(477e) Use of Forward Osmosis in Treatment of Hyper-Saline Produced Water

Oilfield produced water forms the largest waste stream in the production of petroleum. The quality of produced water varies greatly, with some waters being close to potable while others are oily or extremely saline. Hyper-saline produced water is an exceptionally challenging problem as there are few treatment options available.  Desalination of such hyper-saline water is extraordinarily expensive because we are limited to evaporative processes which have enormous energy requirements. Conventional desalination technologies, like reverse osmosis are unusable due to the high osmotic pressure which require exceptionally high hydraulic pressure to overcome.

Forward osmosis circumvents this issue by using osmotic pressure instead of hydraulic pressure. Using a high concentration draw solution, osmotic potential can force osmotic flow from even the highest salinity solutions. The draw solution can then be subsequently regenerated (or used) by another process chosen specifically for that draw solute. While forward osmosis will not use less energy than other technologies, it does have the capability of using lower quality energy (low temperature thermal) or operating with other advantages (low fouling propensity, for instance). FO achieves this while relying simply on an osmotic potential gradient generated by a concentrated draw solution across a semi-permeable membrane. In this work, the feasibility of using FO in treatment of produced water with TDS of about 240,000 ppm was studied. Both an asymmetric cellulose triacetate (CTA) and thin film composite (TFC) forward osmosis (FO) membrane from Hydration Technologies Innovation (HTI) were used for this study. Synthetic produced water was used as a feed solution to simulate the composition of the oilfield produced water. We evaluated the individual effect of the different salts in produced water (NaCl, CaCl₂, MgCl₂, MgSO₄, NaHCO₃, and FeCl₃) on the flux performance. Two types of draw solutions were used in this study: 6M ammonia-carbon dioxide draw solution and 4.8M magnesium chloride draw solution. Results show that although NH₃-CO₂ has the advantage of being economically separated and recycled to the FO process, it has high reverse solute flux across the membrane. This reverse solute flux can also have unforeseen effects such as the formation of low solubility salts that cause scale or a pH change (in this case, a rise), which can also impact the solubility of salts. While ammonia is a key solute that crosses the membrane, carbonate can also precipitate readily if it diffuses. The use of MgCl₂ showed promise as it gave higher water flux with less scaling, although the recovery method (membrane distillation or evaporation) would have greater energy demands and higher cost.