(42w) Use of Forward Osmosis- Membrane Distillation Process in Treatment of Hyper-Saline Produced Water

Oilfield produced water (PW) can be considered as a potential water resource if treated efficiently and economically. However, its high salinity and complex chemistry can limit treatment options.   In this work, the feasibility of using a hybrid forward osmosis (FO) - membrane distillation (MD) process for treating of hyper saline produced water with total dissolved salts (TDS) of about 240,000 ppm was studied. MD is applied as a separation method for the draw solution of the forward osmosis process. A TFC membrane from Hydration Technologies Innovation (HTI) was used in the FO experiment and a polypropylene (PP) membrane from 3M with pore size of 0.45 micron was used in the MD test. Three draw solutions (i.e. NaCl, KCl and MgCl₂) at concentrations near to their saturation limits (5, 4, and 4.8 molar respectively) were considered.  Results showed that each draw solution behaved quite differently in both FO and MD due to substantial differences in diffusivity, solubility, activity, and solution viscosity. MgCl₂ solutions gave the highest water fluxes in FO, while NaCl and KCl showed much higher fluxes in MD.  FO water flux appears to be the limiting factor of design, especially with high salinity feeds, and we found that MgCl₂ represent the best draw solution for the treatment of hyper saline PW using FO-MD process. Stable fluxes of about 4 l/m².hr in the FO test and 3 kg/m².hr in the MD test were achieved over 20 hr. of operation. Adjusting the temperature of the draw solution was found to equalize fluxes.  We also found that no oil contaminated the draw solution, assuring that the MD process would be protected from oil fouling.