Hardness Removal (Ca, Mg)
Technical Profile
The objective of ion exchange softening is the removal of calcium and magnesium from water to avoid fouling downstream equipment such as steam generation or membrane separation.
The basis of ion exchange softening is the chemical reaction of ions in the water with the ion exchange resin. The most common form of ion exchange for produced water treatment is either strong acid cation (SAC) or weak acid cation (WAC). The actual ion exchange is performed by functional chemical groups on a polymer. For a strong acid cation resin the basis polymer is polystyrene and the functional groups are sulphonic groups. For weak acid cation resin the polymer is either acrylonitrile or methacrylate and the functional group is carboxylic. In general, SAC resins are held in the sodium form for softening rather than hydrogen form for demineralization. They are mostly regenerated with brine. WAC is generally used in the hydrogen form and regenerated with acid.
What happens upon ion exchange is the swap of the sodium ion for hardness in the water. When the resin is expressed as R- instead of the rather difficult chemical formula the reaction for calcium would be:
2R-Na+ + Ca2+фR22- Ca2+ + 2Na+
For other types of hardness reactions are analogous. As the water passes through the bed of ion exchange resin all hardness ions are swapped for sodium ions.
It can be seen that the reaction is in equilibrium. This means it can be brought back to the left. This is what is done upon regeneration. The saturated SAC resin is brought into contact with such an amount of sodium ions (generally in the form of brine) that the reaction is forced back. Then the brine is loaded with hardness and can be discharged.
SAC resin usually can bring hardness down to 1 mg/l as CaCO3 When background total dissolved solids (TDS) is over approximately 5000 mg/l competition between sodium and hardness is such that even this 1 mg/l is difficult to achieve. When background TDS is over 7000 mg/l, WAC is a better choice. Under proper conditions, WAC resin can go down to 0.5 mg/l hardness as CaCO3 even at TDS levels as high as 30000 mg/l.
