(706f) Using Structure-Performance Relationships to Design Osmotic Membranes: Part 2, Experimental Results and Model Validation

Forward osmosis (FO) is an osmosis-driven process that can be applied to various liquid separations such as drug delivery, small-molecule dialysis, fruit juice and beverage concentration, hydration packs, brackish and ocean water desalination, sludge dewatering, and wastewater reclamation. While FO processes are theoretically and technologically viable; at present, the main obstacle that hinders their application is the lack of an appropriate membrane design. In studying the literature, one approach towards a ?better? osmotic membrane could involve the formation of a highly permeable and selectivity barrier layer (to effect the osmotic separation) formed over a thin, high porosity support layer (to minimize internal concentration polarization, ICP). Internal concentration polarization results from hindered diffusion of feed and/or draw solutes within the porous support layer of a composite membrane. The extent of ICP is theoretically a function of the porous support layer thickness and (macrovoid) porosity and tortuosity.

Following these design principles, a theoretical framework was developed to identify potential structures of high-performance osmotic membranes. This specific goal of this work was to elucidate the inter-relationships between the five known FO membrane structure-performance parameters (barrier layer water permeability, Pw, and salt permeability, Ps; support layer porosity, εs, tortuosity, τs, and thickness, ds) and their influence on PRO power production and FO water production. A parametric study was performed on the five structure-performance parameters required to make a high-productivity FO membrane. In addition, model analyses demonstrate why the current generation of commercially available reverse osmosis (RO) membranes does not perform well in FO applications.

Preliminary results suggest that for a given porous support layer increasing the barrier layer water permeability directly increases productivity; however, if salt permeability increases proportionately (i.e., selectivity drops) the gain in productivity is completely lost and the draw solute will be wasted. Hence, FO membranes must become more permeable to water, while maintaining high salt selectivity--like that of the current generation of commercial seawater RO membranes. For a given barrier layer, support layer tortuosity appears the most important parameter; as tortuosity approaches unity, FO membrane permeability is minimally affected by large changes in support layer porosity and thickness. These results provide new insights into the structural changes required to produce viable FO membranes. In part 1 of this paper, model development and results of the parametric study will be presented. In part 2 of this paper, a series of FO membrane structures were synthesized and tested to validate model predictions.