Blog

Ion exchange processes have been in use for many years to soften water, their disadvantage being that they are batch operations, with a regeneration stage between each softening stage, the regeneration requiring the use of another chemical to restore the soluble ion content of the ion exchange materials.

The basic process involves passage of the feed liquid through packed beds of ion exchange materials in granular form (originally natural zeolites, nowadays synthetic polymers with appropriate ion-bonding structures). By contact with a cation exchange resin, the cations on the resin exchange with the cations in the feed liquid. Similarly feed liquid anions are exchanged on a bed of anion exchange resin. Eventually the beds will become saturated with the unwanted ions, as is indicated by a breakthrough of these ions into the product stream. The feed flow is then stopped, and the bed is regenerated by a flow of liquid carrying the soluble ions in concentrated solution, and the unwanted ions are carried away in the waste from this regeneration stage.

Sensibly, the exchange beds are arranged two in parallel, so that one bed can be run with raw water feed, while the other bed is being regenerated, their respective functions being interchanged as the first becomes saturated. As foreign matter in
the form of suspended solids would interfere with the ion exchange process, a fine filter is usually mounted ahead of the beds of resin.

The standard water softening ion exchange system uses cation resins only, to exchange soluble ions such as sodium for the hardness-causing calcium and magnesium. The system then uses salt to supply the regenerating ions, which is generally
satisfactory where sodium’s presence in the product water is acceptable. However, this softening process does not change the number of dissolved ions, and where a reduction in dissolved material is required, as in high quality boiler feed water, complete deionization (as shown in Figure 4.19) must be used, with a strong acid cation bed,

exchanging hydrogen ions for the incoming cations, and a strong base anion resin, exchanging hydroxyl ions for the incoming anions. Figure 4.19 shows a system with each unit in duplicate, including an influent sand filter. A system like this
produces a reasonably strong acid waste, and a corresponding alkali waste, which must both be neutralized before disposal.

Look for more information, please click here.