Pressure Retarded Osmosis Process Research Articles

Experimental study of the potential of concentrated NaCl solutions for use in pressure-retarded osmosis process

Pressure retarded osmosis is a process that enables useful work generation from the salinity difference of solutions. The literature most often considers using pressure retarded osmosis with natural sodium chloride (NaCl) solutions, such as seawater, dedicated for open systems. To explore the full potential of this process, however, optimized, highly concentrated solutions of various compounds can be used. The presented research is focused on evaluating the impact of increasing draw solution temperature and concentration on the permeate flow in the osmotic process. The permeate flow is directly related to achievable work in this process, therefore, it is important to find feed and draw solution parameters that maximize it. An experimental setup developed in this study provides full control over the process parameters. Furthermore, the performance characteristics of the membrane over process time were investigated, as it became evident during preliminary experiments that the membrane impact is significant. The studies were conducted without back-pressure, in a configuration typical of the forward osmosis process, with solution circulation on both sides of the membrane. The obtained results show a clear positive impact of both the temperature and concentration increase on the potential output of a pressure retarded osmosis system. The membrane behaviour study allowed for correct interpretation of the results, by establishing the dynamics of the membrane degradation process.

Feasibility and challenges of high-pressure pressure retarded osmosis applications utilizing seawater and hypersaline water sources

Pressure retarded osmosis (PRO) harnesses salinity gradient energy through the mixing of freshwater and saltwater, addressing the demand for sustainable energy sources. PRO typically utilizes river water or secondary wastewater as the feed solution, paired with seawater reverse osmosis (SWRO) brine as the draw solution. However, the limited availability of low-saline water presents a significant obstacle to energy generation. Therefore, the feasibility of a high-pressure PRO process utilizing seawater as the feed solution and hypersaline water as the draw solution was assessed to generate sustainable blue energy. Seawater has the potential to achieve the maximum extractable Gibbs-free energy through high-pressure PRO by maximizing the feed/draw ratio. The performance of the high-pressure PRO was theoretically evaluated, resulting in a tenfold increase in specific energy production compared to conventional SWRO-PRO due to the higher feed/draw ratio. However, high hydraulic pressure increased the membrane structural parameter and further reduced water flux and power density due to significant internal concentration polarization. Nevertheless, the high-pressure PRO process can achieve a power density exceeding 84 W/m2 when the structural parameter remains below 100 μm. The implications of the high-pressure PRO were further discussed to advance the concept of a blue circular economy, enhance environmental resilience, and promote sustainability.

Comparison of 2D and 3D materials on membrane modification for improved pressure retarded osmosis (PRO) process

Pressure retarded osmosis (PRO) is a sustainable process that convert Gibbs free energy to osmotic energy by mixing two solutions of different salinities. The main challenges in the design of PRO membranes are obtaining a membrane with high water permeability and low salt permeability but also very high mechanical strength because the PRO process involves high pressure on the draw solution. Commercially available RO membranes with potential utility in a PRO system exhibit a high salt rejection rate but low water permeability and mechanical stability. Surface modification is a promising strategy for tuning the fundamental properties of the membranes (e.g. hydrophilicity, surface charge and thickness) that can improve the filtration performance of the membranes. The coating layer can also improve the mechanical stability of the membranes. Therefore, in this work, various types of modification materials were applied to the commercial available RO membranes to enhance their performance.With the assistance of hydrophilic materials (e.g. polydopamine – PDA), filtration performance of the membranes can be increased through membrane modification by 2D materials with high charge intensities (e.g. polyelectrolytes and graphene oxides) and by 3D mesoporous materials (e.g. zeolites), which increases the thickness of the membrane that can be beneficial in mechanically reinforcing the membrane. In this work, we modified commercial RO membrane with PDA, polyelectrolytes, graphene oxide and zeolites (ZSM-5). Improved filtration performance (increased water permeability and maintained salt permeability) of the modified membrane was observed. Tensile tests showed enhanced mechanical strength of the modified membranes, especially following 3D zeolites modification (up to 35 % of higher tensile strain was reported). Interestingly, a lower concentration of PDA (2 mg/mL) and zeolites resulted in higher mechanical strength of the modified membranes. Such results were likely due to a more homogenous coating layer when a low modifier concentration was applied. The thin and uniform layer can better absorb energy when membranes are under high pressure.

Multiple staging of pressure retarded osmosis: Impact on the energy generation

As pressure retarded osmosis (PRO) is getting more attention for sustainable energy generation, further investigation to enhance its efficiency is required. Applying a multiple-stage PRO scenario is one suggestion to boost the generated energy in the PRO process. This research evaluates the benefit of multiple stages of pressure retarded osmosis on specific energy generation. Single, dual, ternary, and quaternary pressure retarded osmosis stages were investigated for power generation using four membrane modules in the PRO process. Eight designs were proposed to cover the arrangement of the membrane modules in a single, dual, ternary and quaternary PRO process and estimate the net-specific energy generation of pressure retarded osmosis. For each design, the variation in the total energy input and the total energy output of single and multistage pressure retarded osmosis processes were calculated. The water flux and the power density enhanced with the number of stages for the investigated multistage PRO scenarios. The maximum water flux and power density resulted in four PRO stages of values equal to 6 L/m2.h and 15 W/m2, respectively. For the Dead Sea-reverse osmosis brine salinity gradient, a 33.96 % rise in the net specific energy generation is observed in the quaternary-stages pressure retarded osmosis scenario. The Dead Sea and wastewater salinity gradients produced the highest net specific energy generation of 0.82 kW.h/m3 over the other investigated types of salinity gradients.

The Potential of Multi-stage Pressure Retarded Osmosis for Blue Energy Harvesting

Objective: This study aims to investigate the potential benefits of using different multi-stage and multi-pass flow configurations for improving the efficiency of the pressure-retarded osmosis (PRO) process, which generates energy by utilizing the difference in osmotic pressure between two salinity solutions. Methods: To achieve the objective, the study used a numerical simulation based on the solution diffusion model. A flat sheet forward osmosis membrane was simulated to investigate the potential of the proposed multi-stage PRO configurations. Results: The results of the simulation showed that the multi-stage PRO arrangement generated 18% more power than the single-stage PRO system. Additionally, the multi-pass arrangement resulted in a 58% increase in power output. Conclusion: This study provides important insights into the development of optimized PRO membrane and module configurations to improve the efficiency of energy generation. The findings highlight the potential benefits of multi-stage and multi-pass flow configurations, which can help to advance the field of renewable energy generation using the PRO process.

Salinity gradient power using in the Black Sea regions (in frame of the blue growth development)

Problem Statement. Today, humanity is in search of new sources of energy to make the economy more sustainable, as well as the need for a transition to energy that works on the principles of Carbon-Free Technology. For the Black Sea, this is expressed in the desire for successful implementation of the program Blue Growth Accelerator, which is aimed at the introduction of innovative technologies and alternative energy sources in the energy sector of the Black Sea countries, for the development of the "Blue Economy" and the achievement of its healthy, productive and sustainable state. Salinity gradient power (SGP) is one of the new renewable energy sources. The most studied methods for obtaining SGP energy are technologies based on: Reverse Electrodialysis and Pressure Retarded Osmosis. The interaction of fresh and salt water can provide, in fact, unlimited, free and clean energy. The basis for the generation of such energy is the so-called salinity gradient that occurs when two types of water are mixed. After decades of work and numerous experiments, scientists have developed a way to use the energy of the salinity gradient to generate electricity. This type of electricity is also called "Blue Energy" by association with the color of mixing freshwater and salt water when rivers flow into the ocean. Places (estuaries or deltas), where rivers flow into the oceans and seas, have a truly enormous energy potential. The aim of this study is to identify sites in the northwestern Black Sea region with the necessary conditions for the development of Salinity Gradient Power energy, as well as to assess their potential using the example of estimating the maximum theoretical power of the Pressure Retarded Osmosis process. Research Methodology. In a PRO system, the less concentrated solution flows towards the more concentrated solution due to the positive osmotic pressure difference as long as this difference remains greater than the hydrostatic pressure difference. It is by this principle that osmotic power is produced. Theoretically available amount of energy released when mixing 1 m3 of saturated brine (5 mol/l NaCl solution) and 1 m3 of sea water (0.5 mol/l NaCl) at 293 K is 10 MJ. In the northwestern Black Sea region, along the coast between the Danube and Dnieper rivers, there are 21 limans (lagoons) of which some can be used to generate of Salinity Gradient Power. Results. The results of calculating the maximum net power showed that highest values obtained in the summer months, when the salinity in limans reaches its maximum and, consequently, its difference with the salinity of sea (river) water increases. Proceeding from maximum net power, obtained for the Western Sivash, where the salinity is maintained artificially at certain values, it can be seen that the annual amplitude has a smaller value, which provides more stable conditions. There are objects in the northwestern Black Sea region, in the waters of which, as soon as technologies become available, it will be possible to implement SGP projects. The Kuialnyk Liman, Sasyk- Sivash lake and Western Sivash have the most favorable conditions, where the highest power indicators are shown when using the sea water – hypersaline solution scheme, in which freshwater is not consumed.

Emerging membrane technologies for low-cost desalination

Reverse osmosis (RO) has become an attractive option to produce potable water to cater the global needs. Despite the potential advantages of the RO process, it has limitations too. To address such limitations membrane distillation (MD), forward osmosis (FO) and pressure-retarded osmosis (PRO) are now becoming emerging areas of research for seawater desalination. With significant advances in FO and MD for desalination, the entire operation to be conducted at low hydraulic pressure, which reduces fouling and improves membrane life. PRO is an osmotically driven process that produces green energy from the brine stream. However, hybrid desalination processes in combination with RO, are found to be efficient compared to stand-alone technology, with a reduced desalination cost. Although a few studies are available, practical integration of the current state of the art and its gradual progress is rarely addressed. In this context, emerging FO, MD, PRO processes, and the development of membranes for energy minimisation with improved performance are presented herein. Hybrid processes in combination with renewable energy resources are evaluated. Significant technological development and emergence of new materials is anticipated to play a pivotal role so that membrane-based desalination technologies become more efficient and economical in near future.

Fouling behaviours and mitigation in pressure-retarded osmosis processes with geothermal water/brine-based draw solutions

In this study, we examined pressure-retarded osmosis (PRO) performance using naturally high temperature NaHCO3-enriched geothermal water (i.e., capturing CO2 waste gas) and geothermal brine. The results showed that increasing draw solution temperature from 20 °C to 60 °C, NaHCO3-enriched geothermal water facilitated improving water flux (Jw) without compromising reverse salt flux (Js), leading to lower Js/Jw ratios compared to NaCl-based solution. With NaHCO3-enriched geothermal water (60 °C) as draw solution, periodically physical cleaning (clean water flushing with/without air sparging, osmotic backwashing) displayed ∼17 % lower permeability recovery ratio (p < 0.05), dissimilar foulant morphology and inorganic compositions compared to chemical cleaning (acid, base, NaClO); while the average permeability levels under both cleaning conditions were comparable (p > 0.05). However, the recovery ratio decreased with extending cleaning cycle, resulting in an almost comparable permeability to that without cleaning after 60-h operation. In addition, the presence of Ca2+ at 5 mM in the NaHCO3-enriched geothermal water (60 °C) led to more significant water flux drop and less reverse flux decrease compared to those with Ca2+ at 0 and 30 mM, possibly due to its dense fouling layer formed by smaller-sized CaCO3 precipitates. When geothermal brine (low or high salinity, pH = 3 or 9.7, 60 °C) was used as draw solution, it was noticed that (1) with low salinity brine, basic pH levels led to higher water fluxes; (2) with high salinity brine, the water flux was impacted by the ionic compositions of draw solutions, instead of pH levels.

A multi-objective honey badger approach for energy efficiency enhancement of the hybrid pressure retarded osmosis and photovoltaic thermal system

The pressure retarded osmosis process is emerging as one of the most promising technologies for extracting renewable energy from salinity gradient resources. The hybrid system of the pressure retarded osmosis and photovoltaic/thermal systems is capable of effectively improving overall energy efficiency. The prime research focus of hybrid energy systems is to produce environment-friendly solutions by minimizing energy consumption and improving energy efficiency. Due to the increasing demand for intelligent computing, the challenges of maximum power extraction and efficient energy management have attracted the attention of researchers around the world. Thus, a Honey Badger Approach based energy management strategy is proposed to solve the problem of performance enhancement in the pressure retarded osmosis and photovoltaic/thermal system by maximizing the multi-variable functions. The Honey badger approach uses global sight to tackle the optimization of multiple objectives and to achieve optimal solutions. The proposed energy management method is tested with various types of operational environments, including variations in the operational temperature, solar irradiance and ambient temperature in regard to detrimental effects. The practical hourly data of Perth for a year is utilized to simulate the actual operating conditions. The simulation results indicate that the proposed energy management method overwhelms other competing approaches with better energy efficiency, thereby ensuring the continuity of energy supply and improving the reliability of the system.

Mathematical Modeling of a Hollow Fiber Module Used in Pressure-Retarded Osmosis Process

Pressure retarded osmosis (PRO) can be considered as one of the methods for utilizing osmotic power, which is a membrane-based technology. Mathematical modeling plays an essential part in the development and optimization of PRO energy-generating systems. In this research, a mathematical model was developed for the hollow fiber module to predict the power density and the permeate water flux theoretically. Sodium chloride solution was employed as the feed and draw solution. Different operating parameters, draw solution concentration (1 and 2 M), the flow rate of draw solution (2, 3, and 4 L/min), and applied hydraulic pressure difference (0 - 90 bar) was used to evaluate the performance of PRO process of a hollow fiber module. The effect of these operational parameters was investigated on the theoretical permeate water flux and power density. According to the theoretical results, the permeate water flux and the power density increased with increasing the concentration of draw solution and the flow rate of the draw solution. While decreased with increasing the feed solution concentration. By increasing the applied hydraulic pressure on the draw solution, the water flux decreased and the produced power density increased. The maximum power density and the corresponding permeate water flux of 2 M NaCl draw solution was approximately 16.414 W/m2 and 11.818 LMH respectively, which occurs at an applied hydraulic pressure of 50 bar.

A systematic methodology for targeting the thermodynamic limit of pressure-retarded osmosis with non-zero driving force

A thermodynamically oriented method is developed to identify the theoretical and practical limits of pressure-retarded osmosis (PRO) on converting salinity gradient power to mechanical energy under constant operating pressure with infinite and finite mass exchange area, respectively. It is proved that pinch analysis can be used to find the mass exchange limit of PRO, and according to which draw and feed streams can be plotted on a graph of net osmotic pressure - water exchange. Based on the graphical method, an expression of optimal pressure change is derived that achieves the maximum work extraction of a PRO process with single draw/feed stream. Moreover, problem table method is presented that serves as an analytical procedure to determine the maximum extractable work of the conceptualized multi-stream PRO process. Examples of both a PRO process with a draw stream (0.6 M NaCl seawater) and a feed stream feed (0.015 M NaCl river water) and a multi-stream PRO are included to illustrate the proposed methods. Results of the former example show that the counter-current design achieves 30.85% and 31.49% increase, respectively, in the maximum theoretical and practical work output, compared with the co-current design. The latter example shows that the problem table method produces a 20.47% increase in the work output, compared with the standalone design.

Novel Thin-Film nanocomposite hollow fiber membranes in modules with reduced reverse solute flux for pressure retarded osmosis

Osmotic energy is released when mixing solutions with different salinities. Pressure retarded osmosis (PRO) is a promising membrane technology to harvest this osmotic energy. However, the current polymeric PRO membranes suffer from the high reverse solute fluxes during PRO processes, which would dramatically diminish the membrane performance, especially in big membrane modules. In order to control the reverse solute flux, two cup-like calix[n]arenes, sulfocalix[4]arene (SCA4) and sulfothiacalix[4]arene (STCAss) were incorporated into the polyamide network to enhance the PRO performance of thin-film nanocomposite (TFN) hollow fiber membranes in 1-inch modules. Due to the unique structures and the enhanced molecular-sieving abilities of both macrocyclic molecules, they were able to effectively improve the selectivity of the resultant TFN hollow fiber membranes. After optimizing the concentration of each nano-filler, it was found that the TFN membrane with 0.20 wt% of STCAss produced a PRO power density of 15.0 W/m2 and a reverse salt flux of 28.3 gMH, while the other containing 0.10 wt% of SCA4 produced a PRO power density of 14.2 W/m2 and a reverse salt flux of 31.6 gMH. In long-term PRO tests at 20 bar, the SCA4-incorporated membrane was more efficient in controlling the concentration polarization (CP) and maintaining the power density comparing to the STCAss membrane because the former has a smaller cavity opening than the latter. This research work may provide useful insights to further design PRO membranes and membrane modules for osmotic power generation.

Study of Pressure Retarded Osmosis Process in Hollow Fiber Membrane: Cylindrical Model for Description of Energy Production

A new mathematical model was developed to predict the cylindrical effect of the membrane performance in the pressure retarded osmosis process. The cylindrical membrane transport layers (the draw side boundary and the porous membrane) were divided into very thin sublayers with constant mass transport parameters, among others with a constant radius in every sublayer. The obtained second-order differential mass balance equations were solved analytically, with constant parameters written for every sublayer. The algebraic equation system involving 2N equations was then solved for the determinant solution. It was shown that the membrane properties, water permeability (A), salt permeability (B), structural parameter (S) and the operating conditions (inlet draw side solute concentration and draw side mass transfer coefficient) affect the water flux strongly, and thus the membrane performance, due to the cylindrical effect caused by the variable surface and volume of the sublayers. This effect significantly depends on the lumen radius. The lower radius means a larger change in the internal surface/volume of sublayers with ΔR thickness. The predicted results correspond to that of the flat-sheet membrane layer at ro = 10,000 μm. At the end of this manuscript, the calculated mass transfer rates were compared to those measured. It was stated that the curvature effect in using a capillary membrane must not be left out of consideration when applying hollow fiber membrane modules due to their relatively low lumen radius. The presented model provides more precise prediction of the performance in the case of hollow fiber membranes.

Evaluation of the Specific Energy Consumption of Sea Water Reverse Osmosis Integrated with Membrane Distillation and Pressure-Retarded Osmosis Processes with Theoretical Models.

In this study, theoretical models for specific energy consumption (SEC) were established for water recovery in different integrated processes, such as RO-PRO, RO-MD and RO-MD-PRO. Our models can evaluate SEC under different water recovery conditions and for various proportions of supplied waste heat. Simulation results showed that SEC in RO increases with the water recovery rate when the rate is greater than 30%. For the RO-PRO process, the SEC also increases with the water recovery rate when the rate is higher than 38%, but an opposite trend can be observed at lower water recovery rates. If sufficient waste heat is available as the heat source for MD, the integration of MD with the RO or RO-PRO process can significantly reduce SEC. If the total water recovery rate is 50% and MD accounts for 10% of the recovery when sufficient waste heat is available, the SEC values of RO, RO-PRO, RO-MD and RO-MD-PRO are found to be 2.28, 1.47, 1.75 and 0.67 kWh/m3, respectively. These critical analyses provide a road map for the future development of process integration for desalination.

Predicting the performance of spiral-wound membranes in pressure-retarded osmosis processes

A process simulator has been developed to model and predict the performance of spiral-wound membrane modules in pressure retarded osmosis processes. This has involved automation of generalized protocols for the numerical integration of the solvent and solute flux equations (in conjunction with a suitable electrolyte equation of state) along the surface area of a spiral-wound membrane leaf. Performance equations are solved for discrete area elements and the spiral-wound character of the module as a whole is realized through the programmed sequence in which discrete elements are evaluated. This arrangement allows for mirroring the parabolic flow pattern of the feed stream in the spiral-wound membrane leaf. The total permeation (and, by extension, power density) is thus calculated in a manner that accounts for the driving force profile consistent with flow patterns specific to spiral-wound membranes. This effective treatment of each discrete element as a flat-sheet membrane enables the transferability of membrane parameters characterized in standard, coupon-scale experiments to the simulation of spiral-wound modules. This transferability is illustrated through comparisons of model predictions with published pilot-scale PRO data.

Simulation of forward osmosis and pressure retarded osmosis membrane performance: Effect of TiO2 nanoparticles loading on the semi-permeable membrane

Forward osmosis (FO) has made significant progress in recent years as an emerging membrane technology for the treatment of water and wastewater. Differential osmotic pressure is used to extract pure water across a semi-permeable membrane in this process. CFD simulations were used to investigate the performance of flux changes in membrane systems in this study. This paper simulates the performance of a thin film composite (TFC) membrane with a titanium dioxide (TiO2) modified substrate in terms of FO and pressure retarded osmosis (PRO). The results showed how the simulation results (CFD) can be used to detect flux changes with additive particles. It was discovered that even a small amount of TiO2 nanoparticles can alter water flux. In the FO and PRO systems, adding 1 wt% TiO2 to the membrane matrix increased water flux by about 15% and 14%, respectively. Nevertheless, the increased solute flux was insignificant in simulation, which indicates the positive effect of adding TiO2 nanoparticles. Finally, a comparison of the results for different membranes indicated the best correlation between the simulation and experimental results, when 0.75 wt% TiO2 was used, and a relative error of 0.3% has been recorded in PRO process. Also, the maximum pure water flux rate (8·974×10−4molm2·s)obtained in the simulation of thin film nanocomposite (TFN) 1 and PRO process.

Impact of hydrodynamic conditions on optimum power generation in dual stage pressure retarded osmosis using spiral-wound membrane

The Dual Stage Pressure Retarded Osmosis technique is considered for power generation. The influence of feed flow rates, hydraulic pressure, and pressure drop on mass transfer and solute diffusion in a full-scale membrane model was investigated for the first time to maximize power generation. Dead Sea-seawater, Dead Sea-reverse osmosis brine, reverse osmosis brine-wastewater, and seawater-wastewater salinity gradient resources were investigated for power generation. Results revealed a 71.07% increase in the specific power generation due to the dual-stage pressure retarded osmosis process optimization using Dead Sea-seawater salinity gradient resources. The increase in the specific power generation due to the dual-stage pressure retarded osmosis optimization was 108.8%, 63.18%, and 133.54%, respectively, for Dead Sea-reverse osmosis brine, reverse osmosis brine-wastewater, and seawater-wastewater salinity gradient resources. At optimum operating conditions, using the dual-stage pressure retarded osmosis process as an alternative to the single pressure retarded osmosis process achieved up to a 22% increase in the energy output. Interestingly, the hydraulic pressure at optimum operating conditions was slightly higher than the average osmotic pressure gradients in the dual-stage pressure retarded osmosis process. The study also revealed that power generation in the dual-stage pressure retarded osmosis process operating at constant mass transfer and solute resistivity parameters was overestimated by 2.8%.

Temperature effects on salinity gradient energy harvesting and utilized membrane properties – Experimental and numerical investigation

Salinity gradient energy (SGE) is a source of renewable energy which utilizes the osmotic pressure difference between two water streams of different salinities. The present paper investigates the effect of environmental water temperature variations on the performance of the pressure retarded osmosis (PRO) process employed to harvest SGE. Using experimental and numerical approaches, the effect of water temperature on water and salt permeabilities as well as membrane structure parameter have been measured using a bench scale system and a commercially available FO membrane. The water and salt permeability coefficients have been found to increase with the water temperature while the membrane structure parameter decreases. This variation in the membrane properties has enhanced the maximum power density of the PRO process by about 90% when the water temperature increases from 10 to 35 °C. New correlations to predict the membrane properties at different temperatures have been developed and validated against the tested membrane as well as other membrane properties data available in the literature. This study highlights the importance of understanding the effect of water stream temperature to accurately predict the performance of PRO process that is utilized for harvesting the salinity gradient energy.

Assessing the potential of integrally skinned asymmetric hollow fiber membranes for addressing membrane fouling in pressure retarded osmosis process

Thin-film composite (TFC) membranes are generally preferred over integrally skinned asymmetric (ISA) membranes in pressure retarded osmosis (PRO) process due to their much higher water permeability, but they cannot operate in the active layer facing feed solution (AL-FS) orientation because of potential delamination of the rejection layer under the high applied pressure. However, operating under AL-FS orientation is preferred as a strategy for better fouling control. Hence, in this study, three ISA hollow fiber membranes were fabricated using a dry-jet wet-spinning method, followed by chemical cross-linking to obtain membranes for PRO application. The best-performing membrane was subjected to fouling test in two orientations. Our results demonstrate that the AL-FS orientation is better for a sustainable long-term PRO operation, given the stable performance throughout the two-day testing. Also, the current use of power density per membrane area as a performance benchmark may not be practical for large-scale PRO operation, and the volumetric power density per module is more relevant. In terms of volumetric power density for a hypothetical 5-in. module, our best-performing membrane is predicted to generate a 3.6 to 10 times better power density than commercial modules, rendering our ISA hollow fiber membranes promising for scaling-up to bigger modules.

Membrane distillation & pressure retarded osmosis hybrid system using thermally rearranged nanofibrous membranes

To increase the efficiency of a closed-loop hybrid system consisting of membrane distillation (MD) and pressure retarded osmosis (PRO) processes, thermally rearranged nanofibrous membranes (TR-NFMs) via electrospinning were proposed in this study. Two fluorine-based modifications were performed on TR-NFMs to optimize their affinity with water. Hydrophobicity of TR-NFM was enhanced by atmospheric plasma coating (TR-NFM-Rx) to increase liquid entry pressure of water (LEPw) for MD applications. On the other hand, for PRO applications, gas-phase direct fluorination was applied on TR-NFM (TR-NFM-Fx) to improve its hydrophilicity by replacing the C–H bond of a TR polymer with a C–F bond. Before measuring the performance of the hybrid system, the MD performance was calculated from the mass transfer model based on the temperature and concentration of the solution to confirm the operating conditions of the MD-PRO hybrid system. The resulting TR-NFM-R40 showed an excellent water flux of 51.5 L/m2 hr at 70 °C despite a 3 M NaClaq operating solution that is unfavorable for MD performance. In the PRO process, thin film composite membrane (TR-TFC-F5) also exhibited outstanding power density of 120 W/m2 at 27 bar using 3 M NaClaq and D.I. water as operating and feed solutions, respectively. During 180 h continuous operation of MD-PRO hybrid system, the resulting average power density was about 90 W/m2 and average MD water flux was about 45–50 L/m2 hr at 70 °C using 3 M NaClaq operating solution. In conclusion, the developed TR membranes for MD-PRO hybrid system provides an insight into the future of energy harvesting technology.