Alkalinity in water is present in four different forms - dissolved Carbon dioxide (CO2), Bicarbonate (HCO3), Carbonate (CO3) and Hydroxyl (OH).
Raw water having high alkalinity, even after softening, causes foaming and carryover of solids that leads to embattlement of boiler steel. The decomposition of bicarbonates due to heat produces CO2, which causes corrosion.
The alkalinity needs to be removed and monitored to prevent scale and corrosion to minimise total operating costs.
The following Ion exchange processes can be used for reducing alkalinity:
Dealkalisation with Weak Acid Cation (WAC) Exchange Resin (Carboxylic Resins)
Dealkalisation, using weak acid cation exchanger in hydrogen ion (H+) form, is extremely efficient and cost effective to remove hardness from influent water, when the value of hardness equals alkalinity.
Cations associated with alkalinity are exchanged for H+ ions. Degasser giving alkalinity free soft water, removes the carbonic acid formed. Salts of mineral acids or neutral salts are not exchanged.
Split Stream De-alkalisation with Strong Acid Cation Exchange Resins
This process is preferred when alkalinity is high compared to hardness and is present as sodium alkalinity.
In this process, two beds of strong acid cation (SAC) resin are operated in parallel. One bed contains strong acid cation exchange resin in Na+ form and other in H+ form.
The feed water flow is split between two vessels/beds. The output of these two streams is then blended together and put over a degassifier, which removes CO2 (created by FMA). By controlling the percentage of each flow in the blended water, the level of alkalinity in the final effluent stream can be controlled.
Exhausted strong acid cation resin in H+ form is regenerated by 0.7 - 4.0% of mineral acids like HCl or H2S04. Where the exhausted strong acid cation resin in Na+ form resin is regenerated by 5.0 - 10.0% NaCl.