All natural waters contain, in various concentrations, dissolved salts which dissociate in water to form charged ions.  Positively charged ions are called cations; negatively charged ions are called anions.  Ion exchangers exchange one ion for another, hold it temporarily, and then release it to a regenerant solution.  In an ion exchange system, undesirable ions in the water supply are replaced with more acceptable ions.  The ion exchange process is based on a water soluble matrix to which acidic or basic group are fixed whose mobile counter ions come to equilibrium with a surrounding solution.
 

Demineralization

 This contains the exchange of all cation against hydrogen ions and of all anions against hydroxyl ions. Hydrogen and hydroxyl ions combine to form water. The demineralization can be performed in a cation exchange tower followed by an anion exchange tower (2B2T), or in mixed bed tower (MB). In a mixed bed tower, the two bed systems working in series. Technical ion exchange process require cyclical operation, or a continuous alteration between water production and regeneration the resins.

 2B2T(2-Bed, 2-Tower) System
 2B3T(2-Bed, 3-Tower) System
 Mixed Bed Tower System

2B2T (2-Bed, 2-Tower) System
   CATION EXCHANGER + ANION EXCHANGER

2B3T (2-Bed, 3-Tower) System
    CATION EXCHANGER + DEGASIFIER + ANION EXCHANGER 

CATION EXCHANGER

The first stage of the 2B3T process is carried out in the cation exchange resin. This unit is essentially a cylindrical steel pressure vessel, lined with rubber, containing a column of strong acid cation exchange resin. The pressure vessel is fitted with a number of pads to which are connected the external pipework and valves for directing flows. Internally the vessel is fitted with a top inlet pipe for the incoming water, an inlet distributor positioned above the bed of ion exchanger resin for acid, and a bottom collecting system through which the water passes out of the unit to the degasifier. The top inlet pipe also serves as a collector for wash water, and the bottom collecting system distributes the wash water during regeneration. These internal fittings are constructed from corrosion resisting materials. As the water passes downwards through the column of cation exchange resin, calcium, magnesium and sodium ions in the water are exchanges for hydrogen ions given up by the resin. When the exchange capacity of the cation exchange resin is exhausted i.e., the resin has given up all its exchangable hydrogen ions, it must be regenerated with dilute acid. During the regeneration process the reverse exchange action then takes place: hydrogen ions from the dilute acid are taken up by the cation exchange resin and unwanted calcium, magnesium and sodium ions are expelled. After regeneration the cation exchanger is then ready to treat a further quantity of water. 

DEGASIFIER

The purpose of the degasifier is to remove most of the carbon dioxide from the cation treated water before it caters the anion exchanger thus reducing the load on the anion exchange resin. The tower made of steel, contains a bed of polypropylene netrings supported on a grid. The tower is superimposed on a degassed water collecting tank. Low pressure air, produced by a motor driven air blower enters the tower below the grid and passes upwards through the rings, whilst the cation treated water enters the top, flows downward and collects in the tank. The scrubbing action created between the opposing streams of air and water releases most of the cation dioxide and this is expelled to atmosphere through a vent at the top of the tower.

ANION EXCHANGER

The 3rd stage of the 2B3T process takes place in the anion exchanger. This unit contains a bed of strong base anion exchange resin. The construction of this unit is similar to the cation exchanger. As the degasfied water flows downwards the anion resin bed the acids produced by treatment in the cation exchanger together with silica and residual carbon dioxide are exchanged for hydroxyl ions. These hydroxyl ions, together with free hydrogen ions already present, combine to form demineralized water almost free from impurities. When the anion exchange resin is exhausted, the electrical conductivity of the demineralized water will rise to the instrument alarm set point. The resin is then regenerated with caustic soda solution.

REGENERATION EQUIPMENT FOR CAUSTIC SODA (ACID)
The caustic soda (acid) solution for the regeneration of the anion exchange resin is prepared in batches. The caustic soda (acid) solution is injected into the anion (cation) exchanger in a similar manner to that previously described for acid(soda). As the caustic soda (acid) solution is drawn into the soda (acid) injector it is diluted.  

Applications

General Water Treatment
   Removal of hardness
   Demineralization (Deionization)
   Removal of F-

Manufacture of high purity water
   Demineralization of condensate
   Nuclear power generation
   Semiconductor industry
   Drug and pharmaceutical industries

Other applications
   Effluent treatment and recovery of valuable materials
   Foodstuffs and food additives
   Purification of chemicals
   Separation and purification of antibiotics