Ion Exchange Softeners and Dealkalizers

Ion exchange resins are used extensively in commercial and industrial water treatment to improve the quality of water prior to its intended use. Water softeners are used in homes, laundries, hospitals and manufacturing plants to remove hardness from water. In washing operations soft water produces a pleasing soap lather, rinses cleanly and saves on detergent. Soft water is routinely used as boiler makeup to minimize scale build up on heat transfer surfaces and in other systems, such as cooling towers, where water hardness may interfere with the operating efficiency of heat transfer equipment.

Ion exchange systems are used in the production of dealkalized or demineralized water. These waters are of higher purity than soft water. They are required in applications such as high pressure boilers, semiconductor and electronics manufacturing, pharmaceuticals and metal finishing.

Ion exchange applications are also found in the treatment of waste waters. In particular, ion exchange columns are used to recover metals from plating operations and in the treatment of cooling tower blowdown prior to discharge or recycle.

This paper discusses the role ion exchange plays in the treatment of water for industrial use. In this discussion we will focus on the following topics

  • Water quality – hardness, alkalinity and pH

  • A brief history of ion exchange

  • Types of ion exchange resins

  • How does the ion exchange process work

  • Regeneration procedures

  • Ion exchange systems

  • Troubleshooting

  • Safety issues

WATER QUALITY – HARDNESS, ALKALINITY AND pH

Water impurities exist as dissolved or suspended solids. As water flows over the ground or percolates through the earth’s crust, it picks up impurities. Inorganic minerals like limestone and quartz dissolve into water adding to its dissolved solids content. These are primarily calcium, magnesium and sodium compounds. Other common impurities include iron, manganese and silica. In this way, the quality of a particular water supply is strongly influenced by local mineral deposits.

Water also contains suspended solids. Suspended solids, as the name implies, are not dissolved in water, but exist as insoluble particles and colloids. These include sand, clay, and silt. Surface waters contain higher levels of suspended solids than ground waters, but wells located in sandy areas can contain higher levels of insoluble material.

Ion exchange is used to remove dissolved impurities from water supplies. Two common applications are softening and dealkalization. The softening process removes hardness. Dealkalization is used to remove or reduce alkalinity.

Total Hardness refers to the calcium and magnesium content of the water. By definition, the calcium and magnesium concentration determines the water hardness. The water may also contain sodium, iron and silica, but these substances are not defined as hardness, just the calcium and magnesium.

Total Alkalinity is defined as the amount of bicarbonate (HCO3), carbonate (CO3) and hydroxide (OH) alkalinity in the water. Most ground and surface waters contain bicarbonate alkalinity in equilibrium with carbon dioxide (CO2). Carbon dioxide is a soluble gas. A few waters may have some naturally occurring carbonate alkalinity, in which case, there will be no free carbon dioxide. A water sample can not contain all three forms of alkalinity (bicarbonate, carbonate and hydroxide) at the same time. Other dissolved ions like chloride, sulfate and phosphate do not contribute to total alkalinity.

pH is the measurement of the hydrogen ion concentration of the water. The pH scale runs from 0 to 14 with pH 7 being neutral. pH values below 7 are termed acidic. pH’s above 7 are basic. pH is related to the total alkalinity of the water, but pH and alkalinity are not the same. Generally, the higher the total alkalinity, the higher the pH.

All substances dissolve in water to form ions. The ions have a positive or negative charge. Positively charged ions are called cations. Calcium (Ca ++) and magnesium (Mg ++) hardness are positively charged ions and are, therefore, cations.

Negatively charged ions are called anions. Bicarbonate (HCO3 ), carbonate (CO3 -2), and hydroxide (OH ) alkalinities are all negatively charged, and are therefore, anions.

The sum total of all cations must equal the sum total of all anions to maintain electrical neutrality in the water.

COMMON CATIONS AND ANIONS

Cations

Anions

H+

OH

Na+

Cl

Ca++

HCO3

Mg++

CO3

Fe++

SO4

Fe+++

PO4

A BRIEF HISTORY OF ION EXCHANGE

As early as 1845, H. S. Thompson noted that garden soils had the ability to exchange calcium for ammonia when a solution of liquid manure was poured through them. Later, in the 1850’s, this “base exchange” property of soil was attributed to the presence of zeolites, a class of naturally occurring minerals consisting primarily of silica and alumina oxides.

In 1905, Robert Gans discovered that zeolites could be used to remove calcium and magnesium from water. Several natural and synthetic zeolites were identified and used in commercial water softening equipment. Shortly thereafter, stabilized greensand, another naturally occurring mineral, was shown to be effective in removing calcium, magnesium, iron and manganese from water. Greensand, although not as efficient as zeolite, was more durable and, thus, was the mainstay of ion exchange technology for over 20 years.

In 1934 and 1935, several new ion exchange materials were developed that offered significant performance improvements over greensand. This work culminated in the development of a synthetic ion exchange material made by the sulfonation of a resin produced by the copolymerization of styrene and divinylbenzene. This material rapidly replaced greensand because of its higher exchange capacity, more efficient regenerant consumption and improved hydraulic characteristics.

Today, synthetic ion exchange materials are used almost exclusively for water softening, dealkalization and demineralization. Because of their past history, however, modern synthetic ion exchange resins are still commonly called zeolites or greensand.

TYPES OF ION EXCHANGE RESINS

Ion exchange resins are small, bead-like particles manufactured with a styrene-divinylbenzene copolymer (DVB) backbone. These gel-type resins are made with an 8% or 10% crosslinking of the copolymer structure. The degree of crosslinking determines the “strength” or durability of the resin.

Four fundamental types of ion exchange resins are used in water treatment. All of the resins use the same DVB chemical backbone. The primary difference between the resin types is in the functionality of the ion exchange sites located on the DVB backbone.

Strong acid cation resins contain a sulfonic acid group on the exchange site. They are capable of removing all cations (positively charged ions) associated with strong and weak acid salts. These resins are used in a wide variety of applications, but are commonly found in sodium ion exchangers (water softeners) used for routine hardness removal, hydrogen dealkalizers, and for cation removal in demineralization systems.

Weak acid cation resins are capable of removing calcium and magnesium hardness associated with alkalinity. Non-carbonate hardness is not removed by weak acid cation resin. The primary advantage of weak acid resins is their higher regeneration efficiency as compared to strong acid resins. Weak acid resins are frequently used ahead of strong acid resins to reduce the cost of producing demineralized water.

Strong base anion resins are capable of removing the anions (negatively charged ions) of strongly and weakly dissociated salts. Used in either the chloride form (regenerated with salt brine) or the hydroxide form (regenerated with caustic soda), strong base resins are used in chloride dealkalizer systems and demineralizer trains to remove or reduce alkalinity and silica.

Anion resins are further classified as either Type I or Type II. Type I and Type II resins differ in the functional groups located at the exchange site. Type II resins have a higher exchange capacity and regeneration efficiency than Type I resins. But Type II resins are not as durable as Type I resins. The regeneration efficiency of Type II resins tends to degrade rather rapidly. Eventually, the operating performance of Type II resins degrade to the point where they match the exchange characteristics of Type I resins.

Weak base anion resins are used to exchange all anions except the weakly dissociated silica anion. Weak base resins regenerate more efficiently than strong base resins. As a result, weak base resins are commonly used where complete silica removal is not required. They are also used in combination with a strong base exchanger to improve the efficiency of multi-bed demineralizers.

Ion Exchange Resin Suppliers

Four manufacturers of ion exchange resins exist in the United States. Each offers an extensive product line consisting of several types and grades of ion exchange materials. Other companies market imported resins, or serve as a distributor of resins that are marketed under their own trade names.

MAJOR U.S. SUPPLIERS OF ION EXCHANGE RESIN

Company

Trade Name

Dow Chemical

Dowex

Miles, Inc.

Wofatit and Lewatit

Purolite Co.

Purolite

Resin Tech

ResinTech

Rohm and Haas Co.

Amberlite

Sybron Chemicals, Inc.

Ionac

Thermax

Thermax

Mitsubishi Kasei

Diaion

Although some physical and chemical differences exist between the various brands of ion exchange resin, these differences are often minor and do not affect the overall performance of the resin in most applications. The following table offers a comparison between the product equivalents marketed by the four major U.S. resin manufacturers.

ION EXCHANGE RESIN PRODUCT EQUIVALENTS

Cation Resins

Amberlite

Dowex

Ionac

Purolite

IR-120 Plus

HCR-S

C-249

C-100

IR-122

HGR

C-250

C100X10

IR-200

MSC-1

CFP-110

***

Anion Resins

Amberlite

Dowex

Ionac

Purolite

IRA-400

SBR

ASB-1

A-600

IRA-402

SBR-P

ASB-1P

A-400

IRA-410

SAR

ASB-2

A-300

HOW DOES ION EXCHANGE WORK?

Ion exchange resins selectively remove cations (positively charged ions) and anions (negatively charged ions) from water by replacing the ion located at the exchange site on the resin for the ion dissolved in the water. A cation resin exchanges cations and an anion resin exchanges anions.

The exchange sites are active molecular sites located along the styrene divinyl benzene backbone of the resin. The functional units for cation resins are sulfonic acid and carboxylic acid. For anion resins, it’s a quaternary amine group. In both cases, the functional sites serve as the source of the exchangeable ions.

For example, a strong acid cation resin in the sodium form (common water softening resin), has a sodium ion (a cation) located at the exchange site. As water flows past the exchange site, the sodium is exchanged for other cations that are dissolved in the water such as calcium and magnesium hardness. Iron, another cation, is also exchanged for sodium at the exchange site. This produces a treated effluent containing no hardness, i.e. soft water.

If the cation resin is in the hydrogen form, meaning that hydrogen (a cation) is located on the exchange site; cations will be exchanged for hydrogen. This is the scenario for a hydrogen dealkalizer system. Calcium, magnesium, iron and sodium are exchanged at this site for hydrogen. The treated effluent has a high concentration of hydrogen ions, but no hardness. Recall from the discussion on pH, water that has a high hydrogen ion content has a low pH. The pH from a hydrogen dealkalizer is below 3.0. Water with a pH below 4.3 contains no carbonate or hydroxide alkalinity. All of the carbonates have reacted with hydrogen to produce carbonic acid and free mineral acidity (FMA). Treated water from a hydrogen dealkalizer is effectively softened and dealkalized, but the water cannot be used without subsequent neutralization of the FMA and corresponding upward pH adjustment as low pH water is corrosive to most metals.

Anion resins are used in either the chloride form or the hydroxide form. Here the dissolved anions are exchanged for either chloride or hydroxide. Strong base anion resins in the hydroxide form exchange hydroxide for strongly dissociated (ionized) anions like sulfate and chloride. Other weakly ionized anions like carbonate, bicarbonate and silica are also exchanged by strong base anion resins. Weak base resins also exchange anions, but are unable to remove weakly ionized anions like silica.

REMOVAL OF IONS BY ION EXCHANGE RESIN

Resin

Removes the following ions…..

Strong acid

Removes all cations (positive ions)

Weak acid

Calcium, magnesium and sodium associated with carbonate alkalinity

Strong base

Removes all anions (negative ions) including silica and carbon dioxide

Weak base

Removes chloride and sulfate

ION EXCHANGE CAPACITY

Ion exchange resins do not have an infinite capacity for ion exchange. As the ion exchange process continues, all of the exchange sites are used up or exhausted by the dissolved ions in the water. At this point the resin is no longer capable of exchanging ions and must be restored to its original ionic form by regenerating with a strong solution of salt brine, acid or caustic soda.

The ion exchange capacity of the resin is a measure of the amount of dissolved ions that can be exchanged by the resin between regenerations. The exchange capacity is a function of the resin type, amount of regenerant used per cubic foot of resin, and regenerant flow rate. The resultant exchange capacity is expressed in Kilograins per cubic foot of resin (Kgr/ft3), or milliequivalents per gram (meq/gr).

One grain is equal to 1/7000th of a pound. Most water analyses report hardness and alkalinity values in units of parts per million (ppm) or milligrams per liter (mg/l). In this case, concentrations reported in ppm are equal to mg/l, that is, ppm and mg/l can be used interchangeably. Ion exchange manufacturers still work with water analyses expressed in grains per gallon, however, instead of ppm or mg/l. To convert water quality data from ppm or mg/l to grains per gallon, simply divide ppm by 17.1. For example, water having a total hardness of 171 ppm contains 10 grains per gallon of hardness.

Resin manufacturers report the exchange capacity of the ion exchange media in Kilograins per cubic foot of resin. (1 Kilograin is equal to 1000 grains). This provides an estimate of the amount ofcations or anions that can be removed by the resin between regenerations.

TYPICAL ION EXCHANGE CAPACITIES

Resin

Regeneration Level

Exchange Capacity

Strong acid cation

5 lbs Salt per ft3

17.8 Kgr/ft3

Strong acid cation

5 lbs Acid per ft3

12.5 Kgr/ft3

Strong base anion Type I

4 lbs Caustic per ft3

11 Kgr/ft3

Strong base anion Type II

4 lbs Caustic per ft3

21 Kgr/ft3

The suggested regenerant strength, regeneration level and resultant exchange capacity are available from the resin supplier, equipment manufacturer, or water consultant.

Increasing the regeneration level will provide a higher exchange capacity. This is not a linear relationship, however. Doubling the salt dosage does not double the exchange capacity. The most efficient salt dosage for industrial softeners is between 6 and 8 pounds of salt per cubic foot of resin.

REGENERATION PROCEDURES

Once the exchange capacity of the softener has been exhausted, it is necessary to regenerate the unit to remove the accumulated ions and restore the ion exchange resin back to its original chemical form. Various types of regeneration chemicals are used for this purpose depending on the water treatment requirements. Cation resins are regenerated with sodium chloride (salt), sulfuric acid or hydrochloric acid. Anion resins are regenerated with sodium chloride, caustic soda or ammonium hydroxide.

The regeneration level is a measure of the amount of regenerant required per cubic foot of resin. Typically, strong acid cation units are regenerated with 6 pounds of sodium chloride per cubic foot of resin, or 6 to 8 pounds of sulfuric acid (as 100% acid) per cubic foot.

The regeneration level for anion resin is typically 4 pounds of caustic soda (as 100% caustic) per cubic foot of resin.

These regeneration levels may vary from one installation to another. Since the exchange capacity of the resin increases with increasing regeneration levels, some plants use more acid and caustic per cubic foot to extend the run times of the ion exchange equipment. This is at the expense of efficiency, since the Kilograins of dissolved solids removed per pound of regenerant decreases at higher regeneration levels. In other cases, the resin may be old and require a higher regeneration level to meet the water quality specifications.

The chemical regenerants are mixed with water prior to entering the ion exchange equipment. For softening equipment, the salt is added to a brine tank to produce a saturated brine solution. Saturated brine contains 2.5 pounds of salt per gallon of brine. This is equivalent to approximately 25% to 26% salt. The saturated brine is then pumped or educted into a dilution water flow to produce a minimum 8% brine solution. Under ideal conditions the resin bed should be regenerated with a minimum 8% brine solution for at least 20 minutes. Enough brine is introduced into the ion exchange bed to achieve the required regeneration levels.

Sulfuric acid is used to regenerate strong acid cation units in the hydrogen form. Bulk sulfuric acid (93%) is pre-diluted in a day tank to yield a 20% acid solution. This is then educted or pumped into a dilution water line where it is diluted to the proper concentration prior to entering the cation unit. Normally the regeneration is carried out in a stepwise fashion. First a 2% acid regenerant is introduced to remove most of the calcium from the bed. Then a 4% acid is used to complete the regeneration. This prevents the formation of unwanted calcium sulfate precipitants that can foul the resin. In cases where calcium sulfate precipitation is of particular concern, the acid is introduced in 2%, 4%, and 6% stages. If hydrochloric acid is used instead of sulfuric acid, the stepwise regeneration procedure can be eliminated. Hydrochloric acid is more expensive than sulfuric acid, however. In either case, enough acid must be dosed per cubic foot of resin to achieve the desired regeneration level of 6 to 8 pounds per cubic foot.

Anion resins in the chloride form are regenerated with sodium chloride (salt). Caustic soda is used for anion resins in the hydroxide form. Here bulk liquid caustic soda (50%) is mixed in a day tank to achieve a 20% working solution. The caustic is educted or metered into a dilution water line where it is diluted to a 4% solution prior to entering the anion vessel. Sufficient caustic is used to achieve a regeneration level of about 4 pounds per cubic foot of resin.

OPERATION SEQUENCE

Ion exchange equipment is operated in one of three modes

  • Service

  • Regeneration

  • Standby

Service runs vary depending on the feedwater quality, bed volume, and regeneration level. For example, a sodium softener containing 86 cubic feet of ion exchange resin is operated at a regeneration level of 10 pounds of salt per cubic foot of resin. This produces an exchange capacity of 25 Kilograins per cubic foot. The total exchange capacity of the softener is (25 Kgr/ft3 X 86 cubic feet) or 2,150 Kilograins softening capacity.

If the feedwater to this softener has a total hardness of 300 ppm (300 ppm / 17.1) or 17.5 grains per gallon, the total service capacity of the softener is (2,150,000 grains / 17.5 grains per gallon ) = 122,857 gallons between regenerations. For practical purposes, most softeners are not kept in service until completely exhausted. The softener is frequently removed from service at 80 to 90% of full capacity. In this example, 85% of full capacity would be 104,500 gallons.

A similar calculation can be performed on a hydrogen dealkalizer to determine the estimated service run. The total exchange capacity of the bed is calculated based on the acid regeneration level and resultant total exchange capacity of the bed just as with the sodium softener.

Regeneration of ion exchange equipment consists of:

  • Backwashing

  • Chemical injection

  • Slow rinse

  • Fast rinse

Backwashing: Ion exchange resin is an excellent filter media. Suspended solids in the feedwater are readily trapped in the resin bed where they can cause fouling if not removed. The purpose of the backwash step is to remove these unwanted foulants. Backwashing also lifts and expands the resin prior to regeneration to insure optimum contact between the resin and the regenerant chemicals.

The backwash step is carried out from the bottom up, counter to the direction of the service flow. The flow of the backwash water lifts, suspends and expands the resin bed. This backwash flow carries the suspended solids, resin fines and other debris down the drain.

The backwash flow rate is regulated to expand the bed by 50 to 100% of it service volume. If the backwash rate is too high, however, some of the resin will be washed out of the vessel along with the dirt and debris. If the backwash water temperature changes, the flow rate must be adjusted to prevent this from occurring. As the water becomes colder (more dense) the flow rate must be reduced. Warmer water requires higher flow rates. The technical literature supplied with the resin contains detailed information on the required backwash flow rates for that particular type and grade of resin.

Chemical Injection: The regeneration of ion exchange resin is accomplished by the introduction of chemicals that remove the adsorbed ions from the exchange sites, and restores the resin to its original chemical form. Cation resins are restored to the sodium (salt regeneration) form or hydrogen (acid regeneration) form. Anion resins are restored to the chloride or hydroxide form.

The concentrated regenerants in the storage tank are diluted to the proper concentration prior to entering the ion exchange vessel. This must be precisely controlled or the resin will not be regenerated properly. Too low of a regenerant concentration will reduce the exchange capacity of the bed. Too much regenerant wastes chemical and decreases the regeneration efficiency.

During the regeneration step, samples of dilute regenerant are collected to determine the percent concentration. This is easily accomplished by using a hydrometer which measures percent brine or specific gravity. The results of the hydrometer measurements are then used to make adjustments to the acid or brine strength.

Slow rinse: At the conclusion of the chemical injection step, the regenerant flow is stopped and the dilution water flow is used to “push” the remainder of the chemical regenerant through the resin bed. This completes the regeneration and makes certain that all of the regenerant is utilized efficiently.

Fast rinse: After all the regenerant has passed through the resin bed, the unit is rinsed under full flow conditions. This removes any last traces of regenerant and makes sure the water quality meets the finished product specifications. In high-purity applications, like demineralized water systems, the fast rinse continues until the water meets a minimum conductivity standard. In other cases, the fast rinse continues for a preset time period.

Standby Mode is utilized if the softener or dealkalizer is not to be placed into service immediately at the conclusion of the regeneration. If an ion exchange unit sits in standby for a prolonged period (more than a day), the exchange equilibrium tends to reverse at the exchange sites. This can sometimes adversely affect the quality of the product water. For this reason, ion exchange units are placed into a service rinse mode for 5 to 15 minutes, or until the product water meets a minimum standard before it is placed into service. This rinse cycle flushes any unwanted dissolved solids from the resin bed and guarantees that the product water is acceptable for use. If not, the unit is placed into standby mode again until the problem can be corrected.

After a successful service rinse, the softener or dealkalizer is placed back into service.

ION EXCHANGE SYSTEMS

Design engineers use various combinations of ion exchange units to produce a final product water of any desired quality. Some of the systems are very elaborate and incorporate a series of weak acid, strong acid, weak base, or strong base exchange beds. Most, however, are very simple in concept and design. The more common system designs are for water softening, dealkalization, and demineralization.

Water softeners are used in a wide variety of industrial applications. Here a strong acid cation resin in the sodium form is used to remove calcium and magnesium. The calcium and magnesium hardness is exchanged for sodium, which does not contribute to the water hardness. In this case, the total dissolved solids of the water remains the same, since softening does not reduce the amount of mineral solids in the water.

Softening systems are comprised of an ion exchange vessel ( two or more vessels, if an uninterrupted supply of soft water is required). A salt brine tank is provided, and a control system to regulate the regeneration process. The regeneration can be controlled by a timer that initiates the regeneration at a preset time and day, or by a water meter that starts the regeneration after a preset number of gallons have passed through the softener.

Hydrogen dealkalizers are used to remove hardness and alkalinity from the raw feedwater. Here the hydrogen form of the cation resin is used to convert natural bicarbonate (HCO3) alkalinity to carbonic acid and free mineral acidity (FMA). Once formed, the carbonic acid (H2CO3) readily breaks down to release free carbon dioxide (CO2) and water (H2O), which can be easily removed by simple aeration. Since the strong acid resin replaces all the cations with hydrogen, the effluent from the dealkalizer is both softened and acidic. Generally, the pH of the effluent is less than 3.0.

Water from the hydrogen dealkalizer can be neutralized by blending with effluent from the sodium softener. Any desired alkalinity and pH can be achieved by regulating the percent blend of hydrogen dealkalized and sodium softened water. After blending, the water is passed over an aerating tower (or decarbonator) to remove the free carbon dioxide released in the neutralization step.

Chloride anion dealkalizers are used as an alternative to hydrogen dealkalizers. These units are regenerated with salt brine just like sodium cation softeners. The carbonate and bicarbonate alkalinity is removed by the anion resin and replaced with chloride. Frequently, a sodium cation softener is combined with a chloride-anion dealkalizer to produce a final product water that is softened and dealkalized. The total dissolved solids have not been reduced by this method, but the softened, dealkalized water is suitable for use as makeup in many boiler feedwater applications.

The primary advantage of dealkalization, whether by hydrogen or chloride anion methods, is the reduction of carbonate alkalinity in the boiler feedwater. Under boiler temperatures and pressures, carbonate alkalinity breaks down to release carbon dioxide into the steam. This depresses the pH by forming carbonic acid in the steam condensate. Eliminating the carbonate alkalinity from the boiler feed reduces the CO2 in the steam and helps protect the condensate system from corrosion attack.

Demineralization or deionization achieves complete removal of all ions. The result is high –purity water with extremely low conductivity (high resistivity). This is accomplished by used a strong acid cation unit in the hydrogen form in series with a strong base anion unit in the hydroxide form. The cation unit exchanges hydrogen (H+) for all the cations, producing an effluent that has free mineral acidity (FMA) and a pH of around 3. The effluent from the cation unit is passed through the anion exchanger where the anions are exchanged for hydroxide (OH). The hydroxide (OH) in the anion effluent reacts with the hydrogen (H+) from the cation unit to produce water (H+ + OH = H2O). In this way, all the mineral salts are removed from the feedwater producing a final product water of exceptional purity.

The effluent from the cation unit will contain free carbon dioxide (CO2). Although carbon dioxide can be removed by the strong base anion exchanger, it is more economical to remove this gas in a degasifier. This also extends the service runs on the anion unit. For these reason, a degasifier is often installed between the cation and anion units on waters that are high in carbonate alkalinity.

TROUBLESHOOTING ION EXCHANGE PROBLEMS

Ion exchange design engineers take every precaution to guard against operating problems. Nevertheless, problems can and do occur. The more common problems have the following symptoms:

  • Loss of thruput capacity

  • Reduction in water quality

  • Increased pressure drop across the bed

Loss of Thruput Capacity

A loss of capacity is indicated when the amount of water produced between regenerations declines. These shortened service runs are caused by a variety of factors. Here are some things to check when this problem occurs.

  • Did the feedwater quality change? Higher dissolved solids in the feedwater will give shorter runs.

  • Has the bed level decreased? A loss of resin during the backwash cycle or because of a broken underdrain lateral will reduce the exchange capacity of the bed.

  • Was the last regeneration sequence completed properly? Check the regenerant levels in the storage tanks, the regenerant concentrations, flow rates and times.

  • What is the condition of the resin? Resin that has degraded chemically or physically will not produce acceptable water quality. Collect a sample for laboratory analysis.

Reduction in Water Quality

The ion exchange system is unable to produce water of acceptable quality. The reasons for this often baffle the experts, but here are some things to check.

  • Raw water leaking past a valve. Valves that do not seat properly, or leak because of wear and tear will allow raw, untreated water to contaminate the treated effluent. Check each valve for proper operation.

  • Depending on the type of unit, old resin can suffer from ion “leakage”. Check the chemical and physical condition of the resin. Calcium precipitation in the cation unit can also be a problem.

  • Fouling of the resin bed by organics, iron, oil, microbiological growths or dirt. These contaminants will adversely affect the regeneration of the resin and alter the flow through the bed.

  • Poor quality or off-spec regeneration chemicals. Verify that the chemical regenerants meet your specifications.

Increased Pressure Drop Across the Bed

An ion exchange bed exerts a resistance to flow that is measured as a pressure decrease across the vessel. Some pressure drop is a normal condition, but excessive loss of head and reduced flow is not. Here are some things to check.

  • Is there a blockage in the inlet to the vessel?

  • Has the resin degraded? Ion exchange resin can be degraded by high temperatures and chemical oxidation. Chlorine, a strong oxidizing agent, can cause decrosslinking of the resin resulting in swelling and clumping of the resin. These conditions increase the resistance to water flow and hence a loss of head.

  • Fouling of the resin with dirt, iron, calcium deposits, or corrosion by-products will restrict water flow and increase the pressure drop across the unit.

  • Is there a problem with the underdrain system? Normally this is difficult to check without removing the resin and any support media. Check this last.

Many of the problems associates with ion exchange equipment are difficult to diagnose. If the cause of the problem is not readily apparent, consult an expert in the field for further advice and recommendations.

SAFETY ISSUES

Working with ion exchange systems presents hazards associated with the handling of strong chemicals, ion exchange resins, electrical equipment, confined spaces, and related equipment such as fans, blowers and motors. The best safety practice is to read and become familiar with the instruction manuals that are provided with the ion exchange system. Read and follow all instructions as outlined by the manufacturer. If you have any questions, contact the equipment manufacturer or knowledgeable expert before proceeding.

SUMMARY

Ion exchange is an effective method for improving water quality by removing troublesome impurities like hardness and alkalinity. These impurities, if not removed, will cause problems in boilers and heat exchangers such as scale deposition and fouling.

Since the acceptance of synthetic ion exchange resin as a replacement for zeolites and greensand, many types and grades of ion exchange material are used in the design of water treatment plants. The more popular systems include softeners for removing calcium and magnesium hardness’ and dealkalizers for removing alkalinity. These systems exhibit improved properties such as high exchange capacity, efficient regenerant utilization, and long life.

It is clear that ion exchange serves a useful and necessary role in the improvement in water quality for industrial, commercial and residential use.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s