In a demineralization (DM) plant, strong acid cation (SAC) exchange resin plays a crucial role in removing cations from water, leading to changes in pH, FMA, and sodium levels.

 In a demineralization (DM) plant, strong acid cation (SAC) exchange resin plays a crucial role in removing cations from water, leading to changes in pH, FMA, and sodium levels. The pH will generally decrease as the resin exchanges cations for hydrogen ions, and the free mineral acidity (FMA) increases initially due to the formation of acids from the exchanged cations. Sodium levels will be significantly reduced as the resin exchanges sodium ions for hydrogen, although some slippage (the presence of sodium in the outlet water) can occur due to factors like incomplete regeneration or resin damage. 

Elaboration:

pH Change:

The primary function of the SAC resin is to exchange cations (like calcium, magnesium, and sodium) for hydrogen ions (H+). This process releases hydrogen ions into the water, making it more acidic, thus lowering the pH. 

FMA:

When cations are exchanged for hydrogen, they react with the anions present in the water (like chloride or sulfate) to form acids (e.g., hydrochloric acid, sulfuric acid). These acids are referred to as free mineral acidity (FMA). The outlet of the cation exchanger will initially have a higher FMA compared to the inlet. 

Sodium Reduction:

The SAC resin effectively removes sodium ions from the water, reducing the overall sodium concentration. However, some sodium slippage can occur, meaning a small amount of sodium may still pass through the resin and into the outlet water. This can happen due to factors like incomplete regeneration of the resin or damage to the resin beads, causing them to not fully exchange ions. 

Resin Regeneration:

To maintain the effectiveness of the SAC resin, it needs to be regenerated. This involves passing a strong acid (like hydrochloric acid) through the resin bed, which reverses the exchange process and removes the accumulated cations. 

Mixed Bed Unit:

In a DM plant, the water is often passed through a mixed bed unit, which contains both cation and anion exchange resins. This unit removes any remaining ions and produces highly purified water.