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.