(5y) The Ammonia – Carbon Dioxide Forward Osmosis Desalination Process: Performance and Modeling

McGinnis, R. L., Yale University
Elimelech, M., Yale University

Forward osmosis (FO) has recently received a good deal of attention concerning its use as a viable membrane-based desalination and water treatment process. The process utilizes a concentrated draw solution which causes the natural transport of water across a semipermeable membrane that is impermeable to salt. The draw solute is then either utilized or removed and recycled. Our process utilizes a draw solution composed of dissolved ammonia and carbon dioxide gases. These gases, once dissolved, form highly soluble salts which can generate large osmotic pressures. They may also be stripped from the solution with relative ease at low temperatures and recycled. A major hindrance to the performance of the FO desalination process is the membrane. Previous studies on FO have concluded that current generation polymeric membranes do not perform well in the FO process. These membranes, designed for pressure-driven flow, have a thin separating (active) layer supported by a porous polymer support layer cast upon fabric. The culprit for the poor flux performance with these membranes is the prevalence of internal concentration polarization (ICP). This phenomenon is similar to concentration polarization except that it takes place within the protective confines of the porous support and fabric layers. Here, the boundary layer thickness is not controlled by crossflow and hence significantly impacts the osmotic driving force. Using a commercially available asymmetric FO membrane which lacks a fabric backing layer, this study examines the performance of the NH3-CO2 forward osmosis desalination process and elucidates the ICP phenomenon by observing flux behavior under varying system conditions. The bench scale system was found to perform well in terms of salt rejection and water flux. Water fluxes, however, were far lower than expected based on the available osmotic driving forces. Internal concentration polarization was determined to be the cause of the lower water fluxes. The severity of the ICP can be characterized by determining the solute resistance to diffusion within the support layer, K, which is a function of the thickness, tortuosity, and porosity of the support layer as well as the solute's diffusion coefficient. This term was incorporated into a model to predict the water flux of this membrane at different temperatures for a variety of feed and draw solution concentrations. The model was found to correspond closely with experimental data, suggesting that it may be reliable as a stand-alone tool for predicting water flux at different experimental conditions. The model also was useful in determining how changing membrane structural properties, and hence K, would affect water flux performance.