Strong Acid Cation
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.
Weak Acid Cation
Weak and strong acid cation resins can be placed in different vessels or they can be placed in two distinct layers in the same vessel. The regeneration efficiency of a weak acid resin is very high compared to that of a strong acid resin. Therefore, it is possible to utilize the regenerant acid stream from the strong acid unit to regenerate the weak acid unit. When weak and strong acid cation resins are loaded into the same vessel, the strong acid resin settles on the bottom of the unit after backwash because of the density difference between the two resins. Because the weak acid resin contains some strong acid sites, after regeneration with sulfuric acid, a 10% brine solution must be passed through the unit. The brine solution exhausts any strong acid sites in the weak acid resin and regenerates the strong acid resin in the sodium form. If this is not done when raw water enters the weak acid resin, noncarbonate hardness exchanges at the strong acid sites. FMA exits the weak acid resin and prevents the exchange of residual noncarbonate hardness in the strong acid resin. Normally, a weak acid resin produces FMA for 40-60% of its service cycle. This combination would not be suitable for higher pressure boiler applications because of the presence of excess sodium in the effluent from the sodium-form strong acid resin.
Regeneration of a weak acid cation resin with sulfuric acid must be carefully monitored to insure that the acid concentration during the regeneration does not exceed 0.7%. Higher concentrations of sulfuric acid can react with the Ca++ in the exchange sites of the exhausted resin and result in the precipitation of calcium sulfate (CaSO4). Calcium sulfate, or gypsum, is insoluble even in the concentrated form of many acids. Often, mechanical removal is the only satisfactory way to rid the resin of this contaminant. From an operational standpoint, it is objectionable because it produces a pressure drop across the unit. read more.......