO3), chromic acid (H2CrO4), and HCl can also cause resin deterioration. Dissolved oxygen by itself does not usually cause any significant decline in performance, unless heavy metals and/or elevated temperatures are also present to accelerate degradation, particularly with anion exchange resins.

Although weak base anion resins are more stable than strong base anion resins, they can oxidize and form weak acid groups. When this occurs, the resin tends to retain sodium and requires a greater than normal volume of rinse water following regeneration.

Chemical attack on a cation exchange resin usually results in the destruction of the polymer crosslinks, resulting in an increase in water retention capacity and a decrease in the total wet volume exchange capacity.

Fouling
It is a irreversible sorption or the precipitation of a foulant within resin particles can cause deterioration of resin performance. Common foulants for resins are Silica and Iron. It is better to prevent fouling by removing the foulant before the water flows through the resin beds, rather than try to clean the foulant from the resin. Where fouling conditions are prevalent, proper resin selection can minimize resin fouling.

Osmotic Shock
Exposure of resins to high and low concentrations of electrolytes can cause resin bead cracking and splitting due to the alternate contraction and expansion of the bead. Over time, there may be significant reduction in particle size and an increase in resin fines, causing increased pressure drop across the resin bed during system operation and subsequent resin losses during backwash and regeneration. Ion exchange resin particle size is an important factor related to osmotic shock. Smaller beads are more resistant to breakage than larger particles.

Physical Degradation
Bead breakage due to mechanical attrition can occur when the resin is subjected to unusual mechanical forces, such as a crushing valve, a pump impeller, or an abrasive action during the movement of resin particles from one vessel to another. The broken beads will maintain the same operating capacity as whole perfect beads, but they are more prone to fluidization during backwash, and may be lost. In addition, the small fragments will fill the void spaces between the whole resin beads, resulting in increased pressure drop across the bed. Large beads are more subject to mechanical attrition than smaller ones.

Radiation
Since ion exchange resins are organic polymers, they can be affected by radiation. Generally, cation exchange resins are adequately stable for almost all reasonable applications involving radioactivity. Anion exchange resins are less stable although generally adequate for use in radiation fields.

A new innovation in resin production is now in full blast..the Shallow Shell Technology. It creates high efficiency ion exchange resins. Under a microscope, “Shallow Shell” resins look very different from other resins because the resins have inert core. Only the outer shell is functionalized which shorten the ion exchange diffusion path. This leads to more efficient ion exchange and regeneration and better handling of iron and organic foulants.

With increasing demand for better performing resin coupled with lower operating costs, shallow shell resins is seen as a solution. These resins enable a more complete regeneration and provide a higher, more efficient utilization of the regenerant, lower leakage, and reduced rinse water requirements. When compared to conventional softening or demineralization resins, regenerant cost are seen to reduce by 20 to 50%, without sacrificing capacity.

Advantages of Shallow Shell Resin:

• Higher Recovered Capacity


• Lower Leakages at All Regenerant Levels


• Better Iron Removal


• Lower Rinse Requirements


• No equipment Modifications Needed


• Excellent For High TDS Waters


• Shorter Regeneration Cycles


• Superior Physical Strength


• More Resistant to Oxidation


• Lower Iron and Organic Fouling

To regenerate ion exchange without the use of commercial salt is highly desirable. For ion exchange water softeners treating brackish water feed to RO plants, such a solution already exists. By using shallow shell softening resin and some engineering, the reject from the RO can be used as “free regenerant” brine to efficiently regenerate this unique resin while adding no extra salt to the environment.

(Shallow Shell Technology)SST resin exhibit much higher regeneration efficiency than standard resin, permitting the use of more dilute brine concentrations and lower salt dosages than recommended for standard resins. Reason to this higher regeneration level is the unique outer shell and inner core structure of the resin bead. Ion exchange takes place only in the shell area with the core being totally inert. The diffusion path for cations is therefore shorter than that for standard resin, divalent cations (e.g. calcium, magnesium, barium, strontium) are not exchanged deep in the core of the beads unlike standard resin in which divalent cations migrate deep into the center of the resin beads. The efficiency of removal of these deeply trapped divalent cations essentially determines how well the resin performs during the next service cycle. With SST resin, the time for the brine to diffuse to the shell-core interface is lower, resulting in more highly regenerated beads.

Pilot studies showed that brine concentrations as low as 1% can be used to regenerate
shallow shell resin.

Resin Longevity: Expected Life Span of Ion Exchange Resins

The operating life of ion exchange resin depends on several factors. Degradation of resin can be attributed to mechanical, osmotic or thermal shock; temperature; dissolved oxygen; and chemical oxidation due to attack of chemical like chlorine.

DI resins usually last for many years. In general, cation resin for water softening and demineralization may last 5 to 10 years. Anion resins last anywhere from 3 to 5 years and are dependent on operational conditions. Some of the resin beads break during the swell cycle when regenerating. Moreover, resin life is partially dependent on the number of regenerations and partially on the quantity of oxidizers passed through the column.

You can prolong the life of your resin provided the following circumstances are met:

- Adequate pre-treatment is in place, i.e. organics and suspended solids are removed and kept to minimum.

- Chlorine content in feed water is zero to undetected (determined through water quality analysis). In the presence of chlorine or any oxidant, ion exchange resins will breakdown prematurely.


- Low levels of iron in feed water. Cation resin removes ferrous iron but removing the iron off of the resin is a difficult which will result in loss of capacity overtime due to iron being embedded into the cation bead.

- There is no sudden and significant increase in raw water quality that would affect the performance of pre-treatment system which will in turn affect the quality of feed water to the ion exchange resin. Ion exchange resin system is designed based on feed water quality, therefore include safety factor in the design to handle occasional “spikes” in feed water quality.


- Operators are aware of the proper operation and maintenance of ion exchange system.

Because many variables and factors are involved, it's difficult to predict the life span of a resin and we can only provide you with what is the expected life span given ideal conditions; in critical applications it’s best to start analyzing and benchmarking the resin at least once per year.