Date of Award
Doctor of Philosophy (PhD)
Scott M Husson
Amod A Ogale
Eric M Davis
David A Ladner
My research goal was to understand the role of membrane mechanical properties (e.g. strength and stiffness) in the transport of water and salt through polymer-based thin-film composite (TFC) membranes used for osmotic processes (OP). OP are membrane processes in which the main driving force is a concentration difference of solute(s) in the solutions in contact with the two sides of a semipermeable membrane. OP applications may include removing water from products/contaminants, harvesting energy from salinity gradients, and lowering costs of seawater desalination. The study system for my research was a set of TFC reverse osmosis (RO) membranes designed for rejecting salts in desalination. These TFC RO membranes have thick supporting layers (~150 μm), which increases the diffusion pathway for salts within the membranes. This decreases the effective salinity gradient between the two surfaces of the membrane active layer, which ultimately decreases the process productivity (i.e., water flux). I aimed to provide guidelines for the improved design of TFC membranes for OP, considering the trade-off between membrane mechanical integrity and productivity.
My research approach comprised measurements and analysis of the individual mechanical properties of the three polymeric layers (active, porous support, and backing) that comprise TFC RO membranes, and correlation of these mechanical properties with TFC membrane transport properties and performance in OP. Initially, I studied helically coiled and multiwall carbon nanotubes as additives to create nanocomposite porous supports with improved mechanical properties. The results support the idea that increasing the mechanical stiffness of TFC membrane nanocomposite supports is an effective strategy for enhancing water production in desalination operations. Secondly, I evaluated woven polyester mesh as a backing layer and analyzed the role of mesh opening size on burst strength and mass-transfer resistance of TFC membranes used for OP. The findings show that mass-transfer resistances in OP are an additive effect of the multiple layers that compose a membrane and can be reduced by using more open (yet functional) backing layers such as polyester woven mesh in place of standard nonwoven mesh. Thirdly, I aimed to correlate the reduction in stiffness of the active layer with the change in water permeance of five commercial TFC membranes after contact with five different C1-C4 monohydric alcohols. The motivation was to explain the improvements in water flux after alcohol contact, which is used to pre wet membranes before OP operation. Correlation of results suggests that the mixing of water with the alcohols facilitates penetration of the alcohols into the active layer, likely by disrupting inter-chain hydrogen bonds, thus increasing the active layer free volume for water permeation. Finally, I studied water and sodium chloride transport through TFC membranes that were subjected to known degrees of mechanical strain. I demonstrated the importance of knowing the stress-strain curve of the membrane and highlighted that stiffer membrane structures are desirable to avoid reaching a strain above the reported onset fracture strain of the selective layer. With this information, I introduced a deformability coefficient and a solution-diffusion model with defects to guide the design of membranes and modules for pressurized osmotic processes. In closing the dissertation, I provided recommendations and research opportunities that I envision would improve TFC membrane productivity in OP.
Idarraga-Mora, Jaime Andres, "Mechanical Properties of Thin-Film Composite Membranes and Their Role in Osmotic Processes" (2020). All Dissertations. 2588.