Ion exchange softening, removal of calcium and magnesium is a key step in the recycle process of produced water. It is generally considered necessary to reduce hardness levels to less than 1 mg/L (as CaCO3) and preferably less than 0.2 mg/L. In an effort to avoid boiler fouling, the trend is to specify even lower hardness limits – sometimes as low as 0.1 mg/L.
Weak acid cation (WAC) resins are normally utilized for softening high TDS produced water. The main disadvantage of the WAC process is that the resins require regeneration with hydrochloric acid followed by sodium hydroxide.
Strong acid cation (SAC) ion exchange processes employing only brine regeneration (i.e. no acid or caustic) are much simpler and less expensive, however the SAC process is normally restricted to low TDS (<3000 mg/L) waters.
A new ion exchange softening flow-sheet that uses brine regeneration has been developed based upon the Recoflo® short-bed ion exchange process. Recoflo® makes it possible to use extremely high service flow rates through very short (15 cm) resin beds, while achieving low hardness leakages. The key to applying this to high TDS produced water is the use of high purity brine for resin regeneration.