Title: Advancing Membrane Technologies for Sustainable Production of Energy and Water
Description: Increasing global needs for clean water and renewable energy are key challenges in the 21st century. Membrane-based technologies offer cost-effective separations and show great promise of providing sustainable solutions to challenges in the water-energy nexus. This dissertation aims to advance membrane technologies for sustainable production of energy and water. The work advances the novel vapor pressure-driven osmosis (VPDO) technology for efficient energy conversion from low-temperature heat resources, proposes and develops an innovative cascading osmotically mediated reverse osmosis (COMRO) for energy-efficient desalination of high-salinity brines, and provides a framework for better understanding of the permeability-selectivity tradeoff relationship in thin-film composite polyamide (TFC-PA) membranes widely used in aqueous separations. Abundant low-temperature (< ≈100 ℃) heat resources remain untapped in industries and the natural environment. The emerging vapor pressure-driven osmosis (VPDO) membrane technology enables direct conversion of the low-temperature heat to energy.

However, enhanced fundamental understanding of the process is still needed to bring the technology to practical applications. Work in this dissertation presents a theoretical model to understand mass and heat transfer in VPDO. The performance of two hydrophobic nanoporous membranes, polypropylene (PP) and polytetrafluoroethylene (PTFE), of different chemical and structural properties were evaluated. Knudsen diffusion was demonstrated to dominate the mass transfer in VPDO, and the work showed that evaporative heat transfer is significantly greater than conductive heat losses. A large amount of high-salinity brines (>≈70,000 ppm total dissolved solids, TDS, approximately 2× seawater salt concentration) are generated from various industrial processes and operations. Prevailing evaporation-based methods used to desalinate the hypersaline brines are highly energy intensive. Reverse osmosis (RO) is the most efficient method for seawater desalination, but is unsuitable for treating high salinities.

This dissertation proposes and develops an innovative cascading osmotically mediated reverse osmosis (COMRO) technology to achieve energy-efficient desalination of high-salinity brines. Theoretical analysis in this work showed that COMRO requires only 68.3 bar to desalinate hypersaline feed of 70,000 ppm TDS, whereas conventional RO needs the exceedingly high pressure of 137 bar. Furthermore, up to −17% energy saving can be attained by COMRO. To develop the COMRO technology, this dissertation presents systematic investigations of transport and structural properties of osmotic membranes in COMRO during high-salinity desalination. The impacts of hydraulic pressures and high salinities on water and solute permeabilities of membranes were studied, and the membrane structural parameter was demonstrated to be consistent at different salinities. Governing equations for water and salt fluxes in COMRO were established and the transport model was validated with experimental results. The solution-diffusion (S-D) theory is currently the most widely accepted transport framework for salt-rejecting membranes.

Importantly, it predicts a tradeoff relationship between permeability and selectivity in membrane transport. This study presents a framework to better understand the first principles governing the permeability-selectivity tradeoff in thin-film composite polyamide (TFC-PA) membranes used in aqueous separations. Tradeoff trends predicted by the conventional S-D was observed for nonelectrolyte solutes of different molecular sizes, and the thesis identified a second transport regime which deviates from the current understanding of S-D. The work showed that general principles of the S-D framework are applicable to TFC-PA membranes, and solute size is a principal factor governing the conventional S-D transport. Transition between the “deviation” regime and the conventional S-D transport was demonstrated to be governed by the solute size, and transport features of the “deviation” regime were also analyzed. Further, the dissertation elucidates the roles of structural properties of the polyamide layer in the tradeoff.

The work showed that the tradeoff cannot be completely explained by the changes in free volume sizes of the PA layer, and the shortening of the effective transport pathway also accounts for the permeability-selectivity behavior of permeants. The dissertation proposes mechanisms to elaborate how the membrane structural properties affects the permeability-selectivity behavior of permeant with a certain size. Overall, this dissertation presents pioneering advancements of membrane technologies for sustainable production of energy and water. Fundamental understanding of the heat and mass transport in VPDO was advanced, and the study highlights future directions of developing the technology for power generation. An innovative COMRO technology is proposed for hypersaline desalination, and the work established foundations for understanding the transport phenomena of the process. The dissertation sheds light on fundamental transport mechanisms of the thin-film composite polyamide membranes, and serves to improve understanding of all the osmotic membrane processes.

Findings of the work provide important insights into future design and development of high-performance membrane materials for water-energy application.