ION EXCHANGE RESIN TECHNOLOGY

Ion exchange is used in water treatment, including water softening, industrial demineralization, condensate polishing, ultrapure water production, and wastewater treatment. It has special utility in chemical synthesis, manufacturing, food processing, mining, power generation, agriculture, and a variety of applications  and other industries.

The ion exchange technology is used in different water treatment process applications:

  • Softening (removal of hardness)
  • De-alkalization (removal of bicarbonate)
  • Cationization (removal of all cations)
  • Combined de-alkalization and softening
  • Demineralization (removal of all ions)
  • Mixed bed polishing
  • Nitrate removal
  • Selective removal of various contaminants
Common components of a basic IX vessel include:
  • IX resin
  • Inlet distribution system
  • Regenerant distribution system
  • Retention elements
  • PLC, control valves and piping
Operation Cycles

By definition, ions are charged atoms or molecules. When an ionic substance is dissolved in water, its molecules dissociate into cations (positively charged particles) and anions (negatively charged particles). Taking advantage of this characteristic, IX selectively replaces ionic substances based on their electrical charges. This is accomplished by passing an ionic solution through an IX resin that serves as a matrix where the ion exchange reaction is allowed to take place.

Over time, the resin becomes saturated with the contaminant ions, and it must be regenerated or recharged. This is accomplished by flushing the resin with a regenerant solution. Typically consisting of a concentrated salt, acid, or caustic solution, the regenerant reverses the IX reaction by replenishing the cations or anions on the resin surface, and releasing the contaminant ions into the waste water.

Cationic resins

Cation exchangers can be classified as either strong acid cation (SAC) resins or weak acid cation (WAC) resins, both of which are extensively used for demineralization. SAC resins are also commonly used for softening, while WAC resins are used for de-alkalization applications. Contaminants removed by cation resins typically include:

  • Calcium (Ca2+)
  • Chromium (Cr3+and Cr6+)
  • Iron (Fe3+)
  • Magnesium (Mg2+)
  • Manganese (Mn2+)
  • Radium (Ra2+)
  • Sodium (Na+)
  • Strontium (Sr2+)
Anionic resins

Anion exchangers can be classified as either strong base anion (SBA) resins or weak base anion (WBA) resins. SBA resins are frequently used for demineralization, while WBA resins are often used for acid absorption. Contaminants removed by anion resins typically include:

  • Arsenic
  • Carbonates (CO3)
  • Chlorides (Cl–)
  • Cyanide (CN–)
  • Fluoride
  • Nitrates (NO3)
  • Perchlorate (ClO4-)
  • Perfluorooctane sulfonate anion (PFOS)
  • Perfluorooctanoic acid (PFOA)
  • Silica (SiO2)
  • Sulfates (SO4)
  • Uranium
Specialty Resins

While specialty IX resins are highly effective for specific industrial applications, their greater specificity generally means greater expense and narrower adoption than conventional IX resins. Chelating resins, for example, are used extensively for concentration and removal of metals in dilute solutions, such as Cobalt (Co2+) and Mercury (Hg and Hg2+). Similarly, magnetic ion exchange (MIEX) resins are often deployed for removal of natural organic matter from feed water.

Industrial Ion Exchange

Ionizable groups attached to the resin bead determine the functional capability of the resin. Industrial water treatment resins are classified into four basic categories:

  • Strong Acid Cation (SAC)
  • Weak Acid Cation (WAC)
  • Strong Base Anion (SBA)
  • Weak Base Anion (WBA)
Strong Acid Cation resins

Strong Acid Cation (SAC) resins behave similar to strong acids.
Strong Acid Cation resins are available in two forms: hydrogen (R-SO3H) or sodium (R-SO3Na).

The typical strong acid cation exchange reaction:

2(R-SO3Na) + CaCl2 = (R-SO3)2Ca + 2NaCl

Cross-linking level of the Strong Acid Cation resins is 8-10%.
The ion exchange capacity of Strong Acid Cation resins does not depend on the solution PH.
Strong Acid Cation resins are used for water softening and demineralization.
The exhausted Strong Acid Cation resins may be regenerated.
Regeneration in hydrogen (acid) form is performed by a strong acid (e.g., HCL). Regeneration in sodium (salt) form is performed by sodium chloride solution (NaCl).

Weak Acid Cation resins

Weak Acid Cation (WAC) resins behave similar to weak acids.
Weak Acid Cation resins are available in hydrogen form (R-COOH).
Weak Acid Cation resins have high affinity for hydrogen ions therefore they are easily regenerated by stoichiometric amount of acid.
The ion exchange capacity of Weak Acid Cation resins increases with an increase of the solution PH. WAC resins are not used for treatment acidic (PH<6) solutions.
Weak Acid Cation resins are used for demineralization and de-alkalization of water.

Strong Base Anion resins

Strong Base Anion (SBA) resins behave similar to strong bases.
Strong Base Anion resins are available in hydroxide form: (R-NH3OH).

The typical strong base anion exchange reaction:

R-NH3OH + HNO3 = R-NH3NO3 + H2O

Strong Base Anion resins are used for demineralization and de-alkalization of water.
The exhausted Strong Base Anion resins may be regenerated by concentrated sodium hydroxide (NaOH).

Weak Base Anion resins

Weak Base Anion (WBA)resins behave similar to weak bases.

The typical weak base anion exchange reaction:

R-NH2 + HNO3 = R-NH3NO3

The ion exchange capacity of Weak Base Anion resins increases with a decrease of the solution PH. WBA resins are not used for treatment basic (PH>6) solutions.
Weak Base Anion resins sorb only anions of strong acids (chlorides, nitrates, sulfates).
Weak Base Anion are easily regenerated by small amounts of weak bases (such as ammonia or sodium carbonate), which neutralize the acid taken up by the resin.

WATER TREATMENT WITH IX RESIN

Ion exchange resins are used for many separating processes in of water treatment industry such as

1-WATER SOFTENING.

Hard water contains ions of calcium (Ca2+) and magnesium (Mg2+). Sodium type Strong Acid Cation resins (e.g., R-SO3Na) are used for water softening. The ions of calcium and magnesium dissolved in water are bound by the resin exchanging the equivalent amount of sodium ions (Na+), which are released from the resin to water. PH of water does not change in softening process. Anions are not removed in the process therefore anion base resins are not used. The exhausted resin is regularly regenerated. The regeneration process includes the following stages:

  • Removing suspended hard particles by reverse flow of water.
  • Passing a solution containing high concentration of sodium ions (commonly a strong solution of NaCl) for replacing the calcium and magnesium ions with fresh sodium ions.
  • Rinsing the resin with water in order to remove the regenerating solution.
DEMINERALIZATION (DEIONIZATION ) OF WATER

Both types of ions cations and anions are removed from the water in the demineralization process. Therefore two resin types are used:

  • Strong Acid Cation resins (SAC), The cation resins are used in hydrogen form (e.g., R-SO3H)
  • Strong Base Anion resins. (SBA), the anion resins are used in hydroxide form (e.g., R-NH3OH).

1-The water first passes through the acid cation resin where the dissolved cations are bound by the resin and replaced by the equivalent amount of ions of hydrogen (H+). The water becomes slightly acidic.

2- Then the water passes through the base anion resin where the dissolved anions are replaced with the hydroxide ions, which are released to the water.

Hydroxide and hydrogen ions react and form neutral water. The hydrogen type strong acid cation resin is regenerated by solutions of strong acids (hydrochloric or sulfuric). The strong base anion resin is regenerated by solutions of sodium hydroxide

Demin Treated Water Quality

Treated Water Quality

 

 

Conductivity (Co -Flow Regen)

 

5-25µs/cm

Conductivity (Reverse-Flow Regen)

˂1 µs/cm

Silica Residual (Co-Flow )

 

50-200 µg/l

Silica Residual (Reverse -Flow )

 

5-40 µg/l

Sodium Residual

 

µg/l

pH Value

In principle 7

Demineralized water is completely free of ions, except a few residual traces of sodium and silica, because the SAC and SBA resins have their lowest selectivity for these. With a simple demineralization line regenerated in reverse flow, the treated water has a conductivity of only about 1 µS/cm, and a silica residual between 5 and 50 µg/L depending on the silica concentration in the feed and on regeneration conditions.
Note that the pH value should not be used as a process control, as it is impossible to measure the pH of a water with less than say 5 µS/cm conductivity.

1-HOT ZEOLITE SOFTENING

Zeolite softeners can be used to remove residual hardness in the effluent from a hot process lime or lime-soda softener. The hot process effluent flows through filters and then through a bed of strong acid cation resin in the sodium form . The equipment and operation of a hot zeolite softener is identical to that of an ambient temperature softener, except that the valves, piping, controllers, and instrumentation must be suitable for the high temperature (220-250°F). Standard strong cation resin can be used at temperatures of up to 270°F, but for a longer service life a premium gel or macroreticular resin is recommended.

2-DEALKALIZATION

Often, boiler or process operating conditions require the removal of hardness and the reduction of alkalinity but not the removal of the other solids. Zeolite softening does not reduce alkalinity, and demineralization is too costly. For these situations, a dealkalization process is used. Sodium zeolite/hydrogen zeolite (split stream) dealkalization, chloride-anion dealkalization, and weak acid cation dealkalization are the most frequently used processes.

3-COUNTERFLOW AND MIXED BED DEIONIZATION

Due to increasing boiler operating pressures and the manufacture of products requiring contaminant-free water, there is a growing need for higher water quality than cation-anion demineralizers can produce. Therefore, it has become necessary to modify the standard demineralization process to increase the purity of the treated water. The most significant improvements in demineralized water purity have been produced by counterflow cation exchangers and mixed bed exchangers.

4-MIXED BED EXCHANGER

A mixed bed exchanger has both cation and anion resin mixed together in a single vessel. As water flows through the resin bed, the ion exchange process is repeated many times, “polishing” the water to a very high purity. During regeneration, the resin is separated into distinct cation and anion fractions . The resin is separated by backwashing, with the lighter anion resin settling on top of the cation resin. Regenerant acid is introduced through the bottom distributor, and caustic is introduced through distributors above the resin bed. The regenerant streams meet at the boundary between the cation and anion resin and discharge through a collector located at the resin interface. Following regenerant introduction and displacement rinse, air and water are used to mix the resins. Then the resins are rinsed, and the unit is ready for service.

Counterflow and mixed bed systems produce a purer water than conventional cation-anion demineralizers, but require more sophisticated equipment and have a higher initial cost. The more complicated regeneration sequences require closer operator attention than standard systems. This is especially true for a mixed bed unit. Mixed bed polishing produces a water with less than 0.1 µS/cm conductivity. With sophisticated design and appropriate resins, the conductivity of pure water (0.055 µS/cm) can be achieved. Residual silica values can be as low as 1 µg/L.
The pH value should not be used as a process control, as pH meters are unable to operate at 1 µS/cm conductivity or below.

APPLICATIONS
  • Treatment of water pre-demineralized with ion exchange resins
  • Polishing of reverse osmosis permeate
  • Polishing of sea water distillate
  • Treatment of turbine condensate in power stations
  • Treatment of process condensate in various industries
  • Production of ultra-pure water for the semiconductors industry
  • Service de-ionization (with off-site regenerated columns)
1-DECARBONATORS AND DEGASSERS

Decarbonators and degassers are economically beneficial to many demineralization systems, because they reduce the amount of caustic required for regeneration. Water from a cation exchanger is broken into small droplets by sprays and trays or packing in a decarbonator. The water then flows through a stream of air flowing in the opposite direction. Carbonic acid present in the cation effluent dissociates into carbon dioxide and water. The carbon dioxide is stripped from the water by the air, reducing the load to the anion exchangers. Typical forced draft decarbonators are capable of removing carbon dioxide down to 10-15 ppm. However, water effluent from a decarbonator is saturated with oxygen.

In a vacuum degasser, water droplets are introduced into a packed column that is operated under a vacuum. Carbon dioxide is removed from the water due to its decreased partial pressure in a vacuum. A vacuum degasser usually reduces carbon dioxide to less than 2 ppm and also removes most of the oxygen from the water. However, vacuum degassers are more expensive to purchase and operate than forced draft decarbonators.

2-CONDENSATE POLISHING

Ion exchange uses are not limited to process and boiler water makeup. Ion exchange can be used to purify, or polish, returned condensate, removing corrosion products that could cause harmful deposits in boilers.

Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carry-over of boiler water into the steam. Condensate polishers filter out the particulates and remove soluble contaminants by ion exchange.

Most paper mill condensate polishers operate at temperatures approaching 200°F, precluding the use of anion resin. Cation resin, which is stable up to temperatures of over 270°F, is used for deep bed condensate polishing in these applications. The resin is regenerated with sodium chloride brine, as in a zeolite softener.

In systems requiring total dissolved solids and particulate removal, a mixed bed condensate polisher may be used. The temperature of the condensate should be below 140°F, which is the maximum continuous operating temperature for the anion resin. Additionally, the flow through the unit is generally reduced to approximately 20 gpm/ft².

SELECTIVE REMOVAL OF VARIOUS CONTAMINANTS

Selective removal of metals and other contaminants is mainly used for drinking water and for waste. Many of these applications require special resins: chelating resin making stable metal complexes, for instance.

  • Removal of boron (boric acid) from drinking water
  • Removal of nitrate from drinking water (shown above)
  • Removal of perchlorate from drinking water
  • Removal of heavy metals from waste: Cd, Cr, Fe, Hg, Ni, Pb, Zn

In many of these applications, a residual concentration in the µg/L range is possible. Some contaminants are difficult to remove with ion exchange, due to a poor selectivity of the resins. Examples: As, F, Li.

NanoWater provides high-capacity cation and mixed-bed ion exchange polishers and combined options that maximize condensate polishing performance and minimize operating costs by removing both soluble and insoluble contaminants from boiler feed-water.  Whichever type of Ion Exchange System  you may need, our team of specialists can help you.