Principles of Zeolite Softening

Principles of Zeolite Softening

Sodium zeolite softeners use exchange resins made of polystyrene.  These resins have sodium ions loosely attached and will readily give up sodium for more desirable ions such as calcium and magnesium.  This exchange is only for cations (positively charged ions).  This is why sodium zeolite resin is referred to as a cation exchange resin (Figure 2).

 

Figure 2 – Sodium zeolite resin gives up sodium ions for calcium and magnesium

The water to be softened passes through the vessel containing resin.  Calcium and magnesium ions are exchanged for the sodium ions in and on the resin beads.  The sodium then takes the place of the calcium and magnesium with the appropriate anion (negative component).

A plot of the softener effluent profile shows a low, nearly constant effluent hardness level until the ion exchange resin nears exhaustion.  At this point, the hardness level usually increases quite rapidly and regeneration is required (Figure 3).

Figure 3 – As the resin nears exhaustion, the hardness level increases rapidly.

Regeneration is achieved by reversing the softening reactions.  Exhausted resin is exposed to a concentrated solution of sodium chloride.

Normally, zeolite resin more readily releases sodium in exchange for calcium and magnesium.  However, with the high concentration of sodium in the brine, the sodium ions displace the calcium and magnesium ions attached to the beads.  Thus, the high concentration of salt in the regenerant supplies the driving force to replace the hardness cations.  The calcium and magnesium are removed from the softening unit through the wave brine and rinse streams.

The frequency of regeneration needed depends on the rate or quantity of water, the calcium and magnesium content of the raw water, the quantity of exchange resin in the softener and the amount of salt used per regeneration.  The operating plant usually controls only the flow and the amount of regenerant.  The other parameters are fixed by the system design and the raw water hardness level