Nanofiltration and calcium sulfate limitation for top brine temperature in Gulf desalination plants

Abstract

All MSF desalination plants have been operated at a top brine temperature below 120°C. That is because of sulfate scaling phenomena, which usually occurs when the temperature is higher than 120°C. This type of scaling is hard scale that cannot be removed easily by chemical or mechanical cleaning. In the recent years desalination technology has created a new type of membranes that can overcome sulfate scaling. These new type of membranes is a nanofiltration membrane (NF). NF membranes have the ability to remove the sulfate and calcium ions in seawater feed, as a result the risk for sulfate scaling is eliminated. This paper mainly aims to study the scaling potential of CaCO 3, CaSO 4, BaSO 4, SrS0 4, silica and CaF at different productivity with a nanofiltration product water and compare it with the scaling potential of normal seawater. This paper also studies the maximum possible percentage inhibition for CaSO 4 using nanofiltration product water, if this water is used as a feed for a MSF unit. Acidification process has been also tried at different dosing to increase the % inhibition for CaCO 3 scaling. The result indicated a great percentage inhibition for CaSO 4 scale using a nanofiltration membrane. The inhibition reached 99.1% at 35% productivity and decreased to about 96.98% at 75% productivity without addition of any antiscalant or chemicals. The result also show a low prevention for CaCO 3 scaling started from 23.41 % inhibition at 10% productivity and decrease to about 5.3% at 75% productivity. Also for all other scaling constitutes, a great scaling inhibition was found — about 97.7% for BaS0 4, 99.8% for SrSO 4 scaling and 94.86% inhibition for CaF scaling at 50% conversion. This study proved the reliability of a nanofltration system in preventing all types of scaling in the Gulf seawater, which can be considered as a revolution in the MSF plants when applied at high operation temperature.