Ion-exchange pretreatment for scale prevention in desalting systems

Abstract Ion exchange pretreatment can protect dependably against scaling from calcium sulfate and alkaline deposits in desalting equipment, and against acid corrosion. When a feedwater is “softened” by ion exchange to replace most of its calcium by sodium, use of the sodium-rich reject brine from desalination to regenerate the exchanger makes the treatment chemically self-sufficient. An inexpensive vessel construction using salt-resistant reinforced concrete appears suitable for large-scale use. For moderate concentration factors (up to around 5 for seawater feed), and moderately low sulfate/chloride ratios, regeneration can be carried out in fixed-bed mode, preferably counterflow (upflow). At higher sulfate levels, precipitation occurs, and regeneration must be done in fluidized- (expanded-) bed mode to keep calcium sulfate from collecting in the bed. When weak-acid cation exchange with acidified exchanger is used to eliminate bicarbonate, feedwater acidity excursions are avoided, and the total-dissolved solids level is reduced rather than increased.