Effect of mechanical strain on the transport properties of thin-film composite membranes used in osmotic processes

Abstract

In this work, we studied the mechanical behavior of commercial thin-film composite membranes and measured water and salt transport through membranes that were subjected to known degrees of strain. Our aim was to correlate linear strain with transport properties. Firstly, we showed that the global transport properties of the membranes did not change significantly after being subjected to linear strain values that are typical of pressure-retarded osmosis (PRO) operations. Secondly, using a newly developed osmotically-driven burst pressure test for flat sheet membranes, we theorized that the increased salt passage through the membranes was attributable to local deformation and defect formation in the membrane region along the border of the feed spacer opening. Using laser microscopy, we were able to pinpoint the area on the membrane with increased deformation, and to measure the deformation profile. We defined a deformability coefficient to estimate the membrane strain at a known pressure in terms of easily attainable characteristics like opening size, membrane thickness and secant modulus and used it to postulate a solution-diffusion model that accounts for defects by considering the deformability of the membrane in the experimental setup. By incorporating membrane deformation into the boundary layer equations used to describe water and salt flux in osmotic processes (OP), the model can describe the observed dependence of salt flux with applied pressure. The model was used to fit our PRO experimental data and numerous data reported in the literature, which revealed that salt passage increases as membrane deformation increases. Along with this effect, there is a lowered mass-transfer resistance, which constitutes the trade-off between mechanical deformation and mass-transfer resistance observed in pressurized OP. Our findings show that the deformability coefficient and our solution-diffusion model with defects can serve as guidelines for the design of membranes and modules for pressurized OP such as PRO.