Often, boiler or process operating conditions require the removal of hardness and the reduction of alkalinity but not the removal of the other solids. Water Softener does not reduce alkalinity, and demineralization is too costly and can create problems of metal erosion, etc. in piping & equipment. For these situations, a dealkalization process is used. Water Softener/Strong Acid Cation Exchanger (split stream) dealkalization, and weak acid cation dealkalization are the most frequently used processes.
Water Softener/Strong Acid Cation Exchanger (Split Stream) Dealkalization
In a split stream dealkalizer, a portion of the raw water flows through a Water softener operating in Sodium Cycle. The remainder flows through a hydrogen-form strong acid cation exchanger unit. Effluent from the Sodium Cycle Softener is combined with the Effluent from Strong Acid Cation Exchange unit. The effluent from the Strong Acid Cation exchange Unit contains carbonic acid, produced from the raw water alkalinity, and free mineral acids. When the two streams are combined, free mineral acidity in the Strong acid cation effluent converts sodium carbonate and bicarbonate alkalinity in the sodium cycle softener effluent to carbonic acid as shown below :
Carbonic acid is unstable in water. It forms carbon dioxide gas and water. The blended effluents are sent to a degasser, where the carbon dioxide is stripped from the water by a countercurrent stream of air. Figure 8-10 shows a typical split stream dealkalization system.
In addition to reducing alkalinity, a split stream dealkalizer reduces the total dissolved solids of the water. This is important in high alkalinity waters, because the conductivity of these waters affects the process and can limit boiler cycles of concentration.
Weak Acid Cation Dealkalization
Another method of dealkalization uses weak acid cation resins. Weak acid resins are similar in operation to strong acid cation resins, but only exchange for cations that are associated with alkalinity, as shown by these reactions :
where Z represents the resin. The carbonic acid (H2CO3) formed is removed by a decarbonator or degasser as in a split stream system.
The ideal influent for a weak acid cation system has a hardness level equal to the alkalinity (both expressed in ppm as CaCO3). In waters that are higher in alkalinity than hardness, the alkalinity is not removed to its lowest level. In waters containing more hardness than alkalinity, some hardness remains after treatment. Usually, these waters must be polished by a sodium zeolite softener to remove hardness.
DEION Offers Various capacities of Dealkalisers starting from 1 m3/Hr. capacity up to 100 m3/Hr. Capacity and with Pressure Vessel Material of construction in FRP / MSRL.