Desalination of brine and produced water by direct contact membrane distillation at high temperatures and pressures

Direct contact membrane distillation (DCMD)-based desalination process is a thermally driven separation process where a hydrophobic microporous membrane separates a hot brine feed and a cold distillate which condenses the water vapor from the hot brine passing through the membrane pores. So far, DCMD has been explored for hot brines and other aqueous solutions below 100°C. For feed solutions above 100°C, DCMD has an extra advantage over other conventional separation processes like reverse osmosis (RO) which requires cooling of the feed solution requiring additional energy; further RO cannot utilize the heat available in the feed solution. Produced water obtained from steam assisted gravity drainage (SAGD) process is one such example where DCMD can potentially be a very useful process. In this paper, the DCMD technique is explored in the range of 80-130°C for brines containing 10,000ppm sodium chloride with porous flat sheet polytetrafluoroethylene (PTFE) membranes. The pressure of the solution went up to 2-3atm. The performance of this membrane has also been explored with a saline feed containing 3000ppm NaCl, 45ppm phenol, 45ppm cresol and 10ppm naphthenic acid, simulating the composition of hot produced water obtained from the SAGD process. There was no leakage of salt under any conditions. The highest water vapor flux achieved was 195kg/m ²h which is a few times larger than that for seawater RO process. The water generated by the DCMD process may be used for steam generation in the SAGD process.