Viability of integrating forward osmosis (FO) as pretreatment for existing MSF desalting unit

Abstract

The feed to all Recirculation-multi-stage flash (R-MSF) desalting units in Qatar is pretreated with high temperature additive, which limits its top brine temperature (TBT) to 110°C. The daily capacity of these R-MSF units is about one Million cubic meters (Mm³/d). These units should continue their operation through their life time, (20–30 years). The capacity and performance of these units can be enhanced if Forward osmosis (FO) membrane system is used as pretreatment. The FO membranes can remove the scale constituents in the feed water and allows rising the TBT up to 135°C, and thus increases the capacity of these units. The viability of using FO as feed water pretreatment to an existing operating R-MSF unit is discussed in this paper. A suggested arrangement to use FO as MSF feed pretreatment is presented. Part of the cooling seawater leaving the heat rejection is directed to the FO pretreatment unit as feed solution (FS). The flashing brine leaving the last stage with the maximum brine salt concentration (about 1.5 times of the seawater feed) is directed to the FO unit as draw solution (DS) that absorbs an amount permeate water (D) from the FS while the Ca²⁺, , Mg²⁺, and are rejected. The diluted DS enters the last stage condenser of the heat recovery section. This arrangement allows to increase the TBT and thus the unit distillate capacity. The potential of calcium sulfate deposite index inside MSF condenser tubes is calculated at different TBTs and different FO recovery ratio. The calculations show that Skillman index in reference MSF (operates at TBT = 110°C) plant is greater than one which indicates the potential of scale deposit formation; however, antiscalant is used to disperse the crystalized scale. The simulation results showed that the potential of scale deposit is decreased as the FO recovery ratio increases due to the increase in divalent ions removal. The MSF can operate at TBT = 135°C safely without calcium sulfate scale at FO recovery ratio of 40%. On the other hand, the increase in the FO recovery ratio, will reduce the osmotic pressure difference across the membrane which requires higher membrane surface area. The impact of feed salinity is investigated, and the results showed that lowering the feed salinity gives better performance of FO unit. The cost of the FO membranes is a major factor in applying the suggested use of the FO as pretreatment for the R-MSF unit. The decision to apply this method depends on the availability and reasonable cost of the FO membranes.

Keywords:

• Desalination
• MSF
• FO
• Scale formation