Reactions for Hydrogen Cation Exchanger Resins Regeneration Reactions

Calcium, Magnesium and/or Sodium-Cation Exchanger (Insoluble) + Sulfuric Acid (Soluble) = Hydrogen Cation Exchanger (Insoluble) + Calcium, Magnesium and/or Sodium Sulfates (Soluble).

Reactions for Hydrogen Cation-Exchanger Resins -Reactions with Sulfates or Chlorides

[Ca, Mg, Na2] [S04, Cl2] + H2Z = [Ca, Mg, Na2]Z + H2S04 or HCl

Calcium, Magnesium and/or Sodium as Sulfates and/or Chlorides (Soluble) + Hydrogen Cation Exchanger (Insoluble) = Calcium, Magnesium and/or Sodium-Cation Exchanger (Insoluble) + Sulfuric Acid and!or Hydrochloric Acid (Soluble).

mixed-bed vessels help to separate cation and anion resins during backwash. Advantages of inert resins include: (1) Classify cation and anion resins so that little or no mixing of cation or anion resin occurs before regeneration, and a buffering mid-bed collection zone exists; (2) Improve regeneration efficiency, thereby reducing resin quantities needed; (3) Protect against osmotic shock since the inert layer effectively prevents the exposure of cation resin to the caustic regenerant solution and the exposure of anion resin to the acid regenerant solution.

ION EXCHANGE SOFTENING (SODIUM ZEOLITE SOFTENING)

This is one of the ion-exchange processes used in water purification. In this process, sodium ions from the solid phase are exchanged with the hardness ions from the aqueous phase. Consider a bed of ion-exchange resin having sodium as the exchangeable ion, with water containing calcium and magnesium hardness allowed to percolate through this bed. Let us denote the ion-exchange resinous material as RNa, where R stands for resin matrix and Na is its mobile exchange ion. The hard water will exchange Ca. and Mg ions rapidly, so that water at the effluent will be almost completely softened. Calcium and magnesium salts will be converted into corresponding sodium salts.

The reaction will proceed toward the right-hand side to its completion until the bed gets completely exhausted or saturated with Ca. and Mg ions. In order to reverse the equilibrium so that the reaction proceeds toward the left-hand side, the concentration of sodium ions has to be increased. This increase in sodium ions is accomplished by using a brine solution of sufficient strength so that the total sodium ions present in the brine are more than the total equivalent of Ca and Mg in the exhausted bed. This reverse reaction is carried out in order to bring the exhausted resin back to its -sodium form. This process is known as regeneration. When the softener with the fresh resin in sodium form is put in service, the sodium ions in the surface layer of the bed are immediately exchanged with calcium and magnesium, thereby producing soft water with very little residual hardness in the effluent. As the process continues, the resin bed keeps exchanging its sodium ions with calcium and magnesium ions until the hardness concentration increases rapidly and the softening run is ended.

This softening process can be extended to a point where the hardness coming in and going out is the same. When this condition is reached, the bed is completely exhausted and does not have any further capacity to exchange ions. This capacity is called the total breakthrough capacity. In practice, the softening process is never extended to reach this stage as it is ended at some predetermined effluent hardness, much lower than the influent hardness. This capacity is called the operating exchange capacity. After the resin bed has reached this capacity, the resin bed is regenerated with a brine solution.

The regeneration of the resin bed is never complete. Some traces of calcium and magnesium remain in the bed and are present in the lower-bed level. In the service run, sodium ions exchanged from the top layers of the bed form a very dilute regenerant solution which passes through the resin bed to the lower portion of the bed. This solution tends to leach some of the hardness ions not removed by previous regeneration. These hardness ions appear in the effluent water as leakage. Hardness leakage is also dependent on the raw water characteristics. If the Na/Ca ratio and calcium hardness are very high in the raw water, leakage of the hardness ions will be higher.

Reactions for Hydrogen Cation-Exchanger Resins - Reactions with Bicarbonates

Calcium, Magnesium and/or Sodium Bicarbonates (Soluble) + Hydrogen Cation Exchanger (Insoluble) = Calcium, Magnesium and/or Sodium Cation Exchanger (Insoluble) + Water + Carbon Dioxide (Soluble Gas).

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