Influence of nanofiltration pretreatment on scale deposition in multi-stage flash thermal desalination plants

Abstract

Scale formation represents a major operational problem encountered in thermal desalination plants. In current installed plants, and to allow for a reasonable safety margin, sulfate scale deposition limits the top brine temperature (TBT) in multi-stage flash (MSF) distillers up to 110-112?C. This has significant effect on the unit capital, operational and water production cost. In this work, the influence of nanofiltration (NF) pretreatment on the scale deposition potential and increasing TBT in MSF thermal desalination plants is modeled on the basis of mass transfer with chemical reaction of solutes in the brine. Full and partial NF-pretreatment of the feed water were investigated. TBT can be increased in MSF by increasing the percentage of NF-treated feed. Full NF pretreatment of the make-up allows TBT in the MSF plant to be raised up to 175?C in the case of di hybrid NF-MSF and up to 165?C in the case of tri hybrid NF-RO-MSF. The significant scale reduction is associated with increasing flashing range, unit recovery, unit performance, and will lead to reduction in heat transfer surface area, pumping power and therefore, water production cost.