Engineered osmosis (EO) involves the contact of two solutions with different chemical potentials of water across a membrane interface to generate a water flux. This difference in chemical potential is obtained by a difference in concentration or/and pressure. EO applications use spacers with large openings to reduce concentration polarization. When pressure is applied in processes such as osmotically-assisted reverse osmosis, pressure-assisted osmosis, and pressure-retarded osmosis (PRO), membranes undergo a mechanical deformation as they are compressed against the spacer. This deformation causes an increased reverse salt (RS) flux as the hydrostatic pressure increases.<br/>We incorporated membrane deformation into boundary layer equations to describe water and RS flux in EO. We deformed a commercial TFC membrane using a tensile testing machine with the goal of correlating the linear membrane strain with water permeance and salt reverse flux obtained via direct-flow permeation. We measured cross-flow PRO water flux and used similar literature data to train the mathematical model for predicting changes in RS flux with increased pressure. Laser measuring microscopy revealed the area on the membrane with increased deformation, and confirmed that the deformation profile follows a parabolic trajectory. We refined our model and correlated our PRO data with the opening size and relative open area of feed spacer.<br/>Results showed tht the linear membrane strain reaches values above 15%, which are greater than the reported strain-to-fracture values for these membranes. Water and RS were not statistically different when measured using increasing and decreasing pressure profiles, suggesting that active layer deformation is partially reversible. Best fits were obtained using a model that incorporated diffusion through a film comprising intact polymer, deformed polymer, and occasional fracture sites. Deformations in more than 2% of the membrane area lead to exponential increments in the RS flux as pressure increases. This work introduces membrane deformation as a factor to consider in the design of membranes and modules for engineered osmosis applications.

Incorporating membrane deformation into the boundary layer equation to model water and reverse salt flux in engineered osmosis

2019 Spring National Meeting

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