Development of Electrodialysis for Shale Gas ...

 

 

 

This report covers Task 8 of the project, “BARNETT AND APPALACHIAN SHALE WATER MANAGEMENT AND REUSE TECHNOLOGIES.” The purpose of this effort is to evaluate electrodialysis for the desalinization of flowback water. A ten cell pair, semi-batch electrodialysis Eurodia pilot plant was used to investigate a number of suspected challenges under laboratory conditions. The energy efficiency goal of 0.1 to 0.15 kWh per pound of salt removed was consistently achieved in waters containing between 30,000 and 72,000 mg/l TDS (as NaCl plus CaCl2) by limiting the applied stack potential to 5 volts. Ion flux was improved by 24% by increasing the electrolyte concentration to an ionic strength similar to, or greater than, the ionic strength of the process water. Another increment of ion flux improvement of 19% was achieved by adjusting the pH of the electrolyte to reduce the electrochemical overvoltage at the electrodes. Soluble calcium of 1,000 to 4,000 mg/l in the process water caused process inhibition. This challenge was successfully overcome by replacing the single boundary membrane between the final diluate cell and the cathode with a divalent cation exclusionary membrane. One effect of the protection of the cathode by the exclusionary membrane was a rejection of >80% of the flux of calcium into the electrolyte. This produced an overall improvement in ion flux of 29–39% compared to operation without the cathode protection. A second effect of the cathode protection seems to be the inability of calcium within the electrolyte to foul the posterior side (within the electrode cell) of the cathodic membrane. These and other modifications to the electrodialysis process have led to reducing energy demands of the process from 0.20 kWh/lb salt removed observed with conventional ED processing to less than 0.12 kWh/lb salt removed observed in most of the improved process runs, an energy savings of about 40%.