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Abstract Pressure-retarded membrane distillation (PRMD) is an emerging membrane process to recover energy from low-grade heat sources. The applied hydraulic pressure on the cold-water side in PRMD may strongly affect both energy conversion efficiency and membrane performance. Here, we report the first systematic study on this critical issue. A commercial nanoporous polytetrafluoroethylene membrane was evaluated as a general membrane sample over a range of applied pressures from 0 to 10 bar at a temperature difference of 40 °C. Our results show that the theoretically projected constant water vapor flux decreases significantly with the increase of the applied pressures, which can be attributed to the severe membrane deformation induced by pressures. The membrane in the active-layer-facing-hot-solution orientation is mechanically unstable with the complete loss of water vapor flux under 2 bar. In contrast, the membrane in the active-layer-facing-cold-solution orientation can still work under 10 bar. Combining theoretical analysis and detailed characterization of membrane physical structures, we show that the properties of membrane active layers (i.e. , pore size, porosity, and thickness) deteriorate under elevated pressures. Deformed membranes have lower permeability and higher temperature polarization in PRMD, resulting in the observed lower water vapor fluxes. Our results suggest that improving the mechanical stability of membranes would be the first critical step in realizing practical applications of PRMD for low-grade heat recovery. Potential research directions for developing novel PRMD membranes are also proposed. Graphical abstract Image 1 Highlights • Water vapor flux decreases with the increase of applied hydraulic pressure in pressure-retarded membrane distillation for waste heat recovery. • Membranes in the active layer facing cold permeate solution orientation have much better mechanical stability. • Structural properties of the membrane active layer deteriorate under elevated pressures. • The loss in water vapor flux can be attributed to decreased membrane permeability and severe temperature polarization. [ABSTRACT FROM AUTHOR]

In recent years, pressure retarded osmosis (PRO) has attracted increasing interest in the harvesting of the renewable osmotic power. However, its performance can be significantly influenced by the membrane deformation in the operation when the PRO membrane is lack of sufficient mechanical strength. In this study, we fabricated three different fabric-reinforced thin-film composite (TFC) flat-sheet PRO membranes for osmotic power harvesting. These membranes were prepared through integrating three different types of fabric reinforcement (i.e., tricot fabric, woven fabric and nonwoven fabric) in the membrane substrate layer. It was found that the fabric reinforcement plays an important role in the membrane structural property and mechanical property, both of which can significantly influence the PRO performance. The nonwoven-fabric-reinforced membrane had the greatest structural parameter and thus exhibited the lowest performance. Although the tricot-fabric-reinforced membrane and the woven-fabric-reinforced membrane had similar performance in the forward osmosis (FO) condition ( ∆P =0), the former showed superior performance in the PRO condition ( ∆P >0). This is mainly because the tricot-fabric-reinforced membrane had better mechanical resistance to the multi-directional tensile stretching, which rendered it less prone to changes in structural and separation properties in the PRO operation. This further suggests that the tricot fabric has high potential for future PRO membrane fabrication. The current study also elaborates the coupled effects of compression and stretching on PRO membrane deformation and performance. The results obtained in this study may provide important insights into reinforced PRO membrane design. [ABSTRACT FROM AUTHOR]

Forward osmosis (FO) processes, due to internal concentration polarisation, are limited with regards to flux. Improved flux performance will allow FO to compete with fluxes achieved by hydraulically driven membrane processes. Pressure assisted osmosis (PAO) is proposed to enhance FO performance, by adding hydraulic pressure (0.1–0.8 bar) on the feed side. An FO mass transport model (active layer to feed side orientation) incorporating pressure was developed to describe the fluxes in PAO. Continuous and discontinuous PAO operations on laboratory scale were proposed and evaluated using draw solutions equivalent to 24 bar osmotic pressure. The fluxes increased with increasing hydraulic feed pressures for all PAO experiments, including activated sludge feeds, owing to the increased driving force and membrane deformation. Discontinuous PAO was found to have an adverse effect on the salt fluxes, due to the subsequent pressure release. This study emphasizes the benefits of PAO for use in innovative membrane systems, while illustrating the importance of developing more rigid membranes and better support designs. [ABSTRACT FROM AUTHOR]

Pressure-retarded osmosis (PRO) has the potential to convert salinity gradient energy to hydraulic energy via a semi-permeable membrane. The performance of an osmotic membrane is usually evaluated using a crossflow membrane test cell with symmetric channels. However, it is not possible to test a PRO membrane under conditions of large differences in hydraulic pressure because the membrane cannot be supported in the channel inlet and outlet with a cavity. In this study, we present a newly designed PRO crossflow test cell that features asymmetric channels on both sides of the membrane. This asymmetric design prevents membrane bulge and rupture in the channel inlet and outlet even at an applied hydraulic pressure difference greater than 54.5 bar. Excellent performance of this novel PRO cell was demonstrated using two different membranes at various operating pressures. To prevent membrane deformation at open spaces between spacer strands, a feed channel spacer should be selected carefully. Accordingly, the effects of tricot feed spacers with different voidages and thicknesses on PRO performance were also investigated. [ABSTRACT FROM AUTHOR]

Abstract: The use of forward osmosis (FO) is of growing interest for water desalination, due to its potential energy savings. However, its industrial implementation is still limited by its actual performance limitation in water permeation and reverse salt diffusion, due to membrane properties. Assisted forward osmosis (AFO) is a new concept, aiming at pressurising the feed solution to enhance water permeation through synergising osmotic and hydraulic driving forces. This paper presents the impact of hydraulic pressure on the FO membrane properties and the overall performances of the system in order to validate the interest of AFO. When 6bar was applied on the feed side of the process, the membrane water permeability (A) was observed to double, mainly due to the membrane deformation against the spacers. Under those conditions, the additional driving force provided resulted in 70% increase in permeation flux, despite the more severe concentration polarisation. More interestingly, the observed reverse salt diffusion was significantly lower than expected by the solution diffusion model, confirming the interest of AFO in tackling current limitations of FO technology. This study also revealed the relative limitations of the current methodology used for the determination of membrane solute and water permeabilities, which currently fail to consider membrane deformation that could arise in pressure retarded osmosis and AFO systems. [Copyright &y& Elsevier]

Abstract: Pressure retarded osmosis (PRO) is a promising technology for extracting renewable salinity-gradient energy. However, its performance can be significantly influenced by membrane deformation at the high applied pressures when feed spacers are used in the operation. This study systematically investigated the effect of feed spacer geometry on membrane deformation and the resulted PRO performance. It was observed that feed spacers with larger openings induced more severe membrane deformation at higher applied pressures. A theoretical mechanical model further reveals that the extent of membrane deformation is proportional to the applied pressure and the square of the opening size of the feed spacer and inversely proportional to the membrane mechanical strength. Moreover, a modified RO method was developed to measure the separation properties (i.e., the water permeability (A value), solute permeability (B value) and solute/water selectivity (B/A value)) of the deformed membrane. The measured A, B and B/A values increased with the extent of the membrane deformation. It was further found that the severe membrane deformation not only drastically reduced the PRO water fluxes but also substantially increased the hydraulic pressure loss in the feed flow channel. For feed spacers with large openings, the reduced PRO water fluxes together with the increased pumping need for feed flow severely decreased the net power output. In contrast, the use of feed spacers with small openings resulted in significant improvement of PRO performance. In the current study, the use of commercial RO permeate carrier as the PRO feed spacer (opening ratio ~0.35) lead to a significantly enhanced power density of 5.8W/m2 at the optimal applied pressure of 17.2bar due to the minimized membrane deformation. The current study can provide important implications for PRO studies. [Copyright &y& Elsevier]

Osmotically-driven membrane process (ODMP), the low energy process that has attracted significant attention recently, has a relatively short history of development and there is a large room for improvement in membrane module design. Especially, it is significant but neglected how much membrane area should be packed into the module housing without undermining structural integrity and performance of the membrane. In this regard, this study aims to experimentally investigate the variation of osmotic membrane performance under varying spacer-induced mechanical compression and determine the critical compression ratio. 3D FEM (Finite element method) mechanical stimulation was employed to analyze the variation of stress and strain of membrane under mechanical compression and critical stress and strain were determined based on the experimental results. The study found that the performance of commercial osmotic membrane did not decrease significantly up to 30–40% of compression ratio and 45.5%–69.6% of the packing density of spiral-wound membrane module can be saved. The findings and methods suggested in this study will facilitate the structural optimization of ODMP membrane modules. Also, the discrepancy of the membrane performance between the lab-scale experiment (non-compressed) and commercial-scale membrane module (compressed) validated in this study emphasizes the importance of caution in the reporting of membrane performance. [Display omitted] • Osmotic membrane performance varies under spacer-induced mechanical compression. • Selectivity of membrane drastically decreases at a certain point of compression ratio. • Critical compression ratio, stress, and strain were found for membrane and spacer type. • The commercial osmotic membrane has a 30–40% of critical compression ratio. • The packing density of the commercial membrane module can be improved up to 69.6%. [ABSTRACT FROM AUTHOR]

Membrane deformation is a common phenomenon in pressurized membrane processes. It alters the transport and structural characteristics of membranes and hence can lead to a lower than estimated process performance. Therefore, it is essential to accurately characterize the membrane under representative operating conditions. This will allow for both an understanding of the underlying mechanisms for the change of membrane performance, and an optimization of the design and operation of pressure- and osmotically driven membrane processes. A novel membrane characterization method is proposed, validated and tested in this study. Using the osmotic-resistance filtration model, the membrane's water and solute permeability (A and B), as well as its structural parameter S , can be accurately determined using the method. The method is named the integrated two-stage (ITS) ABS method, as the membrane can be fully characterized at any given pressure using a single continuous test that is divided into two stages; each stage uses a different feed or draw concentration. A , B and S are calculated from the experimentally determined water and solute fluxes by performing a least-squares non-linear regression. The proposed method is robust, simple and offers more accurate predictions of the membrane's transport and structural properties than the currently most widely used reverse osmosis–forward osmosis (RO–FO) characterization method. [Display omitted] • Simple and robust method to determine the A , B and S values of deformed membranes. • ABS determined for Aquaporin and BW30 membranes at pressures up to 60 bar. • Cyclic-pressure tests show the elastic/plastic deformation behaviour of membranes. • The proposed method can be used to characterize any type of membrane or module. [ABSTRACT FROM AUTHOR]

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. Image 1 • Transport properties measured for TFC membranes subjected to known values of strain. • New osmotically-driven burst pressure test introduced for flat sheet membranes. • Deformability coefficient used for estimating membrane deformation against spacers. • Revised solution-diffusion model accounts for pressure-dependent salt passage in PRO. • Trade-off established between mechanical deformation and mass-transfer resistance. [ABSTRACT FROM AUTHOR]