Dealkalization

One of the troublesome constituents in water used as a source of makeup for boiler systems is alkalinity. The so-called total alkalinity of a water is the sum of the CO₃ and OH⁺ found in that solution. The CO₃ portion of the total alkalinity is especially troublesome. As raw water is processed through a strong cation unit in a demineralizer train, the Ca or Mg normally associated with the CO₃ is exchanged for H⁺ and the cation effluent contains H₂CO₃. This acid, called carbonic acid, is very unstable. It disassociates into carbon dioxide (CO2) and water very rapidly. For this reason, many strong acid cation units are followed by a mechanical device called a decarbonator. A decarbonator is nothing more than a vessel filled with pall or Raschig rings supported on a grid over a plenum. A fan blows atmospheric air up through the fill and out a vent at the top of the tank or vessel. Carbon dioxide, which breaks out of the strong acid cation effluent stream, exists at the top. The effluent from the decarbonator normally contains 10 ppm CO₂.

Decarbonization of the strong acid cation effluent can also be accomplished by passing the acidic solution through a strong base anion resin that has been regenerated with caustic. A more complete discussion of this process follows.