Seawater Sulfate Removal

Oil rig in seawater where sulphate removal is necessarySynder NFW Nanofiltration Membrane
Background 
The oil and gas industries have come under increased pressure to find cost effective methods to control the formation of scale in their use of seawater.  The removal of sulfate, which has roughly an 8% concentration in sea water, is the key step for scaling reduction at offshore platforms.  If untreated seawater is mixed with formation water, which contains barium and strontium, significant barium and strontium sulfate scaling and reservoir souring can occur.  The use of nanofiltration membranes with high sulfate rejection and monovalent and divalent ion selectivity would be a cost effective method for achieving sulfate removal.  Synder’s NFW membrane was evaluated against a leading competitor’s NF membranes for sulfate removal in synthesized seawater.  

Feed Solution, Membrane, Operating Parameters 
Table 1: Composition of Synthesized Water 

 Ions  Concentration 
 Sulfate  2500-2800 ppm
 TDS  3%-5% to simulate salt composition in sea water

Table 2: Operating Conditions and Membrane Specifications

 Feed Solution  
 Material  Synthesized Seawater
 pH  6.8-7.6
 Synder Membrane  
 Element  NFW-Spiral Element
 Membrane  Polyamide-TFC
 Competitor’s Membrane  
 Element  NF-Spiral Element 
 Membrane  Polyamide-TFC
 NF Standard Operating Parameters  
 Pressure (PSI)  110
 Temperature (C)  20-25

Results 
The membranes were tested for permeate flux, sulphate rejection, permeate sulphate concentration, and overall salt rejection (based on conductivity).  Based on the information displayed in Table 3 below, the two membranes have similar water flux and MgSO4 rejection under the same conditions (110 psi, 25 °C, ~5 GPM crossflow rate, 2000 ppm MgSO4). 

Table 3: Performance Results

Filtration Results NFW Leading Competitor
Average Permeate Flux (GFD) 28 20
Sulfate Rejection (%) >99 <97.5
Sulfate Concentration in Permeate (mg/L) <30 >50
Overall Salt Rejection (%) 20 26

Conclusion   
In this trial, Synder’s NFW membrane outperformed the leading competitor in removing sulfate from synthetic seawater.  While the membranes showed similar water flux and MgSOrejection under the same operating conditions, the NFW showed greater than 99% sulfate removal efficiency with less than 30 mg/L SO4   in the permeate.  In addition to the higher  rejection and selectivity, the NFW membrane displayed a 40% higher steady state flux than the competitor’s NF membrane.  This combination of greater flux and improved rejection suggests that Synder’s NFW is well suited for seawater sulfate removal.  

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